Mingde Fan

Montmorillonite-supported magnetite nanoparticles for the removal of hexavalent chromium [Cr(VI)] from aqueous solutions.

Montmorillonite-supported magnetite nanoparticles were prepared by co-precipitation and hydrosol method. The obtained materials were characterized by X-ray diffraction, nitrogen adsorption, elemental analysis, differential scanning calorimetry, transmission electron microscopy and X-ray photoelectron spectroscopy. The average sizes of the magnetite nanoparticles without and with montmorillonite support are around 25 and 15 nm, respectively. The montmorillonite-supported magnetite nanoparticles exist on the surface or inside the interparticle pores of clays, with better dispersing and less coaggregation than the ones without montmorillonite support. Batch tests were carried out to investigate the removal mechanism of hexavalent chromium [Cr(VI)] by these synthesized magnetite nanoparticles. The Cr(VI) uptake was mainly governed by a physico-chemical process, which included an electrostatic attraction followed by a redox process in which Cr(VI) was reduced into trivalent chromium. The adsorption of Cr(VI) was highly pH-dependent and the kinetics of the adsorption followed the Pseudo-second-order model. The adsorption data of unsupported and clay-supported magnetite nanoparticles fit well with the Langmuir and Freundlich isotherm equations. The montmorillonite-supported magnetite nanoparticles showed a much better adsorption capacity per unit mass of magnetite (15.3mg/g) than unsupported magnetite (10.6 mg/g), and were more thermally stable than their unsupported counterparts. These fundamental results demonstrate that the montmorillonite-supported magnetite nanoparticles are readily prepared, enabling promising applications for the removal of Cr(VI) from aqueous solution.

Removal of hexavalent chromium (Cr(VI)) from aqueous solutions by the diatomite-supported/unsupported magnetite nanoparticles

Diatomite-supported/unsupported magnetite nanoparticles were prepared by co-precipitation and hydrosol methods, and characterized by X-ray diffraction, nitrogen adsorption, elemental analysis, differential scanning calorimetry, transmission electron microscopy and X-ray photoelectron spectroscopy. The average sizes of the unsupported and supported magnetite nanoparticles are around 25 and 15 nm, respectively. The supported magnetite nanoparticles exist on the surface or inside the pores of diatom shells, with better dispersing and less coaggregation than the unsupported ones. The uptake of hexavalent chromium [Cr(VI)] on the synthesized magnetite nanoparticles was mainly governed by a physico-chemical process, which included an electrostatic attraction followed by a redox process in which Cr(VI) was reduced into trivalent chromium [Cr(III)]. The adsorption of Cr(VI) was highly pH-dependent and the kinetics of the adsorption followed a pseudo-second-order model. The adsorption data of diatomite-supported/unsupported magnetite fit well with the Langmuir isotherm equation. The supported magnetite showed a better adsorption capacity per unit mass of magnetite than unsupported magnetite, and was more thermally stable than their unsupported counterparts. These results indicate that the diatomite-supported/unsupported magnetite nanoparticles are readily prepared, enabling promising applications for the removal of Cr(VI) from aqueous solution.

A combined study by XRD, FTIR, TG and HRTEM on the structure of delaminated Fe-intercalated/pillared clay

Fe-PILC samples were synthesized by the reaction between Na(+)- and/or Ca(2+)-montmorillonite (Mt) and base-hydrolyzed solutions of Fe(III) nitrate. Different from the known usual microporous pillared structure, a meso-microporous delaminated structure containing intercalated or pillared fragments was found in the respective resulting Fe-intercalated or -pillared clays. XRD patterns of Na(+)-Mt-based Fe-intercalated/pillared clays show one large d-spacing above 6.4 nm corresponding to the mesoporous delaminated part, whereas another d-spacing of ca. 1.5 nm was indicative of the microporous pillared part. Fe-intercalated/pillared clays based on Ca(2+)-Mt lead to similar results, but with a d-spacing less than 6 nm and a second low intense d-spacing less than 1.5 nm. In the delaminated Fe-intercalated clays, NO(-)(3) anions were retained even after thorough washing process. They play as counterions to neutralize the positive-charged iron aggregates in the delaminated structure, and can be exchanged by heteropolyanions as [PW(12)O(40)](3-). The delaminated Fe-pillared clays show good thermal stability at 500 degrees C and exhibit at this temperature dramatically higher specific surface area and porosity than the starting montmorillonites. However, calcination at a higher temperature leads to the formation of nanocrystalline hematite. Air-drying after ethanol extraction (EAD) method has an advantage over air-drying (AD) method in preserving the delaminated structure.

CHANGES IN STRUCTURE, MORPHOLOGY, POROSITY, AND SURFACE ACTIVITY OF MESOPOROUS HALLOYSITE NANOTUBES UNDER HEATING

The objective of the present study was to investigate changes in the structural, textural, and surface properties of tubular halloysite under heating, which are significant in the applications of halloysite as functional materials but have received scant attention in comparison with kaolinite. Samples of a purified halloysite were heated at various temperatures up to 1400°C, and then characterized by X-ray diffraction, electron microscopy, Fourier-transform infrared spectroscopy, thermal analysis, and nitrogen adsorption. The thermal decomposition of halloysite involved three major steps. During dehydroxylation at 500–900°C, the silica and alumina originally in the tetrahedral and octahedral sheets, respectively, were increasingly separated, resulting in a loss of long-range order. Nanosized (5–40 nm) γ-Al2O3 was formed in the second step at 1000–1100°C. The third step was the formation of a mullite-like phase from 1200 to 1400°C and cristobalite at 1400°C. The rough tubular morphology and the mesoporosity of halloysite remained largely intact as long as the heating temperature was <900°C. Calcination at 1000°C led to distortion of the tubular nanoparticles. Calcination at higher temperatures caused further distortion and then destruction of the tubular structure. The formation of hydroxyl groups on the outer surfaces of the tubes during the disconnection and disordering of the original tetrahedral and octahedral sheets was revealed for the first time. These hydroxyl groups were active for grafting modification by an organosilane (γ-aminopropyltriethoxysilane), pointing to some very promising potential uses of halloysite for ceramic materials or as fillers for novel clay-polymer nanocomposites.

Facile preparation of hierarchically porous carbon using diatomite as both template and catalyst and methylene blue adsorption of carbon products

Hierarchically porous carbons were prepared using a facile preparation method in which diatomite was utilized as both template and catalyst. The porous structures of the carbon products and their formation mechanisms were investigated. The macroporosity and microporosity of the diatomite-templated carbons were derived from replication of diatom shell and structure-reconfiguration of the carbon film, respectively. The macroporosity of carbons was strongly dependent on the original morphology of the diatomite template. The macroporous structure composed of carbon plates connected by the pillar- and tube-like macropores resulted from the replication of the central and edge pores of the diatom shells with disk-shaped morphology, respectively. And another macroporous carbon tubes were also replicated from canoe-shaped diatom shells. The acidity of diatomite dramatically affected the porosity of the carbons, more acid sites of diatomite template resulted in higher surface area and pore volume of the carbon products. The diatomite-templated carbons exhibited higher adsorption capacity for methylene blue than the commercial activated carbon (CAC), although the specific surface area was much smaller than that of CAC, due to the hierarchical porosity of diatomite-templated carbons. And the carbons were readily reclaimed and regenerated.

Effects of Inherent/Enhanced Solid Acidity and Morphology of Diatomite Templates on the Synthesis and Porosity of Hierarchically Porous Carbon

The inherent or enhanced solid acidity of raw or activated diatomite is found to have significant effects on the synthesis of hierarchically porous diatomite-templated carbon with high surface area and special porous structure. The solid acidity makes raw/activated diatomite a catalyst for the generation of porous carbon, and the porous parameters of the carbon products are strongly dependent on the solid acidity of diatomite templates. The morphology of diatomite also dramatically affects the textural structure of porous carbon. Two types of macroporous structures in the carbon product, the partially solid pillars and the ordered hollow tubes, derive from the replication of the central and the edge pores of diatom shell, respectively. The hierarchically porous carbon shows good capability for the adsorption of solvent naphtha and H(2), enabling potential applications in adsorption and gas storage.

A CRITICAL TEXTURAL EVOLUTION STUDY OF ZEROVALENT IRON/MONTMORILLONITE NANOSIZED HETEROSTRUCTURES UNDER VARIOUS IRON LOADINGS

Heterostructures formed by nanoparticles hybridized with porous hosts are of great potential in many practical applications such as catalysis, adsorption, and environmental remediation, based on their intrinsic properties. The objectives of this study were to synthesize zerovalent iron nanoparticles/montmorillonite heterostructures and to investigate their textural evolution under different Fe loadings. Iron nanoparticles were hybridized with montmorillonite by impregnation of montmorillonite by ferric ions followed by chemical reduction with sodium borohydride in solution. These hybridized Fe nanoparticles were well dispersed on the montmorillonite surface, size adjustable, and resistant to oxidation under the protection of native Fe-oxide shells. The textural evolution of these heterostructures under various Fe loadings was investigated using nitrogen physisorption, X-ray diffraction, electron microscopy, and elemental analyses. As the Fe loadings increased, the total pore and mesopore volumes were almost unchanged; the total, micropore, and external surface areas as well as the micropore volume decreased; and the average pore diameter increased. These textural changes could be attributed to the filling of the interparticle pores of montmorillonite by a variable amount of Fe nanoparticles. In addition, with increasing Fe loadings, the mesoporous character was enhanced for these heterostructures. These fundamental results are important in understanding the structure of these heterostructures as well as in developing some novel applications in related fields.