Membrane behavior and diffusion properties of sand/SHMP-amended bentonite vertical cutoff wall backfill exposed to lead contamination

Abstract

ABSTRACT This paper presents a laboratory experimental investigation on the membrane behavior and diffusion properties of sand/sodium hexametaphosphate (SHMP)-amended bentonite backfill (referred as SHMP-SB backfill) in vertical cutoff walls for containment of lead-contaminated groundwater. A multi-stage chemico-osmotic test was conducted on the SHMP-SB backfill sample using lead nitrate (Pb(NO3)2) solution with concentrations varying from 0.5\u202fmM to 50\u202fmM. The chemico-osmotic efficiency coefficient (ω), effective diffusion coefficient (D*), and retardation factor (Rd) of the backfill were calculated. Both water activity method and van t Hoff equation were adopted to compute the maximum chemico-osmotic pressure difference across the backfill used for determination of ω. The results demonstrated that the SHMP-SB backfill acted as a semipermeable membrane material in the Pb(NO3)2 solutions. The ω of the backfill and Rd of lead ions (Pb2+) decreased with increasing concentration of Pb(NO3)2 solution, whereas D* of Pb2+ increased. When Pb(NO3)2 concentration was less than 50\u202fmM, the maximum error in the ω calculated using the van t Hoff equation relative to the water activity method was found to be as high as 22.2%. A comprehensive comparison in ω, D*, and Rd was made between the results for the backfill in this study and those for the other types of bentonite-based engineered barrier materials including geosynthetic clay liners (GCLs), a compacted soil-bentonite liner (CSBL), and soil bentonite (SB) backfills exposed to calcium chloride (CaCl2), potassium chloride (KCl) or sodium chloride (NaCl) solutions reported in previous studies. It was found the barrier material type (backfills, GCLs or CSBL), cation concentration, cation valence (di-valent versus mono-valent), and cation hydrated radius had considerable effect on the ω, D*, and Rd for the range of studied bentonite-based engineered barrier materials. A linear equation is proposed to quantify the relationship of ω and D* for Pb2+, Ca2+, K+ and Na+ on semi-logarithmic scale for all the bentonite-based engineered barriers reported in this study and previous studies.