Article pubs.acs.org/est
Effect of Select Organic Compounds on Perchlorate Formation at Boron-doped Diamond Film Anodes Adrienne Donaghue† and Brian P. Chaplin*,‡ †
Department of Civil and Environmental Engineering and Villanova Center for the Advancement of Sustainable Engineering (VCASE), Villanova University, Villanova, Pennsylvania 19085, United States ‡ Department of Chemical Engineering, University of Illinois at Chicago, 810 S. Clinton St., Chicago, Illinois 60607, United States S Supporting Information *
ABSTRACT: Rates of ClO 4 − formation from ClO 3 − oxidation were investigated in batch experiments as a function of organic compounds (p-nitrophenol, p-benzoquinone, pmethoxyphenol, and oxalic acid) and current density using boron-doped diamond film anodes. Excluding organics, ClO4− formation rates ranged from 359 to 687 μmoles m−2 min−1 for current densities of 110 mA cm−2. The presence of psubstituted phenols inhibited ClO4− formation rates between 13.0 and 99.6%. Results from a reactive-transport model of the diffuse layer adjacent to the anode surface indicate that competition between organics and ClO3• for OH• within a reaction zone (0.020.96 μm) adjacent to the anode controls ClO4− formation. Under kinetic-limited conditions (1.0 mA cm−2), organics reach the anode surface and substrates with higher OH• reaction rates demonstrate greater inhibition of perchlorate formation (IPF). At higher current densities (10 mA cm−2), organic compound oxidation becomes mass transferlimited and compounds degrade a small distance from the anode surface (∼ 0.26 μm for p-methoxyphenol). Therefore, OH• scavenging does not occur at the anode surface and IPF values decrease. Results provide evidence for the existence of desorbed OH• near the anode surface and highlight the importance of controlling reactor operating conditions to limit ClO4− production during anodic treatment of organic compounds.
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INTRODUCTION Boron-doped diamond (BDD) film electrodes are one of the most efficient anode materials to oxidize recalcitrant and complex waste streams. The high oxidation power of BDD is due to the production of hydroxyl radicals (OH•) via water oxidation and direct electron transfer reactions occurring at the BDD surface.1−6 Experimental results indicate that OH• react at near diffusion-limited rates with a wide range of organic compounds.7 BDD anodes have been utilized in various water treatment applications including the treatment of landfill leachate, reverse osmosis (RO) concentrate, industrial wastewater, and electrochemical disinfection of cooling tower waters, drinking water, wastewater, swimming pools, and spas.8−13 However, the promise of BDD electrodes in water treatment applications is hindered by the formation of perchlorate (ClO4−)9,14−18 and chlorinated organic compounds,8,19−23 which are unwanted byproducts produced during the electrolysis of chloride-containing waters. Perchlorate formation is problematic because it is resistant to further oxidation and its consumption is associated with known health risks, which include disruption of the thyroid gland and carcinogenic potential.24−26 In response to health risks, the Environmental Protection Agency (EPA) issued a health advisory level of 15 parts-per-billion (pbb) for drinking water sources,27 and © 2013 American Chemical Society
Massachusetts and California implemented drinking water standards of 2 and 6 pbb, respectively.24,28,29 Recent studies indicate that mass transfer rates of ClO4− precursors (e.g., ClOx− species) to the electrode surface15,16 and competition with other ions (e.g., Cl−)15,16,18,30 are primary factors affecting ClO4− production. Low mass transfer rates of ClOx− species increase ClO4− production,15 and competitive ions like Cl− can limit the production of ClO4−, likely as a result of competition for OH• and electrode reaction sites.15,30,31 Previous research has shown that ClO4− forms via a multistep oxidation pathway starting with chloride, as shown in reaction 1: Cl− → OCl− → ClO−2 → ClO−3 → ClO−4
(1)
where the rate-determining step is the oxidation of ClO3− to ClO4−.16,32 Experimental and density functional theory (DFT) modeling studies have shown that the conversion of ClO3− to ClO4− is a two-step process.14 The first step of ClO4− Received: Revised: Accepted: Published: 12391
July 17, 2013 September 24, 2013 September 25, 2013 September 25, 2013 dx.doi.org/10.1021/es4031672 | Environ. Sci. Technol. 2013, 47, 12391−12399
Environmental Science & Technology
Article
formation involves ClO3− reacting on the electrode surface via a direct electron transfer (DET) reaction (reaction 2).14
ClO−3 → ClO•3 + e−
The 100 mM KH2PO4 electrolyte was used to eliminate electrostatic migration of ClO3− and ClO4− during experiments and thus simplify the mathematical model. To avoid intermediate and product accumulation that may interfere with ClO4− formation, ClO3− removal (
