Hybrid Ion Exchange Desalination (HIX-Desal) of Impaired Brackish

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Hybrid Ion Exchange Desalination (HIX-Desal) of Impaired Brackish Water Using Pressurized Carbon Dioxide (CO2) as the Source of Energy and Regenerant Hang Dong, Chelsey S Shepsko, Michael S German, and Arup K. Sengupta Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.8b00487 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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Environmental Science & Technology Letters

Hybrid Ion Exchange Desalination (HIX-Desal) of Impaired Brackish Water Using Pressurized Carbon Dioxide (CO2) as the Source of Energy and Regenerant Hang Dong1, Chelsey S. Shepsko1, Michael German1 and Arup K. SenGupta1* 1

Department of Civil and Environmental Engineering, Lehigh University, 1 W. Packer Ave, Bethlehem, PA, 18015

ABSTRACT Reverse osmosis (RO) and electrodialysis (ED) are truly the only water desalination processes currently in practice for the entire range of total dissolved solids (TDS) from 400 – 40,000 mg/L. For high recovery of 80% or more, membrane processes are energy intensive even for a feed water with a TDS of 1000 mg/L and demand significant pretreatment to avoid precipitation and consequent membrane fouling. In this study, we present for the first time a hybrid ion exchange desalination (HAIX-Desal) process that does not require any semi-permeable membrane and can desalinate lean brackish water (TDS≤1500 mg/L) using CO2 as the sole source of energy and chemical regenerant. Hybrid anion exchanger with dispersed ZrO2 nanoparticles (HAIX-NanoZr) and a shellcore weak-acid cation exchange (SC-WAC) resin form the heart of the process. Carbon dioxide or CO2 at 10 atmosphere pressure is the only chemical needed to sustain the process. In contrast to conventional deionization plant, the anion exchanger, i.e., HAIXNanoZr precedes the cation exchanger or SC-WAC to take advantage of the unique carbonate chemistry for desalination. CO2 serves concurrently as both an acid (i.e., H2CO3) and a base (HCO3-) for the HIX-Desal process. Municipal secondary wastewater (Bethlehem, PA) and synthetic brackish water were used in the experimental study to validate the basic premise of the process.

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INTRODUCTION Increased population and a gradual decrease in fresh water supply are propelling societies globally to rethink strategies to sustain potable, industrial and agricultural water supply to meet the current and future demands. Climate variability has aggravated the crisis further by adversely impacting the reliability of existing fresh water resources. Treated municipal wastewater in urban regions, inland brackish groundwater in arid areas and industrial cooling tower blowdowns are water resources that are large, near to the consumption centers and quite insulated from climate change effects. Transforming this huge ‘impaired water’ body into ‘usable’ water offers new opportunities to mitigate water shortages globally. For most municipal wastewaters, the total dissolved solids (TDS) ranges from 400-1000 mg/L. Also, many underground lean brackish waters and large volumes of cooling tower blowdowns from electric power utilities and process industries often contain TDS around 1000 mg/L of TDS1-5. These impaired waters are worthy candidates to replenish the diminishing fresh water supply through appropriate technology innovation. For all waters with TDS above 400 mg/L, semi-permeable RO membrane processes are universally practiced but at high recovery, they suffer from two major shortcomings. For over 80% recovery, the solute concentration at the membrane interface is likely to be nearly 10 times greater than feed water due to solute rejection associated with concentration polarization. Consequently, osmotic pressure to be overcome at the membrane interface will correspondingly rise, thus greatly increasing the energy for desalination at high recovery. Secondly, sulfate to chloride ratio is significantly greater in brackish water compared to sea water. Thus, poorly soluble inorganic salts (e.g., calcium


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or barium salts of sulfate, carbonate, and also phosphate for municipal waste water) are likely to precipitate at the membrane-water interface resulting in membrane fouling. Feed water pre-treatment including dosing of anti-scaling agent is, therefore, a common practice for brackish water desalination when high recovery is to be achieved. In California, Orange County Water District (OCWD) and the city of San Diego currently treat and recover municipal wastewater with total dissolved solids (TDS) below 1000 mg/L. In OCWD, the reverse osmosis step of the treatment plant is primarily responsible for an energy consumption of 1.2-1.5 kWh per 1000 liters of treated water5. Understandably, an alternate process, that avoids extensive pre-treatment and offers energy efficiency at high recovery, is highly desirable. The general objective of the paper is to present a novel hybrid ion exchange desalination (HIX-Desal) process that can achieve high recovery using carbon dioxide (CO2) alone and thus producing no secondary waste. For municipal wastewater, the proposed process can also recover phosphate from municipal wastewater with high purity.

Hybrid Ion Exchange Desalination (HIX-Desal): Underlying Scientific Concept Upon dissolution in water, carbon dioxide or CO2 produces carbonic acid (H2CO3) which can further dissociate, as shown below, into bicarbonate (HCO3-), often referred to as alkalinity due to its acid neutralizing capacity: CO2 (g) + H2O ↔ H2CO3

pKH = 1.41 at 20oC

(1)

H2CO3 ↔ H+ + HCO3-

pKa1 = 6.38 at 20oC

(2)

Thus, in principle, CO2 produces a unique opportunity to be used concurrently as both an acid (i.e., H2CO3) and a base (HCO3-) through process innovation. In a traditional



demineralization process with ion exchange resins, a cation exchanger in H-form is followed by an anion exchanger in OH-form and the process warrants use of mineral acids (HCl or H2SO4) and alkali (NaOH) for regeneration. In HIX-Desal process, a recently developed hybrid anion exchanger with dispersed zirconium oxide nanoparticles (HAIXNanoZr) possessing dual functional groups, is used6-9. While covalently attached quaternary ammonium functionalities provide anion exchange sites, zirconium oxide nanoparticles can selectively remove phosphate, a trace anion present in every municipal wastewater. Fluoride or arsenic, if present as contaminants in brackish groundwater, may also be selectively removed by HAIX-NanoZr. For selective ion exchange and desorption, intraparticle diffusion is the critical ratelimiting step9,10. A shell-core weak-acid cation resin (SC-WAC) has functional groups residing primarily in the periphery and thus intraparticle diffusion path length is relatively short. An innovative combination of SC-WAC and HAIX-NanoZr may offer a treatment synergy that can significantly reduce TDS with minimal energy requirement while simultaneously removing and recovering phosphate from the wastewater. Pressurized carbon dioxide (CO2) is the only chemical needed to sustain the process. Contrary to traditional deionization, an anion exchanger precedes a cation exchanger in HIX-Desal.


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FIGURE 1. A) A schematic of HIX-Desal process illustrating both desalination and CO2 regeneration step; B) Preferred sorption of phosphate onto zirconium oxide sorption sites through formation of inner sphere complexes.



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The schematic of a two-bed HIX-Desal process that can attain dissolved solids reduction and selective phosphate removal with CO2 as the sole regenerant is illustrated in Figure 1A. Considering sulfate and calcium as the representative anion and cation present in secondary waste water, the process- both service and CO2-aided regenerationare presented as follows: Service cycle: Column 1: Anion ―

2R + HCO3― + SO42- (aq) + Ca2+ (aq) → (R + )𝟐SO24 + 2HCO3- (aq) + Ca2+ (aq)

(3)

Column 2: Cation 2RCOOH + Ca2+(aq) + 2HCO3- (aq) → (RCOO ― )𝟐Ca2 + + 2H2O + 2CO2 ↑

(4)

Service overall: ―

2R + HCO3― + 2RCOOH + SO42- + Ca2+ →(R + )𝟐SO24 + (RCOO ― )𝟐Ca2 + + 2H2O + 2CO2 ↑ (5) Regeneration cycle: 2CO2 (g) + 2H2O ↔ 2H2CO3 Column 2: Cation

(RCOO ― )𝟐Ca2 + + 2H2CO3 ↔ 2RCOOH + Ca2+ + 2HCO3-

(6)

Column 1: Anion ―

(R + )𝟐SO24 + 2HCO3- ↔ 2R + HCO3― + SO42-

(7)

Regeneration overall: ―

(RCOO ― )𝟐Ca2 + + (R + )𝟐SO24 + 2CO2 (g) + 2H2O ↔ 2RCOOH +2R + HCO3― + Ca2+ + SO42- (8) 6 ACS Paragon Plus Environment

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Note that electrolytes – both cations and anions - are removed from the aqueous phase during the service cycle (equation 5) and desorbed by CO2 alone as the sole regenerant (equation 8). In a typical treated municipal wastewater after secondary treatment, phosphate is a trace contaminant (often less than 5 mg/L) responsible for eutrophication and must be removed for potable or industrial applications. Hydrated zirconium oxide (HZrO) nanoparticles dispersed within HAIX are selective toward anionic ligands including phosphate9. Phosphate sorption mechanism onto HZrO surfaces through concurrent Lewis Acid-Base and Coulombic interaction is presented in Figure 1B. Salt-free softening and demineralization by CO2, although conceived and investigated earlier with traditional ion exchange resins and ion exchange fibers, suffer from major shortcomings11-15. Also, inability to remove phosphate or other contaminants selectively and poor regeneration efficiency of commercial strong- and weak-acid cation exchange resins with CO2 have been the primary impediments for desalting impaired water and wastewater sources. In this study, treated wastewater from Bethlehem, PA and synthetic brackish water were used for a prolonged laboratory investigation. Specific objectives of the study are to demonstrate that the HIX-Desal process: i) may consistently remove dissolved solids and does not require any other source of energy or chemical besides pressurized CO2 to sustain the process; ii) does not produce any secondary waste stream containing other chemicals; and iii) can additionally remove and recover phosphate from municipal wastewater.



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EXPERIMENTAL Bethlehem Wastewater, Chemical Analysis and Resin Characterization The influent wastewater was collected in a 20 L batch from the secondary effluent at the Bethlehem Wastewater Treatment Plant (Bethlehem, PA). After collection, the influent batch was filtered through an 11 µm retention filter and then stored in a refrigerator. Calcium and sodium were analyzed by a Perkin Elmer AAnalyst200 Atomic Absorption Spectrometer (AAS). Sulfate, chloride and nitrate were analyzed using Dionex Ion Chromatography (IC model ICS-1000) with an IonPac® AS14 column. Phosphorus was analyzed using a Perkin Elmer Optima 2100DV ICP-OES. TDS was measured through a conductivity meter (FisherbrandTM accumetTM AP75).

To monitor the

progression of phosphate loading in HAIX-NanoZr beads between service cycles and regeneration, slices of the beads (parent and exhausted) were prepared using microtomy and characterized by scanning electron microscopy with energy dispersive x-ray (SEMEDX) spectroscopy (Model Hitachi JSM-4300).

The CO2 cylinder was placed on a

weighing scale (Mastercool 98210-A) and CO2 consumption was monitored during the process. PROCESS CONFIGURATION, RESULTS & DISCUSSION TDS Reduction and Regenerability for Multiple Cycles The bench top stainless-steel set-up in a scaffold with 2-300 mL columns and 2-10 L CO2 regeneration tanks along with the CO2 cylinder, pump and fraction collector is shown in Supplementary Information (SI) Figure S1. All columns and fittings containing HAIX-NanoZr and SC-WAC resins were designed to withstand pressure up to 20 bar. Results of three consecutive cycles of operation with the filtered secondary wastewater from Bethlehem, PA with a


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TDS of approximately 500 mg/L are in Figure 2A. Note that for three consecutive cycles, with the proposed process using CO2 as the sole regenerant, more than 50% TDS removal was consistently achieved for well over 200 bed volumes. Equally important, as shown in Supplementary Information (SI) Figure S2, individual species of interest, namely, calcium and sulfate, often responsible for membrane fouling and equipment scaling, were removed efficiently.



FIGURE 2. A) TDS removal or desalination over three consecutive service cycles using Bethlehem secondary wastewater. Influent composition and operation condition: 2.4 mg/L phosphate as P, 130 mg/L Cl-, 70 mg/L NO3-, 50 mg/L SO42-, 100 mg/L HCO3-, TDS 484 mg/L, pH 7.44, SLV 0.41 m/hr, EBCT 4.65 min; B) TDS elution during CO2 regeneration of both columns. Regenerant and operation condition: CO2 sparged water under 150 psi CO2 partial pressure, EBCT 30 min. (EBCT= Empty bed contact time) The elution curves for two consecutive regeneration cycles using CO2 are presented in Figure 2B; regeneration efficiency was consistent and the three consecutive service cycle runs have nearly identical effluent histories with the same TDS reduction as shown in Figure 2A. SI Figure S3 provides sulfate and calcium elution profiles during CO2 regeneration. Results of three consecutive service runs for a synthetic feed water with a TDS of 1250 mg/L with CO2 regenerations in between are included in SI Figure S4. Note that nearly 50% reduction in TDS was achieved for almost 200 bed volumes with nearcomplete removal of sulfate and calcium. No bacterial growth was observed in the columns during the multiple cycles of desalination and CO2 regeneration. Process Scalability and Phosphate Recovery In an attempt to reconcile the experimental results with the underlying scientific hypothesis, calcium loaded SC-WAC was regenerated with three different CO2 pressures. Figure 3A demonstrates how the efficiency of regeneration or calcium desorption improved with an increase in partial pressure of CO2. The HIX-Desal process can thus be rationally scaled up at higher CO2 pressures and the system design streamlined for different feed conditions. Also, the CO2 requirement for regeneration, as measured through the weighing scale below the cylinder, agreed well with the open system saturation concentration of CO2 at the operating pressure. For the column runs and regenerations presented in Figures 1A and 1B, CO2 consumption was approximately 4.0 gram per liter of water treated. 10 ACS Paragon Plus Environment

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Two successive phosphate effluent histories for the Bethlehem plant wastewater are shown in Figure 3B; note that phosphate removal is very efficient and continues for a much longer duration: over 1000 bed volumes. Phosphate is sorbed onto ligand exchange sites of HZrO nanoparticles and not amenable to desorption during CO2 regeneration. Thus, HAIX-NanoZr may intermittently be regenerated with KOH to recover nearly 95% of the phosphorus as potassium phosphate (K2HPO4), (SI Figure S5), a potential high value fertilizer without generating any secondary waste stream. In contrast to previous phosphate recovery studies from wastewater, HIX-Desal process achieves concurrent salinity reduction or hardness removal during the process.16-19

FIGURE 3. (A) Effluent calcium profiles under different CO2 pressure, 10.2 atm (150 psi), 6.8 atm (100 psi) and 3.4 atm (50 psi), respectively. Regenerant and operation condition: CO2 sparged water, SLV 0.28 m/hr, EBCT 30 min. (B) Evidence of selective phosphate sorption during service cycle. Influent composition and operation condition: 2.4 ml/L phosphate as P, 130 mg/L Cl-, 70 mg/L NO3-, 50 mg/L SO42-, 100 mg/L HCO3-, Conductivity 968 µs, pH 7.44, SLV 0.41 m/hr, EBCT 4.65 min.



To further reinforce the underlying sorption mechanism of phosphate from other commonly present bulk anions such as sulfate and chloride, slices of both exhausted and parent HAIX-NanoZr were characterized in Figure 4 by SEM-EDX mapping which shows i) Zr (purple) in the parent exchanger; ii) phosphorus in the exhausted bead; and iii) sulfur and chloride in the exhausted bead. Note that the presence of Zr in the parent resin and P in the exhausted resin merge with each other implying phosphate is adsorbed almost solely by zirconium oxide nanoparticles. On the contrary, sulfate and chloride are distributed throughout the bead, thus confirming that quaternary ammonium groups of the parent anion exchanger are the primary sorption sites for sulfate and chloride anions.

FIGURE 4. SEM-EDX mapping of Zr, P, S and Cl. Sustainability Impact and Relevance to Water-Energy Nexus Between RO and HIX-Desal, there lies a distinctive difference in the underlying scientific principles to achieve desalination. For RO, as the percentage (or fractional recovery) is increased beyond 60%, the prevailing osmotic pressure at the membrane interface increases


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exponentially due to the concentration effect and so increases the specific energy requirement. Supplementary Information (SI Figure S6) illustrates how the concentration of the reject brine stream, CB, at the membrane interface increases with an increase in fractional recovery (Y) for a feed water concentration, CF. An increase in recovery from 80% to 90% (i.e., Y goes from 0.8 to 0.9) will cause 100% increase in osmotic pressure at the membrane interface without taking into consideration additional increase caused by concentration polarization. Furthermore, the likelihood of precipitation of sparingly soluble salts (e.g., calcium sulfate, phosphate and others) will be greatly enhanced at 90% recovery, thus warranting pre-treatment of the feed water. In comparison, the HIXDesal process, as schematically presented in Figure 1A, is driven solely by exchange of HCO3- and H+ with other dissolved solutes present in the impaired brackish water. Thus, an increase in recovery does not have a direct bearing on energy consumption for brackish water desalination. For example, long-term investigation is currently underway to increase the recovery from 80% to 90% for the wastewater containing around 8001000 mg/L TDS in OCWD, CA. In such high recovery processes, HIX-Desal has specific advantages over RO desalination as long as feed water TDS is less than 1500 mg/L. The process configuration of HIX-Desal process is characteristically different from conventional demineralization, i.e., here the anion exchanger (i.e., HAIX-NanoZr) precedes the cation exchanger (i.e., SC-WAC) during service cycles. In principle, all the anions (e.g., SO42-, Cl-) are exchanged favorably into HCO3- into the anion exchanging column while the subsequent weak-acid column preferentially sorbs cations (e.g., Ca2+, Na+) releasing equivalent amount of H+, thus achieving desalination or TDS reduction.



Thermodynamically favorable association of HCO3- and H+ and subsequent stripping of CO2, as presented below, drive the desalination step forward: HCO3- + H+ ↔ H2CO3

Ka1 = 106.38

H2CO3 ↔ H2O + CO2 (g)

KH = 101.41

Overall: HCO3- + H+ ↔ H2O + CO2 (g)

Koverall = 107.79

ΔGo = -RT ln Koverall = -43.72 KJ/mol However, the reverse reaction (i.e. regeneration) is not thermodynamically favorable and this process step requires external energy in terms of pressurized CO2 and Figure 3A shows clear evidence of higher regeneration efficiency with higher pressure. Since CO2 is universally available under pressure in large quantities and fairly inexpensive (e.g., $25-100 per ton depending on the location), the process is easy to implement from an application viewpoint. It has been emphasized in a recent review20 that RO processes need more selectivity toward target trace solutes than enhanced permeability. In this context, HIX-Desal has more flexibility because the functional groups of ion exchanging materials can be tailored to selectively remove target contaminants like perchlorate, fluoride, nitrate, arsenic, phosphate, boron and certain groups of pharmaceuticals for brackish waters with TDS less than 1500 mg/L. Field scale investigations are recommended to demonstrate the attributes of HIX-Desal process in comparison with RO processes for lean brackish water.


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Supporting Information Available Figures S1 – S6. This

information

is

available

free

of

charge

via

the

Internet

at

http://pubs.acs.org/journal/estlcu

Corresponding Author *Arup K. SenGupta. *E-mail: [email protected]. Phone: 610 758 3534.

Notes The authors declare no competing financial interest.

Acknowledgements Support from the Water Innovation Network for Sustainable Small Systems (WINSSS) at the University of Massachusetts- Amherst (2017-2018) is gratefully acknowledged. MG received fellowship through Global Innovation Initiative (GII) from the US State Department.



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17. Greenleaf, JE.; Lin, JC.; Sengupta, AK. Two novel applications of ion exchange fibers: Arsenic removal and chemical‐free softening of hard water. Environ. Prog. Sustainable Energy. 2006, Dec 1;25(4):300-11. 18. Sendrowski, A.; Boyer, TH. Phosphate removal from urine using hybrid anion exchange resin. Desalination, 2013, 322, 104-112. 19. Sengupta, S.; Pandit, A. Selective removal of phosphorus from wastewater combined with its recovery as a solid-phase fertilizer. Water Res. 2011, 45(11), 3318-3330. 20. Werber, JR.; Deshmukh, A.; Elimelech, M. Critical need for increased selectivity, not increased water permeability, for desalination membranes. Environ. Sci. Technol. Lett. 2016, 3, 112-120.

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Hybrid Ion Exchange Desalination (HIX-Desal) of Impaired Brackish

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