Effects of sewage sludge application on heavy metal leaching from mine tailings impoundments

Column experiments were conducted to investigate the removal of heavy metals from two mine tailings (El Arteal and Jaravías) using sewage sludge as a reactive material. When sewage sludge is used as a reactive material on the El Arteal tailings (sample SA), Fe, Mn, Zn and Pb are removed and Cu and Ni are mobilized. The experiments carried out on the Jaravías tailings give similar results, showing the retention of Cu, Pb, Fe and Mn and the mobilization of Ni and Zn. An analysis performed using the PHREEQC numerical code suggests that the retention of Fe in the sewage sludge may be caused by the precipitation of Fe(OH)2.7Cl0.3 and possibly pyrite, and that the retention of Pb at high pH may be caused by the formation of stable phase minerals such as Pb(OH)2 and PbS in these conditions. Ni mobilization in the column experiments with the two tailings samples may be caused by the presence of significant amounts of leachable Ni in the sewage sludge. The complexation of metals with dissolved organic matter, calculated with the Minteq model, may be moderate.     

Integrated biological process for the treatment of a Chilean complex gold ore

Abstract: A process for gold recovery from a complex Chilean ore from Burladora (IV Region) which integrates concentration by flotation, bacterial leaching and cyanidation was studied at a laboratory scale. The chemical composition of the ore is 8.2% Fe, 0.78% Cu, 0.88% As and 3.5 g/t Au, with pyrite, hematite, covelite, arsenopyrite and chalcopyrite as the main metal-bearing minerals. The initial gold recovery by conventional cyanidation on a crushed ore sample was only 54%. The ore was ground and concentrated by flotation with a gold recovery of only 56%. The gold content of the concentrate is 17 g/I Au. Concentrate samples were leached in 1.5 l stirred reactors at 10% pulp density in 1000 ml of acid medium (pH 1.8). Some experiments were inoculated with harvested bacteria previously isolated from mining solutions. Dissolved metals, pH and bacteria concentration in the leaching solutions were periodically determined. In the presence of bacteria, oxidation of the ferrous ion produced by acid dissolution of the concentrate was observed, and after 4 days of leaching 100% of the dissolved iron was present as ferric ion. Gold recovery by cyanidation increased from 13% for the initial concentrate to 34% after 10 days of chemical acid leaching and 97% after 10 days of bacterial leaching. To increase the total gold recovery, the flotation tailings were submitted to cyanidation. A complete flowsheet of the process and a first economical evalualion are proposed. As a possible alternative process, heap bacterial leaching and further cyanidation of the ore are suggested.     

Multiple roles of siderophores in free-living nitrogen-fixing bacteria

Free-living nitrogen-fixing bacteria in soils need to tightly regulate their uptake of metals in order to acquire essential metals (such as the nitrogenase metal cofactors Fe, Mo and V) while excluding toxic ones (such as W). They need to do this in a soil environment where metal speciation, and thus metal bioavailability, is dependent on a variety of factors such as organic matter content, mineralogical composition, and pH. Azotobacter vinelandii, a ubiquitous gram-negative soil diazotroph, excretes in its external medium catechol compounds, previously identified as siderophores, that bind a variety of metals in addition to iron. At low concentrations, complexes of essential metals (Fe, Mo, V) with siderophores are taken up by the bacteria through specialized transport systems. The specificity and regulation of these transport systems are such that siderophore binding of excess Mo, V or W effectively detoxifies these metals at high concentrations. In the topsoil (leaf litter layer), where metals are primarily bound to plant-derived organic matter, siderophores extract essential metals from natural ligands and deliver them to the bacteria. This process appears to be a key component of a mutualistic relationship between trees and soil diazotrophs, where tree-produced leaf litter provides a living environment rich in organic matter and micronutrients for nitrogen-fixing bacteria, which in turn supply new nitrogen to the ecosystem.     

The effects of Fe(II) and Fe(III) concentration and initial pH on microbial leaching of low-grade sphalerite ore in a column reactor

In this study the effects of initial concentration of Fe(II) and Fe(III) ions as well as initial pH on the bioleaching of a low-grade sphalerite ore in a leaching column over a period of 120 days with and without bacteria were investigated. Four different modifications of medium were used as column feed solutions to investigate the effects of initial concentration of Fe(II) and Fe(III) ions on zinc extraction. The experiments were carried out using a bench-scale, column leaching reactor, which was inoculated with mesophilic iron oxidizing bacteria, Acidithiobacillus ferrooxidans, initially isolated from the Sarcheshmeh chalcopyrite concentrate (Kerman, Iran). The effluent solutions were periodically analyzed for Zn, total Fe, Fe(II) and Fe(III) concentrations as well as pH values. Bacterial population was measured in the solution (free cells). Maximum zinc recovery in the column was achieved about 76% using medium free of initial ferrous ion and 11.4 g/L of ferric ion (medium 2) at pH 1.5. The extent of leaching of sphalerite ore with bacteria was significantly higher than that without bacteria (control) in the presence of ferrous ions. Fe(III) had a strong influence in zinc extraction, and did not adversely affect the growth of the bacteria population.     

Biological leaching of sulfide minerals with the use of shake flask,aerated column,air-lift reactor,and percolation techniques

Four different experimental approaches were used to evaluate the microbiological leaching of ore material containing metal sulfides (Fe, Zn, Ni, Cu, Co) and aluminum silicates. A shake flask technique required the shortest contact time for the complete solubilization of the most readily leachable metals (Ni and Zn). Air-lift reactors and aerated column reactors required longer contact times and complete solubilization of either zinc or nickel was not achieved. The air-lift reactor approach was somewhat more effective than the aerated slurry technique. A percolation system was the least effective and yielded the lowest recoveries. Shake flasks (easily autoclavable) offered the advantage of comparison of the microbiological and solely chemical leaching. Aseptic conditions could not be maintained with the air-lift and aerated column reactors because of contamination via aerosol formation. In a relative scale the leaching patterns were similar in that the precipitation of Fe(III) occurred regardless of the technique; zinc and nickel sulfides were solubilized more quantitatively than those of copper and cobalt; aluminum concentrations, although high, indicated low leaching yields relative to aluminum silicates in the ore material; and the pH reached similar final values in the presence of bacteria.     

Uptake of Trace Metals and Rare Earth Elements from Hornblende by a Soil Bacterium

Analysis of trace elements released from hornblende between pH 6.5 and 7.5 in the presence of Arthrobacter sp. shows that Fe, Ni, V, Mn, and, to a lesser extent, Co are preferentially released into solution relative to bacteria-free experiments. This enhanced release into solution could be due to contributions from the slightly lowered pH, the presence of low molecular weight organic acids (LMWOAs), or the presence of a catecholate siderophore in experiments with bacteria. The best explanation for enhanced metal release is siderophore complexation at the mineral surface followed by release to solution. However,the relative rates of metal release to solution in these experiments do not strictly follow the trend predicted by the relative ordering of metal hydrolysis, which might be predicted for siderophore-promoted dissolution. For some of these metals, release to solution is fast initially in biotic experiments, but concentrations in solution reach a steady state value or decrease with time as the bacteria cell numbers increase exponentially. Lack of enhanced release to solution for some metals and decreases in release rate with time for others may be explained by uptake into bacteria. Many of the metals predicted to strongly complex with siderophore (including Al, Ti, Fe, Cu) are heavily taken up into cellular material. The relative ordering of organic ligand-element complexation may therefore partially explain the relative ordering of uptake of trace metals and rare earth elements into cell material. Fractionation of heavy rare earth elements taken up into cellular material is also very strong, and increases from Ho to Lu. Strong fractionation in uptake of some elements by bacteria may create biological signatures either in the mineral substrate or in any mineral precipitates associated with the cellular material.     

Siderophores mediate reduced and increased uptake of cadmium by Streptomyces tendae F4 and sunflower (Helianthus annuus), respectively

Aims: As a toxic metal, cadmium (Cd) affects microbial and plant metabolic processes, thereby potentially reducing the efficiency of microbe or plant‐mediated remediation of Cd‐polluted soil. The role of siderophores produced by Streptomyces tendae F4 in the uptake of Cd by bacteria and plant was investigated to gain insight into the influence of siderophores on Cd availability to micro‐organisms and plants. Methods and Results: The bacterium was cultured under siderophore‐inducing conditions in the presence of Cd. The kinetics of siderophore production and identification of the siderophores and their metal‐bound forms were performed using electrospray ionization mass spectrometry. Inductively coupled plasma spectroscopy was used to measure iron (Fe) and Cd contents in the bacterium and in sunflower plant grown in Cd‐amended soil. Siderophores significantly reduced the Cd uptake by the bacterium, while supplying it with iron. Bacterial culture filtrates containing three hydroxamate siderophores secreted by S. tendae F4 significantly promoted plant growth and enhanced uptake of Cd and Fe by the plant, relative to the control. Furthermore, application of siderophores caused slightly more Cd, but similar Fe uptake, compared with EDTA. Bioinoculation with Streptomyces caused a dramatic increase in plant Fe content, but resulted only in slight increase in plant Cd content. Conclusion: It is concluded that siderophores can help reduce toxic metal uptake in bacteria, while simultaneously facilitating the uptake of such metals by plants. Also, EDTA is not superior to hydroxamate siderophores in terms of metal solubilization for plant uptake. Significance and Impact of the Study: The study showed that microbial processes could indirectly influence the availability and amount of toxic metals taken up from the rhizosphere of plants. Furthermore, although EDTA is used for chelator‐enhanced phytoremediation, microbial siderophores would be ideal for this purpose.     

Bacterially Induced Weathering of Ultramafic Rock and Its Implications for Phytoextraction

The bioavailability of metals in soil is often cited as a limiting factor of phytoextraction (or phytomining). Bacterial metabolites, such as organic acids, siderophores, or biosurfactants, have been shown to mobilize metals, and their use to improve metal extraction has been proposed. In this study, the weathering capacities of, and Ni mobilization by, bacterial strains were evaluated. Minimal medium containing ground ultramafic rock was inoculated with either of two Arthrobacter strains: LA44 (indole acetic acid [IAA] producer) or SBA82 (siderophore producer, PO₄ solubilizer, and IAA producer). Trace elements and organic compounds were determined in aliquots taken at different time intervals after inoculation. Trace metal fractionation was carried out on the remaining rock at the end of the experiment. The results suggest that the strains act upon different mineral phases. LA44 is a more efficient Ni mobilizer, apparently solubilizing Ni associated with Mn oxides, and this appeared to be related to oxalate production. SBA82 also leads to release of Ni and Mn, albeit to a much lower extent. In this case, the concurrent mobilization of Fe and Si indicates preferential weathering of Fe oxides and serpentine minerals, possibly related to the siderophore production capacity of the strain. The same bacterial strains were tested in a soil-plant system: the Ni hyperaccumulator Alyssum serpyllifolium subsp. malacitanum was grown in ultramafic soil in a rhizobox system and inoculated with each bacterial strain. At harvest, biomass production and shoot Ni concentrations were higher in plants from inoculated pots than from noninoculated pots. Ni yield was significantly enhanced in plants inoculated with LA44. These results suggest that Ni-mobilizing inoculants could be useful for improving Ni uptake by hyperaccumulator plants.     

Effects of atmospheric deposition,soil pH and acidification on heavy metal contents in soils and vegetation of semi-natural ecosystems at Rothamsted Experimental Station,UK

The effects of acidification on the soil chemistry and plant availability of the metals Pb, Cd, Zn, Cu, Mn and Ni in new and archived soil and plant samples taken from the >100-year-old experiments on natural woodland regeneration (Geescroft and Broadbalk Wildernesses) and a hay meadow (Park Grass) at Rothamsted Experimental Station are examined. We measured a significant input of metals from atmospheric deposition, enhanced under woodland by 33% (Ni) to 259% (Zn); Pb deposition was greatly influenced by vehicle emissions and the introduction of Pb in petrol. The build up of metals by long-term deposition was influenced by acidification, mobilization and leaching, but leaching, generally, only occurred in soils at pH<4. Mn and Cd were most sensitive to soil acidity with effective mobilization occurring at pH 6.0–5.5 (0.01 M CaCl₂), followed by Zn, Ni and Cu at pH 5.5–5.0. Pb was not mobilized until pH<4.5. Acidification to pH 4 mobilized 60–90% of total soil Cd but this was adsorbed onto ion exchange surfaces and/or complexed with soil organic matter. This buffering effect of ion exchange surfaces and organic matter in soils down to pH 4 was generally reflected by all the metals investigated. For grassland the maximum accumulation of metals in herbage generally corresponded to a soil pH of 4.0. For woodland the concentration of Pb, Mn and Cd in oak saplings (Quercus robur) was 3-, 4- and 6-fold larger at pH 4 than at pH 7. Mature Oak trees contained 10 times more Mn, 4 times more Ni and 3 times more Cd in their leaves at pH 4 than at pH 7. At pH values <4.0 on grassland the metal content in herbage declined. Only for Mn and Zn did this reflect a decline in the plant available soil content attributed to long-term acid weathering and leaching. The chief cause was a long-term decline in plant species richness and the increased dominance of two acid-tolerant, metal-excluder species     

Review of characteristics of mercury speciation and mobility from areas of mercury mining in semi-arid environments

The speciation of mercury—including most phase minerals, secondary phases, gaseous and aqueous species—is very important for evaluating the environmental impact and mobilization of this contaminant. Mining activities produce mercury mine waste, which includes several types of material (mainly mine waste and calcines) with varying mercury content and speciation depending on the ore deposit and processing technology. The main phase minerals are cinnabar, metacinnabar, metallic Hg⁰, corderoite, livingstonite, calomel and schuetteite. The aqueous mobilization of mercury is controlled by complex formation, pH-Eh conditions, the primary phase mineral of mercury, and organic-matter and iron oxyhydroxide content. The possibility of colloidal transport of mercury from mine waste is influenced by the atmospheric emission of metallic Hg⁰ and the leaching of waste by episodic high-intensity precipitations. In these climatic conditions, mercury can be mobilized to pore water, surface water or groundwater by the dissolution of metallic Hg⁰ and cinnabar in acidic conditions, and by the colloidal transport. The presence of Hg-soluble phases (chlorides and oxychlorides) may enhance the mobilization of mercury. In the semi-arid conditions of the Valle del Azogue (SE Spain) the atmospheric emissions of the Hg⁰ present in calcines and mine waste may be significant and the dissolution of Hg⁰ and metal-sulfate salts during episodic runoff events may explain the mobilization of Hg, Fe, Pb, Zn and other heavy metals.     

Siderophore production by using free and immobilized cells of two pseudomonads cultivated in a medium enriched with Fe and/or toxic metals (Cr, Hg, Pb)

Pseudomonads are serious candidates for siderophore production applied to toxic metal (TM) solubilization. The bioaugmentation of contaminated soils by these TM-solubilizing bacteria combined with phytoextraction is an emerging clean-up technology. Unfortunately, siderophore synthesis may be drastically reduced by soluble iron in soils and bacteria can suffer from TM toxicity. In this study, we compared siderophore production by Pseudomonas aeruginosa and Pseudomonas fluorescens by using free and immobilized cells in Ca-alginate beads incubated in a medium containing Fe and/or TM (mixture of Cr, Hg, and Pb in concentrations which represented the soluble fraction of a contaminated agricultural soil). Free cell growth was stimulated by Fe, whatever the microorganism, the inoculum size and the presence or not of TM might have been. P. aeruginosa was less sensitive to TM than P. fluorescens. By comparison with free cells, immobilization with the high inoculum size showed less sensitivity to TM most probably because of lower metal diffusion in beads. Indeed, a maximum of 99.1% of Cr, 57.4% of Hg, and 99.6% of Pb were adsorbed onto beads. The addition of iron in the culture medium reduced significantly siderophore production of free cells while it led only to a low decrease with their immobilized counterparts, in particular with P. aeruginosa. In culture medium enriched with Fe and/or TM, siderophore-specific production of immobilized cells was higher than for free cells.     

Impact of Simulated Acid Rain on Trace Metals and Aluminum Leaching in Latosol from Guangdong Province,China

Acid rain is one of the most serious ecological and environmental problems worldwide. This study investigated the impacts of simulated acid rain (SAR) upon leaching of trace metals and aluminum (Al) from a soil. Soil pot leaching experiments were performed to investigate the impacts of SAR at five different pH levels (or treatments) over a 34-day period upon the release of trace metals (i.e., Cu, Ni, Pb, Zn, and Fe) and Al from the Latosol (acidic red soil). The concentrations of trace metals in the effluent increased as the SAR pH level decreased, and were highest at the SAR pH = 2.0. In general, the concentrations of Cu, Pb, Fe, and Al in the effluent increased with leaching time at the SAR pH = 2.0, whereas the concentrations of Zn, Fe, and Al in the effluent decreased with leaching time at the SAR pH ≥ 4.0. The increase in electrical conductivity (EC) with leaching time at five different SAR pH levels was primarily due to the concentrations of Al and Fe in the effluent. There were good linear correlations between the effluent Al concentrations and the effluent pH at the SAR pH = 2.0 (R² = 0.87) and pH = 3.0 (R² = 0.83). More soil trace metals and Al were activated and released into the soil solution as the SAR pH level decreased.     

Metal Reduction at Low pH by a Desulfosporosinus species: Implications for the Biological Treatment of Acidic Mine Drainage

We isolated an acid-tolerant sulfate-reducing bacterium, GBSRB4.2, from coal mine-derived acidic mine drainage (AMD)-derived sediments. Sequence analysis of partial 16S rRNA gene of GBSRB4.2 revealed that it was affiliated with the genus Desulfosporosinus. GBSRB4.2 reduced sulfate, Fe(III) (hydr)oxide, Mn(IV) oxide, and U(VI) in acidic solutions (pH 4.2). Sulfate, Fe(III), and Mn(IV) but not U(VI) bioreduction led to an increase in the pH of acidic solutions and concurrent hydrolysis and precipitation of dissolved Al³⁺. Reduction of Fe(III), Mn(IV), and U(VI) in sulfate-free solutions revealed that these metals are enzymatically reduced by GBSRB4.2. GBSRB4.2 reduced U(VI) in groundwater from a radionuclide-contaminated aquifer more rapidly at pH 4.4 than at pH 7.1, possibly due to the formation of poorly bioreducible Ca-U(VI)-CO₃ complexes in the pH 7.1 groundwater.     

Actinide and metal toxicity to prospective bioremediation bacteria

Bacteria may be beneficial for alleviating actinide contaminant migration through processes such as bioaccumulation or metal reduction. However, sites with radioactive contamination often contain multiple additional contaminants, including metals and organic chelators. Bacteria-based bioremediation requires that the microorganism functions in the presence of the target contaminant, as well as other contaminants. Here, we evaluate the toxicity of actinides, metals and chelators to two different bacteria proposed for use in radionuclide bioremediation, Deinococcus radiodurans and Pseudomonas putida, and the toxicity of Pu(VI) to Shewanella putrefaciens. Growth of D. radiodurans was inhibited at metal concentrations ranging from 1.8 microM Cd(II) to 32 mM Fe(III). Growth of P. putida was inhibited at metal concentrations ranging from 50 microM Ni(II) to 240 mM Fe(III). Actinides inhibited growth at mM concentrations: chelated Pu(IV), U(VI) and Np(V) inhibit D. radiodurans growth at 5.2, 2.5 and 2.1 mM respectively. Chelated U(VI) inhibits P. putida growth at 1.7 mM, while 3.6 mM chelated Pu(IV) inhibits growth only slightly. Pu(VI) inhibits S. putrefaciens growth at 6 mM. These results indicate that actinide toxicity is primarily chemical (not radiological), and that radiation resistance does not ensure radionuclide tolerance. This study also shows that Pu is less toxic than U and that actinides are less toxic than other types of metals, which suggests that actinide toxicity will not impede bioremediation using naturally occurring bacteria.     

Microbial leaching of metals from sulfide minerals

Microorganisms are important in metal recovery from ores, particularly sulfide ores. Copper, zinc, gold, etc. can be recovered from sulfide ores by microbial leaching. Mineral solubilization is achieved both by 'direct (contact) leaching' by bacteria and by 'indirect leaching' by ferric iron (Fe(3+)) that is regenerated from ferrous iron (Fe(2+)) by bacterial oxidation. Thiobacillus ferrooxidans is the most studied organism in microbial leaching, but other iron- or sulfide/sulfur-oxidizing bacteria as well as archaea are potential microbial agents for metal leaching at high temperature or low pH environment. Oxidation of iron or sulfur can be selectively controlled leading to solubilization of desired metals leaving undesired metals (e.g., Fe) behind. Microbial contribution is obvious even in electrochemistry of galvanic interactions between minerals.     

Metal mobilization from soils by phytosiderophores – experiment and equilibrium modeling

Aims

To test if multi–surface models can provide a soil-specific prediction of metal mobilization by phytosiderophores (PS) based on the characteristics of individual soils.

Methods

Mechanistic multi-surface chemical equilibrium modeling was applied for obtaining soil-specific predictions of metal and PS speciation upon interaction of the PS 2’-deoxymugineic acid (DMA) with 6 soils differing in availability of Fe and other metals. Results from multi-surface modeling were compared with empirical data from soil interaction experiments.

Results

For soils in which equilibrium was reached during the interaction experiment, multi-surface models could well predict PS equilibrium speciation. However, in uncontaminated calcareous soils, equilibrium was not reached within a week, and experimental and modeled DMA speciation differed considerably. In soils with circum-neutral pH, on which Fe deficiency is likely to occur, no substantial Fe mobilization by DMA was predicted. However, in all but the contaminated soils, Fe mobilization by DMA was observed experimentally. Cu and Ni were the quantitatively most important metals competing with Fe for complexation and mobilization by DMA.

Conclusion

Thermodynamics are unable to explain the role of PS as Fe carrier in calcareous soils, and the kinetic aspects of metal mobilization by PS need to be closer examined in order to understand the mechanisms underlying strategy II Fe acquisition.     

The Fe Chelator Proferrorosamine A Is Essential for the Siderophore-Mediated Uptake of Iron by Pseudomonas roseus fluorescens

Pseudomonas roseus fluorescens produces, besides the Fe chelator proferrorosamine A, Fe -chelating compounds, called siderophores. The production of proferrorosamine A and siderophores by P. roseus fluorescens appears to be controlled in a similar way by the concentration of available iron and by the concentration of dissolved oxygen. The higher the concentration of iron available for the microorganism, the lower the production of both chelating compounds. However, the production of siderophores was much more sensitive to iron availability than was proferrorosamine A production. Proferrorosamine A and siderophores were only produced in minimal medium C if the concentration of dissolved oxygen ranged from 4.5 to 2.0 ppm. At higher or lower concentrations, none of the iron-chelating compounds were produced. Furthermore, it has been shown that proferrorosamine-negative Tn5 mutants of P. roseus fluorescens were able to form siderophores only under iron-limiting conditions when proferrorosamine A was added to the medium. Our data suggest that proferrorosamine A production is essential for siderophore synthesis by P. roseus fluorescens; the production of siderophores occurred only when proferrorosamine A was present.     

Rhamnolipid biosurfactant-enhanced soil flushing for the removal of arsenic and heavy metals from mine tailings

Mine tailings containing high contents of arsenic and heavy metals are potential environmental contamination sources. Column experiments were conducted in this study to evaluate the feasibility of using a rhamnolipid biosurfactant (JBR425) to enhance the removal of arsenic and heavy metals from an oxidized mine tailings sample collected from Bathurst, Canada. Capillary electrophoresis (CE) analyses indicated that arsenate [As(V)] was the only extractable arsenic species in the mine tailings. The addition of rhamnolipids did not change the oxidation state of arsenic. It was found that a 0.1% rhamnolipid solution (initial pH adjusted to 11) could significantly enhance the removal of arsenic and heavy metals (i.e., Cu, Pb and Zn) simultaneously. The accumulative removal of arsenic, Cu, Pb and Zn reached 148, 74, 2379, and 259 mg/kg after a 70-pore-volume flushing, respectively. Moreover, the mobilization of arsenic and heavy metals by rhamnolipids was found to be positively correlated with that of Fe, and the mobilization of arsenic was also positively correlated to that of the heavy metals. The mobilization of co-existing metals, to some extent, might enhance arsenic mobilization in the presence of rhamnolipids by helping incorporate it into aqueous organic complexes or micelles through metal-bridging mechanisms.     

Transmission Electron Microscopic Observation of Mercury-Bearing Bacterial Clay Minerals in a Small-Scale Gold Mine in Tanzania

Mercury is a toxic substance that is widely distributed throughout the hydrosphere, biosphere, and lithosphere. Mine waste environments and mine waters support a wide diversity of microbial life. The microbial ecology of environments where mine waters are polluted with heavy metals is poorly understood. Here, we describe the features of bacteria in mercury-contaminated gold panning ponds in a small-scale gold mine (Geita) near Lake Victoria, Tanzania using energy filtering transmission electron microscopy (EF-TEM) and scanning transmission electron microscopy equipped with energy dispersive X-ray spectroscopy (STEM-EDX). Most bacteria in the panning pond showed thick exopolysaccharides (EPSs), and many clay minerals attached onto the surface of EPSs. The clay minerals and EPSs might act as protective layers for the bacteria against toxic materials. The clay minerals were composed of smectite, halloysite, and kaolinite associated with calcite and goethite. Scanning electron microscopy equipped with energy dispersive X-ray spectroscopy indicated that the bulk soil samples contained abundant Si, Al, K, Ca, and Fe with heavy metals such as Au, Ti, and Ag. The results indicate that Hg pollution from panning ponds is caused by not only volatilization of Hg from Au-Hg amalgams, but Hg is also released into the air as dust mixed with dry fine clays, suggesting high long-term environmental risks. Mercury-resistant bacteria associated with clay minerals may have a significant effect on the weathering processes of the ore during long-term bioremediation. The clay mineral complexes on the surface of bacterial cell walls are a stimulator for Hg-resistant bacterial growth in mud ponds contaminated with the Au-Hg materials.     

Bioleaching of spent fluid catalytic cracking catalyst using Aspergillus niger

The use of the fungus Aspergillus niger for the bioleaching of heavy metals from spent catalyst was investigated, with fluid catalytic cracking (FCC) catalyst as a model. Bioleaching was examined in batch cultures with the spent catalysts at various pulp densities (1-12%). Chemical leaching was also performed using mineral acids (sulphuric and nitric acids) and organic acids (citric, oxalic and gluconic acids), as well as a mixture of organic acids at the same concentrations as that biogenically produced. It was shown that bioleaching realised higher metal extraction than chemical leaching, with A. niger mobilizing Ni (9%), Fe (23%), Al (30%), V (36%) and Sb (64%) at 1% pulp density. Extraction efficiency generally decreased with increased pulp density. Compared with abiotic controls, bioleaching gave rise to higher metal extractions than leaching using fresh medium and cell-free spent medium. pH decreased during bioleaching, but remained relatively constant in both leaching using fresh medium and cell-free spent medium, thus indicating that the fungus played a role in effecting metal extraction from the spent catalyst.