Effect Of Natural Organic Matter On The Photooxidation Of Arsenite Induced By Ferric Ions

Arsenic pollution has always been one of the most important environmental issues in the world.It exists in various forms with water and soil environment.The environmental behaviors and effects of arsenic,such as toxicity,motility and bioavailability,are closely related to its chemical form.As arsenic pollution has become an urgent form of the current serious environmental and health problems in the world,Arsenic morphology research is an important basis for carrying out pollution diagnosis,environmental health risk assessment and As pollution remediation.The interaction between iron,natural organic matter（NOM）and As is important for the migration and transformation of As in the environment.However,the ternary mixed system of Fe,NOM and As is more complicated,and its interaction effect and the photooxidation effect of As（Ⅲ）have not been well explained.In this paper,the effects of the addition of NOM on the iron induced photooxidation of As（Ⅲ）in the ternary system of NOM/Fe（Ⅲ）/As（Ⅲ）under the near neutral condition and under the irradiation of UVA or UVB were investigated.Starting from carboxylic acid or phenol,and then to humus,the effects of the type and dosage of NOM on As（Ⅲ）oxidation were studied respectively.The process and mechanism of As（Ⅲ）photooxidation in ternary system were studied and elucidated in combination with the formation of complexes,the calculation of coordination stability constant and competitive coordination effect.The main research contents and conclusions are as follows:（1）UVA and UVB photoreaction experiments were carried out respectively,and NOM was divided into two groups:small molecule carboxylic acid or phenol and large molecule humus.Under the condition of p H 7.0 and different dosage of NOM,the first-order reaction rate constant k_obs of As（Ⅲ）oxidation was calculated.The kinetic parameters of photooxidation of As（Ⅲ）induced by different NOM and iron were determined.The experimental results show that,whether it is UVA or UVB irradiation,or small molecule or large molecule NOM,k_obs decreases with the increase of NOM dosage（except for high concentration oxalic acid）,that is,the addition of NOM inhibits the photooxidation of As（Ⅲ）.（2）In order to investigate the formation of complexes in the ternary system of NOM/Fe（Ⅲ）/As（Ⅲ）,the UV-Vis absorption spectra of the complexes in the binary system of NOM and NOM/Fe（Ⅲ）were carried out first.The equilibrium constants of Fe-NOM complexes were determined by the calculation of related parameters and competitive coordination experiments.The competitive desorption experiments were used to obtain the different coordination ability of different NOM with As（Ⅲ）.（3）The possible reaction process and mechanism of NOM/Fe（Ⅲ）/As（Ⅲ）ternary system were analyzed.Because of its special chemical composition and structural characteristics,macromolecular humus has strong light shielding effect,while micromolecule carboxylic acid or phenol does not have light shielding effect,which only reflects the competitive coordination between NOM and As（Ⅲ）.Therefore,the total inhibition consists of two parts.Quantitative structure-activity relationship（QSAR）analysis was made for the influence of NOM properties on As（Ⅲ）photochemistry.The results show that the molecular weight of NOM,the content of total acid functional group and the conditional stability constant log K_Fe-NOM are significantly related to the photooxidation of As（Ⅲ）.Therefore,QSAR fitting formula can better predict the inhibition rate of other NOM on photooxidation of As（Ⅲ）under the same experimental conditions.The results of this study expand the understanding of environmental behavior and biogeochemical process of arsenic species,and provide basic scientific support for predicting as transfer and transformation in the environment,its bioavailability and as pollution control.