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Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesa20

Impacts of natural organic matter on perchlorate removal by an advanced reduction process a

Yuhang Duan & Bill Batchelor

a

a

Zachry Department of Civil Engineering , Texas A&M University , College Station , Texas , USA Published online: 12 Feb 2014.

Click for updates To cite this article: Yuhang Duan & Bill Batchelor (2014) Impacts of natural organic matter on perchlorate removal by an advanced reduction process, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 49:6, 731-740, DOI: 10.1080/10934529.2014.865462 To link to this article: http://dx.doi.org/10.1080/10934529.2014.865462

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Journal of Environmental Science and Health, Part A (2014) 49, 731–740 C Taylor & Francis Group, LLC Copyright  ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934529.2014.865462

Impacts of natural organic matter on perchlorate removal by an advanced reduction process YUHANG DUAN and BILL BATCHELOR

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Zachry Department of Civil Engineering, Texas A&M University, College Station, Texas, USA

Perchlorate can be destroyed by Advanced Reduction Processes (ARPs) that combine chemical reductants (e.g., sulfite) with activating methods (e.g., UV light) in order to produce highly reactive reducing free radicals that are capable of rapid and effective perchlorate reduction. However, natural organic matter (NOM) exists widely in the environment and has the potential to influence perchlorate reduction by ARPs that use UV light as the activating method. Batch experiments were conducted to obtain data on the impacts of NOM and wavelength of light on destruction of perchlorate by the ARPs that use sulfite activated by UV light produced by lowpressure mercury lamps (UV-L) or by KrCl excimer lamps (UV-KrCl). The results indicate that NOM strongly inhibits perchlorate removal by both ARP, because it competes with sulfite for UV light. Even though the absorbance of sulfite is much higher at 222 nm than that at 254 nm, the results indicate that a smaller amount of perchlorate was removed with the UV-KrCl lamp (222 nm) than with the UV-L lamp (254 nm). The results of this study will help to develop the proper way to apply the ARPs as practical water treatment processes. Keywords: Perchlorate, NOM, ultraviolet light, reduction, ARPs.

Introduction Perchlorate (ClO4 -) is one of the major emerging contaminants of concern and has been found in soil and water systems throughout the United States. Ingestion of perchlorate can inhibit the uptake of iodine into the thyroid, where it is used to produce hormones that are needed for metabolic processes throughout the body.[1–3] The US EPA has not set regulatory levels of perchlorate in drinking water and only some guidance or reference levels exist.[1,4] In 2005, the US EPA proposed an official reference dose (RfD) of 0.7 µg kg−1 day−1 by assuming that the uptake of perchlorate is from both water and food sources.[5] In 2008, the US EPA issued an interim health advisory level of 15 µg L−1 for drinking water. Maintaining concentrations at or under this level is believed to be protective of all subpopulations.[6] In 2011, the US EPA’s final determination indicates that perchlorate will be regulated with a National Primary Drinking Water Regulation (NPDWR).[7] Perchlorate is a highly persistent species at room temperature due to its slow redox kinetics, which extremely Address correspondence to Yuhang Duan, Zachry Department of Civil Engineering, Texas A&M University, College Station, TX 77843-3136, USA; E-mail: [email protected] Received August 8, 2013. Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lesa.

limits the reactivity of perchlorate at room temperature, even though it is a strong oxidant thermodynamically.[1,8] Ion exchange is an effective way to remove perchlorate, but perchlorate collects on the ion exchange resins without any degradation. Additional treatment prior to disposal of the resins is needed. High resin affinity for perchlorate will result in difficult regeneration, causing ion exchange treatment to be relatively expensive compared to other technologies.[1,3] Biological treatment technologies, such as bioreactors and microbial fuel cells, are able to convert ClO4 - to Cl-, which is a relatively non-toxic anion.[1,3,9] However, there is a potential to grow undesirable microorganisms and to produce toxic by-products, which are the primary concerns in applying bioremediation processes. Furthermore, biological processed are susceptible to environmental conditions such as temperature.[1,3,10] Various types of reductants at elevated temperature or when combined with a catalyst are able to degrade perchlorate, including Fe0, Ti (III), Cr (II), V (II), Re (V), and Mo (III), but only at slow rates.[8,11] A new set of treatment technologies, advanced reduction processes (ARPs) has the potential to increase rates of perchlorate reduction. ARPs are similar in concept to advanced oxidation processes (AOPs), but ARPs produce highly reactive reducing free radicals by combining reductants and activating methods. These reducing radicals can donate an unpaired electron to a target contaminant such as perchlorate and thereby chemically reduce it. The high

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732 reactivity of the radicals can accelerate the rate of perchlorate degradation and overcome the disadvantages of conventional treatment technologies. Some reductants that can be used for ARPs include dithionite, sulfite, sulfide, and ferrous ion, while the activating methods include UV light, electron beam, ultrasound, and microwave.[8,12] The results of a discovery project in our laboratory indicate that the combination of sulfite with ultraviolet light produced by low-pressure mercury lamps (UV-L, 254 nm) is an effective and promising way to remove perchlorate.[8] Sulfite is a commercially applied bleaching agent and has been used in water treatment processes. It is known that UV irradiation of sulfite solutions generates hydrated electrons and sulfite radical anions, which are highly reactive reductants, although the sulfite radical anion can also be an oxidant.[13–16] However, sulfite will not produce radicals without UV irradiation.[13] Many wastewater treatment plants are now disinfecting using ultraviolet light, which is also a primary activating method in AOPs. Therefore, the sulfite/UV ARP is a promising treatment technology for destruction of perchlorate in water. However, natural organic matter (NOM) is a problem in many waters, since it is responsible for adding a brownish yellow color and for being a precursor of harmful disinfection by-products (DBPs).[17–19] NOM is a large, poorly defined group of organic compounds that comes from decay of plants and animals in the environment. NOM is able to absorb ultraviolet light and its absorbance at 254 nm is often used to characterize NOM.[17,18] Absorption of UV light initiates a series of photochemical reactions in which the higher molecular weight NOM molecules are broken into smaller fragments that are available for further degradation.[20,21] Therefore, NOM may inhibit the ability of ARPs to degrade perchlorate and there is a need for more data to evaluate the extent to which this occurs. In this research, experiments were conducted to obtain data on the influence of natural organic matter on destruction of perchlorate by sulfite/UV ARP, in order to support development of the processes as a practical water treatment method. Experiments were conducted with a low pressure mercury lamp (UV-L) that produces light primarily at 254 nm and with a narrow band KrCl excimer lamp (UV-KrCl) that produces light primarily at 222 nm.

Materials and methods Reagents All reagents were used as received. Perchlorate standard solution (997 ± 20 µg mL−1) was purchased from Inorganic Ventures, Inc. Sodium sulfite (anhydrous, 98.6%) was purchased from Avantor Performance Materials (Center Valley, PA, USA). Potassium hydrogen phosphate (anhydrous, 98%) was purchased from Alfa Aesar (Ward Hill, MA, USA). Suwannee River natural organic matter powder (Catalog No. 1R101N, RO isolation) was purchased

Duan and Batchelor from International Humic Substances Society (Denver, CO, USA). The elemental carbon of the dry, ash-free Suwannee River NOM was 52.47% (w/w) (details are in Appendix Tables A1–A3). Experimental procedure All irradiation experiments and related work were conducted in an anaerobic chamber (Coy Laboratory Products Inc., Grass Lake, MI, USA) that was filled with a gas mixture (95% nitrogen and 5% hydrogen, PRAXAIR Distribution Inc., Byran, TX, USA) and equipped with an analyzer for oxygen and hydrogen, fan boxes and a palladium catalyst STAK-PAK (Coy Laboratory Products Inc.) that scavenges oxygen. The anaerobic chamber was vacuumed and refilled with the gas mixture as required to keep the anaerobic condition inside the chamber. All UV irradiation experiments were carried out in 17mL, cylindrical, UV-transparent quartz reactors (Starna Cells, Inc., Atascadero, CA, USA). The UV-L light source was produced by Phillips (Phillips Model TUV PLL36W/4P, Somerset, NJ, USA) and it emitted UV radiation with a peak at 253.7 nm. The other source was a KrCl-excimer lamp from the Institute of High Current Electronics, SB Russian Academy of Science and produced UV radiation with a peak at 222 nm. The light intensity at the top of the reactor with both light sources was measured with a UV digital light meter (General Tools, Model No. UV 512C, New York City, NY, USA), which was calibrated by modified ferrioxalate actinometry.[22] Batch experiments were conducted with different levels of NOM and the extent of perchlorate degradation was measured. First of all, a set of control experiments was conducted. Reagent control experiments were conducted with sulfite, but without irradiation. Irradiation control experiments were conducted with each UV light source, but without sulfite. NOM control experiments were conducted with UV light only and with both UV light and sulfite. In the kinetic experiments, sulfite and NOM were added into the perchlorate solution and buffered at pH 11, which is the optimal condition.[8] The initial concentrations of perchlorate and sodium sulfite were 0.1 mM and 11 mM, respectively, for all kinetic experiments. The initial concentrations of NOM were set to 0, 2.5, 5, 10, 30, and 50 mg C L−1. The incident light intensities of the UV-KrCl lamp and the UV-L lamp were 13 mW cm−2 and 9.5 mW cm−2, respectively. A total of seven to eight samples were taken for each experiment that used the UV-L lamp (254 nm) and a total of four to five samples were taken for each experiments conducted with the UV-KrCl lamp (222 nm). The concentration of perchlorate in the samples was measured by ion chromatography and the presence of NOM was characterized by measuring the UV absorbance. Analytical methods Ion chromatography for perchlorate analysis was conducted using a Dionex DX 500 IC (Thermo Fisher

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Perchlorate reduction by advanced reduction process

Fig. 1. Effects of NOM on perchlorate reduction by sulfite/UV-KrCl ARP (a) Relative perchlorate concentration ([perchlorate]initial = 0.1 mM), (b) NOM-absorbance ([NOM]initial = 50 mg L−1 as carbon), (c) Relative sulfite concentration ([sulfite]initial = 11 mM).

Scientific Inc., Bannockburn, IL, USA) with a CD 20 Conductivity Detector and GP40 Gradient Pump. Before the analysis of perchlorate in samples with NOM, the samples were passed through a 0.45-µm cellulose nitrate membrane filter (25 mm-diameter, Whatman, Piscataway, NJ, USA) and collected in 0.5-mL vials (Dionex) with caps.

The samples contained in these vials were analyzed by an ion chromatograph equipped with AS-16 column and AS40 automated sampler following Standard Method 4110.[23] A UV-Visible spectrophotometer (Heλios, Thermo Spectronic, Cambridge, UK) was used to measure the concentration of sulfite and NOM. NOM also absorbs light at

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Duan and Batchelor

this wavelength, so a procedure was developed to correct for this. The absorbance at 270 nm was used to characterize the concentration of NOM in mixtures with sulfite, since the absorbance of sulfite is negligible compared to the absorbance of NOM at 270 nm (Fig. A1 in Appendix). The absorbance at 240 nm was used to measure sulfite concentration, but it was corrected for the absorbance due to NOM. The absorbance at 240 nm due to NOM was estimated by measuring the absorbance at 270 nm, calculating NOM concentration with a calibration curve and then calculating the absorbance due to NOM at 240 nm with another calibration curve. Characterizing NOM by measuring its UV absorbance does not measure the concentration of organic carbon remaining as NOM, because the features of NOM that absorb UV can be destroyed without destroying the organic carbon content of NOM. Therefore, the measurement will be reported as “NOM-absorbance.”

Results and discussion Reagent and irradiation control experiments Little perchlorate was removed in the reagent and irradiation control experiments (Fig. A2 in Appendix). Effects of NOM on perchlorate reduction by sulfite/UV-KrCl ARP Figure 1a shows the relative concentrations of perchlorate for the experiment with UV-KrCl at different concentrations of NOM. More than 50% of perchlorate was removed after 9 h by the sulfite/UV-KrCl without NOM at base conditions. A hypothesis was developed to describe the perchlorate removal mechanism and it is described by Eqs. (1)–(4). First, the sulfite radical is formed by photolysis of sulfite (Eq. (1)). Then, the sulfite radical reacts with perchlorate by extracting an oxygen atom and producing chlorate and a sulfate radical (Eq. (2)). The sulfate radical and the sulfite radical are quenched by the aqueous electron (Eq. (3) and (4)). This mechanism is analogous to the mechanism for sulfite extracting oxygen to reduce chlorite or chlorate.[24–28] The chlorate that is formed by reaction with the sulfite radical (Eq. (2)) could be easily reduced to chloride ion.[29] •− − SO2− 3 + hv = SO3 + eaq

(1)

− •− − SO•− 3 + ClO4 = SO4 + ClO3

(2)

2− − SO•− 4 + eaq = SO4

(3)

2− − SO•− 3 + eaq = SO3

(4)

Table 1. Pseudo–first-order rate constant (kobs ) and R2 values for perchlorate removal at different concentrations of NOM ([perchlorate]initial = 10 mg L−1, [sulfite]initial = 11 mM; UV-KrCl lamp intensity = 13 mW cm−2, pH = 11). NOM Concentration as C (mg L−1) 0 2.5 5 10 30

R2

0.087 ± 0.039 0.063 ± 0.011 0.053 ± 0.019 0.042 ± 0.015 0.030 ± 0.013

0.98 0.99 0.97 0.97 0.95

of perchlorate within 9 h. At the smallest concentration of NOM used in these experiments (2.5 mg L−1 as carbon), approximately 45% of perchlorate was removed, which is only 5% less than that in the experiment without NOM. A simple model, pseudo–first-order model, was applied to compare reaction rates. Table 1 shows the rate constants (kobs ) that were obtained by non-linear regression on the experimental data using Matlab function “nlinfit.” The rate constants decrease as the concentration of NOM increases. This result indicates that NOM inhibits perchlorate reduction with sulfite and the UV-KrCl lamp, even at low concentration. The ability of NOM to inhibit removal of perchlorate by ARP may be due to it competing with sulfite for the absorption of UV light, which is used to produce reactive radicals that reduce perchlorate. It has been reported that NOM absorbs UV light from 220 nm to 280 nm and measurements of this absorbance in this range could be used for characterization of NOM.[18] Additionally, NOM could photolyze to smaller molecules under irradiation by UV light.[20] Studies [19–21] suggest that AOPs can effectively remove NOM from solution by oxidizing it with hydroxyl radicals.[19, 30] Therefore, there is a possibility that NOM would also react with radicals produced from sulfite (sulfite radical anion and hydrated electron) in these ARP. Experiments were conducted to investigate the loss of NOM, which was measured by its UV absorbance (NOMabsorbance) at 270 nm. Figure 1b shows results of an experiment where more than 50% of initial NOM-absorbance was removed within 7 h of irradiation with 222 nm light at Table 2. Quantum yield (φ sulfite ) for loss of sulfite by irradiation with a UV-KrCl lamp under different conditions ([perchlorate]initial = 10 mg L−1, [sulfite]initial = 11 mM, [NOM] = 50 mg L−1 as carbon; UV-KrCl lamp intensity = 13 mW cm−2, pH = 11). Experimental Conditions

Figure 1a shows that the rate of perchlorate reduction decreases as the concentration of NOM increases. The experiment with 30 mg L−1 (as carbon) NOM removed only 20%

kobs (h−1)

Sulfite/UV Sulfite/UV/perchlorate Sulfite/UV/NOM Sulfite/UV/perchlorate/NOM

φ sulfite (mol einstein−1) 0.018 ± 0.0018 0.016 ± 0.0008 0.013 ± 0.0031 0.010 ± 0.0038

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Perchlorate reduction by advanced reduction process

Fig. 2. Effects of NOM on perchlorate reduction by sulfite/UV-L ARP (a) Relative perchlorate concentration ([perchlorate]initial = 0.1 mM), (b) NOM-absorbance ([NOM]initial = 50 mg L−1 as carbon), (c) Relative sulfite concentration ([sulfite]initial = 11 mM).

pH 11. But when sulfite was added into the reactor and irradiated, over 95% of the initial NOM-absorbance was removed within 5 h. These results demonstrate that NOM can absorb UV light at 222 nm and photolyze, but that the process is more rapid when sulfite is present and producing reactive species. The impact of sulfite suggests that it, or reactive species produced by its photolysis, reacted with NOM.

Figure 1b also shows that perchlorate has no effect on the loss of NOM-absorbance with sulfite and UVKrCl irradiation at pH 11, because it shows that the lines for sulfite/UV/NOM almost coincide with those for sulfite/UV/perchlorate/NOM. With perchlorate in the system, there was still more than 95% of the initial NOMabsorbance removed within 5 hours of irradiation with the UV-KrCl lamp.

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736 Figure 1c shows concentrations of sulfite over time during sulfite/UV-KrCl experiments under different conditions. These results show that the presence of perchlorate in the system has little effect on sulfite photolysis with the UV-KrCl lamp. All of the initial sulfite was consumed in both experiments within 9 h. However, when NOM was added, the rate of loss of sulfite was decreased to a large extent. Only 60% of the initial sulfite was removed within 7 h. This result further demonstrates that NOM inhibits the reaction of sulfite with UV light, possibly due to NOM competing with sulfite for UV light. The data on sulfite removal was used to calculate the quantum yield of the photolysis of sulfite to produce the sulfite radical ion and the aqueous electron (Eq. 1). The rate of this reaction is equal to the product of a quantum yield and the rate of light absorption by the reagent. Equations 5 and 6(details are in Appendix A) describe how the reagent concentration changes with time. Equation 5 applies when the reagent is the only light-absorbing compound and Equation 6applies when several compounds absorb light.  I0  dC  (5) = −φ 1 − e−ε CL dt L  φεi Ci I0  dCi  −αall L (6) = 1 − e  dt αall L where C is the concentration of light-absorbing reagent (mol L−1); φ is the quantum yield at wavelength λ (mol einstein−1); I0 is the photon flux at wavelength λ entering the solution (einstein cm−2 s−1); L is the length of the light path through the solution (cm); ε‘ is the base e molar absorptivity at wavelength λ of the light-absorbing reagent (L mol−1cm−1); and α‘all is the summation of absorption coefficients for water and all dissolved compounds that     = αwater + εi Ci ). substantially absorb light (αall Table 2 shows the quantum yield for sulfite for each experiment. These quantum yields were calculated by doing non-linear regressions using sulfite data fitted to Eqs. 5 or 6. Equation 6was used when sulfite and NOM were present to absorb UV light and Eq. 5 was used when only sulfite was present. These equations were solved numerically with the Matlab function “ode45.” Table 2 shows that the quantum yields in the presence of perchlorate are a little lower (10–25%) than those in its absence. The quantum yields for sulfite removal in the presence of NOM are about 30–40% lower than in its absence.

Duan and Batchelor rate reduction decreases with increasing concentration of NOM. There was no perchlorate removal within 14 h in the experiment with 50 mg L−1 (as carbon) NOM. After 25h irradiation, less than 10% of the initial perchlorate had been removed. For the smallest concentration of NOM that was applied in the experiment (2.5 mg L−1 as carbon), only 55% of perchlorate was removed, which is approximately 25% less than that in the experiment without NOM. Pseudo–first-order rate constants for perchlorate reduction with NOM were calculated with non-linear regressions and the results are listed in Table 3. Again, kobs decreases as the concentration of NOM increases. The observed pseudo–first-order rate constant for the experiment with NOM at 2.5 mg L−1 is less than half of that for the experiment without NOM. This result indicates that NOM significantly inhibits perchlorate reduction with the UV-L lamp, even at low concentrations. The loss of NOM-absorbance during irradiation with the UV-L lamp was measured under different conditions. Figure 2b shows the results of experiments at pH 11 in solutions with NOM-only, with sulfite/NOM, and with sulfite/perchlorate/NOM. UV-L irradiation of the solution with only NOM removed only 20% of the initial NOMabsorbance within 12.5 h. But when sulfite was added to the system, almost 100% of the initial NOM-absorbance was removed within 12.5 h, so it can be concluded that sulfite significantly enhances the degradation of NOM during irradiation by a UV-L lamp at pH 11. This result also indicates that radicals produced by photolysis of sulfite may react with NOM. In contrast with the results with UV-KrCl, Figure 2b shows that to some extent, perchlorate inhibited the loss of NOM-absorbance in the presence of sulfite with UVL. With perchlorate in the system, only 70% of the initial NOM-absorbance was removed within 13.5 hours under irradiation with the UV-L lamp, which is 30% less than that in the sulfite/NOM experiments. However, NOMabsorbance was nearly completely removed in the sulfite/perchlorate/NOM experiment after irradiation with the UV-L lamp for 25 h. The reasons for different behavior with the two lamps could be the higher light intensity

Table 3. Pseudo–first-order rate constant (kobs ) and R2 values for perchlorate removal in the presence of NOM ([perchlorate]initial = 10 mg L−1, [sulfite]initial = 11 mM; UV-L lamp intensity = 9.5 mW cm−2, pH = 11).

Effects of NOM on perchlorate reduction by sulfite/UV-L ARP

NOM Concentration as C (mg L−1)

The influence of NOM on perchlorate removal was examined by adding different levels of NOM in experiments conducted with the UV-L lamp irradiating solutions of 10 mg L−1 perchlorate and 11 mM sulfite. All experiments were conducted under same conditions except for the concentration of NOM. Figure 2a shows that the rate of perchlo-

0 2.5 5 10 30 50

kobs (hr−1)

R2

0.11 ± 0.03 0.039 ± 0.011 0.034 ± 0.014 0.025 ± 0.0074 0.0090 ± 0.0040 0.0033 ± 0.0024

0.97 0.95 0.91 0.95 0.87 0.73

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Perchlorate reduction by advanced reduction process (13 mW cm−2 compare to 9.5 mW cm−2) that was applied to the reactor with the UV-KrCl lamp or it could be due to the higher absorbance of NOM at 222 nm (Fig. A1 in Appendix). Both would lead to higher rates of absorption of UV light by NOM. The quantum yields (NOM ) calculated for loss of NOM-absorbance by irradiation with a UV-KrCl lamp is 0.0041 ± 0.0007 mol/einstein, which is more than three times that for NOM irradiation with a UV-L lamp (0.0012 ± 0.0002 mol/einstein). This indicates that not only was more UV light absorbed at 222 nm, it was more effective in destroying NOM-absorbance. Figure 2c shows sulfite concentrations over time under different conditions. The rate of loss of sulfite during irradiation without perchlorate is a little larger than that with perchlorate, which indicates that perchlorate lowers the rate of sulfite photolysis to some extent under irradiation of UV-L lamp. The quantum yields for sulfite/UV-L experiment (0.050 ± 0.0067 mol/einstein) is a little larger than that of the sulfite/UV-L/perchlorate experiment (0.037 ± 0.0061 mol/einstein). But, eventually, 100% of the sulfite was consumed in both experiments with and without perchlorate within 26 h. Nevertheless, the presence of NOM (50 mg L−1 as C) resulted in about 15% loss of sulfite over the first 13–14 h. NOM is competing for UV light with sulfite. In addition, some of intermediates produced by degradation of NOM in the first 14 h may absorb UV light at the wavelength used to measure sulfite, which would interfere with the measurement of sulfite concentration. Overall, these results demonstrate that NOM strongly inhibits perchlorate removal by the sulfite/UV-L ARP. Comparison of sulfite/UV-L and sulfite/UV-KrCl ARPs The rate of perchlorate reduction with the UV-L (254 nm) lamp was faster than the rate of perchlorate removal with the UV-KrCl lamp, even though the KrCl lamp produced higher incident light intensity. When experiments were conducted at the same light intensity (9.5 mW cm−2), the rate constant for perchlorate removal was 0.059 ± 0.01 h−1 (R2 = 1.00) with UV-KrCl and 0.11 ± 0.03 h−1 (R2 = 0.97) for UV-L. Consumption of sulfite by photolysis with the KrCl lamp was so rapid that there was no sulfite remaining in the solution after 9 h, which stopped perchlorate reduction. Degradation of sulfite by photolysis with the UV-L lamp was slower, with more than 40% of the initial sulfite remaining in the solution after 10 h (Fig. 2c, sulfite/UV-L/perchlorate). The remaining sulfite would keep producing sulfite radicals, which would continue to reduce perchlorate until the sulfite was consumed after about 26 h. Theoretically, the 222-nm UV light produced by the KrCl lamp should cause more rapid removal of perchlorate than 254-nm light produced by the UV-L lamp, because sulfite absorbs light more effectively at 222 nm than at 254 nm (Fig. A1 in Appendix), so more sulfite radicals should be generated.

However, the quantum yield for loss of sulfite (sulfite ) was calculated to be 0.016 ± 0.0008 mol einstein−1 for the 222-nm light, which is less than half the value (0.037 ± 0.0061 mol einstein−1) for the 254-nm light. This indicates that a smaller fraction of 222-nm photons that were absorbed by sulfite were able to convert it to reactive species than for 254-nm photons. Therefore, although more photons were being absorbed at 222 nm, fewer radicals were being produced and fewer perchlorate molecules were being reduced.

Conclusions This study has provided information about the influence of NOM and UV wavelength on perchlorate removal by an ARP processes using sulfite and UV. NOM strongly inhibits perchlorate removal by either the sulfite/UV-KrCl or the sulfite/UV-L ARP because NOM is a competitor with sulfite for UV light and a scavenger for free radicals. Even though more UV light was absorbed at 222 nm than at 254 nm, more perchlorate was removed at 254 nm (UVL). The effectiveness of photons in producing radicals was higher with light produced by the UV-L lamp than with the UV-KrCl lamp, even though fewer photons were absorbed. The results of this study will provide information to support development of the processes as a practical water treatment method.

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Duan and Batchelor Environmental occurrence, interactions, and treatment, Gu, B., Coates, J.D., Eds.; Springer Science+Business Media Inc.: New York, 2006; 373–387. Liu, X.; Yoon, S.; Batchelor, B.; Abdel-Wahab, A. Photochemical degradation of vinyl chloride with an advanced reduction process (ARP)-Effects of reagents and pH. Chem. Eng. J. 2013, 215–216, 868–875. Pemberton, R.S.; Depew, M.C.; Heitner, C.; Wan, J.K.S. Some mechanistic insights into a model bleaching process for quinones by bisulfite and dithionite: An ESR-CIDEP study. J. Wood Chem. Technol. 1995, 15 (1), 65–83. Devonshire, R.; Weiss, J.J. Nature of the transient species in the photochemistry of negative ions in aqueous solution. J. Phys. Chem. 1968, 72 (11), 3815–3820. Chawla, O.P.; Arthur, N.L.; Fessenden, R.W. An electron spin resonance study of the photolysis of aqueous sulfite solutions. J. Phys. Chem. 1973, 77 (6), 772–776. Jeevarajan, A.S.; Fessenden, R.W. ESR studies of eaq - in liquid solution using photolytic production. J. Phys. Chem. 1989, 93 (9), 3511–3514. Barrett, S.E.; Krasner, S.W.; Amy, G.L. Characterization and control in drinking water: An overview. In Natural organic matter and disinfection by-products; Barrett, S.E., Krasner, S.W., Amy, G.L., Eds.; American Chemical Society: Washington, DC, 2000; 2–14. Matilainen, A.; Gjessing, E.T.; Lahtinen, T.; Hed, L.; Bhatnagar, A.; Sillanpaa, M. An overview of the methods used in the characterization of natural organic matter (NOM) in relation to drinking water treatment. Chemosphere 2011, 83 (11), 1431–1442. Matilainen, A.; Sillanpaa, M. Removal of natural organic matter from drinking water by advanced oxidation processes. Chemosphere 2010, 80 (4), 351–365. Goslan, E.H.; Gurses, F.; Banks, J.; Parsons, S.A. An investigation into reservoir NOM reduction by UV photolysis and advanced oxidation processes. Chemosphere 2006, 65 (7), 1113–1119. Buchanan, W.; Roddick, F.; Porter, N.; Drikas, M. Enhanced biodegradability of UV and VUV pre-treated natural organic matter. Water Sci. Technol. Water Supply 2004, 4 (4), 103–111. Murov, S.L.; Carmichael, I.; Hug, G.L. Handbook of photochemistry, 3rd Ed.; Springer: New York, 1993. Method 4110-Determination of anions by ion chromatography. In Standard methods for the examination of water and wastewater, 20th Ed.; Clesceri, L.S., Greenberg, A.E., Eaton, A.D., Eds.; American Public Health Association: Washington, DC, 1999; 541–548. Hayon, E.; Treinin, A.; Wilf, J. Electronic spectra, photochemistry, and autoxidation mechanism of the sulfite-bisulfite-pyrosulfite systems: The SO2 -, SO3 -, SO4 -, and SO5 - radicals. J. Amer. Chem. Soc. 1972, 94 (1), 47–57. Ermakov, A.N.; Poskrebyshev, G.A.; Purmal, A.P. Sulfite oxidation: The state-of-the-art of the problem. Kinet. Catal. 1997, 38 (3), 295–308. Zuo, Y.; Zhan, J.; Wu, T. Effects of monochromatic UV-visible light and sunlight on Fe (III)-catalyzed oxidation of dissolved sulfur dioxide. J. Atmos. Chem. 2005, 50 (2), 195–210. Halperin, J.; Taube, H. Oxygen atom transfer in the reaction of chlorate with sulfite in aqueous solution. J. Am. Chem. Soc. 1950, 72, 3319–3320. Halperin, J.; Taube, H. The transfer of oxygen atoms in oxidationreduction reaction. IV. The reaction of hydrogen peroxide with sulfite and thiosulfate, and of oxygen, manganese dioxide and of permanganate with sulfite. J. Am. Chem. Soc. 1952, 74 (2), 380–382. Taube, H. Observations on atom-transfer reactions. ACS Symposium Series 1982, 198, 151–179. Crittenden, J.C.; Trussell, R.R.; Hand, D.W.; Howe, K.J.; Tchobanoglous, G. Water treatment principles and design, 2nd Ed.; John Wiley and Sons Inc.: Hoboken, NJ, 2005.

Appendix Absorbance spectra of NOM and sulfite Figure A1 shows that NOM and sulfite both absorb at wavelengths between 210 nm to 260 nm. However, the absorbance of sulfite is negligible compared to the absorbance of NOM at 270 nm. Therefore, the absorbance at 270 nm of a mixed solution (NOM and sulfite) was used to characterize the concentration of NOM. The absorbance at 240 nm was used to measure sulfite concentration, but it was corrected for the absorbance due to NOM.

Fig. A1. Absorbance spectra from 210 nm to 300 nm for 11 mM sulfite and different concentrations of NOM (pH = 11).

Reagent and irradiation control experiments Figure A2 shows that perchlorate in pH-buffered solution was barely removed either by sulfite without UV irradiation or by UV irradiation without sulfite.

Quantum yield The data on sulfite removal was used to calculate the quantum yield of the photochemical reaction of sulfite to produce the sulfite radical ion and the aqueous electron. The rate of this reaction is equal to the product of a quantum yield and the rate of light absorption by the reagent. The rate of light absorption is the product of the molar extinction coefficient, the molar concentration of reagent and the photon flux. r1,local = φε C I

(A.1)

where r1,local is the rate of photolysis of light-absorbing reagent (mol cm-3 s−1); φ is the quantum yield at wavelength λ (mol einstein−1); I is the photon flux at wavelength λ at that point in the solution (einstein cm−2 s−1); ε‘ is the base e molar absorptivity at wavelength λ of the light-absorbing reagent (L mol−1 cm−1), (note that ε‘ = 2.303 ε, where ε is the base 10 molar absorptivity); and C is the concentration of light-absorbing reagent (mol L−1).

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Perchlorate reduction by advanced reduction process

Fig. A2. Removal of perchlorate (ClO4 -) with and without sulfite and UV light from a) UV-KrCl (intensity = 13 mW cm−2) and b) UV-L (intensity = 9.5 mW cm−2), ([perchlorate]initial = 0.1 mM, [sulfite]initial = 11 mM, pH = 11).

Equation (A.1) describes the rate observed at an individual point in the reactor where the photon flux is specified. However, measurements of changes of concentration in the reactor system were conducted on well-mixed samples and provide information on average rates of removal across the entire reactor. The rate can be averaged over the reactor by integrating across the reactor and applying the Beer–Lambert law to describe the change with photon flux with distance. For a system in which light is absorbed by compounds other than the reagent, the rate of photolysis can be expressed by Eq. A.2.  φε C I0   −αall L (A. 2) 1 − e r1 =  αall L where α‘all is the summation of absorption coefficients for water and all dissolved compounds that substantially absorb light (α‘all = α‘water + ε‘i Ci ); and L (cm) is the length of the light path through the solution. Equation (A.2) can be simplified, when only the reagent absorbs light (α‘all = ε‘C)  I0   r1 = φ 1 − e−ε CL (A. 3) L

A material balance on the reagent in these experiments (sulfite) can be combined with the model equation for the rate of photolysis to give a result that describes how the reagent concentration changes with time. Equation (A.4) applies when several compounds absorb and Eq. (A.5) applies when the reagent is the only light-absorbing compound.  φεi Ci I0  dCi  −αall L = 1 − e (A. 4)  dt αall L  I0  dC  (A. 5) = φ 1 − e−ε CL dt L The incident photon flux in these equations (I0 ) can be calculated from the light energy flux (I∗ ), that was measured by UV meter by applying Planck’s equation. I0 = I ∗ ×

λ Na × h × c

(A. 6)

where I∗ is the light energy flux or irradiance (J cm−2-s−1); λ is the wavelength of UV light (m); Na is Avogadro’s number (6.02 × 1023 mol−1); h is Planck’s constant (6.626 × 10–34 J-s); and c is the speed of light (3 × 108 m s−1).

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Suwannee River NOM

Appendix A1. Elemental compositions of Suwannee River NOM. SampleAquatic NOM Suwannee River

Cat. No.

H2 O

Ash

C

H

O

N

S

P

1R101N

8.15

7.0

52.47

4.19

42.69

1.10

0.65

0.02



H2 O content is the%(w/w) of H2 O in the air-equilibrated sample; Ash is the%(w/w) of inorganic residue in a dry sample; C, H, O, N, S, and P are the elemental composition in%(w/w) of a dry, ash-free sample.

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Appendix A2. 13C NMR estimates of carbon distribution in Suwannee River NOM. SampleAquatic NOM Suwannee River ∗

Cat. No.

Carbonyl 220–190 ppm

Carboxyl 190–165 ppm

Aromatic 165–110 ppm

Acetal 110–90 ppm

Heteroaliphatic 90–60 ppm

Aliphatic 60–0 ppm

1R101N

8

20

23

7

15

27

The values are the electronically integrated peak area percentages for selected ranges of chemical shift.

Appendix A3. Acidic functional groups of Suwannee River NOM. SampleAquatic NOM

Cat. No. Carboxyl Phenolic

Suwannee 1R101N River

9.85

3.94



Q1

Log K1

n1

Q2

Log Q2

n2

10.57

3.94

3.60

2.61

9.74

1.19

Number Root Meanof data Square points Error 112

0.0725

Q1 and Q2 are the maximum charge densities of the two classes of binding sites, Log K1 and Log K2 are the mean log K values for proton binding by the two classes of sites, and n1 and n2 are empirical parameters that control the width (in log K) of a class of proton binding sites. The fitting parameters  with  a modifiedHenderson–Hasselbalch equation:  were obtained QT OT = 1 + (KQ1[H]1/n1 + 1 + (KQ2[H]1/n2 1

2

Impacts of natural organic matter on perchlorate removal by an advanced reduction process.

Perchlorate can be destroyed by Advanced Reduction Processes (ARPs) that combine chemical reductants (e.g., sulfite) with activating methods (e.g., UV...
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