Hydroxides of iron

Bernal, J.D. Mackay, A.L. (1965) Topotaxy. Tschermaks mineralogische und petrogra-phische Mitteilungen 10 331-340 Bernal, J.D. Dasgupta, D.R. Mackay, A.L. (1959) The oxides and hydroxides of iron and their structural interrelationships. Clay Min. Bull. 4 15-29... 

The objective of the synthesis would be to maximise the first reaction (leading to ferrlslllcates) and suppress the formation of hydroxides of iron by the latter reaction by operating at low pH, using aluminium free source of Si and adjusting the reaction conditions to maximise the concentration of monomeric/short chain silicate species. Usually the... 

In crystalline oxides and hydroxides of iron (III) octahedral coordination is much more common than tetrahedral 43). Only in y-FegOs is a substantial fraction of the iron (1/3) in tetrahedral sites. The polymer isolated from nitrate solution is the first example of a ferric oxyhydroxide in which apparently all of the irons are tetrahedrally coordinated. From the oxyhydroxide core of ferritin, Harrison et al. 44) have interpreted X-ray and electron diffraction results in terms of a crystalline model involving close packed oxygen layers with iron randomly distributed among the eight tetrahedral and four octahedral sites in the unit cell. In view of the close similarity in Mdssbauer parameters between ferritin and the synthetic poljmier it would appear unlikely that the local environment of the iron could be very different in the two materials, whatever the degree of crystallinity. Further study of this question is needed. 

Double decomposition by NH4OH. A metallic hydroxide may precipitate, insoluble in excess of ammonium hydroxide or ammonium salts. Examples in analytical chemistry are the precipitation of the hydroxides of iron and aluminum... 

The preparation of lyophilic sols is easy and most of the time a mixture of the dispersion medium and the substance to be dispersed need only be stirred. Gelatine, for example, disperses almost spontaneously in water. The hydroxides of iron, aluminium, chromium and zirconium as well as vanadium pentoxide and silicic acids all belong to the group of hydrophilic colloids. 

In the presence of potassium hydroxide, however, the insoluble hydroxides of iron and nickel are produced. The processes of discharge and charge may be represented as follows ... 

The MPF and TMGM prospecting methods are based on the use of metallo-organics (fulvates and humates of metals) and oxides of iron and manganese (metals bound in oxides and hydroxides of iron and manganese). These forms of metals are the result of the secondary fixation of the movable fomis in rocks and have features such as (1) increased concentration coefficient and (2) only a weak bond with their initial geological source (in comparison, for example, with the movable forms collected in CHIM and MDE). Samples for MPF are taken from the humus-enriched layer at a depth of 5-10 cm, and samples for TMGM are taken from the sand-clay layer at a depth of 15-20 cm. 

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). 

By the precipitation of iron hydroxide the equilibrium shifts in favor of further rust formation. But rust is not pure iron hydroxide - it is a mixture of oxides and hydroxides of iron. 

Another X-ray and electron diffraction study of ferritin (92) also led to the conclusion that the structure was not similar to that of natural oxides or hydroxides of iron. The diffraction pattern could also be indexed... 

Soil-leaching studies indicate that some silica is released from soil rather rapidly. McKeague and Cline (20) have shown that in soil—water mixtures at 100% water saturation, the silica in solution after 5 minutes was approximately half as great as that after 10 days. After the first day or two the silica concentration increased very slowly. They also demonstrated that pH has a marked effect on silica concentrations in soil solutions (21). These authors attributed the control of silica concentration to pH-dependent adsorption and indicated that, of the common soil minerals, iron and aluminum oxides have appreciable adsorption capacity. Jones and Handreck (22, 23) studied the effects of iron and aluminum oxides on silica concentrations in soil solutions and concluded that both caused a significant reduction in dissolved silica, with aluminum oxides being most effective. Minimum silica concentrations occurred at pH 9-10 in solutions in contact with iron and aluminum oxides. Harder and Flehmig (24) reported that the hydroxides of iron, aluminum, and other elements could remove silica from solutions containing as little as 0.5 mg/liter SiOo. 

Acid antimoimte of potash Hydrated carborrate of zinc Hydroxide of iron... 

Because oxides and hydroxides of iron and aluminum exist in diverse forms. Mo can be adsorbed to them in various ways. This may involve covalent bonding or an exchange with the surface hydroxyls (Ellis and Knezek, 1973). Compared with adsorption on hydrous Fe oxides, the strength of adsorption to A1 oxide is much weaker (Jones, 1956, 1957). 

A study was conducted on Mississippi River sediment material under conditions of controlled pH (5.0, 6.5, and 8.0) and redox potential (-150, 50, 250, and 500 mV) to determine the effect of these parameters on chemical forms and distribution of added zinc. The results of this study indicate that adsorption by or coprecipitation with oxides and hydroxides of iron and manganese was the important regulatory process governing the availability of zinc in this sediment-water system. 

Retention of added zinc by sediment solids was 56-60% at pH 5.0, >97% at pH 6.5, and essentially 100% at pH 8.0. Most of the zinc was present in the exchangeable and reducible fractions. Only a small proportion of added zinc was associated with insoluble organic material, as evidenced by the low recovery in DTPA and residual organic fractions (Table 12.3). The reducible phase is believed to consist of Zn strongly adsorbed to or coprecipitated with oxides and hydroxides of iron... 

---