Electrochemical Properties of Na–Attapulgite

Analytical surface charge (σa), electrophoretic, and physicochemical properties of an homoionic form of Na–attapulgite show some similarity with those exhibited by homoionic sodium forms of the platy clay minerals kaolinite and illite. Both electrophoretic mobility (ue) and analytic or titratable surface charge show dependency on pH and electrolyte concentration (c). Transmission electron microscopic examination indicated that particles were 1000 nm long, lath-like, and of aspect ratios (length:width:thickness) 100:3:1. Estimates of (hydrodynamic) equivalent spherical particle diameter (dse), made with photon correlation spectroscopy (PCS), were found to be ∼400 nm and reflect extreme nonisometric particle shape. Failure of PCS at pH < 6.4 suggests the onset of colloidal instability in this region. The correlation betweenueand σawas found to be approximately linear in the range 3 < pH < 6.5 with more complex behavior at pH > 6.5. Apportionment of titratable charge to particle edges suggests a charge site density that is reasonable for the structure of this clay mineral.