Advanced Materials Research Vols. 71-73

Abstract: Acidithiobacillus thiooxidans was grown in the presence of uranium and its redox processes were studied in aerobic conditions for the first time. Results so far obtained may suggest an indirect uranium reduction, mediated by metabolically produced reducing compounds.

Abstract: The role of Acidithiobacillus group of bacteria in acid generation and heavy metal dissolution was studied with relevance to some Indian mines. Microorganisms implicated in acid generation such as Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans were isolated from abandoned mines, waste rocks and tailing dumps. Arsenite oxidizing Thiomonas and Bacillus group of bacteria were isolated and their ability to oxidize As (III) to As (V) established. Mine isolated Sulfate reducing bacteria were used to remove dissolved copper, zinc, iron and arsenic from solutions.

Abstract: Sludge generated in automotive and related industries often contains heavy metals. Bioleaching is an attractive alternative for the treatment of metal containing solids. Bacteria of the genus Acidithiobacillus are the most important microorganisms applied to metal solubilisation. These microorganisms are able to produce sulphuric acid from the aerobic oxidation of elemental sulphur. The biogenerated sulphuric acid can be applied to the solubilisation of metals from a solid matrix. In this paper we present the results of our experiments aimed at the removal of nickel and zinc from sludge generated in the water treatment plant of an automotive industry. Acidithiobacillus thiooxidans cells were immobilised on sulphur pearls in a column reactor. The effects of sulphur pulp density and the dilution rate on the production of sulphuric acid were studied. In a second stage, sulphuric acid was used to solubilise the nickel and zinc from the sludge. The effects of different sludge pulp densities and initial acid pH were studied. High recoveries of zinc and nickel were obtained when the pH value of the sulphuric acid solution was lower than 2.0 for 1 and 2 % of pulp density.

Abstract: In previous studies it has been showed that bacterially produced sulphuric acid is a good leaching agent for laterite tailings. In this work we evaluated heavy metals leaching from low grade laterite ore for cobalt and nickel extraction using sulphuric acid produced in situ by Acidithiobacillus thiooxidans under different culture conditions. In studies where that material was initially added to the cultures, considerable percentages of metals were leached (100 % Mn, 70 % Co, 7.5 % Ni, less than 5 % of Cr and Fe) after 18 days of incubation at low pulp densities (1 % and 2.5 %) of overburden. The maximum percentages were reached when cultures pH was approximately or below than 1.5. At higher pulp densities material was added to the cultures after different pre-cultivating times; also higher sulphur amounts were assayed; in such way about 100 % Mn, 60 % Co, 9 % Ni and Fe and 2.5 % Cr were leached. Although toxic metals were not completely leached, sequential extractions results indicate that these metals are not readily available. All studies finally suggest that bioleaching is a suitable technology for recovery of valuable metals as Co and remediation of mining residues by extraction of heavy metals.

Abstract: Spent catalysts represent a large amount of refinery solid waste. In particular, hydro-processing catalysts contain base valuable metals, such as nickel, vanadium and molybdenum and, for their toxic component, these wastes have been classified as hazardous by the Environmental Protection Agency in the USA. The development of an innovative eco-sustainable process for the valorisation of such wastes would undoubtedly give significant advantages also taking into account primary resources preservation. This paper deals with bioleaching of metals from hazardous spent hydro-processing catalyst by means of iron/sulphur oxidizing bacteria. The exhaust catalyst was rich in nickel (45 mg/g), vanadium (44 mg/g) and molybdenum (94 mg/g). Before bioleaching, the solid was washed by means of a mixture of Tween 80 and ethyl alcohol, for hydrocarbon removal. The effects of elemental sulphur, ferrous iron and actions contrasting a possible metal toxicity (either the presence of powdered activated charcoal or the simulation of a cross current process by means of filtration stages in series) was investigated. Ferrous iron resulted to be essential for metal extraction and for bacteria adaptation. Nickel and vanadium were successfully bioleached in the presence of iron, reaching extraction yields of 83% and 90%, respectively; on the other hand extractions around 50% for nickel and vanadium were observed both in biological systems in the absence of iron and in the chemical controls with iron. As concerns molybdenum, the highest extraction yields experimentally observed was about 50%, after 26 days bioleaching in the presence of iron, while a maximum extraction of 25 % was observed in the other treatments. In conclusion, a bio-oxidative attack with iron could successfully extract nickel, vanadium and partially molybdenum. Further actions aimed at contrasting a possible metal toxicity resulted not to be effective.

Abstract: This work compares gold bioleaching from e-waste containing gold and copper by Chromobacterium violaceum and Pseudomonas fluorescens. The effect of pulp density (ranging from 0.5 to 8%w/v) was examined. Although C. violaceum produced more cyanide than P. fluorescens in the absence of e-waste, P. fluorescens showed higher growth rate, cyanide production and gold leaching efficiency at all pulp densities. Pretreatment with biooxidation of the e-waste using Acidithiobacillus ferrooxidans resulted in the removal in excess of 80% of the copper present in the waste, and increased the gold/copper ratio in the residual solid. Bioleaching the biooxidised e-waste significantly improved gold recovery, especially by C. violaceum, particularly at high pulp density. For example, at pulp densities of 2 and 4% w/v, gold recovery from non-biooxidzed e-waste was 0.22 and 0.14% respectively. Higher gold recovery, at 8%, was obtained for bioleaching of the biooxidised e-waste at both these pulp densities. The ratio of gold/copper in leachates after bioleaching of the biooxidized e-waste was also found to be increased.

Abstract: . This work reports on the bioleaching of spent NiMo catalyst using Aspergillus niger. As-received and decoked catalysts of various sizes (as-received, 180-120 µm, 120-45 µm and <45 µm) were examined. Chemical analysis of the spent catalyst confirmed the presence of heavy metals including Al (38.2%), Ni (3.36%), and Mo (12.9%). SEM-EDX analysis revealed a change in the metal distribution within the spent catalyst following the decoking process. In one-step bioleaching where the fungus was inoculated together with the catalyst, fungal growth was not observed. In 2-step bioleaching where the catalyst was added three days after the start of the fungal incubation, the highest Mo (87.3%) and Al (17.20%) leaching efficiencies were obtained from the catalyst of smallest particle size (i.e. decoked grounded; <45µm) while the highest Ni leaching efficiency (94%) was achieved from the as-received spent catalyst.

Abstract: This paper deals with biohydrometallurgy applied for the remediation of sediments contaminated by heavy metals. Both metal mobilization by bioleaching and metal stabilization by sulfate reducing bacteria stimulation have been investigated. In the first case, leaching experiments were performed with different microbial strains: i) autotrophic Fe/S-oxidizing bacteria, ii) heterotrophic Fe reducing bacteria, and iii) a mix of them. The highest extraction yields were 90% for Cu, Cd, Hg and Zn and were achieved with a consortium of the autotrophic and the heterotrophic strains. In the second case, anaerobic microcosm experiments were performed according to a full factorial experiment where the main factors were: i) acetate, ii) inoculum of alloctonous sulfate-reducing bacteria and iii) treatment time. Significant changes on metal partitioning were observed in all the investigated treatments: in particular, Cu, Pb and Zn concentrations in the mobile fraction were reduced and the ones in the oxidizable fraction significantly increased. Anaerobic processes where hypothesized to lead to the formation of metal sulfides, relatively stable and less bioavailable than mobile fractions. The obtained results open new perspectives for biohydrometallurgy applied in the context of remediation strategies for sediments contaminated by heavy metals.

Abstract: The aim of the present work was to study the feasibility of using bioleaching for the solubilisation of metals from solid waste streams and by-products of copper, steel and recycling industries. Optimal process conditions were tested using iron and sulphur oxidising acidophiles in shake flasks at 25°C. The effects of inoculum, ferrous iron, sulphur, chloride ion, pH, and the type of waste material on metal solubilisation were evaluated. Solubilisation of metals was mainly achieved through acid attack due to the formation of sulphuric acid by sulphur oxidising bacteria. Addition of ferrous iron and chloride ion did not enhance solubilisation.

Abstract: Limestone is commonly used for neutralization of acid mine drainage (AMD). Its main advantages are its lower price, sustained generation of alkalinity and production of low sludge volumes. Nevertheless, armouring of limestone by ferric hydroxides is a problem in oxic limestone drains and in active limestone treatment systems, reducing the efficiency of the process. Due to these disadvantages, there is a permanent search for cheaper and more effective neutralization agents. Many alkaline industrial wastes are gaining importance in the treatment of AMD. The possibilities to use two different industrial by-products, red mud from a bauxite exploitation and low grade magnesium hydroxide from a magnesite mine, as neutralizing and bacterial inhibiting agents, and the comparison with conventional limestone treatment has been studied in this paper. An AMD from Rio Tinto mine site with an initial pH of 2.4 and a ferric concentration of 1 g/L was used. Comparative test were done percolating the AMD in a continuous form with a peristaltic pump through three different columns filled with limestone, red mud and low grade magnesite, during one month and in same conditions of flow rate and amount of each compound used to fill the columns. The evolution of pH, iron and heavy metals, sulphates and microbial populations in the percolate were monitored at different times. The results showed that the best neutralization capacity was obtained with low grade magnesite during the month treatment. By contraire limestone and red mud loosed their neutralization capacity after 10 and 13 days respectively. The control of microbial populations showed that there is an inhibition of chemolithotropic bacteria as long as the materials maintain their neutralization capacity, reverting to the initial conditions when this capacity was loosed.