Over the past few decades, various groundwater remediation techniques have been applied, among which the permeable reactive barrier (PRB) technology has been widely adopted for the passive remediation of diffusing contaminant plumes in groundwater (Sakr et al., 2023; Alok et al. 2020). PRBs have many advantages such as low price, low energy consumption, and continued efficient treatment throughout their lifetime (Zhang et al., 2022; Kim et al., 2023). PRB systems, which consist of a barrier filled with an adsorbent for the target contaminant, are placed in the migration of the contaminant plume. As the contaminant plume migrates through the PRB under a natural hydraulic gradient, the contaminants react with the reactive barrier, leading to sorption and chemical reactions (Yan et al., 2024). Overall, the design of the PRB depends on the environmental compatibility and hydraulic conductivity at the site (Falciglia et al., 2020). The site geology and characteristics of the aquifers should also be considered during the early design stage (Choong et al., 2022). In addition to site characterization, the reactive materials used in the PRB play a critical role in determining PRB design (Kim et al., 2023; Kwak et al., 2024). Physicochemical properties such as the removal efficiency, kinetics (reactivity), hydraulic conductivity, and stability should be considered when selecting the reactive material. The longevity and effectiveness of PRBs are significantly affected by the properties of the reactive material (Zhang et al., 2022). Specifically, the hydraulic conductivity of the reactive material affects the longevity and effectiveness, which may affect the long-term performance.
The PRB installation process is complex because it is directly or indirectly influenced by geology, groundwater hydraulics, and geochemistry. PRB construction below the water table requires detailed analysis of the factors affecting aquifer conductivity (Kwak et al., 2024). At this point, the PRB must have a higher permeability coefficient than an aquifer so that it does not block the ambient flow during contaminant treatment (Liu et al., 2019). However, traditional reactive materials, such as granules or powders, have been found to be problematic in the field owing to their physical properties, which can lead to clogging and hinder groundwater flow. Previous field surveys have indicated that the reactive materials utilized may raise concerns for PRB operation because low mechanical strength and aggregation can partially cause PRB clogging. Notably, a practical problem arises when the reactive material is filled because the PRB is prone to collapse and solid losses during handling. To overcome these problems, sand or gravel is generally mixed with reactive materials to improve PRB permeability (Nyarko et al., 2014; Nyarko et al., 2022). The results showed that employing sufficient reactive materials within the barrier to increase the thickness of the PRB is difficult. Therefore, the PRB must sufficiently increase the longevity of the reactive materials and consider the resulting clogging effect on the permeability.
Benzene, toluene, ethylbenzene, and xylene (BTEX) are among the most common hydrocarbon contaminants released through petroleum oil leakage from contaminated groundwater. BTEX are aromatic hydrocarbons with high toxicity (Anjum et al., 2019; Wang et al., 2022). These characteristics include a high mobility, solubility, and volatility (Choi et al., 2007; Choi et al., 2009; Lee et al., 2021). The presence of contaminants in groundwater poses serious risks to the environment and human health. Various reactive materials have been investigated for contaminant removal (Memetova et al., 2023). Moreover, the reactive materials used in PRBs that have been reported in the literature include zero-valent iron, activated carbon (AC), zeolite, limestone, and activated alumina (Han et al., 2016; Song et al., 2021). The mechanical strength and permeability are important properties to consider when satisfying the PRB installation requirements. Among these reactive materials, AC is preferred because of its large source of raw materials and high adsorption efficiency (Zhang et al., 2021; Lan et al., 2023). AC has been demonstrated to have a high adsorption capacity with an extensive surface area compared with conventional materials (Feizi et al., 2021). Moreover, AC has been widely used in various applications, including the in-situ treatment of groundwater contaminants.
AC is a carbonaceous material (such as wood or coal) that can be fabricated in various forms such as granular, powder, spherical, and fabric (Egbosiuba et al., 2020; Zhao et al., 2024). Depending on the application, a pellet form based on AC has been developed to improve its physical properties, such as mechanical strength and durability (Yu et al., 2020; Tang et al., 2020; Tian et al., 2021; Desai and Aravamudan, 2023). Changes in the morphology of reactive materials alter their ease of handling during PRB installation. In addition, core–shell structures are attracting attention because they are composed of two different material layers (Cai et al., 2020; Dao et al., 2021). The preparation of spherical core–shell structures not only result in a large specific surface area but also high mechanical strength and durability. Therefore, a novel core–shell sphere was designed, and AC balls were prepared using zeolite as the core and powdered AC (PAC) and calcite (CaCO3) as the shell. In this study, AC balls were used to remove BTEX from the solution. The adsorption was evaluated using different carbon contents (5–30%), experimental times, and initial concentrations in the batch system. Adsorption data were evaluated using isotherm and kinetic models. Regeneration experiments were performed using chemical regeneration (methanol) and microwave (MW) heating to confirm their applicability.