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4.5.9 Ex-sito technologies (SRT)

Contents
4.5.9.1 Physical ex-situ techniques
4.5.9.1.1 Physical segregation
4.5.9.1.2 Segmented gate systems
4.5.9.1.3 Soil washing
4.5.9.2 Chemical ex-situ techniques
4.5.9.2.1 Chemical/Solvent extraction
4.5.9.2.2 Heap leaching
4.5.9.2.3 Enhanced soil washing
4.5.9.2.4 Chemical precipitation
4.5.9.2.5 Ion exchange
4.5.9.2.6 Adsorption
4.5.9.2.7 Aeration
4.5.9.2.8 Ozonation and peroxide application
4.5.9.3 Biological ex-situ techniques
4.5.9.3.1 Land farming and bio-piles
4.5.9.3.2 Bio-reactors
4.5.9.3.3 Bio-leaching
4.5.9.3.4 Bio-sorption
4.5.9.4 Thermal ex-situ techniques
4.5.9.4.1 Distillation
4.5.9.4.2 Incineration
4.5.9.4.3 Pyrolysis
4.5.9.4.4 Thermal desorption

Ex-situ treatment is the maximum intervention option. These technologies rely on bringing the waste or radioactively contaminated material to the remediation technology, rather than the other way around. The aims of ex-situ processing are to ensure a more consistent standard of clean-up, and avoid the difficulties inherent with in-situ techniques. Such techniques may not be suitable for the very low concentrations of activity likely for widespread contamination problems, due to small concentration gradients [IAEA-1999].

Ex-situ treatments of materials radioactively contaminated by non-radioactive substances such as oil, solvents, heavy metals and other chemicals have been applied on an industrial scale. The technologies adopted include soil washing, solidification, biological treatment and incineration.

Technologies to clean-up ground and surface waters contaminated with hazardous waste usually rely on pumping followed by ex-situ above ground treatment. The technologies applied closely resemble traditional water treatment technologies used to treat industrial and municipal wastewater.

If the waste is not diluted in the water, these ex-situ technologies are based on the effectiveness of the pumping system in capturing the wastes and bringing them to the surface with the groundwater for treatment. If pumping cannot remove the particles with the adsorbed contaminants from the aquifer, the ex-situ treatment technologies do not have an opportunity to treat them.

Enhancements to traditional pump and treat technologies include pulsed pumping, reinjection, and chemical extraction. These enhancements promote more efficient removal/treatment of less mobile contaminants in less homogeneous, less permeable aquifers.

Extraction of radioactively contaminated groundwater for treatment can be achieved by extraction wells or trenches. The regulatory authorities normally set criteria that must be met before the treated groundwater can be released or re-injected into the environment. The residual wastes from a groundwater treatment system may be radioactive enough to require disposal. If natural flushing is the appropriate procedure for aquifer restoration, the groundwater clean-up period may be shortened using gradient manipulation to direct the flow, injection wells to increase the flow rate, and limited extraction, treatment and re-infection.

Overall, ex-situ techniques are just a potential component in an overall waste/remediation strategy. Even if they are not applied directly to the waste, as in the groundwater examples above, they are of benefit for the treatment of secondary wastes generated by other treatment techniques. (An example is the use of solidification for conditioning of ion exchange resins used in the treatment of groundwater).

The main ex-situ treatment technologies for all wastes fall into three categories: physical, chemical and biological. A separate section is also addressed to thermal treatment methods which were difficult to place into the mentioned categories.

4.5.9.1 Physical ex-situ techniques

These technologies rely on the physical properties of the materials to achieve separation or to fix the contamination to prevent the spread of activity. However, since physical separation of radionuclides is almost always associated with the removal of the clay fraction of the soil matrix, the process will result in a decrease in soil fertility. If the land is to be used for crop production, addition of soil conditioners such as fertilizers will be necessary to restore land fertility after the remedial activity.

Physical separation may be used with chemical extraction to produce fractions with higher concentration of contaminants in smaller volumes. The physical separation technologies may also be suitable for removing radionuclides which have been deposited as solid particulate in the soil.

4.5.9.1.1 Physical segregation

Contamination is often associated with particular size fractions or mineral phases of a soil. Separation and segregation of the contaminated fraction will greatly reduce the amount of material requiring further treatment and disposal, while freeing the reminder for reuse.

A variety of separation techniques have been borrowed from mineral processing, including mechanical sieving and screening, hydraulic size fractionation in, for example, settling tanks or hydro-cyclones, specific gravity separators such as shaking tables or sluices, surface chemistry related processes such as froth floatation, and processes based on the different magnetic susceptibilities of different minerals. A combination of these techniques may be required to isolate the relevant fractions. Segregation is often the first step before one of the above chemical extraction methods is applied. The latter are also referred to as soil washing, if they form part of an extraction procedure.

Liquid-particle separation involves removal and collection of dispersed or colloidal solid particles in a fluid suspension. Liquid-particle separation categories include: screening, membrane filtration, cycloning, flotation, thickening/sedimentation, filtration and centrifugation.

Among these, filtration is the most widely used liquid-particle separation process applied to groundwater treatment from radionuclides and heavy metal contaminants.

4.5.9.1.2 Segmented gate systems

The segmented gate system (SGS) is a characterization and sorting technology that measures the radioactivity of soil, sand, dry sludge or any material that can be transported by conveyor belts, and mechanically separates radioactive contaminated material into clean and contaminated waste streams. This is accomplished by passing the material on a conveyor belt under an array of sensitive, rapidly reacting, radiation detectors that measure radionuclide concentrations. Material above the desired clean-up limits is automatically diverted into a separate waste stream. In this system, contaminants are isolated and removed by locating small particles of dispersed radioactive material, thus significantly reducing the overall amount of material requiring disposition as radioactive waste.

A variety of sensors can be utilized for detection of specific contaminants (i.e., sodium iodide, calcium fluoride or high purity germanium). Typical radionuclides that can be measured by segmented gate systems include 137Cs, 60Co, 226Ra, 232Th, 238U and 241Am. While the detection level for the system depends on the ambient radiation background, conveyor belt speed, thickness of the material layer on the conveyor, and contaminant gray energy and abundance, lower limits of detection, 0.074 Bq/g for 241Am and 0.185 Bq/g for 226Ra, have been successfully demonstrated.

4.5.9.1.3 Soil washing

This ex-situ technique uses pH controlled solutions with the addition of acids or bases, surfactants to dissolve, desorb and remove contaminants. Organic solvents may be used for organic contaminants. A preceding size fractionation improves efficiency and reduces the volumes of material to be treated.

Soil washing techniques are promising for an application to soils contaminated with a wide variety of heavy metal, radionuclide and organic contaminants. Complex mixtures of contaminants in the soil, such as a mixture of metals, non-volatile organic compounds and semi-volatile organic compounds, and heterogeneous contaminant compositions throughout the soil make it, however, difficult to formulate a single suitable washing solution that will consistently and reliably remove all of the different types of contaminant. For such cases, sequential washing, using different washing formulations and/or different soil to washing fluid ratios, may be required. Soil washing is a media transfer technology, i.e., the resulting contaminated water or other solvents need to be treated with a suitable technique and disposed of. The technique offers the ability for recovery of metals and can clean coarse grained soils from a wide range of organic and inorganic contaminants:

  • Aliphatic hydrocarbons, i.e., mineral oils;
  • Polycyclic aromatic hydrocarbons (PAHs);
  • Heavy metals such as Cu, Zn, Pb, Cd, Cr, Hg, Co, Ni and Sn;
  • Pesticides such as insecticides, herbicides and fungicides;
  • Other organic halogenated compounds (e.g., polychlorinated biphenyls) or phenolic compounds;
  • Inorganic contaminants, such as arsenic or cyanide compounds (free or complexed).

A major disadvantage of soil washing is that in many cases it will destroy the (biological) functionality of the soil, in particular when applied to topsoil. The functionality of topsoils depends on the mixture between different grain sizes, the clay and humus contents, and the indigenous microbial flora and fauna. Often a sterile product results, as the latter two constituents are removed or destroyed. Experiments are under way in various countries to reconstitute functionality by adding compost to the soil before returning it to nature.

4.5.9.2 Chemical ex-situ techniques

Ex-situ chemical methods are based on chemical or physico-chemical extraction of soils treated with inorganic and organic solvents to dissolve and selectively remove metals, including radionuclides or to enhance physical separation. For liquids, the aim is to reduce the volume of material to be handled by selective removal of contaminants.

4.5.9.2.1 Chemical/Solvent extraction

The effectiveness and efficiency of a given solvent will depend on the type of binding between the radionuclide and the soil substrate and on the chemical species of the radionuclide. The choice of chemicals that can be applied is much more varied than for in-situ treatment, given the better control of the processes and the fact that the operation can be carried out in closed reaction vessels. Considerations of environmental impact from the remediation operation, for example unwanted effects on the groundwater and aquifers, are restricted to considerations that apply to similar industrial operations.

This method uses a chemical extractant to remove the contamination from the waste, with the aim of concentrating the activity into a separate liquid stream, which can then be treated/disposed separately. The conditions for chemical extraction (temperature, contact time, etc.) will have a significant affect on the efficiency of extraction. The various applicable chemical extraction techniques for solids include extraction with:

  • Water;
  • Inorganic salts;
  • Mineral acids, and
  • Complexing agents.

For liquids, solvent extraction is more usual, where a solvent is used to selectively remove the contaminant from the wastewater stream. Solvent extraction is viable when there is a high concentration of contaminant to be removed.

However, care must be taken in the selection of the solvent; firstly so that regeneration of the solvent is possible by stripping out the contaminant (to avoid creation of an additional waste), and secondly that the residual solvent in the cleaned wastewater will not result in adverse effects (e.g., non-radioactive pollution of the aquifer or enhanced mobilization of residual activity) [IAEA-1999].

4.5.9.2.2 Heap leaching

The contaminated material (generally soil) is excavated and placed (heaped) on an impermeable pad on the surface of the ground. The pad is sloped towards a sump at the bottom edge of the heap. The selected leaching reagent(s) are pumped to the top of the heap and distributed with a drip irrigation system or aerial sprayers. The reagent travels down through the soil, solubilizing and mobilizing the contaminants. The leachate is collected from the sump and pumped to a leachate treatment and regeneration system. The principle of the method is displayed in Figure 4.28.
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Figure 4.28 A heap leaching system
Figure 4.28 A heap leaching system

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4.5.9.2.3 Enhanced soil washing

This method combines the physical separation of soil washing with chemical extraction. The net result is a concentration of the waste material into the fines fraction, and reduced loadings of contaminants in the coarse fractions. The enhancement of standard soil washing improves the decontamination of the cleaned material for return to site.

Additional processes may be added to the basic soil washing process (e.g., crushing, froth flotation, activated carbon addition) and the wash medium may operate with chemical additives to enhance performance, e.g., pH adjusters, detergent addition, coagulants/flocculants, etc.

4.5.9.2.4 Chemical precipitation

Chemical precipitation is used to remove soluble activity from liquids, both as a volume reduction method and to permit it to be disposed of separately. This also includes related techniques such as coagulation and floe precipitation. Because of concentration effects and solubility limits, these techniques are more effective for high concentrations of contaminants.

The precipitation has long been a primary method of treating metal laden industrial waste and drinking waters. Because of the success of this process in these applications, the technique is often considered and selected for use in groundwater remediation containing heavy metals, including their radioactive isotopes. In groundwater treatment, the metal precipitation process may be used as a pre-treatment for other treatment techniques (such as chemical oxidation or air stripping) where the presence of metals would otherwise interfere with the other treatment processes.

In the specific case of radium contamination, barytes (BaSO4) can be used to co-precipitate radium from the water, as radium can substitute for barium in the mineral structure. Attempts have been made to clean radium contamination from mining waters. In addition, barytes is a desired admixture in any radioactively contaminated materials due to its effective attenuation of gamma radiation [IAEA-1999].

4.5.9.2.5 Ion exchange

Ion exchange is the complement to chemical removal. This removes soluble activity from liquid wastes and concentrates it onto a solid ion exchange material. The ion exchange materials function at lower concentrations of activity than chemical removal. Selection of the correct ion exchange material is important. Certain materials are used to hold the activity for disposal, others can be regenerated using an eluting agent (normally a mineral acid or similar). In the latter case, the concentrated eluant may then be treated by chemical methods.

Ion exchange can remove dissolved metals and radionuclides from aqueous solutions. Other compounds that have been treated include nitrate, ammonia and silicate. There are a number of factors that affect the applicability and effectiveness of the process:

  • Oil and grease in water may clog the ion exchange media.
  • A suspended solid content higher than 10 ppm may cause resin binding.
  • Low pH values of the influent may lead to effective competition of the protons with the contaminant ions for binding sites and, hence, a reduction in the efficiency of the process.
  • Strong oxidants in the water may damage the ion exchange resin.
4.5.9.2.6 Adsorption

This method uses adsorption of the contaminant by various media, such as granular activated carbon, which is a common medium for drinking water treatment; activated alumina, which can be used for the treatment of some radioactive compounds; and selective complexes, which essentially complex the contaminant and are not regenerable. It is therefore similar to the use of ion exchange.

The physico-chemical process of adsorption can be used to remove contaminants from liquids, slurries or gases. The process is based on the affinity of some constituents for certain types of surface. An adsorbent, for example certain types of clay, zeolites and granulated activated carbon, is brought into contact with a contaminated medium. After saturation has been reached, the adsorbent with the contaminant attached is removed for further processing. The contaminant is either desorbed, i.e., the adsorbent is ‘regenerated’, or the adsorbent is conditioned and treated, for example cemented into drums, for storage and disposal.

The most common adsorbent is granular activated carbon. Other natural and synthetic adsorbents include: activated alumina, forage sponge, lignin, sorptive clays and synthetic resins [IAEA-2006b].

Adsorption can also be used for radon if decontamination of slowly released gas is required. Polyethylene coated activated carbon is used to adsorb the radon gas, as the polyethylene coating is permeable for radon diffusion but can stop any other gas or vapours which can reduce the adsorption quality of carbon.
The target contaminants for adsorption processes are most organic contaminants and selected inorganic contaminants from liquid and vapour streams. Factors that may limit the applicability and effectiveness of these processes include:

  • Poor sorption of water soluble organic compounds and monovalent ions;
  • High costs if used as the primary treatment on waste streams with high contaminant concentrations;
  • Typically not applicable to sites with high levels of oily substances;
  • Not practical where the concentrations of contaminants are so high that sorption capacities are quickly reached and frequent replacement of the adsorption unit is necessary.
4.5.9.2.7 Aeration

Aeration is used to remove volatile compounds from wastewater. Generally, this is to remove organic compounds with the potential to complex radionuclides. In the context of radioactively contaminated wastewaters, aeration can be used to sparge out radon, which can then be treated. In addition, aeration can be used to alter the redox potential of the wastewater prior to subsequent chemical treatment, to facilitate removal of certain radionuclides (e.g., uranium).

4.5.9.2.8 Ozonation and peroxide application

Oxidation processes including ultra-violet radiation, ozone and/or hydrogen peroxide are used to destroy organic contaminants as water flows into a treatment tank. If ozone is used as the oxidizer, an ozone destruction unit is used to treat collected off-gases from the treatment tank and downstream units where ozone gas may collect, or escape.

Ultraviolet oxidation is a destruction process that oxidizes organic and explosive constituents in water by the addition of strong oxidizers and irradiation with ultra-violet light. Oxidation of target contaminants is caused by direct reaction with the oxidizers, ultra-violet photolysis and the synergistic action of ultra-violet light, in combination with ozone (O3) and/or hydrogen peroxide (H2O2). If complete mineralization is achieved, the final products of oxidation are carbon dioxide, water and salts.

The main advantage of ultra-violet oxidation is that it is a destruction process, as opposed to air stripping or carbon adsorption, for which contaminants are extracted and concentrated in a separate phase. Ozonation is routinely applied in waterworks to disinfect raw water during the production of drinking water.

Similarly, hydrogen peroxide is a strong oxidant that has been used to disinfect water and to oxidize organic contaminants. Peroxide can also be applied to slurries or soils made into slurries. The disadvantages are relatively high costs and the fact that a considerable portion of the peroxide is consumed by the soil organic matter. An unwanted side effect is that a largely sterile soil will result due the latter effect.

Practically any organic contaminant that is reactive with the hydroxyl radical can potentially be treated by oxidation and ultra-violet oxidation. A wide variety of organic and explosive contaminants are susceptible to destruction, including petroleum hydrocarbons; chlorinated hydrocarbons used as industrial solvents and cleaners, and explosive compounds such as tri-nitro-toluene (TNT), cyclo-trimethylene-trinitramine (RDX) and high melting point explosive, cyclo-tetramethylene-tetranitramine (HMX). In many cases, chlorinated hydrocarbons that are resistant to bio-degradation may be effectively treated by ultra-violet oxidation. Typically, easily oxidized organic compounds, such as those with double bonds (e.g., tri-chloro-ethylene (TCE), per-chloro-ethylene (PCE) and vinyl chloride), as well as simple aromatic compounds (e.g., toluene, benzene, xylene and phenol), are rapidly destroyed in ultra-violet oxidation processes.

4.5.9.3 Biological ex-situ techniques

These methods use the same generic treatment processes as for in-situ remediation (see Section 4.5.8.2). Unlike the in-situ processes, however, for ex-situ treatment the contaminated material, micro-organisms and nutrients are added to a suitable mixing vessel. Conditions are then optimized to degrade the contaminants. Most biological treatment is aimed at degradation of organic materials, and so will have value with mixed (hazardous/radioactive) contamination.

The use of mobilizing micro-organisms (siderophores, bio-mimetic analogues, etc.) is also feasible. These use bio-chemistry to convert the radionuclides to a soluble form. The process results in a leach solution that is treated to remove and concentrate the contaminants. The treated leach solution is then recycled to minimize costs and secondary wastes.

The main advantage of ex-situ soil treatment is that it generally requires shorter time periods than in-situ treatment, and there is more certainty about the uniformity of treatment because of the ability to homogenize, screen and continuously mix soils. An advantage over thermal treatment is that no volatile radionuclides need to be contained. However, ex-situ treatment requires excavation of soils, leading to increased costs, equipment engineering requirements, possible permission needs, and material handling and worker exposure considerations.

A difficulty with the use of biological methods is their viability, both in terms of maintaining a viable bio-culture (nutrient supply, temperature variations, absence of biocides in the material to be treated), and with the low tolerance of certain micro-organisms to high radiation fields.

This group of techniques usually involves spreading excavated contaminated soils in a thin layer on the ground surface and stimulating aerobic microbial activity within the soils through aeration and/or the addition of minerals, nutrients and moisture [IAEA-2006b].

Examples of biological ex-situ treatment methods are:

  • Land farming and bio-piles;
  • Bio-reactors;
  • Bio-leaching:
  • Bio-sorption.
4.5.9.3.1 Land farming and bio-piles

Land farming (Figure 4.29) has been proven effective in reducing concentrations of nearly all the constituents of petroleum products. Petroleum products generally contain constituents that possess a wide range of volatility. In general, gasoline, kerosene and diesel fuels contain constituents with sufficient volatility to evaporate from a land farm. Lighter (more volatile) petroleum products (e.g., gasoline) tend to be removed by evaporation during land farm aeration processes (i.e., tilling or plowing) and, to a lesser extent, to be degraded by microbial respiration. Depending upon the regulations for air emissions of volatile organic compounds, these emissions may need to be controlled, for example by putting the land farm under a tent. The midrange hydrocarbon products (e.g., diesel fuel and kerosene) contain lower percentages of lighter (more volatile) constituents than gasoline. Bio-degradation of these petroleum products is more significant than evaporation. Heavier (non-volatile) petroleum products (e.g., heating oil and lubricating oils) do not evaporate during land farm aeration; the dominant mechanism that breaks down these petroleum products is bio-degradation. However, higher molecular weight petroleum constituents, such as those found in heating and lubricating oils, and, to a lesser extent, in diesel fuel and kerosene, require a longer period of time to degrade than do the constituents in gasoline.
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Figure 4.29 Land farming to treat organic wastes
Figure 4.29 Land farming to treat organic wastes

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While the technological and process control requirements are not very sophisticated, a large land area may be required for larger quantities of contaminated soil. Typical land farms are uncovered and, therefore, exposed to climatic factors including rainfall, snow and wind, as well as ambient temperatures. Rainwater that falls directly onto, or runs onto, the land farm area will increase the moisture content of the soil and may cause erosion. During and following a significant precipitation event, the moisture content of the soils may be temporarily in excess of that required for effective bacterial activity. On the other hand, during periods of drought, the moisture content may be below the effective range and additional moisture may need to be added. Erosion of land farm soils can occur during windy periods and particularly during tilling or plowing operations. Wind erosion can be limited by plowing soils into windrows and applying moisture periodically. In colder regions the length of the land farming season typically ranges from 7 to 9 months. In very cold climates, special precautions can be taken, including enclosing the land farm within a greenhouse type structure or introducing special bacteria (psychrophiles) that are capable of activity at lower temperatures. In warm regions, the land farming season can last all year.

The technical arrangements for land farming or bio-piles may include the construction of leachate capture and treatment systems as well as vapour and odour control (Figure 4.29 and Figure 4.30). Control of soil moisture, for example by drainage, may also be required to provide optimum growth conditions. Soils may need to be pre-treated to adjust pH to the optimum, circum-neutral range for most organisms. Growth can be stimulated by addition of nutrients, for example nitrogen and phosphorus, or essential elements, if respective deficiencies exist in the soils to be treated. Cattle or chicken manure is a typical additive, which also introduces additional micro-organisms. Microbial strains specialized to particular contaminants may be obtained as inoculants from commercial suppliers.
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Figure 4.30 The principle of bio-pile arrangements
Figure 4.30 The principle of bio-pile arrangements

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4.5.9.3.2 Bio-reactors

The principles of the treatment process in bio-reactors are rather similar to those of land farming except that the process takes place in a closed vessel and is, therefore, amenable to tighter process control.

Slurry phase biological treatment involves the controlled treatment of excavated soil in a bio-reactor (Figure 4.31). The excavated soil is first processed to physically separate stones and rubble. The soil is then mixed with water to a predetermined concentration dependent upon the concentration of the contaminants, the rate of bio-degradation and the physical nature of the soils. Some processes prewash the soil to concentrate the contaminants. Clean sand may then be discharged, leaving only contaminated fines and wash water to bio-treat. Typically, a slurry contains from 10 to 30 % solids by weight [IAEA-2006b].
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Figure 4.31 A typical bio-reactor arrangement
Figure 4.31 A typical bio-reactor arrangement

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The solids in a reactor vessel are maintained in suspension and mixed with nutrients and oxygen. If necessary, an acid or alkali may be added to control pH. Micro-organisms also may be added if a suitable population is not present. When bio-degradation is complete, the soil slurry is dewatered. Dewatering devices that may be used include clarifiers, pressure filters, vacuum filters, sand drying beds and centrifuges. Slurry phase bio-reactors may be classified as short to medium term technologies.

A variety of bio-remediation methods have been developed for ex-situ metal recovery. These methods may range from complex, process controlled sets of bio-reactors to relatively simple heap leaching arrangements (see below). Such methods can have the added value of recovering metals in relatively high purity making them a marketable commodity that would help to pay for the treatment [IAEA-2006b].

Factors that may limit the applicability and effectiveness of the slurry phase bio-treatment process include [IAEA-2006b]:

  • Excavation of contaminated media is required, except for lagoon implementation.
  • Sizing of materials prior to putting them into the reactor can be difficult and expensive. Non-homogeneous soils and clayey soils can create serious material handling problems.
  • Dewatering soil fines after treatment can be expensive.
  • An acceptable method for treating and disposing of non-recycled wastewater is required.
4.5.9.3.3 Bio-leaching

Bio-leaching occurs naturally when micro-organisms assist in the slow weathering of out-cropping sulphide ore bodies. Bio-leaching is an established bio-technological process for the dissolution and hence mobilization of valuable metals from ores by micro-organisms. Metals for which this technique is mainly employed are copper, cobalt, nickel, zinc, gold, silver and uranium. It is estimated that about 20 – 30 % of the world’s copper production originates from bio-leaching; in the case of uranium it is judged to be about 5 – 10 %. Bio-leaching has also been promoted as a cost efficient method for metal value recovery in developing countries, and its applicability in this context has recently been reviewed [IAEA-2004b].

Bio-leaching also has scope for application in reworking waste material from mining for enhanced recovery of metals, including radionuclides, which has the potential to reduce the environmental burden. The method has been explicitly applied to the remediation of uranium and other mining legacies. The pathways of the resulting contaminated waters have to be carefully controlled, for example by arrangements similar to those for heap leaching (see Section 4.5.9). Microbially mediated leaching processes frequently have the unwanted side effect of acid mine drainage (AMD) generation, for example by pyrite oxidation [IAEA-2004b].

The types of ore that are amenable to bio-leaching comprise sulphides, carbonates and oxides. The groups of micro-organisms involved are mainly bacteria and fungi. In some cases algae and lichens may also play a role. Various mechanisms are involved, depending on the type of ore in question.

In the case of sulphidic minerals the predominant dissolution causing micro-organisms are acidophilic (meaning organisms living between pH0 and pH5) bacteria of the sulphur and iron cycles, namely Acidithiobacillus (abbreviation A., former name Thiobacillus) ferro-oxidans, A. thio-oxidans, Leptospirillum (abbreviation L.) ferro-oxidans, A. caldus, Metallogenium sp., Sulfobacillus thermosulfido-oxidans, Sulfolobus sp., Acidianus brierleyi and several others. The species of Acidithiobacillus live in the moderate temperature range (0 – 45 ºC), Metallogenium and Sulfobacillus thrive at elevated temperatures (40 – 65 ºC) and Sulfolobus and Acidianus are thermophiles growing from 65 to 90 ºC.
The dissolution is generally effected by two mechanisms, depending on the type of mineral to be dissolved:

  • Pyrite and molybdenite and a few other minerals of the same structure can only be dissolved by an oxidizing attack on their crystal lattice, owing to their electronic configuration (non-bonding outer orbitals) [IAEA-2004b]. The bacteria able to do this are the Fe(II) oxidizing A. ferro-oxidans, L. ferro-oxidans and Acidianus sp. This mechanism is known as the thio-sulphate mechanism.
  • All other sulphidic minerals possess bonding outer orbitals and thus are more or less dissolvable by a hydrolytic attack involving protons. In addition, Fe(III) ions further the dissolution by an oxidizing attack. These minerals may consequently be dissolved by all the above mentioned bacteria of the sulphur and iron cycles. This dissolution process is known as the polysulphide mechanism.

In both cases, the dissolution of the mineral is mainly effected by bacteria attached to the surface of the respective mineral. The compounds mediating such attachment are exopolysaccharides (EPS) (‘slimes’). The exopolysaccharides consist, from a chemical point of view, mainly of lipids, carbohydrates, sugar acids (uronic acids) and complexed, inorganic ions such as Fe(III) ions. The distance between the bacterial cell and the mineral substrate surface is of the order of 20 to 50 nm. This space is filled with the exopolysaccharide, creating a reaction space with unknown conditions of pH, redox and ion concentrations; the reaction space is an extension of the radius of action of the cell, thus allowing it to augment its food supply. As a consequence, biological leaching becomes considerably accelerated (sometimes more than 100-fold) compared with the purely chemical process utilizing Fe(III) ions and/or protons only. In the latter process the freely suspended planktonic cells also have to be considered, since their effect is mainly the re-oxidation of the iron ions in solution. Bio-leaching is thus an interface process and belongs to the area of nano-bio-technology.

Final products of dissolution are metal cat-ions, Fe(III) ions, sulphate and/ or sulphuric acid. The energy of the oxidation is partially conserved by the bacteria for metabolic purposes and growth. The bacteria possess specialized cell components allowing them to conserve some of the energy in a utilizable form (adenosine tri-phosphate (ATP), nicotinamide adenine dinucleotide (NADH)). Furthermore, they need only carbon dioxide from the air to build up their cell mass and inorganic trace elements. These are therefore very specialized organisms; this type of metabolism is called litho-autotrophy.

The above mentioned bacteria are generally not important for carbonate and/or oxide ores. Bacteria and fungi are used for dissolving such minerals, which, due to an unbalanced metabolism, excrete organic acids. This requires an ample supply of exogenous carbon sources, which they metabolize, and as a consequence of either too much substrate, or a lack of essential nutrients or trace elements such as nitrogenous compounds or minerals, excrete partly in an intermediate oxidation state. Excreted acids are, for example, citric, oxalic, succinic, malic, acetic and/or formic acid, and sugar (uronic acids) or amino acids. These acids dissolve and/or complex metal cat-ions and thus solubilize them.

The bio-leaching technique is employed in several forms:

  1. In the case of low grade ores that for economic reasons cannot be processed by conventional roasting or other similar processes, a heap leaching process is applied (see Section 4.5.9.2.2). In the majority of cases in which this technique has been applied to date, the ore contained copper, zinc and trace elements. A limited number of experiments of this type have been performed for extracting uranium from low grade ores. One experiment was carried out near Ronneburg, Germany, by Wismut in the 1980s, another one at Elliott Lake in Canada. For this purpose large amounts of low grade ores are placed on leach pads (plastic liners) or dumped in valleys with a known and impermeable geological strata and sprinkled regularly with acidified bacteria-containing solution (which originates from similar operations or from acid mine waters). The dissolved metals and sulphate plus sulphuric acid are left to accumulate to a concentration at which extraction processes such as solvent extraction, ion exchange and/or electro-winning become viable. Residence times for such operations range from several months to a few years. If these heaps are constructed without consideration of the underlying geology and/or abandoned without care, acid mine-rock drainage (ARD) or acid mine drainage (AMD) may result. Abandoned mines, mine shafts, open pits, etc., might also produce and release acid mine-rock drainage (ARD). Owing to the acidity combined with dissolved heavy metals, this might result in serious environmental damage and/or even create new ecosystems (as in Rio Tinto, Spain).
  2. Bio-leaching has been employed experimentally at the field scale in Germany to treat heavy metal contaminated sediments [IAEA-2004b]. The generic scheme from the raw sediment to a viable soil substrate is illustrated in Figure 4.32.
  3. In the case of sulphidic concentrates, bio-leaching is increasingly used for extracting precious metals. In recent years several plants have gone into operation that use acidophilic leaching bacteria for extracting gold, nickel and cobalt. The operation usually consists of stirred tanks (bio-reactors) with volumes of up to 1000 m3 in continuous operation. Residence times are in the range of 3 to 7 days.
  4. In the case of radioactive minerals, there may also be another, unwanted effect: an enhanced emission of radon. Comparison of the radon emission rates and bio-leaching activity at the above mentioned leaching waste heaps near Ronneburg, Germany, has shown that high cell numbers of leaching bacteria were found at sites with high radon emissions, whereas at sites with low emissions only low cell numbers occurred. An explanation for this effect comes from the mineralogy of the ore. At Ronneburg the uranium is embedded in pyrite. Once this pyrite has been attacked by bio-leaching, radon is liberated and may escape into the atmosphere. This causes an additional exposure for the local population and requires measures to reduce or even inhibit the biological process.
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Figure 4.32 The experimental process from raw contaminated sediment to reconditioned soil [IAEA-2004b]
Figure 4.32 The experimental process from raw contaminated sediment to reconditioned soil [IAEA-2004b]

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4.5.9.3.4 Bio-sorption

Micro-organisms, such as bacteria or fungi, have been used as minute biological reactors that can efficiently and economically carry out specialized operations. Microbial biomass, whether living or not, has been shown to selectively sequester and retain elements from dilute aqueous solutions via a process named bio-sorption.

Through the process of bio-sorption the bio-sorbed species are selectively removed from the solution and are retained inside the microbial cells (biomass) in concentrations that are several orders of magnitude higher than those in the original solution. Heavy metals and radionuclides are taken up into cellular components such as the cell walls of certain micro-organisms, which then can be harvested, carrying along the sequestered radionuclides. Bio-sorption is being explored in hydrometallurgy to concentrate metal bearing solutions, for example from heap leaching, and in the treatment of contaminated mining effluents [IAEA-2004b].

Engineering developments in the area of bio-sorption have led to the design of engineered bio-sorbents, microbial biomass cells or cellular components immobilized on or within various matrices, thus acquiring the form of small particles such as those of conventional adsorbents (e.g., activated carbon) or ion exchange resins. Organic cellular material derived from higher plants or algae have also been proposed as the basic material for manufacturing bio-sorbents that can be used for the extraction of metals, including radionuclides [IAEA-2004b].

Bio-sorption methods are largely ex-situ methods applicable for diluting contaminated solutions such as groundwaters or seepage. The contaminated solution is pumped into engineered reactors, in which it contacts the immobilized microbial biomass under optimized conditions (solution pH, flow rate, etc.). The contaminants are retained in an insoluble form by the biomass and the treated solution is let out of the reactor. The process of bio-sorption is reversible under certain conditions, which means that after the bio-sorbent is exhausted it could potentially be used for regeneration, releasing the previously held radionuclides in a small volume of the regenerating solution. Alternatively the bio-sorbent could be used once through and then disposed of appropriately.

Bio-sorption is an equilibrium process, with solution pH playing the role of the master variable, since it defines the speciation of the elements in the solution. This also means that the key driving force that dictates the bio-sorptive uptake capacity of the biomass in terms of mass of bio-sorbed species per unit mass of bio-sorbent (also referred to as the loading capacity) is the residual concentration of the contaminants after treatment and not the initial contaminant concentration [IAEA-2004b].

The optimal bio-sorption pH depends on the biomass used and on the elements being removed; for example, the bio-sorption of uranium by the fungus Rhizopus arrhizus appears to be optimal at pH4, with significant reduction of the metal uptake capacity as the pH drops to pH2. The increased concentration of hydrogen ions at the acidic pH along with the chemical effects on the cell walls of the micro-organisms is responsible for this reduction in capacity. However, increasing the pH towards neutral values may again create operational problems, depending on the composition of the contact solution. The hydrolysis and subsequent precipitation of ferric ions which adsorb on to (coat) the bio-sorbent adversely affect the bio-sorption process [IAEA-2004b].

The bio-sorption of metals by algal biomass is another example in which the sequestering of metals such as lead, zinc or copper by micro-organisms such as Chlorella vulgaris, Chlorella regularis or Chlamydomonas sp. is optimal in the range of pH6 to pH8. The bio-sorption of oxy-anions such as chromates or selenates by the same type of algae has an optimal bio-sorption pH in the acidic range of pH2 to pH3 [IAEA-2004b].

Bio-sorption of 226Ra by several types of micro-organisms, such as Rhizopus arrhizus, Aspergillus niger and Streptomyces niveus, exhibited an optimal contact pH in the neutral to alkaline range, with corresponding radium equilibrium uptake capacities in the range of tens of MBq/g. It is therefore obvious that optimization of bio-sorption processes should be made on a case-by-case basis and requires increased care so that the process will perform satisfactorily.

Considerable efforts have been made to understand the underlying mechanisms of bio-sorption and to improve the process efficiency. The available information has shown that cell walls are the major bio-sorption functional sites for heavy metals, uranium and thorium. It has also been shown that extracellular polymeric substances (EPS) play a significant role in bio-sorption [IAEA-2004b]. The molecular level understanding of the bio-sorptive processes is still limited to selected pairs of metals and micro-organisms. The microbial biomass provides ligand groups on to which the metal species bind. In addition, sorptive and hydrolysis processes play a role. Three major classes of microbial bio-polymers (proteins, nucleic acids and polysaccharides) provide bio-sorption sites. Different ionic species of a given element might exhibit preference for a different binding site. Should the preference of one metal ion for a ligand be similar to that of another ion, a bio-sorption competition effect might be observed if both elements are simultaneously present in the contact solution.

A model of bi-sorption competition effects that is based on Pearson’s classification of metals has been reported as a basic tool for understanding such effects. On the basis of this model, significant ionic competition effects can be observed for metals belonging to the same Pearson classification class. Elements belonging to different classes demonstrate limited competition, while elements belonging to the Pearson’s classification borderline class are affected by the presence of competing co-ions. Additional systematic work for the mechanistic understanding of bio-sorptive processes and the associated ionic competition effects is required [IAEA-2004b].

Numerical simulation techniques play an important role in designing and assessing remediation processes, including those using bio-technological methods [IAEA-2004b]. Although bio-sorption using inactive microbial biomass has been demonstrated to be effective in substantially removing (and in some cases recovering) targeted radionuclides such as uranium, radium and thorium from contaminated solutions, a full scale commercial application is not yet available. The use of living micro-organisms in innovative reactor configurations has recently been under investigation for the same purposes as conventional bio-sorption. This approach to the biological sequestering of metals has substantially different requirements and operating conditions than conventional inactive biomass bio-sorption. This alternative bio-technological approach is often referred to as bio-accumulation or bio-precipitation and is showing excellent potential.

4.5.9.4 Thermal ex-situ techniques

Excavated contaminated soils or sludge’s are also the subject of heat treatment ex-situ when other methods are not applicable. Described methods below include:

  • Distillation as a heat chemical separation technique;
  • Incineration as a destruction method to transfer contaminants from soil and sludge to a safer state;
  • Pyrolysis used for decomposing organic contaminants in excavated soil or sludge by heat in anaerobic conditions;
  • Thermal desorption of soil and waste to temperatures in which organic contaminants become volatile and desorb;
  • Vitrification for destroying or removing organic compounds and immobilizing most inorganic compounds in contaminated soil or sludge (method described in Section 4.5.3);
  • Fluid bed steam reforming to destroy organic contaminants by high temperature steam.
4.5.9.4.1 Distillation

Basically, distillation is a chemical separation process involving vaporization and condensation that is used to separate components of varying vapour pressures (volatilities) in a liquid or gas waste stream. Simple distillation involves a single stage operation in which heat is applied to a liquid mixture in a still, causing a portion of the liquid to vaporize. These vapours are subsequently cooled and condensed to a liquid product termed the distillate or overhead product. The distillate is enriched with the higher volatility components. Conversely, the mixture remaining in the still is enriched with the less volatile components. This mixture is termed the bottoms product. Multiple staging is utilized in most commercial distillation operations to obtain better separation of organic components than is possible in a single evaporation and condensation stage.
Most organic contaminants and certain radionuclides (210Pb and 210Po), heavy metals (Hg) and cyanide are volatile. The volatility increases with temperature so that such contaminants can be driven off by heating the soils concerned and recovering the gaseous contaminants. Distillation is also a side effect of the various in-situ thermal treatment methods discussed in Section 4.5.8.6. Ex-situ, the process can be made more efficient, if carried out in a vacuum. The variation in boiling points between various hydrocarbons and other volatile contaminants can be used to drive off and recover selectively the various compounds (fractionation distillation).

4.5.9.4.2 Incineration

During incineration, high temperatures, 870 – 1200 °C, are used to volatilize and combust (in the presence of oxygen) halogenated and other refractory organic compounds from contaminated soils or wastes. Auxiliary fuels are often employed to initiate and sustain combustion. The destruction and removal efficiency for properly operated incinerators exceeds 99.99 % for hazardous and toxic organic compounds. Incinerator off-gases require treatment by an air pollution control system to remove particulates and neutralize and remove acid gases (HCl, NOx and SOx). Baghouses, venturi scrubbers and wet electrostatic precipitators remove particulates; packed bed scrubbers and spray driers remove acid gases. The end products are CO2, water and ash.

Typical incinerator designs include circulating bed combustors, fluidized bed combustors, infrared combustion combustors and rotary kilns:

  1. Circulating bed combustors (CBCs) use high velocity air to entrain solids and create a highly turbulent combustion zone that destroys toxic hydrocarbons. These combustors operate at lower temperatures than conventional incinerators (790 – 880 °C). Effective mixing and the low combustion temperature of circulating bed combustors reduce operating costs and potential emissions of such gases as nitrogen oxide and carbon monoxide.
  2. Circulating fluidized beds use high velocity air to circulate and suspend the waste particles in a combustion loop, and operate at temperatures up to 880 °C.
  3. The infrared combustion technology is a thermal processing system that uses electrically powered silicon carbide rods to heat organic materials and wastes to combustion temperatures. Wastes are fed into a primary chamber and exposed to infrared radiant heat (up to 1010 °C) provided by silicon carbide rods above the conveyor belt. A blower delivers air to selected locations along the belt to control the oxidation rate of the waste feed. Any remaining combustible substances are incinerated in an afterburner.
  4. Commercial incinerator designs are rotary kilns, equipped with an afterburner, a quench, and an air pollution control system. The rotary kiln is a refractory lined, slightly inclined, rotating cylinder that serves as a combustion chamber and operates at temperatures up to 980 °C.

Incineration is used to remediate soils contaminated with explosives and hazardous wastes, particularly chlorinated hydrocarbons, polychlorinated biphenyls (PCBs) and dioxins. Factors that may limit the applicability and effectiveness of the process include:

  • There are specific feed size and materials handling requirements that can have an impact on the applicability or cost at specific sites.
  • Heavy metals can produce a bottom ash that requires stabilization.
  • Volatile heavy metals and radionuclides, such as lead, cadmium, mercury and arsenic, as well as 210Po and 137Cs, will collect in the off-gas scrubbers and will require treatment and disposal.
  • Metals can react with other elements in the feed stream, such as chlorine or sulphur, forming more volatile and toxic compounds than the original species. Such compounds are likely to be short lived reaction intermediates that can be destroyed in a caustic quench.
  • Sodium and potassium form low melting point ashes that can attack the brick lining and form a sticky particulate that fouls gas ducts.
  • Some organic compounds require rather high temperatures to be broken down completely along with careful process control in the cooling phase.
  • The formation of dioxins and furans is a well known problem resulting from poor process control and too low temperatures during combustion.

Flameless combustion in electrical furnaces with better temperature gradient control may overcome this problem [IAEA-2006b]. These problems, together with the high energy demands and the resulting sterile material when applied to soils, have generally resulted in incineration finding disfavour in many countries.

In addition to conventional flame or flameless incineration, interest in microwave methods for (radioactive) waste treatment is increasing, (see Section 4.5.8.6). With organic materials a volume reduction of 90 % can be achieved, the residuals being glass-like slag’s or molten metals. Again, off-gas treatment for volatile constituents is needed. Owing to the absence of a hot combustion gas stream, however, the volumes to be treated are lower.

4.5.9.4.3 Pyrolysis

Pyrolysis is a form of incineration that chemically decomposes organic materials by heat in the absence of oxygen. Pyrolysis occurs under pressure and at operating temperatures above 430 ºC. In practice, it is not possible to achieve a completely oxygen-free atmosphere. Because some oxygen is present in any pyrolysis system, a small amount of oxidation occurs.

In pyrolysis systems, organic materials are transformed into gases, small quantities of liquid, and a solid residue containing carbon and ash. The off-gases are typically treated in a secondary thermal oxidation unit. Particulate removal equipment is also required, which can include scrubbers and high efficiency particulate air (HEPA) filtration.

Several types of pyrolysis units are available, including rotary kilns, rotary hearth furnaces and fluidized bed furnaces. These units are similar to incinerators except that they operate at lower temperatures and with less air supply.

Pyrolysis is not effective in destroying or physically separating inorganic compounds, including radionuclides, from the contaminated medium. Volatile metals in the off-gas stream must be captured in a scrubbing unit. Residuals containing heavy metals may require chemical stabilization before final disposal. When the off-gases are cooled, liquids will condense producing an oil/tar-like residue and contaminated water. These oils and tars may be hazardous and require further treatment prior to disposal.

The target contaminant groups for pyrolysis are semi-volatile organic compounds (SVOC) and pesticides. Pyrolysis is applicable to the separation of organic compounds from refinery wastes, coal tar wastes, wood treatment wastes, soil contaminated with creosote and hydrocarbons, mixed (radioactive and hazardous) wastes, synthetic rubber processing wastes and paint wastes. Factors that may limit the applicability and effectiveness of the process include:

  • There are specific feed size and materials handling requirements that affect applicability or cost at specific sites.
  • Soil requires drying to achieve a low moisture content (< 1 %);
  • Highly abrasive feed can potentially damage the processor unit;
  • High moisture content increases treatment costs;
  • Treated media containing heavy metals may require stabilization.
4.5.9.4.4 Thermal desorption

Thermal desorption physically removes volatile hazardous and toxic organic compounds and volatile heavy metals (cadmium, lead and mercury) and radionuclides (210Pb, 210Po and 137Cs) from contaminated soil and wastes by application of heat. The target contaminant groups are non-halogenated volatile organic compounds (VOC), semi-volatile organic compounds (SVOC), polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), pesticides and fuels. Thermal desorbers are designed to heat soil and wastes to temperatures sufficient to cause contaminants to volatilize and desorb. Although they are not designed to decompose/destroy organic constituents, thermal desorbers can, depending upon the specific organic compounds present and the operating temperature, cause some of the constituents to completely or partially decompose. The vaporized organic compounds are generally treated in a secondary treatment unit (e.g., an afterburner, catalytic oxidation chamber, condenser or carbon adsorption unit) prior to discharge to the atmosphere. Afterburners and oxidizers destroy organic constituents. Condensers and carbon adsorption units trap organic compounds for subsequent treatment or disposal.

Some pre- and post-processing of soil and wastes is necessary when using thermal desorption. Soil must be screened to remove large (greater than 5 cm diameter) objects, which may be sized (e.g., crushed or shredded) and then re-introduced back into the feed material. Waste streams may also be ground in a homogenizer mill to a size less than 5 mm before treatment. After leaving the desorber the soil is cooled, remoistened to control dust, and stabilized (if necessary) prior to disposal or reuse.

Thermal desorption is applicable to constituents that are volatile at temperatures as high as 650 ºC. Most desorbers operate at temperatures of 150 – 540 ºC. They are constructed of special alloys that can operate at temperatures up to 650 ºC. More volatile constituents (e.g., gasoline) can be desorbed in the lower operating temperature range, while semi-volatile contaminants (e.g., diesel fuel) generally require temperatures in excess of 370 ºC, and relatively non-volatile contaminants (e.g., lubricating oils) require even higher temperatures.

Thermal desorption systems fall into two general classes: stationary facilities and mobile units. Contaminated soil is excavated and transported to stationary facilities; mobile units are operated directly on-site. Desorption units are available in a variety of process configurations including rotary desorbers, thermal screws and conveyor furnaces.

The presence of moisture in the soil and wastes to be treated will determine the residence time required and the heating requirements for effective removal of contaminants. In order for desorption of organic constituents to occur, most of the moisture must be evaporated in the desorber. This can require significant thermal input to the desorber and excessive residence time. Soil and wastes with excessive moisture contents (> 20 %) must be dewatered prior to treatment. Typical dewatering methods include air drying, mixing with drier soil and mechanical dewatering.

The presence of metals can have two implications:

  1. Limitations on disposal of residual solid wastes;
  2. Limitations on metal concentrations due to air emission requirements.

However, at normal operating temperatures, heavy metals and most radionuclides are not likely to be significantly separated from soils.

Factors that may limit the applicability and effectiveness of the process include:

  • There are specific particle size and material handling requirements that can have an impact on applicability or cost at specific sites.
  • Dewatering may be necessary to achieve acceptable soil moisture content levels.
  • Highly abrasive feed can potentially damage the processor unit.
  • The presence of chlorine can affect the volatilization of some metals, such as lead.
  • Heavy metals in the feed may produce a treated solid residue that requires stabilization.
  • Clay, silty soils and high humid content soils increase reaction time as a result of binding of contaminants.
4.5.9.4.5 Fluid bed steam reforming

Steam reforming destroys the hazardous organic portion of mixed wastes by exposing it to high temperature steam [IAEA-2006b]. The process occurs in two phases. In the first phase, waste streams are exposed to steam at moderate temperatures. This volatilizes the organic components and separates them from the inorganic components of the waste stream (similar to thermal desorption). The volatilized organic compounds are transported to another reaction chamber for the second phase treatment, where the gaseous organic compounds are exposed to very high temperature steam, which destroys the organic compounds (Figure 4.33). The radionuclides and non-volatile heavy metals remain in the primary reaction chamber in their solid form. Fluid bed steam reforming uses superheated steam and co-reactants in a fluidized bed to evaporate liquids, destroy organic compounds, convert nitrates, nitrites and nitric acid into nitrogen gas and immobilize heavy metals, including radionuclides. To provide high nitrate and mineral conversion rates, steam reformers are operated in a strongly reducing environment. Carbon and iron based additives (reductants) are used to convert nitric acid, nitrates and nitrites directly to nitrogen gas in the reformer. Clay or other inorganic co-reactants are added to the waste feed, or bed, to convert the radionuclides, alkali metals, sulphate, chloride, fluorine, phosphate and non-volatile heavy metals into an immobilized mineral product. The final waste form is highly stable and leach resistant [IAEA-2006b].

Gases and fine particulate matter entrained in the gases from the reformer are treated in a secondary unit that can also absorb metal fumes from any volatile metals in the waste stream. When treating waste containing any radioactivity, high efficiency particulate air (HEPA) filtration is provided. The only significant gaseous releases are carbon dioxide and water vapour emissions.

Fluid bed steam reformers are operated at 600 – 800 ºC under a small vacuum. The fluidized bed material is generally a granular product solid that accumulates in the bed during processing. Small units can be heated electrically. For production scale units, the energy is supplied by the incoming superheated steam and the introduction of oxygen with the steam to provide oxidation of the organic compounds and carbon from the wastes.
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Figure 4.33 Simplified flow diagram of fluidized bed steam reforming process [IAEA- 2006b]
Figure 4.33 Simplified flow diagram of fluidized bed steam reforming process [IAEA- 2006b]

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Wastes and contaminated materials that can be effectively treated by steam reforming include: radioactive waste with/without hazardous constituents, organic solvents, spent activated carbon, sludge’s, off-gas scrubber recycle streams, decontamination solutions, oils, polychlorinated biphenyls (PCB), ion exchange media and resins, plastics, sodium hydroxide solutions and wastes with high concentrations of Cl, F, S, P and heavy metals, where the final waste must be stabilized to meet heavy metal and radionuclide leach resistance and disposal site performance criteria.

During operation, the contaminated material is introduced into the system at the bottom of the fluid bed. Water in the wastes is evaporated and superheated to the bed temperature by the large mass of hot fluidized product solids. As the water in the waste feed evaporates, the temperature of dried waste solids rises to reaction temperatures. Organic compounds in the wastes are volatilized and pyrolysed upon contact with the hot bed solids. The volatile organic compounds are subjected to steam reformation in the bed. The nitric acid, nitrates and nitrites are converted to nitrogen gas when they come into contact with the reducing agents in the bed.

Alkali metals, non-volatile heavy metals, radionuclides, S, Cl, F, P and other inorganic constituents combine with co-reactants such as clay to form stable, high melting point, crystalline minerals that become the final solid product. The superheated steam, residual acid gases and fine particulates are carried into secondary units for further treatment. The accumulated product solids are semi-continuously removed from the bottom of the reformer as a fully immobilized, water insoluble, product.

The main energy requirements include: evaporation and superheating any incoming water in the waste feed, heating the organic and inorganic constituents, and supplying the heat of reaction for endothermic reformation reactions of steam with carbon and organic compounds. The main sources of energy for the reformer are the superheat of the incoming steam fluidizing gas, the reaction of nitrates with reductants to form nitrogen gas and the oxidation of organic compounds and carbon reductants in the bed.