2. ENVIRONMENTAL ChEMISTRy & TEChNOLOGy 2.1. Lectures

Chem. Listy, 102, s265–s1311 (2008) Environmental Chemistry & Technology
P61 MICRObIOLOGIAL REMEDIATION OF
METAL-CONTAMINATED SOIL
VALéRIA SnOPKOVá
Department of Biotechnology, Institute of Geotechnics the
Slovak Academy of Sciences,
Watsonova 45, 043 53 Košice, Slovak Republic.
snopkova@saske.sk
Introduction
Soil contamination with anthropogenic heavy metals,
which mainly comes from industrial activity, atmospheric
deposition and land application of sewage sludge, has received
much attention in the recent years. The anthropogenic
heavy metals are to be easily accumulated in the topsoil1,
resulting in potential problems such as toxicity to plants and
animals 1,2 , accumulation in food chain, perturbation of the
ecosystem and adverse health effects 4,5 . Metals, which are
significantly toxic to human beings and ecological environment,
include chromium (Cr), cooper (Cu), lead (Pb), mercury
(Hg), zinc (Zn), manganese (Mn), cadmium (Cd), nickel
(ni), arsenic (As) and iron (Fe), etc. 6 .
Contaminated soil is notoriously difficult to treat because
the contaminants are often tightly bound to the soil particles.
Conventional remediation technologies are becoming less
popular due to the high treatment costs and bioremediation
processes to improve the contaminant removal efficiency
and cost effectiveness. However, as an innovative technology,
there are many factors to be investigated with the future
development 7 .
bioremediation
Bioremediation is defined as a method using living organisms,
or their particles (enzymes) to reduce, eliminate, fixate
or transform contaminants presented in the soil, in sediments,
in waters or in the air 8,9 . In the bioremediation processes
bacteria, fungi, yeasts and plants present the most imposed
organisms. Recently, the ability of algaes and planktons are
being researched. Some technologies are based on the use
of general or genetically modificated organisms 8 . Bioremediation
works by either transforming or degrading contaminants
to non-hazardous or less hazardous chemicals. These
processes are called, respectively, biotransformation and
biodegradation. Biotransformation is any alteration of the
molecular or atomic structure of a compound by microorganisms.
Biodegradation is the breaking down of organic substance
by microorganisms into smaller organic or inorganic
components 9 .
Metal – Microbe Interactions
Microorganisms, especially bacteria, exist in complex
biogeochemical matrices in subsurface sediments and soils 9
and are known to mediate many geochemical processes 10,11 .
They can interact with metals via many mechanisms, some of
which may be used as the basis of the potential bioremediation
strategies 9 . Microbes can mobilize metals through autot-
s454
rophic and heterotrophic leaching, chelation by microbial
metabolites and siderophores, and methylation, which can
result in volatilization. Conversely, immobilization can result
from sorption to cell components or exopolymers, transport
into cells and intracellular sequestration or precipitation as
insoluble organic and inorganic compounds, e.g. oxalates 12,13
sulphides or phosphates 14,15 .
M e t a l M o b i l i z a t i o n
bioleaching. Metals can be leached from solid matrices
via autotrophic and heterotrophic leaching. Chemolitotrophic
and heterotrophic bacteria have the major role.
Most autotrophic leaching is carried out by chemolithotrophic,
acidophilic bacteria which fix carbon dioxide and
obtain energy from the oxidation of ferrous iron or reduced
sulfur compounds, which causes the solubilization of metals
because of the resulting production of Fe(III) and H 2 SO 4 16,17 .
The microorganisms involved include sulfur-oxidizing bacteria,
e.g., Acidihiobacillus thiooxidans, iron- and sulfur-oxidizing
bacteria, e.g., Acidithiobacillus ferrooxidans and ironoxidizing
bacteria, e.g., Leptospirillum ferrooxidans 18,19 .
In the case of oxide, carbonate and silicate ores, limits
are set for the use of thiobacilli. For such ores, research is
being done on the use of heterotrophic bacteria. In this case,
metals are dissolved by organic acids, or complexing, or chelating
agents produced by the bacteria. Heterotrophic bacteria
require organic supplement for growth and energy supply.
Among the bacteria, members of the genus Bacillus are most
effective in metal solubilization 19 .
Siderophores are low molecular weight Fe(III) coordination
compounds that are excreted under iron-limiting conditions
by microorganisms, particularly bacteria and fungi,
to enable accumulation of iron from the environment 20,21 .
Although primarily produced as a means of obtaining iron,
siderophores are also able to bind other metals such as magnesium,
manganese, chromium (III), gallium (III) and radionuclides
such as plutonium (IV) 22,23 .
biomethylation of Hg, As, Se, Sn, Te and Pb can be
mediated by a range of bacteria under aerobic and anaerobic
conditions. Methyl groups are enzymatically transferred to
the metal, and the given species may transform a number of
different metal(-loid)s. Methylated metal compounds formed
by these processes differ in their solubility, volatility and
toxicity 23 .
M e t a l I m o b i l i s a t i o n
bioacumulation and biosorption. Bacteria can physically
remove heavy metals from solution through association
of these contaminants with biomass. Bioaccumulation is the
retention and concentration of substance within an organism.
In bioaccumulation, metals are transported from the outside
of the microbial cell trough the cellular membrane, into the
cell cytoplasm, where the metals are sequestred 9,24 . Biosorption
describes the association of soluble substances with the
cell surface. Sorption does not require an active metabolism.
The amount of metal biosorbed to the exterior of bacterial