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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-
bacterial cell
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 [1].
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.
[1] Rohwerder et al. (2003) Appl Microbiol
Biotechnol 63, 239-248.
[2] link to Mark Dopson "Importance and…"
[3] Gehrke et al. (1998) Appl Environ Microbiol
64, 2743-2747.
[4] Kinzler et al. (2003) Hydrometallurgy 71, 8388.
[5] Barreto et al. (2005) Appl Environ Microbiol
71, 2902-2909.