Adsorption characteristics and mechanism analysis of heavy metal Zn2+by cement-soil and alkali activated slag-bentonite-soil

With the increasing emphasis on low-carbon environmental protection, alkali activated cementitious materials are gradually being used as a substitute for cement in vertical cutoff walls for polluted sites. The composition of barrier materials significantly impacts the adsorption of heavy metals, thereby affecting the service life of cutoff walls. However, the adsorption characteristics and mechanisms of traditional cement-soil (CS) and alkali activated slag-soil (AASS) for heavy metals are still unclear. This study investigates the adsorption characteristics of Zn2+ using different mix ratios of barrier materials. Various mix proportions were designed, and qualitative and quantitative analyses were conducted using X-ray diffraction and thermogravimetric tests, respectively. The research results indicate that the adsorption of Zn2+ by barrier materials prepared from CS and slag-bentonite-soil (AASBS) is mainly chemical adsorption. The adsorption rate of heavy metals can be enhanced by increasing the cement content in the CS mix ratio. This is primarily because an increase in OH- in the hydration products encourages the formation of heavy metal precipitation. The generation of C-S-H and ettringite in the hydration products and ion exchange of heavy metals improve the adsorption of heavy metals. Increasing slag and alkali activator dosage, along with decreasing alkali activator modulus, promotes C-A-S-H generation and increases OH-, enhancing heavy metal precipitation. Increasing the content of bentonite in the AASBS mixture promotes the generation of C-A-S-H, and the interlayer structure of bentonite promotes the ion exchange of heavy metals. The findings of the study may offer a theoretical foundation for the installation of AASBS vertical cutoff walls in contaminated areas.