Article pubs.acs.org/est
Synergistic Integration of Ion-Exchange and Catalytic Reduction for Complete Decomposition of Perchlorate in Waste Water You-Na Kim and Minkee Choi* Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Korea S Supporting Information *
ABSTRACT: Ion-exchange has been frequently used for the treatment of perchlorate (ClO4−), but disposal or regeneration of the spent resins has been the major hurdle for field application. Here we demonstrate a synergistic integration of ion-exchange and catalytic decomposition by using Pd-supported ion-exchange resin as an adsorption/catalysis bifunctional material. The ion-exchange capability of the resin did not change after generation of the Pd clusters via mild ethanol reduction, and thus showed very high ion-exchange selectivity and capacity toward ClO4−. After the resin was saturated with ClO4− in an adsorption mode, it was possible to fully decompose the adsorbed ClO4− into nontoxic Cl− by the catalytic function of the Pd catalysts under H2 atmosphere. It was demonstrated that prewetting the ion-exchange resin with ethanol significantly accelerate the decomposition of ClO4− due to the weaker association of ClO4− with the ion-exchange sites of the resin, which allows more facile access of ClO4− to the catalytically active Pd-resin interface. In the presence of ethanol, >90% of the adsorbed ClO4− could be decomposed within 24 h at 10 bar H2 and 373 K. The ClO4− adsorption-catalytic decomposition cycle could be repeated up to five times without loss of ClO4− adsorption capacity and selectivity.
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INTRODUCTION Perchlorate (ClO4−) is mainly used as an oxidizing agent for manufacturing explosives and solid fuel rocket propellants in aerospace and defense industries, and also for producing commercial products ranging from electronics to pharmaceuticals.1 ClO4− can spread widely in surface and groundwater systems due to its high water solubility, low adsorptive properties and slow natural degradation rate. It is known that high dosing of ClO4− causes detrimental effects to the human body by interfering with iodine uptake into the thyroid gland. Because the thyroid gland plays an essential role for regulation of metabolic activity, normal central nervous system development, and growth in the human body, exposure to ClO4− is especially harmful to pregnant women and fetuses.2 The U.S. EPA has listed ClO4− on the drinking water contaminant candidate list.3 An official reference dose (RfD) of ClO4− has been established as 0.0007 mg/kg/day,4 which corresponds to a drinking water equivalent level (DWEL) of 24.5 μg/L.5 Various treatment technologies for ClO4− removal have been reported so far, which can be classified into three general categories: (1) physical adsorption, (2) bioremediation with microorganisms, and (3) catalytic reduction.1 In the physical adsorption method, ClO4− is removed from wastewater by adsorption. Activated carbons and anion-exchange resins have been most widely studied as adsorbents due to their commercial availability. Activated carbons have large surface area and very low cost, but their too small adsorption capacity and weak ion-selectivity for ClO4− in neutral pH conditions have acted as significant hurdles to practical application.6,7 © 2014 American Chemical Society
Various surface modification methods such as preadsorption with cationic surfactants8 and N-doping7,9 have been reported to improve ClO4− adsorption capacity. In contrast, ionexchange processes using ClO4−-selective anion-exchange resins have shown significant benefits with large adsorption capacities (orders of magnitude larger than those of activated carbon) and high ion-selectivity to ClO4− even at lowconcentrations.10−15 Because ClO4− is too strongly adsorbed on these ion-exchange resins, however, the regeneration of spent resin is particularly challenging and costly.15−17 Gu et al.17 reported that ClO4− ions were desorbed from an anionexchange resin by displacement with tetrachloroferrate (FeCl4−) anions using a concentrated FeCl3 - HCl solution. The same group also reported that ClO4− anions in the regenerant solution could be completely reduced to Cl− with excess amount of Fe(II) as a reductant at above 443 K.18,19 In general, the physical adsorption techniques can provide fast, economical methods for ClO4− removal, but less energyintensive and more environmentally benign disposal and regeneration processes for the ClO4−-saturated adsorbents should be developed. In contrast to physical adsorptions, bioremediation and catalytic reduction methods aim to permanently decompose ClO4− to nontoxic Cl−. From a thermodynamic viewpoint, Received: Revised: Accepted: Published: 7503
February 27, 2014 May 30, 2014 June 4, 2014 June 4, 2014 dx.doi.org/10.1021/es501003m | Environ. Sci. Technol. 2014, 48, 7503−7510
Environmental Science & Technology
Article
ClO4− is a strong oxidizing agent having a redox potential of +1.38 V20 and hence can be permanently reduced to Cl−. However, the reduction is kinetically retarded because of its high activation energy (120 kJ/mol).18 Therefore, overcoming this energy barrier by using catalysts (bio- or artificial catalysts) can permanently reduce ClO4− to Cl− in reductive conditions. In bioremediation, enzymatic reduction functions of microorganisms are used to reduce ClO4− to Cl− in the presence of various reducing agents (e.g., H2, acetate, and lactate).21 However, biological processes can be costly for the treatment of water containing low-concentration ClO4− because a highly reducing environment is required.6 Extensive efforts have also been devoted to developing artificial catalysts that can increase the reduction rate of ClO4−. Abu-Omar and co-workers reported the high catalytic activity of Re-based homogeneous catalysts in the presence of H3PO222 and organic sulfide23−26 as reducing agents. Re(V) complex can react relatively rapidly with ClO4− by an oxygen transfer reaction to form a Re(VII) complex, which can be reduced back to the Re(V) complex by the reducing agents. However, such a homogeneous catalyst with a soluble phosphorus or sulfur reducing agent is not readily compatible with water purification systems. As a heterogeneous version of Re catalysts, a supported Pd−Re bimetallic catalyst was also developed.27 The catalyst, smartly combining the hydrogen activation ability of Pd and the oxygen transfer ability of Re, demonstrated reasonably fast reduction of ClO4− with H2 at acidic pH (pH
