Heavy metal binding by the polymeric organic fractions of sewage sludges

Organic complexes in sewage sludge play an important role in the speciation and transformation of metals into potentially more toxic and bioavailable forms. Two organic fractions, bacterial extracellular polymer and fulvic acid, were extracted from mixed liquor and digested sewage sludge by methods established as the most appropriate. The homogeneity of the extracts was verified using gel permeation chromatography. Conditional stability constants and complexation capacities of the organic fractions from the sludges with copper, cadmium, nickel and zinc have been determined using equilibrium dialysis titration. Organic fractions extracted from digested sludge demonstrated a greater capacity to complex metals over mixed liquor extracts. Copper formed stronger complexes than nickel with the mixed liquor biomass and cadmium exhibited the greatest affinity for digested sludge organic matter.