Adsorption Behavior And Mechanism Of Heavy Metal Ions In Geopolymers

With the rapid development of industrialization in China,the production wastewater and waste slag are increasing.Toxic pollutants such as heavy metal ions,organic matter and inorganic matter are discharged into the natural environment together with these wastes,causing serious damage to human life and health and the stability of ecological system.At present,adsorption method as one of the most economical and effective methods to deal with heavy metal ion pollution.Geopolymers are widely used in the removal of heavy metal ions from wastewater and solid wastes because of their cheap and easy raw materials,simple preparation process,low energy consumption,and advantages of large pores,specific surface area and no secondary pollution.Therefore,this thesis adopts the first-principles calculation method based on density functional theory to investigate the geometric and electronic structure,surface properties and adsorption properties of geopolymers.The main results are as follows:1.The first-principles study shows that the process of geopolymer adsorption of heavy metal ions is a spontaneous exothermic process.The terpolymer trimer is an indirect band gap,and its value is 4.269 e V.Compared with the other three ions,N-A-S-H has the best adsorption effect on Sr²⁺,Mn²⁺and Zn²⁺.The partial density of states of heavy metal ions and near-neighbor geopolymers indicates that there is a formant between O atoms and heavy metal ions,indicating that hybridization occurs between their orbitals.The charge density difference results show that there is a charge transfer between heavy metal ion and N-A-S-H,and the charge is transferred from heavy metal ions to N-A-S-H.Mulliken’s analysis showed that chemical bonds were formed between heavy metal ions and N-A-S-H,and the number of electrons loss by heavy metal decreased with the decrease of its ionic radius.2.The research on the structural model of geopolymer basic structural units Si（OH）₄ and Al（OH）₄ adsorbed on the surface of Hg Cl₂ crystal（010）found that the adsorption sites of basic units were obtained by molecular orbital linear analysis using Dmol³ module,which were H atom and O atom respectively.The adsorption results show that the adsorption of Hg Cl₂ by Si（OH）₄ is mainly through physical adsorption.In the adsorption of Al（OH）₄,H atom as the adsorption site showed physical adsorption,while O atom as the adsorption site showed chemical adsorption.When O atom is used as the initial adsorption site,strong hybridization occurs between p and s orbitals of O atom in Al（OH）₄ molecule and d orbitals of Hg atom on the surface of（010）,indicating that Hg-O bond has strong bonding properties.Al（OH）₄ has a stronger affinity for the surface of Hg Cl₂ crystal（010）than Si（OH）₄.3.The study on the basic structural unit Al（OH）₄ adsorbed on the surface of CaSO₄crystal（010）shows that:when Al（OH）₄ adsorbed on the surface of CaSO₄ crystal,indicating that charge transfer occurs between the adsorbent and the adsorption substrate.Al（OH）₄ is mainly adsorbed on the surface of CaSO₄ crystal（010）by chemisorption,and the adsorption energy is the maximum when S is the adsorption site,indicating that the system adsorption is the most stable when S is the adsorption site.The analysis of density of state showed that the resonance between the two was significant when S was used as the adsorption site,which was consistent with the Mulliken settlement results.The strong hybridization between the p and s orbitals of the O atom in Al（OH）₄ molecule and the p orbitals of the O atom on the surface of（010）indicates that the two atoms have strong bonding properties.The charge density difference results analysis shows that when Al（OH）₄ is adsorbed on Ca6,the charge is transferred from Al（OH）₄ to CaSO₄,indicating the charge transfer mechanism between the two.Therefore,Al（OH）₄ has a strong adsorption affinity on the surface of CaSO₄.