NaFeEDTA Decomposition and Hematite Nanoparticle Formation in

Hyeon-Cheol Lee, Jong-Hwa Kim, Jung-Hyun In, and Chang-Ha Lee*. Department of Chemical Engineering, Yonsei University, Seoul 120-749, Korea...
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Ind. Eng. Chem. Res. 2005, 44, 6615-6621

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NaFeEDTA Decomposition and Hematite Nanoparticle Formation in Supercritical Water Oxidation Hyeon-Cheol Lee, Jong-Hwa Kim, Jung-Hyun In, and Chang-Ha Lee* Department of Chemical Engineering, Yonsei University, Seoul 120-749, Korea

Metal complex ethylenediaminetetraacetic acid (NaFeEDTA) was effectively decomposed by supercritical water oxidation (SCWO) with hydrogen peroxide in a plug-flow reactor. The effects of the oxidant amount and temperature on the decomposition rate and efficiency were more significant than that of pressure. The amount of oxidant significantly affected the decomposition rate and efficiency at lower temperatures. The decomposition kinetics of NaFeEDTA as the total organic carbon in the SCWO process was described using a global rate expression. In addition, as a byproduct, hematite (R-Fe2O3) in the range of 50-450 nm with regular shape was obtained from the decomposition of NaFeEDTA by the SCWO process. The size and shape of the metal oxide was controlled by the SCWO condition. Introduction Ethylenediaminetetraacetic acid (EDTA) is widely used in industrial processes such as metal plating, water softening, photography, textiles, and paper manufacturing because it can control the action of different metal ions through complexation.1,2 As a powerful chelating agent, EDTA in wastewater and even in natural water is generally present as a metal complex. Indeed, because of its ability to complex metals, it has been used extensively in processes related to the removal of metal oxides from heat-transfer surfaces.3 Moreover, EDTA is used as an important decontaminating agent in the nuclear industry. Because its presence in decontamination waste is able to cause complexation of the radioactive cations, such complexation is hardly removed by various treatment processes such as chemical and photochemical degradation, ion exchange, and ultrasonic treatment.1,3,4 It is also reported that FeIIIEDTA is degraded photochemically in surface waters with a half-life of several hours.5 Although many processes demonstrate promise in treating contaminants, owing to their ability to destroy a large variety of organic pollutants,6 complete decomposition is difficult because secondary pollutants are often produced in the process and the reaction time is too long to be efficient. Supercritical water oxidation (SCWO) has recently received attention for treating wastewater contaminated with different kinds of refractory materials because of its decomposition ability.7,8 In the SCWO process, a reaction can occur in a single phase and mass-transfer resistance becomes negligible because organic compounds and gases become miscible.9,10 Additionally, using supercritical water with an oxidant, hazardous organic compounds can be completely oxidized and converted to CO2 and H2O.11 In general, the desired decomposition and removal efficiency can be achieved in a few seconds or minutes by way of SCWO.12-15 Moreover, supercritical water can provide an excellent reaction environment for the hydrothermal crystallization of metal oxide particles because of the drastic change in the properties of water around the critical * To whom correspondence should be addressed. Tel.: (822) 361-2762. Fax: (822) 312-6401. E-mail: [email protected].

point.16 Therefore, it is not surprising that metal oxide would be the byproduct when metal-EDTA is decomposed under supercritical water conditions. In the present study, because NaFeEDTA is discharged as one of the metal-EDTAs from nuclear and other power plants, the decomposition of NaFeEDTA by the SCWO process was studied as a way of minimizing secondary pollutants and improving the decomposition efficiency. Experiments in the decomposition of NaFeEDTA were conducted with an oxidant (100-400% H2O2) under a range of supercritical water conditions (387-500 °C and 23-28 MPa) in a tubular plug-flow reactor. On the basis of the results of these experiments, a kinetic model using total organic carbon (TOC) was proposed for the decomposition of NaFeEDTA by the SCWO process. At the same time, to achieve two benefitssthe powerful treatment of pollutants and the recovery of useful materialssthe effects of the supercritical water conditions on the morphology of the produced metal oxide were analyzed. Experimental Section Materials. NaFeEDTA‚3H2O (99+% pure, Aldrich Chemical Co. Ltd.) and hydrogen peroxide (H2O2; 30 wt % purity, Junsei Chemical Co. Ltd.) were used as received. Deionized water was also used after an N2 purge to remove dissolved O2. Apparatus and Procedure. A schematic diagram of the apparatus is shown in Figure 1. The reactor, made by a 1/2-in. stainless steel (SS 316) tube, was a U-type tube of 20.8-mL volume with 9.4-mm i.d. The heatexchanging cooler with a volume of 50 mL, made by a SS 316 tube, was equipped after the reactor. Viswanathan et al.17,18 pointed out that simply varying the concentration or the flow rate of the feed did not affect the particle size of zinc oxide in supercritical water. In this study, to prevent the oxide particles, which are obtained from the decomposition of NaFeEDTA by SCWO, from blocking the reactor, a U-type reactor with a relatively large diameter was used and low flow rates were applied to the experiments. The two preheaters for an aqueous NaFeEDTA solution and H2O2 were made from 1/8-in. stainless steel (SS 316) tubings of 2 m in length. The experimental ap-

10.1021/ie050115h CCC: $30.25 © 2005 American Chemical Society Published on Web 07/07/2005

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Figure 1. Schematic diagram of a SCWO system: 1, NaFeEDTA; 2, H2O2; 3, high-pressure pump; 4, check valve; 5, molten salt bath; 6, preheater; 7, reactor; 8, heat controller; 9, cooler; 10, line filter; 11, backpressure reactor; 12, separator; 13, liquid sample; 14, gas vent; 15, sample tray; 16, reservoir filter.

paratus was washed out using 6 vol % H2O2 at 500 °C and 28 MPa for 2 h. After two high-pressure pumps (Lab alliance prep. 100; Pmax ) 40.8 MPa) delivered the oxidant and the NaFeEDTA solution to the preheaters, the mixture was supplied to the reactor at the T joint. The preheaters and reactor were immersed in a molten salt bath, where the reaction temperature was kept within a range of (1 °C. The reactor temperature was measured using a K-type thermocouple, which was placed at the mixing point in the reactor. The pressure was measured at two different points with an accuracy of 0.25 MPa and controlled using a backpressure regulator (Tescom; Pmax ) 40.8 MPa). After the treated solution passed instantly through a cooler, it was depressurized to atmospheric pressure through a backpressure regulator. The final temperature of the treated solution was less than 25 °C, and the liquid sample was collected in the capped vial periodically. The experimental conditions in this study are listed in Table 1. The experimental temperature range in SCWO was 380-500 °C at a pressure of 23-28 MPa. The feed concentration of NaFeEDTA was fixed at 10 mM, which corresponds to 1.0 × 10-2 mol/L TOC. The percentage of H2O2 supplied to the reactor was based on the stoichiometric demand of oxygen for the complete oxidation of NaFeEDTA, which was needed to convert the carbon content in the feed to carbon dioxide. The amount of H2O2 used changed from 100% to 400% depending on the stoichiometric demand. Because a certain amount of NH4+-N concentration was reported in the SCWO of EDTA,19 the following reaction equation was used to calculate the demand of the oxidant in the SCWO of NaFeEDTA.

2NaFeC10H16N2O2 + 60H2O2 f 20CO2 + 74H2O + NH4+(NO3) + Na2O + 2FeO3 + N2 (1) The flow rates of each influent (oxidant and NaFeEDTA solutions) were adjusted to 2.0, 3.0, 4.0, and 5.0 mL/min at 0.1 MPa and 25 °C, simultaneously. The corresponding residence time in the experimental range was in the range of 17.4-276.4 s in the reactor. The oxidant at the conditions of the same flow rate and pressure as those of the NaFeEDTA solution passed through the preheater with a residence time of 12.7201.8 s. Because the residence time was obtained from the ratio of the reactor volume to the volumetric flow rate, the density and volume properties of the super-

critical water had to be taken into consideration in the calculation of its flow rate. The theoretical values for density and volume at various conditions were derived from a steam table.5 The TOCs of the feed stream and treated effluent were analyzed using a TOC analyzer with a detection limit of 9.3 × 10-6 mol/L (Shimadzu TOC-5000A total organic carbon analyzer). The samples were analyzed until three analyzed results within (0.5% were obtained from the TOC. Also, the average values of the TOC were used to evaluate the decomposition efficiency of NaFeEDTA using SCWO. An inline cyclone-type (0.5-µm-size filter) reservoir filter was placed before the line filter. After the experiments, the particles were collected from the U-type reactor and both filters. The particles were not observed in the liquid sample tray. The morphology of the obtained particles was observed using scanning electron microscopy (SEM) analysis, and the main components of the particles were traced using energy-dispersive X-ray spectroscopy (EDS; Hitachi S-4200) of SEM by element mapping and line scan. The X-ray diffraction (XRD) pattern of the particles was checked with an X-ray diffractometer (Rigaku D/max-3A), and single silicon powder was used as an external standard. Results and Discussion Decomposition Efficiency of NaFeEDTA at SCWO. Without an oxidant, EDTA under severe supercritical water conditions was converted to various low molecular materials in liquid.19 However, the TOC concentration of the treated solution without any oxidant was almost the same as that of the feed at 1.0 × 10-2 mol/L. Therefore, measuring the remaining TOCs is a more reasonable way of representing the decomposition efficiency of NaFeEDTA under SCWO because of the initial conversion effect before addition of the oxidant.19 Figure 2 shows the effects of the temperature and supplied H2O2 on NaFeEDTA decomposition according to the residence time. The decomposition efficiency increased markedly with an increase in the temperature and H2O2. Within 48.4 s, the decomposition efficiency approached 99.8% (based on the TOC) at 400% H2O2, 500 °C, and 25 MPa. In addition, even with a low amount of oxidant at 500 °C and 25 MPa, a decomposition of higher than 97.4% was achieved at the same residence time. As shown in parts a and b of Figure 2, the effect of the temperature on decomposition was significant at 100-200% H2O2. In contrast, the role of the temperature in the improvement of the decomposition efficiency was relatively small at 400% H2O2 (Figure 2c) because excess oxidant contributed to the decomposition of NaFeEDTA. The decomposition efficiency at near-critical temperature (380 °C) did not improve much as the amount of H2O2 was increased from 100% to 200%. However, under the same residence time conditions, a significant improvement in the decomposition efficiency was observed as the temperature was increased from 380 to 400 °C or from 450 to 500 °C under each of the oxidant conditions. Figure 3 shows the effects of the pressure and temperature on NaFeEDTA decomposition at 200% H2O2 according to the residence time. The effect of the pressure on the decomposition efficiency in the SCWO process was much smaller than the effect of the tem-

Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6617 Table 1. Decomposition of NaFeEDTA under Various Conditionsa T (°C)

P (MPa)

feed flow rate (mL/min)b

residence time (s)

H2O2 concnc (stoichiometric %)

decomposition efficiency as the TOC (%)

380 400 450 500 380 400 450 500 380 400 450 500

25 25 25 25 23 23 23 23 28 28 28 28

2.0-5.0 2.0-5.0 2.0-5.0 2.0-5.0 2.0-5.0 2.0-5.0 2.0-5.0 2.0-5.0 2.0-5.0 2.0-5.0 2.0-5.0 2.0-5.0

97.3-243.2 35.9-89.8 23.5-58.8 19.4-48.4 45.1-112.8 28.9-72.2 20.7-51.8 17.4-43.5 110.6-276.4 56.0-140.0 28.3-70.8 22.6-56.5

100-400 100-400 100-400 100-400 200 200 200 200 200 200 200 200

72.3-92.0 75.9-93.2 80.0-95.5 82.1-99.8 67.2-73.9 76.4-79.8 84.9-90.7 88.0-96.4 90.4-93.1 90.5-93.3 92.5-97.8 96.9-98.5

a FeEDTA. The initial feed concentration is 1.0 × 10-2 mol/L NaFeEDTA (corresponding to 1072 mg/L as the TOC). b Based on 0.1 MPa and 25 °C. c Based on a stoichiometric amount.

of NaFeEDTA under the SCWO process was analyzed by the TOC. For design engineering purposes, it is usually sufficient to develop a global rate model to express the reduction of waste components in SCWO. Thus, in the development of the kinetic models, the global rate equation was adopted for the overall oxidation reaction.20-24 The following equation was used to calculate the decomposition rate of NaFeEDTA as the TOC in the SCWO process using H2O2:

rate ) A exp(-Ea/RT)[NaFeEDTA as the TOC]a [H2O2]b[H2O]c (2)

Figure 2. Decomposition efficiencies of NaFeEDTA as the TOC with the temperature (380-500 °C), oxidant amount [(a) 100% H2O2, (b) 200% H2O2, and (c) 400% H2O2], and residence time at 25 MPa.

perature. The decomposition efficiency at near-critical temperatures (380 and 400 °C) showed some differences among the various pressure conditions. However, at 450 °C or higher, the effect of the pressure decreased significantly. In addition, it is noteworthy that there was relatively little improvement in the decomposition efficiency because of an increase in the residence time at this temperature range or at higher pressure conditions, in comparison to the effects of the temperature as shown in Figure 2. Reaction Kinetics of NaFeEDTA Decomposition by SCWO. The reaction kinetics for the decomposition

In eq 2, [NaFeEDTA as the TOC], [H2O2], and [H2O] represent the concentration of each component (mol/L). The reaction orders of NaFeEDTA as the TOC, H2O2, and H2O are represented by a, b, and c, respectively. Ea, R, T, and A are the activation energy, gas constant, reaction temperature, and Arrhenius preexponential factor, respectively. The reaction constant, k, can be defined by k ) A exp(-Ea/RT). Water seems to participate in the reaction as a reactant via a multistep reaction mechanism. Because the reaction medium always consisted of more than 99% water in this study, its impact on the NaFeEDTA disappearance rate, if any, may be hidden. In other words, the experiments were not designed to evaluate the effect of the water concentration. In this way, we considered zero the reaction order for the water due to excess water.19,21-24 When x and τ are defined as x ) 1 - (NaFeEDTA)/ (NaFeEDTA)0 and the residence time, where (NaFeEDTA)0 is the NaFeEDTA concentration at the reactor entrance, eq 2 can be solved analytically with the initial condition x ) 0 at τ ) 0 to clarify the relationship between NaFeEDTA as the TOC and relevant process variables.

(1 - x)1-a - 1 ) (a - 1)k [NaFeEDTA as the TOC]0a-1[H2O2]0bτ if a * 1 (3) ln(1 - x) ) -k[H2O2]0bτ

if a ) 1

(4)

We used a nonlinear regression analysis to fit the experimental NaFeEDTA conversions in Table 1 to eq 3 and thereby determine optimized values for the parameters a, b, A, and Ea. The objective function minimized in the regression method was the sum of the

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Figure 3. Decomposition efficiencies of NaFeEDTA as the TOC with the pressure (23-28 MPa), temperature [(a) 380 °C, (b) 400 °C, (c) 450 °C, and (d) 500 °C], and residence time at 200% H2O2.

squares of the differences between the values of x calculated from eq 3 and those measured experimentally. From the nonlinear regression, the preexponential (A) and energy activation (Ea) values are simultaneously obtained. In this study, to obtain the reasonable four parameters (a, b, A, and Ea) from the nonlinear regression, the initial value of each parameter was evaluated. At first, after the value of k was fixed by using the reference value,21,22 the values of the reaction orders were optimized. Then, the rate constant, k, using the obtained reaction orders was evaluated at each experimental condition. By using these values, the activation energy, which varied in the amount of the supplied oxidant, was obtained from the Arrhenius plot. Finally, the average value of each parameter was used as the initial value in the nonlinear regression analysis. The optimal values of the kinetic parameters lead to the global rate law of eq 5 to describe NaFeEDTA oxidation.

45036 ( 8765 RT 1.8 0.28 [NaEDTA as the TOC] ( [H2O2]0.61(0.19 (5)

Figure 4. Comparison of the experimental and calculated NaFeEDTA conversions.

The rate is in mol/L‚s, all concentrations are in mol/L, the temperature is in K, and the activation energy is in J/mol. The uncertainties expressed in eq 5 are the 95% confidence intervals. Figure 4 shows the parity plot between the experimental NaFeEDTA conversions and those calculated from eq 3 with the optimized rate law parameters. This plot verifies that the estimated parameters provide a reasonable description of the experimental data. It is noted that the experimental results obtained from 380

°C, which is the near-critical temperature of water, show a relatively large deviation in the parity plot. On the contrary, the deviations of the experimental results at 380 °C and 400% H2O2 were comparable to those at the other experimental conditions. Formation of Nanosized Particles from NaFeEDTA Decomposition by SCWO. Figure 5 shows the representative XRD profiles of particles obtained by the SCWO process from NaFeEDTA decomposition. The XRD patterns show that the obtained particles are

(

rate ) (1.54 ( 0.36) × 104 exp -

)

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Figure 5. XRD profiles of the R-Fe2O3 particles obtained: (b) R-Fe2O3; (a) 400 °C, 25 MPa, and 100% H2O2; (b) 450 °C, 25 MPa, and 100% H2O2; (c) standard peak of R-Fe2O3.

Figure 7. SEM micrographs of R-Fe2O3 particles obtained from experiments with different operating conditions. The initial concentration of FeEDTA is 10 mM.

Figure 6. EDS spectrum of R-Fe2O3 particles obtained from decomposition of FeEDTA at 400 °C, 25 MPa, and 100% H2O2.

R-Fe2O3, known as hematite. In addition, the EDS spectrum in Figure 6 demonstrates that the main components of (R-Fe2O3) that were obtained were O and Fe. The atomic percentages of O and Fe were 32.43 and 44.75, respectively. Na presented in Figure 5 was removed when the particles were rinsed with hot water. Thus, after rinsing with hot water, the molar ratio of the particle consisted of only O and Fe. From the results of XRD and EDS, (R-Fe2O3) was obtained from NaFeEDTA decomposition by the SCWO process as a byproduct after decomposition of EDTA. Figure 7 shows the SEM images of R-Fe2O3 particles produced under various SCWO experimental conditions. In the case of the R-Fe2O3 particles prepared under different pressure settings at 450 °C and 100% H2O2,

the particle size decreased with an increase in the pressure. The particles obtained from near-critical pressure, 23 MPa, showed an irregular shape, with the diameter being 1-1.5 µm (Figure 7a). However, the particle size at 250 and 28 MPa was less than 200 nm, and the shape of each particle was nearly uniform. Parts b, d, and e of Figure 7 show the SEM images of the particles obtained from different temperatures at the same pressure, 25 MPa, and oxidant quantity, 100% H2O2. The particle size formed at 400 °C was 50-150 nm. On the other hand, when the temperatures were increased to 450 and 500 °C, the particle size became larger with a wide size distribution, showing a platelike shape. Parts e-g of Figure 7 show the SEM images of the R-Fe2O3 particles obtained from varying amounts of oxidants and pressure at 500 °C. When the SCWO condition for NaFeEDTA decomposition was changed from 100% H2O2 to 200% H2O2 at 25 MPa, the particle size decreased 70-150 nm with a uniform shape. On the other hand, when NaFeEDTA decomposition was performed at 28 MPa and 200% H2O2, the particle size was almost the same as that in Figure 7f, but a small amount of the large particle was also formed. As results across the range of experiments demonstrate, the particle size generally decreased with in-

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creasing pressure and oxidant quantity and as the temperature likewise decreased. However, small particles with a regular morphology could be obtained at the specific conditions. EDTA has been known to be easily decomposed without the need for oxidant using the thermal effect. In this case, the efficiency of EDTA thermal decomposition improved dramatically at temperatures higher than 400 °C and 25 MPa.19 Therefore, it is expected that NaFeEDTA was instantly decomposed by the oxidant at 400 °C and 25 MPa. Then, small particles of R-Fe2O3 formed at the reactor, transforming rapidly from NaFeEDTA to another form of nitrogen during SCWO. On the other hand, at fixed temperature, the optimal conditions for an increase in the decomposition rate of NaFeEDTA resulted in the formation of fine R-Fe2O3 particles.

a, b, c ) reaction orders of the target material, H2O2, and water, respectively C ) effluent liquid concentration (mol/L) C0 ) influent liquid concentration (mol/L) Ea ) activation energy (kJ/mol) [H2O2] ) hydrogen peroxide concentration (mol/L) [H2O] ) water concentration (mol/L) k ) reaction rate constant (s-1) [NaFeEDTA] ) NaFeEDTA concentration (mol/L) [NaFeEDTA as the TOC] ) NaFeEDTA concentration as the TOC (mol/L) P ) pressure (MPa) R ) gas constant T ) temperature (°C, K) x ) decomposition effiency (C/C0)

Conclusion

Literature Cited

The decomposition of NaFeEDTA and the formation of nanosized R-Fe2O3 under the SCWO process were studied under various conditions such as temperature, pressure, residence time, and oxidant quantities. NaFeEDTA was effectively oxidized by a mixture of supercritical water and H2O2 in a plug-flow reactor. The effects of the oxidant amount and temperature on the decomposition rate and efficiency were significant, while the effect of the pressure was relatively small with regard to the TOC. The reaction temperature in the SCWO process played a key role in increasing the decomposition rate and the efficiency of NaFeEDTA as the TOC. However, the improvement in the decomposition efficiency was not directly proportional to any temperature increase above the critical temperature. The disappearance of NaFeEDTA during SCWO proceeds with a global rate law that is 1.81 ( 0.28 orders in NaFeEDTA and 0.61 ( 0.19 orders in oxygen. The activation energy is 45.036 ( 8.765 kJ/mol, and the Arrhenius preexponential factor is (1.54 ( 0.36) × 104 M-1‚s-1. The decomposition kinetics of NaFeEDTA in the SCWO process was accurately described using a global rate expression. When NaFeEDTA was decomposed under the SCWO process, R-Fe2O3 was formed concurrently as a byproduct, which is used to remove pollutants in groundwater such as Mn, F, and methyl tert-butyl ether. Particle morphology was controlled by the SCWO conditions. The shape of the particles obtained from near-critical conditions was irregular with 1-1.5 µm, while the particles under more severe conditions became uniform shape and nanosized. The particle size decreased with a decrease in the temperature and an increase in the pressure or oxidant amount. The R-Fe2O3 particles obtained from NaFeEDTA decomposition at 400 °C, 25 MPa, and 100% H2O2 had nearly regular shape with 50-150 nm. Acknowledgment This subject is supported by Ministry of Environment as “The Eco-technopia 21 project”, and the authors are thankful for their support. Notation A ) Arrhenius preexponential factor

Greek Letters τ ) residence time (s, min)

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Received for review January 29, 2005 Revised manuscript received June 1, 2005 Accepted June 9, 2005 IE050115H