Impact of Simulated Solar Irradiation on Disinfection Byproduct

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Environ. Sci. Technol. 2008, 42, 5586–5593

Impact of Simulated Solar Irradiation on Disinfection Byproduct Precursors A L E X T . C H O W , † D I N A M . L E E C H , * ,‡ TREAVOR H. BOYER,‡ AND PHILIP C. SINGER‡ College of Environmental Sciences and Engineering, South China University of Technology Guangzhou, 510641, P.R. China, Department of Environmental Sciences and Engineering, University of North Carolina-Chapel Hill, Chapel Hill, North Carolina, North Carolina 27599

Received February 14, 2008. Revised manuscript received April 25, 2008. Accepted May 5, 2008.

The Sacramento-San Joaquin Delta is the major drinking water source for 23 million California residents. Consequently, many studies have examined disinfection byproduct (DBP) formation in relation to Delta dissolved organic carbon (DOC) concentration. However, DOC characteristics within the Delta are not the same as those entering downstream water treatment facilities. As water is transferred to Southern California through the California Aqueduct, a 714.5 km-open channel, it is exposed to sunlight, potentially altering DBP precursors. We collected water from three sites within the Delta and one near the California Aqueduct, representing different DOC sources, and irradiated them in a solar simulator at a dose equivalent to that received during four days conveyance in the aqueduct. Photolytic changes in DOC were assessed by measuring CO2 and organic acid production, fluorescence, and ultraviolet absorbance over time. Trihalomethane (THM) and haloacetic acid (HAA) formation potentials, as well as the distribution of hydrophobic, transphilic, and hydrophilic acid fractions were determined at exposures equivalent to one and four days. Solar irradiation significantly decreased ultraviolet absorbance and fluorescence intensity, produced organic acids, and increased the hydrophilic fraction of waters. These changes in DOC caused a shift in bromine incorporation among the THM and HAA species. Our results are the first to demonstrate the importance of sunlight in altering DOC with respect to DBP formation.

1. Introduction The Sacramento-San Joaquin Delta (hereafter referred to as the Delta) is the major drinking water source for 23 million people in California. Water quality in the region is of great concern because of the significant increases in organic carbon concentration as water passes through the Delta and into downstream water treatment plants (1–3). During chlorination, dissolved organic carbon (DOC) acts as a major precursor in the formation of carcinogenic disinfection byproducts (DBPs), such as trihalomethanes (THMs) and haloacetic acids (HAAs) (4). Many studies have evaluated * Corresponding author phone: 252-726-6841; e-mail: dmleech@ email.unc.edu. † South China University of Technology Guangzhou. ‡ University of North Carolina-Chapel Hill. 5586

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the reactivity of DOC in forming DBPs by collecting water samples from upstream rivers feeding the Delta and from within the central Delta (2, 5–8). However, the majority of Delta waters used for municipal water supply purposes are delivered 714.5 km downstream to southern California through the concrete open channel California Aqueduct. Given the potential for photolytic alterations to DOC during conveyance, DBP precursors found in upland Delta sources could be quite different from those at downstream water treatment plant intakes. The Delta is a complex ecosystem, with a variety of carbon sources contributing DOC (2, 3). A mass balance calculation indicates that the two major carbon sources are tributaryborne loads and in situ phytoplankton production. These two sources contribute an annual average of 270 and 47 Mg per day of total organic carbon, equivalent to 69 and 12% of the annual average organic carbon load, respectively (3). Peat soil is also considered a major organic carbon contributor to Delta waterways, through agricultural drainage returns and wetland outflows (5, 8). Drainage from about 1011 km2 of peat soils was estimated to contribute an average of 36 Mg of total organic carbon per day (3). Large areas of Delta peat soil are being considered for conversion to wetland habitats, which could substantially increase organic carbon releases (9). Urban runoff within the Delta contributes less than 1% of the organic carbon in Delta waters, and the effect of this carbon source on DBP formation is relatively minimal (2, 3). Studies have demonstrated that organic carbon in river water, wetland outflow, and agricultural drainage are reactive precursors for DBP formation during drinking water treatment (2, 5, 8, 9). However, before entering water treatment facilities, organic materials in the water are exposed to solar radiation for an average of 3-4 days in the aqueduct. Many studies have shown that humic substances and DOC can be broken down into smaller, more labile organic carbon moieties by solar irradiation (10, 11). Despite this, solar effects on the reactivity of DOC with respect to DBP formation have not been studied. Our objectives were to evaluate the effects of sunlight on the properties and reactivity of DOC from different Delta sources, particularly with respect to their impact on DBP formation.

2. Materials and Methods 2.1. Sample Collection and Preparation. Grab samples were collected in five 2 L polyethylene bottles on January 4-5, 2006 at (1) the Sacramento River at Freeport; (2) a drainage ditch on Twitchell Island; (3) the outflow of a United States Geological Survey experimental wetland on Twitchell Island; and (4) the California Aqueduct at Banks Pumping Station (Figure 1). Water collected at Freeport represented upstream inputs to the Delta, drainage water represented typical agricultural runoff, the outflow from the experimental wetland represented waters from rehabilitated wetlands, and water collected at the Banks Pumping Station represented outflow from the Delta to the Aqueduct. All water samples were filtered through 0.22 µm polycarbonate filters (Millipore) upon return to the laboratory. Filtered samples were stored in sterilized polyethylene bottles at 4 °C. Within two weeks of collection, samples were transported to the University of North Carolina at Chapel Hill for the solar exposure experiments and DBP analysis. To ensure sterility, all samples were again sterile-filtered before the exposure to simulated sunlight, and 500 mL aliquots were transferred into sterile 1 L quartz tubes (5 cm diameter, 33 cm length). Filtrates were analyzed for DOC, ultraviolet absorbance at 254 nm 10.1021/es800206h CCC: $40.75

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FIGURE 1. Sampling locations in the Sacramento and San Joaquin Delta. River water was collected at the Sacramento River at Freeport (Site 1). Agricultural drainage and wetland outflow were collected, respectively, in a drainage ditch (Site 2) and a rehabilitated wetland (Site 3) in the Twitchell Island. Water representative of the California Aqueduct was collected in the aqueduct next to the Harvey O. Banks Pumping Plant. UVA254, and other general water quality parameters, including pH, bromide, ammonia, electrical conductivity (EC), following standard analytical procedures (12). 2.2. Solar Simulator Exposure. Two quartz tubes with 500 mL samples were placed horizontally in an Atlas Suntest XLS+ solar simulator (Atlas Material Testing Technology, Chicago, IL, www.atlas-mts.com/shop/product?ID)8) at a distance of approximately 23 cm from the filter covering the xenon arc lamp. The lamp was set at a constant intensity of 650 W m-2, and each water sample was exposed for 6 and 24 h independently. During the course of exposure, gas and water samples were withdrawn at regular time intervals to examine CO2 and organic acid production, as well as to characterize the DOC by UV absorbance at 254 nm and fluorescence measurements. An exposure of 6 h is equivalent to one full, clear, summer day at 36°N, with a total dose of 14,031 kJ m-2; an exposure of 24 h is equivalent to 4 days of sunlight exposure. A third tube was wrapped in aluminum foil and placed in a dark incubator to serve as a control. All tubes were kept at 27 °C throughout the duration of the experiment. After 6 and 24 h of exposure, water samples

were analyzed for DBP formation potential and the residual organic carbon was subjected to chemical fractionation. 2.3. CO2 Production. CO2 samples were taken aseptically through the septum of a small port in the quartz tubes with 1 mL syringes and measured within 1 h of sampling on a Shimadzu gas chromatograph equipped with a Supelco 80/ 100 porapak Q column (6 ft × 1/8 in), a methanizer (475 °C), and a flame ionization detector. The oven temperature was set at 35 °C with a runtime of 7 min. 2.4. Organic Acid Production and Fluorescence. Three mL of water was also aseptically removed from each tube at regular intervals. One mL was used to measure the production of selected low molecular weight organic acids on a Waters high pressure liquid chromatograph fitted with a UV absorbance detector set at 250 nm and a Biorad organic acid column (BioRad Corporation). Operating conditions included a 25 mM H2SO4 mobile phase, 20 µL sample injection, 0.6 mL min-1 flow rate, and 70 min runtime. Two mL was placed in a 1 cm (3) quartz cuvette and used to measure fluorescence on a Hitachi F-4500 fluorescence spectrophotometer (exVOL. 42, NO. 15, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Characteristics of Raw Watersa

a

parameters

river

drainage

wetland

aqueduct

pH electrical conductivity (µs) DOC (mg L-1) ultraviolet absorbance at 254 nm (cm-1) SUVA (L mg-1 m-1) Br- (mg L-1) total nitrogen (mg L-1) NH3-N (mg L-1) hydrophobic acid (%)

7.74 550 2.3 0.053 2.32 1 mg L-1), and low salt content (∼500 µs). In contrast, water collected from the drainage ditch at Twitchell Island, which is mainly composed of soil leachates from the cultivated peat lands, contained 60 mg L-1 DOC and 16 mg L-1 TN, and had a conductivity of 1730 µs, which is typical of drainage water in the area (1, 8). Water at the wetlands outflow contained 6.5 mg L-1 DOC, 0.3 mg L-1 TN, and 940 µs EC. The rehabilitated wetlands are perennially flooded and planted with different types of vegetation (9). Water samples collected at the aqueduct had a DOC of 3.0 mg L-1, a TN of 1.1 mg L-1, and an EC of 640 µs, typical of water quality in the aqueduct (1, 20). The California Aqueduct is

FIGURE 2. Changes in UV absorbance due to simulated solar irradiation. one of the artificial outflows from the Delta and is a mixture of different source waters, including upstream rivers, agricultural drainage, wetland outflows, and urban runoff (1, 3). Our data reflects a net increase in DOC, TN, and EC as water passes through the Delta from the Sacramento River (North) to the Aqueduct (South). Indeed, field studies and computer models have demonstrated that agricultural drainage and wetland outflows within the Delta are the major sources of DOC, nutrients and salts in waters emanating from the Delta (1). The characteristics of the organic carbon from the agricultural drainage and wetland outflow are different from those in the Sacramento River. A large portion of DOC in the Sacramento River is autochthonous, produced within the water channel by macrophytes and algal biomass (3, 19). The aromaticity, in terms of the specific UV absorbance (SUVA) of the water, was 2.32 L mg-1 m-1 and is comparable to organic carbon found in other rivers in the area (18). Importantly, the Sacramento River water had the lowest DOC, SUVA, and hydrophobic organic acid (HPOA) fraction (59%) compared to the samples collected within the Delta. The high SUVA values and high levels of hydrophobic carbon found in the agricultural drainage and wetlands are mainly attributed to the 1011 km2 of peat soils in the Delta (1, 5). Leachates from these organic-rich soils contain a large portion of humic substances, which are highly hydrophobic and aromatic in nature (7, 8). The SUVA of 3.60 L mg-1 m-1 and 66% HPOA in our drainage sample are typical of the Delta’s agricultural drainage water. Furthermore, perennial flooding in the wetlands may limit organic carbon oxidation, resulting in the highest SUVA and HPOA fraction among all the samples collected, with an average of 4.71 L mg-1 m-1 and 70%, respectively (8, 9). Water at the aqueduct contains different portions of Sacramento River water, agricultural drainage, and wetland runoff, depending on the hydrologic conditions. Our aqueduct sample had a SUVA of 2.91 L mg-1 m-1 and a HPOA content of 63%, representing the mixing of river, drainage, and wetland waters. The fluorescence data (see below) support this finding. Regarding the DBP formation potential of these water sources, the agricultural drainage had the highest THMFP and HAAFP, forming 1438 µg L-1 and 1269 µg L-1 of THMs and HAAs, respectively (Table 2). The highest formation in the drainage water is due to its high levels of DOC and bromide. Surprisingly, when the THMFP and HAAFP are normalized with respect to their DOC concentrations, the reactivity of DOC in the drainage water is by far the lowest among all samples, with an average specific THMFP of 23.7

µg mg-1 and an average specific HAAFP of 20.9 µg mg-1, respectively. The drainage water also had the lowest ratio of Cl2 consumed per mg DOC, reflecting the less reactive properties of the DOC in the water toward chlorine compared to the other samples. Despite the general belief that agricultural drainage waters have a higher propensity to react with chlorine to form halogenated DBPs, these apparently contradictory findings have also been observed by others. A study by the USGS also showed that peat island agricultural drainage water had the lowest tendency to form THMs than river water and other water sources in the Delta (6). Interestingly, organic carbon in the wetland outflow had the highest reactivity, producing more than 70 µg THMs and HAAs per mg of DOC and consuming 2 mg of Cl2 per mg DOC. The existence of relatively high concentrations of bromide in both the drainage water and wetland runoff is also an important factor in DBP formation, but this does not appear to have been responsible for the marked differences in specific DBP yields for the two sources of water. As noted above, the increased specific THMFP and HAAFP yields in the aqueduct water compared to the river water are a result of organic precursors generated within the Delta. 3.2. Impacts of Simulated Solar Irradiation on DOC Characteristics. 3.2.1. UV Absorbance. Our results showed that the overall DOC concentration did not significantly change after 24 h of exposure to solar irradiation. However, the UV absorbance at 254 nm was decreased by 35, 24, 25, and 38% in water samples from the river, drainage, wetlands, and aqueduct, respectively (Figure 2), with first-order decay constants of 1.74 × 10-2, 1.09 × 10-2, 1.16 × 10-2, and 1.85 × 10-2 hr-1. These rate constants suggest that 30-45% of the DOC from the Delta is altered at the south end of the California Aqueduct if the travel time is 4 days with 8 h full sun exposure and photodegradation is the only degradation pathway. Furthermore, these rate constants indicate that DOC in the drainage and wetland was relatively recalcitrant to photodegradation compared to DOC in the aqueduct and river. This is consistent with the fact that the DOC in the drainage and wetland waters had a higher fraction of aromatic carbon and humic material as reflected by the higher SUVA and hydrophobic organic carbon in these waters (Table 1). 3.2.2. Fluorescence. Fluorescence data showed two distinct peaks associated with humic material (Figure 3). Ex 240 Em 420 (fulvic acid-like) and Ex 320 Em 420 (humic acidlike) were greatest for the agricultural drainage, followed by the wetland, aqueduct, and river water. The intensity significantly decreased in the two peaks after exposure, VOL. 42, NO. 15, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Fluorescence patterns for water samples before and after exposure to simulated sunlight. The two distinct peaks associated with humics were Ex 240 Em 420 (fulvic acid-like) and Ex 320 Em 420 (humic-acid like). indicative of photodegradation of humic substances. These observations are consistent with the reduction in UV absorbance at 254 nm. No change in fluorescence for either peak was observed in the dark controls. 3.2.3. XAD Fractionation. Because the distribution of DOC was dominated by hydrophobic organic acids (Table 1), it was difficult to discern any differences in the distribution of the various organic fractions after the simulated solar irradiation as a result of analytical uncertainties associated 5590

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with the DOC measurements. No consistent change in the hydrophobic acid fraction was evident, but all of the waters exhibited an increase in the hydrophilic acid (HPIA) fraction as a result of irradiation, consistent with the fluorescence and UV measurements. 3.2.4. CO2 Production. CO2 production due to solar irradiation increased with increasing exposure to simulated sunlight (Figure S1 in the Supporting Information). The extent of CO2 production was proportional to the DOC concentration

FIGURE 4. Impact of UV irradiation on reactivity of DOC with respect to (a) THM and (b) HAA formation. of the four waters and was greatest for the agricultural drainage, followed by water from the wetland, Aqueduct, and river. When CO2 production was normalized by the DOC content of the water, the organic carbon in the agricultural drainage had the lowest propensity in forming CO2 and the organic carbon in the wetland produced the most CO2 per mg DOC (Figure S1 in the Supporting Information). In all cases, the amount of CO2 produced was less than 8% of the DOC content of the waters, reflecting a low degree of DOC mineralization despite the significant reduction in UV absorbance. It is interesting to note that CO2 production in the drainage and wetland waters were quite different while the changes in UV absorbance were almost identical over the 24 h exposure period (Figure 2). These findings suggest that relatively large amounts of small organic carbon molecules, which do not contribute to UV absorbance, were oxidized to CO2 in the wetland water due to solar irradiation (21). While DOC in the drainage is composed primarily of soil humic substances, wetland DOC may contain small organic carbon fragments from its autochthonous sources, such as plants, insects, and animals (22). 3.2.5. Organic Acid Production. The increase in the hydrophilic acid fraction and the decrease in humic characteristics was attributed to the production of small organic acids (21). Four organic acids (succinic, oxalic, citric, and maleic acids) were produced during the course of irradiation (Figure S2 in the Supporting Information). The production rate appeared greatest in the first 6 h. Succinic acid production was the highest among the four acids measured. On a molar carbon basis, these organic acids combined represented 2, 1.7, 0.24, and 0.13% of the DOC for the river, wetland, agricultural drainage, and aqueduct waters, respectively, at the end of the 24 h exposure period. The production patterns of these organic acids were similar to the pattern of CO2 production (Figure S1 in the Supporting Information), with the lowest organic acid and CO2 production coming from the agricultural drainage sample. In addition, the wetland

FIGURE 5. Change in bromine incorporation for (a) THMs, (b) dihaloacetic acids, and (c) trihaloacetic acids among water samples before and after UV irradiation. sample had the greatest CO2 production but a lower organic acid production than river and aqueduct samples after 24 h of irradiation. No organic acids were produced in the dark controls. 3.2.6. DBP Formation. Figures 4a and 4b present the THM and HAA formation potentials, respectively, normalized with respect to the DOC content of the waters before and after 6 and 24 h of simulated solar irradiation. The specific THMFP and HAAFP of the original samples ranged from 23 to 71 µg mg-1 (2 to 6 mmol mol-1). There was no clear trend in reactivity with respect to THM formation among the four water samples as a result of irradiation, whereas the reactivity with respect to HAA formation tended to decrease with irradiation time for three of the four waters examined. The river and wetland samples exhibited the greatest change, with the HAAFP decreasing by 50 and 30%, respectively, after 24 h of exposure. The decrease in HAA formation potential is consistent with earlier work suggesting that a majority of HAA precursors are aromatic in nature, whereas THM precursors include both aromatic and aliphatic organic material (23, 24). Accordingly, the HAA formation potential decreased because solar irradiation decreased the aromaticity of the DBP precursors. There was a net increase in the bromide ion concentration as water passed through the Sacramento-San Joaquin Delta. Before water entered the Delta, the bromide concentration VOL. 42, NO. 15, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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in the Sacramento River was below detection limits (Table 1). Because of the relatively high concentrations of bromide in the agricultural drainage and the wetlands (0.36 and 0.32 mg L-1, respectively), the bromide concentration increased to 0.15 mg L-1 at the outflow from the Delta at the Aqueduct. As a result, THM and HAA speciation changed in the different water samples. The Sacramento River, with its low bromide concentration, had a relatively high DOC to Br- ratio, which resulted in a low bromine incorporation factor (BIF) for the THMs, dihaloacetic acids (X2AA), and trihaloacetic acids (X3AA) (Table 2). The BIF for any class of DBPs is equal to the moles of bromine atoms incorporated into the DBP class divided by the total number of halogens incorporated into that class (25). As the DOC to Br- ratio decreased, as in the wetland and Aqueduct samples, the BIF correspondingly increased (Table 2). In addition, solar irradiation increased the relative degree of bromine incorporation into THMs and HAAs for all four water samples (Figure 5). In most cases, the BIF increased with increasing irradiation time. This is consistent with theoretical expectations (23), i.e. bromine substitutes to a greater degree into hydrophilic carbon relative to hydrophobic carbon. Because the simulated solar irradiation decreased the concentration of humic material and increased the concentration of hydrophilic acids, a greater degree of bromine substitution would be expected with increased radiation as seen in Figure 5. In summary, river water, agricultural drainage, wetland outflow, and aqueduct waters in the Sacramento-San Joaquin Delta were exposed to simulated sunlight to evaluate the impact of transport in open channels on DBP precursors. In general, solar irradiation did not significantly change the overall DOC concentrations of these waters but did decrease their UV absorbance at 254 nm, their corresponding SUVA values, and their humic fluorescence intensities. A small portion of the DOC was degraded to citric, maleic, oxalic, and succinic acids, and less than 8% was mineralized to CO2. The hydrophilic characteristics of the waters tended to increase. The shift of DOC from humic-like to hydrophilic character resulted in a decrease in the specific HAA formation potential of the waters and an increase in the extent of brominated THM and HAA formation during chlorination. Agricultural drainage and wetland outflow had a comparable decrease in UV absorbance due to irradiation, but the reactivity in forming DBPs and organic acids, and producing CO2 were different, suggesting the presence of small organic fragments in wetland waters which did not contribute to UV absorbance but which were reactive in forming DBPs and in producing CO2. The characteristics of the water at the intake to the California Aqueduct reflected the blend of Sacramento River water with agricultural drainage and wetland outflow from the Delta.

Acknowledgments We thank Robin Miller of the U.S. Geological Survey for providing wetland water samples. National Institutes of Health Institutional Research and Academic Career Development awards given to A.T.C. and D.M.L. funded this study.

Supporting Information Available Figures S1 and S2 illustrate the impact of solar irradiation on CO2 and organic acid production during the solar simulation study. This material is available free of charge via the Internet at http://pubs.acs.org.

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(23) 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 (13), 2920– 2928. (24) Singer, P. C.; Weinberg, H. S.; Brophy, K.; Liang, L.; Roberts, M.; Grisstede, I.; Krasner, S. W.; Baribeau, H.; Arora, H.; Najm, I., Relative Dominance of Haloacetic Acids and Trihalomethanes

in Treated Drinking Water. AWWA Research Foundation and American Water Works Association: Denver, CO, 2002. (25) Obolensky, A.; Singer, P. C. Halogen substitution patterns among disinfection byproducts in the information collection rule database. Environ. Sci. Technol. 2005, 39 (8), 2719–2730.

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