Catalytic destruction of perchlorate in ferric chloride and hydrochloric acid solution with control of temperature, pressure and chemical reagents

Abstract

A method is described to decompose perchlorate in a FeCl3/HCl aqueous solution such as would be used to regenerate an anion exchange resin used to remove perchlorate. The solution is mixed with a reducing agent, preferably an organic alcohol and/or ferrous chloride, and can be heated to accelerate the decomposition of perchlorate. Lower temperatures may be employed if a catalyst is added.

Description

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a method for the destruction of perchlorate in a ferric chloride and hydrochloric acid aqueous regenerant solution used for the regeneration of anion exchange resins when the latter of which have been used for ground water remediation. More particularly, it relates to a destruction technique for perchlorate that does not change the chemical properties of the ferric chloride and hydrochloric acid regenerant solution accomplished by using controlled temperature/pressure and chemical reagents. The regenerated elution mixture may then be recycled or reused for the regeneration of anion-exchange resins loaded with perchlorate.

[0004] 2. Background of the Art

[0005] Perchlorate (ClO4−) has been widely used as a rocket propellant and in munitions in the United States and abroad, and improper disposal of perchlorate-containing materials has resulted in a significant new threat to groundwater and drinking water supplies. Its distribution is widespread, as documented by Damian, Environmental Protection, Jun. 24, 1999, and Urbansky, Bioremediation Journal 2, 81 (1998). Because ClO4− ions are nonvolatile, highly soluble, and kinetically inert in dilute aqueous solution, they cannot be effectively removed from water by conventional carbon filtration or sedimentation methodologies or by chemical reduction with reducing agents commonly used for environmental remediation such as elemental iron or dithionite. On the other hand, treatment by ion exchange, using highly selective anion exchange resins in particular, has proven to be an effective technology for removing ClO4−, especially at low concentrations. The ion-exchange technique has been viewed by many as the preferred method for the removal of perchlorates, and various resins have been studied for removal of perchlorates. For example, Kawaski et al., Radiochimica Act 63, 53 (1993) reported adsorption on DOWEX™ 1-X8. Gu et al., in Perchlorate in the Environment, pp 165-176 (2000) found that one bed volume of a bifunctional resin (Purolite™ D-3696) was able to treat>100,000 bed volumes of groundwater before a significant breakthrough of ClO4− occurred (with an initial ClO4− concentration of˜50 μg/L or˜0.5 μM/L ).

[0006] Once the resin reaches its loading capacity, the spent resin is regenerated for reuse. In other words, the entire treatment process involves the use of selective anion exchange resins to remove prechlorate in water and the regeneration of spent resin bed. However, because ClO4− anions are selectively and strongly sorbed as a result of their low hydration energy and large size, regeneration of spent resin becomes a particularly challenging and costly task. Indeed, a major operating cost for the ion exchange process is the disposal of spent regenerant solution. For example, Calgon Carbon Corporation's ISEP process produces approximately 1% of the volume of the treated groundwater as regenerant wastes (NaCl brine), which must be disposed of. The combined brine and its disposal costs are well over $350 per acre foot of water treated on the basis of a 1% rejection rate.

[0007] Our previous U.S. patent application (Ser. No. 09/491,242), incorporated herein by reference, disclosed a novel methodology to regenerate spent perchlorate-loaded resins using a combination of ferric chloride and hydrochloric acid solutions (Gu et al., Environ. Sci. Technol. 35, 3363-3368, 2001; Gu et al., Remediation, March 2002). Tetrachloroferrate (FeCl4−) anions, formed in a solution of ferric chloride and hydrochloric acid (e.g., 1 M FeCl3 and 4 M HCl), were found to effectively regenerate spent resins loaded with perchlorate. A mass-balance analysis indicated that nearly 100% recovery of ion-exchange sites was achieved by washing with as little as˜2 bed volumes of the regenerant solution in a column flow-through experiment. There was no significant deterioration of the resin's performance with respect to ClO4− removal after repeated loading and regeneration cycles. Our new regeneration method produces a regenerant waste of less than 0.005% volume of the treated groundwater and, therefore, offers a more cost-effective means to regenerate ClO4−-loaded resins with improved regeneration efficiency, recovery, and waste minimization than conventional NaCl brine regeneration techniques.

[0008] However, despite the fact that our new regeneration technique offers greatly improved efficiency and waste minimization of the volume of the regenerant, the production of hazardous perchlorate-containing waste regenerant solutions, and their disposal, remain to be issues of great concern. The need exists, therefore, for a method to completely destroy perchlorate in the ferric chloride-hydrochloric acid regenerant solutions 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-destraction process is efficient and cost-effective, while not being subject to difficult-to-maintain operating conditions, nor generating any secondary wastes. The method should also be suitable for both the treatment of large aquifers or bodies of standing water as well as for localized facilities to provide potable water on site.

[0009] Li and Coppola, EPA STAR Report, 2000, disclosed a hydrothermal/thermal technique to decompose perchlorate in a 7% NaCl brine solution with added reducing agents and promoters. They reported that only partial destruction of perchlorate could be obtained by the additions of ferric salts to sodium chloride brine and essentially no perchlorate decomposition was obtained with or without other promoting agents. Their Integrated Thermal Treatment Process included a reverse osmosis step to concentrate the brine and the nitrate and sulfate rejection systems, but the process is believed to be ineffective for the destruction of perchlorate in the ferric chloride and hydrochloric acid solution used in the process of U.S. Ser. No. 09/491,242

BRIEF SUMMARY OF THE INVENTION

[0010] It is an object of this invention to provide a method for the decomposition of perchlorate eluted from an anion exchange column using a ferric chloride and hydrochloric acid regenerant solution. It is a further objective of this invention to provide a process for recovering and recycling a regenerant solution used to regenerate a spent anion exchange column which has been used to remove perchlorate from water. It is yet a further objective of this invention to provide a system for routinely regenerating an anion exchange column used for perchlorate removal and to recycle the regenerant solution in a cost efficient manner.

[0011] These and other objects of this invention may be achieved by a process which removes perchlorate and regenerant from an anion exchange column, introduces a small quantity of a non-toxic organic solvent (<2% vol/vol), ferrous ions, and/or a catalyst to the mixture, provides sufficient energy to drive a redox reaction to completion, and purges and holds the regenerant for recycle. More specifically, the eluted perchlorate together with regenerant FeCl3/HCl solution are diverted to a reactor vessel (either a batch reactor or a continuous flow-through system), mixed with an organic solvent or a catalyst and maintained at a relatively low temperature (from ambient up to˜300° C.) and pressure (generally <500 psi) that is sufficient to decompose perchlorate. Ferrous chloride and organic alcohols are preferred chemical reagents in view of availability, cost, and effectiveness. Subsequently, the solution is cooled and ready for reuse when a resin column needs regeneration.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a generic flow diagram of a typical fixed resin bed (or column) treatment system for removing perchlorate from contaminated groundwater or surface water.

[0013] FIG. 2 is a schematic of the water purification and regeneration system according to this invention.

[0014] FIG. 3 is a plot of perchlorate remaining in a ferric chloride-hydrochloric acid regeneration solution versus time at different temperatures and using different organic solvents.

[0015] FIG. 4 shows ion chromatograms indicating a decrease in perchlorate concentration with increased heating time. Note that no byproducts such as chlorate or chlorite were identified, which would otherwise shown in the chromatograms (with a shorter retention time).

[0016] FIG. 5 shows 13C-liquid NMR spectra of regenerate solutions after 4 hours and 20 hours of heat treatment in the presence of 3% ethanol. No organic byproducts were identified except ethanol.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Our previous U.S. patent application (Ser. No. 09/491,242) disclosed a novel methodology to regenerate spent perchlorate-loaded resins using a combination of ferric chloride and hydrochloric acid solutions (Gu et al., Environ. Sci. Technol. 35, 3363-3368, 2001; Gu et al., Remediation, March 2002). The new methodology has been proven to be highly efficient with minimized secondary waste generation. In this new methodology, the regenerant is composed of 0.1-1 M FeCl3 and about 1-4 M HCl, in which tetrachloroferrate ions form according to the following chemical equilibrium

FeCl3+Cl−→FeCl4−

[0018] Tetrachloroferrate ions are large, poorly-hydrated anions, and like perchlorate, there is preferential sorption of these anions by strong-base anion-exchange resins. They can be used to displace perchlorate anions from the spent resin (loaded with perchlorate). Once perchlorate ions have been displaced, the resin column is washed with dilute HCl solution or water, in which tetrachloroferrate dissociates according to the following chemical equilibria:

FeCl4−Fe3++4Cl−

FeCl4−FeCl2++3Cl−

FeCl4−FeCl2++2Cl−

[0019] These positively charged ferric ion species and excess Cl− anions are readily eluted off the resin column by charge repulsion, and the anion exchange resin is therefore regenerated to its original state with Cl− as the counter ion by charge balance.

[0020] The entire regeneration process produces a relatively small quantity of regenerant wastes with a high perchlorate concentration. As indicated, the need still exists for a method to completely destroy perchlorate in this acidic regenerant solution, because of the hazardous nature and volume of the waste. It also is desirable to recycle the regenerant solution, provided that the perchlorate-destruction methodology does not change the properties of the regenerant solution. Therefore, the secondary waste production may be eliminated.

[0021] We have discovered that the perchlorate-containing regenerant may be treated to destroy perchlorate while not affecting the FeCl3/HCl active components. This allows repeated recycle of the regenerant while producing no toxic liquid or solid wastes. The destruction is accomplished by a redox reaction using a water-miscible organic solvent or ferrous chloride.

[0022] A non-toxic catalyst such as the V(II)/V(III)/V(IV) redox couples could be used to accelerate the destruction of perchlorate at a relatively low temperature. The overall chemical reactions and equilibrium are described in the following equations (using ethanol as an example of the organic solvent). Note that none of the equations are stoichoimetrically balanced for the simplicity of illustration of these reactions.

[0023] In the absence of catalysts, ethanol (CH3CH2OH) is completely oxidized by ferric ions to water and carbon dioxide (CO2). The partial redox reactions may be written as follows:

CH3CH2OH→CO2+12e−  (1)

Fe3++e−→Fe2+  (2)

[0024] The ethanol could be oxidized stepwise with CO2 and water as its end byproducts:

CH3CH2OH→CH3CHO→CH3COOH→CH3OH→HCHO→HCOOH→CO2+H2O  (3)

[0025] The net reaction for reduced ferrous Fe2+ is oxidation by perchlorate, which is reduced simultaneously to Cl− and water, as shown in following equations:

ClO4−+Fe2+→Fe3++Cl−+H2O  (4)

ClO4−+Fe2+ClO3−+Fe(IV)O  (5)

[0026] This reaction likely involves a net two electron, oxygen atom transfer to form a Fe(IV)oxo (ferryl) species as an intermediate. The Fe(IV) species will react rapidly with Fe(II) to form Fe(III).

Fe(IV)O+Fe(II)→2 Fe(III)  (6)

[0027] Therefore, the overall reaction is the oxidation of ethanol and the reduction of perchlorate catalyzed by the Fe(III)/Fe(II) couple, with byproducts of Cl−, CO2, and H2O, none of which are hazardous. Ferrous ions (such FeCl2) can be used to directly reduce perchlorate (without addition of organic alcohols), with byproducts of ferric ions. An important aspect of this invention is that the chemical properties and compositions of the regenerant solution remain essentially intact after treatment because both ferric ions and chloride are the components of the regenerant solution while CO2 is volatilized (when organic alcohol is used). This allows reuse of the regenerant solution for multiple cycles.

[0028] Another important aspect of this invention is that only a small and calculated amount of ethanol or other organic solvents or ferrous chloride may be used under controlled temperature so that the perchlorate destruction process does not generate unwanted byproducts. For example, with excess amounts of ethanol, ferrous ions, or by using other organic solvents, some intermediate organic byproducts may form as illustrated in Equation 3. More importantly, excess amounts of Fe2+ will also be reduced, with subsequent precipitation of a mixture of Fe3+/Fe2+ hydroxides. Therefore, control of the course of the reaction by monitoring and adjusting the amount of reducing agent is necessary.

[0029] In the presence of vanadium catalysts, the reaction may be accelerated, and a lower heating temperature may be used because of the ability of vanadium to extract oxygen from oxyanions such as perchlorate. The overall reaction may follow the order of (1) ferric Fe3+ is first reduced by ethanol, (2) V(IV) is concurrently reduced by ethanol, and (3) perchlorate is reduced by V(III). The first reaction has been illustrated in Equations (1) and (2), and the second in reactions (7) and (2). The net reaction of perchlorate with V(IV) is shown in equation (8) below:

VO2++e−+2H+→H2O+V3+  (7)

ClO4−+V3+→V(IV)O2+Cl−+H2O  (8)

[0030] Isopropanol appears to be more effective in degrading perchlorate. A number of other water soluble reducing agents are suitable for this application. Alcohols such as a methanol, ethanol, n-propanol, ISO propanol, n-butanol, sec-butanol, tert-butanol, and water soluble homologues, glycols such as ethylene glycol, propylene glycol, glymes such EGMME and EGDME, and amino alcohols such as ethanolamine are usable.

[0031] The reduction of perchlorate proceeds slowly at room temperature but with increasing rapidity at elevated temperatures and in the presence of catalysts. Higher temperatures result in faster rates but increased pressures and a more aggressive mixture. For this reason, the decomposition of perchlorate according to this method is best performed either in a pressure vessel/reactor or in a continuous-flow reactor within which a coiled tube is maintained at or heated to the desired temperature so that perchlorate is completely decomposed as the spent regenerant solution passes through the coiled reactor or in a continuous flow stirred tank reactor. The coiled reactor tube must have a long enough flow path to allow a sufficient reaction time (typically a few hours) to drive the decomposition of perchlorate to completeness or near completeness. The flow-through reactor can also be used in a recycle mode as well in which unreacted perchlorate-containing solution is returned for a second pass through the reaction zone. In the case of a batch reactor, a jacketed reactor is preferred. Because the hot mixture of HCl and ClO4− is very aggressive, the metal reactor is preferably lined with glass or a fluorocarbon such as PTFE. The reactor may be heated using resistance wire, steam or hot oil, the later being preferred for a batch reactor.

[0032] FIG. 2 shows one embodiment of a perchlorate removal system with resin regeneration and regenerant recovery. Contaminated water such as groundwater or well water enters the system through pipe 5 and is pumped to column 3 which is packed with resin. Switch valve 9 admits the water or regenerant. Treated water passes through valve 11 into drain pipe 13 for subsequent use.

[0033] When the resin is to be regenerated, valve 9 is switched to open pipe 47, pump 7 is turned off or the contaminated water is redirected to another column. Valve 11 is switched to direct effluent to pipe 15. Pump 45 is activated to pump regenerant solution from tank 43 into the column 3. The effluent passes through pipe 15 to a holding tank 17. The contents of tank 17 may be pumped using pump 19 and valves 49 and 51 to refill tank 43. Simultaneously or alternatively, the regenerant may be pumped via pipe 21 and valve 23 into lined reactor 25. Reducing agent is pumped into the reactor via line 31 and valve 33 from tank 27 using pump 29 and pipe 31. The reactor can be heated using inlet 35 and outlet 37, which may be electrical wires, steam or oil pipes. The source of heat is not shown. If used, a stirring motor 55 is activated. In the case of a batch reactor, the mixture of regenerant and reductant are allowed to react for a period of time. If a coiled tube is used, flow rate is controlled to achieve the desired residence time. The treated regenerant is preferably cooled and drained through valve 39 and pipe 41 to tank 43, where it may be held until further needed and maybe adjusted using FeCl3 and HCl to obtain optimal concentrations for regeneration.

[0034] The tanks, pipes and valves may be lined to reduce corrosion. When two or more columns are used, the regeneration system may be piped to serve all columns on a rotating basis. The regeneration system may be mounted on a skid or trailer and taken to different locations where contaminated water is being treated. Monitoring equipment such as UV-vis spectrophotometer or ion chromatographs may be permanently or temporarily installed for process control.

EXPERIMENTALS

[0035] Samples of spent FeCl3/HCl regenerant solution were used for the study. This regenerant solution was obtained from the regeneration of a bifunctional anion exchange resin (Purolite D-3696) which had been used for groundwater treatment of perchlorate at Edwards Air Force Base, Calif. (Disclosed in Gu et al. Remediation, March 2002). The resin is the one described in U.S. Pat. No. 6,059,975 and the regeneration procedure is that described in U.S. patent application Ser. No. 09/491,242. Perchlorate concentration in the regenerant solution ranged from ˜6500 to 10,000 mg/L.

[0036] Aliquots of 0.5-10 ml of spent regenerant solution were mixed with an organic solvent at about 0.5 to 3% vol/vol or with ferrous chloride in order to study the relative effectiveness of these organic solvents in destroying perchlorate. These organic solvents included ethanol, isopropanol, citric acid, and acetate. Each mixture was placed either in an acid digestion bomb or in a glass tube. Samples were either held at ambient temperature (ca 25° C.) or heated in an oven to a specified temperature for a specified period of time.

[0037] The samples were then cooled, and an aliquot of sample was analyzed for ClO4− content by ion chromatography. These data were used to calculate the extent of perchlorate destruction at varying experimental conditions as outlined above. The results are shown in Table 1. Isopropanol appeared to be one of the most effective organic solvents in decomposing perchlorate. About 100% decomposition of perchlorate was observed after heating the sample at ˜170° C. for about 28 hours. Black precipitates were also observed in some cases, suggesting the reduction of Fe3+ and the formation of Fe3+/Fe2+-mineral precipitates. In the presence of ethanol, a slightly higher temperature or a longer heating time is required to completely decompose perchlorate under the same experimental conditions. In the absence of an organic solvent or ferrous ion, the decomposition of perchlorate is extremely slow. Similarly, without ferric Fe3+, no significant decomposition of perchlorate was observed, suggesting that the Fe(III)/Fe(II) may be acting as a redox couple in facilitating the reduction of perchlorate, as illustrated in Equations 2 and 4.

[0038] To study the rate of perchlorate decomposition at varying temperatures or in the presence of different organic solvents, the protocol above was repeated with samples taken at various times. The results are shown in FIG. 3 under conditions of (1) without organic solvent and (2) in the presence of ethanol or isopropanol at a temperature of either 170 or 200° C.

[0039] To verify the completeness of perchlorate decomposition and any byproduct formation for the reaction, ion chromatography was used for the analysis of perchlorate (ClO4−), chlorate (ClO3−), and chlorite (ClO2−) (shown in FIG. 4). In addition, liquid-state 13C-nuclear magnetic resonance (NMR) spectroscopy was used to identify organic reactants or byproducts (if any). FIG. 5 shows the NMR spectra of regenerant sample solutions that had been heated at ˜190° C. for 4 and 20 hours in the presence of 3% of ethanol (vol/vol). Prior to analysis, samples were neutralized with NaOH after heat treatment in order to remove ferric ions, which may interfere with the analysis and identification of organic byproducts by NMR technique. The peaks corresponding to the methyl (—CH3) and ethylene (—CH2—) carbon atoms are seen to be diminished as the ethanol is converted to CO2 and out gassed from the liquid.

INDUSTRIAL APPLICABILITY

[0040] The process of this invention is useful in the treatment of surface water (such as lakes or rivers) and groundwater plumes (such as well water) for either drinking purposes by mammals or agricultural irrigation. The method is also useful for the remediation of contaminated areas prior to commercial or residential development. It may be applied also in the decommissioning of the Air Force base or army ammunition plant sites where the soil or groundwater are contaminated with perchlorate.

[0041] Changes and departures in this invention may be made by those skilled in the art without departing from the spirit and scope thereof. Therefore, the invention is not intended to be limited by the description and figures but are those as set forth in the following claims.

Claims

1. A method for the decomposition of perchlorate in an aqueous solution of ferric chloride and hydrochloric acid, the method comprising: mixing the aqueous solution with an organic solvent or ferrous chloride; and, maintaining the mixture in a closed container at a temperature and for a time sufficient to decompose the perchlorate.

2. A method for the decomposition of perchlorate according to claim 1 wherein the organic solvent is selected from the group consisting of organic alcohols and amino alcohols.

3. A method according to claim 2 wherein the organic alcohols are selected from the group consisting of methanol, ethanol, n-propanol, i-propanol, n-butanol, sec-butanol, t-butanol, glycols and alkyl ethers of glycols.

4. A method according to claim 1 wherein said organic solvent concentration is from 0 to 5% (vol/vol).

5. A method according to claim 1 wherein said ferrous chloride concentration is from 0.01 to 20% (wt/vol).

6. A method according to claim 1 wherein the temperature is maintained from ambient temperature up to about 300° C.

7. A method according to claim 6 wherein the heating time depends upon the temperature above ambient temperature.

8. The method according to claim 1 further comprising a catalyst.

9. The method according to claim 6 wherein the catalyst is selected from the group consisting of V(IV) and V(III).

10. A method according to claim 1 which is performed in a batch process.

11. A method according to claim 1 which is performed in a continuous flow reactor.

12. A method according to claim 1 which is performed in a continuous flow reactor recycle.

13. A method according to claim 1 which is used to process a regenerant solution from ion exchange resin column used to treat perchlorate-contaminated water.