Effect Of Black Carbon On The Bioaccumulation, Biodegradation, And Genotoxicity Of Hydrophobic Organic Compounds In Sediment

Sorption of hydrophobic organic compounds (HOCs) by sediment plays an important role in their transportation, bioavailability and ecological effects. Black carbon, a special component in sediment organic matter (SOM), is considered as supersorbent for HOCs, and the sorptive capacity of black carbon could be 10-1000 times higher than those of other forms of SOM. Therefore, the study about the effect of black carbon on bioavailability (e.g. bioaccumulation, biodegradation, and subleathal toxicity) of HOCs in sediment will help us to make an accurate prediction for HOCs risk assessment as well as reasonable sediment quality criteria.In the first part of this research, black carbon was isolated by thermal oxidation method from West Lake sediment, and four typical HOCs, including two PAHs (phenanthrene (PHE) and pyrene (PYE)), pentachlorophenol (PCP), and one pyrethroid (permethrin (PM)), were selected as the target contaminants. Equilibrium batch sorption experiment and a three-step iterative calculation were used to evaluate the contribution from black carbon to the sorption of HOCs by sediment. In the second part of this research, the effect of black carbon (biochar, charcoal, and single-walled carbon nanotube) on the bioavailability of HOCs has been investigated from three aspects of bioaccumulation, biodegradation, and sublethal toxicity (genotoxicity). Some mimic methods, i.e. Tenax desorption, solid phase micro-extraction (SPME)-disposal Polydimethylsiloxane (PDMS) fiber, and comet assay, were employed to quantify the bioavailability. Some models also have been modified and some statistical methods (such as multivariable linear regression) were adopted, which would provide further understanding about the underlying mechanism.The sorption capacity of black carbon toward PHE, PYE, and PCP was much stronger than those of other forms of sediment organic carbon, and the values of KBC were 21.9,34.7, and 17.0 times of KTOC values. The sorption capacity of black carbon toward PM was comparable to that of other forms of sediment organic carbon, and the value of KBC was only 1.2 times of KTOC value. As the surface sorption sites of black carbon became saturated, the contribution from black carbon to the sorption of HOCs in sediment decreased with the increase of aqueous concentrations of sorbates. When the aqueous concentrations of PHE and PYE were less than 8.9μg/l and 0.8μg/l, the contribution from black carbon to sorption were higher than 50%, with the highest values of 84% and 63%, respectively. Sorption toward PCP showed a similar trend with the highest contribution of 30%. The averaged contribution from black carbon to PM sorption was 7.5±1.1%, indicating the sorption capacity of black carbon toward PM was not so significant as that toward PAH compounds.The desorption rate of PHE in the Tenax desorption test was significantly reduced when the sediment was amended with black carbon. The desorbed fraction of PHE decreased from 67.4% to 28.6% after black carbon amendment, and the rapid desorption pool (Frapid) derived from three-phase kinetic model also dropped from 0.265 to 0.131. In contrast, there is no significant effect on PM desorption rate after black carbon addition. The bioaccumulation of PHE in Chironomus tentans was decreased after black carbon addition, and the biota-sediment accumulation factor (BSAF) decreased 72%, but there is no significant influence observed on the PM bioaccumulation. A multivariate linear regression model was developed, and model analysis result showed that, besides chemicals (PHE and PM) in the rapid desorption pool, part of chemicals in slow desorption pool were also available to Chironomus tentans, which meant chemicals, even associated with black carbon particles, could also be accumulated by Chironomus tentans.The biodegradation of PHE by Mycobacterium vanbaalenii PYR-1 was inhibited by addition of black carbons (single-walled carbon nanotube (SWCNT), biochar, and charcoal), and the inhibitory effect of SWCNT was stronger than those of biochar and charcoal. The inhibitory effect of black carbons observed was partly due to their suppression on the bioavailability of PHE, as revealed by measurement of the freely dissolved concentration (Cfree) in the sediment using solid-phase microextraction (disposal PDMS fiber). The Cfree dropped 10%-60% after the addition of biochar and charcoal, and about 85%-95% decrease was observed when SWCNT was amended into sediment. A desorption-biodegradation coupled model was used to investigate the microbial availability of black carbon-associated PHE. Model analysis showed that the PHE sorbed with SWCNT could be directly utilized by PYR-1, which could be attributed to the relatively larger surface area and smaller particle size of SWCNT. The surface area and micropore volume of SWCNT were decreased when coated with dissolved organic matters, which also introduced polar fuctional groups onto SWCNT surface, resulting in the reduction of sorption capacity toward PHE, and the inhibitory effect on the biodegradation rate of PHE.On the sediment particle-size scale (sand, silt, and clay), the distributions of PYE and biodegradation rates of PYE were mainly driven by black carbon contents. The distributions of PYE were significantly positively correlated with black carbon and total organic carbon content in various sized fractions. The biodegradation rate of PYE were significantly negatively correlated with black carbon content, total organic carbon content, and surface areas of various sized fractions. A biodegradation model was modified by imbedding a two-phase desorption relationship describing sequential Tenax extractions. Model analysis showed that PYE sorbed on silt and clay aggregates was directly utilized by the degrading bacteria. The enhanced bioavailability was attributed to the higher PYE concentration, or larger surface area in the silt and clay fractions, which appeared to overcome the reduced bioavailability of PYE due to sorption, making PYE on the silt and clay particles readily available to degrading microbes.The genotoxicity induced by PYE and PCP in Eisenia fetida, which was quatified by comet assay, was inhibited after addition of black carbon-biochar. The genotoxicity of PYE and PCP was decreased 50% and 80%, respectively, when the biochar content increased from 0% to 5%. However, the 10% biochar treatment had a higher genotoxic effect than that in the 5% biochar-added exposure. This may have been caused by the genotoxic compounds and high alkalinity contained in the biochar, which means that the biochar probably could induce genotoxic effect.