LEAD PRECIPITATION IN THE PRESENCE OF SULFATE AND CARBONATE - TESTING OF THERMODYNAMIC PREDICTIONS

Lead precipitation tests from aqueous solutions containing 10 mg l(-1) Pb and 10(-2) M sulphate were performed in the pH range 3.9-11.3, at inorganic carbon concentrations from background levels (approximate to 3 x 10(-5) M) to 1.5 x 10(-2) M. Filterable lead and inorganic carbon concentrations, pH, and X-ray diffraction of the precipitate were measured during long-term ageing of precipitate suspensions in sealed bottles. Even though CO2 exchange with the atmosphere could not be completely prevented in the experimental procedure used, quasi equilibrium conditions between soluble and solid phases were achieved a few hours or days after reagents blending. Residual filterable lead concentrations greater than or equal to 0.1 mg l(-1) were obtained in the aged precipitate suspensions. A satisfactory agreement between solubility predictions by the equilibrium speciation model MINTEQA2 and filterable lead concentrations measured in aged samples may be obtained only with a critical selection of the solid phases considered in the speciation model. X-ray diffraction analysis showed the formation of three different solids: anglesite (PbSO4(s)), cerussite (PbCO3(s)), and hydrocerussite (Pb-3(CO3)(2)(OH)(2(s))). Other solid phases, even though stable from a thermodynamic point of view, seem to be hindered by kinetic limitations from precipitating at room temperature, and are not appropriate solid phases in modelling lead precipitation from aqueous solutions. The results of this work may be useful in predicting maximum achievable efficiencies in the neutralization/precipitation treatment of lead-bearing wastewaters.