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 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(NO₃)₂ solutions. The ω of the backfill and R_d 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 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 (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 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 Pb²⁺, Ca²⁺, K⁺ and Na⁺ on semi-logarithmic scale for all the bentonite-based engineered barriers reported in this study and previous studies.

本文对砂/六偏磷酸钠（SHMP）改性膨润土回填剂（简称SHMP-SB回填剂）在垂直防渗墙中的膜行为和扩散特性进行了室内实验研究，以控制铅污染的地下水。使用浓度为0.5 mM至50 mM的硝酸铅（Pb（NO ₃）₂）溶液对SHMP-SB回填样品进行了多阶段化学渗透测试。化学渗透效率系数（ω），有效扩散系数（D *）和延迟因子（R _d的回填）。同时采用水活度法和van't Hoff方程计算用于确定ω的回填过程中的最大化学渗透压差。结果表明，SHMP-SB回填在Pb（NO ₃）₂溶液中充当半透膜材料。随着Pb（NO ₃）₂溶液浓度的增加，回填的ω和铅离子（Pb ²⁺）的R _d减小，而Pb ^2+的D *增大。当Pb（NO ₃）₂当浓度小于50 mM时，相对于水分活度法，使用van't Hoff方程计算的ω的最大误差高达22.2％。在这项研究的回填结果与其他类型的基于膨润土的工程阻隔材料（包括土工合成粘土衬里（GCL），压实的土-膨润土衬里）的结果之间，对ω，D *和R _d进行了全面比较。（CSBL）和土壤膨润土（SB）补土暴露于氯化钙（CaCl ₂），先前研究中报道的氯化钾（KCl）或氯化钠（NaCl）溶液。发现阻隔材料的类型（回填，GCL或CSBL），阳离子浓度，阳离子化合价（二价相对于一价）和阳离子水合半径在该范围内对ω，D *和R _d有相当大的影响。研究的基于膨润土的工程阻隔材料。提出了一个线性方程式，以量化该研究和先前研究中报道的所有基于膨润土的工程屏障的Pb 2 ⁺，Ca 2 ⁺，K ⁺和Na ⁺的ω和D *的关系，对数尺度。