nZVI induced cellulose nanocrystals/poly(acrylic acid) cross-linked hydrogel for enhanced diclofenac adsorption

Conventional adsorbents face significant challenges when treating wastewater contaminated with organic pollutants due to their high operating costs, limited removal efficiency, and risk of secondary pollution. In this study, a sustainable nanoscale zero-valent iron (nZVI) induced cellulose nanocrystals/poly(acrylic acid) cross-linked hydrogel (Fe@CP) was prepared by free radical polymerization and ion-crosslinking. The nZVI crosslinker is gradually oxidized to Fe³⁺ by the free radicals generated by the initiator. The released Fe³⁺ coordinated with the negatively charged carboxyl and sulfate half ester groups on the CP chains, thereby forming a uniform crosslinked network. The adsorption kinetics of diclofenac (DCF) onto Fe@CP followed the pseudo-second-order model (R² > 0.963), suggesting chemical adsorption plays a dominant role in the process. Furthermore, the adsorption isotherm for DCF on Fe@CP were well-described by Langmuir model, indicating that monolayer adsorption occurs on the adsorbent surface. The maximum adsorption capacity and rate constant of Fe@CP are 801.5 mg·g^-1 and 0.065 L·mg^-1, higher than CP hydrogel crosslinked directly with Fe³⁺ or Fe₃O₄. The Fe@CP hydrogel shows excellent DCF adsorption performance due to nZVI crosslinking, interconnected channels and abundant active sites. In addition, the wide pH adaptability, high ionic strength resistance and ionic coexistence provide great potential for pharmaceutical wastewater applications.