Biosorption of heavy metals by Sphaerotilus natans: an equilibrium study at different pH and biomass concentrations

In the present work a lyophilised cell suspension of Spherotilus natans was studied as biosorbent material for cadmium and copper removal from aqueous solutions. The biomass was firstly characterised by potentiometric titration and evaluation of the major ionic content. The experimental data highlight that the biomass cell wall contains two main acidic groups with a total amount of 5 meq/g. Equilibrium biosorption trials of cadmium and copper were carried out to investigate the effects of two important experimental factors, pH and biomass concentration. As expected, both cadmium and copper biosorption extent was repressed by pH decrease. The effect of the biomass concentration changes both with the equilibrium pH value and the kind of metal adsorbed. In the case of copper biosorption at pH>5, the increase of biomass concentration causes a diminution of the maximum specific metal uptake due probably to cell aggregation phenomena; whereas at acidic pH values the previous trend is inverted perhaps because of the effect of partial hydrolysis of the bacterial cell wall constituents. A different behaviour was observed for cadmium biosorption in relation to the microbial cell concentration: the effect of biomass concentration is less evident and opposite with respect to copper at pH 6 and 3 and no definitive explanation was found for this case. Equilibrium modelling was performed for both metals by using the most used equations reported in the literature. A comparison of the biosorption characteristic of S. natans has been also performed with respect to some results reported by other researchers.