Talanta ELSEVIER
Talanta 44 (1997) 781-786
Europium complexation by an aquatic fulvic acid - effects of competing ions Maria Nord6n, James H. Ephraim, Bert Allard Department o[ Water and Environmental Studies, Link6ping University, S-581 83 Linkdping, Sweden
Received 15 December 1995; received in revised form 10 September 1996; accepted 16 September 1996
Abstract Effects of competing ions, Fe2+/Fe 3+ and Al3+, o n E u 3 + complexation with an aquatic fulvic acid (FA), have been investigated using an ion exchange technique. The influence of different concentrations ( 1 0 - 6 10--4 M) of the competing ions on the distribution coefficient for Eu was measured, and the overall complex formation function, ~ov, was resolved for the Eu systems with Fe and A1. All systems showed pH-dependent/~o~-functions. The presence of 10 - 4 M concentration of competing ion reduced the resolved complex formation function (log flo,) for Eu complexation with fulvic acid by 0.6 and 0.4 log units at pH 5 for Fe and A1, respectively. This indicates that Fe has a more perturbing effect on E u - F A complexation than AI. In similar competition studies Sr and Eu were found not to perturb each others complexation with fulvic acid, suggesting therefore that the two metals probably bind to different sites on the fulvic acid molecule. © 1997 Elsevier Science B.V. Keywords: Competing ions; Europium; Fulvic acid; Ion exchange technique
I. Introduction Humic substances are ubiquitous in natural waters and have significant effects on metal speciation and mobility in aquatic environments [1]. The metal complexes formed with humic substances are influenced by pH, ionic strength, concentrations of metal and humic matter and presence of competing ions. Few studies that investigate the effects of competing metals have been carried out [2 5]. The influence of Ca 2 + [2-4], Mg 2+ [2,3] and Cd 2 + [5] on Cu 2 + binding by naturally occurring dissolved organic matter are some of the few investigations where competition effects are considered. Usually in these studies only minor
effects are observed. In a more recent work A13 + was found to compete with Cu 2 + for binding sites in a fulvic acid [6]. The overall objective of this study has been to quantify the perturbing effects of Fe, AI and Sr on the Eu-FA complexation. Europium was chosen as a model element for trivalent radionuclides that might be released into the environment from, e.g. radioactive waste in geologic deposits [7]. Aluminium and Fe are c o m m o n metals in the environment, and therefore likely competitors. Aluminium can be found in fresh waters in the concentration range of 10 -6 10 . 4 M and Fe in the concentration range of 1 0 7 - - 1 0 - 4 M [1]. Strontium is present in radioactive waste and its
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competition with Eu is therefore of interest. The conditions of the experiments (e.g. FA concentration, ionic strength) were chosen to resemble the experimental conditions in an earlier paper [8] in order to facilitate a meaningful comparison (Table 1). It has not been the goal to determine the selectivity coefficients for Eu, A1 and Fe over the ion-exchange resin.
2. Experimental 2. I. M a t e r i a l s
A well-characterised aquatic fulvic acid (FA), extracted from the surface water of a bog area (Bersbo, Sweden), was used in all experiments. The potentiometric properties of the F A molecule have previously been described by assuming five predominant acid sites [9]. The Fe and A1 content of the Bersbo FA was 0.18 and 1.87 mg g-~, respectively (by Atomic Absorption Spectrophotometry). Other characteristics of the Bersbo FA are given elsewhere [8,9]. Radionuclides (152Eu and 85Sr; from Amersham) and analytical grade chemicals with Milli-Q water were used for all solutions. The non-radioactive Eu and Sr were prepared from E u 2 0 3 and Sr(NO3) 2 (Merck), respectively, and the A1 and Fe mixtures from A1C 3 × 6H20 and FeC12 × 4H20. The sodium form of a Dowex 50WX8 (mesh 50-100) cation exchange resin was used as adsorbent (see below). A radiometer p r i M 82 standard pH-meter (precision + 0.02 pH units) and a pH-combination glass electrode, G K 2401C, were employed for pH determinations. Calibration was carried out with two buffer solutions and the p H values are given as concentrations. Radioactivity measurements were made using a LKB (Wallac) 1282 Compugamma counter. 2.2. Procedures
The ion exchange distribution experiments were carried out batch-wise by a radiotracer technique (152Eu and 85Sr). The additional competing ions (Fe, A1, Eu or Sr) were non-active (no radio-
tracer), and the activities in the solutions with and without competing ions were compared. The ion exchange procedure for studies of metal complexation with humic substances, and the mode of computation of the overall complex formation function, flov, from the distribution coefficient, D, have been outlined in a previous work [8]. Constant ionic strength (0.10 M NaC104), constant total concentration of FA (120 mg 1-1, corresponding to a total acid capacity of 5.6 × 10 -4 eq 1-1 and 6.9 × 10 -5 M with Mn = 1750) and constant concentrations of the complexed elements Eu and Sr were selected. The competing elements (Fe, A1, Eu or Sr) were added without absolute exclusion of air, and therefore some of the Fe would be oxidised to the trivalent state in the absence of FA and consequently Fe(III) hydrolysis products will form with increasing pH. Since FA is known to reduce Fe(III) to Fe(II) it is expected that the Fe(II) will be maintained in the divalent state in the FA systems, even in the presence of air [10-14]. The metal and metal-FA solutions were pH adjusted with HC10 4 or N a O H and left to equilibrate for 20 _+ 3 h prior to addition to the ion exchange resin. This mode of pH adjustment is different from the one used in an earlier paper [8], where the pH was adjusted after aliquots of bulk solutions had been added to the resin. The pH adjustment was altered to accomplish an equilibrium between metal ions and ligands before the ion exchange reaction with the resin. The pH values that were determined after the equilibrium with the resin were used in calculations of log D and log flov. Samples of 5 ml were Table 1 Experimental systems (a) Complexing metal Eu, 5.0 × 10 - 9 M (including 152Eu) Ligand FA, 5.6x 10 4 eq 1-I (120 mg 1 i) Competing metal Fe; 10 6, 10 4 M Sr; 10 s 10-6, 1 0 - 4 M Ion exchange resin 1.0 g l-t (b) Complexing metal Ligand Competing metal Ion exchange resin
Sr, 3.0 )<10 6 M (including 8SSr) FA, 5.7× 10 - 4 eq 1-1 (122 mg l -I) Eu; 10 8, l0 6 10-4 M 10.0 g I-~
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M. Nordbn et al. / Talanta 44 (1997) 781 786
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5
5
4
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3
3
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4
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A.
1
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I
6
8
10
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4
6
8
10
pH
pH 6
6 C
d
5
5
4
4 3 81~
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2
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4
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Fig. 1. The effect of different Fe and A1 concentrations on the distributions of Eu over a cation exchange resin in the absence and presence of FA. [] Eu; II, E u + F A ; (a) Fe(10 - 6 M), ~ Eu; • E u + F A : (b) AI(10 6 M), A Eu; • E u + FA, (c) Fe(10 4 M), © Eu; • E u + FA; and (d) AI(10 - 4 M), + E u ; - E u + F A .
transferred from each container to three tubes (duplicate samples and one control with no resin). The tubes were equilibrated for 20 + 3 h on a shaking-table ( ~ 110 rpm) at ambient temperature ( 2 0 + I°C). After equilibrium, the pH of each metal-resin mixture was determined. The mixture was centrifuged for about 10 min at 2000 rpm ( ~ 3 5 0 g), and 1 ml of the clear solution was withdrawn for radioactivity measurement. Duplicate samples were" taken. The experimental systems are summarised in Table 1.
3. Results and discussion
The distribution of Eu over the ion exchange resin is illustrated in Fig. 1. Data from the Eu and E u + F A series (without competing ions) are given in all diagrams for comparison. The distribution coefficient, D, was independent of pH in the pH-range 3 - 9 for Eu in the absence of FA. This observation differs form an earlier observation where log D decreased with an increase in pH [8]. The discrepancy in the results is attributable to the difference in the method of pH adjustment.
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In the Eu + FA series the distribution coefficient decreased from 2.8 log units in the pH range 3 - 6 to a constant value of 0.8 log units at pH > 6.
3.1. The effects of Fe on the Eu distribution The effects of Fe at 10 - 6 and 10 4 M, respectively, are shown in Fig. la and c. In the presence o f 10 _6 M Fe the distribution coefficient of Eu was the same as in the absence of Fe in the pH range of 3 to 5.5. However, at p H > 5 . 5 the decrease in log D (0.1 to 0.4 log units) was increasing with an increase in pH (Fig. la); i.e. more Eu stayed in solution in presence of 10 6 M Fe at pH > 5.5. The Fe would be present predominantly in the trivalent state, and the decrease may be caused by the formation of colloidal Fe(OH)3 that can adsorb Eu and keep it in solution. Studies of Eu adsorption on colloidal Fe(OH)3 have previously shown a pronounced Eu adsorption at pH > 5 [15]. The present centrifugation procedure to separate the ion exchange resin from the solution would not be sufficient to completely separate colloids from the true solution [15]. For the higher Fe concentration (10 -4 M) the distribution coefficient was increasing at pH > 5 6 (Fig. lc). This increase reached a maximum of 2.2 log units at a pH around 7. This would be due to aggregation of colloids and the formation of solid Fe(OH)3 which together with the resin can serve as the adsorbing surface for Eu. Additionally, the increase in log D at higher pH could be attributable to the interaction of the solid Fe(OH)3 with the ion exchange resin to produce a new surface with a higher affinity for Eu. Calculations of the amount of solid Fe(OH)3 at the two total concentrations (10 - 6 and 10 4 M Fe) indicate that at the low concentration the solid phase would be only approximately 0.01% (pH-range 5-9) of the weight of the ion exchange resin, while at the higher concentration the corresponding value would be 1.0% for the same pH range ( a s s u m i n g Ksp = 4 x 10 -38) [16].
3.2. The effbcts of Al on the Eu distribution Aluminium seemed to influence the adsorption of Eu on the resin in a similar way as Fe (Figs, 1b
and d). In absence of FA 10 6 M A1 decreased log D of Eu by 0.3 log units in the pH interval 3 to 9. At the higher A1 concentration (10 -4 M) a sharp increase in the distribution coefficient was obtained at pH > 6 (Fig. ld), with a maximum of 2.1 log units at a pH around 7.5. The effect was similar to that of Fe(10 4 M). A plausible explanation to the increase can be the formation of solid AI(OH)3 (similar to the formation of Fe(OH)3) that can act as an adsorbent for the Eu. Calculations indicate that the formation of solid AI(OH)3 at the low A1 concentration would be up to 0.007% (pH-range 5.5-9) of the weight of the ion exchange resin. For the higher concentration in the same p H range the formation of AI(OH)3 would be 0.7% of the resin weight (assuming K s p = 2 × 10 -32) [16].
3.3. The effects of competing metals on Eu-FA and Sr-FA complexation No difference in distribution coefficient was discernible for the Eu-FA system in the presence of 10-6 M Fe (Fig. l a). This could imply that 10 6 M Fe does not affect the complexation of Eu with FA. However, it could also imply that since there is excess of FA both Fe and Eu can be complexed without effecting each other. At the higher concentration of Fe (10-4 M) there was a slight increase in l o g D (0.1 to 0.6 log units) between pH 4 and 7 which may be explained by the competition of Fe for the FA binding sites (Fig. lc). In the presence of FA at the lower A1 concentration (10 6 M) no influence was discernible (Fig. lb), just as for Fe at the same concentration. With FA and 10-4 M A1 in the system there was a slight increase in log D (0.i to 0.4 log units) in the pH range 4 - 7 (Fig. ld). In another competition study the influence of Sr on the Eu-FA complex, but also the influence of Eu on the Sr-FA complex, were investigated (Fig. 2). In one set of experiments inactive Sr 2+ w a s added to active Eu 3+ plus FA solution, in a typical ion exchange experiment as described earlier [8]. In the other set inactive Eu 3 + was added to active Sr 2 + plus FA solution, in a similar ion exchange distribution experiment. In the case of
785
M. Nord~n et al. ~Talanta 44 (1997) 781 786
Table 3 Comparison of formation constants for iron, aluminium and europium fulvic acid complexes
5 4 A¢¢
3
.A.¢
.
¢
Metal ion
pH
logfl (1 eq ~1
Reference
Fe(ll)
4.5 6.0 3.5 5.0 2.35 n.g. a 4.5 4 6 4 7 2 . 7 6.5 3.0 3.5 4.5 5.0 6.O
5.4 5.6 5.06 5.77 3.7 5.44 6.90 6.65 8.15 5.10 6 6.06 6.43 7.17 7.54 8.28
[17]
•
m
2
AI(III) 1
Eu(Ill) o
I
I
I
I
4
6
8
10
pH Fig. 2. The effect of different Sr concentrations on the distribution of Eu and the effect of different Eu concentrations on the distribution of Sr, over a cation exchange resin in the presence of FA. 0 E u + FA; • E u + Sr(10-8 M ) + FA; [] Eu+Sr(10 -6 M ) + F A ; • Eu+Sr(10 -4 M ) + F A ; © S t + F A , • S r + E u ( 1 0 - 8 M ) + F A ; & Sr+Eu(10 6 M ) + F A ; • Sr+ Eu(10 4 M ) ~ FA. E u - F A c o m p l e x a t i o n Sr a f f e c t e d the o b s e r v e d log D o f E u o n l y w h e n its c o n c e n t r a t i o n w a s i n c r e a s e d to 1 0 - 4 M, s u g g e s t i n g t h a t c o m p e t i t i o n was o n l y significant at t h a t c o n c e n t r a t i o n level. W h e n E u a c t e d as t h e c o m p e t i n g i o n n o effects o f i n c r e a s i n g c o n c e n t r a t i o n w e r e n o t i c e d f o r t h e dist r i b u t i o n o f Sr o n the i o n e x c h a n g e resin. T h i s i n d i c a t e s t h a t n e i t h e r E u n o r Sr i n f l u e n c e s the others complexation with FA. T h e r e l a t i o n s h i p b e t w e e n the o v e r a l l c o m p l e x f o r m a t i o n f u n c t i o n , flov, a n d p H , c a l c u l a t e d f r o m d i s t r i b u t i o n d a t a are s u m m a r i s e d in T a b l e 2. I r o n
[181 [19] [2(I] [21] [22]
[231 [8]
fl implies both conditional stability constants and overall complex formation functions. a n.g., not given. at 1 0 - 4 M d i s t u r b s t h e E u - F A b i n d i n g m o r e t h a n A1 at t h e s a m e c o n c e n t r a t i o n in the p H i n t e r v a l 4 to 6, e.g. l o g flov = 4.60 f o r F e a n d 4.86 f o r A1, as c o m p a r e d w i t h the o r i g i n a l v a l u e o f 5.22. A t a c o n c e n t r a t i o n o f 1 0 - 6 M b o t h F e a n d A1 seem to d i s t u r b t h e E u - F A b i n d i n g to an e q u a l l y l o w d e g r e e , logfl,,v = 5.03 a n d 5.04 f o r F e a n d A1, respectively. It is difficult to m a k e a m e a n i n g f u l c o m p a r i s o n o f t h e stability c o n s t a n t s o f Fe, A1 a n d E u c o m plexes w i t h fulvic acids b e c a u s e o f a n u m b e r o f
Table 2 The overall complex formation function, flov, for five E u + F A systems described by polynomials (120 mg 1 ~ FA, I=0.10 M NaC104 M) System Eu+FA Eu+Fe(10 -6 Eu+AI(10 -6 Eu+Fe(10 -4 Eu+AI(10 -4
M)+FA M)+FA M)+FA M)+FA
pH
r
n
log flov (PH) b
log []~, IpH = 5)
3 9 3 9 3-7 3-7 3 7
0.997 0.998 0.998 0.992 0.996
30 28 30 30 30
10.683--6.267 pH+2.000(pH)2-0.2452(pH)3+0.01043(pH) 4 15.159-10.084 pH+3.109(pH)2-0.3813(pH)3+0.01637(pH) 4 22.221-16.804 pH+5.359(pH) z 0.7000(pH)3+0.03259(pH) 4 5.593-1.233 pH+0.2861(pH) 2 0.01584(pH) 3 7.225-2.257 pH+0.5341(pH) 2 0.03544(pH) 3
5.22 5.03 5.04 4.60 4.86
r, correlation coefficient and n, number of experimental points.
b For the best fit of the polynomials (indicated by r) to the experimental points all given decimal points of the coefficient must be used. c As an example the log flo, function described with a polynomial is used for calculating log flo, at pH = 5.
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M. Nord~n et al. / Talanta 44 (1997) 781 - 786
factors, e.g. pH, ionic strength, method, m o d e o f c o m p u t a t i o n and the origin o f the FA. However, in Table 3 an attempt is made to c o m p a r e literature values for Fe 2÷ and A13 ÷ fulvic acid complexes with the E u - F A complexation data. The conditional stability constants for the E u - F A complex are in general apparently greater than the constants for the Fe 2 ÷ and A13 ÷ complexes. The conditional stability constant for Fe 2÷ yields a m e a n value o f 5.5 in the p H interval 3.5-6.0, which is considerably lower than m o s t values for E u - F A in the same p H interval (Table 3). However, the concentration o f Fe ( 1 0 - 4 M) in the present study is m u c h greater than for Eu (5 × 10 - 9 M ) which can explain the observed competition f r o m Fe.
4. Conclusions The experimental results f r o m this w o r k corr o b o r a t e the observation that an increased p H augments the complexation between F A and Eu, but that the Sr complexation is insensitive to p H (in the p H range 3 - 8 ) [8]. I r o n and AI exhibited some influence on the E u - F A complexation. However, the effect was m i n o r at 10 - 6 M. The concentration 10 4 M gave a clear effect (e.g. reduction o f log flov by 0.6 and 0.4 log units at p H 5 for Fe and A1, respectively). K n o w n f o r m a t i o n constants for F A with Fe 2 +, m l 3 + and E u 3 + suggests that Eu 3 + forms stronger complexes with F A than Fe 2 ÷ and A13 +. This m a y explain why as m u c h as 10 - 4 M o f the competing ions is needed to affect a clear disturbance o f the E u - F A complex. A n o t h e r feasible conclusion for the high concentration o f competing ions required is that Eu and, Fe and AI, do not necessarily complex to the same site on the F A molecule. Since Sr did not influence the EuF A complex, nor did Eu influence the S r - F A complex, it is also postulated that Sr and Eu p r o b a b l y bind to different sites on the F A molecule. However, these conclusions need to be supported by further experiments, e.g. investigation o f the effect o f different F A concentrations. Such exercises are in progress.
Acknowledgements This w o r k was supported by the Swedish N u clear Fuel and Waste M a n a g e m e n t C o m p a n y and the Swedish N a t u r a l Science Research Council. The assistance by M r Anders Diiker in performing the metal analysis o f the fulvic acid is gratefully acknowledged.
References [1] J. Buffle, Complexation Reactions in Aquatic Systems, Ellis Horwood, Chichester, 1988. [2] W.G. Sunda and P.J. Hanson, in E.A. Jenne (Ed.), Chemical Modeling in Aqueous Systems, American Chemical Society, Washington DC, 1979, p. 147. [3] S.E. Cabaniss and M.S. Shuman, Geochim. Cosmochim. Acta, 52 (1988) 185. [4] J.G. Hering and F.M.M. Morel, Environ. Sci. Technol., 22 (1988) 1234. [5] W. Fish, Ph.D. Modeling the interactions of trace metals and humic materials, Dissertation, MIT, Cambridge, MA, 1984. [6] S.E. Cabaniss, Environ. Sci. Technol., 26 (1992) 1133. 17] International Atomic Energy Agency, IAEA Yearbook 1991, Vienna, 1991. [8] M. Nord6n, J.H. Ephraim and B. Allard, Talanta, 40 (1993) 1425. [9] J.H. Ephraim, H. Bor6n, C. Pettersson, I. Arsenie and B. Allard, Environ. Sci. Technol., 23 (1989) 356. [10l D.T. Waite and F.M.M. Morel, Anal. Chim. Acta, 162 (1984) 263. [11] C.H. Langford, R. Kay, G.W. Quance and T.R. Khan, Anal. Lett., 10 (1977) 1249. [12] R.K. Skogerboe and S.A. Wilson, Anal. Chem., 53 (1981) 228. [13] J.H. Ephraim, A.S. Mathuthu and J.A. Marinsky, SKB TR 90-28, Swedish Nuclear Fuel and Waste Mangement, Stockholm, 1990. [14] C.J. Miles and P.L. Brezonik, Environ. Sci. Technol., 15 (1981) 1089. [15] A. Ledin, S. Karlsson, A. Diiker and B. Allard, Radiochim. Acta, 66/67 (1994) 213. [16] D.A. Skoog and D.M. West, Fundamentals of Analytical Chemistry, Holt-Saunders, Philadelphia, 1982. [17] R.L. Malcolm, in B.W. Nelson (Ed.), Environmental Framework of Coastal Plain Estuaries, Memoir 133, The Geological Society of America, Boulder, 1972, p. 79. [18] M. Schnitzer and S.I.M. Skinner, Soil Sci., 102 (1966) 361. [19] M. Schnitzer and E.H. Hansen, Soil Sci., 109 (1970) 333. [20] M. Adhikari and G. Chakrabarti, J. Indian Chem. Soc., 54 (1977) 573. [21] E.L. Bertha and G.R. Choppin, J. Inorg. Nucl. Chem., 40 (1978) 655. [22] J.H. Ephraim, Sci. Tot. Environ., 108 (1991) 261. [23] G. Bidoglio, I. Grenthe, P. Qi, P. Robouch and N. Omenetto, Talanta, 38 (1991) 999.