The Role of Chloride Ions during the Formation of Akaganéite Revisited

Abstract

Iron(III) hydrolysis in the presence of chloride ions yields akaganéite, an iron oxyhydroxide mineral with a tunnel structure stabilized by the inclusion of chloride. Yet, the interactions of this anion with the iron oxyhydroxide precursors occurring during the hydrolysis process, as well as its mechanistic role during the formation of a solid phase are debated. Using a potentiometric titration assay in combination with a chloride ion-selective electrode, we have monitored the binding of chloride ions to nascent iron oxyhydroxides. Our results are consistent with earlier studies reporting that chloride ions bind to early occurring iron complexes. In addition, the data suggests that they are displaced with the onset of oxolation. Chloride ions in the akaganéite structure must be considered as remnants from the early stages of precipitation, as they do not influence the basic mechanism, or the kinetics of the hydrolysis reactions. The structure-directing role of chloride is based upon the early stages of the reaction. The presence of chloride in the tunnel-structure of akagenéite is due to a relatively strong binding to the earliest iron oxyhydroxide precursors, whereas it plays a rather passive role during the later stages of precipitation.

Nucleation of Hematite: A Nonclassical Mechanism

“…[25] During the prenucleation stage, only ligand exchange and olation bridging processes take place and lead to the formation of ad ynamic population of olation PNCs. [25,27] Phase separationo ccurs on upwards bending of the curve, with significant deviation from the initially linear profile (fine dottedl ine in Figure 1). [25,27] The prenucleation stage is as tate of (meta)stable equilibrium, as is evident from experiments in which the additiono f iron is stopped (also see below) and the pH remains constant without counter-titration with base.…”

“…As established elsewhere, at 25 8Ct he titration experimentsy ield akaganØite in ap Hr ange of approximately 2.1-2.7. [27] Here, the titrationc urve ( Figure 1) can be subdivided into at least two distinctr egionst hat can be regardeda sp re-and postnucle-ation stages. [25] During the prenucleation stage, only ligand exchange and olation bridging processes take place and lead to the formation of ad ynamic population of olation PNCs.…”

From Earth to Outer Space: Laser cleaning semiprecious quartz and a novel application for meteoritic metal

“…In order to evaluate the efficacy of this treatment for chlorine removal, compositional analysis of a cross section is required to monitor the chemical depth profile for chlorine removal. Previous studies have determined that effective bulk chloride removal in iron alloys can only be achieved at temperatures in excess of 400°C [16,17]. Therefore, given the surface sensitivity of laser cleaning, chloride removal is limited to the uppermost micrometers of the sample surface.…”

ZVI (Fe0) Desalination: Stability of Product Water

“…Chlorides have been demonstrated to be preferentially bound to the freshly-formed amorphous (hydrated) iron hydroxide precursors (e.g., Figure I2) during the initial hydrolysis of Fe 3+ [148]. This amorphous material can eventually crystallize as akaganeite [146,148], but may (during forced hydrolysis) crystallize as hematite (α-Fe 2 O 3 ) [149].…”

“…This amorphous material can eventually crystallize as akaganeite [146,148], but may (during forced hydrolysis) crystallize as hematite (α-Fe 2 O 3 ) [149].…”