Rezumat
The surface acidity of raw and acid activated palygorskite clay were studied by acid-base potenti- ometric titration. The Gran plot method was applied for the hydroxide titration and the total surface site (Hs) and the average number of protons reacted per surface site (Z) of palygorskite samples at a given ionic strength were calculated. Acid treatment increases the clay acidity and modifies its surface charge. The point of zero charge value, determined by the common crossing point of Z vs pH curves performed at different ionic strengths, decrease from 8.8 to 3.5 with acid treatment period. For illustrating the acidic characteristics of treated and untreated palygorskite surface, three surface protonation models are tested : a/ the one site-one pKa model, ≡SOH ≡SO- + H+, b/ the two sites-two pKas model, ≡SIOH ≡SIO- + H+ and ≡SIIOH ≡SIIO- + H+ and c/ the one site-two pKa model, ≡SOH ≡SO- + H+ and ≡SOH + H+ ≡SOH2+. The three surface protonation models describe sufficiently of the surface properties and their evolution with acid treatment.