INTERAKCE BAYERITU A KYSELINY KŘEMIČITÉ V SLABĚ KYSELÉM PROSTŘEDÍ ZA NORMÁLNÍCH PODMÍNEK
Interaction between bayerite, Al(OH)3, and silicic acid, H4SiO4, was studied in slightly acid solutions (pH ~ 4.2) at three Si/Al molar ratios (1:1, 1:3 and 1:5) under using of stirred batch reactors. The initial concentrations of H4SiO4 were 1.65x10-3 mol l-1 in all solutions. Two different concentrations of alkaline metals (K, Na), zero and 2x10-3 mol l-1, were adjusted in the solutions. At the initial stage of experiment (0-2 days), the H4SiO4 concentrations quickly decrease to the roughly limit values, between 1.55x10-3 and 1.60 x10-3 mol l-1. This decrease, relatively slight (from 5x10-5 to 1x10-4 mol) and rapid (from -8.10-6 to -7.10-5 mol l-1 day-1), was linearly dependent on the bayerite content; it was interpreted as a (chemi)sorption of H4SiO4 on bayerite surface. At the advanced stages of experiment (2 200 days), the concentrations of H4SiO4 stagnated (the slopes of kinetic curves range from -7.10-8 to +2.10-7 mol l-1 day-1). Therefore, despite the permanent supersaturation with respect to kaolinite (SIkaolinite < 3.60), K-mica (SI(K-mica) < 4.49 in the solutions containing K), and Na-beidellite (SINa-beidellite < 3.27 in the solutions containing Na), any expected reaction (some bayerite conversion into kaolinite, smectite or their precursors) was not proved. If such reaction runs, it is extremely slow under given conditions. Despite the limit values of aqueous Al in the majority of solutions, the bayerite saturation was not reached in any solution (the SIbayerite values ranged from -0.35 to -0.81 at the end of experiments). This phenomenon was interpreted as a modification of bayerite solubility product by the sorption of H4SiO4. Both the initial slopes of kinetics curves for aqueous Al and the times needed for reaching the limit values of aqueous Al show that the rate of bayrite dissolution increases in the solutions containing Na and K ions.
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