Semi-Unzipping of a Bio-Derived and Recoverable Carboxymethyl Cellulose–Chitosan Hydrogel for Cd(II) and As(III) Removal from Contaminated Soil

Abstract

Simultaneous removal of cationic metal and oxyanion-forming metalloid contaminants remains challenging because of their contrasting chemical behaviors. Polyzwitterionic hydrogels containing both cationic and anionic functional domains are promising for such cocapture, yet intranetwork electrostatic pairing often restricts site accessibility. Herein, carboxymethyl cellulose–chitosan polyzwitterionic hydrogel (CMC-CS) was used as a bio-derived precursor to construct Fe3+-crosslinked CMC-CS (Fe3+@CMC-CS) via Fe3+-induced semi-unzipping, followed by in situ reduction of the Fe3+ sites to generate recoverable nanoscale zero-valent iron (nZVI)-functionalized CMC-CS (nZVI@CMC-CS). This structural regulation loosened the compact charge-paired network and enlarged the average pore diameter from 5.14 to 19.47 nm, thereby improving the accessibility of interfacial binding sites. With the subsequent formation of nZVI-functionalized reactive interfaces, the capture capacities for Cd(II) and As(III) increased by 2.50- and 10.01-fold, respectively, compared with the pristine CMC-CS hydrogel. High uptake was also maintained in binary systems, reaching 91.95 mg/g for Cd(II) and 63.16 mg/g for As(III). Mechanistic analyses revealed that accessible interfacial binding sites favored Cd(II) capture through cation exchange, electrostatic attraction, and surface complexation, whereas Fe-based reactive interfaces promoted As(III) uptake via oxidation-associated immobilization, precipitation, and complexation. In a 30-day soil incubation experiment, nZVI@CMC-CS reduced the total contents of Cd and As by 3.87–13.62% and 4.24–9.11%, respectively, while altering their fraction distributions and maintaining high material recovery. These findings demonstrate that network unzipping offers an effective strategy for engineering recoverable reactive hydrogel interfaces for the cocapture of chemically distinct contaminants in soil.