Preparation Of ZnIn₂S₄@Bi₂WO₆ Binary Visible Light Composite Catalyst And Its Degradation Mechanism Of Fluvastatin

Fuvastatin is one of the statins to treatment of hypercholesterolemia.Due to the difficulty of complete removal of fluvastatin by traditional wastewater treatment technology,it widely exists in the aqueous environment and poses potential threat to human life and health.Photocatalytic technology is an effective way to remove statins from water.Bi₂WO₆is a typical bismuth-based photocatalyst with excellent response performance of visible light.However,the conduction band of Bi₂WO₆is positive,which restricts the chain reaction to generate·O₂^-through reduction of O₂by electrons,and then affects the generation of multiple reactive radicals.Meanwhile,the narrow band gap structure causes rapid recombination of photogenerated electrons and holes,which leads the decrease of photocatalytic efficiency.In this study,the composite photocatalysts were prepared based on the staggered band alignment of ZnIn₂S₄and Bi₂WO₆to solve the problems of limited generation of multiple reactive radicals and rapid electron-hole recombination of Bi₂WO₆.The effects of compounding ratio,hydrothermal reaction time and hydrothermal reaction temperature on the structure and performance of the composite photocatalysts were investigated to obtain the optimal preparation conditions.The effects of catalyst dosage,pollutant concentration,p H,humic acid,ion and light source on the removal effect of fluvastatin were investigated using the composite catalysts prepared under the optimal conditions.The qualitative and quantitative analysis of active radicals in the composite photocatalyst system,mineralization analysis,degradation pathway and intermediate product analysis were used to reveal the enhancement mechanism of composite photocatalyst performance.The main conclusions are as follows.（1）The ZlS@BW-30%showed the best photocatalytic performance which was prepared at hydrothermal reaction time of 720 min,hydrothermal reaction of 160°C,and ZnIn₂S₄doping of 30%.The ZlS@BW-30%had a core-shell structure consisting of Bi₂WO₆microspheres as a shell covering a ZnIn₂S₄core,the specific surface area was37.97 m²/g,the band gap was 3.1 e V,and showed the maximum transient photocurrent,the smallest impedance radius.Under xenon lamp irradiation,the degradation efficiency of fluvastatin reached 75.47%,and the removal rate of fluvastatin was significantly improved compared with the single-component Znln₂S₄and Bi₂WO₆systems.（2）When the ZlS@BW-30%dosage was 0.2 g/L,the concentration of fluvastatin was10 mg/L and the initial p H=9 of the solution,the photocatalytic degradation effect was the best,and the removal rate of fluvastatin reached 75.47%after 120 min of light irradiation;The degradation of fluvastatin was inhibited in the presence of anions and cations,and the order of inhibitory effects were:Mg²⁺>Ca²⁺>Na⁺>K⁺.CO₃^2->SO₄^2->HCO₃^->Cl^-;the effect of humic acid on degradation of fluvastatin was significant,the removal rate of fluvastatin was reduced by 20%when the initial concentration of humic acid reached 1 mg/L,and only 29.49%was removed when the concentration of humic acid increased to 10 mg/L;the best degradation of fluvastatin was achieved under UV light irradiation,the removal rate reached 78.45%;ZlS@BW-30%possessed high stability,and the removal rate of fluvastatin only decreased by 1.84%in four batches of cyclic degradation experiments.（3）In the photocatalytic reaction,h⁺played key role in oxidation,while·OH and·O₂^-played secondary role.The concentration of·O₂^-and·OH was 1.36×10^-5mol/L and 2.68×10^-7mol/L,respectively.The ZlS@BW-30%composite photocatalyst generated reactive radicals such as h⁺,·O₂^-and·OH under light illumination.The C-C bond,C=C double bond and C-N bond on the long-chain compounds of fluvastatin were attacked under the synergistic oxidation of reactive radicals such as h⁺,·O₂^-and·OH to form short-chain organic compounds,which further oxidized and decomposed to form H₂O and CO₂.The mechanism for the photocatalytic performance enhancement of ZlS@BW included two points as follow.Firstly,the well-matched staggered band alignment of ZnIn₂S₄and Bi₂WO₆was utilized to form Z-scheme heterojunction.The h⁺in the valence band of ZnIn₂S₄was recombine with e^-in the conduction band of Bi₂WO₆to prolong the lifetime of e^-in the conduction band of ZnIn₂S₄and h⁺in the valence band of Bi₂WO₆,which provided more opportunities for generation of reactive radicals.Secondly,the synchronous generation of multiple ROS was achieved based the band structure of ZnIn₂S₄and Bi₂WO₆,the synergistic oxidation of h⁺,·O₂^-and·OH was contributed to improved the performance of the composite photocatalyst.