An experimental study of illite dissolution kinetics as a function of pH from 1.4 to 12.4 and temperature from 5 to 50°C

Dissolution rates of natural illite (Illite du Puy) were measured from Si release rates during closed system experiments at pH ranging from 1.4 to 12.4 and temperatures ranging from 5 to 50degreesC. Experiments performed at 4<pH<11 exhibited reactive fluid Si/Al concentration ratios that were inconsistent with stoichiometric illite dissolution likely due to secondary phase precipitation. In contrast, after an initial preferential release of aluminum relative to silicon, the reactive fluid Si/Al concentration ratio evolution was consistent with stoichiometric illite dissolution for all experiments conducted at 4>pH>11. Si release rate decreased with time during all experiments; for those experiments performed at 4>pH>11 this observation can be attributed to either 1) changing reactive surface area; 2) the effect of initial fine particle dissolution; or 3) a negative order of the illite dissolution reaction with respect to aqueous Al and/or Si. Measured dissolution rates exhibited a typical variation with pH; rates decrease with increasing pH at acid conditions, minimize at near to neutral pH and increase with increasing pH at basic conditions. An empirical expression describing rates obtained in the present study is given by r = A(H) exp (-EAH+/RT)a(H+)(0.6) + A(H2O) exp(-E-AH2O/RT) + A(OH) exp(-EAOH-/RT)a(OH-)(0.6) where A(H), A(H2O) and A(OH) equal 2.2 X 10(-4), 2.5 X 10(-13) and 0.27 mol m(-2) s(-1), E-Ai stands for activation energies equal to 46, 14 and 67 kJ mol(-1), R designates the gas constant, T refers to absolute temperature, and a(i) signifies the activity of the subscripted aqueous species. Copyright (C) 2003 Elsevier Ltd.