Evaluation of chemical immobilization treatments for reducing heavy metal transport in a smelter-contaminated soil

Three chemical immobilization materials, agricultural limestone (AL), mineral rock phosphate (RP), and diammonium phosphate (DAP), were evaluated using solute transport experiments to determine their ability to reduce subsurface heavy metal transport in a smelter contaminated soil. Percent reductions in metals transported were based on comparison with cumulative totals of metal species eluted through 60 pore volumes from an untreated soil. Reductions of metal eluted from the AL treatment were 55% for Cd, 45.2% for Pb, and 21.9% for Zn. Rock phosphate mixed with soil at 60 and 180 g kg-1 was generally ineffective for reducing Cd, Pb, and Zn elution with <27% reduction for Cd, Pb, and Zn. Rock phosphate placed under contaminated soil as a reactive barrier (i.e. layered RP) at 180 g kg-1 reduced Cd 53% and Zn 24%, and was the most efficient treatment for reducing Pb (99.9%) transport. DAP treatments were superior to all other materials for reducing Cd and Zn elution with reduction >77% for Zn and >91% for Cd from the 90 g DAP kg-1 treatment. Increasing DAP from 10 to 90 g kg-1 increased total arsenic released from 0.13 to 29.5 mg kg-1 and total P eluted from 2.31 to 335 mg. DAP at 10 g kg-1 was the most effective treatment for immobilizing the combination of Cd, Pb, and Zn, with reductions of 94.6, 98.9, and 95.8%, respectively. - Diammonium phosphate was the most effective treatment for immobilizing heavy metals