Use of Diammonium Phosphate to Reduce Heavy Metal Solubility and Transport in Smelter‐Contaminated Soil

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Abstract
Phosphate treatments can reduce metal dissolution and transport from contaminated soils. However, diammonium phosphate (DAP) has not been extensively tested as a chemical immobilization treatment. This study was conducted to evaluate DAP as a chemical immobilization treatment and to investigate potential solids controlling metal solubility in DAP‐amended soils. Soil contaminated with Cd, Pb, Zn, and As was collected from a former smelter site. The DAP treatments of 460, 920, and 2300 mg P kg−1 and an untreated check were evaluated using solute transport experiments. Increasing DAP decreased total metal transported. Application of 2300 mg P kg−1 was the most effective for immobilizing Cd, Pb, and Zn eluted from the contaminated soil. Metal elution curves fitted with a transport model showed that DAP treatment increased retardation (R) 2‐fold for Cd, 6‐fold for Zn, and 3.5‐fold for Pb. Distribution coefficients (Kd) increased with P application from 4.0 to 9.0 L kg−1 for Cd, from 2.9 to 10.8 L kg−1 for Pb, and from 2.5 to 17.1 L kg−1 for Zn. Increased Kd values with additional DAP treatment indicated reduced partitioning of sorbed and/or precipitated metal released to mobile metal phases and a concomitant decrease in the concentration of mobile heavy metal species. Activity‐ratio diagrams indicated that DAP decreased solution Cd, Pb, and Zn by forming metal–phosphate precipitates with low solubility products. These results suggest that DAP may have potential for protecting water resources from heavy metal contamination near smelting and mining sites.