Effect Of Dissociation Characteristics Of Dissolved Organic Matter On The Formation Of Photoactive Oxygen Species

Dissolved organic matter（DOM）is a natural photosensitizer that exists in the natural environment.DOM contains a large number of aromatic ketones and quinone compounds,which can absorb light energy to produce singlet oxygen（¹O₂）and hydroxyl radical（·OH）and other reactive oxygen species（ROS）.These ROS play an important role in the degradation of environmental organic pollutants.Therefore,further research on the influencing factors of ROS generation efficiency in the process of DOM photosensitization is of great significance for studying the natural depletion phenomenon of pollutants in the environment.However,due to the complex structure of DOM,there are many factors that affect the photogenerated ROS,and the mechanism of its influence is still not clear,so further investigation is still needed.The previous research of this research group found that standard humic acid（SHA）grouped according to different dissociation degrees and polyphenolic acid DOM model compounds with similar structures but different dissociation coefficient（p K_a）have different ROS generation capabilities.The ROS produced by DOM photosensitization is essentially the result of light-induced DOM electron transfer,which is an electron transfer reaction between an excited electron donor and an excited electron acceptor.Therefore,this paper selects polyphenolic acid model compounds with similar structures but different functional group content to explore their ability to induce ROS formation and verifies the effect of dissociation on ROS generation by means of additional charge-induced electron transfer.Different DOM obtained from rice and wheat straw under aerobic,anaerobic,microbial inoculation and other different culture（humification）methods are used as the research objects to study the relationship between their photosensitized ability to produce ROS and their p Ka and electron transfer capacity（ETC）（including electron accepting capacity（EAC）and the electronic donating capacity（EDC））,to further illustrate the relationship between the dissociation characteristics of DOM and the generation of ROS.The results of the study show that five model compounds with similar structures but different types and numbers of functional groups have different ROS production abilities in the neutral reaction system.Model compounds with different types of functional groups have a positive correlation between their ROS production ability and the number of functional groups with strong electron-donating ability.For model compounds with different numbers of functional groups,their ability to produce ¹O₂ is positively correlated with the total number of electron-withdrawing groups and electron-donating groups contained therein.The ability to produce·OH is related to the number of functional groups but is also affected by the binding sites of functional groups.After that,the electron transfer of the system was changed by applying a stable current of 10m A,and it was found that the ROS production ability of the DOM model compound was significantly improved in the reaction system with current applied under natural light in the room compared to the system without current.This shows that applying current to the reaction system can promote the photolysis of the compounds in the system,and it also shows that changing the electron transfer of the reaction system will affect the generation of ROS;then,by studying the electron transfer ability of DOM obtained by different humification methods,it is found that the composition of the oxygen-containing functional group of DOM affects the electron transfer ability of DOM.The ETC of different DOM ranges from 13.71-19.50μmol_e-（g C）^-1.The functional groups of different DOM are mainly composed of methoxy,hydroxyl,carboxyl and carbonyl,but the content is different.The EAC size and aromaticity of DOM are positively correlated,and the ETC size and molecular weight are negatively correlated.Then we studied the photo-generated ROS of different DOM and found that although the apparent p K_a value of DOM has no obvious linear relationship with its ability to produce ROS,but the EAC of DOM is positively correlated with the ability to produce ¹O₂,and negatively correlated with the ability to produce·OH,shows that the dissociation characteristics of DOM will affect the generation of ROS.Finally,through the photodegradation experiment of organic dyes,it was found that the DOM obtained by different humification methods all had a promoting effect on the degradation of bisphenol A.The addition of DOM can effectively shorten the photodegradation cycle of BPA,the difference in the degradation promotion effect of BPA is mainly caused by the difference in the structure and functional group composition of different DOM.The research results provide basic data for the in-depth understanding of the corresponding mechanism of the dissociation characteristics of DOM on the generation of ROS.At the same time,it provides information for improving the stable yield of ROS and enhancing the photochemical degradation of organic pollutants in water by affecting the formation conditions of DOM.The water self-purification ability provides a theoretical basis,and has important research value for the application of environmental pollution control and pollution remediation technology in the enrichment of wetland systems.