SOLUBILITY OF HEAVY-METALS IN A CONTAMINATED SOIL - EFFECTS OF REDOX POTENTIAL AND PH

To assess the mobilities of Pb, Cd, and Zn from a contaminated soil, t he effects of redox potential and pH value on metal solubilities were investigated. Both redox potential and pH were found to greatly affect heavy metal solubility in the soil. Results showed that the soil susp ension under continuous oxygen aeration for 21 days resulted in increa ses of redox potential from 290 to 440 mV and pH value from 6.9 to 7.0 , respectively. Soluble concentrations of Pb, Cd, and Zn varied with t ime, and were all lower than 1 mg kg(-1). When the soil suspension was aerated with nitrogen, final redox potential was -140 mV, and pH valu e of 7.1. The soluble metal concentrations were slightly higher than t hose aerated with oxygen. The equilibrium solubility experiments were conducted under three different pH values (3.3, 5.0, 8.0) and three re dox potential (325, 0, -100 mV). Results showed that metals were spari ngly soluble under alkaline conditions (pH = 8.0). Metal solubilities were higher when under slightly acidic conditions (pH = 5.0), and incr eased drastically when pH was kept at 3.3. When solubilities were comp ared under same pH values, it was observed that metal solubilities inc reased as redox potential decreased. Generally speaking, acidic and re ducing conditions were most favorable for metal solubilization, and th e effect of pH was more significant than that of redox potential. It w as proposed that heavy metals were mostly adsorbed onto Fe-Mn oxyhydro xides. The pa-dependent metal adsorption reaction and the dissolution of Fe-Mn oxyhydroxides under reducing conditions was the mechanism con trolling the release of heavy metals from soils.