Magnetite surface charge studies to 290 degrees C from in situ pH titrations

The proton-induced surface charge of magnetite was investigated in 0.03 and 0.30 molal sodium trifluoromethanesulfonate solutions from 25 degrees C to 290 degrees C by potentiometric titrations using a stirred hydrogen electr ode concentration cell. Pure magnetite with excellent crystallinity was pro duced by reaction with the Ni/NiO/H2O hydrogen fugacity buffer at 500 degre es C. Inflection points in the 0.03 molal proton sorption isotherms (pH(inf l)) at 6.50, 6.24, 5.65, 5.47, 5.31 and 5.55 at temperatures of 50 degrees C, 100 degrees C, 150 degrees C, 200 degrees C, 250 degrees C and 290 degre es C, respectively, were used as estimates of the pristine point of zero ch arge (pH(ppzc)) for modeling purposes. These pH(infl) values parallel 1/2 p K(w) and agree within the assigned uncertainty (+/-0.3 pH units) at all tem peratures with independent estimates of the pH(ppzc) calculated from an ext ension of 88the revised MUSIC model. The surface charging can be adequately described by a one-pK model with a surface protonation constant fitted to the pH(infl) values, and giving the standard state thermodynamic properties log K-H,K-298 = 7.00, Delta H(289)degrees = -32.4 +/- 0.8 kJ/mol and const ant Delta C-p = 128 +/- 16 J K-1 mol(-1), with Delta S(298)degrees assumed to be equal to that of rutile protonation (25.5 +/- 3.4 J K-1 mol(-1). The 0.03 and 0.30 molal proton sorption isotherms also exhibit pHs of common in tersection (pH(cip)) at 6.33, 5.78, 5.37, 4.82, 4.62 and 4.90 at 50 degrees C, 100 degrees C, 150 degrees C, 200 degrees C, 250 degrees C and 290 degr ees C, respectively. The difference between the pH(cip) and pH(ppzx) congru ent to pH(infl) values can be related to specific binding of Na+ on the neg atively charged surface, which increases with increasing temperature, altho ugh the pH(infl) values may also be affected by dissolution of the solid. T he electrical double layer model includes a basic Stern layer capacitance, with specific cation and anion binding at the Stern layer, and a fixed diff use layer capacitance computed from Guoy-Chapman theory. To fit the steepne ss and asymmetry of the charging curves above the pH(ppzc), an additional c ation binding constant was invoked, which allows the cation to experience t he surface potential. Significant kinetically controlled dissolution of mag netite was observed below the pH(ppzc), which may be a result of leaching o f Fe2+ from the surface, to produce a magnetite + hematite assemblage, desp ite the high hydrogen partial pressures (ca. 10 bars) used in these experim ents. (C) 2000 Elsevier Science B.V. All rights reserved.