Acid-base properties of Tunisian palygorskite in aqueous medium

Suspension of a Tunisian palygorskite is studied by acid-base potentiometer and mass titrations in order to determine the point of zero charge (PZC). The Gran plot method, commonly used to determine the equivalence points, is applied for the hydroxide titration with the purpose to calculate the average number of protons reacted per surface site (Z) and then the surface site density of palygorskite suspension at a given ionic strength. The dissociation coefficients of palygorskite surface are calculated and consequently the surface constants acidities are determined by graphical extrapolation method. The studied samples, performed at different ionic strengths, presented Z vs. pH curves with a common crossing point at pH = 9.8 (pHPZC). For the purpose of eliminating the influence of the palygorskite dissolution and the hydrolysis of soluble ions, the net number of surface reacted protons per surface site (Znet) is determined. Znet vs. pH leads to PZC of 8.8 witch is in agreement with the mass titration results. For illustrating the acidic characteristics of palygorskite surface, two surface protonation models are tested: the one site-one pKa, ≡ SOH ⇆ SO− + H+, pKa 6.2–6.88; and the two sites-two pKas model, ≡SIOH ⇆ ≡SIO− + H+, pKa1 5.26–5.89 and ≡SIIOH ⇆ ≡SIIO− + H+, pKa2 9.15–9.71. Both of them give a good description of the experimental data.