**3. Results**

Where, Ceq is the liquid phase concentration of DCAU/DCAG; b is Langmuir constant; Qmax is maximum DCAU/DCAG uptake; K is constant; n is the number of metal reactive sites and Q

Samples of 1 g biosorbent loaded with target MxCN was eluted using desorbing agent (1-3 N NaOH) in concentrated form and analysed. Following the elution, biosorbent was washed with DW and then again conditioned to appropriate optimum pH to use in next adsorption/

Two types of effluents were procured from silver and gold plating industry. Both effluents were subjected to characterization using Standard methods (APHA-AWWA-WEF, 1998). The proximate analysis of the samples is shown in Table 8 and 9. Batch equilibration method was followed as mentioned earlier. Rice husk (0.1 g) and *Eichhornia* root (0.1 g) biomass was contacted with 10 ml of gold-cyanide and silver-cyanide effluents, respectively. Prior to sorption, the gold- and silver-cyanide effluents were adjusted to desired optimum biosorption pH. All the batch sorption experiments were carried out under optimum conditions as given in Table 1. After contact, the contents of the flasks were filtered and then analysed for residual metal (i.e. gold and silver) and cyanide. Appropriate controls were run simultaneously.

**Parameters For DCAU experiments For DCAG experiments**

*Eicchornia* **root biomass (L-cysteine treated)**

**(L-cysteine treated)**

**2.8. Continuous biosorption studies using fixed bed column at laboratory level**

Scale-up studies in fixed bed continuous mode at laboratory level for biosorption of gold- and silver-cyanide was carried out in two separate fabricated glass columns of height 44 cm, internal diameter 1.3 cm and filter media height being 30 cm. The total volume of the column

1 was filled with 21 g rick husk biomass pretreated with L-cysteine, while the glass column no. 2 was filled with 24 g *Eicchornia* root biomass also pretreated with L-cysteine. The target effluents were passed through the columns in upward direction in continuous mode at a flow

, while the working volume was 39.80 cm3 (figure not shown). Glass column no.

pH 4.0 6.0 Temperature (°C) 30 30 Biomass quantity (w/v) 1.0 2.0 Contact time (min) 60 60 Rotation speed (rpm) 150 150

is the specific metal uptake.

260 Applied Bioremediation - Active and Passive Approaches

desorption cycle.

was 58.37 cm3

**2.6. Adsorption/desorption of DCAU and DCAG**

**Biomass Rice husk**

**Table 1.** Optimum conditions used for biosorption experiments

**2.7. Biosorption of Au- and Ag-cyanide from industrial wastewaters**

#### **3.1. Screening of low-cost waste biomass for DCAU and DCAG sorption**

Data in Table 2 summarizes the results obtained for DCAU and DCAG sorption under optimal pH conditions. The results showed that optimum sorption in terms of Q (i.e. μmol MxCN sorbed per gram biomass) of 0.02 mM DCAU and 0.1 mM DCAG for most of the waste biomass/ sorbents tested were at pH 4.0 and 6.0, respectively. It was observed that biosorption of DCAU and DCAG was less above pH 7.0 for all the biomass tested. In acidic pH conditions, sorption of DCAU and DCAG increased significantly. The table also shows that other than activated charcoal (chosen as reference material) which showed highest biosorption capacity, biomass of Rice husk (3.65 μmol/g) and *Eichhornia* roots (3.56 μmol/g) were efficient biosorbents for DCAU sorption; while *Eichhornia* roots (4.76 μmol/g) and Tea powder waste (4.73 μmol/g) were efficient biosorbents for DCAG. The overall Q values observed for all the waste sorbents tested for DCAU and DCAG were in the range of 2.69 - 3.65 μmol/g and 2.74 - 4.76 μmol/g, respectively. The observed Q values for efficient biomass were found to be marginally below the Q values obtained for activated charcoal (3.80 - 5.00 μmol/g). As far the optimum pH for sorption was concerned, DCAU uptake was maximal at pH 4.0 for all the biomass tested, while DCAG uptake for majority of the biomass was at pH 5.0 to 6.0. There was no loss of DCAU or DCAG in the control flasks without sorbent during the tested time period.

Table 2 also shows the data on pH values of all unconditioned biomass. Other than the reference materials, the lowest pH observed was that of coconut fibers (pH 4.24), while the highest pH was of mixed algae biomass (pH 7.61). pH of unconditioned Rice husk, Tea powder waste and *Eichhornia* root biomass observed were 5.94, 4.94 and 7.01, respectively, while their optimum pH of biosorption was 4.0 (for DCAU biosorption), 5.0 - 9.0 (for DCAG biosorption) and 4.0 (for DCAU biosorption) and 7.0 – 9.0 (for DCAG biosorption).

**Sr. No. Biosorbent pH of unconditioned**

**(C) Municipal solid waste components**

**(D) Fungal and Bacterial waste/biomass**

**(F) Photosynthetic trees/plants waste**

**(E) Algae biomass**

**(G) Reference materials**

**biomass**

Resource Recovery from Industrial Effluents Containing Precious Metal Species Using Low-Cost Biomaterials…

6. Dairy waste sludge 6.88 2.91 (4.0) 4.71 (6.0) 7. Saw dust 5.59 3.52 (4.0) 4.64 (6.0) 8. Sugarcane Bagasse 5.92 3.16 (4.0) 4.03 (6.0) 9. Tea powder waste 4.94 2.94 (4.0) 4.73 (5.0-9.0)

10. Nirmalya (Waste flowers) 6.20 3.43 (4.0) 4.60 (6.0) 11. Compost 7.28 3.09 (4.0) 3.22 (6.0) 12. Vegetable waste 6.77 2.91 (4.0) 3.88 (6.0)

13. *Ganoderma* sp. 6.04 3.01 (4.0) 4.07 (6.0) 14. Yeast biomass 4.39 2.69 (4.0) 3.06 (5.0) 15. *Mucor heimalis* 4.45 1.97 (4.0) 2.29 (6.0) 16. *Penicillium* waste 4.26 3.08 (4.0) 3.99 (6.0) 17. *Streptomyces* waste 4.86 3.00 (4.0) 2.78 (6.0) 18. *Streptoverticillium* waste 4.67 2.77 (4.0) 3.52 (6.0) 19. Wood rotting fungi 6.04 3.18 (4.0) 4.21 (6.0) 20. Bacterial consortium 6.83 3.12 (4.0) 4.00 (6.0)

21. Mixed algae biomass 7.61 3.29 (4.0) 4.16 (6.0)

22. *Parthenium* sp. 6.69 3.07 (4.0) 4.22 (6.0) 23. *Eichhornia* leaves 5.57 3.20 (4.0) 4.62 (6.0) 24. *Eichhornia* roots 7.01 3.56 (4.0) 4.76 (7.0-9.0) 25. *Eichhornia* stem 5.58 3.38 (4.0) 4.66 (6.0) 26. Runners 6.52 3.06 (4.0) 4.67 (6.0) 27. *Tectona grandis* leaves 5.40 3.42 (4.0) 4.63 (6.0) 28. *Lantana camara* leaves 6.59 2.98 (4.0) 2.74 (5.0)

29. Activated charcoal 5.59 3.80 (4.0) 5.00 (6.0)

**Q (µmol MxCN sorbed per gram biomass)**

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263

**DCAU DCAG**

On the basis of maximum DCAU/DCAG uptake values obtained under optimum pH condi‐ tions, Rice husk and *Eichhornia* root biomass were selected for DCAU sorption, while *Eichhor‐ nia* root and Tea powder waste biomass were selected for DCAG sorption for further experiments. Activated charcoal acted as a reference material.

#### **3.2. Influence of temperature on biosorption of DCAU and DCAG**

It was observed that biosorption of DCAU and DCAG by the selected biomass did not had any significant impact with the change in temperature of the system from 5-45°C.

#### **3.3. DCAU and DCAG loading capacity**

Table 3 and 4 depicts the data on DCAU and DCAG loading capacity of pre-conditioned (at pH 4.0 for DCAU and pH 5.0-7.0 for DCAG) biosorbents selected on the basis of maximum sorption under optimum pH, as described earlier. Also the results were compared with the unconditioned biomass (i.e. the original pH of the biomass itself). It could be seen that Rice husk biomass had the maximum loading capacity for DCAU (7.63 μmol/g) sorption among the two tested biomass; and was followed by *Eichhornia* root biomass (7.04 μmol/g). It was also observed that the loading capacity of activated charcoal was found relatively lower when compared with the Rice husk. While the loading capacity of unconditioned biomass dropped by 7.6% and 43% for Rice husk and *Eichhornia* root biomass, respectively.

In case of DCAG, *Eichhornia* root biomass showed highest loading capacity (9.74 μmol/g) followed by Tea powder waste (9.41 μmol/g). Loading capacity values of *Eichhornia* root biomass was highly competitive and comparable with activated charcoal (9.95 μmol/g), which was used as reference material. Furthermore, the loading capacity of unconditioned biomass was not affected when compared with the conditioned biomass (Table 4).


optimum pH of biosorption was 4.0 (for DCAU biosorption), 5.0 - 9.0 (for DCAG biosorption)

On the basis of maximum DCAU/DCAG uptake values obtained under optimum pH condi‐ tions, Rice husk and *Eichhornia* root biomass were selected for DCAU sorption, while *Eichhor‐ nia* root and Tea powder waste biomass were selected for DCAG sorption for further

It was observed that biosorption of DCAU and DCAG by the selected biomass did not had

Table 3 and 4 depicts the data on DCAU and DCAG loading capacity of pre-conditioned (at pH 4.0 for DCAU and pH 5.0-7.0 for DCAG) biosorbents selected on the basis of maximum sorption under optimum pH, as described earlier. Also the results were compared with the unconditioned biomass (i.e. the original pH of the biomass itself). It could be seen that Rice husk biomass had the maximum loading capacity for DCAU (7.63 μmol/g) sorption among the two tested biomass; and was followed by *Eichhornia* root biomass (7.04 μmol/g). It was also observed that the loading capacity of activated charcoal was found relatively lower when compared with the Rice husk. While the loading capacity of unconditioned biomass dropped

In case of DCAG, *Eichhornia* root biomass showed highest loading capacity (9.74 μmol/g) followed by Tea powder waste (9.41 μmol/g). Loading capacity values of *Eichhornia* root biomass was highly competitive and comparable with activated charcoal (9.95 μmol/g), which was used as reference material. Furthermore, the loading capacity of unconditioned biomass

**biomass**

1. Coconut fibers 4.24 3.32 (4.0)\* 4.62 (5.0)\* 2. Cow dung cakes 7.73 3.07 (4.0) 4.64 (6.0) 3. Groundnut shells 5.49 3.22 (4.0) 4.62 (6.0) 4. Rice husk 5.94 3.65 (4.0) 4.68 (6.0) 5. Rice straw 6.13 3.25 (4.0) 4.69 (6.0)

**Q (µmol MxCN sorbed per gram biomass)**

**DCAU DCAG**

any significant impact with the change in temperature of the system from 5-45°C.

and 4.0 (for DCAU biosorption) and 7.0 – 9.0 (for DCAG biosorption).

experiments. Activated charcoal acted as a reference material.

**3.3. DCAU and DCAG loading capacity**

262 Applied Bioremediation - Active and Passive Approaches

**3.2. Influence of temperature on biosorption of DCAU and DCAG**

by 7.6% and 43% for Rice husk and *Eichhornia* root biomass, respectively.

was not affected when compared with the conditioned biomass (Table 4).

**Sr. No. Biosorbent pH of unconditioned**

**(A) Agricultural waste/by-products**

**(B) Industrial waste/by-products**



**3.4. Influence of biosorbent quantity**

**3.5. Rate of DCAU and DCAG uptake**

**3.6. Adsorption isotherm models**

**3.7. Equilibrium models to fit experimental data**

models with the regression value >0.98.

The effect of biomass quantity (% w/v) on DCAU and DCAG biosorption was studied at optimal pH values. Varying amount of biomass ranging from 0.1 to 5.0 g were used keeping the volume of both the metal-cyanides (MxCNs) solution constant (10 ml); thereby giving the solid-to-liquid ratio in the range of 0.01 to 0.5. The results showed that the biomass quantity increased the % biosorption of both DCAU and DCAG also increased. Maximum uptake in terms of Q (3.84 μmol/g) was observed at 1% (w/v) of Rice husk biomass for DCAU sorption. However, from 1 to 5 % (w/v) there was no significant increase. In case of DCAG sorption, *Eichhornia* root biomass showed highest Q for biomass quantity from 2.0 to 5.0% (w/v).

Resource Recovery from Industrial Effluents Containing Precious Metal Species Using Low-Cost Biomaterials…

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265

The effect of contact time on DCAU and DCAG biosorption was studied at their optimum pH (pH 4.0 and pH 6.0 for DCAU and DCAG sorption, respectively), temperature (30°C) and biomass quantity of 1% (w/v) and 2% (w/v) for DCAU and DCAG, respectively. 10 ml of precious MxCN solution having concentration 0.02 mM (in case of DCAU) and 0.1 mM (in case of DCAG) was contacted with respective biomass (Rice husk and *Eichhornia* root biomass for DCAU and DCAG, respectively) for the period upto 180 min. The time intervals chosen for study were 0 to 180 minutes. Periodically the flask contents were removed by filtration and

It was observed that rate of both the MxCN uptake was maximum in the 15-20 minutes, with over 80% of biosorption. Later, the sorption rate slowed down until it reached a plateau after 30-40 min, indicating equilibration of the system. Maximum sorption of both the precious

The effect of initial concentration provides an important driving force to overcome all mass transfer resistance of target inorganic ion between the aqueous and solid phases. The biosorp‐ tion of both DCAU and DCAG were carried out at different initial concentrations ranging from 0.01 mM to 1.0 mM (corresponding to approximately 10 to 1000 μmol) at pH 4.0 and 6.0 using 1 and 2% (w/v) of Rice husk and *Eichhornia* root biomass, respectively. It was found that the equilibrium sorption capacity of the sorbent increased with increasing initial concentration of MxCNs from 0.01 mM to 1.0 mM, due to the increase in the number of ions competing for the available binding sites in the biomass. The uptake of MxCNs approached towards plateau

above 0.5 mM. There was a significant increase in the specific uptake of both MxCNs

To examine the relationship between sorption isotherm models are widely employed for fitting the data. Langmuir and Freundlich were used to describe the equilibrium between the two MxCNs sorbed on Rice husk and *Eichhornia* root biomass and MxCNs in solution. Data obtained show that MxCNs uptake values could be well fitted to the Langmuir and Freundlich isotherm

MxCNs was observed at 40 min (88.2% for DCAU and 94.3% in case of DCAG).

the filtrates were analyzed for Au, Ag and cyanide concentration.

All the values in table are average of two readings; \*Values in parentheses indicates optimum pH (Gaddi and Patil, 2011; Patil, 2012)

**Table 2.** Biosorption of DCAG and DCAU at optimum pH


All the values presented in table are average of two readings

**Table 3.** DCAU loading capacity of selected biosorbents


All the values presented in table are average of two readings

**Table 4.** DCAG loading capacity of selected biosorbents

Considering the above results, selection of the biosorbent was further narrowed down to Rice husk and *Eichhornia* root biomass for DCAU and DCAG biosorption, respectively, (using conditioned biomass) for further experiments.

### **3.4. Influence of biosorbent quantity**

**Sr. No. Biosorbent pH of unconditioned**

264 Applied Bioremediation - Active and Passive Approaches

**Table 2.** Biosorption of DCAG and DCAU at optimum pH

All the values presented in table are average of two readings

All the values presented in table are average of two readings

**Table 4.** DCAG loading capacity of selected biosorbents

conditioned biomass) for further experiments.

**Sorbent / Biosorbent Loading capacity (µmol/g of biomass)**

**Conditioned biomass (at optimal pH)**

**Table 3.** DCAU loading capacity of selected biosorbents

**Sorbent / Biosorbent Loading capacity (µmol/g of biomass)**

**Conditioned biomass (at optimal pH)**

2011; Patil, 2012)

**biomass**

30. Bagasse Fly ash 8.75 3.40 (4.0) 4.70 (6.0)

Control (without biomass) - 0 (2.0) 0 (6.0)

All the values in table are average of two readings; \*Values in parentheses indicates optimum pH (Gaddi and Patil,

Rice husk 7.63 (4.0) 7.05 (5.94) Moderately affected *Eichhornia* roots 7.04 (4.0) 4.38 (7.01) Significantly affected Activated charcoal 7.61 (4.0) 7.58 (5.59) Not affected

*Eichhornia* roots 9.74 (7.0) 9.77 (7.01) Not affected Tea powder waste 9.41 (5.0) 9.40 (4.94) Not affected Activated charcoal 9.95 (6.0) 9.94 (5.59) Not affected

Considering the above results, selection of the biosorbent was further narrowed down to Rice husk and *Eichhornia* root biomass for DCAU and DCAG biosorption, respectively, (using

**Unconditioned biomass (at original biomass pH)**

**Unconditioned biomass (at original biomass pH)**

**Q (µmol MxCN sorbed per gram biomass)**

**Remarks**

**Remarks**

**DCAU DCAG**

The effect of biomass quantity (% w/v) on DCAU and DCAG biosorption was studied at optimal pH values. Varying amount of biomass ranging from 0.1 to 5.0 g were used keeping the volume of both the metal-cyanides (MxCNs) solution constant (10 ml); thereby giving the solid-to-liquid ratio in the range of 0.01 to 0.5. The results showed that the biomass quantity increased the % biosorption of both DCAU and DCAG also increased. Maximum uptake in terms of Q (3.84 μmol/g) was observed at 1% (w/v) of Rice husk biomass for DCAU sorption. However, from 1 to 5 % (w/v) there was no significant increase. In case of DCAG sorption, *Eichhornia* root biomass showed highest Q for biomass quantity from 2.0 to 5.0% (w/v).

#### **3.5. Rate of DCAU and DCAG uptake**

The effect of contact time on DCAU and DCAG biosorption was studied at their optimum pH (pH 4.0 and pH 6.0 for DCAU and DCAG sorption, respectively), temperature (30°C) and biomass quantity of 1% (w/v) and 2% (w/v) for DCAU and DCAG, respectively. 10 ml of precious MxCN solution having concentration 0.02 mM (in case of DCAU) and 0.1 mM (in case of DCAG) was contacted with respective biomass (Rice husk and *Eichhornia* root biomass for DCAU and DCAG, respectively) for the period upto 180 min. The time intervals chosen for study were 0 to 180 minutes. Periodically the flask contents were removed by filtration and the filtrates were analyzed for Au, Ag and cyanide concentration.

It was observed that rate of both the MxCN uptake was maximum in the 15-20 minutes, with over 80% of biosorption. Later, the sorption rate slowed down until it reached a plateau after 30-40 min, indicating equilibration of the system. Maximum sorption of both the precious MxCNs was observed at 40 min (88.2% for DCAU and 94.3% in case of DCAG).

#### **3.6. Adsorption isotherm models**

The effect of initial concentration provides an important driving force to overcome all mass transfer resistance of target inorganic ion between the aqueous and solid phases. The biosorp‐ tion of both DCAU and DCAG were carried out at different initial concentrations ranging from 0.01 mM to 1.0 mM (corresponding to approximately 10 to 1000 μmol) at pH 4.0 and 6.0 using 1 and 2% (w/v) of Rice husk and *Eichhornia* root biomass, respectively. It was found that the equilibrium sorption capacity of the sorbent increased with increasing initial concentration of MxCNs from 0.01 mM to 1.0 mM, due to the increase in the number of ions competing for the available binding sites in the biomass. The uptake of MxCNs approached towards plateau above 0.5 mM. There was a significant increase in the specific uptake of both MxCNs

### **3.7. Equilibrium models to fit experimental data**

To examine the relationship between sorption isotherm models are widely employed for fitting the data. Langmuir and Freundlich were used to describe the equilibrium between the two MxCNs sorbed on Rice husk and *Eichhornia* root biomass and MxCNs in solution. Data obtained show that MxCNs uptake values could be well fitted to the Langmuir and Freundlich isotherm models with the regression value >0.98.

#### **3.8. Influence of cationic and anionic moieties on DCAU and DCAG sorption**

It was observed that biosorption of both the metal-cyanides were not significantly affected by the presence of various metal cations and anions in majority of the cases. Biosorption of DCAU was affected by zinc, chromium and cadmium up to certain extent (33-40%). In case of DCAG, biosorption was affected significantly by the presence of cadmium, zinc, iron and chromium (37-67%). Biosorption in the presence of other metals cations (copper, nickel and silver) and anions (phosphates, sulphates and chlorides) was consistently above 80%.

**DCAU + Chemicals used for pre-treatment % DCAG**

Resource Recovery from Industrial Effluents Containing Precious Metal Species Using Low-Cost Biomaterials…

DCAU without sorbent (control) 0 0 DCAU + *Eichhornia* root without pretreatment 94.0 100 DCAU + *Eichhornia* root (treated with boiled water) 83.0 88.3 DCAU + *Eichhornia* root (treated with 1% L-cysteine) 100 106.4 DCAU + *Eichhornia* root (treated with 1 N NaOH) 2.5 0.5 DCAU + *Eichhornia* root (treated with 1 N Formaldehyde) 89.4 95.1 DCAU + *Eichhornia* root (treated with acid i.e. HCl) 81.7 86.9 DCAU + *Eichhornia* root (treated with acetate) 64.9 69.0 DCAU + *Eichhornia* root (treated with methanol) 34.5 36.7 DCAU + *Eichhornia* root (treated with acetic anhydride) 56.6 60.2 DCAU + *Eichhornia* root (treated with acetone) 59.0 62.7

**Biosorbent Loading capacity (µmol/g of biomass)**

Rice husk (for DCAU biosorption) 7.60 (100%) 13.34 (175%) *Eichhornia* roots (for DCAG biosorption) 9.72 (100%) 13.62 (140%)

The loaded DCAU and DCAG on Rice husk and *Eichhornia* root biomass, respectively, could be desorbed with more than 97% efficiency using 1 N sodium hydroxide solution. Final concentrations of MxCNs in the eluent were 28-30 folds of initial concentration of DCAU and 22-25 fold of the initial concentration of DCAG. However, during the second cycle of MxCN

The gold-cyanide and silver-cyanide from the effluents procured from the industries could be effectively biosorbed/treated by Rice husk and *Eichhornia* root biomass which were pretreated with L-cysteine. Table 8 and 9 depicts the data on gold-cyanide and silver-cyanide before and after biosorption along with their percentage removal. Gold and cyanide removal efficiency from gold-cyanide effluent was 91.53% and 82.69%, respectively. However, the cyanide content in the treated effluent after biosorption although very less (0.59 and 0.78 mg/l for Auand Ag-cyanide effluents, respectively) but was not complying with the prescribed Indian

adsorption, the loading capacity of the biosorbent decreased by 10-15%.

**3.10. Biosorption of DCAU and DCAG from industrial wastewaters**

**Table 6.** Impact of pretreatment on DCAG biosorption by *Eichhornia* root biomass

All the values presented in table are average of two readings

**Table 7.** Loading capacity of untreated and L-cysteine treated biomass

**3.9. Adsorption-desorption of DCAU and DCAG**

**biosorption**

**Biomass without pretreatment L-cysteine treated biomass**

**Relative % biosorption** 267

http://dx.doi.org/10.5772/56965

When the low-cost biomass was pre-treated with different chemicals, it was found that (Table 5 and 6) there was greater degree of variation in the biosorption of DCAU and DCAG using Rice husk and *Eichhornia* root biomass, respectively. Rice husk and *Eichhornia* root biomass treated with 1% L-cysteine enhanced the biosorption capacity of both the MxCNs. In contrast, the NaOH pretreated biomass significantly hampered the biosorption process. It was 0% in case of DCAU sorption and 2.5% in case of DCAG biosorption.


**Table 5.** Impact of pretreatment on DCAU biosorption by Rice husk

The experiment on pretreatment of biomass with L-cysteine clearly showed enhanced biosorption of DCAU and DCAG from solutions. It was therefore thought worthwhile to find out the loading capacity of both the biomass pretreated with L-cysteine. Experiment on loading capacity of Rice husk and *Eichhornia* root biomass was performed as mentioned earlier (section 2.4). It could be seen from Table 7 that the loading capacity of Rice husk and *Eichhornia* root biomass enhanced the biosorption of DCAU and DCAG up to 175% and 140%, respectively compared to untreated biomass (i.e. in absence of L-cysteine loaded biomass).


**Table 6.** Impact of pretreatment on DCAG biosorption by *Eichhornia* root biomass

**3.8. Influence of cationic and anionic moieties on DCAU and DCAG sorption**

266 Applied Bioremediation - Active and Passive Approaches

anions (phosphates, sulphates and chlorides) was consistently above 80%.

case of DCAU sorption and 2.5% in case of DCAG biosorption.

**Table 5.** Impact of pretreatment on DCAU biosorption by Rice husk

It was observed that biosorption of both the metal-cyanides were not significantly affected by the presence of various metal cations and anions in majority of the cases. Biosorption of DCAU was affected by zinc, chromium and cadmium up to certain extent (33-40%). In case of DCAG, biosorption was affected significantly by the presence of cadmium, zinc, iron and chromium (37-67%). Biosorption in the presence of other metals cations (copper, nickel and silver) and

When the low-cost biomass was pre-treated with different chemicals, it was found that (Table 5 and 6) there was greater degree of variation in the biosorption of DCAU and DCAG using Rice husk and *Eichhornia* root biomass, respectively. Rice husk and *Eichhornia* root biomass treated with 1% L-cysteine enhanced the biosorption capacity of both the MxCNs. In contrast, the NaOH pretreated biomass significantly hampered the biosorption process. It was 0% in

**DCAU + Chemicals used for pre-treatment % DCAU**

DCAU without sorbent (control) 0 0 DCAU + Rice husk without pretreatment 90.1 100 DCAU + Rice husk (treated with boiled water) 86.5 96.0 DCAU + Rice husk (treated with 1% L-cysteine) 100 110.9 DCAU + Rice husk (treated with 1 N NaOH) 0 0 DCAU + Rice husk (treated with 1 N Formaldehyde) 87.2 96.7 DCAU + Rice husk (treated with acid i.e. HCl) 79.0 87.7 DCAU + Rice husk (treated with acetate) 73.6 81.7 DCAU + Rice husk (treated with methanol) 27.1 30.1 DCAU + Rice husk (treated with acetic anhydride) 49.2 54.6 DCAU + Rice husk (treated with acetone) 53.7 59.6

The experiment on pretreatment of biomass with L-cysteine clearly showed enhanced biosorption of DCAU and DCAG from solutions. It was therefore thought worthwhile to find out the loading capacity of both the biomass pretreated with L-cysteine. Experiment on loading capacity of Rice husk and *Eichhornia* root biomass was performed as mentioned earlier (section 2.4). It could be seen from Table 7 that the loading capacity of Rice husk and *Eichhornia* root biomass enhanced the biosorption of DCAU and DCAG up to 175% and 140%, respectively

compared to untreated biomass (i.e. in absence of L-cysteine loaded biomass).

**biosorption**

**Relative % biosorption**


**Table 7.** Loading capacity of untreated and L-cysteine treated biomass

#### **3.9. Adsorption-desorption of DCAU and DCAG**

The loaded DCAU and DCAG on Rice husk and *Eichhornia* root biomass, respectively, could be desorbed with more than 97% efficiency using 1 N sodium hydroxide solution. Final concentrations of MxCNs in the eluent were 28-30 folds of initial concentration of DCAU and 22-25 fold of the initial concentration of DCAG. However, during the second cycle of MxCN adsorption, the loading capacity of the biosorbent decreased by 10-15%.

#### **3.10. Biosorption of DCAU and DCAG from industrial wastewaters**

The gold-cyanide and silver-cyanide from the effluents procured from the industries could be effectively biosorbed/treated by Rice husk and *Eichhornia* root biomass which were pretreated with L-cysteine. Table 8 and 9 depicts the data on gold-cyanide and silver-cyanide before and after biosorption along with their percentage removal. Gold and cyanide removal efficiency from gold-cyanide effluent was 91.53% and 82.69%, respectively. However, the cyanide content in the treated effluent after biosorption although very less (0.59 and 0.78 mg/l for Auand Ag-cyanide effluents, respectively) but was not complying with the prescribed Indian Sstandards (0.2 mg/l). Overall, the results indicated that both Rice husk and *Eichhornia* root biomass were very effective in treating the effluents by biosorption process.


All the figures given in the table are in mg/l, except pH; BDL: Below detectable limits

**Table 8.** Biosorption of gold-cyanide from industrial effluent in batch mode using rice husk pretreated with L-cysteine

**Parameter Biodegradation \*BIS Standards % Removal**

**Table 9.** Biosorption of silver-cyanide from industrial effluent in batch mode using *Eichhornia* root biomass pretreated

All the figures given in the table are in mg/l, except pH; BDL: Below detectable limits

**Physicochemical parameters Before biosorption After biosorption % Removal efficiency**

Resource Recovery from Industrial Effluents Containing Precious Metal Species Using Low-Cost Biomaterials…

Color Colorless Colorless - Turbidity slightly turbid Clear pH 7.16 6.44 - Total cyanide 5.02 0.78 85.0 Gold - - - Silver 7.29 0.98 86.55 Copper 1.56 0.31 80.12 Nickel BDL - - Zinc 0.92 0.27 70.65 Iron 0.18 BDL 100.0 Phosphates 117.5 115.0 2.12 Sulfates 94.1 95.2 0 Chlorides 199.6 193.3 3.15 Chemical oxygen demand (COD) 47 29 38.29

pH 6.99-7.04 7.05-7.12 5.5 – 9.0 - Total cyanide 0.59 mg/l 0.04 0.2 93.22% Chemical oxygen demand (COD) 102 mg/l 23 mg/l 250 77.45% Gold 0.11 BDL NA -

pH 6.95-7.03 7.07-7-11 5.5 – 9.0 - Total cyanide 0.78 mg/l 0.05 0.2 93.58% Chemical oxygen demand (COD) 98 mg/l 19 mg/l 250 80.61% Silver 0.98 BDL NA -

**Table 10.** Treatment of residual gold-cyanide and silver-cyanide by a cyanide and thiocyanate degrading

\*BIS- Bureau of Indian Standards; BDL-Below Detectable Limits; NA-Not Available; All the values presented in table are

**Gold-cyanide**

with L-cysteine

**Silver-cyanide**

average of two readings

heterotrophic bacterial consortiumg

**Before After**

**Efficiency**

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269

In order to treat the residual (unrecoverable) cyanide remaining in the solutions after biosorp‐ tion were subjected to biodegradation process using bacterial consortium under optimized conditions in further experiments.

#### **3.11. Biodegradation of unrecoverable (residual) metal-cyanides**

Typical residual concentrations of gold and cyanide in gold-cyanide effluent after biosorption were 0.11 and 0.59 mg/l, respectively. Similarly, the residual silver and cyanide concentration in the silver-cyanide effluent were 0.98 and 0.78 mg/l, respectively. When these solutions were subjected to biodegradation using bacterial consortium under optimum conditions as men‐ tioned earlier, it was observed that the consortium could degrade the said cyanide from both effluents with an efficiency exceeding 90% within a period of 3-4 h. pH, cyanide and chemical and oxygen demand (COD) of the treated effluent were within the permissible limits prescri‐ bed by statutory agencies in India (Table 10). Percent cyanide removal efficiency was >90% for both types of effluents. Gold and silver metals were not detected in bacterial free treated solutions. Findings indicated that biodegradation could be used as a polishing step in the treatment of MxCNs containing wastewaters.

Resource Recovery from Industrial Effluents Containing Precious Metal Species Using Low-Cost Biomaterials… http://dx.doi.org/10.5772/56965 269


All the figures given in the table are in mg/l, except pH; BDL: Below detectable limits

Sstandards (0.2 mg/l). Overall, the results indicated that both Rice husk and *Eichhornia* root

**Physicochemical parameters Before biosorption After biosorption % Removal efficiency**

**Table 8.** Biosorption of gold-cyanide from industrial effluent in batch mode using rice husk pretreated with L-cysteine

In order to treat the residual (unrecoverable) cyanide remaining in the solutions after biosorp‐ tion were subjected to biodegradation process using bacterial consortium under optimized

Typical residual concentrations of gold and cyanide in gold-cyanide effluent after biosorption were 0.11 and 0.59 mg/l, respectively. Similarly, the residual silver and cyanide concentration in the silver-cyanide effluent were 0.98 and 0.78 mg/l, respectively. When these solutions were subjected to biodegradation using bacterial consortium under optimum conditions as men‐ tioned earlier, it was observed that the consortium could degrade the said cyanide from both effluents with an efficiency exceeding 90% within a period of 3-4 h. pH, cyanide and chemical and oxygen demand (COD) of the treated effluent were within the permissible limits prescri‐ bed by statutory agencies in India (Table 10). Percent cyanide removal efficiency was >90% for both types of effluents. Gold and silver metals were not detected in bacterial free treated solutions. Findings indicated that biodegradation could be used as a polishing step in the

biomass were very effective in treating the effluents by biosorption process.

268 Applied Bioremediation - Active and Passive Approaches

All the figures given in the table are in mg/l, except pH; BDL: Below detectable limits

**3.11. Biodegradation of unrecoverable (residual) metal-cyanides**

conditions in further experiments.

treatment of MxCNs containing wastewaters.

Color Colorless Colorless - Turbidity Clear Clear pH 6.87 4.12 - Total cyanide 3.41 0.59 82.69 Gold 1.30 0.11 91.53 Silver 0.48 0.03 93.75 Copper 0.95 0.18 81.05 Nickel BDL - - Zinc 0.50 0.10 80.00 Iron 0.11 BDL 100.00 Phosphates 97.9 76.1 22.26 Sulfates 63.5 61.3 3.46 Chlorides 173.0 155.2 10.28 Chemical oxygen demand (COD) 42 31 26.19

**Table 9.** Biosorption of silver-cyanide from industrial effluent in batch mode using *Eichhornia* root biomass pretreated with L-cysteine


\*BIS- Bureau of Indian Standards; BDL-Below Detectable Limits; NA-Not Available; All the values presented in table are average of two readings

**Table 10.** Treatment of residual gold-cyanide and silver-cyanide by a cyanide and thiocyanate degrading heterotrophic bacterial consortiumg

#### **3.12. Biosorption of gold- and silver-cyanide effluent in packed bed column**

Biosorption studies on gold-cyanide and silver-cyanide effluents were performed in continu‐ ous mode in two separate packed bed glass columns consisting of Rice husk (column 1) and *Eichhornia* root biomass (column 2), respectively. Biosorption results showed that break‐ through point observed for gold and cyanide in column 1 was 60 h, while the breakthrough time observed for silver and cyanide in column 2 was 40 h (figures not shown). The total effluent passed through the column 1 and 2 was equivalent to 50 and 34 bed volumes, respectively. Column 1 and 2 got completely saturated after 90 and 70 h, respectively.

In the light of above, the present research work was focused to study removal and recovery of Au- and Ag-cyanide from effluents using low-cost biosorbents (using waste biomass from various sources); followed by biodegradation (using active bacterial consortium). It was contemplated that if an efficient process for removal and recovery could be developed, then precious MxCN or metals could be conserved, which according to the project investigators

Resource Recovery from Industrial Effluents Containing Precious Metal Species Using Low-Cost Biomaterials…

It is well-known that certain type of microbial or waste biomass has high degree of competency to adsorb heavy metals. This sorption is solely due to the chemical composition of biomass (Volesky, 2003). With biosorption applications in mind it makes sense to screen variety of biomass types that are readily available in large quantities. There are basically two types of biomass sources that can practically be considered with low costs and availability in mind. First, the industrial waste biomass generated as a by-product of large scale (for example fermentation industry) with virtually no uses for it and disposal is a problem. Secondly, the biomass generated in large quantities from water environment (for examples unwanted plants like *Eichhornia* sp. and algae). It can be easily collected or harvested as raw material for biosorbents. Also, there are many other sources from where low-cost biomass could be procured especially in a developing country like India. These include the vegetable waste, yard wastes, waste flowers and coconut fibres from temples, etc. Energy generation potential from biomass and MSW have been reported by Saini et al (2012). In order to find the right biosorbent candidate, it is imperative to screen variety of biomass occurring in human environment.

Free cyanide and thiocyanate, metal-cyanides and metal-thiocyanates can occur in the wastewaters in various forms depending upon the chemical nature of the compounds and the concentration of metal, cyanide and thiocyanate, provided if metal moiety is bound to cyanide and thiocyanate. For example, free cyanide and thiocyanate can occur in the waters in its

copper-thiocyanate complex occur in waste waters in various forms like Cu(SCN)2

their recovery by adsorption on low-cost biomass procured from various places. Activated

It is well known that the process of biosorption is regulated by aqueous solution pH (Puranik and Paknikar, 1997). The first step in present study was therefore determination of optimum solution pH for biosorption of gold-cyanide and silver-cyanide. It was found that biosorption of both the MxCNs (for all the low-cost biomass) increased with pH and then declined rapidly with further increase in pH. As seen from the Table 2 that maximum sorption of DCAU was at pH 4.0 while DCAG sorption by most of the biomass was at pH 6.0. Sorption decreased in the alkaline pH. It was found that other than activated charcoal (which was used as reference material) biomass like Rice husk & *Eichhornia* roots and *Eichhornia* roots and Tea powder waste were efficient biosorbents for DCAU and DCAG sorption. There was no auto-oxidation loss of both the MxCNs in controls without sorbent confirming that (bio)sorption is the only mechanism by which MxCNs are being removed from solution. In the previous study carried out by Niu and Volesky (2000) found that the maximum adsorption of DCAU by biomass was

charcoal, a conventional material, was used for obtaining comparative data.

, respectively. While metal-cyanide like for example - copper-

2-, Cu(CN)4

2-, etc. Similarly,

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271


, Cu(CN)3


2-, etc. Therefore, it was thought worthwhile to explore the possibility of

opinion would be an innovative approach of resource recovery.

anionic form like CN-

2-, Cu(SCN)4

Cu(SCN)3

and SCN-

cyanide occur in water in various forms such as Cu(CN)2

#### **4. Discussion**

Review of literature show that biosorption of heavy metal cations from aqueous solutions have been studied widely (Paknikar *et al*., 2003). Studies have also been carried out on biosorption of anionic metal species like chromates (Basha *et al*., 2008; Itankar and Patil, 2012), free cyanide (Azab *et al*., 1995) and metal-cyanides (Patil and Paknikar, 1999a) using microbial biomass, especially the waste fungal biomass obtained from fermentation industry and laboratory cultivated biomass. In contrast, biodetoxification of metal-cyanides and thiocyanate using live bacterial consortium was also studied (Patil, 2006; Patil, 2008a; Patil, 2008b; Patil, 2011; Patil and Paknikar, 2000; Patil and Paknikar, 2001). Safety aspects of cyanide use in mining indus‐ tries have been well emphasized by Patil and Kulkarni (2008). Prashanth and Patil (2007) have also studied the impact of free cyanide on edible fish *Catla catla*.

Another important and precious chemical species that are normally encountered in the industrial effluents emanating from mining, electroplating, printing circuit board manufac‐ turing, photography units, etc. are gold-cyanide and silver-cyanide. These species are active and important members of cyano-group chemicals that occur in water environment. Some research has been carried out on the removal of gold-cyanide and silver-cyanide species by biodegradation/biodetoxification method (Karavaiko et al. 2000; Kiruthika and Shrinithya, 2008). However, very little information is available on the removal and recovery of goldcyanide and silver-cyanide from high volume low tenor effluents (Gaddi and Patil, 2011). Much work has been restricted to the removal of metal-cyanides and thiocyanates using anion exchange resins and activated charcoal (Kononova *et al.,* 2007) and polyurethane foam (Hasany, 2001). Some papers on removal of free thiocyanate and metal-thiocyanate have also been published using low-cost materials (Namasivayam, 2007; Thakur and Patil, 2009). Overall literature survey shows that very little work has been carried out on the removal of anionic species like Au-cyanide i.e. [Au(CN)2] - (Dicyanoaurte-DCAU) (Niu and Volesky, 2000) and Ag-cyanide i.e. [Ag(CN)2] - (Dicyanoargentate-DCAG) (Gaddi and Patil, 2011) from waste solutions using low-cost materials emanated either by natural or manmade activities. Since all the cyano-group chemicals like free cyanide and thiocyanate, metal cyanides and metal thiocyanate are toxic to all classes of living cells their removal and recovery from waste prior to discharge in environment is the key.

In the light of above, the present research work was focused to study removal and recovery of Au- and Ag-cyanide from effluents using low-cost biosorbents (using waste biomass from various sources); followed by biodegradation (using active bacterial consortium). It was contemplated that if an efficient process for removal and recovery could be developed, then precious MxCN or metals could be conserved, which according to the project investigators opinion would be an innovative approach of resource recovery.

**3.12. Biosorption of gold- and silver-cyanide effluent in packed bed column**

Biosorption studies on gold-cyanide and silver-cyanide effluents were performed in continu‐ ous mode in two separate packed bed glass columns consisting of Rice husk (column 1) and *Eichhornia* root biomass (column 2), respectively. Biosorption results showed that break‐ through point observed for gold and cyanide in column 1 was 60 h, while the breakthrough time observed for silver and cyanide in column 2 was 40 h (figures not shown). The total effluent passed through the column 1 and 2 was equivalent to 50 and 34 bed volumes,

respectively. Column 1 and 2 got completely saturated after 90 and 70 h, respectively.

Review of literature show that biosorption of heavy metal cations from aqueous solutions have been studied widely (Paknikar *et al*., 2003). Studies have also been carried out on biosorption of anionic metal species like chromates (Basha *et al*., 2008; Itankar and Patil, 2012), free cyanide (Azab *et al*., 1995) and metal-cyanides (Patil and Paknikar, 1999a) using microbial biomass, especially the waste fungal biomass obtained from fermentation industry and laboratory cultivated biomass. In contrast, biodetoxification of metal-cyanides and thiocyanate using live bacterial consortium was also studied (Patil, 2006; Patil, 2008a; Patil, 2008b; Patil, 2011; Patil and Paknikar, 2000; Patil and Paknikar, 2001). Safety aspects of cyanide use in mining indus‐ tries have been well emphasized by Patil and Kulkarni (2008). Prashanth and Patil (2007) have

Another important and precious chemical species that are normally encountered in the industrial effluents emanating from mining, electroplating, printing circuit board manufac‐ turing, photography units, etc. are gold-cyanide and silver-cyanide. These species are active and important members of cyano-group chemicals that occur in water environment. Some research has been carried out on the removal of gold-cyanide and silver-cyanide species by biodegradation/biodetoxification method (Karavaiko et al. 2000; Kiruthika and Shrinithya, 2008). However, very little information is available on the removal and recovery of goldcyanide and silver-cyanide from high volume low tenor effluents (Gaddi and Patil, 2011). Much work has been restricted to the removal of metal-cyanides and thiocyanates using anion exchange resins and activated charcoal (Kononova *et al.,* 2007) and polyurethane foam (Hasany, 2001). Some papers on removal of free thiocyanate and metal-thiocyanate have also been published using low-cost materials (Namasivayam, 2007; Thakur and Patil, 2009). Overall literature survey shows that very little work has been carried out on the removal of anionic


solutions using low-cost materials emanated either by natural or manmade activities. Since all the cyano-group chemicals like free cyanide and thiocyanate, metal cyanides and metal thiocyanate are toxic to all classes of living cells their removal and recovery from waste prior

(Dicyanoaurte-DCAU) (Niu and Volesky, 2000) and

(Dicyanoargentate-DCAG) (Gaddi and Patil, 2011) from waste

also studied the impact of free cyanide on edible fish *Catla catla*.

species like Au-cyanide i.e. [Au(CN)2]

to discharge in environment is the key.


Ag-cyanide i.e. [Ag(CN)2]

**4. Discussion**

270 Applied Bioremediation - Active and Passive Approaches

It is well-known that certain type of microbial or waste biomass has high degree of competency to adsorb heavy metals. This sorption is solely due to the chemical composition of biomass (Volesky, 2003). With biosorption applications in mind it makes sense to screen variety of biomass types that are readily available in large quantities. There are basically two types of biomass sources that can practically be considered with low costs and availability in mind. First, the industrial waste biomass generated as a by-product of large scale (for example fermentation industry) with virtually no uses for it and disposal is a problem. Secondly, the biomass generated in large quantities from water environment (for examples unwanted plants like *Eichhornia* sp. and algae). It can be easily collected or harvested as raw material for biosorbents. Also, there are many other sources from where low-cost biomass could be procured especially in a developing country like India. These include the vegetable waste, yard wastes, waste flowers and coconut fibres from temples, etc. Energy generation potential from biomass and MSW have been reported by Saini et al (2012). In order to find the right biosorbent candidate, it is imperative to screen variety of biomass occurring in human environment.

Free cyanide and thiocyanate, metal-cyanides and metal-thiocyanates can occur in the wastewaters in various forms depending upon the chemical nature of the compounds and the concentration of metal, cyanide and thiocyanate, provided if metal moiety is bound to cyanide and thiocyanate. For example, free cyanide and thiocyanate can occur in the waters in its anionic form like CN and SCN- , respectively. While metal-cyanide like for example - coppercyanide occur in water in various forms such as Cu(CN)2 - , Cu(CN)3 2-, Cu(CN)4 2-, etc. Similarly, copper-thiocyanate complex occur in waste waters in various forms like Cu(SCN)2 - , Cu(SCN)3 2-, Cu(SCN)4 2-, etc. Therefore, it was thought worthwhile to explore the possibility of their recovery by adsorption on low-cost biomass procured from various places. Activated charcoal, a conventional material, was used for obtaining comparative data.

It is well known that the process of biosorption is regulated by aqueous solution pH (Puranik and Paknikar, 1997). The first step in present study was therefore determination of optimum solution pH for biosorption of gold-cyanide and silver-cyanide. It was found that biosorption of both the MxCNs (for all the low-cost biomass) increased with pH and then declined rapidly with further increase in pH. As seen from the Table 2 that maximum sorption of DCAU was at pH 4.0 while DCAG sorption by most of the biomass was at pH 6.0. Sorption decreased in the alkaline pH. It was found that other than activated charcoal (which was used as reference material) biomass like Rice husk & *Eichhornia* roots and *Eichhornia* roots and Tea powder waste were efficient biosorbents for DCAU and DCAG sorption. There was no auto-oxidation loss of both the MxCNs in controls without sorbent confirming that (bio)sorption is the only mechanism by which MxCNs are being removed from solution. In the previous study carried out by Niu and Volesky (2000) found that the maximum adsorption of DCAU by biomass was in the acidic pH ranging from 2.0 to 4.0. These results corroborates with the results obtained in our studies.

that more such biomass screening programmes are needed in search of right candidate for

Resource Recovery from Industrial Effluents Containing Precious Metal Species Using Low-Cost Biomaterials…

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273

In the present study loading capacity of conditioned biomass was also compared with that of unconditioned biomass (Table 3 and 4). For DCAU sorption, the unconditioned biomass showed lowered loading capacity compared to conditioned biomass. This reduction in loading capacity might be due to pH at which the loading capacity was determined. For conditioned biomass, the optimum pH for sorption was 4.0 as against the pH of sorption of unconditioned biomass i.e. the pH of original biomass (pH of Rice husk 5.94; pH of *Eichhornia* root biomass 7.01). In case of DCAG biosorption, it was observed that pH of unconditioned and conditioned biomass did not have any effect on the loading capacity of *Eichhornia* root and Tea powder waste biomass. This could be illustrated by the fact that original pH (unconditioned) of both *Eichhornia* root (pH 7.01) and Tea powder waste (pH 5.94) were similar to the obtained optimum pH values of our experiments. This result is very important from the view point of actual use of the biosorption process at commercial scale is concerned. Use of unconditioned biomass at commercial scale will save both time and money thereby making the cost of treatment economical which otherwise would have required for conditioning the biomass. Considering these results, selection of biosorbent was further narrowed down to Rice husk

and *Eichhornia* root biomass for DCAU and DCAG biosorption, respectively.

between binding sites at higher quantities (de Rome and Gadd, 1987).

column volumes ensuring efficiency and economy.

For cost effective treatment of industrial effluents, it is imperative to discern the biomass quantity (i.e. solid-to-liquid ratio) required. In our experiments, it was found that as the biomass quantity increased the % biosorption of both the MxCNs also increased. Maximum uptake in terms of Q (3.84 μmol/g) was observed at 3% (w/v) of Rice husk biomass for DCAU sorption. However, from 1 to 5 % (w/v) there was no significant increase. In case of DCAG sorption, *Eichhornia* root biomass showed highest Q value for the biomass-to-sorbent quantity from 2.0 to 5.0% (w/v). However, as the concentration of biomass was further increased the MxCN uptake did not increase the biomass loading which is attributable to the interference

Process of biosorption is fundamentally a surface interaction and is characterized by rapid uptake of ions by biomass surfaces. Rapidity of the process makes it a worthy candidate for use in effluent treatment on a commercial scale. Kinetics showed that rate of uptake of both the MxCN was maximum in first 15-20 minutes with over 80% of biosorption. Later, the sorption rate slowed down until it reached a plateau after 35-40 min, indicating the equilibra‐ tion of system. Maximum sorption of DCAU and DCAG was 88% and 94% in 40 min. The quick equilibrium time may be attributed to the particle size. The effective surface area is high for small particles. Such type of result is typical for biosorption of metals involving no energymediated reactions, where metal removed from solution is due to purely physico-chemical interactions between the biomass and metal in solution. Basha *et al.* (2008) observed similar results in case of biosorption of oxyanion species viz. chromium using seaweed *Cystoseira indica*. The rapid kinetics has significant practical importance as it will facilitate smaller reactor/

The influence of starting DCAU and DCAG concentration on biosorption by Rice husk and *Eichhornia* roots biomass showed that equilibrium sorption capacity of the sorbent increased

efficient sorption.

Increased sorption under acidic conditions may be due to the protonation of the functional groups acquiring net positive charges. Probably therefore, the formation of species such as H + -AuCN2 and H+ -AgCN2 on the biomass might have taken place thereby accommodating more number of MxCN species on the biosorbent sites. Waste biomass from natural origin contains large number of surface functional groups like hydroxyl, carbonyl, carboxyl, sulphydryl, amine, imine, amide, phosphonate, phosphodiester, etc. Probably some of these functional groups might have played the crucial role in the sorption of DCAU and DCAG from aqueous solution.

Matheickal and Yu (1996) have reported that pH dependence of cationic and anionic adsorp‐ tion can largely be related to type and ionic state of these functional groups and the chemistry of target compound in solution. DCAU and DCAG in our studies could be compared with anionic metal species like hexavalent chromium (an oxyanion) and arsenic. At low pH values, cell wall ligands are protonated and compete significantly with metal binding. With increasing pH, more ligands such as amino and carboxyl groups, would be exposed leading to attraction between these negative charges and the metals and hence increases in biosorption on to cell surface (Aksu, 2001). As the pH increased further, the overall surface charge on the cells could become negative and biosorption decreased (Aksu, 2001). Patil and Paknikar (1999) have reported the optimum pH of 4.0 for the sorption of copper- and nickel-cyanide from aqueous solutions using *Cladosporium cladospoiroides* biomass.

Free cyanide (CN- ) bearing effluents are highly alkaline in nature and have pH ranging from 9.5 to 12.5, whilst MxCN effluents have pH in range of 6.0 to 10.0. Obviously, suitable pH alterations of the effluents would be required before biosorption. Unlike free cyanide, MxCNs does not evolve potent hydrogen cyanide (HCN) gas because of their high stability constants (APHA-AWWA-WEF, 1998; Sharpe, 1976). Therefore, biosorption under acidic conditions would be a safe procedure. On the basis of screening studies under optimum pH conditions, Rice husk and *Eichhornia* root biomass were selected for DCAU sorption, while *Eichhornia* roots and Tea powder waste biomass were selected for DCAG sorption for further experiments.

The DCAU and DCAG loading capacity of the biosorbent could be taken as an equivalent measure of binding sites present. It was found that Rice husk biomass had the maximum loading capacity for DCAU (7.63 μmol/g) sorption among the two tested biomass; and was followed by *Eichhornia* root biomass (7.04 μmol/g). Loading capacity of Activated charcoal was less (7.61 μmol/g) when compared with Rice husk. In case of DCAG biosorption, *Eichhornia* root biomass showed highest loading capacity (9.74 μmol/g) followed by Tea powder waste (9.41 μmol/g). Loading capacity of *Eichhornia* root biomass though marginally less, but was highly competitive and comparable with that of activated charcoal (9.95 μmol/g). This unlocks newer opportunities of developing an efficient biosorption process for the removal and recovery of anionic species like gold-cyanide and silver-cyanide from low tenor waste solutions. In the study carried out by Patil (1999) it was found that the biomass of *C. clado‐ sporoides* had higher loading capacity (34-40 μmol/g) than activated charcoal (27.5-30 μmol/g) for the sorption of metal-cyanides viz. copper- and nickel-cyanide. These results also indicate that more such biomass screening programmes are needed in search of right candidate for efficient sorption.

in the acidic pH ranging from 2.0 to 4.0. These results corroborates with the results obtained

Increased sorption under acidic conditions may be due to the protonation of the functional groups acquiring net positive charges. Probably therefore, the formation of species such as H

number of MxCN species on the biosorbent sites. Waste biomass from natural origin contains large number of surface functional groups like hydroxyl, carbonyl, carboxyl, sulphydryl, amine, imine, amide, phosphonate, phosphodiester, etc. Probably some of these functional groups might have played the crucial role in the sorption of DCAU and DCAG from aqueous

Matheickal and Yu (1996) have reported that pH dependence of cationic and anionic adsorp‐ tion can largely be related to type and ionic state of these functional groups and the chemistry of target compound in solution. DCAU and DCAG in our studies could be compared with anionic metal species like hexavalent chromium (an oxyanion) and arsenic. At low pH values, cell wall ligands are protonated and compete significantly with metal binding. With increasing pH, more ligands such as amino and carboxyl groups, would be exposed leading to attraction between these negative charges and the metals and hence increases in biosorption on to cell surface (Aksu, 2001). As the pH increased further, the overall surface charge on the cells could become negative and biosorption decreased (Aksu, 2001). Patil and Paknikar (1999) have reported the optimum pH of 4.0 for the sorption of copper- and nickel-cyanide from aqueous

9.5 to 12.5, whilst MxCN effluents have pH in range of 6.0 to 10.0. Obviously, suitable pH alterations of the effluents would be required before biosorption. Unlike free cyanide, MxCNs does not evolve potent hydrogen cyanide (HCN) gas because of their high stability constants (APHA-AWWA-WEF, 1998; Sharpe, 1976). Therefore, biosorption under acidic conditions would be a safe procedure. On the basis of screening studies under optimum pH conditions, Rice husk and *Eichhornia* root biomass were selected for DCAU sorption, while *Eichhornia* roots and Tea powder waste biomass were selected for DCAG sorption for further experiments. The DCAU and DCAG loading capacity of the biosorbent could be taken as an equivalent measure of binding sites present. It was found that Rice husk biomass had the maximum loading capacity for DCAU (7.63 μmol/g) sorption among the two tested biomass; and was followed by *Eichhornia* root biomass (7.04 μmol/g). Loading capacity of Activated charcoal was less (7.61 μmol/g) when compared with Rice husk. In case of DCAG biosorption, *Eichhornia* root biomass showed highest loading capacity (9.74 μmol/g) followed by Tea powder waste (9.41 μmol/g). Loading capacity of *Eichhornia* root biomass though marginally less, but was highly competitive and comparable with that of activated charcoal (9.95 μmol/g). This unlocks newer opportunities of developing an efficient biosorption process for the removal and recovery of anionic species like gold-cyanide and silver-cyanide from low tenor waste solutions. In the study carried out by Patil (1999) it was found that the biomass of *C. clado‐ sporoides* had higher loading capacity (34-40 μmol/g) than activated charcoal (27.5-30 μmol/g) for the sorption of metal-cyanides viz. copper- and nickel-cyanide. These results also indicate

on the biomass might have taken place thereby accommodating more

) bearing effluents are highly alkaline in nature and have pH ranging from

in our studies.


272 Applied Bioremediation - Active and Passive Approaches

solutions using *Cladosporium cladospoiroides* biomass.

+ -AuCN2 and H+

solution.

Free cyanide (CN-

In the present study loading capacity of conditioned biomass was also compared with that of unconditioned biomass (Table 3 and 4). For DCAU sorption, the unconditioned biomass showed lowered loading capacity compared to conditioned biomass. This reduction in loading capacity might be due to pH at which the loading capacity was determined. For conditioned biomass, the optimum pH for sorption was 4.0 as against the pH of sorption of unconditioned biomass i.e. the pH of original biomass (pH of Rice husk 5.94; pH of *Eichhornia* root biomass 7.01). In case of DCAG biosorption, it was observed that pH of unconditioned and conditioned biomass did not have any effect on the loading capacity of *Eichhornia* root and Tea powder waste biomass. This could be illustrated by the fact that original pH (unconditioned) of both *Eichhornia* root (pH 7.01) and Tea powder waste (pH 5.94) were similar to the obtained optimum pH values of our experiments. This result is very important from the view point of actual use of the biosorption process at commercial scale is concerned. Use of unconditioned biomass at commercial scale will save both time and money thereby making the cost of treatment economical which otherwise would have required for conditioning the biomass. Considering these results, selection of biosorbent was further narrowed down to Rice husk and *Eichhornia* root biomass for DCAU and DCAG biosorption, respectively.

For cost effective treatment of industrial effluents, it is imperative to discern the biomass quantity (i.e. solid-to-liquid ratio) required. In our experiments, it was found that as the biomass quantity increased the % biosorption of both the MxCNs also increased. Maximum uptake in terms of Q (3.84 μmol/g) was observed at 3% (w/v) of Rice husk biomass for DCAU sorption. However, from 1 to 5 % (w/v) there was no significant increase. In case of DCAG sorption, *Eichhornia* root biomass showed highest Q value for the biomass-to-sorbent quantity from 2.0 to 5.0% (w/v). However, as the concentration of biomass was further increased the MxCN uptake did not increase the biomass loading which is attributable to the interference between binding sites at higher quantities (de Rome and Gadd, 1987).

Process of biosorption is fundamentally a surface interaction and is characterized by rapid uptake of ions by biomass surfaces. Rapidity of the process makes it a worthy candidate for use in effluent treatment on a commercial scale. Kinetics showed that rate of uptake of both the MxCN was maximum in first 15-20 minutes with over 80% of biosorption. Later, the sorption rate slowed down until it reached a plateau after 35-40 min, indicating the equilibra‐ tion of system. Maximum sorption of DCAU and DCAG was 88% and 94% in 40 min. The quick equilibrium time may be attributed to the particle size. The effective surface area is high for small particles. Such type of result is typical for biosorption of metals involving no energymediated reactions, where metal removed from solution is due to purely physico-chemical interactions between the biomass and metal in solution. Basha *et al.* (2008) observed similar results in case of biosorption of oxyanion species viz. chromium using seaweed *Cystoseira indica*. The rapid kinetics has significant practical importance as it will facilitate smaller reactor/ column volumes ensuring efficiency and economy.

The influence of starting DCAU and DCAG concentration on biosorption by Rice husk and *Eichhornia* roots biomass showed that equilibrium sorption capacity of the sorbent increased with increasing starting concentration of MxCNs from 0.01 to 1 mM (10 to 1000 μmol). This might be due to the increase in number of ions competing for available binding sites in the biomass. Uptake of MxCNs at various concentrations reached a plateau when the concentration was in the range of 0.5 mM (500 μmol). This might be due to the saturation of binding sites, which clearly showed that MxCN uptake by Rice husk and *Eichhornia* root biomass was a chemically equilibrated and saturable phenomenon. The higher starting concentration of target compound offers increased driving force to overcome all mass transfer resistance of target chemical ions between the aqueous and solid phases resulting in higher probability of collision between MxCN ions and the biosorbent. This results in higher uptake of the target compound. Moreover, the biomass cell membrane comprises host of functional groups made of polysaccharides, proteins, lipids that have the potential of binding to MxCN ions.

*nia* root biomass with 1% L-cysteine enhanced the biosorption capacity of both the MxCNs, while the NaOH pretreated biomass significantly hampered the biosorption process. Based upon the results obtained, it was thought worthwhile to determine the loading capacity of Lcysteine pretreated biomass as well. It was observed that the loading capacity of Rice husk and *Eichhornia* root biomass enhanced the biosorption of DCAU and DCAG upto 175% and 140%, respectively compared to untreated biomass (i.e. in absence of cysteine loaded biomass). These result corroborated with the findings obtained by Niu and Volesky (2000). This could be explained by the fact that in the acidic pH (pH 4.0 to 6.0 in our study), weak base groups either on cysteine or on the biomass becomes protonated and acquires a net positive charge. Roberts (1992) had reported the p*K* ranging from 3.5 to 6.0 of the positively charged weak base amine groups. Carboxyl group on the biomass could be protonated in their neutral for as the p*Ka* is in the range of 3 to 5 (Buffle, 1988). In acidic pH range of 2.0 to 6.0, some of the carboxyl groups on cysteine may still be dissociated since the dissociated constant of carboxyl group on cysteine is 1.90, whereas the amino group is protonated and with a positive charge. This allows the cysteine binding to biomass through the integration/combination of negative cysteine carboxyl groups with some of the positively charged biomass functional groups. Thus, the positively charged cysteine amino group were available for binding of anionic MxCN species like

Resource Recovery from Industrial Effluents Containing Precious Metal Species Using Low-Cost Biomaterials…

] which are the target compounds in our studies. In other words, the

] adsorbed by ionizable functional groups on cysteine

http://dx.doi.org/10.5772/56965

275

[Au(CN)2


to the user industry.

anionic species [Au(CN)2

] and [Ag(CN)2

(Waste Biomass --- Cysteine --- H+

(Waste Biomass --- Cysteine --- H+



loaded biomass carrying a positive when protonated.

] and [Ag(CN)2


) --- Au(CN)2

) --- Ag(CN)2



When the target compound is rare and costly, it is always desirable to recover the target compound from industrial effluents having low concentration and high volumes. For an effective and viable biosorption technology, elution methods for the recovery of target compound should be highly efficient, economical and should not cause damage to the biomass. Several eluting agents have been reported in the literature which includes mainly mineral acids, alkalis, organic acids, etc. In the present study, the loaded DCAU and DCAG on Rice husk and *Eichhornia* root biomass, respectively, could be desorbed with more than 95% efficiency using 1 N sodium hydroxide solution. Final concentration of DCAU and DCAG in the concentrated eluent was 28-30 and 22-25 folds, respectively, of the starting concentration. Such high tenor solution of recovered gold-cyanide and silver-cyanide may be recycled back

The next major task in the study was to test the selected biomass viz. Rice husk and *Eichhor‐ nia* root biomass for the removal of gold-cyanide and silver-cyanide from their respective industrial effluents in batch mode. As mentioned earlier that the project investigator encoun‐ tered great difficulty in procuring the effluent samples from industries. In the end, third party intervention helped the investigator to get the sample. In developing country like India, most of industrial personnel are reluctant to give any information regarding toxic chemical waste like cyanide. Moreover, they don't allow the outsider to invade into their industry mainly due to the risk and threat that is associated with cyanide disposal. With the stricter statutory limits imposed by statutory agencies, the conventional physic-chemical methods for the treatment

It is well known that biosorption resembles physical adsorption process and follows an adsorption type isotherm (Tsezos, 1990). Adsorption isotherms are the plots of solute concen‐ tration in the adsorbed state as a function of its concentration in the solution at constant temperature. Equilibrium sorption isotherms give useful evidence for selection of an adsorbent and facilitate evaluation of adsorption process for a given application (Weber, 1985). Isotherm indicates the relative affinity of biosorbent for target ions and the adsorption capacity of biosorbent. Also, the sensitivity of biosorption to changes in target compound concentration can be determined by the relative steepness of the isotherm line. Some of the important equilibrium models developed to describe adsorption isotherm relationships include single layer adsorption (Langmuir, 1918; Freundlich, 1926) and multilayer adsorption (Branauer *et al*., 1938).

Adsorption isotherms are known to have been largely used for projected industrial applica‐ tions (Tsezos and Volesky, 1981). In the present study, it was decided to fit the DCAU and DCAG sorption data with two most widely accepted adsorption models viz. Freundlich and Langmuir. Linear transformation of the adsorption data using Freundlich and Langmuir models (R2 = >0.96) allowed computation of the MxCN adsorption capacities. Experimental data was found to obey the basic principles underlying these models, that is, heterogeneous surface adsorption and monolayer adsorption at constant adsorption energy, respectively (Langmuir 1918; Freundlich 1926).

Other than the MxCN species many additional cations and anions are normally encountered in the effluents emanated from industries like metal mining, electroplating, photofinishing units, printed circuit board manufacturing, etc. These species might inhibit the removal of DCAU and DCAG from aqueous solutions. The impact of commonly occurring cations and anions was therefore studied on biosorption of DCAU and DCAG by Rice husk and *Eichhor‐ nia* root, respectively. It was observed that MxCNs were not significantly affected in most of the cases. However, biosorption of DCAU reduced by 33-40% in the presence of zinc, chromi‐ um and cadmium. In case of DCAG, sorption reduced by 37-67% by the presence of cadmium, zinc, iron and chromium. Biosorption in the presence of other metals cations (copper, nickel and silver) and anions (phosphates, sulphates and chlorides) was consistently above 80%.

Pretreated Rice husk and *Eichhornia* biomass with variety of chemicals showed greater degree of variation in the biosorption of DCAU and DCAG. Pretreatment of Rice husk and *Eichhor‐* *nia* root biomass with 1% L-cysteine enhanced the biosorption capacity of both the MxCNs, while the NaOH pretreated biomass significantly hampered the biosorption process. Based upon the results obtained, it was thought worthwhile to determine the loading capacity of Lcysteine pretreated biomass as well. It was observed that the loading capacity of Rice husk and *Eichhornia* root biomass enhanced the biosorption of DCAU and DCAG upto 175% and 140%, respectively compared to untreated biomass (i.e. in absence of cysteine loaded biomass). These result corroborated with the findings obtained by Niu and Volesky (2000). This could be explained by the fact that in the acidic pH (pH 4.0 to 6.0 in our study), weak base groups either on cysteine or on the biomass becomes protonated and acquires a net positive charge. Roberts (1992) had reported the p*K* ranging from 3.5 to 6.0 of the positively charged weak base amine groups. Carboxyl group on the biomass could be protonated in their neutral for as the p*Ka* is in the range of 3 to 5 (Buffle, 1988). In acidic pH range of 2.0 to 6.0, some of the carboxyl groups on cysteine may still be dissociated since the dissociated constant of carboxyl group on cysteine is 1.90, whereas the amino group is protonated and with a positive charge. This allows the cysteine binding to biomass through the integration/combination of negative cysteine carboxyl groups with some of the positively charged biomass functional groups. Thus, the positively charged cysteine amino group were available for binding of anionic MxCN species like [Au(CN)2 - ] and [Ag(CN)2 - ] which are the target compounds in our studies. In other words, the anionic species [Au(CN)2 - ] and [Ag(CN)2 - ] adsorbed by ionizable functional groups on cysteine loaded biomass carrying a positive when protonated.

(Waste Biomass --- Cysteine --- H+ ) --- Au(CN)2 -

with increasing starting concentration of MxCNs from 0.01 to 1 mM (10 to 1000 μmol). This might be due to the increase in number of ions competing for available binding sites in the biomass. Uptake of MxCNs at various concentrations reached a plateau when the concentration was in the range of 0.5 mM (500 μmol). This might be due to the saturation of binding sites, which clearly showed that MxCN uptake by Rice husk and *Eichhornia* root biomass was a chemically equilibrated and saturable phenomenon. The higher starting concentration of target compound offers increased driving force to overcome all mass transfer resistance of target chemical ions between the aqueous and solid phases resulting in higher probability of collision between MxCN ions and the biosorbent. This results in higher uptake of the target compound. Moreover, the biomass cell membrane comprises host of functional groups made

of polysaccharides, proteins, lipids that have the potential of binding to MxCN ions.

*al*., 1938).

models (R2

(Langmuir 1918; Freundlich 1926).

274 Applied Bioremediation - Active and Passive Approaches

It is well known that biosorption resembles physical adsorption process and follows an adsorption type isotherm (Tsezos, 1990). Adsorption isotherms are the plots of solute concen‐ tration in the adsorbed state as a function of its concentration in the solution at constant temperature. Equilibrium sorption isotherms give useful evidence for selection of an adsorbent and facilitate evaluation of adsorption process for a given application (Weber, 1985). Isotherm indicates the relative affinity of biosorbent for target ions and the adsorption capacity of biosorbent. Also, the sensitivity of biosorption to changes in target compound concentration can be determined by the relative steepness of the isotherm line. Some of the important equilibrium models developed to describe adsorption isotherm relationships include single layer adsorption (Langmuir, 1918; Freundlich, 1926) and multilayer adsorption (Branauer *et*

Adsorption isotherms are known to have been largely used for projected industrial applica‐ tions (Tsezos and Volesky, 1981). In the present study, it was decided to fit the DCAU and DCAG sorption data with two most widely accepted adsorption models viz. Freundlich and Langmuir. Linear transformation of the adsorption data using Freundlich and Langmuir

data was found to obey the basic principles underlying these models, that is, heterogeneous surface adsorption and monolayer adsorption at constant adsorption energy, respectively

Other than the MxCN species many additional cations and anions are normally encountered in the effluents emanated from industries like metal mining, electroplating, photofinishing units, printed circuit board manufacturing, etc. These species might inhibit the removal of DCAU and DCAG from aqueous solutions. The impact of commonly occurring cations and anions was therefore studied on biosorption of DCAU and DCAG by Rice husk and *Eichhor‐ nia* root, respectively. It was observed that MxCNs were not significantly affected in most of the cases. However, biosorption of DCAU reduced by 33-40% in the presence of zinc, chromi‐ um and cadmium. In case of DCAG, sorption reduced by 37-67% by the presence of cadmium, zinc, iron and chromium. Biosorption in the presence of other metals cations (copper, nickel and silver) and anions (phosphates, sulphates and chlorides) was consistently above 80%. Pretreated Rice husk and *Eichhornia* biomass with variety of chemicals showed greater degree of variation in the biosorption of DCAU and DCAG. Pretreatment of Rice husk and *Eichhor‐*

= >0.96) allowed computation of the MxCN adsorption capacities. Experimental

(Waste Biomass --- Cysteine --- H+ ) --- Ag(CN)2 -

When the target compound is rare and costly, it is always desirable to recover the target compound from industrial effluents having low concentration and high volumes. For an effective and viable biosorption technology, elution methods for the recovery of target compound should be highly efficient, economical and should not cause damage to the biomass. Several eluting agents have been reported in the literature which includes mainly mineral acids, alkalis, organic acids, etc. In the present study, the loaded DCAU and DCAG on Rice husk and *Eichhornia* root biomass, respectively, could be desorbed with more than 95% efficiency using 1 N sodium hydroxide solution. Final concentration of DCAU and DCAG in the concentrated eluent was 28-30 and 22-25 folds, respectively, of the starting concentration. Such high tenor solution of recovered gold-cyanide and silver-cyanide may be recycled back to the user industry.

The next major task in the study was to test the selected biomass viz. Rice husk and *Eichhor‐ nia* root biomass for the removal of gold-cyanide and silver-cyanide from their respective industrial effluents in batch mode. As mentioned earlier that the project investigator encoun‐ tered great difficulty in procuring the effluent samples from industries. In the end, third party intervention helped the investigator to get the sample. In developing country like India, most of industrial personnel are reluctant to give any information regarding toxic chemical waste like cyanide. Moreover, they don't allow the outsider to invade into their industry mainly due to the risk and threat that is associated with cyanide disposal. With the stricter statutory limits imposed by statutory agencies, the conventional physic-chemical methods for the treatment of metal-cyanide bearing effluents are proving to be expensive and also inadequate to meet the required standards. This techno-economic impasse has led to closure of several industries especially the plating industries.

working life of the column is over and the "breakthrough point" occurs marking the usable column "service time". These two parameters are very important from the process design point of view because they directly affect the feasibility and economics of the sorption process

Resource Recovery from Industrial Effluents Containing Precious Metal Species Using Low-Cost Biomaterials…

http://dx.doi.org/10.5772/56965

277

After successfully treating both the industrial effluents in batch mode using Rice husk and *Eichhornia* root biomass, further biosorption studies were carried out in continuous mode using packed bed column. It was found that the service time offered by the column beds for goldcyanide (from column 1) and silver-cyanide (from column 2) effluents were 60 h and 40 h, respectively. In other words, these itself were the breakthrough points. For both the columns the transfer zone observed was of 30 h each. The total effluent passed through the column 1 and 2 was equivalent to 50 and 34 bed volumes, respectively, while the complete saturation occurred after 90 and 70 h, respectively. Continuous study clearly showed that both the effluents were biosorbed and treated successfully in the packed bed columns for the removal of both precious and toxic species. Further, in these studies the project investigator did not immobilized any of the biomass primarily because the present work was focused on low tenor effluents containing precious gold and silver and toxic chemical species like cyanide (all below 10 mg/l). Secondly, the results obtained through batch and continuous studies showed that both the biomass were efficient enough to sorb and treat the effluents and therefore the project investigator felt that immobilization of the biomass probably is not required in this case.

Thus, it could be concluded that the waste biomass used in the present study has immense potential "as biosorbents" for the removal/management of low tenor precious and toxic pollutants, as evident from the example of gold-cyanide and silver-cyanide management in the present study. Further, biosorption technology used could also become an economical, non-destructive and reliable alternative to the conventional processes for the management of

Apart from the removal and recovery of precious heavy metal species from industrial effluents, passive bioremediation technology (PBT) can also be employed for some newer type of wastes

A novel approach of combined biosorption-biodegradation processes was used by Patil and Paknikar (1999) for the removal and recovery of copper- and nickel-cyanide from electroplat‐ ing effluents. *Cladosporium cladosporioides* biomass was found to be highly efficient sorbent in this case. The unrecoverable (residual) metal-cyanides after biosorption was subjected to biodegradation process using bacterial consortium. The treated effluent was free of cyanide

The problem of waste photovoltaic cells was addressed by Paknikar et al (1997) by way of recovering and recycling of expensive metals like silver, cadmium and tellurium. In this study, the researchers used scrapings of waste photovoltaic cells, which were dissolved in suitable

**5. Application of biosorption to some newer wastes and products**

and metals and complied with the statutory limits (Patil and Paknikar, 1999).

industrial effluents employed on the commercial scale.

and products that have emerged in the recent times.

(Volesky, 2003).

The gold-cyanide and silver-cyanide from the effluents procured from the industries could be effectively biosorbed by Rice husk and *Eichhornia* root biomass which were pretreated with Lcysteine. Gold and cyanide removal efficiency from gold-cyanide industrial effluent was 91.53% and 82.69%, respectively. However, the cyanide level in the biosorbed treated effluent although very less (0.59 mg/l) but was not below the standard limits prescribed by Indian statutory agencies, which is 0.2 mg/l. Similarly, the cyanide concentration after biosorption treatment to silver-cyanide effluent was also not complying with the standards prescribed by Indian statutory agencies. Overall, the studies on industrial effluents indicated that both the biomass viz. Rice husk and *Eichhornia* root biomass were very effective in treating both the effluents by biosorption process. Therefore, it is possible to employ Rice husk and *Eichhornia* root biomass for the treatment of industrial effluents on commercial scale. The residual (unrecoverable) cyanide remaining in the solutions after biosorption were subjected to biodegradation process using bacterial consortium under optimized conditions.

When the residual gold-cyanide and silver-cyanide biodegradation experiment was run under optimized conditions in batch mode, it was found that the live bacterial consortium previously isolated by Patil (2008) could degrade the cyanide present in the solution within a period of 5 h with an efficiency of >90% for both types of effluents. The resulting treated solution could comply with the disposal standards prescribed by statutory agencies in India. These findings indicated that biodegradation could be used as a polishing step in the treatment of precious MxCNs containing industrial waste waters.

In process applications, the most effective apparatus for sorption/desorption and making the most effective use of the reactor volume, is a fixed-bed column. The column makes optimum use of the concentration gradient between the solute sorbed by the solids and that remaining in the liquid phase thereby providing the driving force for the biosorption process. The process of biosorption (metals and their related species) is governed by three key regimes: (i) the sorption equilibrium, (ii) the sorption particle mass transfer and (iii) the flow pattern through the packed bed. These three regimes determine the overall performance of the sorption column which is judged by its 'service time'. Service time is the length of time until the sorbed species breaks through the bed to be detected at a given concentration in the column effluent. The breakthrough point indicates that the column is saturated practically and could be taken out of operation for some kind of its regeneration (Volesky, 2003).

The column bed is being saturated at inflow concentration which represents equilibrium concentration for the part of the bed upstream from the transfer zone. The saturation of the bed/column varies from zero to the full saturation. This zone of partial saturation moves the column in the direction of the flow at a certain velocity which is predominantly determined by the biomass loading, sorbent capacity and the feed rate to column. The column is operational until this zone reaches the end of the column. Until that time the effluent leaving the column has no trace of the sorbate in it. When the transfer zone reaches the column end, the sorbate concentration in the effluent starts to gradually increase and for all practical purposes, the working life of the column is over and the "breakthrough point" occurs marking the usable column "service time". These two parameters are very important from the process design point of view because they directly affect the feasibility and economics of the sorption process (Volesky, 2003).

of metal-cyanide bearing effluents are proving to be expensive and also inadequate to meet the required standards. This techno-economic impasse has led to closure of several industries

The gold-cyanide and silver-cyanide from the effluents procured from the industries could be effectively biosorbed by Rice husk and *Eichhornia* root biomass which were pretreated with Lcysteine. Gold and cyanide removal efficiency from gold-cyanide industrial effluent was 91.53% and 82.69%, respectively. However, the cyanide level in the biosorbed treated effluent although very less (0.59 mg/l) but was not below the standard limits prescribed by Indian statutory agencies, which is 0.2 mg/l. Similarly, the cyanide concentration after biosorption treatment to silver-cyanide effluent was also not complying with the standards prescribed by Indian statutory agencies. Overall, the studies on industrial effluents indicated that both the biomass viz. Rice husk and *Eichhornia* root biomass were very effective in treating both the effluents by biosorption process. Therefore, it is possible to employ Rice husk and *Eichhornia* root biomass for the treatment of industrial effluents on commercial scale. The residual (unrecoverable) cyanide remaining in the solutions after biosorption were subjected to

biodegradation process using bacterial consortium under optimized conditions.

When the residual gold-cyanide and silver-cyanide biodegradation experiment was run under optimized conditions in batch mode, it was found that the live bacterial consortium previously isolated by Patil (2008) could degrade the cyanide present in the solution within a period of 5 h with an efficiency of >90% for both types of effluents. The resulting treated solution could comply with the disposal standards prescribed by statutory agencies in India. These findings indicated that biodegradation could be used as a polishing step in the treatment of precious

In process applications, the most effective apparatus for sorption/desorption and making the most effective use of the reactor volume, is a fixed-bed column. The column makes optimum use of the concentration gradient between the solute sorbed by the solids and that remaining in the liquid phase thereby providing the driving force for the biosorption process. The process of biosorption (metals and their related species) is governed by three key regimes: (i) the sorption equilibrium, (ii) the sorption particle mass transfer and (iii) the flow pattern through the packed bed. These three regimes determine the overall performance of the sorption column which is judged by its 'service time'. Service time is the length of time until the sorbed species breaks through the bed to be detected at a given concentration in the column effluent. The breakthrough point indicates that the column is saturated practically and could be taken out

The column bed is being saturated at inflow concentration which represents equilibrium concentration for the part of the bed upstream from the transfer zone. The saturation of the bed/column varies from zero to the full saturation. This zone of partial saturation moves the column in the direction of the flow at a certain velocity which is predominantly determined by the biomass loading, sorbent capacity and the feed rate to column. The column is operational until this zone reaches the end of the column. Until that time the effluent leaving the column has no trace of the sorbate in it. When the transfer zone reaches the column end, the sorbate concentration in the effluent starts to gradually increase and for all practical purposes, the

especially the plating industries.

276 Applied Bioremediation - Active and Passive Approaches

MxCNs containing industrial waste waters.

of operation for some kind of its regeneration (Volesky, 2003).

After successfully treating both the industrial effluents in batch mode using Rice husk and *Eichhornia* root biomass, further biosorption studies were carried out in continuous mode using packed bed column. It was found that the service time offered by the column beds for goldcyanide (from column 1) and silver-cyanide (from column 2) effluents were 60 h and 40 h, respectively. In other words, these itself were the breakthrough points. For both the columns the transfer zone observed was of 30 h each. The total effluent passed through the column 1 and 2 was equivalent to 50 and 34 bed volumes, respectively, while the complete saturation occurred after 90 and 70 h, respectively. Continuous study clearly showed that both the effluents were biosorbed and treated successfully in the packed bed columns for the removal of both precious and toxic species. Further, in these studies the project investigator did not immobilized any of the biomass primarily because the present work was focused on low tenor effluents containing precious gold and silver and toxic chemical species like cyanide (all below 10 mg/l). Secondly, the results obtained through batch and continuous studies showed that both the biomass were efficient enough to sorb and treat the effluents and therefore the project investigator felt that immobilization of the biomass probably is not required in this case.

Thus, it could be concluded that the waste biomass used in the present study has immense potential "as biosorbents" for the removal/management of low tenor precious and toxic pollutants, as evident from the example of gold-cyanide and silver-cyanide management in the present study. Further, biosorption technology used could also become an economical, non-destructive and reliable alternative to the conventional processes for the management of industrial effluents employed on the commercial scale.
