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The European Journal of Mineral Processing and Environmental Protection
Vol.3, No.1, 1303-0868, 2003, pp. 58-66
Review
Basic concepts on heavy metal soil bioremediation
C. Garbisu
1
* and I. Alkorta
2
1
Department of Agrosystems and Animal Production, NEIKER, A.B.-Basque Institute of Agricultural Research
and Development, c/ Berreaga, 1; E-48160 Derio, Spain
2
Unidad de Biofísica, Centro Mixto CSIC-UPV/EHU, P. O. Box 644; E-48080 Bilbao, Spain
Received 30 June 2002; accepted 12 January 2003
ABSTRACT
The utilization of organisms, primarily microbes, to clean up contaminated soils, aquifers, sludges,
residues, and air, known as “bioremediation”, is a rapidly changing and expanding area of environmental
biotechnology, that offers a potentially more effective and economical clean-up technique than
conventional physicochemical methods. Although it is certain that up to now the technologies employed
are not technically complex, considerable experience and expertise is required to design and implement a
successful bioremediation program. As a matter of fact, and since bioremediation frequently addresses
multiphasic, heterogenous environments (i.e., soils), successful bioremediation is dependent on an
interdisciplinary approach involving such disciplines as microbiology, engineering, ecology, geology, and
chemistry. The bio-enthusiasm of the early years that followed the initial promising research results and
inspired the creation of many remediation companies has ended in a more realistic and sometimes even
sceptical view of bioremediation since it has now become clear that results obtained in the laboratory do
not necessarily indicate what may happen actually in the field, since it is not possible to simulate all the
changing conditions of a real situation.
Most traditional remediation methods do not provide acceptable solutions for the removal of metals
from soils. Microorganisms that use metals as terminal electron acceptors, or reduce metals as a
detoxification mechanism can be used for the removal of metals from contaminated environments. In some
cases, phytoextraction of metals is a cost-effective approach that uses metal-accumulating plants to clean
up metal polluted soils. © 2003 SDU. All rights reserved.
Keywords: Soil; Metals; Bioremediation; Phytoremediation; Environmental
1. INTRODUCTION
The quality of life on Earth is linked inextricably to the overall quality of the environment. It
is very difficult to define soil quality, as soil composition can vary from place to place. Soil
quality is concerned with more than the soil’s constituents and composition, but how it
functions in a specific environment. The major functions of a soil are generally recognized to
include the ability to protect water and air quality, the ability to sustain plant and animal
productivity, and the ability to promote human health (Doran and Parkin, 1994; Chen and Mulla,
1999).
The release of contaminants into the environment by human activities has increased
enormously over the past several decades. In fact, although a few decades ago, man’s greatest
challenge resided in speeding up the industrialization process, today man attempts to find ways
to deal with the growing industrialization and the associated problems (Thassitou and
Arvanitoyannis, 2001). The relatively sudden introduction of pollutants into the recipient
ecosystems has clearly overwhelmed their self-cleaning capacity and, as a consequence,
resulted in the accumulation of pollutants. Soil pollution has recently been attracting
* Corresponding author. E-mail: cgarbisu@neiker.net
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considerable public attention since the magnitude of the problem in our soils calls for immediate
action.
The large-scale production of a variety of chemical compounds, such as organic solvents,
fuels and fuel additives, pesticides, plasticizers, pigments, dyes, plastics and chemical
feedstocks, has caused global deterioration of environmental quality (Garbisu and Alkorta,
1997; Iwamoto and Nasu, 2001).
Contaminated soils are a common environmental problem all over the world. The various
countries confronted with contaminated soil differ considerably in awareness of the problem
and in the policies and the technologies to tackle it (Rulkens et al., 1998). Nonetheless,
intensive exchange of experiences gained with the management and remediation of polluted
soils is taking place among the various countries.
As a matter of fact, increasingly widespread pollution has caused vast areas of land to
become non-arable and hazardous for both wildlife and human populations. Contaminated
lands generally result from past industrial activities when awareness of the health and
environmental effects connected with the production, use, and disposal of hazardous
substances were less well recognized than today (Vidali, 2001). Unfortunately, the enormous
costs associated with the removal of pollutants from soils by means of traditional
physicochemical methods have been encouraging companies to ignore the problem (Alkorta
and Garbisu, 2001).
In addition to minimizing the impact of future incidents by means of controlling soil pollution
input (developing a long-term perspective of pollution amelioration measures that focus on
slowing the rate of pollution increase), it is imperative to deploy innovative technologies which
could economically remediate toxic wastes adversely impacting our environment, thereby
reducing the threat to human health and the environment (Garbisu and Alkorta, 1997).
In the last few years, disquiet among ordinary people has grown and the public is now
strongly demanding clean-up measures to be urgently introduced. In this context, governmental
recognition of the accumulating hazards has resulted in legislative restrictions on uncontrolled
discharges of wastes and actions mandating environmental restoration of hazardous waste
sites. This recent environmental awareness has highlighted the need for new technologies for
the treatment of these wastes.
2. WHAT IS BIOREMEDIATION?
Traditional physicochemical processes for remediation of soil polluted sites are expensive
and often do not permanently alleviate the pollution hazard. The most common conventional
techniques used for remediation are: (i) excavation and disposal to a landfill, and (ii) to cap and
contain the contaminated areas of a site.
Apart from the fact that it is very difficult and increasingly expensive to find new landfill
sites for the final disposal of the material, the first method simply moves the contamination
elsewhere (with the possibility of creating risks during the excavation, handling, and transport of
hazardous material), while the second is only an interim solution since the contamination
remains on site, requiring monitoring and maintenance of the isolation barriers (Vidali, 2001).
Other methods such as incineration lack public acceptance since they can increase the
exposure to contaminants of both the workers at the site and nearby residents. Some other
techniques that are in various stages of development are the following: extraction of pollutants
with organic solvents or CO2
, oxidation of organic pollutants under subcritical or supercritical
conditions, vitrification, electroreclamation, dehalogenation of chlorinated organic compounds
using an alkali polyethylene glycol, chemical reduction or oxidation of contaminants, steam
stripping, plasma torch techniques, microwave heating, solidification/stabilisation, and so on
(Rulkens et al., 1998).
Bioremediation is a general concept that includes all those processes and actions that take
place in order to biotransform an environment, already altered by contaminants, to its original
status. Bioremediation uses primarily microorganisms or microbial processes to degrade and
transform environmental contaminants into harmless or less toxic forms. Although strictly
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Vol.3, No.1, 1303-0868, 2003, pp. 58-66
speaking, bioremediation is the use of “organisms” to degrade pollutants, it is mainly concerned
with the use of “microorganisms”. In this context, phytoremediation, defined as the use of green
plants to remove environmental contaminants or to render them harmless (Cunningham and
Berti, 1993; Raskin et al., 1994, 1997; Salt et al., 1995, 1998), is recently being considered as a
highly promising technology for the remediation of polluted sites. This topic of
phytoremediation has recently been reviewed by the authors in more detail elsewhere (Alkorta
and Garbisu, 2001; Garbisu and Alkorta, 2001; Garbisu et al., 2002).
3. BIOREMEDIATION OF HEAVY METALS
The term “heavy metal” is arbitrary and imprecise. Some authors (Raskin et al., 1994), for the
sake of simplicity, defined “heavy metal” as any element that has metallic properties (ductility,
conductivity, density, stability as cations, ligand specificity, etc.) and an atomic number greater
than 20. A more biologically relevant classification of metals based on ligand-forming
properties
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