Experimental study on effect of phosphates on sedimentation behavior of lead-contaminated soil-bentonite slurry wall backfills

Soil-bentonite (SB) slurry walls are widely used in controlling migration of the contaminants in groundwater. Adding two types of phosphate to lead contaminated SB backfills may be advantageous to maintain the deflocculated structure of the bentonite in backfill, which is beneficial to enhance contaminant retarding ability of the backfill. A series of sedimentation and scanning electron microscope (SEM) tests are conducted to investigate changes in sedimentation behavior of SB backfills with various lead concentrations and phosphate contents/types, and the relevant mechanism is given. The results reveal that lead results in flocculation of the SB backfill and a 36% increase in the sediment volume. A certain amount of phosphate makes sedimentation curve of the contaminated backfill change from flocculation sedimentation type to accumulation sedimentation type, and the sediment volume of the backfill decreases significantly. Addition of phosphate enhances dispersity of the SB backfill due to steric stabilization of the phosphate and increased negative charge of the clay particle surfaces. The soil particles present a paralleled arrangement. Sodium hexametaphosphate has higher dispersibility compared with sodium pyrophosphate, because the backfills amended with sodium hexametaphosphate have smaller sediment volume. Optimum content of the phosphate is 0.1%, 0.5% and 2% while lead concentration in the backfill ranges in 0~0.1, 1~2, 6 mmol, respectively. The results obtained in this study may provide a meaningful guidance for improving dispersity of contaminated SB backfills.