Metal immobilization and phosphorus leaching after stabilization of pyrite ash contaminated soil by phosphate amendments

Abstract

In this study we would like to show the importance of a holistic approach to evaluation of chemical stabilization using phosphate amendments. An extensive evaluation of metal stabilization in contaminated soil and an evaluation of the leaching of phosphorus induced after treatment were performed. The soil was highly contaminated with Cu (2894 mg kg⁻¹), Zn (3884 mg kg⁻¹), As (247 mg kg⁻¹), Cd (12.6 mg kg⁻¹) and Pb (3154 mg kg⁻¹). To immobilize the metals, mixtures of soil with phosphate (from H₃PO₄ and hydroxyapatite (HA) with varying ratios) were prepared with a constant Pb:P molar ratio of 1:10. The acetic acid extractable concentration of Pb in the mixture with the highest amount of added phosphoric acid (n(H₃PO₄):n(HA) = 3:1) was reduced to 1.9% (0.62 mg L⁻¹) of the extractable Pb concentration in the untreated soil, but the content of water extractable phosphorus in the samples increased from 0.04 mg L⁻¹ in the untreated soil sample up to 14.3 mg L⁻¹ in the same n(H₃PO₄):n(HA) = 3:1 mixture. The high increase in arsenic mobility was also observed after phosphate addition. The PBET test showed phosphate induced reduction in Pb bioavailability. In attempting to stabilize Pb in the soil with the minimum treatment-induced leaching of phosphorus, it was found that a mixture of soil with phosphate addition in the molar ratio of H₃PO₄ :HA of 0.75:1 showed the most promising results, with an acetic acid extractable Pb concentration of 1.35 mg L⁻¹ and a water extractable phosphorus concentration of 1.76 mg L⁻¹. The time-dependent leaching characteristics of metals and phosphorus for this mixture were evaluated by a column experiment, where irrigation of the soil mixture with the average annual amount of precipitation in Slovenia (1000 mm) was simulated. The phosphorus concentration in the leachates decreased from 2.60 mg L⁻¹ at the beginning of irrigation to 1.00 mg L⁻¹ at the end.