Article pubs.acs.org/est
Elucidation of the Mechanism of Reaction between S2O82−, Selenite and Mn2+ in Aqueous Solution and Limestone-Gypsum FGD Liquor Hiroyuki Akiho,*,†,‡ Shigeo Ito,† Hiromitsu Matsuda,† and Toshiaki Yoshioka‡ †
Energy Engineering Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-6-1 Nagasaka, Yokosuka, Kanagawa 240-0196, Japan ‡ Graduate School of Environmental Studies, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan ABSTRACT: The mechanism of reaction between peroxodisulfate ion (S2O82−), selenite (Se(IV)O32‑) and Mn2+ as an inhibitor of selenite oxidation was studied using aqueous solutions composed of commercial reagents, as well as limestonegypsum flue gas desulfurization (FGD) liquors sampled from coal fired power plants. The oxidation of selenite to selenate (Se(VI)O42−) is promoted by the sulfate ion radical (SO4−) which results from decomposition of S2O82−. In the presence of Mn2+, selenite oxidation was prevented due to the difference in rates of reaction with SO4−. The ratio of the oxidation rate constants of selenite and Mn2+ with SO4− was determined over a temperature range of 40−60 °C, and was found to be little influenced by the various coexisting components in FGD liquors.
1. INTRODUCTION In a coal fired power plant, selenium in coal is vaporized during combustion1 and is partially absorbed onto fly ash.2−4 If the plant is equipped with an electrostatic precipitator (ESP) and a wet flue gas desulfurization (FGD) unit, the seleniumcontaining fly ash is captured by the ESP, and the remaining gaseous selenium (as Se(IV)O2) passes the ESP to be dissolved into the FGD liquor,3−7 where selenium primarily exists as selenite (Se(IV)O32−) and selenate (Se(VI)O42−). Selenite is generally removed through a conventional wastewater treatment such as a coagulation-sedimentation process, but selenate is released into the environment unless a reduction process takes place.8,9 The limestone-gypsum process using calcium carbonate (CaCO 3 ) as a desulfurization agent is currently the predominant wet FGD process for coal-fired power plants in many countries. The process is extremely complex, encompassing many simultaneous reactions such as the dissolution of sulfur dioxide (SO2) gas, oxidation of sulfite ion (SO32−) to sulfate ion (SO42−), dissolution of CaCO3 and gypsum formation by the reaction between Ca2+ and SO42−. In many cases, air is supplied into the FGD liquors to improve the oxidation of SO32− (the forced oxidation FGD process), and the amount of air supplied is varied according to the concentration of SO2 in the combustion flue gas and the type of absorber (spray tower, jet bubbling reactor, double contact flow scrubber, etc.). Since the presence and form of various components in an actual FGD liquor depends on the operating conditions, the behavior of selenium in an actual FGD unit is not well understood. Akiho et al. 10,11 indicated that peroxodisulfate ion (S2O82−) is present in limestone-gypsum FGD liquor as the dominant oxidizing agent, and oxidizes © 2013 American Chemical Society
selenite to selenate easily. The oxidation of several organic and inorganic substrates (e.g., Tl+, Fe2+, Mn2+, Ce3+, SO32−, HSO3−, and C2O42−) by S2O82− has been extensively studied,12−19 but the reaction of selenite has not been investigated. Akiho et al.10,11 also revealed that addition of Mn2+ prevented the oxidation of selenite because the S2O82− selectively oxidized Mn2+ to MnO2 instead. The control of selenite oxidation in FGD liquor using Mn2+ may eliminate selenate reduction in wastewater treatment processes. The present work is an investigation of the reaction kinetics between selenite, S2O82− (as a promoter of selenite oxidation) and Mn2+ (as an inhibitor of selenite oxidation) in aqueous solutions and FGD liquors. All experiments were performed under non-FGD conditions because the purpose of this work is to understand the fundamental selenite oxidation reaction in the liquid phase as a preliminary step to clarifying the behavior of selenium in an actual FGD unit.
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Aqueous solutions containing S2O82−, selenite, and Mn2+ were prepared using potassium peroxodisulfate (K2S2O8), selenium dioxide (SeO2), and manganese standard reagent (Mn in 0.1 mol/L of HNO3) (Kanto Chemical Co., Ind.). The initial concentrations of these components are shown in Table 1. At condition A, the decomposition characteristics of S2O82− were evaluated using S2O82− solutions of varying concentrations and temperatures. Received: Revised: Accepted: Published: 11311
October 20, 2012 July 5, 2013 September 9, 2013 September 9, 2013 dx.doi.org/10.1021/es3042302 | Environ. Sci. Technol. 2013, 47, 11311−11317
Environmental Science & Technology
Article
Table 1. Experimental Conditions sample
condition
S2O82− (mmol/L)
selenite (mmol-Se/L)
Mn2+ (mmol/L)
pH (-)
temperature (°C)
reaction time (h)
aqueous solution
A B C D
0.51−5.4 1.4−1.5 1.5−1.6 0−2.6
0 0.012−0.12 0 0.0022−0.21
0 0 0.089−0.91 0.036−0.36
4.3−5.0 3.6−4.2 2.1−3.0 2.5−3.4
25, 40, 50, 60 50 50 40,50,60
96−120 4−30 12−70 15−84
FGD filtrate
E F G H
0.21−2.0 0.17−1.3 0.54−2.2 0.099−2.2
0 0.012−0.015 0 0.011−0.016
0 0 0.060−0.11 0.16−0.18
4.1−7.8 3.8−7.5 6.1−7.1 3.3−7.3
50 50 50 50
72−129 3−12 12−48 48
At conditions B and C, either selenite or Mn2+ was added to the S2O82− solution to evaluate the oxidation characteristics of each. At condition D, the effect of Mn2+ on the prevention of selenite oxidation was investigated using a mixture of S2O82−, selenite, and Mn2+. Twelve FGD liquors (pH 3.2−6.8; oxidation reduction potential (ORP) = 230−690 mV; temperature = 44.0−49.2 °C; [S2O82−] = 19.1−421 mg/L) were obtained from four coal fired power plants in Japan which adopt the forced oxidation FGD. FGD liquors include a variety of components (such as SO42−, Cl−, F−, Na+, K+, Ca2+, Mg2+, Al3+, Mn2+, and Se (primarily as selenate)) at various concentrations, but have little selenite. The FGD liquors were filtered using a 0.45 μm membrane filter to remove solid components such as gypsum and fly ash particles, and the filtrates were used under the conditions shown in Table 1. At condition E, the decomposition characteristics of S2O82− were evaluated without the addition of any reagents. At condition F, only selenite was added to the filtrates to evaluate the oxidation characteristics of selenite. At condition G, the oxidation characteristics of Mn2+ were evaluated using only three FGD filtrates previously containing Mn2+ (without addition of Mn2+). Then, both selenite and Mn2+ were added to the filtrate to investigate the effect of Mn2+ on the prevention of selenite oxidation using condition H. Initial concentrations of S2O82−, selenite and Mn2+ under conditions D and H were set at values generally encompassing those of FGD liquors. The molar ratio of selenite to Mn2+ was set in the range from 0.014 to 2.1 under condition D. The initial pH values of the test solutions depended on the amount of reagents used to prepare the solutions, and the pH of some of the FGD liquors exceeded 7.0 due to dissolution of residual CaCO3. All of the test solutions were prepared in enclosed glass impingers and kept in a water bath set at either 25, 40, 50, or 60 °C. The pH of the solutions was not controlled during the tests. A portion of the solutions was sampled at predetermined intervals and filtered again using a 0.45 μm membrane filter, because MnO2 precipitates in the presence of S2O82− and Mn2+. The filtrates were diluted with deionized water and were stored in a refrigerator at 10 °C until analysis was begun. 2.2. Analysis. Total selenium (SeT) in the stored samples was determined via ICP mass spectrometry (ICP-MS, Shimadzu Corporation, ICPS8500). The Se(IV) content of the same samples was also determined via ICP-MS through hydride generation treatment which reduces only Se(IV) to selenium hydride (H2Se) gas before injection into the analyzer. The Se(VI) content was calculated as the difference between SeT and Se(IV) contents. Petrov et al.20 reported that selenosulfate (SeSO32−), selenocyanate (SeCN−) and additional unknown selenium species other than selenite and selenate exist in FGD liquor, especially those produced by
inhibited oxidation FGD units, which do not feed air into the FGD liquors. In this work, all of the FGD liquor samples were taken from forced oxidation FGD units, where the ratios of SeSO32−, SeCN− and unknown species are very low. Additionally, our previous work using X-ray absorption fine structure (XAFS) analysis revealed that S2O82− causes oxidation from selenite to selenate.11 Therefore, in this work the difference between the SeT and Se(IV) content is regarded to be the complete Se(VI) content. The amount of Mn2+ in the sample was determined by ICP atomic emission spectrometry (ICP-AES, Shimadzu Corporation, ICPS7500). The concentration of S2O82− and SO42− in the filtrate were determined using an ion chromatograph equipped with a guard column (SHOWA DENKO K.K., Shodex IC IA-G) and separation column (SHOWA DENKO K.K., Shodex IC I-524A) for the nonsuppressor system. A mixed solution of 5.0 mmol/L phthalic acid and 4.6 mmol/L tris-aminomethane was used as the eluent for the ion chromatograph measurements.
3. RESULTS AND DISCUSSION 3.1. Decomposition Characteristic of S2O82−. The wellknown overall decomposition reaction of S2O82− in aqueous solution without acid catalysis is shown in eq 1.12−14 S2 O82 − + H 2O → 2SO4 2 − + 2H+ +
1 O2 2
(1)
It has been suggested that the first step of the reaction is one of the following two reactions:12−14 k1a
S2 O82 − → 2SO4 − k1b
S2 O82 − + H 2O → SO4 2 − + SO4 − + H+ + OH
(2)
(3)
where k1a and k1b are the rate constants of eq 2 and eq 3, respectively. In eq 2, two sulfate ion radicals (SO4−) are formed by the decomposition of S2O82−. Another reaction forms only one SO4− as shown in eq 3. Since SO4− is highly reactive, it attacks a substrate in the solution immediately and is converted to SO42−. In this study, the decomposition characteristics of S2O82− were evaluated at different concentrations and temperatures. Four aqueous solutions and eight FGD filtrates containing S2O82− at different concentrations were kept at 50 °C (FGD temperature) for about 100 h. S2O82− concentrations decreased gradually with the passage of time in all cases. Since eq 2 is a first-order reaction and eq 3 can be assumed to be a pseudo-first-order reaction, the rate of S2O82− decomposition is given as eq 4 on the bases of eqs 2 and 3. 11312
dx.doi.org/10.1021/es3042302 | Environ. Sci. Technol. 2013, 47, 11311−11317
Environmental Science & Technology
Article
d[S2 O82 −] = −k1[S2 O82 −] dt
(4)
By integrating eq 4, eq 5 is obtained: ln
[S2 O82 −] [S2 O82 −]i
= −k1t
(5) 2−
where k1 is k1a or k1b[H2O], [S2O8 ]i is the initial concentration of S2O82−, and t is the reaction time. The relationship between ln([S2O82−]/[S2O82−]i) and reaction time obtained from the S2O82− decomposition experiment using aqueous solutions is almost linear, with a slope of 1.2 × 10−6 s−1 (R2 = 0.993), as shown in Figure 1. For the FGD filtrates, Figure 2. Decrease in S2O82− concentration at different temperatures in aqueous solutions.
Figure 1. Relationship between ln([S2O82−]/[S2O82−]i) and reaction time in aqueous solutions (condition A) and FGD filtrates (condition E) at 50 °C. Figure 3. Relationship between lnk 1 and 1/T in the S2 O82− decomposition.
the plots ranged within 15% of the relationship, as shown in the figure. It is known that Ag+ accelerates the decomposition of S2O8.2−14 FGD filtrates used in the test contained various components such as SO42− (>1000 mg/L), Cl− (>1000 mg/L), Ca2+ (>100 mg/L), Mg2+ (>100 mg/L), F− (>10 mg/L), B (>10 mg/L), Na+ (>10 mg/L), K+ (>10 mg/L), and Al3+ (1− 100 mg/L). Some of them also contained Mn2+ (
