The Crystal Structure Of Ground And Acidic Kaolinite And Their Interfacial Effects On Cu²⁺

Kaolinite(Al₄Si₄O₁₀（OH）₈),being abundant in nature environment,widely used in ceramics,paper,rubber,plastics,cosmetics,and among catalyst.As a low-cost adsorbent,it has attracted great interest in the field of the environment.However,the study shows that the adsorption capacity of natural kaolinite is so low due to the nonexchangeable cations between kaolinite and low specific surface area.Therefore,the study of the activation of kaolinite to improve the adsorption properties of kaolinite,and its application in environmental remediation is of great significance.In this study,the kaolinite was activated by mechanical grinding and acid treatment.XRD,FT?IR,TG?DTG,SEM,N₂ physical adsorption isotherm,29 Si and27Al MAS NMR spectroscopy,ICP-AES were adopted to investigate the effect of activation on particle size distribution,morphology,microstructure the dehydroxylation process,the Al and Si coordination environment,and Al and Si dissolution of Maoming kaolinite.Besides,the adsorption capacity of kaolinite before and after ball milling and acidification was studied by absorbing Cu²⁺ ions.The conclusions of the paper are as follows:（1）The high crystallinity Maoming kaolinite was used as the raw material,respectively,for ball milling 1,2 and 4 h.The characterization results showed that the hexagonal layer of kaolinite was crushed quickly in the first grinding stage,companied with the decrease of particle sizes and the increase of specific surface area until reached the maximum value（43.8 m2/g）in 1 h.The further grinding produced spheroidal aggregation of fine particles,and caused the decrease of the specific surface area.Grinding lead to extensive dehydroxylation and the adsorption water content increase.New signals at-100.5 ppm and 14.8 ppm 29 Si NMR and 27 Al NMR spectra respectively,were attributed to a Q3 Si?OH+?Al structure formed via protonation of the top oxygen interconnected between tetrahedral Si and octahedral Al.Grinding led to a gradual transformation from Al^Ⅵ（5.8 ppm）to AlIV（56.5 ppm）.A transition structure of AlV（31.2 ppm）was formed through dehydroxylation and evidenced this transformation.（2）The Maoming kaolinite was activated by 2 mol L^-1 and 8 mol L^-1 sulfuric acid at room temperature.The results showed that the acid treatment did not induce obvious change of the morphology or structure of kaolinite.The structure of the lamellae remains,the variation of pore structure is not obvious,while the infrared results showed that the kaolinite phase displays more ordered after acid treatment.With the acid activation progressed,the adsorption of water on the surface of kaolinite decreased,trace amount of Al ions is removed and a small part of the structure of hydroxyl loss.Activation of kaolinite with high concentrations of sulfuric acid favors the dissolution of Al and retards the dissolution of Si.（3）The ground kaolinite rebelled as MKG1 H,MKG2H and MKG4 H were activated by 2 mol L^-1 and 8 mol L^-1 sulfuric acid under ambient condition.Grinding activation destroys the structure of kaolinite,and then acid activation promotes the dissolution of polished kaolinite,accelerating its structure collapse and transition to amorphous state.Structure hydroxyl loss particles become loose porous,and the amount of water increase.27 Al spectrum at ca.15 ppm appears a newly signal corresponds to Al^Ⅵ site,and ca.-102 ppm in 29 Si spectrum corresponding to the signal of [Si（OSi）3OH（Q31OH）] attributes to the ?OH connected by Al,which formed by protonation.In acid activation process,prolonging the acid activation time and increasing the concentration of sulfuric acid can effectively promote the dissolution of kaolinite,strengthen the the protonation of the top oxygen which between the tetrahedral and the octahedron,and produce more amorphous SiO₂.The high concentration of acid promotes the dissolution of Al but inhibits the dissolution of Si in kaolinite.The theory of protonation of bridging oxygen can explain this phenomenon.The acid activation of high defect kaolinite exhibits greater performance than that of low-defect kaolinite.（4）Adsorption of Cu²⁺ on the ground kaolinite and its activated products at pH value of 5.5.The results showed that the adsorption capacity of ground kaolinite MKG1 H was the best due to its hugest specific surface area.In the acid-activated product of kaolinite,the adsorption of kaolinite acid activated by 2 mol L^-1 acids for 72 h was better than that of 8 mol L^-1,which might be attributed to the larger amount of the Al ions and the less active sites for the adsorption of Cu²⁺.Besides,the specific surface area of 8 mol L^-1 series samples was lower than 2 mol L^-1,which also resulted in the poor adsorption effect.After acid activation,the Zeta potential of the kaolinite samples was significantly reduced and the electronegativity was enhanced.