Enhanced visible-light photodegradation of ciprofloxacin via Co²⁺-doped Zn-Al LDH with persulfate activation

Pharmaceutical residues, particularly ciprofloxacin (CIP), are frequently detected in aquatic environments and pose significant risks to ecosystems and human health. This study investigates the visible-light photocatalytic degradation of CIP using Co²⁺-modified Zn₀.₆₆₇Al₀.₃₃₃-layered double hydroxide (LDH) materials, with persulfate (PS) as a co-activator. The hydrotalcite-like compounds Zn₀.₆₆₇Al₀.₃₃₃(OH)₂(CO₃)₀.₁₆₇•0.5H₂O, denoted as nCoZnH, were synthesized by co-precipitation.These were further calcined at 500 °C for 5 hours to obtain nCoZnH500. Both uncalcined and calcined materials were characterized structurally and physicochemically. Results revealed that nCoZnH maintained the typical hydrotalcite layered structure, with Co²⁺ ions effectively incorporated into brucite-like layers through isomorphic substitution of Zn²⁺. Upon calcination, partial retention of the LDH structure was observed, accompanied by the formation of ZnO and Co₃O₄ phases. The BET surface area of nCoZnH500 was markedly higher than that of uncalcined counterparts. Co²⁺ modification also significantly reduced the bandgap energy, thereby enhancing visible-light-induced photocatalytic activity. Among all samples, 1.0CoZnH and 2.0CoZnH500 exhibited the highest degradation efficiencies, 78.0 ± 1.87% and 70.9 ± 2.31%, respectively, for 10 ppm CIP under visible light. Furthermore, the photocatalytic activity of the synthesized materials was influenced by the Co: Al molar ratio, initial CIP concentration, persulfate (S₂O₈²⁻) dosage, and solution pH. The 2.0CoZnH500 sample demonstrated superior stability compared to 1.0CoZnH, with only a 9.1% reduction in the CIP removal efficiency after four consecutive cycles, maintaining a degradation efficiency of 59.7%. These findings indicate that the synergy between Co²⁺ doping and PS activation effectively boosts CIP degradation. The developed materials offer promising potential for the treatment of pharmaceutical wastewater under visible light irradiation.