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Hybrid nitrate selective resin (NSR-NanoZr) for simultaneous selective removal of nitrate and phosphate (or fluoride) from impaired water sources
Journal of Environmental Chemical Engineering 8 (2020) 103846
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Hybrid nitrate selective resin (NSR-NanoZr) for simultaneous selective removal of nitrate and phosphate (or fluoride) from impaired water sources
T
Hang Dong*, Chelsey S. Shepsko, Michael German, Arup K. SenGupta Department of Civil and Environmental Engineering, Lehigh University, 1 W. Packer Ave, Bethlehem, PA, 18015, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Hybrid ion exchange Selective removal Nitrate Phosphate Fluoride
Reverse osmosis (RO) has been increasingly applied in municipal wastewater reuse facilities. Nitrate (N) and phosphate (P) -laden reject streams from RO demand near-complete NP removal before inland brine disposal to avoid serious eutrophication effects. In addition, elevated nitrate and fluoride levels in groundwater is a threat to public drinking water safety. Selective removal of both nitrate and phosphate, or fluoride through a single adsorbent is promising because of its small footprints, minimum sludge production, and potential NP recovery. This study presents a new ion exchange adsorbent, NSR-NanoZr, for simultaneous selective removal of both nitrate and phosphate, or fluoride. NSR-NanoZr is fabricated by doping zirconium oxide nanoparticles (ZrO2Nano) into a nitrate selective resin (NSR), which tributyl quaternary ammonium functional groups contribute to the nitrate selectivity and Lewis acid-base interactions between ZrO2-Nano and ligands result in the high affinity towards phosphate and fluoride. Lab-scale studies showed that NSR-NanoZr exhibited a completely reversal selectivity and removed both NP selectively from municipal secondary wastewater for multiple cycles. Synergistic application of a weak-acid cation exchanger (WAC) enhanced the phosphate capacity of NSR-NanoZr due to the amphoteric sorption behavior of the ZrO2-Nano. Field-scale studies demonstrated the potential of NSR-NanoZr in removing fluoride together with nitrate.
1. Introduction
in agricultural land shows clear evidence of an increase in nitrate concentration in ground water [14]. The crisis is further worsened by already existing naturally contaminated water with fluoride, which is responsible for skeletal and dental fluorosis of people drinking from such ground water sources [15]. An increase in nitrate concentration beyond the permissible level of 45 mg/L nitrate by WHO poses special challenges because of the need for removal of both fluoride and nitrate. Understandably, biological denitrification [16] or catalytic reduction [17] will not attain any fluoride removal. Similarly, activated alumina, which is universally used for fluoride removal, lacks ability to remove nitrate [18]. Ideally, a single sorbent, which is robust, chemically stable, selective and regenerable, will be appropriate if both nitrate and phosphate, or fluoride can be removed concurrently in the presence of other commonly occurring anions, namely, sulfate and chloride. However, with many emerging materials being increasingly reported [19–28], there are still in lack of materials with excellent selectivity and regenerability for co-removal of nitrate and phosphate, or fluoride. The primary objective of this study is to present a hybrid ion exchange material that is: first, selective toward nitrate, phosphate and fluoride; and second, amenable to regeneration and reuse. Additionally, these
Municipal wastewater after secondary treatment invariably contains a significant amount of nitrate and phosphate that are receiving close scrutiny primarily for two reasons: first, as limiting nutrients, they pose threats of eutrophication including algal blooms into receiving bodies of water [1–4] and second, added difficulties in waste water recovery and reuse for inland plants [5–8]. Especially in arid areas with relatively low flow in natural water bodies, the adverse impact of discharge of nitrate and phosphate is more pronounced due to algal blooms and consequent drops in the dissolved oxygen concentration. For inland municipal wastewater plants away from coastal areas, water reuse encounters very challenging, if not insurmountable, problems because the reject from reverse osmosis (RO) process is very rich in nitrate (N) and phosphate (P) [9]. From a drinking water perspective, the presence of nitrate ions in drinking water has been regulated by the USEPA with a maximum limit of 10 mg/L as N for safe drinking water [10]. Nitrate in drinking water is a serious concern for the public health leading to increased risk of infant methemoglobinemia, or “blue-baby syndrome” [11–13]. Excess application of N-based fertilizer around the world for enhanced fertility
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Corresponding author. Present address: 443 Via Ortega, Room SB27, Stanford, CA, 94305, United States. E-mail address: [email protected] (H. Dong).
https://doi.org/10.1016/j.jece.2020.103846 Received 1 January 2020; Received in revised form 18 February 2020; Accepted 9 March 2020 Available online 10 March 2020 2213-3437/ © 2020 Elsevier Ltd. All rights reserved.
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sorbents would be forgiving to fluctuations in influent concentrations i.e., the treated water quality pertaining to target contaminants will remain unaffected during the fixed-bed process through occasional changes in the influent quality.
constant values (KNO3/SO4) for different alkyl groups are also inserted in the figure [32]. Nitrate selective resins (NSR) with butyl ammonium functional groups are thus developed but NSRs do not exhibit high affinity neither for phosphate nor fluoride because the sole electrostatic interaction between ammonium groups and adsorbates is non-specific and phosphate and fluoride adsorptions are highly competed by other anions. Recent studies have demonstrated that integration of metal oxide nanoparticles with different host materials can be a promising strategy to develop multifunctional composites [33–38], especially by introducing nanoscale zirconium oxide particles into polymeric ion exchangers, sorption affinity for anionic ligands including phosphate can be greatly enhanced [39,40]. Thus, an NSR with dispersed ZrO2-Nano (NSR-NanoZr) has two characteristically distinctive sorption sites: tributyl quaternary ammonium functional groups that can sorb nitrate in preference to divalent sulfate and surface sorption sites of ZrO2-Nano that can selectively bind phosphate and fluoride through Lewis acid-base (LAB) interaction i.e., formation of inner-sphere complexes. Fig. 1B provides an illustration of the NSR-NanoZr with dual functional groups and illustrates surface sorption sites of ZrO2 nanoparticles for phosphate through formation of monodentate or bidentate complexes. Sulfate and Chloride, on the contrary, can only form outer sphere complexes and thus, may not compete with phosphate. It is pertinent to note that once exhausted, both nitrate and phosphate can be simultaneously desorbed from NSR-NanoZr with relatively dilute KOH (2% m/v) solution. The primary objectives of the paper are to prepare NSR-NanoZr and characterize its salient properties in relation to concurrent nitrate and phosphate removal from a real secondary wastewater. Its selective fluoride removal capacity is also to be demonstrated with field tests.
1.1. Underlying concept and development of concurrent nitrate and phosphate selectivity Since sulfate, a divalent anion, is invariably present in every treated wastewater, nitrate/sulfate selectivity is an important parameter in the design of an ion exchange process for selective nitrate removal. For a typical strong-base anion exchanger with electrostatic interaction as the primary binding mechanism, the selectivity sequence for common anions stands as follows: SO42− > NO3− > Cl− Thus, the presence of sulfate in the water greatly suppresses nitrate removal capacity, thus impairing the performance of the process. Also, phosphate in the wastewater, either in the form of H2PO4− or HPO42-, has lower affinity for anion exchangers than sulfate. Thus, we need an appropriately tailored anion exchanger with relatively high affinity for nitrate and phosphate over sulfate for a viable recovery process. Previous studies [29–31] have demonstrated that both the matrix and the functional group influence monovalent/divalent selectivity for anion exchange resins. Specifically, for polystyrene matrix, by changing the alkyl groups of the quaternary ammonium functionality from methyl to ethyl to propyl to butyl as shown in Fig. 1A, the nitrate/sulfate selectivity is greatly enhanced i.e., monovalent nitrate is increasingly favored over divalent sulfate, because divalent ion adsorption is thermodynamically unfavorable when the distance between two neighboring adsorption sites is increased with longer chained alkyl groups. [32] The gradual increase in nitrate/sulfate ion exchange equilibrium
Fig. 1. (A) Nitrate/sulfate selectivity (KNO3/SO4) values with changes in the alkyl groups of the quaternary ammonium functionality. (B) An illustration depicting preferred sorption of phosphate through formation of mono- and bidentate inner sphere complexes at zirconium oxide surface; and a schematic representation of an anion exchange resin with tributyl ammonium functional groups dispersed with zirconium oxide nanoparticles for selective removal of both nitrate and phosphate. 2
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Fig. 2. (A) Nitrate selective resin beads doped with zirconium oxide nanoparticles (B) TEM image showing zirconium nanoparticles on the polymer phase and (C) enlarged zirconium nanoparticles. (D) SEM image showing the macroporous structure of the NSR-NanoZr resin (E) EDX spectra of the NSR-NanoZr resin illustrating presence of elemental zirconium inside the beads (F) Size distribution of the doped particles.
2. Materials and experimental methods
in diameter and 250 mm in length) were used, along with constant-flow stainless steel pumps (Fluid Metering International, Syosset, NY) and fractional collectors (Eldex, Napa, CA). Experimental setup was shown in Figure S1 including a single column process illustrating both contaminant removal and regeneration cycles, and a two-column train setup when two different resins were applied. Field-scale fluoride mitigation studies were carried out inside a school premise in Nalhati village, in Birbhum district in West Bengal, India (N 24°17.68320′, W 087°49.55640′).
2.1. Materials A nitrate selective resin, A520E, commercially available from Purolite Co. (headquartered in Bala Cynwyd, PA) was used as the parent resin to make NSR-NanoZr. A520E resin carries tributyl quaternary ammonium groups and has a polystyrene-divinylbenzene matrix. Salient properties of A520E are listed in Supporting Information Table S1. The zirconium oxide material was obtained from MEL Chemicals (Flemington, New Jersey). Zirconium oxide nanoparticle loading procedures were carried out by following the recipe described earlier [40]. Briefly, nitrate selective resins were soaked into zirconium solution (10 % w/v zirconium oxide dissolved in 25 % v/v sulfuric acid) first, followed by mixing the decanted resins with 5% NaOH solution to precipitate hydrated zirconium oxide onto the pore structure of the resin. Fig. 2A shows the NSR-NanoZr resin beads at approximately 0.30.6 mm in diameter. High resolution transmission electron microscopy (HRTEM) (JEOL JEM-2010 F) of NSR-NanoZr in Fig. 2B and C shows the zirconium oxide nanoparticles inside the polymer phase. They were characterized by scanning electron micrograph (SEM) (Model Hitachi JSM-4300) with energy dispersive X-ray spectroscopy accessories (EDX). The SEM image in Fig. 2D shows the macroporous structure of the parent A520E resin bead. An approximate 10 % zirconium loading rate by mass was confirmed by EDX as shown in Fig. 2E and F shows the size distribution of the ZrO2 nanoparticles inside the NSR-NanoZr that indicating the nano-scale metal oxide particles.
2.3. Chemical analyses and isotherm test at varying pH Anions (i.e. chloride, nitrate and sulfate) were analyzed using Dionex Ion Chromatography (IC model ICS-1000) equipped with an IonPac AS14 column. Phosphate was analyzed by HACH UV–vis spectrophotometer (model DR 5000). Calcium and sodium were analyzed by a Perkin Elmer AAnalyst 200 Atomic Absorption Spectrometer (AAS). Total dissolved solids (TDS) of the water was measured using a conductivity detector (Fisherbrand accumet AP75). Zeta potential and the zero-point charge (ZPC) for zirconium oxide precipitates were measured using the Malvern Zeta-sizer Nano ZS with MPT-2 Titrator using NaNO3 at 0.1 M ionic strength. A batch isotherm test was conducted to determine the phosphorus uptake capacity of NSR-NanoZr at varying pH. Isotherm tests were completed via shaker at 25 °C and equilibrium concentrations were measured after 2–3 days, which was determined based on preliminary kinetic experiments to ensure equilibrium. 3. Results and discussion
2.2. Bethlehem wastewater, fixed-bed column experiments and field-scale fluoride mitigation
3.1. Reversal of nitrate and phosphate Selectivity
Bethlehem wastewater was used as the influent for fixed-bed column experiments. The influent wastewater was collected in a 50 L batch from the secondary effluent at the Bethlehem Wastewater Treatment Plant (Bethlehem, PA) (N 40°37′2.433″, W 75°20′4.585″) and filtered through an 11 μm filter before use. Glass columns (11 mm
Figs. 3A and 3B provide the effluent histories of two column runs using secondary wastewater from the Bethlehem Wastewater Treatment Plant (BWTP) under identical conditions except that the first run (Fig. 3A) used traditional strong-base anion exchange resin, SBA (Purolite A502), while NSR-NanoZr was used for the second run (Fig. 3B). 3
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Fig. 3. (A) Effluent sulfate, nitrate and phosphate histories for traditional strong base anion exchange resins (SBA) using secondary wastewater from the Bethlehem Wastewater Treatment Plant in Bethlehem, PA. (Influent composition: 2.4 mg/L P, 100 mg/L NO3−, 20 mg/L SO42-, 69 mg/L Cl−, 50 mg/L Ca2+, 466 mg/L TDS, pH 7.0, EBCT 4.5 min.) (B) Effluent sulfate, nitrate and phosphate histories for NSR-NanoZr using the same influent. (C)Separation factor of phosphate over sulfate (αP/S) and nitrate over sulfate (αN/S) for SBA and NSR-NanoZr. (D) SEM maps of zirconium (Zr) from a parent NSR-NanoZr bead and phosphate (P), sulfate (S) and chloride (Cl) following exhaustion.
The Y-axis represents the normalized concentration of an individual anion with respect to its influent concentration (C0). Note that the breakthrough sequence of phosphate, nitrate and sulfate were completely reversed from Fig. 3A–B. With SBA (Fig. 3A), phosphate broke through first followed by nitrate and sulfate, confirming that divalent sulfate (SO42−) is preferred over nitrate and phosphate. However, based on the breakthrough sequence in Fig. 3B, the preference or the selectivity sequence for NSR-NanoZr is just the opposite and stands as follows:
αN/ S =
For Fig. 3B, sulfate was well removed for 300 BVs due to efficient anion exchange before resin capacity depletion. However, sulfate breakthrough occurred much earlier than nitrate and phosphate and underwent chromatographic elution, i.e., the concentration of sulfate after breakthrough is greater than the influent (C/C0 > unity), implying the accumulated sulfate on resin was eluted by a more resin-preferred ion, e.g., nitrate in this case. This observation confirmed higher nitrate selectivity of NSR-NanoZr over sulfate [32]. A separation factor (αi/j) is a dimensionless, relative affinity parameter for two competing ions i and j; between phosphate and sulfate, the separation factor or αP/S is defined as
yP x S xP yS
(2)
Using effluent histories in Figs. 3A and 3B, separation factors, αP/S and αN/S, were computed for both SBA and NSR-NanoZr [41,42]. A separation factor higher than unity indicates higher phosphate or nitrate selectivity than sulfate, and lower than unity indicates higher sulfate selectivity. Fig. 3C shows: αP/S and αN/S are less than unity for SBA and greater than unity (3.1 and 1.7) for NSR-NanoZr. A reversal of separation factor values of αP/S and αN/S from less than unity to greater than unity indicates higher phosphate and nitrate selectivity than sulfate by NSR-NanoZr, which is the underlying reason for the reversal of the breakthrough sequence for phosphate, nitrate and sulfate between conventional SBA and NSR-NanoZr. Note that due to the higher phosphate and nitrate selectivity of NSR-NanoZr than SBA, NSR-NanoZr exhibited three times higher phosphate capacity and two times higher nitrate capacity than SBA. In order to further validate the sorption mechanism of sulfate and phosphate at two different sites of NSR-NanoZr, SEM-EDX mapping was carried out using slices of both parent and exhausted exchanger. The mappings presented in Fig. 3D show: (i) Zr (green) in the parent exchanger, (ii) phosphorus (red) in the exhausted bead, and (iii) sulfur (representing sulfate) and chloride in the exhausted bead. Note that P distribution merges with Zr in the annular portion of the NSR-NanoZr bead but P is absent near the core of the bead where Zr is absent as well. This observation provides strong evidence that phosphate is adsorbed almost solely by zirconium oxide nanoparticles. In comparison, sulfate and chloride are distributed throughout the bead including the core confirming that quaternary ammonium groups are the primary sorption sites for sulfate and chloride, which both ions are subject to only
Phosphate > Nitrate > Sulfate
αP/ S =
yN x S xN yS
(1)
where yP and yS denote the equivalent fraction of phosphate or sulfate in the exchanger phase, respectively, and xP and xS denote the corresponding equivalent fractions in the aqueous phase. Similarly, the nitrate/sulfate separation factor is equal to 4
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Fig. 4. Effluent history plots of (A) nitrate and (B) phosphorus with secondary wastewater from the Bethlehem Wastewater Treatment Plant for consecutive column runs (Influent composition: 2.4 mg/L P, 100 mg/L NO3−, 32 mg/L SO42-, 69 mg/L Cl−, 50 mg/L Ca2+, 466 mg/L TDS, pH 7.0, EBCT 4.5 min.) And successive regeneration elution curves for (C) nitrate and (D) phosphate using 2% KOH as the regenerant solution.
desorbed from the zirconium sites due to strong expelling by the negatively charged surface sorption sites. Thus, very efficient regeneration is attainable using an alkaline solution, KOH, as shown in Fig. 3D. Zeta potential and corresponding capacity tests in Fig. 5A demonstrated that an increase of pH gradually transformed the zirconium surfaces negatively charged. Consequently, a decrease in phosphate removal capacity was exhibited with an increase in pH as observed in experimental results of Fig. 5A. For municipal wastewater in general, including Bethlehem wastewater used in the tests, alkalinity (HCO3−) is invariably present as a major dissolved constituent. Passage of treated wastewater through a weak-acid cation exchanger (WAC) will constantly generate a stream with an adjusted pH in the range 4–5 without changing the anion composition in the water. In addition, introduction of the WAC reduces the total dissolved solids and hardness removal. Column tests with and without WAC in front NSR-NanoZr were conducted and pH and phosphate effluent histories were recorded. Fig. 5B shows that, at constant feed wastewater with pH around 7 without the aid of WAC, effluent history of phosphate gradually reached 20 % breakthrough at less than 500 bed volumes. With WAC in front of the column and an adjusted pH around 4.5, the phosphate effluent history lasted for over 1200 bed volumes until 20 % breakthrough was achieved. Note that WAC can be regenerated using pressurized carbon dioxide (CO2) without needing any mineral acid. [12,44] Supplementary Figure S2 provides experimental validation of efficient desorption of hardness by using CO2 at 10.2 atmosphere. Sorption reaction leading to hardness removal and desorption with CO2 are represented by the following reactions: Sorption:
electrostatic interaction with the resin. Nitrate mapping is not shown because of the abundant presence of nitrogen in the background due to amine functional groups of the parent nitrate-selective resin. 3.2. Fixed-bed column runs with secondary wastewater and regenerability Figs. 4A and 4B show effluent histories of nitrate and phosphate using NSR-NanoZr resin with Bethlehem wastewater. Note that both nitrate and phosphate breakthrough at significantly long running lengths, 300 and 1000 bed volumes (BV), respectively. After each cycle, the column was regenerated with 2% KOH and 3 cycles of effluent histories were recorded. Fig. 4C and D show nitrate and phosphate elution profiles for two consecutive cycles of regeneration with nearly identical effluent histories for both ions. More than 90 % recovery of nitrate and phosphate were achieved in less than 6 bed volumes. Successful and repeatable service and regeneration data demonstrate that NSR-NanoZr is efficient for both sorption and desorption of nitrate and phosphate and sustainable for long-term application. The fact that phosphate breakthrough remained nearly the same for 3 consecutive cycles provided evidence that there was no zirconium loss from the anion exchanger. Earlier work [43] also confirmed absence of zirconium particles in the treated water. Note that nitrate and phosphate were highly concentrated during regeneration, along with potassium in the regenerant, generating a nutrient rich stream containing NPK, a potential liquid fertilizer recovered from wastewater. 3.3. Effect of pH and synergy of using a weak acid cation exchanger Zirconium oxide surfaces are amphoteric; they are positive at pH ≤ 5.0 and thus exhibit strong affinity toward phosphate, which is essentially an anionic ligand [15,39] and subject to both electrostatic and Lewis acid-base interactions. While at pH ≥ 11, phosphate will be
2RCOOH + Ca2 + (aq) + 2HCO3 − (aq) → |(RCOO−)|2 Ca2 + + 2H2 O + 2CO2 ↑ Desorption: 5
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Fig. 5. (A) Phosphorus removal capacity of NSR-NanoZr and zeta potential of the hydrated zirconium oxide particles with changes in pH (B) Phosphate column runs using NSR-NanoZr resin with the same Bethlehem secondary wastewater with and without the aid of WAC under otherwise identical conditions and corresponding changes in pH.
Fig. 6. (A) onsite fluoride influent and effluent histories (B) efficient regeneration with dilute solution of 2% NaOH and 2% NaCl (C) SEM-EDX mapping of exhausted NSR-NanoZr.
the treated water has consistently been less than 1.5 mg/L (acceptable limit as per WHO or World Health Organization standards); after about one year of operation, fluoride gradually exceeded 1.5 mg/L and the hybrid anion exchanger, NSR-NanoZr, was regenerated; and, fluoride in the treated water during the subsequent service run again dropped below 1.5 mg/L for over a year. Fig. 6B demonstrates the high efficiency of regeneration with dilute solution of 2% NaOH and 2% NaCl: over 95 % of fluoride was eluted during the regeneration process. Again, slices of exhausted NSR-NanoZr were characterized by SEM-EDX mapping and Fig. 6C demonstrates i) Zr (pink) in the parent exchanger; and ii) fluoride (red) in the exhausted bead. Note that similar to the observations made with phosphate, the presence of Zr in the parent resin and F in the exhausted resin merge with each other, implying that fluoride is adsorbed almost solely by zirconium oxide nanoparticles. Thus, NSR-NanoZr offers opportunities for treating impaired groundwater contaminated with both fluoride and nitrate around the world.
2CO2 (g) + 2H2 O↔ 2H2CO3
(RCOO−)2 Ca2 + + 2H2CO3 ↔ 2RCOOH + Ca2 + + 2HCO3− Note that the process is reversible and sustainable in an environmentally sustainable manner without needing any mineral acid. 3.4. Simultaneous removal of fluoride as a co-contaminant Over 200 million people in the world are threatened with exposure to high concentrations of fluoride in naturally contaminated groundwater, especially in Eastern Africa, Central America and Asia [45]. An elevated nitrate concentration in groundwater is being detected in increasing number of locations, including many fluoride-contaminated areas [14,18] around the world due to over-application of ammoniabased fertilizer. Nitrate-selective resins are appropriate sorbents for removing nitrate alone from groundwater. In a separate field-scale study, NSR-NanoZr was used to validate its fluoride removal capacity and regenerability in a fluoride-contaminated groundwater on a school premise in the Birbhum district of West Bengal, India (N 24°17.68320′, W 087°49.55640′). The Global Innovation Initiative (GII) project under the aegis of the US State Department and the Government of India (GoI) Department of Science and Technology (DST) provided resources to establish fluoride-safe water systems in a village school; Supplementary Figures (S3 A, B and C) show the photographs [18]. Fig. 6A provides the fluoride influent and effluent histories of the field-scale plant for over two years of operation and the following observations are noteworthy: first, while the influent fluoride is 17 mg/L,
4. Major observations and path forward Among various sorbents, ion exchange resins are durable, cost effective, selective and regenerable. However, for specific applications, such as the emerging municipal wastewater reuse, their specific affinities need to be modified and/or tailored. Nitrate selective resins have been designed to be monovalent selective through engineered tributyl quaternary ammonium functional groups. However, lack of phosphate selectivity impedes its application for simultaneous nitrate and phosphate removal and their possible recovery from impaired water and wastewater streams. Polyvalent metal oxides, such as Zr (IV) oxides, 6
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offer high sorption affinity toward anionic ligands including phosphate and fluoride. This hybrid ion exchange sorbent has two distinctive sorption sites different from each other: one for relatively hydrophobic monovalent nitrate and the other for anionic ligands. Major findings of the study can be summarized as follows:
[9] [10]
• Nitrate and phosphate selectivity of NSR-NanoZr greatly exceed • • •
[11] [12]
divalent sulfate, thus offering a realistic opportunity to remove and concentrate both N and P from wastewater in the presence of sulfate and chloride. NSR-NanoZr can be regenerated using a single regenerant, namely KOH, thus producing a spent regenerant rich in N, P and K with practically no chloride and sulfate. For ground waters naturally contaminated with fluoride (F−) and with gradually increasing nitrate beyond the maximum contaminant level (MCL) in drinking water, NSR-NanoZr can be an appropriate sorbent to mitigate the crisis. NSR-NanoZr is durable, chemically stable, selective and amenable to excellent reuse over multiple cycles of operation.
[13]
[14]
[15]
[16]
[17]
CRediT authorship contribution statement
[18]
Hang Dong: Conceptualization, Investigation, Visualization, Data curation, Writing - original draft. Chelsey S. Shepsko: Investigation. Michael German: Writing - review & editing. Arup K. SenGupta: Writing - review & editing, Supervision, Funding acquisition.
[19]
[20]
Declaration of Competing Interest
[21]
There are no conflicts to declare. [22]
Acknowledgements [23]
Three-year grant from the US State Department through Global Innovation Initiative is gratefully acknowledged. This work was also partially supported by the Pennsylvania Infrastructure Technology Alliance (PITA) from the Commonwealth of Pennsylvania.
[24]
Appendix A. Supplementary data
[25]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jece.2020.103846.
[26]
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Copalcúa, Nitrate and phosphate removal by chitosan immobilized Scenedesmus, Bioresour. Technol. 99 (2008) 1274–1279, https://doi.org/10.1016/j.biortech.2007.02.043. G. Hua, M.W. Salo, C.G. Schmit, C.H. Hay, Nitrate and phosphate removal from agricultural subsurface drainage using laboratory woodchip bioreactors and recycled steel byproduct filters, Water Res. 102 (2016) 180–189, https://doi.org/10. 1016/j.watres.2016.06.022. Y.S. Kim, Y.H. Lee, B. An, Sung-A. Choi, J.H. Park, J.S. Jurng, S.H. Lee, J.W. Choi, Simultaneous removal of phosphate and nitrate in wastewater using high-capacity anion-exchange resin, Water Air Soil Pollut. 223 (2012) 5959–5966, https://doi. org/10.1007/s11270-012-1331-1. S.D. Alexandratos, From ion exchange resins to polymer-supported reagents: an evolution of critical variables, J. Chem. Technol. Biotechnol. 93 (2018) 20–27, https://doi.org/10.1002/jctb.5457. V. Sinha, S. Chakma, Advances in the preparation of hydrogel for wastewater treatment: a concise review, J. Environ. Chem. Eng. 7 (2019) 103295, , https://doi. org/10.1016/j.jece.2019.103295. M.T.H. Siddiqui, S. Nizamuddin, H.A. Baloch, N.M. Mubarak, M. Al-Ali, S.A. Mazari, A.W. Bhutto, R. Abro, M. Srinivasan, G. Griffin, Fabrication of advance magnetic carbon nano-materials and their potential applications: a review, J. Environ. Chem. Eng. 7 (2019), https://doi.org/10.1016/j.jece.2018.102812. A.S.K. Kumar, S.J. Jiang, Chitosan-functionalized graphene oxide: a novel adsorbent an efficient adsorption of arsenic from aqueous solution, J. Environ. Chem. Eng. 4 (2016) 1698–1713, https://doi.org/10.1016/j.jece.2016.02.035. G.A. Guter, Nitrate Removal from Contaminated Groundwater by Anion Exchange, Technomic Publishing Co. Inc., Lancaster, PA, 1995. D. Clifford, W.J.W Jr, The determinants of divalent/monovalent selectivity in anion exchangers, React. Polym. Ion Exch. Sorbents 1 (1983) 77–89. A.K. SenGupta, Ion Exchange Technology: Advances in Pollution Control, CRC Press, 1995. A.K. SenGupta, Ion Exchange in Environmental Processes, John Wiley & Sons, 2017, https://doi.org/10.1002/9781119421252. H. Wan, M.S. Islam, N.J. Briot, M. Schnobrich, L. Pacholik, L. Ormsbee, D. Bhattacharyya, Pd/Fe nanoparticle integrated PMAA-PVDF membranes for chloro-organic remediation from synthetic and site groundwater, J. Membr. Sci. 594 (2020) 117454, , https://doi.org/10.1016/j.memsci.2019.117454. L.N. Pincus, A.W. Lounsbury, J.B. Zimmerman, Toward realizing multifunctionality: photoactive and selective adsorbents for the removal of inorganics in water treatment, Acc. Chem. Res. (2019), https://doi.org/10.1021/acs.accounts. 8b00668. B. Pan, D. Chen, H. Zhang, J. Wu, F. He, J. Wang, J. Chen, Stability of hydrous ferric oxide nanoparticles encapsulated inside porous matrices: effect of solution and matrix phase, Chem. Eng. J. 347 (2018) 870–876, https://doi.org/10.1016/j.cej. 2018.04.130. P.S. 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Removal, Lehigh Univ. Diss., 2019, p. 2019. [42] M.S. German, H. Dong, A. Schevets, R.C. Smith, A.K. SenGupta, Field validation of self-regenerating reversible ion exchange-membrane (RIX-M) process to prevent sulfate and silica fouling, Desalination 469 (2019) 114093, , https://doi.org/10. 1016/j.desal.2019.114093. [43] J. Li, Hybrid Ion Exchange Processes for Sustainable Softening and Marcellus Flowback Treatment Hybrid Ion Exchange Processes for Sustainable Softening and Marcellus, Lehigh Univ. Diss., 2018. [44] A.K. Sengupta, H. Dong, M. German, C. Shepsko, Contaminants Removal with Simultaneous Desalination Using Carbon Dioxide Regenerated Hybrid Ion Exchanger Nanomaterials, U.S. Patent US20180273401A1, 2018. https:// patentimages.storage.googleapis.com/43/56/6f/635126869a4c19/ US20180273401A1.pdf. [45] K. Cherukumilli, T. Maurer, J.N. Hohman, Y. Mehta, A.J. Gadgil, Effective remediation of groundwater fluoride with inexpensively processed indian bauxite, Environ. Sci. Technol. 52 (2018) 4711–4718, https://doi.org/10.1021/acs.est. 7b05539.
[37] T.A. Saleh, M. Tuzen, A. Sari, Magnetic activated carbon loaded with tungsten oxide nanoparticles for aluminum removal from waters, J. Environ. Chem. Eng. 5 (2017) 2853–2860, https://doi.org/10.1016/j.jece.2017.05.038. [38] S. Agarwal, I. Tyagi, V.K. Gupta, A.R. Bagheri, M. Ghaedi, A. Asfaram, S. Hajati, A.A. Bazrafshan, Rapid adsorption of ternary dye pollutants onto copper (I) oxide nanoparticle loaded on activated carbon: Experimental optimization via response surface methodology, J. Environ. Chem. Eng. 4 (2016) 1769–1779, https://doi.org/ 10.1016/j.jece.2016.03.002. [39] S. Padungthon, M. German, S. Wiriyathamcharoen, A.K. Sengupta, Polymeric anion exchanger supported hydrated Zr(IV) oxide nanoparticles: a reusable hybrid sorbent for selective trace arsenic removal, React. Funct. Polym. 93 (2015) 84–94, https:// doi.org/10.1016/j.reactfunctpolym.2015.06.002. [40] A.K. Sengupta, S. Padungthon, Hybrid anion exchanger impregnated with hydrated zirconium oxide for selective removal of contaminating ligand and methods of manufacture and use thereof, U.S. Patent No. 9,120,093. 1 Sep. 2015., 2015. [41] C. Shepsko, Advancement of the Hybrid Ion Exchange Nitrogen and Phosphate (HIX-NP) Recovery from Waste Water and a New Approach to Brine-free Nitrate
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Hybrid nitrate selective resin (NSR-NanoZr) for simultaneous selective removal of nitrate and phosphate (or fluoride) from impaired water sources
T
Hang Dong*, Chelsey S. Shepsko, Michael German, Arup K. SenGupta Department of Civil and Environmental Engineering, Lehigh University, 1 W. Packer Ave, Bethlehem, PA, 18015, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Hybrid ion exchange Selective removal Nitrate Phosphate Fluoride
Reverse osmosis (RO) has been increasingly applied in municipal wastewater reuse facilities. Nitrate (N) and phosphate (P) -laden reject streams from RO demand near-complete NP removal before inland brine disposal to avoid serious eutrophication effects. In addition, elevated nitrate and fluoride levels in groundwater is a threat to public drinking water safety. Selective removal of both nitrate and phosphate, or fluoride through a single adsorbent is promising because of its small footprints, minimum sludge production, and potential NP recovery. This study presents a new ion exchange adsorbent, NSR-NanoZr, for simultaneous selective removal of both nitrate and phosphate, or fluoride. NSR-NanoZr is fabricated by doping zirconium oxide nanoparticles (ZrO2Nano) into a nitrate selective resin (NSR), which tributyl quaternary ammonium functional groups contribute to the nitrate selectivity and Lewis acid-base interactions between ZrO2-Nano and ligands result in the high affinity towards phosphate and fluoride. Lab-scale studies showed that NSR-NanoZr exhibited a completely reversal selectivity and removed both NP selectively from municipal secondary wastewater for multiple cycles. Synergistic application of a weak-acid cation exchanger (WAC) enhanced the phosphate capacity of NSR-NanoZr due to the amphoteric sorption behavior of the ZrO2-Nano. Field-scale studies demonstrated the potential of NSR-NanoZr in removing fluoride together with nitrate.
1. Introduction
in agricultural land shows clear evidence of an increase in nitrate concentration in ground water [14]. The crisis is further worsened by already existing naturally contaminated water with fluoride, which is responsible for skeletal and dental fluorosis of people drinking from such ground water sources [15]. An increase in nitrate concentration beyond the permissible level of 45 mg/L nitrate by WHO poses special challenges because of the need for removal of both fluoride and nitrate. Understandably, biological denitrification [16] or catalytic reduction [17] will not attain any fluoride removal. Similarly, activated alumina, which is universally used for fluoride removal, lacks ability to remove nitrate [18]. Ideally, a single sorbent, which is robust, chemically stable, selective and regenerable, will be appropriate if both nitrate and phosphate, or fluoride can be removed concurrently in the presence of other commonly occurring anions, namely, sulfate and chloride. However, with many emerging materials being increasingly reported [19–28], there are still in lack of materials with excellent selectivity and regenerability for co-removal of nitrate and phosphate, or fluoride. The primary objective of this study is to present a hybrid ion exchange material that is: first, selective toward nitrate, phosphate and fluoride; and second, amenable to regeneration and reuse. Additionally, these
Municipal wastewater after secondary treatment invariably contains a significant amount of nitrate and phosphate that are receiving close scrutiny primarily for two reasons: first, as limiting nutrients, they pose threats of eutrophication including algal blooms into receiving bodies of water [1–4] and second, added difficulties in waste water recovery and reuse for inland plants [5–8]. Especially in arid areas with relatively low flow in natural water bodies, the adverse impact of discharge of nitrate and phosphate is more pronounced due to algal blooms and consequent drops in the dissolved oxygen concentration. For inland municipal wastewater plants away from coastal areas, water reuse encounters very challenging, if not insurmountable, problems because the reject from reverse osmosis (RO) process is very rich in nitrate (N) and phosphate (P) [9]. From a drinking water perspective, the presence of nitrate ions in drinking water has been regulated by the USEPA with a maximum limit of 10 mg/L as N for safe drinking water [10]. Nitrate in drinking water is a serious concern for the public health leading to increased risk of infant methemoglobinemia, or “blue-baby syndrome” [11–13]. Excess application of N-based fertilizer around the world for enhanced fertility
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Corresponding author. Present address: 443 Via Ortega, Room SB27, Stanford, CA, 94305, United States. E-mail address: [email protected] (H. Dong).
https://doi.org/10.1016/j.jece.2020.103846 Received 1 January 2020; Received in revised form 18 February 2020; Accepted 9 March 2020 Available online 10 March 2020 2213-3437/ © 2020 Elsevier Ltd. All rights reserved.
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sorbents would be forgiving to fluctuations in influent concentrations i.e., the treated water quality pertaining to target contaminants will remain unaffected during the fixed-bed process through occasional changes in the influent quality.
constant values (KNO3/SO4) for different alkyl groups are also inserted in the figure [32]. Nitrate selective resins (NSR) with butyl ammonium functional groups are thus developed but NSRs do not exhibit high affinity neither for phosphate nor fluoride because the sole electrostatic interaction between ammonium groups and adsorbates is non-specific and phosphate and fluoride adsorptions are highly competed by other anions. Recent studies have demonstrated that integration of metal oxide nanoparticles with different host materials can be a promising strategy to develop multifunctional composites [33–38], especially by introducing nanoscale zirconium oxide particles into polymeric ion exchangers, sorption affinity for anionic ligands including phosphate can be greatly enhanced [39,40]. Thus, an NSR with dispersed ZrO2-Nano (NSR-NanoZr) has two characteristically distinctive sorption sites: tributyl quaternary ammonium functional groups that can sorb nitrate in preference to divalent sulfate and surface sorption sites of ZrO2-Nano that can selectively bind phosphate and fluoride through Lewis acid-base (LAB) interaction i.e., formation of inner-sphere complexes. Fig. 1B provides an illustration of the NSR-NanoZr with dual functional groups and illustrates surface sorption sites of ZrO2 nanoparticles for phosphate through formation of monodentate or bidentate complexes. Sulfate and Chloride, on the contrary, can only form outer sphere complexes and thus, may not compete with phosphate. It is pertinent to note that once exhausted, both nitrate and phosphate can be simultaneously desorbed from NSR-NanoZr with relatively dilute KOH (2% m/v) solution. The primary objectives of the paper are to prepare NSR-NanoZr and characterize its salient properties in relation to concurrent nitrate and phosphate removal from a real secondary wastewater. Its selective fluoride removal capacity is also to be demonstrated with field tests.
1.1. Underlying concept and development of concurrent nitrate and phosphate selectivity Since sulfate, a divalent anion, is invariably present in every treated wastewater, nitrate/sulfate selectivity is an important parameter in the design of an ion exchange process for selective nitrate removal. For a typical strong-base anion exchanger with electrostatic interaction as the primary binding mechanism, the selectivity sequence for common anions stands as follows: SO42− > NO3− > Cl− Thus, the presence of sulfate in the water greatly suppresses nitrate removal capacity, thus impairing the performance of the process. Also, phosphate in the wastewater, either in the form of H2PO4− or HPO42-, has lower affinity for anion exchangers than sulfate. Thus, we need an appropriately tailored anion exchanger with relatively high affinity for nitrate and phosphate over sulfate for a viable recovery process. Previous studies [29–31] have demonstrated that both the matrix and the functional group influence monovalent/divalent selectivity for anion exchange resins. Specifically, for polystyrene matrix, by changing the alkyl groups of the quaternary ammonium functionality from methyl to ethyl to propyl to butyl as shown in Fig. 1A, the nitrate/sulfate selectivity is greatly enhanced i.e., monovalent nitrate is increasingly favored over divalent sulfate, because divalent ion adsorption is thermodynamically unfavorable when the distance between two neighboring adsorption sites is increased with longer chained alkyl groups. [32] The gradual increase in nitrate/sulfate ion exchange equilibrium
Fig. 1. (A) Nitrate/sulfate selectivity (KNO3/SO4) values with changes in the alkyl groups of the quaternary ammonium functionality. (B) An illustration depicting preferred sorption of phosphate through formation of mono- and bidentate inner sphere complexes at zirconium oxide surface; and a schematic representation of an anion exchange resin with tributyl ammonium functional groups dispersed with zirconium oxide nanoparticles for selective removal of both nitrate and phosphate. 2
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Fig. 2. (A) Nitrate selective resin beads doped with zirconium oxide nanoparticles (B) TEM image showing zirconium nanoparticles on the polymer phase and (C) enlarged zirconium nanoparticles. (D) SEM image showing the macroporous structure of the NSR-NanoZr resin (E) EDX spectra of the NSR-NanoZr resin illustrating presence of elemental zirconium inside the beads (F) Size distribution of the doped particles.
2. Materials and experimental methods
in diameter and 250 mm in length) were used, along with constant-flow stainless steel pumps (Fluid Metering International, Syosset, NY) and fractional collectors (Eldex, Napa, CA). Experimental setup was shown in Figure S1 including a single column process illustrating both contaminant removal and regeneration cycles, and a two-column train setup when two different resins were applied. Field-scale fluoride mitigation studies were carried out inside a school premise in Nalhati village, in Birbhum district in West Bengal, India (N 24°17.68320′, W 087°49.55640′).
2.1. Materials A nitrate selective resin, A520E, commercially available from Purolite Co. (headquartered in Bala Cynwyd, PA) was used as the parent resin to make NSR-NanoZr. A520E resin carries tributyl quaternary ammonium groups and has a polystyrene-divinylbenzene matrix. Salient properties of A520E are listed in Supporting Information Table S1. The zirconium oxide material was obtained from MEL Chemicals (Flemington, New Jersey). Zirconium oxide nanoparticle loading procedures were carried out by following the recipe described earlier [40]. Briefly, nitrate selective resins were soaked into zirconium solution (10 % w/v zirconium oxide dissolved in 25 % v/v sulfuric acid) first, followed by mixing the decanted resins with 5% NaOH solution to precipitate hydrated zirconium oxide onto the pore structure of the resin. Fig. 2A shows the NSR-NanoZr resin beads at approximately 0.30.6 mm in diameter. High resolution transmission electron microscopy (HRTEM) (JEOL JEM-2010 F) of NSR-NanoZr in Fig. 2B and C shows the zirconium oxide nanoparticles inside the polymer phase. They were characterized by scanning electron micrograph (SEM) (Model Hitachi JSM-4300) with energy dispersive X-ray spectroscopy accessories (EDX). The SEM image in Fig. 2D shows the macroporous structure of the parent A520E resin bead. An approximate 10 % zirconium loading rate by mass was confirmed by EDX as shown in Fig. 2E and F shows the size distribution of the ZrO2 nanoparticles inside the NSR-NanoZr that indicating the nano-scale metal oxide particles.
2.3. Chemical analyses and isotherm test at varying pH Anions (i.e. chloride, nitrate and sulfate) were analyzed using Dionex Ion Chromatography (IC model ICS-1000) equipped with an IonPac AS14 column. Phosphate was analyzed by HACH UV–vis spectrophotometer (model DR 5000). Calcium and sodium were analyzed by a Perkin Elmer AAnalyst 200 Atomic Absorption Spectrometer (AAS). Total dissolved solids (TDS) of the water was measured using a conductivity detector (Fisherbrand accumet AP75). Zeta potential and the zero-point charge (ZPC) for zirconium oxide precipitates were measured using the Malvern Zeta-sizer Nano ZS with MPT-2 Titrator using NaNO3 at 0.1 M ionic strength. A batch isotherm test was conducted to determine the phosphorus uptake capacity of NSR-NanoZr at varying pH. Isotherm tests were completed via shaker at 25 °C and equilibrium concentrations were measured after 2–3 days, which was determined based on preliminary kinetic experiments to ensure equilibrium. 3. Results and discussion
2.2. Bethlehem wastewater, fixed-bed column experiments and field-scale fluoride mitigation
3.1. Reversal of nitrate and phosphate Selectivity
Bethlehem wastewater was used as the influent for fixed-bed column experiments. The influent wastewater was collected in a 50 L batch from the secondary effluent at the Bethlehem Wastewater Treatment Plant (Bethlehem, PA) (N 40°37′2.433″, W 75°20′4.585″) and filtered through an 11 μm filter before use. Glass columns (11 mm
Figs. 3A and 3B provide the effluent histories of two column runs using secondary wastewater from the Bethlehem Wastewater Treatment Plant (BWTP) under identical conditions except that the first run (Fig. 3A) used traditional strong-base anion exchange resin, SBA (Purolite A502), while NSR-NanoZr was used for the second run (Fig. 3B). 3
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Fig. 3. (A) Effluent sulfate, nitrate and phosphate histories for traditional strong base anion exchange resins (SBA) using secondary wastewater from the Bethlehem Wastewater Treatment Plant in Bethlehem, PA. (Influent composition: 2.4 mg/L P, 100 mg/L NO3−, 20 mg/L SO42-, 69 mg/L Cl−, 50 mg/L Ca2+, 466 mg/L TDS, pH 7.0, EBCT 4.5 min.) (B) Effluent sulfate, nitrate and phosphate histories for NSR-NanoZr using the same influent. (C)Separation factor of phosphate over sulfate (αP/S) and nitrate over sulfate (αN/S) for SBA and NSR-NanoZr. (D) SEM maps of zirconium (Zr) from a parent NSR-NanoZr bead and phosphate (P), sulfate (S) and chloride (Cl) following exhaustion.
The Y-axis represents the normalized concentration of an individual anion with respect to its influent concentration (C0). Note that the breakthrough sequence of phosphate, nitrate and sulfate were completely reversed from Fig. 3A–B. With SBA (Fig. 3A), phosphate broke through first followed by nitrate and sulfate, confirming that divalent sulfate (SO42−) is preferred over nitrate and phosphate. However, based on the breakthrough sequence in Fig. 3B, the preference or the selectivity sequence for NSR-NanoZr is just the opposite and stands as follows:
αN/ S =
For Fig. 3B, sulfate was well removed for 300 BVs due to efficient anion exchange before resin capacity depletion. However, sulfate breakthrough occurred much earlier than nitrate and phosphate and underwent chromatographic elution, i.e., the concentration of sulfate after breakthrough is greater than the influent (C/C0 > unity), implying the accumulated sulfate on resin was eluted by a more resin-preferred ion, e.g., nitrate in this case. This observation confirmed higher nitrate selectivity of NSR-NanoZr over sulfate [32]. A separation factor (αi/j) is a dimensionless, relative affinity parameter for two competing ions i and j; between phosphate and sulfate, the separation factor or αP/S is defined as
yP x S xP yS
(2)
Using effluent histories in Figs. 3A and 3B, separation factors, αP/S and αN/S, were computed for both SBA and NSR-NanoZr [41,42]. A separation factor higher than unity indicates higher phosphate or nitrate selectivity than sulfate, and lower than unity indicates higher sulfate selectivity. Fig. 3C shows: αP/S and αN/S are less than unity for SBA and greater than unity (3.1 and 1.7) for NSR-NanoZr. A reversal of separation factor values of αP/S and αN/S from less than unity to greater than unity indicates higher phosphate and nitrate selectivity than sulfate by NSR-NanoZr, which is the underlying reason for the reversal of the breakthrough sequence for phosphate, nitrate and sulfate between conventional SBA and NSR-NanoZr. Note that due to the higher phosphate and nitrate selectivity of NSR-NanoZr than SBA, NSR-NanoZr exhibited three times higher phosphate capacity and two times higher nitrate capacity than SBA. In order to further validate the sorption mechanism of sulfate and phosphate at two different sites of NSR-NanoZr, SEM-EDX mapping was carried out using slices of both parent and exhausted exchanger. The mappings presented in Fig. 3D show: (i) Zr (green) in the parent exchanger, (ii) phosphorus (red) in the exhausted bead, and (iii) sulfur (representing sulfate) and chloride in the exhausted bead. Note that P distribution merges with Zr in the annular portion of the NSR-NanoZr bead but P is absent near the core of the bead where Zr is absent as well. This observation provides strong evidence that phosphate is adsorbed almost solely by zirconium oxide nanoparticles. In comparison, sulfate and chloride are distributed throughout the bead including the core confirming that quaternary ammonium groups are the primary sorption sites for sulfate and chloride, which both ions are subject to only
Phosphate > Nitrate > Sulfate
αP/ S =
yN x S xN yS
(1)
where yP and yS denote the equivalent fraction of phosphate or sulfate in the exchanger phase, respectively, and xP and xS denote the corresponding equivalent fractions in the aqueous phase. Similarly, the nitrate/sulfate separation factor is equal to 4
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Fig. 4. Effluent history plots of (A) nitrate and (B) phosphorus with secondary wastewater from the Bethlehem Wastewater Treatment Plant for consecutive column runs (Influent composition: 2.4 mg/L P, 100 mg/L NO3−, 32 mg/L SO42-, 69 mg/L Cl−, 50 mg/L Ca2+, 466 mg/L TDS, pH 7.0, EBCT 4.5 min.) And successive regeneration elution curves for (C) nitrate and (D) phosphate using 2% KOH as the regenerant solution.
desorbed from the zirconium sites due to strong expelling by the negatively charged surface sorption sites. Thus, very efficient regeneration is attainable using an alkaline solution, KOH, as shown in Fig. 3D. Zeta potential and corresponding capacity tests in Fig. 5A demonstrated that an increase of pH gradually transformed the zirconium surfaces negatively charged. Consequently, a decrease in phosphate removal capacity was exhibited with an increase in pH as observed in experimental results of Fig. 5A. For municipal wastewater in general, including Bethlehem wastewater used in the tests, alkalinity (HCO3−) is invariably present as a major dissolved constituent. Passage of treated wastewater through a weak-acid cation exchanger (WAC) will constantly generate a stream with an adjusted pH in the range 4–5 without changing the anion composition in the water. In addition, introduction of the WAC reduces the total dissolved solids and hardness removal. Column tests with and without WAC in front NSR-NanoZr were conducted and pH and phosphate effluent histories were recorded. Fig. 5B shows that, at constant feed wastewater with pH around 7 without the aid of WAC, effluent history of phosphate gradually reached 20 % breakthrough at less than 500 bed volumes. With WAC in front of the column and an adjusted pH around 4.5, the phosphate effluent history lasted for over 1200 bed volumes until 20 % breakthrough was achieved. Note that WAC can be regenerated using pressurized carbon dioxide (CO2) without needing any mineral acid. [12,44] Supplementary Figure S2 provides experimental validation of efficient desorption of hardness by using CO2 at 10.2 atmosphere. Sorption reaction leading to hardness removal and desorption with CO2 are represented by the following reactions: Sorption:
electrostatic interaction with the resin. Nitrate mapping is not shown because of the abundant presence of nitrogen in the background due to amine functional groups of the parent nitrate-selective resin. 3.2. Fixed-bed column runs with secondary wastewater and regenerability Figs. 4A and 4B show effluent histories of nitrate and phosphate using NSR-NanoZr resin with Bethlehem wastewater. Note that both nitrate and phosphate breakthrough at significantly long running lengths, 300 and 1000 bed volumes (BV), respectively. After each cycle, the column was regenerated with 2% KOH and 3 cycles of effluent histories were recorded. Fig. 4C and D show nitrate and phosphate elution profiles for two consecutive cycles of regeneration with nearly identical effluent histories for both ions. More than 90 % recovery of nitrate and phosphate were achieved in less than 6 bed volumes. Successful and repeatable service and regeneration data demonstrate that NSR-NanoZr is efficient for both sorption and desorption of nitrate and phosphate and sustainable for long-term application. The fact that phosphate breakthrough remained nearly the same for 3 consecutive cycles provided evidence that there was no zirconium loss from the anion exchanger. Earlier work [43] also confirmed absence of zirconium particles in the treated water. Note that nitrate and phosphate were highly concentrated during regeneration, along with potassium in the regenerant, generating a nutrient rich stream containing NPK, a potential liquid fertilizer recovered from wastewater. 3.3. Effect of pH and synergy of using a weak acid cation exchanger Zirconium oxide surfaces are amphoteric; they are positive at pH ≤ 5.0 and thus exhibit strong affinity toward phosphate, which is essentially an anionic ligand [15,39] and subject to both electrostatic and Lewis acid-base interactions. While at pH ≥ 11, phosphate will be
2RCOOH + Ca2 + (aq) + 2HCO3 − (aq) → |(RCOO−)|2 Ca2 + + 2H2 O + 2CO2 ↑ Desorption: 5
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Fig. 5. (A) Phosphorus removal capacity of NSR-NanoZr and zeta potential of the hydrated zirconium oxide particles with changes in pH (B) Phosphate column runs using NSR-NanoZr resin with the same Bethlehem secondary wastewater with and without the aid of WAC under otherwise identical conditions and corresponding changes in pH.
Fig. 6. (A) onsite fluoride influent and effluent histories (B) efficient regeneration with dilute solution of 2% NaOH and 2% NaCl (C) SEM-EDX mapping of exhausted NSR-NanoZr.
the treated water has consistently been less than 1.5 mg/L (acceptable limit as per WHO or World Health Organization standards); after about one year of operation, fluoride gradually exceeded 1.5 mg/L and the hybrid anion exchanger, NSR-NanoZr, was regenerated; and, fluoride in the treated water during the subsequent service run again dropped below 1.5 mg/L for over a year. Fig. 6B demonstrates the high efficiency of regeneration with dilute solution of 2% NaOH and 2% NaCl: over 95 % of fluoride was eluted during the regeneration process. Again, slices of exhausted NSR-NanoZr were characterized by SEM-EDX mapping and Fig. 6C demonstrates i) Zr (pink) in the parent exchanger; and ii) fluoride (red) in the exhausted bead. Note that similar to the observations made with phosphate, the presence of Zr in the parent resin and F in the exhausted resin merge with each other, implying that fluoride is adsorbed almost solely by zirconium oxide nanoparticles. Thus, NSR-NanoZr offers opportunities for treating impaired groundwater contaminated with both fluoride and nitrate around the world.
2CO2 (g) + 2H2 O↔ 2H2CO3
(RCOO−)2 Ca2 + + 2H2CO3 ↔ 2RCOOH + Ca2 + + 2HCO3− Note that the process is reversible and sustainable in an environmentally sustainable manner without needing any mineral acid. 3.4. Simultaneous removal of fluoride as a co-contaminant Over 200 million people in the world are threatened with exposure to high concentrations of fluoride in naturally contaminated groundwater, especially in Eastern Africa, Central America and Asia [45]. An elevated nitrate concentration in groundwater is being detected in increasing number of locations, including many fluoride-contaminated areas [14,18] around the world due to over-application of ammoniabased fertilizer. Nitrate-selective resins are appropriate sorbents for removing nitrate alone from groundwater. In a separate field-scale study, NSR-NanoZr was used to validate its fluoride removal capacity and regenerability in a fluoride-contaminated groundwater on a school premise in the Birbhum district of West Bengal, India (N 24°17.68320′, W 087°49.55640′). The Global Innovation Initiative (GII) project under the aegis of the US State Department and the Government of India (GoI) Department of Science and Technology (DST) provided resources to establish fluoride-safe water systems in a village school; Supplementary Figures (S3 A, B and C) show the photographs [18]. Fig. 6A provides the fluoride influent and effluent histories of the field-scale plant for over two years of operation and the following observations are noteworthy: first, while the influent fluoride is 17 mg/L,
4. Major observations and path forward Among various sorbents, ion exchange resins are durable, cost effective, selective and regenerable. However, for specific applications, such as the emerging municipal wastewater reuse, their specific affinities need to be modified and/or tailored. Nitrate selective resins have been designed to be monovalent selective through engineered tributyl quaternary ammonium functional groups. However, lack of phosphate selectivity impedes its application for simultaneous nitrate and phosphate removal and their possible recovery from impaired water and wastewater streams. Polyvalent metal oxides, such as Zr (IV) oxides, 6
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offer high sorption affinity toward anionic ligands including phosphate and fluoride. This hybrid ion exchange sorbent has two distinctive sorption sites different from each other: one for relatively hydrophobic monovalent nitrate and the other for anionic ligands. Major findings of the study can be summarized as follows:
[9] [10]
• Nitrate and phosphate selectivity of NSR-NanoZr greatly exceed • • •
[11] [12]
divalent sulfate, thus offering a realistic opportunity to remove and concentrate both N and P from wastewater in the presence of sulfate and chloride. NSR-NanoZr can be regenerated using a single regenerant, namely KOH, thus producing a spent regenerant rich in N, P and K with practically no chloride and sulfate. For ground waters naturally contaminated with fluoride (F−) and with gradually increasing nitrate beyond the maximum contaminant level (MCL) in drinking water, NSR-NanoZr can be an appropriate sorbent to mitigate the crisis. NSR-NanoZr is durable, chemically stable, selective and amenable to excellent reuse over multiple cycles of operation.
[13]
[14]
[15]
[16]
[17]
CRediT authorship contribution statement
[18]
Hang Dong: Conceptualization, Investigation, Visualization, Data curation, Writing - original draft. Chelsey S. Shepsko: Investigation. Michael German: Writing - review & editing. Arup K. SenGupta: Writing - review & editing, Supervision, Funding acquisition.
[19]
[20]
Declaration of Competing Interest
[21]
There are no conflicts to declare. [22]
Acknowledgements [23]
Three-year grant from the US State Department through Global Innovation Initiative is gratefully acknowledged. This work was also partially supported by the Pennsylvania Infrastructure Technology Alliance (PITA) from the Commonwealth of Pennsylvania.
[24]
Appendix A. Supplementary data
[25]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jece.2020.103846.
[26]
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