Study On The Mechanism Of Tetracycline Phototransformation Mediated By Polystyrene Microplastics/Rubber Microparticles

Antibiotics and microplastics/rubber microparticles are pollutants that widely distributed in the water environment.Microplastics/rubber microparticles can be used as efficient carriers of antibiotics due to the characteristics of large specific surface area,small volume and strong hydrophobicity,so that a large amount of antibiotics will be absorbed on their surface,which could further trigger greater ecological threats.Photodegradation is considered to be one of the important ways to affect the conversion of antibiotics in water environment.A series of aging effects can occur on microplastics/rubber microparticles in the environment under light conditions,which not only changes their physical and chemical properties,but also indirectly induces the formation of reactive oxygen species（ROS）,thus affecting the photodegradation of antibiotics.Nevertheless,the current research with respect to microparticles pollution mainly focuses on thermoplastic materials,while the research on elastic rubber particles has not been extensively concerned.Meanwhile,studies on the combined pollution of microplastics/rubber microparticles and antibiotics mainly focus on the adsorption of antibiotics by them,whereas the study on the photodegradation process of antibiotics by these two different microparticles is still lack of in-depth understanding.For this paper,tetracycline（TC）is taken as the research object and polystyrene（PS）and rubber microparticles are selected as the representatives of the two types of plastics in view of the different characteristics of thermoplastic material microplastics and elastic material microparticles.Hence,the key role of microparticles of these two different materials on the photodegradation behaviour of TC was explored respectively.Also,microparticles exposed to the natural environment are easy to form biofilms,thus changing the adsorption process of antibiotics.The rubber microparticles have strong adsorption effect on antibiotics,but the mechanism of photodegradation on the biofilm-developed rubber microparticles to antibiotics remains indistinct.Therefore,the effect of biofilm formed on the surface of commercial tire rubber particles in natural environment on the adsorption and photodegradation process of TC and its mechanism were revealed.This study provides a crucial scientific basis for further assessing the environmental behaviour and ecological risks of combined pollution between antibiotic and microparticles.The main conclusions obtained are as follows:（1）The results showed that the protonation state of TC had an effect on the participation of PS in the photodegradation process of TC,and the proportions of HTC^-and TC^2-forms in deprotonation increased with the increase of p H value,accelerating the photodegradation rate of TC.PS also aged during TC photodegradation,and the intensity of O/C increased from 2.07%to 17.92%with the extension of light time.Additionally,PS could produce ROS in the photoaging process,which enhanced the photodegradation of TC.The free radical of singlet oxygen radical（¹O₂）and superoxide radical(O₂^·-)were the main contributors and the contribution rates were 15.25±2.21%and 11.69±1.84%,respectively.Also,it was verified that the photodegradation effect of TC was related to the aging degree of PS.The formation of environmental persistent free radicals（EPFRs）led to the production of more ROS in highly aged PS,which significantly accelerated the photodegradation of TC,and the route for PS to promote TC photodegradation was proposed.Based on the determination of intermediate products,the photodegradation of TC mainly included hydrolysis,hydroxylation,deamination,demethylation and decarbonylation.（2）Secondly,the effects of different structure rubber microplastics including butadiene rubber（BR）,styrene butadiene rubber（Costa et al.）and nitrile butadiene rubber（NBR）on the photodegradation of TC were investigated.The results exhibited that BR,SBR and NBR could promote the photodegradation of TC（SBR>NBR>BR）.Meanwhile,it was found that the degree of SBR breakage was more obvious after light exposure compared with BR and NBR,forming stronger oxygen-containing functional groups.The carbonyl index（CI）value of SBR was higher,indicating that the aging process of SBR was more significant,which could be related to the chemical composition and surface characteristics of rubber particles.The role of ROS produced by BR,SBR and NBR in TC photodegradation was determined by radical quenching and probe experiments.SBR possessed high ROS generation capacity in TC photodegradation system,and hydroxyl radical（·OH）and ¹O₂ as the main ROS accelerated TC photodegradation,which was mainly due to the abundant photoactive oxidation functional groups on the surface of SBR.The electron donating capacity of BR,SBR and NBR was further determined by electrochemical experiments,among which SBR was the strongest.Finally,the density functional（DFT）theory was used to predict the reactive sites of BR,SBR and NBR.In the ground state（S0）,SBR containing benzene ring structure was more easily excited and the minimum gap of SBR in the single excited state（S1）was 2.426 e V,which indicated that SBR had the strongest promoting effect on TC photodegradation due to the highest reactivity,compared with BR and NBR.（3）Finally,the main effects of commercial tire wear particles（TWPs）on TC adsorption and photodegradation were analyzed.Based on the incubation experiment of TWPs,the quantitative characterization of biofilm（modified crystal violet method）and high-throughput sequencing（16S r RNA）analysis indicated that the formation of biofilm on the surface of TWPs increased the specific surface area and pore size of TWPs after 45 days of incubation,and promoted the accumulation of oxygen-containing organic functional groups.The adsorption capacity of biofilm-developed TWPs on TC（1.430 mg/g）was higher than that of the original TWPs（1.0 mg/g）.Compared with direct TC photodegradation,both the virgin TWPs and biofilm-developed TWPs promoted TC photodegradation with rate constants of0.0232±0.0014 and 0.0152±0.0010 h^-1,respectively（an increase of 2.5-3.7 times）.It was found that the virgin TWPs promoted TC photodegradation faster than the biofilm-developed TWPs.Radical quenching and probe analysis showed that·OH and O₂^·-as the main ROS promoted the photodegradation process of TC.Due to the stronger electron transport ability and photosensitization of the virgin TWPs,the virgin TWPs had a stronger ability to generate ROS,which was more conducive to the photodegradation of TC.The pathways of TC photodegradation induced by the virgin and biofilm-developed TWPs were basically the same,including hydrolysis,deamination,hydroxylation,demethylation,decarbonization and ring-opening processes.In summary,this paper systematically studied the effects of thermoplastic microplastics/rubber microparticles on TC photodegradation process under light conditions,and revealed the influencing factors and mechanism of microplastics/rubber microparticles on TC photodegradation.The results of this study are helpful to improve the understanding of migration and transformation characteristics of microparticles and antibiotics under the condition of combined pollution,and provide theoretical basis for accurately assessing the ecological and environmental risks of microparticles and antibiotics and preventing and controlling combined pollution.