Environmental Toxicology and Chemistry, Vol. 33, No. 8, pp. 1739–1746, 2014 # 2014 SETAC Printed in the USA
Environmental Challenges in China EFFECTS OF DISSOLVED ORGANIC MATTER FROM A EUTROPHIC LAKE ON THE FREELY DISSOLVED CONCENTRATIONS OF EMERGING ORGANIC CONTAMINANTS YI-HUA XIAO,yz QING-HUI HUANG,*y ANSSI V. VÄHÄTALO,x FEI-PENG LI,y and LING CHENy yKey Laboratory of Yangtze River Water Environment of the Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai, China zDepartment of Environmental Sciences, University of Helsinki, Helsinki, Finland xDepartment of Biological and Environmental Science, University of Jyväskylä, Jyväskylä, Finland (Submitted 4 November 2013; Returned for Revision 5 December 2013; Accepted 23 April 2014) Abstract: The authors studied the effects of dissolved organic matter (DOM) on the bioavailability of bisphenol A (BPA) and chloramphenicol by measuring the freely dissolved concentrations of the contaminants in solutions containing DOM that had been isolated from a mesocosm in a eutrophic lake. The abundance and aromaticity of the chromophoric DOM increased over the 25-d mesocosm experiment. The BPA freely dissolved concentration was 72.3% lower and the chloramphenicol freely dissolved concentration was 56.2% lower using DOM collected on day 25 than using DOM collected on day 1 of the mesocosm experiment. The freely dissolved concentrations negatively correlated with the ultraviolent absorption coefficient at 254 nm and positively correlated with the spectral slope of chromophoric DOM, suggesting that the bioavailability of these emerging organic contaminants depends on the characteristics of the DOM present. The DOM–water partition coefficients (log KOC) for the emerging organic contaminants positively correlated with the aromaticity of the DOM, measured as humic acid–like fluorescent components C1 (excitation/emission ¼ 250[313]/412 nm) and C2 (excitation/emission ¼ 268[379]/456 nm). The authors conclude that the bioavailability of emerging organic contaminants in eutrophic lakes can be affected by changes in the DOM. Environ Toxicol Chem 2014;33:1739–1746. # 2014 SETAC Keywords: Dissolved organic matter (DOM)
Emerging organic contaminant
Bioavailability
Eutrophication
organic-bound forms) is not directly related to the bioavailability of the contaminant. The freely dissolved concentration (also known as effective concentration) is the concentration of an organic contaminant that is freely dissolved in water, and it is not bound to DOM, colloids, or particles. The toxicity of an organic contaminant depends primarily on the freely dissolved concentration. Heringa and Hermens [7] published a review in which they clearly pointed out that the interaction of organic contaminants with DOM decreases the freely dissolved concentrations and the bioavailability of organic contaminants. Several methods have been developed to separate freely dissolved organic contaminants from DOM-bound species, including equilibrium dialysis, ultracentrifugation [8], reversephase separation, solid-phase microextraction [9], competing ligands [10], and ultrafiltration. For example, cross-flow ultrafiltration followed by gas chromatography-mass spectrometry has been used to determine the partitioning of organic contaminants between river colloids and the dissolved phase [11]. This method was found to be applicable to low concentrations of organic contaminants and was not limited to measurements of the optical properties of the contaminants, which have been widely used in recent studies [12]. Most studies of the partitioning of emerging organic contaminants between DOM and the aqueous phase have used commercially available humic substances or DOM extracted from soil or natural water samples [3]. There are 2 main sources of DOM in natural waters: autochthonous DOM, which is produced in situ by plankton and through microbial decomposition, and allochthonous DOM, which originates in surface runoff and wastewater discharges [5]. A few processes such as algal blooms related to eutrophication increase the abundance of autochthonous DOM and change the abundance and composition of the DOM [13]. We hypothesized that dynamic changes in
INTRODUCTION
Endocrine-disrupting chemicals and pharmaceutical and personal care products are increasingly being detected in natural waters because they are widely produced and used [1]. Some of these chemicals are of emerging concern because they are persistent, bioaccumulative, and toxic. Such emerging organic contaminants are not destroyed by wastewater treatments and, therefore, are emitted into natural water bodies; and a few may be carcinogenic or have endocrine-disrupting effects on aquatic organisms and humans [2]. These adverse effects depend on the bioavailability and bioaccessibility of the contaminants to aquatic organisms. The fate and toxicity of emerging organic contaminants in aquatic environments can be altered by their interactions with organic ligands, including mixtures such as dissolved organic matter (DOM) [3]. Dissolved organic matter is defined as a mixture of organic compounds that can pass through a 0.2-mm or a 0.45-mm filter [4]. The optically active part of the DOM is called chromophoric DOM, and it plays important roles in biogeochemical cycles and photochemical transformations in aquatic environments [5]. Chromophoric DOM can act as a natural sensitizer, leading to the indirect photochemical transformation of contaminants. Organic contaminants in aquatic environments can be bound to DOM, preventing them from being captured and taken up by organisms or from escaping into the atmosphere [6]. The total concentration of an organic contaminant (including both the freely dissolved and the
All Supplemental Data may be found in the online version of this article. * Address all correspondence to [email protected] Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/etc.2625 1739
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the abundance and composition of DOM in eutrophic water affect the interactions between DOM and organic contaminants. The present study aimed to assess changes in the characteristics of natural DOM and its effects on the bioavailability of organic contaminants during the in situ processing of DOM in a month-long mesocosm experiment in a eutrophic lake. Bisphenol A (BPA) and chloramphenicol were selected as typical emerging organic contaminants because they are widely used, frequently found in freshwater environments, and potentially harmful to humans [14,15]. MATERIALS AND METHODS
Reagents and chemicals
Bisphenol A and chloramphenicol were purchased from Sigma-Aldrich and were >98% pure. All of the solvents used, including methanol and acetonitrile, were of high-performance liquid chromatography (HPLC)–grade and purchased from Merck. High-purity deionized water was produced using a Milli-Q unit (Millipore). Sampling and experimental design
The experiment was performed in Lake Qianwei (31.728N, 121.518E), which is a shallow freshwater lake on Chongming Island, China. Total surface area and average depth of the lake are 5.2 ha and 1.3 m, respectively. Lake Qianwei is maintained as landscape scenery by pumping water into it from nearby channels that are connected to the Yangtze River. The total nitrogen concentration in the lake water ranges from 0.7 mg L1 to 2.8 mg L1, and the total phosphorus concentration ranges from 0.08 mg L1 to 0.26 mg L1, indicating that the lake is eutrophic [16,17]. Green algae (Volvox, Ankistrodesmus, and Ulothrix) and diatoms (Melosiraceae) are the dominant species in spring and autumn, cyanobacteria (Microcystis) are dominant in the summer, and diatoms are dominant in the winter [18]. Lake water, without fish, was isolated in a mesocosm 1.5 m long, 0.4 m wide, and 1.5 m high, made using polyvinyl chloride sheet walls extending to the bottom of the lake. The bottom of the mesocosm was open and inserted into the lake sediment. The mesocosm was established on 26 April 2010, and water samples were collected from the mesocosm every day for the following 25 d. The 25 DOM samples collected from the mesocosm were used in DOM–water partitioning experiments in the laboratory (Figure 1). Water quality and DOM measurement
The mesocosm water temperature, pH, dissolved oxygen concentration, and conductivity were measured every day using a portable multiparameter analyzer (HQ40D; Hach). The concentration of chlorophyll a was determined as soon as a sample was collected using a Phyto-PAM phytoplankton analyzer (Walz) [19]. The fluorescence intensity of chlorophyll a was calibrated to represent the chlorophyll a concentration measured using the hot ethanol extraction method [20]. Total phosphorus was measured using spectrofluorometric methods [21]. Trophic state index (TSI) was calculated as the mean of TSItotal phosphorus and TSIchlorophyll a [22], as shown in Equation 1 TSI ¼ ½ð14:42 ln ½TP þ 4:15Þ þ ð9:81 ln ½Chl a þ 30:6Þ=2
ð1Þ
where [TP] is the total phosphorus concentration (mg L1) and [Chl a] is the chlorophyll a concentration (mg L1).
Figure 1. Experimental design and analytical scheme. DOM ¼ dissolved organic matter; DOC ¼ dissolved organic carbon; GFC ¼ gel filtration chromatography; CFUF ¼ cross-flow ultrafiltration; EOC ¼ emerging organic contaminants; FDC ¼ freely dissolved concentration; SPE ¼ solidphase extraction; HPLC ¼ high-performance liquid chromatography. [Color figure can be viewed in the online issue which is available at wileyonlinelibrary.com]
Each water sample that was collected for DOM and chromophoric DOM measurements was filtered through a 0.22-mm polyvinylidene difluoride (PVDF; Millipore) membrane. The molecular weight distribution of the DOM was measured using gel filtration chromatography (GFC-10ADVP; Shimadzu); Milli-Q water was used as the mobile phase, and polyethylene glycol was used to calibrate the system. Samples for dissolved organic carbon (DOC) analysis were acidified with HCl to approximately pH 2.2, and the DOC was measured by 680 8C combustion and catalytic oxidation using a total organic carbon analyzer (TOC-VCPH; Shimadzu). Chromophoric DOM analyses
The spectral absorbance (at 200–700 nm) of chromophoric DOM was determined using an ultraviolet (UV)–visible spectrophotometer (UV-2450; Shimadzu); Milli-Q water was used as a reference sample. The absorption coefficient at wavelength l, al (per meter), was calculated by dividing the absorbance at wavelength l (Al) by the length of the absorption path (L ¼ 0.01 m), as shown in Equation 2. al ¼ 2:303 Al L1
ð2Þ
The spectral slope coefficient (S) was obtained from the natural logarithms of the absorbance in the spectral ranges 275 nm to 295 nm (S275–295) and 350 nm to 400 nm (S350–400) [23]. The slope ratio (SR) was defined as the S275–295 to S350–400 ratio [23]. The specific UV absorption coefficient (L mg1 m1) was defined as the UV absorption coefficient at a wavelength of 280 nm normalized to the DOC concentration (mg L1), which was modified from the reference [24]. Excitation–emission matrix spectroscopy was used to measure the chromophoric DOM, and a 1-cm quartz sample cell was used in an F-4500 fluorescence spectrophotometer
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equipped with a xenon lamp (Hitachi High-Technologies). The excitation–emission matrix spectra were generated using excitation wavelengths at 3-nm intervals from 250 nm to 400 nm and emission wavelengths at 2-nm intervals from 300 nm to 550 nm, using a 5-nm bandwidth in both excitation mode and emission mode and a scanning speed of 12 000 nm min1. The measurements and data calibrations were performed following the methods published by Stedmon and Bro [25]. The Raman-normalized Milli-Q excitation–emission matrix spectra were subtracted from the normalized spectra to remove relatively low-intensity Raman scattering effects. The fully processed chromophoric DOM excitation–emission matrix spectra were in Raman units (per nanometer). Parallel factor analysis was used to study the individual components of the chromophoric DOM excitation–emission matrix (n ¼ 50). Parallel factor analysis was performed using the DOM fluorescence (DOMFluor) toolbox and Matlab R2010a software [25]. The data array was split into 2 halves for parallel factor analysis modeling, and the parallel factor analysis algorithm was run stepwise on each half independently, giving 3 to 10 components. Once the number of components was chosen, split-half validation and random initialization analyses were conducted to ensure that the model was valid [25]. The fluorescence maximum (Fmax, in Raman units) of each component was reported.
where [BPAfree] is the recovery-corrected freely dissolved concentration in the permeate (kg L1), [DOMBPAbound] is the DOM-bound BPA concentration (the difference between the total and freely dissolved BPA concentrations), [DOM] is the DOC concentration (kg L1), and KOC is the organic carbon partition coefficient (L kg1) normalized to the DOC concentration. Freely dissolved concentration and KOC are each reported as the mean standard deviation of 3 replicate DOM–water partitioning experiments, and they were corrected for the DOC and emerging organic contaminant recoveries for the whole system. The DOC recovery percentage of our cross-flow ultrafiltration system was 86% to 93%. The retention recovery percentages of BPA and chloramphenicol were 91 3.2% and 89 5.1%, respectively.
DOM–water partitioning experiment
Statistical analyses
Water samples (2 L) were filtered through 0.22-mm PVDF membranes, and the high-molecular weight DOM fraction (>5 kDa) was isolated from each filtered sample using a crossflow ultrafiltration unit (LabscaleTM TFF; Millipore). The crossflow ultrafiltration unit consisted of a Millipore Pellicon XL 50 ultrafiltration cassette containing a 5-kDa composite regenerated cellulose membrane (with a filtration area of 50 cm2), a 500-mL acrylic reservoir with pressure gauges, and a diaphragm pump. A 500-mL DOM sample was circulated through the cross-flow ultrafiltration unit for 30 min before the ultrafiltration process was performed. The circulated DOM sample was discharged, and the reservoir was refilled with another 500-mL sample aliquot. Ultrafiltration was then started using a feed pressure of 30 psi and a backflow pressure of 10 psi. An ultrafiltration concentration factor of 4, by volume, was achieved [11]. The high–molecular weight DOM that was collected was used in BPA and chloramphenicol DOM–water partitioning experiments (Figure 1). For the partitioning experiments, 75 mL of high–molecular weight DOM solution was spiked with BPA and chloramphenicol to give a contaminant concentration of 1 mg L1. The DOM and BPA/chloramphenicol mixture was stirred at room temperature (25 8C) in the dark at 0.64 g for 24 h. The solution was then ultrafiltered again using the cross-flow ultrafiltration unit to separate the freely dissolved BPA and chloramphenicol (in the permeate, with a concentration factor of 3) from the DOM–bound species (in the retentate). Bisphenol A and chloramphenicol were extracted from the permeate by passing through a 500-mg ENVI-18 solid-phase extraction column (Supelco), and the analytes were eluted from the column with 1 mL methanol. Bisphenol A and chloramphenicol in the methanol extract were determined using HPLC with a 5-mm C18 VP-octadecyl silica column (Prominence LC-20AD; Shimadzu) and a UV detector. The mobile phase flow rate was 1.0 mL min1, column temperature was 25 8C, and injection volume was 20 mL. The detection wavelengths were 227 nm for BPA and 278 nm for
Linear regression analyses between freely dissolved concentration (and log KOC) and chromophoric DOM parameters were performed using the Statistical Package for the Social Sciences software (SPSS 15.0; SPSS). The level of significance was set at p ¼ 0.05.
chloramphenicol. The average recoveries of BPA and chloramphenicol from spiked samples using Milli-Q as references were 91% and 86%, respectively. The association between the DOM and BPA or chloramphenicol can be described using Equations 3 and 4 DOM þ BPAfree $ DOMBPAbound K OC ¼
½DOMBPAbound ½DOM½BPAfree
ð3Þ ð4Þ
RESULTS
Changes in physiochemical and biochemical properties of water samples
The water temperature in the mesocosm was 16.3 8C to 24.8 8C during the experiment, and the water was consistently weakly alkaline with pH 7.7 to 8.4. The surface water had a mean conductivity of 1040 25 ms cm1 and a mean dissolved oxygen concentration of 7.3 1.2 mg L1. The chlorophyll a concentration increased by 20%, from 21.9 mg L1 to 26.3 mg L1, in the first 18 d of the experiment (Figure 2); it increased rapidly after day 18, and the highest concentration (67.5 mg L1, 208% increase from the
Figure 2. Chlorophyll a (Chl a) concentration, total phosphorus (TP) concentration, and trophic state index (TSI) in the mesocosm over the experimental period.
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from 0.079 mg L1 to 0.171 mg L1 (Figure 2). The trophic state index increased from 64.8 on day 1 to 71.7 on day 21 (10.6% increase) and then decreased to 62.6 on day 25 (Figure 2). Changes in DOM properties
The DOC concentration increased from 6.19 mg L1 to 19.4 mg L1 over the experimental period (Figure 3A). The high–molecular weight DOM (>5 kDa fraction) also increased over the experimental period, from 76.6% of the total DOM on day 1 to 99.5% on day 25 (Supplemental Data, Figure S1). The chromophoric DOM absorption coefficients (a254, a350, and a440) increased by an average of 52.1% over the experimental period (Supplemental Data, Table S1). The specific UV absorption coefficient increased from 1.02 L mg1 m1 to 1.32 L mg1 m1 (29.4% increase) over the experimental period (Figure 3A). The spectral range S275–295 and slope ratio (SR) decreased over the experimental period: S275–295 from 23.9 mm1 on day 1 to 17.4 mm1 on day 25 (27.2% decrease) and SR from 1.16 to 0.98 (15.5% decrease) (Figure 3B). Parallel factor analysis modeling of the excitation–emission matrix spectra identified 3 fluorescent components in the DOM (Figure 4). Components 1 and 2 (C1, excitation/emission ¼ 250 [313]/412 nm; C2, excitation/emission ¼ 268[379]/456 nm) exhibited spectra that resembled those of humic acid–like substances, with broad emissions at wavelengths higher than 400 nm. Component 3 had a single emission maximum at
Figure 3. Dissolved organic carbon (DOC) concentration, specific UV absorbance at 280 nm (SUVA280), absorption spectral slopes (S275–295), and slope ratios (SR) for chromophoric dissolved organic matter.
concentration on day 1) was observed on day 22. After that, the chlorophyll a concentration decreased rapidly; it had decreased to 16.5 mg L1 at the end of the experiment (day 25). Total phosphorus concentration throughout the experiment ranged
Figure 4. Three fluorescent components identified using the parallel factor analysis model and the maxima of the fluorescence intensities (Fmax) of the components over the experimental period. Em ¼ emission; Ex ¼ excitation; R.U. ¼ Raman units. [Color figure can be viewed in the online issue which is available at wileyonlinelibrary.com]
Figure 5. Freely dissolved concentrations (FDC) and log organic carbon partition coefficient (KOC) values of the organic contaminants after they had reacted with the dissolved organic matter. Data are presented as the mean standard deviation (n ¼ 3). BPA ¼ bisphenol A; CAP ¼ chloramphenicol.
Dissolved organic matter and emerging organic contaminants
340 nm and a single excitation maximum at 283 nm and appeared to be a protein-like fluorescent component [26]. The value of Fmax for the 3 fluorescent components increased over time (Figure 4); for C1, C2, and C3, Fmax values were 28.4%, 42.8%, and 85.0% higher, respectively, on day 25 than on day 1. The total fluorescence intensity for chromophoric DOM (the sum of the C1, C2, and C3 intensities) was 49.4% higher, at 0.83 Raman units, on day 25 than on day 1, when it was 0.56 Raman units (Figure 4). Also, C1, C2, and C3 contributed 25.6%, 50.0%, and 24.4%, respectively, to the total fluorescence intensity on day 1 and 22.0%, 47.8%, and 30.2%, respectively, on day 25. Freely dissolved emerging organic contaminant concentrations
The freely dissolved concentrations of BPA and chloramphenicol, which had been added to the high–molecular weight DOM that had been isolated from the mesocosm, decreased as the experiment progressed (Figure 5A). The BPA freely dissolved concentration decreased by 72.3%, from 0.83 mg L1 to 0.23 mg L1, and the chloramphenicol freely dissolved concentration decreased by 56.2%, from 0.89 mg L1 to 0.39 mg L1, between day 1 and day 25. The log KOC of BPA increased by 16.4%, from 4.51 to 5.25, and the log KOC of chloramphenicol increased by 16.6%, from 4.21 to 4.91, between day 1 and day 25 (Figure 5B). The BPA and chloramphenicol freely dissolved concentrations negatively correlated with the chromophoric DOM a254 but positively correlated with the S275–295 (p < 0.05; Figure 6), suggesting that the BPA and chloramphenicol freely dissolved concentrations depended on DOM characteristics that were associated with its spectral properties. Log KOC of BPA and chloramphenicol significantly and positively correlated with the
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specific UV absorption coefficient (p < 0.05; Figure 7) and Fmax of the humic acid–like fluorescent components C1 and C2 (p < 0.05; Figure 8A–D). Log KOC correlated positively but not significantly with the protein-like fluorescent component C3 (Figure 8E–F). DISCUSSION
Accumulation of autochthonous DOM in the mesocosm
The high trophic state index found in our mesocosm (always >60; Figure 2) suggested that the lake water was in a eutrophic state [27]. The DOC concentration increased by a factor of 2 during the experimental period, indicating that DOM accumulated to a large degree (Figure 3A). The mesocosm largely excluded allochthonous DOM inputs; therefore, the increases in the DOM were caused mainly by in situ production by phytoplankton in the water column and partly through a benthic flux from sediment [5]. The increase in the chromophoric DOM (Supplemental Data, Table S1) was not synchronous with the increase in the chlorophyll a concentration (Figure 2), and no correlation was found between the absorption coefficients and the chlorophyll a concentration, suggesting that the increase in chromophoric DOM in the mesocosm was not directly caused by phytoplankton. The present results agree with the results of several previous studies, which also led to the conclusion that phytoplankton is not a direct source of chromophoric DOM [28]. Sasaki et al. [29] found that an increase in chromophoric DOM absorption lagged behind an increase in chlorophyll a concentration in Funka Bay (Japan) during spring bloom and suggested that the chromophoric DOM increase was the result of phytoplankton degradation that started after the spring bloom. The later part of
Figure 6. Plots of freely dissolved concentrations (FDC) of organic contaminants against 2 chromophoric dissolved organic matter parameters, absorption coefficient at 254 nm (a254) and spectral slope (S275–295). BPA ¼ bisphenol A; CAP ¼ chloramphenicol.
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performed by Boehme and Coble [32]. The Fmax value for another humic acid–like component, C2 (excitation/emission ¼ 244[325]/414 nm), increased more than the Fmax for C1, suggesting that C2 is a mainly autochthonous source of humic acid–like chromophoric DOM [31]. The highest increase in Fmax was for the protein-like C3 (excitation/emission ¼ 283/340 nm), indicating that this component was an autochthonous product of chromophoric DOM produced by phytoplankton and microbial degradation [26]. Effects of DOM on the bioavailability of emerging organic contaminants
Figure 7. Plots of the specific ultraviolet absorbance of chromophoric dissolved organic matter at 280 nm (SUVA280) against log organic carbon partition coefficient (KOC) values. BPA ¼ bisphenol A; CAP ¼ chloramphenicol.
the chromophoric DOM increase seen in the present study (after day 22; Supplemental Data, Table S1) may have been caused by the microbial transformation of phytoplankton-derived, noncolored DOM such as polysaccharides [28,29]. A low S is typical of high–molecular weight chromophoric DOM with a high degree of photochemical reactivity and of “newly” produced chromophoric DOM, whereas a high S indicates that the chromophoric DOM has a low molecular weight, is photochemically transformed, and is highly degraded [5]. Both S and SR decreased with time in the present study (Figure 3B), indicating that the water was becoming enriched with newly produced high–molecular weight chromophoric DOM during the experiment (Supplemental Data, Figure S1). The specific UV absorption coefficient increased over time, suggesting that the aromaticity of the DOM also increased with time [30]. The positions of the excitation and emission peaks of the 3 fluorescent components identified by the parallel factor analysis model were similar to the positions of fluorescence peaks that have been found in previous studies [26]. The humic acid–like component C1 (excitation/emission ¼ 268[379]/456 nm) in the present study was a combination of peaks A and C that were found by Coble et al. [31], who regarded it as being a humic substance from the terrestrial environment. In the present study, this humic acid–like component appears to have been derived mainly from the in situ transformation of DOM, which agrees with the results of a study of degradation in river water that was
We found that the transformation of DOM in the eutrophic lake water caused the freely dissolved concentrations of BPA and chloramphenicol to decrease and log KOC to increase (Figure 5). These results suggest that in-lake changes in the DOM abundance and composition could alter the bioavailability of the emerging organic contaminants. We explored the relationship between the freely dissolved concentration and the optical properties of the chromophoric CDOM, and found a significant negative correlation between freely dissolved concentration and a254, suggesting that the emerging organic contaminant freely dissolved concentrations were negatively related to the abundance of chromophoric DOM (Figure 6A and B). We found a significant positive relationship between freely dissolved concentration and S275–295 (Figure 6C and D). The spectral range S275–295 is a useful parameter that can be used to represent the DOM molecular weight, and it was found to be negatively correlated with the DOM molecular weight and positively correlated with the low molecular weight to high molecular weight ratio of the DOM [23]. The correlation between freely dissolved concentration and S275–295 suggests that the accumulation of newly produced high–molecular weight DOM in eutrophic water could cause the freely dissolved concentration of emerging organic contaminants to decrease. The BPA log KOC values (4.51–5.25; Figure 5B) were similar to the value (4.5) found in earlier studies [33,34], but log KOC was increased a little at the end of the present experiment. Log KOC significantly and positively correlated with the specific UV absorption coefficient, which indicates the degree of aromaticity in the DOM (Figure 7). This result agrees with the results of previous studies, suggesting that KOC strongly correlates with the aromatic carbon content and the specific UV absorption coefficient of the DOM [3,35]. An increase in the aromaticity of DOM, coupled with an increase of the number of conjugated double bonds, will increase the number of sites with van der Waals attraction forces, which are some of the most important forces involved in interactions between DOM and organic contaminants [36]. There were significant and positive correlations between log KOC and the humic acid–like fluorescent components C1 and C2 in the chromophoric DOM (Figure 8), suggesting that the organic contaminants preferentially bind to the humic acid–like fraction of the DOM. This fraction represents the more aromatic DOM fractions. The chemical structures of DOM (e.g., the functional groups, aromaticities, and elemental compositions of the components) and the characteristics of the organic contaminants (e.g., the solubility, polarity, and number of functional groups) are crucial factors in the interactions between DOM and organic contaminants [36,37]. Yamamoto et al. [36] found that the KOC of endocrine disruptors (including 17b-estradiol, estriol, and 17a-ethynylestradiol) correlated well with their UV absorptivities at 272 nm and the phenolic group abundance in the DOM, and suggested that the sorption mechanism is closely
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Figure 8. Plots of the maxima of the fluorescence intensities (Fmax) of the 3 fluorescent components (FC1FC3) in chromophoric dissolved organic matter against log organic carbon partition coefficient (KOC) values. BPA ¼ bisphenol A; CAP ¼ chloramphenicol; R.U. ¼ Raman units.
related to interactions between p-electrons and hydrogen bonds. We found a significant positive correlation between log KOC and DOM aromaticity (indicated by the specific UV absorption coefficient), and this agreed with the results of Yamamoto et al. We also found a significant positive correlation between log KOC and the hydrophobic humic acid–like components, suggesting that the interactions between DOM and BPA/chloramphenicol are related to hydrophobic interactions. The role of DOM in the cycling of organic contaminants in eutrophic waters
More and more freshwater ecosystems are suffering from eutrophication because of human-induced pressures, such as increasing population and intense industrialization. The effects that accompany eutrophication include the overgrowth of
phytoplankton, which results in increased primary production [38]. This is coupled with the increasing abundance of DOM and increasing trophic state of the water, which was found in the present study and a few previous studies [13]. Natural DOM greatly affects the transport, fate, and bioavailability of organic contaminants. The results of the present study show that DOM controlled the fate of contaminants in the eutrophic lake that was used. Other researchers previously have found that high biomass concentrations in eutrophic ecosystems cause the dilution of contaminants and low freely dissolved concentrations [39]. There will be a net flux of contaminants that are associated with particles to the sediment because organic particulate matter will have a high sedimentation rate in a eutrophic system [40]. Our results show that, in addition to pronounced losses to the
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sediment, the bioavailability of the emerging organic contaminants can be regulated by DOM in eutrophic water. SUPPLEMENTAL DATA
Table S1. Figure S1. (109 KB DOC). Acknowledgment—This work was funded by the National Natural Science Foundation of China (41071301 and 40601095), the Fundamental Research Funds for the Central Universities (0400219216), and the Kone Foundation of Finland. We thank 3 anonymous reviewers and the editor for their constructive comments on the manuscript.
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Y.-H. Xiao et al. 19. Jakob T, Schreiber U, Kirchesch V, Langner U, Wilhelm C. 2005. Estimation of chlorophyll content and daily primary production of the major algal groups by means of multiwavelength-excitation PAM chlorophyll fluorometry: Performance and methodological limits. Photosynth Res 83:343–361. 20. Jespersen AM, Christoffersen K. 1987. Measurements of chlorophyll-a from phytoplankton using ethanol as extraction solvent. Arch Hydrobiol 109:445–454. 21. Huang XF. 1999. Lake Ecological Investigation and Analysis. China Standards Press, Beijing, China. 22. Carlson RE, Simpson J. 1996. A Coordinator’s Guide to Volunteer Lake Monitoring Methods. North American Lake Management Society, Madison, WI, USA. 23. Helms JR, Stubbins A, Ritchie JD, Minor EC, Kieber DJ, Mopper K. 2008. Absorption spectral slopes and slope ratios as indicators of molecular weight, source, and photobleaching of chromophoric dissolved organic matter. Limnol Oceanogr 53:955– 969. 24. Weishaar JL, Aiken GR, Bergamaschi BA, Fram MS, Fujii R, Mopper K. 2003. Evaluation of specific ultraviolet absorbance as an indicator of the chemical composition and reactivity of dissolved organic carbon. Environ Sci Technol 37:4702–4708. 25. Stedmon CA, Bro R. 2008. Characterizing dissolved organic matter fluorescence with parallel factor analysis: A tutorial. Limnol Oceanogr 6:572–579. 26. Stedmon CA, Markager S. 2005. Tracing the production and degradation of autochthonous fractions of dissolved organic matter by fluorescence analysis. Limnol Oceanogr 50:1415–1426. 27. Carlson RE. 1977. A trophic state index for lakes. Limnol Oceanogr 22:361–369. 28. Rochelle-Newall EJ, Fisher TR. 2002. Production of chromophoric dissolved organic matter fluorescence in marine and estuarine environments: An investigation into the role of phytoplankton. Mar Chem 77: 7–21. 29. Sasaki H, Miyamura T, Saitoh S, Ishizaka J. 2005. Seasonal variation of absorption by particles and colored dissolved organic matter (CDOM) in Funka Bay, southwestern Hokkaido, Japan. Estuar Coast Shelf Sci 64:447–458. 30. Chin YP, Aiken G, Oloughlin E. 1994. Molecular weight, polydispersity, and spectroscopic properties of aquatic humic substances. Environ Sci Technol 28:1853–1858. 31. Coble PG, Del Castillo CE, Avril B. 1998. Distribution and optical properties of CDOM in the Arabian Sea during the 1995 southwest monsoon. Deep-Sea Res Part II 45:2195–2223. 32. Boehme JR, Coble PG. 2000. Characterization of colored dissolved organic matter using high-energy laser fragmentation. Environ Sci Technol 34:3283–3290. 33. Heemken OP, Reincke H, Stachel B, Theobald N. 2001. The occurrence of xenoestrogens in the Elbe River and the North Sea. Chemosphere 45:245–259. 34. Patrolecco L, Capri S, De Angelis S, Pagnotta R, Polesello S, Valsecchi S. 2006. Partition of nonylphenol and related compounds among different aquatic compartments in the Tiber River (central Italy). Water Air Soil Pollut 172:151–166. 35. Sun WL, Ni JR, Xu N, Sun LY. 2007. Fluorescence of sediment humic substance and its effect on the sorption of selected endocrine disruptors. Chemosphere 66:700–707. 36. Yamamoto H, Liljestrand HM, Shimizu Y, Morita M. 2003. Effects of physical–chemical characteristics on the sorption of selected endocrine disrnptors by dissolved organic matter surrogates. Environ Sci Technol 37:2646–2657. 37. Niederer C, Schwarzenbach RP, Goss KU. 2007. Elucidating differences in the sorption properties of 10 humic and fulvic acids for polar and nonpolar organic chemicals. Environ Sci Technol 41:6711– 6717. 38. Suikkanen S, Pulina S, Engstrom-Ost J, Lehtiniemi M, Lehtinen S, Brutemark A. 2013. Climate change and eutrophication induced shifts in northern summer plankton communities. Plos One 8:e66475. 39. Dachs J, Eisenreich SJ, Hoff RM. 2000. Influence of eutrophication on air–water exchange, vertical fluxes, and phytoplankton concentrations of persistent organic pollutants. Environ Sci Technol 34:1095– 1102. 40. Gunnarsson J, Bjork M, Gilek M, Granberg M, Rosenberg R. 2000. Effects of eutrophication on contaminant cycling in marine benthic systems. Ambio 29:252–259.
Environmental Challenges in China EFFECTS OF DISSOLVED ORGANIC MATTER FROM A EUTROPHIC LAKE ON THE FREELY DISSOLVED CONCENTRATIONS OF EMERGING ORGANIC CONTAMINANTS YI-HUA XIAO,yz QING-HUI HUANG,*y ANSSI V. VÄHÄTALO,x FEI-PENG LI,y and LING CHENy yKey Laboratory of Yangtze River Water Environment of the Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai, China zDepartment of Environmental Sciences, University of Helsinki, Helsinki, Finland xDepartment of Biological and Environmental Science, University of Jyväskylä, Jyväskylä, Finland (Submitted 4 November 2013; Returned for Revision 5 December 2013; Accepted 23 April 2014) Abstract: The authors studied the effects of dissolved organic matter (DOM) on the bioavailability of bisphenol A (BPA) and chloramphenicol by measuring the freely dissolved concentrations of the contaminants in solutions containing DOM that had been isolated from a mesocosm in a eutrophic lake. The abundance and aromaticity of the chromophoric DOM increased over the 25-d mesocosm experiment. The BPA freely dissolved concentration was 72.3% lower and the chloramphenicol freely dissolved concentration was 56.2% lower using DOM collected on day 25 than using DOM collected on day 1 of the mesocosm experiment. The freely dissolved concentrations negatively correlated with the ultraviolent absorption coefficient at 254 nm and positively correlated with the spectral slope of chromophoric DOM, suggesting that the bioavailability of these emerging organic contaminants depends on the characteristics of the DOM present. The DOM–water partition coefficients (log KOC) for the emerging organic contaminants positively correlated with the aromaticity of the DOM, measured as humic acid–like fluorescent components C1 (excitation/emission ¼ 250[313]/412 nm) and C2 (excitation/emission ¼ 268[379]/456 nm). The authors conclude that the bioavailability of emerging organic contaminants in eutrophic lakes can be affected by changes in the DOM. Environ Toxicol Chem 2014;33:1739–1746. # 2014 SETAC Keywords: Dissolved organic matter (DOM)
Emerging organic contaminant
Bioavailability
Eutrophication
organic-bound forms) is not directly related to the bioavailability of the contaminant. The freely dissolved concentration (also known as effective concentration) is the concentration of an organic contaminant that is freely dissolved in water, and it is not bound to DOM, colloids, or particles. The toxicity of an organic contaminant depends primarily on the freely dissolved concentration. Heringa and Hermens [7] published a review in which they clearly pointed out that the interaction of organic contaminants with DOM decreases the freely dissolved concentrations and the bioavailability of organic contaminants. Several methods have been developed to separate freely dissolved organic contaminants from DOM-bound species, including equilibrium dialysis, ultracentrifugation [8], reversephase separation, solid-phase microextraction [9], competing ligands [10], and ultrafiltration. For example, cross-flow ultrafiltration followed by gas chromatography-mass spectrometry has been used to determine the partitioning of organic contaminants between river colloids and the dissolved phase [11]. This method was found to be applicable to low concentrations of organic contaminants and was not limited to measurements of the optical properties of the contaminants, which have been widely used in recent studies [12]. Most studies of the partitioning of emerging organic contaminants between DOM and the aqueous phase have used commercially available humic substances or DOM extracted from soil or natural water samples [3]. There are 2 main sources of DOM in natural waters: autochthonous DOM, which is produced in situ by plankton and through microbial decomposition, and allochthonous DOM, which originates in surface runoff and wastewater discharges [5]. A few processes such as algal blooms related to eutrophication increase the abundance of autochthonous DOM and change the abundance and composition of the DOM [13]. We hypothesized that dynamic changes in
INTRODUCTION
Endocrine-disrupting chemicals and pharmaceutical and personal care products are increasingly being detected in natural waters because they are widely produced and used [1]. Some of these chemicals are of emerging concern because they are persistent, bioaccumulative, and toxic. Such emerging organic contaminants are not destroyed by wastewater treatments and, therefore, are emitted into natural water bodies; and a few may be carcinogenic or have endocrine-disrupting effects on aquatic organisms and humans [2]. These adverse effects depend on the bioavailability and bioaccessibility of the contaminants to aquatic organisms. The fate and toxicity of emerging organic contaminants in aquatic environments can be altered by their interactions with organic ligands, including mixtures such as dissolved organic matter (DOM) [3]. Dissolved organic matter is defined as a mixture of organic compounds that can pass through a 0.2-mm or a 0.45-mm filter [4]. The optically active part of the DOM is called chromophoric DOM, and it plays important roles in biogeochemical cycles and photochemical transformations in aquatic environments [5]. Chromophoric DOM can act as a natural sensitizer, leading to the indirect photochemical transformation of contaminants. Organic contaminants in aquatic environments can be bound to DOM, preventing them from being captured and taken up by organisms or from escaping into the atmosphere [6]. The total concentration of an organic contaminant (including both the freely dissolved and the
All Supplemental Data may be found in the online version of this article. * Address all correspondence to [email protected] Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/etc.2625 1739
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the abundance and composition of DOM in eutrophic water affect the interactions between DOM and organic contaminants. The present study aimed to assess changes in the characteristics of natural DOM and its effects on the bioavailability of organic contaminants during the in situ processing of DOM in a month-long mesocosm experiment in a eutrophic lake. Bisphenol A (BPA) and chloramphenicol were selected as typical emerging organic contaminants because they are widely used, frequently found in freshwater environments, and potentially harmful to humans [14,15]. MATERIALS AND METHODS
Reagents and chemicals
Bisphenol A and chloramphenicol were purchased from Sigma-Aldrich and were >98% pure. All of the solvents used, including methanol and acetonitrile, were of high-performance liquid chromatography (HPLC)–grade and purchased from Merck. High-purity deionized water was produced using a Milli-Q unit (Millipore). Sampling and experimental design
The experiment was performed in Lake Qianwei (31.728N, 121.518E), which is a shallow freshwater lake on Chongming Island, China. Total surface area and average depth of the lake are 5.2 ha and 1.3 m, respectively. Lake Qianwei is maintained as landscape scenery by pumping water into it from nearby channels that are connected to the Yangtze River. The total nitrogen concentration in the lake water ranges from 0.7 mg L1 to 2.8 mg L1, and the total phosphorus concentration ranges from 0.08 mg L1 to 0.26 mg L1, indicating that the lake is eutrophic [16,17]. Green algae (Volvox, Ankistrodesmus, and Ulothrix) and diatoms (Melosiraceae) are the dominant species in spring and autumn, cyanobacteria (Microcystis) are dominant in the summer, and diatoms are dominant in the winter [18]. Lake water, without fish, was isolated in a mesocosm 1.5 m long, 0.4 m wide, and 1.5 m high, made using polyvinyl chloride sheet walls extending to the bottom of the lake. The bottom of the mesocosm was open and inserted into the lake sediment. The mesocosm was established on 26 April 2010, and water samples were collected from the mesocosm every day for the following 25 d. The 25 DOM samples collected from the mesocosm were used in DOM–water partitioning experiments in the laboratory (Figure 1). Water quality and DOM measurement
The mesocosm water temperature, pH, dissolved oxygen concentration, and conductivity were measured every day using a portable multiparameter analyzer (HQ40D; Hach). The concentration of chlorophyll a was determined as soon as a sample was collected using a Phyto-PAM phytoplankton analyzer (Walz) [19]. The fluorescence intensity of chlorophyll a was calibrated to represent the chlorophyll a concentration measured using the hot ethanol extraction method [20]. Total phosphorus was measured using spectrofluorometric methods [21]. Trophic state index (TSI) was calculated as the mean of TSItotal phosphorus and TSIchlorophyll a [22], as shown in Equation 1 TSI ¼ ½ð14:42 ln ½TP þ 4:15Þ þ ð9:81 ln ½Chl a þ 30:6Þ=2
ð1Þ
where [TP] is the total phosphorus concentration (mg L1) and [Chl a] is the chlorophyll a concentration (mg L1).
Figure 1. Experimental design and analytical scheme. DOM ¼ dissolved organic matter; DOC ¼ dissolved organic carbon; GFC ¼ gel filtration chromatography; CFUF ¼ cross-flow ultrafiltration; EOC ¼ emerging organic contaminants; FDC ¼ freely dissolved concentration; SPE ¼ solidphase extraction; HPLC ¼ high-performance liquid chromatography. [Color figure can be viewed in the online issue which is available at wileyonlinelibrary.com]
Each water sample that was collected for DOM and chromophoric DOM measurements was filtered through a 0.22-mm polyvinylidene difluoride (PVDF; Millipore) membrane. The molecular weight distribution of the DOM was measured using gel filtration chromatography (GFC-10ADVP; Shimadzu); Milli-Q water was used as the mobile phase, and polyethylene glycol was used to calibrate the system. Samples for dissolved organic carbon (DOC) analysis were acidified with HCl to approximately pH 2.2, and the DOC was measured by 680 8C combustion and catalytic oxidation using a total organic carbon analyzer (TOC-VCPH; Shimadzu). Chromophoric DOM analyses
The spectral absorbance (at 200–700 nm) of chromophoric DOM was determined using an ultraviolet (UV)–visible spectrophotometer (UV-2450; Shimadzu); Milli-Q water was used as a reference sample. The absorption coefficient at wavelength l, al (per meter), was calculated by dividing the absorbance at wavelength l (Al) by the length of the absorption path (L ¼ 0.01 m), as shown in Equation 2. al ¼ 2:303 Al L1
ð2Þ
The spectral slope coefficient (S) was obtained from the natural logarithms of the absorbance in the spectral ranges 275 nm to 295 nm (S275–295) and 350 nm to 400 nm (S350–400) [23]. The slope ratio (SR) was defined as the S275–295 to S350–400 ratio [23]. The specific UV absorption coefficient (L mg1 m1) was defined as the UV absorption coefficient at a wavelength of 280 nm normalized to the DOC concentration (mg L1), which was modified from the reference [24]. Excitation–emission matrix spectroscopy was used to measure the chromophoric DOM, and a 1-cm quartz sample cell was used in an F-4500 fluorescence spectrophotometer
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equipped with a xenon lamp (Hitachi High-Technologies). The excitation–emission matrix spectra were generated using excitation wavelengths at 3-nm intervals from 250 nm to 400 nm and emission wavelengths at 2-nm intervals from 300 nm to 550 nm, using a 5-nm bandwidth in both excitation mode and emission mode and a scanning speed of 12 000 nm min1. The measurements and data calibrations were performed following the methods published by Stedmon and Bro [25]. The Raman-normalized Milli-Q excitation–emission matrix spectra were subtracted from the normalized spectra to remove relatively low-intensity Raman scattering effects. The fully processed chromophoric DOM excitation–emission matrix spectra were in Raman units (per nanometer). Parallel factor analysis was used to study the individual components of the chromophoric DOM excitation–emission matrix (n ¼ 50). Parallel factor analysis was performed using the DOM fluorescence (DOMFluor) toolbox and Matlab R2010a software [25]. The data array was split into 2 halves for parallel factor analysis modeling, and the parallel factor analysis algorithm was run stepwise on each half independently, giving 3 to 10 components. Once the number of components was chosen, split-half validation and random initialization analyses were conducted to ensure that the model was valid [25]. The fluorescence maximum (Fmax, in Raman units) of each component was reported.
where [BPAfree] is the recovery-corrected freely dissolved concentration in the permeate (kg L1), [DOMBPAbound] is the DOM-bound BPA concentration (the difference between the total and freely dissolved BPA concentrations), [DOM] is the DOC concentration (kg L1), and KOC is the organic carbon partition coefficient (L kg1) normalized to the DOC concentration. Freely dissolved concentration and KOC are each reported as the mean standard deviation of 3 replicate DOM–water partitioning experiments, and they were corrected for the DOC and emerging organic contaminant recoveries for the whole system. The DOC recovery percentage of our cross-flow ultrafiltration system was 86% to 93%. The retention recovery percentages of BPA and chloramphenicol were 91 3.2% and 89 5.1%, respectively.
DOM–water partitioning experiment
Statistical analyses
Water samples (2 L) were filtered through 0.22-mm PVDF membranes, and the high-molecular weight DOM fraction (>5 kDa) was isolated from each filtered sample using a crossflow ultrafiltration unit (LabscaleTM TFF; Millipore). The crossflow ultrafiltration unit consisted of a Millipore Pellicon XL 50 ultrafiltration cassette containing a 5-kDa composite regenerated cellulose membrane (with a filtration area of 50 cm2), a 500-mL acrylic reservoir with pressure gauges, and a diaphragm pump. A 500-mL DOM sample was circulated through the cross-flow ultrafiltration unit for 30 min before the ultrafiltration process was performed. The circulated DOM sample was discharged, and the reservoir was refilled with another 500-mL sample aliquot. Ultrafiltration was then started using a feed pressure of 30 psi and a backflow pressure of 10 psi. An ultrafiltration concentration factor of 4, by volume, was achieved [11]. The high–molecular weight DOM that was collected was used in BPA and chloramphenicol DOM–water partitioning experiments (Figure 1). For the partitioning experiments, 75 mL of high–molecular weight DOM solution was spiked with BPA and chloramphenicol to give a contaminant concentration of 1 mg L1. The DOM and BPA/chloramphenicol mixture was stirred at room temperature (25 8C) in the dark at 0.64 g for 24 h. The solution was then ultrafiltered again using the cross-flow ultrafiltration unit to separate the freely dissolved BPA and chloramphenicol (in the permeate, with a concentration factor of 3) from the DOM–bound species (in the retentate). Bisphenol A and chloramphenicol were extracted from the permeate by passing through a 500-mg ENVI-18 solid-phase extraction column (Supelco), and the analytes were eluted from the column with 1 mL methanol. Bisphenol A and chloramphenicol in the methanol extract were determined using HPLC with a 5-mm C18 VP-octadecyl silica column (Prominence LC-20AD; Shimadzu) and a UV detector. The mobile phase flow rate was 1.0 mL min1, column temperature was 25 8C, and injection volume was 20 mL. The detection wavelengths were 227 nm for BPA and 278 nm for
Linear regression analyses between freely dissolved concentration (and log KOC) and chromophoric DOM parameters were performed using the Statistical Package for the Social Sciences software (SPSS 15.0; SPSS). The level of significance was set at p ¼ 0.05.
chloramphenicol. The average recoveries of BPA and chloramphenicol from spiked samples using Milli-Q as references were 91% and 86%, respectively. The association between the DOM and BPA or chloramphenicol can be described using Equations 3 and 4 DOM þ BPAfree $ DOMBPAbound K OC ¼
½DOMBPAbound ½DOM½BPAfree
ð3Þ ð4Þ
RESULTS
Changes in physiochemical and biochemical properties of water samples
The water temperature in the mesocosm was 16.3 8C to 24.8 8C during the experiment, and the water was consistently weakly alkaline with pH 7.7 to 8.4. The surface water had a mean conductivity of 1040 25 ms cm1 and a mean dissolved oxygen concentration of 7.3 1.2 mg L1. The chlorophyll a concentration increased by 20%, from 21.9 mg L1 to 26.3 mg L1, in the first 18 d of the experiment (Figure 2); it increased rapidly after day 18, and the highest concentration (67.5 mg L1, 208% increase from the
Figure 2. Chlorophyll a (Chl a) concentration, total phosphorus (TP) concentration, and trophic state index (TSI) in the mesocosm over the experimental period.
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from 0.079 mg L1 to 0.171 mg L1 (Figure 2). The trophic state index increased from 64.8 on day 1 to 71.7 on day 21 (10.6% increase) and then decreased to 62.6 on day 25 (Figure 2). Changes in DOM properties
The DOC concentration increased from 6.19 mg L1 to 19.4 mg L1 over the experimental period (Figure 3A). The high–molecular weight DOM (>5 kDa fraction) also increased over the experimental period, from 76.6% of the total DOM on day 1 to 99.5% on day 25 (Supplemental Data, Figure S1). The chromophoric DOM absorption coefficients (a254, a350, and a440) increased by an average of 52.1% over the experimental period (Supplemental Data, Table S1). The specific UV absorption coefficient increased from 1.02 L mg1 m1 to 1.32 L mg1 m1 (29.4% increase) over the experimental period (Figure 3A). The spectral range S275–295 and slope ratio (SR) decreased over the experimental period: S275–295 from 23.9 mm1 on day 1 to 17.4 mm1 on day 25 (27.2% decrease) and SR from 1.16 to 0.98 (15.5% decrease) (Figure 3B). Parallel factor analysis modeling of the excitation–emission matrix spectra identified 3 fluorescent components in the DOM (Figure 4). Components 1 and 2 (C1, excitation/emission ¼ 250 [313]/412 nm; C2, excitation/emission ¼ 268[379]/456 nm) exhibited spectra that resembled those of humic acid–like substances, with broad emissions at wavelengths higher than 400 nm. Component 3 had a single emission maximum at
Figure 3. Dissolved organic carbon (DOC) concentration, specific UV absorbance at 280 nm (SUVA280), absorption spectral slopes (S275–295), and slope ratios (SR) for chromophoric dissolved organic matter.
concentration on day 1) was observed on day 22. After that, the chlorophyll a concentration decreased rapidly; it had decreased to 16.5 mg L1 at the end of the experiment (day 25). Total phosphorus concentration throughout the experiment ranged
Figure 4. Three fluorescent components identified using the parallel factor analysis model and the maxima of the fluorescence intensities (Fmax) of the components over the experimental period. Em ¼ emission; Ex ¼ excitation; R.U. ¼ Raman units. [Color figure can be viewed in the online issue which is available at wileyonlinelibrary.com]
Figure 5. Freely dissolved concentrations (FDC) and log organic carbon partition coefficient (KOC) values of the organic contaminants after they had reacted with the dissolved organic matter. Data are presented as the mean standard deviation (n ¼ 3). BPA ¼ bisphenol A; CAP ¼ chloramphenicol.
Dissolved organic matter and emerging organic contaminants
340 nm and a single excitation maximum at 283 nm and appeared to be a protein-like fluorescent component [26]. The value of Fmax for the 3 fluorescent components increased over time (Figure 4); for C1, C2, and C3, Fmax values were 28.4%, 42.8%, and 85.0% higher, respectively, on day 25 than on day 1. The total fluorescence intensity for chromophoric DOM (the sum of the C1, C2, and C3 intensities) was 49.4% higher, at 0.83 Raman units, on day 25 than on day 1, when it was 0.56 Raman units (Figure 4). Also, C1, C2, and C3 contributed 25.6%, 50.0%, and 24.4%, respectively, to the total fluorescence intensity on day 1 and 22.0%, 47.8%, and 30.2%, respectively, on day 25. Freely dissolved emerging organic contaminant concentrations
The freely dissolved concentrations of BPA and chloramphenicol, which had been added to the high–molecular weight DOM that had been isolated from the mesocosm, decreased as the experiment progressed (Figure 5A). The BPA freely dissolved concentration decreased by 72.3%, from 0.83 mg L1 to 0.23 mg L1, and the chloramphenicol freely dissolved concentration decreased by 56.2%, from 0.89 mg L1 to 0.39 mg L1, between day 1 and day 25. The log KOC of BPA increased by 16.4%, from 4.51 to 5.25, and the log KOC of chloramphenicol increased by 16.6%, from 4.21 to 4.91, between day 1 and day 25 (Figure 5B). The BPA and chloramphenicol freely dissolved concentrations negatively correlated with the chromophoric DOM a254 but positively correlated with the S275–295 (p < 0.05; Figure 6), suggesting that the BPA and chloramphenicol freely dissolved concentrations depended on DOM characteristics that were associated with its spectral properties. Log KOC of BPA and chloramphenicol significantly and positively correlated with the
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specific UV absorption coefficient (p < 0.05; Figure 7) and Fmax of the humic acid–like fluorescent components C1 and C2 (p < 0.05; Figure 8A–D). Log KOC correlated positively but not significantly with the protein-like fluorescent component C3 (Figure 8E–F). DISCUSSION
Accumulation of autochthonous DOM in the mesocosm
The high trophic state index found in our mesocosm (always >60; Figure 2) suggested that the lake water was in a eutrophic state [27]. The DOC concentration increased by a factor of 2 during the experimental period, indicating that DOM accumulated to a large degree (Figure 3A). The mesocosm largely excluded allochthonous DOM inputs; therefore, the increases in the DOM were caused mainly by in situ production by phytoplankton in the water column and partly through a benthic flux from sediment [5]. The increase in the chromophoric DOM (Supplemental Data, Table S1) was not synchronous with the increase in the chlorophyll a concentration (Figure 2), and no correlation was found between the absorption coefficients and the chlorophyll a concentration, suggesting that the increase in chromophoric DOM in the mesocosm was not directly caused by phytoplankton. The present results agree with the results of several previous studies, which also led to the conclusion that phytoplankton is not a direct source of chromophoric DOM [28]. Sasaki et al. [29] found that an increase in chromophoric DOM absorption lagged behind an increase in chlorophyll a concentration in Funka Bay (Japan) during spring bloom and suggested that the chromophoric DOM increase was the result of phytoplankton degradation that started after the spring bloom. The later part of
Figure 6. Plots of freely dissolved concentrations (FDC) of organic contaminants against 2 chromophoric dissolved organic matter parameters, absorption coefficient at 254 nm (a254) and spectral slope (S275–295). BPA ¼ bisphenol A; CAP ¼ chloramphenicol.
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performed by Boehme and Coble [32]. The Fmax value for another humic acid–like component, C2 (excitation/emission ¼ 244[325]/414 nm), increased more than the Fmax for C1, suggesting that C2 is a mainly autochthonous source of humic acid–like chromophoric DOM [31]. The highest increase in Fmax was for the protein-like C3 (excitation/emission ¼ 283/340 nm), indicating that this component was an autochthonous product of chromophoric DOM produced by phytoplankton and microbial degradation [26]. Effects of DOM on the bioavailability of emerging organic contaminants
Figure 7. Plots of the specific ultraviolet absorbance of chromophoric dissolved organic matter at 280 nm (SUVA280) against log organic carbon partition coefficient (KOC) values. BPA ¼ bisphenol A; CAP ¼ chloramphenicol.
the chromophoric DOM increase seen in the present study (after day 22; Supplemental Data, Table S1) may have been caused by the microbial transformation of phytoplankton-derived, noncolored DOM such as polysaccharides [28,29]. A low S is typical of high–molecular weight chromophoric DOM with a high degree of photochemical reactivity and of “newly” produced chromophoric DOM, whereas a high S indicates that the chromophoric DOM has a low molecular weight, is photochemically transformed, and is highly degraded [5]. Both S and SR decreased with time in the present study (Figure 3B), indicating that the water was becoming enriched with newly produced high–molecular weight chromophoric DOM during the experiment (Supplemental Data, Figure S1). The specific UV absorption coefficient increased over time, suggesting that the aromaticity of the DOM also increased with time [30]. The positions of the excitation and emission peaks of the 3 fluorescent components identified by the parallel factor analysis model were similar to the positions of fluorescence peaks that have been found in previous studies [26]. The humic acid–like component C1 (excitation/emission ¼ 268[379]/456 nm) in the present study was a combination of peaks A and C that were found by Coble et al. [31], who regarded it as being a humic substance from the terrestrial environment. In the present study, this humic acid–like component appears to have been derived mainly from the in situ transformation of DOM, which agrees with the results of a study of degradation in river water that was
We found that the transformation of DOM in the eutrophic lake water caused the freely dissolved concentrations of BPA and chloramphenicol to decrease and log KOC to increase (Figure 5). These results suggest that in-lake changes in the DOM abundance and composition could alter the bioavailability of the emerging organic contaminants. We explored the relationship between the freely dissolved concentration and the optical properties of the chromophoric CDOM, and found a significant negative correlation between freely dissolved concentration and a254, suggesting that the emerging organic contaminant freely dissolved concentrations were negatively related to the abundance of chromophoric DOM (Figure 6A and B). We found a significant positive relationship between freely dissolved concentration and S275–295 (Figure 6C and D). The spectral range S275–295 is a useful parameter that can be used to represent the DOM molecular weight, and it was found to be negatively correlated with the DOM molecular weight and positively correlated with the low molecular weight to high molecular weight ratio of the DOM [23]. The correlation between freely dissolved concentration and S275–295 suggests that the accumulation of newly produced high–molecular weight DOM in eutrophic water could cause the freely dissolved concentration of emerging organic contaminants to decrease. The BPA log KOC values (4.51–5.25; Figure 5B) were similar to the value (4.5) found in earlier studies [33,34], but log KOC was increased a little at the end of the present experiment. Log KOC significantly and positively correlated with the specific UV absorption coefficient, which indicates the degree of aromaticity in the DOM (Figure 7). This result agrees with the results of previous studies, suggesting that KOC strongly correlates with the aromatic carbon content and the specific UV absorption coefficient of the DOM [3,35]. An increase in the aromaticity of DOM, coupled with an increase of the number of conjugated double bonds, will increase the number of sites with van der Waals attraction forces, which are some of the most important forces involved in interactions between DOM and organic contaminants [36]. There were significant and positive correlations between log KOC and the humic acid–like fluorescent components C1 and C2 in the chromophoric DOM (Figure 8), suggesting that the organic contaminants preferentially bind to the humic acid–like fraction of the DOM. This fraction represents the more aromatic DOM fractions. The chemical structures of DOM (e.g., the functional groups, aromaticities, and elemental compositions of the components) and the characteristics of the organic contaminants (e.g., the solubility, polarity, and number of functional groups) are crucial factors in the interactions between DOM and organic contaminants [36,37]. Yamamoto et al. [36] found that the KOC of endocrine disruptors (including 17b-estradiol, estriol, and 17a-ethynylestradiol) correlated well with their UV absorptivities at 272 nm and the phenolic group abundance in the DOM, and suggested that the sorption mechanism is closely
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Figure 8. Plots of the maxima of the fluorescence intensities (Fmax) of the 3 fluorescent components (FC1FC3) in chromophoric dissolved organic matter against log organic carbon partition coefficient (KOC) values. BPA ¼ bisphenol A; CAP ¼ chloramphenicol; R.U. ¼ Raman units.
related to interactions between p-electrons and hydrogen bonds. We found a significant positive correlation between log KOC and DOM aromaticity (indicated by the specific UV absorption coefficient), and this agreed with the results of Yamamoto et al. We also found a significant positive correlation between log KOC and the hydrophobic humic acid–like components, suggesting that the interactions between DOM and BPA/chloramphenicol are related to hydrophobic interactions. The role of DOM in the cycling of organic contaminants in eutrophic waters
More and more freshwater ecosystems are suffering from eutrophication because of human-induced pressures, such as increasing population and intense industrialization. The effects that accompany eutrophication include the overgrowth of
phytoplankton, which results in increased primary production [38]. This is coupled with the increasing abundance of DOM and increasing trophic state of the water, which was found in the present study and a few previous studies [13]. Natural DOM greatly affects the transport, fate, and bioavailability of organic contaminants. The results of the present study show that DOM controlled the fate of contaminants in the eutrophic lake that was used. Other researchers previously have found that high biomass concentrations in eutrophic ecosystems cause the dilution of contaminants and low freely dissolved concentrations [39]. There will be a net flux of contaminants that are associated with particles to the sediment because organic particulate matter will have a high sedimentation rate in a eutrophic system [40]. Our results show that, in addition to pronounced losses to the
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sediment, the bioavailability of the emerging organic contaminants can be regulated by DOM in eutrophic water. SUPPLEMENTAL DATA
Table S1. Figure S1. (109 KB DOC). Acknowledgment—This work was funded by the National Natural Science Foundation of China (41071301 and 40601095), the Fundamental Research Funds for the Central Universities (0400219216), and the Kone Foundation of Finland. We thank 3 anonymous reviewers and the editor for their constructive comments on the manuscript.
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