Homogeneous and Heterogeneous (Fe_x, Cr_1–x)(OH)₃ Precipitation: Implications for Cr Sequestration

The formation of (Fe, Cr)­(OH)₃ nanoparticles determines the fate of aqueous Cr in many aquatic environments. Using small-angle X-ray scattering, precipitation rates of (Fe, Cr)­(OH)₃ nanoparticles in solution and on quartz were quantified from 0.1 mM Fe­(III) solutions containing 0–0.25 mM Cr­(III) at pH = 3.7 ± 0.2. Concentration ratio of aqueous Cr­(III)/Fe­(III) controlled the chemical composition (x) of (Fe_x, Cr_1–x)­(OH)₃ precipitates, solutions’ supersaturation with respect to precipitates, and the surface charge of quartz. Therefore, the aqueous Cr­(III)/Fe­(III) ratio affected homogeneous (in solution) and heterogeneous (on quartz) precipitation rates of (Fe_x, Cr_1–x)­(OH)₃ through different mechanisms. The sequestration mechanisms of Cr­(III) in precipitates were also investigated. In solutions with high aqueous Cr­(III)/Fe­(III) ratios, surface enrichment of Cr­(III) on the precipitates occurred, resulting in slower particle growth in solutions. From solutions with 0–0.1 mM Cr­(III), the particles on quartz grew from 2 to 4 nm within 1 h. Interestingly, from solution with 0.25 mM Cr­(III), particles of two distinct sizes (2 and 6 nm) formed on quartz, and their sizes remained unchanged throughout the reaction. Our study provided new insights on homogeneous and heterogeneous precipitation of (Fe_x, Cr_1–x)­(OH)₃ nanoparticles, which can help determine the fate of Cr in aquatic environments.