Abstract
A typical sodium activated calcium bentonite (SACaB) was treated with carboxymethyl cellulose (CMC) polymer, called CMC-treated SACaB (CMC-SACaB), and it was investigated for its hydraulic conductivity and enhanced chemical compatibility. Index property and hydraulic conductivity tests were conducted on CMC-SACaB and SACaB with deionized water (DIW), heavy metals-laden water, and actual landfill leachate. Lead-zinc mixed (Pb-Zn) solution and hexavalent chromium (Cr(VI)) solution were selected as target heavy metals-laden water, and calcium (Ca) solution was tested for comparison purposes. The hydraulic conductivity (kMFL) was determined via the modified fluid loss (MFL) test. Liquid limit and swell index in DIW, heavy metal-laden water, and Ca solution increased with increasing CMC content. CMC treatment effectively decreased the kMFL of SACaB when exposed to Pb-Zn solutions with a metal concentration of 1 to 20 mmol/L and landfill leachate. An insignificant change in kMFL of CMC-SACaB occurred with exposure to Pb-Zn solutions with metal concentrations of 1 to 10 mmol/L, Cr(VI) and Ca solutions with metal concentration of 1 to 20 mmol/L, and landfill leachate. A slight increase in kMFL of CMC-SACaB was observed when Pb-Zn concentration increased to 20 mmol/L, and such an increment was more noticeable when the CMC content was lower than 10%. In the DIW, the measured kMFL values of CMC-SACaB and SACaB with a given range of void ratio were consistent with those obtained from the flexible-wall permeameter test.
Highlights
Engineered barriers are extensively used in the containment of groundwater seepage and landfill leachate for the purpose of risk control of contaminated sites and landfill sites [1,2,3]
Evaluating the hydraulic conductivity using the modified fluid loss (MFL) test has the following advantages over that directly measured using a flexible wall permeameter (FWP): (1) the evaluation of hydraulic conductivity via the MFL test can be done within only 24 h, but it would likely take weeks to months to accomplish for a FWP test [28], and (2) the MFL test generally yields a comparable or conservative hydraulic conductivity as compared with that measured via the FWP test when the bentonites are permeated with distilled-deionized water or chemical solutions, though very limited data reported in the literature showed that the MFL test could result in a lower hydraulic conductivity [27,28,29]
Table presents the properties of the carboxymethyl cellulose (CMC)-sodium activated calcium bentonite (CaB) (SACaB) and compares content is higher than 0.6%
Summary
Engineered barriers are extensively used in the containment of groundwater seepage and landfill leachate for the purpose of risk control of contaminated sites and landfill sites [1,2,3]. Only limited studies have investigated the chemical compatibility of NaB, SACaB and bentonite-based engineered barriers using real contaminants, such as heavy metal solutions, low-level radioactive waste leachates, and coal combustion product leachates [5,11,13,14]. The research motivation for this study is summarized as follows: (1) how SACaB can be treated by using a polymer material to improve its chemical compatibility when exposed to ground water with heavy metal contaminants or landfill leachate; and, (2) how to optimize the polymer content in the SACaB for an effective containment barrier with enhanced chemical compatibility With these questions in mind, carboxymethyl cellulose (CMC) is used to treat SACaB and investigate the index properties and chemical compatibility of CMC-treated SACaB (CMC-SACaB) and compare these results with those of the CMC-treated NaB (i.e., HYPER clays). Evaluating the hydraulic conductivity using the MFL test has the following advantages over that directly measured using a flexible wall permeameter (FWP): (1) the evaluation of hydraulic conductivity via the MFL test can be done within only 24 h, but it would likely take weeks to months to accomplish for a FWP test [28], and (2) the MFL test generally yields a comparable or conservative hydraulic conductivity as compared with that measured via the FWP test when the bentonites are permeated with distilled-deionized water or chemical solutions, though very limited data reported in the literature showed that the MFL test could result in a lower hydraulic conductivity [27,28,29]
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