Volatilization process

The iodide or van Arkel-de Boer process is a volatilization process involving transfer of an involatile metal as its volatile compound. It is used for the purification of titanium. The reaction of iodine gas with impure titanium metal at 175°C yields gaseous titanium iodide and leaves the impurities in the sohd residue. 

Despite the difficulties, there have been many efforts in recent years to evaluate trace metal concentrations in natural systems and to compare trace metal release and transport rates from natural and anthropogenic sources. There is no single parameter that can summarize such comparisons. Frequently, a comparison is made between the composition of atmospheric particles and that of average crustal material to indicate whether certain elements are enriched in the atmospheric particulates. If so, some explanation is sought for the enrichment. Usually, the contribution of seaspray to the enrichment is estimated, and any enrichment unaccounted for is attributed to other natural inputs (volcanoes, low-temperature volatilization processes, etc.) or anthropogenic sources. 

The enhanced volatilization process is operated by putting contaminated soil in contact with clean air in order to transfer the contaminants from the soil into an air stream. The air stream is further treated through the use of carbon canisters, water scrubbers or afterburners to reduce air emission impacts. Four methods are available that can achieve this effect19 ... 

Finally, the third major input information required is external (i.e., extrinsic to the compound itself) the environmental physical conditions (see Fig. 2). Temperature and water regimes are often the most determinant factors which affect the mobility of chemicals in the environment by accelerating volatilization or sorption processes. Solar radiation is also crucial in the chemicals fate since it is strongly related to photodegradation and volatilization processes as well. 

Although the use of enclosures is conceptually the simplest approach, some particular problems arise in their use in studies of NH3 loss. These are associated with the chemical reactivity of the gas, particularly its reactivity with water, and to the strong influence of environmental factors on the volatilization process (11). Matching conditions within the enclosure to those prevailing outside is a difficult task and much of the data obtained using enclosures is open to question. However, the problems associated with enclosures can be overcome if the air speed through the enclosure is controllable to within the same range as that of wind speed at the experimental site (9, 12). 

Apparent hysteresis also may be caused by other phenomena. During the consecutive extractions and dilution steps used as a common technique in desorption studies, weathering of the sorbent may occur, resulting in a possible increase of contaminant sorption and decrease in its release. Degradation of the contaminant induced by physicochemical or biological factors, or a volatilization process leading to a decreased contaminant concentration in solution, are additional factors affecting a true hysteresis result. 

The volatilization process changes the contaminant from a sohd or hquid state, where the molecules are held together by intermolecular forces, into a vapor phase. The molar heats of fusion (A//p, volatilization (AH), and sublimation (AH) are related according to the Bom-Haber cycle by... 

The differential volatilization of neat kerosene components from a liquid phase, directly into the atmosphere during volatilization up to 50% (w/w), is presented in Fig. 8.8. Ten kerosene components were selected, and their composition was depicted as a function of gas chromatograph peak size (%), which is linearly related to their concentration. It may be seen that the lighter fractions evaporate at the beginning of the volatilization process. Increasing evaporation causes additional components to volatilize, which leads to a relative increase in the heavier fractions of kerosene in the remaining liquid. 

In the subsurface, kerosene volatilization is controlled by the physical and chemical properties of the solid phase and by the water content. Porosity is a major factor in defining the volatilization process. Galin et al. (1990) reported an experiment where neat kerosene at the saturation retention value was recovered from coarse, medium, and fine sands after 1, 5, and 14 days of incubation. The porosity of the sands decreased from coarse to fine. Figure 8.9 presents gas chromatographs obtained after kerosene volatilization. Note the loss of the more volatile hydrocarbons by evaporation in all sands 14 days after application and the lack of resemblance to the original kerosene. It is clear that the pore size of the sands affected the chemical composition of the remaining kerosene. For example, the fractions disap-... 

Fig. 8.8 Major remaining components of kerosene during the volatilization process (Yaron et al. 1998)...

Volatilization of an organic mixture of contaminants, distributed vertically in the subsurface, may induce not only a decrease in the component concentrations but also an enrichment of the deeper layers during the volatilization process. Figure 8.13 shows the actual content of three representative hydrocarbons—m-xylene (C ), n-decane (Cj ,), and hexadcane (Cj )—which originated from the applied kerosene found along a 20 cm soil column, 18 days after application on dry soil. Roughly 30% of the initial content of m-xylene still remained in the soil after 18 days. Furthermore, the content of m-xylene increased somewhat after the third day a similar trend was found for the n-decane distribution. Hexadecane was partially removed from deeper layers and redistributed near the soil surface. 

Spencer, W. F., M. M. Cliath, and S. R. Yates. Soil-pesticide interactions and their impact on the volatilization process, in Environmental Impact of Soil Component Interactions—Natural and Anthropogenic Organics, Vol. 1, CRC Press, Boca Raton, FL, 1995, pp. 371-381. 

Tc-99, which has a half life of 2.12 x 10 years, can be recovered from nuclear fission waste in kilogram quantities. Solvent extraction, ion exchange, and volatilization processes are employed to separate it from the numerous other fission products. Because of its long half life and its emission of a soft (low energy) beta particle, it can be safely handled in milligram quantities. Almost all chemical studies of the element have been carried out with this isotope. 

If this trend is extrapolated to lower temperatures it is clear that at temperatures varying between -78 and -150°C for carbon—hydrogen bonds on various types of hydrocarbons, AF becomes zero and then endothermic. Under these conditions (AG < 0) the reaction with elemental fluorine will not and should not proceed at all Wfe have observed this in many cases experimentally. At temperatures as low as -150°C the reaction of fluorine with all studied organic compounds either does not occur or is extremely slow. We have occasionally reported reactions in the cryogenic reactor as low as -120°C. It should be explained that the — 120°C reported is the temperature of a single trap in a multizone reactor rather than that for the entire reactor. It is possible that fluorination occurs during the volatilization process at such low temperatures. 

A key parameter used to estimate or model volatilization processes is the pesticide vapour pressure a fundamental property of the chemical agent which is uniquely defined by the temperature. This parameter is readily and reproducibly measured in the laboratory. Two... 

The emphasis in these examples is on volatilization processes and the fate of vapors, although equally impressive advances have been made in understanding particulate drift phenomena. 

When the substituent R stabilizes radicals as in (A) and (C), chain scission is more likely than termination by coupling. Radicals (C) then propagate the depolymerization process with volatilization of polypropylene and polystyrene at a temperature at which these polymers would not give significant amounts of volatile products when heated alone. Moreover, unsaturated chain ends such as (B) would also initiate the volatilization process because of the thermal instability of carbon-carbon bonds in P position to a double bond (Equation 4.23). 

Volatilization processes, combined with gas adsorption chromatographic investigations, are well established methods in nuclear chemistry. Fast reactions and high transport and separation velocities are crucial advantages of these methods. In addition, the fast sample preparation for a-spectroscopy and spontaneous fission measurements directly after the gas-phase separation is a very advantageous feature. Formation probabilities of defined chemical compounds and their volatility can be investigated on the basis of experimentally determined and of predicted thermochemical data, the latter are discussed in Part II of this chapter. 

All deposition and volatilization processes of single atoms or single molecules (the nearest case to zero surface coverage) are basically adsorption and desorption processes, respectively. Two fundamentally different types of reversible processes can occur in the gas adsorption chromatography [7] ... 

Current multimedia models are inadequate in many respects. Description of intermedia transport across the soil-air and unsaturated soil-saturated soil zones suffers from the absence of a suitable theory for multiphase transport through the multiphase soil matrix. These phenomena are crucial in describing pollutant migration associated with hazardous chemical waste sites. Existing unsaturated-zone soil transport models fail to include mass transfer limitations associated with adsorption and desorption and with absorption and volatilization processes. Rather, most models assume equilibrium among the soil-air, soil-solid, solid-water, and soil-contaminant phases. 

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