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Removal of bromide and natural organic matter by anion exchange
water research 44 (2010) 2133–2140
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Removal of bromide and natural organic matter by anion exchange Susan Hsu a,b,1, Philip C. Singer a,* a
University of North Carolina at Chapel Hill, Department of Environmental Sciences and Engineering, Campus Box 7431, Chapel Hill, NC 27599-7431, USA b Los Angeles County Sanitation Districts, 1955 Workman Mill Road, Whittier, CA 90601, USA
article info
abstract
Article history:
Bromide removal by anion exchange was explored for various water qualities, process
Received 21 August 2009
configurations, and resin characteristics. Simulated natural waters containing different
Received in revised form
amounts of natural organic matter (NOM), bicarbonate, chloride, and bromide were treated
9 December 2009
with a polyacrylate-based magnetic ion exchange (MIEX) resin on a batch basis to evaluate
Accepted 14 December 2009
the effectiveness of the resin for removal of bromide. While bromide removal was achieved
Available online 22 December 2009
to some degree, alkalinity (bicarbonate), dissolved organic carbon (DOC), and chloride were shown to inhibit bromide removal in waters with bromide concentrations of 100 and
Keywords:
300 mg/L. Water was also treated using a two-stage batch MIEX process. Two-stage treat-
Ion exchange
ment resulted in only a slight improvement in bromide removal compared to single-stage
Bromide
treatment, presumably due to competition with the high concentration of chloride which is
Disinfection by-products
present along with bromide in natural waters. In view of the relatively poor bromide
Drinking water
removal results for the MIEX resin, a limited set of experiments was performed using
Natural organic matter
polystyrene resins. DOC and bromide removal were compared by treating model waters
Magnetic ion exchange (MIEX)
with MIEX and two polystyrene resins, Ionac A-641 and Amberlite IRA910. The two polystyrene resins were seen to be more effective for bromide removal, while the MIEX resin was more effective at removing DOC. ª 2009 Elsevier Ltd. All rights reserved.
1.
Introduction
Bromide and dissolved organic carbon (DOC) are the two principal precursors in the formation of halogenated disinfection by-products (DBPs) resulting from chlorination of drinking water. Bromide is readily oxidized to hypobromous acid (HOBr) by free chlorine (HOCl). HOBr and HOCl are both strong oxidants which react with NOM to form halogenated DBPs, including trihalomethanes (THMs) and haloacetic acids (HAAs), which currently are regulated by the US Environmental Protection Agency (EPA) under the Stage 1 Disinfectants and Disinfection By-Products (D/DBP) Rule (US EPA, 1998). At high
Br/DOC and Br/Cl2 dose ratios, more brominated DBP species have been shown to form (Oliver, 1983; Symons et al., 1993). Many of these bromine-containing DBPs tend to be more harmful than their fully chlorinated counterparts (Attias et al., 1995; Plewa et al., 2002). A common strategy to control DBP formation is to remove the precursors prior to chlorination. However, these efforts, using processes such as enhanced coagulation, activated carbon adsorption, and nanofiltration, have been directed at the removal of DOC which tends to increase the Br/DOC ratio and the corresponding Br/Cl2 dose ratio, leading to preferential formation of brominated DBPs. Hence bromide removal,
* Corresponding author. Tel.: þ1 919 966 3865; fax: þ1 919 966 7911. E-mail addresses: [email protected] (S. Hsu), [email protected] (P.C. Singer). 1 Tel.: þ1 562 908 4288 x 2145; fax: þ1 562 699 4515. 0043-1354/$ – see front matter ª 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2009.12.027
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as well as DOC removal, is desired for effective DBP control to protect public health. Bromide can be removed by membrane processes, such as reverse osmosis, but this is an expensive technology. It can also be removed by anion exchange, which is the focus of this paper. A magnetic ion exchange (MIEX) resin, marketed by Orica Watercare of Victoria, Australia, was developed specifically for the removal of DOC from raw drinking waters. The MIEX resin is a strong-base resin with iron oxide integrated into a macroporous, polyacrylic matrix, and is typically used with chloride as the exchangeable ion. The iron oxide provides the resin with magnetic characteristics that facilitate aggregation and settling. The average MIEX resin particles are 180 mm in diameter, 2–5 times smaller than traditional resins. The increased surface area-to-volume ratio allows for faster ion exchange kinetics and decreased resin fouling due to shorter NOM diffusion paths within the resin (Bourke et al., 2001). Unlike larger, traditional ion exchange resins that tend to be used in packed beds downstream of solid-liquid separation processes, the MIEX resin is used either in a slurry contactor or in a fluidized bed at the head of the treatment train. The MIEX process presents a very promising method to reduce DBP formation when used upstream of chlorination. Several studies have shown very effective NOM removal using the MIEX resin (Singer and Bilyk, 2002; Drikas et al., 2003; Fearing et al., 2004; Morran et al., 2004; Boyer and Singer, 2005; Allpike et al., 2005). Reduction of THM and HAA formation potential from MIEX treatment has also been shown to be greater than that achieved through enhanced coagulation (Singer and Bilyk, 2002; Drikas et al., 2003; Morran et al., 2004; Boyer and Singer, 2006). The effectiveness of DOC removal is affected by NOM characteristics and the presence of other anions which compete with DOC for the anion exchange sites on the resin. In general, DOC is more effectively removed from waters with higher SUVA values (Singer and Bilyk, 2002; Boyer and Singer, 2005). Waters with high total dissolved solids exhibit poor removal of DOC by the MIEX resin (Singer et al., 2007a). The separation factors for Suwanee River Fulvic Acid (SRFA) and bicarbonate relative to chloride have been found to be 5.7 and 0.76, respectively (Boyer and Singer, 2008), for the MIEX resin, reflecting the MIEX resin’s strong preference for DOC, and it’s weak preference for bicarbonate. Boyer and Singer (2008) also showed that NOM was removed more effectively with polyacrylic resins compared to polystyrene resins. In addition to DOC removal, bromide has also been observed to be removed to a significant extent by the MIEX resin (Singer and Bilyk, 2002; Johnson and Singer, 2004; Humbert et al., 2005), but the effectiveness of bromide removal strongly depends on water quality. High alkalinities (bicarbonate concentrations) and, especially, high sulfate concentrations (>50 mg/L) have been shown to inhibit bromide removal (Singer and Bilyk, 2002; Johnson and Singer, 2004; Boyer and Singer, 2006; Singer et al., 2007a). However, if configured appropriately, the MIEX process may be able to remove both bromide and NOM and is therefore a promising technology for reducing not only overall DBP formation but also formation of the more hazardous brominated species. Accordingly, the objective of this study was to examine the effect of water quality, process configuration, and resin
characteristics on bromide and DOC removal by anion exchange. The effects of alkalinity (bicarbonate) and DOC on bromide removal were examined on a batch basis by treating model waters containing various alkalinities and DOC concentrations with the MIEX resin. The model waters were also treated using a two-stage batch MIEX process in order to determine the feasibility of engineering a two-stage configuration to improve bromide removal. The first stage was expected to remove competing anions such as bicarbonate and DOC, allowing for better removal of bromide in the second stage. A limited set of experiments was conducted with Ionac A-641 and Amberlite IRA910, two macroporous, polystyrene resins, to compare bromide and DOC removal by polystyrene and polyacrylic (MIEX) resins.
2.
Materials and methods
2.1.
Experimental methodology
Simulated natural waters were prepared with bromide concentrations of approximately 100 and 300 mg/L Br using sodium bromide (NaBr; Fisher Chemical, Fairlawn, NJ). These levels were chosen to reflect concentrations found in highbromide natural waters in the USA. Sodium chloride (NaCl; Fisher Chemical, Fairlawn, NJ) was added to give a Cl to Br ratio of 333:1 (by weight), a ratio commonly found in natural waters (Amy et al., 1994). At each bromide concentration, the waters were made with two different alkalinities and three different DOC concentrations. Sufficient sodium bicarbonate (NaHCO3; Fisher Chemical, Fairlawn, NJ) was added to give alkalinities of 24 and 120 mg/L as CaCO3 at pH 8. Suwanee River (SR) NOM, extracted by reverse osmosis, was obtained from the International Humic Substances Society (IHSS), and was used as the source of DOC for experiments using the MIEX resin. Waters were prepared to contain approximately 0, 4, and 10 mg/L DOC. (The actual concentrations measured are shown with each of the results.) Lake Drummond Fulvic Acid (LDFA), extracted by Chang (1990) from a lake in southeastern Virginia, was used at a concentration of 10 mg/L DOC for experiments comparing the effectiveness of MIEX, A-641 and IRA910 resins due to exhaustion of the Suwanee River concentrate. Both the Suwanee River and Lake Drummond are organic-rich bodies of water located in the southern US that have high concentrations of NOM that is dominated by hydrophobic organic carbon. SR NOM and LDFA had average measured SUVA values of 5.2 and 4.3 L/mg-m, respectively. Each model water was prepared with organic-free deionized water (OFDW) and was adjusted to pH 8 using either 0.1 M sodium hydroxide (NaOH) or 0.1 M hydrochloric acid (HCl) before being treated. All water samples were stored at 4 C and brought to room temperature (22 C) before testing. The MIEX resin used in this study was virgin (unused) resin provided by Orica Watercare (Denver, CO). The resin was rinsed with OFDW to wash away the fines and stored in OFDW before use. In the single-stage jar tests, 500 mL samples of each water were treated at MIEX resin doses of 1, 2, 4 and 6 mL/L using a Phipps and Bird six-place gang stirrer (Phipps and Bird, Richmond, VA). MIEX resin was measured out in 10 mL graduated cylinders using Pasteur pipettes and
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3.
Results and discussion
3.1.
Bromide removal in single-stage jar tests
The effect of DOC concentration on bromide removal in low alkalinity model water is shown in Fig. 1 for two different bromide concentrations. The error bars in Fig. 1b represent the range of the results from duplicate MIEX treatments and show good agreement between the duplicate runs. For the same MIEX resin doses, bromide removal is greater in the absence of DOC (represented by the solid curve in Fig. 1a and b), and tends to decrease with increasing DOC concentration. However, the impacts of DOC on bromide removal are relatively minor, presumably due to the large number of exchange sites on the MIEX resin (see below). The difference between bromide removed in the DOC-free and high DOC waters is around 10
a 80
60
40
0 mg/L DOC 3.5 mg/L DOC 9.4 mg/L DOC
20
Analytical methods
pH was measured using an Accumet AB15 pH meter from Fisher Scientific. The pH meter was calibrated before each use. UV254 was measured using a 1 cm quartz cell on a Hitachi U2000 spectrophotometer according to Standard Method 5390 (APHA, 1998). DOC and DIC concentrations were determined using a Shimadzu TOC-VCPH Total Organic Carbon Analyzer; DOC was analyzed according to Standard Method 5310B (APHA, 1998). 2 N HCl was used to acidify samples instead of phosphoric acid in accordance with Shimadzu instrument specifications. Bromide and chloride were measured using suppressed conductivity detection on a Dionex Ion
0 0
1
2
3
4
b
6
7
200
160
120
80 0 mg/L DOC 3.6 mg/L DOC 8.6 mg/L DOC
40
0
Table 1 – Resin characteristics.
Polymeric matrix Exchange capacity (eq/L) Effective size (mm)
5
MIEX resin dose (mL/L)
Bromide removed (µg/L)
2.2.
Chromatograph with an IonPac AG4A-SC guard column and an AS4A-SC analytical column according to U.S. Environmental Protection Agency Method 300.0 (USEPA, 1999). The detection limit associated with this method is 10 mg/L. Each sample was analyzed in duplicate and the relative difference was less than 15%.
Bromide removed (µg/L)
transferred to water samples, using OFDW to rinse all of the resin out of the graduated cylinder. Samples were mixed at 100 rpm for 30 min, then allowed to settle for 30 min. The first 10 mL of sample from the sampling port was discarded and a 250 mL sample was taken and filtered using a 0.45 mm Supor450 membrane filter (Pall, Ann Arbor, MI) to remove any remaining resin particles. The filters were pre-rinsed with 250 mL of OFDW before each use. Samples were analyzed for bromide, DOC and ultraviolet absorbance at 254 nm (UV254). For some samples, residual chloride and dissolved inorganic carbon (DIC) concentrations were also measured. Duplicate runs were performed for two waters containing 24 mg/L as CaCO3, and 300 mg/L Br to examine experimental repeatability. In the two-stage MIEX resin treatments, 4 L of water was first treated in two 2-L square jars with 1 mL/L of the MIEX resin. Samples were mixed at 100 rpm for 30 min and allowed to settle for 30 min. 1.5 L of the treated water from each jar was decanted and combined. A 250 mL sample was taken and filtered for bromide, DOC and UV254 analysis. The rest of the water was treated with 1, 2, 4 and 6 mL/L of MIEX resin. 500 mL samples were treated following the same procedure used for the single-stage jar tests, and filtered samples were analyzed for bromide, DOC and UV254. In view of the relatively poor bromide removal results for the MIEX resin, a limited set of experiments was performed using polystyrene resins. Ionac A-641 and Amberlite IRA910 were obtained from Sybron Chemicals Inc. and Rohm and Haas, respectively. Treatment using the two polystyrene resins was performed in the same manner as for the MIEX resin except that 100 mL samples were taken after contact times of 30 min and 5 h. The resins were prepared according to published procedures (Bolto et al., 2002; Humbert et al., 2005) and stored in OFDW before use. Characteristics of the three resins are given in Table 1. Each sample was filtered before being analyzed for bromide, DOC and UV254.
0
MIEX
A-641
IRA910
Polyacrylic 0.44 180
Polystyrene 1.0 450
Polystyrene 1.1 460
1
2
3
4
5
6
MIEX resin dose (mL/L)
Fig. 1 – Impact of Suwanee River DOC concentration on bromide removal in model waters with an alkalinity of 24 mg/L as CaCO3. (a) Br0 z 100 mg/L, (b) Br0 z 300 mg/L.
7
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0.021 meq/L was removed for water with an initial DOC concentration of 3.5 mg/L (Fig. 2). This corresponds to 51% DOC removed. For the water containing 3.5 mg/L DOC and approximately 100 mg/L Br, 1 mL/L of MIEX resin removed only 35 mg/L or 0.00044 meq/L Br, corresponding to a bromide removal of 35% (Fig. 1a). The reason that bromide is not inhibited to a greater degree by DOC is believed to be a result of the large concentration of exchange sites on the MIEX resin (0.52 meq/mL; Boyer and Singer, 2008). Bicarbonate was also found to have an adverse impact on bromide removal. Fig. 3a and b show bromide removal for DOC-free waters at two different alkalinities and two different initial bromide concentrations of approximately 100 and 300 mg/L, respectively. At a MIEX resin dose of 1 mL/L, 46 mg/L or 0.00058 meq/L of bromide was removed from the 100 mg/L bromide water with an alkalinity of 24 mg/L as CaCO3 compared to 29 mg/L or 0.00036 meq/L of bromide removed when the alkalinity was 120 mg/L CaCO3 (Fig. 3a). For the range of resin doses used, more bromide was consistently removed for waters with an alkalinity of 24 mg/L as CaCO3 compared to waters with an alkalinity of 120 mg/L as CaCO3. In waters containing DOC (not shown), bicarbonate showed the same inhibitory effect on bromide exchange. The difference between bromide removed for the high alkalinity waters and low alkalinity waters appears to be around 20 mg/L at both
a 80 Bromide removed (µg/L)
and 20 mg/L in Fig. 1a and b, respectively, corresponding to 7–10% of the initial bromide concentration. The concentration of bromide removed was greater in the waters with a higher bromide concentration, as expected, because higher bromide concentrations can more effectively compete for exchange sites. The same trends shown in Fig. 1 were also seen for waters with a higher alkalinity (not shown); less bromide was removed as the DOC concentration increased. The influence of DOC on bromide removal is expected due to the MIEX resin’s reported high affinity for NOM and low affinity for bromide (Boyer and Singer, 2008), and the low bromide concentrations in these experiments. Because NOM has been shown to be removed predominantly through ion exchange rather than adsorption onto the MIEX resin (Fu and Symons, 1990; Boyer and Singer, 2008), DOC is expected to compete with bromide for exchange sites on the resin, and bromide removal will be lowered, as seen in Fig. 1. Fig. 2 shows the corresponding removal of SR NOM using the MIEX resin in waters with an alkalinity of 24 mg/L as CaCO3 and an initial bromide concentration of about 100 mg/L (as in Fig. 1a). Around 50% of the DOC was removed from waters with an initial DOC concentration of 3.5 and 9.4 mg/L at resin doses of 1 and 2 mL/L, respectively. These removals are representative of those seen in other waters with various bromide and DIC concentrations (Singer et al., 2007b). Analysis of the bromide and DOC concentrations on an equivalent basis provided insight into the competition between the two species. A charge density of 11.8 meq/g C (Boyer and Singer, 2008) for the DOC was used to calculate the equivalent concentration of exchangeable DOC. Accordingly, for the waters in Fig. 2, DOC concentrations of 3.5 and 9.4 mg/L correspond to 0.041 and 0.11 meq/L, respectively. In comparison, bromide concentrations of 100 and 300 mg/L correspond to 0.0013 and 0.0038 meq/L, respectively, which are more than one order of magnitude lower than the DOC concentrations. Consequently, the combination of high DOC concentrations relative to bromide and the MIEX resin’s strong affinity for DOC allows DOC to be removed more effectively over bromide and to thereby inhibit bromide removal. This is seen when comparing the amount of DOC and bromide removed on an equivalent basis. At a resin dose of 1 mL/L, 1.8 mg/L DOC or
60
40
20 24 mg/L CaCO3 120 mg/L CaCO3 0 0
1
2
3
4
5
6
7
MIEX resin dose (mL/L)
b
9
200
Bromide removed (µg/L)
DOC removed (mg/L)
8 7 6 5 4 3 2
120
80
40
3.5 mg/L DOC 9.4 mg/L DOC
1
160
24 mg/L CaCO3 120 mg/L CaCO3
0
0 0
1
2
3 4 5 MIEX resin dose (mL/L)
6
7
Fig. 2 – Suwanee River DOC removal in model waters with an alkalinity of 24 mg/L as CaCO3 and Br0 z 100 mg/L.
0
1
2
3
4
5
6
MIEX resin dose (mL/L)
Fig. 3 – Effect of alkalinity on bromide removal in water with no DOC. (a) Br0 z 100 mg/L, (b) Br0 z 300 mg/L.
7
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3.2.
Bromide removed (µg/L)
a
80
60
40
20 Single-Stage Two-Stage 0 0
2
4
6
8
Total MIEX resin dose (mL/L)
b
200 b)
Bromide removed (µg/L)
bromide concentrations (Fig. 3a and b), which is of the same order as those seen for the high DOC concentrations shown in Fig. 1. These results are consistent with previous studies showing bromide removal to be adversely affected by alkalinity (Singer and Bilyk, 2002; Johnson and Singer, 2004; Boyer and Singer, 2005). Despite MIEX and other strong-base anion exchange resins having a low affinity for bicarbonate, DIC species compete with bromide for exchange sites because they are present at relatively high concentrations compared to bromide. Considering that DIC exists primarily as bicarbonate at pH 8, alkalinities of 24 and 120 mg/L as CaCO3 correspond to 0.48 and 2.4 meq/L, respectively. These concentrations are orders of magnitude higher than the corresponding bromide and DOC concentrations in our experiments and can therefore drive bicarbonate removal. Measurements of residual DIC concentrations for waters with an alkalinity of 120 mg/L as CaCO3 and 3.7 mg/L DOC showed that 3.3 mg/L or 0.27 meq/L of DIC was removed at a MIEX resin dose of 1 mL/L. On an equivalent basis, the quantity of DIC removed is one order of magnitude greater than the amount of DOC removed (0.021 meq/L) for this same water. Therefore, DOC and bicarbonate were able to inhibit bromide removal to a similar degree despite the resin’s lower affinity for bicarbonate than for DOC.
Bromide removal in two-stage jar tests
160
120
80
40
Single-Stage Two-Stage
0 0
2
4
6
8
Total MIEX resin dose (mL/L)
Fig. 4 – Comparison of bromide removal using single-stage and two-stage jar tests in model waters with an alkalinity of 120 mg/L as CaCO3 and a Suwanee River DOC concentration of approximately 4 mg/L. (a) Br0 z 100 mg/L, (b) Br0 z 300 mg/L.
300
Bromide removed ( µg/L)
Because of the reduced bromide removal observed in the presence of DOC and DIC, two-stage treatment was expected to improve bromide removal. During the first stage of treatment, DOC, which competes with bromide for exchange sites, would be removed to an appreciable degree and increased bromide removal would be expected in the second stage of treatment. A MIEX resin dose of 1 mL/L was used for the first stage treatment. Fig. 4a and b show bromide removal for waters treated with single-stage (solid line) and two-stage batch MIEX processes with an initial DOC concentration of about 4 mg/L and initial bromide concentrations of approximately 100 and 300 mg/L, respectively. For the two-stage treatments, the total resin doses shown are the sum of the resin doses used in the first and second stages. At both bromide concentrations, bromide removal improved using the two-stage process. The improvement is not seen, however, until the two-stage resin dose reaches 3 mL/L and does not appear to be significant, especially for the water with the higher bromide concentration (Fig. 4b). A MIEX resin dose of 3 mL/L used in two-stage treatment removed about 10 mg/L more bromide compared to the single-stage treatment (solid line) in the water containing 300 mg/L Br. An important factor limiting the effectiveness of the twostage process to remove bromide is believed to be the presence of chloride, which tends to be found in natural waters at a concentration 333 times that of bromide (Amy et al., 1994). The model waters used in our experiments, with bromide concentrations of 100 and 300 mg/L, also contained approximately 33 and 100 mg/L of chloride, or 0.93 and 2.8 meq/L, respectively. Fig. 5 shows bromide removal in waters containing only Suwanee River NOM and bromide. Without chloride and DIC, bromide was readily removed despite the
250 200 150 100 50
87 µg/L Br 273 µg/L Br
0 0
1
2
3
4
5
MIEX dose (mL/L)
Fig. 5 – Bromide removal in model waters containing approximately 4 mg/L Suwanee River DOC in the absence of chloride and bicarbonate.
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3.3.
Comparison of MIEX resin with polystyrene resins
Parallel jar tests were performed using MIEX resin (a polyacrylic structure) and two macroporous polystyrene resins to compare bromide and NOM removal using polystyrene and polyacrylic resins. For this portion of the study, we used fulvic acid that was extracted from Lake Drummond, VA (Chang, 1990) because our SR concentrate was exhausted. Waters containing Lake Drummond Fulvic Acid at 10 mg/L DOC, an alkalinity of 120 mg/L as CaCO3, and a bromide concentration of 300 mg/L were treated with MIEX, Ionac A-641 and Amberlite IRA910 (see Table 1). In order to observe mass transfer limitations for the larger polystyrene resins, samples were taken after contact times of 30 min and 5 h. Fig. 7a and b show bromide removal after contact times of 30 min and 5 h, respectively. Resin doses were normalized for the different exchange capacities of the three resins. The three resins showed comparable bromide removal after 30 min. At a resin dose of 2.6 meq/L (6 mL/L) of MIEX resin, 176 mg/L of Br was removed after 30 min, compared to 186 mg/L and 174 mg/L removed by 2 meq/L (2 mL/L) of A-641 and
Bromide removed ( g/L)
100
80
60
40
20
19 mg/L Cl 31 mg/L Cl
0 0
1
2
3 4 5 MIEX resin dose (mL/L)
6
7
Fig. 6 – Impact of chloride on bromide removal in DOC-free model waters with an alkalinity of 24 mg/L as CaCO3.
a
280
Bromide removed (µg/L)
240 200 160 120 80 A641 IRA 910 MIEX
40 0 0
1
2
3
4
5
6
7
Resin dose (meq/L)
b
280 240
Bromide removed (µg/L)
presence of 3 mg/L DOC. This suggests that chloride is a major inhibitor of bromide removal. Fig. 6 shows that decreasing chloride concentrations increased the amount of bromide removed. At each MIEX resin dose, bromide removal appears to increase by approximately 10 mg/L as chloride concentration decreases from 31 to 19 mg/L or 0.87 to 0.54 meq/L. This is similar to the difference in bromide removal observed between waters with alkalinities of 24 and 120 mg/L CaCO3 (Fig. 3a). Chloride concentration is clearly an important factor in determining the extent of bromide removal. In the two-stage process, chloride concentration would increase in the first stage as bromide, DOC and bicarbonate are exchanged, adding to the chloride concentration already present in the water (333 times the initial bromide concentration). Hence, the amount of bromide that can be removed in the second stage would be limited to a significant degree, even when effective DOC removal is seen in the first stage.
200 160 120 80 A641 IRA 910 MIEX
40 0 0
1
2
3 4 Resin dose (meq/L)
5
6
7
Fig. 7 – Bromide removal by MIEX, IRA910 and A-641 resins in model waters containing approximately 10 mg/L Lake Drummond fulvic acid as C and 300 mg/L bromide, with an alkalinity of 120 mg/L as CaCO3. (a) 30 min contact time, (b) 5 h contact time.
2.2 meq/L (2 mL/L) of IRA910, respectively. This corresponds to 64%, 62% and 62% removal for the A-641, IRA910, and MIEX resins, respectively. After a contact time of 5 h, bromide removal by IRA910 and A-641 improved, while the MIEX resin exhibited reduced bromide removal. 2 meq/L of A-641 and 2.2 meq/L of IRA910 removed 222 mg/L and 203 mg/L of bromide or 76% and 73% of the initial bromide concentrations, respectively. The increased amount of bromide removed by these polystyrene resins suggests that additional bromide diffused into the inner pores of the polystyrene resin over time and was removed at less accessible exchange sites. In contrast, the amount of bromide removed by 2.6 meq/L of the MIEX resin was only about 174 mg/L or 62% of the initial bromide concentration. This suggests that DOC and bicarbonate, which are preferentially removed relative to bromide, may have displaced some of the bromide initially removed by the resin. Also, the additional chloride released from the resin may have competed with bromide for exchange sites. These findings suggest that the two polystyrene resins have higher selectivities for bromide compared to the polyacrylic MIEX resin, possibly because of size exclusion considerations
water research 44 (2010) 2133–2140
associated with the polystyrene resins which limit DOC competition with bromide (Boyer and Singer, 2008). DOC removal after 30 min and 5 h is shown in Fig. 8a and b, respectively. The polyacrylic MIEX resin is shown to be much more effective at removing DOC than the polystyrene A-641 and IRA910 resins. This is consistent with other studies which showed that macroporous, polyacrylic resins tend to be more effective at removing NOM than macroporous, polystyrene resins (Fu and Symons, 1990; Bolto et al., 2002). Also, the smaller MIEX resin beads allow for faster DOC exchange kinetics. After 30 min, 0.44 meq/L of MIEX resin removed 4.4 mg/L DOC compared to the 1.3 and 0.7 mg/L of DOC removed by 1 meq/L of A-641 and 1.1 meq/L IRA910, respectively. The doses for each of the three resins correspond to 1 mL/L. These removals correspond to 46%, 13% and 7% for the MIEX, A-641 and IRA910 resins, respectively. After 5 h, an increase in DOC removal was seen for each resin, with the largest increase occurring for the A-641 resin at high resin doses. However, DOC removal by the polyacrylic MIEX resin was still greater than that achieved by the two other resins. This increase in DOC removal over time suggests that the rate of DOC removal was limited by diffusion into the resin matrix,
a
10 9
DOC removed (mg/L)
8 7 6 5
especially for the polystyrene resins that have lower surface area-to-volume ratios.
4.
Conclusions
In model waters containing NOM, bicarbonate, bromide and chloride, the MIEX resin was shown to remove DOC effectively and bromide to a lesser extent. DOC, bicarbonate and chloride were shown to compete with bromide and lower the extent of bromide removal. Two-stage MIEX treatment showed only a slight improvement in bromide removal over single-stage treatment. The high chloride concentration of waters containing bromide limits bromide removal, even after the first stage of treatment in which a significant portion of the DOC has been removed. The two polystyrene resins, A-641 and IRA910, were seen to be more effective for bromide removal, while the polyacrylic MIEX resin was more effective at removing DOC. Increasing contact time allowed more DOC and bromide to be removed by the polystyrene resins, suggesting that ion exchange was limited by solute diffusion into the resin pores. While IRA910 and A-641 were effective at removing bromide, they would not be effective for DBP control due to the long contact time required to remove NOM and the lesser degree of NOM removal compared to the polyacrylic MIEX resin. From the findings of this study, the MIEX resin provides a potentially effective technology for controlling the formation of chlorinated and brominated DBPs because it can remove appreciable amounts of DOC and can lower bromide concentrations, but its effectiveness for the latter will be limited by the presence of other anions in the water.
4 3 A641 IRA 910 MIEX
2 1
Acknowledgements
0 0
b
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3 4 5 Resin dose (meq/L)
6
7
The authors thank Treavor Boyer for his assistance in the laboratory and Orica Watercare for funding this research.
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references 9
DOC removed (mg/L)
8 7 6 5 4 3 A641 IRA 910 MIEX
2 1 0 0
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Fig. 8 – DOC removal by MIEX, IRA910 and A-641 resins in model waters containing approximately 10 mg/L Lake Drummond fulvic acid as C and 300 mg/L bromide, with an alkalinity of 120 mg/L as CaCO3. (a) 30 min contact time, (b) 5 h contact time.
Allpike, B.P., Heitz, A., Joll, C.A., Kagi, R.I., Abbt-Braun, G., Frimmel, F.H., Brinkmann, T., Her, N., Amy, G., 2005. Size exclusion chromatography to characterize DOC removal in drinking water treatment. Environmental Science & Technology 39 (7), 2334–2342. American Public Health Association, American Water Works Association, and Water Environment Federation (APHA), 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, Washington, DC. Amy, G.L., Siddiqui, M., Zhai, W., Debroux, J., Odem, W., 1994. Survey of bromide in drinking waters and impacts on DBP formation. American Water AWWA Research Foundation Report, Denver. Attias, L., Contu, A., Loizzo, A., Massiglia, M., Valente, P., Zapponi, G.A., 1995. Trihalomethanes in drinking water and cancer risk: risk assessment an integrated evaluation of available data, in animals and humans. The Science of the Total Environment 171 (1), 61–68.
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Bourke, M., Harrison, S., Long, B., Lebeau, T., 2001. MIEX resin pretreatment followed by microfiltration as an alternative to nanofiltration for DBP precursor removal. In: AWWA Membrane Technology Conference Proceedings. Bolto, B., Dixon, D., Eldridge, R., King, S., Linge, K., 2002. Removal of natural organic matter by ion exchange. Water Research 36 (20), 5057–5065. Boyer, T.H., Singer, P.C., 2005. Bench-scale testing of a magnetic ion exchange resin for removal of disinfection by-product precursors. Water Research 39 (7), 1265–1276. Boyer, T.H., Singer, P.C., 2006. A pilot-scale evaluation of magnetic ion exchange treatment for removal of natural organic material and inorganic anions. Water Research 40 (15), 2865–2876. Boyer, T.H., Singer, P.C., 2008. Stoichiometry of removal of natural organic matter by ion exchange. Environmental Science & Technology 42 (2), 608–613. Chang, S.D., 1990. Factors Affecting Ozone-induced Destablization of Particulate Materials in Model Aquatic Systems and in Natural Waters. Doctoral thesis, University of North Carolina at Chapel Hill. Drikas, M., Chow, C.W.K., Cook, D., 2003. The impact of recalcitrant organic character on disinfection stability, trihalomethane formation and bacterial regrowth: an evaluation of magnetic ion exchange resin (MIEX) and alum coagulation. Journal of Water Supply: Research & Technology 52 (7), 475–487. Fearing, D.A., Banks, J., Guyetand, S., Eroles, C.M., Jefferson, B., Wilson, D., Hillis, P., Campbell, A.T., Parsons, S.A., 2004. Combination of ferric and MIEX for the treatment of a humic rich water. Water Research 38 (10), 2551–2558. Fu, P.L.K., Symons, J.M., 1990. Removing aquatic organic substances by anion exchange resins. Journal of AWWA 82 (10), 70–77. Humbert, H., Gallard, H., Suty, H., Croue, J.P., 2005. Performance of selected anion exchange resins for the treatment of a high DOC content surface water. Water Research 39 (9), 1699–1708.
Johnson, C.J., Singer, P.C., 2004. Impact of a magnetic ion exchange resin on ozone demand and bromate formation during drinking water treatment. Water Research 38 (17), 3738–3750. Morran, J.Y., Drikas, M., Cook, D., Bursill, D.B., 2004. Comparison of MIEX treatment and coagulation on NOM character. Water Science & Technology: Water Supply 4 (4), 129–137. Oliver, B.G., 1983. Dihaloacetonitriles in drinking water: algae and fulvic acid as precursors. Environmental Science & Technology 17 (2), 80–83. Plewa, M.J., Kargalioglu, Y., Vankerk, D., Minear, R.A., Wagner, E.D., 2002. Mammalian cell cytotoxicity and genotoxicity analysis of drinking water disinfection byproducts. Environmental and Molecular Mutagenesis 40 (2), 134–142. Singer, P.C., Bilyk, K., 2002. Enhanced coagulation using a magnetic ion exchange resin. Water Research 36 (16), 4009–4022. Singer, P.C., Schneider, M., Edwards-Brandt, J., Budd, G.C., 2007a. Magnetic ion exchange for the removal of disinfection by-product precursors: pilot plant findings. Journal of AWWA 99 (4), 128–139. Singer, P.C., Boyer, T.H., Holmquist, A., Morran, J., Bourke, M., 2007b. Integrated analysis of NOM removal by magnetic ion exchange. In: AWWA WQTC 2007 Proceedings. Symons, J.M., Krasner, S.W., Simms, L.A., Sclimenti, M.J., 1993. Measurement of THM and precursor concentrations revisited: the effect of bromide ion. Journal of AWWA 85 (1), 51–62. U.S. Environmental Protection Agency, 1998. National primary drinking water regulations: disinfectants and disinfection byproducts. Federal Register 63 (241), 69289. United States Environmental Agency (USEPA), 1999. EPA Method 300.0, Revision 2.2: Methods for the Determination of Inorganic Anions by Ion Chromatography. US Environmental Protection Agency, Cincinnati, OH.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Removal of bromide and natural organic matter by anion exchange Susan Hsu a,b,1, Philip C. Singer a,* a
University of North Carolina at Chapel Hill, Department of Environmental Sciences and Engineering, Campus Box 7431, Chapel Hill, NC 27599-7431, USA b Los Angeles County Sanitation Districts, 1955 Workman Mill Road, Whittier, CA 90601, USA
article info
abstract
Article history:
Bromide removal by anion exchange was explored for various water qualities, process
Received 21 August 2009
configurations, and resin characteristics. Simulated natural waters containing different
Received in revised form
amounts of natural organic matter (NOM), bicarbonate, chloride, and bromide were treated
9 December 2009
with a polyacrylate-based magnetic ion exchange (MIEX) resin on a batch basis to evaluate
Accepted 14 December 2009
the effectiveness of the resin for removal of bromide. While bromide removal was achieved
Available online 22 December 2009
to some degree, alkalinity (bicarbonate), dissolved organic carbon (DOC), and chloride were shown to inhibit bromide removal in waters with bromide concentrations of 100 and
Keywords:
300 mg/L. Water was also treated using a two-stage batch MIEX process. Two-stage treat-
Ion exchange
ment resulted in only a slight improvement in bromide removal compared to single-stage
Bromide
treatment, presumably due to competition with the high concentration of chloride which is
Disinfection by-products
present along with bromide in natural waters. In view of the relatively poor bromide
Drinking water
removal results for the MIEX resin, a limited set of experiments was performed using
Natural organic matter
polystyrene resins. DOC and bromide removal were compared by treating model waters
Magnetic ion exchange (MIEX)
with MIEX and two polystyrene resins, Ionac A-641 and Amberlite IRA910. The two polystyrene resins were seen to be more effective for bromide removal, while the MIEX resin was more effective at removing DOC. ª 2009 Elsevier Ltd. All rights reserved.
1.
Introduction
Bromide and dissolved organic carbon (DOC) are the two principal precursors in the formation of halogenated disinfection by-products (DBPs) resulting from chlorination of drinking water. Bromide is readily oxidized to hypobromous acid (HOBr) by free chlorine (HOCl). HOBr and HOCl are both strong oxidants which react with NOM to form halogenated DBPs, including trihalomethanes (THMs) and haloacetic acids (HAAs), which currently are regulated by the US Environmental Protection Agency (EPA) under the Stage 1 Disinfectants and Disinfection By-Products (D/DBP) Rule (US EPA, 1998). At high
Br/DOC and Br/Cl2 dose ratios, more brominated DBP species have been shown to form (Oliver, 1983; Symons et al., 1993). Many of these bromine-containing DBPs tend to be more harmful than their fully chlorinated counterparts (Attias et al., 1995; Plewa et al., 2002). A common strategy to control DBP formation is to remove the precursors prior to chlorination. However, these efforts, using processes such as enhanced coagulation, activated carbon adsorption, and nanofiltration, have been directed at the removal of DOC which tends to increase the Br/DOC ratio and the corresponding Br/Cl2 dose ratio, leading to preferential formation of brominated DBPs. Hence bromide removal,
* Corresponding author. Tel.: þ1 919 966 3865; fax: þ1 919 966 7911. E-mail addresses: [email protected] (S. Hsu), [email protected] (P.C. Singer). 1 Tel.: þ1 562 908 4288 x 2145; fax: þ1 562 699 4515. 0043-1354/$ – see front matter ª 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2009.12.027
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as well as DOC removal, is desired for effective DBP control to protect public health. Bromide can be removed by membrane processes, such as reverse osmosis, but this is an expensive technology. It can also be removed by anion exchange, which is the focus of this paper. A magnetic ion exchange (MIEX) resin, marketed by Orica Watercare of Victoria, Australia, was developed specifically for the removal of DOC from raw drinking waters. The MIEX resin is a strong-base resin with iron oxide integrated into a macroporous, polyacrylic matrix, and is typically used with chloride as the exchangeable ion. The iron oxide provides the resin with magnetic characteristics that facilitate aggregation and settling. The average MIEX resin particles are 180 mm in diameter, 2–5 times smaller than traditional resins. The increased surface area-to-volume ratio allows for faster ion exchange kinetics and decreased resin fouling due to shorter NOM diffusion paths within the resin (Bourke et al., 2001). Unlike larger, traditional ion exchange resins that tend to be used in packed beds downstream of solid-liquid separation processes, the MIEX resin is used either in a slurry contactor or in a fluidized bed at the head of the treatment train. The MIEX process presents a very promising method to reduce DBP formation when used upstream of chlorination. Several studies have shown very effective NOM removal using the MIEX resin (Singer and Bilyk, 2002; Drikas et al., 2003; Fearing et al., 2004; Morran et al., 2004; Boyer and Singer, 2005; Allpike et al., 2005). Reduction of THM and HAA formation potential from MIEX treatment has also been shown to be greater than that achieved through enhanced coagulation (Singer and Bilyk, 2002; Drikas et al., 2003; Morran et al., 2004; Boyer and Singer, 2006). The effectiveness of DOC removal is affected by NOM characteristics and the presence of other anions which compete with DOC for the anion exchange sites on the resin. In general, DOC is more effectively removed from waters with higher SUVA values (Singer and Bilyk, 2002; Boyer and Singer, 2005). Waters with high total dissolved solids exhibit poor removal of DOC by the MIEX resin (Singer et al., 2007a). The separation factors for Suwanee River Fulvic Acid (SRFA) and bicarbonate relative to chloride have been found to be 5.7 and 0.76, respectively (Boyer and Singer, 2008), for the MIEX resin, reflecting the MIEX resin’s strong preference for DOC, and it’s weak preference for bicarbonate. Boyer and Singer (2008) also showed that NOM was removed more effectively with polyacrylic resins compared to polystyrene resins. In addition to DOC removal, bromide has also been observed to be removed to a significant extent by the MIEX resin (Singer and Bilyk, 2002; Johnson and Singer, 2004; Humbert et al., 2005), but the effectiveness of bromide removal strongly depends on water quality. High alkalinities (bicarbonate concentrations) and, especially, high sulfate concentrations (>50 mg/L) have been shown to inhibit bromide removal (Singer and Bilyk, 2002; Johnson and Singer, 2004; Boyer and Singer, 2006; Singer et al., 2007a). However, if configured appropriately, the MIEX process may be able to remove both bromide and NOM and is therefore a promising technology for reducing not only overall DBP formation but also formation of the more hazardous brominated species. Accordingly, the objective of this study was to examine the effect of water quality, process configuration, and resin
characteristics on bromide and DOC removal by anion exchange. The effects of alkalinity (bicarbonate) and DOC on bromide removal were examined on a batch basis by treating model waters containing various alkalinities and DOC concentrations with the MIEX resin. The model waters were also treated using a two-stage batch MIEX process in order to determine the feasibility of engineering a two-stage configuration to improve bromide removal. The first stage was expected to remove competing anions such as bicarbonate and DOC, allowing for better removal of bromide in the second stage. A limited set of experiments was conducted with Ionac A-641 and Amberlite IRA910, two macroporous, polystyrene resins, to compare bromide and DOC removal by polystyrene and polyacrylic (MIEX) resins.
2.
Materials and methods
2.1.
Experimental methodology
Simulated natural waters were prepared with bromide concentrations of approximately 100 and 300 mg/L Br using sodium bromide (NaBr; Fisher Chemical, Fairlawn, NJ). These levels were chosen to reflect concentrations found in highbromide natural waters in the USA. Sodium chloride (NaCl; Fisher Chemical, Fairlawn, NJ) was added to give a Cl to Br ratio of 333:1 (by weight), a ratio commonly found in natural waters (Amy et al., 1994). At each bromide concentration, the waters were made with two different alkalinities and three different DOC concentrations. Sufficient sodium bicarbonate (NaHCO3; Fisher Chemical, Fairlawn, NJ) was added to give alkalinities of 24 and 120 mg/L as CaCO3 at pH 8. Suwanee River (SR) NOM, extracted by reverse osmosis, was obtained from the International Humic Substances Society (IHSS), and was used as the source of DOC for experiments using the MIEX resin. Waters were prepared to contain approximately 0, 4, and 10 mg/L DOC. (The actual concentrations measured are shown with each of the results.) Lake Drummond Fulvic Acid (LDFA), extracted by Chang (1990) from a lake in southeastern Virginia, was used at a concentration of 10 mg/L DOC for experiments comparing the effectiveness of MIEX, A-641 and IRA910 resins due to exhaustion of the Suwanee River concentrate. Both the Suwanee River and Lake Drummond are organic-rich bodies of water located in the southern US that have high concentrations of NOM that is dominated by hydrophobic organic carbon. SR NOM and LDFA had average measured SUVA values of 5.2 and 4.3 L/mg-m, respectively. Each model water was prepared with organic-free deionized water (OFDW) and was adjusted to pH 8 using either 0.1 M sodium hydroxide (NaOH) or 0.1 M hydrochloric acid (HCl) before being treated. All water samples were stored at 4 C and brought to room temperature (22 C) before testing. The MIEX resin used in this study was virgin (unused) resin provided by Orica Watercare (Denver, CO). The resin was rinsed with OFDW to wash away the fines and stored in OFDW before use. In the single-stage jar tests, 500 mL samples of each water were treated at MIEX resin doses of 1, 2, 4 and 6 mL/L using a Phipps and Bird six-place gang stirrer (Phipps and Bird, Richmond, VA). MIEX resin was measured out in 10 mL graduated cylinders using Pasteur pipettes and
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3.
Results and discussion
3.1.
Bromide removal in single-stage jar tests
The effect of DOC concentration on bromide removal in low alkalinity model water is shown in Fig. 1 for two different bromide concentrations. The error bars in Fig. 1b represent the range of the results from duplicate MIEX treatments and show good agreement between the duplicate runs. For the same MIEX resin doses, bromide removal is greater in the absence of DOC (represented by the solid curve in Fig. 1a and b), and tends to decrease with increasing DOC concentration. However, the impacts of DOC on bromide removal are relatively minor, presumably due to the large number of exchange sites on the MIEX resin (see below). The difference between bromide removed in the DOC-free and high DOC waters is around 10
a 80
60
40
0 mg/L DOC 3.5 mg/L DOC 9.4 mg/L DOC
20
Analytical methods
pH was measured using an Accumet AB15 pH meter from Fisher Scientific. The pH meter was calibrated before each use. UV254 was measured using a 1 cm quartz cell on a Hitachi U2000 spectrophotometer according to Standard Method 5390 (APHA, 1998). DOC and DIC concentrations were determined using a Shimadzu TOC-VCPH Total Organic Carbon Analyzer; DOC was analyzed according to Standard Method 5310B (APHA, 1998). 2 N HCl was used to acidify samples instead of phosphoric acid in accordance with Shimadzu instrument specifications. Bromide and chloride were measured using suppressed conductivity detection on a Dionex Ion
0 0
1
2
3
4
b
6
7
200
160
120
80 0 mg/L DOC 3.6 mg/L DOC 8.6 mg/L DOC
40
0
Table 1 – Resin characteristics.
Polymeric matrix Exchange capacity (eq/L) Effective size (mm)
5
MIEX resin dose (mL/L)
Bromide removed (µg/L)
2.2.
Chromatograph with an IonPac AG4A-SC guard column and an AS4A-SC analytical column according to U.S. Environmental Protection Agency Method 300.0 (USEPA, 1999). The detection limit associated with this method is 10 mg/L. Each sample was analyzed in duplicate and the relative difference was less than 15%.
Bromide removed (µg/L)
transferred to water samples, using OFDW to rinse all of the resin out of the graduated cylinder. Samples were mixed at 100 rpm for 30 min, then allowed to settle for 30 min. The first 10 mL of sample from the sampling port was discarded and a 250 mL sample was taken and filtered using a 0.45 mm Supor450 membrane filter (Pall, Ann Arbor, MI) to remove any remaining resin particles. The filters were pre-rinsed with 250 mL of OFDW before each use. Samples were analyzed for bromide, DOC and ultraviolet absorbance at 254 nm (UV254). For some samples, residual chloride and dissolved inorganic carbon (DIC) concentrations were also measured. Duplicate runs were performed for two waters containing 24 mg/L as CaCO3, and 300 mg/L Br to examine experimental repeatability. In the two-stage MIEX resin treatments, 4 L of water was first treated in two 2-L square jars with 1 mL/L of the MIEX resin. Samples were mixed at 100 rpm for 30 min and allowed to settle for 30 min. 1.5 L of the treated water from each jar was decanted and combined. A 250 mL sample was taken and filtered for bromide, DOC and UV254 analysis. The rest of the water was treated with 1, 2, 4 and 6 mL/L of MIEX resin. 500 mL samples were treated following the same procedure used for the single-stage jar tests, and filtered samples were analyzed for bromide, DOC and UV254. In view of the relatively poor bromide removal results for the MIEX resin, a limited set of experiments was performed using polystyrene resins. Ionac A-641 and Amberlite IRA910 were obtained from Sybron Chemicals Inc. and Rohm and Haas, respectively. Treatment using the two polystyrene resins was performed in the same manner as for the MIEX resin except that 100 mL samples were taken after contact times of 30 min and 5 h. The resins were prepared according to published procedures (Bolto et al., 2002; Humbert et al., 2005) and stored in OFDW before use. Characteristics of the three resins are given in Table 1. Each sample was filtered before being analyzed for bromide, DOC and UV254.
0
MIEX
A-641
IRA910
Polyacrylic 0.44 180
Polystyrene 1.0 450
Polystyrene 1.1 460
1
2
3
4
5
6
MIEX resin dose (mL/L)
Fig. 1 – Impact of Suwanee River DOC concentration on bromide removal in model waters with an alkalinity of 24 mg/L as CaCO3. (a) Br0 z 100 mg/L, (b) Br0 z 300 mg/L.
7
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0.021 meq/L was removed for water with an initial DOC concentration of 3.5 mg/L (Fig. 2). This corresponds to 51% DOC removed. For the water containing 3.5 mg/L DOC and approximately 100 mg/L Br, 1 mL/L of MIEX resin removed only 35 mg/L or 0.00044 meq/L Br, corresponding to a bromide removal of 35% (Fig. 1a). The reason that bromide is not inhibited to a greater degree by DOC is believed to be a result of the large concentration of exchange sites on the MIEX resin (0.52 meq/mL; Boyer and Singer, 2008). Bicarbonate was also found to have an adverse impact on bromide removal. Fig. 3a and b show bromide removal for DOC-free waters at two different alkalinities and two different initial bromide concentrations of approximately 100 and 300 mg/L, respectively. At a MIEX resin dose of 1 mL/L, 46 mg/L or 0.00058 meq/L of bromide was removed from the 100 mg/L bromide water with an alkalinity of 24 mg/L as CaCO3 compared to 29 mg/L or 0.00036 meq/L of bromide removed when the alkalinity was 120 mg/L CaCO3 (Fig. 3a). For the range of resin doses used, more bromide was consistently removed for waters with an alkalinity of 24 mg/L as CaCO3 compared to waters with an alkalinity of 120 mg/L as CaCO3. In waters containing DOC (not shown), bicarbonate showed the same inhibitory effect on bromide exchange. The difference between bromide removed for the high alkalinity waters and low alkalinity waters appears to be around 20 mg/L at both
a 80 Bromide removed (µg/L)
and 20 mg/L in Fig. 1a and b, respectively, corresponding to 7–10% of the initial bromide concentration. The concentration of bromide removed was greater in the waters with a higher bromide concentration, as expected, because higher bromide concentrations can more effectively compete for exchange sites. The same trends shown in Fig. 1 were also seen for waters with a higher alkalinity (not shown); less bromide was removed as the DOC concentration increased. The influence of DOC on bromide removal is expected due to the MIEX resin’s reported high affinity for NOM and low affinity for bromide (Boyer and Singer, 2008), and the low bromide concentrations in these experiments. Because NOM has been shown to be removed predominantly through ion exchange rather than adsorption onto the MIEX resin (Fu and Symons, 1990; Boyer and Singer, 2008), DOC is expected to compete with bromide for exchange sites on the resin, and bromide removal will be lowered, as seen in Fig. 1. Fig. 2 shows the corresponding removal of SR NOM using the MIEX resin in waters with an alkalinity of 24 mg/L as CaCO3 and an initial bromide concentration of about 100 mg/L (as in Fig. 1a). Around 50% of the DOC was removed from waters with an initial DOC concentration of 3.5 and 9.4 mg/L at resin doses of 1 and 2 mL/L, respectively. These removals are representative of those seen in other waters with various bromide and DIC concentrations (Singer et al., 2007b). Analysis of the bromide and DOC concentrations on an equivalent basis provided insight into the competition between the two species. A charge density of 11.8 meq/g C (Boyer and Singer, 2008) for the DOC was used to calculate the equivalent concentration of exchangeable DOC. Accordingly, for the waters in Fig. 2, DOC concentrations of 3.5 and 9.4 mg/L correspond to 0.041 and 0.11 meq/L, respectively. In comparison, bromide concentrations of 100 and 300 mg/L correspond to 0.0013 and 0.0038 meq/L, respectively, which are more than one order of magnitude lower than the DOC concentrations. Consequently, the combination of high DOC concentrations relative to bromide and the MIEX resin’s strong affinity for DOC allows DOC to be removed more effectively over bromide and to thereby inhibit bromide removal. This is seen when comparing the amount of DOC and bromide removed on an equivalent basis. At a resin dose of 1 mL/L, 1.8 mg/L DOC or
60
40
20 24 mg/L CaCO3 120 mg/L CaCO3 0 0
1
2
3
4
5
6
7
MIEX resin dose (mL/L)
b
9
200
Bromide removed (µg/L)
DOC removed (mg/L)
8 7 6 5 4 3 2
120
80
40
3.5 mg/L DOC 9.4 mg/L DOC
1
160
24 mg/L CaCO3 120 mg/L CaCO3
0
0 0
1
2
3 4 5 MIEX resin dose (mL/L)
6
7
Fig. 2 – Suwanee River DOC removal in model waters with an alkalinity of 24 mg/L as CaCO3 and Br0 z 100 mg/L.
0
1
2
3
4
5
6
MIEX resin dose (mL/L)
Fig. 3 – Effect of alkalinity on bromide removal in water with no DOC. (a) Br0 z 100 mg/L, (b) Br0 z 300 mg/L.
7
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3.2.
Bromide removed (µg/L)
a
80
60
40
20 Single-Stage Two-Stage 0 0
2
4
6
8
Total MIEX resin dose (mL/L)
b
200 b)
Bromide removed (µg/L)
bromide concentrations (Fig. 3a and b), which is of the same order as those seen for the high DOC concentrations shown in Fig. 1. These results are consistent with previous studies showing bromide removal to be adversely affected by alkalinity (Singer and Bilyk, 2002; Johnson and Singer, 2004; Boyer and Singer, 2005). Despite MIEX and other strong-base anion exchange resins having a low affinity for bicarbonate, DIC species compete with bromide for exchange sites because they are present at relatively high concentrations compared to bromide. Considering that DIC exists primarily as bicarbonate at pH 8, alkalinities of 24 and 120 mg/L as CaCO3 correspond to 0.48 and 2.4 meq/L, respectively. These concentrations are orders of magnitude higher than the corresponding bromide and DOC concentrations in our experiments and can therefore drive bicarbonate removal. Measurements of residual DIC concentrations for waters with an alkalinity of 120 mg/L as CaCO3 and 3.7 mg/L DOC showed that 3.3 mg/L or 0.27 meq/L of DIC was removed at a MIEX resin dose of 1 mL/L. On an equivalent basis, the quantity of DIC removed is one order of magnitude greater than the amount of DOC removed (0.021 meq/L) for this same water. Therefore, DOC and bicarbonate were able to inhibit bromide removal to a similar degree despite the resin’s lower affinity for bicarbonate than for DOC.
Bromide removal in two-stage jar tests
160
120
80
40
Single-Stage Two-Stage
0 0
2
4
6
8
Total MIEX resin dose (mL/L)
Fig. 4 – Comparison of bromide removal using single-stage and two-stage jar tests in model waters with an alkalinity of 120 mg/L as CaCO3 and a Suwanee River DOC concentration of approximately 4 mg/L. (a) Br0 z 100 mg/L, (b) Br0 z 300 mg/L.
300
Bromide removed ( µg/L)
Because of the reduced bromide removal observed in the presence of DOC and DIC, two-stage treatment was expected to improve bromide removal. During the first stage of treatment, DOC, which competes with bromide for exchange sites, would be removed to an appreciable degree and increased bromide removal would be expected in the second stage of treatment. A MIEX resin dose of 1 mL/L was used for the first stage treatment. Fig. 4a and b show bromide removal for waters treated with single-stage (solid line) and two-stage batch MIEX processes with an initial DOC concentration of about 4 mg/L and initial bromide concentrations of approximately 100 and 300 mg/L, respectively. For the two-stage treatments, the total resin doses shown are the sum of the resin doses used in the first and second stages. At both bromide concentrations, bromide removal improved using the two-stage process. The improvement is not seen, however, until the two-stage resin dose reaches 3 mL/L and does not appear to be significant, especially for the water with the higher bromide concentration (Fig. 4b). A MIEX resin dose of 3 mL/L used in two-stage treatment removed about 10 mg/L more bromide compared to the single-stage treatment (solid line) in the water containing 300 mg/L Br. An important factor limiting the effectiveness of the twostage process to remove bromide is believed to be the presence of chloride, which tends to be found in natural waters at a concentration 333 times that of bromide (Amy et al., 1994). The model waters used in our experiments, with bromide concentrations of 100 and 300 mg/L, also contained approximately 33 and 100 mg/L of chloride, or 0.93 and 2.8 meq/L, respectively. Fig. 5 shows bromide removal in waters containing only Suwanee River NOM and bromide. Without chloride and DIC, bromide was readily removed despite the
250 200 150 100 50
87 µg/L Br 273 µg/L Br
0 0
1
2
3
4
5
MIEX dose (mL/L)
Fig. 5 – Bromide removal in model waters containing approximately 4 mg/L Suwanee River DOC in the absence of chloride and bicarbonate.
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3.3.
Comparison of MIEX resin with polystyrene resins
Parallel jar tests were performed using MIEX resin (a polyacrylic structure) and two macroporous polystyrene resins to compare bromide and NOM removal using polystyrene and polyacrylic resins. For this portion of the study, we used fulvic acid that was extracted from Lake Drummond, VA (Chang, 1990) because our SR concentrate was exhausted. Waters containing Lake Drummond Fulvic Acid at 10 mg/L DOC, an alkalinity of 120 mg/L as CaCO3, and a bromide concentration of 300 mg/L were treated with MIEX, Ionac A-641 and Amberlite IRA910 (see Table 1). In order to observe mass transfer limitations for the larger polystyrene resins, samples were taken after contact times of 30 min and 5 h. Fig. 7a and b show bromide removal after contact times of 30 min and 5 h, respectively. Resin doses were normalized for the different exchange capacities of the three resins. The three resins showed comparable bromide removal after 30 min. At a resin dose of 2.6 meq/L (6 mL/L) of MIEX resin, 176 mg/L of Br was removed after 30 min, compared to 186 mg/L and 174 mg/L removed by 2 meq/L (2 mL/L) of A-641 and
Bromide removed ( g/L)
100
80
60
40
20
19 mg/L Cl 31 mg/L Cl
0 0
1
2
3 4 5 MIEX resin dose (mL/L)
6
7
Fig. 6 – Impact of chloride on bromide removal in DOC-free model waters with an alkalinity of 24 mg/L as CaCO3.
a
280
Bromide removed (µg/L)
240 200 160 120 80 A641 IRA 910 MIEX
40 0 0
1
2
3
4
5
6
7
Resin dose (meq/L)
b
280 240
Bromide removed (µg/L)
presence of 3 mg/L DOC. This suggests that chloride is a major inhibitor of bromide removal. Fig. 6 shows that decreasing chloride concentrations increased the amount of bromide removed. At each MIEX resin dose, bromide removal appears to increase by approximately 10 mg/L as chloride concentration decreases from 31 to 19 mg/L or 0.87 to 0.54 meq/L. This is similar to the difference in bromide removal observed between waters with alkalinities of 24 and 120 mg/L CaCO3 (Fig. 3a). Chloride concentration is clearly an important factor in determining the extent of bromide removal. In the two-stage process, chloride concentration would increase in the first stage as bromide, DOC and bicarbonate are exchanged, adding to the chloride concentration already present in the water (333 times the initial bromide concentration). Hence, the amount of bromide that can be removed in the second stage would be limited to a significant degree, even when effective DOC removal is seen in the first stage.
200 160 120 80 A641 IRA 910 MIEX
40 0 0
1
2
3 4 Resin dose (meq/L)
5
6
7
Fig. 7 – Bromide removal by MIEX, IRA910 and A-641 resins in model waters containing approximately 10 mg/L Lake Drummond fulvic acid as C and 300 mg/L bromide, with an alkalinity of 120 mg/L as CaCO3. (a) 30 min contact time, (b) 5 h contact time.
2.2 meq/L (2 mL/L) of IRA910, respectively. This corresponds to 64%, 62% and 62% removal for the A-641, IRA910, and MIEX resins, respectively. After a contact time of 5 h, bromide removal by IRA910 and A-641 improved, while the MIEX resin exhibited reduced bromide removal. 2 meq/L of A-641 and 2.2 meq/L of IRA910 removed 222 mg/L and 203 mg/L of bromide or 76% and 73% of the initial bromide concentrations, respectively. The increased amount of bromide removed by these polystyrene resins suggests that additional bromide diffused into the inner pores of the polystyrene resin over time and was removed at less accessible exchange sites. In contrast, the amount of bromide removed by 2.6 meq/L of the MIEX resin was only about 174 mg/L or 62% of the initial bromide concentration. This suggests that DOC and bicarbonate, which are preferentially removed relative to bromide, may have displaced some of the bromide initially removed by the resin. Also, the additional chloride released from the resin may have competed with bromide for exchange sites. These findings suggest that the two polystyrene resins have higher selectivities for bromide compared to the polyacrylic MIEX resin, possibly because of size exclusion considerations
water research 44 (2010) 2133–2140
associated with the polystyrene resins which limit DOC competition with bromide (Boyer and Singer, 2008). DOC removal after 30 min and 5 h is shown in Fig. 8a and b, respectively. The polyacrylic MIEX resin is shown to be much more effective at removing DOC than the polystyrene A-641 and IRA910 resins. This is consistent with other studies which showed that macroporous, polyacrylic resins tend to be more effective at removing NOM than macroporous, polystyrene resins (Fu and Symons, 1990; Bolto et al., 2002). Also, the smaller MIEX resin beads allow for faster DOC exchange kinetics. After 30 min, 0.44 meq/L of MIEX resin removed 4.4 mg/L DOC compared to the 1.3 and 0.7 mg/L of DOC removed by 1 meq/L of A-641 and 1.1 meq/L IRA910, respectively. The doses for each of the three resins correspond to 1 mL/L. These removals correspond to 46%, 13% and 7% for the MIEX, A-641 and IRA910 resins, respectively. After 5 h, an increase in DOC removal was seen for each resin, with the largest increase occurring for the A-641 resin at high resin doses. However, DOC removal by the polyacrylic MIEX resin was still greater than that achieved by the two other resins. This increase in DOC removal over time suggests that the rate of DOC removal was limited by diffusion into the resin matrix,
a
10 9
DOC removed (mg/L)
8 7 6 5
especially for the polystyrene resins that have lower surface area-to-volume ratios.
4.
Conclusions
In model waters containing NOM, bicarbonate, bromide and chloride, the MIEX resin was shown to remove DOC effectively and bromide to a lesser extent. DOC, bicarbonate and chloride were shown to compete with bromide and lower the extent of bromide removal. Two-stage MIEX treatment showed only a slight improvement in bromide removal over single-stage treatment. The high chloride concentration of waters containing bromide limits bromide removal, even after the first stage of treatment in which a significant portion of the DOC has been removed. The two polystyrene resins, A-641 and IRA910, were seen to be more effective for bromide removal, while the polyacrylic MIEX resin was more effective at removing DOC. Increasing contact time allowed more DOC and bromide to be removed by the polystyrene resins, suggesting that ion exchange was limited by solute diffusion into the resin pores. While IRA910 and A-641 were effective at removing bromide, they would not be effective for DBP control due to the long contact time required to remove NOM and the lesser degree of NOM removal compared to the polyacrylic MIEX resin. From the findings of this study, the MIEX resin provides a potentially effective technology for controlling the formation of chlorinated and brominated DBPs because it can remove appreciable amounts of DOC and can lower bromide concentrations, but its effectiveness for the latter will be limited by the presence of other anions in the water.
4 3 A641 IRA 910 MIEX
2 1
Acknowledgements
0 0
b
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2
3 4 5 Resin dose (meq/L)
6
7
The authors thank Treavor Boyer for his assistance in the laboratory and Orica Watercare for funding this research.
10
references 9
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2 1 0 0
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Resin dose (meq/L)
Fig. 8 – DOC removal by MIEX, IRA910 and A-641 resins in model waters containing approximately 10 mg/L Lake Drummond fulvic acid as C and 300 mg/L bromide, with an alkalinity of 120 mg/L as CaCO3. (a) 30 min contact time, (b) 5 h contact time.
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