Selective Oxidation of Arsenite by Peroxymonosulfate with High Utilization Efficiency of Oxidant Zhaohui Wang,* ,†,‡ Richard T. Bush, † Leigh A. Sullivan, † Chuncheng Chen,* ,§ and Jianshe Liu ‡ † Southern Cross GeoScience, Southern Cross University, Lismore, NSW 2480, Australia ‡ College of Environmental Science and Engineering, Donghua University, Shanghai, 201620, China § Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, China * S Supporting Information ABSTRACT: Oxidation of arsenite (As(III)) is a critical yet often weak link in many current technologies for remediating contaminated groundwater. We report a novel, efficient oxidation reaction for As(III) conversion to As(V) using commercial available peroxymonosulfate (PMS). As(III) is rapidly oxidized by PMS with a utilization efficiency larger than 90%. Increasing PMS concentrations and pH accelerate oxidation of As(III), independent to the availability of dissolved oxygen. The addition of PMS enables As(III) to oxidize completely to As(V) within 24 h, even in the presence of high concentrations of radical scavengers. On the basis of these observations and theoretical calculations, a two-electron transfer (i.e., oxygen atom transfer) reaction pathway is proposed. Direct oxidation of As(III) by PMS avoids the formation of nonselective reactive radicals, thus minimizing the adverse impact of coexisting organic matter and maximizing the utilization efficiency of PMS. Therefore, this simple approach is considered a cost-effective water treatment method for the oxidation of As(III) to As(V). ■ INTRODUCTION Arsenic is an element that raises much concern over environmental quality and human health. The greatest threat to public health from arsenic originates from contaminated groundwater. 1 As(III) and As(V) are the two most common naturally occurring oxidation states of dissolved arsenic, with As(III) exceedingly more harmful due to its much higher mobility and toxicity. 2 Therefore, oxidizing As(III) to As(V) is potentially an effective strategy to reduce the impacts of arsenic on society. In addition, its oxidation is usually a prerequisite step for most subsequent arsenic removal technologies like coagulation, sorption, and membrane filtration. 3 Overcoming the slow oxidation rate of As(III) in air- saturated solutions is one of the major technical challenges. 4 Numerous methods based on the advanced oxidation processes (AOPs) including Fenton (like) reaction, 5 zerovalent iron oxidation 6,7 and photocatalysis, 8 have been developed to enhance the kinetics of As(III) oxidation. While these advanced AOPs are effective for the oxidation of As(III), there are significant inherent limitations to their practical application for the remediation of contaminated groundwater. For example, all these oxidation reactions are known to be initiated by the nonselective radicals such as • OH radicals. 9 These reactive radicals tend to oxidize the common coexisting nontoxic organic matter species such as humic acids, besides the targeted As(III), which leads to much lower utilization efficiency (UE) of the oxidant. In addition, the use of UV irradiation or addition of excessive amounts of metal catalysts (e.g., Fe(II)) is evidently unfavorable for the treatment of As(III)-contami- nated groundwater. 5,8 The oxidation of pollutants by using peroxydisulfate (S 2 O 8 2- ) and peroxymonosulfate (HSO 5 - , PMS) as the main parent oxidants is one of the emerging AOPs that is gaining importance in water treatment applications. 10-13 In these systems, the dominant active species was conventionally considered to be the sulfate radicals (SO 4 •- , E 0 = 2.5-3.1 V vs NHE), which are formed through activation of the peroxosulfates by heat, 14 UV radiation, 15,16 or transition metal catalysts 10,17 (eq 1). + → + − − •− • −• S O /HSO initiator SO (HSO , SO , OH) 2 8 2 5 4 5 4 2 (1) Peroxosulfate-based AOPs have recently been introduced into the oxidation of As(III). The UV/peroxydisulfate and Fe(II)/peroxydisulfate systems, for example, can rapidly oxidize the As(III) over a broad pH range (2.0-8.0). 16 In these systems, the oxidation of As(III) is apparently initiated by highly reactive sulfate radicals with As(IV) as the intermediate Received: April 18, 2013 Revised: March 2, 2014 Accepted: March 3, 2014 Published: March 3, 2014 Article pubs.acs.org/est © 2014 American Chemical Society 3978 dx.doi.org/10.1021/es405143u | Environ. Sci. Technol. 2014, 48, 3978-3985