Sequential extraction procedure in columns. Part 1: Development and description of a new method

Abstract

A sequential extraction procedure was carried out in columns using reagents that are known to be reliable from batch tests. The intention was to distinguish between different chemical forms of iron and heavy metals in samples from reduced porous aquifers, which demands anaerobic conditions for the extraction procedure and the determination of small amounts of reactive mineral phases in a quartz dominated sediment system. By means of the developed method, anaerobic conditions can be guaranteed in the columns, which could not be realized to full satisfaction in batch tests that were carried out in a glove box. In order to distinguish between the fractions that were water soluble, exchangeable, bound to carbonates and bound to hydroxides, different reagents were pumped through the sediments and sampled after passage of the columns. Sediment samples of 10 kg each were investigated in this way. The extraction steps were known to be complete when analyses revealed that no further major and trace elements were leached out of the columns. This approach enabled well-adjusted amounts of reagents to be used. By means of the sequential extraction procedure in columns the composition of even small amounts of reactive mineral phases can be determined successfully, which contributes to a deeper understanding of the hydrogeochemical processes in aquifers. In batch tests this accuracy cannot be reached because of the surplus of the extraction solution in relation to the amount of sediment (higher solution-sediment ratio). Furthermore, larger samples are much more representative of the composition of the aquifers than smaller ones and the heterogeneity of the sediment does not limit the accuracy of the results as much as in batch tests. In addition, the technique of flushing sediment in a column is much more typical for the situation in an aquifer than suspending a few grams of a sample in the extracting reagents in batch extraction tests. In order to demonstrate the methodical improvements and field applications, the newly developed method was used to investigate the changing binding forms and mobility of iron and trace metals in samples from a lignite overburden dump, which are influenced by pyrite oxidation processes (acidification) followed by the addition of crushed limestone (neutralization) (see "Sequential extraction procedure in columns. Part 2: Application of a new method").