Research paperEffects of humic acid release from sepiolite on the interfacial and rheological properties of alkaline dispersions
Graphical abstract
Introduction
Sepiolite (Sep) is one of the most important industrial magnesium-rich 2:1 clay minerals, and it differs from other layer silicates because of the lack of continuous octahedral sheets (Brigatti et al., 2013). Discontinuous octahedral sheets extend only in one direction and tetrahedral sheets divided into ribbons form a chain-layer molecular structure with 3.6 × 10.6 Å of very large open channels or tunnels and 320 m2/g of corresponding high surface areas (Sabah et al., 2002).
A unique fibre structure exactly determines the hydrophobicity and anisotropic characteristics in half-cell formula Mg4Si6O15(OH)2.6H2O. Electron acceptor and donor components of Sep fibres were previously calculated as γS+ = 0.149 and γS− = 20.09 (Benli et al., 2012). Sep was also identified as either hydrophobic or hydrophilic, and then it was found that hydrophilic repulsion is dominant according to van Oss's (1994) classification. However, computational molecular simulation results of the same study clearly showed that Sep basal surfaces, even talc-like surfaces, are hydrophobic (Benli et al., 2012). The results were also compared with contact angle measurements via the capillary rise technique. The implications behind hydrophobic material are not enough to explain why the electron donor component of Sep surfaces is much greater than its electron acceptor component. A possible explanation for the larger γS− value of Sep could be attributed to extractable components such as soil organic matter because HA have a large number of free and bound phenolic –OH and carboxylic –COOH groups arising from the natural environment and groundwater (Arnarson and Keil, 2000, Wall and Choppin, 2003, Tombacz et al., 2004, Feng et al., 2005).
Soil organic matter adsorption on clay mineral surfaces and their influence on the solution chemistry (e.g. ionic strength, pH, and soluble cations) have been extensively explored (Arnarson and Keil, 2000, Feng et al., 2005). However, direct measurements focusing on the relationships between viscosity and HA content are scarce; only a study on the effect of grinding on the rheology of Sep dispersions in acid and alkaline media (Çınar et al., 2009) and a few observations on the macromolecular structures of HA (Caceci and Billon, 1990, Pokorna et al., 2001, Zhao et al., 2013) are available. Çınar et al. (2009) showed that Sep dispersions exhibit viscosities practically close to zero at pH 12. This kind of change in viscosity was ascribed to the disruption of the particle network. On the other hand, Pokorna et al. (2001) suggested that carboxylic groups in HA with acidic pH are largely nonionic (R-COOH), whereas with alkaline pH carboxylic R-COO− groups are more dominant. Various groups ionised in HA and relatively more negative charges could lead to more hydrophilic Sep surfaces. Such interesting behaviour led us to investigate the role of HA in the flow behaviour of Sep with alkaline and highly alkaline pH values.
The aim of this study was therefore to show if HA content in the Sep matrix can modify the rheological and interfacial properties of Sep dispersions in highly alkaline media. The understanding of the mechanisms controlling clay mineral response to pH changes is important for evaluating the rheological and interfacial behaviour of the surrounding media: in this case, controlling the viscosity of Sep dispersions depending on the extent of hydrophobicity or wettability in industrial applications.
Section snippets
Materials
All Sep samples were obtained from the AEM Company in the Sivrihisar region of Turkey and labelled SepTT, SepKS and SepSG for Turktaciri, Kurtşeyhi and Sıgırcık, respectively. Quantitative chemical analyses were carried out by ICP (inductively coupled plasma) spectrophotometry in the ACME Analytical Lab., Canada. Table 1 represents the main constituents of chemical analysis and mineralogical characterization of the Sep samples with a Shimadzu XRD-6000 (Shimadzu Corp., Tokyo, Japan) equipped
Rheological properties of sepiolite dispersions at alkaline media
The rheological measurements of three different Sep-water dispersions at different pH values are shown in Fig. 1. It is obvious that Sep dispersions behave very similarly to those in other studies of natural environments (~ pH 8.5) (Tunç et al., 2008, Çınar et al., 2009). However, their flow behaviours and also their shear stresses sharply decreased when pH conditions were changed from natural environment to pH 11.5 (Fig. 1a–c).
In alkaline media, the flow type of Sep dispersions was non-Newtonian
Conclusions
Rheological properties of Sep dispersions indicate that alkaline solutions cause a sharp decrease in viscosity values as a result of the disruption of the particle network, and dissolutions of Sep surfaces as well as contribution of HA release from natural Sep matrix.
Increasing the pH of the dispersions from alkaline to highly alkaline, their rheological behaviours changed from non-Newtonian and pseudoplastic represented by the Herschel–Bulkley viscoplastic behaviour to Newtonian. Released Mg2 +
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