Toxic Metals in the Environment: Thermodynamic Considerations for Possible Immobilization Strategies for Pb, Cd, As, and Hg

The contamination of soils by toxic metals is a widespread, serious problem that demands immediate action either by removal or immobilization, which is defined as a process which puts the metal into a chemical form, probably as a mineral, which will be inert and highly insoluble under conditions that will exist in the soil. If metals are to be immobilized, this might be achieved by the addition of sufficient amounts of the anion or anions which can form the inert mineral. A serious complication arises from the fact that all soils have several other cations that can and do react with the anions. This paper is a review of the equilibrium-state chemistry for the possible immobilizations of four metals: lead, cadmium, arsenic, and mercury. The anions which might precipitate these metals include: oxide, hydroxide, chloride, sulfate, sulfide, phosphates, molybdate, and carbonate. The metal ions which can interfere with these precipitation reactions are calcium, magnesium, iron, aluminum, and manganese. All of the probable combinations are reviewed, and possible immobilization strategies are evaluated from the point of view of thermodynamic stability using free energies of formation scattered throughout the literature. The systems are examined from the point of view of the phase rule and stoichiometric consideration over the 2–12 pH range.