Effect of ph on Stability Constants of Sr(II)-Humate Complexes

Transcription

1 Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 35, No. 8, p (August 1998) Effect of ph on Stability Constants of Sr(II)-Humate Complexes Mohammad SAMADFAM, Takashi JINTOKU, Seichi SATOt and Hiroshi OHASHI Division of Quantum Energy Engineering, Graduate School of Engineering, Hokkaido University* (Received July 14, 1997), (Revised December 4, 1997) The apparent stability constant, b, for the complexation of Sr(II) with humic acid (HA) was determined by a dialysis method in the ph range from 4 to 10 at the ionic strength of 0.1mol/dm3 (NaClO4) at 298K under N2 atmosphere. It was found that log b increased with ph from 2.5 at ph 4.0 to 4.1 at ph The logb tended to level off at higher phs, indicating that saturation was being reached. Further, the apparent ligand to metal ratio was found to be Dissociation of carboxylic and phenolic groups of HA with ph is probably the main factor that gives rise to the changes in stability constants. KEYWORDS: strontium, humic acid, stability constant, complexation, radioactive waste I. Introduction Binding of radionuclides to humic substances, naturally existing organic compounds in soil and groundwater, may affect the transport of radionuclides in natural aquifer systems(1)-(3). Therefore, the interaction of radionuclides with humic substances has been the subject of extensive investigations(4)-(15). Strontium-90 is included in spent fuel, HLW, and more or less in LLW. It has a relatively long half-life of 29.1 y and may contribute to radiotoxicity of waste for several hundred years after disposal. Apparent stability constants for the complexation of Sr(II) with humic acid (HA), one of the humic substances, have already been reported at only one or two ph(4)-(6). Due to the polyelectrolytic nature of HA, it is generally expected that its binding with metal ion will be dependant on ph, ionic strength, and more or less on metal loading(1)(16). Such a claim is supported by numerous experimental results(1)(7)-(12) No one has systematically investigated the ph-dependence of the stability constant of Sr(II)- HA. Carlsen(5) has reported the stability constant at only two different ph values (logb=3.12 at ph 4.5 and logb=3.32 at ph 7) and suggested that the stability constant was only slightly ph-dependent. Such conclusion is questionable because, in addition to the lack of data, the ionic strength was different between the two measurements (0.05 at ph 4.5 and 0.1 at ph 7). The purpose of the present study is to determine the ph-dependence of the apparent stability constant for complexation of Sr(II) with HA. The apparent stability constant was determined in the ph range from 4 to 10 at the ionic strength of 0.1mol/dm3 (NaClO4) at 298K under N2 atmosphere. II. Experimental 1. Materials Humic acid (HA), obtained from Aldrich Co., was purified and its ash content was less than 1%. The carboxylic group capacity of the purified HA sample was 4.9 milli-equivalents/g of HA, determined by direct titration of 0.2g/l of HA solution at 298K. The maximum slope of the titration curve was chosen as the end point of dissociation of the carboxylic groups(16). A typical titration curve and its first derivative are shown in Fig. 1. More detailed preparation and characterization of the HA sample is provided elsewhere(4). Strontium-85 solution, 1.0x10-4dm3 of SrCl2 in 0.5mol/dm3 HCl solution with a specific activity of 4.63x1011Bq/g of strontium and a 85Sr concentration of 1.85x1011Bq/dm3, was obtained from E. I. Du Pont * Kita 13, Nishi 8, Kita-ku, Sapporo Corresponding author, Tel , t Fax , sato qe.eng.hokudai.ac.jp Fig. 1 Titration curve of HA ( ) and its first derivative plot ( œ) 579

2 580 M. SAMADFAM et al. de Nemours & Co. Inc. Other chemicals used in the experiments were reagent grade, and obtained from Wako Chemical Industries Ltd. or Kanto Chemical Co. Inc. Stock strontium solution was prepared by dissolving SrCl2 6H2O in 0.1mol/dm3 NaClO4 and 85Sr solution was then added. Humic acid stock solutions of different concentrations were prepared in 0.1mol/dm3 NaClO4 solution. The ph of the stock solutions were adjusted to approximately 6.0 with 1.0mol/dm3 NaOH or 1.0mol/dm3 HClO4. 3. Determination of Stability Constants If the concentration of Sr(II) ions is much smaller than that of HA, as in this study, the complexation of Sr(II) with HA can be expressed by the reaction, Sr(II)+iHA=Sr(II)(HA)i. (1) Then, the apparent stability constant of the Sr(II)-HA complex is defined by the relationship, 2. Procedure The dialysis method was used to determine the stability constants of Sr(II)-HA complexes at HA concentrations of 50, 100, 200, and 400ppm, and at the ionic strength of 0.1mol/dm3 (prepared with NaClO4) in the ph range of 4 to 10. Each experimental batch used three cellulose ester dialysis tubes (Spectra Pore 1000MWCO) which were rinsed several times with deionized water; two with 1.0x10-2dm3 of the same HA solution, and one containing 1.0x10-2dm3 of the HA-free blank solution with 0.1mol/dm3 NaClO4. The dialysis tubes were placed in a polypropylene-lined separable flask and dialyzed against 0.1mol/dm3 NaClO4 solution of 3.5x10-1dm3 under N2 atmosphere at 298K for about 2h. Then, the Sr(II) solutions were added to the exterior solutions in the separable flasks to yield (1) a final Sr(II) concentration of 1.0x10-7mol/dm3 over the entire (interior+exterior solutions) volume and (2) an activity of at least 100kBq/dm3 over the entire solution. Then, the ph of the solutions was adjusted to the desired values by the addition of 0.1mol/dm3 or 1mol/dm3 solutions of HClO4 or NaOH. The exterior solutions were gently stirred for at least 3 days in an N2 atmosphere at 298K. Preliminary kinetic experiments revealed that the equilibrium was established within this period. After equilibration, the ph of the solutions were measured and duplicate samples of both the interior and exterior solutions were removed for the radiometry of 85Sr. The activity of 85Sr was measured with an Aloka NaI (Tl) well-type scintillation counter. At least 20,000 gross g- ray were counted with a standard gross error of less than 1%, and correction was made for the radioactive decay +- of 85Sr during the measurement. The concentration of HA in both the interior and the exterior solutions were measured to establish the extent of the HA transfer from the interior to the exterior solution during the dialysis. Some correction of the stability constant might be necessary due to the slight transfer of HA to the exterior solution. The concentration of HA was measured at 365nm(17) with a Shimadzu (UV-160) UV-VIS spectrophotometer. Measuring the 85Sr activity in HA-free blank NaClO4 solution sample (one of the three interior solutions in each experimental batch) confirmed that the metal ions freely permeate through the membrane. (2) where i is the ligand-to-metal ratio. In Eq. (2), [HA] is the concentration of free HA ligands capable of binding with Sr(II), and is almost constant and equal to the total concentration of HA ligands, because the concentration of Sr(II) is very low compared with that of HA. The actual concentration of HA ligands is a very complicated function of various parameters including ph, and it would be convenient to assign it as an invariable quantity. This was done by using the total concentration of carboxylic groups of HA, instead of the free HA ligand concentration. It must be emphasized that the potential binding sites of HA is not limited to the carboxylic groups, although they are likely to be responsible for most of the metal binding. Then Eq. (2) may be rewritten as: Now, if the numerical values of [Sr(II)(HA)i]/[Sr(II)] are obtained as a function of the concentration of HA at a fixed value of ph, the plot of log([sr(ii)(ha)i]/[sr(ii)]) vs. the logarithm of the concentration of HA results in a straight line, whose slope and intercept give the values of i and log b, respectively. However, as the HA concentrations are usually very low, there may be a large uncertainty included in the b values (but not in those of i) which are graphically obtained by this method. Therefore, it would be more reliable if i is first graphically determined from the slope of the log ([Sr(II)(HA)i]/[Sr(II)]) vs. [HA] plot, and then is directly obtained from Eq. (3). In addition, if the apparent ligand-to-metal ratio, i, shows only a slight ph dependence, it is acceptable to assume a constant mean value for i over the experimental ph range and use it to determine b. At any ph and HA concentration, the term [Sr(II)(HA)i]/[Sr(II)] in Eq. (3) can be obtained from, where Cin and Cex are the concentrations of radioisotope 85Sr in interior and exterior solutions, respectively. As mentioned above, a set of "log ([Sr(II)(HA)i]/ [Sr(II)]) vs. the logarithmic HA concentration" data at a fixed ph value is necessary to estimate the corresponding values of b and i. It is difficult to adjust the ph of the solutions in different vessels precisely to the same ph value, even with ph buffers. Further, a correction (3) (4) JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

3 Effect of ph on Stability Constants of Sr(II)-Humate Complexes 581 must be made for the experimental data if ph buffers are used. Avoiding these difficulties, we adopted the following method: (1) "log ([Sr(II)(HA)i]/[Sr(II)]) vs. ph" sets of data were separately obtained for each of four HA concentrations, and a smooth curve was fitted to each set of data and then, (2) the "log ([Sr(II)(HA)i]/[Sr(II)]) vs. the log [HA]" sets of the data at fixed ph values were obtained graphically. Since the experiments were carried out under N2, atmosphere, carbonate complexes did not expect to be formed. In addition, it was confirmed from the stability constants of Sr(II) hydrolysis species(18) that they could not form under the range of ph in our study. Therefore, we made no allowance, in determination of the apparent stability constant of Sr(II)-HA, for side reactions involving carbonate and hydroxide formations. III. Results and Discussion Figure 2 shows the log [Sr(II)(HA)i]/[Sr(II)] obtained by Eq. (4) as a function of ph at four different concentrations of HA, along with the fitting curves. Each point on Fig. 2 is the average of two measurements (one from each of the two dialysis tubes containing HA solutions). The fitting curves are second order polynomial functions obtained by the least squares method. Data for the dialysis tubes where the final concentration of HA in the interior solutions decreased more than 5%, were discarded. Each data set shows little dispersion with a standard derivation of less than They were corrected in advance, for the slight transfer of HA to the exterior solution by the following procedure. The concentration of the HAbound Sr(II) in the exterior solution, [Sr(II)(HA)i]ex, was calculated by where A is the isotopic ratio of 85Sr, the ratio of the 85Sr concentration to the total strontium concentration. The value of i was calculated from uncorrected experimental data and was used to calculate the first estimate of [Sr(II)(HA)i]ex by Eq. (5). Once the [Sr(II)(HA)i]ex was known, it was used to correct Cex as: The Ccorrex was then used to estimate the [Sr(II)(HA)i]/ [Sr(II)] term in Eq. (5), and finally the second estimate of i was obtained. The first estimates of Ccorrex resulted in up to 8% changes in the values of [Sr(II)(HA)i]/[Sr(II)], but eventually had no effect on the i values. The second estimates of Ccorrex were rarely more than 0.5% different from the first. The data points in Fig. 1 are corrected values of log [Sr(II)(HA)i]/[Sr(II)], based on the first estimates of Ccorrex. We did not correct the data for the sorption of Sr(II) ions onto the membrane and container wall, because the concentration change resulting from the sorption does not affect the value of [Sr(II)(HA)i]/[Sr(II)]. Meanwhile, the mass balance over Sr(II) showed that no more than 10% of the total Sr(II) ions were adsorbed on dialysis tube and container wall. All the curves in Fig. 2 change similarly, their slope decreases with ph. Therefore, if the values of i do not change with ph, Eq. (3) indicates that log b would also show a similar ph dependency. Figure 3 was obtained graphically from the cross points between the vertical lines at 7 phs and the log ([Sr(II)(HA)i]/[Sr(II)]) curves in Fig. 2. The straight lines in Fig. 3 were obtained by the least squares method. The values of i, obtained from the slope of these straight lines, show a negligible ph dependency, varying from 1.10 to 1.17 in the ph range from 4 to 10. The values of i (6) (5) Fig. 2 Log ([Sr(II)(HA)i]/[Sr(II)]) as a function of ph at different humic acid (HA) concentrations and an ionic strength of 0.1 Fig. 3 Log ([Sr(II)(HA)i]/[Sr(II)]) vs. the logarithmic concentration of humic acid (HA) at constant ionic strength 0.1 and different phs VOL. 35, NO. 8, AUGUST 1998

4 582 M. SAMADFAM et al. larger than 1 implies that 1:2 complexes may also form under the experimental condition, that is, part of the Sr(II) ions may bind with two complexing sites on HA. The average value of was used for i. This average value of i together with the experimental data given in Fig. 2, was used to obtain b from Eq. (3). The values of log b as a function of ph are given in Fig. 4. Log b increases with ph, but tends to level off at higher phs, indicating that saturation is being reached. The results show a correlation between log b and ph; logb=( )+( )ph-( ) (ph)2, obtained by the least squares method. The correlation was made only for the sake of easy incorporation of data into geochemical modeling codes and has no chemical significance. The values of log b and the ligand to metal ratio i at ph 5, obtained in the present study (3.1 and 1.15, respectively), are very similar to those reported in our previous work(4) (3.2 and 1.2, respectively), despite the fact that different experimental methods were used. The apparent stability constant given by Eq. (2) quantitatively describes the strength of metal-humate binding in the simplest way. It was assumed that HA behaves as well-defined low-molecular-weight monomer ligands. However, in reality, HA is an extremely heterogeneous ligand with respect to molecular weight and content of functional groups(2)(16). One should expect that the apparent stability constants show a significant ph-dependence because, (1) a change in ph profoundly affects the ionization of acidic groups and thereby the number of binding sites on HA(16)(19) and, (2) the ph may also affect the structural configuration of HA molecules(16)(20). It can be inferred from the above discussion that the ionization of acidic functional groups of HA is the primary factor, responsible for the observed ph-dependence of the apparent stability constant. Unfortunately, ma- Fig. 4 Stability constants for complexation of Sr(II) with humic acid (HA) as a function of ph at ionic strength 0.1 jor difficulties are encountered in quantitative assessment of the individual contributions of the ionization of acidic functional groups of HA to the overall stability constant. Because, determination of the types and concentrations of functional groups of HA is not usually straightforward and there are many discussions in the literature regarding this point(20). In addition, the ionization of various functional groups may overlap significantly. It is generally accepted that carboxylic and phenolic groups are two major acidic groups of HA (the content of carboxylic groups is likely higher than that of phenolic groups)(2)(16). However, due to the polyelectrolytic nature of HA, it is very unlikely that any two binding sites (even of same type) will be exactly identical in a given sample(20). In actual practice, the concentrations of functional groups are operationally defined and only a portion of the titration curve is analyzed(21). The following discussion is limited to the effect of ionization of the carboxylic groups and it is assumed that the ionization of the carboxylic groups does not overlap with that of the phenolic groups. The ionization of a carboxylic functional group of HA may proceed as; HA=A-+H+ a=[a-]/[ha]0, (7) where [HA]0 is the total concentration of the acidic group and, a is degree of dissociation of the acidic group and can be obtained from titration curve in Fig. 1. The reaction between the ionized acidic groups and Sr(II) ions can be expressed by Combining Eqs. (7) and (8) finally leads to the equation, (8) logb=logksr+i(log a). (9) Equation (9) clearly shows the ph-dependence of the apparent stability constant, however, it fails to fit to experimental data in Fig. 3 if KSr is set to be a constant. An increase in ph from 4 to 8 corresponds to an increase in a from 0.2 to 1. If KSr does not change with ph, then logb would only be increased about 0.9, less than 1.4 obtained from the experiment. Therefore, the increase in apparent stability constant with ph can be attributed to (1) an increase in the number of binding sites due to ionization of the acidic functional groups of HA and, (2) an increase in apparent binding constant of Sr(II) with dissociated acidic groups, KSr. Apparently, the increase in the number of binding sites plays a major role in increasing logb than the change in KSr. The increase in KSr might be caused by a change in the structural configuration of HA. The random coil model which is the most favored structural model for humic substances at present, consider humic macromolecule in aqueous solution as a loosely wound, constantly flexing molecular strand capable of expanding or shrinking caused by intermolecular or in- JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

5 Effect of ph on Stability Constants of Sr(II)-Humate Complexes 583 tramolecular charge effects(22). As the ph increases, it is thought that the structural configuration of HA changes from a tightly coiled, cross-linked configuration, where the metal binding sites are not readily available, to a more open configuration(23). Such a change in structural configuration with ph was attributed to electrostatic repulsion, caused by build-up of negative charge on HA, because of increasing ionization of acidic groups(16). As the repulsive forces enhance through ionization, the ionization of remaining acidic groups become increasingly difficult, resulting in an apparent ionization constant that decreases with an increase in degree of ionization of the macromolecule(16)(23). The saturation behavior of the apparent stability constant might be attributed to a continuous decrease in apparent ionization constant Kapp (a continuous decrease in da/d(ph)) and saturation of ionized groups at higher phs. Although the apparent stability constants may not be particularly representative of reality, they can be easily incorporated into geochemical modeling codes. Fish et al.(24) and Turner et al.(25) concluded that they may be the best for quantitative analysis. IV. Conclusion The apparent stability constant, b, for complexation of Sr(II) ions with humic acid determined by the dialysis method was found to increase from at ph 4.0 to at ph A saturation behavior was observed for logb vs. the ph curve. It is very likely that the observed ph-dependence of the Sr-HA stability constant is caused by the ionization of acidic functional groups of HA with increasing ph. ACKNOWLEDGMENT Some of the experiments were carried out at the Central Institute of Isotope Science, Hokkaido University. This work was supported by a Grant-in-Aid of Scientific Research for the Ministry of Education, Science, Sports and Culture, Government of Japan, and also supported by Mitsubishi Materials Corp. REFERENCES- (1) Carlsen, L.: EUR EN, (1989). - (2) Choppin, G. R.: Radiochim. Acta, 44/45, (1988). (3) Moulin, V., et al.: Radiochim. Acta, 58/59, (1992). (4) Samadfam, M., et al.: Radiochim. Acta, 73, (1996). (5) Carlsen, L.: European Appl. Res. Rept. Nucl. Sci. Technot., 6, (1985). (6) Ibarra, J. V., et al.: Ann. Quim. 77, (1977). (7) Esteban, L., et al.: J. Inorg. Nucl. Chem., 40, (1978). (8) Takahashi, Y., et al.: J. Radioanal. Nucl. Chem., Lett., 186 [2], (1994). (9) Kim, J. I.. Sekine, T.: Radiochim. Acta, 55, (1991). (10) Maus, A., et al.: Radiochim. Acta, 52/53, (1991). (11) Torres, R. A., Choppin, G. R.: Radiochim. Acta, 35, (1984). (12) Nash, K. L., Choppin, G. R.: J. Inorg. Nucl. Chem., 42, (1980). (13) Niitsu, Y., et al.: J. Nucl. Mater., 248, (1997). (14) Zachara, J. M., et al.: Geochim. Cosmochim. Acta., 58, (1994). (15) Shanbhag, P. M., Choppin, G. R.: J. Inorg. Nucl. Chem., 43, (1981). (16) Stevenson, F. J.: "Humus Chemistry", (2nd ed.), Chap. 15 & 17, John Wiley & Sons, (1994). (17) Allard, B., et al.: SKB Technical Rep., TR 90-22, (1990). (18) Nilsson, K., et al.: European Appl. Res. Rept.-Nucl. Sci. Technol., 7 [1], (1985). (19) Marinsky, J. A., et al.: J. Colloid Interface Sci., 89 [2], (1982). (20) Wood, S. A.: Ore Geol. Rev., 11, 1-31 (1996). (21) Perdue, R. S.: "Humus Substances in Soil, Sediments, and Water", (Eds. Aiken, G. R., et al.), Chap. 20, John Wiley & Sons, (1985). (22) Swift, R. S.: "Humus Chemistry II," (Eds. Hayes, M. H. B., et al.), Chap. 15, John Wiley & Sons, (1989). (23) Choppin, G. R.: J. Inorg. Nucl. Chem., 70, (1978). (24) Fish, W., et al.: Environ. Sci. Technol., 20, (1986). (25) Turner, D. R., et al.: Geochim. Cosmochim. Acta, 50, (1986). VOL. 35, NO. 8, AUGUST 1998