The heavy metal contaminated industrial sites not only pollute the environment but seriously restrict regional sustainable development and urbanization. Soil-bentonite cutoff wall is considered as an in-situ vertical engineered barrier used for controlling contaminant transport and enhancing risk control ability. Thus, it is a vital demand for China that contributes to raise capabilities of integrated treatment and risk management.
After a review on testing results of engineering properties of soil-bentonite cutoff wall, it can be concluded that studies on the application of soil-bentonite cutoff wall still exist two main limitations as follows: (1) Bentonites available in China are generally sodium actived calcium-bentonites, whose swell potential and hydraulic conductivity are noticeably lower than natural sodium-bentonite used in all previous studies. However, the material composition, workability, compressibility and permeability of a soil-bentonite backfill created using sodium actived calcium-bentonite remain unknown. (2) Nontoxic salt solutions like potassium chloride and calcium chloride are popularly used in chemical compatibility testing of engineered barriers. None has been conducted on soil-bentonite backfills when exposed to heavy metal contaminants like lead, zinc and chromium.
As a result, the main purpose of this study is to investigate the engineering properties and chemical compatibility of soil-bentonite backfills created using sodium actived calcium-bentonite, including clayey soil-bentonite, sand-bentonite and samd-natural clay- bentonite backfill. This study is financially supported by National Natural Science Foundation of China (Grant No. 41330641, 51278100), Natural Science Foundation of Jiangsu Province (BK2012022) and Scientific Research Foundation of Graduate School of SoutheastUniversity (Grant No. YBJJ1343). Main results are incorporated ad follows:
(1) A series of workability testing is conducted to investigate the required bentonite content for bentonite-water slurry (BCS) and backfill water content range for construction (wB). Methods for predicting the workability parameters are proposed based on the correlations between workability parameters (i.e., BCS and wB) and physical index properties.
(2) The compressibility and consolidation of the three types of soil-bentonite backfills are investigated via oedometer tests. Bentonite content and initial water content are two controlling factors that affect the compressibility. It is found that there exists a unique relationship between void ratio of σ’ = at 1 kPa and compression index, in which void ratio of σ’ = at 1 kPa reflects a compositive influence of initial water content, bentonite content, bentonite quality, and in-situ soil nature on the compression index of soil-bentonite backfills. According to this result, a method for predicting the compression index of various types of soil-bentonite backfills is proposed using initial water content and liquid limit.
(3) Bentonite content required for achieving hydraulic conductivity limit regulatory (k < 10-9 m/s) is investigated. In addition, the influences of bentonite content, bentonite quality, zeolite content, natural clayey soil content, initial water content, and testing methods on the hydraulic conductivity are understood. Methods for predicting hydraulic conductivity of natural clayey soils in literature are adopted to predict the hydraulic conductivity of soil-bentonite backfills. An empirical method based on the framework of Kozeny-Carman equation is proposed to predict the hydraulic conductivity of the clayey soil-bentonite backfills. A new characteristic index, modified clay fraction void ratio, is proposed for predicting hydraulic conductivity of both sand-bentonite and sand/natural clay-bentonite backfills. The modified clay fraction void ratio reflects the influences of bentonite content, bentonite quality, and natural clayey soil content on the hydraulic conductivity.
(4) Impact of presence of lead contamination in clayey soil-bentonite backfills on the compression behavior is investigated, which leads to a 12%~53% decrease in the compression index. The correlation between physical index properties and compression index (Cc) of both inorganic salts and organic compounds contaminated remolded clays, including clayey soil-bentonite backfills tested in this study, is understood. Liquid limit is an important index property that can reflect the chemical compatibility in terms of compressibility. Moreover, it is found that the unique relationship between the void ratio of σ’ at 1 kPa (e1) and compression index can be used for both inorganic salts and organic compounds contaminated remolded clays. An empirical equation is, therefore, proposed for predicting the compression index of clean, inorganic salts, and organic compounds contaminated remolded clays. In addition, the compression behaviour of inorganic salts and organic compounds contaminated remolded clays is also estimated via a Disturbed State Concept (DSC) model. A simplified form of the general model is proposed for inorganic salts and organic compounds contaminated remolded clays, in which one single parameter is used for describing the impacts of presence of contaminants in soil.
(5) Impact of typical heavy metal contaminants on the permeability and swell potential of bentonites is investigated via modified fluid loss test and swell index test. A considerable increase in hydraulic conductivity of bentonites is found when the metal concentration increases to 10 mmol/L. Moreover, such trend is more noticeable in Na-bentonite compared with the results of Na activied Ca-bentonite. The different formations in metcal ions significantly affects the impact of metal concentration on hydraulic conductivity of bentonites. Metal cation obviously leads an increase in hydraulic conductivity. In contrast, metal exists in anionic complex, such as hexavalent chrome, could hardly change the hydraulic conductivity. In addition, it is found that swell index can well describe the influence of inorganc salt on the hydraulic conductivity of various bentonites.
(6) The chemical compatibility of soil-bentonite backfills exposed to heavy metal contaminants, in terms of hydraulic conductivity, is understood. The increase in the hydraulic conductivity due to the presence of heavy metal contaminants with metal concentration upto 500 mmol/L is less than 20-times. The resulting hydraulic conductivity would underestimate the impact of heavy metal on the hydraulic conductivity of soil-bentonite backfills. The hydraulic conductivity of soil-bentonite backfills permeated with lead nitrate-zinc nitrate mixtures is 2.9-times higher than that permeated with calcium chloride for a given total metal concentration. Based on the methods for predicting the hydraulic conductivity of clean soil-bentonite backfills, empirical equations developed from the Kozeny-Carman equation and modified clay fraction void ratio are proposed for predicting the hydraulic conductivity of clayey soil-bentonite and sand-bentonite backfills when permeated with heavy metal solutions, respectively.
(7) Carboxymethyle cellulose (CMC) treatment is applied to enhance the chemical compatibility of sodium actived calcium-bentonite in terms of swell potential and hydraulic conductivity when subjected to heavy metal contaminants. The mechanism of CMC treatment is understood via Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction analysis. Several series of index property tests, modi?ed fluid loss tests and flexible-wall hydraulic conductivity tests are conducted using CMC treated Ca-bentonite with deionized water, heavy metals-laden water, and actual landfill leachate. The result indicates that CMC treated Ca-bentonite yields a relatively lower hydraulic conductivity compared to the untreated Ca-bentonite under a given contamination condition. In addition, the modified fluid loss test can be considered as a quick method for estimating the chemical compatibility of bentonites in terms of hydraulic conductivity.
(8) The transport parameters, hydrodynamic dispersion coefficient and retardation factor of heavy metal contaminants during their transport through soil-bentonite backfill are estimated. The order of the mobility of lead, zinc, and hexavalent chromium is: Cr(VI) > Zn > Pb. Factor analysis is conducted to understand the influences of transport parameters, wall thickness, and concentration criterion for breakthrough time on the breakthrough time according to van Genuchten’s analytic solution. Based on this, required wall thickness values for achieving either short-term or long-term service are given. In additon, a simplified method is developed based on the van Genuchten’s analytic solution for the calculation of the required wall thickness under different concentration criterion as well as service life for a given wall thickness.