Use of diammonium phosphate to reduce heavy metal solubility and transportin smelter-contaminated soil

Phosphate treatments can reduce metal dissolution and transport from contam inated 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 pote ntial solids controlling metal solubility in DAP-amended soils. Soil contam inated with Cd, Pb, Zn, and As was collected from a former smelter site. Th e DAP treatments of 460, 920, and 2300 mg P kg(-1) and an untreated check w ere evaluated using solute transport experiments. Increasing DAP decreased total metal transported. Application of 2300 mg P kg(-1) was the most effec tive for immobilizing Cd, Pb, and Zn eluted from the contaminated soil. Met al elution curves fitted with a transport model showed that DAP treatment i ncreased retardation (R) 2-fold for Cd, 6-fold for Zn, and 3.5-fold for Pb. Distribution coefficients (K-d) 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 K-d values with additional DAP treatment indicat ed reduced partitioning of sorbed and/or precipitated metal released to mob ile metal phases and a concomitant decrease in the concentration of mobile heavy metal species. Activity-ratio diagrams indicated that DAP decreased s olution Cd, Pb, and Zn by forming metal-phosphate precipitates with low sol ubility products. These results suggest that DAP may have potential for pro tecting water resources from heavy metal contamination near smelting and mi ning sites.