Construction of a novel NiO-bentonite composite for enhanced tetracycline degradation under visible light irradiation

Abstract

In this study, a novel ceramic composite based on NiO and bentonite (NiO/BEN) was successfully synthesized via the hydrothermal method for the effective photocatalytic degradation of tetracycline (TC) in aqueous solutions. The synthesized composites were characterized using XRD, SEM/EDS, UV–Vis DRS, PL, XPS, BET, and FTIR techniques. The analyses revealed that NiO and bentonite formed a well-integrated heterostructure, and NiO/BEN had absorption in both the UV and visible light regions, with stronger absorption in the UV region. BET analysis confirmed a mesoporous structure with a specific surface area of 9.66 m²/g, considerably higher than those typically reported for pristine NiO, indicating improved accessibility of active sites. FTIR spectra exhibited characteristic peaks at 1042.8, 1012.9, 790.05, and 466.33 cm⁻¹, confirming the coexistence of NiO and bentonite phases with preserved structural integrity. The composite had a band gap of 4.05 eV, which effectively promoted electron-hole pair generation, while the shallow and deep defects in the structure facilitated efficient charge separation. In photocatalytic degradation experiments, NiO-bentonite composite showed much better performance compared to pristine NiO (19.51%), while increasing the NiO: bentonite ratio from 1:0.2 (52.41%) to 1:0.5 (85.58%) significantly enhanced the tetracycline removal efficiency after 120 min of visible‑light irradiation at neutral pH. The composite also exhibited strong photocatalytic activity across different water matrices, achieving 85.58% removal in distilled water and maintaining appreciable efficiencies of 69.52% in tap water and 70.67% in drinking water, where competing ions and dissolved organics are present. Reusability tests showed a gradual decline in efficiency from 85.58% in the first cycle to 69.09%, 50.65%, 39.34%, and 24.34% over five consecutive cycles, likely due to the accumulation of non-degradable by-products on active sites. The degradation reaction was found to be driven primarily by photogenerated holes, with superoxide radicals also playing an important role. Under sole visible-light irradiation and without added oxidants, the NiO–bentonite composite removed 85.58% of tetracycline at neutral pH, 93.61% at pH 3, and 78.47% at pH 10—each value representing a more than fourfold improvement over pristine NiO (19.51%) and surpassing all NiO/clay systems reported to date—thereby offering an energy-efficient, environmentally friendly, and readily scalable photocatalyst for next-generation water-treatment technologies targeting antibiotic contamination.