Estimating sludge loadings to land based on trace metal sorption in soil: effect of dissolved organo-metallic complexes

This paper describes the results of research examining the effect of dissolved organo-metallic complexes of copper (Cu) and zinc (Zn) from sewage sludge leachate on sorption by a humic-gley soil A-horizon, and the influence of such complexes on resultant sludge loading estimates. Sorption was described with Linear, Freundlich or Langmuir equations, and compared between a sample of sludge leachate (containing 97.4% of Cu and 63.2% of Zn as dissolved organo-metallic complexes) and a reference solution (which mimicked the leachate, except for a lack of dissolved organic material). This comparison revealed that dissolved organo-metallic complexes significantly depressed Cu and Zn sorption in the study soil. The isotherm equations were then used to estimate sludge-derived Cu and Zn loadings to soil in order to result in an “allowable” output concentration from the soil solution to the surrounding environment. These loadings, together with soil bulk density and “availability” of sludge Cu and Zn, were incorporated in a preliminary model to estimate sludge application rates which are acceptable in terms of off-site movement of these metals through leaching losses. In the absence of dissolved organo-metallic complexes (sorption from the reference solution), levels of Cu and Zn sorption in the study soil indicated a sludge application rate of approximately 3500 kg/ha. However, when Cu and Zn sorption from the sludge leachate with dissolved organo-metallic complexes was considered, calculated loading rates were reduced to approximately 690 kg-sludge/ha. This suggests that for sludge loading estimates based on soil sorption characteristics to be relevant to environmental protection, the sorption depressing effect of dissolved organo-metallic complexes should be quantitatively considered.