VUV: The roles of additives, mechanisms, influencing factors, and reaction products

Water Research 161 (2019) 89e97

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Removal of urea from swimming pool water by UV/VUV: The roles of additives, mechanisms, influencing factors, and reaction products Liangchen Long a, Yinan Bu a, Baiyang Chen a, *, Rehan Sadiq b a b

Shenzhen Key Laboratory of Organic Pollution Prevention and Control of Harbin Institute of Technology, Shenzhen, 518055, China School of Engineering, University of British Columbia Okanagan Campus, Kelowna, BC, V1V1V7, Canada

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 February 2019 Received in revised form 5 May 2019 Accepted 29 May 2019 Available online 1 June 2019

To discover an applicable technology for urea abatement from swimming pool water (SPW), this study compared the performances of seven ultraviolet (UV)-based technologies on urea removal, including UV alone, UV coupled with hydrogen peroxide (UV/H2O2), sulfite (UV/Na2SO3), potassium persulfate (UV/ K2S2O8), a combination of UV and vacuum UV (UV/VUV), and UV/VUV in tandem with either H2O2 (VUV/ H2O2) or potassium persulfate (VUV/K2S2O8). Among them, UV and UV/Na2SO3 showed little removal ability, and UV/H2O2 removed only 12.8% of urea within 3-h experiments, while UV/VUV degraded 71.7% of urea without introducing substantial total dissolved solids (TDS). Therefore, UV/VUV was considered as a promising technology for further exploration. In comparison, although UV/K2S2O8 exhibited higher urea removal than UV/VUV, it caused dramatic increases of TDS, which made the regulatory threshold for the TDS increment difficult to maintain. Within UV/VUV studies, some common components in SPW (e.g., cyanuric acid, humic acid, nitrate, and bicarbonate) inhibited the removal process, whereas chloride and sulfate facilitated it, while free chlorine at doses
3 mg-Cl2/L and pH levels from 6.8 to 8.0 imposed little impact on urea degradation. Overall, UV/VUV degraded 40.0% and 22.2% of urea from tap water and SPW, respectively; both were lower than the efficiency observed in ultrapure water. As for reaction byproducts, urea phototransformation via UV/VUV yielded nitrate and ammonia as the key products with the mass balance of nitrogen element being met. However, the contents of organic carbon decreased at a rate slightly lower than urea degradation, suggesting that urea was mostly mineralized and slightly converted to unknown organic compounds. The results hence demonstrate that UV/VUV is an effective alternative for urea removal from SPW. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Urea Vacuum ultraviolet Swimming pool water Photolysis 185 nm

1. Introduction Swimming is a popular healthy activity globally. To protect swimmers from pathogens, swimming pool managers usually apply certain chlorine-based disinfectants (e.g., sodium hypochlorite, calcium hypochlorite, dichloroisocyanurate, and trichloroisocyanuric acid (C3O3N3Cl3) to disinfect swimming pool water (SPW) (WHO, 2006). However, a common problem associated with the use of chlorine-based disinfectants is that they inevitably react with dissolved organic matter (DOM) and form undesirable byproducts such as disinfection by-products (DBPs) (Yang et al., 2018a). Among DBPs, trichloramine has a strong odor that people can easily inhale (Catto et al., 2012), causing respiratory

* Corresponding author. E-mail address: [email protected] (B. Chen). https://doi.org/10.1016/j.watres.2019.05.098 0043-1354/© 2019 Elsevier Ltd. All rights reserved.

symptoms or antipathy (Seys et al., 2015). According to a literature, urea (H2NCONH2, namely carbamide or carbonyl diamide) is likely to yield more trichloramine than many other nitrogenous compounds such as formamide, glycine, histidine, creatinine, and ammonium (Schmalz et al., 2011). Therefore, urea is an important trichloramine precursor. In addition, as a product of protein metabolism, urea is abundantly present in human body fluids and is refractory to biodegradation. It was reported that urea accounts for an average of 84% and 68% of nitrogenous matter in human urine and sweat, respectively (WHO, 2006). The concentrations of urea in SPW ranged widely in many studies, from 0.07 to 18.73 mg/L (Zhang et al., 2018), 0.01e4.02 mg/L (Blatchley and Cheng, 2010), 0.12e3.60 mg/L (De Laat et al., 2011), and 0.69e2.11 mg/L (Schmalz et al., 2011). To alleviate the risks posed by urea, the Chinese government has currently established a maximum contaminant limit (MCL) for urea in SPW of 3.5 mg/L (CJ/T 244e2016). Hence, motives exist for exploring robust and applicable technologies to remove

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urea from SPW. Before preventive measures can be taken, the sources of urea must be understood. Although urine and sweat are two major sources of urea in SPW (Yang et al., 2018a), urea-containing moisturizer might serve as another source (Doi et al., 2009). If we assume the average concentrations of urea as 10,240 mg/L and 680 mg/L in urine and sweat, respectively (WHO, 2006), and each swimmer releases 25e30 mL (WHO, 2006) and 150e400 mL at 29  C (Keuten et al., 2014) of urine and sweat into SPW, respectively, each swimmer would emit 256e307 mg urea via urine and 102e272 mg urea via sweat. Because sweating is unavoidable in swimming activity, eliminating urination and having shower prior to swimming still cannot completely avoid urea occurrence in SPW. Therefore, pool managers must take actions to prevent urea from exceeding the MCL, either by diluting SPW with pristine tap water (TAP) or by reducing urea from SPW with professional on-site treatment facilities. Currently, several technologies have been assessed for urea removal. For example, although urea was once converted into ammonia and carbon dioxide via pyrolysis at high-temperature (>120  C) and high-pressure conditions (Mahalik et al., 2010; Sahu et al., 2010), pyrolysis is inappropriate for treating a large volume of water because of its high cost. Chlorine oxidation degraded only 10%, 20%, and 50% of urea with 1.8, 3.6, and 20.6 mg/L chlorine, respectively, within 24 h at an initial urea concentration ([Urea]0) of 3 mg/L, a pH of 7.3, and a temperature of 25.0  C (De Laat et al., 2011), indicating low efficiency of urea treatment. In addition, unwanted inorganic chloramines formed during the reaction between chlorine and urea (Blatchley and Cheng, 2010; Schmalz et al., 2011). Similarly, although electrochemical oxidation might remove urea rapidly, it requires a large amount of salt as the
ndez et al., 2014; electrolyte (Carlesi Jara et al., 2008, Cataldo Herna Cho and Hoffmann, 2014), which can dramatically increase the quantity of total dissolved solids (TDS), a term also regulated by governments (i.e., ANSI/APSP-11 2009 in USA: TDS increment
1500 mg/L; CJ/T 244e2016 in China: TDS increment
1000 mg/L). Regarding biodegradation, earlier studies mostly targeted to removal of urea from municipal wastewater (Gupta and Sharma, 1996). Because SPW has markedly different water qualities from wastewater, especially the presence of residual disinfectant, the results of urea control in wastewater cannot be readily translated to SPW. Therefore, pyrolysis, chlorine oxidation, electrochemical oxidation, and biodegradation all appear impracticable or inefficient for SPW treatment. Recently, some studies have demonstrated the potential of ultraviolet (UV) and UV-derived technologies in disinfecting and degrading DBPs in water (Hansen et al., 2013; WHO, 2006; Zare Afifi and Blatchley, 2016), suggesting that UV technology is a robust tool for micropollutant control. However, UV generated by mediumand low-pressure mercury lamps could not degrade urea rapidly (Zare Afifi and Blatchley, 2016), suggesting UV alone is not costeffective in treating urea. Furthermore, UV emitted by a 1200-W mercury vapor lamp combined with hydrogen peroxide (UV/ H2O2) or persulfate (UV/S2O2 8 ) transformed 75% or 97%, respectively, of urea in distilled water within 24 h (Bronk et al., 2000), thereby showing the potential of a combined use of UV and additives. However, these results were not verified in real SPW. Meanwhile, vacuum UV (VUV) irradiation is more robust than UV in degrading organic compounds (Kim and Tanaka, 2009). A VUV lamp with 3% energy emitting a 185-nm light can convert 95e100% of urea ([Urea]0 ¼ 50 mg/L) into nitrate (NO 3 ) when VUV was combined with potassium persulfate (K2S2O8, 4 g/L) (Roig et al., 1999). Hence, a comparison of UV, VUV, and many other UV or VUV-based technologies (e.g., VUV/H2O2, VUV/S2O2 8 ) may merit further attention and in-depth exploration.

Hence, this study carried out a series of experiments to understand the potentials of different UV and VUV-based technologies for urea mitigation under typical SPW conditions. These results are intended to 1) compare the efficiencies of UV alone, UV/H2O2, UV/ K2S2O8, UV coupled with sodium sulfite (UV/Na2SO3), VUV, VUV/ H2O2, and VUV/K2S2O8 in removing urea from ultrapure water (UPW); 2) evaluate the impacts of typical and common water  components, such as pH, chloride (Cl), NO 3 , bicarbonate (HCO3 ), 2 sulfate (SO4 ), and DOM, on urea degradation performances by VUV; and 3) identify to the maximum extent possible reaction mechanisms, pathways, and products of the VUV irradiation process. In order to differentiate different lamps and to make the descriptions concise, we herein define the experiments using the lamp with both 254 and 185 nm lights as “VUV” while using the lamp with 254 nm light only as “UV” in this study. 2. Materials and methods 2.1. Samples and chemicals All chemicals and reagents used herein are of analytical grade and were purchased from Sigma-Aldrich and Aladdin, Inc. The stock solutions of urea, H2O2, and K2S2O8 were prepared with UPW generated by a Millipore water generator (Direct-Q3, Millipore, France). The real SPW samples were collected from a large public indoor swimming pool nearby the laboratory (~1.5 km) between 3 and 4 p.m., and the pool was disinfected with a typical disinfectant C3O3N3Cl3. The pH, TDS, and free chlorine of real samples were analyzed immediately after delivery to the laboratory. The concentrations of urea, non-purgeable organic carbon (NPOC), Cl, 2 NO 3 , and SO4 were measured within 24 h after sampling. The concentrations of urea ranged from 1.50 to 3.86 mg/L in collected SPW. To allow experimental consistency and comparison among tests, the [Urea]0 of all samples was adjusted to 5 mg/L. That is, additional urea was added to SPW to achieve an initial urea concentration of 5 mg/L for all tests. 2.2. Analytical methods Urea was determined colorimetrically using diacetyl monoxime according to a Chinese standard analytical method (GB/T 18204.2, China) with a method detection limit (MDL) of 0.1 mg/L. The method is based on the principle that a colored compound with a characteristic wavelength of 460 nm was formed after reaction of urea with diacetyl monoxime and antipyrine at 100  C in the presence of sulfuric and acetic acids. The absorbance of the colored solution was measured with a spectrophotometer (TA-98, Shenzhen Sinsche Technology Co., Ltd., China). Because residual oxidants, such as H2O2 and K2S2O8, can significantly interfere with the accuracy of urea measurement (Fig. S1), Na2SO3 was spiked into target solutions after UV or VUV irradiation to remove remaining oxidants, then sulfuric acid was added to the samples to reduce pH to < 1.5 while the samples were bubbled with air to eliminate excessive Na2SO3 before urea determination. Via these efforts, the accuracies of the urea measurement for samples with
5 mM of H2O2 were improved to > 91% of samples without H2O2 (Fig. S2a), meaning that the influences of H2O2 on the urea measurement were eliminated. However, K2S2O8 is unreactive with Na2SO3 at room temperature; therefore, the interference of K2S2O8 on urea detection appeared non-eliminable (Fig. S2b). In this context, accurate urea detection can be achieved only with a K2S2O8 concentration of
0.1 mM. This means that either the initial dosage of K2S2O8 should be
0.1 mM or it should be largely consumed by UV irradiation prior to urea measurement.

L. Long et al. / Water Research 161 (2019) 89e97

In this context, this study evaluated only the impacts of K2S2O8 with a concentration of
0.1 mM on urea degradation (Fig. S1b).  2 Typical anions in water such as Cl, nitrite (NO 2 ), NO3 , and SO4 were analyzed by an ion chromatographer (CIC-DI120, Qingdao Shenghan Chromatography Technology Co., Ltd., China) equipped with an anion exchange column (Shenghan SH-AC-5, 250 mm  4.6 mm) and a conductivity detector. The mobile phase was made of 20 mM potassium hydroxide (KOH) and run at a flow rate of 0.8 mL/min. The MDLs of these anions were
0.01 mg/L. H2O2 and formic acid were measured by another ion chromatographer (IC-2010, Tosoh Inc., Japan) equipped with not only a conductivity detector but also a UV detector (SPD 20A, Shimadzu, Japan), which gives MDLs of them as 0.01 mg/L. Its mobile eluent was made of 20 mM KOH solution and ran at a flow rate of 0.5 mL/ min according to an established method (Song et al., 2017). NHþ 4 and free chlorines were detected by a spectrophotometer using Nessler's reagent colorimetric method (No. 8038) and DPD colorimetric method (No. 8021) as provided by Hach Co., USA, with their MDLs as 0.02 mg-N/L and 0.02 mg-Cl2/L. TDS of water was determined by an electrical conductivity meter (SX 713e02, Shanghai San-Xin Instrumentation Inc., China) with a MDL of 0.1 mg/L. Formaldehyde was detected by a gas chromatographer with an electron capture detector (GC-9720, Zhejiang Fuli Analytical Instrument Co. Ltd., China) according to EPA method 556 with a MDL of 1 mg/L. NPOC and total organic carbon (TOC) in water were determined by a TOC analyzer (TOC-LCPH, Shimadzu, Japan) with a MDL of 0.1 mg-C/L. The amount of TOC was calculated by subtracting inorganic carbon from total carbon, while NPOC was measured directly after a sample acidic stripping process. The pH was determined by a pH monitor (PHS-3E, Shanghai INESA Scientific Instrument Co., Ltd., China). For quality assurance and control, the allowable relative standard deviations (RSDs) of measured parameters were set as < 10%. In case it was exceeded, more tests were carried out.

2.3. Reactor and experimental conditions The experiments were carried out in two parallel, stainless, column-shaped sterilizers with a water thickness of 1.4 cm, and a volume of 400 mL (Fig. S3). For VUV tests, a low-pressure mercury lamp emitting both 254-nm and 185-nm lights (12W, GPH265T5VH, UV-Tec Co., Ltd., China) was placed in the middle of the reactor, and the lamp was shielded by a quartz sleeve. The irradiation intensity of VUV 185 nm was estimated to be 0.32 ± 0.1 mW cm2 according to an actinometrical method (Yang et al., 2018b). For UV tests, a low-pressure mercury lamp with identical power inputs (12W) as VUV but emitting UV at 254 nm only (GPH265T5L, UV-Tec Co., Ltd., China) was used. Using H2O2 as the actinometer, the irradiation intensity of the UV254 lamp was estimated to be 14.3 ± 0.6 mW cm2. Aliquots were retrieved at designated time intervals. Blank control and duplicate experiments were conducted in all tests. During experiments, the initial dosages of H2O2 and Na2SO3 were both 1 mM while that of K2S2O8 was 0.1 mM. In addition, as shown in Fig. S4b, given that H2O2 can be rapidly consumed by UV and VUV, an additional 1 mM of H2O2 was added every h to supplement its loss. Regarding Na2SO3, to prevent oxygen (O2) from disturbing UV/Na2SO3 performance, the concentration of dissolved oxygen (DO) in water was adjusted to
1.7 mg/L by bubbling nitrogen (N2) into the water before the experiment, and then DO was maintained during experiments by blowing N2 on the water surface.

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3. Results and discussions 3.1. Comparison of methods of urea removal Fig. 1 demonstrates the removals of urea in UPW by seven UVbased methods. In general, the orders of their efficiency were ranked as VUV/K2S2O8 > UV/K2S2O8 > VUV > VUV/H2O2 > UV/ H2O2 > UV z UV/Na2SO3 under the experimental conditions of this study. Specifically, UV alone removed little urea (~0.7%) within 3 h, meaning that direct photolysis with 254 nm can hardly degrade urea, which is consistent with the results reported in an earlier study (Zare Afifi and Blatchley, 2016). Similarly, UV/Na2SO3 as a typical advanced reduction process reduced urea by < 8.0% within 3 h, meaning that urea was not susceptible to reductive radicals such as hydrogen atom (H·), hydrated electron (e aq), or sulfite radical (SO3·-) formed in UV/Na2SO3 (Eq. (1) and Eq. (2)), although they degraded other compounds well (Vellanki et al., 2013). In literature, the rate constant of reaction between urea and H· was 3.0  104 M1s1 (Schuler et al., 1971), smaller than that between 5 1 1 urea and e aq (3.0  10 M s ) (Hart et al., 1967). UV/H2O2 decreased 12.8% of urea within the same time, suggesting that hydroxyl radical (HO·) generated by UV/H2O2 (Eq. (5)) was weak in removing urea too. In literature, the rate constant of reaction between urea and HO· was 7.9  105 M1s1 (Minemura et al., 1980), slightly greater than those reductive radicals. Previously, another UV/H2O2 study reported much higher urea removal (75%) than this study, probably because it had a much higher power input (1200W), a lower water volume photolyzed (20 mL), a longer irradiation time (24 h), and a higher dosage of H2O2 (6.5 mM) (Bronk et al., 2000) than this study (which featured 12W, 400 mL, 3 h, and 3.0 mM at most). In contrast, UV/K2S2O8 exhibited more rapid and higher urea removal (67.8% within 1 h) than other UVbased methods, although K2S2O8 was dosed only once at 0.1 mM level in the beginning. This phenomenon might partially be attributed to the strong oxidation ability of sulfate radical (SO4·-) as generated by UV/K2S2O8 (Eq. (7)) (Moussavi et al., 2014) and partially to the selective susceptibility of urea. Once K2S2O8 was used up (i.e., after 1 h), no further urea degradation was observed, further indicating that SO4·- played the key role in urea degradation. This finding tends to support another study that SO4·- (E0SO4·-/ 0 SO42 ¼ 2.5e3.1 V) was stronger than HO· (E HO·/H2O ¼ 1.9e2.7 V) in transforming compounds with an amide group (-CONH2) (Li et al., 2017).

Fig. 1. Comparison of seven UV-based methods on the removal of urea from water ([Urea]0 ¼ 5.0 mg/L, [K2S2O8]0 ¼ 0.1 mM, [H2O2]0 ¼ 1.0 mM added every h, and [Na2SO3]0 ¼ 1.0 mM).

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L. Long et al. / Water Research 161 (2019) 89e97

UV  , SO2 3 ƒƒ!eaq þSO3



F

e aq



¼ 0:116

(1)

UV

, HSO 3 ƒƒ!H$þSO3

(2)

H2 NCONH2 þ H$/Products

k1 < 3  104 M1 s1

(3)

H2 NCONH2 þ e aq /Products

k2 ¼ 3  105 M1 s1

(4)

UV

H2 O2 ƒƒ!2HO $

FðHO$Þ ¼ 1:0

(5)

of urea were
3% in a 43  C water bather in darkness within 3 h (Fig. S5b). Thus, thermal hydrolysis was excluded as a major contributor to urea removal. Similarly, H2O2 or K2S2O8 alone hardly removed any urea under 43  C in the dark (Fig. S5b), proving that chemical oxidations of urea by H2O2 or K2S2O8 were also negligible. In terms of reaction mechanisms, the homolysis (Eq. (11)) and ionization of water (Eq. (12)) are known VUV photolysis process (Gonzalez et al., 2004) resulting in formation of a list of radicals such as HO·, H·, and e aq. Based upon the abovementioned results, the photodegradation of urea by VUV-based methods seems more likely to undergo two processes: direct 185-nm photolysis and indirect oxidation by oxidative radicals like HO· and SO4·-. VUV

H2 NCONH2 þ HO$/Products

5

1 1

k3 ¼ 7:9  10 M

s

H2 O / HO$ þ H$

FðHO$Þ ¼ 0:33

(6) VUV

þ H2 Oƒƒƒ!e aq þ HO$þH

UV

$ S2 O2 8 / 2SO4



(12)

(7)

Compared to the UV-based processes, VUV and VUV-based methods appeared to be more robust and efficient. The removal of urea in 3 h by VUV reached 71.7%, much higher than UV. The dramatic difference in urea removal between UV and VUV indicates that the removal of urea was mostly attributed to 185-nm light. Meanwhile, this finding confirms that compounds with amide group, such as carbamazepine and tetracycline, were more susceptible to VUV (Zoschke et al., 2014). Additionally, spiking K2S2O8 at the 0.1-mM level promoted the removal of urea (95.8%), suggesting that both SO4·- and 185-nm light had contributed to urea degradation. However, the removal of urea by VUV/H2O2 (51.2%) was surprisingly lower than that by VUV. The addition of H2O2 at this level (1 mM each h for 3 h) was somehow detrimental to urea photodegradation. The reason of this adverse effect is likely attributed to light screening or radical competition or both. Since H2O2 is not only a HO· precursor but also a HO· scavenger with very rapid reaction rate constant (2.7  107 M1s1, Eq. (8)) (Buxton et al., 1988). As shown in Eq. (9) and Eq. (10), the rate of reaction between HO· and H2O2 (2.7  104  [HO·] s1) is greater than rate of reaction between urea and HO· (66  [HO·] s1), the inhibition of urea degradation by H2O2 was most likely due to scavenging of HO·. Based on the water molar absorption coefficient at 185 nm (εH2O, 185 1 1 nm ¼ 1.0 or 1.8 M cm ) (Duca et al., 2017; Weeks et al., 1963) and the concentration of water (55 M), the absorbance of water was calculated to be 55 or 99 cm1, while the absorbance of H2O2 (εH2O2, 1 1 185 nm ¼ 293e341 M cm ) (Duca et al., 2017; Weeks et al., 1963) at 1.0 mM was only 0.294e0.341 cm1 at 185 nm. Therefore, most of 185-nm photons were absorbed by water and addition of H2O2 was less likely to compete for 185-nm light.

H2 O2 þ HO$/H2 O þ HO2 $



F e aq ¼ 0:045

(11)

k4 ¼ 2:7  107 M1 s1

 rateH2 O2 þHO$ ¼ k4 ½H2 O2 ½HO$ ¼ 2:7  107  0:001  ½HO$ ¼ 2:7  104  ½HO$ s1 rateureaþHO$ ¼ k3 ½urea½HO$ ¼ 7:9  105  8:3  105  ½HO$ ¼ 66  ½HO$ s1

(8)

(9)

(10)

To distinguish the potential effects of other mechanisms on urea removal, we monitored the temperature changes along the photolysis process and evaluated the contributions of thermal hydrolysis and chemical oxidation on urea removal at an elevated temperature. As shown in Fig. S5a, the water temperature was increased to ~43  C, then maintained constant afterward for 3 h during the UV and VUV irradiation processes. However, the losses

3.1.1. Comparison of methods on TDS increments In addition to urea removal, the decision whether to use these methods should consider potential side-effects. Fig. S6 shows that the TDS increments derived from UV/K2S2O8 (90.0 mg/L) and VUV/ K2S2O8 (104.5 mg/L) were far greater than those of other advanced oxidation process (AOP) methods (
26.2 mg/L), meaning that they cannot be used more than 10 or 15 times in SPW treatments to meet the regulatory thresholds in China or USA (CJ/T 244e2016 in China: TDS increment
1000 mg/L; ANSI/APSP-11 2009 in USA: TDS increment
1500 mg/L); therefore, they should be applied with caution. Although the TDS increments from UV and UV/H2O2 were minor, they hardly degraded any urea, making them inappropriate for use. In comparison, the TDS increments introduced by VUV and VUV/H2O2 fell into a lower range (< 26.2 mg/L); therefore, they showed better applicability not only in alleviating urea but also in causing low TDS increment. In this context, VUV was regarded as a preferred and promising technology for urea control in SPW, in addition to its capacity in disinfection and photodegradation of micropollutants (Zoschke et al., 2014). 3.2. Influencing factors 3.2.1. Effects of typical DOM In real SPW, C3O3N3Cl3 is often used as a disinfectant, and its transformation product, cyanuric acid (C3H3O3N3), usually acts as a stabilizer of free chlorine. Their reversible reactions are presented below (Eq. (13)): C3H3O3N3 combines with free chlorine to form chlorinated isocyanurates (C3H(3-x)O3N3Clx, x ¼ 1,2,3) when free chlorine concentration is high, while C3H(3-x)O3N3Clx can be transformed to free chlorine and C3H3O3N3 when free chlorine concentration is low. According to the World Health Organization (WHO) guideline, the level of C3H3O3N3 SPW should be kept between 50 and 100 mg/L, and the MCL of cyanuric acid in China is set as 30 mg/L for indoor swimming pools and 100 mg/L for outdoor swimming pools (CJ/T 244e2016). In addition, according to a WHO survey, the cyanuric acid levels were sometimes greater than 100 mg/L (WHO, 2006). In addition to C3H3O3N3, DOM in TAP is another source of organic matter in SPW. Therefore, we evaluated C3H3O3N3 and a selected humic acid as representative DOMs to understand their impacts on VUV photolysis.

C3 H3 O3 N3 þ xHClO#C3 Hð3xÞ O3 N3 Clx þ xH2 O ðx ¼ 1; 2; 3Þ (13) Fig. 2 shows that both C3H3O3N3 and humic acid restrained the

L. Long et al. / Water Research 161 (2019) 89e97

93

20.8 mg/L for SO2 4 in SPW. Therefore, we examined their effects at a range of concentrations. Fig. 3 demonstrates that increasing Cl and SO2 4 promoted the photodegradation of urea, which was unexpected because they are both low in 185-nm light absorptivity (Table 1) (Weeks et al., 1963). To confirm these phenomena, we spiked Cl intermittently into the irradiated sample at 30 min (3 mg/L) and 60 min (7 mg/L) and compared the impacts with a sample without Cl dosing and another sample dosed with 10 mg/L Cl only once at the beginning. Immediately after spiking time, we clearly observed enhancements of urea degradation (Fig. S9a), thereby confirming the facilitating role of Cl. In theory, Cl might react with HO· to form chlorine radical (Cl·) within the UV/H2O2 process (Liao et al., 2001; Yu and Barker, 2003). Therefore, the enhanced degradation of urea with increasing Cl was likely due to the formation of Cl· by VUV-induced HO·. Similarly, we spiked SO2 4 intermittently into irradiated sample at 30 min (5 mg/L) and 60 min (15 mg/L) and observed similar enhancements of urea degradation (Fig. S9b). An earlier study found SO4·- formation upon the reaction of SO2 4 with HO· (De Laat and Le, 2005). Therefore, the promotion of urea removal by the addition of SO2 4 was likely attributed to HO·-induced SO4·-. Notably, since 185-nm light has a very high energy, we cannot exclude the possibility that Cl· and SO4·- might also be directly formed from the VUV irradiation of Cl and SO2 4 .

HO$ þ Cl #ClOH$

k5 ¼ ð3  4:3Þ  109 M1 s1

ClOH$ þ Hþ #Cl$ þ H2 O

(14)

 k6 ¼ 2:6  3:2Þ  1010 M1 s1 (15)



Fig. 2. Effects of typical organic matter including a) humic acid and b) cyanuric acid (a SPW stabilizer) on urea degradation by VUV ([Urea]0 ¼ 5.0 mg/L).

photodegradation rates of urea by VUV. The inhibitory effect of C3H3O3N3 increased with concentration increases from 25 to 200 mg/L. Because the rate constant of reaction between cyanuric acid and HO· (k ¼ 3  107 M1s1) (De Laat et al., 1994) was higher than that of urea with HO· (k ¼ 7.9  105 M1s1), and because C3H3O3N3 level was higher than urea (5 mg/L), C3H3O3N3 essentially acted as a scavenger of HO· and inhibited urea degradation. In contrast, low levels of humic acid (< 5.0 mg/L) showed little impact on urea degradation unless the contents of the humic acid became  5.0 mg/L. Previously, the impact of humic acid on photolysis appears mixing: sometimes DOM enhanced the photolysis effect via DOM sensitization and radical formation (Lam et al., 2003; Lester et al., 2013), but sometimes inhibited photodegradation due to its competition for reactive species (Ngouyap Mouamfon et al., 2011). This study seemed acknowledge the role of competition instead of facilitation from humic substance. In literature, the rate constants for reactions between DOM isolates and HO· ranged at 1.39e2.18  108 M1s1 (Westerhoff et al., 2007), much greater than that between urea and HO·. In addition, NPOC decreased with elevated dosage of humic acid (Fig. S7), supporting that DOM has been degraded in the reaction. 3.2.2. Effects of typical anions 2  Cl, NO 3 , SO4 , and HCO3 are common anions in TAP and SPW, and based upon a preliminary survey, Fig. S8 shows their mean concentrations as 8.6 mg/L for Cl, 6.6 mg/L for NO 3 , and 7.2 mg/L for SO2 in TAP and 56.7 mg/L for Cl, 17.7 mg/L for NO 4 3 , and

SO2 4 ,

NO 3

HCO 3

In contrast to Cl and and clearly inhibited the photodegradation of urea. From the literature, the molar absorption  coefficients of NO 3 and HCO3 at 185 nm were 5568 and 1 1 269 M cm , respectively, which are greater than that of water (1 or 1.8 M1cm1) (Duca et al., 2017; Weeks et al., 1963). Although NO 3 can sometimes generate HO· during photolytical process (Tugaoen et al., 2017), its quantum yields of HO· (<0.09 at l  254 nm) (Mack and Bolton, 1999) were usually too low to be important; therefore, its prominent effect was to compete for 185nm light with urea and resulted in lower degradation of urea. As for HCO 3 , although it is not a strong 185-nm light absorber, it is a known quencher of HO· (k ¼ 8.5  106 M1s1) (Buxton et al., 1988), so it was also reasonable to obtain a decreased urea removal with elevated HCO 3. 3.2.3. Effects of chlorine, pH, DO, and [Urea]0 According to SPW rules, free chlorine must be kept between 0.2 and 1.0 mg/L (WHO, 2006). Fig. 4a indicates that the urea removals were similar for samples dosed with free chlorine from 0.2 to 3.0 mg/L. The results were unexpected because UV-initiated chlorine can yield ClO· radicals (Li et al., 2017), a reactant likely helpful for urea degradation. The urea removal efficiency discrepancy between the ClO· generated by Cl2 and the Cl· generated by Cl may partially be explained by their inner reactivities, the concentration difference between Cl ( 10 mg/L) and chlorine (
3 mg/L), which might not yield sufficient radicals. According to SPW rules, pH must be maintained within a range of 7.2 and 7.8 (WHO, 2006). Regarding its influence, increasing the pH value from 6.8 to 8.5 did not markedly alter the degradation rates of urea (Fig. 4b). Although hydroxide (OH) can scavenge HO· (k ¼ 1.3  1010 M1s1) rapidly (Rabani and Zehavi, 1971), The concentration of OH at neutral pH was too low to play an important role on the light competition and radical scavenging effects. As shown in Fig. 4c, with the rising [Urea]0 from 1 mg/L to

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L. Long et al. / Water Research 161 (2019) 89e97

Fig. 3. Effects of the anions a) chloride, b) nitrate, 3) sulfate, and 4) bicarbonate on urea degradation by VUV ([Urea]0 ¼ 5.0 mg/L).

Table 1 The molar absorption coefficients of compounds at 185 nm (ε185) and their reaction rate constants with hydroxyl radical (kHO·). Compounds

ε185 (M1cm1)

Reference

Urea Water

NA 1 1.8 293 341 269 5,568 240 5 NA

Duca et al. (2017) Weeks et al. (1963) Weeks et al. (1963) Duca et al. (2017) Duca et al. (2017) Duca et al. (2017) Weeks et al. (1963) Weeks et al. (1963)

H2O2 HCO 3 NO 3 2SO4 Cl C3H3O3N3

kHO$ M1s1

Reference

7.9  105 NA

Minemura et al. (1980)

2.7  107

Buxton et al. (1988)

8.5  106 NA 3.5  105 4.3  109 3.0  107

Buxton et al. (1988) De Laat and Le (2005) Yu and Barker (2003) De Laat et al. (1994)

NA: not available from literature.

10 mg/L, the rate constants of urea degradation were decreased from 9.95  103 min1 to 4.53  103 min1 but the absolute quantity of urea removed increased from 0.8 mg/L to 5.4 mg/L in 3h test. This result suggests that a good practice of this technology is perhaps to turn on VUV treatment facility when the urea concentration approaches the MCL of urea (e.g., 3.0 mg/L), and then turn off it when the urea level was low (e.g., 2.0 mg/L). In an early exemplary study, the degradation of cyanide by VUV was promoted by blowing air into the solution (Moussavi et al., 2016), indicating that DO may serve an important role in the photolysis process. Therefore, the effect of DO on urea photolysis was examined by blowing N2 or O2 in water. Fig. 4d shows that the urea removal was remarkably decreased at a DO of 1.6 ± 0.2 mg/L but remained unchanged for samples with a DO of 5 or 7 mg/L. This positive effect is likely due to the formation of peroxyl radicals, which has a very important role in the further transformation and mineralization (Gonzalez et al., 2004). In addition, the reason is also likely attributed to the presence of e aq, which can consume HO·

rapidly (k ¼ 3.0  1010 M1s1) in the absence of DO but can be converted to superoxide anion radical (O2·-, E0 ¼ 0.33 eV) at a rate of 2.2  1010 M1s1 in the presence of DO, which has lower reactivity and kinetics (k ¼ 8.0  109) than e aq in reacting with HO· (Buxton et al., 1988; Gonzalez et al., 2004).

3.2.4. Effects of real water matrix Fig. 5 presents the effects of real water matrix on the removals of urea in real SPW and tap samples. The urea removals by VUV were apparently lower in SPW and TAP than in UPW. It might be that the enhancement effects derived from Cl and SO2 4 were lower than  the inhibition effects derived from NO 3 , HCO3 , NOM in TAP. Compared to TAP, SPW showed an even larger inhibitory effect, because SPW used TAP as the water source, and some substances intentionally added to SPW (e.g., C3H3O3N3) and accidently brought in by swimmers (e.g., sweat) might act as extra inhibitors for urea photodegradation. No matter SPW and TAP, such decreases in urea removal were strongly related to the dilution factors, which are

L. Long et al. / Water Research 161 (2019) 89e97

95

Fig. 4. Effects of a) pH, b) free chlorine, c) initial concentration, and d) dissolved oxygen on urea degradation by VUV ([Urea]0 ¼ 5.0 mg/L).

3.3. Reaction products and pathways

Fig. 5. Effects of real water on urea degradation by VUV ([Urea]0 ¼ 5.0 mg/L).

reasonable because the concentrations of all compounds in water were decreased by dilution and therefore the overall inhibitions were reduced. Based upon the kinetics from above results, the electrical energies required for one order (EEO) of contaminant degradation for these methods are summarized in Table S1. The EEO for VUV irradiation of urea in UPW (168.5 kWh/m3) was greater than other micropollutants treated by most of AOP processes (Miklos et al., 2018). Meaning that urea is a very refractory compound in water. All EEOs of methods treating real SPW were higher than those of UPW, meaning that more efforts should be made to enhance the efficiency of urea degradation in SPW.

In terms of reaction products, Fig. 6a shows that the concenþ trations of NO 3 and NH4 continued to increase throughout the VUV irradiation process, while the concentration of NO 2 was almost undetectable. The amount of total nitrogen (TN) remained unchanged, implying that no other types of nitrogen-containing compounds were substantially formed. Meanwhile, Fig. S11  shows that VUV was unable to convert NHþ 4 rapidly into NO3  within 3 h, meaning that the formation of NO3 was obtained by direct oxidation of urea instead of indirect oxidation of NHþ 4. Regarding carbon-containing compounds, formaldehyde and formic acid were analyzed but not detected at levels above their MDLs (1 mg/L and 10 mg/L, respectively), meaning that these two compounds were not the major photolysis products. Fig. 6b shows that NPOC, TOC, and urea decreased simultaneously, and the rates of NPOC and TOC reductions were slightly lower than that of urea decrease, suggesting that urea was mostly mineralized and slightly converted to volatile, nitrogen-free organic matter. Meanwhile, the study observed a continuous pH decrease throughout the VUV irradiation process (Fig. S12). After considering all reactants, products, and influencing factors, the following two dominant reaction pathways are proposed for the urea photolysis with VUV, one forming nitrate and the other forming ammonia, but both yielding carbon dioxide. Although formation of NHþ 4 increases pH, which does not agree with the acidification of solution, formation of NO 3 decreased pH significantly. As seen from Fig. 6a, þ formation of NO 3 was preferred over formation of NH4 , which explains why the solution pH eventually decreased in the photolysis process.

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L. Long et al. / Water Research 161 (2019) 89e97

(83.1%) > TAP (40.0%) > SPW (22.2%). The major VUV photolysis products of urea were found to be þ þ NO 3 , NH4 , H , and CO2. Because the mass balance of total nitrogen was met whereas total carbon decreased, VUV photolysis of urea was postulated to mainly undergo oxidative mineralization. The information hence demonstrates that VUV is an effective alternative on urea treatment for SPW. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We appreciate the financial support from the Shenzhen Science and Technology Innovation Commission (JCYJ20180306171820685) and the technical assistance from our coworkers in the laboratory including Fei Yang, Yulin Zhang, Yi Chen, Lei Wang, etc. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.watres.2019.05.098. References

Fig. 6. Fate and transformation of a) nitrogen- and b) carbon-containing compounds during the urea degradation process by VUV ([Urea]0 ¼ 5.0 mg/L).

VUV

þ H2 NCONH2 þ4O2 ƒƒƒ!CO2 þ 2NO 3 þ H2 O þ 2H VUV

 H2 NCONH2 þ 3H2 Oƒƒƒ!CO2 þ 2NHþ 4 þ 2OH

(16) (17)

4. Conclusions In general, this study compared the efficiencies of UV, UV/H2O2, UV/K2S2O8, UV/Na2SO3, VUV, VUV/H2O2, and VUV/K2S2O8 on urea degradations in UPW. UV/K2S2O8, VUV, VUV/H2O2, and VUV/K2S2O8 showed superior capabilities to other UV and VUV-based methods. Although the combinations of K2S2O8 with UV or VUV were more robust than other approaches in removing urea, they featured greater risks of increasing TDS, thus making K2S2O8 less suitable for meeting regulatory requirements. In comparison, VUV appears to be a more promising alternative due to its robustness in degrading urea without introducing high TDS. In terms of influencing factors, typical components in SPW such  as C3H3O3N3, humic acid, HCO 3 , and NO3 inhibited urea degradation in the VUV irradiation process. In contrast, increasing the SO2 4 and Cl contents enhanced the urea removal. Free chlorine at a dosage of
3 mg/L and pH at a range of 6.8e8.5 imposed little changes, while low [Urea]0 exhibited higher photodegradation kinetics. Overall, the influences of the real water matrix on the urea removal were negative for the VUV process, with an order of UPW

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