Water removal in acid solutions

The different applications of these zeolites in pervaporation include alcohol dehydration water removal in acid solutions organic dehydration separations such as water/ tetrahydrofuran (THF), water/dioxane, or water/dimethyl-formamide (DMF) removal of organics from water and organic/organic separations such as methanol/methyl-tert-butyl ether (MTBE) or p-xylene/o-xylene. In the following subsections, the mechanism of pervaporation in zeolite membranes will be briefly described and we will provide the details about the different applications. 

Chemical reduction of an ore usually gives metal that is not pure enough for its intended use. Further refining of the metal removes undesirable impurities. Several important metals, including Cu, Ni, Zn, and Cr, are refined by electrolysis, either from an aqueous solution of the metal salt or from anodes prepared from the impure metal. To give one example, ions, obtained by dissolving ZnS or ZnO in acidic solution, can be reduced while water... 

In water, hydrogen cyanide and cyanide ion exist in equilibrium with their relative concentrations primarily dependent on pH and temperature. At pH <8, >93% of the free cyanide in water will exist as undissociated hydrogen cyanide (Towill et al. 1978). Hydrogen cyanide is hydrolyzed to formamide which is subsequently hydrolyzed to ammonium and formate ions (Callahan et al. 1979). However, the relatively slow rates of hydrolysis reported for hydrogen cyanide in acidic solution (Kreible and McNally 1929 Kreible and Peiker 1933) and of cyanides under alkaline conditions (Wiegand and Tremelling 1972) indicate that hydrolysis is not competitive with volatilization and biodegradation for removal of free cyanide from ambient waters (Callahan et al. 1979). 

When thiocyanate ions are added to nitrous acid in water, a pink colouration develops which is believed to be due to the formation of nitrosyl thiocyanate (equation 34), which is too unstable to be isolated but which can be used as a nitrosating agent in aqueous solution. Because the equilibrium constant for ONSCN formation81 is quite large (30 dm6mol 2) at 25 °C, thiocyanate ion is an excellent catalyst for aqueous electrophilic nitrosation. The well established82 series is Cl- < Br < SCN < (NH2)2CS. Thiocyanate ion is also a sufficiently powerful nucleophile to react in acid solution with nitrosamines in a denitrosation process (equation 35), which can only be driven to the right if the nitrosyl thiocyanate is removed by, e.g., reaction with a nitrite trap such as hydrazoic acid. 

This thin-film-composite membrane has been found to have appreciable resistance to degradation by chlorine in the feed-water. Figure 2 illustrates the effect of chlorine in tap water at different pH values. Chlorine (100 ppm) was added to the tap water in the form of sodium hypochlorite (two equivalents of hypochlorite ion per stated equivalent of chlorine). Membrane exposure to chlorine was by the so-called "static" method, in which membrane specimens were immersed in the aqueous media inside closed, dark glass jars for known periods. Specimens were then removed and tested in a reverse osmosis loop under seawater test conditions. At alkaline pH values, the FT-30 membrane showed effects of chlorine attack within four to five days. In acidic solutions (pH 1 and 5), chlorine attack was far slower. Only a one to two percent decline in salt rejection was noted, for example, after 20 days exposure to 100 ppm chlorine in water at pH 5. The FT-30 tests at pH 1 were necessarily terminated after the fourth day of exposure because the microporous polysul-fone substrate had itself become totally embrittled by chlorine attack. 

The Unipnre Environmental, Unipnre process technology is a unique iron co-precipitation method for removal of heavy metals from waste streams or groundwater. It can act as a primary metal-removal system or as a polishing step to an existing treatment system. The reactor mod-nle replaces the nentrahzation tank in a conventional wastewater treatment system. The process prodnces solids that are extremely insolnble in water and mild acid solutions. 

Carbon Disulphide.f — Thoroughly mix 50 cc. of benzene with 50 gin. of alcoholic potassium hydroxide solution (11 gm. of potassium hydroxide in 90 gm. of absolute alcohol), and allow the mixture to stand several hours at a temperature of about 20° C. Shake with about 100 cc. of water, remove the aqueous solution from the benzene, neutralize the latter with acetic acid, and add copper-sulphate solution. No precipitate should form. 

The decomposition of nitrosyl complexes has frequently been observed in acid solution, giving nitrous oxide, nitrogen or even ammonia. Here protonation must occur at the oxygen atom which is then removed as water. Further reaction gives gaseous products. An example9 is shown in equation (26). 

In the aldohexose series, 5-(benzylamino)-l,2-0-cyclohexylidene-5-deoxy-L-idurononitrile gives, on acid hydrolysis, an almost quantitative yield of N-benzyl-2-cyano-5-hydroxypyridinium chloride. On partial, catalytic hydrogenation of this aminonitrile, 5-(benzylamino)-I,2-0-cyclohexylidene-5-deoxy-L- do-hexodialdo-l,4-furanose is obtained. This compound is reducible with sodium borohydride to crystalline 5-(benzylamino)-l,2-0-cyclohexylidene-5-deoxy-L-idofur-anose which, on removal of the cyclohexylidene group with acid, forms the intermediate 5-(benzylamino)-5-deoxy-L-idopyranose this then loses three molecules of water per molecule, to give N-benzyl-5-hydroxy-2-(hydroxymethyl)pyridinium chloride. It is therefore clear that the transformation of 5-aminoaldoses into pyridine derivatives in acid solution is not prevented by the monoalkylation of the amino group. 

If water cannot be excluded, for example, due to the presence of water of crystallization in the carboxylic acid, or when the purity of the CDI is not precisely known, a small excess of CDI can be used. Nevertheless, unreacted CDI should be removed by a short washing of the mixture with water at —5°C prior to the addition of the amino component. CDI is immediately hydrolyzed by water while the activated carboxylic acid is considerably more stable and survives this washing routine. Traces of water remaining in the solution usually do not reduce yields in the coupling step.b ... 

For the estimation of diph4nytamim the powder may be saponified as above, and then distilled until all the diphen amtne has passed over. The distillate is extracted with ether, which is separated, dried with calcium chloride and evaporated. The diphenylamine may also be estimated by conversion to tetrabromo diphcnylamine. For this purpose the ethereal solution is evaporated with atcohalcoholic solution with stirring. The tetrabromo-diphenylamine separates out, and the supernatant liquor should be dear and should smell distinctly of bromine. Water is added and the whole is boiled to remove alcohol and bromine. The bromine compound is filtered oiF, washed with warm water and dried in a steam oven. The bromination may also be ejected by potassium bromate and bromide in acid solution. 

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