**10.1. Effect of pH**

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 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].
