The synergistic mechanism and effect of pyrophosphate-hypophosphite leaching agent for arsenic and lead remediation in soil

Purpose The purpose of the study is to develop remediation technology for soil with arsenic and lead composite pollution by leaching. Arsenate carries a negative charge in soil, while lead carries a positive charge in soil, causing soil remediation more challenging. There is an urgent need to develop a remediation method that can efficiently remove both arsenic and lead simultaneously. Methods A complex agent composed of potassium pyrophosphate (PP) and potassium hypophosphite (PHP) was applied to the Soil A (As soil samples) and B (As-Pb soil samples). The research methods identified the optimal condition of As and Pb removal efficiency by concentration, molar ratio, liquid‒solid ratio, washing time and pH. The mechanism of the experiment is studied through Wenzel/BCR method analysis, I R and M F analysis, leaching toxicity and soil physicochemical properties analysis. Results The results show that the removal of arsenic from Soil A and B was 71.53% and 68.27% by 0.5 mol·L −1 PP and 0.2 mol·L −1 PHP, L/S = 10:1(L/S is liquid–solid ratio, volume ratio of agent to soil), t = 10 h, pH = 5.0. The removal of Pb in Soil B reached 74.89% by 0.6 mol·L −1 PP and 0.3 mol·L −1 PHP, L/S = 15:1, t = 8 h, pH = 7.0. According to Wenzel method, the complex agent converts specific adsorption into nonspecific adsorption state of As by Fe–P complexes through complexation and reductive dissolution of iron oxide. In addition, PP increases the efficiency of lead leaching by preventing the formation of Ca 2 Pb 3 (PO 4 ) 3 Cl, and PHP promotes the reductive dissolution of galena (PbS). Conclusions The mechanism involves competitive adsorption, inner-sphere complexation, reduction dissolution and electron transfer. The effect of pH on removal efficiency was significant. The optimal removal pH for As is weakly acidic, while the Pb removal efficiency is optimized under neutral conditions. There is synergy between PP and PHP in regard to removal of As and Pb in soil.