| Summary: | 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 ion exchange.
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