Stability of Ferrate(VI) in 14 M NaOH-KOH Mixtures at Different

Chapter 9

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Stability of Ferrate(VI) in 14 M NaOH-KOH Mixtures at Different Temperatures Virender K. Sharma,1,* Shavi Tolan,2 Václav Bumbálek,3 Zuzana Macova,3 and Karel Bouzek3 1Department

of Environmental and Occupational Health, School of Public Health, Texas A&M University, 1266 TAMU, College Station, Texas 77843, United States 2Chemistry Department, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901, United States 3Department of Inorganic Technology, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic *E-mail: [email protected]

The stability of ferrate(VI) (FeVIO42-, Fe(VI)) as a function of time in NaOH-KOH mixtures at different molar ratios (3:11, 7:7, 11:3, and 14:0) of 14 M ionic strength and temperature ( 30 °C, 45 °C, and 60 °C) was determined. Measurements of stability were performed under two concentrations of Fe(VI) in mixtures. One concentration of Fe(VI) was less than its solubility (S) (i.e., [Fe(VI)] < S) while other concentration was more than the S (i.e., [Fe(VI)] > S) under different conditions of molar ratios and temperature in the mixture solutions, which resulted in homogeneous and heteogeneous phases, respectively. The homogeneous phase had only dissolved Fe(VI), while the heterogeneous phase consisted of both dissolved Fe(VI) and precipitated Fe(VI). Results of stability were compared by evaluating decay rates of different mixtures. Decay of dissolved Fe(VI) was generally lower in heterogeneous phase than in homogeneous phase. Effect of temperature on stability in homogeneous solution had no role in mixed solution, except 3:11 molar mixture solution. Implication of results in electrochemical synthesis is briefly discussed.

© 2016 American Chemical Society Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Introduction In recent years, ferrate(VI) (FeVIO42-, Fe(VI)) has emerged as an environmentally-friendly chemical in novel processes to treat water and wastewater (1–4). Examples include oxidation of micropollutants, inactivation of chlorine resistant microorganisms, and removal of metals and nutrients (5–17). However, a wide range of applications of Fe(VI) has not been realized due to inefficient synthesis of Fe(VI) compounds. Sodium ferrate(VI) (Na2FeO4) and potassium ferrate(VI) (K2FeO4) are the most commonly applied chemicals, which have been synthesized in the laboratory set-up by three approaches: electrochemical, thermal, and chemical (18–23). Of these three synthesis methods, an electrochemical technique has advantages over other approaches since it uses an electron as a so-called clean chemical to obtain Fe(VI) compounds of high purity. Moreover, the electrochemical method can be easily adopted by on-line synthesis for applications of Fe(VI) in treatment processes (24–26). The most common electrochemical Fe(VI) synthesis processe has used concentrated NaOH solution to produce well soluble Na2FeO4, which cannot be applied for treatments (2, 20, 27, 28). Furthermore, soluble Na2FeO4 is stable only for few hours and, therefore, more stable solid K2FeO4 is needed (29, 30). A use of KOH as an anolyte in the electrochemical cell to synthesize Fe(VI) decreases the synthesis efficiency because of direct contact of solid K2FeO4 to the anode surface, which inhibits the generation of Fe(VI) (20, 31). However, a use of mixture of NaOH and KOH has advantage of the low solubility of Fe(VI) in a K+ ion with no decrease in of Fe(VI) synthesis (20, 31). The synthesis of Fe(VI) in a NaOH-KOH mixture would depend on the solubility and stability of Fe(VI) at different temperature. Recently, we have measured the solubility of Fe(VI) in a NaOH-KOH mixture (32). This paper presents the measurements on the stability of Fe(VI) at different temperatures in order to understand the conditions under which efficient synthesis of Fe(VI) could occur. The measurements on the stability of Fe(VI) in the NaOH-KOH mixture were performed at 14 M ionic strength. This selection of ionic strength was based on our earlier study on the electrochemical synthesis of ferrate(VI) at I = 14 M (32). Synthesis experiments were done at different temperatures (32), hence, a stability study was also carried out in the temperature range of 30-60 °C.

Experimental Section Chemicals Chemicals used in the study were of reagent grades (Sigma-Aldrich, Saint Louis, Missouri, USA) and were used without further purification. Water to prepare solutions had been distilled and passed through an 18 MΩ Milli-Q cm water purification system. Crystals of solid K2FeO4 of high purity (98 % plus) were prepared by the wet method (33). 242 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Procedure


In the experiments, K2FeO4 crystal was added into 2.5 mL of the studied hydroxide solutions. Solutions were prepared with KOH and NaOH with molar ratios of 3:1, 7:7, 11:3, and 14:0 at ionic strength of 14 M. Two different amounts of K2FeO4 were added into the mixture solutions in order to keep the concentration of Fe(VI) either below or above the solubility in the studied mixture solutions. The solubility in the mixture (S*) at each temperature was determined using the following equation (1) (32).

where XK+ is the mole fraction of K+ ions in the solution mixtures and T is temperature in Kelvin. The mixture solution in a small beaker was then placed into a temperature controlled cell and continuously stirred with a magnetic stirrer. The temperature of the cell was controlled within ± 0.1 ºC by a Fischer Scientific Isotemp (Hampton, New Hampshire, USA3016 circulating water bath. Concentrations of dissolved Fe(VI) and total Fe(VI) (i.e. dissolved and precipitated Fe(VI)) in the mixture solutions at different time intervals were determined. The concentrations of the solutions ([Fe(VI)] < S) were determined using the spectrophotometric method (34). Before performing the absorbance measurement, the solution was diluted by adding 100 μL of the solution into 5.0 × 10-3 L of 1.0 × 10-3 M Na2B4O7.10H2O/5.0 × 10-3 M Na2HPO4 (pH 9.0). Absorbance at 510 nm was measured and the molar absorption coefficient ε510nm = 1150 M-1 cm-1 at pH 9.0 was used to calculate the concentration of Fe(VI) (34). In the mixture where Fe(VI) was more than S, two different measurements were carried out. In one case, the solution was filtered with a 0.45 μm filter and concentration of Fe(VI) in the filtrate was determined using the spectrophotometric technique as described above. In other case, concentration of Fe(VI) in mixture solution was directly determined without filtration and Fe(VI) was analyzed using the chromite method (35).

Results and Discussion In the electrochemical synthesis process, concentration of Fe(VI) in the NaOH-KOH mixture increased with time, which involved initial dissolved (or soluble) Fe(VI), followed by precipitation of K2FeO4. The process can thus be considered as the homogeneous phase (dissolved Fe(VI)), followed by the heterogeneous phase (dissolved Fe(VI) and precipitated Fe(VI)). In the experimental set-up, stability measurements were carried out under conditions of both phases. When [Fe(VI)] < S, homogeneous phase existed while heterogeneous phase was present at [Fe(VI)] of > S. 243 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Concentrations of dissolved Fe(VI) in the mixed solution of KOH and NaOH in homogeneous and heterogeneous phases at different temperatures were determined; sample analyses were performed at least twice. Results from the experiment with the KOH and NaOH mixture ([KOH] : [NaOH] = 11 : 3) as an example is shown in Figure 1. In the figure, [Fe(VI)]t represents dissolved molar Fe(VI) concentration at time t and [Fe(VI)]o is the initial dissolved molar concentration of Fe(VI). Results from the experiments with other molar ratios at different temperatures are presented in Appendix (Figures A1-A3). The total concentrations of Fe(VI) in the heterogeneous phase were also determined and are shown as a relative data in Appendix (Figure A4). Stability of Fe(VI) in the NaOH and KOH mixtures of different cations ratios at different temperatures was evaluated by determining decay rates (i.e., slopes) of plots in Figure 1 and Figures A1-A4. The calculated slopes are presented in Figure 2. Plots represent comparative stability of dissolved Fe(VI) in homogeneous and heterogeneous phases. The calculated slopes are shown for dissolved concentration of Fe(VI) in homogeneous and heterogeneous phases along with total Fe(VI) concentration in heterogeneous phase (Figure 2). The concentration of K+ ion greatly affected the stability of Fe(VI) in both phases. In the KOH and NaOH mixture solution ([KOH] : [NaOH] = 3 : 11), decay of ferrate(VI) in the dissolved phase was similar in both phases. In other mixture solutions, decay of dissolved Fe(VI) was lower in the heterogeneous phase than that in the homogeneous phase. In 14 M KOH solution, the difference in the decay of ferrate(VI) was more pronounced at all temperatures. Temperature had no effect on the stability of Fe(VI) in the homogeneous solution within experimental errors, except for the mixture solution with the molar ratio of 3:11. The stability of ferrate(VI) decreased with increase in temperature in both phases at this ratio. The effect of temperature on the stability of ferrate(VI) decreased with increase in K+ ions in the solution and finally had not affected the stability in 14 M KOH solution in the heterogeneous phase. The reason for this behavior is twofold. Decreasing Fe(VI) solubility with increasing K+ ions content in the solution reduces the driving force for the homogeneous Fe(VI) decomposition. Moreover, decomposed Fe(VI) molecules are continuously replaced by new molecules originating from the dissolving heterogeneous phase. Second aspect consists in the fact that the heterogeneous, i.e., solid Fe(VI) is protected from decomposition, at least in its bulk phase.

244 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 1. Decay of dissolved Fe(VI) in 11M KOH and 3 M NaOH mixture at different temperatures. (Different colored symbols represent repetition of experiments) (see color insert)



Figure 2. Decay rate of Fe(VI) in KOH-NaOH mixtures at different temperatures (circle symbol, solid line – dissolved Fe(VI) in homogeneous phase; square symbol, dash line – dissolved Fe(VI) in heterogeneous phase; triangle symbol, dot line – total Fe(VI) in heterogeneous phase) (see color insert)

Conclusions The results of the stability study have implications in pursuing the synthesis of Fe(VI) electrochemically. They indicate the possibility to improve synthesis efficiency by producing directly solid Fe(VI) while eliminating inhibition of the anode surface by precipitated product. Higher temperature reduce impact of gradual passivation/inhibition of the anode by the surface Fe oxo-hydroxide layer on the process, which allows appropriate conditions to synthesize Fe(VI). Based on the current data, a most promising conditions are a 14 M NaOH and KOH mixture with the molar ratios of 7 : 7 to 3 : 11 and at temnperature of 60 °C. An important feature to follow represents, in this respect, properties of the solid Fe(VI) phase, mainly morphology of the crystals, and their implications for a 246 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

solid phase separation from the anolyte. An important space for optimization can be considered here.


Appendix

Figure A1. Decay of dissolved Fe(VI) in 3 M KOH and 11 M NaOH mixture at different temperatures. (Different colored points represent repeated experiments) (see color insert)



Figure A2. Decay of dissolved Fe(VI) in 7 M KOH and 7 M NaOH mixture at different temperatures. (Different colored points represent repeated experiments) (see color insert)



Figure A3. Decay of dissolved Fe(VI) in 14 M KOH at different temperatures. (Different colored points represent repeated experiments) (see color insert)



Figure A4. Decay of relative total Fe(VI) content in KOH-NaOH mixtures at different temperatures (circle – 30 °C; square – 45 °C; triangle – 60 °C) (see color insert)

Acknowledgments V.K. Sharma would like to acknowledge the support of United States National Science Foundation (CHE 0706834). This work was performed during the tenure of V.K. Sharma at the Florida Institute of Technology. We thank Ms. Meagan Strouse in conducting some of the experiments. We also thank Professor Hyunook Kim for his comments, which improved the chapter greatly.


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Stability of Ferrate(VI) in 14 M NaOH-KOH Mixtures at Different

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