Research On In-situ Remediation Of Mixture-contaminated Sediment In Aquaculture Ponds And Effect Evaluation Based On Bioavailability

In recent years,with the growth of the population and the acceleration of industrialization,a large amount of domestic sewage,industrial wastewater and solid waste has flowed into the environment,causing heavy metals and organic substances in the environment to maintain high levels for a long time.Pollutants in the environment enter the aquaculture water through various methods such as precipitation and runoff and gradually accumulate in the sediments.Besides causing the compound pollution of sediment,it can also seriously threaten the quality of aquatic products.As a large aquaculture country,China has a huge demand for aquaculture products.In order to ensure human health and sustainable development of the aquaculture industry,it is necessary to develop environmentally-friendly and low-cost sediment remediation technology.In this paper,the sediment collected from aquaculture ponds was selected as the remediation object.Two types of typical pollutants,heavy metals?HMs?and hydrophobic organics pollutants?HOCs?were selected as target pollutants.V.philippinaram was a test organism,and passive sampling technology based on Diffusive Gradients in Thin-films?DGT?and polydimethylsiloxane?PDMS?were introduced at the same time to explain the effects of natural zeolite and magnetic activated carbon on bioaccumulation and bioavailability;the feasibility and reliability of PDMS in the prediction of remediation were also evaluated.The results were shown as follows:?1?Based on the previous investigation and research of our research group,the physical and chemical properties of the sediment and the concentration and composition of main pollutants were analyzed.There were some differences in the particle size distribution,specific surface area,and organic matter content of the three sediment samples,which might be closely related to the soil conditions in the areas and the surrounding environments.Four heavy metals?Cu,Pb,Cd,Cr?and four HOCs?OCPs,PAHs,PEs,PBDEs?were detected in the sediments,Cu and Pb were the main ones;PAHs and PBDEs were at higher concentrations and more types of organic substances detected in sediments.Pollution of heavy metals and HOCs in the sediments of selected sample sites was slightly more serious than in other regions,and some pollutants had potential ecological risks.?2?Based on the excellent role of natural zeolites in controlling the release of nitrogen and phosphorus and other heavy metals in sediments,the stabilizing effect of natural zeolites on hydrophobic organic pollutants in sediments was studied.The results showed that natural zeolites had the potential to stabilize HOCs in sediments:After adding natural zeolites?10%dry weight of sediment?to the sediments,the bioaccumulation of four HOCs?OCPs,PAHs,PEs,and PBDEs?in V.philippinaram decreased by 10%?38%,The freely dissolved concentration(C_free)of HOCs in pore water measured by PDMS passive sampling also decreased by 6.8%?20%.It was also found that the physical and chemical properties?particle size and specific surface area?of the sediments and the K_ow value of HOCs were important factors affecting the stabilization effect of zeolites.The higher the K_ow of HOCs was,the greater the C_freedecline rates was;the larger the sediment size and the smaller the specific surface area was,the more obvious the C_free declined.The calculation model established by the PDMS passive sampling method was found can effectively predict the bioaccumulation of PAHs and PBDEs in the sediments of the natural zeolite amend group,and the calculated values were no more than 5 times the measured values.?3?In order to strengthen the stabilizing effect on heavy metals and PAHs in the sediments,MAC was prepared from common coal-based activated carbon?PAC?by co-precipitation method,and the adsorption properties of MAC and PAC to Cd²⁺and PHE were further studied.The scanning showed that the surface properties of the synthesized MAC were uniform,the magnetic material was well loaded,and rapid separation can be achieved by an external magnetic field in water.The pseudo-second adsorption kinetics well described the adsorption process of Cd²⁺and PHE by MAC,the chemical action was controlled and mainly occurs on the MAC surface.The isothermal adsorption processes of Cd²⁺and PHE by MAC were in accordance with the Langmuir and Freundlich models,respectively,indicating that the adsorption processes were monomolecular layer adsorption and multimolecular layer adsorption,respectively.Adsorption mechanism,but its affinity for PHE adsorption decreased.?4?The effect of MAC for stabilizing heavy metals and PAHs in sediments was studied.The results showed that the addition of 3%MAC reduced the bioaccumulation of Cu,Cr,Pb,and Cd by 47%,45%,52%,and 31%,respectively.The effective concentration decreased by 55%,36%,39%,and 40%.The difference between the two was mainly related to their properties and the distribution of heavy metals in the sediment.Magnetization can enhance the control effect of activated carbon on four heavy metals,of which Pb was the most obvious one.This was mainly due to the strong chelating ability of iron oxide on the surface of MAC and Pb.different doses of MAC reduced the accumulation of PAHs in clams by 22%to 77%,C_free in sediment pore water,which was significantly related to bioaccumulation,also decreased by 20%to 80%,and the C_freedecline rate of high-ring PAHs was lower than that of low-ring PAHs.The dose-effect model established in this study was found can accurately predict the repair effect of 0%?3%MAC in the sediment,but when the added MAC exceeded 3%,the prediction result of the dose-effect model was significantly different from the actual.Nevertheless,this model has important guiding significance in designing suitable MAC dosage for PAHs-contaminated sludge on-site restoration projects.After the repair experiment was completed,the MAC in the sediment was recovered by an external magnetic field.The average recovery rate was about 72%.MAC also significantly reduced the consumption risk of aquatic products,in which the non-carcinogenic risk?THQs?of heavy metals and the carcinogenic risk?ILCRs?of PAHs were reduced by 50%and 47%,respectively.After magnetic carbon recovery,the content of heavy metals and PAHs in the sediment was also reduced by 4.8%?13%and 2%?60%.This showed that MAC has the potential to completely remove pollutants from the sediment,thereby avoiding potential ecological risks caused by a secondary release of pollutants after remediation.