In Situ Immobilization of Arsenic and Lead in Smelter Contaminated Soil
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The efficacy of in situ fixing of arsenic (As) and lead (Pb) in a highly contaminated soil was examined using chemical extractions for lability, bioavailability, and bioaccessibility accompanied by spectroscopic verification. Soil Samples from a former smelting site in Stockton, Utah, contained 7,520 mg kg^-1 total As and 66,400 mg kg^-1 total Pb, more than 100 times background levels. The high total Pb and As concentrations coupled with high bioaccessibility (Physiologically Based Extraction Test: 246 mg kg^-1 As; 20,900 mg kg^-1 Pb) raise concerns of environmental and human health effects. Goethite, ferrihydrite, and high-iron water treatment residuals (Fe-WTR) were tested as amendments. The Fe-WTR applied at a 10:1 Fe:As mole ratio was the most effective, reducing water soluble As and Pb by 95% and 72%, respectively. However, when soluble P was added to the soil to immobilize Pb, As lability significantly increased due to the similar chemistries of phosphate and arsenate. When soluble P and Fe-WTR were added in sequence, 90% reduction of bioaccessible As and 60% reduction in plant available As (uptake by Pteris quadriaurita) were observed. Speculation that the efficacy of Fe-WTR as an in situ immobilizing agent was due to the presence of high surface area and highly reactive, poorly crystalline Fe oxides was supported by μ-XRF and μ-XANES analyses. Unamended, contaminated soil was observed to be a mixture of As(III) and As(V) oxides whereas amendment with Fe-WTR resulted in a shift to As(V) solid phases dominated by As(V) oxides and sulfates, all containing Fe(III). Identified minerals included ferric arsenate sulfate [Fev5(AsOv4)v3(SOv4)v3], yukonite [Cav7Fev11(AsOv4)v9Ov10·24.3Hv2O], and scorodite (FeAsOv4·2Hv2O). When soluble phosphate was added as a single amendment, As mineralogy included significant concentrations of As(V) oxide, Asv2Ov5. For Pb, addition of soluble P increased hydropyromophite [Pbv5(POv4)v3OH] content to 70%, but when Fe-WTR was added after soluble P, hydropyromophite never exceeded 40%. Just as the addition of soluble P interfered with in situ fixing of As, Fe-WTR partially prevented the conversion of Pb to pyromorphite minerals. The μ-XRF and μ-XANES analyses give physical evidence of the chemical shifts suggested in the bioaccessibility and chemical lability tests.
Rathnayake, Sewwandi (2018). In Situ Immobilization of Arsenic and Lead in Smelter Contaminated Soil. Doctoral dissertation, Texas A & M University. Available electronically from