Effect Of Cement-based Demolition Waste On Immobiliazation,and Phytoavailability Of Cadmium In Soil And Its Mechanisms

With the rapid development of urbanization and industrialization,soil heavy metal pollution intensifies,among which cadmium pollution is the most serious.The accumulation of heavy metals in soils is an important issue because of long half-life,hysteretic and irreversible,which can pose serious harm to people through dust and food.Demolition waste consisting of cement paste,foam concrete is a waste generated by urban development.This material mainly exists in the form of inorganic salt Ca CO₃which has high p H value and high acid and alkali neutralizing ability.And it can be used as soil amendment agent for soil restoration.The chemical and mineral components,specific surface area,pore size,functional groups,surface characteristics and ion exchange capacity of three cement-based demolition wastes were analyzed by BET,XRD,FTIR,SEM and other chemical analysis methods,and studied by static adsorption test.Removal effect and mechanism of Cd removal in water by three cement-based demolition wastes.The effect of acid rain leaching on the stability of Cd in the soil after repair was studied by column leaching experiments.Then the soil contaminated by Cd was selected as the research object,and the influence mechanism of three kinds of cement-based demolition wastes on soil properties and Cd occurrence patterns after Cd-contaminated soil was investigated.Finally,through the pot experiment,the effects of three cement-based demolition wastes on the availability of Cd plants after application of Cd-contaminated soil were investigated.The main findings are as follows:(1)The surface of the three cement-based demolition wastes contains a large number of mesopores.The cation exchange capacity of the three materials(28.8-48.2 cmol/kg)is similar to that of biochar(3-50 cmol/kg).The content of heavy metals in leach solution is less than the limit value of national standard,which will not cause the secondary heavy metal pollution of soil.It has a high ANC value(6,700-13500 meq/kg)and has a significant advantage in excellent adsorption performance.(2)The adsorption of Cd²⁺by three cement-based demolition wastes is well expressed by Langmuir model and Freundlich model,which is uneven multilayer physical adsorption.The maximum theoretical adsorption capacities of OPC,FC and ZEO were 10.97 mg/g,9.47 mg/g and 4.63 mg/g,respectively.In accordance with the pseudo-second-order kinetic model,the adsorption process is the compound effect of multiple adsorption mechanisms,and Cd is also adsorbed through precipitation,ion exchange and other processes such as carbonic acid groups and alkali metal oxide salts.(3)In the soil column simulation acid rain leaching experiment,the application of three kinds of cement-based demolition wastes increased the p H value of soil leachate,and the p H value of soil leachate increased with the increase of the addition amount of three kinds of cement-based demolition wastes.After 6000 ml simulated acid rain leaching,the0-10cm soil layer of OPC,FC and ZEO stabilized soil column still contained 96.0%?99.8%of Cd locked in contaminated soil.(4)The soil culture test showed that all the treatment groups had a significant increase in soil p H and CEC,and increased with the increase of the applied amount.And it increased the activity of soil urease and FDA hydrolase.It was found that all treatment groups significantly reduced the exchangeable form Cd fraction contents,while the contents of the residual Cd increased significantly,and the bioavailability of Cd was reduced by adsorption fixation.The content of TCLP extracted state of Cd in OPC treatment group decreased by 89.0%,93.4%and 94.7%,respectively;the FC treatment group decreased by 80.3%,87.2%and 87.6%,respectively;the ZEO treatment group decreased by 56.0%,67.0%and 81.0%,respectively,and there were significant differences between them and the control group.(5)the pot experiment results showed that the optimal addition amount of the three materials was 20 g/kg,and the plant height and root length of the three materials reached the optimal length.The content of Cd in the plant decreased with the increase of three kinds of materials.