Sulfur dioxide chemical reduction

Chemical Properties. Anhydrous sodium sulfite is stable in dry air at ambient temperatures or at 100°C, but in moist air it undergoes rapid oxidation to sodium sulfate [7757-82-6]. On heating to 600°C, sodium sulfite disproportionates to sodium sulfate and sodium sulfide [1313-82-2]. Above 900°C, the decomposition products are sodium oxide and sulfur dioxide. At 600°C, it forms sodium sulfide upon reduction with carbon (332). 

Titanium Sulfates. Solutions of titanous sulfate [10343-61-0] ate readily made by reduction of titanium(IV) sulfate ia sulfuric acid solutioa by electrolytic or chemical means, eg, by reduction with ziac, ziac amalgam, or chromium (IT) chloride. The reaction is the basis of the most used titrimetric procedure for the determination of titanium. Titanous sulfate solutions are violet and, unless protected, can slowly oxidize ia coatact with the atmosphere. If all the titanium has been reduced to the trivalent form and the solution is then evaporated, crystals of an acid sulfate 3 Ti2(S0 2 [10343-61-0] ate produced. This purple salt, stable ia air at aormal temperatures, dissolves ia water to give a stable violet solutioa. Whea heated ia air, it decomposes to Ti02, water, sulfuric acid, and sulfur dioxide. 

The electrophilic character of sulfur dioxide does not only enable addition to reactive nucleophiles, but also to electrons forming sulfur dioxide radical anions which possess the requirements of a captodative" stabilization (equation 83). This electron transfer occurs electrochemically or chemically under Leuckart-Wallach conditions (formic acid/tertiary amine - , by reduction of sulfur dioxide with l-benzyl-1,4-dihydronicotinamide or with Rongalite The radical anion behaves as an efficient nucleophile and affords the generation of sulfones with alkyl halides " and Michael-acceptor olefins (equations 84 and 85). 

Method Chemical reduction of hexavalent chromium by sulfur dioxide under acid conditions for the continuous operating system and by sodium bisulfite under acid conditions for the batch operating system. The reduced trivalent form of chromium is subsequently removed by precipitation as the hydroxide. 

Chemical reduction is used to transform a toxic substance with a higher valence to a nontoxic or less-toxic substance with lower valence. The most promising application is the reduction of hexava-lent chromium to trivalent chromium. This method is also applicable to other multivalent metals such as lead and mercury. Commonly used chemical agents for this purpose are sulfite salts, sulfur dioxide, and base metals (e.g., iron and aluminum).22 24... 

Sulfur dioxide is another commonly used chemical for chromium reduction. The reduction occurs when sulfurous acid, produced by the reaction of sulfur dioxide and water, reacts with chromic acid as follows ... 

Effluents from sewage treatment plants are not allowed to contain residual chlorine in excess of tolerable values as determined by water quality standards. For example, in discharges to trout streams, the residual chlorine should not exceed 0.02 mg/L. Thus, chlorinated effluents should be dechlorinated. Sulfur dioxide, sodium sulfite, sodium metabisullite, and activated carbon have been used for dechlorination. Because sulfur dioxide, sodium sulfite, and sodium metabisulfite contain sulfur, we will call them sulfur dechlorinating agents. Dechlorination is an oxidation-reduction reaction. The chemical reactions involved in dechlorination are discussed next. 

A number of chemicals are used as reducing agents. The most common chemicals used for reduction of chromium are sulfur dioxide, sodium metabisulfite, sodium bisulfite, and ferrous salts. Other reducing agents used or which can be potentially used for water and wastewater treatment include sodium borohydride to reduce ionic mercury to metallic mercury and alkali metal hydride to alter the chemical form of lead so that it can be precipitated and also to recover silver. The common chemicals used as reducing agents are discussed on the following sections. 

Technology for large-scale application of chemical reduction is well developed. The reduction of residual chlorine in a chlorination or superchlorination process system is termed dechlorination, which is the most common process in municipal water and wastewater treatment. The reduction of chromium waste by sulfur dioxide is another classic process and is in use by numerous plants employing chromium compounds in operations such as electroplating. 

One limitation of chemical reduction of hexavalent chromium, residual chlorine, and others, is that for high concentrations of chromium the cost of treatment chemicals may he prohibitive. When this situation occurs, other treatment processes are hkely to be more economical. Chemical interference by oxidizing agents is possible in the treatment of mixed wastes, and the treatment itself may introduce pollutants if not properly controlled. Storage and handling of sulfur dioxide is somewhat hazardous. 

The most common chemicals used for chromium reduction and other chemical reduction applications are sulfur dioxide (SO2), sodium metabisulfite (Na2S205), sodium bisulfite (NaHSOg), and sulfuric acid (H2SO4). 

The hexavalent chromium is usually chemically reduced by the addition of sulfur dioxide gas, sodium bisulfite, or sodium metabisulfite. These all form sulfurous acid with water. The undissociated form of sulfurous acid enters into the reduction reaction. Accordingly, the reaction is strongly pH dependent, and is usually carried out at a pH of about 2-3. The pH is controlled by the addition of sulfuric acid. 

To eliminate residual free chlorine from hquid, granular activated carbon adsorption or chemical reduction (with reducing agents, such as sulfur dioxide, sodium bisulfite, and sodium metabisulfite) are the most common processes for dechlorination. Ultraviolet (UV) irradiation process is gaining wider acceptance as a dechlorination process (30,45,46, 60,61). 

Chemical Reduction With Filtration (Sulfur Dioxide, Acid, Caustic)... 

A third emission reduction choice is to remain with the existing front end process, which continues to produce a sulfur dioxide-containing waste gas stream, and move to some system which can effectively remove the sulfur dioxide from this waste gas before it is discharged. Many methods are available, each with features which may make one more attractive than the others for the specific sulfur dioxide removal requirements (Table 3.8). Some of the selection factors to be considered are the waste gas volumes and sulfur dioxide concentrations which have to be treated and the degree of sulfur dioxide removal required. It should be remembered that the trend is toward a continued decrease in allowable discharges. The type of sulfur dioxide capture product which is produced by the process and the overall cost are also factors. Any by-product credit which may be available to offset process costs could also influence the decision. Finally, the type of treated gas discharge required for the operation (i.e., warm or ambient temperature, moist or dry, etc.), also has to be taken into account. Chemical details of the processes of Table 3.8 are outlined below. 

The reverse-flow chemical reactor (RFR) has been shown to be a potentially effective technique for many industrial chemical processes, including oxidation of volatile organic compounds such as propane, propylene, and carbon monoxide removal of nitrogen oxides sulfur dioxide oxidation or reduction production of synthesis gas methanol formation and ethylbenzene dehydration into styrene. An excellent introductory article in the topic is given by Eigenberger and Nieken on the effect of the kinetic reaction parameters, reactor size, and operating parameters on RFR performance. A detailed review that summarizes the applications and theory of RFR operation is given by Matros and Bunimovich. 

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