Geochemical controls on aluminium concentrations in coastal waters

Abstract

Environmental context Aluminium may be released into coastal waters in dissolved and particulate forms from urban runoff, industrial discharges and acid sulfate soils. Aquatic organisms may experience toxic effects from exposure to dissolved and particulate aluminium. Therefore, the current study reports the geochemical controls such as speciation, precipitation and adsorption that influence the exposure to these aluminium forms in the field and the laboratory. Abstract A combination of field and laboratory investigations was conducted in order to gain an understanding of aluminium dynamics in coastal seawater environments. Filterable (<0.025 and <0.45µm) aluminium concentrations in waters collected from an industrialised coastal location at Gladstone, Central Queensland, Australia ranged from 0.8 to 39.4µgL–1. Size-based separation measurements made on field and laboratory-spiked coastal waters showed colloidal (>0.025 to <0.45µm) aluminium species were generally minimal, apart from one field sample collected close to a river mouth where aluminium was associated with iron-containing colloids. In seawater (pH 8.15, 22°C) spiked with small increments of aluminium so as not to attain supersaturation, the solubility limit was ~500µgL–1. However, at higher total aluminium concentrations the solution chemistry became highly dynamic. In the presence of aluminium precipitate it was not possible to measure a solubility limit over the 28-day duration of the experiment because the dissolved aluminium concentration varied with both reaction time and precipitate concentration. For instance, when seawater solutions were spiked with 10000µgL–1 of total aluminium, a pulse of dissolved aluminium up to 1250µgL–1 was sustained for several days before decreasing to below 100µgL–1 after 28 days. The initial precipitate appeared to be solely aluminium hydroxide and transformed over time to contain increasing magnesium, consistent with the formation of hydrotalcite (Mg6Al2CO3(OH)16·4H2O), reaching 21% of the precipitate mass after 28 days. Adsorption studies showed that at anticipated suspended particulate concentrations for coastal waters, natural particulate material has a fairly low affinity for dissolved aluminium. The results of the current study highlight the complex chemistry of aluminium in marine waters and the role of precipitation reactions.