Abstract
Hydraulic conductivity of sand-bentonite (SB) backfills amended with polyanionic cellulose (PAC) to lead nitrate (Pb(NO3)2) solutions was evaluated experimentally in this study. PAC-amended sand-bentonite (PSB) backfills were synthesized by mixing sand-bentonite mixture with 0.3 to 1.2% dry PAC (by total dry mixture mass) and mixed with a certain weight of conventional bentonite (CB) slurry. The rheology properties including the filtrate loss, viscosity, density, and pH testes of slurry with various bentonite dosages were measured to determine the reasonable CB dosage of slurry. The slump tests on PSB backfills with various mass slurries were conducted to determine the corresponding water content of backfills with slump 125 ± 5 mm. Under the applied pressure 100 kPa, the hydraulic conductivity to Pb(NO3)2 solutions (kc) of PSB backfills with various PAC contents was evaluated based on the modified filter press (MFP) tests, to ascertain the optimum PAC content of PSB backfills when permeated with Pb(NO3)2 solutions. Index properties, including the specific gravity (Gs) and liquid limit (wL) of PSB backfills, were measured after MFP tests. The MFP tests for PSB backfills were then conducted under various applied pressures to obtain the relationship between void ratio (e) and hydraulic conductivity of backfills. Finally, the flexible-wall permeability test (FWP test) under osmotic pressure 100 kPa was conducted to verify the effectiveness of the MFP test. The results indicate that slurry with 8% bentonite dosage is the reasonable choice in slurry wall construction. PSB has lower GS and higher wL compared to SB; increasing Pb concentration leads to GS of PSB increased and wL of PSB decreased. PSB with 0.6% PAC content is supposed as the optimum proportion of backfills when permeated with concentrated Pb(NO3)2 solution. PAC adsorbs large amount of bound water, which leads to higher water content (w) and e of PSB backfills, while lead ions (Pb) cause the diffuse double layer (DDL) of bentonite compressed and e of PSB backfills reduced. The kc of PSB-0.6 remains lower than 10−9 m/s and increases less than 10 times though the Pb concentration was up to 500 mM, demonstrating that the hydraulic performance of backfills can be improved effectively in Pb(NO3)2 solution by the additive PAC. The comparison results between k from MFP tests and FWP tests show that the MFP test is an effective and easy evaluation of hydraulic conductivity of backfills.
Highlights
Sand-bentonite (SB) vertical cutoff walls are extensively employed as in situ vertical barriers to prevent pollutants migrating into groundwater [1,2,3,4,5,6,7]. e typical SB cutoff walls are constructed by first excavating a trench with 0.6 to 1.5 m in width while simultaneously filling the trench with bentonite-water slurry to maintain the trench stability. e sand is mixed with bentonite and combined with bentonite-water slurry satisfied rheological properties to prepare homogeneous backfills with optimum slump (∼125 ± 25 mm). e backfills is poured into the slurry trench, forming a barrier with low hydraulic conductivity, k
E conventional bentonite (CB) manufactured by Mu-Feng Co., Ltd was powdered natural calcium bentonite activated with Na2CO3 treatment in order to obtain more superior swelling capacity and lower permeability. e physicochemical properties and mineralogical compositions of CB are summarized in Table 1. e X-ray diffraction phase identification is performed by China Petroleum and Chemical Corporation (Sinopec Corp.), Jiangsu Oilfield Branch
Based on the X-ray diffraction analysis, the dominant clay mineral of CB is found to be montmorillonite; the mineral constituent of CB is shown in Table 2. e properties of polyanionic cellulose (PAC) provided by manufactures are shown in Table 3. e fine sand was chosen to model a typical in situ sandy aquifer, which was obtained from the floodplains of Yangzi River in China. e sand was washed by tap water and air dried and screened with 1 mm sieve. e uniformity coefficient (Cu) is 3.66, and the curvature coefficient (Cc) is 0.76, indicating that the sand exhibits poor grain-size distribution
Summary
Sand-bentonite (SB) vertical cutoff walls are extensively employed as in situ vertical barriers to prevent pollutants migrating into groundwater [1,2,3,4,5,6,7]. e typical SB cutoff walls are constructed by first excavating a trench with 0.6 to 1.5 m in width while simultaneously filling the trench with bentonite-water slurry (typically 6–12% by dry weight of conventional bentonite) to maintain the trench stability. e sand is mixed with bentonite and combined with bentonite-water slurry satisfied rheological properties to prepare homogeneous backfills with optimum slump (∼125 ± 25 mm). e backfills is poured into the slurry trench, forming a barrier with low hydraulic conductivity, k (typically k ≤ 10−9 m/s).e potential for chemical incompatibility between backfills and the contaminated groundwater is a crucial consideration for vertical cutoff walls, which results in an Advances in Civil Engineering increase in k. e k of the SB backfills increases obviously (typically greater than 10 times) when exposed to inorganic solution with ionic strength (SI) greater than 300 mM, resulting in the barriers performance destroyed [8,9,10].Polymers as additives used in modifying bentonite have been investigated for improving the hydraulic performance of bentonite-rich barriers [11,12,13,14,15,16]. Sand-bentonite (SB) vertical cutoff walls are extensively employed as in situ vertical barriers to prevent pollutants migrating into groundwater [1,2,3,4,5,6,7]. E typical SB cutoff walls are constructed by first excavating a trench with 0.6 to 1.5 m in width while simultaneously filling the trench with bentonite-water slurry (typically 6–12% by dry weight of conventional bentonite) to maintain the trench stability. Polymers as additives used in modifying bentonite have been investigated for improving the hydraulic performance of bentonite-rich barriers [11,12,13,14,15,16]. E hydraulic performance of polymer-modified bentonite has been largely improved compared to conventional bentonites. PAC-amended bentonite or sand-bentonite backfills used in hydraulic barrier applications has not been reported
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