**1. Introduction**

The cover system, including compacted clay cover (CCC) and geomembrane (GM), applied in contaminated sites could prevent the migration of volatile or semi-volatile organic compounds (VOCs/SVOCs) and limit the movement of precipitation into the underlying waste [1–5]. However, the published literature [6–8] have shown that the cover system has some problems, including geomembrane defects, and desiccation cracks of the CCC, which would lead to the degradation of the barrier performance (Figure 1). Rowe et al. [6] presented that the geomembrane defects are inevitable. The monitoring results of 205 sites show that the percent of complete geomembrane is less than 30%. In addition, more than 50 percent of geomembranes have more than five loopholes per ha. The barrier performance of the cover system would be degraded by geomembrane defects. Then,

**Citation:** Wang, M.; Wen, J.; Zhuang, H.; Xia, W.; Jiang, N.; Du, Y. Screening Additives for Amending Compacted Clay Covers to Enhance Diffusion Barrier Properties and Moisture Retention Performance. *Appl. Sci.* **2022**, *12*, 7341. https:// doi.org/10.3390/app12147341

Academic Editor: Bing Bai

Received: 9 June 2022 Accepted: 18 July 2022 Published: 21 July 2022

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the CCC contributes to the role of VOCs/SVOCs and the precipitation barrier. However, desiccation cracks in CCC act as preferential flow paths and affect the barrier performance of CCC, eventually breaking through the cover system [9]. There are various issues resulting in cracks of CCC [7,10], including differential settlements, extreme drought [11], and dry– wet cycle [9]. The resistance to cracking, i.e., the tensile strength or fracture toughness of the soil, also changes upon drying. Finally, the adhesion at interfaces, which is essential in providing the restraint for desiccation cracks to form, changes with moisture content [12,13]. Omidi et al. [8] showed that the hydraulic conductivity increases nearly two orders of magnitude due to the desiccation cracks of CCC, which would have a great impact on the cover system. Albrecht and Benson [14] presented that the hydraulic conductivity of cracked soils is typically several orders of magnitude greater than that of intact soils.

**Figure 1.** Problems of compacted clay cover desiccation and possible migration of VOCs [6–14].

To restrain the desiccation cracks of CCC, moisture retention additives were added to the soil to improve its moisture retention and desiccation cracks inhibition performance. Moisture retention additives could inhibit moisture evaporation and regulate soil temperature by adsorbing water hundreds or even thousands of times its weight [15]. According to different synthetic materials, the moisture retention additives can be divided into four kinds: modified starch, synthetic polymer, modified cellulose, and other natural compounds and their derivatives, blends, and composites [16]. It is cumbersome to prepare the synthetic polymer moisture retention additives. In addition, it is not suitable for practical engineering because the synthetic polymer would be completely degraded in soil within some years [17]. Therefore, natural materials and their derivatives could be selected as amended materials, namely attapulgite, diatomite, and zeolite. However, the natural zeolite cannot meet the requirements due to its small pore size and being easy to block [18]. Finally, attapulgite and diatomite are proposed as additives for amending CCC.

Usually, the migration modes of gases in the soil included advection and diffusion [5]. Conant et al. [19] found that diffusion is the main migration mode of trichloroethylene (TCE) vapor through a detailed, field-scale analysis of the transport behavior of solvent vapors within the unsaturated zone. The unsaturated zone at the site is approximately 3.5 m thick and comprises the upper portion of the sequence of glaciolacustrine sands and silts of the Borden aquifer. You et al. [20] found that although the transient advective flux can be greater than the diffusive flux; under most of the field conditions the net contribution of the advective flux is one to three orders of magnitude less than the diffusive flux. The advective flux contributes comparably with the diffusive flux only when the gas-filled porosity is less than 0.05. The advective transport of VOCs can be induced by the discrepancy in density between gas phase VOCs and clean air [21–23], water table fluctuation at

coastal sites [21,23–25], and atmospheric pressure fluctuation [26,27]. Therefore, in most non-coastal sites and unsaturated zones with low permeability, the VOCs' migration mode in the organic contaminated sites is mainly diffusion [28–30]. Rowe [31] found that the gas barrier should not only prevent water migration, but also the organic contaminants transmission. Therefore, the development of amended CCC with excellent moisture retention and VOCs/SVOCs diffusion barrier performance is significant.

A systematic study of the moisture retention, hydraulic conductivity, and gas barrier properties of amended CCC was investigated through laboratory experiments. In this paper, the optimal dosage and ratio of dual-additives of the amended CCC were studied through the laboratory test. The hydraulic conductivity and gas barrier properties of amended CCC with optimal dosage and ratio were evaluated. Then, the mechanism behind the phenomenon was demonstrated through SEM, BET, MIP, and TGA test analyses.
