**1. Introduction**

The compacted clay cover (CCC), as one of the main horizontal barriers, is widely applied in industrial organics contaminated sites to effectively control upward migration of volatile organic compound (VOC) and semi-volatile organic compound (SVOC) vapors or gases [1]. Studies reveal that gas migration through the CCC in landfills is predominated by diffusion [2]. Diffusion of VOC and SVOC gases through CCC may lead to the emission of toluene gas in landfills, migrating from pollution sources in deep soil to the air [3]. Upward migration of gas in clay occurs via advection [4] and diffusion [5]. Diffusion is the primary mechanism of VOC and SVOC gas migration in clayey soils on most occasions [6,7]. Advection can affect the migration of VOC or SVOC gas only when the temperature and vapor pressure in clays are relatively high, e.g., during the summer season [8,9]. In real projects, the CCC stays unsaturated on most occasions, except for relatively heavy rainfall infiltration. There are three indicators that quantify gas advection in clays when air pressure is high, gas permeability, air-water relative permeability, and moisture content [10]. A critical step in evaluating a CCC's performance against VOCs or SVOCs is to measure the permeation and diffusion of gas while they change with the moisture contents of CCC.

The influence of moisture content in clay is significant for gas migration. In recent years, a geosynthetic clay liner (GCL) has been extensively used as the VOC/SVOC gas

**Citation:** Bi, Y.-Z.; Wen, J.-M.; Wu, H.-L.; Du, Y.-J. Evaluation of Performance of Polyacrylamide-Modified Compacted Clay as a Gas Barrier: Water Retention and Gas Permeability and Diffusion Characteristics. *Appl. Sci.* **2022**, *12*, 8379. https://doi.org/10.3390/ app12168379

Academic Editors: Carlos Morón Fernández and Daniel Ferrández Vega

Received: 2 May 2022 Accepted: 9 June 2022 Published: 22 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

barrier in contaminated sites. However, the bentonite in GCL is usually sodium-activated calcium bentonite in China, and the barrier performance is not as good as sodium bentonite GCL. This is because the high-quality sodium bentonite resources in China are scarce. Previous studies [11,12] found that the gas permeability of the sodium bentonite GCL decreased with increasing moisture content under different pressures. Studies have shown that the gas permeability changes after hydration from 1.0 × <sup>10</sup>−<sup>18</sup> to 1.0 × <sup>10</sup>−<sup>11</sup> <sup>m</sup><sup>2</sup> [13–15]. After drying, the gas permeability of the GCL can range between 0.03 and 0.21 m<sup>2</sup> [16]. The VOC/SVOC gas migration from contaminated soil through GCL into the air is mainly controlled by pressure and concentration gradients. The advection governs gas flow caused by differential pressures, and the gas advection indicator is reflected by the gas permeability [17]. Gas migration due to a concentration gradient is controlled by the gas diffusion coefficient [18]. Rouf et al. [19] demonstrated that when the apparent degree of saturation (ADOS) of the sodium bentonite GCL increased greater than approximately 65%, both the gas diffusion coefficient and gas permeability of the GCL were considerably reduced. The ADOS is defined as the gravimetric water content (*w*) of a GCL at a given time, divided by the maximum gravimetric water content (*w*ref) that the same GCL reaches during hydration under the same applied stress conditions [19]. It can be anticipated that the gas diffusion and gas permeability of CCC increase significantly with the decrease in both gravimetric water content and ADOS [20]. Extremely low moisture content will lead to VOC gas migration into clay by diffusion and advection more easily, thus impairing the CCC gas barrier efficacy. Low moisture contents in compacted clay can cause fissures due to water loss [21]. Ultimately, a crack network may develop from the fissures, becoming the preferential and dominant pathway for the upward advection of VOC or SVOC gas released from the unsaturated contaminated soils. Through laboratory tests, Drumm et al. [22] found that the hydraulic conductivity near soil cracks increased sharply as compared to intact soil that is not intact, confirming the existence of dominant flow paths after the soil cracked.

Adopting modified materials in CCC is one potential approach to suppress cracking caused by water loss and improve the gas barrier performance. Super absorbent polymers (SAP) are a type of polymers with strong water-absorbing capability, composed of many hydrophilic functional groups, such as carbonyls, hydroxyls, and quaternary ammonium salts. One of the most commonly used SAPs is polyacrylamide (PAM), which is usually adopted as a water retention agent in agriculture, due to its long-term water retention capabilities [23,24]. Studies [25–27] have shown that polymers are adsorbed on the surface of soil particles through physicochemical reactions. A polymer is one of the promising materials recently applied to soil stabilization. It has a long chain of monomers connected to each other by sufficiently strong and flexible Van der Waals forces. The polymer can encapsulate the soil particles and connect them through polymer chain expansion, thereby improving the soil's water retention capacity (WRC). As a result, the hydraulic properties, erosion resistance, and gas impermeability of the modified soils can be improved. Qi et al. [28] researched the dried cracking of clay modified by PAM through a constant temperature evaporation test and image processing technology. They found that PAM is effective in mitigating soil cracking and even inhibiting crack formation. In addition, Yu et al. [29] conducted column tests to compare the cracking of the GCL before and after PAM modification at different temperatures. They demonstrated that the number of GCL cracks decreased after PAM modification at 40 ◦C. Therefore, using PAM to modify the CCC is promising, as it may improve the WRC and enhance the gas barrier performance. Nevertheless, previous studies mainly focused on polymer-modified clay's WRC and hydraulic properties. The barrier performance of GCLs/CCCs containing VOC/SVOC gases should be evaluated not only by gas permeability and WRC, but also by the gas diffusion coefficient.

The primary purpose of this study was to investigate the gas barrier performance of PAM-modified CCCs. A series of laboratory tests were conducted, which included liquid limit, water retention, gas permeability, and gas diffusion tests. The results are useful in facilitating CCC design and its application as gas barriers in VOC and SVOC contaminated sites.
