Letter Cite This: Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/estlcu
N‑Nitrosodimethylamine (NDMA) Degradation by the Ultraviolet/ Peroxodisulfate Process Inmaculada Velo-Gala,† María J. Farre,́ † Jelena Radjenovic,†,‡ and Wolfgang Gernjak*,†,‡ †
Catalan Institute for Water Research (ICRA), Emili Grahit 101, 17003 Girona, Spain Catalan Institute for Research and Advanced Studies (ICREA), Passeig Lluís Companys 23, 08010 Barcelona, Spain
‡
Environ. Sci. Technol. Lett. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 01/13/19. For personal use only.
S Supporting Information *
ABSTRACT: This study investigates the photodegradation kinetics of N-nitrosodimethylamine (NDMA) by the ultraviolet (UV)/S2O82− process initiated by a medium-pressure mercury lamp in a collimated beam setup. Experiments were carried out to characterize the kinetics in the absence and presence of bicarbonate, chloride, and sulfate. The kinetic behavior adjusted well to first-order dependence, and the addition of 33.6 μM S2O82− increased the NDMA removal rate by 3−4 times compared to direct photolysis leading to a reduction of 1 order of magnitude for every 8.2 × 103 J m−2 of fluence. In the range investigated, the reaction rate increased linearly with peroxodisulfate concentration. The presence of inorganic carbon and chloride can negatively affect the efficiency of both the UV/S2O82− process and the conventional UV/ H2O2 process, yet given that the reaction rate constants of Cl− and HCO3− with the •OH radical are 1 and 3 orders of magnitude higher, respectively, than with SO4•−, their scavenging effect is less pronounced in the UV/S2O82− process. In conclusion, the UV/S2O82− process efficiently removes NDMA from water even containing a low concentration of HCO3− and Cl− anions such as reverse osmosis permeates.
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INTRODUCTION N-Nitrosodimethylamine (NDMA) is a disinfection byproduct that is formed by the reaction of chloramines with secondary, tertiary, or quaternary amines.1 It is also the most commonly detected nitrosamine in drinking water2,3 and can be typically found in recycled water that previously had been exposed to chloramines.4 NDMA is classified as a Group 2A substance (probably carcinogenic to humans) by the World Health Organization (WHO)5 and as Category 1B (presumed to have carcinogenic potential for humans; largely based on animal evidence) by the European Union.6 The U.S. Environmental Protection Agency (USEPA) classified NDMA as a “B2 carcinogen-reasonably anticipated to be a human carcinogen”,7 and a 10−6 cancer risk level in drinking water at a concentration of 0.70 ng L−1 has been determined.8 NDMA is in Contaminant Candidate List 4 of the USEPA.9 Therefore, it is expected that NDMA will be the subject of additional regulatory action in the near future. The ability of different methods to eliminate NDMA from water has been investigated,10 for example, degradation by O3/ H2O211 or removal by reverse osmosis (RO) membranes.12 However, NDMA degradation by ultraviolet (UV) 254 nm irradiation remains the most effective method for NDMA removal13−17 but remains a costly post-treatment for RO permeate in water reuse. The NDMA UV absorption spectrum exhibits a strong band with a maximum at 228 nm and a broad weak band with a maximum at 332 nm (Figure S1). It was © XXXX American Chemical Society
confirmed that both low- and medium-pressure Hg lamps have virtually identical photonic efficiencies for the direct photolysis of NDMA (fluence-based rate constants of 2.29 × 10−4 and 2.35 × 10−4 m2 J−1, respectively).16 The addition of H2O2 as a source of reactive radicals during the UV process is one of the most commonly applied advanced oxidation processes (AOPs), yet addition of H2O2 changed the NDMA degradation efficiency only marginally as the generated hydroxyl radicals (•OH) are scavenged to a large extent by the H2O2 itself.16,18 The UV/S2O82− process is mainly based on the formation of sulfate radical (SO4•−), which could be even more reactive than •OH at circumneutral pH [E0(SO4•−) = 2.50−3.10 V, and E0(•OH) = 1.89 V].19−22 Most importantly, in the case of the UV/S2O82− process, the generated sulfate radicals react slower with the peroxodisulfate ion than •OH with H2O2. This study is the first one focused on the application of peroxodisulfate to improve the effectiveness of NDMA treatment by a UV-based process, in this case operated with a medium-pressure mercury (MP-UV) lamp. In particular, the study evaluates (i) the NDMA direct photodegradation kinetics by UV-MP radiation, (ii) the NDMA degradation Received: Revised: Accepted: Published: A
December 10, 2018 January 8, 2019 January 10, 2019 January 10, 2019 DOI: 10.1021/acs.estlett.8b00667 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
Letter
Environmental Science & Technology Letters kinetics by the UV/S2O82− process and its dependence on the peroxodisulfate concentration, and (iii) the behavior of this system in the presence of bicarbonate, chloride, and sulfate anions. This study also estimated the bimolecular reaction rate constants of NDMA with the sulfate, carbonate, and dichloride anion radicals.
germicidal factor applied for disinfection. Such a normalization can be carried out as there is only one electronic transition below 300 nm, which results in the quantum yield being independent of wavelength with a Φ value of 0.30 mol einstein−1.16 Experimental Procedures. All NDMA degradation experiments were conducted for 15 min under constant stirring at 21 °C. Under this condition, the oxygen concentration can be assumed to be near constant at the saturation level. HPLC grade water was used to prepare all solutions. The initial concentration of NDMA was 6.75 × 10−9 M (i.e., 500 ng/L). The pH of 6 was adjusted as this value is typically encountered in RO permeate in water reuse. The pH was not controlled but varied only negligibly considering the accuracy of benchtop pH measurement. As the commercial NDMA stock solution was obtained in methanol, 2.48 × 10−6 M methanol was introduced, a concentration similar to the dissolved carbon concentration that can be expected in RO permeate in water reuse. Measured NDMA concentrations were expressed as average values of duplicate experiments adding the mean deviation as the error range. Pseudo-firstorder linear fits were used to describe NDMA removal kinetics supplying the 95% confidence interval of the fitted rate constant using all data points of the duplicate experiments. Different concentrations of S2O82− (6.72 × 10−6, 20.2 × 10−6, and 33.6 × 10−6 M) derived from the addition of K2S2O8 were used to evaluate the effect of [S2O82−]0 in the degradation process of NDMA by UV/S2O82−, and the results compared to those of direct UV photolysis and blank experiments in the dark with 33.6 × 10−6 M S2O82− (Table S1, experiments 1−5). To investigate the influence of frequently present inorganic ions, NDMA degradation experiments were conducted with different concentrations of inorganic carbon (IC, mainly HCO3− and H2CO3), SO42−, and Cl− (Table S1, experiments 6−16). The pH of 6 was adjusted in these experiments with different chemical reagents to maintain the same experimental conditions, using HCl (Table S1, experiments 15 and 16) and HClO4 (Table S1, experiments 6−9). It should be noted that at pH 6, approximately 20% of IC is present as HCO3− in an equilibrium susceptible to changes in pH. Simulation of Reaction Kinetics. The Kintecus modeling software (Kintecus, Windows version 6.70)26 was used to evaluate the contribution of different reactions and radicals to NDMA degradation. Rate constants for sulfate, carbonate, and dichloride anion radicals were not available and were estimated using the experimental results of this study (Figures S3−S7). The reactions included in the kinetic model are shown in Table S2.
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MATERIALS AND METHODS Reagents. The following chemical reagents used were of high-purity analytic grade and supplied by Scharlab: potassium peroxodisulfate (CAS Registry No. 7727-21-1), hydrochloric acid, sodium bicarbonate, sodium sulfate, sodium chloride, sulfuric acid, perchloric acid, and kit LCK310 (HACH). NDMA (CAS Registry No. 62-75-9) (5000 μg mL−1 in methanol) had a purity of >99.9% and was obtained from Supelco. Deuterated d6-NDMA was supplied by Sigma-Aldrich. All solutions were prepared with high-performance liquid chromatography (HPLC) grade water supplied by Scharlab. N-Nitrosodimethylamine and Peroxodisulfate Determination. The procedure for NDMA analysis was modified from the headspace method proposed previously.23 GC-QqQ analysis was performed with a Trace GC Ultra gas chromatograph equipped with a TriPlus autosampler coupled to a TSQ Quantum triple-quadrupole mass spectrometer system (Thermo Fisher Scientific). Solid phase microextraction was applied in the headspace of the sample prior to injection in a gas chromatograph with triple-quadrupole analysis. Chromatographic separation was performed using a Trace GOLD TG5SILMS column from Thermo Fisher Scientific (30 m × 0.25 mm × 0.25 μm). d6-NDMA was used to correct for recovery, and the limit of quantification (LOQ) was 20 ng L−1, which allowed good observation of the degradation experiments, which were carried out with an initial concentration of 500 ng L−1. Details of the method can be found in text S1 of the Supporting Information and in ref 24. To measure peroxodisulfate, commercial kit LCK310 (HACH) was used along with adjustment of the diethyl-pphenylenediamine (DPD) method used to measure free chlorine. Samples were quantified on the basis of the absorbance at 530 nm after a 30 min incubation using the DR2800 spectrophotometer from HACH. UV Collimated Beam Apparatus, Evaluation of Fluence, and Fluence-Based Rate Constants. The quasi collimated beam UV apparatus (designed and constructed by Ecosystem Environmental System S.A.) was equipped with a 1 kW medium-pressure mercury (MP-UV) lamp. Its emission spectrum as recorded with a spectroradiometer (StellarNet Black Comet) is shown in Figure S1. This setup was used to generate a nearly parallel beam of light emitted in the range of 200−600 nm (Figure S1). Degradation experiments were carried out in an open crystallizing dish (diameter of 10 cm, depth of 2.6 cm, volume of 200 mL) placed approximately 79 cm from the lamp. The fluence rate was determined as previously described by Bolton et al.,25 whereby the spectroradiometer mentioned above was used to measure the photon flux at the water surface as a function of wavelength. Further details about the determination of the fluence can be found in text S2 of the Supporting Information. Fluence-based pseudo-first-order rate constants were calculated after normalizing the polychromatic fluence to an equivalent monochromatic fluence at 254 nm taking into account the molar absorption coefficient of NDMA as a function of wavelength in analogy to the application of the
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RESULTS AND DISCUSSION NDMA Degradation by the UV/S2O82− Process. The obtained rate of degradation by photolysis alone with this lamp was (2.56 ± 0.32) × 10−4 m2 J−1 (Table S1, experiment 1), which is similar to the values found in previous studies using low- and medium-pressure Hg lamps (2.30 × 10−4 m2 J−1).16 All of these values are significantly lower than the degradation rate of 10.80 × 10−4 m2 J−1 obtained by a 222 nm KrCl excimer UV lamp27 and a xenon fluorescence lamp (220−280 nm) (4.2 × 10−4 m2 J−1).27 It is noteworthy though that the latter two studies did not apply the normalization procedure that we used to make the results comparable to those of monochromatic illumination at 253.7 nm. For instance, the absorbance at 222 nm is 4.5 times higher than at 253.7 nm, so B
DOI: 10.1021/acs.estlett.8b00667 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
Letter
Environmental Science & Technology Letters Table 1. Rate Constants and Elementary Reactions of the UV/S2O82− Process reaction
rate constant
no.
NDMA + hv → product S2O82− + hv → 2SO4•− SO4•− + SO4•− → S2O82− SO4•− + S2O82− → SO42− + S2O8•− SO4•− + H2O → SO42− + H+ + •OH • OH + H2O2 → HO2• + H2O • OH + •OH → H2O2 SO4•− + •OH → HSO4− + 1/2O2 S2O82− + •OH → HSO4− + SO4•− + 1/2O2 NDMA + SO4•− → products NDMA + •OH → products NDMA + CO3•− → products NDMA + Cl2•− → products
Φ = 0.30 Φ = 0.70 k = 1.60−8.10 × 108 M−1 s−1 k = 1.50 × 103 to 1.20 × 106 M−1 s−1 k < 3 × 103 s−1 k = 2.70 × 107 M−1 s−1 k = 5.50 × 109 M−1 s−1 k = 0.10−1.00 × 1010 M−1 s−1 k = 0.10−1.20 × 107 M−1 s−1 k = 1.2 × 107 M−1 s−1 k = 3.80 × 108 M−1 s−1 k = 2.0 × 104 M−1 s−1 k = 3.0 × 105 M−1 s−1
1a16 1b29 230,31 330,31 432 519 619 730 833 9 (this study) 1034 11 (this study) 12 (this study)
the value of 10.8 × 10−4 m2 J−1 observed by Sakai et al. would be normalized to 2.4 × 10−4 m2 J−1 with our procedure, in line with the photolysis constants observed by us and Sharpless et al.16 To accelerate NDMA degradation, the oxidant S2O82− was added. Reactions with S2O82− are very slow, and consequently, the direct degradation of NDMA by this anion was negligible in the absence of light (Table S1, experiment 2). The MP-UV lamp was used to activate S2O82− to form SO4•− (Table 1, reaction 1).28 The •OH can be formed by reaction 4 in this system (Figure S8). Figure 1 shows the effect of S2O82− concentration on the photodegradation of NDMA. The addition of 33.6 μM S2O82−
The linear increase in degradation kinetics confirms that scavenging of the generated sulfate radical by the radical source itself (Table 1, reaction 3) is negligible in the operational range tested here. In the case of H2O2, self-scavenging of the hydroxyl radicals by hydrogen peroxide (Table 1, reaction 5) is a significant obstacle to increasing reaction rates via addition of H2O2.16 It is also noteworthy that recombination reactions of the hydroxyl radical are generally rapid reactions (Table 1, reactions 6 and 7). Via the application of kinetic modeling, the rate constant of NDMA and the sulfate radical (reaction 9) was estimated to be 1.2 × 107 M−1 s−1 via adjustment of the modeled to the experimental values of experiments 1 and 3−5 (Figure S3). This is very close to the rate constant estimated computationally recently (1.44 × 107 M−1 s−1) and confirms the robustness of our experimental and mathematical approach.35 Effect of Inorganic Water Matrix Constituents. The most obvious application for this process configuration is the treatment of RO permeates in the context of potable reuse scenarios. These effluents usually have a low concentration of inorganic ions with a level of total dissolved solids between 10 and 20 mg L−1.36 However, in view of application of the process to a wider range of source waters, the impact of other commonly present ions was studied [inorganic carbon (IC), Cl− and SO42−] at concentrations of ≤1 mM. As shown in Figure 2, IC and Cl− anions interfered in the NDMA degradation during the UV/S2O82− process, whereas there was no observed impact of SO42− addition (Table S1, experiments 6−14). The degradation rate constant in the presence of SO42− was kobs = (7.71 ± 0.91) × 10−4 m2 J−1, which is similar to the value obtained in HPLC grade water [(9.30 ± 1.92) × 10−4 m2 J−1] (Table S1) considering the confidence intervals of these measurements. NDMA degradation by sulfate radicals under MP-UV radiation was inhibited by the scavenging effect of bicarbonate and chloride anions (Table S2, reactions 21−33). Figure 2 and Figure S12 show a stronger inhibiting effect of bicarbonate than of chloride. Scavenging of sulfate radicals by carbonate species (Table S2, reactions 21 and 22) dominates the fate of sulfate radicals as their concentration is 6 and 2 orders of magnitude higher than that of NDMA and that of methanol, respectively. The conversion of chloride to chlorine radicals through oxidation by sulfate radicals and their subsequent rapid transformation to dichloride anion radicals (Table S2, reactions 33 and 3537) provide a similar sink for sulfate radicals in the presence of chloride. The products of these reactions,
Figure 1. Removal of NDMA as a function of S2O82− concentration. [S2O82−]0 values of 0 μM (purple), 6.72 μM (blue), 20.2 μM (red), and 33.6 μM (green). [NDMA]0 = 6.75 × 10−9 M. pH 6. Incident photon irradiance I = 4.16 W m−2. Lines represent first-order fits, and the error bars on data points are the mean deviations of duplicate experiments.
increased the NDMA removal rate by approximately 3−4 times compared to that of direct photolysis without added peroxodisulfate. The consumption of S2O82− was
