| Arsenic(As) is ubiquitous in air, water, soil and rocks, which is one of the most hazardous polluting elements for humans, caused diseases and chronic arsenic poisoning. For a long time, arsenic compounds enter surface of soil and waters through irrigating, mining, especially drilling well, then induce serious arsenic pollutions and threaten human health.The toxicity, mobility and bioavailability of arsenic depended on its chemical speciation. Studies on arsenic species transformation provided some references on diagnosing pollution, assessing the environmental health risk and remediating arsenic polluted soil and waters. Under the circumstances of ozone depletion and UV-B radiation enhancement, the photochemical process on the surface of earth attracted much attention. However, the knowledge about photochemical transformation of arsenic species on the surface of soils and particles in waters was not enough understood. This dissertation reported on the processes, impact factors and mechanisms of photochemical transformation of arsenic species on the soil minerals (kaolinite, montmorillonite and goethite), and determining the contributions of photochemical reactions for the abiotic transformation of arsenic species. This paper revealed the reaction mechanism of arsenic species on the surface of soil and particles in waters and the influence law of environmental factors under the irradiation of sunlight. It broadened the knowledge about the geobiochemical processes of arsenic and provided some references on predicting the translocation of arsenic in soils and waters, evaluating its bioaccessibility. Main experiment contents and conclusions of this dissertation are as follows.The species transformation of As(III) on the surface of kaolinite, montmorillonite and goethite particles in suspended solution in photoreactor for aqueous phase were investigated. The concentrations of different arsenic species were determined by atomic fluorescence spectrometry. The effects of initial concentration of As(III), initial pH values, mineral dosages, humic acid on species transformation of As(III) were investigated, and the predominant oxidants and its contributions for the transformation were explored by free radical scavenging methods. The species transformation of As(III) on the surface of kaolinite, montmorillonite and goethite layers in photoreactor for solid phase were investigated. The different arsenic species were extracted by the microwave-assisted phosphoric acid-ascorbic acid extraction, then determined by atomic fluorescence spectrometry. The effects of initial concentration of As(III), pH values of layers, environmental humidity, layers thickness, humic acid and additional Fe3+and Fe2+on the species transformation of As(III) were investigated. The predominant oxidants and its contributions for the transformation were analyzed by free radical scavenging methods. (1)Under the irradiation of metal halide lamp, the photooxidation of As(III) by kaolinite in suspended solution was investigated. Photooxidation of As(III) followed well into the Langmuir-Hinshelwood equation. The species transformation was influenced by the initial pH value of suspended solution. Lower pH value favor the dissolution of iron and the Fe(III) complex ion with high photoactivity was present around the pH3.0. At pH5.0, the addition of humic acid ranged from0.0to5.0mg L-1was benefit to As(III) photooxidation. The addition exceeded5.0mg L-1inhibited its photooxidation. Hydroxyl radical was the main oxidant for As(III) oxidation, and the contribution of HO2·/O2-· was negligible.89.6%of As(III) photooxidation was caused by hydroxyl radical which was from the photolysis of free iron in aqueous phase, and10.4%of As(Ⅲ) photooxidation caused by hydroxyl radical which was from photolysis of structural iron.(2) The photooxidation of As(Ⅲ) on the surface of kaolinite clay layer was investigated. The results indicated that As(Ⅲ) photooxidation on the surface of kaolinite layer was fit to Langmuir-Hinshelwood equation. The acidity had the influences on the As(Ⅲ) photooxidation, low pH was in favor of As(Ⅲ) species transformation. Environmental humidity promoted As(Ⅲ) photooxidation through the promotion of dissolution of iron from the particles and enhancement of As(Ⅲ) mobility. The addition of HA improved the As(Ⅲ) photooxidation about10%. Addition of carboxylic acid also promoted the photooxidation. The addition of iron ion greatly promoted the photooxidation. When ferric concentration was1000.0μg g-1, As(Ⅲ) concentration was decreased from100.0μg g-1to11.1μg g-1after48hours irradiation. When ferrous concentration was1000.0μg g-1, As(III) concentration was decreased to47.7after120hours irradiation. A large amount of ferrous ions could compete with As(III) for hydroxyl radical and ferrous ion could reduce the intermediate As(IV) to As(III). Hydroxyl radical was the main oxidant of As(III) on the surface of kaolinite layer, and accounted for the96.5%of As(III) photooxidation, HO2·/O2-· accouted for only3.5%.(3) As(III) photooxidation was also fit to Langmuir-Hinshelwood equation. pH value greatly affected the As(Ⅲ) photooxidtion, the photooxidation efficiency decreased with the increase of pH. The As(III) photooxidation was improved when the addition of HA was lower than the threshold, and inhibited when its addition higher than the threshold. The hydroxyl radical and hydroperoxyl/superoxide radical was produced under the irradiation. Hydroxyl radical directly participated into the oxidation of As(III). However,62.0%of hydroxyl radical was conversed from hydroperoxyl/superoxide radical. The dissolved ferrous ion transformed the electron to dissolved oxygen, then generated hydroperoxyl/superoxide radical. Ferric ion directly produced hydroxyl radical. The structural iron in montmorillonite showed weak directly produced hydroxyl radical. The structural iron in montmorillonite showed weak photo activity.(4) As(III) photooxidation on the surface of montmorillonite layer was also investigated. Lower pH value promoted the dissolution of structural iron, further promoted the photooxidation. As(III) photooxidation was influenced by the thickness of layer, due to the light-shielding effect.16.0%of photooxidation efficiency was risen by the addition of HA. Hydroxyl radical was the predominant oxidant for As(III) photooxidation on the surface of montmorillonite layer, about78.6%of hydroxyl radical was from the transformation of hydroperoxyl/superoxide radical, the remainder of21.4%was generated from the photolysis of ferric ion.(5) The photooxidation of As(III) by natural goethite in suspended solution was studied. The As(III) photooxidation followed to Langmuir-Hinshelwood equation. Lower pH value facilitated the photooxidation, and goethite dosage also had influence on the photooxidation. The photooxidation was prohibited by the addition of HA through its scavenging hydroxyl radical and hydroperoxyl/superoxide radical.12.9%of photooxidation was caused by hydroperoxyl/superoxide radical, and87.1%of photooxidation was caused by hydroxyl radical and holes. The surface reaction on goethite particles accounted for82.3%of the photooxidation and reaction in aqueous phase accounted for17.7%. As(III) which was present in goethite layer could be readily oxidized in the such extremely oxidative micro-environment. |
PDF Full Text Request