Triplet-State Photochemistry of Dissolved Organic Matter: Triplet-State

Feb 13, 2019 - Triplet-State Photochemistry of Dissolved Organic Matter: Triplet-State Energy Distribution and Surface Electric Charge Conditions...
1 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/est

Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Triplet-State Photochemistry of Dissolved Organic Matter: TripletState Energy Distribution and Surface Electric Charge Conditions Huaxi Zhou,† Shuwen Yan,†,‡ Lushi Lian,† and Weihua Song*,†,‡ †

Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, P. R. China Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, P. R. China

‡

Downloaded via MIDWESTERN UNIV on February 20, 2019 at 08:24:58 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Excited triplet states of chromophoric dissolved organic matter (3CDOM*) are highly reactive species in sunlit surface waters and play a critical role in reactive oxygen species (ROS) formation and pollutant attenuation. In the present study, a series of chemical probes, including sorbic acid, sorbic alcohol, sorbic amine, trimethylphenol, and furfuryl alcohol, were employed to quantitatively determine 3CDOM* and 1O2 in various organic matters. Using a high concentration of sorbic alcohol as high-energy triplet states quencher, 3CDOM* can be first distinguished as high-energy triplet states (>250 kJ mol−1) and lowenergy triplet states (250 kJ mol−1). The remaining triplet-state energy between 94 to 250 kJ mol−1 has been determined by monitoring the apparent 1O2 quantum yield. Combined with sorbic alcohol quenching, TMP was also employed to examine the electronic transfer ability of 3 CDOM*. CDOM was found to have dual roles in the phototransformation of organic contaminants.22,23,38−40 Electron acceptors, including aromatic ketones and quinone moieties, are major contributors to photoinduced 3CDOM*, while electron donors, i.e., mostly amine and phenolic constituents in CDOM, show quenching effects on pollutant attenuation in surface waters. Therefore, the relationships between the phenolic contents and the quenching effects on 3CDOM* generation were investigated. Surface electric charge interactions between CDOM and organic contaminants have received great attention.9,41 Our previous studies42 revealed that negatively charged Suwannee River NOM exhibits different reaction rate constants for three conjugated dienes with different charge conditions (sorbic acid, sorbic alcohol, and sorbic amine). The rapid reaction rate constant between the negatively charged triplet states and sorbic amine (positively charged) is due to their electrostatic

■

EXPERIMENTAL SECTION Chemicals. Sorbic alcohol (trans,trans-sorbic alcohol, 97%), sorbic acid (trans,trans-sorbic acid, 99%), 2,4,6trimethylphenol (TMP, 99%), furfuryl alcohol (FFA, 98%), gallic acid (GAC, 98%), 1,4-benzoquinone (98%), 1,4naphthoquinone (97%), plumbagin (97%), sodium anthraquinone-2-sulfonate (98%), acetophenone (99%), 3-methoxyacetophenone (97%), trifluoroacetic acid (TFA, 99%), ammonium acetate (98%), acetic acid (99.7%), and phosphate salts (NaH2PO4, Na2HPO4 (both 99%)) were purchased from Sigma-Aldrich. Benzophenone (99%), 4-benzoylbenzoic acid (99%), 2-hydroxyphenylacetic acid (OPAC, 98%), p-nitroacetophenone (PNAP, 98%), and pyridine (pyr, 99%) were obtained from Tokyo Chemical Industry Co., Ltd. Sorbic amine (trans,trans-sorbic amine) was synthesized from trans,trans-sorbic acid according to the procedure proposed by Matsumoto and co-workers.43 All chemicals were used as received. Reference NOM and WWOM. Nordic Lake natural organic matter (NLNOM, cat. # 1R108N), Suwannee River natural organic matter (SRNOM, cat. # 1R101N), Suwannee River humic acid (SRHA, cat. # 2S101H), Suwannee River fulvic acid (SRFA, cat. # 2S101F), and Pony Lake fulvic acid (PLFA, cat. # 1R109F) were purchased from the International Humic Substances Society (IHSS). WWOM was isolated from wastewater treatment effluents and contaminated surface waters. The effluents were collected from 2 municipal sewage plants near Shanghai, China. Other WWOM (WWOM1− WWOM8) was collected from wastewater-contaminated rivers and lakes located in Shanghai, and the sampling sites are shown in Figure S1 of the Supporting Information (SI). These contaminated locations were mixtures of untreated wastewater and surface water, as reported in our previous study.44 The isolation method previously reported by Bodhipaksha et al. was used with slight modifications.24 The detailed procedure is provided in Text S1 of the SI. The total phenolic contents in various OM isolates were measured using the Folin-Ciocalteu method11,45 and are presented in Table S1 of the SI. Photochemical Experiments. Photochemical experiments were performed using a solar simulator (Suntest XLS+ Atlas) equipped with a 1700 W xenon lamp. A solar filter was employed to block irradiance below 290 nm. For irradiation of λ > 315 nm, a glass filter was used. The chamber temperature was maintained at 25.0 ± 1.0 °C by a temperature control unit (Suncool). The irradiation intensity on the surface of the solutions was set to 40 W m−2 (1.36 × 10−8 Einstein s−1 cm−2) at 290−400 nm. The absolute irradiance spectra of the simulated solar light and real sunlight were recorded using a spectra-radiometer (USB-4000, Ocean Optics Inc.) and are presented in Figure S2 of the SI. p-Nitroanisole/pyridine (PNA-pyr) actinometry was employed to measure the absolute irradiance,46 which was used to calculate the quantum yields of the triplet states and 1O2. All solutions were prepared using B

DOI: 10.1021/acs.est.8b06574 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology

Table 1. Apparent Singlet Oxygen Quantum Yield (Φ1O2), Triplet Quantum Yield Coefficient (f TMP), High-Energy TripletState Quantum Yield (ΦHi‑triplet), and Steady-State Concentration of High-Energy Triplet States ([Hi-triplet]SS) for Different OM under Simulated Solar Irradiationa OM WWOM 1 WWOM 2 WWOM 3 WWOM 4 WWOM 5 WWOM 6 WWOM 7 WWOM 8 EfOM 1 EfOM 2 average SRNOM SRHA SRFA PLFA NLNOM average

Φ1O2b (%) 7.42 ± 7.95 ± 7.46 ± 6.47 ± 6.14 ± 6.11 ± 7.18 ± 7.30 ± 4.55 ± 5.45 ± 6.60 2.46 ± 2.02 ± 2.66 ± 5.29 ± 3.70 ± 3.23

f TMPb (M−1)

0.39 0.30 0.43 0.38 0.32 0.30 0.45 0.48 0.18 0.41

290 258 240 201 218 220 246 266 125 138

0.14 0.12 0.12 0.12 0.12

53 43 61 162 84

ΦHi‑tripletc (%)

±5 ±5 ±8 ±6 ±6 ±5 ±7 ±9 ±2 ±3 220 ±3 ±1 ±2 ±2 ±3 81

7.46 ± 6.78 ± 6.72 ± 5.75 ± 5.33 ± 5.72 ± 6.62 ± 6.84 ± 4.01 ± 4.37 ± 5.96 1.47 ± 0.59 ± 1.70 ± 4.32 ± 1.79 ± 1.97

0.23 0.25 0.35 0.29 0.21 0.34 0.38 0.36 0.21 0.32 0.12 0.03 0.07 0.22 0.16

[Hi-triplet]SS (10−13 M) 2.48 ± 2.06 ± 2.37 ± 2.43 ± 2.83 ± 2.27 ± 3.09 ± 1.40 ± 2.25 ± 2.98 ± 2.42 1.40 ± 0.84 ± 1.68 ± 2.72 ± 1.74 ± 1.68

0.21 0.19 0.19 0.26 0.30 0.21 0.28 0.16 0.19 0.30 0.12 0.07 0.10 0.16 0.14

Reaction conditions: 5.0 mg-C L−1 of air-saturated OM, 5.0 mM phosphate buffer, pH 7.0, irradiation wavelength, λ > 290 nm. bThe concentration of FFA (furfuryl alcohol) and TMP (2,4,6-trimethylphenol) was 10.0 μM. cSorbic alcohol was spiked at 6 different concentrations ranging from 10.2 μM to 510.0 μM. The error bars represent the standard deviations. a

Milli-Q water. Stock solutions of OM (200 mg-C L−1) were prepared in phosphate buffer (5.0 mM, pH 7.0) using magnetic stirring. The solution was then filtered with a 0.22-μm filter and stored at 4.0 °C. The experimental solutions were prepared by diluting stock solutions with phosphate buffer to 5.0 mg-C L−1. Dissolved organic carbon (DOC) and total nitrogen (TN) were measured using a TOC analyzer (Shimadzu L-CPH). The concentration of dissolved oxygen (DO) was measured using a DO meter (WTW, Germany) and kept constant (about 250 μM) during the irradiation. Samples (20 mL) were placed in specially made cylindrical quartz containers (diameter = 6.0 cm, height = 2.0 cm, thickness = 0.2 cm, as described in our previous study42) and irradiated for a given period under ambient conditions. During illumination, aliquots were removed at various time intervals and analyzed by HPLC-UV. The error bars in the corresponding figures represent the standard deviation. Spectroscopic Characterization. Ultraviolet−visible (UV−vis) absorbance spectra of all the samples were collected in a 1 cm quartz cuvette on a spectrophotometer (Cary 60, Agilent) using phosphate buffer (5.0 mM) as a blank and are presented in Figure S3 of the SI (sample concentration: 5.0 mg-C L−1). The 3D excitation−emission matrix (EEM) fluorescence spectra were recorded using a fluorometer (Aqualog, Horiba-Jobin Yvon), as shown in Figure S4 of the SI (sample concentration: 5.0 mg-C L−1). The instrument parameters were set up as follows. The excitation wavelength was incrementally increased from 240 to 550 nm in 1 nm intervals, and the emission was monitored from 240 to 600 nm at 1 nm intervals for each excitation wavelength. Quinine sulfate standards were used to calibrate the EEM spectra, and the fluorescence intensities were expressed in units of quinine sulfate equivalents (QSEs).47 Analytical Methods for Chemical Probes. HPLC analysis was carried out by an HPLC (Agilent 1260) equipped with a photodiode array detector and a C18 column (4.6 × 250 mm2, 5 μm, Phenomenex Luna). TMP, FFA, sorbic alcohol,

and sorbic amine were eluted with an isocratic mobile phase consisting of acetonitrile (ACN) and water acidified with trifluoroacetic acid (TFA, 0.05%) at a flow rate of 1.0 mL min−1. The volumetric ratio of ACN/acidified water was 70:30 for TMP, 30:70 for FFA, 25:75 for sorbic alcohol and 10:90 for sorbic amine. For sorbic acid, the samples were eluted with an isocratic mobile phase consisting of 15% acetonitrile and 85% 30 mM acetate buffer at pH 4.75 and a flow rate of 1.0 mL min−1. The detection wavelength was 220 nm for TMP and FFA, 230 nm for sorbic alcohol and sorbic amine and 254 nm for sorbic acid. The column temperature was 30 °C for TMP, FFA, and sorbic acid and 10 °C for sorbic alcohol and sorbic amine. 1 O2 and Triplet State Measurements. To study the formation of 1O2 from OM, FFA (10.0 μM) was employed to react with singlet oxygen. TMP (10.0 μM) was used as a chemical probe for the electron transfer of the excited triplet state. To explore the energy transfer of 3CDOM* above 250 kJ mol−1, varied concentrations of sorbic alcohol were employed as a probe to monitor the isomerization rate. The details can be found in our previous studies.42 In brief, eq 1 was employed to calculate the triplet-state formation: k′ [probe] [probe] = + S RP FT FTkP

(1)

where [probe] is the concentration of the chemical probe; RP is the triplet-state quenching rate with the probe, which is calculated by the sum of the isomer formation rates; FT is the triplet-state formation rate; k′S is the pseudo first-order rate constant; and kP is the second-order reaction rate constant for the reaction between the high-energy triplet states and probes. Thus, FT =

1 slope

(2)

The kP values can also be determined from eq 3, C

DOI: 10.1021/acs.est.8b06574 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology kP = k S′

slope intercept

which agreed well with their optical properties, as presented in Table S2 of the SI.20,28 The steady-state concentration of singlet oxygen ([1O2]SS) and triplet states ([3CDOM*]SS, measured with TMP) were also investigated and presented in Table S3 of the SI. The [1O2]SS value was approximately equal to [3CDOM*]SS in EfOM/WWOM, while the [1O2]SS value was slightly higher than [3CDOM*]SS value in terrestrial-origin NOM. [Hi-triplet]SS (the steady-state concentration of high-energy triplet states) was then investigated because this factor is pertinent to the photodegradation dynamics of pollutants in aquatic environments. An average [Hi-triplet]SS value of 2.42 × 10−13 M was observed in WWOM. In contrast, the [Hitriplet]SS values were lower for the reference NOM with an average [Hi-triplet]SS value of 1.68 × 10−13 M. A [Hi-triplet]SS value of 2.72 × 10−13 M in autochthonous-origin PLFA was higher than the other four terrestrial-origin NOMs. In general, triplet states with excited state energies up to 250 kJ mol−1 are critical for CDOM photochemistry.37,48 In comparison with terrestrial-origin NOM, the higher [Hi-triplet]SS values observed in autochthonous-origin PLFA and WWOM suggested that contaminants might be degraded more rapidly in autochthonous-origin PLFA and WWOM than the terrestrial-origin NOM. However, the underlying mechanism is still ambiguous. To provide useful evidence and explain this mechanism, the excited triplet-state energies, phenolic constituents and surface charge conditions during triplet-state formation were further explored and are discussed below. Relationship between 3CDOM* and 1O2 from Sunlit OM. Energy transfer from 3CDOM* to dioxygen can result in the generation of 1O2.17,20,24 To gain a better understanding of the relationship between 3CDOM* and 1O2, the correlations between triplet-state quantum yield coefficients for electron transfer (f TMP) and Φ1O2 were examined for all sunlit OM. A linear correlation coefficient of 0.95 was observed for the correlation between f TMP and Φ1O2 values, as presented in Figure S5 of the SI. A similar linear correlation was also observed in previous studies on EfOM and its isolations with diverse polarities.20 Furthermore, the x-intercept of the linear line implied that a triplet-state pool could promote O2 to 1O2 but could not degrade TMP. By employing sorbic alcohol to examine ΦHi‑triplet, we could explore the relationship between Φ1O2 and ΦHi‑triplet. As illustrated in Figure 1, a linear fit with a correlation coefficient of 0.97 was obtained for the relationship between Φ1O2 and ΦHi‑triplet. The slope of the linear line (1.13) was close to 1, suggesting that the varied Φ1O2 values are quantitatively linked to the ΦHi‑triplet values. In other words, the quantum yields of low-energy triplet states (ΦLw‑triplet), which have an energy of less than 250 kJ mol−1, do not contribute to the varied Φ1O2 values from CDOM. ΦLw‑triplet may remain constant in all 3 CDOM*. The x-intercept of the line was 1.39%, suggesting that the triplet-state pool can promote O2 to 1O2 but cannot be identified by sorbic alcohol. This triplet-state pool can be defined as the apparent 1O2 quantum yield resulting from lowenergy triplet state (ΦLw‑1O2). Energy Distributions in the Triplet States of OM Isolates. In addition to a high-energy triplet-state probe, high concentrations of sorbic alcohol can also be employed as a triplet-state scavenger to quench high-energy portions of triplet states (>250 kJ mol−1). Combined with the FFA probe, the apparent 1O2 quantum yield can be distinguished as ΦLw‑triplet. Therefore, the triplet-state energy distribution was successfully

(3)

where k′S is the sum of the oxygen-dependent (kO2[O2]) and oxygen-independent (kTd ) triplet-state decay rate constants. kO2 is the second-order reaction rate constant between 3CDOM* and O2. For various DOM samples, kO2 values are in a narrow range of 8.1−10.0 × 108 M−1 s−1, therefore the average value of 8.9 × 108 M−1 s−1 was employed.36 kTd values show larger variation and the average value of 9.0 × 104 s−1 was used for calculation.36 The direct photodegradation of these chemical probes was negligible in ultrapure water (5.0 mM phosphate buffer, pH 7.0) under our experimental conditions. Singlet Oxygen Yield from Triplet States (f△). The relations between 3CDOM* and 1O2 proposed by McNeill and Canonica were used for reference.17,36 CDOM is initially excited to 1CDOM*, and further ISC results in the formation of 3CDOM*. 3CDOM* can experience oxygen-dependent decay with the formation of 1O2. The yield for this process (f△) is variable and depends on the photosensitizer.17,36 3 CDOM* can also undergo oxygen-independent deactivation pathways (kTd ). Therefore, Φ1O2 and [3CDOM*]SS can be expressed as eqs 4 and 5, respectively: Φ1O2 =

[3CDOM *]SS k O2[O2 ]f△

[3CDOM *]SS =

R abs

(4)

R absΦ3CDOM * k O2[O2 ] + kdT

(5)

Combining eq 4 and eq 5, we can obtain eq 6:

■

Φ3CDOM * k [O ] + kdT = O2 2 Φ1O2 k O2[O2 ]f△

(6)

RESULTS AND DISCUSSION CDOM* and 1O2 Produced from Sunlit CDOM. In airsaturated waters, dissolved O2 is the primary quencher of 3 CDOM*, and a fraction of the O2 quenching process produces 1O2 ( f△).17 The formation of 3CDOM* is considered to be approximately equal to the yield of singlet oxygen divided by f△.17 Thus, all three chemical probes directly and indirectly measure the triplet states. As illustrated in Table 1, the quantitative parameters of the triplet states were examined, including the apparent singlet oxygen quantum yield (Φ1O2), triplet-state quantum yield coefficient for electron transfer (f TMP, equal to kTMP divided by Ra), and triplet-state quantum yield for energy transfer above 250 kJ mol−1 (ΦHi‑triplet). The quantum yield calculation can be found in Text S2 of the SI. The Φ1O2 values range from 4.55% to 7.95%, the f TMP values range from 125 M−1 to 290 M−1 and the ΦHi‑triplet values range from 4.01% to 7.46% in WWOM. These values were statistically higher than those from terrestrialorigin SRNOM samples and isolated SRHA/FA, which presented Φ1O2 values ranging from 2.02% to 2.66%, f TMP values ranging from 43 M−1 to 61 M−1 and ΦHi‑triplet values ranging from 0.59% to 1.70%. These results indicated that WWOM is more efficient in the formation of triplet states than terrestrial-origin NOM. PLFA is the only autochthonous natural OM from Antarctic area, and it has a comparable photochemical reactivity as WWOM with a Φ1O2 value of 5.29%, f TMP value of 162 M−1 and ΦHi‑triplet value of 4.32%, 3

D

DOI: 10.1021/acs.est.8b06574 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology

more remarkable in WWOM than the terrestrial-origin NOM. Moreover, the percentage of high-energy triplet states was calculated in the range from 20% to 38% in the reference NOM with an average value of 33% based on the change in the Φ1O2 values. This average value agreed well with a previous estimation that approximately 37% of triplet states in NOM have excited state energies of up to 250 kJ mol−1.17,37,48 This average value of ΦHi‑triplet increased to 65% in WWOM samples. These data implied that the triplet states in the terrestrial-origin NOM were mainly low-energy, while the high-energy triplet states were dominant in WWOM. In simple terms, WWOM is efficient for photochemical reactions due to the higher percentage of high-energy triplet states relative to that in the terrestrial-origin NOM. Singlet Oxygen Yield from High-Energy Triplet States ( f△‑High). Using the ΦHi‑triplet values in Table 1 and the 1O2 apparent quantum yields resulting from high-energy triplet states in Figure 2, the 1O2 yield from high-energy triplet states (defined as f△‑High) for various OM was then calculated through eq 6 and presented in Table S4 of the SI. A recently reported kO2 value of 8.9 × 108 M−1 s−1 and kTd value of 9 × 104 s−1 developed by Erickson et al. were also engaged,36 the f△‑High values could be obtained in the range of 0.71−1.18 with an average value of 0.95. It indicated that almost all oxygen reacted with high-energy triplet states were quantitatively transferred to 1O2 under air-saturated conditions. It should be noted that the values of kO2 and kTd were obtained through direct measurement of 1O2 phosphorescence.36 Therefore, they refer to the total 3CDOM* pool (>94 kJ mol−1). The kO2 values for high-energy triplet states may be different from the ones for total 3CDOM*. If the kO2 values for high-energy triplet states are greater than 8.9 × 108 M−1 s−1, then f△‑High will be lower. Exploring the Electron Transfer Capacity of Triplet States in OM Isolates. Furthermore, the triplet-state electron transfer properties were also investigated using a combination of TMP as the probe and a high concentration of sorbic alcohol as the quencher for high-energy triplet states. A concentration of 1.0 mM sorbic alcohol was sufficient to quench most of the high-energy triplet states within the electron transfer reactions (Figure S6b of the SI). Similar to Φ1O2, the f TMP values all decreased significantly when excess sorbic alcohol was present, as illustrated in Figure 3. Highenergy triplet states contributed 27% to 55% of the electron transfers (f TMP values), an average value of 46%, in the terrestrial-origin NOM. The high-energy triplet states contributed an average of 72% of the f TMP values in the WWOM, which was comparable to the value measured based on the Φ1O2 contribution (65%, Figure 2). After quenching with excess sorbic alcohol, the f TMP values were almost equal for all OM, which indicated that the low-energy triplet state fractions presented similar electron transfer capacities and 1 O 2 production. These results also showed that a high overlap existed in the triplet states based on triplet states-induced electron transfer and energy transfer reactions from various OM. It seems to indicate that the same pool of chromophores participate in both electron transfer and energy transfer, particularly for the low-energy triplet state fractions. Quenching Effect of Phenolic Constituents on the Formation of Triplet States. To investigate the influence of phenolic constituents on triplet state formation in sunlit OM, the Φ1O2, ΦHi‑triplet, and f TMP values were plotted against phenolic contents for various OM isolates. Similar to the

Figure 1. Quantum yield of high-energy triplet states (ΦHi‑triplet, %) against apparent 1O2 quantum yield (Φ1O2, %) of several OM samples with identical TOC values of 5.0 mg L−1 under simulated solar irradiation. Reaction conditions: 10.0 μM of FFA for 1 O 2 determination, sorbic alcohol spiked at 6 different concentrations ranging from 10.2 μM to 510.0 μM for triplet detection, air saturation, 5.0 mM phosphate buffer, pH 7.0, and irradiation wavelength λ > 290 nm. The error bars represent the standard deviations.

investigated for all OM isolates. Concentration-dependent experiments were first conducted to determine the appropriate dosage of sorbic alcohol to quench the high-energy triplet states, and the results are presented in Figure S6a of the SI. Obviously, 1.0 mM of sorbic alcohol was sufficient to quench most of the high-energy triplet states, and this concentration was further employed in the subsequent studies. As shown in Figure 2, significant decreases were observed in the Φ1O2 values for all tested OM after quenching by excess sorbic alcohol. Interestingly, the ΦLw‑triplet values remained constant in all tested OM. The observed attenuation in Φ1O2 values was

Figure 2. Effect of the triplet-state scavenger on the apparent 1O2 quantum yield (Φ1O2) of several OM samples with identical TOC values of 5.0 mg L−1 under simulated solar irradiation. Reaction conditions: probe concentration (FFA) 10.0 μM, 1.0 mM of sorbic alcohol as a triplet-state quencher to scavenge triplet states with energies up to 250 kJ mol−1, air saturation, 5.0 mM phosphate buffer, pH 7.0, irradiation wavelength, and λ > 290 nm. The error bars represent the standard deviations. The percentages represent the contribution of high-energy triplet states to the formation of 1O2 and were calculated as (Φ1O2 − Φ1O2,Scavenger)/ Φ1O2. E

DOI: 10.1021/acs.est.8b06574 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology

Figure 3. Effect of a triplet-state scavenger on the triplet-state quantum yield coefficient (f TMP, M−1) of several OM samples with identical TOC values of 5.0 mg L−1 under simulated solar irradiation. Reaction conditions: probe concentration (TMP) 10.0 μM, 1.0 mM of sorbic alcohol as a triplet-state quencher to quench triplet states with energies up to 250 kJ mol−1, air saturation, 5.0 mM phosphate buffer, pH 7.0, irradiation wavelength, and λ > 290 nm. The error bars represent the standard deviations. The percentages represent the contribution of high-energy triplet states to f TMP and were calculated as (f TMP − f TMP,Scavenger)/f TMP.

results in previous studies,11 the terrestrial-origin NOM had higher phenolic contents than the autochthonous-origin PLFA and WWOM. As illustrated in Figure 4a, clear negative trends were observed between the phenolic content and the formation of triplet states, indicating that the lower photoquantum yields of triplet states could be attributed to the higher phenolic contents in OM macromolecules. The exponential correlation presented good fitting for all 3 CDOM* subpopulations with phenolic contents (R2 ≥ 0.97). The mechanistic explanation for this mathematic fitting needs further investigation. Nevertheless this negative correlation might be another reason why autochthonous-origin PLFA and WWOM have higher 3CDOM* yields than terrestrial-origin CDOM. To further investigate the quenching effect of phenolic constituents on triplet states, the formation of singlet oxygen was measured in an EfOM/NOM solution spiked with a model phenol compound with and without the addition of sorbic alcohol as a high-energy triplet-state scavenger. Figure 4b demonstrates that an increasing concentration of spiked phenolic compound led to a decrease in 1O2 produced from both the high-energy and low-energy triplet states of SRNOM/ EfOM, respectively. However, the quenching effects were significantly different. The spiked phenolic compound seemed to have a limited quenching effect on 1O2 produced from lowenergy triplet states of EfOM/SRNOM, resulting in only 20% attenuation of 1O2 with 1.0 mM phenolic compound, and this quenching effect remained constant up to 7.0 mM phenolic compound. Both SRNOM and EfOM presented similar inhibition effects on 1O2 produced from low-energy triplet states, which further supported our above-mentioned conclusion that the low-energy triplet states ( 290 nm, 10.0 μM of FFA (furfuryl alcohol), and 10.0 μM of TMP (2,4,6-trimethylphenol), sorbic alcohol spiked at 6 different concentrations ranging from 10.2 μM to 510.0 μM and OPAC spiked at six different concentrations from 0.03 mM to 6.58 mM. Quenching efficiency = (1 −

Φ1o2 − opac Φ1o2

) × 100%. The

error bars represent the standard deviations.

origin NOM. Moreover, the loss rates of FFA and sorbic alcohol in model quinone and aromatic ketone photosensitizers were investigated, and the results are shown in Figure S7 of the SI. The results suggested that quinones mostly photochemically generate 1O2, but the excited triplet states cannot promote sorbic alcohol to its excited state to generate isomers. In other words, most of the triplet-state excited energies of the model quinones are more than 94 kJ mol−1 but less than 250 kJ mol−1. By contrast, the triplet-state excited energies of aromatic ketones are more than 250 kJ mol−1. Their triplet states produce 1O2, also promote sorbic alcohol to sensitized isomerization. These results indicated that the highenergy triplet states in OM mainly contain aromatic ketone compounds, while quinone compounds are the predominant contributors to the low-energy triplet states, which agreed well with the literature.17,35 To obtain further evidence, the quenching effect of phenolic compounds on 1O2 formation F

DOI: 10.1021/acs.est.8b06574 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology was also investigated using benzoquinone and 3-methoxyacetophenone as model photosensitizers, and the results are presented in Figure S8 of the SI. The quenching effect of phenolic compounds on 1O2 produced from benzoquinone was similar to that of the low-energy triplet states in OM, while the behavior of 3-methoxyacetophenone was similar to that of the high-energy triplet states produced from EfOM/NOM (Figure 4b). Thus, we concluded that the external phenolic compound seems to mostly quench aromatic ketone triplet states and possibly has a minor effect on quinone triplet states. Effects of Electric Charge Conditions on Triplet-State Photochemistry. To further examine the role of charge conditions in 3CDOM* photochemistry, three different charged sorbic probes were employed to characteristically react with the triplet states in WWOM and EfOM and compared to the triplet states in reference NOM from different origins. Figure S9 of the SI demonstrates that three approximately parallel straight lines were obtained for all tested OMs. The similar slopes from three sorbic probes have indicated that the same high-energy triplet states formation rates (FT) can be obtained from different electrostatic probes. However, the intercepts differ, suggesting that kP values were varied with different sorbic probes. The discrepancy in the photochemical behaviors of the three chemical probes was more remarkable in terrestrial-origin SRNOM, isolated SRHA/ FA and NLNOM than those measured in autochthonousdominated PLFA, WWOM, and EfOM. Figure 5 shows the bimolecular reaction rate constants (kp) of high-energy triplet states with sorbic acid/alcohol/amine, which were derived from Figure S9 of the SI and using eq 3. In Figure 5a, the kP values for the reaction between the highenergy triplet states of EfOM/WWOM and negatively charged sorbic acid are shown to be much higher than those observed in terrestrial-origin NOM (SRNOM, SRHA/FA, and NLNOM), while the kP value of autochthonous-dominated PLFA was comparable to those in EfOM/WWOM. A plausible explanation for this result is that PLFA and EfOM/WWOM contain less negative charges than terrestrial-origin NOM, resulting in decreasing electrostatic repulsion between the high-energy triplet states of PLFA/EfOM/WWOM and sorbic acid. The IHSS Database provides tabulated values for carboxylate and phenolate contents in the reference NOM isolates, as shown in Table S5 of the SI. It demonstrated that autochthonous CDOM contain lower acid functional group content than the terrestrial-origin CDOM, further supporting our results. Nevertheless the average value of kP of sorbate ion with high-energy 3CDOM* is 5.9 × 108 M−1 s−1, which is one order magnitude lower than the value from the model compound triplet with simple dienes (average 4.4 × 109 M−1 s−1).48 The electrostatic repulsion would also contribute this significant differential. The kP values of high-energy triplet states with the neutral form of sorbic alcohol in the reference NOM were almost equal to those measured in EfOM/ WWOM due to the limited electrostatic interactions, as presented in Figure 5b. Furthermore, Figure 5c demonstrates that the kP values between high-energy triplet states and positively charged sorbic amine in terrestrial-origin NOM were only slightly higher than those detected in autochthonousdominated PLFA and EfOM/WWOM, mainly due to their different electrostatic attraction behavior. It is well-known that EfOM/WWOM contains a high proportion of protein-like substances from biological sources and exhibits characteristic amino acid-like fluorescence peaks in EEM fluorescence

Figure 5. Bimolecular reaction rate constants between high-energy triplet states and chemical probes including (a) negatively charged sorbic acid, (b) neutral sorbic alcohol, and (c) positively charged sorbic amine in various OM. Reaction conditions: 5.0 mg-C L−1 of OM, probes spiked at 6 different concentrations ranging from 8.9 μM to 516.0 μM, air saturation, 5.0 mM phosphate buffer, pH 7.0, and irradiation wavelength, λ > 290 nm. The error bars represent the standard deviations.

spectra.20,49,50 A previous study41 reported that the positively charged amino groups in proteins can neutralize some of the negative charge from carboxyl and phosphate groups, leading to a reduction in the net negative surface charge of sludge flocs. In a similar approach, the amino groups carrying positive charge in peptides are able to neutralize part of the negative charge from carboxyl and phenolic groups and therefore lessen the net negative surface charge of EfOM, which accelerates the G

DOI: 10.1021/acs.est.8b06574 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

■

ACKNOWLEDGMENTS We are thankful for partial funding support from the National Natural Science Foundation of China (21607026 and 21677039). W.S. also acknowledges support from the program for Professor of Special Appointment (Eastern Scholar) at the Shanghai Institutions of Higher Learning. S.Y. appreciates the financial support from the China Postdoctoral Science Foundation (2016M590321).

reaction rate between the high-energy triplet states and sorbate anions and reduces the reaction rate between high-energy triplet states and positively charged sorbic amines. Moreover, the nitrogen content was measured for all the OM, and the values are presented in Table S6 of the SI. The nitrogen contents in WWOM were 1.8−5.8 times those measured in terrestrial-origin SRNOM and its isolated SRHA and SRFA. The autochthonous-dominated PLFA possessed the highest nitrogen content, which agrees well with its higher reaction rate with sorbate anions and slightly lower reaction rate with sorbic amines. Thus, autochthonous-dominated 3CDOM* may present a higher reactivity with negatively charged organic contaminants; however, terrestrial-origin 3CDOM* might be more efficient for degrading positively charged pollutants. Environmental Implications. Our studies clearly demonstrated that the photochemical reactivity of CDOM is highly correlated with the excited triplet-state energy distribution, phenolic constituents, and surface electric charge conditions. The triplet states in the terrestrial-origin NOM were principally low-energy triplet states, while the high-energy triplet states were predominant in autochthonous-dominated PLFA and WWOM. It is interesting that the 1O2 quantum yield and f TMP generated from low-energy triplet states remained constant in all tested organic matters (5 reference OMs from IHSS, 2 EfOMs, and 8 field collected WWOMs). Phenolic constituents show quenching effects on triplet states and tend to have a higher quenching efficiency on aromatic ketone triplet states, which are the main contributors to the triplet states in WWOMs. The amino groups in peptides have positive charges, which neutralize part of the negative charges from carboxyl and phenolic groups and decrease the net negative surface charge of WWOM. This results in an alteration of the reaction rates between the triplet states and different charged probes. 3CDOM* are important reactive intermediates for both organic contaminant attenuation2,9,27,28 and secondary radical formation.51−54 Increasing wastewater discharge into natural waters may alter the fate of organic contaminants such as antibiotics, which have been previously reported as “3CDOM*-labile” compounds.55 Our triplet-state quantitative parameters involve [Hi-triplets]SS, and kP values may be important in determining the second-order reaction rate constant between 3CDOM* and organic contaminants through competition kinetics. This information will improve the models for predicting the role of 3CDOM* in organic contaminant photodegradation in aquatic systems.

■

■

REFERENCES

(1) Porras, J.; Fernández, J. J.; Torres-Palma, R. A.; Richard, C. Humic substances enhance chlorothalonil phototransformation via photoreduction and energy transfer. Environ. Sci. Technol. 2014, 48 (4), 2218−2225. (2) Niu, X.-Z.; Busetti, F.; Langsa, M.; Croué, J.-P. Roles of singlet oxygen and dissolved organic matter in self-sensitized photo-oxidation of antibiotic norfloxacin under sunlight irradiation. Water Res. 2016, 106, 214−222. (3) Chen, Y.; Hu, C.; Hu, X.; Qu, J. Indirect photodegradation of amine drugs in aqueous solution under simulated sunlight. Environ. Sci. Technol. 2009, 43 (8), 2760−2765. (4) Yan, S.; Song, W. Photo-transformation of pharmaceutically active compounds in the aqueous environment: A review. Environ. Sci.: Processes Impacts 2014, 16 (4), 697−720. (5) Fasching, C.; Behounek, B.; Singer, G. A.; Battin, T. J. Microbial degradation of terrigenous dissolved organic matter and potential consequences for carbon cycling in brown-water streams. Sci. Rep. 2015, 4, 7. (6) Chu, C.; Lundeen, R. A.; Remucal, C. K.; Sander, M.; McNeill, K. Enhanced indirect photochemical transformation of histidine and histamine through association with chromophoric dissolved organic matter. Environ. Sci. Technol. 2015, 49 (9), 5511−5519. (7) Latch, D. E.; McNeill, K. Microheterogeneity of singlet oxygen distributions in irradiated humic acid solutions. Science 2006, 311 (5768), 1743−1747. (8) Chu, C.; Lundeen, R. A.; Sander, M.; McNeill, K. Assessing the indirect photochemical transformation of dissolved combined amino acids through the use of systematically designed histidine-containing oligopeptides. Environ. Sci. Technol. 2015, 49 (21), 12798−12807. (9) Li, R.; Zhao, C.; Yao, B.; Li, D.; Yan, S.; O’Shea, K. E.; Song, W. Photochemical transformation of aminoglycoside antibiotics in simulated natural waters. Environ. Sci. Technol. 2016, 50 (6), 2921− 2930. (10) Sun, L.; Qian, J.; Blough, N. V.; Mopper, K. Insights into the photoproduction sites of hydroxyl radicals by dissolved organic matter in natural waters. Environ. Sci. Technol. Lett. 2015, 2 (12), 352−356. (11) Zhang, D.; Yan, S.; Song, W. Photochemically induced formation of reactive oxygen species (ROS) from effluent organic matter. Environ. Sci. Technol. 2014, 48 (21), 12645−12653. (12) Fujii, M.; Rose, A. L.; Waite, T. D.; Omura, T. Oxygen and superoxide-mediated redox kinetics of iron complexed by humic substances in coastal seawater. Environ. Sci. Technol. 2010, 44 (24), 9337−9342. (13) Canonica, S.; Kohn, T.; Mac, M.; Real, F. J.; Wirz, J.; Von Gunten, U. Photosensitizer method to determine rate constants for the reaction of carbonate radical with organic compounds. Environ. Sci. Environ. Sci. Technol. 2005, 39 (23), 9182−9188. (14) Vione, D.; Minella, M.; Maurino, V.; Minero, C. Indirect photochemistry in sunlit surface waters: Photoinduced production of reactive transient species. Chem. - Eur. J. 2014, 20 (34), 10590− 10606. (15) Parker, K. M.; Mitch, W. A. Halogen radicals contribute to photooxidation in coastal and estuarine waters. Proc. Natl. Acad. Sci. U. S. A. 2016, 113 (21), 5868−5873. (16) Parker, K. M.; Reichwaldt, E. S.; Ghadouani, A.; Mitch, W. A. Halogen Radicals Promote the Photodegradation of Microcystins in Estuarine Systems. Environ. Sci. Technol. 2016, 50 (16), 8505−8513.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b06574. Six tables, nine figures, and three texts, including sample collection and extraction (Text S1), calculation of quantum yields (Text S2), and optical properties of CDOM (Text S3) (PDF)

■

Article

AUTHOR INFORMATION

Corresponding Author

*Phone: (+86)-21-6564-2040; e-mail: [email protected]. ORCID

Weihua Song: 0000-0001-7633-7919 Notes

The authors declare no competing financial interest. H

DOI: 10.1021/acs.est.8b06574 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology (17) McNeill, K.; Canonica, S. Triplet state dissolved organic matter in aquatic photochemistry: reaction mechanisms, substrate scope, and photophysical properties. Environ. Sci.: Processes Impacts 2016, 18 (11), 1381−1399. (18) Rosario-Ortiz, F. L.; Canonica, S. Probe compounds to assess the photochemical activity of dissolved organic matter. Environ. Sci. Technol. 2016, 50 (23), 12532−12547. (19) Leenheer, J. A.; Croué, J.-P. Peer reviewed: Characterizing aquatic dissolved organic matter. Environ. Sci. Technol. 2003, 37 (1), 18A−26A. (20) Zhou, H.; Lian, L.; Yan, S.; Song, W. Insights into the photoinduced formation of reactive intermediates from effluent organic matter: The role of chemical constituents. Water Res. 2017, 112, 120− 128. (21) Maizel, A. C.; Li, J.; Remucal, C. K. Relationships between dissolved organic matter composition and photochemistry in lakes of diverse trophic status. Environ. Sci. Technol. 2017, 51 (17), 9624− 9632. (22) Janssen, E. M. L.; Erickson, P. R.; McNeill, K. Dual roles of dissolved organic matter as sensitizer and quencher in the photooxidation of tryptophan. Environ. Sci. Technol. 2014, 48 (9), 4916− 4924. (23) Wenk, J.; Aeschbacher, M.; Sander, M.; Gunten, U. V.; Canonica, S. Photosensitizing and inhibitory effects of ozonated dissolved organic matter on triplet-induced contaminant transformation. Environ. Sci. Technol. 2015, 49 (14), 8541−8549. (24) Bodhipaksha, L. C.; Sharpless, C. M.; Chin, Y. P.; Sander, M.; Langston, W. K.; Mackay, A. A. Triplet photochemistry of effluent and natural organic matter in whole water and isolates from effluentreceiving rivers. Environ. Sci. Technol. 2015, 49 (6), 3453−3463. (25) Guerard, J. J.; Chin, Y.-P.; Mash, H.; Hadad, C. M. Photochemical fate of sulfadimethoxine in aquaculture waters. Environ. Sci. Technol. 2009, 43 (22), 8587−8592. (26) Guerard, J. J.; Miller, P. L.; Trouts, T. D.; Chin, Y.-P. The role of fulvic acid composition in the photosensitized degradation of aquatic contaminants. Aquat. Sci. 2009, 71 (2), 160−169. (27) Ryan, C. C.; Tan, D. T.; Arnold, W. A. Direct and indirect photolysis of sulfamethoxazole and trimethoprim in wastewater treatment plant effluent. Water Res. 2011, 45 (3), 1280−1286. (28) Bahnmüller, S.; von Gunten, U.; Canonica, S. Sunlight-induced transformation of sulfadiazine and sulfamethoxazole in surface waters and wastewater effluents. Water Res. 2014, 57, 183−192. (29) Paul, A.; Hackbarth, S.; Vogt, R. D.; Roder, B.; Burnison, B. K.; Steinberg, C. E. W. Photogeneration of singlet oxygen by humic substances: comparison of humic substances of aquatic and terrestrial origin. Photochem. Photobiol. Sci. 2004, 3 (3), 273−280. (30) Dong, M. M.; Rosario-Ortiz, F. L. Photochemical formation of hydroxyl radical from effluent organic matter. Environ. Sci. Technol. 2012, 46 (7), 3788−3794. (31) Bodhipaksha, L. C.; Sharpless, C. M.; Chin, Y.-P.; MacKay, A. A. Role of effluent organic matter in the photochemical degradation of compounds of wastewater origin. Water Res. 2017, 110, 170−179. (32) Wenk, J.; Eustis, S. N.; McNeill, K.; Canonica, S. Quenching of excited triplet states by dissolved natural organic matter. Environ. Sci. Technol. 2013, 47 (22), 12802−12810. (33) Aeschbacher, M.; Graf, C.; Schwarzenbach, R. P.; Sander, M. Antioxidant properties of humic substances. Environ. Sci. Technol. 2012, 46 (9), 4916−4925. (34) Maizel, A. C.; Remucal, C. K. Molecular composition and photochemical reactivity of size-fractionated dissolved organic matter. Environ. Sci. Technol. 2017, 51 (4), 2113−2123. (35) Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of photochemistry, 3rd ed.; CRC Press: Boca Raton, FL, 2006; p 633. (36) Erickson, P. R.; Moor, K. J.; Werner, J. J.; Latch, D. E.; Arnold, W. A.; McNeill, K. Singlet Oxygen Phosphorescence as a Probe for Triplet-State Dissolved Organic Matter Reactivity. Environ. Sci. Technol. 2018, 52 (16), 9170−9178.

(37) Zepp, R. G.; Schlotzhauer, P. F.; Sink, R. M. Photosensitized transformations involving electronic energy transfer in natural waters: Role of humic substances. Environ. Sci. Technol. 1985, 19 (1), 74−81. (38) Wenk, J.; Canonica, S. Phenolic antioxidants inhibit the tripletinduced transformation of anilines and sulfonamide antibiotics in aqueous solution. Environ. Sci. Technol. 2012, 46 (10), 5455−5462. (39) Page, S. E.; Logan, J. R.; Cory, R. M.; McNeill, K. Evidence for dissolved organic matter as the primary source and sink of photochemically produced hydroxyl radical in arctic surface waters. Environ. Sci.: Processes Impacts 2014, 16 (4), 807−822. (40) Wenk, J.; Von Gunten, U.; Canonica, S. Effect of dissolved organic matter on the transformation of contaminants induced by excited triplet states and the hydroxyl radical. Environ. Sci. Technol. 2011, 45 (4), 1334−1340. (41) Liao, B. Q.; Allen, D. G.; Droppo, I. G.; Leppard, G. G.; Liss, S. N. Surface properties of sludge and their role in bioflocculation and settleability. Water Res. 2001, 35 (2), 339−350. (42) Zhou, H.; Yan, S.; Ma, J.; Lian, L.; Song, W. Development of novel chemical probes for examining triplet natural organic matter under solar illumination. Environ. Sci. Technol. 2017, 51 (19), 11066− 11074. (43) Matsumoto, A.; Fujioka, D.; Kunisue, T. Organic intercalation of unsaturated amines into layered polymer crystals and solid-state photoreactivity of the guest molecules in constrained interlayers. Polym. J. 2003, 35 (8), 652−661. (44) Yao, B.; Yan, S. W.; Lian, L. S.; Yang, X.; Wan, C. L.; Dong, H. T.; Song, W. H. Occurrence and indicators of pharmaceuticals in Chinese streams: A nationwide study. Environ. Pollut. 2018, 236, 889−898. (45) Wang, B.-N.; Liu, H. F.; Zheng, J. B.; Fan, M. T.; Cao, W. Distribution of phenolic acids in different tissues of jujube and their antioxidant activity. J. Agric. Food Chem. 2011, 59 (4), 1288−1292. (46) Laszakovits, J. R.; Berg, S. M.; Anderson, B. G.; O’Brien, J. E.; Wammer, K. H.; Sharpless, C. M. p-Nitroanisole/pyridine and pnitroacetophenone/pyridine actinometers revisited: Quantum yield in comparison to ferrioxalate. Environ. Sci. Technol. Lett. 2017, 4 (1), 11−14. (47) Coble, P. G. Characterization of marine and terrestrial DOM in seawater using excitation-emission matrix spectroscopy. Mar. Chem. 1996, 51 (4), 325−346. (48) Grebel, J. E.; Pignatello, J. J.; Mitch, W. A. Sorbic acid as a quantitative probe for the formation, scavenging and steady-state concentrations of the triplet-excited state of organic compounds. Water Res. 2011, 45 (19), 6535−6544. (49) Shon, H. K.; Vigneswaran, S.; Snyder, S. A. Effluent organic matter (EfOM) in wastewater: Constituents, effects, and treatment. Crit. Rev. Environ. Sci. Technol. 2006, 36 (4), 327−374. (50) Vakondios, N.; Koukouraki, E. E.; Diamadopoulos, E. Effluent organic matter (EfOM) characterization by simultaneous measurement of proteins and humic matter. Water Res. 2014, 63, 62−70. (51) Zhang, Y.; Del Vecchio, R.; Blough, N. V. Investigating the mechanism of hydrogen peroxide photoproduction by humic substances. Environ. Sci. Technol. 2012, 46 (21), 11836−11843. (52) Zepp, R. G.; Wolfe, N. L.; Baughman, G. L.; Hollis, R. C. Singlet oxygen in natural waters. Nature 1977, 267 (5610), 421−423. (53) McKay, G.; Rosario-Ortiz, F. L. Temperature dependence of the photochemical formation of hydroxyl radical from dissolved organic matter. Environ. Sci. Technol. 2015, 49 (7), 4147−4154. (54) Garg, S.; Rose, A. L.; Waite, T. D. Photochemical production of superoxide and hydrogen peroxide from natural organic matter. Geochim. Cosmochim. Acta 2011, 75 (15), 4310−4320. (55) Yan, S.; Yao, B.; Lian, L.; Lu, X.; Snyder, S. A.; Li, R.; Song, W. Development of fluorescence surrogates to predict the photochemical transformation of pharmaceuticals in wastewater effluents. Environ. Sci. Technol. 2017, 51 (5), 2738−2747.

I

DOI: 10.1021/acs.est.8b06574 Environ. Sci. Technol. XXXX, XXX, XXX−XXX