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

Ln 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 titrati on and evaluation of the major ionic content. The experimental data highlig ht 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 wer e carried out to investigate the effects of two important experimental fact ors, pH and biomass concentration. As expected, both cadmium and copper bio sorption extent was repressed by pH decrease. The effect of the biomass con centration 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 bio mass concentration causes a diminution of the maximum specific metal uptake due probably to cell aggregation phenomena; whereas at acidic pH values th e previous trend is inverted perhaps because of the effect of partial hydro lysis of the bacterial cell wall constituents. A different behaviour was ob served for cadmium biosorption in relation to the microbial cell concentrat ion: 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 th e most used equations reported in the literature. A comparison of the bioso rption characteristic of S, natans has been also performed with respect to some results reported by other researchers. (C) 2001 Elsevier Science B.V. All rights reserved.