Chlorinated-Methylsiloxanes in Shengli Oilfield: Their Generation in

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Chlorinated-Methylsiloxanes in Shengli Oilfield: Their Generation in Oil-Production Wastewater Treatment Plant and Presence in the Surrounding Soils Lin Xu,† Shihe Xu,‡ Qiaoli Zhang,§ Shengxiao Zhang,§ Yong Tian,∥ Zongshan Zhao,∥ and Yaqi Cai*,†

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†

State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China ‡ Toxicology & Environmental Research and Consulting (TERC), The Dow Chemical Company, Midland, Michigan 48674, United States § School of Chemistry & Material Science, Ludong University, Yantai, 264025, China ∥ CAS Key Laboratory of Biobased Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Science, Qingdao 266101, China S Supporting Information *

ABSTRACT: In two oil−wastewater treatment stations of Shengli Oilfield, cyclic volatile methylsiloxanes (cVMS, D4−D6) in the wastewater stream were found to undergo chlorination during electro-oxidation process for wastewater containing chlorine ions (16.1−42.0 g/L). Their converted fractions were 4.71−28.0% for monochlorinated D4−D6 and 0.22−7.96% for dichlorinated D4, which were ∼2 orders of magnitude higher than those for hydroxylated products. Furthermore, portions of chlorinated methylsiloxanes retained in excess sludge were released to the surrounding soils. In soil samples (n = 500), chlorinated methylsiloxanes concentrations ( 99%) colorimetric method,24 respectively. 2.2. Standards and Chemicals. 1-chlormethyl-heptamethylcyclotetrasiloxane [D3D(CH2Cl)], 1-chlormethyl-nonamethylcyclopentasiloxane [D4D(CH2Cl)], 1-chlormethyl-undemethylcyclohexasiloxane [D5D(CH2Cl)], and four isomers of dichlorinated D4including 1-dichlormethyl-heptamethylcyclotetrasiloxane [D3D(CHCl2)], 1,2-bis(chlormethyl)-hexamethylcyclotetrasiloxane [D3D(CH 2 Cl) 2 ], 1,3- bis(chlormethyl)-hexamethylcyclotetrasiloxane [D2(D(CH2Cl))2], 1,5-bis(chlormethyl)hexamethylcyclotetrasiloxane [DD(CH2Cl)DD(CH2Cl)] were custom-synthesized (purity >95%) in Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences. 1-hydroxy-heptamethylcyclotetrasiloxane (D3TOH, purity >95%) was custom-synthesized in Toronto Research Chemicals (Toronto, Ontario, Canada), while 1-hydroxy-nonamethylcyclopentasiloxane (D4TOH, purity >95%) and 1hydroxy-undecamethylcyclohexasiloxane (D5TOH, purity >95%) were synthesized in Shandong University, China. Cyclic methylsiloxane standards (D4, D5, D6, purity >98%) were purchased from Sigma-Aldrich (St. Louis, MO). 13Clabeled D4, D5, and D6 were purchased from both Moravek Biochemicals (Brea, CA) and Cambridge Isotope Laboratories (Andover, MA). Methanol, ethyl acetate, and n-hexane were purchased from Fisher Scientific (Fair Lawn, NJ). 2.3. Sample Pretreatment. Aqueous Samples. cVMS as well as chlorinated and hydroxylated cVMS in aqueous samples were extracted by liquid−liquid extraction using the method outlined below: 13 C−D4, 13C−D5, and 13C−D6 were separately used as internal standards for target compounds with the same number of Si moieties per molecule. 100 mL of sample, spiked with acetone solution of internal standards (40 μL, 500 μg/L) was extracted with 25 mL of n-hexane followed by 20 mL of nhexane/ethyl acetate (1:1 by volume). Subsequently, the organic layer was transferred to a flat-bottom flask and concentrated to 8−10 mL by evaporation at 30 °C under a stream of nitrogen. The remaining extract was added into a drying cartridge (one glass pipet filled with 1.0 g of anhydrous sodium sulfate), which was preconditioned with n-hexane/ ethyl acetate (1:1 by volume, 4 mL), and then eluted with the above solvent (3 mL). The total eluent was concentrated to 0.5 mL with the same way mentioned above, and diluted to 1 mL with n-hexane/ethyl acetate (1:1 by volume). Oil, Sludge, Atmospheric TSP, and Soil Samples. Oil (0.2 g), sludge (1.0 g), atmospheric TSP (0.2 g), or soil (1.0 g) were spiked each with an aliquot of the acetone solution of the internal standards (100 μL, 500 μg/L), and then vortexed for 10 min with 10 mL of n-hexane/ethyl acetate (1:1 by volume).

methylsiloxanes has been reported in pulp-wastewater treatment processes with the old chlorine gas bleaching technique.16 Even in the latter case, the migration and degradation of the monochlorinated methylsiloxanes in soil were not investigated. Furthermore, the isomers of dichlorinated methylsiloxanes should have different physicochemical properties than these of monochlorinated cVMS, and hence their environmental behaviors in soil could be different. The objectives of the study were 3-fold. First, to compare the generation and fate of both monochlorinated and dichlorinated cVMS in a conventional oil production wastewater treatment station and two new water treatment stations equipped with electro-oxidation units in the oilfield; Second, to determine the possible effects of such a water treatment operation on the nearby environment by determining the longterm spatial and temporal trends of the concentrations of these chlorinated cVMS compounds in surrounding soils; Finally, to determine the potential of volatilization and degradation of the chlorinated-cVMS homologues and isomers in soil, two key processes in determining the distribution of these compounds in soil at and near the point sources.

2. MATERIALS AND METHODS 2.1. Sampling. Shengli Oilfield, with 24 million tons of annual oil production volume, is the second largest oilfield in China. This oilfield is located mainly in Hekou, Lijin, Kenli, and Dongying District, Shandong Province, China. The studied oil−wastewater treatment stations (Guangli and Shinan) equipped with electro-oxidation units lie in Dongying District (Figure 1), where oil production volume currently accounts for 26% of total volume in Shengli Oilfield. After oil− wastewater mixture from oil wells was separated, raw wastewater in Guangli (11000 m3/d, pH 6.0−6.4) and Shinan stations (7000 m3/d, pH 6.1−6.2) were successively treated in electro-oxidation units (pH 6.7−7.2, working voltage = 3.37− 3.68 v, current density = 55−65 mA/cm2), primary setting tank (pH 6.7−7.3), coagulation reactor (pH 6.7−7.3), secondary setting tank (pH 6.7−7.3), and filtration tank (pH 6.7−7.3). During year 2008−2017, 53 samples were annually grabbed from Dongying District, including (1) three dewatered sludge samples from three oil-production wastewater treatment stations: Guangli, Shinan, and one reference station (Wanggang Station) which had no electro-oxidation but a conventional treatment including primary setting coagulation, and secondary setting filtration process; (2) 50 grab surface soil samples (depth = 0−5 cm) were collected from 20 sites (G1-G20, Figure 1) around Guangli Station, 20 sites (S1−S20, Figure 1) around Shinan Station, and 10 reference sites (R1− R10) in other areas of Dongying District. The distances from sampling sites to oil-production wastewater treatment stations were provided in SI Table S1. In addition, by high volume pumps (1.10 m3/ min) equipped with TSP (total suspended particulates) selective inlets, 24 h samples (n = 56) of atmospheric TSP were collected on glass fiber filters from seven sites (S1, S2, S5, S8, G1, G5, G7) at Year 2010, 2013, 2014, and 2017, that is, two samples were annually collected at each site. Meanwhile, at these four years, 24 h composite samples (n = 112) of wastewater, excess sludge and residual oil from each treatment unit of both Guangli and Shinan stations (Figure 1) were collected manually using 1 L glass tubes at flow proportion mode. Notably, wastewater samples were centrifuged at 12 000 C

DOI: 10.1021/acs.est.8b06993 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

M = Caqueous × Q aqueous + Csolid × Q solid + Coil × Q oil

After centrifugation at 10 000 rpm (11 068 g) for 10 min, the supernatant was transferred to a glass vial. The samples were re-extracted for two times and the extracts were pooled with the first extraction. The pooled supernatant was dewatered by anhydrous sodium sulfate cartridge. The eluent was concentrated to 1 mL under a gentle stream of nitrogen. The target compounds in the final extracts were measured by selected ion monitoring (SIM) of GC-MS analysis using Agilent 7890A gas chromatograph -5975C mass spectrometric detector. Methylsiloxanes as well as their monochlorinated and hydroxylated products were separated with HP-5MS column (30 m× 0.25 mm × 0.25 μm), while four isomers of dichlorinated D4 were separated with a chiral capillary column (DB-200; 30 m × 0.25 mm i.d. × 0.25 μm). The MS was operated in electron ionization mode (EI) at 70 eV, ion source temperature of 230 °C and quadrupole temperature of 150 °C. MS parameters for compounds were summarized in SI Table S3. Especially, the total ion chromatograms for four isomers of dichlorinated D4 were shown in SI Figure S1. 2.4. Quality Assurance/Quality Control (QA/QC). Following previous studies,16,18,25 some measures were taken to avoid the siloxanes contamination during sample collection and analysis: (1) the analyst did not use hand lotions or other consumer products containing siloxanes; (2) prior to use, all glass tubes/pipettes were cleaned with n-hexane, and then heated at 300 °C; (3) prior to use, anhydrous sodium sulfate cartridges were immersed in n-hexane for 4 h, and subsequently rinsed with 10 mL of n-hexane. After rinsing, the cartridges were dried using purified nitrogen and stored in capped glass tubes; (4) the sample pretreatments were performed in dedicated clean room with recycled air system; (5) in nitrogen blowing process, only steel pipes, not silicone tubing, were used; (6) during sampling events, field blanks were collected to assess potential ambient contamination. Mean field blanks of D4−D6 were 3.3−5.6 ng/L in aqueous samples, 14.6−18.3 ng/g dw in crude oil, 3.9−4.8 ng/g dw in solid sludge, 2.7−3.1 ng/g dw in TSP [prepared with soil (diameter 3 Km away from these two stations (S13−S20, G12-G20, Figure 4). Furthermore, in each year, soil concentrations of chlorinated methylsiloxanes roughly followed an exponential decreasing (slope = −5.93∼ −0.19, R2 = 0.11−0.98, sI Figure S2) with respect to the distance from Shinan and Guangli stations. The generation of chlorinated methylsiloxanes in the electrooxidation process and their spatial trends in the surrounding soils strongly suggest Shinan and Guangli stations as the source of release. Because waste-gas from the oil−wastewater treatment stations was incinerated and residual-oil was recycled to refinery in close systems, little of chlorinated methylsiloxanes from gas and oil could be released to surrounding environment. However, the excess sludge contained chlorinated cVMS as discussed in Section 3.1.1, and then the leakage of sludge during its transport from treatment stations to landfill may release these compounds to surrounding soil environment. In addition, before transferring to landfill, the sludge from these stations was air-dried to 40− 50% of water content. During air-drying process, chlorinated methylsiloxanes may be released to atmosphere by volatilization and raising dust, and then they would migrate to soil through deposition of atmospheric TSP. 3.2.2. Spatial Trends in Atmospheric TSP Sampled. In TSP samples collected from seven sites at year 2010, 2013, 2014, and 2017, concentrations of total chlorinated methylsiloxanes (0.111−40.6 μg/g dw, SI Table S19) from sites closer to Guangli or Shinan stations (G1, G5, S1, S2, and S5, 20−300 m away) were 1−2 orders of magnitude higher than those (0.0397−1.67 μg/g dw) from distant sites (G7 and S8, 1000 m away). A rough calculation (mentioned in the SI) indicated that the anticipated concentrations (SI Table S20) of chlorinated cVMSs in soils, arising from air TSP deposition, accounted for 3.87−520% (mean = 93.3%, median = 56.5%) of their actual concentrations. Although the above calculation neglected removal (volatilization and degradation) of chlorinated cVMSs in soil, it was consistent with the speculation that airborne chlorinated methylsiloxanes may contribute to their occurrence in the surrounding soil. Notably, chlorinated cVMSs should have additional migration mechanisms to soil, such as the readsorption of gas-phase chlorinated cVMS by soils (i.e., soil/air partitioning). However, it was difficult to assess this mechanism yet due to lack of the relevant physicochemical properties such as soil/air partition coefficients. Overall, proportions of D3D(CH2Cl), D4D(CH2Cl), D5D(CH2Cl), and ∑dichlorinated-D4 to total chlorinated methylsiloxanes were 7.61−76.5% (mean = 27.7%), 23.5− 65.5% (mean = 48.6%), 0−43.4% (mean = 21.8%), 0−3.91% (mean = 1.18%), respectively in these atmospheric TSP samples. Compared with the chlorinated cVMS in the dewatered sludge samples, there were more low molecule weight species like D3D(CH2Cl) on atmospheric TSP, but G

DOI: 10.1021/acs.est.8b06993 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology Table 1. Volatilization and Hydrolysis Half-Lives (t1/2, Days) Of Chlorinated Methylsiloxanes in Soil D3D(CH2Cl) D4D(CH2Cl) D5D(CH2Cl) D3D(CHCl2) D3D(CH2Cl)2 D2(D(CH2Cl))2 DD(CH2Cl)DD(CH2Cl) D4 D5 D6

volatilization hydrolysis volatilization hydrolysis volatilization hydrolysis volatilization hydrolysis volatilization hydrolysis volatilization hydrolysis volatilization hydrolysis volatilization hydrolysis volatilization hydrolysis volatilization hydrolysis

.TOC = 10 mg/g

TOC = 20 mg/g

TOC = 50 mg/g

TOC = 100 mg/g

TOC = 200 mg/g

4.62 6.92 13.7 14.7 83.6 40.1 135 7.75 178 9.38 183 10.3 194 12.0 2.31 5.64 9.59 9.61 60.3 29.7

6.46 9.41 19.1 27.6 144 72.5 189 12.7 210 17.2 224 24.4 246 30.2 3.70 7.44 14.9 15.8 92.9 40.7

8.80 12.9 25.3 43.5 190 124 253 16.4 268 26.6 294 38.7 323 57.5 5.09 8.76 22.4 22.4 133 52.6

11.7 18.2 32.8 51.3 254 162 321 22.7 343 33.0 369 54.0 400 74.4 7.04 15.1 26.8 28.9 187 80.4

17.3 26.8 40.0 65.9 325 234 271 33.8 380 48.1 385 73.9 461 99.9 10.9 21.1 33.0 38.6 234 151

3.3.1. Degradation. In capped soils with TOC = 10 mg/g, the half-lives of chlorinated methylsiloxanes in nonsterilized soil (SI Figure S4) were not different statistically (p > 0.05, t test) from those in sterilized soil. This result indicated that similar to methylsiloxanes,30−32 abiotic degradation (ringopening hydrolysis) played a dominant role in the degradation of chlorinated methylsiloxanes in soil. Theoretically, in alkaline soil (pH 7.4), chlorinated methylsiloxanes should have faster hydrolysis rates than nonchlorinated methylsiloxanes because −CH2Cl had stronger electrophilicity than −CH3. However, the measured half-lives of monochlorinated methylsiloxanes in capped soils were 6.92−26.8 days for D3D(CH2Cl), 14.7− 65.9 days for D4D(CH2Cl), 40.1−234 days for D5D(CH2Cl), or 1.2−2.4 times larger than those of their paired nonchlorinated methylsiloxanes (5.64−151 days, Table1, SI Figure S3). One explanation was that stronger sorption monochlorinated methylsiloxane in organic matter would reduce the catalysis efficiency of clay minerals for their hydrolysis in soil because these compounds had larger Koc values than methylsiloxanes as discussed in Section 3.1. This speculation was further supported by the experimental results that the decreasing slopes (0.441−0.577, SI Figure S5) of the semilog plots of hydrolysis rates of monochlorinated cVMS versus soil TOC were slightly larger (1.01−1.13 times) than those (0.436−0.513) for cVMS. Degradation rates of dichlorinated D4 in soil were 1.12− 4.46 times lower than monochlorinated D4, which should be also due to stronger sorption to organic matter. Notably, although the apparent Koc values followed the order: D3D(CHCl2) < D3D(CH2Cl)2< D2(D(CH2Cl))2 < DD(CH2Cl)DD(CH2Cl), sorption to organic matter may be not the only reason for D3D(CHCl2) having fastest hydrolysis rates (t1/2 = 7.75−33.8 days), followed by D3D(CH2Cl)2 (t1/2 = 9.38−48.1 days), D2(D(CH2Cl))2 (t1/2 = 10.3−73.9 days), and DD(CH2Cl)DD(CH2Cl) (t1/2 = 12.0−99.9 days). For example, D3D(CHCl2) and D3D(CH2Cl)2, with two chlorinated methyl groups attaching to one Si atom in each molecule, would have stronger polarity than the analogs with

two Si atoms each has one chlorinated methyl groups, that is, D2(D(CH2Cl))2 and DD(CH2Cl)DD(CH2Cl) . 3.3.2. Volatilization. The volatilization rates of target compounds were calculated by subtracting the slopes of semilog plots in the capped soil samples from the values in the paired opened soil samples under the same conditions. Overall, volatilization half-lives of D3D(CH2Cl) (4.62−17.3 days), D4D(CH2Cl) (13.8−40.0 days), and D5D(CH2Cl) (83.6− 325 days), were 1.1−2.0 times longer than those of parent methylsiloxanes (Table 1). In theory, vapor pressures of chlorinated methylslioxanes were lower than those of their parent methylslioxanes due to the increase in the molecular weights by the chlorination. Meanwhile, as discussed in Section 3.1, chlorinated methylslioxanes had 1.01−1.27 times larger apparent log (Koc) values than those of their paired methylsiloxanes, indicating that their stronger sorption by soil organic matter and slower volatilization from soil to air. Compared with D3D(CH2Cl), four isomers of dichlorinated D4 had 15.7−41.9 times lower volatilization rates (Table 1). In detail, D3D(CHCl2) had fastest volatilization rates (t1/2 = 135−271 days), followed by D3D(CH2Cl)2 (t1/2 = 178−380 days), D2(D(CH2Cl))2 (t1/2 = 183−385 days) and DD(CH2Cl)DD(CH2Cl) (t1/2 = 194−461 days), which was same with their elution order in GC−MS (SI Figure S1). Until now, there was no other study about the volatilities of dichlorinated D4 isomers, except one literature reporting orders of their boiling pointsD3D(CHCl2) (118−120 °C) < D3D(CH2Cl)2 (124−126 °C) < D2(D(CH2Cl))2 (128−129 °C) and DD(CH2Cl)DD(CH2Cl) (128−129 °C).33 3.4. Implication. In this study, mono- and dichlorination of methylsiloxanes were found to be generated in electrooxidation processes of oil−wastewater treatment plants. Their potential environmental releases were confirmed by their presence in the surrounding soils near the point sources. Besides in Shengli Oilfield, electro-oxidation treatment of the chloride-containing wastewater with residual PDMS may also found in some other oil industries, for example, fossil oil, palm oil, and vegetable oil processing. Due to effects of various factors such as concentrations of both methylsiloxanes and H

DOI: 10.1021/acs.est.8b06993 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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chloride ions in the wastewater streams as well as operation parameters of electro-oxidation treatment processes, it was difficult to assess generally if methylsiloxanes could undergo chlorination in all related industries. However, environmental risk of chlorinated methylsiloxanes around related industries should be further studied case by case.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b06993. Details of sampling, calculation for TSP deposition of target compounds, concentrations of target compounds in samples, solid/oil−water partition coefficients, and their elimination curves in soil (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone +86 (10) 62849182; fax: 8610-62849182; e-mail: [email protected]. ORCID

Lin Xu: 0000-0002-4681-6457 Shihe Xu: 0000-0003-2528-7063 Zongshan Zhao: 0000-0002-0800-0983 Yaqi Cai: 0000-0002-2805-5535 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21537004, 21876189), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB14010201).

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REFERENCES

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