PREDICTING TRACE-METAL SPECIATION IN SULPHIDIC LEACHATES FROM ANAEROBIC SOLID-WASTE DIGESTERS BY USE OF THE PH2S-VALUE AS A MASTER VARIABLE

The activity of metal ions in sulphidic solutions is buffered heteroge neously due to the formation of sparingly soluble metal-sulphide preci pitates. Assuming a pure metal-sulphide phase (MeS) the metal ion acti vities are controlled by the activity of H2S and the pH: {Me2+} = K(so )/{S2-} = K(so){H+}2/{H2S}K(al)K(a2) = K(s2){H+}2/{H2S}. The total met al ion concentration, however, can be increased by complexing ligands. The interaction between metal ions, H2S and other ligands can be quan tified by use of the two master variables, pH2S (= - log{H2S}) and pH, both of which can be measured easily (pH2S by use of a pH2S electrode cell). A pH2S scale is defined with sulphide as the strongest coordin ation partner for B-type metal ions. Consequently, the coordination te ndency of metal ions in sulphidic solutions is discussed relative to t he activity of sulphide, respectively to that of its protonated form, H2S. This concept is applied on data from an experimental study on tra ce-metal mobility in sulphidic leachate during anaerobic digestion of solid wastes. The fate of Zn, Cd and Fe can be predicted using double- logarithmic pH-pH2S diagrams. A high potential for metal leaching by v olatile fatty acids, produced in high amounts during the anaerobic dig estion process, was observed for Fe, and to a lesser extent for Zn. In contrast, the solubility of Cd is controlled by the intrinsic solubil ity of a sulphidic system, i.e. the presence of hydrogen sulphide comp lexes. In case of Pb and Cu high supersaturations were observed with r espect to the intrinsic solubility, probably due to the presence of un known strong ligands. In this case a procedure is introduced how to es timate the concentration and conditional stability constants of a meta l-ligand complex by use of the three easily measurable variables pH2S, pH and total metal concentration. A method is described to calculate intrinsic saturation indices, where the formation of hydrogen sulphide complexes as an intrinsic property of sulphidic solutions is consider ed. Intrinsic saturation indices can be used as a criterion for the pr esence of metal-leaching ligands in a sulphidic system.