Modification of the textural properties of palygorskite through microwave assisted acid treatment. Influence of the octahedral sheet composition

Abstract Five palygorskite samples from different geological environments including lacustrine and perimarine deposits and hydrothermal fillings were treated by a microwaveassisted acid leaching process inaqueous HNO3 solution (0.2 M). In all cases, the samples have been characterized by X-ray fluorescence (XRF), X-ray diffraction (FTIR), Fourier transform infrared spectroscopy (FTIR), N2 adsorption-desorption at −196 °C and Scanning electron microscope (SEM). The mineralogical study indicates that palygorskite is the predominant mineral although the presence of other associated minerals has been observed including: quartz, calcite, dolomite, feldspars, smectite, illite-mica and traces of chlorite and sepiolite. After the acid treatment the main mineralogical change is the disappearance of the carbonates, not being observed in any case the occurrence of amorphous silica. Palygorskite remains virtually unchanged from acid attack in four of the samples studied. Only in the sample with the highest Mg(II) content (Mace), mineralogical and chemical data evidence the partial solution of the octahedral layer in the fibrous clay mineral due to the release of magnesium. The textural study indicates significant differences in the specific surface area of the samples ranging from 75 to 225 m2 g−1, this variation being related mostly to the purity of the samples. Under similar experimental conditions, palygorskite is more resistant to acid attack than magnesian clay minerals (saponite or sepiolite). In addition, a relationship has been observed between the magnesium content of palygorskite and the susceptibility to microwave-assisted acid attack. In all cases, an increase in the specific surface area value has been observed, reaching a highest SBET value of 354 m2 g−1 for Mace sample after 16 min of microwave-assisted acid treatment as a consequence of the partial leaching of its octahedral sheet, although its fibrous structure is maintained yet.