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

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(NO 3 ) 2 ) solution with concentrations varying from 0.5 mM to 50 mM. The chemico-osmotic efficiency coefficient ( ω ), effective diffusion coefficient ( D *), and retardation factor ( R d ) 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(NO 3 ) 2 solutions. The ω of the backfill and R d of lead ions (Pb 2+ ) decreased with increasing concentration of Pb(NO 3 ) 2 solution, whereas D * of Pb 2+ increased. When Pb(NO 3 ) 2 concentration was less than 50 mM, 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 R d 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 (CaCl 2 ), 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 R d for the range of studied bentonite-based engineered barrier materials. A linear equation is proposed to quantify the relationship of ω and D * for Pb 2+ , Ca 2+ , K + and Na + on semi-logarithmic scale for all the bentonite-based engineered barriers reported in this study and previous studies. • Effect of lead on ω and D * of SHMP-amended bentonite backfill is investigated for the first time. • Amendment of bentonite with SHMP improves chemico-osmotic efficiency coefficient. • Van't Hoff equation results considerable error in estimating ω in Pb(NO 3 ) 2 solutions. • Factors influencing ω, D * and R d of different bentonite-based barrier materials are assessed.