METAL PARTITIONING AND TOXICITY IN SEWAGE-SLUDGE

Partition coefficients (K-D) for Cd, Cu, Pb and Zn in a sludge/water s olution were determined for sludge/water solutions as an operationally defined three-phase system (particulate, colloidal and electrochemica lly available) over a range of pH values, ionic strength, contact time and sludge/water ratios and were compared with K-D values for sludge/ water solutions as a two-phase system (aqueous phase and particulate p hase). Partitioning results were interpreted in terms of metal mobilit y from sludge to colloids and in terms of potential bioavailability fr om colloids to electrochemically available. It was found that the part ition coefficient for a two-phase system, K-D2 and the partition coeff icient between the colloidal metal and the particulate metal in a thre e-phase system, K-D3p/c have similar values, which means that, for sew age sludge, the two-phase partition coefficient reflects the partition ing between particulate metal and colloidal metal, not dissolved metal . In the absence of a particulate phase, Zn and Cd partition between t he colloidal phase and the electrochemically available phase, while Cu and Fb remain in the colloidal phase. Ionic strength had no clear eff ect on K-D while increased pH resulted in lower metal concentrations i n the colloidal and electrochemically available phases. The toxicity o f Pb-spiked sludge (K-Dtox) is well described by its partitioning in a two-phase system, K-D2ads, while the toxicity of Zn is underestimated .