Biosorption of heavy metals by activated sludge and their desorption characteristics

The biosorption of different metals (Cu2+, Cd2+, Zn2+, Ni2+ and Pb2+) was investigated using activated sludge. The optimum pH was 4 for Cd, Cu and Pb sorption and 5 for Ni and Zn. Biomass metal uptake clearly competed with protons present in the aqueous medium, making pH an important variable in the process. Protons consumed by biomass in control tests versus protons exchange in biosorption tests confirmed a maximum exchange between metal cations and protons at pH 2. The study of the influence of biomass concentration revealed that the amount of protons released from biomass increased with biomass concentration. This would confirm the hypothesis of ion exchange between both types of ions. The application of the Langmuir and Freundlich models showed a better fitting of experimental data to the first model. The maximum sorption uptake of the studied metals by the activated sludge showed the following decreasing order: Pb>Cu>CdZn>Ni. Desorption experiments showed that HCl was a good eluent for the five metals tested, particularly at low pH values (1 and 2). At pH 3 or 4 the desorption yield was significantly lower. However, its use did not allow the reuse of biomass in subsequent loading and unloading cycles. EDTA was also a good desorption agent, achieving the total recovery for the five metals tested at a concentration of 1mM, with the advantage that biomass could be reused for three sorption-desorption cycles.