A proposed sequential extraction method for metals optimized for acid sulfate soil materials

Acid sulfate soils are an issue of international concern. Globally they are estimated to affect 24 million ha. Little work has been done on metal behavior in acid sulfate soils especially in Australia. 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 deals with acidity production only. To adequately manage the metal contamination risk some understanding of metal mobility and availability is required. Total metal analyses give little indication of the potential mobility of the metals, and little understanding of how mobility can change under different remediation or management practices. Mobility is strongly linked to solid-phase associations and so sequential extractions are a useful tool to understand geochemical processes. By using a sequence of reagents, operationally defined fractions can be determined, that reflect the potential mobility and bioavailability of the metals stored in the soil. We examine 7-step sequential extraction for metal association and mobility in acid sulfate soil materials. The notable feature of this proposed sequential extraction procedure is that it has been optimised for acid sulfate soil materials. In particular the method will quantify metals bound in the ‘sulfidic' fraction: in many sequential extraction methods for soils and sediments, the sulfidic-associated metal fraction is not able to be determined separately. The method needs to be applicable to a wide range of acid sulfate soil materials (varying from marine gels to peats) as well as soil materials with differing metal loads. The method described here will be used to help determine environmental risk assessment and subsequent management of acid sulfate soil materials. The results and implications for management are discussed.