Fate Of TBBPA In Soil And Effects Of Earthworms And Heavymetals Studied Using C-14 Tracing Model

With the development of chemical industry,the research on environmental fate of organic chemicals?OCs?has received more concerns by the public and government since OCs are widely used and have potential to cause public hazard.For the OCs with high hydrophobicity and low volatility,soil was the major sink.Due to the complex matrix of soil,current research methods could not quantitatively analyze certain OCs in soil,because OCs may be entrapped into soil micro-pores or bound to soil organic matter by covalent bonds.Generally,the recovered OCs in organic extracts can only describe the "dissipation" process of OCs in soil.However,traditional methods could not distinguish the "degradation" and "bound-residue formation" in the "dissipation" part and could not evaluate the environmental risk of the release of "bound residues" Thus,using "dissipation”rate to evaluate the environmental risk of OCs could underestimate the risk.Although using stable/radioactive isotope tracing technique could recover the OCs efficiently from soil,it could not distinguish the "degradation" and "bound-residue formation" either.Moreover,mathematical models have been used to simulate the fate of OCs in soil,but most of the models represent only individual processes of environmental fate separately,such as the "dissipation" or "mineralization".This makes it unrepresented and incomplete for evaluating the fate of OCs in soil.In the present study,we combined the mathematical model and ¹⁴C tracing technique to investigate the fate of a hydrophobic organic compound-TBBPA,in soil as well as the mechanisms of the influence of environmental factors to the fate.The results were shown below.1)The investigation of the degradation and transformation of ¹⁴C-TBBPA in rice paddy soil as well as the establishment of mathematical model of the fate process.According to previous modeling studies,we choose a first-order two-compartment model,which was used to simulating the mineralization of organic matters in soil,as the basic model and then developed the model from single"mineralization" simulation into complex "fate" process,which included"degradation","mineralization","bound residues" formation as well as"dissipation".Combining the modeling results and experimental data,we found that TBBPA could be easily degraded in oxic soil.In the first 5 days of incubation,20.8± 2.8%of TBBPA was transformed to extractable polar metabolites,which including C-C bond cleavaged products,debrominated products and so on.In addition,small amount of non-polar metabolites of TBBPA were generated in the soil,such as O-methylation products.While the polar metabolites were quickly generated,they also formed into bound residues rapidly by bonding to soil organic matter or being sequestrated into soil miro-pore.Only 3.9 ± 0.3%of TBBPA and its metabolites could be degraded into CO2 completely after 30 days of incubation.At the end of incubation,only 9.4 ± 0.6%of extractable TBBPA parent compound was remained in soil,otherwise most of TBBPA was degraded to polar metabolites and formed to bound residues.2)The effect of earthworm Metaphire guillelmi activity on the fate of TBBPA in oxic soil.By using ¹⁴C tracing technique,we investigated how earthworm activity affects the degradation,transformation of TBBPA in oxic soil as well as the bioaccumulation by earthworms.In addition,combining the modeling results,we tried to explain the mechanism of this influence.Our results showed that earthworm do have significant effect on the fate of TBBPA thoroughly.During 30 days of incubation,earthworms decreased the degradation rate of TBBPA and changed its metabolic pathway in soil,which lead to decreased generation of polar metabolites derived from TBBPA and simulated generation of hydrophobic nonpolar methylated products of TBBPA.Due to the higher hydrophobicity of methylated TBBPA,they could be more persistent and hardly bonded to soil organic matter in soil environment.Therefore,this made lower mineralization activity of TBBPA and higher extractable TBBPA in experiment.Then,because of the lower polar metabolites was generated,which contributed the major part of bound residues we mentioned above,and the accumulation effect by earthworm,the formation of bound residues decreased rapidly.During the incubation time,earthworms accumulated 16.6± 3.6 of initial applied ¹⁴C-TBBPA in soil and the bio-soil-accumulation factor?BSAF?value of earthworms could reach to 3.4 ± 0.7.Following the incubation time,the constitution of accumulated ¹⁴C in earthworm body changed from TBBPA parent compound at the beginning of the incubation to metabolites of TBBPA contributed the major of ¹⁴C in earthworm body,which was found only 10%of total ¹⁴C in earthworm body was belong to TBBPA parent compound at the end of incubation.Meanwhile,a large amount of nonpolar metabolites generated during the incubation time resulted in much higher accululation potential of ¹⁴C compared to that of TBBPA only.The results of distribution of ¹⁴C in each earthworm tissues suggested that earthworm intestine tract accumulated the major part of ¹⁴C?62.9 ± 2.8%of total radioactivity in earthworm body?,while only]0.4± 3.2%was accumulated in the skin of earthworm and the concentration was much lower than that in intestine tract.The constitution of TBPPA and its metabolites in different earthworm tissues also showed that more TBBPA parent compound related to total ¹⁴C in each tissues were in intestine tract than that in skin.These results indicated that earthworms accumulated TBBPA through oral-digestion process rather than skin-diffusion process.In addition,relative higher concentration of extractable ¹⁴C was detected in the clitellum region,where most organs of earthworm were assembled.This means that the metabolites in clitellum region could cause biological effect to earthworm more efficiently than that in other tissues,especially for diMeO-TBBPA and polar metabolites which contributed to the major metabolites in clitellum region.3)The influence of co-contamination between TBBPA and heavy metal on the fateof TBBPA in oxic soil.We investigate the influence of three kind of heavy metal?Cu,Zn and Cd?on the fate of TBBPA in two kinds of soil type?Wushan soil and red soil?under the same cation strength.The results showed that the effect of heavy metal cation was much lower than the influence of the soil physical and chemical properties as well as the soil microbial activity.Only under the high level of cation strength?6000 ?mol/kg?,metal cation significantly inhibited the mineralization of TBBPA in soil and the inhibition effect decreased with the decrease of the metal cation strength.During the three kind of heavy metal cation,Cd²⁺ was the most efficient cation.By usitg principal component analysis?PCA?,we decreased the dimension of target index from six to two principal index.The results suggested that:1)soil physical and chemical properties influenced the water extractable ¹⁴C-TBBPA,namely the bioaccessible fraction of TBBPA in soil;2)soil microbial activity influenced the organic extractable and mineralized ¹⁴C-TBBPA,which indicated that soil microbial activity could influence the bioavailability of TBBPA in soil;3)The toxicity of heavy metal cations on soil microorganisms could also influence the fate of TBBPA in soil to some level.Furthermore,we investigated the effect of Cd²⁺ on the fate of TBBPA in soil at the environmentally relevant concentrations.No significant effect was observed until Cd²⁺ concentration reached 500 ?mol/kg dry soil.This could be explained by the low bioavailability of Cd²⁺ in soil,which resulted in low toxicity effect on soil microorganisms.