Chemical associations of heavy metals and metalloids in contaminated soils near former ore treatment plants:: a differentiated approach with emphasis on pHstat-leaching

Application of pH(stat)-experiments on soil samples of two heavily contaminated sites in northern Belgium (Flanders) was used to make long-term risk predictions of environmentally relevant heavy metals (Pb, Cu, Ni, Zn, Cr) and arsenic. Implementation of the results of other techniques (sequential extractions and mineralogical research) and geochemical modeling with MINTEQA2 was used to obtain a refined understanding of the geochemical association and the processes that control pH-dependent leaching characteristics of the contaminants. The soil on the first site (a former arsenic refinery in Reppel) was extremely polluted with As, Cu (percentage level), Co and Ni (>0.5%) and leaching of contaminants, which is already alarming, will further increase in the near future by soil acidification. The second site was contaminated with so-called burnt pyrite (sulfuric acid industry) containing high concentrations of Pb, Cu, As and Zn (>0.1%) and a serious threat of metal leaching will manifest if no measures are taken. The kinetics of buffering reactions were evaluated by mathematically fitting of the titration curves. Thereby, two independent proton buffer reactions (rapid and slow) were differentiated and quantified. In the paper, some aspects of the pH(stat)-procedure were also reconsidered and evaluated. It was shown that one of the critical parameters, namely the duration of the leaching, needs to be extended to 96 h. A 24-h test was observed to be inadequate in assessing heavy metal release properly. (C) 2002 Elsevier Science B.V. All rights reserved.