Susceptibility NMR microimaging of heavy metal uptake in alginate biosorbents

Magnetic

Resonance Imaging, Vol. 14, Nos. 7/8, pp. 905-906, 1996 Copyright 8 1996 Elsevier Science Lnc. Printed in the USA. All rights reserved 0730-725x/96

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Short Communication SUSCEPTIBILITY NMR MICROIMAGING OF HEAVY UPTAKE IN ALGINATE BIOSORBENTS

METAL

N. NESTLE’

AND R. KJMMICH Universitit Ulm, Sektion Kernresonanzspektroskopie, D-89069 Ulm, Germany Susceptibility NMR microimaging is introduced as a new method for quantitative mapping of the paramagnetic-ion concentration in ion exchange materials such as aiginate biosorbents. Sharp ion intrusion fronts are observed, suggesting nonFickian diffusion. Copyright 0 1996 Elsevier Science Inc. Keywords:

NMR microhnaging;

Magnetic susceptibility;

INTRODUCTION

Ion exchange; Alginate.

c=-

3 AB(x, Y> Bo ’ x:

As we have already demonstrated in previous paoffers a very convenient pers, Is2NMR microimaging possibility to study spatial and temporal aspects of ion exchange processes. However, when using conventional NMR-imaging techniques, one obtains images in which contrast is due to relaxation time weighting. Therefore, to give quantitative interpretations of such images, detailed knowledge is required concerning the dependence of relaxation rates on the ion concentration. When one of the ion species involved in the exchange process exhibits a permanent magnetic dipole moment, a much more direct approach to establishing the spatial distribution of ions is possible by mapping of the magnetic susceptibility. For this kind of experiment, we are using the resonance offset imaging (ROI) sequence 3 shown in Fig. 1. In this sequence, resonance frequencies are probed by incrementing an evolution delay before reading out the echo signal. Eight to 32 such phase-encoding increments are usually needed for a reasonable resonance-offset modulation. The local magnetic susceptibility of the ion exchanger is proportional to the respective concentration c of the pammagnetic centers. Therefore, the ion concentration can be calculated from the observed field offsets AB according to

with xu denoting the molar susceptibility of the ions. In our experiments, we used tube-shaped samples of Ca-alginate (2% w/v Kelco Manugel DJX, sample preparation is described in Ref. 2, which were exposed to a 1 mM solution of rare earth ions for about 10 h. Susceptibility images were acquired before the exposition and at the end of the intrusion experiment (Fig. 2). The progress of the ion intrusion was monitored by means of a standard gradient echo sequence which is less affected by flow artifacts. For the ROI-experiments, the circulation was switched off. As the concentration profiles along the direction of intrusion (Fig. 3) demonstrate, there is a surprising difference between Pr and Yb. While Pr shows a rather steep intrusion front which can be described by the shrinking core model (SCM), 4 the intrusion front of Yb is much flatter, thus suggesting that the process can’t be described by means of the SCM. Rather, an appropriate model for this situation must take into account broadening of the reaction zone by Fickian diffusion. These results are not just of academic interest: as rare earth ions are popular models for actinoid ions in ion exchange experiments, the differences we ob-

Address correspondenceto N. Nestle, Universiuit Uhn, Sektion Kernresonanzspektroskopie,D-89069, Ulm, Germany.

’ Present address: N. Nestle, Mpl D-70569 Stuttgart, Germany. 905

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Magnetic Resonance Imaging l Volume 14, Numbers 7/8, 1996

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echotime Fig. 1. Resonanceoffset imaging (ROT)3: phaseencoding of resonanceoffset information by steppingthe time delay (gray). served within the rare earth series suggest that one should be very careful in selecting the right model ions, as different sorption kinetics are to be expected. thank Dr. J. Weis for stimulating discussions and practical help concerning the ROI method and H. Wiringer and A. Beier for assistance in the course of the experiments as well as Kelco GmbH, Hamburg for the alginate samples. Acknowledgment-We

REFERENCES 1. Nestle,N.; Kimmich, R. NMR microscopyof heavy metal absorptionin calciumalginatebeads.Appl. Biochem.Biotechnol. 56:9- 17; 1996. 2. Nestle,N.; Kimmich, R. Proceedingsof the III. Intemational Symposiumon BiochemicalEngineering,Stuttgart; 1995. 3. Weis, J.; Frollo, I.; Budinsy, L. Magnetic field distribution measurementby the modified flash method. Z. Naturforsch.44a:1151-1154; 1989.

Fig. 2. Susceptibilityimagesof rare earthabsorptionin alginate gels:(a) pure Ca-Alginate tube, (b) sametube after 14 h exposition to 1 mM PrC&, (c) difference of the first two images, (d) difference image of an alginate tube exposed for 10 h to 2 mM YbC13.The bright bar correspondsto 1 mm.

0.035 0.03 0.025 cl mmol*ml 0.02 0.015 0.01

0.005 0 -0.005 0.2

0.7

1.2

1.7

Intrusion depth/m Fig. 3. Intrusion profiles of Pr (full line) and Yb (dashed) line into alginategel. Concentrationsas evaluatedfrom the susceptibilityimagesin Fig. 2.

4. Rao, M.G.; Gupta, A.K. Ion exchangeaccompaniedby ionic reactions.Chem. Eng. J. 24:181-190; 1982.