Humic and Fulvic Acids - American Chemical Society

Chapter 16

Role of Humic Substances and Colloids in the Behavior of Radiotoxic Elements in Relation to Nuclear Waste Disposal Confinement or Enhancement of Migration Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: November 14, 1996 | doi: 10.1021/bk-1996-0651.ch016

1

2

3

Valérie M. Moulin , Christophe M. Moulin , and Jean-Claude Dran 1

Commissariat à l'Energie Atomique, Fuel Cycle Division, Department of Waste Storage and Disposal, Service of Nuclear Waste Storage and Disposal Studies, Section of Geochemistry, BP6 92265 Fontenay-aux-Roses Cedex, France Commissariat à l'Energie Atomique, Fuel Cycle Division, DPE/SPEA/SPS/Analytical Laser Spectroscopy Group, 91121 Saclay, France CSNSM/Centre National de la Recherche Scientifique, 91405 Orsay, France 2

3

The potential role of humic substances and colloids on the fate of radiotoxic pollutants should be evaluated in particular around a nuclear waste repository. The different processes involving these entities such as complexation or sorption can strongly affect the behavior of radionuclides. In particular, results on the complexation of actinides with humic substances, investigated by laser induced fluorescence and spectrophotometry will be presented as well as data on the retention of colloids in the presence or not of heavy elements on mineral surfaces measured by Rutherford Backscattering spectrometry. From these studies, the impact of colloids and humic substances on radiotoxic element behavior will be discussed in terms of confinement or enhancement of migration in the geological media. Due to the ubiquitous occurrence of humic substances and colloids (defined as entities of lnm-ΐμιη size) in natural waters, their specific properties, in particular their scavenging capacities towards metallic cations and also their well-established mobility (1-6), these organic and inorganic species could have important effects on the fate and mobility of these cations in natural systems. On one hand, the formation of organic complexes or pseudocolloidal species will modify the speciation (distribution of chemical species) of the cation of interest and its solubility. On the other hand, they can retard cation migration 0097-6156/96/0651-0259$15.00/0 © 1996 American Chemical Society

In Humic and Fulvic Acids; Gaffney, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

HUMIC AND FULVIC ACIDS

Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: November 14, 1996 | doi: 10.1021/bk-1996-0651.ch016

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when sorbed on mineral surfaces or byfiltrationin the porosity of the medium. These features are particularly important in the case of radionuclides which could be releasedfroma radwaste repository in deep geological formations (7). In thisframework,two aspects have been investigated which will be reported here in order to contribute to the answer of the question: whether the interaction of radioéléments with colloids or humic substances reduce or enhance their mobility. Firstly, the complexation behavior of actinide ions [Αη(ΠΙ), An(V) and An(VI)] with humic substances has been studied by means of spectroscopic methods (laser inducedfluorescenceor spectrophotometry) in order to determine the speciation of radionuclides under natural water conditions representative of those encountered in granitic or sedimentary formations. Secondly, the retention of colloids and heavy elements on mineral surfaces has been investigated by ion-beam techniques (Rutherford Backscattering spectrometry) in order to determine the colloid surface coverage, the colloid detachment rate and the behavior of heavy elements in their presence. Hence, the impact of humic substances and colloids on radionuclide behavior will be evaluated and their consequences on the fate of these pollutants in theframeworkof radwaste disposal. Complexation of Actinides by Humic Substances Investigations have been devoted to the study of the complexation behavior of actinides [Αη(ΙΠ), An(V) and An(VI)] with humic substances through the use of spectroscopic methods: • forfluorescentcations such as lanthanides and actinides [Dy(m), Cm(III), U(VI)]: Time-Resolved Laser-Induced Fluorescence (TRLIF). • for non-fluorescent cations such as actinides [Np(V)]: Spectrophotometry (SP). The effect of different physico-chemical parameters, namely pH, ionic strength, presence of a competing cation and cation concentration, has been studied in order to understand complexation mechanisms. Description of the techniques. These two methods (TRLIF and SP) applied to complexation measurements are based on the same principle: titration of the cation (in the nanomolar to micromolar range for TRLIF (Figure 1) and in the micromolar range for spectrophotometry) by the organic ligand (humic substances) at a constant pH and ionic strength, and measurement of the signal characteristic of each technique, namely thefluorescenceintensity (as well as the lifetime and eventually thefluorescencespectrum modification) for TRLIF, and the absorbance for SP. In the case of TRLIF (8-9), applied to a lanthanide [Dy(m)] and an actinide [Cm(m)] cation, an increase of thefluorescence(at the wavelength of cation emission) is observed when adding the ligand, whereas for an hexavalent cation such as U(VI), a decrease of the fluorescence signal is obtained. In the case of SP (10-11), applied to two actinide cations (Am(m), Np(V)), a decrease or an increase of the absorbance corresponding respectively to the free or bound cation is observed. The analysis of such titration curves (Figure 2) permits us to obtain information on the conditional interaction constants (β) and the complexing capacities (W) of the humic materials towards the studied cation under different experimental conditions (varying pH, ionic strength and metal concentration). In Humic and Fulvic Acids; Gaffney, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

16.

MOULIN E T AL.

Humic Substances and Behavior of Radiotoxic Elements

LOD


MULTICHANNEL ANALYSER

ELECTRONIC

•

BE

u Cm Am Eu Tb Sm Dy.

• •

;

13

5xlO- M 5xlO M 10" M 5xlO" M 10" M 10- M 10- M _13

9

,2

10

10

10

Ref 22 23 24 25 25 25 25

J

Figure 1. TRLIF set-up with the limits of detection (LOD) of the technique for actinide and lanthanide trace analysis in appropriate complexing media (22-25).

Fluorescence Intensity (AU)

2.00

-r

0.40 0.20 0

0.001

0.002

0.003

0.004

0.005

[HA] (g/1)

Figure 2. Typical titration curve obtained for the system Cm(III)-Aldrich humic acids by TRLIF; [Cm]=10 M , pH 4, NaC10 0.001M. _7

4


262



The single site model is used to describe the interactions between the cation and the humic acids (11). Since humic substances are heterogeneous and complex molecules, the ligand is defined as a monodentate site (A) with no particular assumption on its chemical nature. Complexes of 1:1 (metalxation) stoichiometry are assumed to be formed according to the equilibrium:

where [AO] = W χ C - [MAC " )*] W = complexing capacity expressed in mmol/g (number of millimoles of cations bound per gram of organic ligand) C = initial concentration of the organic ligand expressed in g/1. The complexing capacity is obtained graphicallyfromthe titration curve and βfroma non-linear regression (using the Marquadt-Newton algorithm) giving thefluorescenceintensity (or the absorbance) as a function of β, W and 2

1

0

Q

Co.

Results and Discussion. From spectroscopic data, important features concerning the interaction of actinides with humic substances can be deduced. In the case of trivalent elements (Am *, Cm , Dy ), the presence of one isobestic point (obtained with Am by spectrophotometry) indicates the presence of one complex (10);fromfluorescencedata, it has been established that energy transfer via the triplet state of the organic molecules occurs explaining the increase offluorescenceintensity during the titration {8-9). In the case of pentavalent elements (NpC^*), the presence of one isobestic point also indicates the existence of a single type of complex with the humate ligand and under our experimental conditions, no reduction of Np(V) to Np(TV) has been observed. In the case of hexavalent cations (U0 ), thefluorescencestudy (11) shows that no reduction of U(VI) to U(IV) by humic acids under our experimental conditions occurs (no modification of the lifetime) and that the interaction of the uranyl ion with the humic molecules induces a static quenching (explaining a decrease of the signal with nofluorescencelifetime modification during the titration). From a chemical viewpoint, trivalent cations form relatively strong complexes with humic acids. The interaction constants determined (Table I) are independent of pH (4-7) and ionic strength (0.1-0.001 M) but strongly dependent on the metal concentration (Figure 3) which may be related to the presence of different kinds of sites: strong and weak sites. This effect has already been observed in (12). The complexing capacity (Table I) reflecting the number of sites interacting with the cation increases with pH and metal concentration but decreases with ionic strength. 3

3+

3+

2+

2


16. MOULIN ET AL.

Humic Substances and Behavior of Radiotoxic Elements

Table L Values of conditional interaction constants β (in I/moI) of tri-, penta- and hexavalent actinides with Aldrich humic acids and maximum complexing capacities (W in mmol/g)).

Am(m) Cm(m)

pH

IV] (M) 3xl0" 3xl0lxlO5xl0lxlO" 8


7

7

5

Dy(m) Np(V) U(VI)

2X10-

log β

I(M)

4.65 0.1 4.2 0.1 4.2-6.9 0.001-0.1 4.2-6.9 0.001-0.1 4.2-6.1 0.1

5

4-6 7 4-5 5

6

5

6xl0" 4xl0' 4x10" 8

7

0.1 0.1 0.1

1.5 ±0.3 0.9 ±0.3 1.3 ±0.2 1.2 ±0.3 1.3 ±0.3 1.0

0.14 0.25 0.7

M

7.0 ± 0.3 9.1 ±0.2 8.3 ±0.4 7.4 ± 0.3 6.6 ±0.4 7.5 ±0.2 4.6 ±0.2 7.8 ±0.2 7.2 ±0.3

In the case of pentavalent cations, the conditional interaction constant (Table I) obtained for Np0 (log β = 4.6) shows a relatively low affinity of the neptunyl cation with the humic acids as it could be predictedfromthe charge of the ion. The interaction constant obtained for the hexavalent cation (U) with humic acid (Table I) is independent of pH (4-5) in the non-hydrolysis pH-range but some variation with uranium concentration is observed as for trivalent cations. Moreover, the complexation of uranium to humic substances is of the same order of magnitude than the complexation of trivalent actinides which corroborates chemical analogy between both cations. +

2

log β

9.5 9 8.5 8 7.5 7 6.5 6 -8

• Cm (TRLIF) • Cm (TRLIF) 1=0.001 M

•

A Dy (TRLIF) • Am(SP)

g D

A •

•

-7

-6

-5

-4

log [M] Figure 3. Effect of metal concentration on the interaction constant of trivalent actinides with Aldrich humic acids (at a ionic strength of 0.1M). TRLIF: TimeResolved Laser-Induced Fluorescence, SP: Spectrophotometry.


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Summary. Spectroscopic techniques, in particular laser induced fluorescence, appear as a good analytical tool for complexation measurements since they are non-intrusive methods, they allow to work at low level of cation concentration (in the case of TRLIF), in particular below the solubility limits permitting to cover a large range of pH and they also allows the use of low humic acid concentrations to avoid aggregation phenomena. From the conditional interaction constants measured by these techniques, the following trend of actinides for humic acids is deduced: +

Np0 (log β = 4.6) « U0 Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: November 14, 1996 | doi: 10.1021/bk-1996-0651.ch016

2

Νρ

2+ 2

3+

(log βυ = 7.2-7.8) - Am (log βΑ« = 7.1-9.1)

Consequences on actinide speciation. Speciation calculations of the actinides upon interest for radwaste disposals have been performed by considering hydrolysis, carbonate and organic complexation but under conditions relevant to geological formations (either granitic or sedimentary) which could be retained for nuclear waste disposals (Table Π) (11). It appears that for trivalent cations (Am), organic complexes dominate their speciation in watersfromgranitic or sedimentary formations, except at the lowest humic acid concentration and highest partial pressure of C0 . In the case of penta- and hexavalent actinides (Np, U), only inorganic complexes are present (hydroxide or carbonate species) in these waters. It should be emphasised that, if these speciation calculations are performed up to pH 7, organic complexation will predominate over inorganic complexation (whatever the partial pressure of C0 and for humic concentration equal or higher than 1 mg/1) for trivalent and hexavalent actinides. For pentavalent cations, organic complexes will control their speciation only at very high humic acid concentration (100 ppm). Moreover, it should be pointed out that if mixed complexes (organic ligand-cation-inorganic ligand (OH, C0 )) are supposed to be formed under these conditions (Table Π), the speciation will be entirely changed in favor of these latter. Hence, it shows the necessity of studying the formation of organic complexes under conditions closer to real systems (in particular high pH) which implies a need for very sensitive techniques in order to work with actinides in these alkaline media. Moreover, these kinds of investigations should also permit to confirm the existence of mixed complexes which should then modify the actinide speciation. Their existence has been reported in (13) in the case of Eu . 2

2

3

3+

Retention of Colloids and Heavy Elements on Mineral Surfaces Laboratory experiments with model systems under static conditions have been aimed at the determination of the retention mechanisms of colloids and pseudocolloids (association of a heavy element with a colloid) onto mineral surfaces. This will give a better understanding of the fate of radioéléments associated with colloids upon interaction with mineral surfaces as it will occur in the water flow acrossfissuresandfracturesaround a radwaste repository. In these studies, polished cm-sized monoliths are used to simulate macroscopic surfaces of fine particles or as mineral surfaces. Rutherford Backscattering Spectrometry (RBS) is the technique chosen to determine accurately the amount of elementsfixedon the monolith. In Humic and Fulvic Acids; Gaffney, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

16. MOULIN ET AL.

Humic Substances and Behavior of Radiotoxic Elements 265

Table Π. Main characteristics of waters representative of granitic and sedimentary formations (11) pH

logpC0 (atm) 2

DOC* (mg/l)

CRYSTALLINE FORMAHONS

mine waters evolved waters

7-8 8-9

Boom clay

-8.5

-3 -» -2 -5^-4

1 - 10 1 - 10


SEDIMENTARY FORMAnoNS

~ -3

~ 150

*DOC = Dissolved Organic Carbon; the humic acid concentration has been calculated from these data assuming that 50% of the organic carbon is constituted by humic substances and that these latter contain 50% of carbon. Description of the technique. The choice of the RBS analytical technique derivesfromits highly quantitative character together with its good sensitivity for heavy elements (15). Moreover, it is a non-destructive method permitting an elemental depth profiling with a good depth resolution (

Humic and Fulvic Acids - American Chemical Society

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