**9. Desorption and the regeneration of biosorbents**

In order to keep the process costs down and for recovery of valuable metal ions after the biosorption, it is crucial for regeneration of the biosorbent [152]. The primary objective of desorption is to retain the adsorption capacity of the biosorbent. The process of desorption should be such that the metal can be recovered in the concentrated form (in case of metals of economic value), and the biosorbent needs be restored to the original state with undiminished biosorption capacity for reuse [8]. Hence an appropriate eluent for desorption should meet the following requirements [112]

• low cost;

further enhance the biosorption capacity. *Saccharomyces cerevisiae* treated with glutaldehyde increased the biosorption of Cu (II) ions [138]. The autoclaving of cells increases the surface area caused by cell rupture resulting in higher binding capacity compared to the normal cells. The treatment of autoclaved *Aspergillus niger* biomass treated with various chemicals increased the biosorption capacity for chromium from 2.16 to 86.88% when compared with the untreated biomass [139]. Hence, different pretreatments modify the surface functional groups (by masking or exposing) that influence biosorption capacity. The masking of carboxylic and amine groups present on the surface of *Saccharomyces cerevisiae* biomass by esterification and methylation decreased the biosorption capacity for Cu (II) ions which indicates that those functional groups are involved in the biosorption of metal ions and the study showed the better fit with the Freundlich isotherm model [138]. Various studies reported the use of treated biomasses for the removal of metal ions with high absorption rates was given in **Table 8**.

A major consideration for any biosorption is the separation of solid and liquid phases. Centrifugation and filtration are the routinely used techniques but not recommended at the industrial level. A continuous system with the biosorbent attached to a suitable bed is advantageous [149]. The use of free microbial cells as a biosorbent in continuous system is associated with many disadvantages such as the difficulty in separation of biomass, loss of biosorbent after regeneration, low strength, and little rigidity [150]. Microbial biomass can be immobilized by using a biopolymeric or polymeric matrix. The technique of immobilization is a key element that improves the performance of the biosorbent by increasing the capacity, improving mechanical strength and resistance to chemicals, and facilitating easy separation of biomass from a solution containing pollutants [151]. The process of immobilization is well suited for non-destructive recovery. Immobilization of the biosorbent into suitable particles can be done by using techniques like entrapment (in a strong but permeable matrix) or encapsulation (within a membrane-like structure) [152]. A number of matrices have been employed for immobilization including sodium or calcium alginate, polyacrylamide, silica, polysulfone, and polyurethane. It is very important to use a suitable immobilization matrix since it determines the mechanical strength and chemical resistance of the biosorbent particle targeted for biosorption while the matrix should be cheap and feasible to operate [153]. The use of an immobilized biosorbent is also associated with some disadvantages like increase in the cost of the biosorbent and an adverse effect on the mass transfer kinetics. This is because immobilization reduces the number of binding sites that are accessible to metal ions as majority of the sites are embedded within the bead [154]. The live and heat-inactivated *Trametes versicolor* immobilized within carboxyl methylcellulose (CMC) beads were efficient in the removal of Cu (II), Pb (II), and Zn (II) from the aqueous solution. The biosorption capacity were found to be 1.51 and 1.84 mmol, 0.85 and 1.11 mmol, and 1.33 and 1.67 mmol for Cu, Pb and Zn of both live and heat-inactivated biosorbents, respectively. The study shows the best fit with the Langmuir isotherm model [155]. **Table 9** gives the examples of various immobilization

**8. Immobilization of biosorbent**

86 Biosorption

matrices used for the biosorption of metal ions.


The possible eluents are dilute mineral acids (HCl, H<sup>2</sup> SO<sup>4</sup> and HNO<sup>3</sup> ), organic acids (citric, acetic and lactic acids), and complexing agents (EDTA, thiosulphate, etc.) for the recovery of the biosorbent and metal recovery. Desorption efficiency can be determined by the S/L ratio, that is, solid to liquid ratio. The solid represents the biosorbent and liquid represents the eluent (volume) applied. For complete elution and to make the process economical, high S/L values are desirable [3]. Although, desorption is considered advantageous, in some instances,


ions are closely associated with the active ligands of the biosorbent and therefore, there exists a competition between the protons and metal ions for the binding sites [172]. At higher pH, there exists lower number of H+ ions, and the number of active sites of the functional groups is free and exposed (negative charge) which results in increased biosorption by attracting positive charged metal ions. At higher pH, the metal might begin to precipitate and form hydroxides and as a consequence hinder the biosorption process [108]. The increase in pH from 1 to 4 increased the biosorption of Cr (VI) from wastewaters by *Saccharomyces cerevisiae* biomass [173]. For biosorption of Cr by pretreated *Aspergillus niger* the optimum pH was found to be 3 [166]. An increase in pH from 2.0 to 4.5 increased the biosorption of cadmium by *Rhizopus cohnii* biomass and thereafter it reached a plateau in the pH range from 4.5 to 6.5 [89].

Application of Biosorption for Removal of Heavy Metals from Wastewater

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Temperature deals with the thermodynamics of the process and kinetic energy of the metal ions [82]. The temperature can have a positive or negative effect on biosorption at certain intervals. An increase or decrease in temperature causes a change in the biosorption capacity of the biosorbent. High/increasing temperature enhances the biosorptive removal of biosorbates but it is associated with the limitation of structural damage to the biosorbent [38]. Hence, optimum temperature for efficient biosorption has to be chosen for the maximum binding of metal ions. In this context, a maximum biosorption of 86% for cadmium ions was achieved with *Saccharomyces cerevisiae* at 40°C [173]. A rise in incubation temperature from 25 to 40°C sharply increased the biosorption rates of Cr (VI) by *Streptococcus equisimilis* [174].

The mass transfer resistance between the liquid and solid phases can be overcome by the initial concentration of metal ion [175]. The biosorption capacity (quantity of biosorbed metal ions per unit weight of the biosorbent) of the biosorbent increases initially with the increase in metal ion concentration and then reaches a saturation value. However, the biosorption efficiency of the biosorbent decreases with increase in metal ion concentration. The higher biosorption efficiency at low metal concentration is due to the complete interaction of ions with the available binding which sites results in higher rates of efficiency. At higher concentrations, the number of metal ions remaining unbound in the solution is high due to the saturation of available binding sites [176]. The effect of different initial concentration (25–500 mg/L) of Cd ions on the biosorption of *Hypnea valentiae* was studied. It was found that highest biosorption efficiency (86.8%) was observed with a Cd concentration of 25 mg/L from simulated wastewaters [177]. The biosorption efficiency of the cashew nut shell decreased from 86.03 to 76.17%

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.

with the increase in copper ion concentration from 10 to 50 mg/L [178].

**10.2. Effect of temperature**

**10.3. Effect of initial metal concentration**

**10.4. Effect of biosorbent dose**

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

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 eluents for the desorption of metal ions from different biosorbents.
