Determination Of Compound-specific Carbon Isotope Of Polycyclic Aromatic Hydrocarbons In Marine Sediments From The South China Sea

Polycyclic aromatic hydrocarbons(PAHs) are a class of organic pollutants with low content and "three- induced" effects, which can access to the sea by run-off, atmospheric dry and wet deposition, sewage discharge and other ways. Eventually, PAHs get into the sediments and pose a serious threat to the marine ecological environment. It is extremely necessary to control pollution by tracing the sources of pollution.Compound-specific isotope analytical technique is widely used in source apportionment of organic pollutants. However, due to its difficult sample preparation techniques, scarcity of standard material of individual isotope and the isotopic fractionation happened in the experimental process, the application of this technology is restricted. To solve this problem, after optimizing various conditions for sample pre-treatment, gas chromatography and gas chromatography- isotope ratio mass spectrometry, a set of systematical analytical method for determination of compound-specific carbon isotope of polycyclic aromatic hydrocarbons was developed. PAHs in Daya Bay core sediments and surface sediments from the western region of South C hina Sea were detected. The main research work and conclusions are listed below.(1) The time and temperature of soxhlet extraction are investigated by spiking. It showed that dichloromethane was efficient for PAHs extraction at 70 ℃ for 48 hour.(2) When developing solvent was composed of n- hexane and chloroform(9:1, v/v), the optimal efficiency was obtained for the separation of mixed reference materials(polycyclic aromatic hydrocarbons and alkanes) with the high performance thin-layer plates.(3) The isotope values of 16 PAHs have been tested by elementary analysis-isotope ratio mass spectrometers(EA-IRMS). High performance thin- layer chromatography, gas chromatography sample volume, purge time and sample concentration of the isotope fractionation have been investigated by GC-C-IRMS with mixed 16 PAHs standards. The results showed that there was no significant isotopic fractionation in optimized process and carbon isotopic compositions in sedime nt could be corrected by concentration calibration curves.(4) The method detection limits of 16 PAHs were 1.11 ng ~6.41 ng by ten replicate measurements of sample blank. The RSDs were 0.67%~9.00% by five replicate measurements of mixed 16 PAHs standards. The sample recoveries were within 72.8 %~112.3%.(5) The proposed methods of pretreatment and instrumental analysis of polycyclic aromatic hydrocarbons have been used to detect the samples of Daya Bay core sediments and surface sediments from the western region of South C hina Sea. Some conclusions are as follows.(1)In addition to ΣPAHs, there were positive correlations between the regular sediment parameters. The C/N ratios were from 0.77 to 6.37 in the sediments from western region of South China Sea surface and varied from 3.22 to 7.46 in Daya Bay core sediments. The C /N ratios suggested that PAHs pollution wa derived from marine organisms, including mariculture, shipbuilding and other maritime mobile pollution.(2)Concentrations of total PAHs(ΣPAHs) were from 139.1 ng/g to 785.7 ng/g in the surface sediments from western region of South C hina Sea, with mean values of 362.7 ng/g. The 3 ring, 4 ring and 6 ring of PAHs are the dominant monomers in surface sediments, and the concentrations of high molecular PAHs(4~6 ring) were higher than low molecular(2 ~ 3 ring) PAHs. The compound-specific carbon isotope(δ13C) of polycyclic aromatic hydrocarbons were from-33.90 ‰ to-16.98 ‰, with mean values of-25.45 ‰. The carbon isotope values of high molecular PAHs were lower than low molecular PAHs. PAHs contaminations were due to a mixed input sources, including combustion of coal and wood, fossil fuels burning, gasoline and diesel exhaust emissions and petrogenic source.(3)Concentrations of ΣPAHs are from 99.3 ng/g to 676.5 ng/g in Daya Bay core sediments, with mean values of 303.6 ng/g. The high ring(4~6 ring) PAHs were the dominant monomers in core sediments. The pyrogenic source is the dominant pollution source in this area before 1960 s. The pyrogenic and petrogenic mixed sources were the dominant pollution sources after 1960 s, which are probably due to motor vehicle exhaust, navagation and oil spills.The compound-specific carbon isotopes(δ13C) of polycyclic aromatic hydrocarbons in the core sediments with 60 cm depth from Daya Bay, were between 35.0‰ and-16.1‰. The carbon isotope values of high molecular(high ring) PAHs were lower than low molecular(low ring) PAHs. The mean of the compound-specific carbon isotope(δ13C) of sixteen polycyclic aromatic hydrocarbons decreases with the increased depth. There was a decrease trend of the carbon isotope values in early 1957, the pollution source of PAHs could be derived from the incomplete combustion of coal and wood. There was a increase trend of carbon isotope values in the late 1957 and reached peak in 1960. In addition to wood, coal and other incomplete combustion, the pollution sources could due to motor vehicle exhaust emissions, navagation and oil spills and other sources.This study provided the compound-specific carbon isotope(δ13C) for sixteen polycyclic aromatic hydrocarbons, which enriched the pollution data of the sediments from South China Sea and Daya Bay.