Salirian Claff (PhD Student)
Southern Cross University

Project Title
The assessment of metal geochemistry in acid sulfate soils

Biography
Salirian completed her Ba Applied Science (Environmental Resource Management) Hons (First Class) at SCU, graduating in 2002. Since then she has worked in the community education and extension field, initially as the Soils, Habitats and Biodiversity Project Officer with the Nature Conservation Council of NSW, and then working with the stakeholder groups associated with the Moreton Bay Receiving Water Quality Model (at the University of Qld).

Start Date
January 2007

Project Details
The aim of this work is to propose a new seven step sequential extraction scheme that can distinguish the metal fraction bound to the reactive organic matter from that bound up in sulfide form, allowing for some assessment of the metal risk associated with a given acid sulfate soil to be determined.

Acid sulfate soils are soils containing pyrite, and are an issue of international concern. The oxidation of sulfidic soils has a number of environmental and economic ramifications, including the mobilisation of ferrous iron and sulfuric acid, acidification of waterways and soil profiles, fish kills and loss of agricultural productivity (Johnston et al., 2004; Kroon, 2004).

Many researchers have looked at the processes of, and the subsequent impacts of pyrite oxidation as well as issues with the reclamation and remediation of acid affected sites. Little work however, has been done on metal behaviour in acid sulfate soils, especially in Australia. Elevated trace metal concentrations in soils are known to cause plant toxicity, reducing agricultural productivity (Schols and Schnabel, 2006). The sulfuric acid, formed from the pyrite oxidation, has the potential to mobilise metals which were bound in previously geochemically stable forms (Hinwood et al., 2006). The management of metals potentially raise a number of issues in the use and remediation of acid sulfate soils, however current risk assessment and management of these soils tends to deal with acidity cycle only.

Metals can be associated with soil in a number of ways, including complexation, adsorption and precipitation of metal ions within the soil particle and sorption to a mineral surface (Usero et al., 1998). The availability (in terms of the toxicity and mobility) of a given metal is linked to its geochemical form. Because of this total metal concentrations give little indication as to the bioavailability, and hence toxicity of a metal (Gleyzes et al., 2002). By gaining an understanding of how solid phase metals are stored in the soil some predictions can be made as to the present and future mobility and toxicity, and thus an assessment of potential environmental risk can be made. A sequential extraction can be a useful tool in assessing phase associations of metals and other toxicants of interest.

The aim of a sequential extraction is remove metals using a series of reagents that capitalize on the fact that different solid phases are reactive in different extraction conditions (Gleyzes, 2002). Reagents are added to a single sample in a sequence aimed at removing the most mobile to the least mobile. The extractants can range in intensity from distilled water to a hot acid digest. When choosing reagents it is important to consider the potential affects on other non-targeted mineral phases.

For an acid sulfate soil, a fraction which it is important to isolate as much as possible is the sulfidic fraction. Most sequential extractions in use though, are derivations of the Tessier et al. (1979) method, and so optimized for oxic soils. Although some studies do consider anoxic soils and sediments (eg Dollar et al., 2001), the sulfides and organic matter are generally removed in the one extraction step. Although there is a certain degree of non-specificity in any reagent chosen, through systematic examination, reagents which most suit the soil type studied can be chosen.