Chapter 20
Downloaded by NORTH CAROLINA STATE UNIV on October 7, 2012 | http://pubs.acs.org Publication Date: November 19, 1996 | doi: 10.1021/bk-1996-0649.ch020
Bromate Ion Removal by Electric-Arc Discharge and High-Energy Electron Beam Processes 1
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Mohamed S. Siddiqui , Gary L. Amy , and William J. Cooper 1
Department of Civil Engineering, University of South Alabama, Mobile, AL 36688 Department of Civil, Environmental, and Architectural Engineering, University of Colorado, Boulder, CO 80309 Drinking Water Research Center, Florida International University, Miami, FL 33199
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Proposed drinking water regulations in the U.S. will specify a maximum contaminant level (MCL) of 0.01 mg/L for bromate ion (BrO ). The work reported herein involves removing BrO after its formation using electric arc discharge and high energy electron beam (HEEB) process, when other removal and minimization strategies are not effective. The electric arc discharge method destroyed 12-45% bromate for doses ranging from 130-1300 mW-s/cm and a dose of 100 krads was sufficient to reduce 70% of BrO from an initial concentration of 100 μg/L using HEEB process in NOM-free water. The addition of hydrogen gas and the removal of dissolved oxygen enhanced BrO removal during electric arc discharge. During HEEB process the presence of electron scavengers such as hydrogen peroxide and nitrate significantly reduced BrO removal whereas the addition of OH radical scavenger such as t -butanol did not affect the removal of BrO indicating that aqueous electrons (e -) are mainly responsible for BrO - destruction. The presence of natural organic matter (NOM) reduced BrO reduction efficiency. 3-
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INTRODUCTION Bromate (Br0 ") has been shown to cause kidney, and possibly other tumors in laboratory animals. However, the mechanism of action and whether Br0 ~ is direct or indirect acting has yet to be resolved (1). Br0 " is also genotoxic in vitro and in vivo although this is primarily confined to causing physical damage to chromosomes (2). The World Health Organization (WHO) has proposed a provisional guideline of 25 /ig/L for BrCV; proposed drinking water regulations in the U.S. will specify a maximum contaminant level (MCL) of 10 /ig/L for Β1Ό3· and a Best Available Technology (BAT) of pH adjustment and ammonia addition (1). 3
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0097-6156/96/0649-0366$15.00A)
© 1996 American Chemical Society In Water Disinfection and Natural Organic Matter; Minear, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
20. SIDDIQUI ET AL.
Bromate Ion Removal by Electric-Arc Discharge
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Ozone is a strong oxidant and it oxidizes B r in water to Br0 " through two different pathways. Br0 ~ can form through both a molecular ozone (0 ) pathway and a hydroxyl radical (OH) pathway, depending on the dissolved organic carbon (DOC), B r content, and pH of the source waters. By the molecular ozone pathway, B r is first oxidized by dissolved ozone (D0 ) to hypobromite ion (OBr) which is then further oxidized to Br0 " (3)(OH radicals are produced as a result of the decomposition of molecular ozone at pH > 8. Molecular ozone (O3) is highly selective as an oxidant whereas OH radicals are non-selective. The reaction rates of OH radicals with bromine species are many times faster than with molecular ozone (18)). This reaction is pH-dependent since OBr is in equilibrium with hypobromous acid (HOBr). The molecular ozone theory suggests that Br0 " formation is directly driven by dissolved ozone (D0 ) and the OH radical theory indicates that dissolved ozone plays only an indirect role by decomposing 0 to produce radicals which further react with bromine species to produce ΒιΌ ~ (4). 3
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Downloaded by NORTH CAROLINA STATE UNIV on October 7, 2012 | http://pubs.acs.org Publication Date: November 19, 1996 | doi: 10.1021/bk-1996-0649.ch020
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Br0 " control strategies have been predicated upon minimization and removal strategies. ΒιΌ " minimization (during ozonation) approaches have highlighted pH depression, ammonia addition, hydrogen peroxide addition, and modified ozone contactor design and operation (5,6,7,8). Adjustment to pH 6.0 prior to ozonation will significantly reduce Br0 ~ formation; however, acid addition may not be viable or cost effective for high alkalinity waters. Ammonia addition can theoreticallytieup bromine as monobromamine; however, the complexity of ammonia-ozone chemistry has yielded mixed Br0 ~ formation results. Removal approaches have focused on ferrous iron addition, UV irradiation, and activated carbon (9). If the proposed Br0 ~ MCL in the U.S. is lowered further, a combination of minimized production and subsequent removal may be required as a Br0 " control strategy. Recent work has shown that electron beam merits further consideration (9). 3
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The HEEB process and electric arc system can potentially be used at various points in the process train, as either a pre-oxidant or a post-disinfectant. In one scenario, these processes could be used in place of pre-ozonation; here, they would provide CT credit, aid coagulation, destroys synthetic organic compounds and reduce DOC while not contributing to the formation of DBPs. In another scenario, HEEB would be used later in the process train following a pre-ozonation step; here it can be used to provide additional CT requirements while destroying ozonation by-products such as Br0 ". HEEB irradiation can destroy halogenated disinfection by-products such as trihalomethanes (THMs)(10) and may provide effective inactivation of microbes of present or future regulatory interest, including enteric viruses and Giardia. 3
This paper discusses the use of an innovative treatment processes, high energy electron beam irradiation and electric arc discharge, in drinking water treatment to remove Br0 ~. The effect of dose, Br0 ~ concentration, pH, natural organic matter (NOM) and alkalinity on Br0 ~ removal is evaluated. 3
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In Water Disinfection and Natural Organic Matter; Minear, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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WATER DISINFECTION AND NATURAL ORGANIC MATTER
Downloaded by NORTH CAROLINA STATE UNIV on October 7, 2012 | http://pubs.acs.org Publication Date: November 19, 1996 | doi: 10.1021/bk-1996-0649.ch020
EXPERIMENTAL AND ANALYTICAL METHODS SOURCE WATERS. Source waters evaluated included California State Project Water, CA (SPW), Colorado River Water, CA (CRW), Silver Lake Water, CO (SLW),Biscayne Aquifer Water, FL (BAW) and DOC-free (
