DISSOLUTION KINETICS OF HEULANDITE AT PH 2-12 AND 25-DEGREES-C

The dissolution rate of heulandite depends strongly on pH. Based on silica release, the logarithm of the steady-state dissolution rate at pH 2 is -13.1 mol cm-2 s-1. The logarithm of the rate decreases to -15.8 mol cm-2 s-1 at pH 7.2 and increases again to -14.6 mol cm-2 s-1 at pH 12.2. At low pH, Al is released preferentially to silica; but at intermediate and high pH, the release of silica appears to be congruent relative to Al. The change in dissolution rate with pH indicates that at low pH, the dissolution mechanism is controlled by the detachment of a positively charged Al species, >Al-OH2+. Below pH 5, however, a silica-rich surface layer is formed requiring diffusion through the layer. At intermediate and high pH, it is likely that the dissolution rate is controlled by the detachment of a negatively charged silica species, >Si-O-. The reaction order of the hydrogen ion under low pH conditions is 0.7, and the reaction order of the OH- ion is 0.3 at high pH. The measured dissolution rates indicate that a 1 mm heulandite crystal would dissolve in 300,000 yrs if the solution composition is maintained undersaturated.