Mechanisms And Model Of HPCD Extraction Technique To Assess PAHs Bioavailability

Polycyclic aromatic hydrocarbons (PAHs) are typically hydrophobic organic contaminants, ubiquitously distributed in the soils, and infamous for their mutagenicity, teratogenicity and carcinogenicity. In order to evaluate bioavailability of and further assess the risk for ecosystems posed by PAHs, hydroxypropyl-p-cyclodextrin (HPCD) extraction, one of the most effective bioavailability assessment technologies has been employed. Due to the monotypic species targeted by previous researches as model pollutants, however, the chemical diversity among pollutants is not well characterized, thus the mechanisms of bioavailability assessment via HPCD extraction not sufficiently revealed. In this study, inclusion complexation, desorption and bioaccumulation in earthworms of 15 types of PAHs with HPCD, were investigated, using field soils. The experiments were conducted for the effects of host compounds on speciation, environmental transfer, and apparent bioavailability of structurally different organic pollutants, to reveal mechanism of HPCD extraction in organic pollutants bioavailability processes and to build a prediction model between HPCD extraction and accumulation in earthworms.Accelerated Solvent Extraction method was adopted to assess the total concentration of PAHs in 31 types of soils. The results of double-ratios method suggested that soilborne PAHs are mainly from fossil fuel combustion along the highways, coal tar oil area, farmland, petroleum volatilization and coal/wood combustion in the industrial area. PAHs in the harbor area were mainly from a composite source. Based on regression analysis of total PAHs concentration and soil physicochemical properties, the results showed that the content of total PAHs is positively related to soil organic carbon content, surface area, clay fraction and silt fraction (CPAHs= 0.48X1+0.33X2+0.36X5+0.20X7(R2=0.95,n=31)). The content of a single PAH is related to soil organic carbon content, surface area, pore volume, pore diameter, particle size fraction and weather index (R2=0.54～0.94). The results indicated that different types of PAHs have different occurrences in soils.Accumulation of PAHs in earthworms was investigated. The results indicated that the mean value of unadjusted BSAF of PAHs is between 0.06±0.04 and 0.18±0.10, while the corrected value is between 0.12±0.05 and 0.35±0.11 (Nap). Depending on SPSS regression model, the unadjusted BSAF of different kinds of PAHs are dependent on organic carbon content, molecular polarization, clay fraction, silt fraction, molar volume and biodegradation rate (log BSAF=-0.98X3+0.35X5+0.48X8+0.15X9-1.00X11+0.45X15-0.28X16(R2= 0.80, n=390)). In these dependence factors, the most contribution to research of predictability is due to organic carbon content and molecular polarization. The results showed that the difference in accumulation ability of earthworm is the result of physicochemical properties of PAHs accompanies with the different occurrences in soils.Desorption experiment was conducted, with HPCD extraction to investigate the mechanisms of effectiveness of HPCD extraction, based on which a prediction model was established. Depending on SPSS correlation analysis, the ratio of HPCD extraction content of PAHs to total content and unadjusted BSAF of different kinds of PAHs are all related to soil organic carbon content, biodegradation rate, clay fraction and silt fraction, while organic carbon content contributes most to the research of predictability. The soil particle-size decreases while the surface area of soils increased during HPCD treatments. These changes correspond in a way to the variations of the accumulation of earthworm. Hence, the HPCD extraction process may be mechanistically relevant with and suitable to mimic the accumulation process of earthworm.Individual prediction models between the individual PAH concentration inside earthworms and that extracted with HPCD in 26 soils were built with SPSS software (Cworm=((CHPCD+k)/a)0.5+h)(R2=0.88～-0.95,n=26)). For a more universal, compound class-specific approach, the study used one equation to predict concentration of earthworm of all PAHs that uses the PAHs concentration of HPCD extracted (Cworm=((CHPCD+91.58)/0.0049)0.5-136.96(R2=0.96,n=270)). Eighteen soils were used to build up the model, and 8 additional soils were used to verify it. The result showed that the compound-class specific model was suitable to predict the PAHs concentration of earthworm (y=1.21x-0.13(R2=0,93,n=120)). The prediction model between the total PAHs molar concentration of earthworm and HPCD extracted in 26 soils was built (Cworm=((CHPCD-0.018)/1.48)0.5-0.239)(R2=0.92,n=26)). By combining the prediction model of molar concentration with the criterion of earthworm for four different effects (critical concentration, hazardous concentration to 5% of all species, no-observed-effect and Median lethal aquatic concentration), the corresponding PAHs molar concentration of HPCD extracted for four different effects obtained in this study is 0.11 mmol/kg OC,0.37 mmol/kg OC,69 mmol/kg OC and 3735 mmol/kg OC respectively. And the corresponding PAHs molar concentration of total extracted is 0.15 mmol/kg OC,0.52 mmol/kg OC,57 mmol/kg OC and 5261 mmol/kg OC respectively. The results of this study may provide valuable information for the establishment of ecosystem risk assessment criteria for soilborne PAHs.