Ind. Eng. C h e m . Res. 1994,33, 515-518
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Oxidative Decomposition of Nitrogen-Sulfur Oxides David Littlejohn and Shih-Ger Chang' Energy & Environment Division, Lawrence Berkeley Laboratory, Berkeley, California 94720
The oxidative decomposition of nitrogen-sulfur oxides, formed by the reaction of nitrite ion and hydrogen sulfite ion in aqueous solution, has been investigated. The oxidants used in this study included hydrogen peroxide, nitrogen dioxide, and ozone. There was a wide range of susceptibility to oxidation among the compounds studied. Ozone was a better oxidant than nitrogen dioxide and hydrogen peroxide under the conditions used. The rate expression for the reaction of hydrogen peroxide with hydroxylamine was found to be -d[H202l/dt = Iz[H202l[NH2OHl, where lz = (7.3 f 0.5) X lo4 M-l s-1 at 25 "C.
Introduction Aqueous solution-based systems for simultaneous removal of nitrogen oxides (NO,) and sulfur dioxide (Sod from flue gases are currently under development (Chang andLee, 1992). Usually,the scrubbing solutions are mildly acidic (pH 4-6). At these conditions, SO2 is very soluble in solution and the dissolved SO2 is primarily present in the form of hydrogen sulfite ion (HSO3-). The primary components of NO, are nitric oxide, NO, and nitrogen dioxide, NOz. NO is relatively insoluble in aqueous solutions, and additives are often used to enhance its solubility. NO2 is moderately soluble in solution and reacts with water to form nitrous acid, HONO, and nitric acid, "03 (Lee and Schwartz, 1981). Nitrous acid reacts with hydrogen sulfite ion to form a number of nitrogen-sulfur oxides, or nitrogen sulfonate compounds (Chang et al., 1982). These include hydroxyimidodisulfate (also referred to hydroxylaminedisulfonate, HIDS, or HADS), hydroxysulfamate (hydroxylaminemonosulfonate, HSA, or HAMS), nitridotrisulfate (aminetrisulfonate, NTS, or ATS), and imidodisulfate (aminedisulfonate, IDS, or ADS) (Chang et al., 1982). Hydroxylamine and sulfamate are formed by hydrolysis of HAMS and ADS, respectively (Littlejohn et al., 1989; Doyle and Davidson, 1949). The chemistry associated with these compounds is fairly complex, as shown in Figure 1, and has been described previously. Nitrogen sulfonates can build up in scrubbing liquors and could potentially complicate the disposal of the spent scrubbing liquor. They are moderately stable in aqueous solutions at conditions typically found in scrubbing liquors. An effective treatment strategy for control of the nitrogen sulfonates is needed. Previously, precipitation of these compounds had been investigated (Littlejohn and Chang, 1991). A number of the compounds are fairly soluble, making precipitation an unsuitable control strategy. An alternative control strategy is to chemically convert these compounds to innocuous materials. Oxidation of nitrogen sulfonates could convert them to nitrate and sulfate ions. We report here the investigation of the oxidation of nitrogen sulfonates. The three oxidants studied were hydrogen peroxide, ozone, and nitrogen dioxide. Concentrated hydrogen peroxide is available commercially, and ozone can be readily produced with ozone generators. Ozone is finding increasing use for water treatment by municipal water suppliers. The LBL PhoSNOX process is a phosphorusbased scrubbing chemistry (Chang and Lee, 1992). When phosphorus reacts with residual oxygen in flue gas, it generates ozone and phosphorus oxides. These compounds can convert NO to NO2. The PhoSNOX process generates these oxidants and contacts them with the scrubbing N
N20 + 5032NHzOH
-
H2NS03-
Figure 1. Reactions of the nitrogen-sulfur oxides.
solution, where they have the potential to react with the nitrogen sulfonates present.
Experimental Section The nitrogen sulfonate compounds were synthesized by standard methods (Rollefson and Oldershaw, 1932; Sisler and Audrieth, 1948; Nast et al., 1952). Reagent grade compounds were used in the syntheses and in the experiments. The oxidants for use in the measurements were prepared as follows: Hydrogenperoxide: Thirty percent H202 (J. T. Baker) was diluted to 0.5 M for contact with the nitrogen sulfonates. Ozone: Ozone was generated by flowing air or oxygen through a homemade ozone generator. Ozone concentrations up to 2% could be obtained using dried house oxygen. Ozone reactivity was tested by generating 1% O3 in 0 2 and flowing it through a fritted tube immersed in the nitrogen sulfonate solution under study. The total gas flow was 3 mL s-1 into 10 mL of solution. The bubbles passed through a solution column of about 5 cm. Nitrogen dioxide: NO2 (2.49%) in N2 (Matheson) was diluted with nitrogen to prepare a mixture of 1000 ppm NO2 in N2. The gas mixture was then flowed over the surface of the nitrogen sulfonate solution under study at 1L min-l. Bubbling the mixture through solutions would have generated substantial concentrations of nitrite and nitrate ions from the reaction of NO2 with water. Five milliliters of solution was placed in a long-necked bulb, and gas was flowed in through the central tube coaxial with the neck, over the solution, and out of the neck. The solution had a surface area of about 6 cm2and was stirred at about 200 rpm with a magnetic stirrer. Less than 20% of the NO2 was absorbed by the solution, as determined with a chemiluminescent NO, analyzer. The nitrogen sulfonate solutions under study were prepared with concentrations of 0.1-0.3 M, except for solutions of ADS and ATS, which have lower solubilities
0888-588519412633-05I5$Q4.50/0 0 1994 American Chemical Society
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516 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 Table 1. Dissociation Constants for the Nitrogen Sulfonates PK, reference HADS2-+ H A D P + H+ HADS- a HADS” + H+ HAMS + HAMS- + H+ NH2OH s ”SOH+ + H+
12.6 this study
