Remediation Of Organoarsenic-Bearing Wastewaters And Investigation Of Their Oxidant Removal Mechanisms

Arsenic is recognized as priority pollutant and introduced into the environment primarily through natural processes and arsenic related anthropogenic activities including metallurgical industries,ceramic industries,pesticides manufacturing industries and wood preservatives.Arsenic contamination frequently occurring in natural environments threatens the health of tens of millions of people across the world,and the World Health Organization has set a maximum allowable contaminant level of 10 ppb for drinking water.The majority of research efforts carried out in the past decades have concentrated on the inorganic arsenic species,which prevalently exists as the stable forms of arsenite?As?III??and arsenate?As?V??in aquatic environment and both forms of arsenic are hypertoxic and carcinogenic.An increasing focus has been placed on detection and analysis of arsenic speciation in food supplies such as rice and seafood.One source of arsenic introduction into environmental systems that has not been rigorously investigated is the use of organoarsenicals in animal husbandry.Around the world,aromatic organoarsenic compounds such as roxarsone?ROX?and p-arsanilic acid?ASA?are most heavily used as feed additives to enhance the feed efficiency,prevent and treat dysentery,and control intestinal parasites.Almost all of the chemicals added in animal feed is excreted unchanged into the manure and accumulated in the poultry bedding,which eventually enters into aquatic or terrestrial environment with washing and run off,or land application of poultry litter.These compounds can potentially transform to more toxic and mobile counterpart inorganic species?i.e.As?III?and As?V??via biotic and abiotic reactions once they are released into aquatic/soil environment.Eventually,their occurrence in natural environment may have detrimental impacts on the human's health and ecosystem.Therefore,a better understanding of the environmental impact and the ultimate fate of organoarsenic compounds in the environment is of significant importance.Arsenic contamination is a significant environmental concern and its removal have been the subjects of growing concerns in the area of environmental remediation.In the present study firstly studied the fate and environmental behaviors of organoarsenic in aquatic environment.The photolysis behaviors of ASA and the toxicity of its metabolites in aqueous solution under UV irradiation was investigated and the effects of pH,humic acid and dissolved oxygen on ASA conversion were evaluated.The results showed that ASA could be perfectly photodegraded and this process was strongly affected by solution pH,and humic acid.Upon UV irradiation for 20 min,ASA could be completely eliminated,but the reduction of TOC was not significant.In addition,NH₄⁺ions and inorganic arsenic including arsenite and arsenate were identified as the predominant end-products.In addition,the ESR technique was employed to explore the reactive oxygen species?ROS?to verify the ASA photolysis mechanisms.The conversion of ASA included both direct and indirect photolysis involving radicals,and its possible photolysis pathways were proposed on the basis of the identified intermediates.Unfortunately,the toxicity of ASA solution increased after UV irradiation due to the higher toxicity of the intermediates.The results of study might be helpful for assessing the environmental persistence and risks of organoarsenic.Furthermore,this study investigates treatment of organoarsenicals wastewater using UV-based treatment processes,namely UV irradiation at 253.7 nm and the UV-H₂O₂ advanced oxidation process.The photochemical degradation kinetics and transformation mechanism of ROX were systematically examined.In this section,the role of^·OH and the effect of H₂O₂dosage,ROX concentration and background water matrix such as inorganic anions,metal cations,and natural organic matter?NOM?were also investigated.It was found that although ROX could be degraded under direct UV irradiation,^·OH contributed much more significantly in UV/H₂O₂.The removal of ROX increased with increasing initial H₂O₂ dosage while excess H₂O₂ acted as an inhibitor of^·OH.The observed UV fluence based pseudo first-order rate constant of ROX decreased while the degradation rate increased with increasing initial ROX concentrations.Presence of common inorganic anions(i.e.,Cl^-,SO₄^2-and NO₃^-)and metal cations(i.e.,Ca²⁺,Mg²⁺and Fe³⁺)did not affect the ROX degradation under current reaction conditions;while Cu²⁺improved slightly the destruction of ROX.On the contrary,the presence of NOM significantly inhibited the removal of ROX.Mineralization of ROX could be obtained with a limited elimination of ROX.Consumption of H₂O₂ was not significant during the degradation of ROX.This study suggests that UV/H₂O₂ AOP is capable of removing organoarsenicals such as ROX from the contaminated water and provides valuable information on the potential application of this process for the removal of ROX in various aquatic compartments.Finally,this study unprecedentedly evaluated the applicability of glow discharge plasma process for simultaneous oxidation of organoarsenic and immobilization of arsenic in one process.The results show that ROX can be effectively oxidized to inorganic arsenic and this performance is evidently dependent on energy input.Adding Fe?II?can significantly enhance the oxidation of ROX mainly because of the additional production of^·OH via Fenton reaction in GDP,accompanied with which the generated arsenic can be simultaneously immobilized in one process.The immobilization of arsenic can be favorably obtained at pH 4.0-6.0 and Fe?II?concentration ranging from 500 to 1000??.Based on the mineral compositions and analysis?XRD/FTIR/XPS?of precipitate,a mechanism can be proposed that the oxidation of Fe?II?by H₂O₂ generated in-situ in GDP significantly accelerates ROX transformation to the ionic As?V?,which can immediately precipitate with Fe?III?ions or be adsorbed on the ferric oxyhydroxides,forming amorphous ferric arsenate-bearing ferric oxyhydroxides.As such,the present study offer a new recipe for rapid decontamination of organoarsenic pollutants,in which the hypertoxic species can be effectively removed from the wastewater.