Thermally Induced Release of Adsorbed Pb upon Aging Ferrihydrite and Soil Oxides

Noncrystalline iron oxides are among the soil components that could limit Pb solubility and bioavailability in soils. Long-term changes in Pb solubility may occur after noncrystalline iron oxides transform into well-crystallized materials. In this study, thermally induced transformation products and the solubility of preadsorbed Pb were compared in a synthetic ferrihydrite and two natural materials containing iron oxides. The adsorbents were characterized initially and after heating for 60 days at 70 °C. Dissolved Pb was measured by differential pulse anodic stripping voltammetry (dpasv) in the supernatants after heating the suspensions for 60 days at 70 °C and compared to the solubility after adsorption for 15 days at room temperature. Lead activities were calculated from dpasv measurements. Surface area, FTIR, XRD analyses, and extraction with oxalate and pyrophosphate were used to characterize the solid phases. Iron oxide transformation products differed among the systems. The laboratory-synthesized ferrihydrite showed the most distinct transformation to goethite and hematite. Goethite formation was also evidenced from one natural (SMS-1) material. The other (SMS-2) material, with a low surface area initially and no FTIR or XRD evidence of crystalline or noncrystalline iron oxide, revealed limited transformation after thermal treatment. Despite these differences, Pb activity increased from all adsorbents after heating for 60 days at 70 °C. The amount of adsorbed Pb had no effect on iron oxide transformation products. Differences in the degree of transformation and in the identity of transformation products may be due to the presence of organic matter, Si, and Al in the pedogenic materials as well as to the initial Fe forms present in the samples. The results suggest that in addition to iron oxide recrystallization, thermally induced changes in other soil constituents (e.g., organic matter, noncrystalline ferro-alumino-silicates) may be responsible for increased Pb activities in solution.