Aluminum Speciation: A Comparison of Five Methods

The objective of this study was to compare five methods with varying chemical approaches to the speciation of Al. The 8-hydroxyquinoline (HQ) and ferron procedures were used to estimate the inorganic, monomeric forms of Al at reaction times of 15 and 30 s, respectively. Two other procedures, an ion exchange column procedure and a chelating resin procedure, were used primarily to measure organically bound forms of Al. These were compared with a F electrode technique which quantifies Al³⁺ activity using equilibrium thermodynamic calculations and measured values of F activity and total F. Stability constants are then used to calculate the speciation of inorganic forms of Al, and organically complexed forms are obtained by subtraction from total Al. Sample solutions containing 5 mmol m⁻³ F, and 7.9, 15.2, 44.5, and 80.4 mmol m⁻³ Al were synthesized, with and without addition of 1 mol m⁻³ (as dissolved organic C) purified fulvic acid extracted from the surface horizon of an Edneytown soil (Typic Hapludults). A soil solution extract was also obtained from the soil and analyzed. Excellent agreement (r = 0.999) between predicted and measured Al³⁺ was obtained for the electrode procedure in solutions without organic matter (OM). The kinetically reactive Al values obtained by the HQ procedure correlated very well with those of the F electrode procedure in predicting [Al³⁺] and toxic Al (Al³⁺ + AlOH²⁺ + Al(OH)⁺₂), (r = 0.91 and 0.90, respectively), but overestimated these Al forms when the Al/organic C ratios were low. The kinetically reactive Al values obtained by the ferron procedure were greater than values obtained by the HQ and electrode procedures since ferron was also able to breakdown Al-F complexes to a greater extent. The column and chelating resin procedures were able to separate organic and inorganic forms of Al only, thus speciation of inorganic complexes was not feasible. Organically bound Al calculated from the electrode procedure was generally lower but consistent with the values obtained by the chelating resin. The ion exchange column proceduure gave the lowest values of Al-OM at the lowest Al/OM ratios, indicating that some degradation of Al-OM complexes may occur during passage through the column. The F electrode procedure, though promising, should not be indiscriminately used over a wide range of pH, Al/F ratios, and DOC contents without more information on the effects of electrode interactions with organic solutes. Furthermore, the procedure is slow and requires the assumption of equilibrium conditions, which may seldom occur for Al in field conditions. It is, however, promising as a tool for the evaluation of other procedures under well controlled experimental conditions.