Investigation of Aluminum Binding to a Datura innoxia Material Using 27 Al NMR HONGYING XIA AND GARY D. RAYSON* Department of Chemistry and Biochemistry, Box 30001 MSC 3C, New Mexico State University, Las Cruces, New Mexico 88003 Al binding to biomaterial derived from cell wall fragments of the plant Datura innoxia has been investigated using solid- state 27 Al NMRspectroscopy. Carboxylate groups have been determined to be the responsible functionalities for Al 3+ binding to this material at pH 3.5. The binding of Al 3+ directly to the polysilicate structure of the immobilized biomaterial was also observed. At pH 5.0, direct binding of the Al 13 7+ polymer ion to the biomaterial was discovered. Carboxylate groups were also determined to be involved in the binding of Al 3+ at pH5.0. The presence of an additional octahedral Al-binding site was also suggested at the higher pH. Introduction Along with the public awareness of the role of metal ions in the environment,biologicallyderived materialsare attracting increased attentions for their low cost, high selectivity and high capacities in extraction of heavy metals from water systems. Specifically, Al toxicity has recently received scientific and public interest. (1, 2)Ofparticular interest has been its proposed contribution in Alzheimer patients (2). A variety of microorganisms including algae, bacteria, fungi, and yeast have been demonstrated to be capable of ac- cumulatingdifferent metals from aqueous solutions (3-13). Certain higher plant tissues and cultured plant cells have also been shown to adsorb metal ions dissolved in aqueous media (14-19). Previous studies in our laboratory have demonstrated the abilityofa Datura innoxia cellwallmaterial to remove heavy metal ions from aqueous solutions (16- 19). This material is derived from a plant belonging to the Solanaceae familyand isnative to southwestern United States and northern Mexico and has exhibited a tolerance to toxic metals. To efficiently apply the D. innoxia material for the reclamation and remediation of contaminated water, an understanding of the fundamental chemistry of the metal- binding processes is required. The identification of func- tionalities involved in the binding process and the chemical form of the bound metal is included in this level of understanding. Such information willenhance the accurate prediction on biomaterial-bindingcharacteristicsunder real- world conditions, provide guidelines for the selection of optimalbindingconditions,and give insight into the possible modification of native biomaterials for improved binding capacity and selectivity. Previous investigations using Eu(III)-luminescence have indicated that carboxylate and sulfate are the responsible functionalities for the metal ion binding to the D. innoxia material (17, 20-23). Using 113 Cd NMR, the involvement of carboxylate groups in the bindingofCd 2+ was demonstrated for thisand other biomaterials(24). In thispaper,thebinding of Al to the D. innoxia material will be characterized using solid-state 27 Al NMR. The 27 isotope ofAl is a favorable nucleus for direct NMR investigation with a 100% natural abundance and a high receptivity. The relative sensitivity of 27 Al is three orders higher than that of the 13 C nucleus. Because of the nuclear spin of 5 / 2, 27 Al possesses a quadrupole moment. Its resonances are therefore broadened, especially relative to those ofspin 1 / 2 nuclei. Fortunately,itsquadrupolemoment is relativelysmall. This results in relativelyhigh sensitivities. Because of this quadrupole moment, the line width of Al resonance signal is also sensitive to the symmetry and arrangement of ligands about the metal nucleus. The 27 Al nucleus exhibits a relatively broad chemical shift window of approximately450ppm (25). The combination ofline width sensitivityand broad chemicalshift window make 27 Al NMR a powerful probe for studying local metal-binding environ- ments. Experimental Section Cultured cell wall fragments from the anther of the plant D. innoxia were obtained by procedures described elsewhere (21). A portion of this material was subjected to an esterification processto remove anycarboxylate groups.This modification procedure has been described in detail else- where (26). Briefly,a 10.0gsample ofthe D.innoxia material wassoaked in 650mLofanhydrousmethanol(VWRScience, 99.8%). A5.4mLvolume ofconcentrated HCl(VWRScience, 36.5-38.9%) was then added to the suspension (0.1 M in HCl). Aliquotsofthe resultingsuspension were then removed periodically over a 3 day period. These samples and the finalproduct were subsequentlywashed with Nanopure water to remove excess HCl and methanol. Each biomaterial sample was then lyophilized for further study. Infrared absorption spectra have indicated no significant degradation ofthe cellwallmaterialother than the removalofcarboxylate groups by this procedure (24). Aportion ofthe D.innoxia cellfragmentswasimmobilized within a polysilicate matrix using a procedure that has also been described elsewhere (19). Briefly, a 6% solution of sodium m -silicate (Fisher)was added to 300 mLof5%H 2SO4 (Mallinckrodt) to adjust the solution to pH 2. Twenty grams of sieved biomaterial (100/200 m) was then added to the mixture. The resulting slurry was vigorously stirred for 1 h. The 6% sodium silicate solution was then slowly added to the slurry until pH 7.0. The rapid formation of a gelatinous polymer was observed. The suspension was stirred for an additional 30 m in and allowed to set overnight in a refrigerator. The biomaterial-containing polymer was then washed with Nanopure water until a barium test failed to produce a visible precipitate. The finalpolymergelwasdried in an oven overnight at approximately 100 °C and subse- quently ground to the desired particle size. Pure silicate polymer wasalso prepared usingthe same procedure without adding the D. innoxia material. A0.05 M Al 3+ stock solution was prepared by dissolution of the sulfate salt (Fisher Scientific Co.) dissolved in 0.1 M MES. The MES buffer was selected because of its demon- strated inabilityto complexwith metalions(27, 28). It should be noted that the MESwas used as a means ofcontroling the ionic strength of the solutions rather than to control the pH ofthe solutions. Concentrated H2SO4 and NaOH were used to adjust the pH ofsolutions to either 3.5 or 5.0. Under these *Author to whom correspondence should be addressed. Environ. Sci. Technol. 1998, 32, 2688-2692 2688 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 18, 1998 S0013-936X(98)00171-0 CCC: $15.00  1998 American Chemical Society Published on Web 07/30/1998