13.3 Chemistry of Heavy Metals in Water

191

13.2

Mechanisms Involved in Biosorption of Toxic

Heavy Metal Ions and Dyes

The elucidation of interaction mechanisms involved in the removal of metal and

metal complex dyes (MCDs) by the biosorbents is very important to enhance the

biosorption process as well as to recover the metal ions and metal-chelated dyes.

The multifaceted structure of biomass indicates that it has different ways to remove

the metal contaminants. However, the mechanism of metal and MCD interaction

with biosorbent is not yet fully understood. Hence, the mechanisms involved in

the bioremediation of both metal and dyes were found to be complicated ones. The

mechanisms of metal biosorption were influenced by various factors such as (i) live

and dead microbial cells, (ii) kinds of microbes used, (iii) characteristics of metal and

metal-complexed dyes, and (iv) physiochemical conditions such as pH and temper-

ature. Biosorption is a metabolic-independent process that is based on the passive

sequestration of metal species on the surface of the dead biomass. The bacterial

biomass consisting of several chemically active groups that tend to attract metal

ions and dyes from aqueous solution get sorbed onto its surface. The sequestration

of metal and MCD on the surface of the biomass was followed by physio-chemical

interaction with the functional groups present on the microbial surface [8].

The bacterial cell wall as already discussed in the previous section mainly con-

sists of several biomolecules namely polysaccharides, proteins, and lipids. The toxic

heavy metals adhere to various functional groups such as phosphate, sulfate, amino,

and carboxyl group by electrostatic attraction, ion exchange, complexation, van der

Waal’s forces, covalent bonding, and micro-precipitation. The magnitude of biosorp-

tion is influenced by several factors such as the valance of the metal ions, bacterial

genus, and variations in cellular constituents. The biosorption process also depends

on several environmental factors such as ionic strength, solubility of the metal ions,

solution chemistry, pH, and temperature conditions [9]. One possible way to increase

the biosorption process is using less hydrophilic molecules which lowers the affinity

between solute and solvent and therefore get biosorbed more easily [10].

13.3

Chemistry of Heavy Metals in Water

In the sorption process, it is very important to study the two phases, solid and liquid.

The solute or sorbate must be dissolved in the solution which ultimately interacts

with the solid phase. The performance of the biosorption process will be affected by

the properties and behavior of both the sorbate and the sorbent. The speciation of

metal ions in the solution is mainly reliant on pH which is an important parameter

in a sorption process. The metal in the solution might be cationic or anionic when

it becomes dissociated in the solution. Most of the metals expose positively charged

cationic species when they are dissolved in the aqueous media. Different form of

metal ionic species, protonation–deprotonation of active sites on bio-sorbents, is also

dependent on the pH of solution [11]. The protonation of functional groups leads to

lowering of cationic metal ion sorption. At low pH, all functional groups present on