Abstract:With the development of industry, groundwater contamination caused by refractory organic pollutants, such as benzene derivatives and chlorinated solvents, has become a growing environmental concern. In situ chemical remediation technologies have been widely applied; however, long-term remediation is often challenging due to the rapid depletion of active components and pollutant rebound. Therefore, sustained-release materials (SRMs) have emerged as a promising solution to address these limitations. In recent years, various SRMs have been developed to enable more durable in situ remediation of contaminated groundwater through controlled release mechanisms. Most SRMs consist of binders and active agents. Although binders do not participate directly in chemical reactions, they are essential for encapsulating active components within specific substrates, thereby facilitating their slow release. Gel-based SRMs, in particular, utilize environmentally friendly materials, such as sodium alginate, colloidal silica, and gelatin, as carriers to encapsulate oxidants, oxygen releasers, or nutrients. These systems effectively mitigate the rapid release of active agents and the associated rebound of pollutant concentrations observed with traditional remediation materials. Due to their mobility, injectability, and biocompatibility, gel-based SRMs have become a research focus in the field of in situ groundwater remediation. Sodium alginate is a commonly used base material owing to its natural non-toxicity and facile cross-linking properties. When combined with materials such as chitosan or polyvinyl alcohol, its sustained-release performance can be significantly enhanced. It is primarily employed to encapsulate nutrients that promote microbial degradation of refractory organics. Colloidal silica, characterized by low initial viscosity and delayed gelation, is suitable for deep groundwater remediation, although it has a relatively short release cycle (20 hours to 3 days). In contrast, binders based on gelatin, xanthan gum, and cellulose are primarily used to encapsulate oxidants, with release durations exceeding 10 days. The release cycle and kinetics of gel-based SRMs are influenced by both material properties (e.g., binder composition, cross-linking agent type and concentration) and environmental factors (e.g., pH, temperature, and ionic composition of groundwater). Release behaviors typically follow the Korsmeyer-Peppas kinetic model, with Fickian diffusion as the dominant mechanism. Although gel-based SRMs have demonstrated promising remediation performance against refractory organic pollutants under laboratory conditions, their release kinetics remain susceptible to interference in complex groundwater environments, representing a key bottleneck for field applications. Future research should prioritize the development of stimulus-responsive hydrogels, the design of adaptive release systems, and the customization of 3D-printed structures to achieve efficient, long-term remediation of pollutants in heterogeneous subsurface conditions. Close-