(School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009)
Attapulgite is an important one-dimensional nano-mineral material. It is of great theoretical and practical significance for the application of attapulgite clay to deeply understand its nano-mineralogical characteristics and surface properties. In this paper, the pure attapulgite samples were finely separated by X-ray powder diffraction analysis and transmission electron microscope detection. On this basis, the specific surface area and pore size distribution, cation exchange capacity, isoelectric point pH value, equilibrium pH value of aqueous solution and the interface between attapulgite and heavy metal ions were determined by BET specific surface area analyzer, method and salt titration respectively, which provided basic data for correctly understanding attapulgite nanoparticles.
Attapulgite; Surface properties; Nanomaterials; Nanomineralogy.
Introduction to the first author: Chen Tianhu (1962-), professor, mainly engaged in environmental mineralogy and mineral environmental materials research. E-mail: chentianhu168 @ vip.sina.com.
I. Introduction
Attapulgite is a magnesia-alumina silicate clay mineral with special structure, morphology and physical and chemical properties. There are two genetic types: sedimentary genesis and hydrothermal genesis. Attapulgite is widely used in clay mineralogy, materials science, physical chemistry, soil science, environmental engineering and archaeology because of its special geological environment and great potential application value [1]. Since the discovery of the Suzhou-Anhui attapulgite clay mineral belt with mingguang city, Lai 'an, Xuyi and Liuhe counties as the main mining areas in Anhui Province in the early 1980s, China has been continuously exploring and developing it, making the Suzhou-Anhui attapulgite clay mineral belt one of the most important attapulgite clay mineral-intensive areas in the world and an important large-scale characteristic nonmetallic deposit in China [2]. For more than 20 years, domestic related units have conducted some research on the processing and application technology of attapulgite clay resources. However, due to the fine particles of attapulgite, the complex composition of clay minerals and the limitation of resolution of previous research methods, there are still some misunderstandings about the physical and chemical properties of attapulgite in Jiangsu and Anhui [3]. The attapulgite used by many scholars in adsorption experiments in the past is actually attapulgite clay, and the samples contain many impurity minerals, such as montmorillonite, illite and carbonate. Moreover, different researchers have different sampling locations and horizons, so the mineral composition must be very different. The interference of these impurities seriously affects the knowledge and understanding of the physical and chemical properties of attapulgite, and also makes the experimental results of various scholars lack comparability. Due to the difficulty of sample purification and separation, there is still a lack of systematic experimental data on the surface properties of pure attapulgite in attapulgite clay in Jiangsu and Anhui provinces. On the basis of fine purification, this paper systematically studies the surface properties of attapulgite, which is of great theoretical and practical significance for understanding the nano-effect of attapulgite, correctly understanding the mineralogical characteristics of attapulgite and further developing the application technology of this nano-material.
Preparation and Characterization of Pure Attapulgite Samples
The mineral types of sedimentary attapulgite clay in Jiangsu and Anhui are attapulgite clay, dolomite attapulgite clay, montmorillonite attapulgite clay, opal attapulgite clay and montmorillonite clay (attapulgite content < 10%). The mineral composition of sedimentary attapulgite clay is complex and changeable. In order to obtain pure attapulgite samples, on the basis of X-ray diffraction (XRD) analysis and transmission electron microscope (TEM) observation, the ore samples containing only attapulgite clay minerals but not illite minerals were selected for separation and purification. See references [4-5] for specific purification methods and operation steps. After XRD analysis and TEM detection, the content of attapulgite in the purified sample reached more than 98%. The purified attapulgite sample was dried, crushed and passed through a 180 mesh sieve for later use. Hydrothermal attapulgite is taken from Dalongshan, Feidong, Anhui Province. The pure ore is hand-selected, crushed and ground through a 200-mesh sieve, and the purity is more than 98% by XRD analysis and TEM inspection. X-ray analysis adopts D/MAX-RB X-ray diffractometer, copper target, voltage 40kV, current 100 mA, scanning speed 4 /min, and the analysis is completed in the Physical and Chemical Center of Hefei University of Technology. Tem study uses JEOL20 10 high-resolution transmission electron microscope with "Islamic State" X-ray energy spectrum analysis system to obtain TEM images, electron diffraction (SAED) and energy spectrum analysis components (EDS), and the analysis is completed in the Department of Earth Sciences of the University of New Mexico.
Three. Results and discussion
Morphological characteristics of (1) two genetic attapulgite crystals
Figure 1 is a transmission electron microscope image of attapulgite from two producing areas. Hydrothermal attapulgite crystals have a diameter of about 50 nanometers and a length ranging from several hundred nanometers to several tens of microns. The diameter of the deposited attapulgite crystal is about 40 nanometers, and the length varies from several hundred nanometers to several microns. Obviously, the crystallinity of deposited attapulgite is lower than that of hydrothermal attapulgite, which is consistent with the results of XRD analysis.
Figure 1 transmission electron microscope image of attapulgite
Feidong hydrothermal attapulgite crystal; B- Guanshan sedimentary attapulgite
Fig. 1A also shows that the attapulgite crystal arches on the carbon film of copper mesh special for transmission electron microscope, which is the most direct evidence of the elasticity of attapulgite. The discovery of this phenomenon has important theoretical and practical significance for understanding the properties of attapulgite materials and studying attapulgite materials. Whether attapulgite is used as adsorbent or as catalyst carrier material, maintaining mesoporous, gas component permeability and high mass transfer performance is a prerequisite for exerting adsorption and catalytic performance. After the attapulgite clay is fully dispersed, the attapulgite crystal bundles can be dispersed into single fibers, and the single attapulgite fibers are randomly stacked to form solid substances. In the process of processing, due to the elasticity of attapulgite crystals, attapulgite products can have high porosity and keep nano-scale pores between grains. In fact, this also explains the origin of attapulgite's excellent adsorption and catalytic performance. The observation and research results of this nanometer scale have important guiding significance for the research on the processing methods of attapulgite materials.
(ii) cation exchange capacity
The cation exchange capacity of attapulgite was determined by adsorption method. Firstly, an accurately weighed soil sample is adsorbed with a solution of 0.1mol/L. After adsorption is balanced, the supernatant is centrifuged to determine the Cu2+ content before and after adsorption, and the adsorption capacity of the soil sample to be detected is calculated to obtain the cation exchange capacity of the sample to be detected. The results show that the ion exchange capacity of sedimentary attapulgite is 10.87mol/ 100g, and that of hydrothermal attapulgite is 8.4mol/ 100g. The results show that the ion exchange capacity of purified attapulgite samples is relatively low, and many scholars believe that the ion exchange capacity of attapulgite clay is high, which is due to the fact that the clay contains more montmorillonite. The ion exchange capacity of montmorillonite is 20 ~ 30 times that of attapulgite. As far as ion exchange capacity is concerned, attapulgite is not superior to montmorillonite. In the past, there were many misunderstandings about the physical and chemical characteristics of attapulgite due to the lack of ion exchange capacity data of high purity samples of sedimentary attapulgite.
(3) pH value of isoelectric point
The isoelectric point is measured by salt titration. The steps are as follows: (1) Add 0.5000g sample equivalent to the dry weight into several 50 mL centrifugal tubes, and add a proper amount of distilled water and 0.0 1 mol/L HCl or NaOH solution to make the final volume of the solution in the tubes 10 mL, and make the pH distribution within a suitable range (pH =/kloc-0 (2) Balance at 25℃ for 3 ~ 4 d, during which it oscillates for 65438+/-0 h every day, and then measure the pH value of the suspension in each test tube, and record it as pH. (3) Add 0.5 mL 2 mL/L NaCl solution into each test tube, oscillate for 4 h, and then measure its pH value, and record it as pHl. ④ Calculate the δ pH of each branch δ pH (δ pH = PHL-pH). Taking pH as the abscissa and δ pH as the ordinate, the corresponding pH value when δ pH = 0 is the charge zero point (PZC) of the sample.
As can be seen from Figure 2, the charge zero point PZC of Guanshan sedimentary attapulgite is 4.43, and that of Feidong hydrothermal attapulgite is 6.98. The great difference of isoelectric point pH between sedimentary attapulgite and hydrothermal attapulgite may be related to the great difference of octahedral cation composition. The former is relatively rich in iron and magnesium while the latter is relatively rich in aluminum [5].
Fig. 2 Diagram of δ pH-pH relationship of attapulgite determined by salt titration.
(4) The specific surface area and pore size distribution of attapulgite
Determination of specific surface area (SBET) of pure attapulgite by BET-N2 adsorption-desorption method. The specific surface area tester is the ASAP20 10 adsorption instrument of Micromiritics company. The test experiment was completed in the College of Engineering, University of New Mexico, USA. The results show that the specific surface area of deposited attapulgite is 204.5 m2/g, and that of hydrothermal attapulgite is106.4 m2/g ... The specific surface area of bet-N2 is much smaller than the theoretical surface area. There may be two reasons: First, N2 molecules are not adsorbed on all attapulgite surfaces, and adsorption is controlled by the distribution of active centers. Secondly, attapulgite mostly presents crystal bundles, and the crystals are connected in parallel, which increases the crystal diameter, resulting in the specific surface area being less than the theoretical value. The specific surface area of hydrothermal attapulgite in Feidong is much smaller than that of precipitated attapulgite in Guanshan. Transmission electron microscope observation and X-ray diffraction analysis show that the crystal diameter of hydrothermal attapulgite in Feidong is much larger than that of chemically precipitated attapulgite in Guanshan. The results show that the specific surface area of BET-N2 is closely related to the crystal diameter of attapulgite, and the BET-N2 test is mainly about the external surface area, because the internal surface area has nothing to do with the crystal diameter.
Fig. 3 is a diagram of pore size distribution of deposited attapulgite, showing that the pore size distribution has two peaks, the main peak is at 10 ~ 100 nm, slightly deviating from the normal distribution, and the secondary peak is at 3 ~ 4 nm. According to the nano-scale observation of transmission electron microscope, the pore size distribution is related to the particle spacing of attapulgite aggregates. The voids of 3 ~ 4 nm may be intergranular voids in attapulgite crystal bundles, and the pores of 10 ~ 100 nm may be intergranular voids formed by random stacking of attapulgite rod crystals. The characteristics of pore size distribution show that the specific surface area of N2 is the surface area of attapulgite crystal.
Fig. 3 d(V)/dlg(D) desorption pore size distribution.
(5) Hydrolysis in aqueous solution
The equilibrium pH of sedimentary attapulgite and distilled water is 9.09, and that of hydrothermal attapulgite and distilled water is 8.63. The results show that due to protonation of attapulgite in aqueous solution, the solution is weakly alkaline, and the alkalinity of deposited attapulgite is stronger than that of hydrothermal attapulgite, which is because deposited attapulgite is richer in magnesium than hydrothermal attapulgite. Therefore, the surface of attapulgite in aqueous solution shows the properties of solid alkali, and the interaction between attapulgite and metal ion solution also illustrates this point. Fig. 4 shows that the pH value of the aqueous solution gradually increases after the attapulgite reacts with the nickel solution with a concentration of 200 mg/L (pH = 5.2) for a long time, and the final pH value depends on the solid-liquid ratio of the system. The solid-liquid ratio is high, the system alkalinity is high, and the final pH value is high. Fig. 5 shows that the long-term interaction between attapulgite and nickel solution with a concentration of 200 mg/L (pH = 5.2) leads to the formation of nickel hydroxide, indicating that the interaction between attapulgite and heavy metal ions can induce the hydrolysis and precipitation of heavy metal ions due to the surface characteristics of attapulgite. Because the surface of attapulgite crystal is negatively charged, the hydrolysate of heavy metal ions is positively charged, and the hydrolysate of heavy metal ions is evenly coated on the surface of attapulgite. In the process of calcination or reduction calcination, attapulgite can control the migration of metal (oxide) particles and obtain ideal attapulgite-metal (oxide) nanocomposites, which is also one of the important surface characteristics of attapulgite. Using this characteristic of attapulgite, attapulgite-metal (oxide) nanocomposites can be prepared conveniently (Figure 5).
Fig. 4 Long-term pH change of attapulgite and nickel solution (left) and induced hydrolysis and precipitation of Ni2+ (right).
Fig. 5 TEM image of attapulgite-metal (oxide) nanocomposites.
Attapulgite-gold composite; Boron-attapulgite-silver composite; Carbon-attapulgite-copper composite; Attapulgite-titanium dioxide composite material
Four. conclusion
After strict screening and separation, two kinds of high-purity attapulgite samples were obtained, and the surface property parameters of attapulgite were obtained through systematic analysis. The specific surface area of sedimentary attapulgite in Jiangsu and Anhui is 204.5 m2/g, and that of hydrothermal attapulgite is106.4 m2/g. The specific surface area measured by BET-N2 adsorption method is the external surface area of attapulgite, and the pore size of the obtained attapulgite is mainly intergranular voids formed by disordered stacking of rod-like crystals. The cation exchange capacity of deposited attapulgite is 10.87mol/ 100g, and that of hydrothermal attapulgite is 8.47mol/ 100g. The pH value of deposited attapulgite is 4.43, and that of hydrothermal attapulgite is 6.98. The equilibrium pH of sedimentary attapulgite and distilled water is 9.09, and that of hydrothermal attapulgite and distilled water is 8.63. Attapulgite has the property of solid alkali, which can induce hydrolysis and precipitation of heavy metal ions. A series of attapulgite-metal (oxide) nanocomposites can be prepared by induced hydrolysis of attapulgite.
refer to
Chen Tianhu, Peng Shuchuan, Xu, Huang Chuanhui. Study on adsorption mechanism of Cu2+ by attapulgite. PEDOSPHERE, 2005, 15(3):334-340.
[2] Chen Tianhu, Xu, Lu Anhuai, Xu, Peng Shuchuan and Yue. Direct evidence of the transformation from smectite to palygorskite: transmission electron microscope study [J]. China Science (Series D), 2004,47 (11): 985-994.
[3] Chen Tianhu. Research Status and Existing Problems of Attapulgite Clay in Jiangsu and Anhui [J]. Journal of Hefei University of Technology, 200 1, 24 (5): 885-889
Chen Tianhu, Peng Shuchuan, Huang Chuanhui, Shi Xiaoli, Feng Youliang. Preparation of Pure Attapulgite from Attapulgite Clay in Jiangsu and Anhui [J]. Journal of Silicate, 2004,32 (8): 965-969.
Chen Tianhu, Xu, Yue. Nanomineralogy and geochemistry of attapulgite clay in Jiangsu and Anhui [M]. Beijing Science Press 2004
Palygorskite nano-mineral materials and their surface properties with metal (metal oxide) nanocomposites
Chen Tianhu, Gao Wei
(School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009)
Abstract: Palygorskite is an important one-dimensional nano-mineral. It is of great theoretical and practical significance to understand the mineralogy and surface properties of palygorskite nano-minerals. Pure palygorskite samples were obtained by fine separation with the help of X-ray diffraction and transmission electron microscope. On this basis, the BET specific surface area, pore size distribution, cation exchange capacity, pHPZC, isoelectric point pH, equilibrium pH of aqueous solution and the interface between palygorskite ions and heavy metals were tested. The experimental results provide basic data for the surface properties of palygorskite and lay a foundation for correctly understanding the mineralogical properties of palygorskite nano-minerals.
Key words: palygorskite, surface properties, nanomaterials, nano-mineralogy.