Effects of ionic strength and fulvic acid on adsorption of Tb(III) and Eu(III) onto clay

Journal of Contaminant Hydrology 192 (2016) 146–151

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Effects of ionic strength and fulvic acid on adsorption of Tb(III) and Eu(III) onto clay Maria Poetsch, Holger Lippold ⁎ Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Permoserstraße 15, 04318 Leipzig, Germany

a r t i c l e

i n f o

Article history: Received 16 October 2015 Received in revised form 28 June 2016 Accepted 15 July 2016 Available online 16 July 2016 Keywords: Fulvic acid Metal binding Linear Additive Model Adsorption Ionic strength

a b s t r a c t High salinity and natural organic matter are both known to facilitate migration of toxic or radioactive metals in geochemical systems, but little is known on their combined effect. We investigated complexation of Tb(III) and Eu(III) (as analogues for trivalent actinides) with fulvic acid and their adsorption onto a natural clay in the presence of NaCl, MgCl2 and CaCl2 up to very high ionic strengths. 160Tb, 152Eu and 14C-labelled fulvic acid were employed as radiotracers, allowing investigations at very low concentrations according to probable conditions in far-field scenarios of nuclear waste repositories. A combined Kd approach (Linear Additive Model) was tested for suitability in predicting solid–liquid distribution of metals in the presence of organic matter based on the interactions in the constituent subsystems. In this analysis, it could be shown that high ionic strength does not further enhance the mobilizing potential of humic matter. A quantitative reproduction of the influence of fulvic acid failed for most systems under study. Assumptions and limitations of the model are discussed. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The storage of radioactive waste demands for evidence of security over a long period. Mainly because of its high sorption capacity as well as favourable geomechanical properties, clay is being explored as one of the potential host rocks for a final repository. In Germany, suitable areas were identified in the north (Jurassic and Lower Cretaceous clay formations) and in the south (Opalinus clay formation, which extends to Switzerland) (BGR, 2007). Different from Opalinus clay, the formations in Northern Germany are in contact with highly saline ground and pore waters (Brewitz, 1982). Salt concentrations up to 4 M are to be considered. The mobility of actinides in ground water is essentially influenced by their interaction with organic and inorganic aquatic colloids as well as with mixtures of both (Kim, 2006). Complexation with dissolved humic matter can dominate the aqueous species distribution of actinides (Dearlove et al., 1991; Choppin, 1992). Depending on the geochemical conditions, humic-bound metals can be kept in solution, or their adsorption can be enhanced (Lippold et al., 2005). Humic substances are deemed to mobilize actinides in the far field of a repository by formation of inner-sphere complexes at moderate pH (Bradbury et al., 2005; Reich et al., 2007; Sachs et al., 2007). However, the influence of high ionic strength on reactions in the ternary system of metal, humic substance and mineral has not been examined so far (see Vilks (2011) for a review). Results obtained for low-salinity systems cannot be transferred to highly saline ground or pore waters (Bossart, 2007). ⁎ Corresponding author. E-mail address: [email protected] (H. Lippold).

http://dx.doi.org/10.1016/j.jconhyd.2016.07.006 0169-7722/© 2016 Elsevier B.V. All rights reserved.

Whereas low salinity does not have a major effect on complexation of humic substances with metal ions (Joseph et al., 2011), competition for binding sites is substantial at high concentrations of electrolytes (Tipping, 2002). High metal loads also influence the adsorption behaviour of humic substances onto clay (Evans and Russell, 1959; Greenland, 1971; Glaus et al., 2005; Joseph et al., 2013) or clay minerals like kaolinite (Samadfam et al., 1998; Křepelová et al., 2007, 2008; Schmeide and Bernhard, 2010), montmorillonite (Takahashi et al., 2002) and illite (Preston and Riley, 1982). Conceptual models of varying complexity have been developed to describe the mobility of metals in ternary systems with organic matter by means of stability constants and distribution coefficients (see Lippold and Lippmann-Pipke (2009) and references therein). Relatively simple approaches are based on a pseudo two-phase system consisting of solid phase and humic substance, between which the metal is partitioned. These models rely on the assumption that dissolved organic matter is not adsorbed, which is, however, not valid under specific geochemical conditions. In the Linear Additive Model (Zachara et al., 1994), adsorption of organic matter is explicitly considered. Solid–liquid distribution of the metal is determined by interactions in three binary subsystems as shown in Fig. 1. In view of their size, dissolved humic substances are regarded as a dispersed pseudo solid phase, i.e., a competing adsorbent that is in turn adsorbed itself. The three elementary interaction processes are assumed to be independent of each other (Zachara et al., 1994; Samadfam et al., 1998; Lippold and Lippmann-Pipke, 2009; Liu et al., 2011). In the present study, we investigated the influence of organic matter on adsorption of Tb(III) and Eu(III) onto clay in the presence of NaCl,

M. Poetsch, H. Lippold / Journal of Contaminant Hydrology 192 (2016) 146–151

Fig. 1. Linear Additive Model for adsorption of metals (M) onto solid surfaces in the presence of humic substances (HS).

MgCl2 or CaCl2, the main constituents of clay pore waters. The trivalent lanthanides were employed as analogues for trivalent actinides such as Am(III) and Cm(III), since their chemical behaviour is known to be similar (Choppin, 1995; Cotton, 2007; Lieser, 2008). We used natural clay material from the Opalinus clay formation (Mont Terri, Switzerland) and fulvic acid as a surrogate of higher-molecular-weight clay organics. Beside a major fraction of low-molecular-weight organic acids (acetate, propionate, lactate), up to 7% of humic-like substances were found in DOC extracted from natural clay materials (Claret et al., 2005; Glaus et al., 2005). Fulvic acid was chosen because the molecular weight of the humic-like compounds was reported to be comparatively low (Courdouan et al., 2007a,b). 160 Tb, 152Eu and 14C-labelled fulvic acid were used as radioactive tracers, allowing reliable measurements in very dilute systems. NaCl, MgCl2 and CaCl2 were added at concentrations up to 4 M, corresponding to conditions related to the Cretaceous and Jurassic clay formations in Northern Germany (thus exceeding salinities found for pore waters in Opalinus clay). Experiments were performed at pH 5 and pH 7. Formation waters from deep boreholes in the Northern German clay show pH values between 6.4 and 6.7 (Brewitz, 1982). Lower pH values may occur in case of advective transport of external waters along fractures. Extending the study to slightly acidic pH was also motivated by the fact that the influence of humic substances on the mobility of metals often undergoes a changeover in this pH range (Lippold et al., 2005). Main objective of this work was to answer the question as to whether the mobilizing potential of natural organic matter may be enhanced at the conditions of highly saline systems. Synergistic as well as antagonistic effects are conceivable. Modelling shall provide information on general principles in the distribution behaviour of metals in ternary systems with clay and humic or humic-like substances. The validity of the Linear Additive Model is a basic prerequiste in common approaches for reactive transport modelling (Lippold and Lippmann-Pipke, 2009). 2. Materials and methods 2.1. Materials 160 Tb (half-life 72.3 d) was produced by neutron activation of natural 159Tb at the TRIGA Mark II reactor of the University of Mainz (Germany) and was used in the form of a stock solution of 10− 4 M [160Tb]Tb(III). [152Eu]EuCl3 in 0.1 M HCl was purchased from Radioisotope Centre Polatom (Poland). An Opalinus clay powder sample of the borehole BHE-241 from the Mont Terri rock laboratory (Switzerland) was used without further pretreatment as a 4.8 g L−1 suspension under oxic conditions. The exact mineralogical composition is given elsewhere (NAGRA, 2002). Major

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mineral phases are illite (23 ± 2 wt.%) and kaolinite (22 ± 2 wt.%), minor mineral phases are quartz (14 ± 4 wt.%), calcite (13 ± 8 wt.%), illite-smectite (11 ± 2 wt.%), and chlorite (10 ± 2 wt.%). The total contents of iron minerals and organic carbon are around 4 wt.% and 1 wt.%, respectively. The specific surface area was determined to be 28 m2 g−1 (N2-BET), reported values for the cation exchange capacity vary between 11.1 and 16.0 meq per 100 g. Fulvic acid was isolated from surface water collected on the moor “Kleiner Kranichsee” (near Carlsfeld, Germany). Purification was carried out according to the recommendations of the International Humic Substances Society (Aiken, 1985). The average molecular weight was determined to be 38 kDa by size exclusion chromatography, the total acidity was found to be 8.46 meq g−1 by potentiometric titration. To eliminate matrix effects in adsorption studies, the fulvic acid was radiolabelled by azo-coupling with [14C]aniline (Mansel and Kupsch, 2007). A detailed description of the procedure is given elsewhere (Lippold and Lippmann-Pipke, 2014). All other chemicals were of analytical grade and were used as received. Solutions were prepared using ultrapure Milli-Q water, and all experiments were carried out at ambient conditions. 2.2. Adsorption experiments Variable amounts of NaCl, MgCl2 or CaCl2 within a range of 0– 4 mol L−1 were added to a clay suspension. The pH was adjusted to 5 or 7 using HCl. Stock solutions of [160Tb]Tb(III) (for pH 5) or [152Eu]Eu(III) (for pH 7) were added to yield a concentration of 10−6 M (42 Bq mL−1 160 Tb, 930 Bq mL−1 152Eu). Fulvic acid solutions, adjusted to pH 5 or 7, were then added resulting in a concentration of 40 mg L−1. After equilibration in an end-over-end rotator (24 h proved to be sufficient in a previous study (Lippold and Lippmann-Pipke, 2014)) and centrifugation (45 min at 7000 rpm), the remaining concentration of [160Tb]terbium or [152Eu]europium in the supernatant was determined with a gamma counter Wizard 3″ (Perkin Elmer, USA). Wall adsorption was found to be negligible. Adsorption experiments with 14C-labelled fulvic acid (40 mg L−1, 150 Bq mL−1) were performed in a similar way. Analyses were carried out with a liquid scintillation analyzer Tri-Carb 3110 TR (Perkin Elmer, USA), using Ultima Gold scintillation cocktail (Perkin Elmer, USA). In blank tests, it was ascertained that no precipitation or wall adsorption of fulvic acid was induced by the high electrolyte contents. 2.3. Complexation experiments Solutions of 40 mg L− 1 fulvic acid, 10−6 M [160Tb]Tb(III) (pH 5) or [152Eu]Eu(III) (pH 7) and variable amounts of NaCl, MgCl2 or CaCl2 (0–4 mol L−1) were left to stand for an hour and then centrifuged at 7000 rpm with “Vivaspin 2” 2 kDa ultrafilters (Sartorius, Germany). The concentration of [160Tb]terbium or [152Eu]europium in the filtrate (non-complexed fraction) was determined by gamma counting. 3. Modelling adsorption in ternary systems The effect of ionic strength on metal adsorption in ternary systems (consisting of metal, fulvic acid and clay) is calculated from adsorption and complexation data determined for the constituent binary systems (Fig. 1), specified by the following equations: The solid–liquid distribution coefficient for direct metal adsorption KM/S (L g−1) is given by Eq. (1) d M=S

Kd

¼

ΓM cM

ð1Þ

with ΓM (mol g−1) denoting the amount of adsorbed metal and cM (mol L−1) denoting the equilibrium concentration of free metal in solution.

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Fig. 2. Adsorption of Tb(III) (pH 5) and Eu(III) (pH 7) onto Opalinus clay in the presence of NaCl, MgCl2 and CaCl2.

The distribution coefficient for adsorption of fulvic acid KFA/S (L g−1) d is defined accordingly: FA=S

Kd

¼

Γ FA cFA

ð2Þ

ΓFA (g g−1) is the adsorbed amount of fulvic acid, and cFA (g L−1) is its concentration. It is assumed that adsorption is not influenced by a small amount of metal complexed to FA. For simplicity, metal-fulvate complexation is likewise quantified as a distribution coefficient KM/FA (L g−1), calculated according to Eq. (3) d M=FA

Kd

¼

cFA Γ FA M M ¼ cM cM cFA

ð3Þ

−1 ). where cFA M is the concentration of metal bound to fulvic acid (mol L According to the Linear Additive Model, these distribution coefficients are independent of each other, and Eqs. (1)–(3) can be combined to Eq. (4) where KLAM (L g−1) is the solid–liquid distribution coefficient d of the metal in the ternary system with fulvic acid:

M=S

¼ K LAM d

Kd

FA=S

M=FA Kd M=FA K d cFA

þ Kd 1þ

cFA

ð4Þ

4. Results and discussion 4.1. Adsorption of Tb(III), Eu(III) and fulvic acid onto clay in binary systems Fig. 2 shows adsorption of Tb(III) (pH 5) and Eu(III) (pH 7) onto Opalinus clay as a function of ionic strength. With increasing salinity, Kd values strongly decrease, most pronounced for the bivalent cations. Mineral phases dominating metal adsorption are kaolinite and illite, owing to their high specific surface areas. Contents of iron minerals and organic matter are too low to affect adsorption. At a pH of 5, metal adsorption is stronger suppressed than at pH 7, which may be explained by the lower adsorption capacity of clay resulting from protonation.

In Fig. 3, adsorption of fulvic acid onto Opalinus clay at pH 5 and 7 in the presence of NaCl, MgCl2 and CaCl2 is shown as a function of ionic strength. Compared to metal adsorption, the results are relatively insensitive to increasing salinity. Adsorbed fractions vary between 90% and 100%. The Kd values are comparable to other studies (Lippold et al., 2005; Weng et al., 2006; Lippold and Lippmann-Pipke, 2009). Adsorption of fulvic acid is slightly higher at pH 5 because repulsion by negative charge on fulvic acid and clay surface is reduced by protonation. Since air was not excluded during preparation of the clay sample, it must be considered that ferric minerals such as pyrite are oxidized, entailing possible alterations in the sorption properties especially in respect of fulvic acid. Concerning the effect of ionic strength, however, major implications are not expected, as these ferric components constitute a very small fraction. 4.2. Complexation of Tb(III) and Eu(III) with fulvic acid The dependence of Tb(III)-fulvate and Eu(III)-fulvate complexation on ionic strength is shown in Fig. 4. For better comparison, these data are likewise given as distribution coefficients KM/FA , considering the d fulvic acid colloids as a pseudo solid phase. Complexation of Tb(III) and Eu(III) is strongly suppressed with increasing ionic strength and is significantly lower at pH 5 than at pH 7, caused by protonation of fulvic acid, which leads to blocking of ligands. Similar to adsorption of Tb(III) and Eu(III) onto clay, the influence of ionic strength is higher for the bivalent cations Ca2+ and Mg2+. Both show strong interference on complexation, explained by competition and/or shielding effects. Differences in the effects of the bivalent cations may be due to the fact that the ionic potential of Mg2+ is much higher than that of Ca2+, entailing a tendency towards covalent bonding (Cartledge, 1928; Fajans, 1967; Huheey et al., 2012). As interactions of Tb(III) or Eu(III) are ionic, a preference of Mg2+ for covalent interaction might reduce its competitive influence, at least in respect of metal-FA complexation. The following sources of experimental errors are to be taken into consideration: (I) The membrane of the ultrafilters may be permeable for small fulvic acid molecules, entailing an underestimation of complexed fractions. (II) Tb or Eu may be adsorbed at the membrane,

Fig. 3. Adsorption of fulvic acid onto Opalinus clay at pH 5 and pH 7 in the presence of NaCl, MgCl2 and CaCl2.

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Fig. 4. Complexation of Tb(III) (pH 5) and Eu(III) (pH 7) with fulvic acid in the presence of NaCl, MgCl2 and CaCl2.

resulting in an overestimation of complexed fractions. (III) A possible perturbation of the complexation equilibrium due to the up-concentration of fulvic acid during the filtration process may affect the results. Regarding issue (I), fractions of fulvic acid passing through the 2 kDa filter were found to be ~3% (based on UV absorption at λ = 254 nm), causing a systematic error in KM/FA that cannot be corrected. This error gains in d importance at high metal loads, concomitantly with the analytical random error due to the low metal concentrations in the filtrate. Effects of shrinking at high ionic strengths were not observed. Issue (II) proved to be insignificant in preliminary tests (provided that stock solutions are acidified when kept for a longer time). Complexation was also investigated by time-resolved laser fluorescence spectroscopy (TRLFS) as an alternative method. Very similar results were obtained, suggesting that issue (III) is not important as well. (An example of comparative measurements is shown in the Supplementary material, Fig. A.1.) The TRLFS studies have also shown that formation of chloro complexes for Tb(III) and Eu(III) at high ionic strength is negligible. Formation of carbonate species is, however, significant at pH 7 (~ 15% for Eu(III) in water at atmospheric CO2 pressure (Plancque et al., 2003)). Dissolution of calcite from the clay material increases the carbonate content. Since metal binding to organic matter is counteracted by carbonate complexation, the degree of interaction in pristine natural clay is probably overestimated under the present ex-situ conditions. 4.3. Adsorption of Tb(III) and Eu(III) onto clay in ternary systems with fulvic acid Fig. 5 shows adsorption of Tb(III) and Eu(III) onto Opalinus clay in the presence and absence of fulvic acid as a function of ionic strength for NaCl, MgCl2 and CaCl2. At pH 5, the presence of fulvic acid leads to increased adsorption of Tb(III) in general. Adsorption of fulvic acid mediates adsorption of bound metal ions at acidic conditions. With increasing ionic strength, metal binding to fulvic acid is suppressed, which results in lower Kd values, but they do not fall below the values obtained in the absence of organic matter. At pH 7, the effect of fulvic acid is reversed: Metal adsorption is decreased compared to the system without fulvic acid. This changeover is a consequence of the decrease in adsorption of the organic carrier, possibly also of the increase in its metal load. At a given pH, there is little

variation in the adsorbed amounts of fulvic acid with increasing ionic strength, while metal binding diminishes (Fig. 4). For this reason, the mobilizing effect of organic matter cannot be enhanced by increasing salinity, visible from the differences in Kd in the absence and presence of FA (Fig. 5, right-hand diagram). (At the lowest (0 M) and highest (4 M) electrolyte concentrations, this difference amounts to 19.33 L g−1 and 18.74 L g− 1, respectively, for NaCl, 40.68 L g−1 and 0.16 L g−1, respectively, for MgCl2, and 24.02 L g−1 and 0.02 L g−1, respectively, for CaCl2.) The Linear Additive Model as described in Chapter 3 was applied in order to examine as to whether the invoked correlations between binary and ternary systems hold quantitatively. Fig. 6 shows the measured distribution coefficients of the metal in the ternary systems together with the corresponding KLAM values calculated according to Eq. (4). d The calculation is based on the experimental KM/S and KM/FA values d d shown in Figs. 2 and 4, respectively. For KFA/S and c , which are affected d FA by the largest experimental uncertainty (Fig. 3), the mean values of each data series were used, since there is no trend in these results. (Implications of uncertainty in KFA/S and cFA on the modelling results can be seen d in Fig. A.2, provided as Supplementary material.) For evaluating the performance of the Linear Additive Model, it must be noted that no parameters were adjusted in this approach. The model is capable of reproducing trends in the experimental data as a function of ionic strength. However, deviations of up to one order of magnitude are observed, especially for adsorption in the NaCl system. Obviously, metal adsorption in the presence of organic matter cannot be quantitatively predicted on the basis of interaction data for the underlying elementary processes as formulated here. While linearity of adsorption or complexation as a function of concentration is not actually a prerequisite of the model, an “additive” (i.e., independent) treatment of the constituent interactions according to Eq. (4) is based on several assumptions: (a) Ternary surface complexes of the kind S-M-FA do not exist. (b) There is no difference in adsorption of FA and FA-M. (c) There is no mutual influence on adsorption of M and FA-M. (d) The stability of the complex in solution is equal to its stability in the adsorbed state. (e) FA can be regarded as one reactant. Real ternary surface complexes where adsorption of FA is mediated by M acting as a bridge are hardly conceivable in view of the sheer size of FA molecules. In a TRLFS study, Takahashi et al. (2002) found

Fig. 5. Adsorption of Tb(III) (pH 5) and Eu(III) (pH 7) onto Opalinus clay in the presence (empty symbols) and absence (full symbols) of fulvic acid as a function of ionic strength for NaCl, MgCl2 and CaCl2 as electrolytes.

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Fig. 6. Adsorption of Tb(III) (pH 5) and Eu(III) (pH 7) onto Opalinus clay in the presence of fulvic acid as a function of ionic strength for NaCl, MgCl2 and CaCl2 as electrolytes. The full symbols show the values calculated according to the Linear Additive Model (Eq. (4)).

that the fluorescence properties of fulvic-bound Cm(III) adsorbed to montmorillonite did not show any difference compared to the fulvic complex in solution, indicating that FA acts as a vehicle for the metal, not vice versa. Assumptions (b) and (c) are certainly valid for the system under study as the concentrations of Tb or Eu are very low, by far exceeded by the electrolyte concentrations. Adsorption sites for M are partly blocked by adsorption of FA-M. Since, however, metal binding to fulvic acid is higher than metal adsorption by orders of magnitude (cf. Figs. 2 and 4), this interference is negligible. Assumption (d) is more critical. Beside the fact that the inner structure of FA molecules will be changed in the adsorbed state, the complexing ligands are partially involved in the adsorption process, which is thus a competitive reaction with regard to metal binding. This effect will be even more pronounced if FA is added prior to the metal because desorption of FA is kinetically hindered (Lippold and Lippmann-Pipke, 2014). Besides, calcite dissolution from the clay material produces additional Ca that is not contained in the systems where metal-fulvic complexation was determined. Concentrations of leached Ca are, however, much lower than the concentrations of the added electrolytes (b 10−2 M). Assumption (e) is probably a major weakness of the model because fulvic acid is not a defined compound but a polydisperse mixture of molecules with variable properties. Selectivities in complexation and adsorption are not taken into account. For instance, preferred adsorption of molecules with high metal loads would cause an underestimation by the model, since it operates with an averaged metal load, determined for the humic substance as a whole. 5. Conclusions Fulvic acid strongly influences adsorption of trivalent lanthanide ions onto clay by formation of metal-fulvate complexes. At low pH, metal adsorption is enhanced by adsorption of the organic carrier, whereas at in-situ pH conditions of clay rock, the presence of fulvic acid brings about a mobilization of metals. High ionic strength, as found for pore waters in clay formations in Northern Germany, suppresses adsorption of metals as well as their complexation with humic-like clay organics, which has significant implications on humicbound metal transport. However, this study has shown that there is no synergism in the mobilizing effects of humic matter and electrolytes at in-situ pH. On the contrary, a mitigating effect of ionic strength was evidenced, based on the fact that metal binding is suppressed while adsorption of humic matter is hardly influenced. Reconstructing the effect of fulvic acid on metal adsorption quantitatively by combining the constituent binary systems according to the Linear Additive Model did not yield satisfactory results. Beside possible differences in stability for dissolved and adsorbed metal-fulvate complexes, selectivities in complexation and adsorption among the multicomponent system of fulvic molecules probably impair the applicability of this approach. Studies with fractionated humic material could serve to clarify the importance of selectivity effects.

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