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1 SUSTAINABILITY OF ION EXCHANGE REGENERATION WITH SODIUM, POTASSIUM, CHLO RIDE, AND BICARBONATE SALTS By GABRIEL MAUL A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUI REMENTS FOR THE DEGREE OF MASTER OF ENGINEERING UNIVERSITY OF FLORIDA 2013
2 2013 Gabriel Maul
3 To my Mom, Pilar, and Dad, Ted
4 ACKNOWLEDGMENTS I acknowledge my advisor, Dr. Boyer, for many years of su pport, guidance, and patience, my thesis committee members, Dr. Chadik and Dr. Delfino for helping to improve the work with their feedback, my research group for the intellectual support my girlfriend, Ana, for reassuring and supporting me, my parents, Pi lar and Ted, for their love and for inspiring me to pursue graduate school.
5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 6 LIST OF FIGURES ................................ ................................ ................................ .......... 7 LIST OF ABBREVIATIONS ................................ ................................ ............................. 8 ABSTRACT ................................ ................................ ................................ ..................... 9 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 11 1.1. Motivation ................................ ................................ ................................ ........ 11 1.2. Ion Exchange Equilibrium and Kinetics ................................ ............................ 12 1.3. Ion Exchange Resin P roperties ................................ ................................ ....... 13 1.4. Ion Exchange Affinity of Na + K + Cl and HCO 3 ................................ ............ 15 1.5. Alternative Regenerants for Ion Exchange ................................ ...................... 17 1.6. Ion Exchange Regenerant Disposal ................................ ................................ 17 1.7. Combined Ion Exchange ................................ ................................ .................. 18 1. 8. Critical Knowledge Needed ................................ ................................ .............. 19 1.9. Goal and Objectives ................................ ................................ ......................... 19 2 MATERIALS AND METHODS ................................ ................................ ................ 21 2.1. Ion Exchange Resins ................................ ................................ ....................... 21 2.2. Regeneration Agents and C hemical C ontaminants ................................ ......... 22 2.3. Regeneration E xper iments ................................ ................................ .............. 22 2.4. Analytical M ethods ................................ ................................ ........................... 24 3 RESULTS ................................ ................................ ................................ ............... 30 4 DISCUSSION ................................ ................................ ................................ ......... 38 4.1. Effect of Resin Properties ................................ ................................ ................ 38 4.2. Economic Considerations ................................ ................................ ................ 41 4.3 Environmental Impacts ................................ ................................ ..................... 43 4.4 Engineering Considerations ................................ ................................ .............. 48 5 CONCLUSION ................................ ................................ ................................ ........ 54 LIST OF REFERENCES ................................ ................................ ............................... 56 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 63
6 LIST OF TABLES Table page 2 1 Properties of selected ion exchange resins ................................ ........................ 26 2 2 Ion exchange resin drying parameters ................................ ............................... 27 2 3 Experimental matrix for b atch equilibrium regenerations of anion exchange resins ................................ ................................ ................................ .................. 28 2 4 Experimental matrix for batch equilibrium regenerations of cation exchange resins ................................ ................................ ................................ .................. 28 2 5 Dosing parameters for regeneration experiments that vary anion exchange resins ................................ ................................ ................................ .................. 29 2 6 Dosing parameters for regeneration experiments that vary cation exchange resins ................................ ................................ ................................ .................. 29 3 1 Paired t tests of ions used to regenerate ion exchange resins ........................... 34 3 2 Paired t tests of ion exchange resins regenerated by N a + and K + ...................... 36 3 3 Paired t tests of ion exchange resins regenerated by Cl and HCO 3 ................ 36 4 1 Economic comparison of ion exchange r egenerants ................................ .......... 50 4 2 Environmental impacts of selected alternative regenerants for ion exchange .... 51 4 3 The effect of metal cations on anaerobic digestion ................................ ............. 53
7 LIST OF FIGURES Figure page 3 1 Regeneration of C100/A520E ion exchange resins with NaCl, NaHCO 3 KCl, and KHCO 3 ................................ ................................ ................................ ......... 32 3 2 Regeneration of paired ion exchange resins with NaCl and KHCO 3 by resin pair ................................ ................................ ................................ ..................... 33 3 3 Regeneration of paired ion exchange resins with NaCl and KHCO 3 by regenerant ion ................................ ................................ ................................ .... 35 3 4 Na + and K + regeneration of C100 cation exchange resin paired with m ultiple anion exchange resins ................................ ................................ ........................ 37 3 5 Regeneration of A520E anion exchange resin with Cl and HCO 3 paired with mu ltiple cation exchange resins ................................ ................................ .......... 37
8 LIST OF ABBREVIATIONS AER Anion exchange resin AER NO 3 Anion exchange resin in nitra te form Ca 2+ Calcium CaCO 3 Calcium carbonate CER Cation exchange resin CER Ca Cation exchange resin in calcium form Cl Chloride DI Deionized HCO 3 Bicarbonate K + Potassium MIEX HCO 3 Magnetic ion exchange in bicarbonate form Na + Sodium NOM Natural organic matter SO 4 2 Sulfate TDS Total dissolved solids
9 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Engineering SUSTAINABILITY OF ION EXCHANGE REGENERATION WITH SODIUM, POTASSIUM, CHLO RIDE, AND BICARBONATE SALTS By Gabriel Maul May 2013 Chair: Treavor H. Boyer Major: Environmental Engineering Sciences Ion exchange (IX) is a water treatment technology that requires concentrated sodium chloride (NaCl) brine for regeneration. IX regeneration with salts that can be disposed with less adverse environmental impact than NaCl has the potential to improve the sustainability of ion exchange. Potassium (K + ) and bicarbonate (HCO 3 ) are pote ntial alternative regenerants that have environmental benefits in soils and aquatic systems. The goal of this research was to compare the regeneration efficiency, economic factors, and environmental impacts of salts of Na + K + Cl and HCO 3 for regenerat ion of pairs of anion exchange resin (AER) and cation exchange resins (CER) that have been exhausted with inorganic contaminants. The specific objectives of this research were (1) compare the regeneration efficiency of NaCl, NaHCO 3 KCl, and KHCO 3 for four anion exchange resins (AER) and three cation exchange resins (CER) that were exhausted with nitrate and calcium, respectively, and (2) compare the salts NaCl, NaHCO 3 KCl, and KHCO 3 in terms of costs, environmental impacts, and other factors. At equivale nt capacities, AER had greater regeneration efficiency than CER. Polystyrene AER had higher regeneration efficiency with Cl while polyacrylic AER had higher regeneration efficiency with HCO 3 The ion exchange resin property that
10 improved Cl regeneration efficiency was gel pore structure, and the resin properties that improved HCO 3 regeneration efficiency were polyacrylic polymer composition, gel pore structure, and a short functional group chain. HCO 3 regeneration data suggested deprotonation of HCO 3 into CO 3 2 in the resin phase. CER showed higher regeneration efficiency with K + than with Na + for each resin tested. The salts from most expensive to least were KHCO 3 >> NaHCO 3 > KCl > NaCl, and the order from most soluble to least 3 >> NaHCO 3 The salts with the least adverse environmental impact in aquatic systems and wastewater treatment systems seem to be NaHCO 3 at low to moderate concentrations due to beneficial effects and NaCl at high concentrations. KCl seems to have the least adverse environmental impact in soils in terms of reducing hydraulic conductivity and is an essential nutrient in agricultural crops although K + tolerance varies between plants.
11 CHAPTER 1 INTRODUCTION 1.1. Motivation Ion exchange (IX) is a robust and low energy demand water treatment technology with a wide range of applications, but more sustainable options are needed for IX regeneration and disposal of the concentrated sodium chloride (NaCl) regenerant brine. Concentrated NaCl inhibits bacterial growth, stresses the development of plants, and adversely impacts soil properties although the exact concentration depends on the system and organisms (Bernstei n, 1975; Erickson, et al., 1996; Kincannon and Gauddy Jr., 1966; Ludzack and Noran, 1965; Mohleji and Verhoff, 1980; Mount, et al., 2009; Kugelman and Mccarty, 1965) Also, NaCl is not economically removed or converted from the brine. Therefore, IX regene ration using NaCl produces a concentrated solution (8 20% NaCl) that can adversely affect the environment and is not economical to remove ( Clifford, et al. 2011) However, alternative ions can be used to regenerate IX resi ns instead of NaCl that are known to be more environmentally beneficial. Potassium (K + ) is an essential nutrient for functioning of plants, animals, and humans, and potassium chloride (KCl) fertilizer is the most important compound for supplying K + to the fertilizer industry, increasing growth and resistance to environmental stresses (Wist, et al. 2009; Marschner, 2012) In aquatic systems, HCO 3 increases alkalinity and is a major pH buffer that is critical to t he maintenance of life (Allan and Castillo, 2007) A lkalinity increases resilience of a water body to acidification, an established environmental problem that is exacerbated by acid precipitation from industrial processes (Allan and Castillo, 2007) Therefore, IX regeneration using alternative chemicals such as salts containing K + and HCO 3 have the potentia l to generate IX
12 brine that has a reduced adverse impact on the environment and may have options for beneficial reuse. 1.2. Ion Exchange Equilibrium and Kinetics A wide range of cationic and anionic contaminants can be selectively removed to very low conce ntrations using IX including calcium (Ca 2+ ), magnesium (Mg 2+ ), radium, fluoride, nitrate, arsenate, chromate, perchlorate, and natural organic matter (NOM). Ca 2+ and Mg 2+ removal (softening) is the most common use of ion exchange ( Clifford, et al. 2011) During treatment, the contaminant ion is exchanged onto the active exchange sites of the IX resin while with the presaturant ion is displaced into the feed water. As active exchange sites become saturated with the contamina nt ion, the resin loses the capacity to remove additional contaminants as it approaches operational capacity (exhaustion). Regeneration restores treatment capacity by driving the presaturant ions back onto the resin using a high concentration of regenerant commonly 8 18% NaCl ( Clifford, et al. 2011) Effective IX treatment and regeneration relies on control of the equilibrium between ions and the resin. Equilibrium is determined by several factors: general selectivity prin ciples, ion concentrations in solution, and resin properties. In dilute solutions, IX resins have a greater affinity for the ion with higher charge, smaller hydrated radius, greater polarizability ( Clifford, et al. 2011) Resins that are more hydrophobic will tend to have higher affinity for hydrophobic ions than resins that are more hydrophilic ( Clifford, et al. 2011) IX equilibrium is also a function of both relative ion concentrations a nd absolute ionic strength. Relative ion concentrations are primary factors that cause a shift in equilibrium between the active exchange sites and solution similar to chemical equilibrium behavior. For example, an increase in NaCl concentration in the sol ution will drive more Na + or Cl onto the active exchange sites.
13 However, IX resins also undergo the phenomenon of electroselectivity in which selectivity of resin for divalent ion over a monovalent decreases as ionic strength of the solution increases and the divalent ion activity coefficient decreases (Helfferich, 1962) At high ionic strength, this may lead to selectivity reversal such as in the sulfate chloride binary system where SO 4 2 typical type I strong base anion resin ( Clifford, et al. 2011) Activity coefficients are difficult to predict for ions in many regeneration solutions beca use ionic strength is often greater than 1M (6% NaCl). In addition to equilibrium principles previously discussed, diffusion principles may affect mobility of ions into active sites into the center of IX resins. Diffusion increases proportionally to small er ion size, smaller valence of ions, lower degree of crosslinking, the molar fraction of a particular ion decreases, and temperature increases (Helfferich, 1962) Small resin sizes and shallow shell resins d ecrease average particle diffusion distances, increasing regeneration efficiency due to improved kinetics (Purolite, 2013; Slunjski, et al. 1999) 1.3. Ion Exchange Resin Properties Key resin properties such as polymer composition, functional groups, pore structure, degree of crosslinking, and water content dete rmine specific affinity for ions. Synthetic ion exchange resins are typically made of styrene cross linked with 3 to 8% divinylbenzene copolymer composition ( Clifford, et al. 2011) Resins can also be made of a more hydrop hilic polyacrylic matrix. The hydrophilic nature of the polyacrylic polymer composition has demonstrated to be beneficial to regeneration of NOM (Baker, et al. 1977) Resins with a polyacrylic matrix have better resistance to organic fouling due to the decreased van der Waals forces between the hydrophilic resin and the
14 hydrophobic organic molecules compared to polystyrene resins (Baker, et al. 1977) An acrylic resin (IRA 958) was able to operate through seven regeneration cycles with no obvious loss in TOC removal (Kim and Symons, 1991) Resins with a polyacrylic matrix have approximately 30 times lower affinity than polystyrene resins for ClO 4 a strongly hydrophobic ion ( Clifford, et al. 2011) The common functional groups consist of four categories: strongly acidic (sulfonate, SO 3 ), weakly acidic ( carboxyla te, COO ), strongly basic (quaternary ammonium, N(CH3) 3 + ), and weakly basic (tertiary amine, N(CH 3 ) 2 ) (Helfferich, 1962) Strong acid sulfonate groups are able to remove both carbonate and non carbonate ha rdness, while weak acid carboxylate groups are only able to remove carbonate hardness ( Clifford, et al. 2011) Quaternary amines commonly have a trimethyl group (Type I) ( N(CH 3 ) 3 + ), but longer chains of triethyl groups ( N(C 2 H 6 ) 3 + ) or tripropyl groups ( N(C 3 H 9 ) 3 + ) are also possible ( Sengupta and Marcus, 2004) Divalent ions prefer resins with closely spaced exchange sites where the balancing of the two charges is more favorable (Clifford and Weber, 1983; Sengupta and Cli fford, 1986; Subramonian and Clifford, 1988) In addition, quaternary amine functional groups can either be type I or type II groups. Type II contain a hydroxyl ( OH) group, making Type II AERs more hydrophilic than Type I AERs (Gregory and Dhond, 1972) S imilar type I or type II strong base anion resins showed no significant difference in NO 3 removal ( Clifford and Liu, 1993) Weak base anion resins have pr imary, secondary, or tertiary amine functional groups all of which have higher affinity for divalent anions compared to strong base anion resins with quaternary amines (Boari, et a l. 1974) Therefore, weak base anion resins are used less frequently because most anion contaminants tend to be
15 ubiquitous and reduce treatment capacity and regeneration ef ficiency. Pore structure is an important factor in determining IX resin ion affinity. Conventional gel resins are a homogeneous polymer exhibiting microporosity with pore volumes typically up to 10 to 15 ngstroms (Wheaton and Lefecre, 1999) Macroporous pore structure consists of pore volumes up to several hundred ngstroms that allow e xchange with larger particles; however, they have poorer regeneration efficiencies and lower capacities (Wheaton and Lefecre, 1999) Resins with macroporous structures offer the advantages of greater chemical oxidation resistance, permanent, large pore diameters, and higher resistance to osmotic shock (Abrams and Milk, 1997) Macroreticular resins consist of very small randomly packed gel microparticles with continuous macropores Many macroporous resins also fit this de finition, so the difference between macroporous and macroreticular may be slight (Abrams and Milk, 1997) The active sites on the gel m icroparticles have higher reaction rate constants than the macroporous sites (Ihm, et al. 1988) Water content may also be a factor in ion exchange selectivity. Anion exchangers of open structure and high water content were found to have the most efficient removal of charged aquatic NOM, regardless of hydrophob icity (Bolto, et al. 2002) Low water content has been hypothesized as creating an increase in diffusional resistance and leading to poor regeneration efficiency in cation exchange resins (Greenleaf and Sengupta, 2009) 1.4. Ion Exchange Affinity of Na + K + Cl and HCO 3 IX resin affinity of various ions and Na + and Cl is well understood in full scale ion exchange regeneration (D. Clifford and Liu, 1993; Liu and Clifford, 1996; Flodman and
16 Dvorak, 2012; C lifford, et al., 2011) IX resin affinity of K + and HCO 3 have been less studied. Selectivity constants for alkali metals increase as atomic weight increases (Li + < Na + < K + < Rb + < Cs + ) (Wist, et al. 2009) At a 2 kg total salt dose for regeneration, K + regenerated 13% less capacity compared to Na + ; however, no significant difference in ca pacity resulted in total salt doses that were greater (Wist, et al. 2009) The selectiv ity sequence for affinity of typical type I strong base anion exchange resins is generally accepted as SO 4 2 > NO 3 > Cl > HCO 3 (Holl and Kiehling 1981) Treatment and regeneration with HCO 3 has been studied with magnetic ion exchange (MIEX) resin for the removal of NOM. Treatment with MIEX in HCO 3 form performed similarly to MIEX in Cl form over 3 regeneration cycles (Rokicki and Boyer, 2011) Long term treatment with HCO 3 form MIEX resin decreased performance 7 18% compared to MIEX Cl over 21 regeneration cycles possibly as a result of fouling of an undetermined type (Walker and Boyer, 2011) A study of HCO 3 form MIEX resin interactions with Cl SO 4 2 and NO 3 showed 20% greater NO 3 removal using MIEX HCO 3 with a regeneration ratio (eq ion supplied/eq resin capacity) of 100 compared to 10 (Rokicki and Boyer, 2011) The regeneration ratio is a measure of the regenerant concentration relative to the resin capacity. Using 35 bed volumes (BV) of 1M NaHCO 3 (8.4% NaHCO 3 ), NO 3 form type I polystyrene resin was able to treat >200 BV of feed water, even with regenerant containing SO 4 2 NO 3 and Cl at concentrations ~200 mg/L (Jelnek, et al. 2004) In the formerly cited study, the difference between virgin brine and simulated used brine was ~20% decrease in tre atable bed volumes. Breakthrough capacities for NO 3 removal were typically less with AER HCO 3 compared to AER Cl in column experiments in both nitrate selective resin (triethylamine functional groups) and in Type I non nitrate
17 selective resin (trimethylam ine functional groups), and elution of NO 3 during regeneration required significantly more bed volumes (Matosic, et al. 2000) Regeneration with CO 2 gas and CaCO 3 yielded partial HCO 3 regeneration of AERs (Holl and Kiehling, 1981) 1.5. Alternative Regenerants for Ion Exchange Several a lternat ive regenerants have been proposed to regenerate NaCl with lower environmental impact, but each has its disadvantages compared to conven tional regeneration. Na 2 CO 3 and Na 2 SO 4 have significantly lower solubility s odium citrate and NaOH are expensive, and seawater decreases feed water quality, decreases treatment capacity; and must be pretreat ed to prevent fouling (Wist, et al. 2009) KCl has been shown to have similar capacity regenerated compared to NaCl, reduce sodium in the effluent, and use regenerant more efficiently at the disadvantage of higher TDS in the effluent due to the higher molecular weight of the salts and higher cost (Wist, et al. 2009) 1.6. Ion Exchange Regenerant Disposal IX brine disposal is recognized as a challenge in water treatment. One of the most common brine disposal methods is sani tary sewer discharge, but NaCl typically passes through untreated into the effluent since conventional wastewater treatment is not designed to remove dissolved inorganic contaminants (Clifford, et al. 2011; Flodman and Dvorak, 2012) IX brine disposal to sewer is limited to water quality permit limits of the wastewater treatment plant. These limits are partly based on toxicity to the receiving water body and may include total dissolved solids (TDS) limits and/or whole effluent toxicity (WET) testing with specific t est organism species such as Ceriodaphnia dubia (Goodfellow, et al., 2000) Even if permit limits are achieved, wastewater
18 discharge containing IX brine increases salinity to the environment. For every 100 mg/L TDS increase above 500 mg/L in water supplies, $95 million are required to repair damage to utility infrastructure, agriculture, and industrial facilities (Gritzuk, 2002) For this reason, large scale IX is typically limited to coastal locations where the brine can be discharged to the ocean to accommodate high NaCl loadings ( Clifford, et al. 2011) Several techniques have been developed to reduce the consumption and dis charge of regenerant salt. Partial regeneration involves using only a fraction of salt required for complete regeneration, but can be accompanied with higher leakage during treatment (Flodman and Dvorak, 2012; Liu and Cliff ord, 1996) Regenerant brine reuse has been researched for IX systems that remove perchlorate, nitrate, arsenic, and NOM ( Clifford, et al. 2011) The practice can reduce salt use significantly at the cost of increased pr ocess complexity, the accumulation of ions stripped from the resin during regeneration that reduced treatment capacity and increase contaminant leakage, and the need to store and handle spent regenerants ( Clifford, et al., 2011; Clifford and Liu, 1993; Flodman and Dvorak, 2012) For some ions, biodegradation in the b rine allows additional brine reuse by removing the accumulated ions in the brine. Denitrified brine from NO 3 removal was reused up to 10 times without increasing nitrate leakage past the MCL (Liu and Clifford, 1996) 1.7. Combined Ion Exchange Combined ion exchange, the practice of using both a cation and anion exchange resin in a mixed reactor has the potential to increase the efficiency of brine use by using both the cation and anion of the salt to regenerate resins. Combined ion exchange treatment of hardness and dissolved organic matter achieved >55% hardness removal
19 and 70% DOC removal using 16 mL/L of MIEX Na and 2 mL/L MIEX Cl with the same brine solution (Apell and Boyer, 2010) 1.8. Critical Knowledge Needed Previous research has provided a plethora of regeneration results using Na + and Cl salts for different resins and contaminants, but regeneration using K + and HCO 3 has only been studied for a few resins (Walker and Boyer, 2011; Wist, et al. 2009) The role of resin properties in regeneration of these ions is not fully understood. The complex factors of ion exchange equilibrium, ion concentrations, and resin properties introduce significant variability and are not fully understood for K + and HCO 3 in ion exchange. Few regeneration experiments have directly compared the regeneration eff iciency of Na + to K + and Cl to HCO 3 Although K + and HCO 3 have recognized benefits over Na + and Cl in soils and aquatic systems, the economic and environmental impacts of these ions have not been fully investigated in the context of IX brine disposal. The knowledge of these impacts is essential to improve the sustainability of IX. Combined ion exchange treatment has been shown to be feasible over several regeneration cycles, but it is not fundamentally known how regeneration efficiency varies between c ation exchange resin and anion exchange resin over a wide range of resins. 1.9. Goal and Objectives The goal of this research was to compare t he regeneration efficiency, economic factors, and environmental impacts of salts with Na + K + Cl and HCO 3 for regeneration of pairs of AER and CER that were exhausted with inorganic contaminants. The specific objectives of this research were to (1) compare the regeneration effic iency of NaCl, NaHCO 3 KCl, and KHCO 3 for different pairs of anion
20 exchange resin and cation exchange resin that were exhausted with nitrate and calcium respectively and (2) compare the salts NaCl KCl, NaHCO 3 and KHCO 3 in terms of costs, environmental impacts and other factors
21 CHAPTER 2 MATERIALS AND METHODS 2.1. Ion Exchange R esins Four anion exchange resins (AER) and three cation exchange resins (CER) were selected for regeneration experiments based on differences in resin polymer composition, pore structure, and functional groups ( Table 2 1 ) NO 3 wa s selected as the contaminant ion for AER because it is an ion with a primary drinking water standard with a 10 mg/L maximum contaminant level. NO 3 was selected over natural organic matter because ion exchange reactions do not interact with Ca 2+ Ca 2+ was selected as the contaminant ion for CER because softening is the most common use for ion exchange. The first two resins were selected based on the contaminants used in every regeneration experiment: C100 was selected because it is a resin often used to re move Ca 2+ and A520E was selected because it is a resin often used to remove nitrate selectively over sulfate. The other resins were selected based on covering major polymer compositions, pore structures and functional groups. The ion exchange capacity ref ers to the minimum wet ion exchange capacity as specified by the manufacturer. Virgin cation resin was preconditioned to Ca 2+ form using CaCl 2 while virgin anion resin was preconditioned to NO 3 form using NaNO 3 Resins were mixed for 3h using a concentrat ion of preconditioning solution equivalent to 10 the resin capacity in batches of 20 mL resin in 1 L solution. After preconditioning, resin was rinsed with deionized (DI) water and excess water was decanted 10 using approximately 500 mL of DI water. Prec onditioned resin was stored in a closed borosilicate glass container in approximately 50 mL of DI water.
22 Preconditioned resin was dried in a desiccator prior to dosing ( Table 2 2 ). Resins were dried for a minimum o f 4 d based on preliminary data that showed resin batches stopped decreasing in weight after 4 d. Triplicate batches of 2 4 mL of resin were measured in a 10 mL graduated cylinder and decanted with the resin into three separate weighboats. The liquid in th e weighboats was drained using a pipette against the bottom of the weighboat to suction out the water. After drying, the weight of the resin in each batch was measured, and the dry wet density (mg dry resin/mL wet resin) was calculated. For every resin, th e coefficient of variation (standard deviation/mean) of the dry wet density remained below 7%. 2.2. Regeneration A gents and C hemical C ontaminants The regenerant and preconditioning solutions were made from ACS grade chemicals and deionized water. The foll owing regeneration agents were used : NaCl (CAS 7647 14 5 ), NaHCO 3 ( CAS 144 55 8) KCl (CAS 7447 40 7 ) KHCO 3 (CAS 298 14 6 ) The resin was preconditioned using the following chemicals: NaNO 3 (CAS 7631 99 4 ) CaCl 2 2H 2 O (CAS 10035 04 8 ) 2.3. Regeneration E xperiments Batch equilibrium regeneration experiments were conducted for pair ed resins using each regenerant with regeneration ratios varying from 1 to 300 ( Table 2 3 and Table 2 4 ). All regeneration ex periments were conducted in triplicate in 250 mL bottles on a Thermo Scientific Variomag Poly magnetic stir plate. Based on preliminary kinetics tests with C100 and A520E resins, the resins for all experiments were mixed for 3 h at 300 rpm to reach equilib rium. Previous studies have shown > 95% of equilibrium was attained in 5 min (Horng and Clifford, 1997)
23 Resin dosing was determined by calculating the weight of dry resin dose to add to each bottle, which was different for ea ch resin (Eq. 2 1 ) ( Table 2 5 and Table 2 6 ). dw (2 1 ) Where W = weight of dry resin dose (mg) D = resin dose (mL wet resin/L water) V = water volume (L) dw = dry wet density (mg d ry resin/mL wet resin) The anion dose for all anion resins was set at an arbitrary 1mL/L. Due to the regeneration focus of the regeneration experiments the cation resin dose was calculated to have an equivalent capacit y as the 1mL/L anion resin dose. The refore, both the CER and AER in each experiment had an equivalent molar ion exchange capacity. Before analytical measurement, HCO 3 regenerated samples were prepared further to avoid interference due to precipitated CaCO 3 After mixing HCO 3 regenerated sa mples, 20 mL from each bottle were pipetted and filtered through a Whatman 0.45 um nylon filter and placed into a vial for NO 3 analysis. After mixing Cl regenerated samples, 20 mL from each bottle were directly pipetted into a vial for NO 3 analysis. Pr ior to calcium analysis, HCO 3 regenerated samples including the blank were acidified to carbonate species by driving off CO 2 in order to avoid interference of alkalinity with the total hardness method. Samples remained mixing as they were pH adjusted to 3 4 with 1N or 6N HCl. At this pH, preliminary experiments showed the Ca 2+ displaced due to H + in the acid was <0.5 mg/L as CaCO 3 or 1.1%. After pH adjusting, all samples were re checked to ensure the pH was below 4 before proceeding, and more
24 acid was adde d as needed. Regeneration was quantified by measuring calcium released from cation resins and nitrate released from anion resins divided by the minimum capacity specified by the manufacturer. Regeneration was quantified by measuring Ca 2+ released from cat ion resins or NO 3 released from anion resins divided by the minimum capacity specified by the manufacturer. 2.4. Analytical M ethods An Accumet AP71 pH meter with a temperature and automatic temperature compensation probe and calibrated with pH 4, 7, and 10 standards was used during pH adjustment and calcium titrations. The extent of regeneration was determined by measuring Ca 2+ displaced from the cation resin and NO 3 displaced from the anion resin. Ca 2+ was measured according to a procedure to measure to tal hardness based on Standard Method 2340 (EDTA Titrimetric Method). The calcium standard was tested by mixing 25 mL of standard with 175 mL of DI water For pH adjusted samples, the NH 3 buffer was inadequate to raise the pH to 10 as specified in the meth od, so 5N NaOH was added after 1 mL NH 3 to adjust the pH to 10 +/ 0.2. The amount of NH 3 buffer and NaOH did not affect the amount of titrant necessary to titrate the calcium in the sample, so the volume of these added solutions was not recorded. All titr ations were performed at room temperature within 5 minutes of pH adjustment to minimize CaCO 3 precipitation. Ca 2+ in the sample was calculated according to Eq. 2 2, which was modified to include dilution of the sample from acid added in HCO 3 regenerated s amples. ( 2 2 ) Where A = mL titrant needed for sample
25 B = mg/L CaCO 3 equivalent/1 mL EDTA titrant Total nitrogen was measured using a Shimadzu TOC V CPH total organic carbon analyzer equipped with a Shimadzu TNM 1 and an ASI V au to sampler. TN calibration standards of 0, 0.1, 0.5, 1, and 2 mg/L N were prepared in DI water using NaNO 3 The coefficient of determination, R 2 was > 0.995 for each calibration curve. The samples were diluted a factor of 10 using the machine dilution fun ction. Check standards were measured at the end of each run from both calibration standards as well as a Ricca standard solution. Paired t tests were conducted using MATLAB based on equilibrium regeneration experiments. The analysis was conducted using wit h meq Ca 2+ or NO 3 displaced/IX resin capacity and the standard deviation from triplicate regeneration experiments.
26 Table 2 1 Properties of selected ion exchange resins Resin, Manufacturer Resin Type Functional group Ion exchang e capacity Polymer composition Pore structure Water Content A520E, Purolite Anion exchange R N + (CH 2 CH 3 ) 3 0.9 meq/mL PS MP 50 56% Dowex 22, Dowex Anion exchange R N + (CH 3 ) 2 (CH 2 OH) 1.2 meq/mL PS MP 48 56% Marathon 11, Dowex Anion exchange R N + (CH 3 ) 3 1.3 me q/mL PS G 48 58% IRA958, Amberlite Anion exchange R N + (CH 3 ) 3 0.8 meq/mL PA MR 66 72% C100, Purolite Cation exchange R SO 3 2.0 meq/mL PS G 44 48% SST60, Purolite Cation exchange R SO 3 1.69 meq/mL* PS (shallow shell) G 36 46% C150, Purolite Cation exch ange R SO 3 1.8 meq/mL PS MP 48 53% PS: polystyrene PA: polyacrylic MP: macroporous G: gel MR: macroreticular *The wet resin capacity (meq/mL) for SST60 is not directly given but calculated by proportion from C100, which the same resin properties except f or the shallow shell. C wet,SST60 = (C dry, SST60 /C dry,C100 )*C wet,C100
27 Table 2 2 Ion exchange resin drying parameters Resin, Manufacturer Average Dry wet density (mg dry resin/mL wet resin) Coefficient of Variation Batches of dr ied resin, N A520E, Purolite 0.363 4.0% 18 Dowex 22, Dowex 0.402 3.3% 10 Marathon 11, Dowex 0.421 1.1% 4 IRA958, Amberlite 0.286 6.9% 4 C100, Purolite 0.518 5.5% 11 SST60, Purolite 0.523 1.5% 3 C150, Purolite 0.459 0.8% 3
28 Table 2 3 Experimental matrix for batch equilibrium regenerations of anion exchange resins Cation Exchange Resin [CER] Paired Anion Exchange Resin [AER] Regenerant Regeneration Ratio ( eq ion supplied/eq resin capacity) Corresponding Regenerant Concentr ation Range* (mg/L) C100 Ca A520E NO 3 NaCl 1x, 10x, 30x, 100x, 300x 53 15,780 NaHCO 3 1x, 10x, 30x, 100x, 300x 76 22,682 KCl 1x, 10x, 30x, 100x, 300x 68 20,389 KHCO 3 1x, 10x, 30x, 100x, 300x 90 27,031 Dowex 22 NO 3 NaCl 1x, 10x, 30x, 100x 300x 70 21,039 KHCO 3 1x, 10x, 30x, 100x, 300x 120 36,041 Marathon 11 NO 3 NaCl 1x, 10x, 30x, 100x, 300x 76 22,793 KHCO 3 1x, 10x, 30x, 100x, 300x 130 39,045 IRA 958 NO 3 NaCl 1x, 10x, 30x, 100x, 300x 47 14,026 KHCO 3 1x, 10x, 30x, 100x 300x 80 24,028 Table 2 4 Experimental matrix for batc h equilibrium regenerations of cati on exchange resins Anion Exchange Resin [AER] Paired Cation Exchange Resin [CER] Regenerant Regeneration Ratio ( eq ion supplied/eq re sin capacity) Corresponding Regenerant Concentration Range* (mg/L ) A520E NO 3 C100 Ca NaCl 1x, 10x, 30x, 100x, 300x 53 15,780 KHCO 3 1x, 10x, 30x, 100x, 300x 90 27,031 SST60 Ca NaCl 1x, 10x, 30x, 100x, 300x 53 15,780 KHCO 3 1x, 10x, 30x, 100x, 300x 90 27,031 C150 Ca NaCl 1x, 10x, 30x, 100x, 300x 53 15,780 KHCO 3 1x, 10x, 30x, 100x, 300x 90 27,031 All concentrations were based on a 1 mL/L anion exchange resin dose
29 Table 2 5 Dosing parameters for regenera tion experiments that vary anion exchange resins Resin Pair [Cation/Anion] Resin Dose (mL/L) Regeneration Capacity (meq/mL resin) Regenerant Volume (L) Weight of Dry Resin Dose (mg) Anion Cation Anion Cation Anion Cation C 100 Ca/ A520E NO 3 1.0 0.45 0 .9 0.9 0.250 90.9 58.6 C 100 Ca/ Dowex 22 NO 3 1.0 0.60 1.2 1.2 0.250 100.4 80.2 C 100 Ca/ Marathon 11 NO 3 1.0 0.65 1.3 1.3 0.250 105.2 85.1 C 100 Ca/ IRA 958 NO 3 1.0 0.40 0.8 0.8 0.250 71.4 52.4 Table 2 6 Dosing parameters for regeneration experiments that vary cation exchange resins Resin Pair [Cation/Anion] Resin Dose (mL/L) Regeneration Capacity (meq/mL resin) Regenerant Volume (L) Weight of Resin Dosed (mg) Anion Cation Anion Cation Anion Cation C100 Ca/ A520E NO 3 1.0 0.45 0.9 0.9 0.250 90.9 58.6 SST60 Ca/ A520E NO3 1.0 0.53 0.9 0.9 0.250 90.9 69.6 C150 Ca/ A520E NO 3 1.0 0.50 0.9 0.9 0.250 90.9 57.3 Resin was measured on the scale within 1 mg of values in table.
30 CHAPTER 3 RESULTS During preliminary experiment s, the first IX pair (C100/A520E) was regenerated with two additional salts: NaHCO 3 and KCl ( Fig ure 3 1 ). Regeneration of the ions resulted in similar isotherm plots regardless of combination of salts (ex. Cl had similar regeneration efficiency when regenerated with NaCl and with KCl) for most salts. Therefore, NaHCO 3 and KCl were eliminated as redundant chemicals to be tested in regeneration isotherms The only exception was that Na + in NaHCO 3 had significantly hi gher regeneration efficiency than Na + in NaCl. This could be due to general laboratory error or error due to the calcium titration method The salts NaCl and KHCO 3 were used to regenerate pairs of anion exchange resins in nitrate form ( AER NO 3 ) and cation exchange resins in calcium form ( CER Ca ) in batch equilibrium experiments with regeneration ratios varying from 1 to 300 ( Fig ure 3 2 ). For all experiments, r egeneration (Ca 2+ or NO 3 released/capacity) increased as regeneration ratio increased (eq ion supplied/eq resin capacity). However, salt use efficiency decreased as regeneration ratio increased, yielding diminished returns for increasing salt input. Regeneration % exceeded 100% for some experiments because t he capacity used to calculate regeneration % was the minimum capacity given by the manufacturer. Each experiment with HCO 3 produced precipitation, likely as a result of CaCO 3 formation from Ca 2+ released from CER during regeneration and high HCO 3 concent rations at neutral pH. Precipitation occurred clearly in experiments with 30 regeneration ratio and above for KHCO 3 regenerant. Paired t tests at the 5% significance level were conducted to compare Na + to K + for CERs and compare Cl to HCO 3 for AERs ( Table 3 1 )
31 AERs had higher regeneration efficiency than CERs, with most AERs achieving complete (> 90%) regeneration at a regeneration ratio of 100 compared with CERs achieving complete regeneration at 300 regeneration ratio. A520 E, Dowex 22, and Marathon 11 were most effectively regenerated by Cl while IRA958 was m ost effectively regenerated by HCO 3 at all regeneration ratios. All three CERs were most effectively regenerated by K + than by Na + Regeneration plots of paired ion e xchange resins with NaCl and KHCO 3 were also compared by ion exchange resin ( Figure 3 3 ). Paired t tests at the 5% significance level were conducted to compare each of the CERs for Na + and K + separately and each of the AERs for Cl and HCO 3 separately ( Table 3 2 and Table 3 3 ) The AER NO 3 were regenerated most effectively by Cl in the sequence: Dowex 22 IRA958 The AER NO 3 were regenerat ed most effectively by HCO 3 in the sequence: IRA 958 > Marathon 11 > Dowex 22 > A520E The CER Ca were regenerated most effectively by both Na + and K + The precision of the method can be assessed u sing results from the pai red CER that was constantly paired with all AERs (C100) and the paired AER that was paired with all CERs (A520E). C100 had similar regeneration efficiency regardless of the paired AER ( Fig ure 3 4 ). Likewise, A520E had similar regeneration efficiency regardless of the paired CER ( Fig ure 3 5 ). Therefore, the method used showed high precision for over all regeneration experiments.
32 A) B ) Fig ure 3 1 Regeneration of C100/A520E ion exchange resins with NaCl, NaHCO 3 KCl, and KHCO 3 A) cations B ) a nions
33 A ) B ) C ) D ) E ) F ) Fig ure 3 2 Regeneration of paired ion exchange resins with NaCl a nd KHCO 3 by resin p air A ) C100/A520E B) C100/Dowex 22 C) C100/Marathon 11 D ) C100/IRA 958 E) C150/A520E F ) SST60/A520E
34 Table 3 1 Paired t tests of ions used to regenerate ion exchange resins Ion Exchange Resin T test: Na + /K + T test: Cl /HCO 3 More Effective Regenerant Ion A520E h=1 Cl Dowex 22 h=1 Cl Marathon 11 h=1 Cl IRA 958 h=1 HCO 3 C100 h=1 K + C150 h=1 K + SST60 h=1 K +
35 A ) B ) C ) D ) Figure 3 3 Regeneration of paired ion exchange resins with NaCl and KHCO 3 by r egenerant i on A ) Na + B ) K + C ) Cl D ) HCO 3
36 Table 3 2 Paired t tests of ion exchange resins regenerated by Na + and K + Cation Exchange Resin Comparison Na + K + t test More Effectively Regenerated Resin t test More Effectively Regenerated Resin C100/C150 h=0 N/A h=0 N/A C100/SST60 h=1 SST60 h=1 SST60 C150/SST60 h=1 SST60 h=1 SST60 Table 3 3 Paired t tests of ion exchange resins regener ated by Cl and HCO 3 Anion Exchange Resin Comparison Cl HCO 3 t test More Effectively Regenerated Resin t test More Effectively Regenerated Resin A520E/Dowex 22 h=0 N/A h=1 Dowex 22 A520E/Marathon 11 h=0 N/A h=1 Marathon 11 A520E/IRA958 h=0 N/A h=1 IRA 958 Dowex 22/Marathon 11 h=1 Marathon 11 h=1 Marathon 11 Dowex 22/IRA 958 h=0 N/A h=1 IRA 958 Marathon 11/IRA 958 h=1 Marathon 11 h=1 IRA 958
37 A ) B ) Fig ure 3 4 Na + and K + r egeneration of C100 cation exchange resin paired with multiple anion exchange resins A ) Na + B ) K + A ) B ) Fig ure 3 5 Regeneration of A520E anion exchange resin with Cl and HCO 3 paired with multiple cation exchange resins A ) Cl B ) HCO 3
38 CHAPTER 4 DISCUSSION 4.1. Effect of Resin Properties Few studies have specifically compared K + form regeneration to Na + form regeneration although general CER affinities are known to be greater for K + (Clifford, et al. 20 11) AER are generally accepted to have a lower affinity for HCO 3 than Cl and SO 4 2 (Holl and Kiehling, 1981) In a previous study, MIEX HCO 3 res in that was regenerated using a regeneration ratio of 10 (~20 mM HCO 3 ) removed NO 3 and SO 4 2 equivalent to ~5% and ~15% respectively of the total resin capacity during subsequent treatment (Rokicki and Boyer, 2011) This implies that a 10 regene ration ratio of HCO 3 regenerated approximately the same resin capacity (20 25% of active exchange sites). These results were similar to 10 regeneration results found in this research (18 40% regeneration) although the presence of sulfate in the previous study had an influence on the capacity that was regenerated. The ionic strength of the regenerant was similar between this research and the previous study as well. For almost every regeneration ratio, AER regeneration efficiency was greater than CER regen eration efficiency CERs have a stronger affinity to Ca 2+ than AERs have to NO 3 because Ca 2+ is a divalent ion and most ion exchangers have a higher affinity for divalent ions than monovalent ions. CERs would be expected to be the resin that determines sa lt dose because they have less regeneration efficiency. In a study of hardness and DOC removal with combined ion exchange, the CER required a 50 regeneration ratio, while the AER only required a 25 regeneration ratio to achieve similar removal (Apell and Boyer, 2010) An increase in the regeneration ratio from 25 to 200 for AER did not result in additional removal of DOM. A direct comparison of
39 regeneration results with full scale plants is not practical because the concentrati on of regenerant in the study was ~2M (120,000 mg/L NaCl) in comparison to the present work that used regenerants < 40,000 mg/L ( Table 2 3 and Table 2 4 ). Selectivity of resins for contaminants changes wi th increasing ionic strength, especially in divalent monovalent ion exchange. Regeneration results provide data to make inferences about resin properties on regeneration. All AERs with polystyrene polymer composition were more efficiently regenerated with Cl compared with HCO 3 Previous studies reported similar findings despite variances in functional groups (Matosic, et al. 2000) IRA 958, the only polyacrylic resin had the best HCO 3 regeneration efficiency and possessed approximately 20% higher water content than the other AERs. H CO 3 has been shown to deprotonate inside the resin matrix, attaching to the active exchange sites as CO 3 2 (Kimura, et al. 1982) A previous study has shown that the resins have the ability to deprotonate polyprotic anions from greatest to least in the order: polyacrylic type 1 >> polystyrene type 2 > polystyrene type 1 (Horng and Clifford, 1997) IRA 958 had similar Cl regeneration efficiency compared with other polystyrene resins. For CERs, SST60 (shallow shell polymer composition) was regenerated significantly more efficiently than both C100 and C150 resin, especially at 100 and 300 regeneration ratios. Therefore, shallow shell CERs may have greater regeneration efficiency at high regenerant concentrations due to shorter diffusion within the r esin. Previous studies suggest that shallow shell resins have a h igher operating capacity at any regeneration level with less fouling (Fries, 2009) However, the greater regeneration efficiency of SST60 was based on a resin capacity that was estimated because the
40 manufacturer did not directly give it. Therefore, it is possible that SST60 regeneration efficiency may only appear greater due to an unintentionally high dose of SST60 resin compared with C100 or C150 resin. Pore structure may have an influence on regeneration efficiency in some cases. Gel pore structure is the only resin property that distinguishes Marath on 11 from other AERs to explain greater Cl regeneration efficiency. Marathon 11 was also the second highest most effectively regenerated AER by HCO 3 Gel resins have showed a higher regeneration efficiency with hydrophobic ions (such as NO 3 ) since the affinity of gel resins for hydrophobic ions is lower than macroporous resins ( Clifford, et al. 2011) However, with CERs, C100 resin (gel) and the C150 resin (macroporous) showed no significant difference in regeneration w ith either Na + or K + Functional groups varied among the AERs tested and may explain differences in regeneration efficiency. Three different types of functional groups were among the tested AERs. For HCO 3 regeneration, the shorter functional group chains, trimethylamine ( R N + (CH 3 ) 3 Type I quaternary ammonium) and dimethylethanol ( R N + (CH 3 ) 2 (CH 2 OH) Type II quaternary ammonium), had greater regeneration efficiency compared with the longer functional group chain triethylamine ( R N + (CH 2 CH 3 ) 3 ). Previous stud ies have shown longer functional groups and subsequent greater spacing of active sites have higher affinity for monovalent ions over divalent ions compared with shorter functional groups ( Clifford and Weber, 1983; Sengupta and Clifford, 1986) The poor regenerati on efficiency of A520E with long functional groups further supports the theory of HCO 3 deprotonation to CO 3 2 in the resin phase. IRA 958 has the same functional group ( R N + (CH 3 ) 3 ) as Marathon 11, but had significantly more effective regeneration with HCO 3 Dowex 22
41 with the Type II functional group containing an OH group did not regenerate HCO 3 better than Marathon 11 and IRA 958, both of which had Type I functional groups that did not have an OH group. It is unclear whether the OH group produced no cha nge in HCO 3 regeneration or less efficient regeneration since a direct comparison cannot be made. Marathon 11 varies from Dowex 22 in both pore structure and functional group, while IRA 958 varies from Dowex 22 in both polymer composition and functional g roup. Type II functional groups produced no change in Cl regeneration efficiency. Combined ion exchange regeneration with stoichiometrically equivalent AER and CER doses demonstrated that every AER had higher regeneration efficiency than the paired CER. H owever, this system did not include SO 4 2 which is the major competing ion to anion contaminant removal in natural waters. Previous studies showed that the majority of active exchange sites can be filled with SO 4 2 instead of NO 3 the target contaminant (Rokicki and Boyer, 2011) Therefore, AER regeneration in a full scale combined ion exchange process would be expected to be more stoichiometrically equivalent to CER regeneration. 4.2. Economic Considerations Price ranges and solubility were comp ared as economic factors for the salts NaCl, NaHCO 3 KCl, KHCO 3 ( Table 4 1 ) All prices are shown as 2012 dollars and f.o.b. (shipping is not included in the costs). The chemicals in order of most expensive to least expensive were : KHCO 3 >> NaHCO 3 > KCl > NaCl. The chemicals from most soluble to least soluble were: NaCl KCl >> KHCO 3 >> NaHCO 3 NaCl costs varied from $36 $175 per ton depending on source: rock salt, solar salt, or vacuum and open pan salt in order of increasing price (Kostick, 2013) For US salt production, 36% of production came from rock salt, 9% vacuum pan, and 8% solar salt. The remainder of production
42 was produced as brine, but was not includ ed in this analysis since salt is unlikely to be shipped as brine due to high transport costs. NaCl for regeneration is typically dosed at 3 12% concentration by weight, and NaCl solubility does not vary significantly with temperature ( Clifford, et al. 201 1; Bharmoria, et al. 2012) NaHCO 3 costs varied from $512 $792 per ton converted from price per 100 lb bag, so larger scale may be more economical (Independent Chemical Informati on Service, 2006) NaHCO 3 costs 4.9 7.5 times more than the average price of NaCl by weight and 7 11 times more expensive by mole. With a maxi mum solubility at 10.3% that can decrease ~30% between 25C and 5 C, NaHCO 3 may as a regenerant may pose oper ational issues with achieving a high salt regenerant concentration ( Clifford, e t al. 2011; Tata Chemicals Europe Limi ted, 2013) KCl costs varied from $979 $1229 per ton converted from price per lb, so large scale costs may be lower than stated (Independent Chemical Information Service, 2006) KCl costs 9.3 11 times more than the average price of NaCl by weight and 12 15 times more expensive by mole. KCl is typically mined and refined from potash ore (a mixture of 30 40% KCl, 52 69% NaCl, and 1 8% water insoluble minerals) (Wist, et al. 2009) KCl solubility is similar to NaCl at 35.5%, but decreases ~15% with a temperat ure change from 25 C to 5C (Bharmoria, et al. 2012) Like NaCl, KCl can dissolve completely in a span of 30 min (Wist, et al. 2009) KHCO 3 costs varied from $2898 $6981 per ton (Armand Products Company, 2011) KHCO 3 costs 28 66 times more than the average price of NaCl by weight and 47 110 times more expensive by mole. The price of KHCO 3 appears to make this chemical prohibitively ex pensive. Solubility is 23% at 20C but is 25% less soluble at 0C (Armand Products Company, 2008)
43 Operational issues may come from the changing solubility of some salts with temperature. Solubility of KCl and NaHCO 3 decrease with temperature, and bridging or recrystallization in the brine tank can lead underapplication of salt use and poorer regeneration than intended (Wist, et al. 2009) Solubility of NaHCO 3 limits cold weather regenerant concentration to approximately 7%. 4.3 Environmental Impacts The environmental impact of chemicals NaCl, NaHCO 3 KCl, and KHCO 3 in aquatic systems, wastewater treatment systems, and soils/agricultural e ffects are summarized ( Table 4 2 ). Cation and anions are not present as individual constituents but are combined with other ions, so the individual toxicity of a certain ion may not be separable from the associated cation or anion (Goodfellow et al., 2000) A phenomenon that may transcend aquatic, wastewater, and soil systems is salt antagonism, the ability of a nontoxic ion to decrease the toxic effect of another ion. Salt antagonism has been documented with several cations such as the influence of Mg 2+ on metal toxicity in plant roots and anaerobic microbes in wastewater digestion (Luo, et al. 2008; Mccarty and Mckinney, 1961) Understanding the environmental impacts of salts with K + compared with Na + and salts with HCO 3 compared with Cl to aquatic systems is a paramount concern for IX brine management. IX systems have the option to discharge brine directly to surface waters or discharge to the local wastewater treatment plant that also commonly discharges to water bodies. Salinity loading from IX brine disposal is a major concern when the plant discharges to freshwater ecological systems. However, dilution in the wastewater c ollection system often significantly decreases the toxicity of ions to freshwater systems. In a regression analysis of 24 h and 48 h acute whole effluent
44 toxicity testing for ceriodaphnia dubia (a common freshwater WET test organism), relative toxicity was greatest in the order: K + > HCO 3 Mg 2+ > Cl > SO 4 2 while Ca 2+ and Na + were not significant factors (Mount, et al. 2009) A study of ceriodaphnia dubia toxicity bioassays showed the salts used in this study were most toxic to least toxic based on a 48 h lethal concentration 50 (LC50) in the sequence: KCl = KHCO 3 > NaHCO 3 > NaCl (Mount, et al. 2009) A 1:1 combination of NaCl:KCl salt resulted in LC50 data more toxic than NaCl; therefore, the salt solution mixture did not result in a reduction in toxicity according to salt toxicity antagonis m, although it would be possible at a lower concentration of K + than 1:1 (Mount, et al. 2009) The other test organisms, Daphnia magna and fathead minnows (Pimephales promelas) were more resilient to salt solutions than ceriodaphnia dubia (Mount, et al. 2009) An increase in copper toxicity is associated with K + in fathead minnows, while Na + was observed to decrease copper toxicity (Erickson, et al. 1996) Na + has shown to stimulate bacterial growth at low concentrations (~0.25 mM) and inhibit growth at high concentrations (>50 mM) (Yang and Drake, 1990) Similar effects of Na + were demonstrated by increased phosphorus uptake in algae (Mohleji and Verhoff, 1 980) Therefore, slight increases in Na + may increase eutrophication in freshwater systems with low background Na + Road salt for snow has been documented to mobilize heavy metals bound in soil due to high NaCl concentrations (Norrstrm and Jac ks, 1998) Therefore, high concentrations of other salts may also increase heavy metals to the environment, although the amount of salt required to release substantial amounts of heavy metals is unknown. Cl has been shown to decrease nitrite toxicity in salmon, which could be an example of salt toxicity
45 antagonism (Perrone and Meade, 1977) HCO 3 could benefit pH of water bodies as it buffers acidification (Kramer, 1982) Wastewater treatment with activated sludge is dep endent on biological processes that are impacted to various degrees by increased salinity. KCl, NaHCO 3 and KHCO 3 have higher molecular weight than NaCl, so the TDS of an IX regenerant waste stream will likely be greater. Activated sludge can treat waters with 30,000 mg/L NaCl with 10 30% less COD removal efficiency and slower kinetics (Kincannon and Gauddy Jr., 1966; Ludzack and Noran, 1965) Other impacts on the activated sludge system at 30,000 mg/L NaCl includes poor flocculation, higher effluent solids, low phosphorus uptake, and 90% d ecrease in nitrification (Ludzac k and Noran, 1965; Panswad and Anan, 1999) Sludge settling and dewatering were significantly deteriorated under conditions where Na + to divalent cation ratio > 2 (H iggins and Novak, 1997) Small concentrations of K + (>2 meq/L) adversely affected activated sludge dewatering properties and effluent quality (Murthy and Novak, 2001) In IX softening processes, Ca 2+ ions may be part of the waste stream and aid in sludge properties. However, K + was found to aid phosphate release in anoxic conditions leadi ng to greater subsequent aerobic phosphorus uptake (Comeau, et al. 1987) The optimal molar K:P ratio for phosphorus uptake was determined to be approximately 0.21 (Rickard and Mcclintock, 1992) K + in land appli ed sludge also adds value as fertilizer (Sommers, 1977) HCO 3 is consumed at a rate of 7.14 g/g N during nitrification, so increased HCO 3 would increase the nitrification capacity of a wastewat er ( Li and Irvin, 2007) HCO 3 could also increase the buffering capacity during sludge digestion (Parkin and Owen, 1986) The effects of major cations are well studied for anaerobic digestion
46 ( Table 4 3 ) (Evans and Furlong, 2003) Low cation concentrations (< 400 mg/L cation) produce a stimulatory effect while moderate to high concentrations produce an inhibitory effect An inhibition study on anaerobic digestion similarly determined inhibition from most toxic to least toxic on a molar basis to be: Mg 2+ > Ca 2+ > K + > Na + (Kugelman and Mccarty, 1965) Low concentrations of ions (~100 400 mg/ L ion) produce a stimulatory effect to anaerobic microbes, while high concentrations (>~2000 mg/L ion) are inhibitory. In a study of salt toxicity antagonism in anaerobic digesters, K + stimulated methane production at a concentration range of 2 100 mM K + and inhibi ted methane at concentrations > 100 mM K + in a 300 400 mM Na + solution (Kugelman and Mccarty, 1965) The impacts of Na + K + Cl and HCO 3 on agricultural soils affects not only the prospect of IX brine application for agricultural purposes, but also the effects of salinity in wastewater effluent t hat is increasingly reused for irr igation. Salinity stress in plants is due to two primary factors: decreased osmotic potential in the root media that inhibits solute transport and specific effects of ion such as decreased enzyme activity (Marschner, 2012; Bernstein, 1975) Plants under salinity stress generally have slower and stunted growth of stems, leaves, and fruits (Bernstein, 1975) However, Na + salinity and K + salinity are not equivalent because K + is an essential nutrient vital for plant growth that is often added as KCl to crops to aid in growth. K + plays a primary role in cell extension, stem el ongation, and gas exchange through the stoma ta (pores on leaves and stems) (Marschner, 2012) K + is the dominant ion in counterbalancing organic acid anions and NO 3 in c ytoplasm and chloroplasts (Marschner, 2012) K + has been frequently observed to increase resistance of plants to stress such as drought,
47 frost, salinity, pathogens, and i nsects (Marschner, 2012) However, agricultural application must be carefully considered because K + fertilizer requirements and tolerance vary not only between plants, b ut also as a function of K + mobility related to water content in the soil (Marschner, 2012) For example, s tress with KCl up to 150 mM decreased growth of sunflower roots shoots, and calli as much as 90% (Santos, et al. 2001) The optimum K + requiremen t for optimal plant growth has been determined to be 20 50 g/kg in vegetative parts, fruits, and tubers (Marschner, 2012) Unlike in aquatic systems, Na + and K + did not s ignificantly affect the EC50 of copper in barley (Lock, et al. 2007) Cl can be a main counter ion to K + depending on plant species and availability, but malate can be synthesized to be substituted as the main counter ion in most plants (Marschner, 2012) Solutions of 0.5% 2% NaHCO 3 and KHCO 3 mixed with horticultural oils have shown to be effective biocompatible fungicides when applied weekly to leaves of euonymus, roses, and sweet red pepper (Box and Dugan, 1993) Application of 0.5% KHCO 3 to plants significantly reduced decay of stored sweet red pepper after h arvesting compared with NaHCO 3 and control groups (Fallik, et al. 1997) In soil, sodium effects have been heavily studied. Sodium adsorption ratio (SAR) quantifies the impact of excess Na + on soil (Eq. 4 1 ) (Wist, et al. 2009) (4 1) SAR > 10 15 can reduce water uptake in plants to various extents depending on species and reduce hydraulic conductivi ty in soils up to 90% in clay soils by plugging pores (Fallik, et al. 1997) K + has a significantly less adverse impact on hydraulic conductivity in soils than Na + ( Chen, et al. 1983) HCO 3 accum ulation in calcareous soils (soils high in calcium carbonate) was determined to be the most important factor in
48 plant (Marschner, 2012) (Mengel, et al. 1984) Iron is a plant micronutrient that participates in key re dox reactions such as in the chloroplasts, and iron chlorosis prevents the plant from producing sufficient chlorophyll (Marschner, 2012) Bench scale tests of irrigation of landscape plants using Cl Ca 2+ K + and Mg 2+ concentrations in proportion to IX waste brine with KCl regenerant concluded that irrigation with KCl waste brine was viable but very dependent on appropriate species (Wu, et al. 1995; Wu, et al. 2008) 4.4 Engineering Considerations The use of K + or HCO 3 salts in regenerating IX has several secondary advantages over NaCl for potable water use. The replacement of NaCl brine for IX regeneration with of KCl would be expected to increase potassium and decrease sodium intake for residents supplied by the drinking water (Wist, et al. 2009) In addition, IX regenerated with HCO 3 form resin would both decrease Cl and increase HCO 3 in the feed water, decreasing iron corrosion according to the Larson Skold index (Larson and Skold, 1958) A decrease in Cl in the feed water can lead to a decr ease in the chloride to sulfate mass ratio and decrease lead release into drinking water (Edwards and Triantafyllidou, 2007) Release of HCO 3 into the treated water by HCO 3 form anion exchange has been also experimentally determined to decrease corrosivity (Takasaki and Yamada, 2007) Although salt concentration s are approximately 10% during IX regeneration, salt concentrations will be significantly decreased by dilution from other flows in the wastewater collection system. An estimation of final concentrations of salts to wastewater treatment plants is important to better estimate environmental impacts. A
49 rough approximation of diluted salt concentration at the wastewater treatment plant can be estimated from the brine use factor (lb NaCl discharged/10 6 gallons produced) of the IX process converted to mg/L. This estimate assumes several ideal conditions: one water treatment plant returning 100% flow to one wastewater treatment plant, and waste brine is discharged continuously and proportionally with flow to maintain greatest dilution possible. In this case, the f low rate of the treatment plant is irrelevant in determining the concentration at the wastewater treatment plant. Brine use factors with and without brine reuse found in the literature ranged from approximately 150 1500 mg/L NaCl for IX nitrate removal a nd 350 600 mg/L NaCl for IX softening ( Clifford and Liu, 1993; Flodman and Dvorak, 2012) An exa mple of optimized brine use factor with brine reuse could be approximately 400 mg/L (Flodman and Dvorak, 2012) Actual concentrations at a wastewater treatment plant are expected to be higher than stated due to < 100% of flows returned to the wastewater treatment plant, other sources of salts in the distribution system, and non ideal dilution. For wa stewater treatment plant effluent at actual concentrations, the potential exists to fail WET toxicity tests for freshwater test organisms (1,770 2,330 mg/L NaCl LC50 for Ceriodaphnia dubia). Anaerobic digestion will likely experience stimulatory concentr ations (100 200 mg/L Na + ) or slightly higher concentrations.
50 Table 4 1 Economic comparison of ion exchange regenerants Regenerant NaCl (Conventional) NaHC O 3 KCl KHC O 3 Price by weight Price Range, 2012 Dollars ($/ton) $36 $175 a $512 $792 b $979 $1229 b $2898 $6981 c Price Index Relative to Average NaCl, $106/ton 0.3 1.7 4.9 7.5 9.3 11 28 66 Price by mol Price Range, 2012 Dollars ($/kmol) $2.32 $11.27 $47.41 $73.33 $81.50 $102.31 $319.83 $770.45 Pri ce Index Relative to Average NaCl, $6.80/kmol 0.3 1.7 7.0 11 12 15 47 110 Approximate Solubility, 25C (g/L) 36.0% d 10.3% d 35.5% d 23.0% e All costs are f.o.b. (shipping not included in cost ) a Does not include salt in brine price (Kostick, 2013) b (Independent Chem ical Information Service, 2006) c (Armand Products Company, 2011) d (Hammond, 2011) e Solubility is a t 25C (Armand Products Company, 2008)
51 Table 4 2 Environmental impacts of selected alternative regenerants for ion exchange Regenerant Aquatic Systems Wastewater Treatment Soils/ Agricultural NaCl Ceriodaphnia dubia (water flea): 48 hour LC50: 1,770 2,330 mg/L 1 Increased Na + can lead to greater eutrophication in low Na + water bodies 2 Na + stimulated bacterial growth at low concentrations (~0.25 mM), but inhibited at high concentrations (>50 mM) 3 Cl decreased nitrite toxicity in coho salmon (salt antagonism) 4 Activated sl udge shocked by 30,000 mg/L NaCl decreased COD removal 30% 8 High salinity (33,000 mg/L NaCl) was characterized by poor flocculation, higher effluent solids, low phosphorus uptake, 90% decrease in nitrification, and 10% loss in COD removal efficiency 9, 10 R atio of Na + to divalent cations >2 decreased sludge settling and dewatering 11 In plants, salinity inhibits solute transport through decreased osmotic potential in the root media and decreases enzyme activity 19 Sodium adsorption ratio (SAR) > 12 15 hinders water uptake ([Na + ]/(1/2([Ca 2+ ]+[Mg 2+ ])) (1/2) 20 SAR >10 30 reduced hydraulic conductivity up to 90% in clay soils by plugging pores 21 Cl readily uptaken to balance K + charge, decreases need to produce malate 22 KCl Ceriodaphnia dubia (water flea): 48 hou r LC50: 580 670 mg/L 1 K + increased copper toxicity in minnows 5 K + used for control of zebra mussels in municipal and industrial processes 6 Cl decreased nitrite toxicity in coho salmon 4 K + more toxic to anaerobic bacteria than Na + but less toxic than C a 2+ and Mg 2+ 12 K + > 2 meq/L may decrease activated sludge dewatering properties in low hardness waters 13 K + but not Na + can stabilize and expel phosphate groups, leading to greater subsequent aerobic phosphorus uptake 14 K + essential in enhanced biologi cal phosphorus removal. Optimum molar K:P ratio was 0.21 15 K + in land applied sludge adds value as fertilizer 16 K + plays essential role in plant growth, gas exchange through stomata, charge balance of organic anions, and increased resistance to drought, frost, salinity, pathogens, and insects 22 Optimum K + requirement varies based on species and K + soil mobility: 20 50g/kg in vegetative parts and fruits 22 K + did not increase EC50 of copper in barley 23 K + reduced hydraulic conductivity less severely than N a + 24 150 mM KCl decreased sunflower growth up to 90% 25 Cl readily uptaken to balance K + charge, decreases need to produce malate 22 NaHCO 3 Ceriodaphnia dubia (water flea): 48 hour LC50: 880 1,1170 mg/L 1 Increased Na + can lead to greater eutrophicati on in low Na + water bodies 2 Na + stimulated bacterial growth at low concentrations (~0.25 mM), but inhibited at high concentrations (>50 mM) 3 HCO 3 buffers acidification 8 Ratio of Na + to divalent cations >2 decreased sludge settling and dewatering 11 HCO 3 is crucial for nitrogen removal: consumed at a rate of 7.14g/g N oxidized during nitrification 17 HCO 3 essential to buffer pH in sludge digestion 18 Sodium adsorption ratio > 15 hinders water uptake ([Na + ]/(1/2([Ca 2+ ]+[Mg 2+ ])) (1/2) 20 Weekly treatment wi th 0.5% 2% NaHCO 3 mixed with horticultural oil was biocompatible fungicide 26 HCO 3 accumulation in calcareous soils (soils high in CaCO 3 ) is primary cause of iron chlorosis (deficiency) by 22, 27
52 Table 4 2. Conti nued. Regenerant Aquatic Systems Wastewater Treatment Soils/ Agricultural KHCO 3 Ceriodaphnia dubia (water flea): 48 hour LC50: 580 670 mg/L 1 K + increased copper toxicity in minnows 5 K + used for control of zebra mussels in municipal and industrial proces ses 6 HCO 3 buffers acidification 7 K + more toxic to anaerobic bacteria than Na + but less toxic than Ca 2+ and Mg 2+ 12 K + > 2 meq/L may decrease activated sludge dewatering properties in low hardness waters 13 K + but not Na + can stabilize and expel phosphate groups, leading to greater subsequent aerobic phosphorus uptake 14 K + essential in enhanced biological phosphorus removal. Optimum molar K:P ratio was 0.21 15 HCO 3 is crucial for nitrogen removal: consumed at a rate of 7.14g/g N oxidized during nitrificat ion 17 HCO 3 essential to buffer pH in sludge digestion 17 K + in land applied sludge adds value as fertilizer 16 K + plays essential role in plant growth, gas exchange through stomata, charge balance of organic anions, and increased resistance to drought, fro st, salinity, pathogens, and insects 22 Optimum K + requirement varies based on species and K + soil mobility: 20 50g/kg in vegetative parts 22 Weekly treatment with 0.5% 2% KHCO 3 mixed with horticultural oil was biocompatible fungicide 26 0.5% KHCO 3 signifi cantly reduced sweet red pepper postharvest decay 28 HCO 3 accumulation in calcareous soils (soils high in CaCO 3 ) use iron 22, 27
53 1 (Mount, et al. 2009) 2 (Mohleji and Ve rhoff, 1980) 3 (Yang and Drake, 1990) 4 (Perrone and Meade, 1977) 5 (Erickson, et al. 1996) 6 (Lewis, et al. 1997) 7 (Kramer, 1982) 8 (Kincannon and Gauddy Jr., 1966) 9 (Ludzack and Noran, 1965) 1 0 (Panswad and Anan, 1999) 1 1 (Higgins and Novak, 1997) 1 2 (Kugelman and Mccarty, 1965) 1 3 (Murthy and Novak, 2001) 1 4 (Comeau, et al. 1987) 1 5 (Rickard and Mcclintock, 1992) 16 (Sommers, 1977) 17 (B. Li and Irvin, 2007) 18 (Parkin and Owen, 1986) 19 (Bernstein, 1975) 20 (Wist, et al. 2009) 21 (Frenkel, et al. 1976) 22 (Marschner, 2012) 23 (Lock, et al. 2007) 24 ( Chen, et al. 1983) 25 (Santos, et al. 2001) 26 (Box and Dugan, 1993) 27 (Mengel, et al 1984) 28 (Fallik, et al. 1997) Table 4 3 The effect of metal cations on anaerobic digestion Cation Stimulatory Moderately Inhibitory Strongly Inhibitory Na + (mg/L) 100 200 3,500 5,500 8,000 K + (mg/L) 200 400 2,500 4,500 12,000 Ca 2+ (mg/L) 100 200 2,500 4,500 8,000 Mg 2+ (mg/L) 75 150 1,000 1,500 3,000 Source: (Evans and Furlong, 2003)
54 CHAPTER 5 CONCLUSION T he regeneration efficie ncy, economic factors, and environmental impacts of salts with Na + K + Cl and HCO 3 were compared for different pairs of anion exchange resin (AER) and cation exchange resins (CER) that were exhausted with inorganic contaminants At equivalent removal c apacities, AER regeneration efficiency for NO 3 was greater than CER regeneration efficiency for almost every regeneration ratio due to the greater affinity of CER for Ca 2+ compared with the affinity of AER for NO 3 The presence of SO 4 2 a divalent anion in real systems may decrease AER regeneration efficiency. The regeneration results for polystyrene strong base AERs agree with the generally accepted affinity sequence: NO 3 > Cl > HCO 3 ; however, regeneration results for polyacrylic strong base AERs sh owed the affinity sequence: NO 3 > HCO 3 > Cl CERs were more effectively regenerated with K + than with Na + CERs results had uncertainty that the calculated capacity of SST60 may be introducing error to the data, but the data suggested that shallow shell pore structure improved regeneration efficiency for both K + and Na + AER results suggested that gel pore structure improved Cl regeneration efficiency. AER results suggested that polyacrylic polymer composition, short functional group chains such as trim ethylamine ( R N + (CH 3 ) 3 ) and gel pore structure improved HCO 3 regeneration efficiency compared with the other resin properties. AER results suggest HCO 3 deprotonation to CO 3 2 inside the AER matrix. Deprotonation explains both high HCO 3 regeneration eff iciency by the polyacrylic resin and poor HCO 3 regeneration efficiency by the resin with long functional group chains. Type II AER (functional group containing OH) produced either no change or poorer regeneration efficiency than Type I AER (functional gro ups without OH).
55 The salts NaCl, KCl, NaHCO 3 and KHCO 3 were compared in terms of costs, environmental impacts, and other factors. Each salt adversely affects aquatic systems, wastewater treatment systems, and soils at high concentrations. NaCl is the most economical of the salts. At high concentrations, NaCl seems to have the lowest impact on aquatic systems compared to the other salts. NaHCO 3 costs 5 8 times more than NaCl. The literature suggests it has t he least environmental impact to aquatic systems of the salts and wastewater treatment systems at low to moderate concentrations because of beneficial buffering but is more harmful than NaCl at high concentrations based on freshwater organism toxicity tests The exact concentrations or loading that NaH CO 3 has a less adverse environmental impact depends on the dilution, species, and water quality in the freshwater system. HCO 3 in the treated water as a result of HCO 3 form anion exchange is expected to decrease corrosion as well. KCl costs 9 11 times more than NaCl. The literature suggests it has the lea st envi ronmental impact to soil systems of any of the other salts and benefits crops by providing K + an essential nutrient. The acceptable salt application to soils is highly dependent on plant species soil characteristics, and land availability KHCO 3 is prohibitively expensive, costing at least 28 times more than NaCl, and none of the environmental systems that were studied would be less adversely impacted as a result of having both K + and HCO 3 in t he salt. Variable solubility with temperature presents operational problems for IX regeneration with NaHCO 3 KCl, and KHCO 3
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63 BIOGRAPHICAL SKETCH The author began research with the University Scholars Program in 2009, estimating the methane generation potential of construction and demolition waste. He received a Bachelor of Science in Environmental Engineering from U niversity of Florida in 2011.