Kinetics of Dissolution of Magnetite in PDCA Based Formulations

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1 Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 34, No. 8, p (August 1997) Kinetics of Dissolution of Magnetite in PDCA Based Formulations Santhanam RANGANATHAN*, Amaladoss Antony Michael PRINCE*, Pandalgudi Seshan RAGHAVAN*, Raghavachary GOPALAN*, Madapuzi Parthasarathy SRINIVASAN** and Sevilimedu Veeravalli NARASIMHAN**,t * Madras Christian College ** Water and Steam Chemistry Laboratory (BARC), Indira Gandhi Centre for Atomic Research Campus (Received January 7, 1997) Magnetite is one of the important corrosion products of pressurised heavy water reactors (PHWRs) where carbon steel is the dominant surface in the primary heat transport system. Designing of formulations capable of dissolving magnetite is important for effective decontamination of such surfaces. The rate of dissolution of synthetically prepared magnetite was studied in low concentrations of PDCA containing acidic formulations. The effect of addition of ascorbic acid, citric acid, Fe2+-PDCA complex on the rate was also studied. The effects of ph and the temperature on the dissolution rate were determined. The PDCA as a complexant has some positive factors like low protonation constant and enhanced stability to radiation. KEYWORDS: magnetite solubility, pyridine dicarboxylic acid, iron complexes, ascorbic acid, citric acid, decontamination, chemical reaction kinetics, dissolution, ph value, temperature dependence I. INTRODUCTION Dissolution of metal oxides by chelating agents has been used for many industrial applications and especially in the nuclear power plants. In the water-cooled nuclear reactors, the main oxides deposited on the primary heat transport (PHT) system surfaces causing radioactivity buildup are magnetite (Fe3O4) and mixed ferrites (nickel and cobalt) impregnated with active Co ions and fission products(1). The best method of removing the radioactive impurities present in these oxides is to dissolve the oxide using organic acids, reductants and complexing agents at low concentrations. Base metal corrosion was kept minimum as a result of low concentration. But the dissolution is by no means simple. It is very sensitive to ph, temperature and the nature of the components of the formulation used. Normally the dissolution process may proceed by either of the two mechanisms, namely, dissolution followed by solution complexation or adsorption followed by dissolution. In the later case, strong complexing ligand gets adsorbed on the surface which weakens the M-O2- bond and causes the release of metal ion. In the former case, surface O2- is attacked by H+ to form OH- or H2O which destabilises the lattice(2)-(4). To a large extent these mechanisms depend upon the ph of the formulation also. In this work, the dissolution of magnetite using PDCA based formulation is investigated. The 2,6-pyridinedicarboxylic acid * T ambaram, Tamil Nadu , INDIA. ** K alpakkam, Tamil Nadu , INDIA. Corresponding author, Tel , t Fax , wscl@igcar.ernet.in (PDCA) is a tridentate ligand forming moderately strong complexes compared with pyridine 2-carboxylic acid (picolinic acid). It is found that the nitrogen protonation capability is very much reduced due to the enhance deactivation of aromatic ring by the second carboxylic acid group. Hence in the ph region of 2-5, PDCA is not picked by cation exchanger and this aspect is important when the reagent is to be regenerated over the cation exchanger. In the present work the effect of varying ph, temperature, concentration of PDCA, CA, AA on the heterogeneous rate constant (kobs) is studied and discussed. II. EXPERIMENTAL The magnetite particles of narrow size distribution were prepared by using a slightly modified procedure which was developed earlier(5). About 675ml of doubly distilled water adjusted to a ph of 3 by sulphuric acid was used for dissolving 58.8g of ferrous ammonium sulphate. After purging with nitrogen for about an hour, 50% by wt/v solution of KOH (purged) was added gradually till the solution reached a ph in the range Then 93.5g of KNO3 was added, and the gel obtained was aged for 4h at 98dc. The precipitate was centrifuged, washed several times with distilled water, and suspended in 1M HNO3 for 1h to remove any unreacted reagents. The resulting solution was then centrifuged, decanted the supernatant, washed the precipitate and dried in an air oven at 85dc. The magnetite was dissolved in minimum quantity of HCl(-13N) and the iron was estimated by -phenanthroline method spectrophotometrically. The 0 magnetite thus obtained was found to be better than 95% pure. Its X-ray diffraction pattern was taken and 810

2 Kinetics of Dissolution of Magnetite in PDCA Based Formulations 811 this confirmed the ferrite pattern in it. Dissolution studies were carried out in a round-bottomed three-necked flask. Through one hole argon gas was bubbled and the other two were used for condenser and thermometer. The dissolution kinetics of magnetite under inert conditions at 85 and 60dc in varying composition of pyridinedicarboxylic acid (PDCA), ascorbic acid (AA), and citric acid (CA) formulations were carried out. The experiment was performed as follows: 500ml of the formulation (PDCA-2.75mM, AA-1.7mM, CA-1.4mM) was deaerated by passing purified argon gas for 1h while heating to the working temperature of 85 or 60dc. A suspension of 53.06mg of magnetite in 10ml of demineralised water (DM water) was added to the formulation, after keeping it in an ultrasonic bath for half an hour to disperse the particles. It was observed that the magnetite samples suspended uniformly by ultrasonic agitation prior to start of the dissolution work gave a higher and constant kobs. Hence in all the experiments the ultrasoniconditioning was provided to disperse the particles. Aliquots were drawn from the reaction vessel at differentime intervals after filtering through a fine stainless mesh filter during sampling. The sample aliquots were checked by the zeta sizer in particle size analysis mode for the absence of particles. All the experiments were carried out with 53.06mg of magnetite while varying the other parameters. Total iron content was determined by the spectrophotometric analysis using 0-phenanthroline method(6). Using the cubic rate law, the rate constant (kobs) was calculated from the initial slope of the plot of [1-(Ct/C0)]1/3vs. time(7). The expression defining the dissolution process is [1-(Ct/C0)]1/3=1-kobst kobs=apparent heterogeneous rate constant inclusive of the influences of ligands, acidity etc. and the initial radius of particle. In all the experiments, the ph was found to be unaltered at 2.62 before and after dissolution. This may be due to the presence of citric acid in the formulation which was maintaining the ph without participating in the complexation or dissolution reaction. III. RESULTS AND DISCUSSION In the previous studies of magnetite dissolution using EDTA, Matijevic et al. have reported that the dissolution of metal oxides in the presence of chelating agents proceeded by two different mechanisms(2)-(4). In all the cases of dissolution, analogous results were obtained showing that adsorption was greatly dependent on ph. The dissolution of oxides containing more than one kind of metal ions was also studied and it was reported that in an oxide containing nickel and iron, nickel was preferentially leached from the ferrite. As the ph increased, the kobs decreased gradually. Gorichev et al. reported that dissolution of magnetite by EDTA was very sensitive to oxygen and an induction period was usually observed in dissolution(8). They found that the optimum ph for dissolution of magnetite in EDTA formulation was 2.3 and the rate constant decreased above and below this ph. 1. Effect of Sonication From the data in Table 1 it was clear that the amount of dissolution in 1h and the kobs were affected to a significant extent when the sample was kept in the ultrasonic bath for a period of half an hour before carrying out the dissolution. (Fig. 1) Increased % dissolution and kobs were observed in comparison to those experiments wherein the dissolutions were carried out without using ultrasonic bath. The larger agglomerates were brokendown effectively by ultrasonic agitation prior to dissolution. Thus the surface areas of magnetite particulates were increased which lead to the observed increase in % dissolution and kobs. Table 1 Effect of sonication on % dissolution of magnetite Formulation: PDCA=2.75mM; CA=1.4mM; AA=1.7mM Fig. 1 Effect of ultrasonic dispersion on dissolution rate of magnetite in PDCA at 85dc VOL. 34, NO. 8. AUGUST 1997

3 812 S. RANGANATHAN et al. 2. Precision in kobs The heterogeneous dissolution rate in the medium under investigation is significantly high. This necessitates sampling at frequent intervals. In the first 1h of each run, the possibility of errors creeping in is quite high. To confirm the reliability of the data collected, the experiment was repeated 5 times with the formulation PDCA (2.75mM)+CA(1.4mM)+AA(1.7mM) at 60dc. The lower and the higher limits of the kobs values for these five runs were found to be 2.76x10-2 and 3.00x10-2min-1 respectively and the standard deviation was found to be 9.4x10-4min-1 which showed that the error in the experiments was within acceptable limits. The % RSD in kobs is about -3.5%. In all the experiments in the present study the extent of dissolution in the range of 60-70% was utilised in the computation of kobs. Hence it is reasonable to assume that all the kobs values computed will have an RSD of 3.5%. Hence the error values are not individually mentioned. The reasons for the non linear behaviour of the (1- Ct/C0)(1/3) vs. time plot are many. The original shrinking core model assumes that all particles involved in dissolution had a fixed and narrow radius distribution. If this were not to be the case then kobs would have a large variation. Since the particulates were being subjected to an ultrasonic dispersion treatment in this study, there is a possibility that the average particle size varied in this study and influenced the rate constant. The room temperature data (30dc) showed that it took about 180min for 40% dissolution and majority of the points obeyed the kinetic plot (Fig. 2). In otherwords linear behaviour was more controlled by the total iron dissolved and in the present set of experiments the initial average particle size did not vary appreciably. In most of the high temperature experiments kinetic plot was considered only up to 60-70% dissolution, during which period the linearity was obeyed. As a result of the experimental conditions chosen, the iron loading of the formulation itself started inhibiting the kinetics beyond 60-70% of the dissolution. This is one of the strong reasons for the absence of correlation between parametric dependence of kobs and percentage dissolution at 1h and beyond as explained earlier. The room temperature dissolution kinetics observations substantiates the method adopted for the calculation of kobs. 3. Effect of Inert Gas Purging The effect of maintaining an inert and aerated condition on the dissolution kinetics of magnetite in PDCA formulation was studied. Table 2 shows that the kobs is very sensitive to oxygen. There was a three fold reduction in kobs when the dissolution was carried out under aerated condition. The decrease in rate constant was probably due to the oxidation of Fe2+ to Fe3+ in solution thereby reducing the concentration of Fe(II)-PDCA complex in solution which acted as a good (kinetically fast) reducing agent for Fe3+ in magnetite lattice. The AA present in solution reduced Fig. 2 Room temperature dissolution of magnetite in PDCA Table 2 Effect of inert atmosphere on dissolution on magnetite Fe(III)-PDCA to Fe(II)-PDCA thereby countering the effect of dissolved oxygen to a certain extent. Since the supply of oxygen was quite high, AA was consumed fully under aerated conditions. This sensitivity of kobs to the oxygen was also reported by Gorichev et al. in the dissolution of magnetite by EDTA(8). 4. Effect of Ascorbic Acid Studies were carried out with and without ascorbic acid in the PDCA formulation to know its influence on the rate of dissolution of magnetite. From the data in Table 3, it is evident that AA is playing a vital role in affecting the kobs. At both the temperatures 85 and 60dc the rate of dissolution was en- JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

4 Kinetics of Dissolution of Magnetite in PDCA Based Formulations 813 Table 3 Effect of ascorbic acid on magnetite dissolution Formulation: PDCA=2.75mM; CA=1.4mM hanced by the addition of AA. The metal release (Fe2+ and Fe3+ release) at 85dc was quite high in the initial stages (<20min) of dissolution of magnetite. This resulted in adequate concentration of Fe(II)-PDCA in solution which was responsible for accelerating further oxide dissolution. Hence at 85dc without AA, the kobs was reasonably high. Upon addition of AA, the increase in kobs was only marginal (3 fold). This was essentially due to the increased production of Fe(II)-PCDA from Fe(III)-PDCA upon reduction by AA. At 60dc the kobs without AA was drastically low and there was also an induction period. In other words, the release of sufficient quantity of Fe2+ had not occurred within 20min at 60dc. By addition of AA, additional conversion of Fe3+ to Fe2+ occurred rapidly which resulted in enhanced rate due to Fe(II)-PDCA. Hence the kobs at 60dc with AA and that at 85dc without AA were of comparable magnitude. (Fig. 3) In the absence of AA, the ratio of Fe2+/Fe3+ in solution at any given time was found to be proportional to that found in oxide. In the experiments carried out with AA in all the samples, the iron was found to be only in +2 oxidation state in solution. Direct reduction of Fe3+ by AA present in the oxide was not expected to be rapid as revealed by experiments with AA only. However, Fe(III)-PDCA complex can be reduced almost instantaneously as and when it was getting released to the solution under which condition the ratio of AA concentration to Fe(III)-PDCA concentration would be very high. Thus having formed Fe(II)-PDCA in solution the dissolution proceeds at a faster rate. This was also observed in the dissolution of magnetite by EDTA(9)(10). 5. The Effect of Fe(II)-PDCA Complex as Reductant In order to study the role of Fe(II)-PDCA complex as a reductant in place of AA in the dissolution kinetics of magnetite, experiments were carried out at 85 and 60dc. Ferrous ammonium sulphate (FAS) and ferrous sulphate (FS) were used to supply the necessary Fe2+ in excess of PDCA to form Fe(II)-PDCA complex and the difference in rate was observed (Fig. 4). AA was not used in these studieṣ From Table 4 it is clear that the presence of Fe(II)- PDCA complex helps to enhance the dissolution rate constant significantly(compare with data without AA as given in Table 3). At 60dc as the concentration of Fe(II)- Fig. 3 Effect of AA on dissolution rate of magnetite in PDCA PDCA increased, the kobs and % dissolution reached a maximum for 0.458mM Fe(PDCA)2 and it started to decrease for higher concentration. The decrease in kobs with 0.687mM Fe(PDCA)2 at 60dc is explained by the fact that the surface adsorption of Fe(II)-PDCA on the metal oxide occurred in preference to the PDCA adsorption. Initially slow leaching has been observed in similar system, where the addition of ferrous ions drastically shortened the induction period(11). The metal release after reduction could be enhanced by increasing the concentration of free PDCA to 5.5mM instead of 2.75mM at the same Fe(PDCA)2 concentration at 0.687mM and the kobs was found to be increased by 2 times. Similar concentration effect due to an increase in PDCA concentration in the absence of Fe(PDCA)2 reducing agent was not noticeable. This was due to the fact that contribution by reductive part was very small. The experiments carried out at 85dc showed that, as the concentration of Fe(II)-PDCA increased in the formulation, the kobs and % dissolution increased contrary to that observed at 60dc in the case of 0.687mM of ferrous. The metal release at 85dc dominates the dissolution rate to overcome the hindrance caused by surface adsorption. Matijevic et al. also reported that the dissolution was very sensitive to temperature. The source of Fe2+ did not affect kobs in any way. VOL. 34, NO. 8, AUGUST 1997

5 814 S. RANGANATHAN et al. carried out with that quantity of magnetite (53.06mg) which would be complexed by only PDCA upon complete dissolution. AA and CA remained free even after complete dissolution. Some amount of AA would have been consumed for reductive purpose. In the present set of experiments, additional magnetite was introduced (beyond the complexing capability of PDCA) to study the effect on kobs as given in Table 5. The data in Table 5 showed a definite decrease in the kobs with increase in the initially added magnetite quantity. Nevertheless, excess magnetite >53.06mg was dissolved due to the complexation by CA, AA and its oxidation products. The reduction in kobs was probably due to increased surface adsorption as a consequence of which there was less availability of the reagent for dissolution. Once the PDCA was fully utilised by complexation, the complexing capability of the remaining formulation was inadequate to maintain the same rate and extent of dissolution. Similar trends were also seen in EDTA+CA+AA formulation. The increase in ph was due to the consumption of CA as a complexant which altered the buffer system. Fig. 4 Effect of Fe(II) on dissolution of magnetite in PDCA at 60dc Table 4 Effect of ferrous PDCA complex as reductant Formulation: PDCA=2.75mM; CA=1.4mM Source of Fe: Ferrous ammonium sulphate t Source of Fe: Ferrous sulphate Free PDCA conc. 5.5mMtt The kinetic behaviour at room temperature showed that in the presence of AA, there was an induction period until 10% oxide dissolution occurred. This was absent when Fe(II)-PDCA was used as the reductant. This further substantiated the reductive role of Fe2+ especially in the beginning stages of dissolution. 6. Effect of Ratio of Initial Magnetite to PDCA Concentration Most of the experiments reported in this work were 7. Effect of ph In all the earlier runs the quantity of Fe3O4 chosen was such that upon complete dissolution only PDCA was used up in complexation. Since CA remained free, the ph of the solution before and after the dissolution did not change. Hence the kobs had no ph contribution in all these runs. However, ph is a very important factor which determines the rate of the reaction. Experiments were carried out by varying ph in dissolution studies of magnetite. (Figs. 5 and 6) From Table 6 it is clear that the ph is playing a very vital role in dissolution. The above data showed that there was an optimum ph 2.62 above and below which the kobs decreased. At lower phs (<2.62) adsorption of PDCA on magnetite was hindered. The PDCA gets adsorbed on magnetite surface by interaction between the Fe2+ or Fe3+ lattice sites and COO- group of PDCA. As the ph is lowered the COO- groups remain as -COOH and hence resist adsorption. Since there is strong evidence in literature to show that the point of zero charge (pzc) for magnetite in decontaminant solution is significantly altered(12), PDCA adsorption is an important step Table 5 Effect of initial magnetite quantity of kobs Formulation: PDCA=2.75mM; CA=1.4mM; AA=1.7mM at 60dc t Average of two measurements JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

6 Kinetics of Dissolution of Magnetite in PDCA Based Formulations 815 Fig. 5 Effect of ph on dissolution of magnetite in PDCA at 60dc for dissolution. If that step is hindered by lowering of ph, kobs decreases. At higher ph, due to the insufficient H+ ion, the bond breaking between oxygen and metal ion was favoured to a lesser extent. So the optimum ph for the dissolution of magnetite in PDCA formulation was When the ph was increased, the % dissolution showed an induction period. At ph 1.04 the induction period was found to be 20min and at phs 3.69 and 4.56 the induction period was found to be 30 and 60min respectively. At still higher ph of 6.45 the induction period was found to be more than 210min. Even though there was an induction period, the increase in the dissolution rate was due to the formation of the Fe(II)-PDCA complex by the initial dissolution, which helped in further dissolution of magnetite by reductive mechanism. As the ph increased, the formation of adequate concentration of Fe(II)-PDCA got delayed. In all the cases of magnetite dissolution at different ph the induction period was determined by the time taken for dissolution of magnetite to form -12% Fe(II)-PDCA complex. (-4.6mg of Fe complexing with PDCA). Even though CA was not showing any influence on the dissolution of magnetite under laboratory conditions, it was playing an important role in maintaining the ph in nuclear power plant during the decontamination. Fig. 6 A plot of ph vs. kobs at 60dc Table 6 Effect of ph on dissolution of magnetite Formulation; PDCA=2.75mM; CA=1.4mM; AA=1.7mM at 60dc t By H2SO4 addition, ttby NaOH addition IV. MECHANISM OF DISSOLUTION A possible mechanism to explain these phenomena could be as shown below. A molecule of PDCA gets adsorbed on particulate Fe3O4 by partly coordinating with a Fe(III) site (>Fe(s)(III)-denotes iron in +3 state in magnetite lattice). Initial adsorption of PDCA on Fe(III) site (indicated by -) >Fe(s)(III)+PDCA->>Fe(s)(III)-(PDCA). (1) VOL. 34, NO. 8, AUGUST 1997

7 816 S. RANGANATHAN et al. A ferrous complex in solution interacts with the above site. Fe(s)(III)-(PDCA)+Fe(II)(PDCA)2-> >Fe(s)(III)-(PDCA)-Fe(II)-(PDCA)+PDCA. An intramolecular electron transfer reaction occurs to reduce the Fe(III) site in the solid. >Fe(s)(III)-(PDCA)-Fe(II)-(PDCA)-> (2) >Fe(s)(II)-(PDCA)-Fe(III)-(PDCA). (3) Release of ferric PDCA complex to solution and complexation of >Fe(II) site >Fe(s)(II)-(PDCA)-Fe(III)-PDCA+PDCA-> >Fe(s)(II)-(PDCA)+Fe(III)(PDCA)2. (4) Release of ferrous PDCA complex from solid to solution. >Fe(s)(II)-(PDCA)+PDCA->Fe(II)(PDCA)2. (5) This overall mechanism thus explains the influence of different parameters on the rate in the present work. In addition to the steps mentioned above, parallel reaction involving acid aided dissolution also proceed. In this study it was difficult to ascertain the relative contributions of these two paths. In all these experiments the extent of dissolution did not reach 100% even after appreciable elapse of time. The quantity of PDCA taken was just sufficient to complex all the iron released from magnetite. Subsequent to dissolution of major portion of magnetite, the balance free PDCA left was inadequate to drive the reaction to completion in 120min. Nevertheless it must be noted that the reaction only slowed down considerably. When the PDCA concentration was increased from 2.75mM to 5.5mM the extent of dissolution reached 92% in 30min as compared to 64% in the same time with 2.75mM of PDCA. The activation energy for the dissolution was determined by measuring kobs as a function of temperature in the range 30-85dc. From the slope of the Arrhenius plot, the activation energy was found to be kJ/mol for magnetite dissolution in PDCA-AA-CA formulation. In all the above studies the kobs was computed by taking into consideration only the initial stages of dissolution (<30% of total oxide dissolution). V. CONCLUSION The nature of the chelant influences the dissolution rate of magnetite to a larger extent. The chelant adsorbed onto the oxide surface through carboxyl group,f orming surface metal complexes, leading to the weakening of metal-oxygen bond of the magnetite, enhances the dissolution. The PDCA is as good a complexing agent as EDTA and produces similar effects on dissolution rate. The rate falls rapidly as the ph rises above 3. (At a ph of 2.62 at 85dc with the PDCA concentration equal to twice that of total iron in Fe3O4, PDCA+AA+CA mixture effectively dissolves magnetite.) However, addition of reducing agents such as Fe(II)-PDCA, AA markedly increased the dissolution rate. Either ascorbic acid or citric acid alone was not found to be a very good dissolution agent at concentrations up to 3.4mM. Further studies are in progress with other complexants and reducing agents in place of PDCA and ascorbic acid. ACKNOWLEDGMENTS This work was sponsored by the Board of Research in Nuclear Sciences, DAE. The support extended by Shri. S. Velmurugan, WSCL, Dr. P. K. Mathur, Head, Analyticalchemistry Division and Dr. P. N. Moorthy, Associate Director, Chemistry Group, Bombay is gratefully acknowledged. We thank the Atomic Energy Society of Japan for the generous support given in publishing this paper. -REFERENCES- (1) Johnson, Jr., Griggs, A. B., Kustas, F. M., Shaw, R. A.: Water Chemistry (11), BNES, 1980, Paper 54. (2) Chang, H. C., Healy, T. W., Matijevic, E.: J. Colloid Interface Sci., 92, 469 (1983). (3) Chang, H. C., Matijevic, E.: J. Colloid Interface Sci., 92, 479 (1983). 4) Rubio, J., ( Matijevic, E.: J. Colloid Interface Sci., 68, 408 (1979). (5) Regazzoni, A. E., Matijevic, E.: Corrosion., 40[5], 257 (1984). (6) Vogel, A. I.: "Quantitative Inorganic Analysis", (3rd ed.), 787 (1962). (7) Segal, M. G., Sellers, R. M.: J. Chem. Soc., Faraday Trans. I, 78, 1149 (1982). (8) Gorichev, I. G., Dukhanin, V. S., Kipriyanov; N. A.: Zh. Fiz. Khim., 54, 774 (1980). (9) Shailaja, M., Narasimhan, S. V.: J. Nucl. Sci. Technol., 28[12], 1107 (1991). (10) Shailaja, M., Narasimhan, S. V.: J. Nucl. Sci. Technol., 28[8], 748 (1991). (11) Baumgartner, E., Blesa, M. A., Marinovich, H., Maroto, A. J.: Inorg. Chem., 22, 2224 (1983). (12) Regazzoni, A. E., Matijevic, E.: Corrosion., 38[4], 212 (1982). JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY