Effect Of Low Molecular Mass Organic Acids On Dissolution And Recrystallization Of Clay Minerals

Smectite clay minerals play an important role in geochemical processes,for example,low molecular weight organic acids to promote their dissolution to provide large amounts of aluminum and silicon elements for the soil to achieve the desilicification and aluminization process of the soil.With chromite mining and rock weathering,the heavy metal chromium is released into the soil,and its environmental hazards are becoming increasingly prominent.Smectite clay minerals have surface and interlayer adsorption capacity to immobilize soil contaminants,and they have the operability and important environmental significance of soil chromium adsorption.In addition,the structural aberration formed by the smectite clay minerals in the process of dissolution and recrystallization can enhance the maximum adsorption of heavy metals on their surfaces and between layers,which has become a research hotspot for the preparation of adsorbent materials.In summary,it is necessary to further explore their dissolution and recrystallization processes to provide theoretical insights into the understanding of geochemical processes and structural aberrations.In this thesis,three typical clay minerals of the smectite,montmorillonite,beidellite and nontronite,were studied in combination with experiments and simulations to illustrate the driving effect of low molecular weight organic acids on the dissolution and recrystallization of clay minerals and the mechanism of structural aberrations formed in this process,and the electronic structure differences were determined from the bonding properties of the three clay minerals for the adsorption of dichromate,and finally,it was confirmed that there is a difference in the amount of dichromate adsorption between the structural distortion and the ordinary pillaring.Based on the above theoretical basis and understanding,the specific research contents and conclusions are as follows.1.Low molecular weight organic acids promote the dissolution and recrystallization of montmorillonite.During the dissolution process,the tetrahedral and octahedral linkage bonds were elongated from 0.160 nm to 0.161-0.182 nm by adsorption of low molecular weight organic acids to the edges of the minerals,resulting in the stripping of the tetrahedron from the octahedron.The negative charge of the carboxyl oxygen atom of the organic acid increases by about 0.2 e,which generates an electrostatic force of strength 1.26 e V with the other crystal layer and closes the distance between the two crystal layers,creating a gradual d₀₀₁.During the recrystallization process,due to the charge redistribution of mineral crystals and organic acid adsorption and electrostatic force pulling at the edge positions,the Si-O tetrahedron were peeled off from the octahedron and precipitation as Si O₂ spheres on the mineral surface occurred,which was confirmed by thermogravimetric analysis and scanning electron microscopy.Meanwhile,simulation calculations revealed that the reaction transition state of the process is the completely dissolved state where the tetrahedral structure becomes disordered.The process of dissolution and recrystallization of montmorillonite driven by low molecular weight organic acids occurs:pillaring occurs and interlayer ions are replaced,adsorption sites are transferred from interlayer to surface and end face,organic acid adsorption leads to tetrahedron stripping,solution cations replace crystal ions,electrostatic interaction forms structural aberration.2.Under experimental conditions,low molecular weight organic acids drive the local dissolution of tetrahedron and octahedron of beidellite and nontronite.Thermogravimetric analysis revealed that none of the reaction systems showed the third heat absorption valley,combined with the shortening of the bond lengths of tetrahedron and octahedron of beidellite and nontronite from 0.184,0.166 nm to 0.175-0.182 nm and 0.165-0.166 nm,confirming that the tetrahedron were not exfoliated.Mulliken charge found that the Al-O tetrahedron of beidellite raised the charge of octahedral aluminum atoms by 0.3 e.Therefore,the decrease of electron giving ability of aluminum atoms leads to local dissolution of crystals.X-ray fluorescence spectroscopy chemical formula analysis indicates the existence of octahedral central atomic vacancies in beidellite and nontronite as a possible cause of local dissolution.In addition,X-ray diffraction and transmission electron microscopy results indicate the presence of 0.23,0.22 e V electrostatic forces in the system to form structural distortions.The process of local dissolution of crystals occurring in beidellite and nontronite:organic acid adsorption at the end face,adsorption leads to crystal deformation and local dissolution,and electrostatic interaction forms structural aberration.3.Based on the differences of experimental phenomena of three clay minerals,experiments and simulations of adsorption of dichromate were performed for montmorillonite,beidellite,nontronite and magnesium-substituted montmorillonite.The adsorption amounts of montmorillonite,beidellite and nontronite were 12,9 and 8 mg/L in solutions with an initial concentration of 30 mg/L.The bond strengths of the four minerals for the adsorption of dichromate were 0.48 e V,0.50 e V～0.52 e V,0.12 e V,-0.47 e V.It was found that beidellite was the most unstable for the adsorption of dichromate,and explained the lowest amount of adsorption.The electron localization function found that montmorillonite,beidellite and nontronite all formed adsorption covalent bonds,while the octahedra of magnesium-substituted montmorillonite formed stable structures with ionic bonds.The electron density of states found that the p-orbital electrons of magnesium ion appeared only at-40 e V,and no energy overlap occurred with the p-orbital electrons of dichromate at-7.5 e V～0 e V,so it was concluded that the two did not form adsorption bonds.The adsorption experiments revealed that the adsorption increment of structural distortion was 7.08 mg/g and the structural distortion provided 0.75 e V electrostatic force to immobilize the dichromate.