Article pubs.acs.org/est
Anion Exchange Resins as a Source of Nitrosamines and Nitrosamine Precursors Riley C. Flowers*,†,‡ and Philip C. Singer† †
Department of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States S Supporting Information *
ABSTRACT: Anion exchange resins are important tools for the removal of harmful anionic contaminants from drinking water, but their use has been linked to the presence of carcinogenic nitrosamines in treated drinking water. In benchscale batch and column experiments, anion exchange resins from a large, representative group were investigated as sources of the nitrosamines Nnitrosodimethylamine (NDMA), N-nitrosodiethylamine (NDEA), N-nitrosodi-npropylamine (NDPA), and N-nitrosodi-n-butylamine (NDBA) and their precursors. Several resins were found to release high levels (up to >2000 ng/L, orders of magnitude above drinking water regulatory levels) of nitrosamines upon initial rinsing with lab-grade water, with levels subsiding within 50−100 bed volumes of rinsing. Resins released similarly high levels of nitrosamine precursors, with spikes in precursor release triggered by regeneration of resins with sodium chloride or by interruptions in flow resulting in prolonged contact times. Free chlorine or preformed monochloramine in feedwater led to the production of nitrosamines. Resins released different nitrosamines and precursors depending on their functional groups, with some resins releasing as many as three different nitrosamines and their precursors. These findings have significant implications for the pretreatment and appropriate use of anion exchange resins by drinking water utilities and for the production of anion exchange resins by manufacturers.
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INTRODUCTION
Early studies found that anion exchange resins used in deionized water systems released NDMA.14−16 Recently, experiments with anion exchange resins used in drinking water treatment have found nitrosamines. In batch contact studies, resins containing TMA or DMEA were found to release appreciable levels of NDMA (up to 140 ng/L).17 In continuous-flow column contact studies, three resins were found to release high levels of NDMA or NDBA precursors (1000−11 000 ng/L) when flow began, with precursor release subsiding quickly and rising briefly with the regeneration of the resins with NaCl. 18 Nitrosamine levels rose with the introduction of oxidants to the feedwater. These studies demonstrated a link between anion exchange resins and nitrosamines but were limited in scope, investigating only three or four different resins out of the wide array of resins used for drinking water treatment. Furthermore, these studies focused on NDMA, even though there are three other carcinogenic nitrosamines that may be associated with the use of anion exchange resins and that have 10−6 cancer risk concentrations at low ng/L in drinking water.19 In this work, the releases of NDMA, NDEA, NDPA, and NDBA and their precursors by a large group of drinking water treatment resins
Nitrosamines are carcinogens that include, among others, Nnitrosodimethylamine (NDMA), N-nitrosodiethylamine (NDEA), N-nitrosodi-n-propylamine (NDPA), and N-nitrosodi-n-butylamine (NDBA). Nitrosamines, especially NDMA, have been detected in several drinking waters.1−4 NDMA has been identified as a byproduct of the disinfection of water with chloramines.5 The mechanism of NDMA formation is a two-step process involving dichloramine and dissolved oxygen reacting with dimethylamine (DMA).6−9 Further study has identified DMA and tertiary amines containing DMA groups as major nitrosamine precursors.10 In the context of drinking water treatment, coagulant polymers containing charged amine groups have been identified as sources of NDMA precursors.11−13 Anion exchange resins are another group of drinking water treatment polymers and are commonly used for the removal of anionic contaminants during drinking water treatment. Resins are typically composed of a cross-linked polymer (polystyrenedivinylbenzene or polyacrylic) matrix that is functionalized with quaternary amine groups to provide positively charged exchange sites. Resins are classified according to functional group. Type I resins have trialkylamines [trimethylamine (TMA), triethylamine (TEA), triproplyamine (TPA), or tributylamine (TBA)] at the charged sites, while type II resins use dimethylethanolamine (DMEA). © XXXX American Chemical Society
Received: January 31, 2013 Revised: April 29, 2013 Accepted: May 7, 2013
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dx.doi.org/10.1021/es4003185 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
Table 1. Characteristics of Resins Investigated
a
resin
functional group
wet exchange capacity (meq/L)
particle size range (μm)
pore structure
polymer backbone
A300Ea,b A400Eb A520Eb A530Ea,b A532Eb A600Ea,b A860Ea,b CalRes 2103a CalRes 2109a,b IRA400b IRA410a IRA910b MIEXa PWA15a,b PWA2a,b PWA5a,b PWA9a SBG2HPa SIR-100a,b SIR-110a,b TAN-1b
DMEA TMA TEA TEA/TBA TEA/TBA TMA TMA TPA TBA TMA DMEA DMEA TMA TMA TBA TEA TMA DMEA TEA TBA TMA
1.45−1.6 1.3 0.9 0.6 0.85 1.6 0.8 0.8 0.6 1.4 1.25 1 0.4 1.4 0.6 1 0.8 1.45 0.85 0.6 0.7
300−1200 300−1200 300−1200 300−1200 550−650 300−1200 800−1300 450−550 450−550 600−750 600−750 530−800 150−180 300−1200 300−1200 300−1200 300−1200 300−1200 300−1200 300−1200 420−1200
macroporous gel macroporous macroporous gel gel macroporous macroporous macroporous gel gel macroporous macroporous gel macroporous macroporous macroporous gel macroporous gel macroporous
styrene styrene styrene styrene styrene styrene acrylic styrene styrene styrene styrene styrene acrylic styrene styrene styrene acrylic styrene styrene styrene styrene
Indicates a resin used in batch experiments. bIndicates a resin used in column experiments.
were investigated. There were 21 resins with different functional groups, pore structures, and polymer matrices from several different manufacturers subjected to a series of batch and continuous-flow column contact experiments to determine nitrosamine and nitrosamine precursor release. Treatment plant operations including regeneration, periodic down-time, and the introduction of low levels of oxidant were simulated. The initial releases of nitrosamines and nitrosamine precursors and the effects of treatment operations on nitrosamine and nitrosamine precursor release were assessed.
tigated are all commercially available for use in drinking water treatment. Batch Contact Experiments. To confirm the release of nitrosamines and nitrosamine precursors, simple batch experiments were conducted. For cleaning, resins (200 mL) were loaded into glass chromatography columns and rinsed with 3 bed volumes (BV) of 10% NaCl solution followed by 17 BV of LGW at an empty bed contact time (EBCT) of 3 min. This procedure was based on manufacturer recommendations for the commission of resins in treatment plants. Column cleaning was not possible with the MIEX resin because of its small size, so 200 mL of resin was swirled in batches of 200 mL of LGW 20 times for cleaning. The cleaned resins were contacted with LGW at a resin concentration of 20 mg/L. The solutions were buffered at pH 7.0 with a 10 mM phosphate buffer. After 1 h of contact, the resins were separated from solution using 0.7 μm pore size borosilicate filters that were prerinsed with 1.0 L of LGW and baked at 400 °C for four hours. The solutions were then analyzed for nitrosamines and nitrosamine formation potential. Continuous-Flow Column Contact Experiments. With no prior cleaning, resins (200 mL BV) were packed into glass chromatographic columns with an inner diameter of 2.5 cm and a height of 60 cm (resin height of 41 cm). LGW containing 10 mM phosphate buffer at pH 7.0 was passed through the columns at a flow rate of 66.7 mL/min, resulting in an EBCT of 3 min, which is typical of contact times used for ion exchange in water treatment practice. The columns were regenerated periodically with 600 mL (3 BV) of a 10% NaCl solution. Column flow was interrupted periodically, and the resins were stored submerged in the columns for 12−14 h. pH 7 buffered feedwater containing free chlorine (0.24 mg/L as Cl2, standardized iodometrically) or preformed monochloramine (0.24 mg/L as Cl2, prepared according to a previously reported method,7 with chloramine speciation confirmed using UV−vis spectrometry8) was introduced at the end of each experiment. Samples (2.0 L, 10 BV) were collected at the beginning of each
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MATERIALS AND METHODS Materials. NDMA, NDEA, NDPA, NDBA, d6-NDMA, and d14-NDPA standards were obtained from Accustandard (New Haven, CT), and d10-NDEA and d18-NDBA were obtained from CDN Isotopes (Pointe Claire, Quebec, Canada). EPA Method 521 method-specific activated carbon solid-phase extraction (SPE) cartridges were purchased from Restek (Bellefonte, PA). Laboratory-grade water (LGW) was prepared using a system consisting of filters, granular activated carbon adsorbers, mixed-bed ion exchange resins, and ultraviolet (UV) treatment. The LGW was analyzed periodically and found to contain levels of nitrosamines and nitrosamine precursors below detection limits. All other chemicals were purchased from Fisher Scientific (Pittsburgh, PA) and were reagent-grade or higher. All glassware was rendered chlorine demand-free, rinsed with acetone, and baked at 400 °C for four hours. Detergent, which often contains TEA, was not used at any time during glassware preparation. Strong base anion exchange resins were obtained from five different manufacturers. Resin characteristics are presented in Table 1. Resin information comes from technical literature available from manufacturers. Functional group identities, which are often reported generically as “quaternary ammonium” in technical literature, were provided by the manufacturers in private communications. The resins invesB
dx.doi.org/10.1021/es4003185 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
Table 2. NDMA Precursor Release by TMA and DMEA Resins During Continuous-Flow Column Experiments resin
functional group
pore structure
first 10 BV NDMA FP (ng/L)
regeneration effect NDMA FP (ng/L)
flow interruption effect NDMA FP (ng/L)
IRA400 A600E A400E A860E PWA15 TAN-1 A300E
TMA TMA TMA TMA TMA TMA DMEA
gel gel gel macroporous gel macroporous gel
>2000 >2000 1880 >2000 333 165 1400
269 184 19 2000 442 400 821 54 380
Table 3. NDMA, NDEA, and NDBA Precursor Release by TEA, TBA, and Bifunctional TEA/TBA Resins During ContinuousFlow Column Experiments first 10 BV NFP (ng/L)
regeneration effect NFP (ng/L)
flow interruption effect NFP (ng/ L)
resin
functional group
pore structure
NDMA
NDEA
NDBA
NDMA
NDEA
NDBA
NDMA
NDEA
NDBA
A530E A532E CalRes 2109 PWA2 SIR-110 A520E PWA5 SIR-100
TEA/TBA TEA/TBA TBA TBA TBA TEA TEA TEA
macro gel macro macro gel macro macro macro
>2000 431 2000 >2000 300 130
>1000 >2000 naa na na >2000 400 300
>1000 38 >2000 113 60 na na na
136 xb
