FeOOH, precipitation

FeOOH precipitates as platy crystals. When formed by fast oxidation of Fe(OH)2 at pH 12 well formed hexagonal plates result, whereas simultaneous precipitation/oxi-dation gives thin plates which are often rolled up (Feitknecht, 1959). Feroxyhyte (5 -FeOOH) produced by rapid oxidation (e.g. with H2O2) of FeQ2 solution at pH 8 also forms thin platy crystals around 100 nm in size (Fig. 4.27, upper). As the pH is lowered, the crystals become smaller and develop a grassy appearance (Carlson Schwertmann, 1980). Natural feroxyhyte from the Clara Mine in the Black Forest occurred as vermiform aggregates (Fig. 4.27, lower) (Walenta, 1997). 

Precipitation of ferric hydroxide gel was also observed in the preparation of spindlelike hematite (a-Fe203) particles in a dilute ferric chloride solution in the presence of phosphate (9). In this case, however, the positive role of the gel was not definite since similar uniform hematite paricles were obtained as well in homogeneous systems in the presence of the same anions (9). Also, Hamada and Matijevic (10) prepared uniform particles of pseudocubic hematite by hydrolysis of ferric chloride in aqueous solutions of alcohol (10-50%) at I00°C for several days. In this reaction, it was observed that acicular crystals of (3-FeOOH precipitated first, and then they dissolved with formation of the pseudocubic particles of hematite. The intermediate P-FeOOH appears to work as a reservoir of the solute to maintain an ideal supersaturation for the nucleation and growth of the hematite. Since the (3-FeOOH as an intermediate and the pseudocubic shape tire not peculiar to the alcohol/water medium... 

To obtain the high selective catalyst, the jj-FeOOH phase was used as the initial iron compound. Its contamination with Cl was so lowered as it was possible by 24 hr. ageing and thorough washing of ji-FeOOH precipitate. 

In the goethite process, the precipitation of iron from solution occurs in the form of hydrated ferric oxide, FeOOH. The commercial development of the process was due to Societe de La Vielle Montagne. The process basically involves the reduction of iron to the ferrous state, and this is followed by oxidation by air at a temperature of around 90 °C and at a pH controlled at around 3.0. The reaction can chemically be shown as ... 

Despite the seeming exactitude of the mathematical development, the modeler should bear in mind that the double layer model involves uncertainties and data limitations in addition to those already described (Chapter 2). Perhaps foremost is the nature of the sorbing material itself. The complexation reactions are studied in laboratory experiments performed using synthetically precipitated ferric oxide. This material ripens with time, changing in water content and extent of polymerization. It eventually begins to crystallize to form goethite (FeOOH). 

As an example of an equilibrium calculation accounting for surface complexation, we consider the sorption of mercury, lead, and sulfate onto hydrous ferric oxide at pH 4 and 8. We use ferric hydroxide [Fe(OH)3] precipitate from the LLNL database to represent in the calculation hydrous ferric oxide (FeOOH /1H2O). Following Dzombak and Morel (1990), we assume a sorbing surface area of 600 m2 g-1 and site densities for the weakly and strongly binding sites, respectively, of 0.2 and 0.005 mol (mol FeOOH)-1. We choose a system containing 1 kg of solvent water (the default) in contact with 1 g of ferric hydroxide. 

In a second example, we calculate how pH affects sorption onto hydrous ferric oxide, expanding on our discussion (Section 10.4) of Dzombak and Morel s (1990) surface complexation model. We start as before, setting the dataset of surface reactions, suppressing the ferric minerals hematite (Fe203) and goethite (FeOOH), and specifying the amount of ferric oxide [represented in the calculation by Fe(OH)3 precipitate] in the system... 

This discrepancy might be explained if after about an hour the reaction approached equilibrium and slowed due to a diminishing thermodynamic drive. If the Fe+++ produced did not precipitate on the hematite surface, and did not form either hematite or goethite (FeOOH), it would accumulate in solution and weaken the drive for uranyl reduction. As the saturation index for hematite reached about 1.7, or about 1.25 for goethite, reaction would cease. 

As pH rises, the metal content of drainage water tends to decrease. Some metals precipitate directly from solution to form oxide, hydroxide, and oxy-hydroxide phases. Iron and aluminum are notable is this regard. They initially form colloidal and suspended phases known as hydrous ferric oxide (hfo, FeOOH n O) and hydrous aluminum oxide (HAO, AlOOH nH.2O), both of which are highly soluble under acidic conditions but nearly insoluble at near-neutral pH. 

The phenomena of surface precipitation and isomorphic substitutions described above and in Chapters 3.5, 6.5 and 6.6 are hampered because equilibrium is seldom established. The initial surface reaction, e.g., the surface complex formation on the surface of an oxide or carbonate fulfills many criteria of a reversible equilibrium. If we form on the outer layer of the solid phase a coprecipitate (isomorphic substitutions) we may still ideally have a metastable equilibrium. The extent of incipient adsorption, e.g., of HPOjj on FeOOH(s) or of Cd2+ on caicite is certainly dependent on the surface charge of the sorbing solid, and thus on pH of the solution etc. even the kinetics of the reaction will be influenced by the surface charge but the final solid solution, if it were in equilibrium, would not depend on the surface charge and the solution variables which influence the adsorption process i.e., the extent of isomorphic substitution for the ideal solid solution is given by the equilibrium that describes the formation of the solid solution (and not by the rates by which these compositions are formed). Many surface phenomena that are encountered in laboratory studies and in field observations are characterized by partial, or metastable equilibrium or by non-equilibrium relations. Reversibility of the apparent equilibrium or congruence in dissolution or precipitation can often not be assumed. 

The process involves seeding tanks containing FeS04 7 H2O solution, alkali and metallic iron with FeOOH, and aerating the stirred suspension. The seeds, which are very fine, are precipitated from Fe" solution at 20-50 °C (depending on the colour required), i. e. 

FeOOH) indicated that a Clostridium species released 55% of the coprecipitated nickel after 40 hours. Similarly, precipitated nickel sulfides in sediment can be mobilized through sulfur oxidation by Thiobacilli (Wood 1987). In this case, the oxidized sulfur may produce H2SO4 and decrease the pH. 

Hydrolysis of FeCf Solutions. Aging of ferric chloride solutions yields, as a rule, either colloidal akageneite (p-FeOOH) or hematite (a-Fe203). However, the two forms are closely related in the formation of the precipitates (95,142). 

Simple cations such as [Fe(H20)6]3"" undergo a certain amount of primary hydrolysis, depending upon the pH of the solution. The ion is in the hexaaqua form only at pH values lower than 2.0. Above that value the hydroxopentaaquairon(III) ion, [Fe(H20)50H]2 +, is predominant. Further increase in the pH of the solution causes more hydrolysis, until a complex solid material sometimes described erroneously as iron(lll) hydroxide is precipitated. The solid does not have the formula Fe(OH)3, but contains iron(III) oxohydroxide (FeOOH) and iron(III) oxide in various states of hydration, Fe203.xH20. 

Orange iron oxide with the lepidocrocite structure (y-FeOOH) is obtained if dilute solutions of the iron(II) salt are precipitated with sodium hydroxide solution or other alkalis until almost neutral. The suspension is then heated for a short period, rapidly cooled, and oxidized [3.22], [3.23],... 

Body-doped pigments contain 1-5% cobalt that is uniformly distributed throughout the bulk of the pigment particles. It is either incorporated during production of the FeOOH precursor or precipitated as the hydroxide onto one of the intermediate products [5.11] using cobalt(II) salts as the cobalt source. 

The selective hydrolysis of metal ions to produce various forms of hydrated oxides is the most widely used form of precipitation. In particular, the removal of iron from hydrometallurgical process streams is a continuing problem. Iron enters the circuit as a constituent of a valuable mineral, such as chalcopyrite (CuFe2), or an impurity mineral, such as the ubiquitous pyrite or pyrrhotite. So far, effective removal of the iron has been achieved by the precipitation of iron(III) as jarosite (MFe3(S04)2(OH)6),401 goethite (FeOOH)402 or hematite (Fe203).403... 

In the first study of hydrolysis of Fe(OEt)3 carried out by Thiessen et al. [1577], the precipitate of cubic crystals Fe203 4H20 was obtained after refluxing of diluted alcoholic solution of iron ethoxide for a few hours. Hydrolysis of iron ethoxide by excess of water results in amorphous precipitate, which on thermal treatment at first gives y-Fe203 and then a-Fe203 (in contrast to the precipitates of (X-FeOOH, which precipitate from aqueous solutions) [1776]. 

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