Processes And Mechanisms Of Arsenic(?) Photooxidation Through Complexation With Iron Species

Contamination of water and soils with inorganic arsenic (including As(?) and As(?)) has been one of the most important topics in the fields of environmental science and technology. Arsenic is famous for its acute and chronic toxicity. The toxicity of arsenic is highly affected by its speciation. Therefore the transformation and transport of arsenic species in the environment and the treatment methods are worth of studying.The interactions between iron and arsenic play a very important role in the environmental behavior and effect of arsenic species. However, the mechanisms of As(?) oxidation in the presence of iron are still unclear due to the complicated speciation of iron and arsenic. In this work, the interactions between As(?) and iron species at acidic and circumneutral conditions were studied. The photooxidation mechanisms of As(?) in the presence of iron species were investigated by considering the interactions and complexation between As(?) and Fe(?).(1) In the first part of this work, photooxidation of As(?) on nascent colloidal ferric hydroxide (CFH) in aqueous solutions at pH 6 was studied to reveal the transformation mechanism of arsenic species. Experiments were done by irradiation using a light-emitting diode lamp (?= 394 nm). Results show that photooxidation of As(?) and photoreduction of Fe(?) occurred simultaneously under oxic or anoxic conditions, while the presence of oxygen could improve the oxidation efficiency of As(?). Quencher experiments showed no obvious effect on As(?) oxidation efficiency at circumneutral conditions, while the presence of competing reagent phosphate inhibited the oxidation reaction. All these results revealed that photooxidation of As(?) in the presence of nascent CFH occurred through electron transfer from As(?) to Fe(?) induced by absorption of radiation into a ligand-to-metal charge-transfer (LMCT) band. The complexation between As(?) and CFH was investigated by UV spectrum analysis. The stability constant (pKf= 3.58) and absorption coefficient (?285= (4.26 ± 0.72) × 103 L mol-1 cm-1) was obtained by fitting the optical density change of CFH-As(?) mixture solution to Benesi-Hildebrand equation. The photooxidation kinetics of As(?) in the presence of CFH fitted well to Langmuir-Hinshelwood equation that the reaction rate constant kLH= 0.6882 ± 0.0150 ?mol L-1 min-1 and the Langmuir adsorption constant K= (1.41 ± 0.12) × 105 L mol -1. Sunlight-induced photooxidation of As(?) also occurred, implying that photolysis of the CFH-As(?) surface complex could be an important process in environments wherein nascent CFH exists.(2) In the second part of this work, the mechanisms of formation and the photochemistry of dissolved Fe(?)-As(?) complexes in acidic aqueous solution were studied. UV spectrum analysis of Fe(?)-As(?) mixture solution confirmed the formation of Fe(?)-As(?) complexes in acidic aqueous solution at high As(?) concentrations. The stability constants and absorption coefficients of the 1:1 and 1:2 Fe(?)-As(?) complexes (pK1= 2.8 and ?1= 3 × 103 L mol-1 cm-1 for [Fe(H2As03)]2+; pK2= 2.5 and ?2= 4.3 × 103 L mol-1 cm-1 for [Fe(H2AsO3)2]+) were obtained by numerical fitting of the optical density change of Fe(?)-As(?) mixture solution. Photooxidation of As(?) in the presence of Fe(?) ions in acidic media was investigated by laser flash and steady-state photolysis. At low As(?) concentration (< 1 mmol L-1), oxidation of As(?) resulted in HO'radical formation via photolysis of the FeOH2+complex. At higher As(?) concentrations (> 10 mmol L-1), As(?) was oxidized by LMCT process. Photoactive Fe(?)-As(?) complexes formed Fe(?) at 308 nm at a quantum yield of 0.01 and at 266 nm at a quantum yield of 0.033. At all arsenite concentrations, white FeAsO4 colloids formed during As(?) photolysis in the presence of Fe(?) ions. The intermediate As(?) was observed by laser flash photolysis at both low and high As(?) concentration conditions. Solid Fe(?)-As(?) complexes were prepared and characterized, interactions between As and Fe was observed by determining the binding energy of each element by X-ray photoelectron spectroscopy (XPS) measurements. The photochemical transformation of As(?) to As(?) in solid Fe(?)-As(?) complexes was confirmed.(3) The third part is a study of the transformation of As(?) under the visible light induced iron(?)/sulfite system. In this work, the oxidation of As(?) to As(?) in an iron(?)/sulfite system under visible light using sunlight or a light-emitting diode lamp (? 404 nm) were investigated. Several kinds of free radical quenchers, nitrogen and complexation competing agent were used for mechanism study. Comparing to a) iron(?) system with irradiation of light and b) iron(?)/sulfite system without irradiation light, our results show a significant enhancement of As(?) oxidation efficiency at pH 6 in iron(?)/sulfite-visible light (LED) system, corresponding to an initial rate constant of 0.196 min-1. Approximately 93% of arsenic was removed from solution by centrifugal treatment after 30 min of irradiation treatment. Under the irradiation of sunlight, As(?) was also oxidized quickly in this system. Sequential addition of sulfite could improve the oxidation efficiency for water having high concentrations of As(?) (i.e.,66.7 ?mol L-1) or sequential addition of As(?). Mechanism investigation revealed that the pathways of As(?) oxidation at circumneutral pH is complicated that involved free radicals (mainly HO-, SO4- and SO5-) and ligand-to-metal charge transfer between As(?) and colloidal ferric hydroxide (CFH) particles. This part of work may contribute to an understanding of the main mechanisms of As(?) removal using an iron-sulfite system and may aid development of new As(?) decontamination methods driven by visible light.