BIOTECHNOLOGY AND METALS
There are many anthropogenic (the result of human activity) sources of
toxic metals and related elements.
The two major uses of microorganisms in aspects of environmental
biotechnology associated with metals are :


Mineral Leaching
Biological Treatment of Heavy Metal Contaminated
Groundwater
There are three general categories of biotechnological processes for
treating liquid wastes containing toxic metals :





Biosorption
Microbial Metal Transformation ( Including Metal Bioleaching,
Sulfur and Sulfur Dioxide Removal )
Extracellular Precipitation (Possibly Including Metal
Transformation)
Uptake / Binding by Purified Biopolymers and Other Specialist
Molecules Derived From Microbial Cells or Their Components
Sulfate Reducing Bacteria ( Involving Metal Interactions )
There are many biotechnological processes currently in use for
alleviating toxic-metal pollution :
BIOSORPTION


Uptake by whole microorganisms (living or dead) via physiochemical mechanisms such as adsoption or ion-exchange
High density reactors using immobilised biomass (bacteria, algae,
higher plants)
Typical examples of these processes are :
AMT-Bioclaim™
Waste Bacillus sp. Biomass from fermentations are treated with alkali to
enhance metal uptake, chemically cross-linked, pelleted by extrusion or
milling and dried to provide a material that has an indefinite shelf life.
The granuated Bacillus preparations undertake non-selective removal of
Cd, Cr, Cu, Hg, Ni, Pb, U and Zn (singly or mixed). Metal loadings are up
to  10% of the dry weight of the product and give a removal efficiency
of  99% with total metal effluent concentrations in the range of 10-50
p.p.b..
AlgaSORB™
AlgaSORB contains algal biomass immobilised in a silica matrix and is
used in batch or column systems. It has been used to successfully
remove Ag, Al, Au, Co, Cu, Cr, Hg, Ni, Pb, Pd, Pt, U and Zn from
contaminated effluents and process streams.
Bio-Fix™
Bio-Fix is a biosorbent using biomass from a variety of sources,
including cyanobacteria (Spirulina), yeast, algae and plants (Lemma sp.
and Sphagnum sp.). The biomass is blended with xanthan and guar
gums to give a consistent product and immobilised as beads using
polysulfone. The loading for Zn2+ is approximately X4 that of ionexchange rsin and the metal ions are eluted with hydrochloric acid or
nitric acid. The bioadsorbent can be reused for more than 120
extraction-elution cycles.
MICROBIAL TRANSFORMATIONS
Many bacteria, algae, fungi and yeast can transform metal and metalloid
species by:


Oxidation ( e.g. Au3+ to Au ; Ag+ to Ag )
Reduction ( e.g. As3+ to As5+ (easier to precipitate than As3+
using Fe3+ ; CrO42- to Cr3+ ; Se4+ to Se )


Methylation ( e.g. Se4+ / Se6+ to volatile organo-selenium
compounds )
Dealkylation ( organometallis compounds to metal ions which
can be removed using a bioadsoptive process )
Metal Bioleaching :
The dissolution of metal sulfides is achieved by a series of direct and
indirect mechanisms.
Dissolution of metal sulfide (MeS) ore occurs in aerobic, ferric sulfate
medium :
MeS + Fe2 (SO4)3

MeSO4
+
(Ferric)
2FeSO4 + S
(1)
(Ferrous)
Metal sulfides may also be the result of direct leaching through
biologicallly mediated oxidation of the ore :
MeS +
2O2
Thiobacillus ferrooxidans

MeSO4
(2)
Thiobacillus ferrooxidans
S + 3O2
+ 2H2O

4H+
+ 2SO42-
(3)
This reaction is dominated by reaction (1) in the presence of ferric ions.
There are other processes which utilise Thiobacillus ferrooxidans.
The general chacteristics of Thiobacillus ferrooxidans are:


Chemolithotrophic - energy for growth and maintenance is
derived from the oxidation of ferrous iron or reduced sulfur
compounds. Some strains are even able to grow by oxidation
of hydrogen
Autotrophic - carbon dioxide is the cellular carbon source.
This is achieved using the Calvin-Benson cycle.

In this cycle, 3 molecules of ATP and two molecules of
NADP(H) are required to fix one molecule of CO2 . Thus, for the
production of one triose, nine ATP and six NADP(H) are
needed. The reduction of NADP+ and NADP(H) is believed to
occur by reverse electron transport. The number of molecules
of ATP needed to favour this thermodynamically unfavorable
flow of electrons is not clear. In addition to the Calvin-Benson
cycle, Thiobacillus ferrooxidans possesses a complete
glycolytic pathway and an incomplete citric acid cycle with 
ketoglutarate dehydrogenase being absent
Desulfurization of Sour Gases and Flue Gases :
In a scrubber :
H2S
+
Fe2(SO4)3

S
+
2FeSO4
+
H2SO4
Sulfur is recovered from the process.
FeSO4 (Ferrous ions) is sent to an aerobic bioreactor where
Thiobacillus ferrooxidans is used to make Fe(SO4)3 (Ferric ions):
4Fe(SO4)
+
2H2SO4
+ O2

2Fe2(SO4)3 + 2H2O
Sulfur Dioxide Removal From Flue Gas :
In a scrubber :
SO2
+
Fe2(SO4)3
+ 2H2O

2Fe(SO4)3 + 2H2SO4
Again Thiobacillus ferrooxidans is used to make Fe(SO4)3 (Ferric ions).
UPTAKE / BINDING BY PURIFIED BIOPOLYMERS AND OTHER
SPECIALIST MOLECULES DERIVED FROM MICROBIAL CELLS OR
THEIR COMPONENTS
Metal Binding Proteins and Polypeptides
Metallothioneins are small cysteine-rich polypeptides that can bind
essential metals such as Cu and Zn as well as non-essential metals
such as Cd.
Metal--Glutamyl peptides (phytochelatins) are short peptides involved
in heavy metal detoxification in algae, plants and some fungi.
Cell Wall Components and Exopolymers
Microbial exopolymers (e.g. capsules, slime layers) have received most
attention. The majority of these exopolymers are comprised of
polysaccharide, gylcoproteins and lipoploysaccharide, which may be
associated with proteins.
In activated sludge, the bacterium Zoogloea ramigera has received
much attention due to its extensive exopolysaccharide production.
Others species capable of this are Klebsiella (Enterobacter) aerogenes,
Arthrobacter viscosus and Pseudomonas.
A correlation normally exists between high anionic charge and metalcomplexing capacity. The carboxyl groups of the peptidoglycan are the
main metal-bindng site in the cell walls if gram-positive bacteria. Many
fungi have high chitin content in their cell walls and this has high
biosorptive capacity,
Fungal melanins contain phenol units, peptides, carbohydrates,
aliphatic hydrocarbons and fatty acids and possess potential metal
binding sites.
SULFATE REDUCING BACTERIA
Hydrogen sulfide is porduced by sulfate reducing bacteria (S.R.B.'s)
such as Desulphovirio and Desulphomaculum.
Carbon sources for growth may be glucose, acetate, lactate and ethanol
with lactate and ethanol being favoured.
Often S.R.B.'s are used in conjunction with methanogenic bacteria:
Ethanol
Acetate
S.R.B

Acetate
Methanogenics

CO2 + CH4
These are strictly anaerobic and operate at very low redox potentials.
Budelco Process
Aim :
Simultaneous Removal of Heavy Metals and Sulfate
Scale :
7,000 m3 / day
Cost :
33,000,000 Dutch Gilders ( = $HK 124,806,000)
Basic Steps :
1. Anaerobic reactor to treat influent containing C.O.D. and Sulfate
(SO42-). Sulfate is converted to Sulfide.
2. Gas from the reactor contains CO2 and CH4 and H2S. This gas stream
is contacted with a Zinc Sulphate solution to convert the sulfide to to
metal sulfide (metal precipitation).
3. Liquid from the reactor contains S2- , some V.F.A. and some SO42(unreacted). Solid from the reactor contains MS (precipitated) and
Biomass.
4. A Submerged Fixed Film Reactor (S.S.F.) is used for the conversion
of S2- in the liquid stream from the reactor to solid S. This is achieved
microbially.
5. A Tilled Plate Settler is used to remove solids which consist of Sulfur
produced within the S.F.F. - Metal Sulfides and Biomass
6. CH4 is flared
7. Solids are roasted (Metal Refining). Metal Roasting = Use of rotary
kilns for oxidising and driving off sulfur and arsenic from various
ores including gold, silver, iron etc at temperatures of 800-1600 K)
8. A biofilter removes any remaining odorous material (usually H2S)
ZnSO4 Solution
CH4
CO2 , H2S , CH4
Compost
Filter
Flare
Scrubber
Tilted Plate
Settler
H2S (l)
Sand
Filter
Fixed Film
Reactor
U.A.S.B.
O2 , CO2
S, MS
Metal Sulfide
Precipitation
INFLUENT
Sludge Treatment
(roasted)
The Buldelco Process
Effluent