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 polluted soil. The ploughed layer was sampled as undisturbed blocks for flooding experiments, and sieved samples were incubated in aqueous suspensions at controlled pH (pH 6.2) under different redox conditions. Redox potential and pH, concentrations of major and trace elements, and organic and inorganic ligands, were measured in the solutions. Their chemical speciation was calculated by using the programme Soilchem. These experiments and calculations have shown that pH variations definitively influence trace metal solubility, whatever they are induced by reductive dissolution, organic acid formation, or other processes, and that strong acidification can be obtained with well buffered soil when about 1% available carbon is anaerobically transformed into organic acids. The organic acids also intervene by complexation, particularly for Pb. On another hand, denitrification can limit these effects by consuming protons and organic substances. 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 rule, hydromorphy in a well-buffered contaminated soil at a first step should increase the mobility of divalent trace metals, by decreasing pH and reducing Mn and Fe oxides, but prolonged flooding can lead to fix trace metals again, rather by re-adsorption or precipitation phenomena than by formation of insoluble sulphides.