Influence of reducing conditions on solubility of trace metals in contaminated soils

Dissolved trace metals Cd, Pb, Zn, and other solutes were determined after reducing conditions have been imposed to samples of an agricultural pollute d soil. The ploughed layer was sampled as undisturbed blocks for flooding e xperiments, and sieved samples were incubated in aqueous suspensions at con trolled pH (pH 6.2) under different redox conditions. Redox potential and p H, concentrations of major and trace elements, and organic and inorganic li gands, were measured in the solutions. Their chemical speciation was calcul ated by using the programme Soilchem. These experiments and calculations have shown that pH variations definitive ly influence trace metal solubility, whatever they are induced by reductive dissolution, organic acid formation, or other processes, and that strong a cidification can be obtained with well buffered soil when about 1% availabl e carbon is anaerobically transformed into organic acids. The organic acids also intervene by complexation, particularly for Pb. On another hand, deni trification can limit these effects by consuming protons and organic substa nces. Given a steady pH, reducing conditions enhance the mobility of trace metals, at first by dissolution of manganic and ferric oxides; Pb appeared more sensitive to these processes than Zn, and finally Cd. As a general rul e, hydromorphy in a well-buffered contaminated soil at a first step should increase the mobility of divalent trace metals, by decreasing pH and reduci ng Mn and Fe oxides, but prolonged flooding can lead to fix trace metals ag ain, rather by re-adsorption or precipitation phenomena than by formation o f insoluble sulphides.