Homogeneous and Heterogeneous (Fe_x, Cr_1–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.

Organic–mineral interfacial chemistry drives heterogeneous nucleation of Sr-rich (Ba x , Sr 1− x )SO 4 from undersaturated solution

“…Here, we advance that understanding by using in situ grazing-incidence small-angle X-ray scattering (GISAXS) to measure heterogeneous nucleation and growth rates of (Ba x , Sr 1−x )SO 4 precipitated on organic films from Sr-rich solutions. As previously demonstrated, GISAXS measures nucleation and growth rates at solid-liquid interfaces with sufficient temporal and spatial resolution (18)(19)(20)(21)(22)(23)(24)(25)(26) to provide unique information about substrate-mineral interactions (24). In our recent study, a range of organic coatings (terminated with -COOH, -SH, and mixed -SH-and-COOH functional groups) were found to significantly affect the heterogeneous nucleation and growth kinetics of barite by controlling the local ion concentrations and the interfacial energies for heterogeneous barite nucleation (24).…”

“…Accordingly, an incident angle of 0.10°was selected to achieve >99% reflectivity of the incident X-rays, so only scattering generated from nanoparticles at the substrate surfaces were probed (43). Before each measurement, a piece of bare or coated SiO 2 substrate was placed into a homemade GISAXS cell (24,25), with the surface aligned vertically with the center of incident X-ray beam. After alignment, 0.7 mL of freshly mixed solution ( Table 1) was injected into the homemade cell and the in situ measurement started.…”

“…SAXS measurements were also recorded every 2 min with an exposure time of 20 s and a sleeping time of 100 s for 1 h. Preliminary tests showed no heterogeneous precipitation or ion adsorption occurred at the bare SiO 2 substrate; thus, the presence of bare SiO 2 substrate does not alter the solution conditions. For SAXS measurement, the sizes of homogeneous (Ba x , Sr 1−x )SO 4 precipitates were larger than the SAXS detectable range (1-50 nm) (22,25), so SEM was employed to characterize the morphology of the homogeneous precipitates at the end of the 1-h precipitation experiments.…”

“…With the assumption of noninteracting spherical nanoparticles with lognormal distributions, the average particle sizes were calculated by fitting the 1D scattering intensity curves. All data analysis was performed with Igor Pro-6.34 (WaveMetrics), and more detailed data analysis can be found in our previous publications (19)(20)(21)(22)(23)(24)(25)(26).…”

Removal of Methylene Blue from Aqueous Solution Using Agricultural Residue Walnut Shell: Equilibrium, Kinetic, and Thermodynamic Studies

“…According to previous work, the WS particles of 0.1 g were suspended in DI water (pH was adjusted from 2 to 12 using 0.1 mol/L HCl or NaOH), and the zeta potential measurements were conducted every 1 min for 30 min. The average values and standard deviations of zeta potential values were calculated after the readings became stable [32,33]. and then shaking on a platform shaker until the equilibrium was achieved.…”