In situ chemical treatments for reducing heavy metal solubility and transport in smelter contaminated soils.

摘要

Scope and method of study. The study investigated and developed chemical immobilization techniques for reducing heavy metal solubility and transport. Solute transport experiments were conducted with metal contaminated soil collected from a defunct smelter. Treatment materials included agricultural limestone, rock phosphate, and diammonium phosphate. Numerical integration of Cd, Pb, and Zn eluted from soil columns allowed for treatment comparisons. Metal elution curves were fit with a transport model to determine retardation (R) and distribution coefficients (Kd). Eluted fractions were speciated with a chemical equilibrium model to investigate heavy metal species controlling transport and potential solids controlling solubility.; Findings and conclusions. In the first study, the 10 g DAP kg--1 application was the most effective for immobilizing heavy metals eluted from the contaminated soil with only 5.4% of Cd, 1.1% of Pb, and 4.2 % of the Zn eluted when compared with untreated soil. Application of DAP released As from the contaminated soil via ligand exchange. Total phosphorus eluted from the DAP treatments was 1 % of the total added for the 10 mg DAP kg--1 treatment and 10.5% of the total added for the 90 mg DAP kg--1 treatment. Layered RP at 180 g kg--1 was the most effective treatment for reducing Pb transport (99.9% reduction). Limestone was moderately effective for reducing Cd (45%) and Pb (54.8%), but was less effective for Zn. In the second study, DAP treatments of 460, 920, and 2300 mg P kg--1 were investigated. Model-fitted metal elution curves showed increased retardation (R) and distribution coefficients (Kd) with increased P application. Chemical speciation revealed decreased total mass of high mobility metal species with DAP treatment. Activity-ratio diagrams showed DAP application shifted the mineral controlled solubility of Cd from CdCO3 and Pb from PbSO4 to metal-phosphate compounds. Phosphate minerals controlled Zn in solution for untreated and DAP amended soils. Chemical immobilization using DAP may serve as a long-term remediation treatment for Cd, Pb, and Zn in smelter contaminated soils. Application schemes with DAP must be planned to reduce As and P elution, and co-application of timing materials may be necessary to offset soil acidification.