| Xiawan Harbour is one of the main channels of wastewater emissions inQingshuitang industrial district of Zhuzhou. After decades of accpitation, heavymetals such as cadmium, lead, zinc, copper and and so on in sediments in XiawanHarbour and and surrounding water had been exceeded the standard. In recent years,along with the large-scale comprehensive treatments of environmental pollutionprojects and local industrial enterprises in the production of quality improvement inZhuzhou, the rate of industrial waste water and waste gas emissions significantlyreduced and th industrial solid wastes were getted the safe disposal basically whichmade regional environment have a certain degree of improvement. However, theinternal pollutions formed heavy metals in the sediments of XiaWan harbor still cannot ignored. Sediments by adsorption and release of heavy metal pollutants directlyaffect the environmental quality of the overlying water; on the other hand, thesediment is the important reservoirs and destination of heavy metals in water, and alsois the main living places and food resources of benthic organisms, which leads toheavy metals directly or indirectly through the food chains and constitutes a seriousthreat to the survival of aquatic organisms and human health. So it is very necessaryto investigate the pollution of heavy metals in sediments and get its appropriatedisposal methods. In this study, a systematic investigation of the sewage situation ofXiawan harbor was carried out. Then, through the in-depth study of pollutioncharacteristics of heavy metals in sediments, the pollution risks were assessedcomprehensively in Xiawan harbor. According to the pollution characteristics ofXiawan harbor sediment pollution, cement-based solidification/stabilization (S/S)with DTCR (dithiocarbamate) as an additive was used to solidify and stabilizesediment containing heavy metals in this research.Twenty-six representative samples are selected from upriver to downriver and theconcentration of heavy metals including Cu, Zn, Pb and Cd are evaluated.Themethods of Index of Geoaccumulation, Nemerow index method and Potentialecological risk evaluation are applied to evaluate the heavy metal elementcontamination of superficial sediment in Xiawan Harbor. Results showed that: pH ofXiawan Harbor’s deposit along its whole length is generally within the range of7.5~7.9, and there is a trend for the value to increase gradually from upstream todownstream; serious compounding pollution of heavy metals is found in the deposit of different parts in Xiawan Harbor, of which Cd and Zn are respectively the top twoones with the highest times of exceed standards; The correlation analysis of heavymetals showed that the significance between Cd and Zn, Zn and Pb, Cu and Pb isranked top three, which may be caused by a common polluting source. The correlationbetween other heavy metal pollutants is not significant, which means the possibilityof the same polluting source is relatively small, and it has something to do with thefact that the size of the wastewater rivers are relatively small and there are manydischarge points; BCR morphometric analysis showed that Cd’s bio-available patternis more than its bio-unavailable pattern; Big changes have happened to Cu’s reduciblepattern, and other patterns basically stay the same, mainly existing as residual; Pb’sstability is not very strong, and can be easily changed to be bio-available, causinghazards; Zn’s patterns do not change greatly in general, and it is mostly existed asresidual. But as the percentage of Zn is too high that its bio-available pattern canreach up to361.8~4136mg/kg, therefore there is still a very high risk of releasing;Risk evaluation research showed that Cu, Zn, Pb, Cd all have reached the level ofheavy pollution, of which Cu, Zn and Pb have reached the pollution level of mediumand above, and Cd in all the parts have exceeded serious pollution level. Cd is thebiggest heavy metal element in the deposit of Xiawan Harbor that constitutes potentialecological hazards.DTCR was determine as the appropriate additive stabilizer by using it and Na2Sas stabilizing agents for treating heavy metal contaminated sediment with the leachingtoxicity as the evaluation index。Cement-based solidification/stabilization (S/S)with DTCR (dithiocarbamate) as an additive was used to solidify and stabilizesediment containing heavy metals. The optimum ratios of cement and DTCR weredetermined by testing the compressive strength of the whole solidified body andleaching toxicity of granular solidified body (particle size9.5mm). And the effect ofS/S was assessed though the leaching toxicity test of granular solidified body andwhole solidified body in the acidic environment (pH=3). The mechanism of S/S wasanalyzed by X-ray diffraction (XRD) and envrionmental scanning electronmicroscope (ESEM). The optimal proportion of cement and DTCR was50%and2%(w/w, dry basis), respectively. Under optimum conditions, the7d compressivestrength of the solidified body was up to1.03MPa and leaching concentrations of Cu,Zn, Pb and Cd in granular solidified body were0.105,4.65,0.232,0.123mg/L,respectively, which could meet the requirements of the security landfill. The leachingeffect in the acidic environment (pH=3) demonstrated that cement-based S/S added DTCR was better than cement-based S/S only. XRD and ESEM analysis revealed thatCa(OH)2, Hydrate Calcium Silicate (C-S-H) and Ettringite (AFt) as hydrationproducts would be formed in hydration reaction which wrapped up heavy metals andform a solidified body with certain strength. |
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