Amorphous iron hydroxide

Pierce, M.L. Moore, C.B. 1982. Adsorption of arsenite and arsenate on amorphous iron hydroxide. Water Research, 16, 1247-1253. 

The immobilized arsenic in the precipitate is bound only by sorption onto the amorphous iron hydroxides. A sustainable immobilization would need additional action. 

Martinez, C.E. McBride, M.B. (1998a) Solubility of Cd, Cu, Pb and Zn in aged co-predpitates with amorphous iron hydroxides. Environ. Sci. Techn. 32 143-148 Martinez, C.E. Sauve, S. Jocobson, A. McBride, M.B. (1999) Thermally induced release of adsorbed Pb upon aging ferrihydrite and soil oxides. Environ. Sd. Techn. 33 2016-2020... 

Van der Woude, J.H.A. De Bruyn, P.L. (1983) Formation of colloidal dispersions from supersaturated iron(III) nitrate solutions. I. Precipitation of amorphous iron hydroxide. Colloids Surfaces 8 55-78... 

Mustafa S, Haq I. 1988. Adsorption of copper, cobalt and nickel on amorphous iron hydroxide from aqueous electrolyte solution. Environ Technol Lett 9 1379-1386. 

Reactions 2a and 2b only yield a negative free energy (AG) in the neutral pH range if a crystalline Fe(III) oxohydroxide (such as goethite) is formed as the reaction product. Inversely, the oxidation of As(III) is thermodynamically favored by reaction with an amorphous iron hydroxide, but not with goethite. The most suitable reductant under the conditions of the anoxic lake hypolimnion is certainly sulfide, which is thermodynamically favored and has been shown to react with As(V) (68, 16) according to reaction 3. 

Figure 4. Solubility of amorphous iron hydroxide as a function of log [Hf] (25°C., I = 3M NaC 101()...

Eventually this process forms the neutral species Fe(H2O)3(OH)30, which precipitates as amorphous iron hydroxide, which may settles out of the water column. Figure 3 shows the predicted effect of pH on the relative concentrations of the various iron hydrolysis species with and without considering the iron hydroxide solid, which dominates the speciation above pH 3.0 at 1 dM total iron. The log of the solubility product of this solid is -38.8, indicating that iron is very insoluble at natural pH values. Over time, this metastable amorphous material converts to more thermodynamically... 

Amorphous iron hydroxide, which precipitates rather spontaneously, is still undersaturated. Maghemite, goethite, and hematite do not usually precipitate spontaneously, but form as secondary mineral phases from hydroxides. That means the trivalent iron mainly remains in solution through complexation reactions. 

Mineral saturation indices for melanterite and amorphous iron hydroxide agree quite well with field occurrences of the same minerals. Jarosite, however, appears to be supersaturated for nearly all of the samples regardless of the presence or absence of the mineral in these streams. Field observations indicate that jarosite precipitation occurs in the microenvironment of bacterial colonies where the chemical conditions may be quite different from the bulk solution. These considerations lead us to suggest that there is a kinetic barrier which hinders jarosite precipitation but does not hinder ferric hydroxide precipitation and that this barrier is overcome by the surfaces of bacterial colonies (both iron-oxidizers and unidentified nonoxidizers ). ... 

In the analysis of the deposition of iron sediments it has already been mentioned that quite likely both iron silicates and carbonates and amorphous iron hydroxide were formed, which could convert to other forms both during the formation of the sediment and in subsequent diagenesis. Reduction of hydroxide could have been controlled by external (atmospheric) or internal (organic matter, free carbon in the sediment) oxidation-reduction buffer systems. All these variants need additional consideration in the thermodynamic analysis of diagenetic processes. 

Diagenesis in oxidizing conditions results mainly in dehydration, condensation, compaction, and crystallization of amorphous iron hydroxide and silica-gel sediments. The stable mineral forms—goethite and quartz—are formed in several stages via intermediate metastable phases. 

In addition to a-FeOOH, (/8, y, 8)-FeOOH have been synthesized. The last varieties apparently are metastable and convert to hematite a-Fe203 when boiled, with a-FeOOH formed as an intermediate phase (Butler and Ison, 1965). All the other mineral species (limonite, turgite, hydrogoethite) are goethite with a variable amount of adsorbed water, or mixtures of goethite, amorphous iron hydroxides, and sometimes hematite. 

We (Vorob yeva and Mel nik, 1977) investigated the phase transformations of freshly precipitated amorphous iron hydroxide at F = 1-9 kbar and T = 100-200°C in acid, neutral, and alkaline environments. It was estab-... 

Freshly precipitated iron hydroxides are unstable their properties (solubility, structure, morphology) vary with time. The physicochemical direction of the aging process consists of transformation of thermodynamically unstable active, i.e. easily reacting, varieties of X-ray-amorphous iron hydroxide into stable, inactive crystalline goethite. 

Diphenyl Mercury Adsorption. Adsorption of DPM from seawater onto amorphous iron hydroxide, manganese oxide and bentonite clay was not detected in this study. A comparison of standard diphenyl mercury solutions in seawater with Identical solutions to which sediment phase had been added and shaken for 48 hours was routinely performed as part of the isotherm determination. There was no significant difference in the concentration of dissolved diphenyl mercury for standard. versus standard plus solid phase for any of the suspensions of amorphous, Fe(OH)-, MnO, or bentonite in seawater, implying no significant adsorption of DPM from seawater onto these phases under the concentrations studied. If lower concentrations of DPM could have been used (ppb or lower) it is possible that adsorption might have been detected. 

Pierce, M. L., and Moore, C. B. (1980). Adsorption of arsenite on amorphous iron hydroxide from dilute aqueous solution. Environ. Sci. Technol. 14, 214-216. 

Davis, J.A., and Feckie, J.O., 1978. Surface ionization and complexation at the oxide/water interface Surface properties of amorphous iron hydroxide and adsorption of metal ions. J. Colloids Interfacial Sci., v. 67, pp. 90-107. 

Iron concentration can be reduced by precipitation of an iron mineral such as siderite (FeC03), jarosite (NaFe3(S04)2(0H) ), or amorphous iron hydroxide (Fe(OH)33). It was unclear what mineral or phase controls iron concentration, especially since precipitation was sensitive to oxidation potential (pe), and the field redox electrode measurements used in the model are not always relevant for specific redox couples. Since the mine workings flooded before the study was begun, samples of the solid phases could not be collected. 

Haen Hfo material was determined as 65 m /g (Kofod et al., 1997), which is quite low compared with the specific surface area of amorphous iron hydroxides. These results showed that the Riedel de Haen Hfo material used in the column experiments more closely resembles goethite than freshly precipitated iron hydroxide. 

The constants for the surface complexation of calcium, sulphate, phosphate and arsenate are included in the file minteq.dat. This data was not sufficient for the modelling of the column experiments performed and had to be augmented. Surface complexation constants for magnesia and chromate for amorphous iron hydroxide were taken from Dzombak and Morel (1990) (see Table 12.3). Van Geen (1994) showed that also carbon dioxide has to be considered for the modelling of adsorption. Carbon dioxide is not mentioned in Dzombak and Morel (1990). The database used contains complexation constants derived from data of Van Geen et al. (1994) and were reoptimized by Dr. C. A. J. Appelo (Amsterdam) for use with PHREEQC2 and amorphous iron hydroxide. This data was transferred from the database file PHREEQC.dat to Minteq.dat. 

According to Dzombak and Morel (1990) only one type of surface site is necessary to model the anion adsorption onto the surface of amorphous iron hydroxide. The input parameters specifying the availability of these sites are total amount of sites (moles/L), specific area (m /g) and mass (g/L). 

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