Near-infrared investigation of folding sepiolite

Abstract

Sepiolite is an industrially important clay mineral of the palygorskite-sepiolite group with alternating 2:1 ribbons and hydrated tunnels. Dry sepiolite, Mg 8 Si 12 O 30 (OH) 4 (OH 2 ) 4 , loses half of its OH 2 content upon further heating and undergoes a structural collapse known as folding. This treatment is considered essential for enhancing the absorptive properties of the clay. In this paper, the folding process is studied by near-infrared (NIR) spectroscopy, mid-infrared attenuated total reflectance (ATR), and thermogravimetric analysis (TGA). The folded state, Mg 8 Si 12 O 30 (OH) 4 (OH 2 ) 2 , reveals a new spectrum of fundamental and higher-order OH 2 vibrations, as well as systematically split doublets of structural and surface O-H vibrations. Detailed assignments for the stretching, combination, and overtone O-H modes are proposed on the basis of the two non-degenerate populations of Mg 3 OH (and SiOH) present in the folded state. It is demonstrated that NIR is of particular diagnostic value in monitoring conveniently and non-invasively the folding process, which appears as a simple transition between well-defined dry-and folded-structures. At the level of elementary sepiolite particles (laths), folding is described as a cooperative process requiring the integrity of the ribbons and the inter-ribbon linkages (moderately acid-leached sepiolite does not fold). This is opposed to the skewed and sometimes complex OH 2 desorption trace observed by high-resolution TGA, which appears to indicate a multimodal distribution of laths. It is proposed that the rate-determining step for a sepiolite (also, palygorskite) lath to fold is the creation of a critical zone at mid-particle length, which is OH 2 -deficient and contains unstable, fivefold-coordinated Mg 2+ .

Catalytic performance of microporous materials for the production of renewable fuels

“…The spectra of the dehydrated samples exhibit six peaks in the region of stretching O-H vibrations. In agreement with the literature [20,[35][36][37][38][39], the peaks at 3740 and 3724 cm −1 are assigned to Si-OH groups of the tetrahedral silicate layer and those at 3691 and 3673 cm −1 to Mg(3)-OH groups in the octahedral sheets of the sepiolite structure. The bands at 3597 and 3531 cm −1 have been attributed to water molecules coordinated to Mg cations.…”

Geochemical and mineralogical characterization of smectites from the Ventzia basin, western Macedonia, Greece

“…in addition to that of a band at 7183 cm –1 ) and several other minor band shifts. This is the typical signature of clay minerals such as palygorskite/sepiolite that distinguishes them from the other common non-modulated phyllosilicates (Gionis et al , 2006; Stathopoulou et al , 2011; Bukas et al , 2013; Tsampodimou et al , 2015). The ~7265 and 4565 cm –1 bands appearing upon drying are assigned to the 2ν, ν + δ modes of the abundant silanol (SiOH) termination groups that decorate the outer surface of the laths.…”

Geochemical processes during the stabilization of potentially toxic elements (PTEs) in contaminated soil by using mineral amendments

“…Compared to the filtrates obtained at natural pH (~5), the NIR spectra of acid-treated PCM filtrates (blank and Pb-loaded) (Figure 5.7, red line) included an additional band at 7312cm -1 . A similar band was observed in an acid-treated sepiolite at ~7320 cm -1 by Tsampodimou et al (2015) and was assigned to the 2vSiOH mode of a new type of SiOH groups that are formed by the attack of the protons on the octahedral sheet and the displacement of Mg (Madejová et al, 2009;Komadel, 2016). The respective alkali-treated filtrates (Figure 5.7, green line) revealed some enhancement of a band at ~5284cm -1 for blank and Pb-loaded filtrates, which was assigned to a water combination mode.…”