Modification On Structure And Surface Property Of Vermiculite From Linshou Hebei And The Mechamism Involved

Vermiculite?Vrm?is a 2:1 type phyllosilicate mineral with special crystal structure and physicaochemical properties and one of the predominant non-metallic mineral resources in China.Because of its unique two-dimensional layered structure,special interlayer space,and a large number of active groups,Vrm displays good surface reactivity,which leads to wide application in various areas such as materials,energy,and environment.“The structure of matter determines its properties”.By effectively controlling regulating the structure of Vrm,the surface acidity,adsorption performance,hydrophobicity could be significantly improved,thus broadening its applications.Despite of the significances in mineral utilization,Vrm is less studied compared with montmorillonite and mica,and many basic data regarding their structures and surface properties are still lacking or controversial.This is mainly attributed to two reasons.On one hand,most of the vermiculite samples studied at present are“interstitial vermiculite”.“Interlayer vermiculite”is an interlayer mineral formed by stacking vermiculite layer and mica layer according to certain regularity,different from Vrm mineral in structure and properties.Therefore,it is not accurate and sufficient to obtain the relevant information about Vrm through the study of“interstitial vermiculite”.On the orther hand,the previous research on the structure regulation of vermiculite minerals is mostly focused on the optimization of preparation conditions and the characterization of surface properties.In constrast,the essential relationship between the structure evolution and surface properties of vermiculite minerals is seldom involved.In order to investigate the structure and surface property and the corresponding mechamism involved of Vrm,the vermiculite minerals from Lingshou,Hebei Province were selected as the research object in our work.Based on the systematic study of the mineralogical characteristics of Vrm,the effects of hydrochloric acid concentration,acidizing time,thermal activation temperature,and organic modifier concentration on the structural changes of vermiculite minerals were investigated.The microstructure changes and the corresponding mechanism of vermiculite minerals during acid activation,thermal activation and organic modification process were revealed,and the relationship between microstructure and surface properties of vermiculite minerals and its microscopic mechanism were expounded.This paper has obtained the following valuable conclusions:1.The mineralogical characteristics of Vrm were systematically studied.Vrm from Lingshou,Hebei province is assigned to the trioctahedral mineral as defined.The average layer charge of Vrm is 0.61 eq/O₁₀?OH?₂?half unit cell?and inhomogeneous,which is balanced by primary Ca²⁺and a small amount of K⁺.Al and Si occupy the tetrahedral sites of vermiculite minerals,with a ratio of 1.07:2.93.The octahedron cations are mainly Mg²⁺and a small amount of Al³⁺and Fe³⁺.The vermiculite mineral has weak expansibility and the d₀₀₂ value varies with different ambient humidity.2.The microstructure evolution of acid-activated vermiculite minerals and the influence on surface acidity and mechanism are expounded.?1?The original structure of vermiculite mineral contains three kinds of Si:Q³?2Al?,Q³?1Al?,and Q³?0Al?.When the concentration of hydrochloric acid is low(?1.00 mol L^-1),the product consists of amorphous SiO₂ and unspoiled vermiculite minerals.Protons can penetrate into the interlayer space of vermiculite minerals,leading to the dissolution of the cations.Moreover,protons react with the structural hydroxyl group,and at the same time are adsorbed on the Si-O_Xb-Al of tetrahedral sheets and the Si-O_Xa-M linked with the octahedral sheets,forming Si-OH-Al and Si-OH-M.With the acid concentration further increasing,the tetrahedral Al dissolves,and the generated Si-OH reacts with partially dissolved Al,to form Q⁴?1Al?structural groups,resulting in the increase of Si-OH-Al.When the concentration of hydrochloric acid is high(>1.00mol L^-1),the products are amorphous SiO₂ containing Al^VI and three-dimensional cross-linked structure of Si-OH.?2?According to the main reactions in all the stages and the structural characteristics and formation mechanism of acidizing products,it is proposed that the acidizing process be divided into four stages:I)Protons attack the SiOH at end surface of the mineral,exchanging with the interlayer cations and hydroxyl reaction,and was then adsorbed on the structural negative-charge positions;II)fracture of Si-O_Xb-Altet and Si-O_Xa-Moct bonds?M is cationic Mg,Al and Fe in octahedron tablets?;III)dissolution of structural cations;IV)Si-OH condensation and structural rearrangement.?3?The amount of vermiculite mineral acid is 0.70 mmol g^-1,with the simultaneouspresence of B and L acid positions.The B acid positions are mainly derived from the absorpted of H₃O⁺at the negative electric field caused by replacement of silicon by aluminum in the silicon-oxygen tetrahedral sheets,interlayer polarized water,and Si-OH in end surface of vermiculite and amorphous SiO₂;while L acid comes from the broken aluminum bonds in the laminate structure.With the increase of hydrochloric acid concentration,the amount of total acid first increased and then decreased,and reached the maximum 1.40 mmol g^-1 when the concentration of hydrochloric acid was1.00 mol L^-1.Moreover,the amount of B acid first increased and then decreased,and the amount of L acid increased with the concentration of hydrochloric acid.?4?A mineral phase with a d₀₀₂ value of about 1.40 nm was formed in the early stage of acid-soluble vermiculite minerals.In the initial stage of acid treatment process,Si and Al began to dissolve.In the hydrochloric acid solution with high concentration,Al takes precedence over Si,while Si takes precedence over Al for dissolution in low concentration hydrochloric acid.Interlayer cations are quickly dissolved at the beginning of the reaction.The progresses of Si dissolution contains two stages:the rapid dissolution stage and the dissolution blocking stage.The structural cationic dissolved in order of is Mg²⁺>Al³⁺>Fe³⁺,which goes through three stages?i.e.,the rapid dissolution stage,the slow dissolution stage,and the dissolution blocking stage?.3.The microstructure,microscopic morphology,and pore evolution characteristics of vermiculite minerals under different heat treatment conditions were revealed.The heat-resistance temperature of the micro morphology and pore structure ofvermiculite minerals can reach 845°C.The dehydration process of vermiculite minerals contains several stages.The phase transformation process of vermiculite minerals during the heat treatment was showed as follows:vermiculite mineral with a d values of 1.43 nm?vermiculite minerals with a d value of 1.43 nm and 1.24 nm?90¹55°C??Interlayer structure phase with d value of 1.00 nm,1.16 nm and 2.4 nm?155²30°C??talc-like structure facieses with collapse interlayer?0.96 nm??600⁸45°C??enstatite?916¹200°C?.4.The non-uniform structure characteristics and formation mechanism of organic vermiculite minerals are demonstrated.With the surfactant increasing?from 0.2 to 3.0CEC?,the loading amount of surfactant on organic vermiculite increases,and the bottom spacing shows a one-step increasing mode(the maximum basal spacing of the modified organic vermiculite in 0.2CEC has reached 2.71 nm,closed to the bottom spacing?2.88 nm?that in 3.0CEC.The uneven distribution of layer charge in vermiculite minerals leads to the non-uniform structural characteristics of the coexistence of"non-polarity"and"polarity"of organic vermiculite.When the amount of surfactant added is less than 1.0CEC,the vermiculite mineral layer with low layer charge has priority organic cationic exchange and the basal spacing is 2.7⁴.5 nm.Some layers of vermiculite mineral laminates with high charge are exchanged by organic cations with the bottom spacing 1.44 nm,and the other part of the high-charge vermiculite minerals still retain the bottom spacing 1.48 nm of the original vermiculite minerals.When the surfactant is more than 1.0CEC,the bottom surface spacing of vermiculite minerals with low layer charge is 2.7⁴.5 nm.The basal spacing of vermiculite mineral laminates with high layer charge is 1.44 nm.The organic phase in organic vermiculite is stacked in an orderly manner between layers in the form of“like solid state”,and the hydrophobicity is enhanced.