Environmental Pollution 223 (2017) 19e30
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Environmental Pollution
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Characteristics of dissolved organic matter (DOM) and relationship with dissolved mercury in Xiaoqing River-Laizhou Bay estuary, Bohai Sea, China*
* Tao Jiang a, b, Ulf Skyllberg b, Erik Bjorn€ d, Nelson W. Green e, Jianhui Tang c, , ** Dingyong Wang a, , Jie Gao a, Chuxian Li a a Key Laboratory of Eco-environments in Three Gorges Reservoir Region, Ministry of Education, Chongqing Key Laboratory of Agricultural Resources and Environment, College of Resources and Environment, Department of Environment Science and Engineering, Southwest University, Chongqing 400716, China b Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umea, SE-90183, Sweden c Key Laboratory of Coastal Environmental Processes and Ecological Remediation, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences (CAS), Yantai 264003, China d Department of Chemistry, Umeå University, SE-90187 Umeå, Sweden e School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States article info abstract
Article history: Because of heterogeneous properties, dissolved organic matter (DOM) is known to control the envi- Received 23 August 2016 ronmental fate of a variety of organic pollutants and trace metals in aquatic systems. Here we report Received in revised form absorptive and fluorescence properties of DOM, in concurrence with concentrations of dissolved mercury 30 November 2016 (Hg), along the Xiaoqing River-Laizhou Bay estuary system located in the Bohai Sea of China. A mixing Accepted 3 December 2016 model consisting of the two end-members terrestrial and aquatic DOM demonstrated that terrestrial Available online 26 January 2017 signatures decreased significantly from the river into the estuary. Quasi-conservative mixing behavior of DOM sources suggests that the variations in the average DOM composition were governed by physical Keywords: Dissolved organic matter processes (e.g., dilution) rather than by new production and/or degradation processes. In contrast to Fluorescence some previous studies of river-estuary systems, the Xiaoqing River-Laizhou Bay estuary system displayed UVeVis absorption a non-significant correlation between DOM and Hg quantities. Based on this and the variation of Hg Bohai Sea concentration along the salinity gradient, we concluded that Hg showed a non-conservative mixing Laizhou Bay behavior of suggested end-member sources. Thus, rather than mixing, Hg concentration variations Mercury seemed to be controlled by biogeochemical̊ processes. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction achieved by quantifying the concentration and functionalities of DOM in parallel with pollutants along river-estuary systems. Environmental pollution and ecological deterioration of river- Further, the techniques for analyzing DOM have advanced over the estuary systems occur due to rapid population growth and indus- last few decades, which provide deeper insight into the heteroge- trial development, threatening these vital ecosystems. The fate of neous properties of DOM (Leenheer and Croue, 2003). Among these environmental pollutants within river-estuary systems is linked to techniques, the low-cost and fast measurements by UV-Vis ab- dissolved organic matter (DOM). It is therefore critical to under- sorption and fluorescence spectroscopies have resulted in exten- stand the biogeochemical characteristics and reactivities of DOM sive use in DOM investigations. The techniques provide important functionalities in these environments (Bianchi, 2007). This may be tools for studies of temporal-spatial distribution, sources, and dy- namics of DOM in a range of aquatic environments, including freshwater and coastal ecosystems (Yang et al., 2013; Helms et al., 2008; Fichot and Benner, 2012; Xie et al., 2012; Spencer et al., * This paper has been recommended for acceptance by Dr. Harmon Sarah 2012; Li et al., 2015). Furthermore, the chemical reactivity of Michele. DOM mostly depends upon its intrinsic chemical structure such as * Corresponding author. ** Corresponding author. its aromatic components, molecular weight, and degree of humi- E-mail addresses: [email protected] (J. Tang), [email protected] (D. Wang). fication. The correlation between “structure and reactivity” may be http://dx.doi.org/10.1016/j.envpol.2016.12.006 0269-7491/© 2016 Elsevier Ltd. All rights reserved. 20 T. Jiang et al. / Environmental Pollution 223 (2017) 19e30 useful to elucidate the role of DOM in a number of reactions of The Xiaoqing River-Laizhou Bay estuary system in Bohai Sea, importance for the environmental fate of pollutants (Weishaar et China, was the object of this study. The aims were to: (1) elucidate al., 2003; Dittman et al., 2009; Burns et al., 2013). Environmental the DOM spectroscopic characteristics and understand the reasons studies to an increasing extent rely upon spectrophotometric ana- for the variation of DOM properties along the river-estuary system; lyses to reveal, through optical properties, spatial and temporal (2) validate the hypothesis that the variations in concentrations of variability in the composition of DOM in aquatic systems. There- dissolved Hg can be explained by DOM properties;(3) further test fore, based upon the optical characterization of DOM, an improved whether optical indices (e.g. SUVA254) could be reasonable pre- understanding of the causal relationships between DOM compo- dictors of dissolved Hg concentrations in Xiaoqing River-Laizhou sition and pollutants may be obtained for river-estuary system. Bay estuary area. As part of a research project on the biogeo- Mercury (Hg) pollution has been recognized globally as a chemical processes of DOM in river-estuary systems of China, re- problem in many aquatic systems and its harmful health effects are sults from this study may explain spatial variations and an impetus for studying Hg fate in various environments (Ullrich et environmental implications of DOM, and also give insights into the al., 2001). Methylmercury (MeHg), which is formed through biotic close relationship between DOM and Hg in the estuary methylation of inorganic Hg, is especially problematic for aquatic environment. organisms and humans due to its high toxicity and bio- accumulation. Across various aquatic systems, river-estuary sys- 2. Materials and methods tems are important ecological environments. In recent years, Hg cycling in estuarine and coastal marine environments have been 2.1. Study sites widely studied (Merritt and Amirbahamn, 2009; Pan and Wang, 2012; Mason et al., 1999; Laurier et al., 2003; Lawson et al., 2001; Laizhou Bay is the southernmost of three bays in the Bohai Sea, a Han et al., 2006; Canario et al., 2008; Choe et al., 2004; Conaway semi-enclosed inner sea of China. The shallow coastal bay, less than et al., 2003). There are only a few reports on the Hg contamina- 10-m in depth, covers approximately 10% of the Bohai Sea area. tion in Chinese estuaries, where high levels of Hg commonly are Since the 1970s, the Bohai Sea has experienced heavy anthropo- encountered, caused by the rapid industrial developments (Wang genic impact due to the rapidly developing urbanization and et al., 2009, Luo et al., 2012; Gao et al., 2014; Ci et al., 2011). It is economy in the area. Laizhou Bay is heavily polluted by anthro- well known that DOM regulates Hg dynamics in aquatic environ- pogenic pollutants imported from more than fifteen rivers and ments through its chemical bonding to reduced sulfur groups numerous waste water pipelines discharging into the bay. In 2013, (Ravichandran, 2004; Skyllberg et al., 2006), by its induction of more than one-fourth of its area failed to meet the Grade IV na- photoreactions (Jeremiason et al., 2015), and by its control of dark tional sea water quality standard (SOA, 2014). Among the rivers redox transformations (Gu et al., 2011, Jiang et al., 2015). Addi- draining into Laizhou Bay, the artificial Xiaoqing River is the most tionally, DOM plays a dual role (i.e., enhancement or inhibition) in important with respect to pollution and runoff (Pan et al., 2010; the production of MeHg due to its effect on Hg speciation and Heydebreck et al., 2015). It is the second largest freshwater input bioavailability (Graham et al., 2012; Zhang et al., 2014; Hsu-Kim et to Laizhou Bay, following the Yellow River, and it drains an area of al., 2013). Although a number of studies have suggested that DOM 10,340 km2 along its 223 km length starting in the north of the Taiyi composition is a critical factor for Hg reactivity and bioavailability Mountains. It passes through heavily urbanized and industrialized in estuaries, and offshore areas (Bergamaschi et al. 2012; Schartup areas of Shandong Province, including Jinan, Zibo, Weifang, and et al., 2013;2015), the relationship between Hg and DOM properties Dongying, and finally discharges into Laizhou Bay. High levels of has not been frequently studied along river-estuary systems. To our trace heavy metals have been a concern in the Bohai Sea with knowledge, only a few studies have determined absorptive and respect to its environmental quality and risk (Gao et al., 2014). fluorescent characteristics of DOM in parallel to Hg quantities along river-estuary gradients (Bergamaschi et al. 2012; Schartup et al., 2015). 2.2. Sampling collection Optical properties of DOM originating from different environ- mental sources have been studied freshwater systems (Dittman et Surface water samples were collected along the river Xiaoqing al., 2009; Burns et al., 2013). Dissolved organic carbon (DOC) and River into the Laizhou Bay estuary in April 2014 (Fig. 1). More DOM quality parameters, derived from the UV-Vis absorption detailed sample information is found in the reference (Heydebreck spectrum (e.g., specific UV absorbance, spectral slope), were used to et al., 2015). Briefly, the study area was divided into two parts, with explain the relationship between DOM and Hg (Dittman et al. 2009; respect to the salinity changes along the river-estuary system, Burns et al., 2013). Although a strong positive correlation between including the freshwater river (upstream of the Xiaoqing River Hg and DOC concentration have been observed, suggesting Hg mouth, with salinity < 5psm) and the outer estuary (beyond mobility in aquatic systems to be associated with DOM (Mierle and Xiaoqing River mouth, with salinity > 5psm). As shown in Fig. 1,a Ingram 1991; Dittman et al., 2009; Bergamaschi et al. 2012), across total of 31 sites were selected for surface water collection, including aquatic systems with diverse geochemical processes and sources of 18 sites (sampling No.1e18) from the Xiaoqing River and 13 sites DOM, the positive correlation between Hg and DOC is not always (sampling No.19e31) from the Laizhou Bay estuary. The river consistent. Furthermore, some studies even have arbitrarily samples were collected below bridges or dams, while the estuarine concluded that no effect of DOM exists, merely because no signif- samples were collected off a fishing boat. A stainless steel bucket icant correlation between DOC and Hg was found. Thus, studying was filled with water from the surface along the channel line of the DOM characteristics and dynamics is important for advancing our river. Three to five buckets of water were mixed in a pre-cleaned understanding of the role DOM plays in the river-estuary system brown glass bottle. An aliquot (approximately 200 mL) of water with respect to Hg. In this study, we use the optical properties of was used for DOM and Hg chemical analysis. The sampling buckets DOM to help us reveal how human activities’ may influence on the and glass bottles were rinsed in situ with river water three times Hg distribution in an river-estuary system. Thus, the DOM sources before use. In the field, in situ water quality parameters including of terrestrial and anthropogenic or microbial/algal origin were used pH, dissolved oxygen (DO), salinity and conductivity were to identify areas with high anthropogenic input signatures, influ- measured using a portable multi-parameter water quality meter encing the Hg distribution in a manner that can be identified. (YSI ProPlus, YSI Inc., USA). T. Jiang et al. / Environmental Pollution 223 (2017) 19e30 21
® determined in filtered (0.45 mm, Millipore polyvinylidene fluoride filter) acid-treated (0.5%HCl) water samples. A procedure of purg- ing and trapping, followed by cold vapor atomic fluorescence spectroscopy (CVAFS) (Model III, Brooks Rand, USA) was used to determine the samples concentration with THgd detected after pre- concentration according to EPA method 1631. For MeHgd, samples were distilled and ethylated according to EPA method 1630 and measured by GC-CVAFS. Method blanks were done for both quality assurance and control (QA/QC). All results here were reported as mean values. More details of measurement and QA/QC procedure were shown in Support Information SI1. Statistical analyses were per- ® formed in OriginPro 2015 and SPSS 19.0 by results cross-check. A t-test was used to determine whether the means of two indepen- dent groups different. Statistical significances (p value) were all Fig. 1. Sampling sites in Xiaoqing River-Laizhou Bay estuary system. noted at 0.05 or 0.01, unless otherwise specified.
2.3. DOC and spectroscopy measurement 3. Results and discussion
Water samples were filtered through a 0.45 mm cellulose acetate 3.1. DOM quantity membrane previously rinsed twice with ultrapure Milli-Q water. The filtrate was kept in high-density polyethylene (HDPE) bottles Ranges of DOC, CDOM, and FDOM properties for the Xiaoqing under a dark condition at 4 C until analysis. DOM concentration River-Laizhou Bay estuary system are shown in Table 1. The range of was represented by dissolved organic carbon (DOC) (mg L 1), DOC concentrations in the river was 6.7e32 mg L 1 and in the which was measured by a Vario MACRO TOC analyzer (Elementar, estuary 2.8e11 mg L 1 for the collected samples. For both the river Germany) and was calculated as the difference between the total and estuary samples, the coefficient of variation averaged 48%, but and inorganic carbon concentrations. We also measured the hy- in spite of this large variance the average means of the DOC con- drophobic fraction of DOC (expressed as mg L 1) after passing centrations of these two end-members were different (p < 0.05). ® acidified DOM samples (pH ¼ 2) through an Amberlite XAD-8 The parameters a(355) and Fn(355) were taken as proxies for the macroporous resin column, based on the IHSS aquatic humic sub- quantities of CDOM and FDOM, respectively. The a(355) ranged stance extraction method. The hydrophobic fraction was calculated from 0.22 to 17 m 1 and the FDOM ranged from 1.1 103 to as the difference between the total DOC and the non-hydrophobic 75 103 a.u. Both a(355) and FDOM were significantly higher in the DOC passing through the column. UVevis and fluorescence spectral Xiaoqing River than in the Laizhou estuary (p < 0.05). This matched ® measurements were done on an Aqualog (Horiba, Japan) the pattern of CDOM and DOC in other river-estuaries, with lower absorption-fluorescence spectrometer equipped with a 150 W values in the estuary/coastal water than in the upstream freshwater ozone-free xenon lamp at a constant 20 ± 3 C in a temperature- (Seritti et al., 1998; Rochelle-Newall and Fisher, 2002; Asmala et al., control room. All works, including sampling and spectrophoto- 2013, 2016). Compared with other studies (Support Information metric measurements, were completed within less than two weeks. SI2, Table S1), DOC and CDOM in this study was most similar to Briefly, UVevis absorption spectra of samples in a 10 cm quartz river-estuary systems influenced by anthropogenic effluences, such cuvette were determined against a Milli-Q water blank over a as Yangtze River estuary (Wang et al., 2014), and at the southern 230e800 nm range with 1 nm intervals. The CDOM abundance was and western coasts of Florida (Green and Blough, 1994). Our con- represented by decadic absorptivity, a(355) (Guo et al., 2007; centrations of DOC and CDOM was lower than in the more pristine Gueguen et al., 2012; Li et al., 2015). For emission-excitation (terrestrial dominated) river-estuary systems in the Baltics (Asmala matrices (EEMs) of fluorescence spectra, the emission spectra et al., 2013, 2016) and in the Arno River-Tyrrhenian Sea estuary were scanned every 3.18 nm interval at wavelengths from 250 to system (Seritti et al., 1998), but higher than in Cadiz Bay, Spain 620 nm, with excitation wavelengths ranging from 230 to 450 nm (Catala et al., 2013), a similar semi-enclosed system located in the in 5 nm interval. The bandpass of excitation and emission was 5 nm. southwest of the Iberian Peninsula. Interestingly, in the Pearl River Scan integration time was 3 s. The EEMs of DOM samples were estuary, in southern China, which is also heavily influenced by corrected for Raman scattering by subtraction from a Raman- anthropogenic effluences, the CDOM was not as high as in the normalized Milli-Q water. The absorption spectra were used to Laizhou Bay estuary (Chen et al., 2004; Hong et al., 2005). correct for inner-filter effects. EEMs were not subjected to parallel When CDOM was normalized to DOC (CDOM/DOC, L mg 1 m 1) factor analysis (PARAFAC) due to the small and diverse sample we observed a decrease from 0.83 in the river to 0.40 in the estuary. dataset. Instead, fluorescent components were identified according Thus the chromophoric DOM decreased approximately 50%. This to Coble (1996) by a traditional “peak-pick” method. The abundance change can be attributed to an increase in the proportion of non- of bulk fluorescent DOM (FDOM) was represented by Fn(355) (Chen chromophoric DOM, either through the degradation of CDOM or et al., 2004; Bergamaschi et al., 2012), which is acquired at excita- additional input of non-absorbing DOM in the estuary. Urban tion wavelength of 355 nm and emission wavelength of 450 nm. All (including industrial) developments near estuaries have been re- DOM samples were processed in triplicates for all measurements ported to add non-chromophoric DOM (Pan et al., 2010; unless otherwise specified. Heydebreck et al., 2015). Both CDOM and FDOM correlated positively with DOC in the Xiaoqing River-Laizhou Bay estuary (Fig. 2), suggesting that these 2.4. Determination of dissolved total mercury (THg) and two measures could be used as proxies for DOC concentrations methylmercury (MeHg) through remote or in situ measurements in this system. Similar observations have been reported in other rivers, estuaries and Dissolved THg and MeHg, denoted THgd and MeHgd, were coastal waters (Blough and Del Vecchio, 2002; Kowalczuk et al., 22 T. Jiang et al. / Environmental Pollution 223 (2017) 19e30
Table 1
Variations inf DOC, decadic a(355) and Fn(355) along the Xiaoqing River-Laizhou Bay estuary transect.