Glaus, M. A.

Hummel, W.

Loon, L. R. Van

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Radiochim. Acta 84, 111-114 (1999) © by Oldenbourg Wissenschaftsverlag, München Rapid Communication Complexation of Radionuclides with Humic Substance: The Metal Concentration Effect By W. Hummel, M. A. Glaus and L. R. Van Loon Paul Scherrer Institut, Waste Management Laboratory, CH-5232 Villigen PSI, Switzerland (Received September 27, 1998; accepted in revised form January 28, 1999) Humic substance / Complexation / Metal concentration / Performance assessment Summary A synopsis of experimental data concerning the complexation of radionuclides with humic substance from various laboratories using different experimental methods reveals a consistent pattern of the metal concentration effect: With decreasing radionuclide concentration (and all other parameters held constant) an in-crease in the complexation of the radionuclide with humic sub-stance is observed. The implications of this effect for model calibration and practical applications are discussed. 1. Introduction The complexation of radionuclides with humic sub-stance has been investigated for three decades by vari-ous laboratories using different experimental methods over a wide range of experimental parameters. The re-ported results, mostly given in terms of "complexation constants" or "stability constants" of some sort, vary considerably. In a recent study of the complexation of Np(V) with humic acid [1] the authors made an inter-comparison of results from different laboratories. They conclude that there are two primary reasons for the observed variation in reported complexation constants : large uncertainties and systematic errors in certain experimental methods, and different approaches of ev-aluating the concentration of free humic acid. We agree that some of the observed scatter in reported complexation constants can be ascribed to artefacts and uncertainties in experimental procedures. Also calculations of the concentration of free humic acid by different authors may be based on different model assumptions and operational definitions. This certainly is a source of sometimes large variations in compari-sons of reported numerical results, even if the original experimental data show very good agreement. How-ever, the main reason for the observed differences is not a matter of definitions and systematic experimental artefacts, but stems from the fact that the measure-ments, while using different experimental methods, span a large range, not only in pH but also in metal concentration. In the following we will visualize the influence of varying metal concentration on the results by a synopsis of experimental data measured by sev-eral different techniques in a number of laboratories [1,2*, 4-14] , 2. Synopsis of experimental data The minimal set of experimental data needed to de-scribe the interaction of a metal ion, M, with a humic substance, HS, comprises [MHS], the concentration of metal bound to a certain humic substance, [M], the concentration of free metal in the aqueous phase, and (HS) total) the total concentration of humic substance. [MHS] and [M] can be quantified on the molar scale [mol/L], but (HSXotai is known only as mass per unit volume (g/L). Note, that [] is used in the following for molarity units and ( ) for mass per unit volume. This minimal set of experimental information can be summarized by a constant CK, defined as the num-ber of moles of metal bound per gram of humic sub-stance divided by the concentration of free metal in solution : C K = ^ M H S ] _ [M] · (HS)total The unit of CK therefore is volume per mass, usually given as L/g. In contrast to simple organic ligands with known molecular structure, "humic substance" serves just as a label for an operationally defined class of a naturally occurring mixture of medium to large size molecules. Any attempt to calculate the concentration of free hu-mic substance (i.e. the concentration of free binding sites) involves additional model assumptions or ad-ditional operational definitions, such as a mean molec-ular weight, a proton exchange capacity, the degree of proton dissociation, or a loading capacity. These dif-ferent assumptions and definitions invariably lead to different values for the concentration of free binding * All experimental raw data published in [2] are also available on the Internet under the address http://www.psi.ch by chos-ing "Departments and Projects" and "Waste Management". 112 W. Hummel, M. A. Glaus and L. R. Van Loon Source Substance, Method • [2] AHA, equilibrium dialysis • [6] AHA, anion exchange o [6] AHA, electrophoresis • [6] GOHY, anion exchange o [6] GOHY, electrophoresis • [5] AHA, UV/vis spectroscopy o [5] LBHA, UV/vis spectroscopy a [4] GOHY, UV/vis spectroscopy » [1] GOHY, UV/vis spectroscopy « JF* a • • * • • t . + » • It" ·*". * * . pH Fig. 1. Synopsis of the pH dependence of published Np(V) com-plexation data of various humic acids obtained by different experimental methods. Abbreviations are: AHA Aldrich humic acid, GOHY Gorleben GOHY-573 humic acid, LBHA Lake Bradford humic acid. In cases where a set of experimental data has been averaged, error bars (1 σ) are shown. sites and thus result in different values for com-plexation constants. Hence, as discussed in detail in [3], the concentration of free binding sites is a model dependent parameter. In order to avoid ambiguities due to this model dependent parameter, for the follow-ing intercomparisons all experimental data were re-evaluated according to Eq. (1) and given as log CK values. In this contribution we will not discuss the effect of different water compositions on radionuclide-humic complexation, i.e. the effect of competing cations like Ca or Al on radionuclide complexation is neglected. Therefore we only compare data measured in NaCl [7], KNO3 [8] or NaC104 solutions (all other refer-ences). 2.1 Np(V) A synopsis of experimental data concerning the com-plexation of Np(V) with humic substance is shown in Fig. 1 as a plot of log CK versus pH. As can be seen in this plot, the data cluster essentially in two regions of log CK. Data in the lower cluster, determined by UV/vis spectroscopy by two different groups [1, 4, 5], show good overall agreement. Data in the upper clus-ter, determined by equilibrium dialysis [2], electropho-retic ion focusing and anion exchange [6], exhibit somewhat larger scatter. In Fig. 1 all data points are recalculated from experimental values according to Eq. (1). Hence, the separation of data into two clusters does not arise due to different definitions of com-plexation constants. In [1] it was concluded that the only alternative explanation for such divergence may lie in the experimental methods. The authors state that 0.5 H -\ • Source Substance, Method • [2] AHA, equilibrium dialysis « [6] AHA, anion exchange α [6] AHA, electrophoresis • [6] GOHY, anion exchange • [5] AHA, UV/vis spectroscopy o [5] LBHA, UV/vis spectroscopy a [4] GOHY, UV/vis spectroscopy » [1] GOHY, UV/vis spectroscopy O -15 -13 log [ M i Fig. 2. Np(V) complexation data of various humic acids obtain-ed by different experimental methods: Metal concentration ef-fect in the pH range 7.0—7.5. Abbreviations are: AHA Aldrich humic acid, GOHY Gorleben GOHY-573 humic acid, LBHA Lake Bradford humic acid. In cases where a set of experimental data has been averaged, error bars (1 a) are shown. the large discrepancy in the results is hardly ascribed to their spectroscopic method and therefore, the ob-served discrepancy may be attributable to large uncer-tainties in the other experimental methods. Indeed, some of the scatter seen in the upper data cluster of Fig. 1 may arise from uncertainties in experimental methods but the main features reflect real chemical effects. Fig. 1 is a two-dimensional projection of a multi-dimensional chemical parameter space. The most important parameter not considered in Fig. 1 is the concentration of the radionuclide. If we take a "slice" of data from Fig. 1 in the pH range 7 to 7.5, where results from all different sources overlap, and plot log CK versus log [M], the concentration of free Np(V), the metal concentration effect is immediate-ly revealed (Fig. 2). All measurements by UV/vis spectroscopy were done at high metal concentration (10~4 M and higher), whereas the measurements by equilibrium dialysis were carried out at 10"8 M Np(V) and the measurements by electrophoretic ion focusing and anion exchange were done in the range of 10~13 M. The consistent picture revealed by Fig. 2 (and similar pictures can be found for any "pH slice") indicates a strong dependence of the experimental CK values on the radionuclide concentration. With decreasing metal concentration a significant increase in the complexation of the metal by humic substance is observed. 2.2 Ca This observation is not restricted to Np(V). Another example is shown in Fig. 3 where experimental data of Ca complexation with humic substance is assembled. Again, data measured by three different methods, equi-librium dialysis [2], Schubert's ion exchange method [7] and measurements with a Ca sensitive electrode [8], reveal a consistent picture. With decreasing metal Complexation of Radionuclides with Humic Substance: The Metal Concentration Effect 113 7 -6 -- 1 . 0 --1.5 -10 -8 -6 -4 -2 log [ M ] Fig. 3. Ca complexation data of humic and fulvic acids obtained by different experimental methods: Metal concentration effect in the pH range 6.0—6.7. Abbreviations are: AHA Aldrich hu-mic acid, BSFA Broubster Sepragen fulvic acid, PPHA Purified peat humic acid. In cases where a set of experimental data has been averaged, error bars (1 σ) are shown. concentration the complexation of Ca with humic sub-stance becomes more pronounced in a very systematic manner. 4 3 2 1 0 - 1 -14 -12 -10 -8 -6 -4 log [ M ] Fig. 4. Eu, Am and Cm complexation data of humic and fulvic acids obtained by different experimental methods: Metal con-centration effect in the pH range 5.0—5.5. Abbreviations are: AHA Aldrich humic acid, BFA Bersbo fulvic acid, GOHY Gor-leben GOHY-573 humic acid, LBHA Lake Bradford humic acid, EDLE Equilibrium dialysis-ligand exchange, TRLFS Time-re-solved laser-fluorescence spectroscopy. In cases where a set of experimental data has been averaged, error bars (1 σ) are shown. The dotted line represents model calculations according to the model discussed and calibrated in [13]. 1.5 • Source Substance, Method • [2] AHA, equilibrium dialysis » [7] BSFA, Schubert's ion exchange • (8] PPHA, Ca ion-selective electrode + Source (Element) Substance, Method • [2] (Eu) AHA, EDLE • [9] (Eu) AHA, equilibrium dialysis «[10] (Eu) AHA, ultrafiltration τ [11] (Eu) BFA, TRLFS o [12] (Am) LBHA, UV/vis spectroscopy • [13] (Am) GOHY, UV/vis and ultrafiltration o [13] (Cm) GOHY, TRLFS «[141 (Cm) AHA, TRLFS % V . 2.3 Eu, Am, Cm In the cases of Np(V) and Ca discussed so far, the overall metal concentration effect in the investigated concentration range is found to be less than two orders of magnitude in CK. However, in the cases of the triva-lent cations Eu, Am and Cm the metal concentration effect is much more pronounced. A synopsis of experi-mental data from different laboratories, measured with different methods and comprising different humic sub-stances reveals a very consistent pattern (Fig. 4). (Note, that the Cm data from [13] were re-evaluated starting with the raw experimental data given in Fig. 3 in [13] and using the same evaluation method as outlined in [11] and used in [14]. The values of [Cm3+] and [CmHA(II)] given in Table 5 of [13], correspond-ing to [M] and [MHS] in our nomenclature, are the result of fitting the raw data to the humic complexation model of [13].) The scatter in experimental data from different sources does not exceed one order of magni-tude in CK (Fig. 4) if data within a certain narrow ran-ge of metal concentration is considered. This is con-sistent with the observations for Ca (Fig. 3) and Np(V) (Fig. 2). However, the overall systematic variation in CK as a function of free metal concentration spans sev-eral orders of magnitude. 3. Conclusions Summarizing our results revealed by a synopsis of experimental data, we conclude that there is ample evi-dence of a metal concentration effect concerning the complexation of radionuclides with humic substance. The observation of such an effect is not surprising if we consider "humic substance" as a mixture of mole-cules. Some binding sites of these molecules are ex-pected to exhibit stronger affinity to metal ions than others. These strong binding sites determine the metal uptake at very low metal concentrations. With increas-ing metal concentration the strong sites reach satu-ration and weaker binding sites determine the metal uptake resulting in an overall decrease of metal-hu-mate interaction with increasing metal concentration. This conceptual view of metal complexation by humic substance is widely accepted in the literature [8, 15, 16] and the discussions generally do not address the metal concentration effect as such but concern the question how this effect could be adequately described by humic binding models. A recently proposed prag-matic approach for application in the field of perfor-mance assessment for repositories of radioactive waste is discussed in detail in [2]. For a review of binding models with respect to the effects of metal concen-tration, pH and ionic strength see [3]. However, any model may underestimate the metal-humate interac-tion by several orders of magnitude (e.g. dotted line in Fig. 4) if the model is calibrated only at high metal concentration and subsequently used for calculations at very low metal concentration. Hence, consideration of the metal concentration effect is of utmost impor-tance in any practical application aiming at an assess-ment of the influence of humic substance on the com-plexation and mobility of traces of radionuclides es-caping an underground repository. Acknowledgements Partial financial support by Nagra is gratefully acknowledged. The authors would like to thank Drs. 114 W. Hummel, M. A. Glaus and L. R. Van Loon F. J. Pearson Jr., J. Hadermann and an anonymous re-viewer for giving useful suggestions to improve the text. References 1. Marquardt, C., Kim, J. I. : Complexation of Np(V) with Hu-mic Acid: Intercomparison of Results from Different Lab-oratories, Radiochim. Acta 80, 129 (1998). 2. Glaus, Μ. Α., Hummel, W., Van Loon, L. R. : Experimental Determination and Modelling of Trace Metal-Humate Inter-actions: A Pragmatic Approach for Applications in Groundwater, PSI Bericht Nr. 97-13, Paul Scherrer Institut, Villigen, Switzerland (1997) pp. 251. 3. Hummel, W.: Binding Models for Humic Substances. In: Modelling in Aquatic Chemistry, Grenthe, I. and Puigdo-mènech, I., Eds., OECD Nuclear Energy Agency, Paris (1997), Chap. V. 4. Kim, J. I., Sekine, T.: Complexation of Neptunium(V) with Humic Acid, Radiochim. Acta 55, 187 (1991). 5. Rao, L., Choppin, G. R.: Thermodynamic Study of the Complexation of Neptunium(V) with Humic Acids, Radio-chim. Acta 69, 87 (1995). 6. Marquardt, C., Herrmann, G., Trautmann, Ν. : Complexation of Neptunium(V) with Humic Acids at Very Low Metal Concentrations, Radiochim. Acta 73, 119 (1996); and Mar-quardt, C. : Wechselwirkung von Neptunium(V) in sehr ge-ringen Konzentrationen mit Huminsäure, Dissertation, Uni-versität Mainz (1994). 7. Higgo, J. J. W., Davies, J„ Smith, B., Din, S., Crawford, M. Β., Tipping, E., Falck, W. E., Wilkinson, Α. Ε., Jones, M. Ν., Kinniburgh, D. : Comparative Study of Humic and Fulv-ic Substances in Groundwaters: 3. Metal Complexation with Humic Substances, British Geological Survey, Technical Report WE/92/12, Wallingford, UK (1992). 8. Benedetti, M. F., Milne, C. J., Kinniburgh, D. G„ van Riemsdijk, W. H., Koopal, L. K.: Metal Ion Binding to Hu-mic Substances: Application of the Non-Ideal Competitive Adsorption Model, Environ. Sci. Technol. 29, 446 (1995). 9. Carlsen, L., Bo, P., Larsen, G. : Radionuclide — Humic Acid Interactions Studied by Dialysis. In: Geochemical Behavior of Disposed Radioactive Waste, Barney, G. S., Navratil, J. D., Schulz, W. W., Eds., American Chemical Society, Wash-ington, DC (1984), p. 167-180. 10. Caceci, M. S.: The Interaction of Humic Acid with Eu-ropium(ni). Complexation Strength as a Function of Load and pH, Radiochim. Acta 39, 51 (1985). 11. Bidoglio, G., Grenthe, I., Qi, P., Robouch, P., Omenetto, N.: Complexation of Eu and Tb with Fulvic Acids as Studied by Time-Resolved Laser-Induced Fluorescence, Talanta 38, 999 (1991). 12. Kim, J. I., Buckau, G., Bryant, E., Klenze, R.: Com-plexation of Americium(III) with Humic Acid, Radiochim. Acta 48, 135 (1989). 13. Kim, J. I., Rhee, D. S., Wimmer, H., Buckau, G., Klenze, R. : Complexation of Trivalent Actinide Ions (Am3+, Cm3+) with Humic Acid: A Comparison of Different Experimental Methods, Radiochim. Acta 62, 35 (1993). 14. Moulin, V., Tits, J., Moulin, C., Decambox, P., Mauchien, P., de Ruty, O. : Complexation Behaviour of Humic Substances towards Actinides and Lanthanides Studied by Time-Re-solved Laser-Induced Spectrofluorometry, Radiochim. Acta 58/59, 121 (1992). 15. Buffle, J., Altmann, R. S„ Fileila, M., Tessier, Α.: Com-plexation by Natural Heterogeneous Compounds: Site Oc-cupation Distribution Functions, a Normalized Description of Metal Complexation, Geochim. Cosmochim. Acta 54, 1535 (1990). 16. Tipping, E., Hurley, Μ. Α.: A Unifying Model of Cation Binding by Humic Substances, Geochim. Cosmochim. Acta 56, 3627 (1992). 

Complexation of Radionuclides with Humic Substance: The Metal Concentration Effect

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