The formation of secondary Al-hydroxysulfates is environmentally important as they regulate aluminum concentrations in acidic environments and may serve as efficient sinks for geogenic toxic elements such as arsenic and selenium. Based on recent studies, nanocrystalline basaluminite Al4(OH)10(SO4) x 5(H2O) appears to be the most frequent Al-phase in such settings. A few studies, however, have postulated that secondary Al-hydroxysulfates include the ɛ-Keggin polyoxocation, Al12(AlO4)(OH)24(H2O)127+, commonly referred to as Al13, and that the entire low temperature geochemistry of Al passes through the Al13 metastability field. To better understand what controls the type of Al-hydroxysulfates forming in natural and anthropogenically affected settings, here we present results from titration experiments leading to the formation of Al-hydroxysulfates from solutions with either presence or absence of aqueous Al13. Both the synthesized precipitates as well as the corresponding supernatant solutions were analyzed by standard and advanced analytical techniques including chemical analyses, 27Al Nuclear Magnetic Resonance (NMR) spectroscopy, synchrotron-based high energy X-ray diffraction and subsequent pair distribution analyses, and a kinetic photometric method for the quantification of aqueous Al13. While the chemical composition of the synthesized precipitates are similar, their structures strongly differ depending on the presence or absence of Al13 in solutions. The synthesis from sulfate-bearing solutions with a large compositional range resulted in instantaneous precipitation of nanocrystalline basaluminite. In contrast, crystalline Al13-sulfate formed in experiments where aqueous Al13 was synthesized from AlCl3 solutions before sulfate was added at a later stage. Based on these results and complimentary geochemical modelling, we conclude that the presence of sulfate leads to a strong complexation of monomeric aluminum. This inhibits the formation of aqueous Al13 and thus the precipitation of Al13-sulfate, while promoting the precipitation of nanocrystalline basaluminite instead. Since sulfuric acid is a primary source of acidity in most natural acidic environments, the formation of aqueous Al13 in nature is likely less important than previously thought. Instead, due to its chemical and structural variability, the formation of basaluminite and its transformation to more stable phases likely control the fate of Al in a large range of natural and anthropogenic settings.
Moradi et al. (Fri,) studied this question.