Chlorine aqueous chemistry

A qualitative similarity to the aqueous chemistry of chlorine will be evident. For each oxoanion of chlorine, there is a corresponding bromine species, although perbromate salts form only under certain strongly oxidizing conditions (e.g., oxidation of bromate ion in alkaline solution with F2 or XeF2) and in fact were unknown until 1968. 

Chlorine-based chemistry is a case in point. In addition to the problem of salt generation, the transport of chlorine is being severely restricted. Moreover, chlorine-based routes often generate aqueous effluent containing trace levels of chlorinated organics that present a disposal problem. 

We have seen that, particularly in the presence of water, hydrogen does readily form ions in which the hydrogen atom has lost its electron, cco (Section 4.3) For example, HC1( ) dissolves in H2O to form a solution of hydrochloric add, HCl(aq), in which the electron of the hydrogen atom is transferred to the chlorine atom—a solution of hydrochloric acid consists largely of H (aq) cuid ClT aq) ions stabilized by the H2O solvent. Indeed, the ability of molecular compoimds of hydrogen with nonmetals to form acids in water is one of the most importcuit aspects of aqueous chemistry. We will discuss the chemistry of acids and bases in detail later in the text, particularly in Chapter 16. 

Carbon dioxide radical anions, C02 , are commonly used in aqueous chemistry as a reducing agent for metalloporphyrins or as intermediate in the formation of superoxide anion. COf has been reported to undergo efficient electron transfer reactions with methyl violo-gen, quinones, alkyl halides, fumarates, nitro and nitrosobenzenes and chlorinated benzaldehydes. With nitrobenzenes and chlorinated benzaldehydes, electron attachment occurs on the nitro and aldehyde groups, respectively. CO2 radicals have also been reported to add to some unsaturated compounds such as acrylamide and pyridin-3-ol. Efficient hydrogen abstraction from mercaptobenzenes have also been reported. 

Chlorine dioxide gas is a strong oxidizer. The standard reversible potential is determined by the specific reaction chemistry. The standard potential for gaseous CIO2 in aqueous solution reactions where a chloride ion is the product is —1.511 V, but the potential can vary as a function of pH and concentration (26) ... 

The starting material for all industrial chlorine chemistry is sodium chloride, obtained primarily by evaporation of seawater. The chloride ion is highly stable and must be oxidized electrolytically to produce chlorine gas. This is carried out on an industrial scale using the chlor-alkali process, which is shown schematically in Figure 21-15. The electrochemistry involved in the chlor-alkali process is discussed in Section 19-. As with all electrolytic processes, the energy costs are very high, but the process is economically feasible because it generates three commercially valuable products H2 gas, aqueous NaOH, and CI2 gas. 

These elements are noble metals and, as such, can be dissolved only with great difficulty. The usual leaching agent is hydrochloric acid, with the addition of chlorine to increase the solution oxidation potential. This strong chloride medium results in the almost exclusive formation of aqueous chloroanions, with, under certain circumstances, the presence of some neutral species. Very seldom are cationic species formed in a chloride medium. However, these elements do possess a range of easily accessible oxidation states and, with the possibility of a number of different anionic complexes that are dependent on the total chloride concentration, this provides a very complicated chemistry. A summary of the most important chloro complexes found in these leach solutions is given in Table 11.6, from which the mixed aquochloro and polynuclear species have been omitted. The latter are found especially with the heavier elements. 

By contrast, membrane U-1 was so severely damaged by chlorine dioxide that reproducible experimental data could not be collected. The response of membranes A-2 and X-2 exposed to 30.0 ppm CIO2 for 40 hours is illustrated in Figures 11 and 12. Both membranes show only slight response at pH 3.0 and 5.8 but are severely damaged at pH 8.6. The chemistry of chlorine dioxide in aqueous solution is evidently very pH dependent. 

Rav-AchaC. 1998. Transformation of aqueous pollutants by chlorine dioxide Reaction, mechanisms and products. In Handbook of environmental chemistry. Vol. 5, 143-175. 

In 1976 he was appointed to Associate Professor for Technical Chemistry at the University Hannover. His research group experimentally investigated the interrelation of adsorption, transfer processes and chemical reaction in bubble columns by means of various model reactions a) the formation of tertiary-butanol from isobutene in the presence of sulphuric acid as a catalyst b) the absorption and interphase mass transfer of CO2 in the presence and absence of the enzyme carboanhydrase c) chlorination of toluene d) Fischer-Tropsch synthesis. Based on these data, the processes were mathematically modelled Fluid dynamic properties in Fischer-Tropsch Slurry Reactors were evaluated and mass transfer limitation of the process was proved. In addition, the solubiHties of oxygen and CO2 in various aqueous solutions and those of chlorine in benzene and toluene were determined. Within the framework of development of a process for reconditioning of nuclear fuel wastes the kinetics of the denitration of efQuents with formic acid was investigated. 

Other species that can initiate this sulfur oxidation chemistry are N03 (discussed in Chapter 7.D.1) and ClJ. The latter radical anion is formed in sea salt particles when atomic chlorine is generated and reacts with chloride ion. In addition, Vogt et al. (1996) have proposed that oxidation of SO2- by HOC1 and HOBr in sea salt particles may be quite important. Table 8.13 summarizes the aqueous-phase chlorine chemistry that occurs in sea salt particles and Table 8.14 the oxidation of S(IV) by reactive chlorine and bromine species in solution. 

TABLE 8.13 Some Aqueous-Phase Chlorine Chemistry... 

TABLE 8.14 Aqueous-Phase Chemistry Involving S(IV) and Reactive Chlorine and Bromine Ions... 

The chemistry of ozone in aqueous solutions and the health effects are complex. It is clear that ozone reacts with water products in the water supply to form numerous disinfection byproducts. However, the general pattern that emerges from most studies is that the reaction byproducts of ozonation appear to be less toxic than those produced by chlorination. 

They studied the chemistry of oxidized Si(lll) surfaces treated at two concentrations of the silane in trichloroethylene solution using angle-dependent X-ray photoelectron spectroscopy (XPS or ESCA). Although these are non-aqueous adsorption studies, sufficient surface silanol or adsorbed water is present for complete hydrolysis to occur because no trace of chlorine is seen in the XPS spectra. The two concentrations studied were 1% v/v, termed saturated, and <1/400% v/v, termed dilute. They lead to two distinct types of molecular bonding to the surface. C( 1 s) XPS spectra of these two situations are shown in Fig. 3. 

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