**7. Effect of pretreatment on biosorption**

Since the process of biosorption relies on the number and availability of functional groups on the surface of the biosorbent, modification by changing the surface characteristics can greatly influence the capacity of biosorbent used for the removal of metal ions [137]. Microbial-derived biosorbents are amenable for modification in order to increase the available binding sites and enhance the biosorption capacity leaving low residual metal concentration. A number of methods have been employed for surface modification of microbial biomass. The physical methods of pretreatment include heating, autoclaving, freeze drying, thawing, and lyophilization. Various chemical methods used for the pretreatment include acid or alkali treatment, washing with detergents, treatment with organic chemicals such as formaldehyde, sodium hydroxide, dimethyl sulfoxide, and cross-linking with organic solvents [3]. Physical- or chemical-treated microbial biomass show altered properties of metal biosorption compared to the original biomass. If the biomass is large in size, they are grounded into fine granules and are treated further for efficient biosorption [8]. The characteristic feature of pretreatment is to modify the surface groups either by removing or masking or by exposing the greater number of binding sites [3]. It is also observed that the longer duration of pretreatment can


a Indicates the dry weight of the biosorbent, \* Indicates batch biosorption experiments at laboratory scale.

**Table 8.** Use of chemically modified (treated) biosorbents for the biosorption of metals.

involved in biosorption was carboxyl, amino, and hydroxyl groups. The Langmuir isotherm model showed the better fit with an ion exchange mechanism for biosorption [89]. **Table 7**

Since the process of biosorption relies on the number and availability of functional groups on the surface of the biosorbent, modification by changing the surface characteristics can greatly influence the capacity of biosorbent used for the removal of metal ions [137]. Microbial-derived

summarizes some more examples of fungi as biosorbents.

**7. Effect of pretreatment on biosorption**

**Biosorbent type Metal ion Biosorption capacity** 

As (III), Hg (II), Cd (II), Pb (II)

As (III), Hg (II), Cd (II), Pb (II)

Cd (II), Zn (II), Pb (II)

Pb (II), Ni (II), Cr (VI)

Ni (II), Pb (II)

(II), Zn (II), Cr (VI)

Indicates the dry weight of the biosorbent; <sup>b</sup>

**Table 7.** Fungal biomass used for biosorption of metals.

*Penicillium canescens*

84 Biosorption

*Penicillium chrysogenum*

*Pencillium purpurogenum*

*Pencillium simpliccium*

*Saccharomyces cerevisiae*

*Phanerochaete chrysosporium*

a

*Pleurotus ostreatus* Cu (II), Ni

experiments at laboratory scale.

**Biosorption capacity/efficiency (mg/g or %)**

26.4, 54.8, 102.7, 213.2 mg/ga\*

35.6, 70.4, 110.4, 252.8 mg/ga\*

52.50, 65.60, 76.90 mg/ga\*

ga\*

270.3, 46.3, 32.6 mg/

*Lentinus sajor* Cr (VI) 18.9 mg/ga\* Langmuir C–O, N–H,

*Pleurotus ostreatus* Cr (VI) 20.71%b\* –COOH,

8.06, 20.4, 3.22, 10.75 mg/ga\*

*Trametes versicolor* Ni (II) 212.5 mg/ga\* Langmuir Carboxyl,

**Isotherm model**

Ni 82.5 mg/ga\* [127]

*Aspergillus niger* Cu (II) 9.53 mg/gb\* [128]

*Aspergillus terreus* Cu (II) 180 mg/ga\* Freundlich [134]

Redlichpeterson and Langmuir

**Functional groups involved**

Langmuir [129]

Langmuir Physical

C–H

–NH<sup>2</sup>

–NH<sup>2</sup>

hydroxyl, amine

55.9, 53.6 mg/gb\* Ion exchange [135]

Langmuir –COOH,

Indicates the wet weight of the biosorbent; \*

Chemical ion exchange

adsorption

Physic-chemical adsorption

Ion exchange, surface complexation and electrostatic interaction

Physicochemical interaction

**Mechanism Reference**

[126]

[130]

[131]

[132]

[133]

[121]

[136]

Indicates batch biosorption

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**.
