Sorption of lanthanum onto clay minerals: a potential tracer for fine sediment transport in the coastal marine environment?

Abstract

AbstractIn order to improve predictions of coastal morphological response to sea-level rise and sustainably manage dredged sediment there is an urgent need to develop a field methodology that can measure accurately transport pathways of the <63 µm sediment fraction in coastal and estuarine environments. Techniques such as sediment trend analysis and sediment tracing using fluorescent sands are well established for the silt and sand fraction but are unsuitable for clay sediments due to their cohesive nature. Geochemically labelled clays have been used as fine sediment tracers in freshwater environments with some success, although little is known about their chemical or physical behaviour once released in saline environments.A number of pure clays and natural estuarine sediments were labelled with La following agitation in a 0.01 M solution of La Cl3. In order to examine the retention of La on the clay mineral surface the labelled sediment was washed sequentially four times using both de-ionised water and artificial seawater. A labelled bentonite retained 43000 µg g-1 La and this was only reduced to 36000 µg g-1 La after washing in seawater. This suggests that retention of La is good even in saline conditions and concentrations of La are high enough to enable detection after considerable signal dilution. Sorption of La is dependent predominantly upon the cation exchange capacity of the sediment.