Article pubs.acs.org/est
Source and Migration of Short-Chain Chlorinated Paraffins in the Coastal East China Sea Using Multiproxies of Marine Organic Geochemistry Zongshan Zhao,†,‡ Huijuan Li,†,‡ Yawei Wang,† Guoliang Li,† Yali Cao,‡ Lixi Zeng,†,§ Jing Lan,∥ Thanh Wang,† and Guibin Jiang*,† †
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China ‡ Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, China § College of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China ∥ College of Chemistry and Pharmaceutical Sciences, Qingdao Agricultural University, Qingdao 266109, P. R. China S Supporting Information *
ABSTRACT: Multiple proxies of terrestrial organic matters (TOM) were introduced to study the migration behaviors of short-chain chlorinated paraffins (SCCPs) in the coastal East China Sea (ECS). The contents of SCCPs in the surface sediment collected from Changjiang (Yangtze) River Delta (CRD) and along the Zhejiang− Fujian coastline ranged from 9.0 to 37.2 ng/g (dry weight, d.w.), displaying a “band-style” distribution trend. Spatial distribution patterns of SCCP congeners presented an increasing trend seaward and southward along the coastline for shorter carbon length (C10 + C11) and lower chlorinated (Cl5 + Cl6 + Cl7) congeners, suggesting a spreading tendency seaward and southward from the CRD and the north of the inner shelf. The significant relationship between ΣSCCPs and total organic carbons (TOC) (r2 = 0.402, p < 0.05) indicated that the migration of SCCPs in sediments was markedly affected by TOC. The spatial patterns of the TOM proxies of TOC δ13C, the contents of ΣC27 + C29 + C31 n-alkanes, terrestrial marine biomarker ratio (TMBR), and terrestrial TOC (T-TOC) were all similar to that of ΣSCCPs. Linear relationships between SCCP contents and both the contents of ΣC27 + C29 + C31 n-alkanes (r2 = 0.537, p < 0.05) and T-TOC (r2 = 0.495, p < 0.05) were also observed. The consistence demonstrated that a major portion of sedimentary SCCPs in the coastal ECS should be from the river input of Changjiang River and deposited in the CRD and along the inner shelf of the ECS, but only a minor fraction was transported to the offshore areas.
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INTRODUCTION
Thence, the introduction of other indicators is necessary and meaningful for study of the sources, migration path, and transformation mechanisms of anthropogenic contaminants, and even their fate in the marine environment. Generally, organic matters buried in the sediments of estuaries and their adjacent areas are mainly composed of MOM from marine primary production and TOM via river input.6 If sharing similar migration paths and deposition characteristics, then the spatial and temporal distributions of river-derived contaminants should resemble those of TOM. The proxies of TOM can then provide valuable information
In marine environments, sediments constitute the most important carriers for marine organic matters (MOM) and terrestrial organic matters (TOM). As the intensity of human activities has increased, more and more anthropogenic contaminants are transported to the ocean environment, buried in the marine sediments, and might threaten the marine ecosystems during the migration processes.1,2 To date, some marine environmental surveys on anthropogenic pollutants have been carried out to investigate their contents and to identify their characteristics.2−4 Geographical features and the proxies of TOC and grain size are important features for studying the source identification and distribution/accumulation characteristics.5 However, some abnormal levels or characteristics of pollutants in some regions are still difficult to understand due to the lack of detailed and comprehensive data. © XXXX American Chemical Society
Received: November 30, 2012 Revised: April 23, 2013 Accepted: April 23, 2013
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ribbon of mud area.27,28 The surface currents of the ECS consist of a northward flow of the warm and saline Taiwan warm current (TWC), relatively cold and brackish southward flowing Jiangsu and Zhejiang−Fujian coastal current (JCC and ZFCC), and the Changjiang diluted freshwater (CDFW) (Figure 1). Three cruises were conducted in July 2010, and
when investigating potential sources, migration path, and environmental fate of these contaminants. The δ13C of organic carbon and C/N ratio of TOC are traditional proxies used to determine the relative amounts of TOM in marine sediments. Usually, TOM has a more negative δ13C value (∼ −27‰) and a higher C/N ratio (>12) than MOM (−20‰ and 6−8 respectively),7 and the values of δ13C and C/N ratio can provide useful information on the relative contributions of TOM to the TOC. Recently, proxies based on biomarker contents and ratios have also been introduced to distinguish TOM from sedimentary TOC and have shown high potential as alternative approaches. The long-chain odd-carbon n-alkanes, long-chain even-carbon n-alkanols, and fatty acids produced by higher plants are the most used TOM biomarkers.8,9 The content ratios of these terrestrial biomarkers to marine phytoplankton biomarkers (such as brassicasterol, dinosterol, and C37 alkenones) can be directly used to estimate the relative contributions of TOM to sedimentary TOC in both open and coastal sea regions.10−12 The East China Sea (ECS), one of the largest shelf seas in the world, is an important terrestrial organic carbon sink. Large amounts of terrestrial particulate matters (∼0.5 Gt per year)13 with ∼2.5 Mt of organic matters14 are discharged into the ECS via the Changjiang River (CR). In recent years, anthropogenic activities have altered the nature and amounts of terrestrial matter delivered by the CR15 and resulted in coastal eutrophication and the burial of marine organic matter in shelf sea sediments.16 Meanwhile, a large number of organic contaminants are transported into the ECS such as polybrominated diphenyl ethers (PBDEs),17 organochlorine pesticides (OCPs),18,19 and polycyclic aromatic hydrocarbons (PAHs),20 etc. Short-chain chlorinated paraffins (C10−C13, SCCPs), with the formula of CnH2n+2‑zClz and chlorine content ranging from 30% to 72% by weight,21 are a group of widely used industrial additives.22 In the past decade, they have attracted increasing attention23−25 as a result of potential properties in long-range transport,23 persistence in the environment,24 and toxicity to aquatic organisms.25 Our previous study has shown the occurrence of SCCPs in the sediment in the ECS, with a general decreasing trend with distance from the coast.26 Spatial distributions and correlation analysis demonstrated that TOC, riverine input, ocean current, and atmosphere deposition might contribute to the accumulation of sedimentary SCCPs. However, their main source and migration path are still not clear due to lack of necessary supporting data in the studied region. This study is a continuation of our previous work to further investigate the possible sources and migration paths of SCCPs in the coastal ECS by introducing organic geochemistry indicators. The studied region is concentrated in the Changjiang River Delta (CRD) and along Zhejiang−Fujian coastline, which is heavily affected by CR after the Holocene. To our knowledge, this is the first work to employ multiproxies of organic geochemistry to study the environmental behaviors of chlorinated paraffins.
Figure 1. Sampling sites in the coastal ECS, modified from ref 26. Mud areas are marked in gray and major surface currents are indicated by large arrows: CDFW (Changjiang diluted freshwater), ZFCC (Zhejiang−Fujian coastal current), TWWC (Taiwan warm current), and KC (Kuroshio current).
July and August 2011. In all, 37 surface sediment samples (0−3 cm) from the CRD to the middle shelf of the ECS (Figure 1) were collected using a Van Veen stainless steel grab sampler. Samples of MZ14, MZ15, MZ16, MZ17, MZ18, and MZ19 are from the same batch of Zeng’s work, but they are reanalyzed in this study. All the samples were immediately stored in a refrigerator at −20 °C until analysis. Extraction and Cleanup. The pretreatment procedure for SCCPs was based on a previous report with some modifications.29 Homogenized dry sediment (2 g) was spiked with the surrogate standard (1 ng of 13C10-trans-chlordane) and extracted by accelerated solvent extraction (ASE) with the extraction solvent of dichloromethane and n-hexane (1:1). Activated copper granules were added to the extraction to remove elemental sulfur. The extract was rotary-evaporated to about 2 mL and then cleaned and fractionated on a multilayer silica−Florisil composite column, which consisted of 3 g of Florisil, 2 g of activated silica gel, 5 g of acid silica gel (30%, w/ w), and 4 g of anhydrous sodium sulfate from bottom to top. The column was precleaned with 50 mLof hexane and the extract was eluted in sequence with 40 mL of hexane (first
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EXPERIMENTAL SECTION Sampling Area and Sample Collection. The ECS is a typical Western Pacific marginal seas open shelf, with the world’s broadest continental shelf. A major portion of the sediments discharged from the CR is ultimately deposited in the CR estuary and southern inner shelf of the ECS, forming a B
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using an EQUITY-5 capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness) with helium as the carrier gas. Oven temperature programming for GC/MS was 60−200 °C at 15 °C/min, 200−300 °C at 2.5 °C/min, and holding at 300 °C for 5 min. All the results are reported on a dry weight (d.w.) basis. Quality Assurance and Quality Control (QA/QC) on SCCPs. Quality assurance and control measures were performed to ensure the identification and quantification of the analysis. Glassware and sodium sulfate were solvent-rinsed and heated at 450 °C prior to use. A procedural blank was processed with each batch of eight samples and no quantifiable SCCPs were detected in these blanks; the results were therefore not blank corrected. The recoveries for the surrogate standard, 13 C10-trans-chlordane, were in the range of 81−106%. TOC and Total Nitrogen (TN) Analysis. Prior to analysis, the homogenized dry sediment samples were decalcified with 4 N HCl at room temperature for 24 h, and then rinsed with deionized water and dried in an oven at 55 °C. The TOC and TN analysis were performed using a Thermal Flash 2000 Elemental Analyzer, with standard deviations of ±0.02 wt % (n = 6) and ±0.002 wt % (n = 6), respectively. The standard used in the EA Analysis is Soil Reference (C = 3.5%, N = 0.37%, Säntis Analytical AG).
fraction) and 100 mL of dichloromethane/hexane (1:1, v/v) (second fraction). The second fraction was concentrated to near dryness and then reconstituted in 50 μL of cyclohexane. Prior to analysis, ε-HCH was added in order to determine the recoveries. For biomarkers, deuterium-substituted C24 n-alkane and C19 n-alkanol were added as internal standards to the homogenized dry sediments (∼2 g), which were then extracted ultrasonically with a 3:1 mixture of dichloromethane and methanol four times. The extracts were first hydrolyzed with 6% KOH in MeOH and then extracted with hexane. The extracts were subsequently separated into fractions using silica gel chromatography. The nonpolar lipid fraction (containing nalkanes) was eluted with 8 mL of hexane and dried under a gentle N2 stream for instrumental analyses. The neutral lipid fraction (containing the three marine phytoplankton biomarkers) was eluted with 12 mL of dichloromethane/methanol (95:5, v/v), dried under a gentle N2 stream, and derivatized using N,O-bis(trimethylsilyl)-trifluoroacetamide (BSTFA) at 70 °C for 1 h before instrumental analysis. Instrument Analysis, Identification, and Quantification. SCCPs analysis was performed on a 7890A gas chromatograph (GC) coupled with a 7000B triple quadruple mass spectrometer (Agilent, USA) as described by a previous study.29 One μL of the final extract was injected with a 7683B Series Injector (Agilent, USA) in splitless mode into a DB-5MS capillary column (30 m length, 0.25 mm i.d., 0.25 μm film thickness) at an injector temperature of 275 °C. Helium was used as carrier gas at a flow of 1.0 mL/min. The oven temperature program was as follows: 1 min isothermal at 100 °C, increased to 160 °C at 30 °C/min, held for 5 min, then ramped to 310 °C at 30 °C/min and held for 17 min. The lowresolution mass spectrometer was employed in the ECNI mode with methane as reagent gas. The transfer line temperature and ion source temperature were set to 275 and 200 °C, respectively. The most abundant isotope was used for quantification and the second-most abundant isotope was used for identification. To ensure the instrument sensitivity, SCCP congeners were divided into four groups by the optimized combinations (C10, C11, C12, and C13) and subjected to analysis by four individual injections. Identification of CP congener groups was performed by comparison of retention time, signal shape, and correct isotope ratio according to Reth and Oehme.30 The actual relative integrated signals for each congener were obtained by correcting the SIM signals of [M − Cl]− ions from isotopic abundance and response factors. Congener group abundance profiles were established using the actual relative integrated signals, followed by internal standard method to determine the relative concentrations of the congener group content in the commercial standards and environmental samples. The quantification of biomarkers analysis was performed on an Agilent 6890N GC with FID detector, using a HP-1 capillary column (50 m × 0.32 mm i.d., 0.17 μm film thickness, J&W Scientific) and hydrogen as the carrier gas at a flow rate of 1.2 mL/min. The oven temperature program was as follows: 1 min isothermal at 80 °C, increased to 200 °C at 25 °C/min, to 250 °C at 4 °C/min, and to 300 °C at 4 °C/min, and then held for 15 min. The identification and structure verification of biomarkers were performed on GC/MS (Thermo) by comparisons with the retention times of the standards. The MS was operated in the electron ionization (EI) mode (70 eV), and the mass scanning ranged between m/z 50 and 650 amu,
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RESULTS AND DISCUSSION Previous studies have indicated that the occurrence of high levels of sedimentary contaminants in the Mud Area Southwest of Cheju Island is mainly attributed to atmospheric deposition.20,26 Terrestrial matters buried in this region are dominated with sediments from the Old Yellow River.31 Therefore in this study, the coastal ECS including the CRD, the Zhejiang−Fujian coastline, and their adjacent regions, which are significantly affected by CR, were chosen as the sampling region to further study the source and the migration path of sedimentary SCCPs (Figure 1). Contents and Spatial Distributions of SCCPs. Similar to our previous study,26 SCCPs were detected in all the surface sediment samples. ΣSCCPs (total SCCP content) and degree of chlorination (Cl%) in all the samples are listed in Table SI-1 in the Supporting Information. Generally, ΣSCCPs varied from 9.0 to 37.2 ng/g d.w., with an average value of 24.0 ng/g d.w. in the sediments, which were much lower than those in sediments from Liaohe River Basin (39.8−480.3 ng/g)32 and the Pearl River Delta in China (320−6600 ng/g).33 Compared with our previous work,26 most of the SCCP contents of the 6 samples from the same batch (MZ14, MZ15, MZ16, MZ17, MZ18, and MZ19) were slightly lower (−13% to 50%), with the average difference of 25%, indicating that the method we used is satisfactory. The relatively lower levels of SCCPs in this area are most likely due to the “dilution effect” of terrestrial matters and marine matters. Previous work reported that about 0.5 Gt/yr terrestrial particulate matters is discharged into the ECS by CR,15 and there is a significant increase of the production and burial of marine organic matters in the shelf sediments due to coastal eutrophication from increased human activities.16 The very high sedimentation rate in the CRD and along the inner shelf of the ECS might be another reason for the “dilution effect” in the specific hydrodynamic conditions.34 Similar results have also been reported for the terrestrial organic matters preserved in the ECS.18,35 The spatial distribution of SCCPs in the coastal ECS displays a “band-type” distribution, which decreased vastly from the C
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found in the soils from the rural areas of the Liaohe River Basin,32 the sediments from the lower industrial areas of the Pearl River Delta,33 marine mollusks from the Chinese Bohai Sea,36 and marine mammals from the remote Arctic,37 but obviously different from those found in the sediments from the highly industrialized areas of the Pearl River Delta;33 the sediments of Lake Ontario;3 soils, sediments, and sewage sludge related to municipal wastewater treatment plants;29,38,39 and marine mammal samples from urbanized and industrialized areas.37 It has been reported that log Kow of SCCPs increases linearly with the increasing carbon atom numbers,40 whereas vapor pressures (VPs) tend to decrease with increasing carbon chain length and degree of chlorination. The shorter carbon chain and lower chlorinated congener group of SCCPs are more prone to migrate to regions far away from the emission sources than those of longer carbon chain and higher chlorinated congener groups.32,33 Therefore, the spatial distributions of different SCCP congener groups can provide useful information on the potential sources and the migration path. Figure 3 shows the spatial patterns of the relative abundance of different SCCP congener groups. The areas with relative higher abundance of long carbon chain (C12 + C13) are concentrated in the CRD and the north of the Zhejiang−Fujian mud area (Figure 3A), while the short carbon chains (C10 + C11) showed an increasing trend seaward and southward along the Zhejiang−Fujian coastline (Figure 3B). The similar pattern for SCCP congener groups based on chlorine groups was also observed, with relatively higher amounts of Cl8 + Cl9 + Cl10 groups in the CRD and the north of the Zhejiang−Fujian mud area (Figure 3C,D). The seaward increasing trend of relative amounts of short carbon (C10 + C11) and lower chlorinated congeners (Cl5 + Cl6) along the profile of sites of YRE, DH2-1, DH2-3, and DH2-5 in the ECS was consistent with our previous study,26 further demonstrating the possible transport path of SCCPs from land to the ocean. Additionally, the southward increasing trend along the Zhejiang−Fujian coastline for short carbon chains (C10 + C11) and lower chlorinated congeners (Cl5 + Cl6) with higher volatility and solubility21 resembles the ZFCC southward flow (Figure 1), suggesting that SCCPs in the CRD or the north of Zhejiang−Fujian mud area could be transported southward along the coastline and then buried in the inner shelf. TOC and TOC-Based Proxies. Many studies have indicated that TOC could be responsible for the distribution/ accumulation of organic pollutants in the aquatic environment,18,33,41,42 although no such correlation was found between TOC and organic contaminants in some regions.17,32,43 Usually, high contents of organic pollutants are related to high TOC levels since these organic pollutants can be removed from the water column and adsorbed onto the particular matters because of the high affinity between these materials.44 In this study, TOC of the sediment samples (Figure 4A, Table SI-1) varied from 0.12% to 0.69%, with an overall trend of higher values along the coastline including the CRD and the inner shelf, but lower values away from the coast. Significant linear relationships were found between the TOC and SCCP contents (r2 = 0.402, p < 0.05) (Figure SI-2) in the coastal ECS, which is consistent with our previous study in the whole ECS,26 whereas it is different from that between TOC and some hydrophobic halogenate organics in the inshore areas of the ECS,17 in the Pearl River Delta,33 and in the Daliao River Estuary45 of China. The consistency further indicated that the distribution or
inner shelf to the outer shelf, but remained consistent in the north−south direction (Figure 2). For example, the areas with
Figure 2. Spatial distributions of SCCP contents (ng/g d.w.) in surface sediments of the ECS. The gradient distributions were calculated using Kriging by Surfer 8.0.
higher contents of ΣSCCPs were located in CRD (such as ES4 (37.2 ng/g d.w.)), the inner shelf of the ECS (such as MZ5 (35.1 ng/g d.w.), MZ6 (32.1 ng/g d.w.), and MZ10 (34.3 ng/g d.w.)), while lower levels of ΣSCCPs was distributed in the middle and outer shelf (such as MZ9 (10.1 ng/g d.w.) and MZ13 (19.5 ng/g d.w.)). This distribution characteristic demonstrated a direct influence of river input by CR and the impact of the proximity to land-based sources of the coast. Also, the pattern with higher ΣSCCPs in the CRD and along Zhejiang−Fujian coastline resemble the southward flowing direction of ZFCC and the elongated inner-shelf mud wedge pattern from the Yangtze mouth into the Taiwan Strait (Figure 1). The results indicated that a major portion of the sedimentary ΣSCCPs along the coastline are most likely from the river input by CR from its basin, which was similar to both the terrestrial inorganic and organic matters preserved in the ECS.18,35 Spatial Distribution Patterns of SCCP Congener Groups. The congener group profiles of all sedimentary SCCPs showed a significant variation in both the different carbon congener groups and degree of chlorine (Figure SI-1). C10 homologue was the most dominant carbon chain group, accounting for 40.5−57.9% of ΣSCCPs, followed by C11 homologue (21.2−32.1%), C12 homologue (11.1−17.8%), and C13 homologue (6.7−12.3%). Based on the chlorine groups, the predominant congeners were of the lower chlorinated congener groups (Cl5−Cl7) at 16.6−36.4%, 30.9− 42.7%, and 16.3−27.3%, respectively, and cumulatively accounted for 76.7−96.4% of ΣSCCPs. The patterns of SCCP congener groups in this study are similar to those D
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Figure 3. Spatial distributions of (SCCP congener groups)% in surface sediments of the ECS: (A) (C12 + C13 groups)%; (B) (C10 + C11 groups)%; (C) (Cl8 + Cl9 + Cl10 groups)%; (D) (Cl5 + Cl6+Cl7 groups)%. The gradient distributions were calculated using Kriging by Surfer 8.0.
Zhejiang−Fujian coastline, forming a depositional mud area in the ECS inner shelf.28,47 Thence, the similar patterns between SCCPs and fine-grained percentage indicated that the surface sedimentary SCCPs in the coastal ECS are mainly from the river input of CR and implied that the grain size should be another key factor influencing the sedimentary SCCPs due to the high affinity between fine-grained minerals and organic matter.14,46
accumulation of SCCPs in the marine environment was significantly affected by TOC. In addition, the spatial distribution of grain size (sand%) also displayed a “bandtype” distribution along the coastline, with
