Performance, Mechanism, and Kinetics of Fe(III)EDTA Reduction by

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Performance, Mechanism, and Kinetics of Fe(III)EDTA Reduction by Thiourea Dioxide Feiqiang He,* Yong Qian, and Jianping Xu School of Chemistry, Biology, and Materials Science, East China University of Technology, Nanchang 330013, P. R. China

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ABSTRACT: Reducing Fe(III)EDTA into Fe(II)EDTA has a very remarkable significance for NO removal using Fe(II)EDTA solution. Here, the reductant thiourea dioxide (TD) was first served for reducing Fe(III)EDTA. The mechanism and kinetics of Fe(III)EDTA reduction with thiourea dioxide has been researched under different thiourea dioxide concentrations, varied pH, and diverse temperatures, in which the Fe(III)EDTA reduction rate increased as the pH or temperature increased. Fe(III)EDTA reduction with thiourea dioxide presented a quasi-second-order reaction on Fe(III)EDTA. Moreover, the activation energy (Ea) and activation entropy (ΔS‡) from Fe(III)EDTA reduction using thiourea dioxide were respectively calculated as 70.644 kJ/mol and 5.588 J/(K·mol). Compared with other reaction systems on reducing Fe(III)EDTA, thiourea dioxide has been proved to be more powerful. The kinetics model describing Fe(III)EDTA reduction by thiourea dioxide in air was discussed. Finally, the simultaneous reduction of Fe(III)EDTA and Fe(II)EDTA-NO using TD was researched. It was shown that Fe(II)EDTA-NO could hinder Fe(III)EDTA reduction, and their reduction rates were close to each other. Our study provides a bridge between foundational research and industrial denitration employing combined Fe(II)EDTA with thiourea dioxide. NO(aq) + Fe IIEDTA2 − → Fe IIEDTA − NO2 −

1. INTRODUCTION All commercial purification technologies for air pollutants expelled with industrial operation (such as selective catalytic reduction on denitrification1,2and limestone gypsum on desulphurization3) for nitric oxide and sulfur dioxide removal are costly with respect to investment and operational expenses. For this reason, there is still an urgent need from industrial activities for developing advanced cost-effective methods to optimize current techniques. Because SO2 is so highly dissolved in a solution that readily absorbs the scrubbing solution for removing NO, the wet process becomes a part of the promising ways for simultaneous desulfurization and denitrification. Namely, the simultaneous NO and SO 2 removal could be resolved by a single step, in which both dissolved NO and SO2 were availably captured with liquidphase reaction sequences. Thus, co-instantaneous removal of NO and SO2 with a wet process as an alternative to separate removal has attracted more attention for industrial applications that require air purification. One such removal process is metal chelate absorption, and the fundamental principles of it have been widely explored in the last 40 years by many laboratory teams.4,5 However, until now, continued related research has continued to be concentrated upon the strategy of the process. Fe(II)EDTA as a classic complex absorbent can simultaneously separate SO2 and NO in flue gas. Nitric oxide was fleetly captured with Fe(II)EDTA, generating a steady ferrous−nitrosyl complex Fe(II)-EDTA-NO (1) whose equilibrium constant is about 107 M−1.6 But, during the absorption process, because 3−9% (v/v) O2 usually exists in exhaust gas from industry applications,7 most of the Fe(II)EDTA is readily oxidized, producing Fe(III)EDTA with O2 in solution (eq 2) and losing complexing ability to NO. © XXXX American Chemical Society

(1)

4Fe IIEDTA2 − + O2 (aq) + 4H+ → 4Fe IIIEDTA− + 2H 2O (2)

Accordingly, the Fe(II)EDTA concentration decreases quickly after the absorption process. Therefore, in order to maintain a high NO absorption efficiency, Fe(II)EDTA regeneration becomes very essential for the whole process with simultaneous desulphurization and denitrification. It is widely reported that sulfo-compounds (sulfite, bisulfite, dithionite ion, Na2S2O8 etc.) were combined with Fe(II)EDTA to enhance nitric oxide absorption.8−11 Reducing agents such as SO32−, HSO3−, and S2O4− were employed for the Fe(III)EDTA reduction but depressed the reducing power of SO32− and HSO3−, and the S2O4− easily oxidizing in the air could hinder Fe(II)EDTA regeneration. Na2S2O8 did not directly transform Fe(III)EDTA into Fe(II)EDTA, but persulfate-derived ions (HSO4−) generated by catalytic action can accomplish this, which leads to the result that Fe(II)EDTA regeneration by Na2S2O8 is limited. In addition, various bacterium, including Escherichia coli FR-2 and Klebsiella sp. strain FD-3, have been widely employed for Fe(II)EDTA regeneration12.13 But, their reduction rate was low, and the requirement and environment for the isolation and culture of the bacteria were rigorous, resulting in expensive costs. Meanwhile, activated carbon, 14 selenium, 15 hydrazine (N2H4),16 metal powder (Zn, Fe, Al),17 polyphenolic compound,18 and other reductants were shown effective for Fe(III)EDTA reduction. But, due to complex procedures, Received: November 1, 2018 Revised: February 27, 2019

A

DOI: 10.1021/acs.energyfuels.8b03820 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

2.2. Mechanism of Fe(III)EDTA Feduction with TD. First, 250 mL of Fe(III)EDTA solution was obtained through blending Na2EDTA and NH4Fe(SO4)2·12H2O and was subsequently added into a flask with a pH meter in a heat-collection, constanttemperature-type magnetic stirrer (Figure S1). Then, it was bubbled with a 3 L min−1 continuous N2 (99.999%) stream to maintain an anaerobic environment. Subsequently, the pH was adjusted to 8.0 using NaOH and H2SO4 solution. The reduction reaction began when TD was added to Fe(III)EDTA solution at 50 °C with a magnetic agitator set at 2500 rpm. Ultimately, the Fe(II)EDTA concentrations of the solution were measured at set time intervals. The reaction products were analyzed and determined. 2.3. Influence of pH upon Reducing Fe(III)EDTA. The pH value was changed from 7.4 to 9.0 using buffer solutions to examine the effect of the pH change on reduction. Then, 0.2703 g of TD was interfused with 250 mL of 0.002 M Fe(III)EDTA solution (50 °C, anaerobic condition). Finally, the Fe(II)EDTA concentrations were tested at different times. 2.4. Influence of Temperature Upon Reducing Fe(III)EDTA. The temperature was increased from 35 to 65 °C for researching the impact of the temperature on reduction, and the pH was maintained at 8.6 with a buffer agent. The, 0.2703 g of TD was interfused with 250 mL of 0.002 M Fe(III)EDTA solution. The Fe(II)EDTA concentration changes over time were analyzed. 2.5. Fe(III)EDTA Reduction in Air. The reduction of Fe(III)EDTA with TD in air was conducted with an original concentration of 0.002 M, a temperature of 50 °C, a pH of 9.0, and with 0.01 M TD. Ultimately, Fe(II)EDTA concentration changes over time were analyzed. 2.6. Relevance of Fe(II)EDTA-NO Reduction. In reference to related literature,21 Fe(II)EDTA-NO was obtained through bubbling an established mixture of nitric oxide and nitrogen into Fe(II)EDTA solution until the nitric oxide concentration from the outlet was equivalent to that from the inlet (measured with a flue gas analyzer, KM950). In this paper, 250 mL of 0.002 M Fe(II)EDTA-NO was employed and maintained with N2 positive pressure. Then, 0.1681 g of Na2EDTA and 0.2411 g of NH4Fe(SO4)2·12H2O were mixed with 0.002 M Fe(II)EDTA-NO to prepare a mixing solution with 0.002 M Fe(III)EDTA and 0.002 M Fe(II)EDTA-NO with adjusting the pH to 9.2. Ultimately, the changes of their concentration with time were analyzed after TD addition. 2.7. Analytical Methods. The data obtained in our work were average values from triplicate experiments with their standard deviations. Fe(II)EDTA concentrations were measured by ophenanthroline colorimetry at 510 nm.8 Fe(II)EDTA-NO concentration was determined using a UV/vis spectrophotometer at 434 nm.22 Fe(III) concentration was calculated from the difference between total Fe and Fe(II).23 The sulfite concentration was measured with iodometry.24 The urea concentration was measured using diacetylmonoxime coloration.25

environmental pollution, and high costs, these methods have not been commercialized successfully. Thiourea dioxide (TD), called formamidine sulfinic acid, has long been used as a reducing agent for papermaking, textile, heavy metal treatment, and printing industries.19 The reduction process with TD mainly translates into less toxic and hazardous sulfite and urea.20 TD exists as two types of isomers in solution, corresponding to isomer I under acidic conditions or room temperature and isomer II under basic conditions or elevated temperatures (Figure 1). Specifically,

Figure 1. Two types of isomers of TD.

thiourea dioxide is quite stable at room temperature in solution. However, it decomposes immediately to yield the strong reductant sulfoxylic acid by heating or alkali catalysis. Therefore, presuming that Fe(III)EDTA could be reduced with TD is reasonable. Meanwhile, the produced sulfite and urea from thiourea dioxide during the reduction process could be recycled as a rich resource. So, Fe(II)EDTA regeneration with TD will be more efficient and a safe, green process in simultaneous desulfurization and denitrification. In this work, to further explore the inherent relevance between Fe(III)EDTA and thiourea dioxide, the reaction mechanism and kinetics on reducing Fe(III)EDTA with TD were researched and analyzed in depth. Ultimately, we preliminarily research the simultaneous reduction of Fe(III)EDTA and Fe(II)EDTA-NO using TD. This research provides assistance and information for industrial denitration employing combined Fe(II)EDTA with thiourea dioxide.

2. EXPERIMENTAL SECTION 2.1. Materials. Na2EDTA (99.0%), FeSO4·7H2O (99.0%), NH4Fe(SO4)2·12H2O (99.0%), and NaNO2 (99.0%) were supplied by Tianjin Kermel Chemical Reagent Co., Ltd., China. TD was obtained from Xinsheng Chemical Co., Ltd., China. N2 (99.999%) and NO (99.99%) were obtained from Guangzhou Yuejia Gas Co., China. All other chemicals were analytical grade, commercially available, and used without further purification.

Figure 2. (a) Fe(II)EDTA concentration change with time under different TD concentrations. (b) The pH change in the process. Conditions: temperature of 50 °C, original Fe(III)EDTA concentration of 0.002 M, and original pH value of 8.0. B

DOI: 10.1021/acs.energyfuels.8b03820 Energy Fuels XXXX, XXX, XXX−XXX

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3. RESULTS AND DISCUSSION 3.1. Mechanism between Fe(III)EDTA and TD. The Fe(II)EDTA concentration variety is presented in Figure 2a with reducing Fe(III)EDTA with TD with diverse original concentrations. It is visible that TD can effectively reduce Fe(III)EDTA under appropriate conditions. As TD concentration increases, the produced Fe(II)EDTA concentration rises, with values of 0.00108, 0.00139, and 0.00187 M at the 30th minute, with addition of 0.01, 0.015, and 0.02 M TD, respectively (Figure 2a). The color change was an important indicator in Fe(III)EDTA reduction. It was found, as shown in Figure 3, that the

(Figure S2). Specifically, the urea concentrations were raised to 0.00044, 0.00068, and 0.00086 M, and the sulfite concentrations were increased by 0.00041, 0.00067, and 0.00085 M within 30 min with respect to 0.01, 0.015, and 0.02 M TD, respectively. The proportion among produced Fe(II)EDTA, generated urea, and generated sulfite were all closely 2:1:1 under three different initial concentrations of TD. As analyzed above, it is proved that sulfite and urea could be considered as the main products of TD oxidation. The pH value variation is connected to Fe(III)EDTA reduction with TD, as illustrated in Figure 2b. It can be seen that the pH values of all solutions decrease during the Fe(III)EDTA reduction process, revealing that OH− was continually depleted when Fe(III)EDTA was reduced with TD. In addition, OH− depletion increases as the TD concentration increases. For example, the pH values decrease from 8.00 to 5.44, 4.97, and 4.35 during the 30 min testing with 0.01, 0.015, and 0.02 M TD, respectively (Figure 2b). Moreover, it was observed from Figure 2a and 2b that Fe(III)EDTA concentration changes were in accordance with OH− consumption by diverse TD concentration. So, OH− was indispensable during Fe(III)EDTA reduction with TD. Therefore, a credible stoichiometry between Fe(III)EDTA and TD was achieved:

Figure 3. Color change during the reduction of Fe(III)EDTA with TD [(A) color of 0.01 M Fe(III)EDTA with pH 8.0; (B) color of Fe(III)EDTA solution after reduction with 0.1 M TD at 50 °C for 30 min; (C) color of solution after nitric oxide and nitrogen mixture was poured into (B)].

(NH 2)2 CSO2 + 2Fe(III)EDTA− + 4OH− → 2Fe(II)EDTA2 − + (NH 2)2 CO + SO32 − + 2H 2O

solution color varied from light yellow Fe(III)EDTA to colorless Fe(II)EDTA, and when the nitric oxide and nitrogen mixture was poured into the colorless Fe(II)EDTA solution, a black-brown Fe(II)EDTA-NO appeared, revealing that the product of reducing Fe(III)EDTA with TD could effectively capture NO. These color changes were consistent with the results from Zhang et al.26 and Xiang et al.15 TD was further proved to translated Fe(III)EDTA into Fe(II)EDTA. Simultaneously, the determination of the products of TD oxidation for deducing the reaction mechanism is critical. Wang et al.27 used TD as a green reductant for preparation of nanometer nickel powder from spent electroless nickel, finding that the reductant TD was transformed into SO32− and urea. Therefore, regarding the Fe(III)EDTA reduction with TD, sulfite and urea are suspected as the main products of TD oxidation. For the sake of verifying our conjecture, the changes of sulfite and urea concentrations were respectively investigated. It can be found that the sulfite and urea were produced and that their concentrations increase with TD concentration

(3)

In addition, to further confirm eq 3, the original mole proportion of TD to Fe(III)EDTA was changed from 0.5:2 to 2:2. In order to make the reaction reach its limit as soon as possible, the verification experiment was carried out at a high temperature and pH value. We found that almost all of the Fe(III)EDTA was translated into Fe(II)EDTA by the ratios of 1:2, 1.5:2, and 2:2. In other words, when the mole proportion of TD to Fe(III)EDTA was lower than 1/2, Fe(III)EDTA can not be completely restored (Figure S3). The stoichiometric proportion of TD to Fe(III)EDTA could be confirmed to be 1:2, and the reaction formula between Fe(III)EDTA and TD was confirmed as eq 3. 3.2. Influence of pH. Because TD normally exhibits strong reducibility under alkaline conditions, and the Fe(III)EDTA proportion transformed into Fe(II)EDTA also deeply relies upon solution alkalinity, the relevant influence of original pH values on reduction was examined with pH 7.4−9.0, in which

Figure 4. (a) Fe(II)EDTA concentration variation under different pH values in the range of 7.4−9.0. (b) The diagrams from 1/[Fe(III)] vs time. Reaction parameters: temperature of 50 °C, 0.01 M TD, and [Fe(III)EDTA]0 = 0.002 M. C

DOI: 10.1021/acs.energyfuels.8b03820 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels log(kobs) = −z(14 − pH) + log(k[TD] y )

aqueous borate buffers were mixed to maintain unchanged pH values for each of the Fe(III)EDTA solutions during the reduction process. From Figure 4a, the Fe(III)EDTA reduction rate increased as pH values increased. For example, Fe(II)EDTA regeneration efficiencies were 28.8%, 54.0%, 82.0%, and 100% during the 30 min, corresponding to pH values of 7.4, 7.8, 8.4, and 9.0, respectively. Besides that, we found that higher alkalinity was propitious to Fe(III)EDTA reduction with TD, further confirming that continual OH− was depleted in the overall reaction (eq 3). 3.3. Kinetic Analysis with Different pH. For inferring kinetics on Fe(III)EDTA reduction with TD, a traditional strategy is that one of the reactants is added to be excessive and the pH value is kept to be unchanged by buffers in the reaction process. In this work, the TD concentration was far greater than that of the Fe(III)EDTA concentration and could be assumed to be unchanged during the whole process. During the reaction stage (15−30 min), because of the extremely low reduction rate, the chemical equilibrium can be achieved. The rapid phase (0−15 min) was then selected as the analytical object for kinetics calculations, and then the Fe(III)EDTA reaction rate was written as follows: r=−

To calculate the reaction order with respect to OH , the plot of log (kobs) against (14 − pH) was fitted, as shown in Figure 5, to give a beeline, and the slope of the beeline was −0.97 with a correlation coefficient (r2) of 0.9864. Therefore, the reaction order of OH− is about 0.97.

Figure 5. Plot of log (kobs) against (14 − pH).

d[Fe(III)EDTA−] dt

= k[Fe(III)EDTA−]x [TD] y [OH−]z

3.4. Influence of the Temperature. Temperature can impose an important impact on reducing Fe(III)EDTA with TD. The effect of temperature on reduction was examined under four different temperatures. Figure 6a shows that the reduction rate increases as the temperature increases. Concretely, Fe(II)EDTA concentrations increase from 0 M to 0.00149, 0.00169, 0.00186, and 0.00197 M within 30 min at 35, 45, 55, and 65 °C, respectively. Similarly, the rapid stage (0−15 min) was also employed for kinetic calculations. The line graph of 1/[ Fe(III)EDTA] vs t were plotted to obtain kobs. It is also found that reducing Fe(III)EDTA with TD shows a quasi-second-order reaction with regard to Fe(III)EDTA with the correlation coefficients (r2) of 0.9913, 0.9713, 0.9987, and 0.9536 under the four different temperatures (Figure 6b). Besides, the correlated observed rate constants (kobs) given from the slopes of the fitting line are increasing with rising temperature, i.e., 39.25, 83.55, 194.14, and 451.72 L mol−1 min−1, corresponding to 35, 45, 55, and 65 °C, respectively. Activation energy can be used to represent the minimum energy required for a chemical reaction to occur.28 The Arrhenius equation is an empirical formula for the relationship between the rate constant of a chemical reaction and the temperature (eq 10).

(4)

where x, y, and z respectively stand for reaction orders of Fe(III)EDTA, TD, and OH−, r for reaction rate, and k for the overall rate constant. [Fe(III)EDTA], [TD], and [OH+] represent concentrations of Fe(III)EDTA, TD, and hydroxyl ions in molarity (M), respectively. In the above analysis, TD concentration was considered as constant. Besides, pH value was almost unchanged with buffer solution throughout the whole process. So, k[TD]y[OH−]z was seen to be constant as kobs. Therefore, eq 4 was given as: r=−

d[Fe(III)EDTA−] = kobs[Fe(III)EDTA−]x dt

(5)

where [Fe(III)EDTA] can be obtained from 0.002 − [Fe(II)EDTA]. The Fe(III)EDTA stoichiometric coefficient is 2 according to eq 3. So, we assumed that the reaction order with Fe(III)EDTA is 2, and we then verified this assumption. From Figure 4b, 1/[Fe(III)EDTA] varies almost linearly with time (t), and the correlation coefficients (r2) were 0.953, 0.977, 0.993, and 0.994 at the pH values of 7.40, 7.80, 8.40, and 9.00, respectively, which confirms that the reaction order of Fe(III)EDTA is 2. In addition, kobs values were obtained from the slopes of the plots, i.e., 9.158, 29.716, 81.326, and 374.172 L mol−1 min−1. Evidently, kobs increases as the pH value increases, ulteriorly revealing that high pH presents a benefit when reducing Fe(III)EDTA with TD. In the last paragraph, we assumed that k[TD]y[OH−]z was seen to be constant (kobs). kobs = k[TD] y [OH−]z

(9) −

k = Ae−Ea / RT

The natural logarithm of eq 10 was taken to obtain ln k = −Ea /RT + ln(A)

Then a logarithm is taken on both sides: (7)

log(kobs) = log(k[TD] y ) + log([OH−]z )

(8)

(11)

where k represents the rate constant. A is the pre-exponential factor, or Arrhenius constant, and the unit is the same as for k. Ea is called the experimental activation energy (kJ mol−1). R stands for molar gas constant (R = 8.314 J/(K·mol)), and T represents the absolute temperature (K). When the rate constant of a reaction is obeyed in the Arrhenius’ equation, its plot of ln(k) versus T −1 would show a beeline. It can be seen from Figure 7 that the diagram from ln(k) versus T −1 in our paper provides a beeline, and the slope

(6)

log(kobs) = log(k[TD] y [OH−]z )

(10)

D

DOI: 10.1021/acs.energyfuels.8b03820 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 6. (a) Fe(II)EDTA concentration variation under different temperatures. (b) The diagrams from 1/[Fe(III)EDTA] vs time. Reaction parameters: pH = 8.6, original TD concentration of 0.01 M, and [Fe(III)EDTA]0 = 0.002 M.

was compared with those reported in other published works. Because of the diversities of experimental parameters used by various researchers, comparisons of the efficiencies of Fe(III)EDTA reduction with that of other research are difficult. In each case, we chose the best one as a comparison, as shown in Table 1. We found that only Zn and Na2S2O4 are superior to TD for the Fe(III)EDTA reduction efficiency. Table 1. Comparison of Reducing Fe(III)EDTA with Diverse Systemsa ref 13

Figure 7. Polts of the Arrhenius and Eyring−Polanyi equations. Conditions: original Fe(III)EDTA concentration of 0.002 M, pH = 8.6, and 0.01 M TD.

12 17

was employed to calculate Ea. Finally, the activation energy was calculated to be 70.644 kJ/mol (Figure 7). The Eyring−Polanyi formula, eq 12, was employed to analyze chemical kinetics for describing the variation on chemical reaction rate with temperature. It can be written as: k=

kBT ΔS‡/ R −ΔH ‡/ RT e e h

29 30 31 32 33

(12)

this paper

It was converted in the following form:

reduction agent Escherichia coli FR-2 Klebsiella sp. strain FD-3 Zn Al Sn Fe Na2S2O4 Na2SO3 Na2SO3/Na2SeSO3 mixture activated carbon/ Na2SO3 TD

reduction efficiency (initial concentration)

required time

87% (6 mM)

9h

52.4% (10 mM)

12 h

100% (50 mM) 60% (100 mM)