Effect Of Dissolved Organic Matter Oxygen-containing Function Groups On Photo-generated Reactive Oxygen Species

At present,a large number of researches have revealed that dissolved organic matter(DOM)contains a mass number of aromatic ketones and quinone compounds,which can produce reactive oxygen species(ROS)such as 1O2,HO·through photosensitivity.These ROS generated under sunlight play an important role in the attenuation of surface water and soil organic pollutants.Therefore,paying attention to the efficiency of DOM-induced ROS generation is a meaningful work for studying the natural exhaustion of pollutants in the environment.However,due to the complexity of the DOM structure,the mechanism of its ROS production and its effect on the photodegradation of PPCPs are not clear,and it remains to be further explored.According to previous research by our research group,we found that the NOM component separated by XAD-8 resin under different pH conditions affects the light generation of ROS.We speculate that this may be related to the degree of dissociation of functional groups.Therefore,from the perspective of the dissociation characteristics of DOM functional groups,this paper selected standard DOM(fractionated according to different dissociation degrees)and polyphenolic organic compounds with similar structures and different pKa as the molecular model of DOM.The influence factors of ROS formation were explored,which directly explained the influence of functional groups.Evidence for the correlation between the dissociation coefficient of DOM and the generation of ROS is initially established,which provides a data basis for the mechanism of indirect photolysis of DOM on pollutants.And taking the DOM produced by rice straw in different cultivation(humusification)as the research object,the dynamic changes of DOM functional groups were discussed.At the same time,the changes of DOM photoinduced ROS during straw humification and their effects on the photodegradation of trace drug paracetamol(APAP)in the water environment were analyzed.Research indicates:The effects of standard humic acid(SHA)on the ROS generation of each component grouped by XAD-8 were different.Low pH components(pH 4.8)have higher humic acid carboxyl content,lower aliphatic functional groups,high oxygen content,and the ability to generate ROS under light radiation is higher than that of high pH components(pH 7.0,pH 11.0)Strong.This shows that the dissociable degree of oxygen-containing functional groups in the DOM structure has a certain effect on the generation of ROS.Model compounds with similar molecular weights and structures but different pKa values have different ROS production abilities in a neutral reaction system.This indicates that the pKa value of the model compound has an effect on the photodegradation of ROS but the degree of influence is not linear,which may also be related to the control of the relative position of the substituent relative to the dominant functional group.The ability of photoinduced 1O2 at pH values of 3.0,7.0,10.0 for the three model compounds DHB2,5(2.97),DHB3,4(4.48)and GUA(9.99)with large differences in pKa values increased with increasing dissociation.The ability of DHB3,4 and GUA photoinduced HO·is also true.The correlation evidence between NOM dissociation coefficient and ROS generation was preliminary established,which explained the influence of functional groups.The DOM produced by humification of rice straw at different culture stages contains aromatic C=C bonds and other chromophores.With the increase of culture time,the molecular weight of DOM increased,the functional groups changed from methoxy and alcoholic hydroxyl groups to carboxyl,phenolic hydroxyl and carbonyl groups,and the aromaticity of DOM increased.The ability of photosensitization to produce 1O2 first increased,then decreased,and final increased,while the ability to generate OH·was just the opposite of it,which was reduced first and then enhanced and final decreased.The dynamic change of rice straw decomposition process promotes the degradation of paracetamol(APAP).The promotion effect of DOM on APAP photodegradation in different culture stages is mainly affected by HO·,and the difference is mainly caused by the structure and functional groups of DOM.The research results provide basic data for deep understanding of the response mechanism of DOM structural characteristics to ROS generation.At the same time,it can provide a theoretical basis for improving the stable yield of DOM to form ROS by influencing the formation conditions of DOM,enhancing the photochemical degradation of pollutants in water,and improving the self-purification ability of water.It also has important research value for the application of environmental pollution control and restoration technologies in rich wetland systems.