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Bioflotation as a new technique for separation of fine particles Thore Rohwerder University Duisburg-Essen, Biofilm Center, D-47057 Duisburg, Germany Flotation is already known for several decades. It is told that miners discovered this process while washing their clothes. Certain chemicals of the washing powder adsorbed at metal sulfides and made them float on the washing solution. Today, flotation is widely used for producing concentrates of raw materials, e. g., for separating metal sulfides of interest from undesired ones or from gangue material. An aqueous suspension of finely grained ore is conditioned with flotation chemicals, making the metal sulfides either more hydrophobic (collector) or more hydrophilic (depressant). Then, small air bubbles are produced, which aggregate with the hydrophobic particles and, thus, make them float foam-like on the suspension whereas the hydrophilic sulfides are depressed, i. e., they are kept in the liquid phase. Although this is a very useful method it is environmentally unfriendly as the currently used depressants such as sulfur dioxide, cyanide, and chromate are very toxic. The answer to this problem could be the employing of bacteria for flotation processes. Of particular interest are the socalled leaching bacteria, acidophilic metal sulfide oxidizing species, which are already used in hydrometallurgy. These bacteria selectively attach to sulfide surfaces (their substrate), forming a biofilm, i. e., a layer of bacterial cells plus excreted extracellular polymeric substances (EPS) [1, 2]. The slimy matrix of EPS consists of polysaccharides and lipids [3-5], sometimes also proteins and nucleic acids are found. Bacterial attachment alters sulfides to become more hydrophilic. Consequently, leaching bacteria such as Acidithiobacillus ferrooxidans have success- 4/29/2017 A B OM EPS PS CM bacterial cell Fe3+ Fe2+ S O 22 3 Pyrite (FeS2) Atomic Force Microscopic image of leaching bacteria attached to pyrite (A) and derived model (B); OM outer membrane, PS periplasmic space, CM cytoplasmic membrane; modified from . fully been employed as a depressant in pilot-scale bioflotation experiments. However, our knowledge on the selectivity of certain bacterial species to certain sulfides is limited. In future, free EPS and related compounds should be tested. The objective is replacing the currently used toxic depressing chemicals by EPS-derived reagents, consisting of the functional units of natural EPS from leaching bacteria.  Rohwerder et al. (2003) Appl Microbiol Biotechnol 63, 239-248.  link to Mark Dopson "Importance and…"  Gehrke et al. (1998) Appl Environ Microbiol 64, 2743-2747.  Kinzler et al. (2003) Hydrometallurgy 71, 8388.  Barreto et al. (2005) Appl Environ Microbiol 71, 2902-2909.