title: Homogeneous and Heterogeneous (Fex, Cr1-x)(OH)3 Precipitation: Implications for Cr Sequestration.
abstract: 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.
authors.affiliation: Department of Civil & Environmental Engineering, University of Houston , Houston, Texas 77004, United States.
authors.forename: Chong
authors.initials: C
authors.lastname: Dai
authors.affiliation: X-ray Science Division, Argonne National Laboratory , Argonne, Illinois 60439, United States.
authors.forename: Xiaobing
authors.initials: X
authors.lastname: Zuo
authors.affiliation: Department of Civil & Environmental Engineering, University of Houston , Houston, Texas 77004, United States.
authors.forename: Bo
authors.initials: B
authors.lastname: Cao
authors.affiliation: Department of Civil & Environmental Engineering, University of Houston , Houston, Texas 77004, United States.
authors.forename: Yandi
authors.initials: Y
authors.lastname: Hu
mesh_terms: D011232:Chemical Precipitation; D002857:Chromium / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D053758:Nanoparticles / Q000737:chemistry*; D011791:Quartz / Q000737:chemistry; D053838:Scattering, Small Angle; D012996:Solutions; D014867:Water / Q000737:chemistry; D014874:Water Pollutants, Chemical / Q000737:chemistry*
chemical_list: D012996:Solutions; D014874:Water Pollutants, Chemical; D014867:Water; D002857:Chromium; D011791:Quartz; D007501:Iron