Combined effects of dissolved organic matter, pH, ionic strength and halides on photodegradation of oxytetracycline in simulated estuarine waters

Abstract

Estuarine waters of variable compositions are sinks for many micropollutants. The varying water properties can impact the photodegradation of organic pollutants. In this study, the combined effects of dissolved organic matter (DOM), pH, ionic strength, and halides on the photodegradation of the model organic pollutant oxytetracycline (OTC) were investigated. Suwannee River natural organic matter (SRNOM) was used as a representative DOM. The results showed that the observed photolysis rate constant (k_obs) of OTC increased rapidly upon increase of pH. SRNOM induced a 11.0–17.9% decrease of the k_obs for OTC. In the presence of SRNOM, the ionic strength and specific halide effects promote OTC photodegradation with a 39.2–84.2% and 7.1–28.8% increase of the k_obs, respectively. The effects of SRNOM, ionic strength and halides on OTC photodegradation are pH-dependent. Direct photolysis half-lives (t_1/2) of OTC were estimated in view of the more important role of direct photolysis compared to indirect photolysis. The estimated t_1/2 values decreased from 187.4–206.6 d to 34.4–36.6 d as the pH increases in the Yellow River estuarine region. The results of this research demonstrate that the photodegradation rate of OTC increases rapidly in the gradient from river water to marine water in estuarine regions.

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Environmental significance

Photodegradation behaviors of organic pollutants would be complex and variable in estuarine waters due to the rapid change of physical and chemical properties and constitution, such as pH, ionic strength, dissolved organic matter (DOM), and halides. The combined effects of these variable factors on the photodegradation of frequently detected antibiotics in estuarine waters are still unclear. Here, we prove that the inhibitory effect of DOM and promotional effects of ionic strength and specific halides on oxytetracycline (OTC) photodegradation are all pH dependent. The combined effects of these factors lead to the decrease of photochemical half-lives for OTC in estuarine regions from river water to marine water.

Introduction

Estuaries are dynamic systems due to oceanic and terrestrial influences.¹ They play important roles in the accumulation and degradation of micropollutants^2,3 which enter estuarine waters via riverine transport, discharge from land-based sources, atmospheric deposition, and mariculture activities.^4,5 Among various micropollutants, antibiotics have received increasing attention in response to their widespread occurrence^6–8 and relatively higher concentrations were detected in estuarine and coastal areas compared to surrounding river and marine areas.^4,9 Alarmingly, antibiotics are able to induce bacterial resistance that is severely harmful to human health.^10,11 It is thus of significant importance to investigate the degradation behavior of antibiotics in estuarine waters.

Photochemical transformation was proven to be the predominant elimination pathway of antibiotics in sunlit surface waters.^12–15 Antibiotics, such as tetracyclines (TCs), can undergo direct photolysis as well as indirect photolysis mediated by singlet oxygen (¹O₂) and hydroxyl radicals (·OH).^16–18 Meanwhile, dissolved organic matter (DOM), which is ubiquitous in natural waters, plays an important role in the photodegradation of organic pollutants.^19–21 Previous studies found that humic acid (HA) inhibits phototransformation of TCs through competitive light absorption or photochemically produced reactive intermediate (PPRI) quenching, but on the other hand the excited triplet HA, and the generated ¹O₂ and ·OH accelerated the photodegradation of TCs.^16,18

The photochemical transformation of ionizable antibiotics was found to be pH dependent.^22,23 It may also be affected by ionic strength and halides in surface waters.²⁴ Notably, pH, ionic strength, and halide concentrations can in turn influence the photochemical properties of DOM. Firstly, the pH of the surrounding environment affects the absorbance of DOM.²⁵ Secondly, both pH and ionic strength heavily influence the formation yields of PPRIs by DOM. Dalrymple et al. found that the quantum yields of ¹O₂ and hydrogen peroxide (H₂O₂) from DOM exhibited a linear dependence on E₂/E₃ (A_{254 nm} divided by A_{365 nm}) which decreases as pH increases.²⁶ The quantum yields of excited triplet DOM (³DOM*) also changed significantly with pH and ionic strength.^27,28 Thirdly, halides can impact photoexcitation and photobleaching of DOM, which has been described in recent reviews.^29,30

In estuarine regions, pH always increases from freshwater to seawater during mixing.³¹ Accompanied by the pH increase, ionic strength and the concentrations of halogen ions (Cl⁻ and Br⁻) within estuary systems also increase from river water to seawater.³² Parker et al. investigated the effect of ionic strength and halides with DOM on the photodegradation of microcystins in estuarine systems and emphasized the important role of halides.³³ The combined effect of these important factors with DOM on the photodegradation of ionizable antibiotics in estuarine waters seems more complex and unpredictable because of the rapid change of the physical and chemical properties during the mixing of river water with seawater. Estuaries are considered to be the most important path for organic pollutant delivery to oceans.⁹ It is hence of great environmental significance to investigate the combined effects of dissolved compounds on the phototransformation of antibiotics, which determines their persistence in estuarine waters.

In this study, oxytetracycline (OTC) was chosen as the target antibiotic due to its frequency of detection and elevated concentration in estuarine waters.³ The structure and pK_a (3.22, 7.46, and 8.94)³⁴ of OTC are shown in Fig. S1 in the ESI.† The effects of DOM on OTC photodegradation were investigated in solutions of varying acidity. The underlying mechanisms by which DOM impacted the photodegradation of four ionic species of OTC [H₃L⁺, H₂L, HL⁻, and L²⁻ (L represents OTC)] were also unveiled. Furthermore, the combined effects of ionic strength and halides with DOM on OTC photodegradation under different pH conditions were also explored. Solutions with different ionic strength, halide concentrations, and acidity were used to mimic the complex and dynamic estuarine environments. To the best of our knowledge, there are to date no other studies investigating the combined effects of DOM, pH, ionic strength, and halides on the photodegradation of OTC.

Materials and methods

Materials

Oxytetracycline dihydrate (OTC, 98%), sodium azide (NaN₃, 98%), 2,4,6-trimethylphenol (TMP, 98%), and sorbic acid (SA, 99%) were purchased from J&K Scientific Ltd. (Beijing, China). All organic solvents with chromatographic purity were purchased from TEDIA (USA). Suwannee River natural organic matter (SRNOM) was purchased from the International Humic Substances Society. Other chemicals, such as sodium chloride (NaCl), sodium bromide (NaBr), and sodium sulphate (NaSO₄), were all analytically pure and purchased from Tianjin Damao Chemical (Tianjin, China). Ultrapure water (18.2 MΩ) was obtained from a purification system produced by Chengdu Ultrapure Technology Co., Ltd. (China).

Irradiation experiments

OTC was dissolved at an initial concentration of 5.0 mg L⁻¹. H₂SO₄ and NaOH were applied to adjust the pH to 2.0, 3.2, 5.0, 7.0, 8.0, 9.0, and 12.0. The fractions of H₃L⁺, H₂L, HL⁻, and L²⁻ in these solutions of different pH are listed in Table S1.† The irradiation experiments were performed in an XPA-7 merry-go-round photochemical reactor (Xujiang Electromechanical Plant, Nanjing, China) with quartz tubes containing the reaction solutions. A water-refrigerated 1000 W xenon lamp equipped with 290 nm filters was used to mimic the UV-A and UV-B portions of sunlight. The light intensity at the surface of quartz tubes and the sunlight radiation spectrum were detected with a TriOS-RAMSES spectroradiometer (TriOS GmbH, Germany), and the results are shown in Fig. S2.† The total radiation intensity from 290 to 450 nm of the light source in the position of the quartz cuvettes was measured to be 11.4 mW cm⁻², which is slightly lower than that of sunlight (12.8 mW cm⁻²) measured at noon in mid-summer in Dalian, China. A water bath unit was employed to maintain the temperature at 25 °C.

Quenching experiments were performed with SA (2.0 mM) and TMP (2.0 mM) as excited triplet quenchers, NaN₃ (2.0 mM) as the ¹O₂ quencher, and isopropyl alcohol (IPA, 20.0 mM) as the ·OH quencher, which were also selected in previous studies.^35–39 SA and TMP quench the excited triplet state through energy transfer and electron transfer, respectively.^36,39 The observed photolysis rate constants (k_obs) of OTC in the presence of SA and TMP were corrected by calculating their screening factors. For some experiments, SRNOM was added in OTC solutions with concentrations of 5.0 mg L⁻¹ and 10.0 mg L⁻¹. Na₂SO₄ (0.020, 0.050, 0.100, and 0.250 mol L⁻¹), NaCl (0.030, 0.075, 0.150, and 0.375 mol L⁻¹) or NaBr (0.075 mol L⁻¹ and 0.80 mmol L⁻¹) was added at different concentrations to investigate the effects of ionic strength and specific halides.

The light screening factor of SRNOM (S_λ) was calculated with the method described by Zhang et al.⁴⁰S_λ and the integrated screening factor for the wavelength range 290–455 nm (S_290–455) were calculated using the following equations:

where α_λ is the light attenuation coefficient of SRNOM (cm⁻¹), ε_λ is the molar absorption coefficient of OTC (cm⁻¹ L mol⁻¹), and l is the light path length (cm) which was calculated with the method described in previous studies.^20,41 The α_λ and ε_λ values in different pH solutions were determined to calculate the S_290–455 of SRNOM. The indirect photolysis rate constant (k_ind) as caused by DOM was calculated using the following equation:

k_ind = k_obs(DOM) − k_obs(non-DOM)S_290–455 (3)

where k_obs(DOM) and k_obs(non-DOM) are the observed photolysis rate constants of the OTC in the presence and absence of SRNOM, respectively.

The k_obs values of the four species of OTC (H₃L⁺, H₂L, HL⁻, and L²⁻) were calculated with the determined k_obs values of OTC in solutions with a pH of 2.0, 3.2, 5.0, 7.0, 9.0, and 12.0 based on the method described by Jin et al.²² (details are shown in Text S1†). The molar absorptivity from 220 nm to 500 nm for different species of OTC was also calculated based on the same method (details in the ESI†). Eqn (1)–(3) were also used in the case of the different OTC species. In eqn (1), the α_λ values of SRNOM at a pH of 2.0, 5.0, 8.0, and 12.0 were used to calculate the S_λ of H₃L⁺, H₂L, HL⁻, and L²⁻, respectively, as these four species are the main existing forms of OTC in these four pH solutions, respectively (as shown in Table S1†). The ε_λ of H₃L⁺, H₂L, HL⁻, and L²⁻ was obtained with the method described in Text S1.† The concentrations of the four species were calculated with eqn (4):

c_i = [OTC] × α_i (4)

where c_i is the concentration of different species and α_i is the fraction of H₃L⁺, H₂L, HL⁻, and L²⁻ at pH 2.0, 5.0, 8.0, and 12.0, respectively.

Analytical methods

A Shimadzu LC-20A HPLC system (Shimadzu, Kyoto, Japan) with a UV-Vis detector and an Ultimate™ AQ-C18 column (250 mm × 4.6 mm, 5 μm, Welch Materials, Maryland, USA) was employed for the quantification of OTC. The detection wavelength for OTC was 355 nm. The mobile phase consisted of an acetonitrile and phosphate aqueous solution (0.1%, pH = 2.3) with a ratio of 30 : 70.

Results and discussion

Effects of DOM on OTC photodegradation under different pH conditions

Photodegradation of OTC followed pseudo first-order kinetics (Fig. S3†). No significant loss of OTC was observed in the dark controls, indicating that light irradiation is the driving force for the degradation of OTC in the experiments. The observed photolysis rate constants (k_obs) of OTC exhibited a positive dependence on the pH values of the aqueous solutions (Fig. 1, Table S2†). This is in accordance with previous findings stating that the direct photolysis rate constant (k_d) of OTC increased with increasing pH, which was attributed to the increased light absorbance at increasing pH.²² A 4.6-fold increase of the k_obs for OTC was observed as the pH increased from 6.0 to 9.0, which represents the pH range of surface waters.⁴²

Fig. 1 Experimental and predicted photolysis rate constants (k_obs) and SRNOM-induced indirect photolysis rate constants (k_ind) of OTC under different pH conditions (The inset figure shows the details of k_obs and k_ind for OTC at pH from 2.0 to 7.0; the error bars represent the 95% confidence interval, n = 3).

Quenching and nitrogen saturation experiments were performed to investigate the involvement of self-sensitized photolysis in OTC photodegradation. Addition of NaN₃ and IPA significantly reduced the k_obs at pH 9.0 and showed no significant effects on the k_obs in pH 5.0 and 7.0 solutions (Fig. S4†). Thus, ¹O₂ and ·OH-initiated self-sensitized oxidation reactions were only observed at pH 9.0 and not at pH 5.0 and 7.0 for the studied photolysis of OTC. Upon nitrogen saturation, the k_obs increased significantly in the solutions at pH 5.0 and 7.0 (Fig. S4†), implying that OTC can undergo direct photodegradation through its excited triplet state as dissolved oxygen (O₂) is a triplet quencher. However, in solutions at pH 9.0, the k_obs decreased with nitrogen saturation. This further confirmed the involvement of self-sensitized oxidation reactions as the quenching of the excited triplet state by O₂ leads to the generation of ¹O₂ that was proven to be involved in the phototransformation of OTC in pH 9.0 solutions.

In the presence of SRNOM (5.0 mg L⁻¹ and 10.0 mg L⁻¹), the k_obs of OTC was significantly decreased (p < 0.05) in different pH solutions (Fig. 1). The S_290–455 of SRNOM in different pH solutions was calculated (Table S2†). With the S_290–455 and k_obs values at different pH, the indirect photolysis rate constants (k_ind) induced by SRNOM were calculated using eqn (3) and the values are listed in Table S2.† As can be seen in Fig. 1, the k_ind values also increased with increasing pH (Table S2†). Overall, SRNOM induced an 11.0–17.9% decrease of the k_obs for OTC in solutions with pH from 6.0 to 9.0.

The k_obs values for the four species of OTC were calculated with the method as shown in Text S1.† The calculated k_obs for H₃L⁺, H₂L, HL⁻, and L²⁻ was 0.20 ± 0.02, 0.62 ± 0.03, 1.32 ± 0.07, and 5.52 ± 0.15 min⁻¹, respectively, which also decreased in the presence of SRNOM (Table 1). With the calculated k_obs and molar absorptivity (Fig. S5†), the SRNOM-induced k_ind of the four species was calculated, and the results are shown in Table 1. The k_ind (10.0 mg L⁻¹ SRNOM) increased from about zero for H₃L⁺ to 1.28 ± 0.15 min⁻¹ for L²⁻. The k_obs and k_ind (10.0 mg L⁻¹ SRNOM) values for OTC at different pH were calculated based on the k_obs and k_ind for the four species and their percentages in different pH solutions. As can be seen in Fig. 1, the calculated k_obs and k_ind of OTC increased with increasing pH and showed good agreement with the determined values at pH 2.0, 3.2, 5.0, 7.0, 8.0, 9.0, and 12.0. Thus, it can be concluded from the results that k_obs in the absence and presence of DOM and the DOM-induced k_ind for ionizable pollutants can be reliably predicted.

Table 1 Calculated k_obs and k_ind for different species of OTC and S_290–455 of SRNOM towards different species of OTC (The unit for k_obs and k_ind is ×10⁻² min⁻¹)^a

Condition	k	H3L^+	H2L	HL^−	L^2−
a The errors of kobs and kind represent 95% confidence levels, n = 3.
Without SRNOM	k obs	0.20 ± 0.02	0.62 ± 0.03	1.32 ± 0.07	5.52 ± 0.15
With 5.0 mg L^−1 SRNOM	k obs	0.19 ± 0.01	0.52 ± 0.01	1.31 ± 0.05	4.99 ± 0.19
	S 290–455	0.736	0.708	0.728	0.702
	k ind	0.04 ± 0.01	0.08 ± 0.03	0.35 ± 0.06	1.11 ± 0.20
With 10.0 mg L^−1 SRNOM	k obs	0.12 ± 0.01	0.49 ± 0.01	1.14 ± 0.07	4.97 ± 0.07
	S 290–455	0.702	0.671	0.703	0.667
	k ind	−0.02 ± 0.02	0.08 ± 0.02	0.21 ± 0.08	1.28 ± 0.15

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To better understand the effects of SRNOM on the photolysis of OTC, quenching experiments were performed in pH 5.0, 7.0, and 9.0 solutions, and the results are shown in Fig. 2. The results showed that in the presence of SRNOM, removal of O₂ (N₂ saturation) increased the k_obs of OTC, implying that O₂ (a triplet quencher) has a negative effect on the photolysis rate. Meanwhile, addition of SA and TMP significantly decreased the k_obs of OTC in solutions at pH 5.0, 7.0, and 9.0 (Fig. 2), implying that excited triplet states of SRNOM (³SRNOM*) were involved in sensitizing the indirect photolysis of OTC. The ³DOM*-sensitized photodegradation was also proven to be involved in the photolysis of other antibiotics.⁴³ Addition of NaN₃ and IPA significantly reduced the k_obs in pH 7.0 and 9.0 solutions (Fig. 2), implying that ¹O₂ and ·OH-initiated indirect photolysis by SRNOM was involved in the OTC photolysis in solutions of high pH.

Fig. 2 Observed first-order photolysis rate constants (k_obs) of OTC in the presence of 10.0 mg L⁻¹ SRNOM under different conditions (* represents significant difference, p < 0.05, n = 3; the error bars represent the 95% confidence interval, n = 3).

Ionic strength and specific halide effects at different pH

According to previous studies, ionic strength can accelerate the photodegradation of antibiotics by increasing the steady state concentrations of their excited triplet states.^24,44 Meanwhile, halides in coastal and estuarine waters can promote the photodegradation of organic micropollutants by generating halogen radicals.^45,46 Thus, ionic strength and specific halide effects were investigated using Na₂SO₄, NaCl, and NaBr. The concentrations of Na₂SO₄ ranged from 0.020 to 0.250 mol L⁻¹ with the corresponding salinity from 2.8‰ to 35.5‰, which can be applied to mimic the salinity change in estuarine regions. As can be seen in Fig. 3a and b, the k_obs of OTC increased with the increasing Na₂SO₄ concentration at pH 7.0 and 9.0. This indicates that ionic strength has a positive effect on OTC photodegradation.

Fig. 3 Observed first-order photolysis rate constants (k_obs) of OTC in Na₂SO₄ (a and b) and NaCl (c and d) solutions at pH 7.0 and 9.0 (* represents significant difference, p < 0.05, n = 3; the error bars represent the 95% confidence interval, n = 3).

In the presence of SRNOM (10.0 mg L⁻¹), the promotional effect of ionic strength is much higher in pH 9.0 solutions (84.2% increase as the concentration of Na₂SO₄ increases from 0.020 mol L⁻¹ to 0.250 mol L⁻¹) than in solutions at pH 7.0 (39.2% increase) and also higher than that in the absence of SRNOM at pH 9.0 (54.5%). As shown in Fig. S6,† SRNOM-induced k_ind values also significantly increased with increasing ionic strength, especially in pH 9.0 solutions. The solutions with high ionic strength can enhance the steady state concentration of ³SRNOM*, leading to the promotion of indirect photolysis initiated by ³SRNOM*, ¹O₂, and ·OH. As discussed above, SRNOM-induced k_ind increased with increasing pH. Thus, the promotional effects of ionic strength on indirect photolysis induced by SRNOM are higher in pH 9.0 solutions than in solutions at pH 7.0. In estuarine environments, the ionic strength increases rapidly from river water to seawater due to seawater encroachment.³² The increase of ionic strength led to a 30.6–54.5% and 39.2–84.2% increase of the k_obs for OTC in the absence and presence of SRNOM, respectively.

Specific Cl⁻ effects were studied using NaCl with concentrations from 0.030 to 0.375 mol L⁻¹. In the absence of SRNOM, the k_obs of OTC was seen to increase with the increasing NaCl concentration (Fig. 3c and d) due to the ionic strength effect as shown above. However, in the presence of SRNOM (10.0 mg L⁻¹), the k_obs of OTC in NaCl solutions with a concentration of 0.375 mol L⁻¹ showed no obvious increase compared to that in NaCl solutions with a concentration of 0.150 mol L⁻¹ at pH 7.0, whereas k_obs decreased significantly at pH 9.0. It can be concluded that aside from the ionic strength effects, specific Cl⁻ effects can also influence OTC photolysis. As shown in Fig. S6,† the SRNOM-induced k_ind of OTC exhibited an increase-decrease trend in NaCl solutions. This was considered to be attributable to the quenching of ³SRNOM* and ·OH by a high concentration of Cl⁻ (0.375 mol L⁻¹).³²

The specific effects of Br⁻ and mixtures of Cl⁻ + Br⁻ were also investigated in solutions in the presence of 10.0 mg L⁻¹ SRNOM and with an ionic strength of 0.075 mol L⁻¹. The values of the k_obs of OTC under different experimental conditions are listed in Table S3.† The value of the SRNOM-induced k_ind of OTC was calculated by means of eqn (3), and the results are shown in Fig. 4. As can be seen in Fig. 4, the presence of 0.075 mol L⁻¹ Br⁻ significantly increased the k_ind at pH 6.0, 7.0, 8.0, and 9.0, and the increase of k_ind is obviously higher than that in the presence of Cl⁻ at the same concentration. This can be interpreted by the lower oxidation potential of Br⁻ (1.6 eV) as compared to Cl⁻ (2.0 eV), which leads to easier generation of bromine radicals.⁴⁷ The k_ind value obviously decreased as the Br⁻ concentration decreased to 0.80 mmol L⁻¹ (the Br⁻ concentration in seawater). However, coexistence of Cl⁻ + Br⁻ with SRNOM further increased the k_ind as compared to the value of k_ind in the presence of 0.075 mol L⁻¹ Cl⁻ or 0.075 mol L⁻¹ Br⁻ with SRNOM. The specific halide effects can lead to a 7.1–28.8% increase of the k_obs for OTC.

Fig. 4 SRNOM-induced indirect photolysis rate constants (k_ind) of OTC in Na₂SO₄, NaCl, and NaBr solutions (* represents significant difference, p < 0.05, n = 3; the error bars represent the 95% confidence interval, n = 3).

Estimation of environmental photodegradation half-lives for OTC

As shown above, SRNOM can influence the photodegradation of OTC through light screening and sensitizing effects. Both ionic strength and specific halides can enhance SRNOM-induced indirect photolysis. Nevertheless, direct photolysis is the most important degradation pathway of OTC in solutions of different pH, ionic strength and halide concentrations. Therefore, quantum yields (Φ) for the direct photolysis of the four species of OTC were calculated with the method described in Text S1.† The Φ values for H₃L⁺, H₂L, HL⁻, and L²⁻ are (1.22 ± 0.12) × 10⁻⁴, (3.39 ± 0.16) × 10⁻⁴, (5.32 ± 0.28) × 10⁻⁴, and (17.5 ± 0.48) × 10⁻⁴, respectively. Among the four factors, pH plays the most important role in increasing the k_obs of OTC. Thus, the Φ values of OTC at different pH were calculated, and the direct photolysis half-lives (t_1/2) of OTC in natural waters were predicted with the models established by Zhou et al.⁴⁸ The t_1/2 values of OTC in river, estuarine, and marine waters with a depth of 8 m (average mixed layer depth of the Bohai Sea and the Yellow Sea in summer⁴⁹) in the Yellow River estuarine region were predicted, and the results are shown in Fig. 5.

Fig. 5 Estimated direct photolysis half-lives (t_1/2) of OTC in river, estuarine, and marine waters in the Yellow River estuarine region.

As can be seen in Fig. 5, the t_1/2 values of OTC decreased rapidly with the increase of pH and are lowest in marine water as compared to estuarine and river water. Only taking the estuarine water into account, the t_1/2 values of OTC decreased from 187.4–206.6 d at pH 6.0 to 34.4–36.6 d at pH 9.0. Besides pH, ionic strength and specific halides can promote the photodegradation of OTC, whereas DOM inhibits the photodegradation. The pH, ionic strength and halide concentrations increased in the estuarine regions from river water to marine water. Meanwhile, the reported concentration of DOM in seawater was lower than that in freshwater.⁴¹ Hence, the sunlight relevant t_1/2 values of OTC must decrease rapidly from river water to seawater in the estuarine regions.

Conclusion

In this study, the combined effects of SRNOM, pH, ionic strength, and halides on the photodegradation of OTC were investigated. The results showed that SRNOM can inhibit the photodegradation of OTC, and pH, ionic strength, and halides promote the photodegradation of OTC. The effects of SRNOM, ionic strength, and halides are all pH-dependent. Meanwhile, ionic strength and specific halides can enhance the indirect photolysis induced by SRNOM, and the enhancement effect is also pH-dependent. Thus, among the four factors investigated, pH plays the most important role in promoting the photodegradation of OTC. In waters of different compositions (different pH, ionic strength, and halide or DOM concentrations), direct photolysis plays a dominant role in OTC photodegradation as compared to indirect photolysis. The direct photolysis half-lives were estimated to be rapidly decreasing in the estuarine regions along the gradient from river water to marine water. This study provided profound insights into the combined effects of SRNOM, pH, ionic strength, and halides on the photodegradation of OTC. It is of great significance to elucidate the photochemical transformation behaviors of ionizable micropollutants in estuarine regions where the environmental conditions (e.g. pH, ionic strength, and halide concentrations) are dynamic.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (21707017, 41877364 and 51479005), the China Postdoctoral Science Foundation (2017M621194), the Fundamental Research Funds for the Central Universities (2412017QD025) and the Jilin Province Science and Technology Development Projects (20180520079JH).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8em00473k

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