Seawater electrolysis

Abdel Ghany, N.A. et al., Oxygen evolution anodes composed of anodically deposited Mn-Mo-Fe oxides for seawater electrolysis, Electrochem. Acta, 48, 21-28 (2002). 

Oxygen evolution reaction (OER) can be considered as one of the most relevant processes in electrochemistry involved in, for instance, chlor-alkali and ozone productions. One of the challenges is seawater electrolysis in order to store pure hydrogen and oxygen gases and prevent CI2 formation. The overall OER,... 

Historically, the primary source for chlorine has been seawater. Electrolysis of NaCl in seawater produces CI2 gas and H2 gas. 

Saline Water for Municipal Distribution. Only a very small amount of potable water is actually taken by people or animals internally, and it is quite uneconomical to desalinate all municipally piped water, although all distributed water must be clear and free of harmful bacteria. Most of the water piped to cities and industry is used for Htfle more than to carry off small amounts of waste materials or waste heat. In many locations, seawater can be used for most of this service. If chlorination is requited, it can be accompHshed by direct electrolysis of the dissolved salt (21). Arrayed against the obvious advantage of economy, there are several disadvantages use of seawater requites different detergents sewage treatment plants must be modified the usual metal pipes, pumps, condensers, coolers, meters, and other equipment corrode more readily chlorination could cause environmental poUution and dual water systems must be built and maintained. 

Platinum Platinum-coated titanium is the most important anode material for impressed-current cathodic protection in seawater. In electrolysis cells, platinum is attacked if the current waveform varies, if oxygen and chlorine are evolved simultaneously, or if some organic substances are present Nevertheless, platinised titanium is employed in tinplate production in Japan s. Although ruthenium dioxide is the most usual coating for dimensionally stable anodes, platinum/iridium, also deposited by thermal decomposition of a metallo-organic paint, is used in sodium chlorate manufacture. Platinum/ruthenium, applied by an immersion process, is recommended for the cathodes of membrane electrolysis cells. ... 

Fluorine comes from the minerals fluorspar, CaF, cryolite, Na3AlF6 and the fluorapatites, Ca,F(P04)3. The free element is prepared from HF and KF by electrolysis, but the HF and KF needed for the electrolysis are prepared in the laboratory. Chlorine primarily comes from the mineral rock salt, NaCl. The pure element is obtained by electrolysis of liquid NaCl. Bromine is found in seawater and brine wells as the Br ion it ts also found as a component of saline deposits the pure element is obtained by oxidation of Br (aq) by Cl,(g). Iodine is found in seawater, seaweed, and brine wells as the I" ion the pure element is obtained by oxidation of I (aq) by Cl,(g). 

A hanging mercury drop electrodeposition technique has been used [297] for a carbon filament flameless atomic absorption spectrometric method for the determination of copper in seawater. In this method, copper is transferred to the mercury drop in a simple three-electrode cell (including a counterelectrode) by electrolysis for 30 min at -0.35 V versus the SCE. After electrolysis, the drop is rinsed and transferred directly to a prepositioned water-cooled carbon-filament atomiser, and the mercury is volatilised by heating the filament to 425 °C. Copper is then atomised and determined by atomic absorption. The detection limit is 0.2 pg copper per litre simulated seawater. 

A typical measurement was performed as follows. The feeder was lowered into the crucible and the sample solution (seawater) was allowed to flow under an inert atmosphere with the suction on. A constant current was applied for a predetermined time. When the pre-electrolysis was over, the flow was changed from the sample to the ammonium acetate washing solution, while the deposited metals were maintained under cathodic protection. Ammonium acetate was selected for its low decomposition temperature, and a 0.2 ml 1 1 concentration was used to ensure sufficient conductivity. At this point the feeder tip was raised to the highest position and the usual steps for an electrothermal atomic absorption spectrometry measurement were followed drying for 30 s at 900 C, ashing for 30 s at 700 °C, and atomization for 8 s at 1700 °C, with measurement at 283.3 nm. The baseline increases smoothly with time as a consequence of an upward lift of the crucible caused by thermal expansion of the material. 

The alkali metals (Group 1A) and the alkaline earth metals (Group 2A) are not found free in nature because they are so easily oxidized. Their primary sources are seawater, brines of their soluble salts and deposits of sea salt. The metals are obtained from the electrolysis of their molten salts. 

Sodium is produced by an electrolytic process, similar to the other alkali earth metals. (See figure 4.1). The difference is the electrolyte, which is molten sodium chloride (NaCl, common table salt). A high temperature is required to melt the salt, allowing the sodium cations to collect at the cathode as liquid metallic sodium, while the chlorine anions are liberated as chlorine gas at the anode 2NaCl (salt) + electrolysis —> Cl T (gas) + 2Na (sodium metal). The commercial electrolytic process is referred to as a Downs cell, and at temperatures over 800°C, the liquid sodium metal is drained off as it is produced at the cathode. After chlorine, sodium is the most abundant element found in solution in seawater. 

Chlorine is produced commercially by the electrolysis of a liquid solution of sodium chloride (or seawater), through which process an electric current is passed though the solution (electrolyte). 

The chlor-alkali process,34 in which seawater is electrolyzed to produce Cl2 and NaOH, is the second most important commercial electrolysis, behind production of aluminum. 

What are some major advantages of using seawater to produce hydrogen from electrolysis ... 

Electro-active labile metal contents have also been measured by using a combination of electro-deposition and analysis by graphite furnace AAS (Batley and Matousek, 1977). Metals (e.g. Pb, Co, Ni, Cr from seawater) are plated on to a short graphite tube by application of a suitable potential. At the end of the electrolysis period, the graphite cell (plus pre-concentrated metal) is placed in an electro-thermal atomiser attached to an AAS spectrometer, and the element content determined. 

In the process used by Norsk Hydro, magnesium hydroxide extracted from seawater with the aid of calcined dolomite is mixed with charcoal and magnesium chloride brine and is heated to 1000-1200°C in the presence of chlorine produced during subsequent electrolysis of magnesium chloride. The main reactions are [266]... 

Chlorine occurs in Nature mainly as sodium chloride in seawater and in various inland salt lakes, and as solid deposits originating presumably from the prehistoric evaporation of salt lakes. Chlorine is prepared industrially mainly by electrolysis of brine ... 

Current economic and ecological analyses of the various processes available for recovery of minerals from seawater (evaporation, solvent extraction, sorption, ion exchange, flotation, fractional precipitation, distillation, electrolysis, electrodialysis, and electrocoagulation) favor ion exchange and sorption technology. 

Electrolysis can be useful to clean historic objects recovered from shipwrecks. Coatings of salts from the seawater on metal objects are removed by an electrochemical process. A voltaic cell is set up with a cathode that is the object itself and a stainless steel anode in a basic solution. Chloride ions are removed when the electric current is turned on. 

Seawater is evaporated by concentrating the seawater in the first evaporation pool transporting to the next evaporation zone, in which calcium sulfate precipitates out and finally crystallizing sodium chloride in a further evaporation zone. The residual brine is rich in potassium and magnesium salts. The salt obtained is too impure to be used in electrolysis. Washing in special units is sufficient to increase the sodium chloride content to > 99%. I m- of seawater yields ca. 23 kg of sodium chloride. 

---