Determining aluminium speciation in seawater from a subtropical estuarine port

The toxicity of aluminium (Al) in aquatic environments is intimately linked to the forms (or species) present and their respective concentrations, rather than the total Al concentration. Reflecting this in water quality... [ view full abstract ]

The toxicity of aluminium (Al) in aquatic environments is intimately linked to the forms (or species) present and their respective concentrations, rather than the total Al concentration. Reflecting this in water quality guidelines is a difficult exercise because of the diversity of species that exist—differentiated on physical (particulate, colloidal and truly soluble) and chemical grounds (simple ionic, polymeric, organically complexed, etc.).
In this study, we adopted a two-step approach. First, we characterised the physical speciation, using nominal cut-offs for filtration of 0.45 µm to separate particulate from dissolved, and a further differentiation of the dissolved fraction into colloidal and soluble using <10-kDa ultra-filter (~0.003-µm pore size). Aluminium concentrations in port seawaters for each size fraction were determined by ICP-MS. Results for estuarine water samples, from a remote, undeveloped estuary in the same region had the same characteristics, with the aluminium concentration in the order: particulate >> soluble > colloidal. Dilution experiments with freshwater of aged (~ 1 month) samples close to seawater salinity revealed no change in this distribution over varying timescales of 30 min, 7 d and 35 d, nor was it influenced by temperatures of 18, 24 and 28°C (reflecting the seasonal cycle for the port seawaters).
The second stage of the work featured analytical procedures to quantify speciation on a chemical basis using chromatographic separation followed by selective detection. An example is provided by the complexation of labile Al with the fluorophore lumogallion, which is subsequently separated from matrix components including other refractory Al species by reversed-phase chromatography and quantitation by fluorimetry. Early results indicate that the chromatographic method with fluorimetric detection of the Al-lumogallion complex yields concentrations in the range 50–80% of the dissolved (<0.45 µm) Al concentration originally determined by ICP-MS.