Metal partitioning and toxicity in sewage sludge

Partition coefficients (K_D) for Cd, Cu, Pb and Zn in a sludge/water solution were determined for sludge/water solutions as an operationally defined three-phase system (particulate, colloidal and electrochemically 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 phase). Partitioning results were interpreted in terms of metal mobility from sludge to colloids and in terms of potential bioavailability from colloids to electrochemically available. It was found that the partition coefficient for a two-phase system, K_D2 and the partition coefficient between the colloidal metal and the particulate metal in a three-phase system, K_D3p/c have similar values, which means that, for sewage sludge, the two-phase partition coefficient reflects the partitioning between particulate metal and colloidal metal, not dissolved metal. In the absence of a particulate phase, Zn and Cd partition between the colloidal phase and the electrochemically available phase, while Cu and Pb remain in the colloidal phase. Ionic strength had no clear effect on K_D, while increased pH resulted in lower metal concentrations in the colloidal and electrochemically available phases. The toxicity of 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.