- Email: [email protected]
A green and robust method to measure nanomolar dissolved organic nitrogen (DON) by vacuum ultraviolet
Chemical Engineering Journal 363 (2019) 57–63
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
A green and robust method to measure nanomolar dissolved organic nitrogen (DON) by vacuum ultraviolet
T
⁎
Yi Chen, Junli Wang, Baiyang Chen , Lei Wang Shenzhen Key Laboratory of Organic Pollution Prevention and Control, State Key Laboratory of Urban Water Resource and Environment of Harbin Institute of Technology (Shenzhen), 518055, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
order to analyze nanomolar dis• Insolved organic nitrogen, VUV was evaluated.
converted ∼90% of dissolved • VUV organic nitrogen species into inorganic nitrogen.
pretreatment enabled a lower • VUV MDL (1.5 μg-N/L) than conventional methods.
If NH • into NO
+ was completely converted 4 − 3 , IC alone can measure DON
in water.
A R T I C LE I N FO
A B S T R A C T
Keywords: Total nitrogen Organic nitrogen VUV Sample pretreatment Photolysis
Quantifying trace dissolved organic nitrogen (DON) in water is currently challenging for conventional methods not only because of the presence of dissolved inorganic nitrogen (DIN) species, which usually dominate in total nitrogen (TN), but also because of the high method detection limits (MDLs) of current TN methods, which are ≥100 μg/L. In order to overcome these barriers, an earlier study has applied electrodialysis to eliminate DINs from water, thereby reducing the interference of DIN. However, it remains difficult to analyze DON at nanomolar level. To address this issue, this study proposes to convert DON completely into DIN by vacuum ultraviolet irradiation (VUV) and then to measure DON by the sum of DINs (i.e., ammonia, nitrite, and nitrate). A number of verification tests show that the proposed method had comparable results with conventional methods for selected model compounds and real samples. The average nitrogen recovery for sixteen model DON compounds (each at 2.0 mg-N/L) with varying structures and molecular weights was 89 ± 16%, similar to those obtained by thermal-activated persulfate oxidation (87 ± 18%) and high temperature catalytic oxidation (86 ± 20%) methods. Moreover, this method features an advantage that it reached a MDL of 1.5 μg-N/L, much lower than conventional TN methods. With a prolonged 12-h irradiation, this approach was able to convert ammonia completely into nitrate, thus enabling detection of DON by using ion chromatography only.
Abbreviations: DON, dissolved organic nitrogen; DIN, dissolved inorganic nitrogen; TN, total nitrogen; TOC, total organic carbon; MW, molecular weight; MDL, method detection limit; VUV, vacuum ultraviolet irradiation; ED, electrodialysis; UV, ultraviolet; IC, ion chromatography; MO, methyl orange; MCAN, chloroacetonitrile; HTCO, high temperature catalytic oxidation; TKN, total Kjeldahl nitrogen; TPO, thermal-activated persulfate oxidation; SPW, swimming pool water; EPA, environmental protection agency; HAN, haloacetonitrile; RSD, relative standard deviation; EDTA, ethylene diamine tetraacetic acid ⁎ Corresponding author. E-mail address: [email protected] (B. Chen). https://doi.org/10.1016/j.cej.2019.01.124 Received 29 October 2018; Received in revised form 15 January 2019; Accepted 18 January 2019 Available online 22 January 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Chemical Engineering Journal 363 (2019) 57–63
Y. Chen et al.
its high MDL, which is usually ≥200 μg-N/L [12]; not to say that nitrate is a strong interferent of the conversion process [20]. Therefore, there are motives to develop new methods to overcome the second barrier [21,22]. In this study, vacuum UV irradiation (VUV) is proposed to provide a green and robust pretreatment method for nanomolar DON analysis. According to literature, UV and hydrogen peroxide (UV/H2O2) were once integrated to analyze TN at trace levels; however, it resulted in lower recovery yet higher variability than HTCO and thermal-activated persulfate oxidation (TPO) methods [19,23]. Therefore, the UV/H2O2 method was not considered as a promising alternative. In contrast, VUV photolysis is known to be robust in degrading or mineralizing aqueous components, either directly by direct light irradiation or indirectly via formation of hydroxyl radical (%OH), a strong non-selective oxidant [24,25], and ozone, a strong selective oxidant [26]. Therefore, VUV is likely applicable to convert DON completely into DIN. Moreover, VUV irradiation is easy to operate and theoretically introduces no external ions into samples; therefore the pretreated solution can be tested by IC. In an earlier study, VUV was already employed in a size-exclusion chromatographer to analyze total organic carbon (TOC) and TN [27], thus demonstrating the promise of this method. However, it requires systemic verification, intensive examination, and more comparison with conventional methods. This study is committed to proving this approach. Firstly, we compared the VUV photolysis method with two conventional methods (i.e., TPO and HTCO) in converting DON into DIN for 16 model DON compounds. Then, in order to optimize the conversion efficiency, we evaluated the influences of several operational and environmental conditions (i.e., pH, light dosage, and typical ions) on VUV performance. Next, for the purpose of handy DON measurement, we converted glycine and its photolysis product, ammonia, into nitrate completely with a prolonged irradiation process. Finally, we demonstrated the calibration curve, application scope, and a comparison of DON recoveries in real tap water between this method and conventional methods.
1. Introduction Dissolved organic nitrogen (DON) is ubiquitously present in marine and terrestrial environments and it is an essential part in the nitrogen element cycles [1,2]. According to literatures, DON may account for up to 70% of total nitrogen (TN) in certain rivers near the Atlantic Ocean [3] or up to 80% in tertiary effluents [4]. The compositions of DON include humic substance, peptides, proteins, amino sugars, and amides [5]. Besides its roles as a nutrient, DON also acts as contaminant in natural environment that can sometimes cause certain environmental issues, such as eutrophication [6], or formation of nitrogenous disinfection byproducts in engineering water system (e.g., drinking water and wastewater treatment processes), which are potentially highly toxic to human and ecosystem [7,8], or be associated with membrane fouling in water treatment processes [9]. So, it is important to measure DON to understand its occurrence or to control it. However, there are two challenges currently facing with DON measurement. The first challenge is that DON is determined by the calculative difference between two sets of nitrogen-containing parameters, including 1) TN and 2) dissolved inorganic nitrogen (DIN) species, i.e., ammonia, nitrite, and nitrate [5]. The approach is problematic for samples with low ratio of DON to TN because the subtraction process can significantly magnify the analytical errors of each parameter’s analysis, making DON data unreliable [7,10]. In certain cases when DIN dominates TN (i.e., DON accounts for only a very small fraction of TN), the calculated DON was obtained even as negative values [11]. The second barrier of DON measurement is associated with the method detection limits (MDLs) of current TN methods, which are ≥100 μg-N/L now, making measurement of nanomolar DON impossible. In an earlier review, it was reported that approximately 30% of surface water, 76% of shallow groundwater, and 81% of deep groundwater samples have DON below the MDL of current TN analytical methods [12]. These two barriers hence hindered our understanding of the occurrence and roles of DON in the environment and engineering systems. In order to overcome the first challenge, an earlier study has applied electrodialysis (ED) to eliminate DIN from water (> 99%) and recover most of DON (mean recovery = 88%) [13], thereby dramatically reducing the analytical errors brought by the subtraction process. However, it remains challenging to detect samples with trace amount of DON (i.e., at nanomolar level) because of the high MDL. Prior to this study, we briefly reviewed three approaches currently used for TN analysis. The first approach involves conversion of TN into nitrate using persulfate-based oxidative methods such as thermal [14], microwave [15], and ultraviolet (UV) activated persulfate [16], and then measurement of nitrate was achieved with spectrophotometry. Due to the uses of persulfate, this approach has very high level of sulfate formation and therefore cannot employ ion chromatography (IC) to analyze nitrate. The concerns come from the interference of sulfate on nitrate and the impact of sulfate on the ion-exchanging capacity of IC column. Since spectrophotometry has higher MDL than IC in detecting nitrate, this approach usually reported MDL of TN > 100 μg-N/L. Notably, certain impurities in persulfate may lead to high blank data [17]. The second approach of analyzing TN is using thermal-based combustion methods, such as high temperature combustion or high temperature catalytic oxidation (HTCO) [18], to convert TN into nitric oxide gas and then detecting the gas by chemiluminescence analyzer. Although convenient and automated, a previous comparison study showed that thermal-based methods were lower in TN recovery than persulfatebased methods [19]. More importantly, the MDLs of thermal-based methods are also ≥100 μg-N/L even per manufacturers’ guidance unless it is done under vacuum conditions. The third approach for analyzing TN is related to the total Kjeldahl nitrogen (TKN) method, which reduces DON compounds into ammonia first and then quantifies DON by subtracting ammonia from TKN. This method was less used now not only because of its extreme complexity in operation but also because of
2. Methods and materials 2.1. Chemicals and sample preparation In verification tests, sixteen DON model compounds with varying molecular weight (MW) and structures were selected. Their chemical structures, MWs, and formula are presented in Table S1. Before experiments, each compound was dissolved in ultrapure water individually as generated by a Millipore water generator (Direct-Q3) and then stored in a refrigerator (4 °C). Sulfuric acid (H2SO4) and sodium hydroxide (NaOH) were used to adjust water pH in case of need. Three real water samples were collected for comparison, including a river water taken from the Dasha River, a tap water collected from the laboratory faucet, and a swimming pool water (SPW) collected from the campus stadium. Prior to use, they were filtered through 0.45 μm nylon fiber membranes and treated with ED to eliminate DINs. 2.2. Analytical methods Ammonia was detected by a spectrophotometer (DR 3900, Hach Co., Ltd.) according to the United States Environmental Protection Agency (EPA) method 350.2 with the MDL as 20 µg-N/L. Nitrate and nitrite were analyzed by an IC (IC2010, Tosoh Inc., Japan) equipped with an AS15 column (Thermofisher, USA), an eluent generator (EDG100, Minghao Chromatographic Technology, China), an eluent suppressor (WLK-6, Reepo analytical instrument Co., Ltd, China), and a 100 μL injection loop. According to the EPA method 300.0 guideline, the MDLs of nitrite and nitrate are estimated to be 1.5 µg-N/L under the conditions of this study. The pH was monitored by an electrode meter (pH100, Extech Instruments Co.). Two conventional TN analytical methods were compared with the 58
Chemical Engineering Journal 363 (2019) 57–63
Y. Chen et al.
new method. The first conventional method was TPO method, in which potassium persulfate was activated at 125 °C using a heater (DIS-1A, Changhong Technology Co., Ltd.) to convert TN completely into nitrate within 40 min; and then nitrate was analyzed by a spectrophotometer (DR 3900, Hach Co., Ltd.). The second conventional TN analytical method was HTCO, which was achieved by a TOC combustor coupled with a TN detector (TOC-LCPH, Shimadzu Co., Ltd). The principle of HTCO method lies in the conversion of TN into nitric oxide gas at 720 °C and then detection of gas with a chemiluminescence detector. The MDLs of TN for both TPO and HTCO are 0.1 mg-N/L per manufacturers’ guidance.
recoveries were observed for MO, melamine, and MCAN for the three tested methods. The nitrogen recoveries from MO were poor for all three methods, confirming that it is either refractory [30–32] or easy to form nitrogenous gases that cannot be captured [33]. The recovery of N by VUV was higher than those achieved by TPO and HTCO for MO, but lower than TPO for MCAN and worse than HTCO for melamine. These results somehow agree with another study that melamine did not react rapidly with %OH in VUV process [34]. As for haloacetonitrile (HAN), an earlier study proved that MCAN was the most refractory compound of all tested HANs while imine (C]N)-containing compounds were the major products of VUV photolysis [35]. In order to distinguish the roles of VUV and UV254, Fig. S1 presents the recovery differences of three typical DON compounds, i.e., urea, MCAN, and trichloroacetamide, irradiated by two different types of lamps with identical power intensity. The results show that these compounds were transformed to greater extents by VUV lamps emitting both 185 nm and 254 nm lights than UV lamps emitting 254 nm light only. Therefore, it confirms that the conversions of DON were mainly triggered by 185 nm light (i.e., VUV). In terms of trends, the recoveries of nitrogen appeared less dependent on the MWs of compounds but more associated with their structural properties. Of 16 model compounds, 13 of them resulted in formation of ammonia as the dominant photolysis products but only 3 of them yielded nitrate as the major product (Table S1). Although it was only a snapshot of speciation, certain patterns can be obtained. The compounds with amide (O]CeNH2) or nitro (eNO2) functional groups were more likely converted to nitrate, while the compounds with carboxyl (eCOOH), imine (C]N, except for melamine), and nitrile (C^N) functional groups were mainly transformed into ammonia. Based on literature, VUV can transform compounds in water via three mechanisms: 1) direct photolysis via 254 and/or 185 nm lights; 2) oxidation via formation of %OH, a non-selective strong oxidant (Eqs. (1) and (2)); and 3) reduction via formation of hydrated electron (e−) and hydrogen atom (H%) (Eqs. (1) and (2)). From this study, it is likely that the amidecontaining compounds mainly underwent oxidation process by %OH oxidation, while other compounds undertook CeN bond cleavages by 185 nm irradiation directly under the conditions of this study. Furthermore, since these experiments were carried out under aerated conditions, H% and e− might react with dissolved oxygen and form other reactive species such as superoxide radical anion (%O2−) and per hydroxyl radical pair (Eqs. (3) and (4)).
2.3. Photoreactor and experimental procedures The VUV photoreactor used in this study consists of a stainless-steel frame holding at most four low-pressure mercury lamps (12 W, UV-Tec Co., Ltd) emitting both 185 nm and 254 nm lights. The photon flux of 185 nm VUV was estimated to be 1.23 × 10−3 μEinstein/s/cm2 according to a method recently introduced in a literature [28], which roughly accounts for 5% of total UV light intensity. For energy calculation convenience, the irradiance of the reactor was obtained by multiplying the power intensity of lamp (i.e., 12 W) by the reaction time and number of lamps, and then dividing it by the volume of treated sample, i.e., power intensity of lamp × number of lamps × reaction time/water volume. A quartz tube (22 cm long, 1.5 cm in diameter, 30 mL in volume, and made of Suprasil quartz) as a water container was placed in the middle of the chamber. The lamps and quartz tube were deployed in parallel and as close as possible to minimize VUV light loss. In addition, a fan was placed on top of the reactor to cool the air and vent out ozone formed in the reactor. More details about the reactor are described elsewhere [29]. 3. Results and discussion 3.1. Model compounds evaluation Fig. 1a shows that 90-min VUV irradiation achieved an average N recovery of 89 ± 16% for 16 model DON compounds, each set at 2 mgN/L and tested separately without pH buffer. The average recoveries were similar to those obtained by TPO (87 ± 18%) and HTCO (86 ± 20%) methods, meaning that VUV can obtain comparable nitrogen recoveries as conventional methods. Of them, relatively low 120 TPO
VUV (
NO
NO
NH
)
HTCO
20
(a)
18 16 DON Conc. (mg-N/L)
DON Recovery (%)
100
(b)
80 60 40
14 12 10
8 6 4
20
2 0
0
Fig. 1. A comparison of DON recoveries using three analytical methods on a) 16 model compounds (C0 = 2.0 mg-N/L, volume = 30 mL, irradiation time = 90 min, total irradiation energy = 1200 kWh/m3) and b) three real water samples pretreated with electrodialysis (volume = 5 mL, irradiation time = 120 min, total light energy = 9600 kWh/m3). 59
Chemical Engineering Journal 363 (2019) 57–63
Y. Chen et al.
120
Urea
Glycine
MCAN
scavengers in water, we evaluated their influences on the VUV treatability for four typical DON compounds. According to Fig. 2b, adding 10 mg/L chloride (which has pH of 6.5) imposed minor effect on the DON recovery by VUV, which is not surprising because it has low UV absorptivity and reactivity with %OH [36,37]. Furthermore, although chloride features high reactivity with %OH, the adduct dissociates readily above pH of 5 and gives back %OH; only when pH falls below 5 chloride can be transformed to lowly reactive chlorine radical ions (Cl2%−) [39]. In contrast, adding 100 mg/L carbonate (which has pH of 10.6) substantially reduced the DON recoveries, an indicator of the presence of inhibitors. The dosages of chloride and carbonate mimicked the water quality condition of a local surface water, and essentially represented a condition with maximum interference to the proposed VUV process because all samples are supposed to be pretreated by ED first, which can significantly reduce the amount of total dissolved solid to below 10 mg/L [13]. To distinguish the roles of carbonate and hydroxide, 7 mg/L sodium hydroxide was added to prepare a solution with identical pH (10.6) as the sample dosed with 100 mg/L sodium carbonate. A comparison of them shows that the samples dosed with carbonate resulted in less DON recovery than those dosed with hydroxide; so the results confirm that both carbonate and hydroxide have suppressed the conversions of DON, either as %OH scavengers [40] or as light competitors. Possibly, in the alkaline condition OH− has reacted with carbonate to form carbonate radical anion, which is a weak oxidizing radical [41].
(a)
MO
DON Recovery (%)
100 80 60 40 20 0 4.0
6.5
8.6
9.2
10.6
11.1
pH 120
(b) DON Recovery (%)
100 80
60 40 20 0 Control
Cl
CO ²
3.3. Influence of light dosage
OH
In order to optimize the process, Fig. 3 presents the effect of light energy input on the DON recovery. It was clear that increasing irradiation energy from 200 to 1600 kWh/m3 significantly promoted the TN recoveries for urea and MO, which achieved almost 100% with an irradiation of 1600 kWh/m3. However, low or no significant enhancement was observed for MCAN and glycine irradiation processes, in which the TN recoveries were only enhanced from 40 to 64% for MO and little impact on MCAN. For glycine, because it can be readily transformed with 200 kWh/m3, no additional benefits were obtained with more irradiation flux.
Fig. 2. The influences of a) pH and b) chloride (10.0 mg/L), carbonate (100.0 mg/L), and hydroxide (7.0 mg/L) on the DON recovery by VUV photolysis (C0 = 2.0 mg-N/L, volume = 30 mL, irradiation time = 60 min, total light energy value = 800 kWh/m3). hν
H2 O→ ∙OH + H∙ hν
(1)
H2 O→ ∙OH + H+ + e−
(2)
e− + O2 → %O2−
(3)
H% + O2 → HO2%
(4)
3.4. Inorganic nitrogen conversion Ideally, if both DON and DINs can be converted completely into nitrate, the amount of DON could be obtained by analyzing nitrate with IC only, which would be convenient. To reach this goal, we hereafter made an extra effort to convert ammonia derived from DON into nitrate. In this part glycine was chosen because it yields mostly ammonium during the VUV photolysis process (Table S1). In addition,
3.2. Influence of pH and coexisting compounds To better understand the VUV efficiency under practical conditions, Fig. 2a shows the TN recoveries for four representative DON compounds at a pH range from 4.0 to 11.1. In general, increasing pH resulted in either unchanged or lower DON recovery although the extents of influence were compound-specific. For example, when pH increased from 4.0 to 8.6, the recoveries of urea and glycine remained constant (> 80%); however, when pH went above 8.6, their recoveries dropped dramatically. Similarly, the recoveries of MCAN and MO were constant from pH 4.0 to 9.2 but became much lower at pH above 9.2. These phenomena indicate that acidic and neutral solutions were preferred over alkaline solution on DON conversion, suggesting that one of the key drivers of the process was %OH, which features lower reactivity in alkaline conditions than that in acidic conditions [36,37]. According to literature, the pKa value of %OH is 11.9 [38], meaning that when pH is > 11.9 most of %OH become less reactive O%−. In this study, the pH were all below 11.9; so, the decrease of DON transformation with increasing pH was likely due to the conversion of HO% into O%− (Eq. (5)) and/or the consumption of %OH by OH−. HO% + OH− → O%− + H2O
120 Urea
Glycine
MCAN
MO
DON Recovery (%)
100 80 60
40 20 0 200
(5)
400 800 Light Energe (kWh/m3)
1600
Fig. 3. The influence of light energy on the recoveries of selected DON compounds (2.0 mg-N/L) by VUV photolysis.
Because chloride and carbonate are two known %OH inorganic 60
Chemical Engineering Journal 363 (2019) 57–63
Y. Chen et al.
1600
Light Energy (kWh/m3) 3200 4800 6400
120
8000
9600 Measured DON ( g-N/L)
0 2.2
Conc. (mg-N/L)
1.8 TN
1.4
NO 1.0
NO
0.6
NH /NH
(a)
Urea: y = 1.0025x + 0.4082 R² = 0.9998
100 80 60
Urea
40 20 0 0
0.2
20
40
60
80 -N/L)
100
120
-0.2 2
4
6 Time (h)
8
10
12
5 Measured DON (mg-N/L)
0
Fig. 4. The conversion of DON into DIN species during VUV photolysis of glycine (C0 = 2.0 mg-N/L, light intensity = 400 kW/m3, pH = 9.2).
because we know from literature that the reaction between ammonia and %OH is slow and the rate increases with increasing pH [29], the following tests were carried out at pH of 9.2 to balance the needs of both DON conversion and ammonia transformation. Fig. 4 shows the DON conversion and DIN interconversions during a 12-h VUV irradiation process using two VUV lamps, which ultimately reached 9600 kWh/m3. The measured TN stayed within 9.8% variance throughout the process, meaning that there was no substantial amount of other nitrogen-containing compounds formed. Ammonium appeared immediately after VUV irradiation and reached peak in 30 min, confirming that glycine was fully converted to ammonia in this period. However, after approximately 2 h, the amount of ammonia gradually decreased whereas the amount of nitrate kept increased over time. Until 12 h, nitrate accounted for about 99% of TN, meaning that nearly all ammonia had been converted to nitrate. During the process, nitrite appeared at the first 1.5 h but decreased later and became undetectable after 4 h. The result agreed well with earlier findings that %OH can convert ammonium slowly into nitrite and nitrate but can convert nitrite rapidly into nitrate [29,42]. As a result, nitrate became reasonably the dominant and final DIN product at the end [29,42] for IC analysis. The results hence demonstrated an example of complete conversion of DON and ammonia into nitrate. Although the time required to convert ammonia into nitrate was still too long under the conditions of this study, this approach might still be favored because it substantially reduce the operational tediousness of measuring multiple DIN parameters and allow a very low MDL of DON. In addition, it holds hopes for further optimization by increasing light dosage, designing better photoreactor, and integrating robust immobilized catalyst [43] into the photoreactor in the future.
4
(b)
Glycine: y = 0.9483x + 0.0369 R² = 0.9985 (0.1-5.0 mg-N/L)
Glycine Urea
3 2 1
Urea: y = 0.9696x + 0.0038 R² = 0.9997 (0.1-2.0 mg-N/L)
0 0
1
2 3 4 Dosed Concentration (mg-N/L)
5
Fig. 5. The calibration curves for a) urea at low concentrations from 0.005 to 0.1 mg-N/L and b) urea and glycine at high concentration ranges from 0.1 to 5.0 mg-N/L (Total light energy = 9600 kWh/m3, samples unbuffered).
DON. That is, when the initial dosage of urea was above 2.0 mg-N/L, the gaps between measured and spiked TN became large. This certainly indicates an inadequacy or uncertainty of VUV photolysis to analyze samples with high DON content. Although irradiation of glycine shows reasonable correlation between measured and spiked TN from 0.1 to 5.0 mg-N/L, the influence of initial concentration on VUV treatability requires extra attention. In addition to calibration curve, the repeatability of this method was checked by using synthetic waters dosed with various amounts of urea. Fig. S2 shows a repeat of six urea measurements at either 2.0 or 0.5 mgN/L for different irradiation periods, and the results showed comparable recoveries among samples (RSD ≤ 6.9%), suggesting that the results are repeatable. 3.6. Real sample evaluation Fig. 1b shows the DON amounts detected in three types of real water, including a tap water, a river water, and a swimming pool water (SPW). In general, the measured DON followed as TPO > VUV > HTCO, but VUV has the lowest RSD of all methods, confirming that VUV method was equivalent to other two conventional TN methods. Prior to comparison, the samples were pretreated with ED to eliminate DINs according to the method introduced in earlier study [13], and the SPW samples were diluted by 10-fold. For the tap water, because residual disinfectant was present and certain non-ionizable inorganic chloramine (such as monochloramine) may be formed, sodium sulfite as a quenching agent was added to convert chlorine and chloramines into ammonia and chloride, therefore all DINs were eliminated. In order to further verify this method, we compared the results of three analytical methods for samples diluted with different amounts of ultrapure water. As shown in Fig. 6a, the DON concentrations decreased proportionally with the dilution ratios, and the correlation curve
3.5. Application scope and MDL Herein we chose urea and glycine as two reference compounds for application scope evaluations because they yielded significantly different DIN products. The slopes, intercepts, R2, and relative standard deviations (RSDs) for the calibration curves are presented in Fig. 5a. The information regarding the slope (1.002), R2 (> 0.999) and RSDs (< 5%) of the concentrations spanning from 5 to 100 μg-N/L support that the VUV method can provide not only accurate but also precise DON measurement at nanomolar level. Because the MDL of DON is essentially dependent on the MDLs of DINs, the MDL of DON via VUV pretreatment may be 20 μg-N/L if the final DIN is ammonia or 1.5 μg-N/ L if the ultimate product is nitrate. At high DON range (i.e., 0.1–5 mg-N/L), Fig. 5b shows that urea did not always show excellent correlations between measured and dosed 61
Chemical Engineering Journal 363 (2019) 57–63
Y. Chen et al.
400
-N/L)
350 300 250 200
TPO 360 300 240 180 120 60 0
VUV
HTCO
completely converted into nitrate, a convenient measurement of DON may be enabled by using IC only. Overall, the study demonstrated VUV as a green and robust tool to detect nanomolar DON, which may help better understand the roles and occurrences of DON in the environment and engineering water systems.
(a)
(b)
y = 330.02x R² = 0.9032
y = 305.44x y = 293.39x R² = 0.9893 R² = 0.8584 0.0
0.5
Acknowledgements
1.0
150
The study was financially supported by the Shenzhen Science & Technology Innovation Commission (JCYJ20170818091859147). Also, we appreciate our crews in the laboratory (Ting Jie and Yinan Bu) for their technical helps.
100 50
0 0.00
-N/L)
800 700
600
700 600 500 400 300 200
0.25
0.34 0.50 Concentration ratio (d)
y = 0.8155x + 351.36 R² = 0.9523 y = 0.8177x + 299.01 R² = 0.9539 0
100
200
1.00
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.01.124.
(c)
y = 1.0948x + 308.16 R² = 0.9787
References
300
[1] J.C. Neff, F.S. Chapin, P.M. Vitousek, Breaks in the cycle: dissolved organic nitrogen in terrestrial ecosystems, Front. Ecol. Environ. 1 (2003) 205–211. [2] D.E. Canfield, A.N. Glazer, P.G. Falkowski, The evolution and future of earth's nitrogen cycle, Science 330 (2010) 192–196. [3] N. van Breemen, Natural organic tendency, Nature 415 (2002) 381. [4] E. Pehlivanoglu-Mantas, D.L. Sedlak, Wastewater-derived dissolved organic nitrogen: analytical methods, characterization, and effects—a review, Crit. Rev. Environ. Sci. Technol. 36 (2006) 261–285. [5] B. Tom, A.B. Deborah, Dissolved organic nitrogen: a dynamic participant in aquatic ecosystems, Aquat. Microb. Ecol. 31 (2003) 279–305. [6] J. Sun, H. Simsek, Bioavailability of wastewater derived dissolved organic nitrogen to green microalgae Selenastrum capricornutum, chlamydomonas reinhardtii, and chlorella vulgaris with/without presence of bacteria, J. Environ. Sci. (China) 57 (2017) 346–355. [7] W. Lee, P. Westerhoff, Dissolved organic nitrogen measurement using dialysis pretreatment, Environ. Sci. Technol. 39 (2005) 879–884. [8] S.W. Krasner, P. Westerhoff, B. Chen, B.E. Rittmann, S.N. Nam, G. Amy, Impact of wastewater treatment processes on organic carbon, organic nitrogen, and DBP precursors in effluent organic matter, Environ. Sci. Technol. 43 (2009) 2911–2918. [9] X. Yue, Y.K.K. Koh, H.Y. Ng, Effects of dissolved organic matters (DOMs) on membrane fouling in anaerobic ceramic membrane bioreactors (AnCMBRs) treating domestic wastewater, Water Res. 86 (2015) 96–107. [10] J.F. Saunders, Y. Yu, J.H. McCutchan, F.L. Rosario-Ortiz, Characterizing limits of precision for dissolved organic nitrogen calculations, Environ. Sci. Technol. Lett. 4 (2017) 452–456. [11] D. Graeber, J. Gelbrecht, B. Kronvang, B. Gücker, Technical note: comparison between a direct and the standard, indirect method for dissolved organic nitrogen determination in freshwater environments with high dissolved inorganic nitrogen concentrations, Biogeosci. (BG) Discuss. (BGD) (2012). [12] P. Westerhoff, H. Mash, Dissolved organic nitrogen in drinking water supplies: a review, J. Water Supply: Res. Technol.-Aqua 51 (2002) 415. [13] A. Zhu, B. Chen, L. Zhang, P. Westerhoff, Improved analysis of dissolved organic nitrogen in water via electrodialysis pretreatment, Anal. Chem. 87 (2015) 2353–2359. [14] R. Benner, B. Von Bodungen, J. Farrington, J. Hedges, C. Lee, F. Mantoura, Y. Suzuki, P.M. Williams, Measurement of dissolved organic carbon and nitrogen in natural waters: workshop report, Mar. Chem. 41 (1993) 5–10. [15] E. Dafner, S. De Galan, L. Goeyens, Microwave digestion of organic substances, a useful tool for dissolved organic nitrogen measurements, Water Res. 33 (1999) 548–554. [16] F.A.J. Armstrong, P.M. Williams, J.D.H. Strickland, Photo-oxidation of organic matter in sea water by ultra-violet radiation, analytical and other applications, Nature 211 (1966) 481. [17] E.S.A. Badr, E.P. Achterberg, A.D. Tappin, S.J. Hill, C.B. Braungardt, Determination of dissolved organic nitrogen in natural waters using high-temperature catalytic oxidation, TrAC Trends in Anal. Chem. 22 (2003) 819–827. [18] T.W. Walsh, Total dissolved nitrogen in seawater: a new-high-temperature combustion method and a comparison with photo-oxidation, Mar. Chem. 26 (1989) 295–311. [19] J.H. Sharp, K.R. Rinker, K.B. Savidge, J. Abell, J. Yves Benaim, D. Bronk, D.J. Burdige, G. Cauwet, W. Chen, M.D. Doval, D. Hansell, C. Hopkinson, G. Kattner, N. Kaumeyer, K.J. McGlathery, J. Merriam, N. Morley, K. Nagel, H. Ogawa, C. Pollard, M. Pujo-Pay, P. Raimbault, R. Sambrotto, S. Seitzinger, G. Spyres, F. Tirendi, T.W. Walsh, C.S. Wong, A preliminary methods comparison for measurement of dissolved organic nitrogen in seawater, Mar. Chem. 78 (2002) 171–184. [20] P.J. Johnes, A.L. Heathwaite, A procedure for the simultaneous determination of total nitrogen and total phosphorus in freshwater samples using persulphate microwave digestion, Water Res. 26 (1992) 1281–1287. [21] J. Ma, L. Adornato, R.H. Byrne, D. Yuan, Determination of nanomolar levels of
500
400 300 200 0
50
100
150 -N/L)
200
300
Fig. 6. A comparison of three methods on the DON measurements for a) the concentration and b) the trendlines of a real tap water and diluted samples; and c) the concentration and d) the trendlines of a real tap water added with various amounts of urea (light energy: 9600 kWh/m3).
between the DON contents and the dilution ratios has a R2 > 0.98 and a maximum RSD of 10.15%. In contrast, the calibration correlation coefficients were 0.90 for TPO method and 0.85 for HTCO method. Although the exact amount of DON in real tap water was unknown, it at least indicates that the VUV pretreatment method could recover them with consistent efficiency. As for DON recovery, Fig. 6b shows reasonable trends (slope = 1.0948) for the VUV method when the tap water was added with varying amounts of urea from 50 to 300 μg-N/L, meaning that urea has been completely converted. In comparison, the concentrations of samples measured by TPO method were higher in RSD, especially when the amount of added urea was 50 μg-N/L, meaning that the TPO method was less suitable to detect DON at low levels. In contrast, the data obtained by HTCO method were generally lower than other methods, suggesting that certain substances cannot be completely converted into nitric oxide gas. 4. Conclusions In order to detect trace DON in water, this study employed VUV pretreatment to convert DON completely into DINs. In general, the study proved that the VUV method has similar recoveries and RSDs as conventional methods, but better than conventional methods in achieving lower MDL of DON (i.e., 1.5 μg-N/L). The recoveries of DON via VUV increased with increasing light dosage, decreasing pH, or lower DON concentration. The favorable pH condition was ≤8.6, and the preferred DON concentration of this method was ≤2 mg-N/L. The presence of chloride at selected dosage imposed minor effect on the conversions, but carbonate and hydroxide have significant suppressing effects on the VUV performance, meaning that ED as an anion removal process is needed prior to VUV irradiation process. If ammonia can be 62
Chemical Engineering Journal 363 (2019) 57–63
Y. Chen et al.
chloroacetonitriles by the UV/H2O2 process, Water Res. 99 (2016) 209–215. [33] T. Tasaki, T. Wada, K. Fujimoto, S. Kai, K. Ohe, T. Oshima, Y. Baba, M. Kukizaki, Degradation of methyl orange using short-wavelength UV irradiation with oxygen microbubbles, J. Hazard. Mater. 162 (2009) 1103–1110. [34] V. Maurino, M. Minella, F. Sordello, C. Minero, A proof of the direct hole transfer in photocatalysis: the case of melamine, Appl. Catal. A 521 (2016) 57–67. [35] P. Kiattisaksiri, E. Khan, P. Punyapalakul, T. Ratpukdi, Photodegradation of haloacetonitriles in water by vacuum ultraviolet irradiation: mechanisms and intermediate formation, Water Res. 98 (2016) 160–167. [36] S. Gligorovski, R. Strekowski, S. Barbati, D. Vione, Environmental implications of hydroxyl radicals (OH), Chem. Rev. 115 (2015) 13051–13092. [37] S. Gligorovski, R. Strekowski, S. Barbati, D. Vione, Addition and correction to environmental implications of hydroxyl radicals (OH), Chem. Rev. 118 (2018) 2296–2296. [38] Y. Nosaka, A.Y. Nosaka, Generation and detection of reactive oxygen species in photocatalysis, Chem. Rev. 117 (2017) 11302–11336. [39] G.V. Buxton, C.L. Greenstock, W.P. Helman, A.B. Ross, Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (OH/O− in aqueous solution), J. Phys. Chem. Ref. Data 17 (1988) 513–886. [40] F.L. Rosario-Ortiz, S. Canonica, Probe compounds to assess the photochemical activity of dissolved organic matter, Environ. Sci. Technol. 50 (2016) 12532–12547. [41] Y. Liu, X. He, X. Duan, Y. Fu, D. Fatta-Kassinos, D.D. Dionysiou, Significant role of UV and carbonate radical on the degradation of oxytetracycline in UV-AOPs: Kinetics and mechanism, Water Res. 95 (2016) 195–204. [42] K. Zoschke, H. Börnick, E. Worch, Vacuum-UV radiation at 185 nm in water treatment – a review, Water Res. 52 (2014) 131–145. [43] Y. Liu, Y. Zheng, B. Du, R.R. Nasaruddin, T. Chen, J. Xie, Golden carbon nanotube membrane for continuous flow catalysis, Ind. Eng. Chem. Res. 56 (2017) 2999–3007.
nutrients in seawater, TrAC Trends Anal. Chem. 60 (2014) 1–15. [22] M.D. Patey, M.J.A. Rijkenberg, P.J. Statham, M.C. Stinchcombe, E.P. Achterberg, M. Mowlem, Determination of nitrate and phosphate in seawater at nanomolar concentrations, TrAC Trends Anal. Chem. 27 (2008) 169–182. [23] J. Golimowski, K. Golimowska, UV-photooxidation as pretreatment step in inorganic analysis of environmental samples, Anal. Chim. Acta 325 (1996) 111–133. [24] C.M. Ma, G.B. Hong, H.W. Chen, N.T. Hang, Y.S. Shen, Photooxidation contribution study on the decomposition of azo dyes in aqueous solutions by VUV-based AOPs, Int. J. Photoenergy (2011). [25] Y.F. Huang, Y.H. Huang, Identification of produced powerful radicals involved in the mineralization of bisphenol a using a novel UV-Na2S2O8/H2O2-Fe(II, III) twostage oxidation process, J. Hazard. Mater. 162 (2009) 1211–1216. [26] M.G. Gonzalez, E. Oliveros, M. Wörner, A.M. Braun, Vacuum-ultraviolet photolysis of aqueous reaction systems, J. Photochem. Photobiol., C 5 (2004) 225–246. [27] S.A. Huber, A. Balz, M. Abert, W. Pronk, Characterisation of aquatic humic and nonhumic matter with size-exclusion chromatography–organic carbon detection–organic nitrogen detection (LC-OCD-OND), Water Res. 45 (2011) 879–885. [28] L. Yang, M. Li, W. Li, J.R. Bolton, Z. Qiang, A green method to determine VUV (185 nm) fluence rate based on hydrogen peroxide production in aqueous solution, Photochem. Photobiol. 94 (2018) 821–824. [29] J. Wang, M. Song, B. Chen, L. Wang, R. Zhu, Effects of pH and H2O2 on ammonia, nitrite, and nitrate transformations during UV254 nm irradiation: implications to nitrogen removal and analysis, Chemosphere 184 (2017) 1003–1011. [30] M. Seifhosseini, F. Rashidi, M. Rezaei, N. Rahimpour, Bias potential role in degradation of methyl orange in photocatalytic process, J. Photochem. Photobiol., A 360 (2018) 196–203. [31] S. Haji, B. Benstaali, N. Al-Bastaki, Degradation of methyl orange by UV/H2O2 advanced oxidation process, Chem. Eng. J. 168 (2011) 134–139. [32] L. Ling, J. Sun, J. Fang, C. Shang, Kinetics and mechanisms of degradation of
63
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
A green and robust method to measure nanomolar dissolved organic nitrogen (DON) by vacuum ultraviolet
T
⁎
Yi Chen, Junli Wang, Baiyang Chen , Lei Wang Shenzhen Key Laboratory of Organic Pollution Prevention and Control, State Key Laboratory of Urban Water Resource and Environment of Harbin Institute of Technology (Shenzhen), 518055, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
order to analyze nanomolar dis• Insolved organic nitrogen, VUV was evaluated.
converted ∼90% of dissolved • VUV organic nitrogen species into inorganic nitrogen.
pretreatment enabled a lower • VUV MDL (1.5 μg-N/L) than conventional methods.
If NH • into NO
+ was completely converted 4 − 3 , IC alone can measure DON
in water.
A R T I C LE I N FO
A B S T R A C T
Keywords: Total nitrogen Organic nitrogen VUV Sample pretreatment Photolysis
Quantifying trace dissolved organic nitrogen (DON) in water is currently challenging for conventional methods not only because of the presence of dissolved inorganic nitrogen (DIN) species, which usually dominate in total nitrogen (TN), but also because of the high method detection limits (MDLs) of current TN methods, which are ≥100 μg/L. In order to overcome these barriers, an earlier study has applied electrodialysis to eliminate DINs from water, thereby reducing the interference of DIN. However, it remains difficult to analyze DON at nanomolar level. To address this issue, this study proposes to convert DON completely into DIN by vacuum ultraviolet irradiation (VUV) and then to measure DON by the sum of DINs (i.e., ammonia, nitrite, and nitrate). A number of verification tests show that the proposed method had comparable results with conventional methods for selected model compounds and real samples. The average nitrogen recovery for sixteen model DON compounds (each at 2.0 mg-N/L) with varying structures and molecular weights was 89 ± 16%, similar to those obtained by thermal-activated persulfate oxidation (87 ± 18%) and high temperature catalytic oxidation (86 ± 20%) methods. Moreover, this method features an advantage that it reached a MDL of 1.5 μg-N/L, much lower than conventional TN methods. With a prolonged 12-h irradiation, this approach was able to convert ammonia completely into nitrate, thus enabling detection of DON by using ion chromatography only.
Abbreviations: DON, dissolved organic nitrogen; DIN, dissolved inorganic nitrogen; TN, total nitrogen; TOC, total organic carbon; MW, molecular weight; MDL, method detection limit; VUV, vacuum ultraviolet irradiation; ED, electrodialysis; UV, ultraviolet; IC, ion chromatography; MO, methyl orange; MCAN, chloroacetonitrile; HTCO, high temperature catalytic oxidation; TKN, total Kjeldahl nitrogen; TPO, thermal-activated persulfate oxidation; SPW, swimming pool water; EPA, environmental protection agency; HAN, haloacetonitrile; RSD, relative standard deviation; EDTA, ethylene diamine tetraacetic acid ⁎ Corresponding author. E-mail address: [email protected] (B. Chen). https://doi.org/10.1016/j.cej.2019.01.124 Received 29 October 2018; Received in revised form 15 January 2019; Accepted 18 January 2019 Available online 22 January 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Chemical Engineering Journal 363 (2019) 57–63
Y. Chen et al.
its high MDL, which is usually ≥200 μg-N/L [12]; not to say that nitrate is a strong interferent of the conversion process [20]. Therefore, there are motives to develop new methods to overcome the second barrier [21,22]. In this study, vacuum UV irradiation (VUV) is proposed to provide a green and robust pretreatment method for nanomolar DON analysis. According to literature, UV and hydrogen peroxide (UV/H2O2) were once integrated to analyze TN at trace levels; however, it resulted in lower recovery yet higher variability than HTCO and thermal-activated persulfate oxidation (TPO) methods [19,23]. Therefore, the UV/H2O2 method was not considered as a promising alternative. In contrast, VUV photolysis is known to be robust in degrading or mineralizing aqueous components, either directly by direct light irradiation or indirectly via formation of hydroxyl radical (%OH), a strong non-selective oxidant [24,25], and ozone, a strong selective oxidant [26]. Therefore, VUV is likely applicable to convert DON completely into DIN. Moreover, VUV irradiation is easy to operate and theoretically introduces no external ions into samples; therefore the pretreated solution can be tested by IC. In an earlier study, VUV was already employed in a size-exclusion chromatographer to analyze total organic carbon (TOC) and TN [27], thus demonstrating the promise of this method. However, it requires systemic verification, intensive examination, and more comparison with conventional methods. This study is committed to proving this approach. Firstly, we compared the VUV photolysis method with two conventional methods (i.e., TPO and HTCO) in converting DON into DIN for 16 model DON compounds. Then, in order to optimize the conversion efficiency, we evaluated the influences of several operational and environmental conditions (i.e., pH, light dosage, and typical ions) on VUV performance. Next, for the purpose of handy DON measurement, we converted glycine and its photolysis product, ammonia, into nitrate completely with a prolonged irradiation process. Finally, we demonstrated the calibration curve, application scope, and a comparison of DON recoveries in real tap water between this method and conventional methods.
1. Introduction Dissolved organic nitrogen (DON) is ubiquitously present in marine and terrestrial environments and it is an essential part in the nitrogen element cycles [1,2]. According to literatures, DON may account for up to 70% of total nitrogen (TN) in certain rivers near the Atlantic Ocean [3] or up to 80% in tertiary effluents [4]. The compositions of DON include humic substance, peptides, proteins, amino sugars, and amides [5]. Besides its roles as a nutrient, DON also acts as contaminant in natural environment that can sometimes cause certain environmental issues, such as eutrophication [6], or formation of nitrogenous disinfection byproducts in engineering water system (e.g., drinking water and wastewater treatment processes), which are potentially highly toxic to human and ecosystem [7,8], or be associated with membrane fouling in water treatment processes [9]. So, it is important to measure DON to understand its occurrence or to control it. However, there are two challenges currently facing with DON measurement. The first challenge is that DON is determined by the calculative difference between two sets of nitrogen-containing parameters, including 1) TN and 2) dissolved inorganic nitrogen (DIN) species, i.e., ammonia, nitrite, and nitrate [5]. The approach is problematic for samples with low ratio of DON to TN because the subtraction process can significantly magnify the analytical errors of each parameter’s analysis, making DON data unreliable [7,10]. In certain cases when DIN dominates TN (i.e., DON accounts for only a very small fraction of TN), the calculated DON was obtained even as negative values [11]. The second barrier of DON measurement is associated with the method detection limits (MDLs) of current TN methods, which are ≥100 μg-N/L now, making measurement of nanomolar DON impossible. In an earlier review, it was reported that approximately 30% of surface water, 76% of shallow groundwater, and 81% of deep groundwater samples have DON below the MDL of current TN analytical methods [12]. These two barriers hence hindered our understanding of the occurrence and roles of DON in the environment and engineering systems. In order to overcome the first challenge, an earlier study has applied electrodialysis (ED) to eliminate DIN from water (> 99%) and recover most of DON (mean recovery = 88%) [13], thereby dramatically reducing the analytical errors brought by the subtraction process. However, it remains challenging to detect samples with trace amount of DON (i.e., at nanomolar level) because of the high MDL. Prior to this study, we briefly reviewed three approaches currently used for TN analysis. The first approach involves conversion of TN into nitrate using persulfate-based oxidative methods such as thermal [14], microwave [15], and ultraviolet (UV) activated persulfate [16], and then measurement of nitrate was achieved with spectrophotometry. Due to the uses of persulfate, this approach has very high level of sulfate formation and therefore cannot employ ion chromatography (IC) to analyze nitrate. The concerns come from the interference of sulfate on nitrate and the impact of sulfate on the ion-exchanging capacity of IC column. Since spectrophotometry has higher MDL than IC in detecting nitrate, this approach usually reported MDL of TN > 100 μg-N/L. Notably, certain impurities in persulfate may lead to high blank data [17]. The second approach of analyzing TN is using thermal-based combustion methods, such as high temperature combustion or high temperature catalytic oxidation (HTCO) [18], to convert TN into nitric oxide gas and then detecting the gas by chemiluminescence analyzer. Although convenient and automated, a previous comparison study showed that thermal-based methods were lower in TN recovery than persulfatebased methods [19]. More importantly, the MDLs of thermal-based methods are also ≥100 μg-N/L even per manufacturers’ guidance unless it is done under vacuum conditions. The third approach for analyzing TN is related to the total Kjeldahl nitrogen (TKN) method, which reduces DON compounds into ammonia first and then quantifies DON by subtracting ammonia from TKN. This method was less used now not only because of its extreme complexity in operation but also because of
2. Methods and materials 2.1. Chemicals and sample preparation In verification tests, sixteen DON model compounds with varying molecular weight (MW) and structures were selected. Their chemical structures, MWs, and formula are presented in Table S1. Before experiments, each compound was dissolved in ultrapure water individually as generated by a Millipore water generator (Direct-Q3) and then stored in a refrigerator (4 °C). Sulfuric acid (H2SO4) and sodium hydroxide (NaOH) were used to adjust water pH in case of need. Three real water samples were collected for comparison, including a river water taken from the Dasha River, a tap water collected from the laboratory faucet, and a swimming pool water (SPW) collected from the campus stadium. Prior to use, they were filtered through 0.45 μm nylon fiber membranes and treated with ED to eliminate DINs. 2.2. Analytical methods Ammonia was detected by a spectrophotometer (DR 3900, Hach Co., Ltd.) according to the United States Environmental Protection Agency (EPA) method 350.2 with the MDL as 20 µg-N/L. Nitrate and nitrite were analyzed by an IC (IC2010, Tosoh Inc., Japan) equipped with an AS15 column (Thermofisher, USA), an eluent generator (EDG100, Minghao Chromatographic Technology, China), an eluent suppressor (WLK-6, Reepo analytical instrument Co., Ltd, China), and a 100 μL injection loop. According to the EPA method 300.0 guideline, the MDLs of nitrite and nitrate are estimated to be 1.5 µg-N/L under the conditions of this study. The pH was monitored by an electrode meter (pH100, Extech Instruments Co.). Two conventional TN analytical methods were compared with the 58
Chemical Engineering Journal 363 (2019) 57–63
Y. Chen et al.
new method. The first conventional method was TPO method, in which potassium persulfate was activated at 125 °C using a heater (DIS-1A, Changhong Technology Co., Ltd.) to convert TN completely into nitrate within 40 min; and then nitrate was analyzed by a spectrophotometer (DR 3900, Hach Co., Ltd.). The second conventional TN analytical method was HTCO, which was achieved by a TOC combustor coupled with a TN detector (TOC-LCPH, Shimadzu Co., Ltd). The principle of HTCO method lies in the conversion of TN into nitric oxide gas at 720 °C and then detection of gas with a chemiluminescence detector. The MDLs of TN for both TPO and HTCO are 0.1 mg-N/L per manufacturers’ guidance.
recoveries were observed for MO, melamine, and MCAN for the three tested methods. The nitrogen recoveries from MO were poor for all three methods, confirming that it is either refractory [30–32] or easy to form nitrogenous gases that cannot be captured [33]. The recovery of N by VUV was higher than those achieved by TPO and HTCO for MO, but lower than TPO for MCAN and worse than HTCO for melamine. These results somehow agree with another study that melamine did not react rapidly with %OH in VUV process [34]. As for haloacetonitrile (HAN), an earlier study proved that MCAN was the most refractory compound of all tested HANs while imine (C]N)-containing compounds were the major products of VUV photolysis [35]. In order to distinguish the roles of VUV and UV254, Fig. S1 presents the recovery differences of three typical DON compounds, i.e., urea, MCAN, and trichloroacetamide, irradiated by two different types of lamps with identical power intensity. The results show that these compounds were transformed to greater extents by VUV lamps emitting both 185 nm and 254 nm lights than UV lamps emitting 254 nm light only. Therefore, it confirms that the conversions of DON were mainly triggered by 185 nm light (i.e., VUV). In terms of trends, the recoveries of nitrogen appeared less dependent on the MWs of compounds but more associated with their structural properties. Of 16 model compounds, 13 of them resulted in formation of ammonia as the dominant photolysis products but only 3 of them yielded nitrate as the major product (Table S1). Although it was only a snapshot of speciation, certain patterns can be obtained. The compounds with amide (O]CeNH2) or nitro (eNO2) functional groups were more likely converted to nitrate, while the compounds with carboxyl (eCOOH), imine (C]N, except for melamine), and nitrile (C^N) functional groups were mainly transformed into ammonia. Based on literature, VUV can transform compounds in water via three mechanisms: 1) direct photolysis via 254 and/or 185 nm lights; 2) oxidation via formation of %OH, a non-selective strong oxidant (Eqs. (1) and (2)); and 3) reduction via formation of hydrated electron (e−) and hydrogen atom (H%) (Eqs. (1) and (2)). From this study, it is likely that the amidecontaining compounds mainly underwent oxidation process by %OH oxidation, while other compounds undertook CeN bond cleavages by 185 nm irradiation directly under the conditions of this study. Furthermore, since these experiments were carried out under aerated conditions, H% and e− might react with dissolved oxygen and form other reactive species such as superoxide radical anion (%O2−) and per hydroxyl radical pair (Eqs. (3) and (4)).
2.3. Photoreactor and experimental procedures The VUV photoreactor used in this study consists of a stainless-steel frame holding at most four low-pressure mercury lamps (12 W, UV-Tec Co., Ltd) emitting both 185 nm and 254 nm lights. The photon flux of 185 nm VUV was estimated to be 1.23 × 10−3 μEinstein/s/cm2 according to a method recently introduced in a literature [28], which roughly accounts for 5% of total UV light intensity. For energy calculation convenience, the irradiance of the reactor was obtained by multiplying the power intensity of lamp (i.e., 12 W) by the reaction time and number of lamps, and then dividing it by the volume of treated sample, i.e., power intensity of lamp × number of lamps × reaction time/water volume. A quartz tube (22 cm long, 1.5 cm in diameter, 30 mL in volume, and made of Suprasil quartz) as a water container was placed in the middle of the chamber. The lamps and quartz tube were deployed in parallel and as close as possible to minimize VUV light loss. In addition, a fan was placed on top of the reactor to cool the air and vent out ozone formed in the reactor. More details about the reactor are described elsewhere [29]. 3. Results and discussion 3.1. Model compounds evaluation Fig. 1a shows that 90-min VUV irradiation achieved an average N recovery of 89 ± 16% for 16 model DON compounds, each set at 2 mgN/L and tested separately without pH buffer. The average recoveries were similar to those obtained by TPO (87 ± 18%) and HTCO (86 ± 20%) methods, meaning that VUV can obtain comparable nitrogen recoveries as conventional methods. Of them, relatively low 120 TPO
VUV (
NO
NO
NH
)
HTCO
20
(a)
18 16 DON Conc. (mg-N/L)
DON Recovery (%)
100
(b)
80 60 40
14 12 10
8 6 4
20
2 0
0
Fig. 1. A comparison of DON recoveries using three analytical methods on a) 16 model compounds (C0 = 2.0 mg-N/L, volume = 30 mL, irradiation time = 90 min, total irradiation energy = 1200 kWh/m3) and b) three real water samples pretreated with electrodialysis (volume = 5 mL, irradiation time = 120 min, total light energy = 9600 kWh/m3). 59
Chemical Engineering Journal 363 (2019) 57–63
Y. Chen et al.
120
Urea
Glycine
MCAN
scavengers in water, we evaluated their influences on the VUV treatability for four typical DON compounds. According to Fig. 2b, adding 10 mg/L chloride (which has pH of 6.5) imposed minor effect on the DON recovery by VUV, which is not surprising because it has low UV absorptivity and reactivity with %OH [36,37]. Furthermore, although chloride features high reactivity with %OH, the adduct dissociates readily above pH of 5 and gives back %OH; only when pH falls below 5 chloride can be transformed to lowly reactive chlorine radical ions (Cl2%−) [39]. In contrast, adding 100 mg/L carbonate (which has pH of 10.6) substantially reduced the DON recoveries, an indicator of the presence of inhibitors. The dosages of chloride and carbonate mimicked the water quality condition of a local surface water, and essentially represented a condition with maximum interference to the proposed VUV process because all samples are supposed to be pretreated by ED first, which can significantly reduce the amount of total dissolved solid to below 10 mg/L [13]. To distinguish the roles of carbonate and hydroxide, 7 mg/L sodium hydroxide was added to prepare a solution with identical pH (10.6) as the sample dosed with 100 mg/L sodium carbonate. A comparison of them shows that the samples dosed with carbonate resulted in less DON recovery than those dosed with hydroxide; so the results confirm that both carbonate and hydroxide have suppressed the conversions of DON, either as %OH scavengers [40] or as light competitors. Possibly, in the alkaline condition OH− has reacted with carbonate to form carbonate radical anion, which is a weak oxidizing radical [41].
(a)
MO
DON Recovery (%)
100 80 60 40 20 0 4.0
6.5
8.6
9.2
10.6
11.1
pH 120
(b) DON Recovery (%)
100 80
60 40 20 0 Control
Cl
CO ²
3.3. Influence of light dosage
OH
In order to optimize the process, Fig. 3 presents the effect of light energy input on the DON recovery. It was clear that increasing irradiation energy from 200 to 1600 kWh/m3 significantly promoted the TN recoveries for urea and MO, which achieved almost 100% with an irradiation of 1600 kWh/m3. However, low or no significant enhancement was observed for MCAN and glycine irradiation processes, in which the TN recoveries were only enhanced from 40 to 64% for MO and little impact on MCAN. For glycine, because it can be readily transformed with 200 kWh/m3, no additional benefits were obtained with more irradiation flux.
Fig. 2. The influences of a) pH and b) chloride (10.0 mg/L), carbonate (100.0 mg/L), and hydroxide (7.0 mg/L) on the DON recovery by VUV photolysis (C0 = 2.0 mg-N/L, volume = 30 mL, irradiation time = 60 min, total light energy value = 800 kWh/m3). hν
H2 O→ ∙OH + H∙ hν
(1)
H2 O→ ∙OH + H+ + e−
(2)
e− + O2 → %O2−
(3)
H% + O2 → HO2%
(4)
3.4. Inorganic nitrogen conversion Ideally, if both DON and DINs can be converted completely into nitrate, the amount of DON could be obtained by analyzing nitrate with IC only, which would be convenient. To reach this goal, we hereafter made an extra effort to convert ammonia derived from DON into nitrate. In this part glycine was chosen because it yields mostly ammonium during the VUV photolysis process (Table S1). In addition,
3.2. Influence of pH and coexisting compounds To better understand the VUV efficiency under practical conditions, Fig. 2a shows the TN recoveries for four representative DON compounds at a pH range from 4.0 to 11.1. In general, increasing pH resulted in either unchanged or lower DON recovery although the extents of influence were compound-specific. For example, when pH increased from 4.0 to 8.6, the recoveries of urea and glycine remained constant (> 80%); however, when pH went above 8.6, their recoveries dropped dramatically. Similarly, the recoveries of MCAN and MO were constant from pH 4.0 to 9.2 but became much lower at pH above 9.2. These phenomena indicate that acidic and neutral solutions were preferred over alkaline solution on DON conversion, suggesting that one of the key drivers of the process was %OH, which features lower reactivity in alkaline conditions than that in acidic conditions [36,37]. According to literature, the pKa value of %OH is 11.9 [38], meaning that when pH is > 11.9 most of %OH become less reactive O%−. In this study, the pH were all below 11.9; so, the decrease of DON transformation with increasing pH was likely due to the conversion of HO% into O%− (Eq. (5)) and/or the consumption of %OH by OH−. HO% + OH− → O%− + H2O
120 Urea
Glycine
MCAN
MO
DON Recovery (%)
100 80 60
40 20 0 200
(5)
400 800 Light Energe (kWh/m3)
1600
Fig. 3. The influence of light energy on the recoveries of selected DON compounds (2.0 mg-N/L) by VUV photolysis.
Because chloride and carbonate are two known %OH inorganic 60
Chemical Engineering Journal 363 (2019) 57–63
Y. Chen et al.
1600
Light Energy (kWh/m3) 3200 4800 6400
120
8000
9600 Measured DON ( g-N/L)
0 2.2
Conc. (mg-N/L)
1.8 TN
1.4
NO 1.0
NO
0.6
NH /NH
(a)
Urea: y = 1.0025x + 0.4082 R² = 0.9998
100 80 60
Urea
40 20 0 0
0.2
20
40
60
80 -N/L)
100
120
-0.2 2
4
6 Time (h)
8
10
12
5 Measured DON (mg-N/L)
0
Fig. 4. The conversion of DON into DIN species during VUV photolysis of glycine (C0 = 2.0 mg-N/L, light intensity = 400 kW/m3, pH = 9.2).
because we know from literature that the reaction between ammonia and %OH is slow and the rate increases with increasing pH [29], the following tests were carried out at pH of 9.2 to balance the needs of both DON conversion and ammonia transformation. Fig. 4 shows the DON conversion and DIN interconversions during a 12-h VUV irradiation process using two VUV lamps, which ultimately reached 9600 kWh/m3. The measured TN stayed within 9.8% variance throughout the process, meaning that there was no substantial amount of other nitrogen-containing compounds formed. Ammonium appeared immediately after VUV irradiation and reached peak in 30 min, confirming that glycine was fully converted to ammonia in this period. However, after approximately 2 h, the amount of ammonia gradually decreased whereas the amount of nitrate kept increased over time. Until 12 h, nitrate accounted for about 99% of TN, meaning that nearly all ammonia had been converted to nitrate. During the process, nitrite appeared at the first 1.5 h but decreased later and became undetectable after 4 h. The result agreed well with earlier findings that %OH can convert ammonium slowly into nitrite and nitrate but can convert nitrite rapidly into nitrate [29,42]. As a result, nitrate became reasonably the dominant and final DIN product at the end [29,42] for IC analysis. The results hence demonstrated an example of complete conversion of DON and ammonia into nitrate. Although the time required to convert ammonia into nitrate was still too long under the conditions of this study, this approach might still be favored because it substantially reduce the operational tediousness of measuring multiple DIN parameters and allow a very low MDL of DON. In addition, it holds hopes for further optimization by increasing light dosage, designing better photoreactor, and integrating robust immobilized catalyst [43] into the photoreactor in the future.
4
(b)
Glycine: y = 0.9483x + 0.0369 R² = 0.9985 (0.1-5.0 mg-N/L)
Glycine Urea
3 2 1
Urea: y = 0.9696x + 0.0038 R² = 0.9997 (0.1-2.0 mg-N/L)
0 0
1
2 3 4 Dosed Concentration (mg-N/L)
5
Fig. 5. The calibration curves for a) urea at low concentrations from 0.005 to 0.1 mg-N/L and b) urea and glycine at high concentration ranges from 0.1 to 5.0 mg-N/L (Total light energy = 9600 kWh/m3, samples unbuffered).
DON. That is, when the initial dosage of urea was above 2.0 mg-N/L, the gaps between measured and spiked TN became large. This certainly indicates an inadequacy or uncertainty of VUV photolysis to analyze samples with high DON content. Although irradiation of glycine shows reasonable correlation between measured and spiked TN from 0.1 to 5.0 mg-N/L, the influence of initial concentration on VUV treatability requires extra attention. In addition to calibration curve, the repeatability of this method was checked by using synthetic waters dosed with various amounts of urea. Fig. S2 shows a repeat of six urea measurements at either 2.0 or 0.5 mgN/L for different irradiation periods, and the results showed comparable recoveries among samples (RSD ≤ 6.9%), suggesting that the results are repeatable. 3.6. Real sample evaluation Fig. 1b shows the DON amounts detected in three types of real water, including a tap water, a river water, and a swimming pool water (SPW). In general, the measured DON followed as TPO > VUV > HTCO, but VUV has the lowest RSD of all methods, confirming that VUV method was equivalent to other two conventional TN methods. Prior to comparison, the samples were pretreated with ED to eliminate DINs according to the method introduced in earlier study [13], and the SPW samples were diluted by 10-fold. For the tap water, because residual disinfectant was present and certain non-ionizable inorganic chloramine (such as monochloramine) may be formed, sodium sulfite as a quenching agent was added to convert chlorine and chloramines into ammonia and chloride, therefore all DINs were eliminated. In order to further verify this method, we compared the results of three analytical methods for samples diluted with different amounts of ultrapure water. As shown in Fig. 6a, the DON concentrations decreased proportionally with the dilution ratios, and the correlation curve
3.5. Application scope and MDL Herein we chose urea and glycine as two reference compounds for application scope evaluations because they yielded significantly different DIN products. The slopes, intercepts, R2, and relative standard deviations (RSDs) for the calibration curves are presented in Fig. 5a. The information regarding the slope (1.002), R2 (> 0.999) and RSDs (< 5%) of the concentrations spanning from 5 to 100 μg-N/L support that the VUV method can provide not only accurate but also precise DON measurement at nanomolar level. Because the MDL of DON is essentially dependent on the MDLs of DINs, the MDL of DON via VUV pretreatment may be 20 μg-N/L if the final DIN is ammonia or 1.5 μg-N/ L if the ultimate product is nitrate. At high DON range (i.e., 0.1–5 mg-N/L), Fig. 5b shows that urea did not always show excellent correlations between measured and dosed 61
Chemical Engineering Journal 363 (2019) 57–63
Y. Chen et al.
400
-N/L)
350 300 250 200
TPO 360 300 240 180 120 60 0
VUV
HTCO
completely converted into nitrate, a convenient measurement of DON may be enabled by using IC only. Overall, the study demonstrated VUV as a green and robust tool to detect nanomolar DON, which may help better understand the roles and occurrences of DON in the environment and engineering water systems.
(a)
(b)
y = 330.02x R² = 0.9032
y = 305.44x y = 293.39x R² = 0.9893 R² = 0.8584 0.0
0.5
Acknowledgements
1.0
150
The study was financially supported by the Shenzhen Science & Technology Innovation Commission (JCYJ20170818091859147). Also, we appreciate our crews in the laboratory (Ting Jie and Yinan Bu) for their technical helps.
100 50
0 0.00
-N/L)
800 700
600
700 600 500 400 300 200
0.25
0.34 0.50 Concentration ratio (d)
y = 0.8155x + 351.36 R² = 0.9523 y = 0.8177x + 299.01 R² = 0.9539 0
100
200
1.00
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.01.124.
(c)
y = 1.0948x + 308.16 R² = 0.9787
References
300
[1] J.C. Neff, F.S. Chapin, P.M. Vitousek, Breaks in the cycle: dissolved organic nitrogen in terrestrial ecosystems, Front. Ecol. Environ. 1 (2003) 205–211. [2] D.E. Canfield, A.N. Glazer, P.G. Falkowski, The evolution and future of earth's nitrogen cycle, Science 330 (2010) 192–196. [3] N. van Breemen, Natural organic tendency, Nature 415 (2002) 381. [4] E. Pehlivanoglu-Mantas, D.L. Sedlak, Wastewater-derived dissolved organic nitrogen: analytical methods, characterization, and effects—a review, Crit. Rev. Environ. Sci. Technol. 36 (2006) 261–285. [5] B. Tom, A.B. Deborah, Dissolved organic nitrogen: a dynamic participant in aquatic ecosystems, Aquat. Microb. Ecol. 31 (2003) 279–305. [6] J. Sun, H. Simsek, Bioavailability of wastewater derived dissolved organic nitrogen to green microalgae Selenastrum capricornutum, chlamydomonas reinhardtii, and chlorella vulgaris with/without presence of bacteria, J. Environ. Sci. (China) 57 (2017) 346–355. [7] W. Lee, P. Westerhoff, Dissolved organic nitrogen measurement using dialysis pretreatment, Environ. Sci. Technol. 39 (2005) 879–884. [8] S.W. Krasner, P. Westerhoff, B. Chen, B.E. Rittmann, S.N. Nam, G. Amy, Impact of wastewater treatment processes on organic carbon, organic nitrogen, and DBP precursors in effluent organic matter, Environ. Sci. Technol. 43 (2009) 2911–2918. [9] X. Yue, Y.K.K. Koh, H.Y. Ng, Effects of dissolved organic matters (DOMs) on membrane fouling in anaerobic ceramic membrane bioreactors (AnCMBRs) treating domestic wastewater, Water Res. 86 (2015) 96–107. [10] J.F. Saunders, Y. Yu, J.H. McCutchan, F.L. Rosario-Ortiz, Characterizing limits of precision for dissolved organic nitrogen calculations, Environ. Sci. Technol. Lett. 4 (2017) 452–456. [11] D. Graeber, J. Gelbrecht, B. Kronvang, B. Gücker, Technical note: comparison between a direct and the standard, indirect method for dissolved organic nitrogen determination in freshwater environments with high dissolved inorganic nitrogen concentrations, Biogeosci. (BG) Discuss. (BGD) (2012). [12] P. Westerhoff, H. Mash, Dissolved organic nitrogen in drinking water supplies: a review, J. Water Supply: Res. Technol.-Aqua 51 (2002) 415. [13] A. Zhu, B. Chen, L. Zhang, P. Westerhoff, Improved analysis of dissolved organic nitrogen in water via electrodialysis pretreatment, Anal. Chem. 87 (2015) 2353–2359. [14] R. Benner, B. Von Bodungen, J. Farrington, J. Hedges, C. Lee, F. Mantoura, Y. Suzuki, P.M. Williams, Measurement of dissolved organic carbon and nitrogen in natural waters: workshop report, Mar. Chem. 41 (1993) 5–10. [15] E. Dafner, S. De Galan, L. Goeyens, Microwave digestion of organic substances, a useful tool for dissolved organic nitrogen measurements, Water Res. 33 (1999) 548–554. [16] F.A.J. Armstrong, P.M. Williams, J.D.H. Strickland, Photo-oxidation of organic matter in sea water by ultra-violet radiation, analytical and other applications, Nature 211 (1966) 481. [17] E.S.A. Badr, E.P. Achterberg, A.D. Tappin, S.J. Hill, C.B. Braungardt, Determination of dissolved organic nitrogen in natural waters using high-temperature catalytic oxidation, TrAC Trends in Anal. Chem. 22 (2003) 819–827. [18] T.W. Walsh, Total dissolved nitrogen in seawater: a new-high-temperature combustion method and a comparison with photo-oxidation, Mar. Chem. 26 (1989) 295–311. [19] J.H. Sharp, K.R. Rinker, K.B. Savidge, J. Abell, J. Yves Benaim, D. Bronk, D.J. Burdige, G. Cauwet, W. Chen, M.D. Doval, D. Hansell, C. Hopkinson, G. Kattner, N. Kaumeyer, K.J. McGlathery, J. Merriam, N. Morley, K. Nagel, H. Ogawa, C. Pollard, M. Pujo-Pay, P. Raimbault, R. Sambrotto, S. Seitzinger, G. Spyres, F. Tirendi, T.W. Walsh, C.S. Wong, A preliminary methods comparison for measurement of dissolved organic nitrogen in seawater, Mar. Chem. 78 (2002) 171–184. [20] P.J. Johnes, A.L. Heathwaite, A procedure for the simultaneous determination of total nitrogen and total phosphorus in freshwater samples using persulphate microwave digestion, Water Res. 26 (1992) 1281–1287. [21] J. Ma, L. Adornato, R.H. Byrne, D. Yuan, Determination of nanomolar levels of
500
400 300 200 0
50
100
150 -N/L)
200
300
Fig. 6. A comparison of three methods on the DON measurements for a) the concentration and b) the trendlines of a real tap water and diluted samples; and c) the concentration and d) the trendlines of a real tap water added with various amounts of urea (light energy: 9600 kWh/m3).
between the DON contents and the dilution ratios has a R2 > 0.98 and a maximum RSD of 10.15%. In contrast, the calibration correlation coefficients were 0.90 for TPO method and 0.85 for HTCO method. Although the exact amount of DON in real tap water was unknown, it at least indicates that the VUV pretreatment method could recover them with consistent efficiency. As for DON recovery, Fig. 6b shows reasonable trends (slope = 1.0948) for the VUV method when the tap water was added with varying amounts of urea from 50 to 300 μg-N/L, meaning that urea has been completely converted. In comparison, the concentrations of samples measured by TPO method were higher in RSD, especially when the amount of added urea was 50 μg-N/L, meaning that the TPO method was less suitable to detect DON at low levels. In contrast, the data obtained by HTCO method were generally lower than other methods, suggesting that certain substances cannot be completely converted into nitric oxide gas. 4. Conclusions In order to detect trace DON in water, this study employed VUV pretreatment to convert DON completely into DINs. In general, the study proved that the VUV method has similar recoveries and RSDs as conventional methods, but better than conventional methods in achieving lower MDL of DON (i.e., 1.5 μg-N/L). The recoveries of DON via VUV increased with increasing light dosage, decreasing pH, or lower DON concentration. The favorable pH condition was ≤8.6, and the preferred DON concentration of this method was ≤2 mg-N/L. The presence of chloride at selected dosage imposed minor effect on the conversions, but carbonate and hydroxide have significant suppressing effects on the VUV performance, meaning that ED as an anion removal process is needed prior to VUV irradiation process. If ammonia can be 62
Chemical Engineering Journal 363 (2019) 57–63
Y. Chen et al.
chloroacetonitriles by the UV/H2O2 process, Water Res. 99 (2016) 209–215. [33] T. Tasaki, T. Wada, K. Fujimoto, S. Kai, K. Ohe, T. Oshima, Y. Baba, M. Kukizaki, Degradation of methyl orange using short-wavelength UV irradiation with oxygen microbubbles, J. Hazard. Mater. 162 (2009) 1103–1110. [34] V. Maurino, M. Minella, F. Sordello, C. Minero, A proof of the direct hole transfer in photocatalysis: the case of melamine, Appl. Catal. A 521 (2016) 57–67. [35] P. Kiattisaksiri, E. Khan, P. Punyapalakul, T. Ratpukdi, Photodegradation of haloacetonitriles in water by vacuum ultraviolet irradiation: mechanisms and intermediate formation, Water Res. 98 (2016) 160–167. [36] S. Gligorovski, R. Strekowski, S. Barbati, D. Vione, Environmental implications of hydroxyl radicals (OH), Chem. Rev. 115 (2015) 13051–13092. [37] S. Gligorovski, R. Strekowski, S. Barbati, D. Vione, Addition and correction to environmental implications of hydroxyl radicals (OH), Chem. Rev. 118 (2018) 2296–2296. [38] Y. Nosaka, A.Y. Nosaka, Generation and detection of reactive oxygen species in photocatalysis, Chem. Rev. 117 (2017) 11302–11336. [39] G.V. Buxton, C.L. Greenstock, W.P. Helman, A.B. Ross, Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (OH/O− in aqueous solution), J. Phys. Chem. Ref. Data 17 (1988) 513–886. [40] F.L. Rosario-Ortiz, S. Canonica, Probe compounds to assess the photochemical activity of dissolved organic matter, Environ. Sci. Technol. 50 (2016) 12532–12547. [41] Y. Liu, X. He, X. Duan, Y. Fu, D. Fatta-Kassinos, D.D. Dionysiou, Significant role of UV and carbonate radical on the degradation of oxytetracycline in UV-AOPs: Kinetics and mechanism, Water Res. 95 (2016) 195–204. [42] K. Zoschke, H. Börnick, E. Worch, Vacuum-UV radiation at 185 nm in water treatment – a review, Water Res. 52 (2014) 131–145. [43] Y. Liu, Y. Zheng, B. Du, R.R. Nasaruddin, T. Chen, J. Xie, Golden carbon nanotube membrane for continuous flow catalysis, Ind. Eng. Chem. Res. 56 (2017) 2999–3007.
nutrients in seawater, TrAC Trends Anal. Chem. 60 (2014) 1–15. [22] M.D. Patey, M.J.A. Rijkenberg, P.J. Statham, M.C. Stinchcombe, E.P. Achterberg, M. Mowlem, Determination of nitrate and phosphate in seawater at nanomolar concentrations, TrAC Trends Anal. Chem. 27 (2008) 169–182. [23] J. Golimowski, K. Golimowska, UV-photooxidation as pretreatment step in inorganic analysis of environmental samples, Anal. Chim. Acta 325 (1996) 111–133. [24] C.M. Ma, G.B. Hong, H.W. Chen, N.T. Hang, Y.S. Shen, Photooxidation contribution study on the decomposition of azo dyes in aqueous solutions by VUV-based AOPs, Int. J. Photoenergy (2011). [25] Y.F. Huang, Y.H. Huang, Identification of produced powerful radicals involved in the mineralization of bisphenol a using a novel UV-Na2S2O8/H2O2-Fe(II, III) twostage oxidation process, J. Hazard. Mater. 162 (2009) 1211–1216. [26] M.G. Gonzalez, E. Oliveros, M. Wörner, A.M. Braun, Vacuum-ultraviolet photolysis of aqueous reaction systems, J. Photochem. Photobiol., C 5 (2004) 225–246. [27] S.A. Huber, A. Balz, M. Abert, W. Pronk, Characterisation of aquatic humic and nonhumic matter with size-exclusion chromatography–organic carbon detection–organic nitrogen detection (LC-OCD-OND), Water Res. 45 (2011) 879–885. [28] L. Yang, M. Li, W. Li, J.R. Bolton, Z. Qiang, A green method to determine VUV (185 nm) fluence rate based on hydrogen peroxide production in aqueous solution, Photochem. Photobiol. 94 (2018) 821–824. [29] J. Wang, M. Song, B. Chen, L. Wang, R. Zhu, Effects of pH and H2O2 on ammonia, nitrite, and nitrate transformations during UV254 nm irradiation: implications to nitrogen removal and analysis, Chemosphere 184 (2017) 1003–1011. [30] M. Seifhosseini, F. Rashidi, M. Rezaei, N. Rahimpour, Bias potential role in degradation of methyl orange in photocatalytic process, J. Photochem. Photobiol., A 360 (2018) 196–203. [31] S. Haji, B. Benstaali, N. Al-Bastaki, Degradation of methyl orange by UV/H2O2 advanced oxidation process, Chem. Eng. J. 168 (2011) 134–139. [32] L. Ling, J. Sun, J. Fang, C. Shang, Kinetics and mechanisms of degradation of
63