Beyond Redox Synergy: Role of Trivalent Cations in Ni-Based Layered Double Hydroxides for Peroxydisulfate Activation

The persistence of fluoroquinolone antibiotics in aquatic environments represents a growing environmental concern and requires efficient and scalable treatment strategies. In this study, ciprofloxacin degradation was investigated through persulfate activation by using Ni-based layered double hydroxides (LDHs). NiFe-LDHs with different molar ratios (1:1, 2:1, and 3:1) were synthesized and compared with NiAl-LDH to elucidate the role of the trivalent metal in the catalytic performance. A 22 factorial design was employed to evaluate the influence of the catalyst dosage and persulfate volume. All catalysts exhibited rapid removal, reaching approximately 57% removal within 20 min and up to 87% removal after 120 min. Increasing the Ni content did not improve the overall removal performance, indicating that activity is not directly governed by the amount of Ni present. Kinetic analysis followed pseudo-second-order behavior, while factorial results indicated that catalyst dosage had a stronger influence than the persulfate concentration. Mass spectrometry confirmed the disappearance of the parent ion (m/z 332), indicating an effective transformation of ciprofloxacin. Radical scavenger experiments indicated a greater contribution of radical species in the Fe-containing materials, whereas NiAl-LDH showed a more adsorption-driven removal pathway. No synergistic enhancement between Ni and Fe was observed, as NiAl-LDH exhibited comparable or superior performance despite containing the non-redox-active Al3+ center. Density functional theory (DFT) calculations revealed that Ni centers play a dominant role in persulfate activation, whereas the trivalent metal primarily modulates the electronic structure and the reaction pathway of the catalyst. These findings challenge the conventional assumption that the presence of multiple redox-active metal centers necessarily enhances the catalytic performance in LDH systems and highlight the critical role of the local electronic environment in designing efficient catalysts for environmental applications.