Metal removal from wastewater by bacterial sorption: kinetics and competition studies

Biosorption has distinct advantages such as high efficiency and cost-effectiveness for removal of metal ions from wastewater. However, most studies of bacterial biosorption have focused on the removal of a single metal ion from aqueous solutions, even though natural water and wastewater rarely contain only one kind of heavy metal. This study investigated competitive biosorption of Co2+, Ni2+ and Cr3+ in different combinations at various concentrations under conditions of co-existence in a synthetic wastewater system utilizing the gram-negative bacterium Pseudomonas aeruginosa. In a binary system with Co2+ and Ni2+, the presence of the same concentration of dissolved Ni2+ led to a significant decrease in the amount of Co2+ adsorbed onto cells when compared with a single Co2+ system in which Ni2+ was absent. This result is most likely due to the similar competitive affinity of these equivalent cations for sorption sites located on the cells of the bacterium. However, the presence of Cr3+ with either Co2+ or Ni2+ led to a strong reduction in the uptake of each of these bivalent ions by cells, whereas Cr3+ adsorption was unhindered. Cr3+ also significantly inhibited the adsorption of both Co2+ and Ni2+ onto cells in a ternary system involving Co2+-Ni2+-Cr3+, with the order of preferential metal uptake being Cr3+ > Co2+ approximately Ni2+. Removal efficiency of the heavy metals reached 100 % at low concentrations ([Cr3+]=[Co2+]=[Ni2+=20 microM) in a multi-metal system. The results of this study indicate that pre-determination of co-existing heavy metals in wastewater and examination of their competition for the sorption sites of biosorbents are required for the efficient removal of inorganic toxicants utilizing biosorption processes.