Aluminum Affects Heterogeneous Fe(III) (Hydr)oxide Nucleation, Growth, and Ostwald Ripening

Heterogeneous coprecipitation of iron and aluminum oxides is an important process for pollutant immobilization and removal in natural and engineered aqueous environments. Here, using a synchrotron-based small-angle X-ray scattering technique, we studied heterogeneous nucleation and growth of Fe­(III) (hydr)­oxide on quartz under conditions found in acid mine drainage (at pH = 3.7 ± 0.2, [Fe³⁺] = 10^–4 M) with different initial aqueous Al/Fe ratios (0:1, 1:1, and 5:1). Interestingly, although the atomic ratios of Al/Fe in the newly formed Fe­(III) (hydr)­oxide precipitates were less than 1%, the in situ particle size and volume evolutions of the precipitates on quartz were significantly influenced by aqueous Al/Fe ratios. At the end of the 3 h experiments, with aqueous Al/Fe ratios of 0:1, 1:1, and 5:1, the average radii of gyration of particles on quartz were 5.7 ± 0.3, 4.6 ± 0.1, and 3.7 ± 0.3 nm, respectively, and the ratio of total particle volumes on quartz was 1.7:3.4:1.0. The Fe­(III) (hydr)­oxide precipitates were poorly crystallized, and were positively charged in all solutions. In the presence of Al³⁺, Al³⁺ adsorption onto quartz changed the surface charge of quartz from negative to positive, which caused the slower heterogeneous growth of Fe­(III) (hydr)­oxide on quartz. Furthermore, Al affected the amount of water included in the Fe­(III) (hydr)­oxides, which can influence their adsorption capacity. This study yielded important information usable for pollutant removal not only in natural environments, but also in engineered water treatment processes