Method of decontaminating a cementitious or a metallic surface

A method for the decontamination of a cementitious or a metallic surface having contaminants is described. The method comprises the steps of supplying to said surface at least one microorganism under conditions conducive to growth of said at least one microorganism; maintaining said conditions conducive to growth of said microorganism and the generation of an acid or other metabolite to degrade said surface to a desired depth; terminating said conditions conducive to growth upon achieving at least said desired depth of degradation; removing the products of said degradation; treating said degradation products; and, disposing of said treated degradation products wherein the at least one microorganism is a citric acid generating microorganism.

The present invention relates to a method for the decontamination of a 
cementitious or a metallic surface, particularly, though not exclusively, 
to the in-situ decontamination of such surfaces. 
The decommissioning of chemical plants and reactors in the nuclear industry 
is an often lengthy and costly process, which desirably takes the site 
from closure to a state where it is free for other unrestricted use. Due 
to the very high costs involved with decommissioning, it is essential that 
decommissioning strategies are as cost effective as possible. 
The decommissioning of, for example, a nuclear reactor or contaminated 
nuclear facilities is carried out in a number of stages. Firstly, fuel and 
accessible loose radioactive materials are removed from reactors and 
plant, then these and associated pieces of equipment are decontaminated as 
appropriate and dismantled; contaminated waste materials are disposed of 
and finally, the remaining buildings are demolished and the debris 
disposed of. 
Contamination, as opposed to activation products, is located at the surface 
or accessible areas such as cracks and pores for example. 
Decontaminating techniques are used within the decommissioning procedure to 
remove contaminants prior to demolition or re-use of buildings and other 
facilities for example. Decontamination is an important procedure because 
it not only reduces radiation dose rates within the working area, but also 
has the potential for reducing the impact of the waste and 
reclassification to a lower and less expensive category so as to achieve 
lower disposal costs. Reclassification of waste is achievable in the case 
of concrete and metal surfaces, for example, because the contaminants are 
concentrated at the exposed surfaces, near the surface or at accessible 
cracks. Therefore if those surfaces can be removed, the bulk of the 
remaining material can be disposed of as lower level waste. 
Conventional methods for the decontamination of concrete surfaces have 
mainly employed physical methods such as shot blasting, scabbling, high 
pressure water jets, high energy processes such as lasers and plasmas, 
milling, sawing and explosives for example. Depending on the 
circumstances, each technique has its own advantages. However, common and 
inherent disadvantages are that these techniques generate comparatively 
large and difficult to manage volumes of waste; they are labour intensive 
and also involve high capital expenditure. Chemical methods employ 
aggressive materials and/or large volumes of liquid waste. 
GB 2 261 316, of common ownership herewith, describes the decontamination 
of cementitious surfaces by the use of microorganisms to degrade the 
surface. The residue produced by such degradation being subsequently 
removed. This document is concerned with the use of sulphur oxidising 
bacteria to produce sulphuric acid to attack the surface. 
However, the toxic materials which are being removed from the contaminated 
surface and the residue generated from the micro organisms or biomass are 
chemically separate and need to be dealt with separately. 
Decontamination of metal surfaces has, in the past, been largely confined 
to washing down of surfaces with suitable chemicals, electrochemical, shot 
blasting, strippable coatings, ultrasonics and detergents for example. 
We have now found that biosystems may be used which have particular and 
unforseen advantages. 
According to the present invention there is provided a method for the 
decontamination of a cementitious or metallic surface having contaminants, 
the method comprising the steps of supplying to said surface at least one 
microorganism under conditions conducive to growth of said at least one 
microorganism; maintaining said conditions conducive to growth of said 
microorganism and the generation of an acid or other metabolites to 
degrade said surface to a desired depth; terminating said conditions 
conducive to growth upon achieving at least said desired depth of 
degradation, removing the products of said degradation; treating said 
degradation products; and, disposing of said treated degradation products, 
the method being characterised in that said at least one microorganism is 
a citric acid generating microorganism. 
The depth of penetration of said contaminants may be up to about 20 mm. 
Conditions conducive to the growth of the microorganisms may include 
aeration, temperature, light and humidity control and the provision of 
suitable nutrients to sustain growth. 
According to the present invention, the generated acid is citric acid. 
Citric acid has the particular advantage of chelating with contaminant 
metal ions to remove them from the surface. Most heavy metal citrates are 
soluble in aqueous solutions, and therefore, are removed from the 
contaminated surface. 
The generation of citric acid may be achieved by the use of strains of 
fungi, yeasts or bacteria, for example. Examples of the former may include 
Aspergillus spp. (such as A niger, A wentii, A carbonarius), Gliocladium 
spp., Trichoderma spp., Scopulariopsis spp., Paecilomyces spp., 
Penicillium spp. and Mucor spp., Saccharomycopsis lipolytica, Arthobacter 
spp. and Rhodococcus spp. 
In the case of biodecontamination of metals such as mild steel, stainless 
steel, copper, aluminium and zircalloy, for example, the microorganisms 
may be applied to the contaminated surface prior to or after initial 
cleaning procedures to remove loose contamination. The microorganisms 
release the contaminants by a corrosion mechanism or by selective 
leaching. The microorganisms may also release contaminants that are held 
within cracks, fissures and/or at grain boundaries (produced during 
working or manufacture of the metal), which tend to be resistant to 
techniques that do not dissolve the surface by a corrosion or selective 
leaching mechanism. In the case of metals, aggressive metabolites (which 
are substances produced by metabolism) produced and held at the surface 
beneath a biofilm have more time to penetrate these surface features. 
Thus, as the contact time between the surface and the metabolite is also 
increased, decontamination will be further facilitated. 
The inoculum of the at least one citric acid producing microorganism may be 
produced in either continuous, semi-batch or batch cultures. A bioreactor 
may be used to produce the inoculum. The inoculum may comprise a single 
microbial species or a consortium of species. In the case of fungal 
species, spores may be cropped from mature sporulating cultures and be 
made up into a spore suspension. Spore suspensions have the advantage that 
they may be kept for long periods at low temperatures. Thus, appropriate 
suspensions may be produced under controlled conditions in a laboratory 
and transported to the site where decontamination is required. 
Immediately prior to the application of the microorganism to the surface to 
be decontaminated, the microbial inoculum and a suitable nutrient media 
may be mixed together to give a known concentration of cells or spores, 
the nutrient media providing nutrients which are not readily available 
from the cementitious or metallic surface. At this stage, the basic feed 
may be incorporated into a carrier medium for application to the surface 
to be decontaminated. Such carrier media may include paint, gel or foam 
matrices which will adhere to the surface to be decontaminated. 
Preferably, the carrier medium may include the nutrient source. The role 
of the carrier medium would also be to encourage initial microbial 
colonisation of the surface. Continued growth of the microorganism would 
be maintained by the supply of nutrient to the carrier and the maintenance 
of a suitable environment with regard to temperature and humidity for 
example. 
In some circumstances it may be desirable to apply an initial pre-treatment 
to the contaminated surface so as to encourage colonisation by the acid 
producing bacteria. Such pre-treatment may take the form of a dilute acid 
or of a biofilm forming microbial culture. The biofilm is the layer which 
contains the bacteria and is effectively the environment in which the 
bacteria grow. 
Application of the feed in its carrier medium may be by various techniques 
which may include: 
(a) spraying the structure to be decontaminated, the spray ranging from 
mist to jet; 
(b) flooding the structure and/or by a wet and dry cycle such as by pulsing 
the feed around the structure surface; 
(c) trickle application of the feed over the surface; 
(d) painting where the feed has been incorporated into a paint medium, the 
paint being applied by spraying as noted above or by brushing; 
(e) where the feed has been produced as a gel, painting or spraying as 
noted above may be used; or 
(f) foam. 
After the basic feed has been applied to the surface it is necessary to 
maintain the process until an acid-producing microbial biofilm is 
established. Once the biofilm has been established, nutrients continue to 
be applied. Such application may be by continuous, semi-batch or batch 
means. 
Recycling of nutrients, for example, may also be appropriate to make the 
most cost effective use of materials. 
During the decontamination process, it may also be necessary to remove 
degradation products and waste effluent from time to time or on a 
continuous basis until the desired amount or depth of degradation has been 
achieved. Alternatively, the degradation products may be removed at the 
end of a desired degradation cycle. 
When the degree of degradation has been achieved, the process must be 
stopped. This can be achieved by removing or stopping the nutrient supply 
or by applying a sterilising process such as heat, pressure, irradiation 
or application of a biocide for example. 
Removal of degradation products from the surface may be effected by various 
techniques including vacuum suction, scraping, scabbling, abrasion 
blasting, washing or by any other known technique appropriate to a wet or 
a dry process as used in any particular circumstances. 
Treatment and separation of dead biomass, unused nutrients, solid and 
liquid waste forms may be achieved by known techniques such as filtration; 
chemical means , e.g. ion-exchange, distillation and precipitation for 
example; biological, e.g. biosorption, biodegradation, bioaccumulation and 
biotransformation for example. 
The ultimate disposal of the treated biomass may be by any known technique, 
for example by cementation to reduce leach rate. 
The method of the present invention may be employed to decontaminate 
surfaces contaminated with various kinds of hazardous material, for 
example, radioactive or non-radioactive heavy metal species. 
Such metal species may include metal compounds and complexes as well as 
metals in elemental, alloy or ionic form. 
Hazardous metal species which can be present as surface contaminants and 
treated in this way may include: 
(i) actinides or their radioactive decay products or compounds thereof; 
(ii) fission products; 
(iii) toxic heavy metals or compounds thereof. 
Actinides are elements having periodic numbers in the inclusive range 89 to 
104. 
The term "fission product" as used herein refers to those elements formed 
as direct products (or so-called "fission fragments") in the fission of 
nuclear fuel and products formed from such direct products by beta decay. 
Fission products include elements in the range from selenium to cerium 
including elements such as Ba, Zr, Te, Cs and Ce. 
Non-radioactive heavy metals which can be present as contaminants on a 
surface treated by the method of the present invention may include toxic 
metals such as cobalt, chromium, lead, cadmium and mercury which are 
commonly found as contaminants around industrial plants. 
In order that the present invention may be more fully understood, an 
example will now be described by way of illustration only with reference 
to the accompanying drawing which shows a schematic flow diagram of a 
process according to the present invention.

An example of the above is the decontamination of concrete where spores of 
Aspergillus carbonarius are applied in a suspension to a pre-treated 
surface, the pre-treatment being with a solution of a nutrient source such 
as a carbon source of sucrose or molasses for example. The citric acid 
produced as the spores germinate and the fungi grow, forms calcium citrate 
with the concrete, the calcium citrate being insoluble, and heavy metal 
citrates with most of the heavy metal contaminants which may be present on 
the surface. The soluble contaminant citrates are removed from the system 
by, for example, circulation of the liquor through a suitable treatment 
plant for removal of the heavy metal citrates. Solid degradation products 
may be removed by known techniques as discussed hereinabove.