Iron hydroxide, sorption

This porous material is a commercial sorption material of the company Dr. Ecker GmbH (GERMANY) which is a composition of calcite, brucite, fluorite and iron hydroxides (RFA 14 % Fe203). 

The mobilization of arsenic from the tailings material seems to be a slow and continuos process attributed to reduction of iron phases. The seepage water of the middle source contains arsenite as well as arsenate in high concentrations and seems to be the only water source in contact with the tailings material. The concentrations of arsenic downstream are still high and the immobilization process by precipitation of iron hydroxide and coprecipitation or sorption of arsenic is incomplete. A reason for this may be the slow kinetics of the oxidation process and the transport of fine grained hydroxide particles. These particles are mobile and can bind the arsenic (mainly as arsenate) too. 

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

The sorption on newly formed iron hydroxides is known to be reversible on short time scales (8). The differences in autoradiographs (B) and (C) of Figure 3 indicate that a large amount of exchangeable activity is associated with the ferromagnesian minerals however, areas other than those of high iron content also show a decrease in activity after the CaCl2 extraction. 

Fig. 9 shows the pH-dependent sorption of metal cations Fig. 10 the same for selected anions on iron hydroxide. 

Fig. 10 pH-dependent sorption of anions on iron hydroxide (after Drever 1997)... 

Fig. 11 Schematic depiction of the pH-dependent sorption behaviors of iron hydroxide surfaces at accretion of the H+ and OH- ions (after Sparks 1986).

Davis, C. C., Knocke, W. R., and Edwards, M., 2001, Implications of aqueous silica sorption to iron hydroxide Mobilization of iron colloids and interference with sorption of arsenate and humic substances Environmental Science Technology, v. 35, p. 3158-3162. 

Results of the adsorption-structural studies on building-up of iron hydroxide particles by aluminium hydroxide followed by calcination have shown that with account of phase and structural changes, the sorption capacity of the samples increases with the calcination temperature and content of Fe20s in their composition (Table 7, samples 5-8). 

This behaviour of the sorption capacities of the samples indicates that the value of the volume shrinkage of thermally treated iron hydroxide is noticeably higher than the accretion in its sorption capacity. As it was mentioned above, this is caused by forming an additional pore volume inside the built-up globule. 

Comparison of sorption capacities Vj of built-up samples with the additive quantities of sorption (Vs add.) shows that depending on the composition and thermal treatment, the latter vary from 0.32 to 0.39 cm /g, i.e., they are substantially lower than the experimental values. However, in reality, in the case of mechanical mixing of individual hydroxides of iron or aluminium Vs values must coincide since the sorption capacity is a statistical mean determined by the particle size of the components mixed and their arrangement. In the present case one of these conditions is violated as the particle size increases substantially as a result of building-up of particles. This is the main reason for the differences between Vs of the samples obtained and the additive sorption capacities (Vs add.). Moreover, shrinkage of hydroxide components in the process of drying and calcination makes a noticeable contribution to this process, which is observed especially distinctly for the samples of iron hydroxide built-up by aluminium hydroxide (Table 7). 

Davis, Ch.C., Chen, H.W., and Edwards, M.. Modeling silica sorption to iron hydroxide. Environ. Sci. Technol., 36, 582, 2002. 

The degree of reversibility can depend on the amount of time that the pollutant has been in contact with the solid. Sorption onto iron hydroxides, organic matter and metal carbonate minerals is often observed to be irreversible over time spans exceeding years (Brady etal. 1999). Immobilization reactions are indicated by slow reaction kinetics as found, for example, for the binding of nickel, zinc, and cadmium to different soil constituents which (Gerth 1985). When in contact with iron oxides, these elements are immobilized by matrix diffusion (Briimmer etal. 1988 Gerth etal. 1993). 

Pb Sorption to iron hydroxides, organic matter, carbonate minerals, formation of insoluble sulfides Iron hydroxide availability pH, alkalinity, and Ca " levels to answer if calcium carbonate is stable. Eh, and if Eh is low, sulfide levels. Organic carbon content... 

Ba Sorption to iron hydroxides, formation of insoluble sulfate minerals Sulfate levels... 

U(VI) Sorption to iron hydroxides, precipitation of insoluble minerals, reduction to insoluble valence states Iron hydroxide availability pH, availability of reducing compound... 

Sr Sorption to carbonate minerals, formation of insoluble sulfates Iron hydroxide availability pH, and levels to answer if calcium carbonate is stable. 

Humic acids are soluble in weak alkaline solutions and are essentially insoluble in water and mineral acids. They may be precipitated from solution by the action of mineral acids and bivalent or trivalent cations, however, they are fairly resistant to the acid hydrolysis. They are dark spherocol-loids with a cross-linked structure which plays a part in their high sorption capacity. They exhibit different degrees of a tendency to aggregation and very different degrees of dispersion. In comparison with other types of natural organic substances, the humic acids are characterized by their extraordinary stability in the soil. This stability is due to their ability to form organomineraJ complexes, particularly with clay minerals and with aluminium and iron hydroxides. 

CC Davis, H-W Chen, M Edwards. Re-examining the role of silica sorption in iron hydroxide surface chemistry. Environ Sci Technol, submitted for publication. 

Samples from the site contained considerable amounts of freshly precipitated iron hydroxides. Their transformation into thermodynamically more stable minerals such as goethite or hematite has a very slow kinetics, thus ferrihydrite was chosen as the major adsorbing surface. The Diffuse Double Layer model (Dzombak and Morel, 1990) was selected to describe surface complexation. The respective intrinsic surface parameters and the reaction constants for the ions competing with uranium(VI) for sorption sites were taken from a database mainly based on Dzombak and Morel, 1990, with the urani-um(Vl) sorption parameters as determined by Dicke and Smith, 1996. The results, based on runs with 1000 varied parameter sets, are summarized in Table 5.2. 

PHREEQC is a computer programme for hydrogeochemical calculations such as speci-ation of water constituents, mineral-water saturation indices, batch reactions, ID-transport (Parkhurst and Appelo, 1999). The recent version of the programme (PHREEQC2 Ver. 2.3) is able to model sorption processes using the surface complexa-tion concept (Dzombak and Morel, 1990). At present PHREEQC2 is distributed with a set of surface complexation constants that are derived from amorphous, freshly precipitated iron hydroxide. The data for the surface complexation reactions in the database of PHREEQC2 are taken from Dzombak and Morel (1990). 

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