Purification of Turkish Bentonites and Investigation of the Contact Angle, Surface Free Energy and Zeta Potential Profiles of Organo-Bentonites as a Function of CTAB Concentration

Abstract

Purification of raw bentonites and organo-bentonite preparations is sometimes required for industrial use. Zeta (electrokinetic) potential (ζ), contact angle (wettability/hydrophobicity), and surface free energy (SFE) are important surface characteristics and vary significantly according to the applied surfactant concentration when preparing organo-bentonite. Changes in these characteristics determine the stability, behavior, and efficiency of organo-bentonites in various applications such as adsorption, composite materials, and drug-delivery systems. Knowing how much surfactant should be used to prepare organo-bentonite is, therefore, critical. The purpose of the present study was to determine the effect of concentration of the cationic surfactant cetyltrimethylammonium bromide (CTAB) adsorbed in organo-bentonite (prepared from two local and commercial raw bentonites with potential for use in adsorbent and composite materials) on the ζ potential, contact angle, and SFE profiles. The raw bentonites were purified using sedimentation and centrifugation techniques prior to preparation of the organo-bentonite. The purification results were evaluated in light of X-ray diffraction (XRD), cation exchange capacity (CEC), free swelling volume (FSV), X-ray fluorescence (XRF), and particle-size analysis data. Most of the gangue minerals (feldspar, calcite, clinoptilolite, opal, quartz, and mica) having particle size >5 μm were removed from the raw bentonites by using a one-stage sedimentation or a Falcon gravity separator (FGS). Higher yields (68.8% and 81.3% for two bentonites) were obtained with the FGS compared to sedimentation while purification levels were almost the same. ζ changed greatly from –35 mV (and –40 mV) toward 38 mV (and 40 mV) with increasing CTAB concentrations. Similar profiles were also obtained for wettability; maximum contact angles for organo-bentonites were measured as ~72–73o, while they were 12.65 and 14.1o for two purified and unmodified bentonites. SFEs were calculated using contact-angle data, and decreased to minimum values of 41.5–43.6 mJ/m2 from 78.6–78.2 mJ/m2 upon treatment of raw bentonites with CTAB. 100–130% CEC concentration was sufficient to prepare organo-bentonites with maximum hydrophobicity and positively charged surfaces.

An Introduction to Montmorillonite Purification

“…For this, 1.0% clay dispersion was prepared as described in the sedimentation process and then left for 10 minutes to allow the coarse particles to settle. The dispersion remaining on the collapsed particles was completely removed and fed to FGC once [1].…”

“…An example of laboratory scale sedimentation process for montmorillonite purification is given below, step by step [1].…”

“…For this reason, sometimes the term bentonite is used instead of montmorillonite. Montmorillonite clay has some superior characteristics such as high surface area, high swelling capacity, good adsorbent ability, high cation exchange capacity, plasticity, high chemical stability, and good mechanical properties [1][2][3][4]. Due to these properties it exhibits, montmorillonite has many uses and research areas, some of which can be listed as pharmaceutical and cosmetic industry, adsorbents, catalysts, drilling mud, and filler in construction industries such as ceramics [2,[5][6][7].…”

“…Figure 2.An example of sedimentation process; (a) raw montmorillonite dispersion, (b) purified montmorillonite, (c) settled gangue minerals[1].…”

Bentonitin Karıştırmalı Ortam Değirmeni ile Kuru Öğütülmesi

“…Montmorillonite, the most abundant smectite mineral in bentonite ores, is aqueous aluminum silicate and has a layered crystalline structure with the general chemical formula of (Na, Ca)0.33(Al, Mg)2 (Si4O10)(OH)2*nH2O. Bentonites have superior properties such as high cation exchange capacity, large surface area, high swelling capacity, plasticity, good mechanical properties, and chemical stability [1][2][3][4][5]. Therefore, bentonites are used in various applications such as adsorption, pharmaceutical and cosmetic industry, drilling mud, food, paper, paint, and ceramics [1,[6][7][8].…”