Biosorption of toxic heavy metals by fungal biomass

Abstract

Research in recent years has established that metal biosorption using microbial biomass can be developed into potentially cost-effective process for removing metals or recovering valuable metals from industrial effluents. Of particular interest are abundant fungal biomass types produced as a waste byproduct of large-scale industrial processes which can be very inexpensive sources of metal biosorbent. Different fungal strains have been selected for studying their ability to remove lead and copper ions from aqueous solution. Batch biosorption experiments were conducted to examine the kinetic and equilibrium of lead and copper biosorption. The biosorption kinetic data shows that both lead and copper biosorption may be divided into two phases: (i) a fast biosorption phase with most of the metal ions taken up from solution within the first 15 minutes, and; (ii) a much slower second phase which continued even after 24 h. For equilibrium studies of lead biosorption, the Langmuir isotherm fit the equilibrium data better than the Freundlich isotherm. Aspergillus niger and Mucor rouxii exhibited exceptionally high lead biosorption capacities, up to 50 % of the fungal biomass dry weight employed. These two fungal strains may be applied to develop potentially cost-effective biosorbents for removing lead from effluents. For equilibrium studies of copper biosorption, the Freundlich isotherm generally fit the equilibrium data better than the Langmuir isotherm. Aspergillus nidulans, Mucor rouxii, Cladosporium cladosporioides and Phycomyces blakeleeanus (+) exhibited moderate copper biosorption capacities (about 1-1.5% of the fungal biomass dry weight employed). Based on these kinetic and equilibrium data, Mucor rouxii has been selected for further studies. The pH was found to have significant effect on lead and copper biosorption. The Mucor rouxii biomass exhibited the highest removal efficiency at pH 6.0 for lead and at pH 5.0 for copper. The effects of chemical treatments on copper biosorption of Mucor rouxii were studied. It was found that while alkaline treatment significantly increased copper biosorption capacity by 100%. acid, heat and formaldehyde treatments reduced copper biosorption capacity. The substitution of mycological peptone with bacto peptone in the growth medium caused the morphological change of the fungal biomass from filamentous form to yeast-like form. However, no improvement of the lead or copper biosorption capacities was observed. The biomass was also found to adsorb a variety of different metal cations. At pH 5.0, the biosorption capacities for metals decreased in the following order: Pb2+ > Zn2+ > Mg2+ > Cr3+ > Cd2- > Ag+ > Ni2+ > Cu2+ > Co2+ > Na+. A linear correlation between biosorption capacity and ionic radius of the metal ions was not observed. However, the metal biosorption capacity correlated much better with Z2/r where Z is the ionic charge and r the ionic radius. Equilibrium batch studies of metal biosorption had been extended to binary metal system (Pb2+ + Cu2+). Scatchard plots of lead biosorption at different initial copper concentrations strongly indicated the presence of multiple lead binding sites on the cell surfaces. Three-dimensional biosorption isotherm surfaces had been used to evaluate the performance of the binary metal biosorption system. The biosorption of lead decreased when copper was present. Three biosorption mathematical models (i.e. competitive, noncompetitive and modified multi-component models) were evaluated for their predictive ability of metal biosorption for the binary metal system. The modified multi-component model had the smallest sum of square residue which indicated a greater match with the experimental data.