Chemical and Ecotoxicological Assessment of Multiple Heavy Metal-Contaminated Soil Treated by Phosphate Addition

In the present study, two different phosphates, namely potassium dihydrogen phosphate (PDP) and dipotassium hydrogen phosphate (DHP), were used to immobilize multiple heavy metals (HMs) in a long-term contaminated soil collected from Hezhang County (China). Chemical and biological methods were used to evaluate the treatment efficiency. The first step of the Community Bureau of Reference sequential extraction (BCR1) method was used to predict the mobility and availability of HMs and showed that PDP and DHP significantly decreased BCR1-extracted Pb (BCR1-Pb) concentration (p < 0.05) by 90.36 and 86.19 %, respectively, when they were applied at a P/HMs (sum of Pb, Cd, and Zn) molar ratio of 5: 1. Two phosphates also reduced BCR1-Cd concentration by up to 36.78 and 37.62 %, respectively, but had no apparent effect on Zn mobility. Microcalorimetric and fluorescein diacetate hydrolysis activity analyses were used to assess the toxicity change of HMs. Soil microbial activity was increased to some extent in phosphate-treated soils, which may result from decreased HMs toxicity, as well as the nutritional effects of added phosphates. Adverse impacts on microbial activity were also observed, which may be due to the increased pH associated with phosphate treatment. This study provides baseline information for establishing remediation strategies for managing multiple HM-contaminated agricultural soils. Future studies should focus on reducing the environmental risk of Zn and decreasing the adverse pH effects of phosphate treatment on soil microbial communities.