Study On The Efficiency And Mechanism Of Solidification Of Heavy Metal Pb And Zn Ions By Alkali-activated Slag Cement Materials

The heavy metals Pb and Zn can cause great harm to a variety of human organs and systems.Therefore,wastes containing Pb and Zn need to be properly disposed of before landfill or resource utilization.The widely applied method of using cement-based materials to solidify/stabilize heavy metal solid waste has the advantages of simple operation and low cost.Alkali-activated slag cementitious materials feature excellent mechanical properties and durability,thus can achieve ideal curing effects for heavy metals that are difficult to be solidified/stabilized by other cement systems,and are more suitable for multiple heavy metal stabilization.In this paper,heavy metals Pb²⁺and Zn²⁺are introduced into the alkali slag system in the form of nitrate,and the influence of preparation parameters such as alkali equivalent,heavy metal content,alkali type,and water glass modulus on the consolidation efficiency is discussed.Modern analysis and testing techniques such as XRD,TG-DSC,FTIR,SEM-EDS,BSE,TEM,ICP,XPS,etc.,are adopted to analyze the changes in the hydration products of alkali slag before and after the addition of heavy metals as well as the main immobilization mechanism.The medium and long-term stability of the alkali slag solidified forms under three simulated erosion environments,i.e.landfill site organic leachate,acid rain erosion,and sulfate erosion were compared.The main laws and conclusions obtained are as follows:Whether NaOH or water glass is used to activate the slag,it can achieve effective immobilization of 1%～3%Pb²⁺and 0.5%～2%Zn²⁺.The 1%content of Pb²⁺can slightly improve the 28d strength of the solidified body,and 1%Zn²⁺content will affect the early strength of the NaOH-activated cured form,and when it reaches 2%,the water glass-activated cured body cannot generate strength.For the cured forms with 3%Pb²⁺content,3%alkali equivalent can achieve high-efficiency curing.For the cured forms with 1%Zn²⁺content activated by water glass,when the alkali equivalent is less than5%,the 3d strength will be seriously affected.When using NaOH to activate Zn²⁺solidified forms,at least 4%of the alkali equivalent is required to ensure that the leaching concentration of each age meets the limit.The main hydration products of alkali slag include C-（A）-S-H gel,hydrotalcite phase,and hydrocalumite.C-（A）-S-H gel plays a major role in immobilize heavy metals Pb²⁺and Zn²⁺.After adding heavy metals Pb²⁺and Zn²⁺,the structure and morphology of C-（A）-S-H will change,indicating that C-（A）-S-H gel has a immobilization effect on Pb²⁺and Zn²⁺.Pb²⁺is equally distributed in C-（A）-S-H gel,while Zn²⁺generates precipitate particles clustering in C-（A）-S-H gel.Heavy metals will form precipitation in the alkaline hydration environment of alkali slag,mainly silicate precipitation and hydroxide precipitation.In addition,carbonate hydroxide precipitation may also be generated,but in this study,author did not find Ca Zn₂（OH）₆·2H₂O reported by other researchers.The precipitates will be concentrated on the surface of the hydration product,or be wrapped by the alkali slag matrix.In a simulated landfill organic leachate environment,the resistance of Pb²⁺solidified forms to acid corrosion is better than that of Zn²⁺solidified forms.Compared with the simulated acid rain environment,sulfate erosion poses a greater threat to the environmental safety of solidified forms.The resistance of Pb²⁺solidified forms activated by water glass is better than that activated by NaOH;and the Zn²⁺solidified forms activated by NaOH has better resistance to sulfate erosion than that activated by water glass.Although the effective diffusion coefficient D_e of each group in the semi-dynamic leaching test belongs to the order of 10^-13～10^-7,considering the ultra-high content of heavy metals in this study,the long-term stability and safety can be guaranteed in practical applications.