Preparation Of Nanosilica Immobilization Agent For Contaminated Heavy Metal Soil And Its Application Performance Research

Heavy metal pollution in soil,especially soil pollution caused by the coexistence of multiple heavy metals,has become one of the major environmental hazards associated with the rapid urbanization and industrialization worldwide.At present research,physical,chemical and biological methods are often used to repair soil contaminated by heavy metals.Among them,in-situ solidification/stabilization technology,as a representative chemical remediation method,can reduce the migration ability and bioavailability of heavy metals and reduce the ecological environment risk of heavy metals in contaminated soils,thereby eliminating pollutants and repairing contaminated soils.In-situ solidification/stabilization technology is simple to operate and easy to scale up,attracting the attention of many researchers.Nano-silica is more and more used for remediation of heavy metal contaminated soils and water due to its large surface area and environmental friendliness.However,nano-silica only has the ability to adsorb heavy metal ions,instead of immobilizing heavy metal ions.In this paper,thiol-modified nano-silica(RNS-SH),mercapto propyltrimethoxysilane-and ferrous sulfate-modified nano-silica(RNS-SF),mercapto-and ethylenediamine-modified nano-silica(RNS-SE)were synthesized by using nano-silica as the carrier and grafting organic compounds containing immobilization groups on the surface.It is hoped that on the basis of the large specific surface area and pore structure of nano-silica,the adsorption-passivation ability of surface fuctionalized nano-silica to heavy metals can be improved.The main content and research results of this article are as follows:1.Preparation and stability research of mercapto propyltrimethoxysilane-and ferrous sulfate-modified nano-silicaThe morphology of the immobilization agent was characterized by transmission and scanning electron microscope.The physical and chemical properties such as loss on ignition,specific surface area and apparent density were determined.Compared with RNS-SH,RNS-SF has higher immobilization efficiency of heavy metals in soils,and the stability of immobilization agent is improved at the same time.The effects of storage state,storage conditions and storage time on the stability of the immobilization agent were evaluated by examining the immobilization performance of the immobilization agent on heavy metals in the soil.The results show that after five months of storage,the immobilization ability of the powdered immobilization agent for heavy metal lead is 99.4%,better than 86.1%of the filter cake state.After 110 days' storage in ambient conditions,the immobilization rate of the immobilization agent for lead is 87.3%,whereas the immobilization agent stored in vacuum and dark conditions maintained an immobilization rate of>91.5%for lead.Moreover,the extension of the immobilization time and increase of water content in soils can further improve the immobilization efficiency of the immobilization agent for heavy metals.2.The immobilization performance and action principle of RNS-SF on heavy metals lead,and cadmium as well as arsenic in contaminated soilThe research results show that the immobilization efficiency of RNS-SF for heavy metals in the soil increases with the increase of the addition ratio.The immobilization agent shows excellent performance for the remediation of the contaminated soil.Specifically,with a dosage of 3.0%(mass ratio)in the soil,it can immobilize bioavailable lead,cadmium,and arsenic by 97.1%,85.0%,and 80.1%,respectively.When 6%immobilization agent is added,all of the EXC and part of the Carb fractions of lead,cadmium were transformed to FeMnOx,OM,and RES fractions.The lead/cadmium immobilizated product has good stability against acid and alkali within the experimental range(pH value 3-10).After addition of 3.0%RNS-SF,98.6%of H2O-As,86.8%of AE-As and 42.3%of Al-As were transformed into stable arsenic in the form of residual precipitates.The immobilized arsenic product has good acid resistance stability(pH value 3-6).The immobilization agent can generate-S-M-S mercapto metal compounds with lead and cadmium,and react with arsenic to generate Fe3(AsO4)2 or FeAsO4 precipitation,thereby immobilizing the lead,cadmium and arsenic.3.Preparation of mercapto and ethylenediamine modified nano-SiO2(RNS-SE)and its immobilization effect on heavy metal in contaminated soilThe experimental results show that the introduction of ethylenediamine can improve the immobilization performance of the immobilization agent for heavy metals in the soil,and reduce the use of thiol functional groups by 5.9%under the same immobilization efficiency,which achieves the purpose of reducing product cost.At the same time,the introduction order of mecapto and amine groups has a great influence on the immobilization performance of the prepared immobilization agent.The simultaneous addition of mecapto and amine groups can lead to the RNS-SE having a higher immobilization efficiency for heavy metais(the immobilization rate for lead,cadmium and copper were 91.4%,95.1%and 98.6%respectively.)When addition of 6%RNS-SE immobilization agent which was surface modified with 1.60 mmol/g mercapto and 3.0 mmol/g ethylenediamine,the immobilization rates of heavy metals such as lead,cadmium and copper,were 98.1%,99.5%and 99.7%respectively.The immobilization agent can transform all of the EXC,part of Carb fractions of lead,cadmium were transformed into and FeMnOx,OM,and RES fractions.