B. Escobar

Interfacial phenomena affecting the adhesion of Thiobacillus ferrooxidans to sulphide mineral surface

Abstract The electrokinetic and hydrophobic properties of the autotrophic bacteria Thiobacillus ferrooxidans as well as selected minerals have been studied by microelectrophoresis, contact angle and hexadecane/aqueous phase partition measurements. Adhesion measurements were conducted on sized fractions of quartz, pyrite, chalcopyrite and other copper sulphides in order to establish the relationships between cell surface properties and cell adhesion to specific mineral surfaces. It was found that the bacteria had an isoelectric point at pH 3.0 and that they were slightly hydrophobic, showing a water contact angle of 20–24 and extraction recoveries into hexadecane in the range 14–22 wt.% Adhesion mechanisms involved in the interaction between Thiobacillus ferrooxidans and mineral surfaces as a function of the electrokinetic and hydrophobic properties of bacteria and minerals are illustrated. Calculated free energies of bacterial adhesion to quartz, chalcopyrite and pyrite indicated that adhesion appears to occur at the secondary minimum according to the Derjaguin—Landau—Verwey—Overbeek theory.

Designing antimicrobial bioactive glass materials with embedded metal ions synthesized by the sol-gel method.

Bioactive glasses (SiO2-P2O5-CaO) having tailored concentrations of different biocide metal ions (copper or silver) were produced by the sol-gel method. All the particles release phosphorous ions when immersed in water and simulated body fluid (SBF). Moreover, a surface layer of polycrystalline hydroxy-carbonate apatite was formed on the particle surfaces after 10 day immersion in SBF as confirmed by X-ray diffraction and scanning electron microscopy (SEM) showing the bioactive materials. Samples with embedded either copper or silver ions were able to further release the biocide ions with a release rate that depends on the metal embedded and the dissolution medium: water or SBF. This biocide ion release from the samples explains the antimicrobial effect of our active particles against Escherichia coli DH5α ampicillin-resistant (Gram-negative) and Streptococcus mutans (Gram-positive) as determined by the Minimum Bactericidal Concentration (MBC) method. The antimicrobial behavior of the particles depends on the bacteria and the biocide ion used. Noteworthy, although samples with copper are able to release more metal ion than samples with silver, they present higher MBC showing the high effect of silver against these bacteria.

Chemical and biological leaching of enargite

Enargite (Cu 3 AsS 4 ) was leached faster by bacteria in sulfuric acid medium (pH 1.6) with added ferric sulfate than by chemical leaching at the same or higher iron concentration. During chemical leaching with ferric iron, the copper dissolution rate decreased from an initial value of 0.03% per hour to a value of 0.002% per hour. Enargite is oxidized to elemental sulfur and dissolved arsenic (As 3+ and As 5+ ). Less than 10% of sulfur is oxidized to sulfate. The arsenic and copper dissolutions observed in bacterial leaching experiments suggest the existence of a specific bacterial action on the leaching of enargite, demonstrated by the ability of bacteria to oxidize enargite at very low concentration of dissolved iron and by the higher dissolution rate obtained in bacterial leaching compared to chemical ferric leaching.

A method for evaluating the proportion of free and attached bacteria in the bioleaching of chalcopyrite with Thiobacillus ferrooxidans

Abstract A method for evaluating the proportion of bacteria attached to mineral sulfides is presented. The method is based on the determination of the rate of ferrous iron oxidation by attached bacteria on the mineral surfaces. The values obtained with this procedure are in agreement with measurements of attached bacteria using bacteria grown with radioactive (C 14 ) NaHCO 3 . The methodology was used to evaluate the fraction of bacteria attached to chalcopyrite during bioleaching of this sulfide. It was observed that a significant fraction of the bacteria grow attached to the mineral.

Influence of lipopolysaccharides on the attachment of Thiobacillus ferrooxidans to minerals

The loss of part of the lipopolysaccharides (LPS) of the outer membrane of T. ferrooxidans negatively influenced the attachment of the bacteria to minerals and the bioleaching process. LPS previously extracted from T. ferrooxidans and which had come into contact with pyrite inhibited the attachment of cells to minerals and also negatively affected the bioleaching. These results suggest that LPS play an important role in the attachment of the microorganisms and therefore, its presence or absence could affect the bioleaching process.

Short communication: Adverse effect of surface-active reagents on the bioleaching of pyrite and chalcopyrite by Thiobacillus ferrooxidans.

Oxidation of Fe(II) iron and bioleaching of pyrite and chalcopyrite by Thiobacillus ferrooxidans was adversely affected by isopropylxanthate, a flotation agent, and by LIX 984, a solvent-extraction agent, each at ≤ 1 g/l. The reagents/l were adsorbed on the bacterial surface, decreasing the bacterias development and preventing biooxidation. Both reagents inhibited the bioleaching of pyrite and LIX 984 also inhibited the bioleaching of chalcopyrite.

Use of acidophilic bacteria of the genus Acidithiobacillus to biosynthesize CdS fluorescent nanoparticles (quantum dots) with high tolerance to acidic pH

The use of bacterial cells to produce fluorescent semiconductor nanoparticles (quantum dots, QDs) represents a green alternative with promising economic potential. In the present work, we report for the first time the biosynthesis of CdS QDs by acidophilic bacteria of the Acidithiobacillus genus. CdS QDs were obtained by exposing A. ferrooxidans, A. thiooxidans and A. caldus cells to sublethal Cd2+ concentrations in the presence of cysteine and glutathione. The fluorescence of cadmium-exposed cells moves from green to red with incubation time, a characteristic property of QDs associated with nanocrystals growth. Biosynthesized nanoparticles (NPs) display an absorption peak at 360nm and a broad emission spectra between 450 and 650nm when excited at 370nm, both characteristic of CdS QDs. Average sizes of 6 and 10nm were determined for green and red NPs, respectively. The importance of cysteine and glutathione on QDs biosynthesis in Acidithiobacillus was related with the generation of H2S. Interestingly, QDs produced by acidophilic bacteria display high tolerance to acidic pH. Absorbance and fluorescence properties of QDs was not affected at pH 2.0, a condition that totally inhibits the fluorescence of QDs produced chemically or biosynthesized by mesophilic bacteria (stable until pH 4.5-5.0). Results presented here constitute the first report of the generation of QDs with improved properties by using extremophile microorganisms.

Effect of Flotation and Solvent Extraction Reagents on the Bioleaching of a Copper Concentrate with Sulfolobus Metallicus

The effect of two flotation reagents: isopropyl- and ethyl-xanthates and two solvent extraction reagents (SX): Lix 984 and Lix 860IC on the bioleaching of a copper concentrate with Sulfolobus metallicus was analyzed. The experiments were performed in 250 ml shake flasks with 100 ml basal medium with an initial pH of 1.6, inoculated with a pure culture of S. metallicus and 0.5 % copper concentrate (38% of Cu) at 70oC. In the bioleaching flasks 0, 50 or 500 μg/mL of flotation reagent and 0, 10 or 100μg/mL of SX reagents was added. All organic reagents caused a strong inhibition of ferrous iron oxidation and growth of planktonic cells of S. metallicus. However, bioleaching in presence of 50 μg/mL of isopropyl-xanthate or 10 μg/mL of Lix 984 reached copper dissolutions of 70% and 72% respectively after 200 hours, compared with 98% in the process without organic reagents. These results suggest that isopropyl-xanthate and Lix 984 at the lowest concentrations used in this study would not affect the mechanism of bioleaching by contact of S. metallicus on the sulphide, however it strongly affected the indirect mechanism. Ethyl-xanthates and Lix 860IC, beginning with 50μg/mL and 10μg/mL respectively, generated a total inhibitory effect on the oxidation activity of S. metallicus on the sulphides.

Activation of bacteria in agglomerated ores by changing the composition of the leaching solution

Abstract This work presents preliminary results of a study to determine the technical feasibility of heap bioleaching of copper sulfides in a mixed ore that had previously undergone a chemical leach to recover the copper from the copper oxides in the ore. The recovery of the copper sulfides is economically and environmentally important. The first step in this study was to determine the presence of bacteria in samples of both the agglomerated ores taken from the heaps and the samples from the irrigation solution used in the leaching process. No bacteria were detected in either sample, probably because of the very low pH, the high concentration of sulfate (200 g/l), and the presence of other ions in the irrigation solution. On the other hand, the cultivation of bacteria from recently crushed, fresh mixed ore (oxide–sulfide), and from cured, agglomerated ore, showed the presence of a small bacterial population, with ferrous iron-oxidizing activity. This indicates that the ore initially contained viable bacteria that were inhibited in the heap. Based on these results, a glass laboratory column containing 700 g of agglomerated ore from a partially, chemically leached heap was set up. The ore in the column was irrigated in a closed circuit with nutrient medium containing 2 g of ferrous sulfate per liter. Bacteria were detected, and the redox potential in the leaching solution began to increase on day 20 of the bioleaching experiment, indicating the beginning of the bioleaching stage of the sulfide mineral. The data indicate that the bacteria initially present on the ore can be recovered after acid curing, and also after prolonged contact with a solution containing a high concentration of sulfate. These results indicate that it is technically possible to induce a change from a chemical leaching process with high ionic strength to a bacterial leaching process, by dilution of the irrigation solution.

Enumeration of Acidithiobacillus ferrooxidans adhered to agglomerated ores in bioleaching processes

In bioleaching processes, bacteria adhered to agglomerated ores are frequently determined in the washing solution after treating the mineral with different techniques like sonification or chemical treatment with SDS, Tween 20, Tritón X-100, or only basal medium to release the adhered cells. In this work we compare the efficiency of these techniques, not only by determination of the number of released cells, but also by establishing their viability. The results indicate that, in spite of the high number of bacteria that can be released from an agglomerated ore, when detergent solutions are used, bacteria are heavily damaged and lose their ferrous-iron oxidation activity. On the other hand, when hand stirring with basal medium is used to release bacteria, a method that does not produce damage to the cells, only a percentage of the total population of active ferrous-iron-oxidizing adhered bacteria is released; therefore, the enumeration or determination of bacteria in the washing solution would be inaccurate. We thus propose that agglomerated ores be monitored directly for the presence of active bacteria, by determination of the ferrous-iron oxidation ability of the attached bacteria.