Characterization of the pores in hydrous ferric oxide aggregates formed by freezing and thawing

Abstract

Hydrous ferric oxides (HFO) are efficient sorbents for inorganic and organic pollutants and therefore have great potentials in environmental science and engineering applications. Freezing and thawing of HFO suspensions leads to the formation of dense HFO aggregates. It facilitates the handling and increases the drying rate of HFO. In this study, we used a combination of pycnometry, gas adsorption (N 2 gas, water vapor), and small-angle neutron scattering (SANS) to characterize the porosity and pore size distribution of dense HFO aggregates formed by freezing dialyzed HFO suspensions at −25 • C and thawing them at room temperature. The crystallinity of the HFO, which was a 2-line ferrihydrite, was not affected by this treatment. Wet sieving and laser diffraction analysis showed that the dense HFO aggregates had a unimodal size distribution with an average diameter of 235 ± 35 µm. Increasing the freezing rate by cooling with liquid N 2 (−196 • C) resulted in much smaller aggregates with an average diameter of 20 µm. Adding NaNO 3 electrolyte to the HFO suspensions prior to freezing also resulted in the formation of smaller aggregates. The dense HFO aggregates formed at −25 • C had a porosity of 0.73 ± 0.02 l l −1 . SANS revealed a unimodal size distribution of pores, with an average pore diameter of 2.0 nm. The diameter of the HFO crystallites was estimated by transmission electron microscopy to be 1.9 ± 0.5 nm. Geometrical considerations taking into account the unit particle and average pore size suggest that the crystallites retain 1-2 layers of hydration water during the coagulation induced by freezing. Analysis by N 2 gas adsorption showed that drying the dense HFO aggregates induced a reduction in porosity by about 25% and shifted the pore size distribution to smaller diameters. Rewetting during water vapor adsorption did not induce significant changes of the aggregate structure. The specific surface area of the dry HFO aggregates was between 320 and 380 m 2 g −1 .

Enhanced arsenate removal by novel Fe–La composite (hydr)oxides synthesized via coprecipitation

“…The XRD patterns of Fe/La 0:1 (hydr)oxide after reaction with arsenate at pH 10.0 had not changed, which implied that no new substance was formed and only adsorption reaction occurred. However, new peaks for (1 2 0), (0 1 2), (À1 0 3) and (1 0 3) appeared, which are the typical XRD pattern of LaAsO 4 (JCPDS 15-756) [26]. This result shows that lanthanum arsenate precipitate was formed at pH 5 and pH 7 in the process of arsenate removal, which confirmed our above assumption.…”

“…3 shows the XRD patterns of the synthesized Fe-La composite (hydr)oxides. The oxide at an Fe/La molar ratio of 1:0, namely the pure Fe (hydr)oxide, has two broad peaks at approximately 35.4°and 62.2°, which are attributed to d spacing of 0.254 and 0.149 nm of poorly ordered two-line ferrihydrite [26]. This indicates that the Fe/La 1:0 was amorphous and consisted of two-line ferrihydrite.…”

Adsorptive removal of arsenic from water by an iron–zirconium binary oxide adsorbent

“…1. It shows two broad peaks at approximately 35.2°and 61.2°, consistent with d spacing of 0.254 and 0.150 nm, respectively, which are similar to those of the poorly ordered two-line ferrihydrite [24,25]. This indicates that the prepared oxide is amorphous and similar to two-line ferrihydrite.…”

Arsenate uptake and arsenite simultaneous sorption and oxidation by Fe–Mn binary oxides: Influence of Mn/Fe ratio, pH, Ca2+, and humic acid

“…1. The pattern of the oxide with a Mn/Fe molar ratio of 0:1, namely the pure Fe oxide, shows the poorly ordered 2-line ferrihydrite pattern with two broad peaks at 34.4°and 62.1°, according to d spacing of 0.260 and 0.149 nm, respectively [24,25]. The patterns of Fe-Mn binary oxides with Mn/Fe molar ratios of 1:9, 1:6, and 1:3 were almost identical to that of the 0:1 oxide.…”