Mechanism Of As(Ⅲ) And As(Ⅴ) Removal By Coagulation Of Ferric Salts And As(Ⅲ) Removal By Simultaneous Photocatalytic Oxidation-coagulation Of Titanium Salts

Arsenic contamination of groundwater has been reported regionally around the world. Long-term exposure to arsenic in drinking water with excessive contents can cause skin lesions and organ cancers, posing a significant threat to human health. Effective removal of arsenic has been important for safe drinking water supply. In natural waters, arsenic exists mostly as inorganic species of arsenite（As（Ⅲ）） and arsenate（As（Ⅴ））. As（Ⅲ） is more toxic than As（Ⅴ）, and is more difficult to remove by conventional water treatment processes. This dissertation proposes a new simultaneous ultraviolet photocatalytic oxidation-coagulation of titanium salts for efficient oxidation and removal of As（Ⅲ）. The mechanism of As（Ⅲ） oxidation in the reaction system has been investigated. At the same time, the influences of size and zeta potential of ferric salts hydrolyzed precipitates on the removal of As（Ⅲ） and As（Ⅴ） by coagulation of ferric salts and subsequent filtration was also investigated.The results revealed that As（Ⅲ） could be oxidized into As（Ⅴ） by the UVC（λ=254nm） photocatalytic oxidation-coagulation of Ti（SO₄）₂（UVC/Ti（SO₄）₂ coagulation） within a short reaction time, with efficient As（Ⅲ） removal at pH 4-6. The As 3d XPS spectra indicated that 84.7% of arsenic on the coagulated precipitate was in the oxidized form of As（Ⅴ） after the UVC/Ti（SO₄）₂ treatment of As（Ⅲ） water solution at pH 5. Arsenic remained in solution was in the oxidized form of As（Ⅴ）. In the pH range 4⁶ and a moderate dose of Ti（SO₄）₂（10 mg/L）, As（Ⅲ） could be almost completely removed（>99%） by UVC/Ti（SO₄）₂ coagulation at an initial As（Ⅲ） concentration of 200 μg/L. Because of the oxidation of As（Ⅲ） to As（Ⅴ）, the As（Ⅲ） removal efficiency by UVC/Ti（SO₄）₂ coagulation was significantly greater than that of Ti（SO₄）₂ coagulation at pH 5, especially at a low dose of Ti（SO₄）₂（2.5mg/L）. In the range 10¹000 μg/L, the intial As（Ⅲ） concentration had little influence on As（Ⅲ） removal by UVC/Ti（SO₄）₂ coagulation. Silicates showed less influence on As（Ⅲ） removal, while phosphates imposed significant effect on As（Ⅲ） removal by UVC/Ti（SO₄）₂ coagulation at pH 5. The adverse effects of silicate and phosphate could be offset by increasing coagulant dosage. The presence of Ca2+ and Mg2+ ions could decrease the negative zeta potential of Ti（SO₄）₂ hydrolyzed precipitates by double layer compression effect, which would reduce the electrostatic repulsion between As（Ⅴ） anionic species and Ti（SO₄）₂ hydrolyzed precipitates and consequently enhanced the removal of As（Ⅲ） by UVC/Ti（SO₄）₂ coagulation at pH 7. The oxidation of As（Ⅲ） to As（Ⅴ） by UV photolysis alone was slow and weak, accounting only for 4.1% after 21 min. This indicated that the major photocatalytic oxidation in UVC/Ti（SO₄）₂ reaciton system was not caused by homogeneous photolysis of As（Ⅲ）. The Ti 2p XPS spectra of the coagulated precipitates after the UVC/Ti（SO₄）₂ treatment of As（Ⅲ） water solutions indicated that the oxidation of As（Ⅲ） to As（Ⅴ） was not via oxidation-reduction between direct electron transfer between As（Ⅲ） and Ti（IV）. Finally, the effect of hydroxyl radical scavengers（TBA, Me OH）, hole scavenger（I-） and electron scavenger（Cu2+） on the oxidation and removal of As（Ⅲ） by UVC/Ti（SO₄）₂ coagulation indicated that the mechanisms of the simultaneous photocatalytic oxidation-coagulation of UVC/Ti（SO₄）₂ involved the formation and reactions of hydroxyl radical（OH·） and superoxide(HO₂^·/O₂^·-).Similar to UVC/Ti（SO₄）₂ coagulation, studies further showed that UVA/Ti（SO₄）₂ coagulation（λ=365nm） could also oxidize and remove As（Ⅲ） efficiently. In the pH range 4⁶ and a low dose of Ti（SO₄）₂（5 mg/L）, As（Ⅲ） removal efficiency by UVA/Ti（SO₄）₂ coagulation was gearter than 96.1% at an initial As（Ⅲ） concentration of 200 μg/L. This was greatly higher than that of Ti（SO₄）₂ coagulation alone under the same condition. As（Ⅲ） removal efficiency by UVA/Ti（SO₄）₂ coagulation declined significantly as pH increased from 7 to 10. The analysis of the arsenic oxidation state revealed that arsenic remaining in solution was all in the oxidized form As（Ⅴ）. As（Ⅲ） removal by UVA/Ti（SO₄）₂ coagulation decreased in the presence of SO2-4in the pH range 8¹0. The presence of F- decreased As（Ⅲ） removal efficiencies throughout the pH range 4¹0, but most significantly influenced at pH 4.This study also investigated systematically the removal of As（Ⅲ） and As（Ⅴ） by ferric salts coagulation followed by filtration based on the influences of size and zeta potential of the ferric salts hydrolyzed precipitates. The results indicated that the sizes of the early-formed precipitates were in the colloidal range（4-755 nm） from pH 5 to 10. The zeta potential of the precipitates was positive at pH < 7, and became negative at pH > 7. Since As（Ⅲ） species exist predominantly as the neutral molecular form in the pH range 5¹0, there was no electrostatic force between As（Ⅲ） and ferric salts hydrolyzed precipitates. Consequently, As（Ⅲ） removal by coagulation of ferric salts had almost no direct correlation with zeta potential of the precipitates. As（Ⅴ） exists generally as anionic oxyanions in the pH range 5¹0. Electrostatic force between As（Ⅴ） oxyanions and hydrolyzed precipitates of ferric salts influence the removal of As（Ⅴ） by ferric salts coagulation-filtration significantly. The electrostatic attraction promoted the adsorption of negatively charged As（Ⅴ） species onto positively charged precipitates when the zeta potentials was positive at pH 5⁷, and the highest As（Ⅴ） removal occurred. The electrostatic replusion between As（Ⅴ） oxyanions and the negatively charged hydrolyzed precipitates would hinder As（Ⅴ） adsorption onto precipitates. This consequently resulted in the significant decline of As（Ⅴ） removal efficiencies as the precipitates became more negatively charged at pH 8¹0. In the pH range of this study, the hydrolyzed precipitates of ferric salts with arsenic could all be removed by microfiltration after coagulation, thus precipitate sizes showed much less effect on the arsenic removal of As（Ⅲ） and As（Ⅴ）. The increase of monovalent electrolyte（Na Cl） concentration and addition of divalent electrolyte（Ca（OH）2, Mg Cl2） imposed a noticeable effect on the size of Fe Cl3 hydrolyzed precipitates by double layer compression effect. Monovalent electrolyte showed only a smaller effect on zeta potential of precipitates. However, the addition of divalent electrolytes evidently reduced the negative zeta potential in the pH range 8¹0. Monovalent electrolyte showed little influence on the removal of As（Ⅲ） and As（Ⅴ） by Fe Cl3 coagulation-filtration. Although divalent electrolyte had little impact on As（Ⅲ） removal its presence enhanced As（Ⅴ） removal significantly in the pH range 8-10. In the presence of HA, the isoelectric point of Fe Cl3 hydrolyzed precipitates shifted from pH 7 to 6 and the negative zeta potential of precipitates increased in the pH range 7¹0. Due to the influence of HA, As（Ⅲ） removal efficiency decreased thoughout the pH range 5-10, the decreased degree of As（Ⅴ） removal increased as pH increased from 5 to 8. The relative importance of the two coagulation removal mechanisms, co-precipitation and adsorption, was found to be dependent on the solution pH.