Process And Influence Mechanisms Of Soil Humus Mediating Sulfadiazine Photodegradation

Sulfonamides are widely used in clinical treatment and livestock breeding farms because of its antimicrobial spectrum.It can cause a cumulative effects on the ecology and environment,and then posing a certain potential hazards to the human health and ecosystem functions.Soil humus,as a kind of photoactive component,can significantly mediate the photochemical behavior of sulfonamides.For this study,sulfadiazine（SDZ）was selected as the research object,and the photolysis properties of soil humus mediating SDZ photodegradation was discussed.Simultaneously,the dominant mechanism of SDZ photodegradation with or without the influence of HA were srevealed,and the contribution of each control components was described.Furthermore,the influence of environmental factors on SDZ photodegradation were be investigated.All results of the study would provide a scientific reference for the health risk assessment of soil environmental system,main conclusions are as follows:（1）SDZ exhibits a good stability under the conditions of dark,while slow degradation occurs with a ultraviolet excitation.The photolysis of SDZ conforms to the first-order kinetics model when it happen in pure water or mediated by different HA.In pure water environment,lower SDZ concentration and neutral pH value can promote the photodegradation of SDZ,but the degradation process was generally carried out slowly,and the degradation rate was only 0.0069 min^-1.Similarly,the photodegradation of SDZ could also have a quicker rate at lower HA concentration(C_HA<15 mg/L)and neutral pH value（pH=7.1）when mediated by different HA.（2）HA,from the peat soil,paddy soil,deciduous soil or market,were mainly composed of C,H,O and N.In terms of structural types,the main structural units of each HA are lipids,aromatic compounds,amino acids and oxygen-containing functional groups such as carboxyl and hydroxyl.In terms of structural content,HA from the market contained more nitrogenous compounds,aromatic compounds,lipids and oxygen-containing functional groups.Furthermore,the purchased HA samples possessed a higher humus degree and a more complex structure,containing more photoactive groups.While,the HA extracted from peat soil,paddy soil or deciduous soil presented a relatively simple component and easier to be mineralized and decomposed.（3）Each of HA could combine the SDZ existed in the solution,respectively.and static quenching was the mainly binding form.The combination of purchased HA and SDZ was dominated by hydrophobic force and hydrogen bond force,HA extracted from deciduous soil was dominated by the hydrogen bond force,while the combination of SDZ and HA extracted from rice and peat soil may involve other binding mechanisms,such asπ-πforce.（4）In the pure water environment,the photolysis of SDZ involved both direct photolysis and self-sensitized photolysis.·OH and ¹O₂ are the controlling components in the self-sensitized photolysis process of SDZ,and the contribution of ¹O₂ was greater than·OH.When it comes to mediation of each HA,purchased HA and paddy soil HA significantly enhanced the photolysis of SDZ compared to the other two.·OH and ¹O₂ were still the dominant mechanisms to promote the photodegradation of SDZ in various systems,and the contribution rates of ¹O₂ to the photodegradation of SDZ（22.67³8.96%）were significantly greater than that of·OH（9.33²6.97%）.Furthermore,the sources of·OH and ¹O₂ in each system were not only from the self-sensitized photolysis of SDZ,but also HA samples.The mediating effect of each HA on SDZ photolysis was related to the capacity of its photogenic·OH and ¹O₂.（5）The photodegradation rate of SDZ in the pure water system was positively correlated with the strength of solution ions.Unexpectedly,the lower ionic strength（50 mmol/L）had a weakening effect on the photodegradation of SDZ when the system was mediated by each HA,which could be related to the quenching effect of halogen ions on the·OH in system.NO₃^-could generate·OH with the effect of light,but the photodegradation rates of SDZ mediated by HA were lower than that of pure water system due to the light filtrating effect and active species quenching effect of HA.Although Fe³⁺could also generate with the effect of light,but the process colud be susceptible to the competition of Fe³⁺,leading to a inhibition effect on SDZ photodegradation in the system of pure water and SDZ-HA.In addition,K⁺,Mg²⁺and Ca²⁺had almost no significant effect on the photodegradation of SDZ in the pure water environment,while showed an inhibition effect in the systen mediated by HA.Moreover,monovalent cations（K⁺）had a more significant inhibition effect on the photodegradation of SDZ compared to the divalent cations(Mg²⁺and Ca²⁺).