**10.4. Effect of biosorbent dose**

a loss in the capacity of the biosorbent to retain the desired metal ion has been reported. The metal Cr (VI) was desorbed almost completely from the *Mucor hiemalis* biomass by using 0.1 N of NaOH. The biomass retained its activity of biosorption and desorption up to five cycles. Experimental data fit well with the Langmuir isotherm model, and FTIR analysis showed that the amino groups are involved in biosorption [165]. **Table 10** summarizes the use of different

Various factors influence the biosorption process namely, biomass concentration, initial metal concentration, and operational factors like pH, temperature, concentration of the initial metal

The pH of the solution is an important factor since it influences the metal chemical speciation, solubility, and the total charge of the biosorbent [82]. At low pH (acidic pH), the hydronium

eluents for the desorption of metal ions from different biosorbents.

**10. Factors affecting biosorption**

ion, and concentration of the biosorbent.

**10.1. Effect of pH**

**Type of biosorbent**

88 Biosorption

*Spirulina sp*

*Aspergillus niger*

*Aspergillus flavus*

*Raw wheat bran*

*Scenedesmus sp*

*Aspergillus niger*

*Rhizopus nigricans*

**Type of eluent**

0.1 M HNO3

0.1 N NaOH

0.1 N HNO3, 0.1 N NaOH

0.01 mol/L HCl, HNO3

0.1 M H2SO4

0.5 N H2SO4

*Montmorillonite* 0.1 M HCl Ni (II),

**Metal ion**

Cr, Cd, Cu

Cd (II), Pb (II)

Mn (II)

Cd (II), Ni (II), Zn (II)

**Table 10.** Use of different eluents for desorption of metal ions.

HNO3 Pb (II),

**% of desorption** **Isotherm model**

98 Langmuir Carboxyl,

Cr 90% Freundlich Carboxyl,

**Functional groups involved**

phosphate, hydroxyl, amine

amide, phosphate, hydroxide

Cu (II) 80% [167]

Zn 99% Freundlich Five [169]

Cr Redox reaction [50]

92.8%, 90% Freundlich Physical

100%, 57% Langmuir Four [168]

90% Five [171]

adsorption

**Mechanism Number** 

Ion exchange [46]

Chemisorption [166]

**of cycles**

Three [170]

**Reference**

Biosorbents provide the binding sites for metal biosorption, and hence its dosage strongly affects the biosorption process [179]. The increase of the biosorbent dose at a given initial metal concentration increases the biosorption of metal ions due to greater surface area which in turn increases the number of available binding sites [179]. At lower concentrations of the biosorbent, the amount of metal biosorbed per unit weight of the biosorbent is high. Conversely, at high concentration of the biosorbent, the quantity of metal ion biosorbed per unit weight decreases. 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 observed in many other studies in respect of the effect of biomass concentration.

[82]. The determination of equilibrium parameters is the basic requirement for designing a good biosorption system. For determination of the best-fitting sorption isotherm, linear regression is frequently used. In order to predict the isotherm parameters, the method of least

Application of Biosorption for Removal of Heavy Metals from Wastewater

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

91

The biosorption capacities of different biosorbents for different pollutants can be best explained by biosorption equilibrium isotherms. Several isotherm models are available to describe the mechanism of the biosorption process and the equilibrium biosorption distribution. Some of the isotherms used in biosorption studies are Langmuir, Freundlich, and Temkin isotherms.

Biosorption isotherm data of Pb (II) and Cu (II) ions onto green algal species, *Spirogyra* and *Cladophora,* were in good agreement with the Langmuir isotherm demonstrating the formation of the monolayer coverage of metal ions on the outer surface of the biosorbent [95]. The Langmuir model fitted well with the biosorption of Pb (II), Zn (II), and Ni (II) ions onto *Bacillus subtilis* [186]. Freundlich isotherm showed the best fit for the biosorption of Cu (II) ions onto lactic acid bacterium, *Enterococcus faecium* [106]. Biosorption of Cr (VI) ions onto

Various types of bioreactors have been investigated for application at the industrial level. A bioreactor is a system used for the production of microorganisms or desired metabolites employing defined and controllable factors. The typical categories of bioreactors used for the biosorption are stirred tank bioreactors (STRs), air lift bioreactors (ALRs), fluidized bed bioreactors (FBRs), and fixed bed bioreactors (FXRs). These reactors can be operated either in batches or in continuous modes or both (fixed bed and stirred tank bioreactors). Factors (pH, temperature, mixing and agitation, and nutrient availability) affecting the process of biosorption in the bioreactor have to be optimized and controlled by using cooling jackets (temperature), baffles/

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

agitators (mixing), feed lines (supplies nutrients), and acid/base addition (pH) [188].

However, the biosortion process may show better fit with a specific isotherm.

*Bacillus thuringiensis* also shows the better fit with Freundlich isotherm [187].

**12. Bioreactors used for biosorption**

**12.1. Fixed bed bioreactors**

in a continuous mode [190].

squares is applied.

#### **10.5. Effect of contact time**

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 equilibrium contact time of 30 min [108].
