Determination of the free-ion concentration of rare earth elements by an ion-exchange technique: implementation, evaluation and limits

Transcription

1 Environ. Chem. 2016, 13, CSIRO 2016 Supplementary material Determination of the free-ion concentration of rare earth elements by an ion-exchange technique: implementation, evaluation and limits Sébastien Leguay, A Peter G. C. Campbell A and Claude Fortin A,B A Institut National de la Recherche Scientifique, Centre Eau Terre Environnement, 490 rue de la Couronne, Québec, QC, G1K 9A9, Canada. B Corresponding author. fortincl@ete.inrs.ca Evaluation of the column equilibration method A necessary condition for determining the free metal ion concentration by using the ion-exchange technique is that equilibrium must be reached between resin-bound metal and the free metal ion concentration in solution. In the column-equilibration approach, the solution is passed through the column with a constant flow rate until the equilibrium is achieved. One of the attractive aspects of the IET is the low cost of this method. The stronger the affinity between the resin and the metal, the higher the required volume to attain the equilibrium. In the case of Eu 3+, a high specific affinity for the resin was anticipated owing to its high valence state (+3). The evaluation of the column equilibration was thus performed in a fairly strong electrolyte in order to reduce the interactions between Eu 3+ and the resin (0.1 M NaNO 3; ph 4.0) at 5 and 2 ml min 1 (n = 3) with a calibration solution containing 7.7 (±0.2) 10 8 mol L 1 of europium. Fig. S1 represents the Eu concentration of the solution coming out (effluent) of the column as a function of volume of the calibration solution passed through the resin. Whatever the flow rate, the Eu concentration rapidly increased from 0 to 15 ml of solution (first step) and then remained constant (from 15 to 300 ml). The observation of a plateau generally indicates that equilibrium has been reached, but this is not the case in the present work because the concentration of the solution coming out is lower than the initial concentration ([ ). ini Eu] tot The rapid increase in the Eu concentration within the first 15 ml of effluent may reflect the presence of two types of sites: the outer sites that are more accessible would be filled first, and then the lessaccessible inner sites would gradually be occupied by the europium in the solution diffusing into the resin pores. Additional experiments (n = 3) implementing the same experimental design (but without resin) have shown that the first step was not caused by europium adsorption onto the tube walls, and seem to confirm our hypothesis of a two-step binding process. Moreover, the proportion of exchanged europium increased as the flow rate decreased, suggesting that the plateaus were due to diffusion-limited Eu exchange from the bulk solution to the resin or through the resin pores (Fig. S2). The decrease in flow Page 1 of 6

2 Environ. Chem CSIRO 2016 rate increases the contact time between the solution (inside the column) and the resin, and improves metal ion exchanges with the resin. However, even at a very low flow rate (0.5 ml min 1 ), the exchange was never complete (Table S1). The use of the column equilibration approach thus appears very tedious, despite the use of a high concentration (0.1 M) of counter-ions (Na + ), probably owing to the slow kinetics of exchange between metal ions in solution and the resin binding sites. Time required to equilibrate the cation exchange resin (batch-approach) 10-7 [Eu] tot (mol L -1 ) Time (H) Fig. S1. Europium remaining in solution as a function of time in the batch approach ([ ~ M ; V = ini Eu] tot 50 ml ; m r ~9 mg) in the absence (black circles) or presence of 2 mg C L 1 of Suwannee River Humic Acid (blue triangles). Page 2 of 6

3 Environ. Chem CSIRO 2016 Table S1. eff tot Proportion of europium remaining in the solution eluting from the column (0.1 M NaNO 3 medium) The Eu was measured after ~50 ml of the calibration solution was passed through the column Flow rate (ml min 1 ) %ΔEu = ([Eu] ini [Eu] eff )/[Eu] ini % % 50 % 60 % [Eu] coming out (%) tot V (ml) Fig. S2. Effluent europium concentration as a function of the volume passed through the resin in the presence of ~9 mg of the polystyrene sulphonate resin with a flow rate of 5 ml min 1 (open squares) and 2 ml min 1 (black circles) (0.1 M NaNO 3 and ph 4.0). Page 3 of 6

4 Environ. Chem CSIRO 2016 Ion exchange technique: calibration data 1000 Eu (L g -1 ) ph Fig. S3. Eu distribution coefficient (λ Eu ) as a function of ph. Page 4 of 6

5 Environ. Chem CSIRO 2016 Model ligands: experimental data Table S2. Experimental conditions for the experiments performed with model ligands (MHSM M NaNO 3) BDL, below detection limit ph [Ligands] (mol L 1 ) Total lanthanide concentration (mol L 1 ) Fraction of free ion lanthanide (%) La Ce Nd Eu La Ce Nd Eu ini eq Ini eq ini eq Ini Eq NTA BDL BDL Malic acid E Page 5 of 6

6 Environ. Chem CSIRO 2016 Correction for the solution equilibrium disturbance [EuHA] eq (mol L -1 ) R 2 = 1.00 y = 0.831x [Eu] eq (mol L -1 ) tot Fig. S4. [EuHA] v. [Eu] as determined by using the repeat-batch approach (ph = 4.0; MHSM M NaNO 3 medium and [Eu] ini = (7.8 ± 0.5) 10 8 M). The solid line represents the best linear regression and the dashed lines define the 95 % confidence interval. Page 6 of 6