The use of flotation techniques in the remediation of heavy metal contaminated sediments and soils: an overview of controlling factors

The potential use of flotation as a remediation technique for heavy metal contaminated sediments and soils is discussed. In general, the flotation process is much less selective for heavy metals in sediments and soils compared to the results obtained in common mineral ore flotation practice. The following factors could be isolated as responsible for this different behaviour: the metal distribution over the different geochemical phases (metal partitioning), the metal distribution over the different size fractions (metal fractionation) and the presence of organic matter (humic substances). These controlling factors were illustrated by performing Denver flotation experiments on a natural canal sediment and column flotation tests on synthetic sediments.It is emphasised that sequential extraction techniques can be used as a tool for determining the original metal partitioning in sediments and soils and as a result helps to select the most appropriate procedure of flotation (e.g. a phase-specific collector). A chemical pre-treatment step (e.g. sulphidisation, hydroxylation) prior to the flotation of sediments and soils can be beneficial depending on the in-situ metal partitioning and serves to transfer heavy metals, which are associated with different speciation sources (oxides, sulphides, clay minerals, organic matter, etc.), into one unique-good floatable-chemical speciation form (e.g. oxides, sulphides). The selective flotation of metal-bearing particles from fine-textured sediments and soils is very difficult and therefore a major challenge for future research is to tackle the problems related with the flotation of these fines. Organic matter, which is rather selectively flotated, has an adverse effect on the metal (sulphide) flotation selectivity due to its adsorption onto mineral surfaces.