Surface-Mineralogical Mechanism Of Acid Disolution, Adsorption, And Desorption For Chain-Layer Mineral: Palygorskite

Based on systemic mineralogical study of two attapulgite clays from Jiangsu and Anhui Province, the pink and gray attapulgite clay which were collected from Guanshan deposit in Mingguang, Anhui Province, were selected to be purified for magnesium-rich and magnesium-poor palygorskite. The polycrystalline X-Ray Diffraction study show that the magnesium-rich palygorskite belongs to orthorhombic system, magnesium–poor palygorskite is a complex of monoclinic and orthorhombic ones. Magnesium-poor one is gray in color, and the other is pink. The R2+/R3+ ratios of octahedral layer are 1.4 for magnesium-poor palygorskite and 3.8 for magnesium-rich one. The FTIR and diffused NIR results has proved that the magnesium-rich palygorskite is a tri-octahedral mineral, and the magnesium-poor one is one kind of mineral intervened between tri-octahedral and di-octahedral.Systemic acid leaching experiments were carried on two magnesium-rich and magnesium -poor palygorskite. The results show that both palygorskite have the same trend for Si4+ leaching, but the different leaching trend of octahedral cations can be seen in those two palygorskite during acid leaching process. As 1 mol/L HCl solution was employed, the Si4+ concentration increased gradually. However at 3 and 5 mol/L HCl, the trends of Si4+ concentration in leachate versus leaching time could be divided into 3 steps. For magnesim-poor palygorskite, the concentrations of octahedral cations are increasing with the increase of leaching time and the concentration of HCl solution. However, the same leaching trend was only seen in 1 mol/L HCl solutions for magnesium-rich palygorskite. It is obvious that three steps can be considered for magnesium-rich one. The conditionality of the leaching behavior of Mg2+ and Si4+ should be considered for the"characteristic three step trend"in 3 and 5 mol/L HCl solution for Si4+ and magnesium-rich one.The Mg2+/(Al3++Fe3+) ratio increases steadily with leaching time and then reaches a constant value of 2 for magnesium-poor palygorskite, and >4 for magnesium-rich palygorskite in leached solution of 1 mol/L HCl. However as it leached by 3 and 5 mol/L HCl, the Mg2+/(Al3++Fe3+) ratio reachs a maximum value in a short time (<1.5 h), and then decreases to a constant value of 2 and 4. The different dissolution among different concentration of HCl solution indicates that the concentration has influenced the extraction rate of Mg2+. Based upon this fact, it is believed that at the earlier stage of dissolution by 1 mol/L HCl, the extraction of the cations is controlled by diffusion mechanism, i. e. the cations diffuse from the crystal surface to the solution, and the H+ from solution to the surface. Initially, not only the octahedral cations, but also the tetrahedral cations with Al3+ in tetrahedron site are exposed on the surface of the crystal. Therefore, the Mg2+/(Al3++Fe3+) ratio is lower. The chemical reaction gradually becomes a leading factor along with the developing of the dissolution. The preferential leaching of Mg2+ results in the gradual increase of Mg2+ concentration. The penetration of the H+ from solution to the inner structural channels of the crystal makes the leaching throughout the channels, and the Mg2+/(Al3++Fe3+) ratio reach a constant value. In a word, two leaching state should be considered: diffusion mechanism and chemical reaction mechanism.The powder XRD results show that the solid leachate is still palygorskite as the activating time less than 7 h. When leaching time is longer enough to 360 h, the diffraction peaks become wider and split into several peaks. Finally, a big and large overlapped peak is formed at range of 20 to 30o. The microtopography of TEM and AFM show that more and more amorphous spherical particulate is formed along with leaching time. There is a large amount of amorphous spherical particulates in 360 h acid leached solid leachate. The amount of SiO2% is bigger than 60% by XRF analysis. There is only absorption peak related to Si-O, Si ? HO: ?Siand Si ? OH:?Al in FTIR spectra. The above-mentioned facts suggest that the amorphous spherical particulate is amorphous silica.The bands near 2300nm become vague and fade away in the diffuse NIR spectra of pink relicts along with leaching time. The bands between 1300 and 1600nm of activated palygorskite are much wider than un-activated one. The diffuse spectra of unactivated and activated gray palygorskte are slightly different. The above-mentioned facts indicate that the different chemical composition brings the different spectra. The pink one is a magnesium-rich palygorskte, and the gray one is a magnesium-poor palygorskite. It is the different contents of Mg2+ and the preferential selectivity of Mg2+ that bring the different FTIR, NIR spectra and different activation mechanism.The EPR, FTIR spectra of a magnesium-rich palygorskite, 4 h acid activated two magnesium-rich ones and one magnesium-poor one indicate that the Cu2+ is wrapped in the micro-channel of crystal structure and in the hexagonal cavities of Si-O sheet and a small fraction penetrate into the octahedral vacancies. XPS spectra show that two valences appear on the surface of un-activated palygorskite, and Cu+ can be found on the surface of activated palygorskite. The further work should be carried to exoatiate on why two valences exist. In a word, there are four different occurrences of Cu ions in palygorskite: 1) adsorbed on the surface, 2) entering into the micro-channel of crystal structure, 3) adsorbing in the hexagonal cavities; and 4) penetrating into the octahedral vacancies.The desorbing experiment of Cu-adsorbed palygorskite indicates that the chemical equilibrium is reached less than 15 minutes. The amount of desorbed Cu2+ is less than 1%. The above-mentioned facts suggest that the Cu2+ ions wrapped in micro-channels, in hexagonal cavities, and in octahedral vacancies are not accessible to H+, difficult to be desorbed. The desorbed Cu2+ ions come from the surface. It seems that the palygorskite is an outstanding materials to solve the polluted water environment.