Aluminum-Cycle Ion Exchange Process for Hardness Removal: A New

From a sustainability viewpoint, sodium exchange softening, although used widely, is under scrutiny due to its production of excess Na-laden spent reg...
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Aluminum-Cycle Ion Exchange Process for Hardness Removal: A New Approach for Sustainable Softening Jinze Li,† Suman Koner,‡ Michael German,† and Arup K. SenGupta*,† †

Department of Civil & Environmental Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, United States Jalpaiguri Govt. Engineering College, Civil Engineering Department, Jalpaiguri 735102, India

‡

S Supporting Information *

ABSTRACT: From a sustainability viewpoint, sodium exchange softening, although used widely, is under scrutiny due to its production of excess Na-laden spent regenerant and subsequent discharge to the environment. Many arid regions are introducing regulations disallowing dumping of concentrated sodium salts, the residuals from popular Na-exchange softening. The sodium content of the softened water is, also, always higher than in the feed, which poses a dietary health concern when used for drinking or cooking. An efficient, easy-to-operate hardness removal process with reduced sodium in both the treated water and in the spent regenerant is an unmet global need. Use of a cation exchange resin in Al3+-form for hardness removal, that is, exchange of divalent Ca2+ or Mg2+ with trivalent Al3+, is counterintuitive, and this is particularly so, because the aluminum ion to be exchanged has higher affinity than calcium. Nevertheless, ion exchange accompanied by precipitation of aluminum hydroxide allows progress of the cation exchange reaction leading to hardness removal. Experimental results demonstrated that calcium can be consistently removed for multiple cycles using a stoichiometric amount of AlCl3 as the regenerant. The process essentially operates at the maximum possible thermodynamic efficiency: removal of one equivalent of Ca2+ corresponds to use of one equivalent of Al3+ as a regenerant. During the Al-cycle process there is no increase in Na+ concentration and partial reduction in the total dissolved solids (TDS) of the treated water. It is noteworthy that the ionexchange resin used, components of the fixed-bed column and operational protocol are nearly the same as traditional softening processes on Na-cycle. Thus, existing Na-cycle systems can be retrofitted into Al-cycle operation without major difficulty.

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INTRODUCTION

2(R − SO−3 )Na + + Ca 2 + → (R − SO−3 )2 Ca 2 + + 2Na + (1)

Many industrial and municipal unit operations require removal of hardness, that is, reduction in dissolved calcium and magnesium concentration, to avoid scaling on heat transfer equipment or membranes and to decrease consumption of detergents and chemicals in cooling and washing water. Removal of hardness from water is a treatment or pretreatment commonly referred to as softening. The two universally practiced processes for softening or hardness removal are lime softening and ion exchange. Lime softening produces voluminous sludge to be settled, dewatered and disposed of, and the softened water requires post-treatment for pH adjustment and turbidity removal. Understandably, the development of single-step Na-cycle cation exchange softening that does not produce any sludge nor need any post-treatment was hailed as a major breakthrough and adopted universally nearly five decades ago.1−5 In cation exchange softening, polymeric strong acid cation exchange resin in Na-form (SACNa) replaces one equivalent of hardness (i.e., Ca2+) with one equivalent of Na+ in accordance with the following reaction: © XXXX American Chemical Society

Thus, the sodium content of the treated water is always higher than in the feed, which poses a health concern when used for drinking or cooking, especially for the growing numbers of people with hypertension.6−8 Concentrated brine (5−12% w/v) is used to regenerate the exhausted cation exchange resin back to Na-form, in accordance with the following reaction (where n = 6−12): (R − SO−3 )2 Ca 2 + + n NaCl → 2(R − SO−3 )Na + + Ca 2 + + (n − 2)Na + + nCl− (2)

The consequent environmental impact caused by the high TDS, spent regenerant, has surfaced as a major concern in Received: June 17, 2016 Revised: August 27, 2016 Accepted: October 4, 2016

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Environmental Science & Technology drought-prone and environmentally sensitive ecosystems. More than 25 communities in California face bans or restrictions on the use of SAC-Na softeners because of the risks to agriculture and downstream water quality caused by the spent brine, especially during recent droughts.9−11 Frequent release of concentrated brine from ion exchange softeners is a global concern, for example, growing high rise complexes in urban areas. Sodium-Free Approaches and Alternatives to NaCycle Softening. In the mid-1960s, the Sirotherm process was introduced as a novel ion exchange process that relied on thermal regeneration,12,13 but the commercial scale-up was a failure. Similar thermal regeneration processes using hydrogels have been investigated, but currently have limited capacity compared to commercial resins.14 Weak-acid cation (WAC) exchange resin can be a possible alternative to minimize Naladen spent regenerant, but efficient regeneration of WAC requires mineral acids. From a process safety viewpoint, transport, storage and handling of acid poses safety hazards that often make the softening process uneconomical under the current regulatory environment. Attempts have also been made to remove hardness through electrocoagulation, but the extent of hardness removal is far less than traditional softeners and is pH sensitive. Scaling on aluminum electrodes and poor performance of iron electrodes also complicates feasibility with low cost electrodes.15,16 In an attempt to identify an environmentally benign regenerant, carbon dioxide-sparged water has been successfully used for removal of temporary hardness using weak acid ion exchange fibers (IX-Fibers).17−19 However, high cost and relative unavailability of IX-fibers are the primary impediments against rapid deployment of this softening process. Many novel inorganic, polymeric and hybrid ion exchangers have been developed and commercially deployed during the last two decades to remove contaminating ions selectively, for example, arsenic, boron, chromate, copper, fluoride, nitrate, phosphate, radium, synthetic organics, trace metals, etc.20−34 However, most of them lack high selectivity, capacity or regeneration efficiency for calcium removal. It is also true that many companies have introduced “salt-free softeners”, advertising nonchemical water softening, but these items often have dubious claims, for example, magnetic softeners.35 An efficient, easy-to-operate hardness removal process with reduced sodium in both treated water and in the spent regenerant is an unmet global need that demands technological innovation to mitigate this water treatment shortcoming. Underlying Scientific Aprroach of Al-Cycle Cation Exchange. In the proposed new cation exchange softening process, typical monovalent Na+ would be replaced by a trivalent cation, e.g., Al3+, as the presaturated form of the resin. Aluminum salts are inexpensive, readily available, exist in the + III oxidation state over a wide range of pH and pe values, and aluminum hydroxide has a very low solubility product (Ksp = 1.3 × 10−33). In addition, the kinetics of labile aluminum hydroxide precipitate formation are very rapid.36 Thus, for dilute solutions undergoing hardness removal, exchange of Ca2+ with Al3+ was expected to proceed with simultaneous precipitation of Al(OH)3(s) and consequent reduction in alkalinity, as follows: 2(R −

SO−3 )3 Al3 +

→ 3(R −

+ 3Ca

SO−3 )2 Ca 2 +

(4)

6H+ + 6HCO−3 → 6H 2O + 6CO2 (g)↑

(5)

Overall 2(R − SO−3 )3 Al3 + + 3Ca 2 + + 6HCO−3 → 3(R − SO−3 )2 Ca 2 + + 2Al(OH)3 (s) ↓ +6CO2 (g)↑ (6)

Note that in the overall ion exchange reaction, aluminum is precipitated as solid Al(OH)3(S) and HCO−3 is partially removed as CO2 gas. Consequently, in accordance with the Le Châtelier Principle, the reaction will proceed to the right, although divalent Ca2+ possesses lower affinity than trivalent Al3+. Once the capacity is exhausted, the cation exchanger is regenerated stoichiometrically with dilute AlCl3 on an equivalent basis, as follows: 3(R − SO−3 )2 Ca 2 + + 2Al3 + → 2(R − SO−3 )3 Al3 + + 3Ca 2 +

(7)

Stoichiometric or highly efficient regeneration is possible because of higher Al3+ affinity, than Ca2+, for cation exchange sites, that is, the aluminum−calcium separation factor is greater than one, αAl/Ca > 1. Figure 1 illustrates individual steps of the conceptualized process and the following points are worth noting:

Figure 1. A schematic of Al3+-Ca2+ exchange followed by Al(OH)3 precipitation with hydrolysis leading to partial TDS reduction.

(i) Spent aluminum regenerant contains no sodium and its disposal is less environmentally stressful than for typical waste sodium brine regenerant; (ii) Hardness removal can happen on an equivalent basis, that is, one equivalent of Ca2+ is removed with an equivalent of Al3+ regenerant; (iii) Sodium concentration in the treated water does not increase; and (iv) There is an overall reduction in TDS of the treated water.

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GOALS OF THE RESEARCH The significant contrast of the proposed concept from traditional practice lies in using a more favorable trivalent cation (Al3+) as the preloaded counterion in the cation

2+

+ 2Al3 +

2Al3 + + 6H 2O → 2Al(OH)3 (s) ↓ +6H+

(3) B

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approximate composition: pH 7.5 ± 0.5, [Na+] = 9.8 mequiv/L, [Ca2+] = 5 mequiv/L, [HCO‑3] = 5 mequiv/L, [SO24−] = 2.1 mequiv/L and [Cl−] = Balance. The exhausted cation exchanger was regenerated using either 3% AlCl3 (w/v) for SAC-Al or 5% NaCl (w/v) for SAC-Na, respectively. A schematic of the laboratory column setup is provided in Supporting Information #2. For hardness removal with SAC-Al resin, synthetic feedwater and regenerant were first passed through the column for four cycles of exhaustion-regeneration to arrive at steady state, after which the calcium effluent history remained consistent during further cycles. Inlet and outlet water chemistry were compared during operation, along with internal changes of morphology of the resin between the service cycle and the regeneration cycle. The waste regenerant was analyzed to determine the efficiency and effectiveness of regeneration. During Al-cycle runs, some carbon dioxide accumulated at the top of the column and was vented at the end of each service cycle. For comparison, cyclic runs were also carried out in Na-form using 5% NaCl as the regenerant. Chemical Analysis and Resin Characterization. Calcium and sodium were analyzed by a PerkinElmer AAnalyst 200 Atomic Absorption Spectrometer (AAS). Aluminum was analyzed using the Eriochrome Cyanine R Method with a Hach DR5000 UV−vis spectrophotometer (Method 8326). Sulfate (0−70 mg/L) was analyzed using SulfaVer4 sulfate powder pillows reagent set by Hach (Method 8051) and the Hach DR5000. Chloride was analyzed using Dionex Ion Chromatography (IC model ICS-1000) with an IonPac AS14 column; the eluent for the IC was 5 mM NaHCO3. To monitor the changes in cation exchanger beads between service cycle and regeneration, slices of Purolite C145 (parent, exhausted, and regenerated) were prepared using microtomy and characterized by scanning electron microscopy with energy dispersive X-ray (SEM-EDX) spectroscopy (Model Hitachi JSM-4300). Calcium and aluminum mapping of sliced beads was carried out for Al-cycle softener resin to determine the progression and reversibility of loading of the beads at different stages of the process.

exchanger for removing divalent hardness ions, for example, Ca2+ and/or Mg2+. Supporting Information #1 provides Al3+Ca2+ isotherm data substantiating higher affinity of aluminum over calcium over a wide range of concentrations. From an application viewpoint, the process configuration of the softening process remains the same, except that the cation exchange resin is used in Al3+-form instead of Na+-form. It is worth noting that Fe3+ may also be a candidate instead of Al3+ for the proposed approach, but the equivalent weight of Fe3+ is double the equivalent weight of Al3+. The objectives of the study were to validate the long-term performance of the Alform softening process over multiple cycles of exhaustionregeneration without excess brine or mineral acid as a regenerant through monitoring: • Regeneration efficiency (RE) of the process versus process stoichiometry, that is, meq of Al3+ needed to remove one meq of Ca2+, in comparison to the current Na+-exchange softening process; • Chemistry of the treated water, especially sodium concentration and TDS; and • Morphology of the cation exchanger beads for consistent Ca2+ and Al3+ content changes during regeneration and treatment.

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MATERIALS AND METHODS Ion Exchange Column Runs and the Cation Resin. Fixed-bed column testing of hardness removal was carried out using 11 mm diameter glass columns (ACE Glass, Inc., Vineland, NJ), constant flow rate pumps (Fluid Metering International, Syosset, NY) and fractional collectors (Eldex, Napa, CA). Purolite Co. (Philadelphia, PA) provided macroporous strong-acid cation exchange (SAC) resins with sulfonic acid functional groups (Purolite C145). However, no endorsement is implied; similar resins are available from other manufacturers. Table 1 provides the salient properties of the Table 1. Salient Properties of Purolite C145, a Macroporous Strong Acid Cation Exchange Resina

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RESULTS Softening by SAC-Na Resins. SAC-Na was used to treat synthetic groundwater over two consecutive cycles of exhaustion-regeneration (5% NaCl). Figure 2A has the calcium and sodium effluent histories for the SAC-Na softening process. For approximately 300 bed volumes (BVs), calcium was removed and sodium was elevated, that is, reduction in calcium is always accompanied by an equivalent increase in sodium concentration. Two more cycles (not included in Figure 2A) of operation yielded identical results and confirmed reproducibility of the process. After approximately 350 BVs, effluent concentrations were identical to the influent, signaling capacity exhaustion of the SAC bed. Figure 2B presents calcium elution during regeneration with 5% NaCl, while Figure 2C depicts the stoichiometric regeneration efficiency (RE), where the dimensionless ratio of the equivalents of regenerant (Na+) used versus equivalents of Ca2+ recovered (Na+/Ca2+) was plotted against bed volumes. Note that 86% of removed calcium was eluted after 10BVs of 5% NaCl regenerant, corresponding to a Na+/Ca2+ recovery ratio of 7.5. It is possible to recover a portion of the spent regenerant with lower calcium concentration and reuse it to attain higher regeneration efficiency. Nevertheless, according to

a

Additional information available from the manufacturer’s technical brochure.

cation exchange resin used in testing. The empty bed contact time (EBCT) and superficial liquid velocity (SLV) were recorded for each experimental column run. Previous studies with polymeric ligand exchangers confirmed that no significant channeling occurs under such operating conditions.37,38 SAC resin was loaded in either aluminum or sodium form by passing the chloride salt of the respective cation. The synthetic feedwater solution used in the study had the following C

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Figure 3. Effluent history of (A) sodium and calcium during SAC-Al treatment over two cycles; and (B) conductivity during SAC-Na and SAC-Al treatment.

Figure 2. Effluent history during operation of SAC-Na resin for (A) sodium and calcium during the service cycle; (B) calcium during the regeneration cycle (5% NaCl); and (C) regeneration efficiency on an equivalent basis.

widely used practices in softening, the ratio of regenerant to recovered hardness is rarely less than 4 and often exceeds 6, depending on the treated water quality requirements.1−3 Softening with SAC-Al. Figure 3A shows two consecutive effluent histories during SAC-Al treatment using Purolite C145. All other conditions were identical as the Na-cycle column runs presented in Figure 2A, namely, influent composition, empty bed contact time (EBCT) and superficial liquid velocity (SLV). Two observations from Figure 3A are worth noting: first, calcium, although removed for nearly 400BVs, broke through early and gradually; second, sodium in the treated water never exceeded the influent concentration. Note, during the first 50BVs the sodium concentration in the treated water was slightly less than that in the feedwater. Figure 3B compares the conductivity of the feed and treated water between the SAC-Na and SAC-Al processes under identical conditions, a surrogate parameter for total dissolved solids. Treated water conductivity was noticeably lower after SAC-Al versus the influent water or after SAC-Na treatment. Conductivity decreased from an influent of 1750 μS to an average of 1300 μS in the SAC-Al treated water- a 25% reduction. Two additional column runs, each followed by AlCl3 regeneration, yielded nearly identical effluent or breakthrough histories with respect to calcium, sodium and conductivity. SAC-Al Regeneration and SEM-EDX Mapping. Figure 4A shows elution of calcium during two successive regener-

Figure 4. Effluent history of (A) calcium and (B) pH during SAC-Al regeneration by 3% AlCl3.

ations by 3% AlCl3 for 6BVs; the bed was then rinsed with 3BVs of the calcium-containing influent solution. During both regenerations, 86% of calcium on the bed was recovered and the effluent pH followed identical trends. As shown in Figure 4B, the pH value dropped to pH 3.5 during regeneration for 6BVs, followed by an increase during rinsing. Note that when pH reached the minimum value (pH 3.5), regeneration was completed: pH can thus be a surrogate indicator for complete regeneration of SAC-Al. In an attempt to reconcile the experimental results with the underlying scientific hypothesis, slices of both exhausted and regenerated Purolite C145 were characterized by SEM-EDX D

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Figure 5. SEM-EDX mapping and spectroscopy of a cation exchange resin bead during Al-cycle operations (A) after exhaustion by synthetic hard water and (B) after regeneration by 3% AlCl3.

Al(OH)3 (s) + 3H+ → Al3 + + 3H 2O

mapping. Figure 5A mapping of the exhausted resin shows Ca (green) throughout the entire resin, while Al predominates in the periphery; this observation was supported by the EDX spectra. In contrast, the regenerated resin in Figure 5B shows an absence of calcium in the SEM-EDX mapping and spectra. Ion exchange sites throughout the exchange cation resin were converted into Al-form through Ca2+-Al3+ exchange. Aluminum precipitates were present in the macropores of the outer periphery of the cation exchange resin beads in both the exhausted and regenerated SAC-Al. But, aluminum precipitation was never observed in the aqueous phase of the column as white precipitated solids or as impeding the effluent flow. The cyclic process was continued for five sorption−desorption cycles after reaching steady state. Regeneration Efficiency and Calcium Removal Capacity. Figure 6A shows the regeneration efficiency, RE, that is, equivalents of regenerant consumed per equivalent of hardness, for both SAC-Na and SAC-Al softening processes. While RE was nearly 7.5 for Na-cycle softening, it was close to unity for the Al-cycle. Stoichiometrically, an RE value of unity is the thermodynamic limit for ion exchange processes and Figure 6A validates that no excess regenerant was required during regeneration with AlCl3. Figure 6B shows the comparison of calcium removal capacities between the Na- and Al-cycle processes. As anticipated from the column effluent histories, the capacity was approximately 20% greater for SAC-Na exchange processes than for SAC-Al. Figure 6C shows the mass balance during AlCl3 regeneration: there was an absence of any aluminum accumulation in the bed, that is, the process was at steady state after several cycles of operation, as there was no buildup of aluminum. Aluminum chloride regenerant had a pH of 2.9−3.0, thus the free hydrogen ion (H+) concentration was sufficient to dissolve previously precipitated aluminum hydroxide present in the macropores. Once dissolved, aluminum ions from the precipitates enhanced regeneration by displacing Ca2+ from the ion exchange sites:

(8)

3(R − SO−3 )2 Ca 2 + + 2Al3 + → 2(R − SO−3 )3 Al3 + + 3Ca 2 +

(9)

The aluminum content of the cation exchange resins was consistent after every exhaustion cycle (80−85 mg Al/g resin). For further visual confirmation about the existence of aluminum hydroxide precipitates in the pores, SEM images were taken of the virgin parent bead and compared with the exhausted resin bead, Supporting Information #3. Comparison of the SEM images show that colloidal aluminum hydroxide precipitates accumulated in the macropores of the SAC-Al resin beads. Minor aluminum leakage, if any, in the beginning of the service cycle was completely arrested by having a small cation exchange column in sodium form as post-treatment, Supporting Information #4; the small SAC-Na column also eliminated calcium leakage.

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DISCUSSION Process Validation. Use of a cation exchange resin in Al3+form for hardness removal is counterintuitive because aluminum ions to be exchanged have higher affinity than calcium for SAC (See Supporting Information #1). However, ion exchange accompanied by precipitation of aluminum hydroxide forced the cation exchange reaction and hardness removal to completion, Figure 3 and 4. Using stoichiometric amounts of AlCl3 as a regenerant, calcium was consistently removed for multiple cycles. Thus, no excess TDS would be discharged to the environment and the process can be operated nearly at the highest possible thermodynamic efficiency. Other associated benefits of the process over traditional Na-exchange softening are (i) no increase in Na+ concentration in the treated water (Figure 3A); (ii) partial reduction of the total dissolved solids (TDS) of the treated water (Figure 3B); and, most importantly, (iii) the complete elimination of Na+ in the spent regenerant. It may be noted that during the early part of the service cycle, Na+ concentration in the treated water was E

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parallel with hardness removal during the early stages of the column run: (R − SO−3 )3 Al3 + + 3Na + + 3HCO−3 ↔ 3(R − SO−3 )Na + + Al(OH)3 (s) ↓ +3CO2 (g)↑ (10)

The SEM-EDX mapping and spectroscopy, and the aluminum mass balance (Figures 5A−C, respectively) demonstrated the dynamics of hardness removal, regeneration and steady state behavior of the process. While aluminum exchanged gradually with calcium from the cation exchange sites during the service cycle, the opposite was true during stoichiometric regeneration with favorable breakthrough. No precipitate formation was observed within the aqueous-phase of the column after multiple cycles. Also, any increase in head loss and consequent decrease in the flow rate during lengthy column runs was absent altogether during multiple cycles of exhaustion and regeneration. The capacity of the cation exchange process during Al-cycle was lower than during Na-cycle and the hardness breakthrough was gradual (Figures 3A and 6B) because the isotherm for calcium−aluminum exchange was unfavorable, that is, αCa/Al < 1.0. This observation of calcium breakthrough is in agreement with the theory of ion exchange chromatography.22,39 For applications where low hardness leakage is a requirement for the treated water, the SAC-Al process could be considered a pretreatment prior to a SAC-Na column in series. Such a hybrid process would still have significantly lower regenerant consumption because the bulk of the hardness would be removed in the first column where stoichiometric regeneration would be possible. The proposed Al-cycle softening process is a viable alternative to the current practice when spent brine disposal poses insurmountable environmental challenge and high sodium in the treated water is unacceptable. Sustainability Issues and New Opportunities. One of the striking findings of the investigation was that a traditional cation exchange resin, presaturated with a trivalent ion (e.g., Al3+), can remove divalent cations (e.g., Ca2+ hardness) and was amenable to efficient regeneration. From a sustainability viewpoint, results of the study are particularly attractive because generation of excess spent regenerant (e.g., SAC-Na processes) or voluminous sludge (e.g., lime softening) was avoided. Equally important, the ion exchange resins, fixed-bed column components and operational protocol are nearly identical to existing SAC-Na processes: existing systems could be retrofitted with minimal expense or effort. Although not presented in the study, the residuals of AlCl3 present in the spent regenerant could be easily neutralized with calcite or limestone, as shown below. The precipitation reaction is irreversible and would eliminate any aluminum in the spent regenerant.

Figure 6. Comparison over multiple cycles of operation of (A) regeneration efficiency between SAC-Na and SAC-Al; (B) ion exchange capacity of SAC-Na and SAC-Al; and (C) mass balance of aluminum regenerant influent and effluent.

2Al3 + + 3CaCO3(s) + 3H 2O → 3Ca 2 + + 2Al(OH)3 (s) ↓ +3CO2 (g)↑

(11)

The use of FeCl3 as a regenerant to create SAC in Fe3+-form is an option, instead of AlCl3. However, the equivalent weight of Al(III) is half of Fe(III), that is, 9 vs 18.6. Thus, the mass of ferric salt needed for regeneration is greater than the amount of Al(III) salt, all other conditions remaining identical. Figure 7 provides an overview of the proposed Al-cycle softening process, namely, the absence of sodium in the spent

Figure 7. Schematic overview of water hardness treatment by SAC-Al and regeneration with AlCl3.

slightly less than that in the influent (Figure 3A). This observation suggests that the following reaction also occurs in F

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(11) California Ban on Salt-Based Water Softeners | Update | Pelican Water. http://www.pelicanwater.com/blog/update-california-ban-saltbased-water-softeners/. (12) Weiss, D. E.; Bolto, B. A.; McNeill, R.; Macpherson, A. S.; Siudak, R.; Swinton, E. A.; Willis, D. The Sirotherm Demineralisation ProcessAn Ion-Exchange Process with Thermal Regeneration. Part I. J. Inst. Engrs. 1965, 37, 193. (13) Jackson, M. B.; Bolto, B. A. Amphoteric composite resins and method of preparing same by polymerization of a two-phase dispersion of monomers. U.S. Patent 4,134,815, Jan. 16, 1979. (14) Custers, J.; Stemkens, L.; Sablong, R.; Asseldonk, D.; Keurentjes, J. Salt-free softening by thermo-reversible ion-adsorbing hydrogels. J. Appl. Polym. Sci. 2014, 131 (9), 40216. (15) Liao, Z.; Gu, Z.; Schulz, M. C.; Davis, J. R.; Baygents, J. C.; Farrell, J. Treatment of cooling tower blowdown water containing silica, calcium and magnesium by electrocoagulation. Water Sci. Technol. 2009, 60 (9), 2345−2352. (16) Schulz, M. C.; Baygents, J. C.; Farrell, J. Laboratory and Pilot Testing of Electrocoagulation for Removing Scale-Forming Species from Industrial Process Waters. Int. J. Environ. Sci. Technol. 2009, 6 (4), 521−6. (17) Greenleaf, J. E.; SenGupta, A. K. Environmentally benign hardness removal using ion-exchange fibers and snowmelt. Environ. Sci. Technol. 2006, 40 (1), 370−6. (18) Greenleaf, J. E.; Lin, J. C.; SenGupta, A. K. Two novel applications of ion exchange fibers: Arsenic removal and chemical-free softening of hard water. Environ. Prog. 2006, 25 (4), 300−11. (19) Padungthon, S.; Greenleaf, J. E.; SenGupta, A. K. Carbon dioxide sequestration through novel use of ion exchange fibers (IXfibers). Chem. Eng. Res. Des. 2011, 89 (9), 1891−900. (20) Cumbal, L.; SenGupta, A. K. Arsenic removal using polymer supported hydrated iron(III) oxide nanoparticles: Role of Donnan membrane effect. Environ. Sci. Technol. 2005, 39 (17), 6508−6515. (21) Sarkar, S.; Blaney, L. M.; Gupta, A.; Ghosh, D.; SenGupta, A. K. Use of ArsenXnp, a hybrid anion exchanger, for arsenic removal in remote villages in the Indian subcontinent. React. Funct. Polym. 2007, 67 (12), 1599−611. (22) SenGupta, A. K.; Lim, L. Modeling chromate ion-exchange processes. AIChE J. 1988, 34 (12), 2019−29. (23) Zhao, D.; SenGupta, A. K.; Stewart, L. Selective removal of Cr (VI) oxyanions with a new anion exchanger. Ind. Eng. Chem. Res. 1998, 37 (11), 4383−7. (24) Padungthon, S.; Li, J.; German, M.; SenGupta, A. K. Hybrid anion exchanger with dispersed zirconium oxide nanoparticles: a durable and reusable fluoride-selective sorbent. Environ. Eng. Sci. 2014, 31 (7), 360−72. (25) Guter, G. A. Adsorption with crosslinked copolymer of monovinyl aryl compound and polyolefin. U.S. Patent 4,479,877, Oct 30, 1984. (26) Hatch, M. J. Removal of Metal Ions from Aqueous Medium using a Cation-Exchange Resin having Water-Insoluble Compound Dispersed Therein. Patent EP0071810 A1. Feb 16, 1983. (27) Zhao, D.; SenGupta, A. K. Ultimate removal of phosphate from wastewater using a new class of polymeric ion exchangers. Water Res. 1998, 32 (5), 1613−25. (28) Blaney, L. M.; Cinar, S.; SenGupta, A. K. Hybrid anion exchanger for trace phosphate removal from water and wastewater. Water Res. 2007, 41 (7), 1603−13. (29) Landry, K. A.; Boyer, T. H. Diclofenac removal in urine using strong-base anion exchange polymer resins. Water Res. 2013, 47 (17), 6432−44. (30) Li, P.; SenGupta, A. K. Genesis of selectivity and reversibility for sorption of synthetic aromatic anions onto polymeric sorbents. Environ. Sci. Technol. 1998, 32 (23), 3756−66. (31) Li, P.; SenGupta, A. K. Sorption of hydrophobic ionizable organic compounds (HIOCs) onto polymeric ion exchangers. React. Funct. Polym. 2004, 60, 27−39. (32) Zhang, H.; Shields, A. J.; Jadbabaei, N.; Nelson, M.; Pan, B.; Suri, R. P. Understanding and modeling removal of anionic organic

regenerant and the partial reduction of TDS and constant sodium concentration in the treated water. This new cation exchange softening process using strong acid cation exchange resin in Al-cycle, as presented in this study, overcomes key shortcomings of the traditional Na-exchange process. Various attributes of the process now warrant further investigation and validation through field-scale studies.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b03021. (1) an overview of ion exchange separation factor and selectivity coefficient between aluminum−calcium; (2) a process flow diagram of the experimental setup; (3) SEM images of virgin SAC-Na resin and exhausted SAC-Al resin; and, (4) SAC-Al effluent treatment by a smaller SAC-Na column (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Partial financial support received from Pennsylvania Infrastructure Technology Alliance (PITA) is gratefully acknowledged. S.K. received a Raman postdoctoral fellowship from the University Grants Commission (UGC), India. Comments from reviewers helped refine and improve the final version of the paper.

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DOI: 10.1021/acs.est.6b03021 Environ. Sci. Technol. XXXX, XXX, XXX−XXX