Synthesis and Application of a Quaternary Phosphonium Polymer

Article pubs.acs.org/est

Synthesis and Application of a Quaternary Phosphonium Polymer Coagulant To Avoid N‑Nitrosamine Formation Teng Zeng,† Joseph J. Pignatello,‡ Russell Jingxian Li,§ and William A. Mitch*,† †

Department of Civil and Environmental Engineering, Stanford University, 473 Via Ortega, Stanford, California 94305, United States Department of Environmental Sciences, The Connecticut Agricultural Experiment Station, 123 Huntington Street, P.O. Box 1106, New Haven, Connecticut 06504-1106, United States § Department of Chemistry, Stanford University, 333 Campus Drive, Stanford, California 94305, United States ‡

S Supporting Information *

ABSTRACT: Quaternary ammonium cationic polymers, such as poly(diallyldimethylammonium chloride) (polyDADMAC) are widely used for coagulating and removing negatively charged particles and dissolved organic matter (DOM) from drinking water. Their use, however, has been linked to the formation of carcinogenic N-nitrosamines as byproducts during chloramine-based drinking water disinfection. In this study, a novel quaternary phosphonium cationic polymer, poly(diallyldiethylphosphonium chloride) (polyDADEPC), was synthesized such that the quaternary nitrogen atom of polyDADMAC was substituted with a phosphorus atom. Formation potential tests revealed that even under strong nitrosation conditions, polyDADEPC and related lower-order P-based compounds formed oxygenated and not nitrosated products. Bench-scale jar tests using three different source waters further demonstrated that polyDADEPC achieved coagulation performance comparable to commercial polyDADMACs for particle and DOM removals within the typical dose range used for drinking water treatment. This work highlights the potential use of a phosphonium coagulant polymer, polyDADEPC, as a viable alternative to polyDADMAC to avoid nitrosated byproduct formation during chloramination.

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(NDMA).11 Regardless of source water quality, increasing doses of polyDADMAC generally led to elevated NDMA levels in treated water from chloraminating utilities.11 Experimental animal studies and human epidemiological data12,13 have suggested a probable linkage between dietary intake of N-nitrosamines, or their formation from reactions between ingested amines and nitrite under acidic conditions in the stomach (i.e., endogenous formation), and increased cancer risk. According to the U.S. Environmental Protection Agency’s (USEPA’s) Integrated Risk Information System, as low as 0.7 ng/L NDMA in drinking water is associated with a 10−6 lifetime excess cancer risk.14 Several states and provinces across North America, such as California,15 have recently established guidelines for NDMA levels in drinking water. With the USEPA considering regulation of N-nitrosamines, six of them were included in the Unregulated Contaminant Monitoring Rule 216 and five on the Contaminant Candidate List 3.17 The confluence of impending regulation and continued reliance on polyDADMAC highlights a need to identify alternative cationic polymer coagulants that minimize N-

INTRODUCTION Treatment of surface water sources for drinking water generally involves coagulation and flocculation followed by sedimentation and filtration to remove suspended and colloidal particles as well as dissolved organic matter (DOM).1 Water utilities in the United States and other parts of the world have long relied on cationic polymers as coagulant aids in conjunction with metallic salts (e.g., aluminum sulfate (alum) or ferric chloride (ferric)) to improve floc size, strength, and settleability.2 Many utilities have also implemented polymer-enhanced coagulation to reduce the concentration of DOM and thereby mitigate the formation of regulated halogenated disinfection byproducts (DBPs), such as trihalomethanes and haloacetic acids.3−5 Cationic polymers for water treatment, such as poly(diallyldimethylammonium chloride) (polyDADMAC), almost exclusively rely on quaternary ammonium groups bearing positive charges distributed along a macromolecular backbone for ion pairing with negatively charged particles or DOM. PolyDADMAC or lower-order amines (e.g., dimethylamine), either existing as unbound impurities or released during degradation of polyDADMAC, can react with chloramines to form Nnitrosamines,6−9 a group of nitrogen-containing DBPs of emerging health and regulatory concerns.10 In a limited survey of U.S. water utilities practicing chloramination, polyDADMAC was identified as a major precursor to N-nitrosodimethylamine © 2014 American Chemical Society

Received: Revised: Accepted: Published: 13392

August 20, 2014 October 14, 2014 October 16, 2014 October 16, 2014 dx.doi.org/10.1021/es504091s | Environ. Sci. Technol. 2014, 48, 13392−13401

Environmental Science & Technology

Article

established inside reaction flasks by the standard Schlenk pump and fill technique31 using a Schlenk manifold that allowed silicone rubber tubes to be connected either to the vacuum pump or to the N2 tank via a three-way valve. Detailed synthetic procedures for both polymers are provided in SI section S2. Characterization of Ammonium and Phosphonium Monomers and Polymers. Nuclear magnetic resonance (NMR) spectroscopy, high-resolution mass spectrometry (HRMS), and elemental analysis were used to confirm the identity and purity of synthesized monomers. Size exclusion chromatography (SEC) and colloidal titration were used to measure the molecular weights and charge densities, respectively, of synthesized and commercial polymers. Detailed methods are provided in SI section S3. NOC Formation Potential of N-Based and P-Based Precursors. Two potential sources of NOCs relevant to cationic polymers and their precursors are chloramination during drinking water treatment and reaction with nitrite under acid conditions (i.e., “acid-nitrite”) in the stomach following consumption of residual cationic polymers. Two sets of formation potential tests were set up to evaluate the nitrosation propensity of N-based and P-based precursors, including the secondary analogues (dimethylamine (DMA), diethylamine (DEA), diethylphosphine (DEP)), the tertiary analogues (allyldimethylamine (ADMA), allyldiethylamine (ADEA), allyldiethylphosphine (ADEP)), the quaternary monomeric analogues (diallyldimethylammonium chloride (DADMAC), diallyldiethylammonium chloride (DADEAC), DADEPC), and the quaternary polymeric analogues (three commercial polyDADMACs as well as synthesized polyDADEAC and polyDADEPC). For chloramination, reaction solutions containing 10 mg/L individual precursors were buffered at pH 6.9 with 10 mM phosphate, dosed with 2 mM preformed monochloramine (NH2Cl), and incubated at room temperature for 10 d.32 For acid-nitrite treatment, reaction solutions were adjusted to pH 3.1, dosed with 10 mM sodium nitrite and incubated at 37 °C for 10 d.33 The extended reaction time and high doses of nitrosating agents were chosen to promote conversion of precursors to NOCs. The goal was to compare the maximum potential for NOC formation, not to predict actual concentrations that might be formed in drinking water or in the stomach. Following incubation, a portion of each sample was screened for specific anticipated NOCs (i.e., NDMA and Nnitrosodiethylamine (NDEA)) as well as major oxygenated phosphine products (i.e., diethylphosphine oxide (DEPO), diethylphosphinic acid (DEPA), and allyldiethylphosphine oxide (ADEPO)) (SI section S4). The remainder of each sample was measured for total NOC (abbreviated as “TONO”) concentrations (see below).34 To complement formation potential tests, the energetics of chloramination and acid-nitrite reaction pathways were assessed for two secondary amines (i.e., dimethylamine and diethylamine) and one secondary phosphine (diethylphosphine) via quantum chemical calculations using Gaussian 09 (revision C.01, Gaussian Inc.) (SI section S5).35 Coagulation Performance of Ammonium and Phosphonium Polymers. Three raw source waters were collected in June 2014 from Ohio (OH), Texas (TX), and California (CA), respectively. Water samples were shipped within 24 h in acid-rinsed high-density polyethylene containers to Stanford University and refrigerated at 4 °C in the dark until use. Benchscale jar tests were conducted using a Yost & Son jar test apparatus following standard protocols currently practiced by participating utilities (see SI section S7 for coagulation details).

nitrosamine formation. One potential strategy is to alter the backbone or functional groups on the quaternary ammonium polymer. However, this strategy may affect the performance of the coagulant, require significant changes in manufacturing procedures, and only alter the variety of N-nitrosamines formed. Recognizing that quaternary ammonium functional groups are employed predominantly because of their permanent positive charge, an alternative solution is to replace the quaternary ammonium with another positively charged functional group. Phosphorus, belonging to the nitrogen group of elements, similarly possesses a formal positive charge upon binding to four alkyl chains as a quaternary phosphonium group.18 Phosphonium monomers and polymers have attracted substantial interest in diverse fields because of their greater antimicrobial activity,19 higher thermal stability,20 and superior ionic conductivity21 as compared with ammonium derivatives. However, the utility of phosphonium polymers for coagulation has been largely overlooked in the literature. A handful of polymer coagulants functionalized with phosphonium groups were reported in earlier studies,22,23 but there has been little evaluation of their application under drinking-water-relevant conditions. Unlike quaternary phosphonium salts, secondary and tertiary phosphines are chemically unstable under oxidizing conditions and readily convert to oxygenated products (e.g., phosphine oxides and oxoacids).24,25 In light of this, we hypothesized that phosphonium polymers are less likely to form nitroso compounds (NOCs) than their ammonium analogues upon chloramination. To test this hypothesis, we developed a quaternary phosphonium cationic polymer by substituting the quaternary N atom of polyDADMAC with a P atom. We further hypothesized that such a minimal alternation to the polyDADMAC structure should effectively preserve its unique coagulation capability. Our synthetic approach began with quaternization of chlorodiethylphosphine with allyl chloride to form a diallyldiethylphosphonium chloride (DADEPC) monomer, followed by free-radical cyclopolymerization of the nonconjugated dienes in DADEPC to produce a quaternary phosphonium polymer (i.e., polyDADEPC). The coagulation efficacy and nitrosation propensity of polyDADEPC were evaluated against its quaternary ammonium analogue as well as commercial polyDADMACs currently used by water utilities.

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MATERIALS AND METHODS Synthesis of Phosphonium Monomer and Polymer. Information on chemical sources and purities as well as reagent preparation is given in the Supporting Information (SI) (section S1). A phosphonium polymer containing dimethyl moieties on the quaternary P atom (i.e., poly(diallyldimethylphosphonium chloride)) would serve as a direct substitute for polyDADMAC. Because of the unavailability of dimethylphosphine-based starting materials, a quaternary phosphonium polymer containing diethyl moieties (i.e., poly(diallyldiethylphosphonium chloride) (polyDADEPC)) was synthesized. A three-step synthesis protocol adapted from previously published methods was used for the preparation of polyDADEPC.26,27 To facilitate comparison of nitrosation propensity and coagulation efficacy, an ammonium polymer containing diethyl moieties on the quaternary N atom (i.e., poly(diallyldiethylammonium chloride) (polyDADEAC)) was synthesized in parallel.28−30 Phosphonium monomer synthesis was conducted under an inert atmosphere of dry, purified nitrogen gas (N2) because of the pyrophoricity of alkyl phosphines. The N2 atmosphere was 13393

dx.doi.org/10.1021/es504091s | Environ. Sci. Technol. 2014, 48, 13392−13401


Article

Table 1. Properties of Ammonium and Phosphonium Monomers and Polymers elemental analysis (wt %); found (calculated) monomer

molecular ion

31

P{1H} (ppm)a

DADEAC

C10H20N

+

N.A.

DADEPC

C10H20P+

22.3

polymer (source) polyDADMAC (OH) polyDADMAC (TX) polyDADMAC (CA) polyDADEAC (Stanford) polyDADEPC (Stanford)

observed mass (m/z)

154.15939 154.15957 (matching score = 99.73) 171.13002 171.13026 (matching score = 98.53)

Mn (kDa)c 51.8 47.6 46.8 134.2 116.1

theoretical mass (m/z)

(±0.2) (±0.1) (±0.1) (±0.3) (±0.3)

Mw (kDa)d 172.0 155.5 147.0 304.5 212.8

(±1.6) (±1.3) (±1.2) (±3.9) (±2.0)

error (ppm)b −1.17 −1.44

C

H

62.57 10.90 (63.31) (10.63) 57.48 9.93 (58.11) (9.75) CD (meq/g)f

PDIe

pH 5

pH 7

3.3 3.2 3.1 2.3 1.8

5.65 (±0.16) 5.15 (±0.10) 5.22 (±0.08) 4.55 (±0.06) 5.20 (±0.06)

5.64 (±0.19) 5.16 (±0.07) 5.29 (±0.07) 4.54 (±0.07) 5.17 (±0.04)

N

P

7.28 (7.38) polyDADMACs > DADMAC (Figure 1a). The relatively high yield of NDMA from ADMA likely resulted from the facile dealkylation of this tertiary amine to form DMA.45 Conversely, the quaternary ammonium, DADMAC, is much less prone to undergo dealkylation8 and formed an order of magnitude less NDMA than ADMA. All three commercial polyDADMACs exhibited a greater tendency than DADMAC to form NDMA, which concurs with prior observations of an enhancement of NDMA formation from quaternary DADMAC resulting from polymerization.9 No evident correlation existed between the NDMA yield and the molecular weight or charge density of polyDADMACs. For DEA-based precursors, only NDEA, not NDMA, was observed. The formation of NDEA followed the same decreasing trend, although the specific yields were lower than for NDMA, likely due to the increased steric hindrance to nitrosation imposed by the larger diethyl groups.46 The TONO assay was employed to capture the formation of unexpected NOCs. Figure 1c shows that the TONO yields from N-based precursors decreased in the order of secondary amine > 13395



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Scheme 1

tertiary phosphines, were quantified in chloraminated and acidnitrite treated samples (SI Figure S5). Upon chloramination (Scheme 1) or acid-nitrite treatment, DEPO accounted for >90 wt % of the total P mass for diethylphosphine, whereas DEPA explained the remaining P mass. In the case of allyldiethylphosphine, ADEPO nearly closed the P mass balance. Less than 1 wt % of DEPO or DEPA was found as side products from allyldiethylphosphine, suggesting that release of diethylphosphine from allyldiethylphosphine via dealkylation was insignificant compared with oxidation reactions. In the case of DADEPC, ADEPO constituted 13−37 wt % of the total P mass, and the majority of the P mass consisted of the parent DADEPC. Likewise, ADEPO dominated over DEPO and DEPA as the product of polyDADEPC. The total yield of oxygenated phosphine products (DEPO, DEPA, and ADEPO) from polyDADEPC was ∼27 and ∼18 wt % upon chloramination and acid-nitrite treatment, respectively. By analogy to DADEPC, the remainder is suspected to be unreacted parent polymer or polymer fragments. These results suggest that P-based precursors would give oxidized rather than nitrosated products even under strong nitrosation conditions, hence minimizing NOC formation. It is also possible that NDEP was actually formed but was highly unstable and decomposed under the experimental conditions. Stoichiometrically balanced equations for nitrosation of secondary amine and phosphine precursors by chloramines or acid-nitrite were modeled using Gaussian 09 to determine whether NOC formation is thermodynamically favored (see SI Table S2 for reactions and calculated ΔGr,(aq)). NDEA formation by the chloramination and acid-nitrite pathways were favorable (ΔGr,(aq) = −38.82 kcal/mol and −38.16 kcal/ mol, respectively), but oxidation to N-diethylhydroxylamine was not (ΔGr,(aq) = +10.38 kcal/mol). Although NDEP formation from diethylphosphine is exergonic with negative ΔGr values by the chloramination (Scheme 1) and acid-nitrite pathways (ΔGr,(aq) = −32.01 kcal/mol and −31.36 kcal/mol, respectively), oxidation reactions to DEPO (ΔGr,(aq) = −50.87 kcal/ mol) and DEPA (ΔG r,(aq) = −104.96 kcal/mol) were considerably more favored. In addition, the nitrosation reactions of DEP are less energetically favorable compared with those of diethylamine. Although these results align with the detection of NDEA, but not NDEP, energies for transition states, relevant to the kinetics of nitrosation, and other competing reaction pathways were not evaluated. Coagulation Performance of Ammonium and Phosphonium Polymers. The coagulation efficacy of polyDA-

tertiary amine > polymer > quaternary ammonium monomer, a pattern similar to that observed for specific N-nitrosamines. The TONO and specific N-nitrosamine yields agreed within 15%. Upon acid-nitrite treatment, the specific N-nitrosamine and TONO yields showed profiles similar to those observed during chloramination (Figures 1b,d), except that NOC yields from secondary and tertiary amines were higher, whereas yields from ammonium monomers and polymers were lower. Upon chloramination and acid-nitrate treatment, NDMA accounted for 2.4 and 7.1 wt % of the total N mass for DMA, respectively, whereas NDEA accounted for 1.1 and 1.0 wt % of the total N mass for DEA, respectively. No attempts were made to identify and quantify reaction products other than NDMA and NDEA. The NOC resulting from DEP-based precursors is anticipated to be P-nitrosodiethylphosphine (NDEP). Unfortunately, an authentic NDEP reference standard is not commercially available, and this compound has never been reported in the literature. Thus, a combined suspect and target MS screening approach was applied to evaluate NDEP formation in chloraminated and acid-nitrite-treated samples.47,48 HRMS scans were performed with an Agilent 6520 Accurate Mass quadrupole time-of-flight mass spectrometer equipped with a dual electrospray ionization source to extract the exact mass of NDEP calculated from its molecular formula for the corresponding m/z of M+, [M + H]+, and [M + Na]+. MS/ MS scans were performed with an Agilent 6460 triple quadrupole mass spectrometer equipped with a jet stream electrospray ionization source to extract two characteristic precursor-to-product transitions, [M + H]+ → [M + H − C2H4]+ and [M + H]+ → [M + H − 2C2H4]+, of NDEP with reference to the fragmentation patterns of NDEA (SI Table S1). No traces of NDEP were found. Following an identical synthetic route for NDEA,49 attempts were made to synthesize NDEP from neat DEP under oxygen-free, extreme acid-nitrite conditions (i.e., 5 M sodium nitrite in glacial acetic acid; SI section S6). The final products consisted predominantly of diethylphosphine oxide and a small fraction of diethylphosphinic acid, according to 31P NMR spectra and HRMS scans (SI Figures S6−S8). These stable phosphine oxidation products are relatively nontoxic.25 In addition, for all P-based precursors, no TONO responses were detected, implying negligible formation of NOCs relative to N-based precursors. To help define a P mass balance during the nitrosation of Pbased precursors, diethylphosphine oxide, diethylphosphinic acid, and allyldiethylphosphine oxide, the three oxygenated products most likely to form from corresponding secondary and 13396



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Figure 2. Coagulation performance of polyDADMACs, polyDADEAC, and polyDADEPC in waters OH, TX, and CA. (a−c) Representative time courses of turbidity removal at a polymer dose of 1.0 mg/L. “Alum/Ferric” indicates that only aluminum sulfate or ferric chloride was dosed to serve as a benchmark. Aluminum sulfate was used for waters OH and CA; ferric chloride was used for water TX. (d−f) DOC, DON, and UV254 percent removals as a function of polymer dose. Zero polymer dose corresponds to the aluminum sulfate or ferric chloride only control. Error bars represent the range of duplicate measurements; where absent, bars fall within symbols.

Figure 3. Nitrosation propensity of waters OH, TX, and CA following coagulation with polyDADMACs, polyDADEAC, or polyDADEPC. (a−c) Specific N-nitrosamine yields in coagulated waters as a function of polymer dose. “Raw” represents N-nitrosamine formation from the chloramination of raw, noncoagulated waters (i.e., no addition of either alum/ferric or polymer). Zero polymer dose corresponds to the aluminum sulfate or ferric chloride only control. (d−f) TONO yields in waters coagulated with a high polymer dose at 2.0 mg/L. “Raw” represents TONO formation from the chloramination of raw, noncoagulated waters. Left-hand-side bars represent the sum of N-nitrosamine yields as the equivalent concentration of NDMA (data taken from the bars corresponding to 2.0 mg/L polymer dose in panels a−c). Right-hand-side bars represent TONO yields as the equivalent concentration of NDMA. Error bars represent the range of duplicate measurements.

and SI section S7 for experimental details). Samples were treated with a constant dose of alum or ferric and supplemented with varying doses of polymers. The performance of

DEPC was evaluated against its ammonium analogue polyDADEAC as well as commercial polyDADMACs by jar tests using three source waters (see SI Table S3 for water quality 13397



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This 22 ng/L increase in NDMA accounted for 85% of the 26 ng/L increase in TONO. Similarly, the 19 ng/L increase in NDEA (equivalent to 13 ng/L increase in NDMA) upon polyDADEAC addition constituted over 85% of the 15 ng/L increase in TONO. In contrast, polyDADEPC did not induce any additional formation of NOCs compared with the alum only control. For water TX, the increase in NDMA and NDEA concentrations reached up to 31 and 12 ng/L (as NDMA), respectively, representing a major portion of the increase in TONO of 34 and 15 ng/L upon the addition of polyDADMAC and polyDADEAC. For water CA, the fractions of increase in TONO that are attributable to NDMA or NDEA formation upon polymer addition were ∼90%. Like nitrogen, phosphorus may promote microbial growth in distribution systems and, when treated water is eventually discharged to receiving waters after passage through a wastewater treatment plant, is a major contributor to eutrophication.55 The total dissolved phosphorus (TDP) and inorganic orthophosphate were measured in coagulated water samples (see SI section S7 for analytical details) to assess to what extent the use of polyDADEPC might contribute to the net P loading. Increasing doses of polyDADEPC increased TDP levels (Figure 4); however, the orthophosphate concentrations

polyDADEPC tracked closely with that of ammonium polymers with respect to the removal of turbidity, DOC, DON, and UV254. Figure 2a−c provides example time courses of turbidity removal in water samples dosed with or without polymers. In contrast to the alum or ferric only controls, turbidity reduction in polymer-dosed samples exhibited a biphasic pattern with an initial rapid decline followed by a more gradual drop. Other tested polymer doses generated similarly shaped turbidity removal profiles but differed in the initial rate of drop (SI Figures S9−11). For all waters, polyDADEPC and polyDADMAC slightly outperformed polyDADEAC within the high settling velocity region (i.e., 0−9 min), even though the residual turbidity eventually converged. Figure 2d−f demonstrates that the removal efficiencies of DOC, DON, and UV254 improved upon polymer addition, but leveled out at the high polymer doses. PolyDADEPC performed similarly to polyDADMAC and polyDADEAC. Depending on the type and doses of polymers, the percent removal of DOC increased by up to 17−25% in comparison with the alum or ferric only controls. The removal of DON exhibited a similar trend, although the increase in percent removal was less pronounced. The percent removal of UV254 was always greater than that of DOC and DON, indicating that aromatic DOM components were more effectively removed. Previous research has suggested that polyDADMAC preferentially removes the more hydrophobic, high molecular weight fractions of DOC and DON.50 The NOC formation potential of waters after coagulation with ammonium and phosphonium polymers was evaluated under uniform formation conditions. No N-nitrosamines were detected in unchloraminated raw waters. NDMA was detected in chloraminated waters with concentrations ranging from 7 to 13 ng/L. NDMA formation potential was not reduced in waters treated by coagulants (Figure 3a−c), implying that precursors present in these waters are not readily removed by coagulation.51,52 The addition of polyDADMAC and polyDADEAC promoted the formation of NDMA and NDEA, respectively, relative to the alum or ferric only controls. NDMA was the only specific N-nitrosamine detected in waters coagulated with polyDADMAC. For polyDADEAC, NDMA remained at the same levels generated in the alum or ferric only controls, but NDEA accumulated, indicating that NDEA formed as a consequence of polyDADEAC addition. Increasing polymer doses spurred higher N-nitrosamine yields in all three waters. Water CA exhibited the largest increase in N-nitrosamine formation, which might be due to its high native bromide (>400 μg/L; SI Table S3), which is known to enhance the yield of Nnitrosamines.53,54 For polyDADEPC, no measurable increases in NDMA or NDEA were found relative to the alum or ferric only controls, and no NDEP was detectable. As measured by the TONO assay, all three waters formed appreciable amounts of NOCs upon chloramination, including raw samples with no coagulant addition (Figures 3d−f). Approximately 38%, 20%, and 34% of the TONO could be explained by NDMA formed in waters OH, TX, and CA, respectively, consistent with previous research indicating that NDMA is not a major component of the total NOCs formed by chloramination.34 Unlike NDMA, 16−20% of TONO was eliminated during coagulation with alum or ferric alone, suggesting that certain NOC precursors were better removed than NDMA precursors by coagulation. The addition of polymers further enhanced the TONO yields. For water OH, the addition of polyDADMAC increased the concentration of NDMA from 7 to 29 ng/L and of TONO from 16 to 42 ng/L.

Figure 4. Total dissolved phosphorus levels in waters coagulated with varying doses of polyDADEPC. Error bars represent the range of duplicate measurements; where absent, bars fall within symbols.

in waters OH, TX, and CA remained, respectively, at the 0.02, 0.05, and 0.04 mgP/L measured in the untreated waters, indicating that the increasing polyDADEPC dose contributed to an increase in organic P. Both quaternary ammonium and phosphonium compounds exhibit biocidal properties. 56 Although further research is needed, the lack of significant increase in orthophosphate, likely the most bioavailable fraction, with polyDADEPC dose suggests that polyDADEPC is unlikely to promote microbial growth in distribution systems. Even at the highest polyDADEPC dose (i.e., 2.0 mg/L), TDP levels were below or slightly above 0.1 mgP/L, a typical guideline value for limiting nutrients in wastewater effluents.55 Thus, even if all of the residual polyDADEPC passes through drinking water and wastewater treatment plants, it is unlikely to result in an exceedance of P regulatory limits. Implications. Many utilities have relied on quaternary ammonium cationic polymers, in particular, polyDADMAC, for coagulation and flocculation. However, if the USEPA considers a maximum contaminant level for N-nitrosamines at a low nanograms per liter level, the use of quaternary ammonium cationic polymers may hinder compliance, especially for utilities that practice chloramine disinfection. A substantial alteration in the quaternary ammonium cationic polymer structure to 13398



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minimize N-nitrosamine formation could present risks in terms of coagulant performance and other operational issues, necessitating substantial pilot and full-scale testing. In this work, we replaced the N atom in polyDADMAC with a P atom but maintained the essential structure of polyDADMAC. The minimal alternation in structure is anticipated to avoid alternations in polymer handling procedures and operational challenges. The newly synthesized phosphonium polymer performed at least as well as commercially available polyDADMACs for turbidity removal and DOM reduction. Furthermore, this phosphonium polymer did not appear to form nitrosated byproducts. The primary obstacle to implementation of the phosphonium-based “polyDADMAC” is its lack of commercial availability. A comprehensive assessment of manufacturing cost and production efficiency is needed prior to commercialization. This would be best addressed by industrial polymer manufacturers. As demonstrated in this work for polyDADEAC and polyDADEPC, the procedures for the synthesis of the polymer from the quaternary ammonium or phosphonium monomers are nearly identical. Thus, if the quaternary phosphonium monomer were commercially available, quaternary phosphonium polymers could be produced within the existing polymer manufacturing facilities. Although quaternary phosphonium monomer synthesis requires oxygen-free conditions (unlike quaternary ammonium monomer synthesis), it is unlikely that this would represent a significant hindrance because quaternary phosphonium derivatives have found a wide variety of important applications (e.g., flame retardants and antimicrobial agents);57 however, simpler and more environmentally benign synthetic routes should be sought. The stability of the phosphonium polymer under typical storage conditions should also be evaluated. Although not the focus of this work, it is noteworthy that the majority of commercial anion-exchange resins similarly rely on the quaternary ammonium functionality, which also contributes to N-nitrosamine formation.9,58,59 The concept of substituting P for the quaternary N would likely apply to these resins as well and hence solve another problem of emerging concern.

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REFERENCES

(1) Crittenden, J. C.; Trussell, R. R.; Hand, D. W.; Howe, K. J.; Tchobanoglous, G. Water Treatment - Principles and Design, 2nd ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2005. (2) Bolto, B.; Gregory, J. Organic polyelectrolytes in water treatment. Water Res. 2007, 41, 2301−2324. (3) Edzwald, J. K.; Tobiason, J. E. Enhanced coagulation: US requirements and a broader view. Water Sci. Technol. 1999, 40, 63−70. (4) Liang, L.; Singer, P. C. Factors influencing the formation and relative distribution of haloacetic acids and trihalomethanes in drinking water. Environ. Sci. Technol. 2003, 37, 2920−2928. (5) Matilainen, A.; Vepsäläinen, M.; Sillanpäa,̈ M. Natural organic matter removal by coagulation during drinking water treatment: A review. Adv. Colloid Interface Sci. 2010, 159, 189−197. (6) Kohut, K. D.; Andrews, S. A. Polyelectrolyte age and Nnitrosodimethylamine formation in drinking water treatment. Water Qual. Res. J. Can. 2003, 38, 719−735. (7) Wilczak, A.; Assadi-Rad, A.; Lai, H. H.; Hoover, L. L.; Smith, J. F.; Berger, R.; Rodigari, F.; Beland, J. W.; Lazzelle, L. J.; Kincannon, E. G.; Baker, H.; Heaney, C. T. Formation of NDMA in chloraminated water coagulated with DADMAC cationic polymer. J. Am. Water Works Assoc. 2003, 95, 94−106. (8) Park, S. H.; Wei, S.; Mizaikoff, B.; Taylor, A. E.; Favero, C.; Huang, C. H. Degradation of amine-based water treatment polymers during chloramination as N-nitrosodimethylamine (NDMA) precursors. Environ. Sci. Technol. 2009, 43, 1360−1366. (9) Kemper, J. M.; Walse, S. S.; Mitch, W. A. Quaternary amines as nitrosamine precursors: A role for consumer products? Environ. Sci. Technol. 2010, 44, 1224−1231. (10) Krasner, S. W.; Mitch, W. A.; McCurry, D. L.; Hanigan, D.; Westerhoff, P. Formation, precursors, control, and occurrence of nitrosamines in drinking water: A review. Water Res. 2013, 47, 4433− 4450. (11) Krasner, S. W.; Mitch, W. A.; Westerhoff, P.; Dotson, A. Formation and control of emerging C- and N-DBPs in drinking water. J. Am. Water Works Assoc. 2012, 104, E582−E595. (12) Jakszyn, P.; González, C. A. Nitrosamine and related food intake and gastric and oesophageal cancer risk: A systematic review of the epidemiological evidence. World J. Gastroenterol. 2006, 12, 4296−4303. (13) Bryan, N. S.; Alexander, D. D.; Coughlin, J. R.; Milkowski, A. L.; Boffetta, P. Ingested nitrate and nitrite and stomach cancer risk: An updated review. Food Chem. Toxicol. 2012, 50, 3646−3665. (14) U.S. Environmental Protection Agency, Office of Research and Development (ORD), National Center for Environmental Assessment. Integrated Risk Information System; U.S. Environmental Protection Agency: Washington, DC, 1993. http://www.epa.gov/iris/ (accessed August 6, 2014). (15) California Department of Public Health. NDMA and Other Nitrosamines − Drinking Water Issues; California Department of Public Health: Sacramento, CA, 2009. http://www.cdph.ca.gov/certlic/ drinkingwater/Pages/NDMA.aspx (accessed August 6, 2014). (16) U.S. Environmental Protection Agency. Unregulated Contaminant Monitoring Rule 2 (UCMR 2); U.S. Environmental Protection Agency: Washington, DC, 2006. http://water.epa.gov/lawsregs/rulesregs/ sdwa/ucmr/ucmr2/basicinformation.cfm#list (accessed August 6, 2014). (17) U.S. Environmental Protection Agency. Contaminant Candidate List 3 (CCL3); U.S. Environmental Protection Agency: Washington, DC, 2009. http://water.epa.gov/scitech/drinkingwater/dws/ccl/ccl3. cfm (accessed August 6, 2014). (18) Engel, R.; Cohen, J. L. I. Organic polycationic salts − Syntheses and applications. Curr. Org. Chem. 2002, 6, 1453−1467. (19) Kanazawa, A.; Ikeda, T.; Endo, T. Novel polycationic biocides: Synthesis and antibacterial activity of polymeric phosphonium salts. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 335−343. (20) Xie, W.; Xie, R.; Pan, W.-P.; Hunter, D.; Koene, B.; Tan, L.-S.; Vaia, R. Thermal stability of quaternary phosphonium modified montmorillonites. Chem. Mater. 2002, 14, 4837−4845.

ASSOCIATED CONTENT

S Supporting Information *

Additional experimental details, tables, schemes, and figures as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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Article

AUTHOR INFORMATION

Corresponding Author

*Phone: (650)725-9298; fax: (650)723-7058; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We greatly appreciate Dr. Ning Dai (University at Buffalo, The State University of New York) for the help with TONO assay and Peter A. Maraccini (Stanford University) for the use of the turbidimeter. We would also like to thank participating utilities for their support. This research was supported by the Water Research Foundation (Project 4452). 13399



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(21) Wang, K.; Zeng, Y.; He, L.; Yao, J.; Suresh, A. K.; Bellare, J.; Sridhar, T.; Wang, H. Evaluation of quaternary phosphonium-based polymer membranes for desalination application. Desalination 2012, 292, 119−123. (22) Nonaka, T.; Makinose, K.; Kurihara, S.Synthesis and properties of water-soluble thermosensitive copolymers having phosphonium groups. In Stimuli-Responsive Water Soluble and Amphiphilic Polymers; McCormick, C. L., Ed. American Chemical Society: Washington, D.C., 2000; Vol. 780, pp 255−266. (23) Song, W.; Zhao, Z.; Zheng, H.; Wang, G. Gamma-irradiation synthesis of quaternary phosphonium cationic starch flocculants. Water Sci. Technol. 2013, 68, 1778−1784. (24) Honaker, M. T.; Hovland, J. M.; Salvatore, R. N. The synthesis of tertiary and secondary phosphines and their applications in organic synthesis. Curr. Org. Synth. 2007, 4, 31−45. (25) Nath, N. S.; Bhattacharya, I.; Tuck, A. G.; Schlipalius, D. I.; Ebert, P. R. Mechanisms of phosphine toxicity. J. Toxicol. 2011, DOI:10.1155/ 2011/494168. (26) Butler, G. B.; Skinner, D. L.; Bond, W. C.; Rogers, C. L. Linear polymers from diene monomers by the cyclic polymerization mechanism. XI. Polyphosphonium salts via cyclopolymerization. J. Macromol. Sci., Part A: Pure Appl. Chem. 1970, 4, 1437−1451. (27) Seyferth, D.; Masterman, T. C. Cyclopolymerization of diallyldiphenylphosphonium salts. Polymeric phosphonium salts, phosphine oxides, and phosphorus ylides. Macromolecules 1995, 28, 3055−3066. (28) Butler, G. B.; Angelo, R. J. Preparation and polymerization of unsaturated quaternary ammonium compounds. VIII. A proposed alternating intramolecular−intermolecular chain propagation. J. Am. Chem. Soc. 1957, 79, 3128−3131. (29) Negi, Y.; Harada, S.; Ishizuka, O. Cyclopolymerization of diallylamine derivatives in dimethyl sulfoxide. J. Polym. Sci., Part A-1: Polym. Chem. 1967, 5, 1951−1965. (30) Liu, L.-H.; Gong, Z.-Q.; Zheng, Y.-J. Synthesis and structure characterization of diethyldiallylammonium chloride. J. Cent. South Univ. Technol. 2003, 10, 347−351. (31) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-Sensitive Compounds, 2nd ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 1986. (32) Mitch, W. A.; Sedlak, D. L. Characterization and fate of Nnitrosodimethylamine precursors in municipal wastewater treatment plants. Environ. Sci. Technol. 2004, 38, 1445−1454. (33) Mirvish, S. S. Formation of N-nitroso compounds: Chemistry, kinetics, and in vivo occurrence. Toxicol. Appl. Pharmacol. 1975, 31, 325−351. (34) Dai, N.; Mitch, W. A. Relative importance of N-nitrosodimethylamine compared to total N-nitrosamines in drinking waters. Environ. Sci. Technol. 2013, 47, 3648−3656. (35) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision C.01; Gaussian, Inc.: Wallingford, CT, 2009. (36) Majewski, P. Investigation of the reaction between dialkylphosphines and carbon tetrachloride. Part I. Phosphorus, Sulfur Silicon Relat. Elem. 1993, 85, 41−47. (37) Kam, S.-K.; Gregory, J. The interaction of humic substances with cationic polyelectrolytes. Water Res. 2001, 35, 3557−3566.

(38) Kvinnesland, T.; Ødegaard, H. The effects of polymer characteristics on nano particle separation in humic substances removal by cationic polymer coagulation. Water Sci. Technol. 2004, 50, 185− 191. (39) Bolto, B.; Abbt-Braun, G.; Dixon, D.; Eldridge, R.; Frimmel, F.; Hesse, S.; King, S.; Toifl, M. Experimental evaluation of cationic polyelectrolytes for removing natural organic matter from water. Water Sci. Technol. 1999, 40, 71−79. (40) Kam, S.-K.; Gregory, J. Charge determination of synthetic cationic polyelectrolytes by colloid titration. Colloids Surf., A 1999, 159, 165−179. (41) Graham, N.; Gang, F.; Fowler, G.; Watts, M. Characterisation and coagulation performance of a tannin-based cationic polymer: A preliminary assessment. Colloids Surf., A 2008, 327, 9−16. (42) Eichholzer, M.; Gutzwiller, F. Dietary nitrates, nitrites, and Nnitroso compounds and cancer risk: A review of the epidemiologic evidence. Nutr. Rev. 1998, 56, 95−105. (43) Shah, A. D.; Mitch, W. A. Halonitroalkanes, halonitriles, haloamides, and N-nitrosamines: A critical review of nitrogenous disinfection byproduct formation pathways. Environ. Sci. Technol. 2012, 46, 119−131. (44) Schreiber, I. M.; Mitch, W. A. Nitrosamine formation pathway revisited: The importance of chloramine speciation and dissolved oxygen. Environ. Sci. Technol. 2006, 40, 6007−6014. (45) Mitch, W. A.; Schreiber, I. M. Degradation of tertiary alkylamines during chlorination/chloramination: Implications for formation of aldehydes, nitriles, halonitroalkanes, and nitrosamines. Environ. Sci. Technol. 2008, 42, 4811−4817. (46) Sun, Z.; Liu, Y.; Zhong, R. Theoretical investigation of reactivities of amines in the N-nitrosation reactions by N2O3. J. Mol. Model. 2011, 17, 669−680. (47) Krauss, M.; Singer, H.; Hollender, J. LC−high resolution MS in environmental analysis: From target screening to the identification of unknowns. Anal. Bioanal. Chem. 2010, 397, 943−951. (48) Moschet, C.; Piazzoli, A.; Singer, H.; Hollender, J. Alleviating the reference standard dilemma using a systematic exact mass suspect screening approach with liquid chromatography−high resolution mass spectrometry. Anal. Chem. 2013, 85, 10312−10320. (49) Love, B. E.; Jones, E. G. An improved synthesis of camphorquinone-3-oxime. Synth. Commun. 1999, 29, 2831−2840. (50) Lee, W.; Westerhoff, P. Dissolved organic nitrogen removal during water treatment by aluminum sulfate and cationic polymer coagulation. Water Res. 2006, 40, 3767−3774. (51) Chung, J.; Yoon, Y.; Kim, M.; Lee, S. B.; Kim, H. J.; Choi, C. K. Removal of radio N-nitrosodimethylamine (NDMA) from drinking water by coagulation and Powdered Activated Carbon (PAC) adsorption. Drinking Water Eng. Sci. 2009, 2, 49−55. (52) Knight, N.; Watson, K.; Farré, M.; Shaw, G. N-nitrosodimethylamine and trihalomethane formation and minimisation in Southeast Queensland drinking water. Environ. Monit. Assess. 2012, 184, 4207− 4222. (53) Le Roux, J.; Gallard, H.; Croué, J.-P. Formation of NDMA and halogenated DBPs by chloramination of tertiary amines: The influence of bromide ion. Environ. Sci. Technol. 2012, 46, 1581−1589. (54) Luh, J.; Mariñas, B. J. Bromide ion effect on N-nitrosodimethylamine formation by monochloramine. Environ. Sci. Technol. 2012, 46, 5085−5092. (55) Maupin, M. A.; Ivahnenko, T. Nutrient loadings to streams of the continental United States from municipal and industrial effluent. J. Am. Water Resour. Assoc. 2011, 47, 950−964. (56) Kanazawa, A.; Ikeda, T.; Endo, T. Synthesis and antimicrobial activity of dimethyl- and trimethyl-substituted phosphonium salts with alkyl chains of various lengths. Antimicrob. Agents Chemother. 1994, 38, 945−952. (57) Rickelton, W. A. Phosphine and its derivatives. In Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc.: Hoboken, NJ,2000. 13400



Article

(58) Kimoto, W. I.; Dooley, C. J.; Carré, J.; Fiddler, W. Role of strong ion exchange resins in nitrosamine formation in water. Water Res. 1980, 14, 869−876. (59) Flowers, R. C.; Singer, P. C. Anion exchange resins as a source of nitrosamines and nitrosamine precursors. Environ. Sci. Technol. 2013, 47, 7365−7372.

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Synthesis and Application of a Quaternary Phosphonium Polymer

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