SPECTROSCOPIC STUDIES OF THE INTERACTION OF Eu(III) WITHTHE ROOTS OF WATER HYACINTH

COLLEEN KELLEY∗, ABIGALE J. CURTIS, JENNIFER K. UNO and COURTNEY L.BERMAN

Department of Chemistry, Northern Arizona University, Flagstaff, AZ 86011-5698, U.S.A.(∗ author for correspondence, e-mail: colleen.kelley@nau.edu)

(Received 30 March 1998; accepted 6 May 1999)

Abstract. The water hyacinth is a plant currently being used throughout the world, including siteswithin 1 km of the Chernobyl nuclear accident, for the removal of toxic metals from water. Wehave recently shown that the roots of water hyacinth will remove large quantities of Eu(III) fromwater. In this study we were able to determine that carboxylic acids are responsible for binding theintracellular proportion of Eu(III) in the roots of water hyacinth using the techniques of NuclearMagnetic Resonance (NMR) and Infrared (IR) spectroscopies.

Keywords: europium, phytoremediation, speciation, spectroscopy, water hyacinth

1. Introduction

The water hyacinth is a plant currently being used throughout the world, includingsites within 1 km of the Chernobyl nuclear accident, for the removal of toxic metalsfrom water (Macaskie, 1991). We are interested in the capacity of an aquatic mac-rophyte, the water hyacinth, to remediate fresh water sources contaminated withthe lanthanide metal, europium. The fate of lanthanide metals in the subsurfaceenvironment has become a focus of attention since the recent mandate for cleanupof waste at nuclear fuel processing sites and weapons productions facilities (Rileyet al., 1992). We have chose to study Eu(III) as a model for trivalent radionuclidesthat might be released into the environment from radioactive waste (InternationalAtomic Energy Agency, 1991). Europium is also useful in NMR spectroscopicstudies as it has the ability to induce dramatic changes in the NMR chemical shiftsof nearby nuclei (Silversteinet al., 1991).

Water hyacinth was chosen for several reasons. Water hyacinth has been shownto accumulate high concentrations of toxic metals (O’Keefeet al., 1984; Leeetal., 1987; Lowet al., 1990; Haoet al., 1993; Delgadoet al., 1993; Lowet al.,1994), and is currently being used to remediate sites contaminated with metals(Wolverton, 1975; Chignoet al., 1982; Muramotoet al., 1983; Gonzalezet al.,1989; Watanabeet al., 1997). In addition, these plants are easy to grow, propagatereadily, and their large biomass facilitates handling and tissue manipulations.

Water, Air, and Soil Pollution119: 171–176, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

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We have recently shown that water hyacinth will accumulate large quantitiesof europium from water (Kelleyet al., 1999). The accumulation of europium wasshown to occur predominantly in the root material. These findings are similar towhat has been found for the biosorption of other metals by roots of water hyacinth(Low et al., 1994; Haoet al., 1993). The objective of this study was to speciatethe extractable Eu(III) in root tissue of water hyacinth using Nuclear MagneticResonance (NMR) and Infrared (IR) techniques.

2. Experimental

2.1. APPARATUS AND REAGENTS

All chemicals were purchased from indicated sources and used without furtherpurification. Infrared (IR) spectra were recorded on a Buck Scientific Inc. Model500 Infrared Spectrophotometer and a Mattson Instruments 2020 Galaxy SeriesFT-IR at Arizona State University in the transmittance mode. Nuclear MagneticResonance (NMR) spectra were obtained on a Varian Gemini 200 MHz NMR, aVarian Unity Plus 400 MHz NMR at Arizona State University, and a Varian Unity500 MHz NMR at Arizona State University.

2.2. WATER HYACINTH EXTRACT

The water hyacinth,Eichhornia crassipes, was obtained from a local nursery andcultivated under aquatic greenhouse conditions (Niret al., 1990). All plants wererinsed thoroughly with deionized, distilled water prior to investigation. Mature,healthy plants were harvested and allowed to dry for 4–6 days. When dried, theroots of the plant were isolated and ground into a powder in preparation for extrac-tion. The ground roots (48.5 g) were placed into a 2 L percolator to be extractedwith 1 L of HPLC grade ethyl acetate (Grecaet al., 1991). After 24 h, the extractwas collected and evaporated to dryness. 0.105 g of extract was obtained for a0.22% yield.1H NMR samples were prepared by dissolving the extract in CD3OD.IR samples were prepared by mixing the extract with nujol.

2.3. CONTAMINATION OF WATER HYACINTH ROOT EXTRACT WITH

Eu(NO3)3·6H2O

Mature, healthy plants of like size and mass were contaminated with Eu(NO3)3·6H2O(Aldrich) as follows: after being rinsed of soil and debris with deionized, distilledwater, each plant was placed in a solution of 90 ppm Eu(NO3)3·6H2O and stirredfor 30 min (Dinget al., 1994). The plants remained in the Eu(NO3)3·6H2O solutionfor six days. At the end of six days, the plant roots were dried and extracted asdescribed above (Section 2.2).1H NMR samples were prepared by dissolving theextract in CD3OD. IR samples were prepared by mixing the extract with nujol.

INTERACTION OF Eu(III) WITH THE ROOTS OF WATER HYACINTH 173

Figure 1. Comparison of the1H NMR (CD3OD) spectra of the extract of water hyacinth root(spectrum B) and the extract of water hyacinth root contaminated with Eu3+ (spectrum A).

3. Results

A comparison of the1H NMR spectra of both the contaminated and uncontam-inated root extracts are shown in Figure 1. The arrows indicate areas that differbetween the two. Comparisons of the IR spectra of the contaminated and un-contaminated root extracts are shown in Figure 2. Significant differences in thestretching frequencies of organic acids (νC=O andνO−H) are indicated.

4. Discussion and Conclusions

It has been demonstrated that water hyacinth will accumulate Eu(III) from water,and that the Eu(III) was shown to be predominantly in the root material (Kelleyetal., 1999). In this paper, we are interested in speciating the extractable root Eu(III),which most likely corresponds to intracellular Eu(III). Our procedure for extractionenables us to look at the Eu(III) bound to the organic molecules found in the cellularmatrix of water hyacinth roots.

A comparison of the IR spectra of both the contaminated (A) and uncontamin-ated (B) root extracts are shown in Figure 2, illustrating the loss of the IR bandsassociated with carboxylic acid functionalities (νC=O andνO−H) upon complexationwith Eu(III). A new, weak band appears at 1642 cm−1 in the spectra of the contam-

174 C. KELLEY ET AL.

Figure 2.Comparison of the IR (nujol) spectra of the extract of water hyacinth root (spectrum B) andthe extract of water hyacinth root contaminated with Eu3+ (spectrum A).

inated extract (A) indicative of metal-carboxylate binding (Mehrotraet al., 1983).A schematic for the formation of an Eu(III)-carboxylate complex in the roots ofwater hyacinth is illustrated in Figure 4.

The 1H NMR data shown in Figure 1 also suggests that organic acids are in-volved in the binding of Eu(III) in the roots of water hyacinth. The differencesfound in the area betweenδ 2 and 3 ppm coincides with the expected region forthe protons on the carbon adjacent to the carboxylic acid functionality to haveresonances in the1H NMR spectra. For example, acetic acid exhibits a singlet atδ 2.15 ppm in its1H NMR spectra (Pouchet, 1983). One can envision a shift and

INTERACTION OF Eu(III) WITH THE ROOTS OF WATER HYACINTH 175

Figure 3.Complexation of europium by an organic acid would result in a shift and broadening in the1H NMR spectrum of the protons adjacent to the carboxylic acid functional group. The1H NMRspectra depicting this are shown in Figure 1.

Figure 4. Complexation of europium by an organic acid would result in a loss of theνC=Oand νO−H stretching frequencies. Complexation would form a Eu-carboxylate complex having ametal-carboxylateνO−C−O stretching frequency.

broadening of this resonance upon complexation with europium, as depicted inFigure 3. The spectra shown in Figure 1 suggest such a phenomenon has occurred.These results coincide with the results found by Choppin in which the protons adja-cent to the carboxylic acid in diethylenetriaminepentaacetate (DTPA) are found asa broad singlet atδ 3.1 ppm. Complexation of DTPA with La(III) results in a shiftand a broadening of this peak to a quartet atδ 3.2–3.5 ppm (Choppinet al., 1979).Similar results can be found in the Eu(III)-DTPA complex in which the protons onthe carbon atom adjacent to the carboxylic acid groups in this complex are foundbetweenδ 2 and 3 ppm (Jenkinset al., 1988).

Future studies include the speciation of the proportion of Eu(III) adsorbed tothe roots of water hyacinth. For these studies we will use X-ray absorption spec-troscopy and scanning electron microscopy.

We can conclude from these studies that the extractable root Eu(III), which mostlikely corresponds to intracellular Eu(III), is complexed to organic acids. This wasconfirmed by NMR and IR spectroscopy.

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Acknowledgements

This work was funded in part by the Department Of Energy through The Historic-ally Black Colleges and Universities/Minority Institutions Environmental Techno-logy Consortium.

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