Assessment of the effects of soil amendments on the leaching of lead and arsenic from contaminated soil

Abstract

Lead has a long history in the life of human beings. Unfortunately, only recently did people start to realize that lead is a toxin. Because of increasing human activity, soil lead pollution has become a serious environmental problem. Phosphorus is an important amendment for the in situ immobilization of lead in contaminated soils. To evaluate the effect of phosphorous on the bioaccessibility of lead in contaminated soil, a Physiologically Based Extraction Test (PBET) was used in this thesis research. Three kinds of lead contaminated solids were used in the in situ immobilization test. In the in situ process, enough phosphate was added to form chloropyromorphite (the least soluble lead-phosphate salt). In the PBET process, chloropyromorphite was oversaturated in the PBET supernatant for lead-contaminated soil, potentially indicating that it could precipitate from the PBET solution itself, resulting in an experimental artifact. To better understand the lead reactions occurring in the process, the formation of chloropyromorphite was studied in HCl- and HNO3- based PBET solution. In the presence of phosphate, lead was not efficiently removed from the PBET solution under either homogeneous or heterogeneous conditions. In homogeneous chloropyromorphite formation tests under different conditions, the lead solubility in the PBET solution was generally greater than predicted for chloropyromorphite. The results indicate that the PBET is an acceptable method to evaluate the bioaccessibility of lead contaminated soils that are treated by phosphate amendments, as almost no lead precipitation occurred in the PBET process. Arsenic (As) is a toxic metalloid that has caused widespread soil contamination problems. While various iron-based amendments have been studied for immobilizing As in contaminated soils, the feasibility of stabilized iron-based nanoparticles has not been reported. This study investigated the effectiveness of using three types of iron-based nanoparticles, including zero-valent iron (ZVI), ferrous sulfide (FeS), and magnetite (Fe3O4), for the immobilization of arsenic in two kind contaminated soils. Different Fe/As molar ratio (5:1 ~100:1) and treatment times (3 and 7 days) were conducted to evaluate the effect of these nanoparticles. Higher amounts of iron particles were more effective in reducing As bioaccessibility and leachability. In addition, magnetite nanoparticles generally worked a little better than FeS and ZVI nanoparticles in reducing the bioaccessibility. For two different types of soils, treatment was more effective on the soil with the lowest iron content and highest As concentration. These results suggest that the immobilization of As and reduction of As bioaccessibility in soil by adding iron-base nanoscale particles maybe an effective method to remediate As contaminated soils.