Environ. Sci. Technol. 2007, 41, 6277-6282
Treatment of Perchlorate-Contaminated Groundwater Using Highly Selective, Regenerable Ion-Exchange Technologies B A O H U A G U , * ,† GILBERT M. BROWN,‡ AND CHEN-CHOU CHIANG§ Environmental and Chemical Sciences Divisions, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, and Calgon Carbon Corporation, Pittsburgh, Pennsylvania 15205
Treatment of perchlorate-contaminated water using highly selective, regenerable ion-exchange and perchloratedestruction technologies was demonstrated at a field site in California. Four treatment and four regeneration cycles were carried out, and no significant deterioration of resin performance was noted in 2 years. The bifunctional resin (Purolite A-530E) treated about 37 000 empty bed volumes (BVs) of groundwater before a significant breakthrough of perchlorate occurred at an average flow rate of 150 gpm (or 1 BV/min) and a feed perchlorate concentration of about 860 µg/L. Sorbed perchlorate (∼20 kg) was quantitatively recovered by eluting with as little as 1 BV of the FeCl3-HCl regenerant solution. The eluted ClO4- was highly concentrated in the third quarter of the first BV of the regenerant solution with a concentration up to 100 000 mg/L. This concentrated effluent greatly facilitated subsequent perchlorate destruction or recovery by precipitation as KClO4 salts. High perchlorate destruction efficiency (92-97%) was observed by reduction with FeCl2 in a thermoreactor, which enabled recycling of the FeCl3HCl regenerant solution, thereby minimizing the need to dispose of secondary wastes containing ClO4-. This study demonstrates that a combination of novel selective, regenerable ion-exchange and perchlorate-destruction and/ or recovery technologies could potentially lead to enhanced treatment efficiency and minimized secondary waste production.
Introduction Perchlorate (ClO4-) has emerged as one of the most widespread contaminants found in sediments, groundwater, and surface water (1-5), and cost-effective remediation technologies are needed to remove trace quantities of ClO4- from contaminated media. Among various treatment options, ionexchange technology has long been used for water treatment because of its simplicity, high capacity, and capability of operating at a relatively high flow rate with a small treatment * Corresponding author phone: (865) 574-7286; fax: (865) 5768543; e-mail:
[email protected]. † Environmental Sciences Division, Oak Ridge National Laboratory. ‡ Chemical Sciences Division, Oak Ridge National Laboratory. § Calgon Carbon Corporation. 10.1021/es0706910 CCC: $37.00 Published on Web 07/28/2007
2007 American Chemical Society
unit. Currently, the two most commonly used ion-exchange technologies for perchlorate removal in contaminated water are (i) selective but non-regenerable strong-base anion exchange and (ii) nonselective or low-selective anion exchange resins with sodium chloride (NaCl) brine regeneration. In the first case, the spent resin cannot be regenerated by desorption from the resin using a conventional brine solution (6-8), so the resin is discarded after it reaches its sorption capacity. The resin bed must be replaced, and the change-out time depends on the feed ClO4- concentration and water quality. Additionally, the spent resin containing perchlorate has to be properly disposed of as hazardous waste. In the second case, the resin can be regenerated by flushing with concentrated NaCl brine solution. However, because of its relatively low selectivity, the resin preferentially removes other common anions such as sulfate and nitrate in water. Because these anions exist in contaminated groundwater or surface water usually at orders of magnitude higher concentrations than ClO4-, they occupy most of ion exchange sites on the resin (>99%), resulting in an extremely low sorption efficiency or capacity for ClO4-. Therefore, the resin has to be regenerated frequently, producing large volumes of secondary brine wastes containing ClO4-. These factors contribute to relatively high capital and operating costs for conventional ion-exchange technologies, which are discussed in numerous reports and publications (2, 6, 8, 9). Alternatively, the bioremediation technique has been successfully demonstrated to remove ClO4- from contaminated water and is cost-effective, especially for treatment of contaminated groundwater with relatively high ClO4- concentrations but poor water quality (e.g., with high organics, co-contaminants, and suspended solids) (2, 3, 10, 11). However, the technique could be ineffective or costly for treatment of large plumes with a low ClO4- (e.g., at tens or hundreds of µg/L concentration levels) because a highly reducing environment has to be maintained for the biodegradation of ClO4-. The major challenge here is to sustain enough biomass to create continuous reducing conditions at a high flow rate without adding substantial amounts of electron donors and/or acceptors (as a food source for microbes). Additionally, many groundwater constituents such as dissolved oxygen, nitrate, Fe(III), and Mn(IV) ions are known to be preferred electron acceptors for biological reduction and have to be reduced before the degradation of ClO4- occurs. Groundwater with extreme pH conditions (either acidic or alkaline) also has to be preconditioned to create a favorable environment for microbes to be functional (11). Furthermore, for drinking-water treatment, posttreatment is usually required to remove added nutrients, other ingredients, and/or potential pathogens (2, 3). This work reports the first field-scale demonstration using highly selective ion-exchange technology for removing ClO 4from contaminated groundwater after four regeneration cycles using novel tetrachloroferrate (FeCl4-) displacement techniques (7). The new technology also enables either quantitative destruction or recovery of eluted perchlorate for possible reuse, thus overcoming problems of conventional throwaway ion-exchange and/or brine regeneration methodologies. The technology, including the bifunctional anion-exchange resin (Purolite A-530E), the ferric chloride-hydrochloric acid (FeCl3-HCl) regeneration, and the perchlorate destruction/ recovery, made it possible to recycle both the spent resin and the regenerant solution, leading to practically no production of secondary wastes containing ClO4-. In brief, the bifunctional resin is composed of two quaternary ammonium groups: the first has a long alkyl chain for higher VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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selectivity, and the second has a shorter alkyl chain for improved reaction kinetics. Both laboratory and small-scale field tests indicate that the resin is highly selective and efficient at removing ClO4- from the contaminated water. For example, the resin was able to treat ∼100 000 bed volumes (BVs) of groundwater before a breakthrough of ClO4- occurred under continuous flow conditions (running at ∼2 BV/min at an influent ClO4- concentration of ∼50 µg/L) (12). The new regeneration technique involves the use of FeCl4- ions, formed in a solution of FeCl3 and HCl, to displace sorbed ClO4- on the spent resin bed (7); it is followed by a rinse with water, in which the sorbed FeCl4- dissociates by chemical equilibrium and thus desorbs so that the resin bed is regenerated to its original state with chloride as counterions by charge balance. This technique has been shown to be highly efficient, and nearly 100% recovery of the exchange sites can be achieved by rinsing with as little as 1 BV of the regenerant solution (this study) and nitrate > chloride (6, 8, 9). The corresponding hydration energy of these anions is in the order of perchlorate (∆G0 ) -205 kJ/mol), nitrate (∆G0 ) -314 kJ/mol), chloride (∆G0 ) -371 kJ/mol) (19, 20). The efficacy of using the FeCl4--displacement technique to regenerate spent resins loaded with ClO4- relies on the fact that the FeCl4- is also a large, poorly hydrated anion and known as one of the most strongly extracted anions from HCl solutions by either liquid-liquid solvent extraction or anion exchange (7, 20-22). However, the FeCl4- ion has a much-desired chemical property: it dissociates in water or dilute acidic solution and forms positively charged Fe(III) species such as Fe3+, FeCl2+, and FeCl2+, which are then readily eluted from the resin by charge repulsion. Therefore, the resin is regenerated to its original state with Cl- as the counterion by charge balance. In practice the resin bed is ready to be reused after regeneration by rinsing with about 10-20 BVs of potable water. Because the rinsewater contains VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Elution profiles of sulfate and nitrate during the regeneration of the spent resin bed. The regenerant solution consisted of 1 M FeCl3 and 4 M HCl, and the flow rate was ∼0.6 gpm. The elution profile of ClO4- (shown in Figure 2) was included for comparisons. practically no detectable amounts of ClO4- ions, it can be readily disposed of via a process drain after neutralization and precipitation of residual Fe3+ in solution (e.g., by passing it through a carbonate gravel bag filter and, if necessary, a small polishing resin canister). The sorption affinity of perchlorate and its desorption by FeCl4- are also illustrated in the elution profiles of nitrate and sulfate during the regeneration of the resin bed (Figure 3). Both nitrate and sulfate are major competing ions sorbed by the resin during the water-treatment phase because their concentrations are orders of magnitude higher than that of ClO4- in groundwater. However, during the regeneration these anions are expected to be eluted off the resin bed more quickly than ClO4- because the bifunctional resin shows relatively low selectivities for nitrate and sulfate (23, 24). Indeed, results indicate that sulfate was eluted and concentrated in the first quarter of the first BV of the regenerant (since the resin bed was drained before regeneration). On the other hand, nitrate was eluted and concentrated in the second quarter BV or between the elution profiles of sulfate and perchlorate. This elution sequence (i.e., sulfate f nitrate f perchlorate) is consistent with previous studies which showed that sulfate is the least strongly sorbed by Type-I polystyrenic anionexchange resins, whereas perchlorate is the most strongly sorbed (24). This finding may also be partially explained by the fact that sulfate has an extremely high hydration energy (∆G0 ) -1103 kJ/mol), as noted earlier. The elution profiles are analogous to chromatographic separation of ions based on their retention time or selectivity in ion-exchange reactions, although the elution profiles of sulfate, nitrate, and perchlorate overlapped. The overlapping of the elution profiles could be partly attributable to a relatively high concentration of the regenerant solution (1 M FeCl3 and 4 M HCl) as well as a low sample frequency (every quarter BV) used during the regeneration. In fact, previous laboratory studies have shown that the sulfate elution profile could be completely separated from that of perchlorate if the regeneration was performed at a slower flow rate and the effluent was sampled more frequently (18). Separation of these anions in the spent regenerant solution is of great interest because removal of nitrate and sulfate is beneficial prior to thermal reduction or destruction of ClO4- (described below) in that a smaller amount of reducing agent is needed for the process. Perchlorate Destruction, Recovery, and Waste Minimization. Because the eluted perchlorate was so concentrated 6280
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TABLE 1. Perchlorate Destruction Performance temperature flow rate inlet ClO4- outlet ClO4(°C) (gph) (mg/L) (mg/L) 190 190
1 1.5
7290 7040
175 553
ClO4- destruction efficiency (%) 97.6 92.1
in the third quarter of the first BV of the regenerant solution (Figure 2), three options are available for subsequent perchlorate destruction, recovery, and/or disposal. First, this small volume or the first BV of the regenerant solution can simply be neutralized and disposed of as hazardous waste. Assuming that the resin bed is able to treat ∼100 000 BVs of contaminated water at influent ClO4- concentrations of 50 µg/L or less (12), this translates only about 0.001% of secondary wastes produced in comparison with the amount of water treated. Note that although about 2-6 BVs of the regenerant solution are typically used to ensure a complete regeneration, any solution in excess of the first BV of the spent regenerant can be reused in subsequent regeneration cycles; the presence of small quantities of ClO4- in the regenerant solution has no significant impact on subsequent regeneration efficiencies (12, 18). Second, perchlorate in the regenerant solution is destroyed by reduction with ferrous chloride at an elevated temperature in a thermoreactor (14). While perchlorate is degraded, the ferrous ion (Fe2+) is oxidized (to Fe3+), and this process renews the FeCl3-HCl regenerant solution because Fe3+ ions are depleted as a result of the sorption of FeCl4- ions during the regeneration. In other words, this process minimizes the need to dispose of the secondary regenerant waste by allowing it to be recycled. This option was evaluated in this study at Calgon Carbon Corporation. Ferrous chloride was used as a reducing agent in accordance with the following chemical stoichiometry (14):
ClO4- + 8Fe2+ + 8H+ f Cl- + 8Fe3+ + 4H2O However, an excess amount of FeCl2 is usually needed due to the presence of other oxidizing agents (such as nitrate and dissolved oxygen) in the spent regenerant solutions (18). The results (Table 1) indicate that perchlorate in spent regenerant solution can be effectively destroyed at an operating temperature of 190 °C and a flow rate of ∼24-36 gpd (or a
residence time of 40 min to 1 h). At an influent ClO4concentration of 7000-7300 mg/L (diluted due to the addition of FeCl2), the effluent ClO4- concentration was found to range from 175-550 mg/L, representing a destruction efficiency of approximately 92-98%. These results clearly demonstrate that perchlorate can be rapidly and effectively destroyed in spent regenerant solution. Similar observations have been made in a small-scale field demonstration at the Edwards Air Force Base in California (18). As noted earlier, although perchlorate was not completely destroyed in this case, the presence of residual amounts of ClO4- should not affect the regeneration efficiency when the treated solution is reused for regenerating the resin bed. The residual ClO4concentration was very low in comparison with the peak ClO4- concentration in spent regenerant solution (up to 100 000 mg/L). An operating temperature of about 190 °C was chosen for obtaining optimum perchlorate-destruction efficiency while preventing potential precipitation of mixed ferric and ferrousoxide solids. Detailed temperature-dependent reaction kinetics between ferrous ions and ClO4- have been reported elsewhere (14). The reaction was found to be slow at relatively low temperatures, despite its favorable thermodynamics, because of the high activation energy (∼120 kJ/mol) required for the degradation of ClO4- to occur in aqueous solutions. The reaction followed a pseudo-first-order-rate law in the presence of excess Fe(II), and the rate increased nearly 3 orders of magnitude when the temperature was increased from 110 to 195 °C. We also note that nitrate in the regenerant solution was completely degraded whereas sulfate was not degraded at all under the same experimental conditions. In the third option, perchlorate in the regenerant solution can be recovered as pure perchlorate salts such as potassium perchlorate (KClO4) because of relatively low solubility of this species in water (Ksp ) 1.05 × 10-2 at 20 °C). At a concentration of 100 000 mg/L ClO4-, theoretical calculations suggest that about 98% of ClO4- could be precipitated as KClO4 solids, assuming that the final K+ concentration is kept at 0.5 M or higher. This option was tested in laboratory studies, and we found that about 13.1 g of KClO4 (or ∼9.4 g of ClO4-) was recovered by mixing 100 mL of the spent regenerant solution with 30 mL of saturated potassium chloride (KCl) solution. This represents a recovery of 94% of perchlorate from the spent regenerant solution. This amount of recovery was slightly lower than that predicted by theoretical calculations, partly because of an increased volume (due to the addition of KCl) and a relatively low K+ concentration. This treatment option could be attractive because of its simplicity, greatly reduced volume of hazardous material, and elimination of the destruction unit. Additionally, one highly significant result of perchlorate recovery is that perchlorate recovered during environmental remediation could be turned from a liability into the reutilization of a valuable material. For example, the recovered KClO4 could potentially be reused in pyrotechnics or munitions. This recovery technology also has been demonstrated successfully in recovering trace quantities of perchlorate in water and sediments for isotopic analysis and environmental forensics studies (25, 26).
Acknowledgments The technical and field support operations provided by Y. K. Ku and H. Yan at Oak Ridge National Laboratory (ORNL) and personnel at Calgon Carbon Corporation are gratefully acknowledged. This research was supported in part by Aerojet and the Environmental Security Technology Certification Program (ESTCP) of the U.S. Department of Defense. ORNL is managed by UT-Battelle, LLC, under contract DE-AC0500OR22725 with the U.S. Department of Energy.
Literature Cited (1) Jackson, W. A.; Anandam, S. K.; Anderson, T.; Lehman, T.; Rainwater, K.; Rajagopalan, S.; Ridley, M.; Tock, R. Perchlorate occurrence in the Texas southern high plains aquifer system. Ground Water Monit. Rem. 2005, 25, 137-149. (2) Urbansky, E. T. Perchlorate chemistry: Implications for analysis and remediation. Bioremed. J. 1998, 2, 81-95. (3) Urbansky, E. T. Issues in managing the risks associated with perchlorate in drinking water. J. Environ. Manage. 1999, 56, 79-95. (4) Damian, P.; Pontius, F. W. From rockets to remediation: the perchlorate problem. Environ. Prot. 1999, 24-31. (5) Aziz, C.; Borch, R.; Nicholson, P.; Cox, E. Alternative causes of wide-spread, low concentration perchlorate impacts to groundwater. In Perchlorate Environmental Occurrences, Interactions, and Treatment; Gu, B., Coates, J. D., Eds.; Springer: New York, 2006; pp 71-91. (6) Batista, J. R.; McGarvey, F. X.; Vieira, A. R. The removal of perchlorate from waters using ion exchange resins. In Perchlorate in the Environment; Urbansky, E. T., Ed.; Kluwer/Plenum: New York, 2000; pp 135-145. (7) Gu, B.; Brown, G. M.; Maya, L.; Lance, M. J.; Moyer, B. A. Regeneration of perchlorate (ClO4-)-loaded anion exchange resins by novel tetrachloroferrate (FeCl4-) displacement technique. Environ. Sci. Technol. 2001, 35, 3363-3368. (8) Tripp, A. R.; Clifford, D. A. The treatability of perchlorate in groundwater using ion exchange technology. In Perchlorate in the Environment; Urbansky, E. T., Ed.; Kluwer/Plenum: New York, 2000; pp 123-134. (9) Gu, B.; Brown, G. M. Recent advances in ion-exchange for perchlorate treatment, recovery and destruction. In Perchlorate Environmental Occurrences, Interactions, and Treatment; Gu, B., Coates, J. D., Eds.; Springer: New York, 2006; pp 209-251. (10) Logan, B. E. Evaluation of biological reactors to degrade perchlorate to levels suitable for drinking water. In Perchlorate in the Environment; Urbansky, E. T., Ed.; Kluwer/Plenum: New York, 2000; pp 189-197. (11) Hatzinger, P. B.; Diebold, J.; Yates, C. A.; Cramer, R. J. Field demonstration of in situ perchlorate bioremediation in groundwater. In Perchlorate Environmental Occurrences, Interactions, and Treatment; Gu, B., Coates, J. D., Eds.; Springer: New York, 2006; pp 311-341. (12) Gu, B.; Brown, G. M.; Alexandratos, S. D.; Ober, R.; Dale, J. A.; Plant, S. Efficient treatment of perchlorate (ClO4-)-contaminated groundwater by bifunctional anion exchange resins. In Perchlorate in the Environment; Urbansky, E. T., Ed.; Kluwer/ Plenum: New York, 2000; Ch. 16, pp 165-176. (13) Gu, B.; Ku, Y.; Brown, G. M. Treatment of perchloratecontaminated water using highly-selective, regenerable ionexchange technology: a pilot-scale demonstration. Fed. Facil. Environ. J. 2003, 14, 75-94. (14) Gu, B.; Dong, W.; Brown, G. M.; Cole, D. R. Complete degradation of perchlorate in ferric chloride and hydrochloric acid under controlled temperature and pressure. Environ. Sci. Technol. 2003, 37, 2291-2295. (15) U.S. Environmental Protection Agency. Record of decision for the Western groundwater operable unit OU-3. U.S. EPA Region 9: San Francisco, CA, 2001; http://www.epa.gov/superfund/new/ awards/rods/aerojet.pdf. (16) Jackson, P.; Laikhtman, M.; Rohrer, J. S. Determination of trace level perchlorate in drinking water and ground water by ion chromatography. J. Chromatogr. 1999, 850, 131-135. (17) Urbansky, E. T.; Gu, B.; Magnuson, M. L.; Brown, G. M.; Kelty, C. A. Survey of bottled waters for perchlorate by electrospray ionization mass spectrometry (ESI-MS) and ion chromatography (IC). J. Sci. Food Agric. 2000, 80, 1798-1804. (18) Gu, B.; Brown, G. M. Field demonstration using highly selective, regenerable ion exchange and perchlorate destruction technologies for water treatment. In Perchlorate Environmental Occurrences, Interactions, and Treatment; Gu, B., Coates, J. D., Eds.; Springer: New York, 2006; pp 253-278. (19) Brown, G. M.; Bonnesen, P. V.; Moyer, B. A.; Gu, B.; Alexandratos, S. D.; Patel, V.; Ober, R. The design of selective resins for the removal of pertechnetate and perchlorate from groundwater. In Perchlorate in the Environment; Urbansky, E. T., Ed.; Kluwer/ Plenum: New York, 2000; Ch. 15, pp 155-164. (20) Moyer, B. A.; Bonnesen, P. V. Physical factors in anion separations. In Supramolecular Chemistry of Anions; Bianchi, A., Bowman-James, K., Garcia-Espana, E., Eds.; VCH: New York, 1997; Ch. 1. (21) Diamond, R. M.; Whitney, D. C. In Ion Exchange, Vol. 1; Marinsky, J. A., Ed.; Marcel Dekker: New York, 1966; pp 277-351. VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(22) Marcus, Y.; Kertes, A. S. Ion Exchange and Solvent Extraction of Metal Complexes; Wiley-Interscience: New York, 1969. (23) Bonnesen, P. V.; Brown, G. M.; Bavoux, L. B.; Presley, D. J.; Moyer, B. A.; Alexandratos, S. D.; Patel, V.; Ober, R. Development of bifunctional anion exchange resins with improved selectivity and sorptive kinetics for pertechnetate. 1. Batch-equilibrium experiments. Environ. Sci. Technol. 2000, 34, 37613766. (24) Gu, B.; Ku, Y.; Brown, G. Sorption and desorption of perchlorate and U(VI) by strong-base anion-exchange resins. Environ. Sci. Technol. 2005, 39, 901-907.
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(25) Bao, W.; Gu, B. Natural perchlorate has its unique oxygen isotope signature. Environ. Sci. Technol. 2004, 38, 5073-5077. (26) Bo¨hlke, J. K.; Sturchio, N. C.; Gu, B.; Horita, J.; Brown, G. M.; Jackson, W. A.; Batista, J. R. Perchlorate isotope forensics. Anal. Chem. 2005, 77, 7838-7842.
Received for review March 20, 2007. Revised manuscript received June 15, 2007. Accepted June 19, 2007. ES0706910
