Ferric-hydroxide-phosphate

Golterman H. L. (1995a) The role of the ferric hydroxide-phosphate-sulphide system in sediment water exchange. Hydrobiologia 297, 43-54. 

To form ferritin, micelles of ferric hydroxide phosphate are included in the apoferritin molecule. If ferritin is reacted with Fe in an HCO3 buffer, a phosphorus-free ferritin is obtained. The reaction between iron and apoferritin requires that the iron be in the ferrous form, but after its inclusion into the apoferritin molecule, the ferrous iron is oxidized to ferric iron by air. 

Haywood and Riley [14] have described a spectrophotometric method for the determination of arsenic in seawater. Adsorption colloid flotation has been employed to separate phosphate and arsenate from seawater [15]. These two anions, in 500 ml filtered seawater, are brought to the surface in less than 5 min, by use of ferric hydroxide (added as 0.1 M FeC 2 ml) as collector, at pH 4, in the presence of sodium dodecyl sulfate [added as 0.05% ethanolic solution (4 ml)] and a stream of nitrogen (15 ml/minutes). The foam is then removed and phosphate and arsenate are determined spectrophotometrically [16]. Recoveries of arsenate and arsenite exceeding 90% were obtained by this procedure. 

Cerium Ce(IV) co-precipitation with ferric hydroxide, dissolution in hydrochloric acid, then passed through a column of bis (2 ethyl hexyl) phosphate on poly(vinylchloride), eluted with 0.3 M perchloric acid Spectrofluorimetry at 350 nm (excitation 255 nm) [626]... 

Uranium coprecipitated with aluminium phosphate, precipitate dissolved in nitric acid Adsorption onto colloidal ferric hydroxide... 

Precipitation can occur if a water is supersaturated with respect to a solid phase however, if the growth of a thermodynamically stable phase is slow, a metastable phase may form. Disordered, amorphous phases such as ferric hydroxide, aluminum hydroxide, and allophane are thermodynamically unstable with respect to crystalline phases nonetheless, these disordered phases are frequently found in nature. The rates of crystallization of these phases are strongly controlled by the presence of adsorbed ions on the surfaces of precipitates (99). Zawacki et al. (Chapter 32) present evidence that adsorption of alkaline earth ions greatly influences the formation and growth of calcium phosphates. While hydroxyapatite was the thermodynamically stable phase under the conditions studied by these authors, it is shown that several different metastable phases may form, depending upon the degree of supersaturation and the initiating surface phase. 

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... 

Table 5.8 SCM input parameters for ferric hydroxide surfaces used for the calculation of phosphate adsorption using the CCM and the adsorption of Pb, Cu, Zn and Cd using the DDLM...

The resulting salt, whilst readily soluble in dilute mineral acids, is insoluble in cold acetic acid, phosphoric acid, and sodium phosphate. It is slightly soluble in citric and tartaric acid solutions, and readily dissolves m neutral aqueous ammonium citrate, yielding a green solution with a brownish tint.3 The salt is insoluble in water, but hot water hydrolyses it, and boiling with excess of ammonia solution converts it into a mixture of ferric hydroxide 4 and ferric phosphate, or if the ammonia is present in great excess the ferric phosphate may be entirely decomposed. Thus —... 

The reaction is reversible, and on boiling a suspension of freshly precipitated ferric hydroxide in an aqueous solution of ammonium phosphate, ammonia is liberated and ferric phosphate remains.5 In consequence of this reversibility, and owing to the volatility of ammonia and the active mass of the ammonium phosphate produced, it is not easy to carry the first-named reaction to completion, namely, in the direction of from left to right in the foregoing equation. 

Sodium ferri-triorthophosphate, NaH5[Fe(P04)3]H20, results when sodium chloride or phosphate is heated on the water-bath with a solution of ferric hydroxide in phosphoric acid. It is a pale red, crystalline powder, sparingly soluble in water. 

These two A s produce comparable concentrations of the ferric ion to precipitate either Fe(OH)3 or FeP04. Thus, at high pH conditions, the phosphate ion would have a big competitor in the form of the hydroxide ion. An Fe available in solution is grabbed by the OH ion to form the ferric hydroxide, leaving less amount of Fe " to precipitate ferric phosphate. The ferric salts are, therefore, a poor performer for removing phosphorus. Also, to be effective requires adjusting the pH to the pH of almost mineral acidity of less than 3. 

Iron absorption takes place predominantly in the duodenum where the acid environment enhances solubility, but also throughout the gut, allowing sustained-release preparations to be used. Most iron in food is present as ferric hydroxide, ferric-protein complexes or haem-protein complexes. Ferrous (Fe " ) iron is more readily absorbed than ferric (Fe ). Thus the simultaneous ingestion of a reducing agent, such as ascorbic acid, increases the amount of the ferrous form ascorbic acid 50 mg increases iron absorption from a meal by 2-3 times. Food reduces iron absorption due to inhibition by phytates, taimates and phosphates. 

Manganese dioxide, sodium nickel hexacynaoferrate, hydrous titanium oxide, zirconium phosphate, ferric hydroxide, different commercial sorbents... 

The flotation or iron as ferric hydroxide has been studied extensively.14 Rotaiion was effective over a pH range of 4.5-8,5 using anionic surfactants (e.g., SDS), and above a pH of 9 using cationic surfactants (e.g., HTA), While SDS is a good choice In terms of cost and toxicity considerations, it is displaced rather ensily from precipitates hy competing anions. The anions most effective in blocking flotation were phosphate, hexaphosphate, arsenate, EDTA, and oxalate. 

Ferric casse is also affected by pH. Indeed, iron has a degree of oxidation of three and prodnces soluble complexes with molecules such as citric acid. These complexes are destabilized by increasing pH to prodnce insolnble salts, snch as ferric phosphates (see white casse ) or even ferric hydroxide, Fe(OH)3. 

When ferric phosphorus is reduced to ferrous phosphorus by iron-reducing bacteria, phosphorus is released. We know that phosphate is occluded in hydrated ferric hydroxide coating. This is released by reduction of ferric iron to ferrous iron. 

The expected influence of silica on other metal oxide adsorbents, such as granular ferric hydroxide, is similar to its negative effect on alumina. Phosphate and fluoride are other strong ligands commonly found in groundwater that exhibit a negative influence similar to that of silicate on the arsenic capacity of alumina and GFH (5,12). Thus, the concentrations of these ions must be known before a reasonable estimate of arsenic capacity can be made. 

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