Hydrogels are attracting significant interest for underwater applications, including wastewater treatment. However, many hydrogels suffer from poor mechanical properties, short-term stability, and reusability issues. Water molecules weaken van der Waals interactions, and complex water environments can damage the hydrogel structure, particularly in the presence of metal ions. Mussel-inspired hydrogels utilizing polydopamine (PDA) offer enhanced underwater mechanical properties due to covalent/non-covalent bond formation. The creation of dynamic bonds is crucial for achieving both high mechanical strength and self-healing capabilities. Pure PDA hydrogels often fall short of desired mechanical properties, and overoxidation of catechol groups further worsens performance. Therefore, an integrative strategy is needed to combine catalytic performance, long-term mechanical stability, and selective permeability for effective wastewater treatment. This research addresses these challenges by developing a cellulose-based hydrogel for dye degradation, incorporating grafted polymers and a dynamic redox catechol system triggered by Pd nanoparticles. The hydrogel is further enhanced by a graphene oxide membrane for enhanced stability and selective permeability.

The authors review existing literature on hydrogels for underwater applications, highlighting their limitations in mechanical properties, stability, and reusability. They discuss mussel-inspired hydrogels based on polydopamine (PDA) and the challenges of achieving sufficient mechanical strength and self-healing properties. The use of dynamic bonds is discussed as a crucial aspect of improving hydrogel performance. The literature also points to the need for a protective membrane to enhance selective permeability and prevent damage from metal ions and other components present in wastewater. Previous research on similar systems is cited, indicating the need for an integrated strategy to combine desirable properties.

The researchers developed a cellulose-based hydrogel for dye degradation by grafting polymers and incorporating Pd nanoparticles. The process involved three steps: (1) immobilizing Pd NPs on cellulose-graft-polydopamine (DACO-PDA) fibers to create DACO-PDA@Pd NPs, rich in hydrophilic groups; (2) converting phenolic hydroxyls on DACO-PDA@Pd NPs into semiquinone radicals, then oxidizing them to quinone/hydroquinone radicals during the reduction of Pd²⁺ to Pd NPs; and (3) mixing SA and AA with DACO-PDA@Pd NPs to create a catechol-conjugated alginate hydrogel. The Pd NPs act as nanoreinforcement and trigger the dynamic redox catechol system, providing self-healing capability. The hydrogel is then coated with a graphene oxide membrane (P-GOM) using PNIPAM chains, improving its hydrophilicity, stability, and selective permeability. The materials used were sodium periodate, sodium chlorite, hydrochloric acid, dopamine hydrochloride, sodium alginate, acrylic acid, graphene oxide, poly(N-isopropylacrylamide), ammonium persulfate, palladium chloride, sodium borohydride, phosphate buffers, and ethanol. The hydrogel's mechanical properties were characterized by compressive and tensile testing. The self-healing ability was evaluated by visual inspection and tensile testing after fracturing. Wettability and selective ion permeability were assessed by water contact angle measurements. Dye degradation performance was tested using Congo red (CR) and methylene blue (MB) in tap water with NaBH₄ as a reducing agent. The reusability of the hydrogel was evaluated by conducting multiple cycles of dye degradation. Pd leaching was monitored using inductively coupled plasma mass spectrometry (ICP-MS). The characterization techniques employed included Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (XRD), UV-Vis spectroscopy, electron spin resonance (ESR), and liquid chromatography-mass spectrometry (LC-MS).

The resulting hydrogel exhibited significantly enhanced mechanical strength compared to pure PAA and SA hydrogels. Compressive and tensile tests demonstrated its high durability and recoverability. The dynamic catechol chemistry endowed the hydrogel with a self-healing ability, with visible fractures disappearing completely within 2 hours. The hydrogel demonstrated a high degradation rate for both anionic (Congo red) and cationic (methylene blue) dyes, achieving over 95% degradation after multiple cycles. The graphene oxide membrane coating enhanced the hydrophilicity and protected the hydrogel structure from metal ion damage, maintaining its performance and preventing degradation over multiple cycles. The Pd NPs were effectively immobilized within the hydrogel, with minimal leaching even after 50 cycles. The study confirmed that Pd NPs served as controllable reaction sites for dye degradation. LC-MS analysis identified the final products of CR degradation as small, less toxic molecules.

The findings demonstrate a successful strategy for creating mechanically robust, self-healing hydrogels suitable for long-term underwater applications. The integration of dynamic catechol chemistry, Pd nanoparticles, and a graphene oxide membrane addresses the limitations of previous hydrogels. The high degradation efficiency and reusability of the hydrogel suggest significant potential for wastewater treatment. The minimal Pd leaching confirms the stability of the catalytic system. The results contribute to the field of materials science by demonstrating a route to create highly durable, efficient, and reusable catalytic hydrogels for environmental remediation.

This study successfully developed a tough, self-healing hydrogel with high catalytic activity for dye degradation in water. The combination of a dynamic catechol redox system, Pd nanoparticle incorporation, and a protective graphene oxide membrane resulted in a material with excellent mechanical properties, reusability, and stability. This research suggests significant potential for this hydrogel in water purification applications. Future research could explore scaling up production, testing with more complex wastewater matrices, and investigating the hydrogel's performance with other pollutants.

While the study demonstrates excellent performance in controlled conditions, further research is needed to assess the hydrogel's long-term stability in real-world wastewater environments, which may contain a wider range of contaminants and potentially interfering substances. The impact of variations in wastewater pH, temperature, and ionic strength on hydrogel performance should be investigated. A comprehensive life cycle assessment of the material's sustainability would also be beneficial.