Effect of Ferrihydrite Crystallite Size on Phosphate Adsorption Reactivity

Abstract

The influence of crystallite size on the adsorption reactivity of phosphate on 2-line to 6-line ferrihydrites was investigated by combining adsorption experiments, structure and surface analysis, and spectroscopic analysis. X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that the ferrihydrite samples possessed a similar fundamental structure with a crystallite size varying from 1.6 to 4.4 nm. N2 adsorption on freeze-dried samples revealed that the specific surface area (SSABET) decreased from 427 to 234 m(2) g(-1) with increasing crystallite size and micropore volume (Vmicro) from 0.137 to 0.079 cm(3) g(-1). Proton adsorption (QH) at pH 4.5 and 0.01 M KCl ranged from 0.73 to 0.55 mmol g(-1). Phosphate adsorption capacity at pH 4.5 and 0.01 M KCl for the ferrihydrites decreased from 1690 to 980 μmol g(-1) as crystallite size increased, while the adsorption density normalized to SSABET was similar. Phosphate adsorption on the ferrihydrites exhibited similar behavior with respect to both kinetics and the adsorption mechanism. The kinetics could be divided into three successive first-order stages: relatively fast adsorption, slow adsorption, and a very slow stage. With decreasing crystallite size, ferrihydrites exhibited increasing rate constants per mass for all stages. Analysis of OH(-) release and attenuated total reflectance infrared spectroscopy (ATR-IR) and differential pair distribution function (d-PDF) results indicated that initially phosphate preferentially bound to two Fe-OH2(1/2+) groups to form a binuclear bidentate surface complex without OH(-) release, with smaller size ferrihydrites exchanging more Fe-OH2(1/2+) per mass. Subsequently, phosphate exchanged with both Fe-OH2(1/2+) and Fe-OH(1/2-) with a constant amount of OH(-) released per phosphate adsorbed. Also in this stage binuclear bidentate surface complexes were formed with a P-Fe atomic pair distance of ~3.25 Å.

Laboratory synthesis of goethite and ferrihydrite of controlled particle sizes

“…However, if we use the back-calculated SSAs from the linear relationship previously found (cf. section 3.4.3) (Wang et al, 2013), of 400 and 494 m 2 /g for AFh and FFh, respectively, we obtain values of 2.43 and 2.12 μmol m -2 for AFh and FFh, respectively. These values suggest that the surface of the aged sample is more reactive than that of the fresh sample, i.e., larger FH particle sizes are more reactive, which is an unexpected result, but shows a similar trend as for goethite.…”

“…Because of this extreme particle aggregation, direct and accurate particle size measurements in ferrihydrites However, as stated before, the obtained BET SSA values are indicative of the exposed surface area of the dry aggregates, not of the individual particles, and do not reflect the SSA values expected under aqueous suspension. To approach the latter, we used the relationships reported by Wang et al (2013) between particle size and phosphate adsorption capacity, and in turn between the latter and BET SSA of a series of ferrihydrites, to cross-calculate the relationship between particle size and BET SSA of our ferrihydrites. The linear relationship obtained is shown in Figure 8, and we may interpolate our BET SSA values, to obtain particle sizes of 3.4 and 4.2 nm for FFh and AFh, respectively.…”

Structure and composition of Fe–OM co-precipitates that form in soil-derived solutions

“…; Arai and Sparks (2001)) was impeded by superposing bands of effluent OM and the absence of distinct shoulders (Figure 3). This was likely related to the freeze-drying of the Fe-OM co-precipitates, which results in a rather broad band at 1036 cm -1 induced by adsorbed phosphate (Wang et al, 2013). Adsorbed As, Si, and sulfate could not be traced in spectra of the Fe-OM co-precipitate-free controls, which points to the interaction of these compounds with the Fe-OM co-precipitates.…”

“…For this reason, ATR-FTIR spectra of filter cakes were used to distinguish between free effluent OM and OM in the Fe-OM co-precipitates. We assume the filter cakes to consist solely of Fe-OM co-precipitates, which is indicated by the absence of COO -stretch) and 1042 cm -1 (C-O-C stretch, C-OH stretch in polysaccharides; adsorbed phosphate (Wang et al, 2013)). A tailing at 1600 cm -1 (asym.…”

Advances in coupling of kinetics and molecular scale tools to shed light on soil biogeochemical processes

“…Over the past decades, solid-phase speciation studies of P have relied largely on the use of spectroscopic techniques, especially Fourier transform infra-red (FTIR) (Tejedor-Tejedor and Anderson, 1990;Persson et al 1996;Arai and Sparks 2001;Elzinga and Sparks 2007;Luengo et al 2006;Antelo et al 2010), 31 P solid-state NMR (Bleam et al 1991;Kim and Kirkpatrick 2004;Li et al 2010Li et al , 2013aKim et al 2011) and synchrotronbased X-ray pair distribution function (PDF) analyses (Wang et al 2013).…”