The application of PHstat leaching tests to assess the PH-dependent release of trace metals from soils, sediments and waste materials

pH is one of the key parameters that determines heavy metal mobility in soils, sediments and waste materials. In many respects leaching behaviour as reflected by the pHstat leaching tests provide a better means of assessing environmental impact than analysis of total elemental composition. This paper discusses the use of pHstat leaching tests as a tool to assess the potential mobilisation of trace metals from soils, sediments and waste materials. The possibilities of pHstat leaching tests are illustrated by means of different examples. Titration curves (ANC=acid neutralizing capacity) obtained in the pHstat tests with continuous setpoint titration and the release of elements as a function of time were described as the sum of two independent first-order reactions, which represent ‘buffers’ or ‘pools’ with a different reactivity. The mathematical description of metal leaching behaviour from soils and sediments enabled a distinction between 5 groups of elements with a different leaching behaviour, which could be related to ‘pools’ with different reactivity. Contrary to single and sequential extractions, where attention is focussed on the reactivity towards a reagent (which is often difficult to link with a natural process), the reactivity and mobility of metals as a consequence of a process (acidification in the case of pHstat leaching) is investigated. Moreover, the potential buffering capacity of the sample and its sensitivity to pH changes as a result of external stresses (e.g. soil acidification, liming) can be estimated.