The REDEQL.EPAK computer model was used to study speciation of Pb, Cd, Zn, and Ca in leachates from dolomitic Pb mine tailings. By allowing or disallowing precipitation of solids and equilibration of the modelled leachate with atmospheric C02, comparison of fresh and aged leachates was made. The effects of treatment of the tailings with phosphate containing fertilizer were studied through addition of P04 (3-) to the modelled solution. Equilibrium constants pertaining to metal ion-humic acid complexation were added to the thermodynamic data base of the model in order to study the effects of decaying plant material on tailings leachate.Initial leachate of the tailings is found to be supersaturated with Cd and Zn. Non-complexed ("free") Cd(2+) and Zn(2+) is predicted to comprise most of the soluble form of these metals in the leachate; Pb is predicted to be present largely as PbCO3 ion pair. Equilibration of the leachate with the atmosphere is predicted to lead to extensive precipitation of CdCO3 and ZnSiO3. Precipitation of Pb5(PO4)3Cl is predicted at high PO4 (3-) concentration and at low pH. Complexation by the humic acid is predicted to compete effectively with other ligands in the leachate for the metal ions. The results are compared with experimental findings.
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