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SOIL CHEMISTRY

Dissolution of Zinc-Cadmium Sulfide Solid Solutions in Aerated Aqueous Suspension

Dep. of Crop and Soil Sciences, Cornell Univ., Ithaca, NY 14853

^* Corresponding author (mbm7{at}cornell.edu).

Anoxic conditions in Zn- and Cd-contaminated soils and sediments result in the formation of highly insoluble metal sulfides. Little is known, however, about the ability of mixed Zn–Cd sulfides, the most likely solid phases to form in these environments, to persist and limit Zn and Cd solubility under intermittently oxidizing conditions that are common with fluctuating water tables in wet soils. We therefore conducted laboratory experiments to measure the zero-order rate constants for oxidative dissolution of synthetic solid solutions of Zn–Cd sulfides under aerated conditions, and the release of SO₄^2–, Zn²⁺, and Cd²⁺ into solution. It was found that ultrafine synthetic metal sulfide particles with moderately high Zn/Cd mole ratios (20) were relatively stable under fully aerated conditions, and oxidized at much slower rates than sulfides with low Zn/Cd ratios. At the same time, high-Zn sulfides were very efficient at retaining Cd in insoluble form even at very high solid-phase Cd concentrations (>10000 mg kg^–1) and low pH (<5), preferentially releasing Zn into solution. The preferential retention of Cd in sulfide particles with high Zn/Cd ratios was indicated by the persistence of much higher Zn/Cd mole ratios in solution than in the solid as the sulfide suspensions were equilibrated with atmospheric O₂. In contrast, Cd-rich Zn–Cd sulfides were readily oxidized under aerated conditions with the release of high concentrations of both Zn and Cd into solution. We concluded that colloidal Zn–Cd sulfide solid solutions high in Cd would be unlikely to limit the solubility of Cd or Zn to levels that would be nontoxic to soil biota or plants, whereas metal sulfides with high Zn/Cd ratios may be sufficiently persistent to retain Cd in an insoluble form in intermittently aerobic as well as anaerobic heavy-metal-contaminated wetlands.

Abbreviations: EC, electrical conductivity • FAA, flame atomic absorption • ICP, inductively coupled plasma