Catalytic decomposition of hydrogen peroxide by ferric ion in dilute sulfuric acid solutions
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(mH,O,) (mFe 3-)
(mu-)
where k = 4.3 • 10-3 s -1 at 25 ~ From 25 ~ to 50 ~ the activation energy is 85.6 kJ/mol. The decomposition of hydrogen peroxide proceeds by a particular redox reaction sequence that depends on the ratio of the concentrations of hydrogen peroxide to free ferric ion. The rate law determined here is consistent with the form derived from the redox sequence for the case where the ratio of hydrogen peroxide to free ferric ion concentration is greater than 1.0. The magnitude of the rate constant indicates that the decomposition of hydrogen peroxide may cause rapid loss of this oxidant in leaching solutions containing ferric ion.
I.
INTRODUCTION
H Y D R O G E N peroxide has been used to provide the oxidizing capacity of lixiviants for the in situ leaching of sandstone-type uranium deposits. In experimental studies that used solutions similar to those expected for in situ leaching conditions, the rate of uraninite (UO2-U3Os) dissolution has been found to be more rapid in both basic ~ and acidic: solutions containing hydrogen peroxide than in solutions containing other oxidants such as dissolved oxygen,3 ferric ion,4 and sodium perchlorate, s Any advantages in uraninite dissolution rate, however, are offset by the fact that during in situ leaching, a considerable portion of the added H202 is likely to be consumed by the oxidation of iron sulfides and organic material that are commonly present in sandstone-type uranium deposits.67 Also, experimental and field observations indicate that H202 rapidly decomposes soon after injection into uranium ores,s-l~ thereby quickly reducing the oxidizing capacity of the lixiviant. Hydrogen peroxide decomposes according to the overall reaction: 2H202(1) = 2H20(1) + O2(g)
[1]
where (1) and (g) refer to the liquid and gaseous molecular species, respectively. The rate of this reaction is catalyzed by metal ions 11 such as Fe 3+, Cu >, and Co 3+. Of these, the ferric ion is undoubtedly the most abundant catalytically active metal ion present during the acidic leaching of uranium ores. Concentrations up to 100 ppm total dissolved iron are reported by Tweeton, et al. p- from an in situ uranium leaching operation that used solutions containing dilute sulfuric acid, pH = 1.5 to 2.0, and hydrogen peroxide. However, H202 decomposition may take place in slightly acidic to strongly basic solutions, as well. It has been shown by Eligwe, et a l ) 3 that the addition of ferrous
L.E. EARY, formerly a Graduate Student with the Department of Geochemastry and Mineralogy, Pennsylvania State University, is now Research Scientist with Battelle Pacific Northwest Laboratory, Rlchland, WA 99352. Manuscript submitted March 20, 1984. METALLURGICAL TRANSACTIONS B
ions to H 2 0 2 solutions with pH between 4.0 and 6.0 caused a significant decrease in the rate of uranium extraction from a New Mexico ore. As suggested by Eligwe, et al., J3 the observed rate decrease was probably a result of the rapid consumption of the H,O2 in reactions with ferrous ions to form ferric species. Although th
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