Abstract
Palygorskite is a natural fibrous clay mineral that can be used in several applications, for which colloidal stability in aqueous suspensions is a key point to improve its performance. In this study, methods of magnetic stirring, high-speed shearing, and ultrasonication, as well as different chemical dispersants, combined with these methods, namely carboxymethylcellulose, alginate, polyphosphate, and polyacrylate, were used to improve the dispersibility and the formation of stable suspensions of palygorskite in different conditions of pH. The stability and particle size of suspensions with a low concentration of palygorskite were evaluated by visual inspection, optical and electron microscopy, dynamic light scattering, and zeta potential measurements. Moreover, the palygorskite used in this work was initially characterized for its mineralogical, chemical, physical, and morphological properties. It was found that more stable suspensions were produced with ultrasonication compared to the other two physical treatments, with magnetic stirring being inefficient in all tested cases, and for higher pH values (pH of 12 and pH of 8, the natural pH of the clay) when compared to lower pH values (pH of 3). Remarkably, combined with ultrasonication, carboxymethylcellulose or in a lesser extent polyphosphate at near neutral pH allowed for the disaggregation of crystal bundles of palygorskite into individualized crystals. These results may be helpful to optimize the performance of palygorskite in several domains where it is applied.
Highlights
Palygorskite is a natural clay mineral with fibrous morphology and an ideal formula (Mg,Al,Fe)5 Si8 O20 (OH)2 (OH2 )4 ·4H2 O, whose structure is based on blocks alternating with cavities
No information was provided for the molecular weight of alginate sample, for which we estimated a value of ca. 280 kDa by rheometry measurements. (ii) As synthetic polyelectrolytes, sodium polyphosphate (Emplura grade, catalogue number 106529) purchased from SigmaAldrich (Merck) and hydrophobically modified poly(sodium acrylate) (HM-PAA) were used
A study was conducted that aimed at developing appropriate conditions for the preparation of stable colloidal aqueous suspensions of palygorskite, a mineral whose fibrous particles have a propensity to aggregate in water to form bundles and large aggregates
Summary
Palygorskite is a natural clay mineral with fibrous morphology and an ideal formula (Mg,Al,Fe) Si8 O20 (OH) (OH2 )4 ·4H2 O, whose structure is based on blocks alternating with cavities (tunnels). The structural blocks contain two tetrahedral silica sheets, with the tetrahedra inverted from one sheet to the other, sandwiching a central octahedral sheet of metal oxide-hydroxide, where the metal ions are mainly magnesium and aluminum, with iron in a lesser extent. Both hydroxyl ions and water molecules are coordinated to metal centers, while the remaining water molecules occupy positions inside the tunnels. The bonding between nonshared oxygens from the tetrahedral silica sheets on the external surface of the particles and hydrogen provides a high density of silanol groups on the particle surface [2]. Several micro- and nanoscale applications for the palygorskite—namely in sorption processes; catalytic, rheological, and environmental; in organo-mineral hybrid composites, films, membranes, and bioplastics; for drug delivery; for tissue engineering; as a source of supported carbonaceous materials; and as a component of sensor devices or bioreactors—
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