Introduction
Phthalate esters (PAEs) are synthetic compounds which have been used as pesticide carriers or insect repellents, as well as in cosmetics, fragrances, lubricants and defoaming agents. However, by far, they are mostly utilized as additives in plastic products. Several PAEs are carcinogenic in animal models (
Huber et al., 1996). In addition, some PAEs and their metabolic products act functionally as antiandrogens during the prenatal period (
Mylchreest et al., 1998;
Moore et al., 2001) and cause reproductive and developmental toxicities in animals (
Agarwal et al., 1986). Recent investigations have shown that several PAEs are environmental hormones (
Kambia et al., 2001). Since they are not chemically but physically bound to the polymer chains, they may be leached into the environment. The most commonly used PAEs (dimethyl-, diethyl-, di-n-butyl-, butylbenzyl-, bis (2-ethylhexyl)- and di-n-octyl phthalate esters) and another plasticizer, the bis(2-ethylhexyl) adipate, have been included in the list of priority pollutants in several countries. For example, the US Environmental Protection Agency (EPA) has established a maximum admissible concentration (MAC) in water of 6 μg/L for bis (2-ethylhexyl) phthalate (DEHP) and 0.4 mg/L for bis(2-ethylhexyl) adipate (
Fukuwatari et al., 2002;
Gomez-Hens and Aguilar-Caballos, 2003).
Therefore, PAE contamination has become a hot issue. Many researchers have paid much attention to the distribution of PAEs in the atmosphere (
Wang et al., 2008b), soil (
VikelsØe et al., 2002;
Li et al., 2006;
Cai et al., 2008;
Zeng et al., 2008a), surface water and sediments (
Yuan et al., 2002;
Sha et al., 2007;
Wang et al., 2008a;
Zeng et al., 2008b), landfill (
Asakura et al., 2004;
Zheng et al., 2007), and sewage sludges (
Cai et al., 2007). However, there are very limited reports about the distribution of PAEs in groundwater. In many areas in China, groundwater is used as drinking water. It is meaningful to study the distribution and source of PAEs in groundwater so as to provide basic knowledge for control.
The Jianghan plain is located in the central and southern area of Hubei Province in the Middle Reach of the Yangtze River, where floods and water-logging occur frequently. This area is low-lying and is characterized by deep alluvial deposits, many smaller rivers and numerous larger and shallow lakes formed by the meandering of the Yangtze River. The alluvial plain is a honeycomb of waterways bordered by natural levees, and the depressional areas encompassed by these waterways are dish-shaped in cross section. The Jianghan plain is one of the important bases of Chinese agricultural productivity
In the present paper, seventeen groundwater samples were collected from the eastern part of the Jianghan plain, Hubei, China. Concentrations of 16 PAEs were analyzed using solid-phase extraction (SPE)-gas chromatography (GC). The distribution of PAEs and the relationship with surface water are discussed.
Experiments
Reagents and chemicals
All solvents, including methylene chloride, hexane, methanol and acetone were of HPLC grade and purchased from TEDIA (USA) and have a purity of at least 99%. AccuBOND C18 SPE sorbent was purchased from Agilent. Sodium sulfate (granular, anhydrous) was purified by heating at 450°C for 4 h in a SX2-8-13 muffle furnace (Wuhan, China) and cleaned with methylene chloride before use.
Twenty PAE standard solutions used in this study, including dimethyl phthalate (DMP), diethyl phthalate (DEP), diisobutyl phthalate (DIBP), di-n-butyl phthalate (DBP), bis (2-methoxyethyl) phthalate (BMEP), bis(4-methyl-2-pentyl) phthalate (BMPP), diamyl phthalate (DAP), bis (2-ethoxyethyl) phthalate (BEEP), hexyl 2-ethylhexyl phthalate (HEHP), dihexyl phthalate (DHP), butyl benzyl phthalate (BBP), bis (2-n-butoxyethyl) phthalate (BBEP), di (2-ethylhexyl) phthalate (DEHP), dicyclohexyl phthalate (DCP), di-n-octyl phthalate (DOP), dinonyl phthalate, benzyl benzoate, diphenyl phthalate (DPP), diphenyl isophthalate (DPIP) and dibenzyl phthalate (DBZP), were purchased from Dr. Ehrenstorfer GmbH (Germany); they included an internal standard (benzyl benzoate), three surrogate standards (500 ng/μL in acetone) and sixteen PAEs (1000 ng/μL in n-hexane).
To avoid contamination, no plastic equipment was used during sampling and processing. All the glass apparatuses were soaked in K2CrO4 sulfuric acid solution at least 12 h, rinsed with organic-free reagent water at least 10 times, and finally baked at 180°C for 4 h.
Sampling
Seventeen groundwater samples were collected in July 2007 from the Jianghan plain (Wuhan-Qianjiang section), Hubei, China. The temperature, electrical conductivity, pH and depth of the samples were recorded during sampling. Sampling sites and basic properties are shown in Fig. 1 and Table 1. All samples were sealed in 4 L glass containers and carried back to the laboratory within the day and stored at 4°C in a refrigerator before further research.
Standards preparation
A stock standard solution of 100 mg/L of 20 PAEs was prepared by diluting the original standard solution with hexane. Calibration standard solutions with concentrations of 0.5, 1, 5, 10, 20, 30, 40, 50 mg/L were prepared by diluting the stock standard solution with hexane. Stock and calibration standard solutions were stored at 4°C in the refrigerator. The calibration curve was established for each PAE, and calibration solutions were replaced every month.
Sample pretreatment
For the water phase, samples were extracted using a 12–port vacuum manifold solid-phase extraction (SPE) system (Supelco, USA). Sample extraction and pretreatment were performed according to EPA Method 3535 and 8061a with modification, respectively. Water samples were passed through a 0.45 μm membrane, and 10 μL 100 mg/L surrogate standards (DPP, DPIP, DBZP) were added to 1 L samples. An SPE cartridge (500 mg×6 mL) (Agilent, USA) was pretreated with 2 mL methylene chloride, 1 mL acetone, 2 mL methanol and 2 mL organic-free water, respectively. Water samples were extracted at a flow rate of 4 mL/min. After being dried for about 3 min, a drying column containing about 8 g of anhydrous sodium sulfate was placed under the SPE cartridge, as well as a 10 mL collection tube below each drip tip of apparatus. Elution solvents were in turn passed through the cartridge. The elution was condensed to 0.2 mL in a N2 blow equipment (Shanghai, China).
Instrumental analysis
The samples were analyzed with Agilent 6890N gas chromatography (PA, USA) and a DB-5 MS capillary column (30 m×250 mm×0.25 mm, Agilent, USA) for chromatographic separation. The detector temperature was maintained at 280°C. The column temperature program was initiated at 80°C for 1.0 min, increased to 280°C at a rate of 6°C/min, and held for 10 min. The flow rate of the carrier gas N2 was kept constant at 1.2 mL/min. The extracts (2.0 μL) were injected onto GC in splitless mode with an inlet temperature of 280°C.
Quantitation was performed using the internal calibration method based on eight-point calibration curve for individual PAEs. Benzyl benzoate was used as the internal standard for the quantification of PAEs.
Quality assurance
For all the samples, a procedural blank and spiked sample consisting of all reagents was run to check for interference and cross contamination. Quantifications of PAEs were done with the calibration curves of which the correlation coefficients were all higher than 0.99. Limits of detection (LODs) of the method with standard solution were 22–341 ng/L under full scan acquisition mode. The spiked recoveries of PAEs were in the range of 61.7%–97.8%. All data were blank and recovery corrected. The typical chromatogram (Fig. 2) showed that all the peaks of the 20 PAEs were separated very well.
Results and discussion
PAEs species in groundwater
Figure 3 presents the detection rate of PAEs in 17 groundwater samples. It shows that 4 PAEs species were detected, i.e. DIBP, DBP, BEEP and DEHP. The detection rate of DEHP was 100%. It was reported that DBP, DIBP and DEHP were dominant in agricultural soils among 16 PAEs (
Zeng et al., 2008a), and DBP and DEHP were the main species among 5 PAEs (DMP, DEP, DBP, DEHP, DOP) in the Yangtze River water and sediments (
Wang et al., 2008b). In the water and sediments of urban lakes in Guangzhou, DMP, DEP, DBP, DIBP, BMPP and DEHP were present in all water and sediment samples, and DBP, DIBP and DEHP were abundant in water (
Zeng et al., 2008a). The similarity of the PAE species in these media indicated that PAEs in groundwater may come from soil and surface water. However, BEEP was not detected frequently in the environment and its concentration was very low even though it was detected elsewhere (
Zeng et al., 2008a, b). The origins of BEEP in groundwater thus merit further investigation.
Total PAE concentrations in groundwater
Concentrations of total PAEs in groundwater samples ranged from 80.12 to 1882.18 ng/L (Fig. 4). It was reported that the total concentrations of 5 PAEs (DMP, DEP, DBP, DEHP, DOP) in the Wuhan Section of the Yangtze River were 0.034–0.456 μg/L and 35.73–91.22 μg/L in the high and low water periods, respectively (
Wang et al., 2008a), whereas in the Xiaolangdi-Dongming Bridge Section of the Yellow River, the total 5 PAE concentrations ranged from 3.99 to 45.45 μg/L in June (
Sha et al., 2007). The total concentrations of 16 PAEs in urban lakes in Guangzhou were from 1.69 to 4.72 μg/L (
Zeng et al., 2008a). Thus, the concentrations of PAEs in surface water are related to the sampling season and vary with the sampling area. If DIBP and BEEP were subtracted from the total PAEs in our results, the concentrations of the total PAEs would be much lower than that in the surface waters.
Interestingly, the lowest concentration of total PAEs occurred at sites W3, W4, and W12, which were farthest from the Yangtze River. It also can be seen that the total PAE concentrations followed the order W16>W15>W13>W12, W10>W17>W7>W2, W1>W3, W16>W14, W6>W4. This means that in the direction vertical to the Yangtze River, the farther from the Yangtze River the sampling sites were, the lower the concentrations of the PAEs. So the distribution of PAEs in the groundwater of the Jianghan plain was seriously affected by the Yangtze River. In this paper, samples were collected in July, which belongs to the high water period when the Yangtze River supplies water underground. Thus, the high concentrations of PAEs in the Yangtze River entered into groundwater along with the water supply. The decreasing trend of PAE concentrations with increasing intervals from the Yangtze River was due to the water supply characteristics, adsorption and degradation in the underground transportation process.
However, sites W1 and W2 showed higher PAE concentrations than W3 and W4, which was mainly because W1 and W2 were close to the Hanjiang River. Thus, the water supply from the Hanjiang River also affected the occurrence of PAEs in groundwater. W8 and W9 are near Honghu Lake. The concentration of PAEs at W8 was higher than that at W9 even though W9 was nearer the Yangtze River, which indicated that Honghu Lake also had an effect on the distribution of PAEs in the groundwater of the Jianghan plain. The concentrations of total PAEs at W5 and W11 were unexpectedly high. There was a Tianhe water plant and a construction material factory at site W5. A steel wire plant was near site W11; and, there was no industry present in the other sites. Higher concentrations of PAEs at W5 and W11 may come from these plants.
Individual PAE concentrations in groundwater
Figure 5 illustrates that concentrations of DIBP, DBP, BEEP and DEHP, which were nd-558.07, nd-331.33, nd-785.34, and nd-56.5-1100.76 ng/L, respectively. According to the national standards of drinking water in China (GB5749-2006), the limits to concentrations of DBP and DEHP for drinking water sources are 0.003 and 0.008 mg/L, respectively. PAEs in the Jianghan plain did not exceed this quality standard limit. However, this should be paid attention to so as to prevent further pollution.
Different PAEs may come from different sources. The contents of the detected four PAEs in the different sampling sites are illustrated in Fig. 5. The concentrations of DIBP, DBP and DEHP were low at sites far from the Yangtze River, such as W1, W2, W3, W4, W12, W13, and high at sites near the Yangtze River, such as W6, W9, W10, W16. The results demonstrated that the concentrations of DIBP, DBP and DEHP in the groundwater of the Jianghan plain were closely related to the water supply of the Yangtze River. One of the most efficient ways to prevent pollution from PAEs in the groundwater of the Jianghan plain is to control further pollution of the surface water.
However, the distribution of BEEP was irregular, which was very different from other PAEs. This may be due to the application of some kinds of products containing BEEP in the related areas.
Relationship between PAE concentration and electrical conductivity
Electrical conductivity (EC) is an important parameter of water quality. EC mainly depends on the species and concentrations of ions, which are related to the properties of the groundwater media and pollution from artificial activities. It was proved that the EC of groundwater is closely related to the contaminant status (
Gao et al., 2006). As is shown in Table 1, there was an obvious difference in the EC of the groundwater among the different sites, which may reflect different inorganic pollutants. However, there was no linear relationship between PAE concentration and EC (Fig. 6), which indicates that the source and transportation mechanism of PAEs and inorganic compounds are different.
Relationship between PAE concentration and sample depth
It is easy to think that deeper groundwater is less polluted. However, PAE concentration was irrelevant to sample depth (Fig. 7). This result indicated that there was no absorption and degradation in the vertical transportation process of PAEs, nor was transverse transportation more important than vertical transportation in this area.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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