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Homogeneous and Heterogeneous (Fex, Cr1–x)(OH)3 Precipitation: Implications for Cr Sequestration
- Dai, Chong, Zuo, Xiaobing, Cao, Bo, Hu, Yandi
- Environmental Science & Technology 2016 v.50 no.4 pp. 1741-1749
- X-radiation, aquatic environment, chemical composition, chromium, iron, nanoparticles, pH, quartz
- 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 (Feₓ, Cr₁–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 (Feₓ, Cr₁–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 (Feₓ, Cr₁–ₓ)(OH)₃ nanoparticles, which can help determine the fate of Cr in aquatic environments.