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Effect of extracellular polymer composition of activated sludge on the removal of heavy metals by biosorption

Yüncü, Bilgen
Activated sludge microorganisms can remove many hazardous substances from wastewater by adsorbing and concentrating them on their surfaces. Biosorption of these substances onto activated sludge surfaces are influenced by the chemical properties of the substance in question as well as the surface properties of the microorganisms. The purpose of this study is to identify the biosorption mechanisms of heavy metals and the effect of extracellular polymer (ECP) composition of activated sludge on the biosorption of Pb(II), Cd(II), Cu(II), Zn(II) and Ni(II). Microorganisms cultured under different growth conditions are expected to have different compositions of ECPs and hence, different biosorption capacities. For this purpose, three sets of reactors with C/N ratios of 9, representing a carbonlimited case; 21, representing conventional municipal wastewater treatment plant activated sludge and 43, representing nitrogen-limited condition, were set up. The semi continuous reactors were fed synthetically and operated at a sludge age of 8 days. Isotherm and kinetic experiments that were held with three different C/N ratios was indicated that the biosorptive capacity of activated sludge was highly dependent on metal species and the C/N ratio. Although, the dependence of biosorptive capacity on C/N ratio was different for each metal, biosorption properties of activated sludge were found to be directly related with ECP composition. Among the heavy metals tested, Pb(II) was the one that was adsorbed at the highest capacity at all C/N ratios. Also, with the purpose of understanding the mechanism of the process, Ca(II) and Mg(II) ions and carbohydrates released into the solution were also monitored and it was indicated that an ion exchange process is involved in the biosorption of heavy metals especially at high metal concentrations but the whole metal removal can not be explained by