Operationally determined speciation of copper and zinc in sewage sludge

Abstract

The distribution of copper and zinc in the different component phases of sewage sludge from a treatment plant has been determined by means of single and sequential extraction methods. The difference of this distribution in mixed fresh, digested and bed sludge has been assessed. The selective extraction procedure has made it possible to demarcate five metal fractions: exchangeable, oxidisable, acid soluble, reducible and residual. It is found that, during treatment, the metals become less and less easy to mobilise and tend to associate with the reducible and residual phases. Copper and zinc associated with the exchangeable and oxidisable phases are, in fact, easily mobilised by ion-exchange reaction and by the decomposition and transformation of organic matter. Based on the data for fractionation of copper and zinc in various phases, the ranking of different forms of copper and zinc in sewage sludge is found as follows.Cu: acid soluble > residual > reducible > oxidisable > exchangeableZn: acid soluble > reducible > residual > oxidisable > exchangeableThe above sequence is found to hold uniformly for all the sludge samples viz. mixed fresh, digested and bed sludge. An estimate of these forms is important for understanding how the metals are bound to the sludge and their mobility in the soil. It can help to explain the process by which metals are eliminated from sewage sludge and to evaluate facilities required for their removal. It can also indicate the impact of the use of sludge on agricultural soils, as amendments. Operationally determined speciation investigated by selective extraction technique suggests the use of sequential extraction rather than single extraction may be of greater value for determining metal profiles in sewage sludges.