Surface Chemistry and Surface Speciation of Palygorskite at Room Temperature

Palygorskite exhibits a complex surface chemistry resulting from its fibrous morphology, modulated ribbon structure, and coexistence of silanol- and octahedral-derived reactive sites. Despite its industrial and environmental relevance, previous studies report contradictory point-of-zero-charge values and inconsistent surface speciation models. In this work, the surface chemistry of a natural Mg-rich palygorskite from Senegal was investigated through mineralogical and crystallochemical characterization combined with potentiometric titrations performed at ionic strengths of 0.1, 0.01, and 0.001 M KCl. X-ray diffraction, scanning electron microscopy, BET and total surface area measurements, and TEM-EDX analyses indicate that the sample corresponds to a Type III magnesian palygorskite. Surface titration data were interpreted using a non-electrostatic surface complexation model implemented in PHREEQC. The results show that palygorskite surface charge is controlled by both permanent exchange charge and pH-dependent edge reactions. Three main reactive surface groups were identified from crystallographic and geochemical constraints: silanol (>SiOH), aluminol bridge sites associated with octahedral Al (>Al-O-Si), and Mg-coordinated edge water groups (>MgOH2). The point of zero net proton charge ranges from pH 8.19 to 8.30. The proposed model provides a crystallographically consistent description of palygorskite surface reactivity in aqueous systems.