Removal of Organic Solutes

Organic matter in unpolluted freshwater is usually in the form of humic acids (Section 13.6), which may discolor drinking water, foul ion-exchange resins, transport toxic metal ions, or generate carcinogenic chlorocarbons if the water is chlorinated (see below). Humic compounds may be removed by coagulation. Low levels of organic solutes can be removed by sorption on activated charcoal. 

In searching for chemical methods of destroying organic pollutants, chemists tend to look to oxidative methods, since the preferred end product is usually CO2, but, as discussed below, special methods are usually necessary to bring about oxidation to acceptable products. In 1995, however, it was shown that elemental (zero-valent) metals such as ordinary iron filings can be effective in the reductive conversion of halocarbon pollutants such as trichloroethene in wastewater to innocuous hydrocarbons over a few days  

As often happens, this is actually a rediscovery rather than new science, since reaction 14.19 has been known for some time in connection with the 

We saw in Section 6.5.2 that ultraviolet irradiation of titanium dioxide promotes electrons into its conduction band and leaves reactive holes in the valence band with a band gap of 3 V (more, for very small Ti02 

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Activated charcoal or carbon is widely used for vapour adsorption and in the removal of organic solutes from water. These materials are used in industrial processes to purify drinking water and swimming pool water, to de-colorize sugar solutions as well as other foods, and to... 

Figure 4.11 shows that ultrafiltration and pervaporation for the removal of organic solutes from water are both seriously affected by concentration polarization. In ultrafiltration, the low diffusion coefficient of macromolecules produces a concentration of retained solutes 70 times the bulk solution volume at the membrane surface. At these high concentrations, macromolecules precipitate, forming a gel layer at the membrane surface and reducing flux. The effect of this gel layer on ultrafiltration membrane performance is discussed in Chapter 6. 

Previous investigations have indicated that the rate-limiting step for removal of organic solutes from dilute aqueous solution by porous active... 

For many years fluorine has been deterrnined by the Willard-Winters method in which finely ground ore, after removal of organic matter, is distilled with 72% perchloric acid in glass apparatus. The distillate, a dilute solution of fluorosiUcic acid, is made alkaline to release fluoride ion, adjusted with monochloroacetic acid at pH 3.4, and titrated with thorium nitrate, using sodium a1i2arine sulfonate as indicator. 

The chemical yield of hydrogen peroxide and the anthraquinone per process cycle is very high, but other secondary reactions necessitate regeneration of the working solution and hydrogenation catalyst, and the removal of organic material from the extracted hydrogen peroxide. 

Adsorption — An important physico-chemical phenomenon used in treatment of hazardous wastes or in predicting the behavior of hazardous materials in natural systems is adsorption. Adsorption is the concentration or accumulation of substances at a surface or interface between media. Hazardous materials are often removed from water or air by adsorption onto activated carbon. Adsorption of organic hazardous materials onto soils or sediments is an important factor affecting their mobility in the environment. Adsorption may be predicted by use of a number of equations most commonly relating the concentration of a chemical at the surface or interface to the concentration in air or in solution, at equilibrium. These equations may be solved graphically using laboratory data to plot "isotherms." The most common application of adsorption is for the removal of organic compounds from water by activated carbon. 

Flow-injection (FI) on-line analyte preconcentration and matrix removal techniques greatly enhance the performance of atomic spectrometry [348], By using USN with membrane desolvation (MDS) as the interface, FI sorbent extraction can be directly coupled with ICP-MS for the analysis of organic solutions [349]. 

Figure 5.5. Kinetics of Zn adsorption on a soil and soil with removals of organic matter and Fe oxides (1 g soil reacted with 20 ml of 10 mg/L Zn solution concentration)...

Solvent extraction has many features that make this separation technique applicable for the removal of organic pollutants from wastewater. The organic solutes can be recovered, and there is thus a potential for economic credit to the operation. In comparison with biological treatment, it is not subject to toxicity instabilities. The disadvantage is that, even with solvents having low solubilities in water, solvent losses can be substantial owing to... 

An important consideration in extending laboratory data to natural waters is the effect of radical scavengers on the removal of the solute of interest. Many naturally occurring species react with their own second-order rate constants, with the reactive species produced in irradiated water. For site remediation, these are generally the natural constituents of the water, while for industrial treatment they may be other organic chemicals not targeted for treatment. The following are the common constituents of natural waters that may affect the efficiency of radiolytic water treatment. 

On the other hand, the progress of wet-processes as preparative techniques of metal oxide films has been remarkable. The so-called soft solution process that provides oxide layers by means of electrochemical oxidation of a metal surface is expanding as a synthetic method of various mixed metal oxides with controlled thickness [2], The two-dimensional (2D) sol-gel process based on the hydrolysis of metal alkoxides at the air/water interface has been reported as a preparative technique of ultrathin oxide films (Fig. 6.1a) [3]. It is also known that LB films of metal complexes of long-chain alkyl carboxylic acid can be converted to metal oxide films after removal of organic component by oxygen plasma [4] and UV-ozone treatments (Fig. 6.1b) [5]. Preparation of metal oxide... 

If the framework structure of a zeolite remains constant, the cation exchange capacity is inversely related to thd Si/Al ratio. Furthermore, fine tuning of the adsorptive and catalytic properties can be achieved by adjustment of the size and valency of the exchangeable cations. Dealumination of certain silica-rich zeolites can be achieved by acid treatment and the resulting hydrophobic zeolites then become suitable for the removal of organic molecules from aqueous solutions or from moist gases. 

It is of interest that the rate constants for adsorption of the various organic pesticides on the 273-micron carbon are remarkably similar. The data suggest that even for a rather broad spectrum of different types of organic pesticides, similar rates of removal from solution should result. Thus, the effectiveness of active carbon as it relates to rate of removal of organic pesticides from solution should be relatively independent of the type of organic pesticide, at least within the classes of compounds and conditions used for the present experiments. 

In a much earlier patent, the removal of organics from exhaust gases by oxidation over a supported uranium oxide catalyst was reported by Hofer and Anderson [39]. The catalyst was 4% U3O8 supported on alumina spheres. The authors used the incipient wetness technique to impregnate alumina with uranyl nitrate solution. In this case the catalyst precursors were calcined at 700°C for 3 h to decompose the uranium salt. The use of other uranium compounds as starting materials was mentioned and these included uranyl acetate, uranium ammonium carbonate and uranyl chloride. The alumina-supported catalyst had a surface area of ca 400m g and further added components, such as copper, chromium and iron, were highlighted as efficient additives to increase activity. 

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