Environmental and Wastewater Treatment Applications of Stimulus-Responsive Hydrogels

Abstract

Stimulus-responsive hydrogels have emerged as versatile materials for environmental and wastewater treatment applications due to their ability to adapt to changing environmental conditions. This review highlights recent advances in the design, synthesis, and functionalization of such hydrogels, focusing on their environmental applications. Various synthesis techniques, including radical polymerization, grafting, and copolymerization, enable the development of hydrogels with tailored properties such as enhanced adsorption capacity, selectivity, and reusability. The incorporation of nanoparticles and bio-based polymers further improves their structural integrity and pollutant removal efficiency. Key mechanisms such as adsorption, ion exchange, and photodegradation are discussed, emphasizing their roles in removing heavy metals, dyes, and organic pollutants from wastewater. Additionally, this review presents the potential of hydrogels for oil-water separation, pathogen control, and future sustainability through integration into circular economy frameworks. The adaptability, cost-effectiveness, and eco-friendliness of these hydrogels make them promising candidates for large-scale environmental remediation.

Keywords: adsorption and photodegradation; hydrogel functionalization; stimulus-responsive hydrogels; sustainable water management; wastewater treatment.

Figure 1

Polymerization techniques for obtaining hydrogels.

Figure 2

A comprehensive classification of hydrogels, highlighting various types based on their chemical composition, structure, and responsiveness to external stimuli.

Figure 3

An overview of hydrogels’ reactions to different stimuli, illustrating their adaptability and functionality in response to environmental changes such as pH, temperature, and light.

Figure 4

A schematic illustration for the adsorption mechanism.

Figure 5

(A) The HPMC-g-poly(AM-co-SPA) hydrogels were examined both before and after freeze-drying, with SEM images revealing their porous morphology and EDX analysis providing elemental composition details; (B) the pH-responsive behavior of the HPMC-g-poly(AM-co-SPA) hydrogels; (C) temperature-responsive behavior of HPMC-g-poly(AM-co-SPA) hydrogels [126].

Figure 6

(A) The SEM images represent the morphological structures of various hydrogels synthesized through radical polymerization: (c) GP/AA-co-AM, (d) GP/GTS/AA-co-AM, (e) GP/PAM, and (f) MCC/AA-co-AM; (B) swelling ratios of hydrogels at different pHs; (C) the competitive adsorption behavior of GP/CTS/AA-co-AM was evaluated for (a) Pb(II), Cd(II), (b) RhB, and MO in mixed solutions [141].

Figure 7

(A) Photodegradation of (a) MB dye and (b) OG dye using pristine g-C3N4 and g-C₃N₄/CMC/SA_2:1 beads; (c) OG dye degradation using g-C₃N₄/CMC/SA_1:1 and g-C₃N₄/CMC/SA_2:1 beads; and (d) OG dye degradation using g-C₃N₄/CMC/SA_2:1 beads over 5 consecutive cycles [Experimental conditions: initial concentration of OG = 10 ppm, MB = 5 ppm, pH = 3.8, temperature = 25 °C, under simulated solar light]; (B) (a) Photodegradation performance of g-C3N4/CMC/SA2:1 beads was evaluated under the influence of various scavengers and reactive agents, including IPA, EDTA, ASC, Ag⁺, IPA/ASC, ASC/Ag⁺, and EDTA/ASC. The experimental setup involved OG dye with an initial concentration of 10 ppm at pH 3.8, a temperature of 25 °C, and simulated solar light irradiation. (b) Adsorption and photodegradation efficiency of MB and OG dyes were assessed in binary-dye solutions, considering their different charge properties and interaction potentials with the composite beads. (c) Similarly, RBBR and OG dyes were tested in binary-dye solutions to explore the composite’s ability to manage competitive adsorption and photodegradation under similar experimental conditions [142].

Figure 8

The practical applicability of stimulus-responsive hydrogels in wastewater treatment and environmental remediation.