In situ stabilization of soil lead using phosphorus and manganese oxide

In addition to the formation of insoluble lead (Pb) compounds, adsorption is another potentially important process controlling the bioavailability of Pb in soils. Less attention has been given to manganese (Mn) oxides, even though they are known to adsorb Pb more strongly than any other metal (hydr)oxides. This study was conducted to evaluate the effects of P and Mn oxide on bioavailable Pb in five metal-contaminated soils or mine spoils from Kansas and Missouri. Cryptomelane was used as the representative Mn oxide. Nine treatments were used: zero P, 5000 mg of P as triple superphosphate (TSP) or phosphate rock (PR), 2500 and 5000 mg of Mn oxide/kg, and combinations of Mn oxide and P as TSP or PR. Changes in bioavailable Pb over time were measured using a modified physiologically based extraction procedure (PBET), and mineralogical changes were observed using X-ray diffraction. The addition of P or cryptomelane reduced bioavailable Pb in all five materials. The addition of P and cryptomelane together was more effective in reducing bioavailable Pb than the addition of either amendment alone in all materials. Reductions in bioavailable Pb in stomach phase extractions upon addition of P or P and cryptomelane ranged from 15 to 41% and 23 to 67%, respectively, compared to the unamended control. Similarly, a modified toxicity characteristic leaching procedure (TCLP) indicated less soluble Pb in materials receiving P and cryptomelane compared to the control, P- or cryptomelane-treated samples. X-ray diffractometry analysis supported the PBET and TCLP results, indicating that more "pyromorphite-like minerals" formed in the presence of both P and Mn oxide compared to the control. This new improved technique to remediate Pb-contaminated soil and mine wastes has advantages over standard methods.