Occurrence Of Microplastics In Aquaculture Water And Their Interaction Mechanism With Typical Disinfectants

Microplastics（MPs）is widely distributed in the global waters,from the equator to the polar regions,from offshore to ocean,and from the surface seawater to deep-sea sediments.Current research on MP pollution has be expanded from the marine environment to the freshwater（rivers,lakes and reservoirs）,domestic wastewater and urban drinking water.The basic data on MP pollution are becoming more and more abundant.However,few studies have focused on MPs in small water bodies,especially aquaculture water.MPs have been detected in fish,shrimp,shellfish and other aquatic products directly consumed by human beings.The MP pollution in these aquatic products is directly related to the aquaculture water environment.However,the basic data of MP pollution in aquaculture water are lacking.In addition,it has been recognized that MPs can inevitably interact with the persistent organic compounds,drugs and personal care products,and natural organic matter.The interaction with photochemically active organic matter may accelerate or inhibit the phototransformation of MPs.However,no studies have explored the interaction between MPs and disinfectants that are artificially put into the aquaculture water environment.There is a lack of comprehensive understanding for environmental behavior of MPs in the actual aquatic environment.In this context,the aquaculture water in fish ponds was selected as the research area of this project.The occurrence and distribution characteristics of MPs in aquaculture water were investigated.The interaction mechanism between MPs and typical disinfectants（non-oxidized povidone iodine（PVP-I）disinfectant and oxidized trichloroisocyanuric acid（TCCA）disinfectant）were explored.（1）A flocculation separation method for MPs in aquaculture water was established,where the characteristics of MPs were investigated based on this method.The results showed that MPs were detected in water samples from all fish ponds at the two experimental stations in the Pearl River Estuary of Guangzhou,with the abundance of 10.3-60.5 particles/L（S1）and 33.0-87.5 particles/L（S2）.Most MPs were colored and fibrous.The main size was less than 1000μm.The main components were polypropylene（PP）and polyethylene（PE）.MPs with size less than 333μm,which was usually neglected in most studies,were detected in aquaculture water of this study（S1:43.7%;S2:33.2%）.In addition,the average abundance of MPs in aquaculture water（42.1 particles/L）was higher than that in inlet of fish ponds（32.1particles/L）.The small-sized MPs（<100μm）in aquaculture water was also more abundant,which indicated that fish ponds have become important accumulation area of MP pollution.（2）The interaction mechanism between MPs and non-oxidizing PVP-I disinfectant was investigated.The results showed that the presence of PP MPs aggravated the loss of triiodide ion（I₃^-）in PVP-I disinfection system.Moreover,the loss increased with the increase of PP MPs concentration（0.1-1.5 g/L）.The loss of I₃^-in the presence of PE MPs was less than that of PP MPs.The smaller the particle size of MPs,the more the loss of I₃^-.The existence of oxygen accelerated the loss of I₃^-,while indoor natural sunlight had little effect on the loss of I₃^-.I₃^-and pentaiodide ion（I₅^-）were iodine species that interacted with PP MPs in the form of hydrogen bonds,which resulted in the adsorption of polyiodide ions（I₃^-and I₅^-）in the PVP-I system to the MP surface and weakened the sterilization ability of the PVP-I disinfection system.At the same time,the interaction changed the surface physicochemical properties of PP MPs,such as the increased hydrophilicity,the change of fluorescent chromophore,and the increased absorption for ultraviolet light.Furthermore,the loss of I₃^-in the presence of aged PP MPs was higher than that of pristine PP MPs.Compared with pure water,the loss of I₃^-in aquaculture water was increased,while the interaction between MPs and polyiodide ions（I₃^-and I₅^-）was weakened.（3）The photochemical transformation process and mechanism for MPs was discussed in presence of the oxidizing TCCA disinfectant.The results showed that the simulated sunlight irradiation accelerated the surface oxidation and carbon chain scission of PP MPs.However,in the presence of TCCA,the peak intensities corresponding to oxygen-containing functional groups and alkyl groups on the surface of PP MPs decreased compared with that of light-irradiated PP MPs.Its contact angle was larger than that of the light-irradiated PP MPs,indicating that the presence of TCCA inhibited the photooxidation of PP MPs.The results under dark conditions are opposite to those under light conditions.At the first day of the reaction period,PP MPs accelerated the photolysis of TCCA and hypochlorous acid（HCl O）.During the subsequent reaction period,the chloride ions（Cl^-）concentration increased slowly and partially adsorbed on the surface of PP MPs.Chloride radicals（Cl·）formed by the consumption of hydroxyl radicals（·OH）by chloride ions（Cl^-）participated in phototransformation on the surface of PP MPs,leading to the formation of the carbon-chlorine bonds（C-Cl）,which competed·OH with Cl^-for active sites and inhibited the photooxidation of PP MPs.（4）The change in the photochemical properties of the derived organic matter in presence of the oxidized TCCA disinfectant and its effects on MP interface photochemical transformation were investigated deeply.The results showed that simulated sunlight irradiation promoted the release of organic matter（PP-DOM）from PP MPs into water.The total organic carbon（TOC）concentration increased from 17.2 mg/L to 29.6 mg/L within a7-day reaction period.The generated PP-DOM had ultraviolet light absorption and fluorescence properties,and its fluorophore composition was simpler than that of natural organic matter（NOM）.With the increase of TCCA concentration,the TOC concentration of the system reduced（42.4-36.6 mg/L）with the decreased UV absorption ability and fluorescence emission intensity.PP-DOM had the ability to generate the reactive oxygen species.In presence of TCCA,PP-DOM could react with HCl O and Cl·,leading to the change in the molecular structure,including the increase in molecular weight,the decrease in aromaticity,and the decrease in the content of hydrophobic components,and the formation of chlorinated Compounds（Cl-DOM）.Cl-DOM consumed·OH for dechlorination,which further inhibited the photooxidation of PP MPs.A complete pathway for the photochemical transformation of PP MPs in the presence of TCCA was formed.To sum up,there was a high level of MP pollution in aquaculture water.MPs could interact with polyiodide ions（I₃^-and I₅^-）in the form of hydrogen bonds,thus reducing the sterilization ability of non-oxidizing PVP-I disinfection system.In the presence of oxidative TCCA disinfectant,Cl·participated in the phototransformation on PP MP surface.Other chlorine species（TCCA,C-Cl bond,HCl O,Cl^-and Cl-DOM）consumed·OH to inhibit the photooxidation on the surface of PP MPs,initiating a new pathway for photochemical transformation of PP MPs in aquaculture water.This paper lays a foundation for the researches on the interaction between MPs and surrounding environmental substances,and also provides a new understanding and knowledge for the environmental behavior of MPs in the actual aquaculture water environment.