Nucleationprecipitation Of Fresh Iron Oxyhydroxides

Over the years, the precipitation of iron oxides and hydroxides from acidic solutions has received considerable attention in various fields, such as catalyst synthesis, environmental sciences and industrial processes [e.g. 22-29]. Freshly formed iron hydroxide particles help to control pollution in aquatic systems, e.g. by fixation and transport of phosphates, heavy metals and other reactive inorganic and/or organic species [30-33]. The high reactivity of these iron phases is mainly due to their small size. The formation and aggregation of iron colloids, which occurs in continental and marine aquatic systems [22, 34-38], is also employed in water and wastewater treatments [35, 39,40].

In aquatic systems, Fe(III)-aquo ions may polymerize by deprotonation to form sparingly soluble colloidal iron oxyhydroxides. Although the crystalline end-products of this reaction, such as goethite (a-FeOOH) and hematite (a-Fe2O3), are well characterized, the structure and shape of the intermediate phases are far from being fully elucidated. The shape and 'porosity' of freshly formed iron colloids and aggregates control their capacity to bind and transport a high number of elements. In some cases, e.g. where the size of the particles is not too small, a fractal approach can be useful to determine the reactivity of the iron colloids and aggregates and to explain their high specific surface and roughness. This approach, however, must be combined with a molecular-scale characterization, since these objects are very small and are not always geometrically self-similar.

General models of nucleation have been proposed [41, 42] based on the concept of similarity between the structure of polymers and the structure of the resulting crystalline phases. Nonetheless, these models have failed to predict the precipitation processes correctly. A generalized approach based on the electrostatic field theory [25, 43] was able to provide hints for understanding the formation of small clusters but could not be applied to larger ones.

5.3.1 Nucleation

Several pioneering studies [41, 42, 44, 45] have developed the general concepts of polymerization, growth and precipitation of Fe(III) ions, but the intermediate polymers and clusters have not been identified experimentally. Structural information was based on the modeling of potentiometric data, not on the observation of the objects, which became possible later by the use of X-ray absorption spectroscopy (XAS) and SAXS [39,40, 46-51].

The structure and size of iron polymers depends on the hydrolysis ratio n = OH/Fe, time t, and nature of the anions. At low hydrolysis ratios, two Fe monomers react to form a dimer in which both octahedra share an edge. Mossbauer spectroscopy has shown that, at n <0.5, hydrolysis of Fe(NO3)3 at 92 °C leads to the formation of Fe2(OH)2+ [52].

Extended edge X-ray absorption fine structure (EXAFS) spectroscopy has revealed the presence of a dimer formed from FeCl3 at n = 1.5 and t = 10 min [50], with the two Fe at a distance of 3.01 A, corresponding to Fe octahedral edge sharing [53]. The dimer could be stabilized by organic ligands such as carboxylates and proteins [45], whereas the growth of Fe species in the absence of 'stabilizers' continues with the formation of a trimer (Figure 5.1) [50].

Figure 5.1 Structure of a planar trimer (A) and a double corner trimer (B) of Fe3(OH)5+.

In addition to the trimers, in Fe(NO3)3 solution, for n = 1.5 and t > 1h, small polymers are detected with Fe-Fe interatomic distances of 2.85 A, 3.06 A, 3.52 A and 3.95 A, which correspond respectively to face, edge, double corner and single corner sharing between Fe octahedra [51, 53]. The unusual Fe-Fe distance of 2.85 A, which is characteristic of Fe octahedra with face sharing [47, 53], has also been observed in freshly formed two-line ferrihydrite [54]. When the Fe concentration is in the decimolar range, XAS and SAXS data can be reconciled with models taking into account a limited number of Fe clusters (Figure 5.2). In more dilute solutions ([Fe] < 10-4 m), the number of edge- and corner-sharing neighbors at pH ~ 2.8 suggests the formation of small Fe polymers that most likely consist of a mixture of dimers, trimers and tetramers [28]. Tetrameric species with edge and double-corner linkages were also shown to form during Cr3+ and Ga3+ hydrolysis [55].

The relevant differences measured in the very first steps of FeCl3 and Fe(NO3)3 hydrolysis are most likely due to the nature and complexing strength of anions. In the early stages of FeCl3 hydrolysis, one or two of the six coordination sites in the Fe octahedra are occupied by chloride anions, whereas only OH and OH2 ligands

DimF + PentaE

DimF + PentaE-DC TrimF + Trim DC

Single corr linkage

DimF + PentaE

DimF + PentaE-DC TrimF + Trim DC

Single corr linkage

Dim E + Tetra SC Trim E + Trim DC Fe23 Ferrihydrite 2L

Dim E + Tetra SC Trim E + Trim DC Fe23 Ferrihydrite 2L

Figure 5.2 Six possible clusters present in Fe(NO3)3 solution at n = 1.5. Dim = dimer, trim = trimer, tetra = tetramer, penta = pentamer, F = Face, E = edge; DC = double corner; SC = single corner. All clusters correspond to substructures of larger minerals. The Fe23 ferrihydrite 2L substructure is described in [54].

Figure 5.2 Six possible clusters present in Fe(NO3)3 solution at n = 1.5. Dim = dimer, trim = trimer, tetra = tetramer, penta = pentamer, F = Face, E = edge; DC = double corner; SC = single corner. All clusters correspond to substructures of larger minerals. The Fe23 ferrihydrite 2L substructure is described in [54].

are detected in Fe(NO3)3 solutions. Consequently, Fe(NO3)3 exhibits more binding possibilities than FeCl3, which likely includes single-corner bonds whose free energy of formation of 105 kJ mol-1 is lower than that of a double-corner bond [43].

Marked differences between the polymerization mechanisms of Fe(NO3)3 and FeCl3 have also been shown to occur at higher hydrolysis ratios (or times). In the case of Fe(NO3)3, the cluster radii increase with hydrolysis ratio [48]. In particular, some authors have suggested that the dodecamer Fe12(OH)3+ may form in solutions of Fe(NO3)3 at n> 2.3 [56], whereas others have postulated the formation of the polycation [FeO4Fe12(OH)24(H2O)12]7+ with a structure similar to that of the Al13 polymer [57]. In contrast, the size of aggregate subunits formed from FeCl3 does not appear to vary with the hydrolysis ratio [58]. On the basis of SAXS, EXAFS and Ar adsorption data, it has been suggested that the polymer Fe24O12(OH)36+, which shows a local structure similar to akaganeite (P-FeOOH), can form [50] (Figure 5.3).

5.3.2 Diffusion, Aggregation, Fractal and Nonfractal Growth

In contrast to FeCl3 hydrolysis, the hydrolysis of Fe(NO3)3 leads to a more chaotic evolution of Fe polymers and Fe nuclei. Two possible mechanisms have been suggested to explain this observation. The first consists of crystal growth or precipitation based on ion diffusion and includes the following steps [59]: (i) diffusion of Fe(III) ions to the surface of the nucleus; (ii) dehydration of Fe at the surface; (iii) adsorption of dehydrated Fe; (iv) diffusion of Fe on the surface to a more energetically favorable position. In contrast to the ion diffusion mechanism, the growth of Fe species appears to be explained better by the aggregation of primary polymers. This second polymerization polymerization

Figure 5.3 XAS Fe K-edge radial distribution function R and structural evolution of Fe polycations during hydrolysis in FeCl3 solution.

mechanism, i.e. the formation of larger polymers or precipitates, which are generally amorphous at ambient temperatures, has been studied using a fractal approach [48, 58].

In the case of FeCl3, particle sizes measured by photon correlation spectroscopy (PCS) and SAXS [58] decrease with time and increase with n at a constant time. For example, at t = 24 h, the hydrodynamic radius rH of the particles is 12 nm at n = 1.5 and 200 nm at n = 2.7. Owing to the small size of Fe aggregates, SAXS curves do not show any fractal behavior at n = 1, whereas suspensions of Fe24 subunits appear arranged in fractal aggregates at n = 2.0 and 2.5 [58]. With increasing n, the sticking of polycations follows the usual rules of clustering as controlled by van der Waals interactions. An increase to n = 3 causes the formation of precipitates that exhibit a characteristic scattering curve with an apparent fractal dimension of Df = 2. In solutions at n = 1, the polycations feature a highly positive charge and the aggregation mechanism depends on long-range dipolar magnetic interactions with each subunit acting as a dipole that can realign in prevalently linear aggregates. More subunits are formed when n increases, but the positive charge at the subunit surface decreases, especially at n> 2.5, i.e. just before the flocculation threshold of the sols. For this condition, electrostatic repulsion decreases and electrostatic attractive interactions become responsible for the fractal arrangement of colloids. The various steps of FeCl3 polymerization are summarized in Figure 5.3.

In the case of Fe(NO3)3, aggregates are thought to take up a linear or semi-linear shape at the local range-order that consists of 4-5, 3-4, 7 and 9 subunits for n = 1.5, 2.0,2.2 and 2.5 respectively [49]. At t < 10 min, greater branching and polydispersity is observed for n = 2.2 and 2.5, whereas aggregates have a fractal geometry with an apparent fractal dimension of Df = 1.75 for n = 2.8, typical of a cluster-cluster aggregation mechanism. Although this value is lower than that measured for FeCl3 solutions at n = 2.7(Df = 2), some authors have suggested that less aggregated and less dense, small particles can form in Fe(NO3)3 solutions compared with FeCl3 solutions [59].

5.3.3 The Effect of Strong Competing Ligands: The Case of Phosphate

When in competition with O and OH ligands, the presence of species such as phosphate, silicate, and organic matter is thought to affect the composition, structure, morphology and reactivity of Fe hydrolysis products [60-67]. PO4- ions hinder the hydrolysis of Fe3+ cations by affecting the size and crystallinity of the particles even at low [PO4]/[Fe] molar ratios [61]. For example, EXAFS spectra at the Fe K-edge have indicated that, for n = 1 and [P]/[Fe] « 0.2, Fe nucleation is extensively blocked at the dimerization step (Figure 5.4) [68] and that Fe-O-P linkages are formed [69]. For n = 1.5, dimers are the major Fe species (Figure 5.4) with P in the second coordination sphere of Fe at a distance of 3.2-3.3 A [65, 67]. Binding of phosphate to the Fe dimers inhibits the binding of a third Fe octahedron to the dimer, unlike what occurs in FeCl3 solutions. For n = 2.0, each phosphate is able to bind three Fe dimers [69] in order to form small aggregates with a size of approximately 100-120 A [58].

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