Degradation Of Typical Sulfonamide Antibiotics From Water By Visible-Light Catalysts Based G-C₃N₄

The antibiotics exsited in water environment have posed a potential threat to human health and ecological risks,so the degradation of sulfonamide antibiotics has been focused by the gloaal researchers.Among all the treatment methods,visible catalytic technology has attracted a global attention due to the advantage of green,mild reaction conditions and low energy consumption.The most common used photocatalysts are Zn O and Ti O2.Most of them were in nano-scale and only could absorb UV light to ensure the high efficient photocatalytic activity,which is a challenge to practical application.Thus,micrometre-scale g-C₃N₄ with visible-light catalytic activity regarded as photocatalysts has attracted an extense attention.However,among the researches of visible catalytic technology based on g-C₃N₄,most of them have been focued on the enhancement of photocatalytic activity.Besides,it is insufficient to study the mechanism of micro-pollutants degradation.Aiming at these above problems,this paper adopts visible-light photocatalysis technique based on g-C₃N₄ as the a treatment method to investigate the photodegradation efficiency of four sulfonamide antibiotics including sulfamethoxazole（SMX）,sulfisoxazole（SSX）,sulfadiazine（SDZ）and sulfamerazine（SMZ）.g-C₃N₄ was synthesized by a thermal treatment of urea or thiourea.Depending on the photodegradation efficiency of SMX,g-C₃N₄ with excellent photocatalytic performance was selected.Then the degradation behavior of the four sulfonamide antibiotics by g-C₃N₄ under visible light irradiation was systematicly studied,including effectiveness,mechanism and pathways.When initical p H was 7.0,the contaminants concentration of was 10 μM,and the dosage of g-C₃N₄ was 50 mg/L,90.1% of SMX could be degradated,and SSX,SDZ and SMZ could be almost completely degradated.The effects of typical water quality parameters,such as solution p H,bicarbonate ion and humic acid,on the photodegradation efficiency were also discussed.It can be found that although the water quality parameters showed different trends on the photodegradation of the four sulfonamides;excellent photocatalytic efficiency could always be achieved.Holes and ·O₂^-were the main oxidative species in the photodegradation process of SMX,SDZ and SMZ;but ·O₂^-and OH·played the dominant role for SSX degradation.The difference of main active specises led to the varied degradation pathways.The similiar pathways shared by the four sulfonamides were the cleavage of S-N bond and hydroxylation of the benzene ring.The specific degradation pathways were also identified as below:（1）the nitration of amino group on the benzene ring in the degradation process of SMX,SDZ and SMZ;（2）the bond cleavage between benzene ring and S in the degradation process of SSX;and（3）the carboxylation of the methyl group in degradation process of SMZ.To enchance the role of ·O₂^-during the degradation process of the four sulfonamide antibiotics,Ag/g-C₃N₄ photocatalysts were synthesized.The properties of Ag/g-C₃N₄ photocatalysts were investigated by adjusting the dosage of PVP and Ag content.The optimum dosage of PVP and the content of Ag NPs were determined as 0.3 g and 5 wt.% by the degradation efficiency of SMX.The photocatalytic experiments revealed that compared with g-C₃N₄,the degradation rates of SMX and SSX could by increased by 32.1% and 10.4%,respectively when 5 wt.% Ag/g-C₃N₄ was exposed to visible light irradiation for 60 min.And SDZ and SMZ could be almost completely degraded by 5 wt.% Ag/g-C₃N₄ under visible light for 30 min.The excellent photodegradation activity could also be achieved with the effect of quality parameters（initical p H,HCO3-and humic acid）and water quality.Ag NPs loaded on the sheet of g-C₃N₄ could enhance the photocataoytic activity due to the SPR effect for increasing the separation of photogenerated pair and the formed Schottky barrier for decreasing the recombination of photogenerated pair.In the photodegradation processes of the four sulfonamide antibiotics,holes and ·O₂^-were the main active species.The main intermediate products of SMX,SDZ and SMZ by 5 wt.% Ag/g-C₃N₄ were similar with that by g-C₃N₄ under visible light irradiation.However,as for SSX,the intermediate products and pathways by the two photocatalysts were different due to the enhanced role of holes in the photodegradation process by 5 wt.% Ag/g-C₃N₄.Holes could oxidize amino group on the benzene ring to form nitration,which was not found in the photodegradation process by g-C₃N₄.Taking the SMX photodegradation by 5 wt.% Ag/g-C₃N₄ as example,the relationship between active species and intermediate products were studied and discussed.The results revealed that ·O₂^-and OH· were beneficial to the mineralization of SMX but holes were unfavorable to the mineralization of SMX.So in construction of gC₃N₄ composites,the role of ·O₂^-and OH· should be enhanced for efficient removal and mineralization of SMX.