Carbon Nanotube Amendment For Treating Dichlorodiphenyltrichloroethane And Hexachlorocyclohexane Remaining In Dong-ting Lake Sediment-an Implication For In-situ Remediation

Organochlorine pesticides(OCPs)were largely sprayed on the floodplain soils before the project of Returning Farmland to Lake in China,dichlorodiphenyltrichloroethane(DDT)and hexachlorocyclohexane(HCH)have been widely used for pest control in agricultural production.Due to the strong bioaccumulation and difficult degradation of OCPs,the residual DDTs and HCHs still remained in the environment even being prohibited from using in a large scale for decades,which caused contamination of sediment in Dong-ting Lake with DDTs and HCHs and posed threats to human health and other organisms even the whole ecosystems.Carbon nanotubes are considered to be promising materials for sediment remediation because of its high affinity and adsorption for HOCs.As one category of nanoparticles,carbon nanotubes have the property of the extremely large specific surface area(SSA),making it widely used in many aspects especially adsorption field.The present study put forward an in-situ amendment strategy using SWCNTs and MWCNTs to treat the residual DDTs and HCHs in Dong-ting Lake sediments,with the expectation to prevent DDTs and HCHs being released from sediments.The measured bottom value of total DDTs was 4.99 ng/g in dry sediment,while HCHs were not detected in the composite sediment sample.In order to compare more clearly the efficacy of carbon nanotubes amendment,certain amount of DDTs and HCHs have to be artificially spiked into the sediments considering its low value in sediments collected in Dong-ting Lake.In this study,single-walled carbon nanotubes(SWCNTs)and multi-walled carbon nanotubes(MWCNTs)at different concentrations of 0.058,0.145 and 0.29 wt%were used to manage residual DDTs and HCHs in Dong-ting Lake sediment.The efficacy was assessed by DDTs and HCHs deriving from the aqueous equilibrium experiment and uptake in semipermeable membrane devices(SPMDs).Desorption experiment and the quiescent flux experiment were conducted as well.The results showed that DDTs and HCHs were released from sediment.The p,p'-DDT was desorbed less readily than its metabolites and similarly a-HCH was desorbed less easily than other HCH isomers from sediment.Carbon nanotubes had great effects on treating DDTs and HCHs.SWCNTs were more effective due to its larger specific surface area(SSA)than MWCNTs.The effectiveness of carbon nanotube amendment was dependent on type,dose and sediment-sorbent contact time,enhancing with the increasing dose and contact time.Carbon nano tubes showed high effectiveness for treating both DDTs and HCHs especially under the optimum condition(sediment treated with 0.29 wt%SWCNTs for 4 months).Enough amounts(0.29 wt%)of carbon nanotubes reached great reductions of DDTs and HCHs aqueous equilibrium concentration as well as DDTs and HCHs uptake in SPMDs even in a relatively short period(1 month).Furthermore,small amounts(0.058 wt%)of carbon nanotubes could also reach similar effects on reducing DDTs and HCHs aqueous equilibrium concentration as well as DDTs and HCHs uptake in SPMDs given enough sediment-sorbent contact time(4 months),which may be implications for in-situ remediation based on addition of carbon nanotubes in an overall consideration of the amendment cost,effectiveness,as well as time effect.In addition,carbon nanotubes being sprinkled on the surface of sediment as a cap and being injected into sediment as a mixture were considered as two effective ways to prevent DDTs and HCHs being released from sediment.Carbon nanotubes can be potentially useful as sorbents in in-situ remediation.