ACID-BASE PROPERTIES OF AQUEOUS ILLITE SURFACES

In this paper, the acid-base properties of illite/water suspensions ar e examined using the constant capacitance surface complexation model. On the basis of results of potentiometric titrations and solubility ex periments, we conclude that the proton reactions in the supernatants o f illite suspensions can be successfully represented by proton reactio ns of Al(H2O)(3+)(6) and Si(OH)(4) in water solutions. For illustratin g the acidic characteristics of aqueous illite surfaces, two surface p rotonation models are proposed: (1) one site-one pK(a) model, =SOH rev ersible arrow =SO- + H+, pK(a)(int) = 4.12-4.23; (2) two sites-two pK( a)s model, =SIOH reversible arrow =SIO- + H+, pK(aI)(int) = 4.17-4.44, and =SIIOH reversible arrow =SIIO- + H+, PKaIIint = 6.35-7.74. Evalua tion of these two models indicates that both of them can give good des criptions of the experimental data of systems with different illite co ncentrations and ionic strengths and that the one site-one pK(a) model can be considered as a simplification of the two sites-two pK(a)s mod el. Since both models assume only deprotonation reactions at the illit e surfaces, they suggest that the surface behavior of the illite is si milar to that of amorphous SiO2. Model assumptions, experimental proce dures, and evaluative criteria are detailed in the paper. (C) 1997 Aca demic Press.