SPECIATION OF AQUEOUS MONONUCLEAR AL(III)-HYDROXO AND OTHER AL(III) COMPLEXES AT CONCENTRATIONS OF GEOCHEMICAL RELEVANCE BY AL-27 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

Aluminum-27 (Al-27) nuclear magnetic resonance (NMR) spectroscopy was used to characterize Al(III)-hydroxo complexes, in aqueous solutions w ith total Al(III) concentrations of 1.0-10 mu M, using a custom-built NMR probe, coil, and sample bottle with low background aluminum impuri ties. Published Al-27 NMR spectroscopy studies have traditionally used total Al(III) concentrations that are generally outside the range of geochemical interest (total [Al(III)] greater than or equal to 1000 mu M). In this study, lower AI(III) concentrations (less than or equal t o 10 mu M) were used to more closely approximate natural conditions, w hile allowing the measurement of mononuclear Al(III) species by Al-27 NMR spectroscopy. The sensitivity of the Al-27 NMR spectroscopy system , as measured by the signal-to-noise ratio (S/N), is S/N = 5 for 1.0 m u M Al(III) at pH 2.00 and S/N = 3 for 10 mu M Al(III) at pH 5.20. Thi s level of sensitivity is within the range of geochemically relevant A l(III) concentrations found in slightly acidic natural waters. Quantit ative models are developed which Link observations of NMR chemical shi fts and linewidth ratios to the calculated equilibrium speciation of m ononuclear Al(III) for 10 mu M Al(III) solutions at pH values 2.00 to 5.20 (prepared by titrating acidic Al(III) solutions with pyridine). L inear-regression best fits of the models to the NMR data are used to d etermine the intrinsic chemical shifts and linewidths of individual mo nonuclear Al(III) species. The intrinsic chemical shift of each Al(III ) species, ''i'' delta(i) (ppm), is (1) delta(Al3+) = 0 for Al3+ (defi ned by convention), (2) delta(Al(OH)2+) = 3.5 (SE = 1.3, N = 10) for A l(OH)(2+), (3) delta(Al(OH)2+)similar or equal to 3.7 (SE = 1.4, N = 1 0) for Al(OH)(2)(+) and (4) delta(Al(OH)4-)) (SE = 0.03, N = 4) for Al (OH)(4-); where positive chemical shifts are ''downfield,'' SE = stand ard error, and N = number of samples. A convention is delineated in wh ich the linewidth of the Al(III) species/peak of interest is normalize d with respect to that of a reference species (Al3+) under the same co nditions. Such linewidth ratios are independent of investigation-speci fic variables such as solution viscosity and temperature. Due to the l arge sample volume (18.8 mL) used here to achieve increased sensitivit y, there is some Line broadening caused by magnetic field inhomogeneit ies; however, this line broadening is constant and reproducible both d uring and between experimental runs, and it was corrected for in the d etermination of linewidths of individual Al(III) species. For an absol ute linewidth of LW(Al3+) = 1.6 Hz for Al3+, the linewidth ratio (ppm/ ppm) of each species ''i'' (LW(i))/(LW(Al3+)), is: (1) (LW(Al3+))/(LW( Al3+)) = 1 for Al3+ (by definition), (2) (LW(Al(OH)2+))/(LW(Al3+)) = 4 95 (SE = 11, N = 8) for Al(OH)(2+), and (3) (LW(Al(OH)2+)/LW(Al3+)simi lar or equal to 450 (SE = 140, N = 10) for Al(OH)(2)(+). The increased sensitivity of this system, and the knowledge of intrinsic Al-27 NMR spectroscopic parameters for Al3+, Al(OH)(2+), Al(OH)(2)(+), and Al(OH )(4)(-) sets the stage for use of Al-27 NMR Spectroscopy to characteri ze these species in natural waters and to study other Al(III) species of geochemical interest.