Rare earth elements: behaviour and fate in estuaries

Dr Rate leads a research group who have shown that rare-earth elements are anomalously enriched in some estuarine sediments, especially those which receive drainage from acid sulfate soils. Rare earth elements are widely used as tracers of important geochemical processes. This project is investigating a life-cycle of rare-earth elements in the Peel-Harvey Estuary, Western Australia, including inputs via rivers, reactions with sulfidic sediments, and re-release to the environment following removal and deposition of dredged sediments.

Accumulation of trace elements in reduced sediments is a widely observed phenomenon, and trace elements are hosted and immobilized both in pyrite in mature sediments and in unstable monosulfide minerals. Exposure of reduced sediments to atmospheric oxygen by sea level change or drainage by humans has the potential to generate acid sulfate soils, a serious environmental issue globally which largely affects near-coastal environments. The formation of acid sulfate soils commonly results in release of sulfide-associated trace elements is dissolved forms, which is of concern because of the potential toxicity of many trace elements. In addition, dredging of aquatic sediments and disposal of dredged material in aerobic environments is also known to oxidise sulfidic materials and release trace element contaminants.

The rare-earth elements (REE) are the lanthanide series of elements (La to Lu); scandium and yttrium which have similar chemical properties are sometimes included. These elements are very important geochemically because their behaviour is very similar, with minor systematic differences across the lanthanide series (e.g., solubility of minerals). This means that a subset of REE may become enriched or depleted during a geochemical process such as weathering or oxidation. This fractionation of REE is consistent enough to allow the REE to be used as geochemical tracers based on recognition and analysis of their fractionation patterns.

It is already well-known that REE can be released during the process of sulfide oxidation to form acid sulfate soils. In addition, REE are fractionated during geochemical reactions relevant to sulfide accumulation and acid sulfate soil oxidation. Such reactions include precipitation within iron oxides in sediments, formation of complexes  of REE with natural organic compounds, and mineral transformations. The assumptions driving this research are that (i) REE accumulation in monosulfides is significant and that (ii) REE will be fractionated during the process of incorporation into monosulfide phases, but these assumptions are currently untested. The Peel-Harvey estuary has unique characteristics (widespread acid sulfate soils; hyper-eutrophication; extreme accumulation of sulfides in sediments) which allow testing of both hypotheses using a combination of field sampling and experimentation.