The Roles of Organics in Nucleation and Growth of Solid Solutions
Solid solutions are likely to form when two isostructural minerals crystallize simultaneously from multicomponent aqueous solutions. The crystallization of solid solutions is essential to many geological, biological, and environmental processes, which are fundamentally linked to the global geochemical cycles, survival of marine organisms, and heavy metal remediation. Under the environmentally relevant conditions, the formation of solid solutions is expected to be dramatically affected by organics, however, the mechanistic bases of the roles of organics in crystallization behaviors of solid solutions (i.e., nucleation and growth) are still limited. Nucleation and growth processes of barium-strontium sulfate ((Ba, Sr)SO4) solid solutions at organic-water interfaces were investigated using grazing-incidence small-angle scattering (GISAXS). Results showed that Sr-bearing barite nucleation was significantly promoted even when the bulk solution was undersaturated, due to the enrichment of cation ions at organic-water interfaces that increased the local supersaturation in the adjacent solution. In supersaturated solutions, this enrichment generates nanometer-sized Sr-rich nuclei on organic films, while Sr-poor barite grows quickly to micrometer-sized crystals in bulk solutions. Theoretical solid-solution calculations explain the distinct Sr incorporation in barites on organic films and in bulk solutions. The findings resolve marine Sr-bearing barite paradox and provide insights into manipulating solid-solution nucleation and growth through the unique chemical environment near organic–mineral interfaces. Iron hydroxides are important scavengers for dissolved chromium (Cr) via coprecipitation processes; however, the influences of organic matter (OM) on Cr sequestration in Fe-Cr-OM ternary systems and the stability of the coprecipitates are not well understood. Herein, acetic acid (HAc), polyacrylic acid (PAA), and natural organic matter (NOM) were selected as model OMs. HAc had insignificant effects on the coprecipitates. Contrarily, large amounts of PAA and NOM were incorporated in the coprecipitates. The complexation capabilities of varied OM with Fe3+/Cr3+ ions, hydrolyzed Fe/Cr species, and newly formed Fe/Cr hydroxides are found to control Cr sequestration, re-dissolution and transportation. This study provides new mechanistic insights of OM in controlling (Fe, Cr)(OH)3 solid solutions, which are essential for Cr remediation and removal in both natural and engineered settings.