Mechanism Of Algal-derived Dissolved Organic Matter Accelerating The Photodegradation Of Histamine H₂ Receptor Antagonists

Histamine H₂-receptor antagonists（HRAs）are a new class of pollutants widely distributed in the aqueous environment,which are difficult to degrade and toxic to aquatic organisms.Currently,traditional biological treatment processes cannot remove HRAs from water effectively.Dissolved organic matter（DOM）is widely exist in the aqueous environment and plays an important role in the photodegradation of organic pollutants.The extracellular polymer（EPS）secreted by algae is a major source of DOM in the aqueous environment,and the photosynthetic pigments rich in algal cells are also important components of DOM.HRAs entering the water column inevitably come into contact with algae-derived dissolved organic matter（AD-DOM）.However,the effect of AD-DOM on HRAs removal is not clear at present.In this study,cimetidine（CIM）and ranitidine（RAN）were selected as typical substances of HRAs.The effect of AD-DOM on the photodegradation of these two target substances under simulated solar illumination was investigated.The environmental factors on the photodegradation of CIM and RAN by AD-DOM were also studied.The photodegradation mechanism of CIM by AD-DOM was investigated in this study.The results of this study help to clarify the transformation of HRAs in the present of AD-DOM in natural water,and provide theoretical support for the self-purification and restoration of polluted water with HRAs.The main results of this thesis are as follows:（1）The degradation of CIM and RAN was promoted by AD-DOM under simulated solar illumination.The degradation percentages of CIM and RAN were 100%and 85%,respectively,after 24 h of irradiation,when the concentration of AD-DOM（calculated as chlorophyll a）was 0.5 mg·L^-1 and the initial concentrations of CIM and RAN were 500μg·L^-1,respectively.The degradation rates of CIM and RAN were increased with the the concentration of AD-DOM（calculated as chlorophyll a）.The reaction of AD-DOM with CIM and RAN fitted the first-order reaction kinetic.The degradation of CIM and RAN is the result of the combined action of chlorophyll,EPS and other substances in AD-DOM under light irradiation.（2）Environmental factors have an effect on the photodegradation of CIM and RAN by AD-DOM.Among them,the degradation rate of CIM and RAN was positively related with light intensity and negatively correlated with the initial concentration of the pollutants.The degradation of CIM and RAN was best under neutral conditions.The salinity and metal ion Fe³⁺inhibited the photodegradation of CIM and RAN,and the inhibition effect increased with the ion concentration.The ions of HCO₃^-and NO₃^-had no significant effect on the degradation of CIM.However,NO₃^-inhibited the photolysis of RAN,and the inhibition rate increased with the concentration of NO₃^-,while HCO₃^-decreased the degradation rate of RAN 9.2%during24 h irradition.In addition,the interaction of NO₃^-and salinity had a significant effect on the degradation of CIM by AD-DOM.（3）Dissolved organic matter of humic acid（HA）and surfactants in the water environment have an impact on the photodegradation of CIM and RAN,among which the surfactant sodium dodecylbenzenesulfonate（SDBS）can significantly improve the degradation efficiency of RAN after 24 h irradidation.The combind of SDBS and AD-DOM could accelerate the photodegradation of CIM and RAN.（4）The dominant reactive oxygen species produced by the photodegradation of AD-DOM was singlet oxygen（~1O₂）,which oxidized with the pollutants.SDBS could increase the generation of ~1O₂ significantly in the solution of AD-DOM,which was reached to 715.3±13.69μmol·L^-1 from 346±6.32μmol·L^-1 during 24 h irradiation.（5）The mechanism of AD-DOM accelerating the photodegradation of CIM was proposed as follow:the reactive oxygen specie of ~1O₂ was produced with AD-DOM under simulated solar irradiation,which oxidized with the S atom in CIM and formed cimetidine sulfoxide,and then the S atom in cimetidine sulfoxide was oxidized with ~1O₂ in further.Finnaly,the C-S bond in CIM was broken and converted to low molecular weight products until complete degradation.