Desalination of Brackish Waters Using Ion-Exchange Media

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Ind. Eng. Chem. Res. 2006, 45, 4752-4756

Desalination of Brackish Waters Using Ion-Exchange Media Jason D. Pless, Mark L. F. Philips, James A. Voigt, Diana Moore, Marlene Axness, James L. Krumhansl, and Tina M. Nenoff*

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Sandia National Laboratories, P.O. Box 5800, MS 1514, Albuquerque, New Mexico 87185-1514

An environmentally friendly method and materials study for desalinating inland brackish waters (i.e., coal bed methane produced waters) using a set of ion-exchange materials is presented. This desalination process effectively removes anions and cations in separate steps with minimal caustic waste generation. The anionexchange material, hydrotalcite (HTC), exhibits an ion-exchange capacity (IEC) of ∼3 mequiv g-1. The cationexchange material, an amorphous aluminosilicate permutite-like material, (Nax+2yAlxSi1-xO2+y), has an IEC of ∼2.5 mequiv g-1. These ion-exchange materials were studied and optimized because of their specific ion-exchange capacity for the ions of interest and their ability to function in the temperature and pH regions necessary for cost and energy effectiveness. Room temperature, minimum pressure column studies (oncepass through) on simulant brackish water (total dissolved solids (TDS) ) 2222 ppm) resulted in water containing TDS ) 25 ppm. A second once-pass through column study on actual produced water (TDS ) ∼11 000) with a high carbonate concentration used an additional lime softening step and resulted in a decreased TDS of 600 ppm. 1. Introduction Current commercial desalination techniques focus on reverse osmosis (RO) or distillation of seawater. The main drawback of these processes is their high energy consumption. In addition, these methods are restricted to coastal water locations for the input of seawater and the disposal of brackish water waste. If breakthroughs in water remediation are to be accomplished, then (1) the use of untapped inland groundwater sources must be pursued and/or (2) the technologies developed must be economically viable. One important source of inland groundwater is the byproduct water from oil and natural gas wells. Large volumes of water are coproduced by drilling companies while accessing the oil and natural gas available in hydrocarbon-bearing formations. This produced water can have high concentrations of dissolved solids and, thus, needs to be treated on-site prior to permitted discharge or trucked off-site to a licensed disposal area. Possible technologies for treating produced water include reverse osmosis, distillation, and ion exchange. We have focused on ion exchange because it is a low energy-intensive process and the media can be either easily regenerated or disposed. Coal bed methane (CBM) producers in the San Juan and Raton Basins of northern New Mexico and southern Colorado face this exact logistical challenge for water disposal. These wells are located in isolated areas with limited infrastructure, and lease operators are shipping produced water from wells not co-located with saltwater disposal (SWD) sites by truck to SWD sites at costs of $1-4 per barrel. Lower process costs could be realized by using an inexpensive, easy-to-install on-site process or portable process. Furthermore, treating produced water and reusing it for agricultural, rangeland, or industrial applications would turn this liability of brackish water disposal into an asset. Ion exchange with highly selective materials is a promising, alternative technique to reverse osmosis for desalinating isolated inland CBM waters.1-6 The optimization of ion-exchange processes requires the development of operations based on lowcost materials with high salt-removal efficiencies. In utilizing * Corresponding author. E-mail: [email protected]. Tel.: (505) 844-0340. Fax: (505) 844-5470.

this approach, the ion-exchange materials need to be highly selective, be robust, be inexpensive, be easy to regenerate, and not result in secondary water pollution. To that end, we have focused our studies on hydrotalcite (HTC) and a permutite-like aluminosilicate which exchange specific anions and cations, respectively.7,8 Hydrotalcites9-13 are layered double hydroxides with the general formula [M(II)1-xM(III)x(OH)2]x+[A]-‚mH2O where M(II) ) Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, and Zn2+; M(III) ) Al3+, Cr3+, Mn3+, Fe3+, Co3+, and Ga3+; and A ) Cl-, Br-, I-, NO3-, CO32-, SO42-, silicate-, polyoxometalate, and/or organic ions. The crystal structure of HTC is related to that of brucite, Mg(OH)2,7 and can be described as positively charged layers of [M(II)/M(III)/(OH)] octahedra. The positive charge is compensated for by anions located between the layers. Ion exchange in HTC occurs via two routes: (1) the classical ion-exchange process and (2) the so-called “memory affect”.11 In the classical ionexchange method, the A-HTC is dispersed in an aqueous solution containing a second ion, A′, and A′ anions partially substitute for A, forming AxA′1-x-HTC. In the “memory affect” method, the HTC is calcined and the structure collapses due to anion and water loss. The collapsed material is dispersed into an aqueous solution containing an anion, and the HTC structure recrystallizes including the “new” anion. Permutite8 is an amorphous aluminosilicate ion-exchange material. Using the permutite described in the literature as a starting point, we have developed a class of cation ion-exchange, permutite-like materials with the nominal formula Nay+xAlxSi1-xO2+y/2.14 (In describing the desalination process, we refer to the cation-exchange material as “permutite”.) Ion exchange occurs through the classical process described above. In order for the permutite to desalinate brackish waters, it must be pretreated with an acid to exchange the charge-balancing Na+ cations with H+. Our recent studies show that aluminosilicate permutite has a strong selectivity for sodium cations in aqueous solutions.14 In this present work, a desalination process15,16 for inland brackish water, in particular CBM produced waters, using ionexchange materials is described. This study includes detailed syntheses, ion-exchange capacity (IEC) measurements, regen-

10.1021/ie060138b CCC: $33.50 © 2006 American Chemical Society Published on Web 05/26/2006

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eration studies, preliminary bench-scale column studies, and economic analyses. 2. Experimental Section 2.1. Synthesis. Hydrotalcite (HTC-baseline) is prepared by a precipitation reaction at room temperature. In a typical reaction, 0.72 mol MgCl2 (99.7%, Fisher) and 0.24 mol AlCl3 (99.8%, Fisher) were dissolved in 800 mL of distilled H2O. The solution was reacted with Na2CO3 (99.1%, Fisher), and the pH was adjusted to >9.5 with an aqueous solution of 1 M NaOH (98.5%, Fisher), precipitating Mg/Al hydroxides and turning the solution into a thick opaque suspension. This mixture was allowed to age for ∼24 h at room temperature. The product was filtered and washed until the conductivity (conductivity units in equivalent NaCl, used for calibration of the instrument) of the free ions in the filtrate was ∼100 µS, a 106-fold decrease. The product was dried overnight in air at 100 °C. Subsequently, a 15 g portion of the dry HTC was calcined at 550 °C for 1 h. Permutite (baseline material) was prepared by a precipitation reaction at room temperature. In a typical reaction, 0.32 mol Al(NO3)3‚9H2O (98%, Alfa Aesar) was dissolved in distilled water, having a concentration of ∼3 M. In a separate beaker, 100 mL of distilled water was added to a 1.08 mol sodium silicate solution (PQ Corporation, Solution RU, 2.40 SiO2/Na2O wt ratio). Permutite was precipitated by the slow addition of the Al3+ solution to the silicate solution with stirring. The pH of the final solution was adjusted with 1 M HNO3 (Fisher) to between 3 and 4. The product was filtered and washed until the conductivity of the filtrate was ∼100 µS, a 106-fold decrease. The sample was dried overnight in air at 100 °C. Subsequently, a 10 g portion of the dry permutite was treated with 12 g of glacial acetic acid (Fisher) and 100 mL of distilled water. The sample was allowed to stir at 70 °C for 15 min and then filtered. The acetic acid treatment was repeated for a second time. Then, the permutite was filtered and washed until the pH of the filtrate was >4. The acid-treated sample was dried overnight in air at 100 °C. 2.2. Synthesis Scale-Up. The HTC synthesis procedure was scaled up (1) by a factor of 8 times the baseline conditions (HTC-8X) and (2) by concentrating the metal salts 3 times the baseline conditions (HTC-3X). In addition, permutite was scaled up by a factor of 5 times the baseline conditions (Permutite5X). The three scale-up syntheses, following the respective procedural steps described above, were performed in a 2 L beaker. Shear and paddle mixers, lowered from above into the beaker, were used for a more complete mixing of the large samples. 2.3. Structural Characterization. Powder X-ray diffraction (XRD) patterns were recorded at room temperature on a Siemens Kristalloflex D 500 diffractometer (Cu KR radiation, 40 kV, 30 mA; 2θ ) 5-60°, 0.05° step size, and 3 s count time) and used for crystalline phase identification. The phases were identified by comparison with the data reported in the JCPDS (Joint Committee of Powder Diffraction Standards) database. Scanning electron microscopy (SEM) were performed for analysis of particle size and morphology. The instrument used was a JEOL JSM-6300V scanning electron microscope. Samples were deposited on carbon tape and coated with 5 nm of gold to prevent charging. 2.4. Ion-Exchange Capacity Measurement. The ionexchange capacity (IEC) for HTC was measured by reacting either 50 mL of 0.1 M Na2(SO4) (aq) or 0.1 M NaCl (aq) with 1 g of calcined HTC for 15 min at 70 °C with stirring. Then, the HTC was filtered and washed. The filtrate was collected

Figure 1. Model desalination system tested with simulant Tularosa Basin (NM) brackish water.

and titrated with 0.1 M HCl (aq). The IEC, expressed as milliequivalents per gram (mequiv g-1), was calculated using the following equation:

IEC ) (MHClVHCl)/massHTC The IEC was measured three times per sample to get an average. Similarly, the IEC for permutite was measured by reacting 1 g of acid-treated permutite with 50 mL of 0.1 M NaOH (aq) for 15 min at 70 °C with stirring. Then, the permutite was filtered and washed. The filtrate was collected and titrated with 0.1 M HCl (aq). The IEC, expressed as milliequivalents per gram (mequiv g-1), was calculated using the following equation:

IEC ) [(MNaOHVNaOH) - (MHClVHCl)]/masspermutite The IEC was measured three times per sample to get an average. 2.5. Chemical Regeneration. Spent HTC was thermally regenerated at 550 and 600 °C in a box furnace under static atmosphere. Spent permutite samples were chemically regenerated by the acid-treatment procedure described above using glacial acetic acid (Fisher). 2.6. Column Studies. The model system for the desalination of brackish water is displayed in Figure 1. Loose powders of the ion-exchange materials were packed into two 40 × 2.5 cm columns. Simulant brackish water (with the following composition (ppm): Ca2+ ) 40; Mg2+ ) 26; Na+ ) 665; Cl- ) 496; SO42- ) 575; CO32- ) 420; total dissolved solids (TDS) ) 2222; pH ) 8.15) was flowed through the serial columns containing ∼50 g of HTC and 50 g of permutite at a flow rate of 4 mL min-1 using a peristaltic pump. (Under this flow rate, 100 mL of simulant was treated in 25 min.) The ion concentration in the treated water was determined by conductivity measurements. A second study was conducted on actual brackish produced water obtained from the San Juan Basin, Farmington, NM (with the following composition (ppm): Ca2+ ) 22; Mg2+ ) 17; Na+ ) 4500; Cl- ) 2200; SO42- ) 7).18 It is important to note that the sequence, anion then cation removal, may be reversed, because it is not the important factor in the desalinating of the water. Several desalination process sequences are possible using cation/anion getters for desalinating water. The best process cycle for a given application depends on several factors, such as the chemistry (particularly carbonate hardness) of the input water and the desired chemistry of the output water. For example, lime softening can be used to pretreat the brackish water to remove carbonate hardness, which precipitates as CaCO3. This pretreatment process considerably decreases the amount of anion-getter materials needed. The treated water then flows through the either the anion- or cation-exchange column, followed by the other. This process was demonstrated using produced water from Farmington, NM (see Figure 3).

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is eliminated. Furthermore, this is particularly attractive to remote-location producers, as it does not need to be monitored or need continued maintenance. Ongoing work is focused on extrudate and pelletizing studies for optimizing column flow through and on-site (well-head) water purification. 4. Conclusions

Figure 3. Model desalination system tested with brackish produced water from the San Juan Basin.17

In a similar process the water can be lime softened but treated with permutite that is exchanged with Na+ rather than H+. The water is treated until the columns are saturated with divalent ions such as Ca2+, Mg2+, and SO42-. If the brackish water contains a substantial amount of Na+ relative to Ca/Mg/Ba, then a relatively small amount of permutite could pretreat a large quantity of water prior to RO desalination, to prevent scaling and extend the membrane life. However, the use of lime softening is not practical in some desalination processes. An example of this is in cases where the Ca/Mg hardness of the water is associated with chloride or sulfate rather than carbonate. In a separate process, the cation-exchange column can be treated with NH4+ rather than H+. This might be done if the treated water is intended for crop watering or rangeland reclamation. The ion-exchange media are inexpensive to synthesize. Economic analysis indicates that this ion-exchange process is competitive with current disposal costs of $1-4 per barrel19,20 for produced water in the San Juan and Raton basins if the TDS is sufficiently low (3 800 mg/L or less).17 Lime pretreatment appears to be preferred as a means of lowering treatment cost, especially for bicarbonate-dominated waters. Costs may be lowered if untreated produced water is cut with the treated water. With the above-mentioned ion-exchange capacities, we can approximate the amount of ion-exchange material needed for treating a given amount of produced water. For the San Juan water (11 000 ppm TDS; mostly NaCl), the amount of sodium is assumed to be 4 328 ppm Na ) 4.23 (g of Na)/(kg of water) ) 0.188 equiv/(kg of water) ) 188 equiv/(metric ton of water), and the metric ton is assumed to weigh 1000 kg. On the basis of the exchange capacities, we approximate spent ion-exchange material to be 75.3 kg of permutite and 62.7 kg of HTC, per metric ton of desalinated water. This process is competitive for the desalination of produced waters because the ion-exchange process is passive and the high energy costs of pumping or heating for reverse osmosis and distillation technologies, respectively, are reduced. Further energy savings are realized because of the high ion-exchange capacity of the ion-exchange materials. The cation exchanger can be regenerated by treating with acid or ammonium salt. Alternatively, the leach-free solids produced can be disposed as landfill, employed as a soil amendment, or used as a raw material for cement production. This may be of considerable importance in the case of produced water, where on-site disposal would be a significant asset. Cost savings can be achieved in remote areas because the process is mobile. Environmental savings are realized because membrane and brackish water waste

We have presented a novel cost- and energy-efficient method of desalinating produced brackish water using inorganic media, hydrotalcite and permutite. Hydrotalcite is used to remove anions, while permutite sequesters cations, in separate steps. The resulting water is potable or can be used in rangeland or industrial applications. These ion-exchange materials exhibit ionexchange capacities (IEC) between 2.5 and 3.0 mequiv g-1. Synthesis scale-up did not adversely affect the IEC of either material. These materials have been shown to desalinate brackish waters containing high concentrations of monovalent and divalent ions. For waters with a high carbonate concentration, lime softening can be incorporated. The additional lime softening step decreases the amount of anion getter needed to treat the waters. Further cost and environmental savings could be realized from the lack of salt and brackish water side-products in this process. The salt removal in this present desalination process results in inert “dirt-like” products that are not caustic to the environment and have other applications and/or sales potentials. Acknowledgment The authors thank M. Hightower, T. Hinkebein, D. Horschel, and A. Sattler for helpful discussions. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DEAC04-94AL85000. Literature Cited (1) Prajapati, M. N.; Gaur, P. M.; Dasare, B. D. Brackish Water Desalination by a Continuous Countercurrent Ion-Exchange Technique. Desalination 1985, 52, 317. (2) Krongold, E.; Vofsi, D. Water Desalination by Ion-Exchange Hollow Fibers. Desalination 1991, 84, 123. (3) (a) Khamizov, R. Kh.; Muraviev, D.; Tikhonov, N. A.; Krachak, A. N.; Zhiguleva, T. I.; Fokina, O. V. Clean Ion-Exchange Technologies. 2. Recovery of High-Purity Magnesium Compounds from Seawater by an IonExchange Isothermal Supersaturation Technique. Ind. Eng. Chem. Res. 1998, 37, 2496. (b) Muraviev, D.; Khamizov, R. Kh.; Tikhonov, N. A.; Morales, J. G. Clean (“Green”) Ion-Exchange Technologies. 4. High-Ca-Selectivity Ion-Exchange Material for Self-Sustaining Decalcification of Mineralized Waters Process. Ind. Eng. Chem. Res. 2004, 43, 1868. (4) Schoeman, J. J.; Steyn, A. Investigation into Alternative Water Treatment Technolgies for the Treatment of Underground Mine Water Discharged by Grootvlei Proprietary Mines LTD into the Blesbokspruit in South Africa. Desalination 2001, 133, 13. (5) Oren, Y.; Rubinstein, I.; Linder, C.; Saveliev, G.; Zaltman, B.; Mirsky, E.; Kedem, O. Modified Heterogeneous Anion-Exchange Membranes for Desalination of Brackish and Recycled Water. EnViron. Eng. Sci. 2002, 19, 513. (6) Baghino, G.; Peretti, R.; Zucca, A.; Serci, A.; Fercia, M. L.; Lonis, R. Use of Natural Zeolites for Wastewater Treatment. Res. J. Chem. EnViron. 2005, 9, 11. (7) Miyata, S. Anion-Exchange Properties of Hydrotalcite-Like Compounds, Clay. Clay Miner. 1983, 31 305. (8) (a) Ramann, E.; Marz, S.; Biesenberger, K.; Spengel, A. Exchange of bases by Silicates. Z. Anorg. Chem. 1916, 95, 115. (b) Ramann, E.; Marz, S.; Biesenberger, K.; Spengel, A. Exchange of alkalis and ammonium by hydrous aluminum-alkali silicates (permutites). Z. Anorg. Chem. 1916, 29, 398. (9) Bontchev, R. P.; Liu, S.; Krumhansl, J. L.; Voigt, J.; Nenoff, T. M. Synthesis, Characterization, and Ion Exchange Properties of Hydrotalcite

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Mg6Al2(OH)16)(A)x(A′)2-x‚4H2O (A,A′ ) Cl-, Br-, I-, and NO3-, 2 g x g 0) Derivatives. Chem. Mater. 2003, 15, 3669. (10) Khan, I. A.; O’Hare, D. Intercalation chemistry of layered double hydroxides: Recent developments and applications. J. Mater. Chem. 2002, 12, 3191. (11) Fogg, A. M.; Green, V. M.; Harvey, H. G.; O’Hare, D. New separation science using shape-selective ion exchange intercalation chemistry. AdV. Mater. 1999, 11, 1466. (12) Parker, L. M.; Milestone, N. B.; Newmann, R. H. Use of hydrotalcite as an anion absorbent. Ind. Eng. Chem. Res. 1995, 34, 1196. (13) Braterman, P. S.; Xue, Z. P.; Yarberry, F. Layered Double Hydroxides. In Handbook of Layered Materialsl; Auerbach, A. M.; Carrado, K. A.; Dutta P. K., Eds.; Marcel Dekker: New York, 2004; p 373. (14) Pless, J. D.; Philips, M. L. F.; Maxwell, R. S.; Axness, M.; Nenoff, T. M. Structure-Property Relationship of Permutite-Like Amorphous Silicates, Nax+yM3+xSi1-xO2+y (M3+) Al, Mn, Fe, Y), for Ion-Exchange Reactions. Chem. Mater. 2005, 17, 5101. (15) Nenoff, T. M.; Phillips, M. L. F.; Pless, J. D. U.S. Patent Pending, filed 2004; Sandia National Laboratories Technical Advance SD-6855. (16) Pless, J. D.; Krumhansl, J. L.; Voigt, J. A.; Moore, D.; Axness, M.; Phillips, M. L. F.; Sattler, A.; Nenoff, T. M. Desalination of Brackish

Groundwaters and Produced Waters Using In-situ Precipitation; Sandia Internal Report, SAND2004-3908; Sandia National Laboratories: Albuquerque, NM, 2004. (17) Produced water provided to Sandia National Labs by Burlington Resources, San Juan Unit 32-9, Central Well disposal unit #5, 2004. (18) Pourbaix, M. Atlas of Electrochemical Equilibria on Aqueous Solutions, 2nd ed.; National Association of Corrosion Engineers: Houston, TX, 1974; p 163. (19) Evans, L.; Miller, J. E. Initial Cost Analysis of a Desalination Process Utilizing Hydrotalcite and Permutite for Ion Sequestration; Sandia Internal Report, SAND2004-5461; Sandia National Laboratories: Albuquerque, NM, 2004. (20) Donahe, R.; Hightower, M. M.; Nenoff, T. M.; Pless, J. D.; Sattler, A. R. Desalination of Produced Water by NoVel Ion-Exchange Processes; Sandia Internal Report, SAND2004-5649C; Sandia National Laboratories: Albuquerque, NM, 2004.

ReceiVed for reView February 2, 2006 ReVised manuscript receiVed April 14, 2006 Accepted April 25, 2006 IE060138B