Research progress on the application of hydrogel adsorbent materials in wastewater treatment: A review

Water is the source of life and plays an important role in human development. Industrial output and residential water consumption are rising in tandem with the rapid economic growth, while the pollution of water resources caused by modern human activities is becoming more and more serious. Numerous pollutants, including heavy metal ions (Wang and Cheng, 2023), organic pollutants (Nabipour et al., 2020, Marchel et al., 2023), oil-containing wastewater (Yang et al., 2024a), and others, are released into water bodies during industrial and agricultural production processes. This has major repercussions, degrades water quality, and makes the global freshwater crisis an urgent global issue (Zheng et al., 2024). Therefore, the study of how to remove these pollutants from water bodies has been receiving attention from countries all over the world (Li et al., 2024a). Currently, a variety of methods have been utilized for wastewater treatment, including biodegradation (Nakarmi et al., 2023), electrochemical methods (Wu et al., 2024), adsorption (Zhao et al., 2023), ion exchange (Bashir et al., 2019), etc. Of the above, the biological method of treating wastewater has a lengthy cycle and requires an extremely strict environment to cultivate certain strains of bacteria, so its application in the actual project is subject to certain limitations; the electrochemical method of treating wastewater has a fast rate of treatment but consumes a lot of power and quickly wears down the pole plate; the ion exchange method of treating wastewater has good results but its high cost prevents its large-scale development. In contrast to alternative approaches, the adsorption method is inexpensive, simple to use, and more efficient. Therefore, one of the key solutions to the issue of pollutant residuals in wastewater is the development of inexpensive adsorbents with high removal effectiveness (Zhang et al., 2023a).

The choice of appropriate adsorbent materials is essential since wastewater has a wide range of characteristics (Ahmaruzzaman et al., 2023). The main means of treating pollutants in water by adsorption are to take advantage of the high porosity and specific surface area of adsorbents materials, or physical or chemical adsorption of pollutants through the functional groups of materials (Li and Lin, 2021). At the moment, activated carbon, molecular sieves, and adsorbent resin are the three primary commercial adsorbents. The adsorption capacity of activated carbon is strong, but it has a short life due to regeneration and replacement issues. Although molecular sieves can overcome this issue with traditional adsorbents, they must be specially designed and processed for practical applications to ensure mechanical strength. Adsorption resin and other materials have lower costs than other materials, but their pore size distribution is uneven and difficult to control, which slows down the adsorption process. Natural mineral materials have been used extensively in wastewater treatment in recent years. It has been discovered that zeolite (Mo et al., 2022), attapulgite (Cui et al., 2012), diatomite (Caliskan et al., 2011) have good heavy metal ion adsorption capacities. A wide spectrum of academics and researchers have also expressed interest in the application of nanotechnology to the treatment of water. Nanomaterials such as carbon nanotubes, metal-organic frameworks (MOFs) nanoparticles and nano zero-valent iron exhibit high specific surface area and excellent adsorption properties (Vesali-Naseh et al., 2021). However, when nanomaterials are exposed to an aqueous environment for a long period of time, their structural stability deteriorates, and they are difficult to separate from the solution after adsorption. These drawbacks limit the application of nanomaterials for adsorption in industry. Therefore, while ensuring the removal efficiency, the selected adsorbents need to be developed in the direction of low cost, good environmental adaptability, and green environmental protection.

In contrast, hydrogel adsorbents can solve problems such as recycling and strengthening modification at the same time and have attracted extensive attention in the field of wastewater treatment. As an adsorption material, the three-dimensional network structure of hydrogel contains more hydrophilic groups, rapidly swells in water, and can absorb and maintain a large amount of water without being dissolved under the swelling state, so it can accommodate a variety of pollutants in its highly porous network structure. In addition, the high specific surface area and porosity of hydrogels after water absorption and expansion are conducive to the diffusion of pollutants in water. Hydrogel adsorbents after modification and other treatments usually contain certain functional groups, such as amino, sulfonate, carboxyl, etc., which will have physical and chemical interactions with pollutants in water, so as to achieve the purpose of adsorption and removal. The earliest application of hydrogel materials as adsorbents in the field of wastewater treatment is natural polymer hydrogels, such as lignin (Albadarin et al., 2017), pectin (Shao et al., 2021), cellulose (Godiya et al., 2022), etc. However, the mechanical properties and adsorption properties of pure natural hydrogels are poor, and some scholars have obtained composite polymer hydrogels by modifying natural polymers to enhance their various properties (Yue et al., 2019, Cao et al., 2021a, Yang et al., 2021, Wittmar et al., 2020). In the related research and application of wastewater treatment, composite polymer hydrogels have gradually dominated the field. Although a lot of research has been carried out on hydrogel adsorbents in recent years, how to prepare hydrogel adsorbents with both high adsorption capacity and high strength is still a serious problem.

The publication trend of hydrogel adsorbents in wastewater treatment has steadily increased over the past decade, reflecting the growing interest and progress in this field. Several reviews have been published on the use of hydrogel adsorbents for wastewater treatment (Fig. 1). For example, Parisa et al (Mohammadzadeh Pakdel and Peighambardoust, 2018). provided a comprehensive review of acrylate-based hydrogels as novel materials for removing contaminants from water, discussing their synthesis and mechanisms. Zhang et al (Zhang et al., 2022a). published another review focusing on hydrogel materials for sustainable water harvesting and treatment, highlighting their potential for environmental applications. Recently, Radoor et al (Radoor et al., 2024). published a review that discussed the recent advances in cellulose- and alginate-based hydrogels for water and wastewater treatment. Despite the existing reviews, there is still a need for a comprehensive and up-to-date review that not only summarizes recent advances in hydrogel adsorbents, but also provides a detailed analysis of their properties, challenges, and future prospects. Therefore, this review is based on an extensive collection of literature from various databases, focusing on the research published in the past ten years to ensure that the latest research findings are included. The synthesis route of hydrogel adsorbents is reviewed, and their unique properties and adsorption mechanism are discussed in detail (Fig. 2). According to the different pollutants in water, they are summarized as heavy metal ion pollution, dye pollution, etc. The application of hydrogel adsorbents in wastewater treatment field in recent years is summarized, and the potential and advantages disadvantages of hydrogel materials as adsorbents in this field, as well as possible solutions, are discussed. In addition, this review introduces the advantages of hydrogel adsorbents compared to other contaminant removal methods, as well as strategies for the recovery and reuse of hydrogel materials after adsorption. This information is intended to provide valuable references for subsequent research.