Validation of an operational procedure for aluminium speciation in soil solutions and surface waters

Based on various methods, this paper proposes an optimized procedure to assess Al speciation in soil and surface waters. A 5-s flash extraction of some Al inorganic species by 8-hydroxyquinoline is proposed to replace the widely used rapid extraction (15 s) procedure. All species of Al relevant to natural waters were for extractibility under the recommended conditions. The speciation of this pool may be computed by equilibrium modelling. The Al-F species were not extracted and must be computed separately by equilibrium modelling without too many uncertainties. They cannot be determined using the free F-selective electrode method, because of interferences with other ions that are too strong. The polymeric and colloidal Al species are derived from the unrecovered Al in ion chromatography using a Dionex CS-3 cationic analytical column, for which the dimeric AlSi species extracted during the 8-hydroxyquinoline extraction step must be added. Organic Al was considered as being the amount of Al not accounted for by these combined procedures. An example of speciation is given with respect to some spring waters from the Vosges mountains in France. Often less than one-third of Al was found as toxic species, which identifies the need of such determinations to assess Al toxicity in acidified ecosystems. Among the non-toxic species, the AlSi complexes were found to sometimes reach up to 50% of the total Al, which emphasizes the great influence of silica on Al toxicity in surface waters.