Bioleaching of metals from soils or sediments using the microbial sulfur cycle

Reduced inorganic sulfur species like elemental sulfur or sulfide are sensitive to changes in oxidative environments. Generally, inorganic reduced sulfur exists in natural environments in a solid phase, whereas its oxidation leads to sulfur solubilization and a production of acidity. This oxidation occurs spontaneously, due to chemical mechanisms, however, its rate can be enhanced by microbes by several orders of magnitude. The acidification which accompanies the sulfur oxidation brings about rather extreme conditions for microbial growth, pH can drop below 2. The microbial oxidation of reduced inorganic sulfur causes several phenomena of environmental concern. The most substantial environmental aspects is the coupling of sulfur oxidative changes with mobility of toxic heavy metals. Oxidation of sulfur and subsequent production of acid leads to a release of cationic metals to the environment. This is happening e.g. in acid mine drainage, acidification and toxic metals release from sediments dredged for nautical reasons, contamination due to flood events, appearance of acid sulfate soils at sites rich in sulfide minerals or at metals-polluted sites which receive acidic leachates.The microbial sulfur oxidation can be also applied in specific technologies. Its use for biohydrometallurgy, i.e. microbial mining of metals from low-grade ores is well known and applied in practise. Another application is microbial desulfurization of coal containing inorganic sulfur inclusions. Recently, its use for decontamination of solid wastes containing toxic metals has been proposed. The bioleaching of heavy metals from anaerobically digested sewage sludge was successfully demonstrated at technological scale.This thesis deals with the microbial leaching of toxic cationic heavy metals from contaminated soils or sediments. The bioleaching of metals from these materials provides a direct advantage if it contains a sufficient amount of reduced sulfur. This applies for freshwater sediments which can accumulate substantial amounts of reduced sulfur in anoxic conditions at bottom of freshwater bodies. Metal sulfides, important form of heavy metals in these materials, are directly attacked by microbial oxidation. Metals in other forms are solubilized by the effects of lowering pH. Therefore, bioleaching is the most natural attempt to solubilize metals in the sediments, since the oxidative changes always occur. However, when the amount of reduced sulfur is insufficient or negligible, like in most aerobic soils, additional sulfur substrate or direct addition of sulfuric acid must be considered to achieve sufficiently low pH.The advantage of using bioleaching or sulfuric acid is particularly in the possible application of other processes of microbial sulfur cycle, i.e. sulfate reduction resulting in a sulfide production and precipitation of heavy metals in the spent leaching liquor and a partial sulfide oxidation leading to the recovery of elemental sulfur. The elemental sulfur can be supplied back to the bioleaching step.Experiments were done to compare the feasibility of microbially produced elemental sulfur (from partial sulfide oxidation) with sulfur flower as a substrate for bioleaching. The results proved that the microbially produced elemental sulfur is a feasible substrate and it seems better available for thiobacilli, than the sulfur flower. This effect is caused by two phenomena: (1) the surface of microbially produced sulfur is more hydrophillic than the orthorhombic sulfur flower and (2) the microbially produced sulfur consists of much smaller particles and provides thus higher specific surface area (2.5 m 2.g -1) than the commercially available orthorhombic sulfur flower (0.1 m 2.g -1). However, the mutual relevance of these two aspects is not fully understood and it may be speculated to which extent would be sulfur oxidation increased, when sulfur flower is pulverized to particles of similar size as the microbially produced sulfur.The growth yields on the two types of sulfur were nearly identical during batch experiments, however, the final biomass and sulfate concentrations with microbially produced sulfur were about twice as high as with the sulfur flower. The maximum sulfur specific oxidation rate in batch cultivation was also about two-times higher with microbially produced sulfur than with the sulfur flower. During microbial sulfur oxidation, the first was consumed the finest fraction, which forms typically up to 40% of the microbially produced sulfur. Subsequently, the larger aggregates are oxidized, however, at lower rate. The lowest pH achieved by thiobacilli in a continuous cultivation on microbially produced sulfur was 1.7. This was observed at a dilution rate of 0.04 h -1. Maximum production rate of sulfuric acid was 1 mmol.L -1.h -1at a sulfur loading of ca. 4 mmol.L -1.h -1. When compared to the maximum conversion rates of the other process of microbial sulfur cycle, i.e. sulfate reduction and partial sulfide oxidation, the production of sulfuric acid proceeds at the lowest rate.Three experimental studies were accomplished to demonstrate the bioleaching of metals from soil and sediments. The first study involved artificially zinc-contaminated clay, silt, and sandy soil, and the leaching behaviour was studied at varying additions of sulfuric acid between pH 1.5-6. The measurement of aluminium solubilization was used to quantify the extent of soil matrix damage at extreme acidity. Zinc solubilization followed a monotonous increase with decreasing pH, being 17-43% at pH 7 and 72-95% at pH 1.5. However, the aluminium solubilization revealed a sharp edge at pH=4. Below this pH, Al-concentrations increased exponentially, indicating major damage to the soil mineral matrix. The study revealed two different possible strategies in leaching: the first, called here intensive, uses high concentrations of sulfuric acid to achieve satisfactory extraction efficiency and high soil/solution ratio. However, a considerable damage of the soil matrix can be expected. The second strategy, called extensive, uses lower concentrations of acid, however, the soil/solution ratio must be properly decreased.The second leaching study aimed at the possible use of sulfur oxidation in-situ, i.e. within the soil profile after artificial contamination of the soil with cadmium. Microbially produced elemental sulfur and orthorhombic sulfur flower were supplied to the soil prior to its placement into the soil pots and the velocity of soil acidification and cadmium solubilization were observed. The microbially produced sulfur proved faster oxidation and acidification than the orthorhombic sulfur flower: immediately after addition of microbially produced sulfur, pH started to decrease. pH-decrease in sulfur flower treatments was observed only after 55 days lag. The solubilization of cadmium into the pore water followed directly the changes of pH. In this study, a vegetative uptake of solubilized cadmium was tested using a common mustard ( Sinapis alba ). With decreasing soil pH, the concentrations of cadmium in biomass increased, however, the biomass yields decreased. When Cd concentrations and biomass yield were combined, an optimum soil pH of 5-5.5 was found for the vegetative removal. However, the overall efficiency of the vegetative removal was rather low.In the third leaching study, a sediment from a wetland receiving mine drainage was used, since this sediment was expected to contain high amounts of reduced sulfur. 150-hours aeration of the sediment resulted in acidification down to pH 4.2, accompanied by the increase in redox-potential from -150 meV to +300 meV and an increase of sulfate concentration to ca. 6 mmol.L -1. At the same time, the solubilization of Cd, Cu and Zn was recorded. Total soluble iron revealed initial increase up to 48 mg.L -1within 50 hours of aeration, followed by decrease below detection limit. This was explained by initial desorption of soluble Fe 2+followed by its oxidation and precipitation of the resulting Fe 3+ion. The minimum pH achieved by aeration of the sediment was not sufficient to achieve satisfiable extraction efficiency for the studied metals. Therefore, addition of sulfuric acid or elemental sulfur was investigated.The study firstly involved the leaching after exposition to varying concentrations of sulfuric acid. Two different processes were observed in the sediment slurry after acid addition: (1) the monotonous pH decrease with time, caused by oxidation of reduced compounds, which was observed at low acid additions, and (2) delayed pH increase at high acid additions due to the scavenging of acidity by various processes of pH-buffering, solubilization of minerals, diffusion etc. Solubility of Cd, Cu, Pb, and Zn was controlled by exposure intensity defined as actual activity of acid multiplied by the time of exposition. Similar to the first leaching study, intensive and extensive leaching strategies could have been distinguished, where intensive leaching involved high concentrations of H 2 SO 4 and short extraction times, and the extensive leaching used no or low amendments of acid, however, at prolonged extraction times. In the second step of experiments, the bioleaching tests were performed. According to the previous studies, microbially produced sulfur proved faster acidification compared to the orthorhombic sulfur flower.The use of microbial sulfur oxidation for the enhancement of toxic metals removal from contaminated soils or sediments is technically feasible. When integrated with the other processes of microbial sulfur cycle, i.e. sulfate reduction and partial sulfide oxidation, it may strongly benefit from relatively easy processing of the spent liquor and sulfur regeneration and re-use. Compared to these processes, the bioleaching is a rate-limiting step.It is possible to perform bioleaching both in fully agitated soil slurries, as well as in heap-leaching or in-situ configurations, using an intensive or extensive leaching strategies. The use of processes with different intensity may be the main advantage of the partial or full use of microbial sulfur cycle to control the toxic metals in the environment. Example of the integration of processes of microbial sulfur cycle is e.g. a wetland or anaerobic pond using sulfate reduction in an extensive and sustainable way to control the pollution of voluminous metals and sulfate containing aqueous stream, followed by a more intensive bioleaching. Other example is the introduction of elemental sulfur in the soil to promote its slow acidification and release of metals into the pore water. The collected water can be further treated by intensive or extensive sulfate reduction.