Dissolution of well and poorly crystallized kaolinites; Al speciation and effects of surface characteristics

This study compared surface characteristics and dissolution behavior of well-crystallized (KGa-1b) and poorly crystallized (KGa-2) kaolinite standards. Atomic force microscopy (AFM) revealed that particles of KGa-1b generally have nicely hexagonal micromorphology and crystallographically controlled microtopographic features. Particles of KGa-2 are also hexagonal, but their micromorphology tends to be more rounded. Basal-plane surfaces tend to be more irregular with fewer clearly crystallographically controlled features. KGa-1b particles tend to be larger in diameter and thicker than KGa-2 particles. Micromorphologic measurements showed that both KGa-1b and KGa-2 have modal edge- to total surface-area ratios of approximately 0.1 (mean ∼0.2), although these measurements did not include the potentially large contribution of basal-plane step edges (additional 20% or more). Dissolution experiments were conducted in oxalic acid and inorganic acids at pH 3, 22°C, I = 0.01 M, under batch dissolution conditions. Dissolution rates (measured as Si release) in 1 mM oxalic acid were approximately twice as fast for KGa-2 as for KGa-1b (2.27 vs. 0.96 nmol/m2·h). Rates for KGa-2 and KGa-1b were similar in HNO3 (0.86 and 1.16 nmol/m2·h, respectively). The comparable rates for these two sedimentary kaolinites and for a hydrothermal kaolinite studied by Wieland and Stumm (1992) suggests that the fundamental structure of kaolinite, rather than specific surface details, exerts the greatest influence on dissolution kinetics. High-performance cation exchange chromatography (HP-CEC) was used to determine the distributions of monomeric Al species over the course of kaolinite dissolution. For dissolution in 1 mM oxalate, Al-oxalates were observed almost exclusively in agreement with results of equilibrium speciation calculations. For dissolution in HNO3, the peak representing uncomplexed Al species, Al1, was predominant but not exclusive, as predicted by calculations. A peak having a retention time characteristic of species with +2 charge may be evidence for an AlOSi(OH)2+3 species, and warrant further investigation.