**12.1. Fixed bed bioreactors**

This is because of lower adsorbate to binding site ratio due to the insufficient amount of solute present for complete distribution onto the available binding sites and possible interaction between binding sites. The biosorption of Cd and Pb ions by *Anabaena sphaerica* was increased with an increase in the biosorbent dose from 0.025 to 0.25 g/100 ml but stabilized at higher biomass dosages because of the formation of aggregates which reduce the effective surface area for biosorption [180]. The biosorption efficiency of *Parthenium hysterophorus* for Cr biosorption increased from 61.28 to 80.81% with an increase in biomass concentration from 0.1 to 1 g because of the availability of more binding sites but the biosorption capacity decreased from 9.43 to 0.37 mg/g due to decreased metal to biosorbent ratio [181]. A similar trend was

The time required to attain maximum biosorption depends on the type of biosorbent, metal ion, and their combination. The rate of biosorption is rapid initially (within an hour) with almost 90% of the metal binding because all the active sites are vacant and available for metal ion biosorption. But with increase in time the rate of biosorption decreases due to increase in percentage saturation by metal ions remaining in the solution [182]. Most of the Cd and Zn ions are biosorbed onto *Aspergillus niger* biomass within the first 6 h and there is no further biosorption after 24 h [183]. *Bacillus cereus* and *Pseudomonas aeruginosa* biosorb Zn ions with an

The increase in agitation speed increases the biosorption capacity of the biosorbent by minimizing its mass transfer resistance. While the added turbulence enhances the sorption of the metal ions [184], it may also lead to the destruction of the physical nature of the biosorbent. A moderate speed ensures the best homogeneity for the suspension with a high capacity of biosorption. High agitation speeds result in the occurrence of vortex phenomenon which results in the loss of the homogenous nature of the suspension. Excessive turbulence may also reduce the time of interaction between the biosorbate and biosorbent, thus decreasing the extent of biosorption [183]. The optimum speed of agitation for the biosorption of Cd and Zn by *Aspergillus niger* was found to be 120 rpm [183]. With an increase in agitation speed from 0

Sorption isotherms explain the equilibrium relationships between biosorbent and biosorbate and the mass of the biosorbed component per unit mass of biosorbent and the concentration of biosorbate in the medium under a given set of conditions (temperature and concentration). It also determines the equilibrium distribution of metal ions and how selective retention takes place when two or more biosorbent components are present [185]. The term "isotherm" can be defined as a curve explaining the retention of a substance on a solid at various concentrations

to 80 rpm, the biosorption efficiency also increased from 32.4 to 65% [62].

observed in many other studies in respect of the effect of biomass concentration.

**10.5. Effect of contact time**

90 Biosorption

equilibrium contact time of 30 min [108].

**11. Biosorption equilibrium isotherms**

**10.6. Effect of agitation speed**

It is designed with the biosorbent fixed onto a bed and a container having the bed within. During biosorption, the water contaminated with heavy metals is passed through the column. The biosorbents biosorb the metal ions until the maximal capacity is reached. The biosorbent is then regenerated for the release of heavy metals. In order to ensure continuous working conditions, the presence of two columns is employed. Biosorption is performed on one column while the regeneration of spent biosorbent on the other by rinsing with a suitable chemical reagent. Most of the biosorption processes have used fixed bed bioreactors. Its advantages include simplicity in construction and operation and possibility to carry out process in a countercurrent flow (a current flowing in opposite direction) [189]. However, it is necessary to examine the pressure drop and the effect of column dimensions when operated in a continuous mode [190].


biosorbent was observed [201]. Flotation is a separation process that can effectively separate the metal-loaded biosorbent suspended in the aqueous solution. The technique of biosorptive flotation was applied for the removal of nickel, copper, and zinc ions from the aqueous solutions using grape stalks as the sorbent. Two feed solutions containing different metal concentrations were prepared. The dilute metal solution was applied followed by the concentrated metal solution in the counter-current mode in order to improve the performance of the biosorbent. The experiments were conducted in 10 L columns and satisfactory metal removal was observed (Cu—95%; Zn—98%; Ni—70%; Ca—82%). The biosorbent after regeneration by using an aqueous mixture of sodium sulfate and sodium citrate can be used for the second cycle [202]. A two-step operation for biosorption and sedimentation was operated in a 200 L pilot plant for the removal of pollutants using biomass of *Cunninghamella elegans* and the obtained results proved

Application of Biosorption for Removal of Heavy Metals from Wastewater

http://dx.doi.org/10.5772/intechopen.77315

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Most biosorbents sequester metal ions by using cell-surface moieties. However, they lack the property of specificity and affinity for metals. By using the available genetic engineering technologies specific tailoring can be done to the microbial biosorbents with required selectivity and affinity for metal ions [204]. Genetic engineering technology involves altering the genetic material of the organism in order to develop an efficient strain for the removal of metal ions against the wide range of contaminants present in the wastewater [205]. One such emerging strategy which has received increased attention in recent times is the use of metal-binding proteins such as metallothioneins and phytochelatins. For example, *E. coli* was modified to express phytochelatin 20 on its surface enhancing the accumulation of Hg by 25 times over that by wild-type strains [204]. The technology also offers the advantage of developing microbial strains that can withstand complex environmental conditions and stressful situations. A major obstacle associated with the molecular approach is that it has been applied to only limited bacterial strains like *Escherichia coli.* Hence, other microorganisms need to be explored using this molecular intervention. **Table 12** shows the list of selected genetically engineered bacteria used for the removal of metal ions.

**Genetically engineered** 

Hg 7.4 96 *E. coli* Hg2+ transporter [206] As 0.05 100 *E. coli* Metalloregulatory protein

Ni 10 uM 15 μmol *E. coli* nixA gene [208]

Hg 77.58 mg/g *Rhodopseudomonas palustris* pSUTP+pGPMT [210]

Cr 10 48–93.8% *Alcaligenes eutrophus* pEBZ141(Cr resistance

**Table 12.** Use of genetically engineered microorganisms for biosorption of metal ions.

**Expressed gene of** 

**Reference**

[207]

[209]

**interest**

ArsR

genes)

**bacteria**

that the biosorption process is effective in treating wastewaters efficiently [203].

**14. Biotechnological intervention: genetically engineered** 

**microorganisms (GEM)**

**Metal ion**

**Initial concentration (ppm)**

**Biosorption efficiency %**

a Indicates the dry weight of the biosorbent; <sup>b</sup> Indicates the wet weight of the biosorbent; \* Indicates batch biosorption experiments at laboratory scale; and # Indicates continuous biosorption experiments.

**Table 11.** Use of different bioreactors for biosorption of metal ions.

#### **12.2. Fluidized bed and air lift bioreactors**

These two reactors almost work on the same principle of separation and can be operated in the batch mode. The reactor contains liquid, gaseous, and solid phases. The solid phase is a biosorbent on solid particles used for the retention of metals. The reactor operates with the idea that the gas allows the liquid containing the metal species to be removed to rise. The liquid then flows upward through the middle of the reactor and comes back down through the edges resembling a fountain [191]. In this the liquid is in continuous movement and moves the entire volume of the column. The metal species then adhere to the biosorbent. Once the biosorbent is harvested, the target molecule is separated. Since the particles are in continuous movement, it is preferred and also reduces the clogging effect of the biosorbent. Fluidized reactors are associated with the low mass transfer [38].

#### **12.3. Stirred tank bioreactors**

Liquid phase can be separated from the solid phase by a membrane system. Though the process is simple, the cost of operation is high due to high energy requirements [192].

The efficiency in the removal of metal ions largely depends on the type of bioreactor, type of biosorbent, and operating conditions. Recent studies evaluated the efficiency of different biosorbents in the removal of metal ions by using various types of bioreactors (**Table 11**).
