| Polycyclic aromatic hydrocarbons featured by strong carcinogenic, teratogenic and mutagenic effects are one of ubiquitous pollutants classes in the environment. Soil are constantly under the huge pressure from various pollutants released into the environment by way of indirect or direct (rain, snow, dust) processes, becoming the largest terrestrial sinks of PAHs, especially urban soils. Shanghai, as the bellwether in the collective group of Yangtze River Delta areas, has the most prosperous economy, massive industries activities and densely populated areas. Large scale of spatial and temporal distribution characteristics of PAHs in contaminated soils under different land use types in Shanghai have already been studied and the results from those studies indicated that soil from various land use types are all featuring different degrees of ecological risk. This paper selects top-soils in typical areas of Shanghai as the research object, using physical separation (size separation) in conjunction with organic matter density separation method to explore how the soil organic matter, organic mineral complex and different types of mineral adsorbents influence PAHs in the environment, and to reveal the natural environment behavior of PAHs in soil. In the meantime, it could act as the scientific basis for urban soil treatment or rehabilitation. Using selected Milli-Q water as the solvent for desorption thermodynamic analysis to reveal how the temperature influence desorption process and to infer the desorption contents of PAHs in lower temperature in the natural environment.S16PAHs range from 5.2μg/g to 125.6μg/g in all samples and the total concentrations among sampling points exhibit significant differences.Σ16PAHs concentrations from industrial land are pretty much higher than that of other land use types.Σ16PAHs concentration of JS, BS, DC is 125.6μg/g,32.6μg/g and 25.7μg/g, respectively, which combined accounted for 73.5%of sum of E16PAHs concentrations in all samples.16 kinds of individual PAHs species showed consistently levels overall, besides JM. Fluoranthene is the highest individual PAHs species with a mean of 17.9%, followed by pyrene (14.3%) and Phenanthrene (12.2%). Naphthalene (1.6%), acenaphthene (0.4%), acenaphthyene (0.5%), fluorene (0.8%) is the four compounds of lowest concentration, respectively. At JM, Phenanthrene accounted for 30.4% of Σ16PAHs, followed by naphthalene (18.1%) and fluoranthene (17.4%). Except JM,4-5 rings PAHs are the highest group in the seven remaining sampling points, followed by 3 rings,6 rings PAHs. The sample from JM, lies in one of Shanghai’s most bustling business district, have convenient transportation and heavily traffic. PAHs in top-soil showed a 4 ring> 3 ring> 2 ring> 5 ring> 6 ring gradient trend,2 rings and 3 rings PAHs concentration combined accounted for above 50%.The relative abundance of different PAHs rings could reflect pyrolysis or petroleum sources, LMW/HMW ratios are all below 1 except for JM, which namely indicate the source is mainly from petroleum and related products. PAHs in the study area are mainly from incomplete combustion of fossil fuel. Petroleum and its products is the pollution source in JM. Four groups of isomers ratio is consistent with the above results, all samples are indicative of oil, coal and biomass burning sources.Soil is divided into five categories of particle size (> 500μm,250-500μm,125-250μm,63-125μm,<63μm) by wet sieving. Soil particle size quality with the particle size decreases, trend further decrease after the first increase. The highest proportion of particle size is <63μm particle group, up to 43.2-56.9%, followed by 125-250μm particle group. The smallest proportion of> 500μm particle group only accounted for 1.1-1.9% of total weight of the original soil.Five categories of soil particle size in the total PAHs concentration distribution is uneven, ranging from 2.04 to 136.34μg/g Maximum values are not always present in a certain size, but mostly appeared at>125um or more proportions which may be largely due to a large particle size containing coal, coke plant debris and other carbonaceous materials have a strong adsorption of PAHs. The lowest Σ16PAHs was observed at at <63um particle size fractions which may be associated with the lowest TOC content. After standardized PAHs by organic carbon, the original PAHs enrichment capability have been changed in terms of size fractions, the mean enrichment capability exhibit a following decline trend as follows:25-250μm,250-500μm,>500μm,63-125μm,<63μm. 125-250μm PAHs group of enrichment of organic matter is the strongest, while that of <63μm PAHs group is the weakest.3 rings and more than 3 rings PAHs combined accounted for the major proportions of T-PAHs in which 4 rings PAHs is predominantly occupied (40%-52%), followed by the 5 rings,3 rings,6 rings. While 2 rings PAHs accounted only for 0.2-8%in T-PAHs which is significant lower than any other rings of PAHs.Each size fractions of the soil are divided into two density fractions. The larger the size fractions, the more coarse particles and consequently the greater the proportion of light components group which are in the range of 1.9-81.1%. The PAHs contents in the organic absorbent of light component group range from 98.9μg/g to 228.6μg/g, of which the highest value appears at 125-250μm size fraction of the light component. A decreasing trend has been found towards the coarse particles and fine particles. The concentration range of PAHs in heavy fractions of inorganic mineral is 1.3-3.7μg/g, of which the highest value appears at>500μm diameter, heavy fractions’PAHs concentration is descending with the decrease of particles size. The concentration of PAHs in light fraction is 32-152 times higher than that of heavy fraction, usually about 1-2 orders of magnitude. After standardized PAHs by organic carbon, light fraction of organic carbon normalized values greater than the corresponding heavy components, The highest organic carbon normalized value still appears at 125-250μm light component and a decreasing trend towards the coarse particles and fine particles. Standardization of organic carbon value is highest in<63μm heavy component and the more coarse size fractions, the lower organic carbon normalized values. It opposed to the various components pattern of PAHs variation. It indicates not only the content of organic carbon, but organic carbon quality is also the main factor affecting PAHs enrichment. It also shows the light fraction organic sorbent has a higher affinity for PAHs compared to the heavy components.TOC content range from 6.9 to 27.3g/kg in different size fractions in all soil samples, BC range from 0.8 to 8.2g/kg. TOC and BC are both positively correlated with PAHs concentration. However, the correlation coefficient is higher between BC and PAHs. PAHs in light components correlated with TOC.There is no correlation between TOC content and PAHs in heavy fractions. The total PAHs were positively correlated with the TOC, the correlation coefficient was 0.451 (P<0.01).Compared to higher ring of PAHs, lower ring PAHs (2-3 rings) have the higher correlation values with TOC.The PAHs were extracted by water with ASE at eight temperature gradients ranging from 25 ℃ to 100℃. Aqueous PAHs concentration increases with the increase of temperature. The concentration of PAHs are 0.3μg/L at 40 ℃ and rise to 12.9μg/L when the temperature rose to 100℃. Desorption enthalpy value of the sample is in the range of 38.9-114.3kJ/mol. The mean of desorption enthalpy value of T-PAHs, Phe, Ant, Fin, Py are 52.5kJ/mol 51.5kJ/mol 58.8kJ/mol,90.9kJ/mol and 96.7kJ/mol, respectively. In particular, the fitting analysis of Py is the best with R2 value is above 0.99, AH respectively, from 86.9-114.3kJ/mol.With the increased molecular weight and hydrophobicity, the desorption enthalpy of PAHs increased correspondingly. Aqueous PAHs concentration of 5-6 rings at low temperatures often below the detection limit, while fit Van’t Hoff equation at high temperatures. AH value of original soil,<63μm and the light fraction soil exhibit consistently regular pattern in the same site, revealing similar adsorption mechanism. In different sampling sites, the mean value of desorption enthalpy of JS is less than that of JM and DC. Except 2 rings (Nap) and 3 rings PAHs (Phe), the mean value of desorption enthalpy of JM and DC are basically at the same level. |
PDF Full Text Request