Degradation of sodium dodecyl benzenesulfonate by vacuum ultraviolet irradiation

Journal of Water Process Engineering 34 (2020) 101172

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Degradation of sodium dodecyl benzenesulfonate by vacuum ultraviolet irradiation

T

Hang Lia, Yanling Yanga, Jingfeng Gaob, Xing Lia, Zhiwei Zhoua,b,*, Nan Wanga, Peng Dua, Tingting Zhanga, Jianyong Fenga a b

College of Architecture and Civil Engineering, Beijing University of Technology, Beijing 100124, China National Engineering Laboratory for Advanced Municipal Wastewater Treatment and Reuse Technology, Beijing University of Technology, Beijing 100124, China

A R T I C LE I N FO

A B S T R A C T

Keywords: SDBS Vacuum ultraviolet (VUV) Ultraviolet (UV) Bathing and laundry wastewater

Sodium dodecyl benzenesulfonate (SDBS) is a common anionic surfactant used in detergents, and a major household pollutant. SDBS released with bathing and laundry wastewater causes eutrophication of water bodies and toxicity in aquatic organisms. We compared the degradation of SDBS in aqueous solution, by vacuum ultraviolet (VUV, 254 nm and 185 nm) and ultraviolet irradiation (UV, 254 nm). Using lab-scale reactors, VUV degraded SDBS more efficiently than UV, also achieving a higher mineralization rate. HO% was the main reactive oxygen species produced by VUV. SDBS concentration affected the distribution of absorbed VUV and UV photons, and influenced the reaction triggered. VUV was more efficient at lower concentrations, when indirect oxidation by HO% was prevalent. The UV process, relied mostly on direct photolysis, and was affected slightly by initial concentration. VUV was more efficient at solution pH of 5 and 9, while UV performed better at alkaline conditions. Temperature increased the degradation rates for both processes. The addition of SO42− slightly promoted SDBS degradation in the VUV process, while Cl− and HCO3− inhibited it. For UV, SO42− and Cl− illustrated no significant difference on the degradation, while HCO3− had a positive impact on the system. We identified the four photoproducts of SDBS degradation by VUV, and proposed a degradation pathway. Finally, the fact the VUV could be efficiently used to remove anionic surfactant from real wastewater, proved that it can be applied efficiently with low energetic consumption.

1. Introduction Surfactants are widely employed in household cleaning detergents, personal care products, and in the industrial, medical, and biological fields. They are one of the main household pollutants released with wastewater [1–5]. The most common surfactants in detergents are anionic linear alkylbenzene sulfonates (LAS) [6,7]. Extensive use of LAS, including the pervasive sodium dodecyl benzenesulfonate (SDBS), has serious environmental consequences, through eutrophication of water bodies and toxic effects to aquatic organisms [8,9]. Various water treatment technologies, including biological degradation, coagulation, advanced oxidation process (AOP), membrane filtration, and their combinations, have been applied to LAS removal, with variable results. While an activated sludge process could remove 95–99% of LAS from sewage [10,11], the process was disturbed around 50 mg/L LAS, and even inhibited at higher concentrations [9].

Coagulation-flocculation by FeCl3 could efficiently remove (99 %) high surfactant concentrations from industrial wastewater, but required large amounts of coagulant [12]. Polyethersulphone or polysulphone membrane filtration has also been used to remove surfactants, but only retained 32%–78%, depending on the concentration [13]. Recently, AOPs have been developed as part of water or wastewater treatment process. They deployed potent oxidants, including Cl2, ClO2, KMnO4, O3, O3/H2O2, and O3/activated carbon; oxidizing irradiation (e.g. UV, ultrasound, and electron beam); and plasma treatments [14–19]. Interestingly, previous reports indicated that SDBS has a high reactivity with HO% formed during O3/H2O2 process, but slow reactivity with Cl2, ClO2, KMnO4, and O3 [18]. This indicates that HO% could be one of the most effective species for surfactant degradation. Recently, vacuum-ultraviolet (VUV) has gained great attention as an efficient and environmentally friendly method to produce HO% during water or wastewater purification [20]. VUV has wavelengths of

⁎

Corresponding author at: Beijing University of Technology, No.100 Xi Da Wang Road, Chao Yang District, Beijing 100124, China. E-mail addresses: [email protected] (H. Li), [email protected] (Y. Yang), [email protected] (J. Gao), [email protected] (X. Li), [email protected] (Z. Zhou), [email protected] (N. Wang), [email protected] (P. Du), [email protected] (T. Zhang), [email protected] (J. Feng). https://doi.org/10.1016/j.jwpe.2020.101172 Received 18 October 2019; Received in revised form 25 January 2020; Accepted 28 January 2020 2214-7144/ © 2020 Elsevier Ltd. All rights reserved.

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2.2. Experimental procedures

100–200 nm, and is photo-chemically active [21]. 185 nm photons are strongly absorbed by water molecules (coefficient of 1.8 cm−1), causing the hemolysis and ionization of water molecules. Powerfully-oxidizing hydroxyl radicals (HO% and H%) are then produced through Eqs. (1) and (2) [22,23]. The VUV process is more efficient than most UV-based AOPs for degrading organic contaminants and inorganic ions, and does not require additional oxidizing chemicals [24]. It has been successfully applied to degrade multiple organic pollutant (e.g. algal organic matter, pesticides, antibiotics, phenolic compound) in aqueous solution [25–30]. High degradation rate (> 90 %) of five typical pesticides (aldicarb, alachlor, chloroneb, methiocarb and atrazine) were achieved under VUV fluence of 12 mJ cm−2 [27]. VUV was technically efficient for degradation of cloxacillin antibiotic, and the addition of Fe2+ was more energy-effective [28]. VUV photo-Fenton process significantly enhanced the degradation and mineralization of sulfamethazine [29]. All these studies have demonstrated the potential of VUV process for the water containing organic contaminants. Since SDBS can be efficiently removed by hydroxyl radicals, an obvious removal effect can be expected under VUV irradiation. H2O + hv185nm→ HO% +H% Ф1 = 0.33 H2O + hv185nm→ H

+

+eaq

−

%

+ HO Ф2 = 0.045

Photochemical experiments were performed in a sealed cylindrical borosilicate glass reactor with approximately 700 mL in volume. The reactor was covered with a 6 mm layer of thick PVC to prevent ultraviolet leakage. VUV (GPH150T5VH/4, Heraeus Co.) or UV (GPH150T5L/4, Heraeus Co.) 6 W low-pressure mercury lamps were placed in the reactor, protected by a quartz tube and at 1 cm from its surface (Fig. S2). The VUV lamp emitted radiation both at 254 nm (∼ 90 %) and 185 nm (∼ 10 %) according to 1:9 [28], with the light intensities of 0.064 mW/cm2 and 0.006 mW/cm2, respectively. The UV lamp emitted at 254 nm, with an intensity of 0.069 mW/cm2. Light intensities were measured with a digital light power meter (CELNP2000-2). To reach a stable light output, the lamps were turned on at least 20 min prior to the photochemical experiments. The pH of the SDBS solution was adjusted to 3, 5, 7, 9, or 11 with 0.1 mol/L HCl or 0.1 mol/L NaOH. The temperature was controlled in a water bath equipped with a thermostat, and ranged from 273 K to 303 K. The effects of inorganic anions (SO42−, Cl−, and HCO3−) on SDBS degradation was investigated at 0, 1, and 2 mM. All results reported are average values of triplicate experiments.

(1) (2)

2.3. Analytical methods

Bathing and laundry wastewater reuse has been limited by the lack of effective purification methods. For example, biodegradation requires long incubation cycles and adsorption has low recycling rates. Moreover, UV-based AOPs require the addition of oxidizing agents to the wastewater pipes, which can be challenging and lead to toxicity of the treated solution. Hence, the VUV process poses as a sustainable and oxidant-free method to remove surfactant, and other micro-pollutants from wastewater, allowing its reuse [31]. Despite the efforts to develop processes for surfactant removal, the mechanisms of VUV and UV photo-induced SDBS degradation have not been investigated before. The aim of this paper was to investigate the mechanisms involved in SDBS oxidation by VUV and UV, and identify the role of reactive species in each process. We also analyzed the effects of different parameters in the systems, including initial SDBS concentration, pH, temperature, and presence of inorganic anions. The pathway of SDBS degradation by VUV was also proposed. Finally, the VUV and UV processwas seperately applied to real bathing and laundry wastewater to evaluate its practicability. The results of this work offer a better understanding of the reaction mechanisms and outcomes of VUV, which can be applied to the degradation of various organic micropollutants.

The pH was determined with a Thermo pH meter (Shanghai, China), calibrated daily using pH buffer solutions. Total organic carbon (TOC) was measured by a vario TOC® cube analyzer (Elementar, Germany). We monitored the time course of benzene ring cleavage by the benzene ring spectral band positioned at 224 nm using UV-vis [32]. SDBS concentration was determined in a UV–vis detector (UV2600, China) at 223 nm [33]. Anionic surfactants were measured in real bathing and laundry wastewater samples as methylene blue active substances using standard methods [34]. Chemical oxygen demand (CODcr) was determined as described before [35]. Intermediates during SDBS degradation were identified with an Agilent 1290 Infinity/6460 LC/QQQ MS equipped with an electrospray ionization (ESI) source, and operated in the negative (ESI)- electrospray ionization mode. The spray voltage (−) was 3.5 kV, and capillary temperature was 300 °C. The mobile phase was a mixture of methanol and ultrapure water (0.1 % formic acid) at a flow rate of 0.2 mL min−1. The elution process was: 0−4 min, 25 %−5 % methanol; 4−5 min, 5 %–25 % methanol; and 5−8 min, 25 % methanol. Inorganic anions were analyzed at 25℃ in a chromatographer (Metrohm, 883 Basic IC plus), and equipped with a Metrosep A Supp (250.0 mm × 4.0 mm) column. The eluent used was a 3.2 mmol/L Na2CO3 and 0.1 mmol/L NaHCO3 solution, flow was 0.700 mL/min, and the sample volume was 20 u L. The EPR spectra were recorded in duplicate or triplicate on a electron-spin resonance spectrometer (JEOL JES-FA200), at room temperature (ca. 25 ℃). DMPO solutions were mixed vigorously in the presence of VUV or UV irradiation, and then loaded into a capillary tube (0.5 mm × 10 cm). The kinetics of SDBS degradation were fitted based on a pseudofirst-order kinetic model [18], where C0 (mg/L) is the initial concentration of SDBS (mg/L), Ct is the concentration at time t (min) and k (min−1) is the pseudo first-order rate constant (Eq. (3)).

2. Materials and method 2.1. Chemicals All chemicals used were of analytical grade or higher, and were used without further purification. SDBS (AR grade, mixture, seen in Fig. S1), sodium salts (NaCl, Na2SO4, and NaHCO3), tert-butyl alcohol (TBA), and formic acid (FA) (AR grade, ≥88 %) were all supplied by Aladdin (Shanghai, China). Stock solutions were prepared with ultra-pure water generated by a Milli-Q Heal Force ultra-pure system (Millipore, USA). The SDBS stock was prepared by mixing 1.0 g into 1000 mL of ultrapure water that had been stored at 4℃. HCl (37 wt%), NaOH (96 wt%), H2SO4 (98 wt%), and methylene blue stocks were prepared with chemicals from Beijing Chemical Works (Beijing, China). 5,5-dimethyl-1pyrrolineN-oxide (DMPO) (COMPANY) was used as a radical spintrapping agent. Methanol (HPLC grade) and formic acid (HPLC grade, ≥98 %), used in UPLC–MS, were obtained from Fisher Scientific (NJ, USA) and Aladdin (Shanghai, China), respectively. Bathing and laundry wastewater were collected from a student’s dormitory located in Beijing, China. The samples were stored in a refrigerator at 4 ℃.

−ln (Ct/C0) = kt

(3)

3. Results and discussion 3.1. SDBS degradation by the VUV and UV processes Fig. 1 shows the degradation of SDBS by the VUV and UV processes over a 120 min period. Overall, VUV was more efficient than UV under 2

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the 224 nm spectral band (Fig. 2(a) and (b)) [32]. The rate of benzene cleavage was almost 3.2 times higher with VUV (91 % at 60 min) than UV (∼77 % at 120 min). Interestingly, the absorption peaks became wider and developed slight shoulders with the progression of both treatments, indicating the presence of intermediate products. VUV also had higher mineralization capacity than UV, as indicated in Fig. 2(c) by the lower TOC content at 120 min (53 % and 68 %, respectively). SO42− concentration followed a similar trend to benzene ring cleavage for both processes. Production of SO42− ions accelerated dramatically after 30 min of VUV, but not UV, reaching a final ion concentration of 1.66 mg/L and 0.63 mg/L, respectively. SO42− ions are produced by the breaking of a sulfonyl bond, and their presence indicates that indirect advanced oxidation and direct photolysis both had effect. These data suggest that indirect advanced oxidation occurred in VUV, leading to more cleavage of SDBS benzene rings and sulfonyl groups than direct photolysis. To identify the primary reactive species produced in each process, we added radical scavengers (TBA or FA) to the reactions (Fig. 3). TBA is a commonly used HO% scavenger, due to its high reaction rate constant (6.0 × 108 M−1s−1) [36]. As shown in Fig. 3(a), TBA inhibited SDBS degradation in a concentration dependent manner, with k values of 0.0400, 0.0142, 0.0116, and 0.0084 min−1, for 0, 1, 10, and 100 mM, respectively. These results demonstrated that HO% was the predominant reactive oxygen species formed by VUV. For UV, the k decreased slightly from 0.0125, to 0.0123, 0.0102, and 0.0094 min−1, for the same concentrations. To verify the generation of HO%, EPR spectra were recorded using DMPO as the spin-trapping agent. VUV led to the formation of fourlined spectra with relative intensities of 1:2:2:1 (Fig. 4(a)), while the characteristic DMPO-OH peaks were inconspicuous for the UV process (Fig. 4(b)). Hence, the EPR analysis confirmed that HO% was produced exclusively by indirect oxidation in the VUV process and not by direct photolysis. As seen in Eq. (4)–(8), FA can simultaneously scavenge HO% and H%,

Fig. 1. SDBS degradation by VUV and UV. Conditions: [SDBS]0 = 10 mg/L, T = 298 K, pH = 8.64, reaction time = 120 min.

the same conditions. Within 60 min of the treatment, 91 % of SDBS had been degraded by VUV, while UV only achieved 67 % at the final time point. Data for both treatments could be fitted with high linearity (R2 > 0.999) with Eq. (3), indicating that they follow pseudo-first-order kinetics. The SDBS degradation rate for VUV (kVUV) and UV (kUV) was 0.0400 and 0.0104 min−1, respectively. Next, we investigated the mechanisms of SDBS degradation by VUV and UV. While the UV process should rely almost exclusively on direct photolysis (by λ = 254 nm) [29], VUV may act by direct photolysis (by λ = 185 nm and λ = 254 nm) and/or indirect oxidation by reactive species (e.g., HO%/HO2%/O2−%). UV–vis spectra, ring cleavage, total organic carbon (TOC), and concentration of SO42- for VUV and VU over 120 min are depicted in Fig. 2. We monitored benzene ring cleavage at

Fig. 2. Mineralization of SDBS by the VUV and UV processes. a) UV–vis spectra, b) ring cleavage, c) mineralization, and d) sulfate concentration with VUV and UV treatments. Conditions: [SDBS]0 = 10 mg/L, pH0 = 8.64, T = 298 K, reaction time = 120 min. 3

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of scavenging HO% capacity. However, when the addition of FA increased to 50 u L, the degradation rate of SDBS decreased significantly. This result suggested that FA absorbed 185 nm photons (ФFA,185 > ФH2O,185), leading to a decrease in HO% production [39]. The UV process was almost unaffected by FA, confirming that it hardly generates radical species, relying on photolysis for SDBS degradation. hv = 185nm

HCOOH(or HCOO−) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H2 O + COΦ FA,185 = 1

(4)

HCOOH + HO% → HCOO% + H2O, k = 1.3 × 108 M−1s−1 %

%

HCOOH + H → HCOO + H2, k = 6.1 × 10 M HCOO HCOO

− −

%

+ HO → COO %

+ H → COO

7

−%

−%

−1 −1

s

9

−1 −1

+ H2O, k = 3.2 × 10 M 8

s

−1 −1

+ H2, k = 2.1 × 10 M

s

(5) (6) (7) (8)

3.2. Effects of various factors on SDBS degradation 3.2.1. Effect of initial SDBS concentration Fig. 5 shows the effect of initial SDBS concentration ([SDBS]0) on the kinetics of its degradation by VUV and UV. Increasing [SDBS]0 steadily reduced the efficiency of the VUV process. Contrarily, the effect was positive but mild with UV. It is likely that [SDBS]0 affected VUV and UV photon absorption in the aqueous solution, which would alter the proportion of direct photolysis and indirect oxidation in the reaction. The photon absorption fraction for each solution component was calculated for [SDBS]0 of 10, 15, and 20 mg/L by Eq. (9) [29]:

Pi, λ (%) =

εi, λ Ci i

∑i = 1 εi, λ Ci

× 100 (9)

Pi, λ and εi, λ are, respectively, the photon absorption fraction (%) and molar absorption coefficient (cm−1M−1) (determined with UV–vis spectrophotometer based on the Lambert-Beer Law) of a specific solution component (i) at a certain wavelength (λ, nm). Ci is the molar concentration (M) of a solution component (i). εi,λCi is the molar absorption of solution component(i) at λ nm wavelength. Note we used a molar absorption coefficient of 190 nm (ε190) instead of 185 nm (ε185) in these calculations, as they were similar, and the spectrophotometer available had a lower limit of 190 nm [29]. We monitored the molar absorptions of SDBS at 190 nm and 254 nm by UV–vis detector. According to the molar absorption values of εi,λCi, we could calculate the fraction of VUV and UV photons with Eq. (9). The molar absorption of SBDS measured by UV–vis at 190 nm was 1.0129, 1.3335, 1.5085 at varied concentrations of 10, 15, 20 mg/L, respectively. VUV photons were mainly absorbed by H2O due to its high molarity (ca. 55.6 M) and high molar absorption coefficient at 185 nm (εH2O,185 = 3.2 × 10−2 cm−1M−1). And the molar absorption of H2O

Fig. 3. Effect of radical scavengers on SDBS degradation by VUV and UV. (a) tert-butyl alcohol (TBA). Conditions: [SDBS]0 = 10 mg/L, T = 298 K, reaction time = 120 min; (b) formic acid (FA). Conditions: [SDBS]0 = 10 mg/L, T = 298 K, reaction time = 120 min. The solid lines represent the best linear fit for the data. The relative standard deviation for all data points was less than 5 %.

and absorb 185 nm photons [37,38]. As for TBA, FA gradually inhibited SDBS degradation by VUV (Fig. 3(b)), with k values of 0.0400, 0.0247 and 0.0195 min−1, for 0, 20 and 50 u L, respectively. At 0–20 u L, the degradation rate of SDBS tended to be stable because of the limitation

Fig. 4. Electron spin resonance (ESR) spectrum of HO% formed in water under VUV(a) and UV(b) processes. 4

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Fig. 5. Effect of initial SDBS concentration on its degradation by VUV and UV. Conditions: [SDBS]0 = 10 mg/L, 15 mg/L, 20 mg/L, T = 298 K, reaction time = 120 min. The solid lines represent the best linear fit to the data. The relative standard deviation for all data points was less than 5 %.

Fig. 7. Effect of initial pH on SDBS degradation by VUV and UV. Conditions: [SDBS]0 = 10 mg/L, T = 298 K, reaction time = 120 min. The solid lines represent the best linear fit to the data. The relative standard deviation for all data points was less than 5 %.

at 190 nm was 1. 7792. The fraction of VUV photons absorbed by SDBS increased with the initial concentration: 36.28 %, 42.84 %, and 45.88 % for 10, 15, and 20 mg/L, respectively (Fig. 6(a)). In diluted SDBS solutions with deionized water, VUV photons absorbed by H2O could trigger the generation of large amounts of reactive species (Eqs. (1) and (2)), and more efficient degradation by indirect oxidation; SDBS absorbed fewer photons and photolysis was limited. Higher initial SDBS concentrations allocated more VUV photons towards the less efficient photolysis. The correspondent degradation rate also decreased: 0.0400, 0.0310, and 0.0269 min−1 with the concentration increasing. Other studies have also shown a reduced rate of contaminant degradation by VUV at higher initial concentrations [40], confirming that indirect oxidation is more efficient than photolysis. UV photons behaved differently than VUV in their absorption and distribution. The molar absorptions of SDBS measured by UV–vis at 254 nm were 0.0116, 0.0179, 0.0225 at varied concentrations of 10, 15, 20 mg/L, respectively. [SDBS]0 affected slightly the adsorption of photons by H2O, likely due to the small molar adsorption coefficient of H2O at 254 nm (εH2O,254 = 2.0 × 10−4 cm−1M−1) [40], and the low energy of photons (U254 = 4.71 × 105 J einstein−1). The molar absorption of H2O was 0.01112 at 254 nm. However, the fraction of UV

photons absorbed by SDBS increased to 51.06 %, 61.68 %, and 66.92 % at 10, 15, and 20 mg/L, respectively (Fig. 6(b)). The degradation rate also increased slightly: 0.0104, 0.0107, and 0.0111 min−1 for the same concentrations. Hence, higher [SDBS]0 slightly enhanced direct UV photolysis due to increases in the utilization rate of 254 nm photons. In summary, the distribution of VUV and UV absorbed photons suggested both indirect oxidation (HO%) and direct photolysis contributed to SDBS degradation. [SDBS]0 considerably limited the efficiency of the VUV process, due to reduction in reactive species generated and increase of SDBS photolysis. Yet, VUV was still much more efficient than UV, confirming indirect oxidation has a bigger role on SDBS degradation. 3.2.2. Effects of initial pH In Fig. 7, we analyzed the effect of solution pH (from 3 to11) on the degradation of SDBS by VUV and UV. pH had variable effects on the VUV process but influenced the UV system steadily. We observed a higher degradation rate by VUV at pH of 5 and 9 (k of 4.56 × 10−2 and 4.34 × 10−2 for pH 5 and 9, respectively). Several explanations are possible: (1) deprotonated SDBS compounds (at pH = 9) could be more vulnerable to hydroxyl radicals than their undissociated forms (at pH = 3) [41]; (2) the presence of H+ could enhance the scavenging of

Fig. 6. Fraction of VUV (a) and UV (b) photons absorbed by each solution component as a function of initial SDBS concentration. Conditions: T = 298 K, reaction time = 120 min. 5

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HO% [42]; (3) the effect could be caused by a reduction in oxidation capacity, as increasing pH from 0 to 7 will inhibit HO% production and reduce its oxidation potential (2.59 V to 2.18 V) [43]. At higher pH, HO% dissociates into the oxygen anion radical (O%-) (Eq. (10)) [44], and the H2O2 equilibrium equation right shifts from H2O2 to HO2- (Eq. (11)) [45], increasing HO% consumption. O2%- could be produced from HO2and HO% (Eqs. (12) and (13)). The lower oxidation potential of O%- and O2%- compared to HO% would reduce the degradation rate of SDBS at pH above the optimal value of 9. Additionally, the aqueous concentration of bicarbonate and carbonate increase with mineralization at high pH, also contributing to a decrease in HO% utilization. The fact that pH 5.0 and 9.0 were optimal indicated that HO% had comparative reactivity to around neutral pH condition and anionic forms of SDBS. Previous studies have shown similar effects of pH on the efficiency of VUV irradiation. Kutschera et al. found that the degradation efficiency of geosmin and 2-methylisoborneol was higher at pH from 4.0–8.0 [45]. Furthermore, Imoberdorf and Mohseni reported that the optimal pH for the elimination of NOM by VUV ranged from 5.0–9.0 [38]. By contrast, degradation by UV increased progressively with the pH value; the highest degradation rate was obtained in alkaline media (k of 1.20 × 10−2 for a pH of 11). The effect is likely due to the presence of a higher fraction of dissociated SDBS at higher pH. This process mimics the photolysis of deprotonated phenolic compounds, which are decomposed almost an order of magnitude faster than their undissociated form [30,46]. Fig. 7 Effect of initial pH on SDBS degradation by VUV and UV. Conditions: [SDBS]0 = 10 mg/L, T = 298 K, reaction time = 120 min. The solid lines represent the best linear fit to the data. The relative standard deviation for all data points was less than 5 %. HO%↔O%− + H+, pka = 11.9 %

%

H2O2 ↔

HO2−

HO + HO ↔ H2O2, pka = 11.6 +

+H

HO2− + HO%→ O2%− +H2O

Fig. 8. Effect of temperature on SDBS degradation by VUV and UV. Conditions: [SDBS]0 = 10 mg/L, reaction time = 120 min. The solid lines represent the best linear fit to the data. The relative standard deviation for all data points is less than 5 %.

(10) (11) (12) (13)

These results indicate that pH can significantly affect target aqueous compound degradation by VUV and UV irradiation. While SDBS photodegradation was enhanced by UV in alkaline media, this condition was adverse for the VUV process. Nevertheless, VUV still led to higher SDBS degradation rates than UV, independent of pH.

Fig. 9. Effects of different ions (IAs, 0–2 mM each) on SDBS degradation by VUV and UV. Conditions: [SDBS]0 = 10 mg/L, T = 298 K, reaction time = 120 min.

3.2.3. Effects of temperature In Fig. 8, we tested the effect of solution temperatures ranging from 273 K to 303 K. The temperature of the VUV and UV lamps was kept stable in these experiments to ensure the irradiation output was not affected. Elevating the temperature caused an obvious acceleration of SDBS degradation in both VUV and UV systems, although with a smaller impact on the UV process (k from 1.58 × 10−2 to 7.16 × 10−2 in VUV, and 0.55 × 10−2 to 1.51 × 10−2 min−1 in UV, from 273 to 303 K, respectively). This suggests that oxidation by HO% could be dramatically enhanced by increasing the temperature of the VUV process. Generally, the effective quantum yield of reactive radicals is determined by their instantaneous quantum yield and the fraction that can escape from the “solvent cage”. There is a positive correlation between temperature and the quantum yield of reactive radicals, due to the reduction in water viscosity and the improvement of diffusioncontrolled dissociation of the photo-fragments [47,48]. Thus, it is likely that higher temperatures led to more efficient VUV SDBS degradation by increasing the quantum yield of HO% through the photolysis of water [49]. To better understand the influence of temperature on the rate of SDBS degradation, we created −ln(kobs) vs. 1/T plots for the four temperatures tested (see insert on Fig. 8). The data had a good fit to the Arrhenius equation, calculated with the activation energy (Ea) for

photolysis of SDBS at 37.23 and 25.32 kJ/mol, for VUV and UV, respectively. 3.2.4. Effects of co-existing inorganic anions Fig. 9 shows the effects of different concentrations (0,1 and 2 mM) of inorganic anions (SO42−, Cl−, and HCO3−) on SDBS degradation by VUV and UV. Under VUV irradiation, adding 1 mM of SO42− accelerated the SDBS degradation by 4 %, while Cl− and HCO3− inhibited it by 3 and 5 %, respectively. The effects were more pronounced at 2 mM: a 7 % and 8 % inhibition with Cl− and HCO3−, and 26 % increase with SO42−. The competition of SO42− with HO% was insignificant due to its slow reaction (Eq. (14)) [50]. Moreover, the SO4%− produced under VUV irradiation could form HO% (Eqs. (15) and (16)) [51], which also promoted SDBS degradation to some extent. Other studies also found that SO42− promoted pesticides degradation by VUV [27,51]. The redox potential of SO4%− is comparable or higher than that of %OH (2.5–3.1 V and 2.8 V, respectively) and SO4%− can oxidize most organics found in wastewater. Its lifespan is relatively long, in the order of 30−40 μs [52], thus it is more stable than %OH (20 ns) with an enhanced chance to reacts with targeted pollutants. Contrarily, Eqs. (17) and (18) [27] show that Cl− and HCO3− are scavengers of HO%. Moreover, the Cl− 6

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H. Li, et al.

and HCO3− can react with HO% to form less reactive species, thus suppressing SDBS degradation. Previous research showed that SO4%−based AOPs are more efficient for the abatement of most micropollutants than HO%-based [53]. Other studies have analyzed the effects of different inorganic anions on the degradation of multiple chemicals by VUV irradiation [27,51]. Cl− and HCO3− competed for VUV photons (low absorption of UV photons at 254 nm) with the other solution components, thus suppressing both direct photolysis and indirect oxidation of SDBS. Moreover, the results obtained in the presence of scavengers indicated that HO% is a more efficient AOP than Cl% or CO3%−. SO42− + HO% → SO4%− + OH−

Table 1 Products of the VUV detected by LC–MS. Product ID

Observations

m/z

P1

315

P2

301

P3

271

P4

177

(14)

hv < 200nm

SO24− ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→SO•4−+e−aq

(15)

SO4%− + H2O → HSO4− + HO% Cl

−

%

%

−

(16) −1 −1

+HO → Cl + OH , k = 3.0 × 10 M

HCO3−+HO%

→ CO3

%−

9

s

7

(17) −1 −1

+ H2O, k = 1.0 × 10 M

s

(18)

For the UV process, SO42− and Cl− did not affect SDBS degradation, though HCO3− promoted it slightly. This phenomenon could be due to changes in solution pH, as UV produces few active species and inorganic anions have low absorption of 254 nm photons. Accordingly, the pH of the solution decreased from 8.64 to 7.42 and 7.28, for 1 and 2 mM of SO42−, and from 8.64 to 7.83 and 7.42, for 1 mM and 2 mM of Cl−. Sodium bicarbonate caused the pH to increase from 8.64 to 9.13 and 9.41, at 1 mM and 2 mM, respectively. This was consistent with the results obtained in the pH studies above (Fig. 7).

cleavage and complete mineralization. Hence, it is important to note that the intermediates identified could represent only part of the SDBS degradation products. There are two main mechanisms for HO% action on SDBS: 1) attack to the alkyl chain; 2) attack to the ortho positions of the alkyl chain on the benzene ring [15,17]. Most of the primary photoproducts identified contained the sulphonate group. This indicated that the carbon at the α position of the aromatic ring was the principal site of HO% radical action that lead to the cleavage of the alkyl chain. Previous studies proposed a pathway leading to HO% addition to the benzene ring at the ortho- site of the alkyl chain, due to the ortho- and meta-directing nature of the alkyl and sulphonate groups, respectively [17]. We could identify the reaction from intermediates P1, P2, and P3 (Table 1. In theory, the attack of HO% on the alkyl chain of SDBS can be expected to occur along the whole alkyl chain, although it is more pausible that it occurs at the α- and β-positions. Yet, it was the branched carbon that was affected, as the molecular weight of the product generated by attacking α-carbon and β-carbon is much smaller than that of the base peak obtained. We used the inferred molecular weight of the products to determine the branch chain position, and concluded that R1 and R2 contained four carbon atoms, and R3 and R4 contained five. This indicated the presence of methyl, ethyl, and propyl groups in the branched chain, which is consistent with the inferred SDBS mixture composition in the initial solution. After comprehensive analysis, we concluded that both methods of action 1) and 2) existed in this reaction. Based on the major indetified by-products mentioned, and the analysis above, we propose the pathway SBDS degradation in Fig. 10. At the beginning, free electrons formed by the action of HO% in the alkyl chain of SDBS, which made the carbon atom active. Those reacted with the oxygen. Then HO% cleaved the alkyl chain at the β-position, which led to the formation of a carbonyl group, generating P3 and P4. Finally, HO% and dissolved oxygen converted the alkyl chain of SDBS into a carboxyl group, generating the P1 and P2.

3.3. Pathway of SDBS degradation by VUV To identify the pathway of degradation by the VUV process, we analyzed a SDBS solution with 50 mg/L, by ESI (-) LC/MS. The fundamental peak in the mass spectrogram of the initial solution had marker ions for four main isomers at [M−H]− = 297, 311, 325, and 339 m/z. Substances with 325 m/z corresponded to the DBS− resulting from SDBS hydrolysis in water, which also releases Na+. SDBS molecules contain sulfonic acid groups, which are easy to esterify with methanol and formic acid, generating molecules with fragment ions 340 and 369 m/z. SDBS molecules lose hydrogen in the ESI (-) mode and become negatively charged by one unit. The ion peak generated is 339 m/z, seen in mass spectrogram signal Peaks 5–8 (Fig. S1). The base peak of Peak 5 was the product of propyl removal ([M−H]- = 297). Another main ion peak present ([M−H]- = 339) corresponded to esterification products. This indicated that the alkyl chain of SDBS had a propyl branch chain. The base peak of Peak 6 is 311 m/z, and four ions were present ([M−H]- 297, 311, 325, and 339 m/z). The marker at 297 m/z was the deethyl product of SDBS to be removed, and at 311 and 339 were its esterification products. This indicated that Peak 6 corresponded to an ethyl branch chain of SDBS. Peak 7 had the base peak at 339 m/z, and two main fragment ion peaks at 311 and 325 m/z. The signal at 339 m/z corresponded to a esterification product of SDBS, 325 m/z to the SDBS molecule or 339 m/z demethylation products, and [M−H]- 311 to SDBS demethylation products. This indicated that the alkyl chain corresponding to Peak 7 had a methyl branch chain. The base peak of signal Peak 8 is 339 m/z, in which there was a main fragment at 325 m/z. This corresponded to SDBS and its esterification products, indicating that there was no branched chain in the alkyl chain of the SDBS isomer. The main chemistry structural formula for SDBS in solution, inferred from these data, is in Fig. S3. From 20–90 min. of VUV, four predominant products were present at high concentrations: [M−H]− = 315, 329, 271, 177 (Figs. S8–S12.). The relative abundance of intermediates increased and then decreased with irradiation time, as seen in the Fig. S1. Continuous attack of aromatic benzene ring derivatives by the HO% radicals can lead to ring

3.4. Application of VUV and UV processes to real bathing and laundry wastewater To elucidate the influence of the water matrix on the degradation of anionic surfactants, we applied VUV and UV to real bathing and laundry wastewater. Both treatments led to the degradation of surfactants (Fig. 11), with VUV performing slightly better than UV. Surfactant 7

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Fig. 10. Proposed pathway of SDBS degradation by VUV.

(1) Indirect oxidation has a higher mineralization capacity than direct photolysis. Cleavage of benzene ring was completed after 60 min of VUV, but uncompleted after 120 min of UV. VUV also achieved a higher SDBS mineralization, with TOCt/TOC0 decreasing to 53 % within 120 min. With UV, it reached only 68 %. SO42− ion concentration, produced by the breaking of a sulfonyl bond, reached 1.66 and 0.63 mg/L, for VUV and UV, respectively. (2) The VUV process was more efficient at solution pH of 5 and 9, while UV performed better at alkaline conditions. Temperature promoted the degradation of SDBS in both processes, but with a higher impact on VUV. Adding SO42− slightly accelerated degradation by VUV, while Cl− and HCO3− inhibited the process. With UV, SO42− and Cl− had no effect, but HCO3− slightly promoted SDBS degradation by increasing the solution pH. (3) Indirection oxidation (predominant in VUV) was more susceptible to changes in external conditions than direct photolysis (predominant in UV). Nevertheless, indirect oxidation was overall more efficient in the degradation of SDBS than direct photolysis. (4) At a high initial concentration, we identified four SDBS isomers. Four possible major by-products (P1, P2, P3, P4) were identified

degradation was accompanied with decreases in CODCr and TOC. In bathing wastewater, COD was reduced from 242.5 mg/L to 207.6 mg/L with VUV and 216.6 mg/L with UV, after 180 min. TOC declined from 69.18 mg/L to 52.91 mg/L and 60.03 mg/L with VUV and UV, respectively. For laundry wastewater, CODCr went from 490.5 mg/L to 439.2 mg/L with VUV and 452.6 mg/L with UV, after 180 min. TOC declined from 158.56 mg/L to 147.93 mg/L and 153.18 mg/L, with VUV and UV, respectively. The degradation rate was inferior for wastewater than for the SDBS-pure water experiments above. This might be due to the scavenging of HO% and UV photons by inorganic anions and organic matter present in sewage. Additionally, sewage high turbidity might impede VUV and UV photons from propagating efficiently.

4. Conclusions In these experiments, both VUV and UV irradiation could decompose SDBS efficiently. Yet, the VUV process had a better removal performance at a same power consumption. The main conclusions of this work were: 8

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Fig. 11. Degradation of anionic surfactants in real bathing and laundry water by VUV and UV. Bathing water conditions: [CODCr]0 = 242.5 mg/L, TOC0 = 69.18 mg/L, turbidity = 52.3 NTU. Laundry water conditions: [CODCr]0 = 490.5 mg/L, turbidity = 56.1 NTU, T = 298 K, reaction time = 180 min.

with the analysis by LC–MS of VUV irradiation. With these data, we proposed the pathway of SDBS degradation by the VUV process. (5) VUV and UV at 180 min irradiation led to the degradation of 66.4 % and 58.0 % of anionic surfactants present in bathing wastewater. For laundry wastewater, 64.9 % and 51.8 % were degraded with VUV and UV, respectively. The VUV process was more efficient and led to a higher mineralization of surfactants in both samples tested. This work will help the development of processes with optimal conditions for degradation of SDBS, and find new ways to treat bathing and laundry wastewater.

Declaration of Competing Interest The authors declared that they have no conflicts of interest to this paper. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the paper submitted.

Acknowledgements The authors acknowledge the financial support of the National Key Research and Development Program of China (2018YFC0406203 in 2018YFC0406200). We would like to give our sincere thanks to the peer-reviews for their suggestions. 9

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