25. colloidal ferric charge

The colloid, as usually prepared, is electro-positive in character, and may be precipitated from solution by electrolysis, by the addition of small quantities of electrolytes, or by the action of an oppositely charged colloid, such, for example, as (negative) arsemous sulphide, whereby the two electrical charges neutralise each other.7 The smallest quantities of a few electrolytes required to precipitate colloidal ferric hydroxide from solution are given in the following table —8... 

It is possible also to prepare colloidal ferric hydroxide with a negative charge. This may be done by adding slowly 100 c.c. of 0 01-normal ferric chloride solution to 150 c.c. of 0-01-normal sodium hydroxide, the mixture being continuously shaken during the process.2... 

The origin of the surface charges, and of the double layers is probably different for different materials. Thus colloidal ferric hydroxide... 

The peptisation of most oxides, sulfides and salts may be explained on the same basis.- According to Graham the peptisation of ferric hydroxide hydrogel takes place on treating it with ferric chloride. One may assume that the ferric ion attaches itself to the ultramicrons of the colloidal ferric hydroxide, thus imparting to the latter the positive charge. 

What was so revolutionary in the papers by Hofmeister and co-workers. First, these were the first extended systematic smdies on specific ion effects beyond the effect of different charges. Second, Hofineister considered quite different systems to derive the specific effects precipitation of proteins, of colloidal ferric oxide, and of sodium oleate, collagen (isinglass), etc. Third, he was the first to draw some general conclusions about specific ion effects that allowed him to order salts according to their water withdrawing capability ... 

Ferric hydroxide coprecipitation techniques are lengthy, two days being needed for a complete precipitation. To speed up this analysis, Tzeng and Zeitlin [595] studied the applicability of an intrinsically rapid technique, namely adsorption colloid flotation. This separation procedure uses a surfactant-collector-inert gas system, in which a charged surface-inactive species is adsorbed on a hydrophobic colloid collector of opposite charge. The colloid with the adsorbed species is floated to the surface with a suitable surfactant and inert gas, and the foam layer is removed manually for analysis by a methylene blue spectrometric procedure. The advantages of the method include a rapid separation, simple equipment, and excellent recoveries. Tzeng and Zeitlin [595] used the floation unit that was devised by Kim and Zeitlin [517]. 

One of the earliest known chemical properties of ferric salts was their ready conversion to ferric hydroxide colloids in solutions. These solutions were intensively studied in the classic period of colloid chemistry, and their properties have been discussed in detail by Weiser (8). Since the focus of these studies was on colloid properties per se, precautions were taken to prepare pure colloids. Generally hydrolyzed solutions would be dialyzed extensively against distilled water to remove foreign ions. Even the purest preparations retained detectable concentrations of anions, consistent with a positive surface charge on the colloidal particles. 

Water treatment by either direct or contact filtration has become common practice for raw water with low turbidity [<3NTU] and low colour. Simple metal salts such as alum or ferric chloride are added to plant inlet water. Hydrolysis takes place with the formation of hydroxylated species, which adsorb, reducing or neutralizing the charge on the colloidal particles in the raw water, promoting their collision and the formation of floes that settle or can be filtered out. 

Vanadium pentoxide sols can be employed to bring about coagulation of positively charged colloids for example, ferric hydroxide and aluminium hydroxide. The amount necessary for the coagulation of a given quantity of the positive colloid is very small in comparison with the required quantities of arsenic trisulphide, antimony trisulphide, and other negative colloids. It appears, therefore, that the colloidal... 

For effective removal of the colloids, as much of the ferric ions should be converted to the solid Fe(OH)3(j). Also, as much of the concentrations of the complex ions should neutralize the primary charges of the colloids to effect their destabilization. Overall, this means that once the solids have been formed and the complex ions have neutralized the colloid charges, the concentrations of the complex ions standing in solution should be at the minimum, which corresponds to the optimum pH for the coagulation process. 

Copper and zinc have been extracted from sea water by sorption colloid flotation. The metal ions are brought to the surface in less than 5 min. using a negatively charged ferric hydroxide collector, the cationic surfactant dodecylamine, and air. 95% copper and 94% zinc could be recovered. Maximum recovery is attained at pH 7.6 88). 

Molybdenum is rapidly extracted from sea water by a sorbing colloid flotation method. Optimum collection by ferric hydroxide takes place at pH 4.0, when the colloid has an apparent maximum positive charge density and is able to sorb molybdenum nearly quantitatively as molybdate anion. From a 500 ml sample of sea water molybdenum is accumulated in the foam on the water surface in 5 min. using sodium dodeeyl sulfate as surfactant and air bubbling through the solution 89,90). 

Destabilisation may be achieved by the enmeshment of the colloid in a precipitate. In this process a metal salt such as aluminium sulphate (alum) or ferric chloride, is added to the water forming positively charged species in the typical pH range of 6 - 7 for clarification. The hydrolysis reaction produces an insoluble gelatinous hydroxide according to the following equations ... 

Inorganic colloids (hematite, 75 nm) did not cause irreversible flux decline. Pretreatment of the solutions using ferric chloride not only prevented flux decline under criticalfouling conditions (high calcium concentration and IHSS HA), but also influenced rejection. The latter depends on the charge of the ferric hydroxide precipitates. Cation rejection increased when positive ferric hydroxide colloids were deposited on the membrane, which the organic rjection decreased. 

At critical fouling conditions ferric chloride successfully prevented fouling at any dosage. At the higher organic concentration the iron oxyhydroxide precipitates are neutralised and the osmotic effects observed are smaller. The impact on rejection of these less positively charged colloids is also reduced. 

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