Kinetic model for the cyanidation of silver ammonium jarosite in NaOH medium

A synthesis of silver ammonium jarosite has been carried out obtaining a single-phase product with the formula: [(NH₄)_0.71(H₃O)_0.25Ag_0.040]Fe_2.85(SO₄)₂(OH)_5.50. The product consists on compact spherical aggregates of rhombohedral crystals. The nature and kinetics of alkaline decomposition and also of cyanidation have been determined. In both processes an induction period followed by a conversion period have been observed. During decomposition, the inverse of the induction period is proportional to [OH⁻]^0.75 and an apparent activation energy of 80 kJ mol^− 1 was obtained; during the conversion period, the process is of 0.6 order (OH⁻ concentration) and an activation energy of 60 kJ mol^− 1 was obtained. During cyanidation, the inverse of the induction period is proportional to [CN⁻]^0.5 and an apparent activation energy of 54 kJ mol^− 1 was obtained; during the conversion period the process is of 0 order (CN⁻ concentration) and an activation energy of 52 kJ mol^− 1 was obtained. Results obtained are consistent with the spherical particle model with decreasing core and chemical control, in the experimental conditions employed. For both processes and in the basis of the behaviour described, two mathematical models, including the induction and conversion periods, were established, that fits well with the experimental results obtained. Cyanidation rate of different jarosite materials in NaOH media have also been established: this reaction rate at 50 °C is very high for potassium jarosite, high and similar for argentojarosite and ammonium jarosite, lower for industrial ammonium jarosite and negligible for natural arsenical potassium jarosite and beudantite. These results confirm that the reaction rate of cyanidation decreases when the substitution level in the jarosite lattice increases.     

Comparative study of inorganic arsenic resistance of several strains of Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans

The growth characteristics of several strains of Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans were studied in the presence of soluble inorganic arsenic(III) and (V) with regard to media pH changes, total bacterial populations and sulfur oxidation rates. Most of these bacteria could reach large populations and have strong sulfur oxidation activity in the absence of arsenic. However, in the presence of up to 120 mM arsenite or arsenate, different strains showed different inorganic arsenic resistance. A. thiooxidans LYS and A. ferrooxidans BY-3 were two of the best performers which showed high arsenite resistance: up to 80 mM and 60 mM, respectively. On the other hand, A. thiooxidans JY and A. ferrooxidans TKY-2 could adapt up to 120 mM and 100 mM arsenate, respectively. These bacteria strains may play key roles in the bioleaching of arsenopyrite or in the bio-oxidation pretreatment of arsenic-bearing refractory gold sulfide ores and concentrates.     

Leaching of chalcopyrite with ferric ion. Part IV: The role of redox potential in the presence of mesophilic and thermophilic bacteria

This paper reports a study on the effect of redox potential in chalcopyrite bioleaching in the presence of iron- and sulphur-oxidizing bacteria. Bioleaching tests were carried out in stirred Erlenmeyer flasks at 180 rpm, with 0.5 g of chalcopyrite mineral, 99 ml of a sulphate solution of Fe³⁺/Fe²⁺ (with the redox potential ranging between 300 and 600 mV Ag/AgCl) at pH 1.8 and 1 ml of a mesophilic (35 °C) or thermophilic (68 °C) culture. The overoxidation of the leaching solution, due to the activity of iron-oxidizing microorganisms (Acidithiobacillus ferrooxidans, Leptospirillum ferrooxidans and Sulfolobus BC), favoured the precipitation of jarosite on chalcopyrite surfaces followed by passivation. Iron- and sulphur-oxidizing microorganisms, such as A. ferrooxidans and Sulfolobus BC adapted for 4 months to elemental sulphur as the sole energy source, recovered their iron-oxidizing ability after being in contact with Fe²⁺.     

Microbial depyritisation of three different coals by Acidithiobacillus ferrooxidans in a batch stirred tank reactor

Abstract

The present study aimed to investigate the depyritisation potential of Acidithiobacillus ferrooxidans on two different types of coals, namely lignite and anthracite collected from three different countries (Korea, China and Indonesia). The experimental work was conducted on a batch mode in a stirred tank reactor. All the batch biooxidation of pyrite in the different coal samples were conducted in a pH controlled condition (pH?=?1·5). The growth medium employed for the batch biooxidation of pyritic coal was free from iron supplement. At. ferrooxidans oxidised mineral pyrite of Korean anthracite at a greater rate (98%) compared to 96 and 92% of pyrite oxidation for Indonesian and Chinese lignite respectively. The ratio of bioleach residue to the feed was reasonably good with range of 8·56–9·06 stating the net mass loss of 9–14. Coal depyritisation was carried out by the available Fe³⁺ ion in the inoculum producing Fe²⁺ ion as a product and this Fe²⁺ ion was further oxidised to Fe³⁺ ion by At. ferrooxidans. This Fe³⁺ ion produced by At. ferrooxidans continued the oxidation of the residual pyrite in the coal until all pyrite content was oxidised completely. The three different coals were found to be feasible for biological depyritisation of the coal could be scaled up for further studies in a continuous stirred tank bioreactor.

L’étude présente avait pour but d’examiner le potentiel d’enlèvement de la pyrite par Acidithiobacillus ferrooxidans dans deux types de charbons, soit le lignite et l’anthracite, récoltés dans trois pays, soit la Corée, la Chine et l’Indonésie. On a effectué le travail expérimental dans un mode en vrac, dans un réacteur agité. On a effectué toute la bio-oxydation en vrac de la pyrite des échantillons de charbon à un pH contrôlé de 1·5. Le médium de croissance utilisé pour la bio-oxydation en vrac du charbon pyritique ne contenait pas de supplément de fer. At. ferrooxydans oxydait le minerai de pyrite de l’anthracite coréen en plus grande proportion (98%) que l’oxydation de la pyrite du lignite indonésien (96%) ou chinois (92%), respectivement. Le rapport de résidu de biolixiviation à l’alimentation était raisonnablement bon, avec une gamme de 8·56 à 9·06, établissant la perte de masse nette de 9 à 14. L’enlèvement de la pyrite du charbon était effectué par l’ion Fe³⁺ disponible dans l’inoculum, donnant lieu à l’ion Fe²⁺ comme produit et cet ion Fe²⁺ était davantage oxydé en ion Fe³⁺ par At. ferrooxidans. Cet ion Fe³⁺ produit par At. ferrooxidans continuait l’oxydation de la pyrite résiduelle dans le charbon jusqu’à ce que toute la pyrite soit complètement oxydée. On a trouvé qu’il était possible d’effectuer l’enlèvement biologique de la pyrite des trois charbons et d’augmenter l’échelle pour des études futures dans un bioréacteur en continu agité.     

(Bio)chemistry of bacterial leaching—direct vs. indirect bioleaching

Bioleaching of metal sulfides is effected by bacteria, like Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, Sulfolobus/Acidianus, etc., via the (re)generation of iron(III) ions and sulfuric acid.According to the new integral model for bioleaching presented here, metal sulfides are degraded by a chemical attack of iron(III) ions and/or protons on the crystal lattice. The primary iron(III) ions are supplied by the bacterial extracellular polymeric substances, where they are complexed to glucuronic acid residues. The mechanism and chemistry of the degradation is determined by the mineral structure.The disulfides pyrite (FeS₂), molybdenite (MoS₂), and tungstenite (WS₂) are degraded via the main intermediate thiosulfate. Exclusively iron(III) ions are the oxidizing agents for the dissolution. Thiosulfate is, consequently, degraded in a cyclic process to sulfate, with elemental sulfur being a side product. This explains, why only iron(II) ion-oxidizing bacteria are able to oxidize these metal sulfides.The metal sulfides galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS₂), hauerite (MnS₂), orpiment (As₂S₃), and realgar (As₄S₄) are degradable by iron(III) ion and proton attack. Consequently, the main intermediates are polysulfides and elemental sulfur (thiosulfate is only a by-product of further degradation steps). The dissolution proceeds via a H₂S*⁺-radical and polysulfides to elemental sulfur. Thus, these metal sulfides are degradable by all bacteria able to oxidize sulfur compounds (like T. thiooxidans, etc.). The kinetics of these processes are dependent on the concentration of the iron(III) ions and, in the latter case, on the solubility product of the metal sulfide.     

Direct versus indirect bioleaching

The dissolution of metal sulfides is controlled by their solubility product and thus, the [H⁺] concentration of the solution, and further enhanced by several chemical mechanisms which lead to a disruption of sulfide chemical bonds. They include extraction of electrons and bond breaking by [Fe³⁺], extraction of sulfur by polysulfide and iron complexes forming reactants [Y⁺] and electrochemical dissolution by polarization of the sulfide [high Fe³⁺ concentration]. All these mechanisms have been exploited by sulfide and iron-oxidizing bacteria. Basically, the bacterial action is a catalytic one during which [H⁺], [Fe³⁺] and [Y⁺] are breaking chemical bonds and are recycled by the bacterial metabolism. While the cyclic bacterial oxidative action via [H⁺] and [Fe³⁺] can be called indirect, bacteria had difficulties harvesting chemical energy from an abundant sulfide such as FeS₂, the electron exchange properties of which are governed by coordination chemical mechanisms (extraction of electrons does not lead to a disruption of chemical bonds but to an increase of the oxidation state of interfacial iron). Here, bacteria have evolved alternative strategies which require an extracellular polymeric layer for appropriately conditioned contact with the sulfide. Thiobacillus ferrooxidans cycles [Y⁺] across such a layer to disrupt FeS₂ and Leptospirillum ferrooxidans accumulates [Fe³⁺] in it to depolarize FeS₂ to a potential where electrochemical oxidation to sulfate occurs. Corrosion pits and high resolution electron microscopy leave no doubt that these mechanisms are strictly localized and depend on specific conditions which bacteria create. Nevertheless, they cannot be called ‘direct’ because the definition would require an enzymatic interaction between the bacterial membrane and the cell. Therefore, the term ‘contact’ leaching is proposed for this situation. In practice, multiple patterns of bacterial leaching coexist, including indirect leaching, contact leaching and a recently discovered cooperative (symbiotic) leaching where ‘contact’ leaching bacteria are feeding so wastefully that soluble and particulate sulfide species are supplied to bacteria in the surrounding electrolyte.     

Adhesion to metal sulfide surfaces by cells of Acidithiobacillus ferrooxidans,Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans

Attachment of four strains of Acidithiobacillus ferrooxidans to pyrite, chalcopyrite, galena, sphalerite or quartz was found to be mineral-selective. The bacterial extracellular polymeric substances (EPS) are responsible for mediating this process. Attachment of cells of A. ferrooxidans as well as of Acidithiobacillus thiooxidans was diminished, when depleted of their EPS. After 5 days of cultivation cells of A. ferrooxidans cover mineral surfaces with a dense biofilm, as visualised by fluorescence microscopy and AFM. Primary attachment was restricted to surface sites with visible defects.Chemical analyses of EPS of A. ferrooxidans, A. thiooxidans and Leptospirillum ferrooxidans indicated neutral sugars, fatty acids and uronic acids. The composition differed with the strain and the growth substrate. IronIII ions were only detectable in EPS of ironII ion- and pyrite-grown cells, but not in EPS of sulfur grown cells. Pyrite oxidation rates correlated with the amount of EPS-complexed ironIII ions in the case of A. ferrooxidans and L. ferrooxidans. Furthermore, pyrite oxidation rates of L. ferrooxidans were correlated with the genetic affiliation of the strains. The data for A. ferrooxidans seem to indicate a similar correlation, however, the results were not as clear-cut as those obtained for L. ferrooxidans. Sulfur oxidation rates of A. thiooxidans did not require EPS complexed ironIII ions.     

Electrochemical noise analysis of bioleaching of bornite (Cu5FeS4) by Acidithiobacillus ferrooxidans

Electrochemical noise (EN) is a generic term describing the phenomenon of spontaneous fluctuations of potential or current noise of electrochemical systems. Since this technique provides a non-destructive condition for investigating corrosion processes, it can be useful to study the electrochemical oxidation of mineral sulfides by microorganisms, a process known as bacterial leaching of metals. This technique was utilized to investigate the dissolution of a bornite electrode in the absence (first 79 h) and after the addition of Acidithiobacillus ferrooxidans (next 113 h) in salts mineral medium at pH 1.8, without addition of the energy source (Fe²⁺ ions) for this chemolithotrophic bacterium. Potential and current noise data have been determined simultaneously with two identical working bornite electrodes which were linked by a zero resistance ammeter (ZRA). The mean potential, E_coup, coupling current, I_coup, standard deviations of potential and current noise fluctuations and noise resistance, R_n, have been obtained for coupled bornite electrodes. Noise measurements were recorded twice a day in an unstirred solution at 30 °C. Significant changes in these parameters were observed when the A. ferrooxidans suspension was added, related with bacterial activity on reduced species present in the sulfide moisture (Fe²⁺, S²⁻). ENA was a suitable tool for monitoring the changes of the corrosion behavior of bornite due to the presence of bacterium.     

Silicate mineral dissolution in the presence of acidophilic microorganisms: Implications for heap bioleaching

Silicate minerals are found with sulfide minerals and therefore, can be present during heap bioleaching for metal extraction. The weathering of silicate minerals by chemical and biological means is variable depending on the conditions and microorganisms tested. In low pH metal rich environments their dissolution can influence the solution chemistry by increasing pH, releasing toxic trace elements, and thickening of the leach liquor. The amenity of five silicate minerals to chemical and biological dissolution was tested in the presence of either ‘Ferroplasma acidarmanus’ Fer1 or Acidithiobacillus ferrooxidans with olivine and hornblende being the most and least amenable, respectively. A number of the silicates caused the pH of the leach liquor to increase including augite, biotite, hornblende, and olivine. For the silicate mineral olivine, the factors affecting magnesium dissolution included addition of microorganisms and Fe²⁺. XRD analysis identified secondary minerals in several of the experiments including jarosite from augite and hornblende when the medium contained Fe²⁺. Despite acidophiles preferentially attaching to sulfide minerals, the increase in iron coupled with very low Fe²⁺ concentrations present at the end of leaching during dissolution of biotite, olivine, hornblende, and microcline suggested that these minerals supported growth. Weathering of the tested silicates would affect heap bioleaching by increasing the pH with olivine, fluoride release from biotite, and production of jarosite during augite and hornblende dissolution that may have caused passivation. These data have increased knowledge of silicate weathering under bioleaching conditions and provided insights into the effects on solution chemistry during heap bioleaching.     

Speciation of vanadium (V) extracted from acidic sulfate media by trioctylamine in n-dodecane modified with 1-tridecanol

The speciation of vanadium (V) extracted from acidic sulfate media by protonated trioctylamine in n-dodecane modified with 5% (wt) 1-tridecanol has been investigated by Fourier transformed infrared spectroscopy (FTIR) and ⁵¹V nuclear magnetic resonance spectroscopy (⁵¹V NMR). In aqueous sulfate solutions, vanadium (V) exists both as VO₂⁺ and VO₂SO₄⁻ ions. The FTIR spectra of 0.2 mol kg^− 1 protonated trioctylamine in n-dodecane modified with 5% (wt) 1-tridecanol, loaded with various concentrations of vanadium (V) by extraction from 1 mol kg^− 1 H₂SO₄, indicate that vanadium (V) exists in organic phases as polyvanadates, likely as decavanadates. The condensed nature of the extracted form of vanadium (V) was neither confirmed nor precluded by ⁵¹V NMR as the micellar structure of these organic phases imposes local conditions which allow the transformation of VO₂⁺ and VO₂SO₄⁻ into polyvanadates, but also modify the chemical shifts compared to the ones observed in bulk aqueous solutions for mononuclear and polynuclear vanadium (V) species.     

Does aporusticyanin mediate the adhesion of Thiobacillus ferrooxidans to pyrite?

The adhesion of Thiobacillus ferrooxidans to pyrite was quantified by electrical impedance measurements. Cells grown on soluble iron adhered specifically and with high affinity to pyrite, exhibiting an equilibrium dissociation constant of 5×10⁻¹⁵ M cells. Purposeful manipulation of individual cells using optical trapping techniques revealed that 92% of the iron-grown cells adhered to pyrite with a force greater than 5.2 pN, the maximum force exerted by the trap. In contrast, cells grown on sulfur adhered to pyrite with lower affinity, and 91% of sulfur-grown cells were dissociated from pyrite with an average force of 3.6 pN. Purified recombinant aporusticyanin and intact cells of T. ferrooxidans showed an identical pattern of adhesion to the same minerals. The addition of ferrous ions or organic chelators to the binding mixture prevented the binding of either aporusticyanin or intact cells to pyrite. Preincubation of either the pyrite alone or both the pyrite and the cells with exogenous aporusticyanin inhibited the adhesion of cells to pyrite by 41% and 60%, respectively. A His85Ala mutant apoprotein bound much less tightly to pyrite than did the wild type aporusticyanin. These observations are consistent with a model where aporusticyanin located on the surface of the bacterial cell acts as a mineral-specific receptor for the initial adhesion of T. ferrooxidans to pyrite. Binding of the apoprotein to solid pyrite is accomplished in part by coordination of the unoccupied copper ligands with an iron atom at the exposed edge of the pyrite crystal lattice.     

Biochemistry of sulfur extraction in bio-corrosion of pyrite by Thiobacillus ferrooxidans

By adding a small amount of the amino acid cysteine to an acidic solution containing Thiobacillus ferroxidans cells and subsequent incubation with synthetic pyrite layers, the duration of the lag phase in the growth of T. ferrooxidans is minimized and the leaching rate of this sulfide is increased three times compared to the normal process without this biochemical additive.In the presence of cysteine, pyrite can be oxidized in the absence of bacteria with a leaching rate comparable with that attained by bacteria under normal leaching conditions.It seems that the sulfhydryl group of cysteine participates in a binding process with pyrite. Free-SH groups from the pyrite surface would be the counterpart for the formation of the corresponding disulfide. This thiol-disulfide reaction means that cysteine is consumed by the pyrite surface with the subsequent release of iron-sulfur species. This result is accounted for by the fact that pyrite without bacteria can be completely oxidized. Bacteria would take advantage of this biochemical corrosion process by uptake and oxidation of the released species which are continuously supplied to the acidic-biotope.The reactivity of cysteine with the pyrite surface, which improves the bacterial leaching rate, suggested the investigation of other mono-thiol molecules with the aim to mask the pyrite surface against bacterial attack. The latter can be relevant for protecting steel against microbiologically induced corrosion (MIC). S-tert-butyl mercaptan ((CH₃)₃C–SH, TBM)) was found to stop the bacterial corrosion by T. ferrooxidans. Permanent disulfide bonds between pyrite and the blocking SH-agent are presumably the cause for this protection.     

Dissolution and structural alteration of phlogopite mediated by proton attack and bacterial oxidation of ferrous iron

Microbiological weathering of a research-grade mica mineral, phlogopite, was studied using ferrous sulfate media that were inoculated with an acidophilic iron-oxidizing bacterium, Thiobacillus ferrooxidans. Weathering due to dissolution was monitored by analysis of Si, Al, Fe, K, Na, Mg, and Ca in the leach solutions and in chemical controls at pH 1.0, 1.5 and 2.0. Structural alterations of phlogopite were analyzed by X-ray diffraction. At pH 2, the oxidation of Fe(II) by T. ferrooxidans was accompanied by the formation of jarosite within 7 days of incubation at 22°C. The precipitation of jarosite was coupled with partial alteration of phlogopite to vermiculite and an interstratified (mixed-layer) phlogopite/vermiculite. Similar results were obtained with chemical controls containing 120 mM ferric sulfate. The data suggested that K incorporated into jarosite was released from interlayer positions in phlogopite; thus, jarosite constituted a sink for K. The formation of jarosite and expansible layer silicate phases was pH-dependent. At pH<1.5, jarosite was not formed and phlogopite weathering was due to chemical dissolution without detectable structural alteration.     

Activity of arsenic in copper

Values have been reported in the literature for the Henrian activity coefficient for arsenic in molten copper ranging from 1.45 × 10^-4 to 5 × 10^-7 at temperatures between 1273 and 1573 K. In this study, that data was reexamined and was found to be in closer agreement than originally reported. The Henrian activity coefficient was found to range from 2.2 × 10^-3 at 1273 K to 5.6 × 10^-3 at 1373 K. The experimental data indicate that at 0.21 < N_As < 0.30 the activity coefficient γ_As can be determined from the following equations: log γ_As = -5.58 N _Cu ² + 1.65(T = 1273 K) log γ_as = -6.22 N_Cu/2 + 2.25(T = 1373 K) This study also examined the extent of the disassociation of tetratomic arsenic vapor as As, As₂ and As₃. The results of the analysis indicate that As₂ is the predominant species when arsenic vapor, equilibrated with metallic arsenic at temperatures below 873 K, is heated to temperatures above 1273 K.     

Process mineralogy of bacterial oxidized gold ore in São Bento Mine (Brasil)

The São Bento deposit (Santa Bárbara, MG) occurs in the middle portion of the Quadrilátero Ferrífero (latu sensu), hosted by the São Bento iron formation. The most important minerals in the deposit are arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, galena, electrum, magnetite, ilmenite, siderite, ankerite, calcite, quartz, chlorite, stilpnomelane and muscovite. A mineralogical characterization of samples from the bacterial oxidation process at the São Bento gold mine (MG) was performed using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS), Mössbauer spectroscopy and electron probe microanalysis (EPMA). Samples were collected in four different dates: one sample from the flotation concentrate and four from the bacterial oxidation system (BIOX) (TK2, TK4/TK68 bioreactors and TK28 thickener). Pyrrhotite was completely oxidized and arsenopyrite, pyrite and chalcopyrite were only slightly oxidized in the BIOX. Siderite occurs in small concentration in BIOX. Quartz, chlorite, and muscovite were slightly affected along the process. In BIOX, amoniumjarosite and hydroniumjarosite in lower abundance are the main phases formed. Native sulfur was detected in significant concentrations. Goethite and hematite are the main oxyhydroxides and an unidentified hydroxide containing up to 10 wt.% MgO was observed. Subordinate phases in BIOX samples are fibroferrite, zykaite, bukovskyite, sarmientite, tooeleite, alunite and gypsum. Chemistry of the iron sulfates shows that there are two amoniumjarosites in the BIOX, high and low-As jarosites. Low-As amoniumjarosite contains 10 wt.% As₂O₅ in average; while the high-As one contains 20 wt.% As₂O₅ in average. As-to-S substitution ratio is 1:1 suggesting a solid solution series toward an “As jarosite”. Their crystallization seems to be controlled by a solvus.     

Attachment of acidophilic bacteria to solid surfaces: The significance of species and strain variations

Sixteen strains of acidophilic bacteria were screened for their abilities to adhere to pyrite ore, glass beads and ferric hydroxysulfates. These were three culture collection and two isolated strains of the iron- and sulfur-oxidizer, Acidithiobacillus ferrooxidans, two each of the sulfur-oxidizer Acidithiobacillus thiooxidans and the iron-oxidizer Leptospirillum ferrooxidans (the type strain and a mine isolate in either case), five heterotrophic acidophiles (four Acidiphilium and one Acidocella sp.) and two moderately thermophilic iron/sulfur-oxidizers (Sulfobacillus thermosulfidooxidans and Sulfobacillus acidophilus). Considerable variations were found between different species of acidophiles, and also between different strains of the same species, in how they attached to the three solid materials tested. Attachment to the solid substrata generally increased with time (over 100 min) though > 99% of one At. ferrooxidans isolate (strain OP14) were attached to pyrite after just 10 min exposure. Most acidophiles attached more readily to pyrite than to glass beads, and attachment to ferric hydroxysulfates was highly variable, though one At. ferrooxidans isolate (strain SJ2) and one heterotrophic acidophile (Acidocella sp. het-4) both attached strongly to ferric iron precipitates (jarosites and schwertmannite) that formed in cultures of At. ferrooxidans grown at pH > 2. The results of these experiments showed that even closely related strains of acidophilic bacteria can display very different propensities to attach to solid materials, an observation that may explain the somewhat disparate results reported on occasions by research groups that have examined single, or limited numbers of strains, of acidophiles (mostly At. ferrooxidans). The significance of differential attachment of mineral-oxidizing and other acidophiles to pyrite and other solids is discussed in the context of biohydrometallurgy.     

Leaching of gold and palladium with aqueous ozone in dilute chloride media

The recycling of gold and palladium from metallic scraps can be carried out by ozone-leaching at ambient temperature and low (∼0.1 M) H⁺ and Cl⁻ concentrations. Rh and Pt remain un-reacted, whereas metals such as Cu, Ni, Ag, can be previously eliminated through O₂/H⁺ and O₂/O₃/H⁺ leaching pretreatments. Gold and palladium are dissolved in O₃/Cl⁻/H⁺ with formation of AuCl₄⁻ and PdCl₄²⁻. Leaching studies showed a passive region, basically located at < 0.01 and < 0.05 M Cl⁻ for Au and Pd, respectively. In the non-passive region, rates were only slightly dependent on either H⁺ and Cl⁻. Secondary formation of chlorine or hypochlorous acid was negligible at ≤ 0.1 M Cl⁻. Kinetics appeared to be controlled by mass transfer of O_3(aq) to the solid–liquid interface, showing first order dependency with respect to [O₃]_aq. Rates increased with temperature up to about 40 °C, but decreased at higher temperatures due to the fall in the O₃ solubility. The ozone mass transfer coefficients showed an activation energy < 20 kJ/mol. Gold leaching rate gradually diminished for pH > 2, as consequence of the influence of the [H⁺] on transfer control. The electric power consumption associated with O₃ generation was in the range 4–8 kWh/kg metal leached.     

Studies on the effect of addition of silver ions on the direct oxidation of pyrite

The nature of the reaction between Ag⁺ and pyrite in 0.25 M H₂SO₄ solutions has been investigated in order to determine whether Ag⁺ can enhance the ferric sulfate leaching of this mineral. Analysis of reacted pyrite particles using scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and low-angle X-ray diffraction (XRD) indicates that elemental silver and elemental sulfur are the primary surface species formed by this interaction. Rest potential measurements of a pyrite electrode immersed in a solution containing 10⁻² M Ag⁺ are also consistent with what is expected for the deposition of metallic silver. Furthermore, the XRD data reveal that, at the most, only minor amounts of Ag₂S are being produced. The presence of Ag₂O has also been detected, but this is due to oxidation of silver after the experiment is complete and while the particles are being transferred for surface analysis. When 1 M ferric sulfate is contacted with pyrite which has been pretreated in a AgNO₃ solution, most of the silver immediately redissolves and does not redeposit while ferric ions are present. This indicates that the kinetics of the transfer reaction between Ag⁺ and pyrite is slower than the reaction between Fe³⁺ and pyrite and suggests that Ag⁺ does not likely enhance the ferric sulfate leaching.     

Electrochemistry ofthiobacillus ferrooxidans interactions with pyrite

A cyclic voltammetry technique was used to study the interactions of mineral-pyrite during bioleaching with the bacteriumThiobacillus (T.) ferrooxidans over its entire growth cycle. Invariably, the pyrite surface drastically changed its properties on the second day of bacterial rowth (bioleaching). After 2 days, the cyclic voltammograms (CVs) were insensitive to convective diffusion produced by stirring. The product layer was examined by scanning electron microscopy (SEM), X-ray diffraction, and chemical analysis. The SEM study revealed an extremely high density of bacteria on the pyrite surface. The high density of bacteria, along with the solid reaction products formed on the pyrite surface, created conditions for crack/pore diffusion, explaining why the CVs became insensitive to convective diffusion (stirring) in solution. X-ray diffraction study confirmed jarosite as a product layer. A mechanism is proposed by whichT. ferrooxidans cells serve as nucleation sites for jarosite formation. Formerly Graduate Student, College of Mines and Earth Resources, University of Idaho.     

Screening of bacterial cells for biosorption of oxyanions: Application of micro-PIXE for measurement of biosorption

An attempt was made to isolate bacterial strains capable of biologically removing tungstate (WO₄²⁻) and perrhenate (ReO₄⁻). Thirty-eight water samples were collected from various areas of Anzali lagoon, Iran. Initial screening of a total of 100 bacterial isolates, resulted in the selection of one isolate with maximum biosorption capacity of WO₄²⁻ and ReO₄⁻. It was tentatively identified as Bacillus sp. according to morphological and biochemical properties and named strain GT-83. WO₄²⁻ and ReO₄⁻ uptake by Bacillus sp. GT-83 involved both inactive and active phenomena. The amount of WO₄²⁻ (initial concentration 184 mg/l) removed from aqueous solution after 16 h by inactive and active phenomena was 26 and 194.5 μg/mg protein, respectively. The strain also removed ReO₄⁻ inactively (23 μg/mg protein). Bacillus sp. GT-83 tolerated high MIC of the oxyanions. The order of toxicity of the oxyanions to the bacterium was WO₄²⁻ > ReO₄⁻ in solid media. The effects of increasing metal concentrations on the growth rate were determined in order to obtain precise patterns of resistance in liquid cultures. From the results of the oxyanions toxicity, inhibitory concentrations in solid media were higher than those in liquid media. Oxyanions biosorption was determined during the course of growth. Bacillus sp. GT-83 was capable of removing WO₄²⁻ and ReO₄⁻ during the active growth cycle with a sorption capacity of 194.5 μg WO₄²⁻/mg protein and 137.1 μg ReO₄⁻/mg protein. In view of the results of oxyanions accumulation experiments, it was concluded that Bacillus sp. GT-83 was not only tolerant to oxyanions, but it also bound considerable amounts of WO₄²⁻ and ReO₄⁻ from the growth medium. The binding of tungsten and rhenium on the cell wall of Bacillus sp. GT-83 was confirmed with micro-PIXE.