Importance of microbiology in the development of sustainable technologies for mineral processing and wastewater treatment

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1 Importance of microbiology in the development of sustainable technologies for mineral processing and wastewater treatment D. Barrie Johnson. School of Biological Sciences, University of Wales, Bangor, LL57 2UW. U.K. Abstract Mining companies have become increasingly aware of the potential of microbiological approaches for recovering base and precious metals from low-grade ores, and for remediating acidic, metal-rich wastewaters that drain from both operating and abandoned mine sites. Biological systems offer a number of environmental and (sometimes) economical advantages over conventional approaches, such as pyrometallurgy, though their application is not appropriate in every situation. There are two major areas in which biological systems are currently utilised in full-scale operations by the mining industries: metal extraction ( biomining ) and treatment of acid mine drainage (bioremediation). A third, much-researched area, biosorption of metals, has yet to become established as a proven biotechnology. Mineral processing using microorganisms has been exploited for extracting gold, copper, uranium and cobalt, and current developments are targeting other base metals. Engineering systems ranging from crude heap leaching systems to temperature-controlled bioreactors have been used, depending on the nature of the ore and the value of the metal product. Recently, there have been major advances on the microbiology these (and related) ecosystems, which will allow greater control (e.g. of redox potentials) of bioleaching operations, and thereby greater efficiencies and faster rates of metal extraction. Biological treatment of acidic mine effluents mostly involves the use of constructed wetlands. More recently, compost bioreactors have been shown to be effective in both generating alkalinity and removing heavy metals (e.g. as sulfides). Although these passive systems generally require little maintenance once constructed, their performance may be variable and they do not allow recovery and reuse of metals from the waste streams. In contrast, active bioremediation systems, based on microbial sulfidogenesis, offer both system control and separation (and recovery) of metals. This paper presents a concise review of current applications and developments in the field of biohydrometallurgy, and of how projected developments could allow future expansion of microbiological applications in the mining industries. 1

2 1. Introduction Microorganisms have had significant impact on the extraction and recovery of metals from ores and wastes long before their roles were recognised. Construction of precipitation ponds at the Rio Tinto mine (southern Spain) and the Parys mine (Anglesey, north Wales) to recover copper from leached rocks by cementation is documented during the th centuries. It was not until the middle of the 20 th century that the first bacteria that accelerate the dissolution of metal-containing sulfide minerals at these (and other) sites were discovered. Realisation that the abilities of these microorganisms to oxidise minerals could be harnessed in more precisely engineered operations led to the emergence of a biotechnology, often referred to as biomining. The advantages of bioprocessing of ores and concentrates over more conventional approaches such as pyrometallurgy have been described elsewhere (e.g. Rawlings, 2002) and include the potential for processing lowgrade deposits and re-processing earlier metal-containing wastes, the production of less chemically-active tailings, lower energy inputs and other environmental benefits (zero production of noxious gases etc.). Current full-scale mining operations that utilise microorganisms involve the processing of sulfidic ores. However, there are other microbiological systems that can liberate metals from oxidised ores (e.g. Ehrlich et al., 1995; Ehrlich, 1997) or be used to remove metal impurities from sands and clays (Dudeney et al., 1999). In a non-metal context, bacteria that solubilise pyrite have been used to pre-treat high sulfur-containing coals, in order to lower the amounts of sulfur dioxide produced when the fossil fuels are combusted. Whilst this is a well researched area of applied microbiology (e.g. Bos et al., 1992) that has resulted in at least one full-scale operation (on Sardinia), the economics on microbial depyritization of coal relative to alternative approaches (most notably scrubbing of flue gases) has resulted in relatively little take up of the biological technology. The other major impact of biological systems on the mining industry is in cleaning up polluted ecosystems and the recovery of metals from solid and liquid wastes. Bioremediation and biorecovery often use the same or similar principles, such as microbial sulfidogenesis. One much-researched area involves the use of biological materials (e.g. biomass produced in fermentation processes) to absorb metals from wastewaters. Though biosorption has been promulgated as being cost-effective (relative to alternative technologies, such as ion exchange resins) for removing (and often recovering) metals when present in low concentrations, this technology has still to be scaled up into a full-scale operation (Tsezos, 1999). In contrast, the use of constructed wetlands and compost bioreactors to remediate waste streams draining from mines is increasingly perceived to be a viable approach for long-term treatment of acid mine drainage waters. Whilst these passive biological systems 2

3 have the drawback of offering little or no system control, more precise and directed (and more expensive) biotechnology is available in the form of sulfidogenic bioreactors. 2. Bioprocessing of sulfidic ores and concentrates The subject area has been described recently in two excellent review articles (Rawlings et al., 2003; Rawlings, 2002), while detailed accounts may be found in a text edited by Rawlings (1997). Bioprocessing of sulfides can be divided into bioleaching, which results in the solubilisation of target metals (e.g. copper from chalcopyrite and covellite) and biooxidation, whereby microbial dissolution of pyrite and arsenopyrite associated with finegrain gold allows extraction of the precious metal by cyanidation. Besides these two metals, biomining has been harnessed to extract uranium and cobalt, and other metals (such as nickel and zinc) are due to be commercially bioprocessed in the future. 2.1 Mineral bioprocessing: mechanisms. Sulfide minerals may be divided into those, such as zinc sulfide (sphalerite), that are acid soluble, and those, such as pyrite and arsenopyrite, that are acid-insoluble. Two different routes (the thiosulfate and polythionate mechanisms) have been proposed for the biological oxidation of these sulfide mineral types (Schippers and Sand, 1999). Acid-soluble sulfides are readily degraded by sulfur-oxidising acidophiles. In this model, the mineral is first subjected to proton-mediated dissolution, forming the free metal and hydrogen sulfide: MS + 2 H + M 2+ + H 2 S The hydrogen sulfide so formed is microbially oxidised to sulfuric acid, allowing the process to continue: H 2 S + 2 O 2 H 2 SO 4 Acid-soluble sulfides may also be attacked by ferric iron, producing ferrous iron and polysulfide: MS + Fe 3+ + H + M H 2 S n + Fe 2+. The latter may be further oxidised by ferric iron producing elemental sulfur which, in turn, is oxidised to sulfuric acid, which will further accelerate mineral dissolution via proton attack: 0.5 H 2 S n + Fe S 8 + Fe 2+ + H S O 2 + H 2 O 2H SO 4 In contrast, sulfides that are resistant to proton attack are oxidised solely by ferric iron, producing thiosulfate as an initial by-product: FeS Fe H 2 O 7 Fe 2+ + S 2 O H +. 3

4 Figure 1. Hypothetical scheme for the oxidation of pyrite by iron- and sulfur-oxidising acidophilic bacteria (Schippers and Sand, 1999). Thiosulfate is unstable in acidic liquors, particularly in the presence of ferric iron, and is oxidised to tetrathionate, which in turn decomposes to a variety of other poythionates, sulfur and sulfate, via a proposed highly-reactive disulfane-monosulfonic acid intermediate (Schippers and Sand, 1999). There has been considerable debate about the proposed direct and indirect mechanisms of sulfide mineral oxidation, the former referring to that mediated by prokaryotes attached to the mineral surface and the latter to that by free-swimming (planktonic phase) microorganisms. These models have largely been superseded by recent advanced in the understanding of how lithotrophic ( rock eating ) acidophiles accelerate the oxidative dissolution of sulfide minerals, as just discussed. For example, acid-insoluble sulfides are attacked by ferric iron produced by iron-oxidising bacteria and archaea (described below) whether they are attached to the mineral surface (in which case the iron is tightly associated with the glycocalyx produced by the sessile cells) or in the liquid phase, where ferric iron oxidises the mineral following transport by diffusion of mass action, as illustrated in Fig. 1. The terms direct and indirect, which used to imply different (but in the case of the former, a largely unknown) mechanisms are now redundant. More appropriate alternatives may be contact and non-contact leaching to indicate that the mechanism is the same, though the proximity of the microbe to the mineral surface may differ. 4

5 2.2 Mineral bioprocessing: microorganisms. The conditions that ensue when sulfide ores and concentrates are subjected to accelerated oxidation include low ph, high concentrations of dissolved metals and, in some cases, elevated temperatures. These conditions limit the diversity of life forms that occurs in commercial bioleaching operations. They are invariably simple, often single-celled organisms, and predominantly prokaryotic (bacteria and archaea). Most live only in extremely acidic liquors (ph <1-4) and are obligate acidophiles, some are thermophilic (to varying degrees) and some fix carbon dioxide (as green plants). The primary microorganisms involved in mineral oxidation are those that catalyse the oxidation of ferrous iron and/or reduced sulfur whilst others contribute to the process indirectly by, for example, removing materials that accumulate during ore dissolution and which are inhibitory to the mineral-oxidisers (Johnson, 2001). The known diversity of bacteria that have been implicated in accelerating sulfide mineral oxidation has expanded beyond Acidithiobacillus ferrooxidans (formerly Thiobacillus ferrooxidans) which was once considered (and still is by some scientists) to be the most significant pyrite-oxidiser. Some of the recent discoveries have extended the range of conditions within which ore bioprocessing may occur. For example, the bacterium Sulfobacillus montserratensis and the archaea Ferroplasma spp. can oxidise pyrite at ph <1 (Yahya and Johnson, 2002; Edwards et al., 2000) and novel halotolerant iron-oxidisers offer the possibility of using saline and brackish waters (the only available sources in some parts of the world) in bioleaching operations (Simons and Norris, 2002). A list of the more significant prokaryotes that have been implicated in mineral bioprocessing operations is given in Table 1. There are, surprisingly, relatively few detailed descriptions of the microbial communities that are present in commercial mineral processing operations, and many of the most active strains present have yet to be characterized (Norris et al., 2000). In one such recent study of a moderately thermophilic (45 C) stirred tank system leaching a polymetallic concentrate, three bacteria and one archaeon were enumerated and isolated in pure culture (Okibe et al., 2003). Total and relative numbers of the prokaryotes were found to change as the concentrate passed through the three in-line reactors (Fig. 2). Two of the more novel isolates (thermotolerant Leptospirillum and Ferroplasma spp.) were studied in detail, and the importance of stable mixed communities of acidophiles in mineral processing highlighted. 2.3 Mineral bioprocessing: engineering processes. Rawlings (2002) categorised the engineering approaches used in biomining as: (i) irrigation-based principles (dump- and heap-leaching, and in situ leaching) and, (ii) stirred tank processes. The earliest engineering technology used ( dump leaching ) was very basic, and involved gathering low-grade (otherwise waste) copper-containing ore of large rock/boulder size into vast mounds or dumps and irrigating these with dilute sulfuric acid to encourage the growth 5

6 Table 1. Acidophilic Prokaryotic Microorganisms Involved in Commercial Mineral Processing Operations Mineral-degrading acidophiles 1a. Iron-oxidizers Leptospirillum ferrooxidans Thermal Classification* Mesophile L. ferriphilum Mesophile L. thermoferrooxidans Moderate Thermophile Ferrimicrobium acidiphilum Ferroplasma acidiphilum Fp. acidarmanus 1b. Sulfur-oxidizers Acidithiobacillus thiooxidans At. caldus Metallosphaera spp. Sulfolobus spp. 1c. Iron- and sulfur-oxidizers Acidithiobacillus ferrooxidans Acidianus spp. Sulfolobus metallicus 1d. Iron-reducers Acidiphilium spp. 1e. Iron-oxidizers/reducers Acidimicrobium ferrooxidans 1f. Iron-oxidizers/reducer and sulfur-oxidizer Sulfobacillus spp. Mesophile Mesophile Mesophile/thermotolerant Mesophile Moderate thermophile Extreme Thermophile Extreme Thermophile Mesophile Extreme Thermophile Extreme Thermophile Mesophile Moderate thermophile Mesophiles and Moderate thermophiles * Mesophiles (T optimum <40 C); Moderate thermophiles (T optimum C). Extreme thermophiles (T optimum >60 C). and activities of mineral-oxidising acidophiles, primarily iron-oxidising mesophiles. Copper was precipitated from the metal-rich streams draining from the dumps using by displacement with scrap iron ( copper cementation ). Later developments on the engineering and hydrometallurgical aspects of biomining have involved the use of thin layer heaps of refractory sulfidic ores (mostly copper, but also gold-bearing material) stacked onto waterproof membranes, and solubilized copper recovered using solvent extraction coupled with electrowinning (SX/EW). In situ bioleaching was developed to scavenge for uranium and copper in otherwise worked out mines. This involves fracturing underground workings using explosives, percolating with acidic leach liquors containing metal-mobilising bacteria, 6

7 Table 2. Evolution of populations of bioleaching microorganisms in a series of in-line stirred tank bioreactors maintained at 45 C (Okibe et al., 2003). pumping the pregnant liquor to the surface and extraction of solubilized metals. More recently, aerated stirred tanks have been used to process sulfidic ore concentrates. These tanks, which may be extremely large (up to 1,350 m 3 ), allow for greater control (e.g. of temperature; sulfide mineral oxidation being an exothermic process) of biooxidation of mineral ores. To date, stirred tank bioreactors used for mineral processing have tended to operate between 40 C and 50 C (i.e. where moderate thermophiles and thermotolerant acidophiles would tend to be of greatest significance). However, a high temperature (>70 C) stirred tank system will be commissioned in the near future at the Chuquicamata copper mine, Chile, to process a mixed copper concentrate (containing enargite, chalcocite, chalcopyrite and covellite; Rawlings et al., 2003). Schematics of heap and stirred-tank bioleaching operations are shown in Fig Bioremediation of metalliferous mine waters Acidic, sulfur-rich wastewaters are the by-products of a variety of industrial operations such as galvanic processing and the scrubbing of flue gases at power plants (Johnson, 2000). The major producer of such effluents is, however, the mining industry. Waters draining active and (in particular) abandoned mines and mine wastes are often net acidic (sometimes extremely so) and typically pose an additional risk to the environment by the fact that they often contain elevated concentrations of metals (iron, aluminium and manganese, and 7

8 (a) copper metal acid irrigation electrowinning copper ore heap solvent extraction (b) mineral concentrate water & nutrients liquid ph adjustment & disposal make-up tank primary aeration tanks secondary aeration tanks settling tank solids to cyanidation & gold recovery Figure 3. Schematic diagrams of (a) heap bioleaching and (b) stirred tank bioleaching of sulfide ores and concentrates (redrawn from Rawlings, 2002). possibly other heavy metals) and metalloids (of which arsenic is generally of greatest concern). The global scale of the environmental pollution caused by mine water discharges is difficult to assess accurately, though in 1989 it was estinmated that ca. 19,300 km of streams and rivers, and ca. 72,000 ha of lakes and reservoirs had been seriously damaged by mine effluents. 8

9 Acid mine drainage may form in underground workings (ground-waters) of deep mines, though this is generally of minor importance when a mine is in active production and water tables are kept artificially low by pumping. However, when mines are closed and abandoned, and the pumps turned off, the rebound of the water table can lead to contaminated groundwater being discharged, sometimes in a catastrophic event, such as happened at the Wheal Jane mine in Cornwall, U.K. in Because the water that refills the mine dissolves any acidic salts that have built up on the exposed walls and ceilings of underground chambers, this initial drainage water tends to be more potentially polluting (in terms of acidity and metal content) than AMD that is discharged subsequently (Clarke, 1995). Acidic, metalrich waters may also form in spoil heaps and mineral tailings, essentially by the same biologically-driven reactions (section 2.1) as in mine shafts and adits. Due the more disaggregated (and concentrated, in the case of tailings) nature of the acid-generating minerals in these waste materials, AMD that flows from them may be more aggressive than that which discharges from the mine itself. Another important consideration here is the potential long-term pollution problem, as production of AMD may continue for many years after mines are decommissioned. 3.1 `Source control vs. `migration control options. Following the axiom that prevention is better than cure, it is generally preferable, though not always pragmatic, to preclude the formation of AMD in the first instance. Such techniques are known collectively as source control measures. However, the practical difficulties entailed in inhibiting the formation of AMD at source mean the only alternative is to minimise the impact that this polluting water has on receiving streams and rivers, and on the wider environment; such an approach involves one or more migration control measures. Quite often, these have been divided into active and passive processes, the former generally (though not exclusively) referring to the continuous application of alkaline materials to neutralise acidic mine waters and precipitate metals, and the latter to the use of natural and constructed wetland ecosystems. Passive systems have the advantage of requiring relatively little maintenance (and recurring costs) than active systems, though they may be expensive and/or impracticable to set up in the first place. In reality, all passive treatment technologies require a certain amount of maintenance costs. A more useful subdivision is between those remediation technologies that rely on biological activities, and those that do not. Within these major groups, there are processes that may be described as either active or passive (Fig. 4). 9

10 Flooding/sealing of underground mines Underwater storage of mine tailings PREVENTION Land-based storage in sealed waste heaps Microencapsulation (coating) technologies Application of anionic surfactants Abiotic "Active" systems: aeration and lime addition "Passive" systems: e.g. anoxic limestone drains REMEDIATION Biological "Active" systems Off-line sulfidogenic bioreactors Accelerated iron oxidation (immobilized biomass) "Passive" systems Aerobic wetlands Compost reactors/wetlands Figure 4. Options for preventing the formation of, and remediating, metalliferous drainage waters from metal and coal mines (adapted from Johnson and Hallberg, 2002). 3.2 Biological remediation strategies: significant biological processes. The basis of bioremediation of AMD derives from the abilities of some microorganisms to generate alkalinity and immobilise metals, thereby essentially reversing the reactions responsible for the genesis of AMD. Whilst, in aerobic wetlands constructed to treat AMD, macrophytes such as Typha and Phragmites spp. are the most obvious forms of life present, their direct roles in improving water quality has been questioned (Johnson and Hallberg, 2002). 10

11 Microbiological processes that generate net alkalinity are mostly reductive processes, and include denitrification, methanogenesis, sulfate reduction, and iron and manganese reduction. Ammonification (the production of ammonium from nitrogen-containing organic compounds) is also an alkali-generating process. Due to the relative scarcity of the necessary materials (e.g. nitrate), some of these processes tend to be of minor importance in AMD-impacted environments. However, since both ferric iron and sulfate tend to be highly abundant in AMD, alkali genesis resulting from the reduction of these two species has potentially major significance in AMD-impacted waters. Photosynthetic microorganisms, by consuming a weak base (bicarbonate) and producing a strong base (hydroxyl ions) also generate net alkalinity: 6HCO - 3 (aq) + 6H 2 O C 6 H 12 O 6 + 6O 2 + 6OH -. Whilst the reduction of soluble ferric iron does not decrease solution acidity, the reduction of solid phase (crystalline and amorphous) ferric iron compounds does: Fe(OH) 3 + 3H + + e - Fe H 2 O, where e - represents an electron donor, which is generally an organic substrate. Bacteria that catalyse the dissimilatory reduction of sulfate to sulfide generate alkalinity by transforming a strong acid (sulfuric) into a relatively weak acid (hydrogen sulfide): SO CH 2 O + 2H + H 2 S + 2H 2 CO 3. Besides the ameliorative effect on AMD brought about by the resulting increase in ph, the reduction of sulfate is an important mechanism for removing toxic metals from AMD, since many (e.g. zinc, copper and cadmium) form highly insoluble sulfides: Zn 2+ + H 2 S ZnS + 2H +. Biological oxidation of ferrous iron to ferric (which is highly insoluble above ph 2.5) is the other major metal-immobilising process that occurs in aerobic wetlands and bioreactors, as described below. 3.3 Biological remediation strategies: aerobic wetlands and compost bioreactors. The majority of bioremediation options for AMD are passive systems, and of these only constructed wetlands and compost bioreactors have so far been used in full-scale treatment systems. The major advantages of passive bioremediation systems are their relatively low maintenance costs, once constructed, and the fact that the solid-phase products of water treatment are retained within the wetland sediments. On the downside, they are often relatively expensive to install and may require more land area than is available or suitable, their performance is less predictable than chemical treatment systems, and the long-term fate and stability (in the case of compost bioreactors) of the deposits that accumulate within them is uncertain (Johnson and Hallberg, 2002). 11

12 Aerobic wetlands are generally constructed to treat mine waters that are net alkaline. This is because the main remediative reaction that occurs within them is the oxidation of ferrous iron, and subsequent hydrolysis of the ferric iron produced, which is a net acid-generating reaction: 4Fe 2+ + O 2 + 4H + 4Fe 3+ +2H 2 O 4Fe H 2 O 4Fe(OH) H +. If there is insufficient alkalinity in the mine water to prevent a significant fall in ph as a result of these reactions, this may be amended by the incorporation of, for example, an anoxic limestone drain. In order to maintain oxidising conditions, aerobic wetlands are relatively shallow systems that operate by surface flow. Macrophytes are planted for aesthetic reasons, and also to regulate water flow and to stabilise the accumulating ferric precipitates (ochre). Also, by causing oxygen flow from aerial parts to their root systems, some aquatic plants may accelerate the rate of ferrous iron oxidation. In contrast to aerobic wetlands, the key reactions that occur in compost bioreactors used to mitigate AMD are anaerobic. The term compost bioreactor is a preferable generic term to describe such systems, as in some installations they are enclosed entirely below ground level and do not support any macrophytes, so that they should not be described as wetlands. Indeed, whether or not macrophytes are used in constructed compost ecosystems is often down to aesthetic considerations alone. Penetrating plant roots may cause the ingress of oxygen into the anaerobic zones, which is detrimental to promoting reductive processes. The microbially-catalysed reactions that occur in compost bioreactors generate net alkalinity and biogenic sulfide, and therefore these systems may be used to treat mine waters that are net acidic and metal-rich, such as AMD from abandoned metal mines. Again in contrast to aerobic wetlands, the reductive reactions that occur within compost wetlands are driven by electron donors that derive from the organic matrix of the compost itself. The choice of bulky organic materials used vary according to local availability as well as their proven effectiveness, though generally the composts are prepared by mixing relatively biodegradable materials (e.g. cow or horse manure, or mushroom compost) with more recalcitrant materials (e.g. sawdust, peat, or straw). The slow biodegradation of the latter is presumed to act as a long-term provision of appropriate substrates for the indigenous ironand sulfate-reducing bacteria (FRB and SRB) that are generally considered to have the major roles in AMD remediation in compost bioreactors. However, there are few quantitative data on the relative significance of iron and sulfate reduction in compost wetlands, and virtually nothing is known about how the microbiology of these systems changes as the ecosystem ages, especially with regard to substrate provision. Besides biologically-mediated 12

13 processes, AMD quality in compost wetlands is improved by filtration of suspended and colloidal materials, and adsorption of metals by the organic matrix. An important engineering variant on the basic compost bioreactor theme is the RAPS (reducing and alkalinity producing system) layout (Younger et al., 2003), which is also known as SAPS (successive alkalinity producing system). In this type of system (Fig. 5), AMD flow is forced downwards through a layer of compost (to remove DO and to promote the reduction of iron and sulfate) and then though a limestone gravel bed (to add additional alkalinity). Usually, water draining a RAPS flow into a sedimentation pond and/or an aerobic wetland, to precipitate and retain iron hydroxides. influent compost Limestone aggregate effluent Figure 5. Schematic of a reducing and alkalinity producing system (RAPS) for remediating acidic mine waters (redrawn from Younger et al., 2003). 3.4 Biological remediation strategies: sulfidogenic bioreactors. Off-line sulfidogenic bioreactors represent a radically different approach for remediating AMD (Boonstra et al., 1999; Johnson, 2000). These engineered systems have three major advantages over passive biological remediation: (i) their performances are more predictable and readily controlled; (ii) they allow heavy metals, such as copper and zinc, present in AMD to be selectively recovered and re-used; (iii) concentrations of sulfate in processed waters may be significant lowered. On the negative side, the construction and operational costs of these systems are considerable. Sulfidogenic bioreactors utilise the biogenic production of hydrogen sulfide to generate alkalinity and to remove metals as insoluble sulfides, which is one of the processes that occurs in compost bioreactors and PRBs. However, off-line sulfidogenic bioreactors are constructed and operated to optimise production of hydrogen sulfide. Since the SRB currently used in these reactors are sensitive to even moderate acidity, the systems have to be engineered to protect the microorganisms from direct exposure to the inflowing AMD. 13

14 At least two technologies using off-line sulfidogenic bioreactors have been described: the Biosulfide and the Thiopaq processes. The Biosulfide system has two components, one biological and one chemical, which operate independently (Rowley et al., 1997). A schematic of the process is shown in Fig. 6. Raw AMD enters the chemical circuit, where it comes into contact with hydrogen sulfide generated in the biological circuit. By careful manipulation of conditions (ph and sulfide concentration), selective separation of a particular metal sulfide is possible; this may then be removed from the partially-processed water ahead of further treatment. Some of the treated AMD enters the biological circuit to provide the sulfate source in the bioreactor, which contains a mixed culture of SRB. For the process to run optimally, additional alkali may be required beyond that produced by the SRB, in which case it is added in chemical form. The Thiopaq system differs from the Biosulfide process in that it utilises two distinct microbiological populations and processes: (i) conversion of sulfate to sulfide by SRB, and precipitation of metal sulfides and, (ii) conversion of any excess hydrogen sulfide produced to elemental sulfur, using sulfide-oxidising bacteria (SOB). The Thiopaq process has been operating successfully in treating zinc-contaminated groundwater at the Budelco zinc refinery in the Netherlands, since The zinc sulfide produced is fed into the refinery, and is incorporated into the metallic zinc product from the plant. Application of this technology to treat AMD has also been demonstrated: a pilot-scale Thiopaq operation at the Kennecott Bingham Canyon copper mine in Utah was able to selectively recover >99% of copper present in a ph 2.6 waste stream (Boonstra et al., 1999). SRB are, in general, heterotrophic bacteria and, unlike the iron-oxidising acidophiles described earlier, require provision of organic material as carbon and energy sources. In the first sulfidogenic bioreactor set up at the Budelco refinery, this was provided in the form of ethanol. However, hydrogen may substitute as electron donor for sulfate reduction: SO H 2 + 2H + H 2 S + 4H 2 O. The use of hydrogen is advantageous since it is a more economical to use for high sulfate loadings, and results in lesser production of bacterial biomass. Hydrogen may conveniently be formed by cracking methanol or from natural gas. In both cases, carbon dioxide is also produced, and some SRB are able to fix this as carbon source (Boonstra et al., 1999). 4. Future potential developments and applications The application of biological systems for ore processing and waste remediation is likely to become increasingly important in the 21 st century. Driving this will be the need to process ores containing increasing small concentration of target metal(s), the potential for reprocessing waste spoils and tailings, economic constraints, and possible legislative changes on the environmental impact of more traditional approaches such as pyrometallurgy. In the 14

15 Figure 6. Flow path of the Biosulphide process for remedying acid mine drainage and recovery of heavy metals (Rowley et al., 1997). immediate future, heap leaching is likely to be a major area of expansion, though new patented processes, such as BioCOP for extracting copper from chalcopyrite in stirred tanks at high temperatures, are also likely to expand. Microbial systems that allow the recovery thereby the reuse of metals that are currently either dumped in oxidised sludge wastes or retained in constructed wetlands, are also likely to be further developed and utilised in future sustainable and integrated approaches to metal extraction, resource conservation and safeguarding the global environment. Acknowledgement The author is grateful to the financial support of the BBSRC and NERC (grant ref. 5/BRM18412) for financial support under the DTI LINK scheme. 15

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