Effect And Mechanism Of BiVo₄-Based Visible Light Catalyst Synergistic With H₂O₂ To Degrade Doxycycline In Water

Doxycycline（DC）,as one of the antibiotics,has been widely used in the field of medicine and breeding.The DC can enter the aquatic ecosystem through the discharge of sewage and wastewater,which poses a threat to the ecosystem and individual organism,and has potential hazards to human health.Therefore,it is urgent to develop the methods to remove the DC from water.Bismuth vanadate（BiVO₄）has the advantage of using sunlight to degrade pollutants,so it is an ideal method to degrade DC in water by using BiVO₄ as visible light catalyst.However,the photogenerated electrons and photogenerated holes produced in the photocatalytic process of BiVO₄ are easy to compound,and the photogenerated carriers are difficult to be utilized effectively.The removal rate of pollutants is not high,which limits the practical application of BiVO₄ in the field of water treatment.Therefore,how to further improve the separation degree of photogenerated electrons and photogenerated holes in BiVO₄ has become the target pursued by scholars.In order to improve the separation degree of photogenerated carriers and improve the phtocatalytic activity of BiVO₄,the following research work was carried out in this paper:BiVO₄ with exposed（0 4 0）facets was obtained by controlling the conditions of hydrothermal synthesis;nano-BiVO₄ was prepared by controlling crystal growth with 1,3-PDO as solvent.nano-BiVO₄@Ag₂O（typeⅠheterojunction）was prepared by combining nano-BiVO₄ with Ag₂O.The visible light/catalyst/H₂O₂ system was constructed by using three kinds of BiVO₄-based catalyst and H₂O₂.The degradation efficiency and mechanism of photocatalytic degradation system were studied by taking DC which was often detected in water as the target pollutant.BiVO₄ with high exposure of（0 4 0）facets was obtained by optimizing hydrothermal synthesis conditions.The synthesis conditions include p H,temperature and cooling methods.The optimized BiVO₄ has the strongest diffraction peak of（0 4 0）facets,the effective separation degree of photoelectron and hole,and the highest photocatalytic activity.Compared with BiVO₄ with low exposure of（0 4 0）facets,the photocatalytic activity of BiVO₄ with high exposure of（0 4 0）facets was increased by about 7 percentage points to 50%.The visible light/BiVO₄/H₂O₂ system was constructed by BiVO₄ and H₂O₂.The removal efficiency and influencing factors of the system were studied with DC as the target pollutant.The results showed that H₂O₂ could promote the system to produce more active species and improve the DC removal rate to 91%.The optimal dosage of H₂O₂ was 4 mmol/L.The removal rate of DC was affected by the p H of water.So,the optimal p H value of water was about 7.5.The effect of water temperature on DC removal rate was not obvious.The increase of BiVO₄ dosage could promote the removal of DC,but when the amount of BiVO₄ was too much,the transmittance of water would be affected.So,the optimal amount of BiVO₄ was182 mg/L.nano-BiVO₄ was prepared by solvothermal method using 1,3-PDO as solvent,and the preparation conditions were optimized.The results showd that the p H of synthetic reaction solution had little effect on the crystal structure,morphology and photocatalytic activity of catalyst.However,the amount of 1,3-PDO had obvious effect on the physical and chemical properties of catalyst.When the amount of 1,3-PDO was 47%（V%）,nano-BiVO₄ could be prepared.Under this condition,the particle size of the catalyst was the smallest,the specific surface area was the largest,the separation degree of photogenerated electrons and holes was greatly improved,and the photocatalytic activity of the catalyst was also obviously improved.Compared with BiVO₄ with highly exposed（0 4 0）facets,the photocatalytic activity of nano-BiVO₄ prepared by solvothermal method was increased by about 8 percentage points to 58%.Visible light/nano-BiVO₄/H₂O₂system was constructed by nano-BiVO₄ and H₂O₂.DC was used as the target pollutant to investigate the removal efficiency and influencing factors.The results showed that the removal rate of DC was 94%in light/nano-BiVO₄/H₂O₂ system.The optimal dosage of H₂O₂ was 4 mmol/L.The p H of water affected the removal rate of DC,and the optimal p H was 5.5-9.5.The influence of water temperature and catalyst dosage in light/nano-BiVO₄/H₂O₂ system was the same as that in light/BiVO₄（0 4 0）/H₂O₂ system.In addition,Cl^-and SO₄^2-had a weak promoting effect on DC removal.Br O₃^-had a significant promoting effect on DC removal.NO₃^-and PO₄^3-had an inhibiting effect on DC removal.River water as a complex matrix background promotes DC removal.In order to further improve the separation degree of photocarriers and improve the utilization efficiency of photocatalyst,nano-BiVO₄@Ag₂O composites with heterojunction（typeⅠ）was prepared by combining Ag₂O and nano-BiVO₄.According to the characterization of XPS,there was heterojunction at the interface between nano-BiVO₄ and Ag₂O.When the mass ratio of Ag₂O to nano-BiVO₄ was10%,nano-BiVO₄@Ag₂O had the higher maximum absorption wavelength,the higher higher separation degree of photogenerated electrons and holes and the stronger photocatalytic activity.The photocatalytic activity of nano-BiVO₄@Ag₂O was 80%and about 22 percentage points higher than that of nano-BiVO₄.The efficiency and influencing factors of DC degradation in visible light/nano-BiVO₄@Ag₂O/H₂O₂ system were investigated.The experimental results showed that the optimal dosage of H₂O₂ was 12 mmol/L,and the removal rate of DC reached 97%.The influence of p H value,water temperature,nano-BiVO₄@Ag₂O dosage,Cl^-,SO₄^2-,NO₃^-,PO₄^3-and water matrix on DC removal in light/nano-BiVO₄@Ag₂O/H₂O₂ system was basically the same as that in light/nano-BiVO₄/H₂O₂ system.Low concentration of Br O₃^-can promote DC removal,while excessive Br O₃^-can inhibit DC removal.The degradation mechanism and degradation pathways of DC in light/nano-BiVO₄@Ag₂O/H₂O₂ system and light/nano-BiVO₄/H₂O₂ system were compared.The results showed that the active species produced in two systems were the same,and the order of contribution rate was h⁺>·O₂^->·OH.However,the production of active species in two systems was different.The active species were more easily produced in light/nano-BiVO₄@Ag₂O/H₂O₂ system,and the proportion of intermediates（m/z<353）was higher,which was 24.4 percentage points higher than that in the light/nano-BiVO₄/H₂O₂ system.The degradation paths of DC in two systems mainly include oxidation,hydroxylation,demethylation,amide oxidation and ring-opening reaction.The catalytic stability of nano-BiVO₄@Ag₂O was investigated by repeated experiments,X-ray diffraction and UV-visible absorption.The UV-visible absorption spectra and crystal structure of nano-BiVO₄@Ag₂O had not changed significantly before and after use.The nano-BiVO₄@Ag₂O could still degrade doxycycline effectively after repeated use for four times.The research results of this paper have important theoretical significance and potential application prospects in the field of water treatment.