| Phenylarsonic compounds are a series of substances containing an arsenic acid group on their aromatic rings. Due to the merits of low toxicity, low residue in animals, improving conversion, and promoting growth, these compounds are widely used as feed additives in food animal production. Phenylarsonic compounds are barely metabolized and residued in animal bodies, and are mostly excreted unchanged into animal wastes. Because of the large volume, expensive transportation and disposal expense of the animal wastes produced in the massive concentrated animal feeding operation, and as well as the high nutrient contents in them, such as N and P, these arsenic containing animal manure are often applied to fertilize surrounding farmlands. With rather high water solubilities, phenylarsonic compounds are easily leached out the manures and soils by irrigation and rain water, ending up in surface water in the vicinity of the farmlands. Although phenylarsonic compounds have low toxicity and are safe for animals at low doses, they can form more toxic arsenite and arsenate through a series of transformation processes in the environment. Photodegradation is one of the most important and common pathways for organic contaminants elimination in surface water bodies, and investigate the photodegradation behaviors of phenylarsonic compounds in aqueous solutions is vitally important for fully understanding the transportation and transformation behaviors of phenylarsonic compounds in the aquatic e nvironment. To date, direct phototransformation of phenylarsonic compounds under natural or simulate sunlight has not been investigated.In this work, the xenon lamp was firstly used to simulate the natural sunlight, and mechanisms of phenylarsonic compounds photodegradation were carefully investigated. Specifically, the formation of inorganic arsenic species and organic intermediates were characterized; the contributions of direct and self-sensitized photolysis to the photodegradation of phenylarsonic compounds were evaluated; effects of reactive oxygen species on the photodegradation of phenylarsonic compounds were estimated; the ability of phenylarsonic compounds to serve as the 1O2 sensitizer were calculated; and the rate constants for 1O2-mediated degradation of phenylarsonic compounds were quantified. Meanwhile, based on the existing literatures and our experimental results, a 1O2-based "heterogonous" kinetic model was developed to fully understand the formation and deactivation of 1O2 during the self-sensitized photodegradation process of phenylarsonic compounds. The impact of natural water matrices on phenylarsonic compounds photodegradation was also studied by measuring their degradation rates in natural water samples under simulated and natural sunlight. In this case, PNA/PYR was used as chemical actinometer to monitor the difference of the light intensity in the simulated and natural sunlight irradiation conditions. Then, main factors that influenced the degradation of phenylarsonic compounds were investigated, based on the composition of natural water samples and the effect single environmental factors on the degradation of phenylarsonic compounds obtained in pure water matrix.Photodegradation of phenylarsonic compounds would result in the formatio n of toxic arsenite and arsenate in the aquatic environment, thereby increasing the potential risks of using phenylarsonic feed additives to the ecological environment and the public health. To minimize the adverse effects of using phenylarsonic feed addit ives from the source, we proposed a method of disposal of the phenylarsonic compounds containing animal manures. The treatment process involved leaching of the phenylarsonic compounds out of the manures by water first, and then disposing of the phenylarsonic compounds containing leachate using Fenton process. The treatment parameters of the Fenton process, including initial p H, the dosages of H2O2 and Fe2+, for the oxidation of p-ASA, and the final solution p H on the arsenic removal from aqueous solution were investigated. The effect of water matrices on the treatment efficiencies of Fenton process were evaluated by conducting the experiments in pure water, natural water samples and in the swine manure leachate. The degradation pathways of p-ASA during the Fenton process was delineated, based on the determined organic and inorganic intermediates, and the formation laws of these degradation byproducts. The main innovative conclusions obtained in this dissertation can be listed as follows:(1) The arsenic acid group was cleaved from the aromatic ring of phenylarsonic compounds in the form of arsenite during their photodegradation, and then the formed As(III) would further be oxidized to As(V) by 1O2 generated in the photochemical system. Dissolved oxygen(DO) plays a critical role on both of the photodegradation of phenylarsonic compounds and the formed As(III) oxidation. DO can direct participate in phenylarsonic compounds photodegradation and the formed As(III) oxidation by the formation of 1O2. The degradation of phenylarsonic compounds was found to occur mainly through self-sensitized photodegradation mechanisms, with 1O2 being the main ROSs generated for both of phenylarsonic photodegradation and the formed As(III) oxidation. Direct photolysis played little role on the degradation of phenylarsonic compounds. 1O2 reacts rapidly with phenylarsonic compounds, with the 1O2-mediated rate constant of p-ASA, 4-HPAA and PAA were 1.91±0.23×109, 7.14±0.24×108 and 1.09±0.15×108 M-1s-1, respectively. The 1O2 quantum yield obtained for p-ASA, 4-HPAA and PAA were 6.95±0.23×10-2, 4.70±0.21×10-2 and 5.99±0.25×10-3.(2) In the self-sensitized photodegradation process, the initially sensitized formed 1O2 and phenylarsonic compounds were generally in close proximity rather than randomly distributed, leading to the generation of spatially correlated pairs/geminate pairs. The spatially correlated molecules can either react directly with each other, or diffuse apart and become more evenly distributed, and finally react with the reactants surrounding with them, as the case in homogeneous system. In addition to react with the phenylarsonic compounds, 1O2 diffused to homogeneous solution can also be quenched by the solvent or the external added 1O2 chemical scavenger, such as Na N3. The reactions occurred between the phenylarsonic compounds and 1O2 in geminate pairs and in homogeneous together restricted the photodegradation of phenylarsonic compounds. Based on the formation and deactivation of 1O2 during the self-sensitized photodegradation process of phenylarsonic compounds, a 1O2 based “heterogeneous†kinetic model was developed to depict the photodegradation of phenylarsonic compounds. The rates for the reactions of phenylarsonic compounds and 1O2 taken place in geminate pairs and in homogeneous solution, were proportionate to the first and second power of the concentration of phenylarsonic compounds, respectively. In dilute solution(when phenylarsonic compounds present at low initial concentration), the contribution of phenylarsonic compounds degradation occurring in homogeneous solution to the overall degradation observed was much less than that in spatially correlated pairs, and the photodegradation of these compounds an also be well fitted by pseudo- first-order kinetics. The photodegradation rate of p-ASA, 4-HPAA, and PAA at an initial concentration of 1.33 μM(765 Wm-2, p H=5.7) obtained by fitting with the pseudo- first-order kinetics were 5.87±0.19×10-2, 3.46±0.15×10-2, and 5.14±0.23×10-3 min-1, respectively.(3) Based on the determined organic and inorganic degradation products, and the formation laws of these intermidiates, the mechanisms and pathways of phenylarsonic compounds photodegradation can be summarized as the following steps. Excited triplet-state is first formed from phenylarsonic molecules after absorbing sunlight, and then the energy transferred from the excited triplet-state molecules to the ground oxygen resulted in the formation of 1O2. The 1O2 formed subsequently attacks phenylarsonic compounds, yielding arsenite and the corresponding hydroxyl substitute products(p-aminophenol, p-hydroquinone, and phenol). Arsenite is then oxidized to arsenate, while p-aminophenol and phenol are further oxidized by 1O2 to p-hydroquinone and p-benzoquinone, along with the release of ammonium. The p-hydroquinone and p-benzoquinone formed can further oxidized to 1,2,4-trihydroxybenzene and hydroxybenzoquinone. The fully mineralization of phenylarsonic compounds is not likely to occur under sunlight irradiation conditions.(4) The rates of p-ASA, 4-HPAA, and PAA photodegradation were highly p H depended, it increases with the rising solution p H, which may explained by the fact that the greater negative charge at higher solution p H enr iches the electron density on the aromatic ring and C-As bond, and thus enhances the attraction towards 1O2. As p-ASA, 4-HPAA, and PAA exist predominately in the forms of monovalent anions in the common p H range(5 to 8) of surface water, their photodegradation rate are expected to be relatively insensitive to p H changes in natural aquatic environment. Indirect photolysis caused by inorganic ions commonly present in natural waters was negligible, but dissolved organic matter could significantly inhibit their photodegradation by competitively absorbing sunlight and quenching of the ROSs formed in the photochemical process. Because of relatively high content of dissolved organic matter present in natural water samples, photodegradation rates of p-ASA, 4-HPAA and PAA were observed to reduced by 22.5-89.6, 44.5-87.6, and 40.9-88.1% compared to these in triple-distilled water. The degradation half- lives of p-ASA, 4-HPAA and PAA increased by 5 times under natural sunlight compared to the simulated sunlight irradiation, due to lower irradiation intensity of natural sunlight compared to that of the Xenon arc lamp(190 vs. 765 Wm-2), and as well as the lower solution temperature in the rooftop experiments than that in lab(19-23 vs.25 oC). The degradation half- lives obtained for p-ASA, 4-HPAA and PAA under natural sunlight were 60 min, 100 min and 10 h, respectively.(5) The optimum conditions for the oxidation of p-ASA and removal of formed As(V) were found to be: 0.53 m M Fe2+ and 2.12 m M H2O2, 30 min reaction for natural waters, and 10.0 m M Fe2+ and 40.0 m M H2O2, 120 min reaction for leachates of animal manure; initial solution p H of 3.0, and final solution p H of 4.0. Under such conditions, over 98% of the p-ASA(at an initial concentration of 10 mg- As L-1) could be oxidized to As(V), while the residual arsenic concentratio ns in the solution phase were less than 70 μg L-1, which meets the recommended limit for arsenic(100 μg L-1) in reclaimed water for the long-term use in irrigation(U.S.EPA, 2004). The dissolved organic matter could significantly inhibit the Fenton oxidation efficiencies of p-ASA, by competitively scavenge the ?OH formed during the Fenton process. Meanwhile, the dissolved organic matter and PO43- commonly present in swine manure leachate, would significantly inhibite the As(V) removal by competitively adsorption on the ferric hydroxide surface.(6) Based on the degradation products detected and the general mechanisms involved in Fenton oxidation, the degradation pathway can be proposed for p-ASA in Fenton treatment.-NH2 is an electron-donating group, and can enhance the electron desensity on its para-position of the aromatic ring(C-As bond). ?OH first attacks p-ASA at the para-position of aromatic ring, leading to the formation of p-aminophenol and arsenite. Arsenite is immediately oxidized to arsenate, while p-aminophenol is further degraded to p-hydroquinone, p-benzoquinone, maleic acid and formic acid, and can be eventually mineralized to CO2, ammonia, and H2 O under the conditions of sufficient oxidants and treatment time. The results of laboratory experiments consistently indicate that Fenton process is promising as a simple and effective method for the treatment of p-ASA-containing wastewaters.In conclusion, photodegradation of phenylarsonic compounds would result in the formation of toxic arsenite and arsenate under natural sunlight and in natural water bodies, thus increasing the potential risks of using phenylarsonic feed additives to both the ecological environment and public health. The photodegradation of phenylarsonic compounds is found to occur through self-sensitized mechanisms, with 1O2 being the main reactive oxygen species generated during the photodegradation process, while direct photolysis plays little role in the degradation. The general rules of phenylarsonic compounds photodegradation under natural sunlight is described, and a 1O2-based “heterogeneous†kinetic model was developed to fully understand the self-sensitized photodegradation process of phenylarsonic compounds in the present work, which are crucially important for the better understanding of the fate and behavior of phenylarsonic compounds in the aquatic environment. Moreover, the results of our work indicated that Fenton process is promising as a simple and effective method for the treatment of various types of phenylarsonic compounds-containing wastewaters by flexible adjusting the Fenton’s reagent dosage and the reaction time, and can be scaled up for reducing the risk of organoarsenic feed additives in animal wastes and in surface water. |
