Environ. Sci. Technol. 2003, 37, 2291-2295
Complete Degradation of Perchlorate in Ferric Chloride and Hydrochloric Acid under Controlled Temperature and Pressure BAOHUA GU,* WEIJIN DONG,† GILBERT M. BROWN, AND DAVID R. COLE Oak Ridge National Laboratory, P.O. Box 2008, MS-6036, Oak Ridge, Tennessee 37831-6036
Despite favorable thermodynamics, the reduction of perchlorate (ClO4-) is kinetically limited in aqueous media because of its high activation energy. In this paper, a new methodology has been presented for degrading ClO4in an FeCl3-HCl solution at an elevated temperature (40 000 bed volumes at an influent concentration of ∼4.5 µmol/L ClO4- before a significant breakthrough of ClO4- occurred (2, 3). However, the exceptionally high affinity of ClO4- for these selective anionexchange resins makes regeneration with conventional NaCl brine extremely difficult and costly for practical applications (4, 5). We therefore developed a new technique for regenerating ClO4--laden anion-exchange resins by displacement * Corresponding author phone: (865)574-7286; fax: (865)576-8543; e-mail:
[email protected]. † Present address: Department of Biological and Environmental Sciences, McNeese State University, Lake Charles, LA 70609. 10.1021/es0262378 CCC: $25.00 Published on Web 04/04/2003
2003 American Chemical Society
with tetrachloroferrate (FeCl4-) anions, formed in a solution of ferric chloride and hydrochloric acid (FeCl3-HCl) (3, 6). The new regeneration scheme is so efficient that nearly 100% recovery of ion-exchange sites could be achieved by washing with as little as 2-3 bed volumes of the FeCl3-HCl regenerant solution through a resin column. In comparison with conventional brine regeneration techniques, the new methodology provides improved regeneration efficiency, recovery, and waste minimization. However, despite the aforementioned advantages of using the new regeneration technique, the production and disposal of hazardous ClO4--containing waste regenerant solutions remain issues of great concern. The need exists, therefore, for a method to completely destroy perchlorate in the FeCl3HCl regenerant solution and, more importantly, to allow the reuse of the regenerant solution. The methodology must not change the properties of the regenerant solution so that the solution can be reused in many regeneration cycles. Ideally, the perchlorate destruction process should be efficient and cost-effective, while not being subject to difficult-to-maintain operating conditions or generating any secondary wastes or adverse environmental consequences. The method should also be suitable for large-scale applications in the field or localized facilities. The present study therefore entails the development of a new method for decomposing ClO4- eluted from an anion-exchange resin bed using the FeCl3-HCl regenerant solution. The new method uses ferrous iron(II) chloride as a reductant to reduce ClO4- at a slightly elevated temperature or pressure. Detailed kinetic studies were performed at varying temperatures, and on the basis of the kinetic experiments, a flow-through reactor was designed and tested for degrading ClO4- at a continuous operational mode.
Materials and Methods Batch kinetic reactions between ClO4- and Fe(II) in the FeCl3HCl solution were performed in flame-sealed glass tubes (4 × ∼120 mm), which are capable of withstanding a static pressure of up to 25-35 atm. The ClO4--laden FeCl3-HCl solution was obtained from the regeneration of a bifunctional anion-exchange resin, which had been used for the treatment of ClO4--contaminated groundwater at Edwards Air Force Base in California (3). Only the first few bed volumes of the waste regenerant solution were used because they were concentrated with ClO4- at ∼10 000 mg/L (or ∼100 mmol/ L), but the Fe(III) concentration was significantly depleted because of its sorption onto the resin bed (as FeCl4-). Solidphase ferrous Fe(II) (as FeCl2‚H2O) was first added to glass tubes at two different concentrations (either “low Fe” at 0.115 g or “high Fe” at 0.23 g), followed by the addition of 1 mL of the waste regenerant solution containing ClO4-. This provided a molar ratio of Fe(II)/ClO4- at approximately 8:1 or 16:1 because complete reduction of 1 mol of ClO4- to Clrequires an equivalent of 8 electrons [or 8 mol of Fe(II)] assuming the stoichiometry of the reaction is
ClO4- + 8Fe(II) + 8H+ f Cl- + 8Fe(III) + 4H2O Therefore, in the later case (high Fe), an excess amount of Fe(II) was added (about twice as much as is needed to completely reduce ClO4-). The initial ClO4- concentrations (before reaction) in the low Fe and high Fe reactant solutions were ∼91 and 87 mmol/L, respectively. The glass tubes were then immediately flame-sealed and transferred to a preheated oven at the desired temperature. VOL. 37, NO. 10, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The oven temperature was controlled at a precision of (0.2 °C by means of a Gemini-K temperature controller (J-KEM Electronics, MO). However, during initial sample loading, the oven had to be opened, and samples had to be heated to reach the desired temperature, resulting in a slight time delay (∼10-15 min). Such an effect generally could be neglected in the calculations of the reaction kinetics when the overall reaction extends over several hours or days. However, the effect became significant when the reaction was performed at a relatively high temperature (e.g., at 195 °C) or that occurred in
