Effect of soil washing on heavy metal removal and soil quality: A two-sided coin

Heavy metal contamination in soil due to human activities is a global environmental problem. To find a washing solution that can significantly decontaminate heavy metals and minimize damage to soil quality, six washing solutions (H₃PO₄, K₂CO₃, CH₃COOK, KH₂PO₄, HNO₃ and KNO₃) were used at different concentrations to treat contaminated soil collected from the field. Furthermore, changes in soil physicochemical properties and heavy metal speciation among prewashed, postwashed and neutralized samples were tested. Additionally, soil enzyme activities and soil microbial diversities in contaminated soil among the prewashed, postwashed and neutralized samples were also measured. Finally, a pot experiment was conducted with Mentha haplocalyx to test the efficiency of soil washing. The results revealed that the optimum washing solution was 1% HNO₃ and that the removal rates of Cd and Pb were 75.7% and 60.6%, respectively, under treatment conditions of 35 °C, 90 min and a solid-liquid ratio of 1:10. The pH, total phosphorous, available potassium, soil enzyme activities and soil microbial diversity decreased significantly after washing. However, after the neutralization of washed soil with Ca(OH)₂, the available phosphorous, total nitrogen and some microorganisms increased significantly compared with those of the soil before washing. After treatment with 1% HNO₃, the chemical forms of Cd and Pb in soil mainly existed as F₁ (exchangeable) fractions, but the main forms of the two metals changed to F₅ (residual) and F₃ (bound to Fe-Mn oxides) fractions after neutralization with Ca(OH)₂. In addition, the plant height, root length, and fresh and dry weight of M. haplocalyx were not significantly affected by soil neutralization, while the Pb, Cu and As concentrations in the aboveground parts significantly decreased. Therefore, although soil washing could effectively remove Pb and Cd in soil, it also resulted in a significant decline in soil quality, but soil neutralization could effectively alleviate the negative effects during soil washing.