Ferric floes

A good example of the use of microelectrophoresis experiments is supplied by the study of ferric floes, which are widely used in municipal water treatment plants. The zeta potentials shown below were derived from the measured floe electromobilities using the Smoluchowski equa-... 

Figure 6.11 The zeta potential of ferric floes as a function of concentration and PH.

Very finely divided minerals may be difficult to purify by flotation since the particles may a ere to larger, undesired minerals—or vice versa, the fines may be an impurity to be removed. The latter is the case with Ii02 (anatase) impurity in kaolin clay [87]. In carrier flotation, a coarser, separable mineral is added that will selectively pick up the fines [88,89]. The added mineral may be in the form of a floe (ferric hydroxide), and the process is called adsorbing colloid flotation [90]. The fines may be aggregated to reduce their loss, as in the addition of oil to agglomerate coal fines [91]. 

With aluminum sulfate, optimum coagulation efficiency and minimum floe solubiUty normally occur at pH 6.0—7.0. Iron coagulants can be used successfully over the much broader pH range of 5.0—11.0. If ferrous compounds are used, oxidation to ferric iron is needed for complete precipitation. This may require either chlorine addition or pH adjustment. 

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. 

Actiflo A flocculation-clarification system for treating potable and waste waters. A variety of flocculating agents, typically ferric chloride, may be used. After flocculation, a fine sand is added and the system mixed, which entraps the floes. A lamellar separator accelerates the settling process, and the floe is separated from the sand in a hydrocyclone. Developed by Kruger Products, a subsidiary of USFilter, and now offered by Vivendi Water Systems. Operated first in France in 1991 by 2002, nearly 100 units were operating worldwide. 

The Prussian Blue Reaction. On addition to an alkaline solution of hydrocyanic acid of a few ml. of ferrous sulphate solution and of ferric chloride solution, then shaking and warming, a blue precipitate of ferric ferrocyanide appears on acidification with hydrochloric acid. In presence of only a trace of hydrocyanic acid, only a greenish-blue colouration appears, due to the formation of a colloidal suspension of the ferric ferrocyanide. On standing (sometimes for as long as 12 hours) the suspension settles to blue floes, leaving the supernatant solution colourless. 

The coupled biogeochemical cycling of iron and arsenic has also recently been studied in a system dominated by the addition of iron as an engineering practice.14 Since 1996, ferric chloride has been added as a coagulant to the Los Angeles Aqueduct in order to control the levels of naturally occurring arsenic that reach the water distribution system for the City of Los Angeles. The iron- and arsenic-rich floe formed by this process is deposited in North Haiwee Reservoir. Sediments and porewaters from this reservoir were examined to determine the extent to which arsenic and iron are remobilized in the sediments and to probe the speciation of arsenic in the solid phase and its possible effects on arsenic remobilization. 

Copper also was removed by adsorbing colloid flotation using ferric hydroxide floe and SDS surfactant. Optirenm removal occurred in the pH range 6-8, depending on ionic strength. The 99.98% removal was achieved at pH 7.5 with an initial copper concentration of SO mg/L. ... 

For pH valnes above 8, precipitate flotation was more efficient in removing zinc than foam separation of soluble zinc species.15 The optimum pH was 9.2. The surfactant was SDS. The adsorbing colloid flotation of zinc used aluminum hydroxide and NLS in the pH range 8.0-8.6 ferric hydroxide floes were not effective. Removal was 99.8% or higher.15 Table 17.3-1 shows the removal of ziec in association with other metals. 

Mix 1 cm3 of gold sol with 9 cm3 of ferric hydroxide sol, shake, and observe for half an hour. The formation of a floe illustrates hcteroflocculation (Chapter 9). 

Figure 4.27 Floe si distribution for two ferric chloride concentrations and HA (5 m L HA in background solution, pH 8) analysis was repeated three times at 100 mgH FeCl).

Figure 4.30 Scattering intensity over scattering vector the slope of the graph is related to the density of the aggregates ferric chloride floes with 5 mgL as DOC HA, pH 8.

Ferric chloride pretreatment could achieve organics removal up to 90%. The removal depends strongly on solution chemistry , organic ty pe and concentration, and the characteristics of the formed floes. Performance is difficult to predict if the number of parameters change rapidly as in a normal surface water. Iron rejection is low at high dosage when the floes are very small. These floes penetrate into the membranes and cause most detrimental flux decline. 

Ferric chloride addition could improve organic rejection to values comparable with UF and NF. However, the process is potentially unreliable due to the dependence of rejection and flux on organic type and concentration, solution chemistry, and floe characteristics. 

The method of solution preparation is described in Chapter 4. The ferric chloride dosages used were adapted from coagulation papers which did not necessarily foresee membrane separation of the floes (Dennett et al. (1996)). 

Effect of Ferric Hydroxide Floes on Flux in the Absence of Organics... 

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