Retention of Pb and Cr(VI) onto slurry trench vertical cutoff wall backfill containing phosphate dispersant amended Ca-bentonite

Abstract This study investigated the potential use of sodium hexametaposphate (SHMP) as an amendment in calcium bentonite (Ca-bentonite) and sand/Ca-bentonite backfill to enhance retention of two heavy metals, lead (Pb) and hexavalent chromium (Cr(VI)), commonly encountered in contaminated groundwater by conducting batch equilibrium experiments. The pH of the equilibrated bentonites and backfills was also measured. The results showed that the SHMP amendment and the presence of source chemicals lowered the equilibrium pH of the bentonites and backfills. All the tested bentonites and backfills exhibited nonlinear retention behavior of Pb and Cr(VI) which was well described by Langmuir and Freundlich models. SHMP amendment increased the Pb retention capacity of the bentonite and backfill by a factor of 1.45 and 1.72 respectively, whilst it helped little on improving Cr(VI) retention capacity of the bentonite and backfill. Moreover, the percent retention (PR) and partition coefficient (Kp) were found to increase first and then decrease with the initial metal concentrations, and the peak PS or Kp occurred at initial concentration of 874 mg/L for Pb retention on all tested bentonites and backfills, whilst 1 and 48 mg/L for Cr(VI) retention on the tested bentonites and backfills respectively. Based on the variations of PR and Kp with initial metal concentration, the enhanced retention of Pb on bentonite and backfill with SHMP amendment was pronounced when the initial Pb concentration was higher than the threshold value of 1642 mg/L. However, the bentonite, even though amended with SHMP, is not an ideal barrier material for Cr(VI) retention.