Sacrificial iron electrodes

A patented electrochemical unit that uses sacrificial iron electrodes to generate the ferrous ion is effective in removing hexavalent chromium as well as other heavy metals. In the electrochemical cell, a direct current is conducted through the cell containing a number of carbon steel plate electrodes. This generates the ferrous ion (Fe ) and hydroxyl ion (OH ). The ferrous ion reduces hexavalent chromium to the trivalent state as follows ... 

Lee BS, Son EJ, Choe EK, Kim JW. Electrochemical treatment of dyeing effluent using sacrificial iron electrodes. J Korean Fiber Soc 1999 36 329-337. 

Mollah, M., Pathak, S., Patil, P., Vayuvegula, M. (2004). Treatment of orange II azo-dye by electrocoagulation (EC) technique in a continuous flow cell using sacrificial iron electrodes. J. Hazard. Mater. 109, 165-171. 

Figure 8.5. Schematic diagram outlining the experimental design and principle reactions and mineral products associated with the electrokinetic stabihzation of Cr( VI) with sacrificial iron electrodes.

A further method of generating a rather special electrokinetic barrier is that proposed by Faulkner, Hopkinson, and Cundy (2005). Using sacrificial iron electrodes, a low permeability iron-rich layer can be built up between the electrodes. Hydraulic conductivities around 10 ni/s are reported. Further details are given in the reference. 

Electrolysis of tetrachloromethane and benzaldehyde in DMF (and a small amount of supporting electrolyte) with a sacrificial iron electrode yielded 60% of 1,1,1 -trichloro-2-phenyl-2-ethanol ... 

Electrochemical coprecipitation with iron electrodes may also be effective in removing As(V) from water (Table 7.1). Recently, Sagitova et al. (2005) developed an electrochemical unit for treating household water. The unit oxidizes As(III) to As(V). A sacrificial anode creates dissolved iron, which coprecipitates As(V) with iron (oxy)(hydr)oxides. Filtration of the iron (oxy)(hydr)oxides can lower the arsenic concentrations of the water to below 10pgL 1. 

Faulkner, Hopkinson, and Cundy, 2005). Because of the adverse effect of OH on soil remediation, due to the immobilization of many metal ions by precipitation in alkalinized soils, and the reduced efficiency of electrokinetic remediation when sacrificial iron-rich electrodes are employed (e.g. Leinz, Hoover, and Meier, 1998), noncorrosive electrodes and techniques to minimize soil alkalinization are generally employed for electrokinetic remediation (e.g. Rohrs, Ludwig, and Rahner, 2002 Virkutyte, Sillanpaa, and Latostenmaa, 2002). However, low adsorption of Cr(VI) in soils occurs in alkaline conditions, whereas high adsorption of Cr(VI) is favored in acidic conditions (Reddy et al, 1997). Furthermore, the reduction of Cr(VI) to Cr(III) by the delivery of iron (Fe°, Fe " ) is fairly well documented (Rai, Sass, and Moore, 1987 Eary and Rai, 1991 Haran et aL, 1995 Powell et aL, 1995 Pamukcu, Weeks, and Wittle, 1997 Batchelor et al., 1998 Reddy et /., 2003). Accordingly,under an applied direct current (DC) electric field, stabilization of Cr(VI)-contaminated soils may potentially be achieved where oxidative dissolution of iron-rich anodic electrodes provides Fe(j,q) to react with the anode-bound migration of Cr(VI). Hence, the use of iron-rich sacrificial electrodes and soil alkalinization may find application in the electrokinetic stabilization of Cr(VI)-contaminated soils. This concept is explained in this chapter based on the results of laboratory stabilization experiments on three Cr(VI)-impacted soils taken from three sites within the UK. 

Electrochemical dissolution of sacrificial anodes, for example, iron, aluminum, or magnesium, has been proposed for phosphate removal from urine. Ikematsu et al. [15] used an electrochemical reactor craisisting of two DSA and one iron electrode for combined nitrogen oxidation and phosphate precipitatirai. First, urea was oxidized at the DSA, then the current direction was changed, and phosphate was precipitated by dissolving the iron electrode. Zheng et al. [29, 30] used synthetic and real fresh urine for their experiments with iron and aluminum electrodes. With both types of electrodes, complete phosphate removal was achieved. At 40 mA cm and a gap width of 5 mm, 1.3 mol Fe. mol had to be dosed to remove 98 % of the phosphate (calculated by assuming a current efficiency... 

In a bipolar arrangement, the sacrificial electrodes are placed between the two parallel electrodes without any electrical connection. The two monopolar electrodes are connected to the electric power source with no interconnections between the sacrificial electrodes. This cell arrangement provides a simple setup, which facilitates easy maintenance. When an electric current is passed through the two electrodes, the neutral sides of the conductive plate will be transformed to charged sides, which have opposite charge compared with the parallel side beside it. The sacrificial electrodes are known as bipolar electrodes. It has been reported that EC cell with monopolar electrodes in series connection was more effective where aluminum electrodes were used as sacrificial and iron was used as anode and cathode. And, electrocoagulation with Fe/Al (anode/cathode) was more effective for the treatment process than Fe/Fe electrode pair (Modirshahla et al. 2007). 

Alternatively, iron-rich sacrificial electrodes, which dissolve under acidic conditions generated at the anode by the application of electric field, may be used. The dissolved iron, in cationic form, migrates toward the cathode and then precipitates as iron-rich mineral phases (ferric iron oxyhydroxides, hematite, goethite, magnetite, and ZVl) near the cathode due to high-pH conditions. Contaminants such as Cr(Vl) can react with this iron and reduce into Cr(III). Cr(VI) transport may be limited by high sorption under low-pH conditions therefore, alkaline solution may be injected from the anode to increase the soil pH, and thereby reduce sorption and increase transport of Cr(Vl) to react with iron. 

Using nickel-2,2 -bipyridine complex as the catalyst, electroreductive coupling of 5-bromoindole gave rise to the bis-indole shown using NaBr as the electrolyte and iron and the sacrificial electrode [219]. 

Sacrificial Anodes Incontrastto the impressed current technique, the use of sacrificial anodes does not depend on the creation of driven electrochemical cell. Rather, a galvanic cell is formed between the structure and the sacrificial anode in which electrons pass spontaneously from the latter to the former (Fig. 9). Thus, the source of the electrons (the sacrificial anode) must have a more negative electrode potential than the structure. It was for this reason that Humphrey Davy chose zinc or iron to protect copper, and it also explains why magnesium, aluminum and zinc alloys are used to protect steel today. 

Galvanized steel is a common example of galvanic coupling where steel (Fe), with a standard electrode potential of —0.440 V vs. SHE, is cathodicaUy protected by zinc, which has a more active standard electrode potential of —0.763 V. Obviously, zinc is not a corrosion-resistant metal and cannot be classified as a barrier coating. It protects steel from corrosion through its sacrificial properties. Because zinc is less noble than iron in terms of the standard electrode potentials, it acts as an anode. The sacrificial anode (zinc) is continuously consumed by anodic dissolution reaction and protects the underlying metal (iron in steel) from corrosion. In practice, sacrificial anodes are comprised of zinc, magnesium alloys, or aluminum. 

Cathodic control protection protects the substrate by coating with a less noble metal, for which the slopes of the cathodic polarization curves are steep. The cathodic overpotential of the surface is increased by the coating therefore, the corrosion potential becomes more negative than that of the substrate. Coating materials used for this purpose are zinc, aluminum, manganese, cadmium, and their alloys. The electrode potential of these metals are more negative than those of iron and steel. When exposed to the environment, these coatings act as sacrificial anodes for the iron and steel substrates. 

When using magnesium as a sacrificial electrode, phosphate can be precipitated as struvite [31]. Stmvite is a preferred product over iron or aluminum phosphates, because it is a better phosphate fertilizer [32]. Without base addition, the process can only be applied to stored urine, because a high pH value is required for struvite formation. 

Rust can also be prevented by placing a sacrificial electrode in electrical contact with the iron. The sacrificial elecfrode musf be composed of a metal that is above iron on the activity series. The sacrificial elecfrode oxidizes in place of the iron, protecting the iron from oxidahon. Anofher way to protect iron from rusting is to coat it with a metal above it in the achvity series. Galvanized nails, for example, are coated with a thin layer of zinc. Since zinc is more active than iron, it oxidizes in place of the underlying iron (just like a sacrificial electrode does). The oxide of zinc is not crumbly and remains on the nail as a protective coating. 

A If a metal more active than iron, such as magnesium or aluminum, is in electrical contact with iron, the metal rather than the iron wiU he oxidized. This principle underhes the use of sacrificial electrodes to prevent the corrosion of iron. 

Which of these metals does not act as a sacrificial electrode for iron ... 

Iron corrosion can be prevented by preventing water contact, minimizing the presence of electrolytes and acids, or using a sacrificial electrode. 

Li et al. used the same approach for locally depositing and visualizing microarrays of copper hexacyanoferrate and iron hexacyanoferrate on glassy carbon electrode, respectively. Dissolution of a sacrificial Cu UME and stripping of Ee predeposited on a platinum UME generated relevant metal cations in the gap between the UME and the GCE, which precipitated some EefCNjs" anions that were generated simultaneously by reduction of EefCNjs" at the GCE. The deposited metal hexacyanoferrate microstructures showed catalytic activity for the oxidation of dopamine and the reduction of hydrogen peroxide, respectively, which were characterized by SECM. 

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