25. colloidal ferric reduction

In oxidized surface waters and sediments, dissolved iron is mobile below about pH 3 to 4 as Fe and Fe(lII) inorganic complexes. Fe(III) is also mobile in many soils, and in surface and ground-waters as ferric-organic (humic-fulvic) complexes up to about pH 5 to 6 and as colloidal ferric oxyhydroxides between about pH 3 to 8. Under reducing conditions iron is soluble and mobile as Fe(II) below about pH 7 to 8, when it occurs, usually as uncomplexed Fe ion. However, where sulfur is present and conditions are sufficiently anaerobic to cause sulfate reduction, Fe(H) precipitates almost quantitatively as sulfides. Discussion and explanation of these observations is given below. Thermodynamic data for iron aqueous species and solids at 25°C considered in this chapter are given in Table A12.1. Stability constants and A//° values computed from these data are considered more reliable than their values in the MINTEQA2 data base for the same species and solids. 

Iron Vanadate is, metallurgically, the most important vanadate. Precipitation of a solution of a vanadate with ferrous sulphate gives rise to a precipitate of indefinite composition, ortlio-, pyro-, meta-, and perhaps a poly-vanadate being present, as well as ferric or ferrous oxide. Reduction of the vanadate to a vanadyl salt may also ensue. The precipitate is usually colloidal and carries down with it some sodium vanadate. The dried powder may be either green, yellow, brown, or red the more nearly the precipitate approximates to a red colour the lower is its vanadium content. An iron vanadate has also been prepared by electrolysis of a solution of sodium vanadate between iron poles.1... 

Fig. 1. Hypothetical soil profile chat has all principal horizons. Not all horizons shown are present in any given profile, but every profile has some of them. Terms used in diagram Eluviation is the downward movement of soluble or suspended material in a soil from the A horizon to the B horizon by groundwater percolation. The term refers especially, but not exclusively, lo the movement of colloids, whereas the term leaching refers lo the complete removal of soluble materials. Illuviation is the accumulation of soluble or suspended material in a lower soil horizon that was transported from an upper horizon by the process of eluviation. Gleying is soil mottling, caused by partial oxidation and reduction of its constituent ferric iron compounds, due to conditions of intermittent water saturation. Process is also called gleizalion (Adapted from USD A diagram)...

F. W. Grover obtained a little by the dialysis of the commercial ferric chlorides. W. Biltz obtained no coDoid by the hydrolysis of soln. of chromic nitrate, owing to the small hydrolysis of the salt as observed by H. W. Woudstra. S. Takegami obtained the colloid at the cathode during the electrolytic reduction of chromic acid and B. Kandelaky, by the hydrolysis of chromic ethylate. 

More recently Tabushi et have used oxygenated Fe (TpivPP)Cl in the presence of HCl and H2-colloidal platinum supported on poly(vinylpyrrolidone) in the presence of acid chlorides as a cytochrome P450 mimic. Under such conditions cyclohexene is converted to its epoxide and slow reduction of the ferric complex followed by formation of the dioxygen ferrous complex completes the catalytic cycle. 

The charge on the suspended solids is often sufficiently negative to yield a stable dispersion that settles slowly and is difficult to filter. In this case, the key to effective colloid removal by sedimentation is reduction of the zeta potential to zero or near-zero. Once the charge is reduced or eliminated, electrostatic repulsive forces are minimized. The gentle agitation of the flocculation basin then causes numerous colloid collisions, first with formation of microflocs and then with visible floe particles, which can easily be settled or filtered. Coagulants such as alum, ferric chloride or cationic polymers all function primarily as charge reducers [4,41,42]. 

The formation of (II) provides a quite selective spot test for palladium. Gold must be removed prior to the test because it will cause the development of a deep ruby red in the spot plate test and a diffused violet spot on the paper, apparently due to the reduction of the gold ions to the colloidal metal. Interference may also arise from 0s04 , Os+, Ru+, and RuCle ions because they have distinct self-colors. Mercurous ion causes partial interference by the reduction of part of the palladium to the elementary state, but a positive response can still be seen. It is possible to detect I part of palladium in the presence of 200 parts of platinum or 100 parts of rhodium. Less favorable ratios should be avoided because of the color of these salts. No interference is caused by mercuric and iridic chloride, but free ammonia, ammonium ions, stannous, cyanide, thiocyanate, fluoride, oxalate, and tetraborate ions do interfere. Lead, silver, ferrous, ferric, stannic, cobaltous, nickel, cupric, nitrite, sulfate, chloride, and bromide ions do not interfere. 

---