Biological treatments, referred to as bioremediation, encompass several tech­niques which can involve the redox transformation, biological accumulation or breakdown of a contaminant. Chemical speciation (oxidation state and com­plex form) is one of the primary controls on the mobility of metal contaminants in the environment, affecting both their solubility and reactivity with surfaces. For example, the metal chromium is mobile and highly toxic in the Cr(vi) state, but is both less mobile and up to 1000 times less toxic as the Cr(iii) oxidation state.59 The radionuclide, 60Co can form a stable and mobile complex with

Can be performed in situ.

No additional nutrients required.

Relatively low cost compared to physiochemical methods. Not governed by physiological constraints of living cells. No secondary waste produced.

Metal recovery is possible, especially from process waters. Specific contaminants can be targeted.

Can be performed in situ.

Relatively low cost compared to physiochemical methods. No secondary waste produced.

Specific contaminants can be targeted e. g. Cs1 transported by K1-uptake processes.

Potential for re-oxidation and re-mobilisation of metals and radionuclides.

Complex groundwater or soil chemistry can complicate or prohibit treatment.

Regular monitoring required to assess effectiveness.

Can only operate in conditions required for cell growth (i. e. limited pH range).

Early saturation can require metals desorption to continue use.

No potential for degradation of compounds.

Targeting certain contaminants may require the cultivation and introduction of species not natively present.

Very limited commercial application.

Requires subsurface conditions favourable for microbial metabolism.

Toxicological effect on cell may inhibit cell metabolism or lead to cell death.

Targeting certain contaminants may require the cultivation and introduction of species not natively present.

Very limited commercial application.

May only operate over a specific pH range in certain cases. Mineral precipitation may clog pore spaces restricting groundwater flow to contaminants further from injection wells.

Treatment is limited to the surface area and depth of the plant roots.

Possibility of contaminants entering the food chain.

Slow growth and low biomass require long-term commitment. Saturation of contaminants may lead to toxicity affecting plant survival.

Technique	Advantages	Disadvantages
Chemical Oxidation	Can be performed in situ. Rapid treatment time. Ability to treat high concentrations of contaminants.	Non-selective High capital and operating costs Most methods operate over a narrow pH range
Sediment Washing	Closed system allows easier control of geochemical conditions. Can treat both organic and inorganic contaminants in the same system. Relatively low cost.	Ex situ technique. Ineffective in removing metals in the residual phase of sediments. Certain chelating agents used present an environmental risk themselves.
Electrokinetic	Ability to treat organic and inorganic contaminants simultaneously. Can operate in zones of low hydraulic flow through induction of electric field. Effective at removing high concentrations of contaminants. Can operate in situ. Contaminants can be removed with electrodes.	Non-selective, problems can arise if target ions are in much lower concentrations than non-target ions. Corrosion of anodes in acidic conditions. Contaminants removed may require further disposal. Precipitation of metals close to electrode can impede process. Requires continued operational costs.
In situ Vitrification	Can treat organic, inorganic and radionuclide contaminants simultaneously. Can be completed in situ with fused glass blocks remaining in place. Compacts original volume of contamination by up to 20-50%. One-step, fast process. Helps prevent leaching of contaminants.	Water in soils affects operational time and costs. Requires special equipment and training. High energy input needed.
Permeable Reactive Barrier	Can be performed in situ. Ability to treat multiple contaminants simultaneously. Typically low capital and operating costs compared with pump and treat systems. Variety of reactive media can be used to target specific contaminants. Long-term efficiency can be improved through adsorption from secondary precipitated minerals. Passive system requiring no ongoing energy input.	Mineral precipitation may passivate certain reactive media. Groundwater flow must be well characterised. Mineral precipitation may reduce permeability of barrier and affect groundwater flow. Limited to shallow depths (< 15.24 metres) due to construc­tion challenges.

Table 5 Continued.

100 Richard Kimber, Francis R. Livens and Jonathan R. Lloyd

ethylenediaminetetraacetic acid (EDTA) in the Co(iii) state, but is less stable and hence less mobile in the Co(ii) state.60

It has long been established that microorganisms are able to reduce metals,61,62 with more recent work showing they are able to use such processes to conserve energy for growth. Focussing on reductive transformations, microbes are able to use some metals as the terminal electron acceptor during anaerobic respiration, in environments where oxygen has been depleted. Thus, stimulating their activity in the subsurface can cause the reduction of high oxidation state metal contaminants to less soluble forms and hence retard their migration. The mechanisms involved in microbial metal and radionuclide reduction are described in detail elsewhere.59,63 Microorganisms are also able to reduce and degrade some organic contaminants through analogous respiratory processes when supplied with a suitable electron donor. For example, almost 98% of tetrachloroethylene (PCE) underwent complete reductive dechlorina­tion to ethane when a laboratory column experiment used Rhine river sediment supplied with lactate as an electron donor.64 Trichloroethylene (TCE), an industrial solvent and common subsurface contaminant,65 was also shown to be degraded by the methanotrophic bacterium Methylosinus trichosporium OB3b in a co-metabolic process in a copper deficient medium.66 The reader is directed towards a recent review by Pant and Pant, for a detailed account on the microbial remediation of TCE.67

Metal and radionuclide transport can also be restricted through precipitation with enzymatically generated ligands, such as sulfide68,69 and phosphate (see Figure 1).63 If supplied with an excess of these ligands then most of the metal should be removed from solution. An advantage to this method is that high concentrations of ligand are generated close to the cell surface which can act as nucleation foci for the onset of metal precipitation. An integrated approach to metal remediation using sulfur-cycling bacteria has been demonstrated.70

Figure 1 Diagram illustrating an integrated approach to bioremediation of metal — contaminated soils. The conditions and inputs required for bioleaching and bioprecipitation are displayed along with the outline reactions for each stage where M21 = target metal ions (considered as divalent cations). (Adapted from C. White, J. A. Sayer and G. M. Gadd, FEMS Microbiol. Rev., 1997, 20, 3-4).

In this study, a number of metals were leached from artificially contaminated soil through the production of sulfuric acid by sulfur-oxidising bacteria. This lea­chate was then applied to a bioreactor containing sulfate-reducing organisms where greater than 80% of the metals were precipitated as solid metal sulfides.

The bacterial strains Rahnella sp. and Bacillus sp. were both shown to be capable of hydrolysing sufficient organophosphate to remove up to 95% of uranium in a simulated groundwater system. The system was most efficient between pH 5.0 and 7.0 with EXAFS spectroscopy identifying the uranyl phosphate precipitate as an autunite/meta-autunite group mineral.71 This builds on earlier work on a Citrobacter (now classified as a Serratia) strain which coupled the efflux of phosphate driven by phosphotase-mediated breakdown of glycerol-2-phosphate to efficient uranium precipitation.72 A case study involving phosphate biomineralisation at the Hanford site is discussed in detail later in this chapter. The biosorption and bioaccumulation of metals may act as a component in metal remediation through sorption of metals to cell surfaces or uptake into the cell. This can occur as a physiochemical, metabolic — independent mechanisms whereby metals sorb onto the surface of biomass or via metabolic-dependent processes in which the metal is taken up into the cell where it may precipitate locally and accumulate. Both processes have been reviewed extensively but a lack of commercial development has weakened continued research into this field.73 76

These techniques can be achieved through several different methods. Bios­timulation involves the addition of key nutrients, such as an electron donor and carbon source, to the subsurface to stimulate the native microorganisms, usually done via injection wells. Advantages of such a method include the sti­mulation of extant bacteria that are already well suited to the environmental conditions and distributed throughout the subsurface. Relying on the local geology and hydrogeology to distribute the nutrients evenly can however, prove to be a disadvantage.

If the native bacteria do not have the metabolic capability to remediate a par­ticular contaminant then bioaugmentation can be employed where by specialised microorganisms are added to the subsurface, along with the required nutrients, in order to remediate the contaminant. A number of reviews are available on the processes involved in bioaugmentation.77,78 Both the aforementioned techniques operate in situ but ex situ bioremediation is also a possibility. Ex situ treatment involves the excavation of contaminated soil or pumping of groundwater into an above ground facility where the biological conditions can be better controlled. Although excavation and pumping is more expensive than in situ treatments, benefits include being able to adjust to aerobic or anaerobic conditions as required. The ability to operate in aerobic conditions allows certain bacteria to utilise organic contaminants, such as petroleum hydrocarbon mixtures and polycyclic aromatic hydrocarbons, as their source of carbon and energy thus potentially degrading the contaminants completely to CO2 and H2O. A further advantage of ex situ remediation is the ability to homogenise and continuously monitor the soil to ensure complete treatment occurs. Numerous studies exam­ining the effectiveness of ex situ bioremediation have been performed.79 81

Phytoremediation, which utilises the ability of plants to degrade or accu­mulate contaminants, can also be employed in the remediation of soil and groundwater. The cost-effectiveness and non-environmentally disruptive nature of phytoremediation offers advantages over other bioremediation techniques. Further advantages include the ability to easily monitor the plants and the possibility of recovering and re-using valuable, accumulated metals. However, there are a number of disadvantages associated with this process which includes remediation being limited to the surface area and depth of the plant roots, the possibility of contaminants entering the food chain and the usually long period of time phytoremediation requires for completion. For further details, the reader is directed to a number of recent reviews.82,83