**7. Factors affecting biosorption**

*6.1.4.1. Biosorption by yeast*

**Sr. No.** sporocarps

32 Biosorption

1. Volvariella volvacea (edible Mushroom) – mycelia,

**Table 5.** Mushrooms and biosorption of different metals [48].

**Metals Yeasts Temperature** 

2. Chromium *Saccharomyces cerevisiae Candida utilis*

4. Copper *Saccharomyces cerevisiae*

*Candida pelliculosa Schizosaccharomyces pombe*

**Table 6.** Yeasts and their biosorption features regarding different metals [48].

**(°C)**

25 25

25 30 25

1. Cadmium *Saccharomyces cerevisiae* 25 7 100 2 2 12.3 [69]

3. Cobalt *Saccharomyces cerevisiae* 25 7 100 2 2 8.2 [162, 163]

5. Lead *Mucor rouxii* 25 5.0 125 15 — 17.13 [166] 6. Mercury *Saccharomyces cerevisiae* 25 7 100 2 2 76.2 [162] 7. Nickel *Saccharomyces cerevisiae* 25 7 100 2 2 14.1 [162] 8. Zinc *Saccharomyces cerevisiae* 25 7 100 2 2 11.8 [162]

5.2 5.5

7 6 4 150 160

100 120 -

The free form of yeast cells is not considered good candidates for biosorption [86]. Free cells face the problem of separation of solid liquid phase. This problem seems to be less effective in flocculating cell [90]. Pretreatment of yeast cells can result in increased surface to volume ration for binding of metal with the metal binding sites. It is reported that pH above 5 optimizes the metal biosorption in yeast cells [91]. According to Abbas et al., [48] in yeasts, higher concentration of heavy metals can be accumulated by bioaccumulation process than biosorption. However, general biosorption is responsible for the major uptake of heavy metals for many filamentous fungi. Biosorption of various metals by different yeasts is given in **Table 6**.

**pH Agitation Time Wt** 

1 1

2 120 96

**(g/L)**

80 1.0

2 13.3 -

**q(mg/g) or % removal**

55.3% 28

29.9 95.04% 74.85

**References**

[162] [162]

[162] [164] [165]

**Sr. No. Mushrooms Metals References**

2. Ganoderma lucidum Chromium [69, 159] 3. Coriolopsis strumosa Copper [160] 4. Daedalea tenuis Copper [160] 5. Lentinus strigosus Copper [160] 6. Lenzites malaccensis Copper [160] 7. Phellinus xeranticus Copper [160] 8. Rigidoporus lineatus Copper [160] 9. Rigidoporus microporus Copper [160] 10. Trametes lactinea Copper [160] 11. Ganoderma lucidum Copper [159, 160] 12. Agaricus macrospores Cadmium, mercury, copper [161]

Cadmium, lead, Copper,

[158]

Chromium

Biosorption process is affected by following factors.

**Temperature**: For efficient removal of metal ions from environment, the optimum temperature needed to be investigated. It is generally assumed that biosorption is carried out between 20 and 35°C. High temperatures above 45°C may results in damage to proteins which in turn affects metal uptake process [48, 93–95].

**pH**: It is a very important parameter. It affects solubility of metal ions and binding sites of biomass. At lower pH, the biosorption of metals is affected [96, 97]. General range of pH for metal uptake is between 2.5–6. Above this limit, metal uptake ability of biosorbent gets compromised [48].

**Nature of biosorbents**: Metal uptake is reported in different forms like biofilms, freely suspended microbial cells or immobilization of microbial cells. It can be altered by physical or chemical treatments. Physical treatments include autoclaving, drying, boiling, sonication, etc. Chemical treatment as the name indicates involves chemicals like acid or alkali to improve biosorption capacity. According to Wang and Chen, [75], the fungal cells are deacetylated which affects the structure of chitin resulting in the formation of chitosan-glycan complexes which have results high metal affinities. Abbas et al., [48] also report about effect of age, growth medium components on biosorption as they might result in cell wall composition, cell size and EPS formation.

**Surface area to volume ratio**: This property plays an important role in efficient removal of heavy metal from medium. The surface area property plays a significant role in case of biofilms [48]. The binding of metal ions with microbial cell wall is previously reported [98]. Although intracellular metal adsorption is energy-consuming process but still microorganisms prefer it over wall adsorption.

**Concentration of biomass**: The concentration of biomass is directly proportional to the metal uptake [48, 98, 99]. It is reported that electrostatic interaction between the cells plays an important role in metal uptake. At a given equilibrium, the biomass adsorbs more metal ions at low cell densities than at high densities [100]. Metal uptake depends on biding sites. More biomass concentration or more metal ions restricts the access of metal ions to binding sites [48, 101].

**Initial metal ion concentration**: The initial concentration provides an important driving force to overcome all mass transfer resistance of metal between the aqueous and solid phases [102]. Increasing amount of metal adsorbed by the biomass will be increased with initial concentration of metals. Optimum percentage of metal removal can be taken at low initial metal concentration. Thus, at a given concentration of biomass, the metal uptake increases with increase in initial concentration [48].

**Metal affinity to biosorbent**: Physical/chemical pretreatment affects permeability and surface charges of the biomass and makes metal binding groups accessible for binding. It can be manipulated by pretreating the biomass with alkalis, acids detergents and heat, which may increase the amount of metal uptake [48, 94].
