Proton adsorption and acid-base properties of Tunisian illites in aqueous solution

Suspensions of illite clay minerals samples of three different origins (American illite from Montana, It(Mo), Tunisian glauconite from Gafsa, It(Ga), and Tunisian illite — chlorite mixed layer from El Hamma, It(Ha)) were prepared. The Fourier Transform Infrared (FTIR) spectroscopy, X-ray Diffractometry (XRD), Electron Scanning Microscopy (SEM) and surface area measurements were used to characterize the three illite samples having different extent of isomorphic substitution. The layers have a permanent negative charge due to isomorphic substitutions and pH dependent charges on the surface hydroxyls on the edges. Surface speciation of these samples was investigated using continuous potentiometric titration and mass titration curves between pH 4 and 11 at 0.001 M, 0.01 M, and 0.1 M NaCl solutions at ambient temperature. The two methods revealed point of zero charge (PZC) of the amphoteric edges sites approximately similar to the purified samples, in the range ～7.5–8.5, ～8.2–8.7, and ～ 9.0–9.3 for It(Mo), It(Ha) and It(Ga), respectively. The PZCs of freshly prepared dispersions are higher than those reported in the literature indicating basic character of these samples (pH of equilibrium suspensions in distilled water were ～7.9–9). In the present study, the focus was on the surface charge characteristics. A simple SCM model approach is presented to explain the illite H⁺ adsorption data. Surface weak acidic sites and surface ionization constants were calculated from titration data using regression methods. Sites with pKa ₁ ^int values of 6–6.7; 5.4–5.8; 6.1–6.6 and sites at pKa ₂ ^int values of 9.2–9.9; 10.2–10.4; 9.3–10 for It(Mo), It(Ga) and It(Ha), respectively, were assigned to amphoteric Al-OH and/or Fe-OH and Si-OH groups on the edges of illite samples.