The accumulation of mine tailings on Earth is a serious environmental challenge. The importance for the recovery of heavy metals, together with the economic benefits of precious and base metals, is a strong incentive to develop sustainable methods to recover metals from tailings. Currently, researchers are attempting to improve the efficiency of metal recovery from tailings using bioleaching, a more sustainable method compared to traditional methods. In this work, the research status of using biological leaching technologies to recover heavy metals from tailings was reviewed. Furthermore, CiteSpace 5.7.R2 was used to visually analyze the keywords of relevant studies on biological leaching of tailings to intuitively establish the current research hotspots. We found that current research has made recent progress on influencing factors and microbial genetic data, and innovations have also been made regarding the improvement of the rate of metal leaching by biological leaching combined with other technologies. This is of great significance for the development of bioleaching technologies and industrial production of heavy metals in tailings. Finally, challenges and opportunities for bioleaching provide directions for further research by the scientific community.
Rapid social and industrial developments have resulted in an increased demand for metals. As a result of rapid industrial development combined with increased exploitation of large quantities of mine resources, the environmental pollution of tailings is expanding at an alarming rate. The environmental impacts of tailings will eventually directly or indirectly threaten the human health (Figure
The harm of tailing pollution.
Heavy metal contamination from tailings is detrimental to the human and environmental health [
Heavy metal pollution from tailings also seriously damages local land resources. Heavy metal mining and processing activities in southern Poland led to heavy zinc pollution and moderate lead pollution of the local agricultural land and thus agricultural production in these areas; as a result, the planting of green leafy vegetables was banned, which has led to significant economic losses [
As discussed above, the environmental risks of heavy metals from mining are very severe. However, tailings are also an important secondary resource. Gold, silver, lead, zinc, sulphur, indium, gallium, cadmium, germanium, selenium, tellurium, and other associated elements recovered from Chinese copper ore during the processing and smelting process account for 44% of the total output value of the raw materials; associated gold accounts for more than 35% of China’s gold reserves, of which 76% of gold and 32.5% of silver is produced by copper mines [
Fortunately, various methods have been developed in recent years to recycle tailings, including the promising technique of bioleaching, chemical extraction remediation techniques, and electrochemical repair techniques, that convert waste into value. Chemical extraction is limited by soil complexity and extractants. Electrochemical repair techniques are expensive and require complex operations. However, bioleaching is a low-cost, green technology for leaching metals from a variety of minerals and waste materials [
Advances in molecular biology techniques and their applications in biosynthesis to detect and identify organisms have expanded our understanding of the interactions of metallic microbes and their important role in metal extraction and recovery [
This review provides an overview of the mechanisms and influencing factors of bioleaching technologies and provides a general understanding of the current status of the bioleaching of copper, iron, and zinc tailings, rare earth elements, and four toxic metals. The challenges and opportunities for the recovery of heavy metals from tailings by biological leaching are also discussed, which will provide a valuable reference for the full exploitation and utilization of secondary resources and the industrial development of the recovery of heavy metals from tailings using biological leaching technologies.
The essence of the biological leaching process is that Thiobacillus acquires the energy needed for growth by oxidizing reduced sulphur compounds, which leads to a decrease in the pH value and changes in the redox environment of the environmental system, thus changing the state of the heavy metals in the system from the original organic matter-bound state to the free state. Since the early 1950s, Fe/S-oxidizing bacteria have been used in industrial-scale processes to extract metals from sulphide ores [
Direct mechanisms: the microorganisms oxidize sulphide minerals through direct attack on the mineral surface [
Indirect mechanisms: metabolites of Fe/S-oxidizing bacteria are used in redox reactions that occur with sulphide minerals, and finally, a redox circulatory system is formed. The sulfuric acid produced reduces the pH of the environmental system to approximately 2.0 and greatly promotes the dissolution of heavy metals [
Another approach is to complement the indirect mechanism by characteristics of bacterial attachment to mineral surfaces, in which the attached cells oxidize ferrous ions into iron ions within a layer of bacteria and extracellular polymeric substances, and the ferric ions within this layer leach the sulphide [
In summary, all previous studies have found that the key to the exploration of the mechanism of biological leaching lies in microorganisms. Only by thoroughly studying the behavior of microorganisms in the leaching process can the mechanism of biological leaching be accurately and systematically interpreted, which will also depend on developments in molecular biology and other disciplines.
Bioremediation technology has attracted increasing attention in the field of environmental protection. At its core, the mechanism and species selection of microorganisms in pollution remediation have become the focus of research. In bioleaching, bacteria play a catalytic role [
Physical and chemical properties of microorganisms.
Strain | Shape | Optimum pH | Optimum temp (°C) |
---|---|---|---|
Acidithiobacillus ferrooxidans | Rod-shaped | 2-3 | 30-35 |
Acidithiobacillus thiooxidans | Rod-shaped | 1.5-2.0 | 28-30 |
Leptospirillum ferrooxidans | Vibrioid-shaped | 1.5-2.0 | 25-35 |
Regarding the selection of microbial species, natural microorganisms (indigenous microorganisms) show better adaptability and have a higher leaching rate; therefore, the majority of studies to date used indigenous microbes [
In terms of the selection of microbial diversity, mixed bacteria leaching is more efficient, and the majority of studies to date focused on the use of mixed bacteria leaching [
The whole process of biological leaching can be summarized from the aspects of microbial species, diversity, bacterial density, activity, and distribution as follows [
In the future, the microorganism selection related to bioleaching needs to be further improved in several aspects [
The commonly used experimental equipment and their functions in biological leaching of heavy metals from tailings are listed in Table
Experimental equipment and functions commonly used in biological leaching experiment.
Instruments or methods | Functions |
---|---|
Inductive coupled plasma atomic emission spectrometry (ICP-OES) | Determination of metal content |
Atomic absorption spectrophotometer (AAS) | Determine the concentration of metal in solution |
pH meter and redox potential analyzer | Determine pH values and redox potential Eh |
Fourier transform infrared spectrometers (FTIR) | Qualitative and quantitative analysis of samples |
X-ray diffractometer (XRD) | Mineralogical analysis |
Phenanthroline spectrophotometry | Determination of ferrous ion and total iron concentration |
16S rRNA gene sequencing | Monitor microbial distribution |
Biological leaching methods for the treatment of electronic waste can be divided into three types according to the type of biomass exposure to the waste: one-step, two-step, and spent-medium [
Technical flow of heavy metals from bioleaching tailings.
The pH is an important physical and chemical parameter in biological leaching. In the study of bioleaching of heavy metals from tailings, iron/sulfur-oxidizing bacteria are more likely to undergo redox reactions under acidic conditions. The effect of pH on the optimum leaching rates of different metals is different. In one study, when the slag particle size was less than 0.83 mm, the smelting slag of lead/zinc on copper, iron, lead, zinc, and other metals was highly dependent on pH, but the pH had little influence on the solubilization of manganese [
A different study showed that the reduction potential of the solution had a great influence on the leaching rate of pyrite, and the influence degree was much greater than the number and activity of bacterial cells [
Temperature is one of the important influencing factors in the process of bioleaching. It mainly affects the microbial activity during the process of biological leaching, which then affects the leaching of heavy metals from tailings. Experiments were carried out to examine the effect of temperature on heavy metal leaching in the temperature range of 7–42°C, in which the growth rate of acidophilic bacteria varied with pH, but the degree of change depended on the temperature [
Previous studies on pulp density are scarce, but current studies show that pulp density mainly affects the leaching of heavy metals by affecting the pH. The larger the pulp density, the greater the pH decline [
A continuous oxygen supply is required during biological leaching. The oxygen and carbon dioxide contents are closely related to the activity of the microorganisms and affect the progress of the redox reaction. The monitoring of dissolved oxygen showed that the demand for oxygen increases with an increase in pulp density [
In addition to the above common factors, there are the following new findings. For example, the leaching rate of heavy metals from tailings decreases with an increase in the solid concentration [
CiteSpace (5.7.R2) provides data visualization and network analysis capabilities [
Research hotspots of bioleach technology (keywords co-occurrence).
Count, centrality, and earliest occurrence year of keywords.
Count | Centrality | Year | Keywords |
---|---|---|---|
26 | 0.10 | 2009 | Bioleaching |
10 | 0.15 | 2009 | Iron |
10 | 0.05 | 2010 | Heavy metal |
10 | 0.31 | 2009 | Acidithiobacillus ferrooxidan |
8 | 0.20 | 2011 | Bacteria |
7 | 0.10 | 2012 | Extraction |
7 | 0.12 | 2009 | Copper |
Subsequently, a cluster analysis was conducted by selecting the label clusters with the indexing term function (Figure
Analysis of label clusters with indexing terms.
Copper was one of the first metals used by humans. Today, it is the most commonly used material for cables, electronics, electrical components, and construction; therefore, the demand for copper will continue to grow. Primary resources have been overexploited, and for sustainable development of resources, the development and utilization of tailings have become urgent. A survey of the copper content in tailings of the Musina mine, an abandoned copper mine in the northern Limpopo Province, showed that the residual copper currently stands at 8,555 tons [
Metallic zinc is not only an irreplaceable material for batteries but also an essential trace element for the human body. Zinc resources mainly exist in the form of lead-zinc ore; thus, it is necessary to recover zinc from secondary resources. The potentially high mobilization and dispersion of zinc found in mine tailings in central Mexico can potentially harm the surrounding ecosystem [
Nickel was used as an excellent iron material in the early days. To control nickel tailing pollution, legume trees were planted on nickel tailings in the tropical area of Zimbabwe. After 20 and 40 years of restoration, different degrees of fertility islands were formed under the canopy of the legume plants [
The contents of rare earth elements in minerals are not as high as those of copper, zinc, iron, and other metals, but the unique optic, catalytic, electronic, and magnetic properties of rare earth elements make them invaluable in a number of advanced technological fields, especially their thermal stability, good electrical conductivity, and corrosion resistance [
China has the world’s largest rare earth element reserves by the mining value. In terms of global production, China (85%) dominates, followed by Australia (10%), Russia (2%), India (1%), Brazil (1%), Malaysia, and Vietnam [
Biological leaching of precious metals occurs mainly through complexation and decomposition mechanisms of cyanogenic bacteria [
Similar to gold, silver has very little associated content, and biological leaching alone does not yield good leaching results. The silver tailings in Coahuila, Mexico, contain a large amount of silver mining waste, and the leaching efficiency of silver was found to be 40–67% using indigenous microbial leaching [
Uranium is radioactive, has a very long half-life, and is mainly used as nuclear fuel. Uranium resources are mainly distributed in the United States, Canada, and South Africa. Although large amounts of uranium exist in the Earth’s crust, it is difficult to exploit these reserves due to technical limitations. However, uranium metal is an important raw material in nuclear physics, and primary uranium mineral resources have been overexploited. Thus, future exploration will be focused on the secondary recovery of waste resources such as tailings. The recovery of uranium from ores and tailings using bioleaching technologies would exceed the economic value of underground mining [
Lead, chromium, and arsenic are toxic heavy metal pollutants. In the past, due to a lack of regulation and environmental protection consciousness, tailings piled up at random and caused widespread pollution of these metals, triggered a wide range of endemic diseases, thus posing a serious threat to human health. Today, pollution control of tailings is therefore mainly focused on these three metals.
In a study of lead concentrations in a 300-year-old abandoned mine tailings dam in Zacatecas, Mexico, the average level of lead was found to be
Treatments of chromium have mainly been developed to reduce hexavalent chromium to trivalent chromium, or to prevent the oxidation of trivalent chromium. Oxides of hexavalent chromium do not degrade by themselves and will accumulate in organisms for a long time. The industrial wastewater standard defines hexavalent chromium as a first-class pollutant, and many countries even prohibit products of hexavalent chromium electroplating from entering the market. The treatment of chromium pollution in tailings using bioleaching technologies is constantly being updated. Chromium-containing tailings are abundant in the Sukinda Valley, India. In a study on bioleaching of chromium, during the bioleaching process, the total chromium was initially extracted in the form of hexavalent chromium due to phosphate in the medium and was subsequently reduced due to hexavalent chromium adsorption and reduction to trivalent chromium [
An investigation of the metal content in agricultural soil near abandoned metal mines revealed that arsenic has a more profound impact on agricultural soil than the migration behavior of cadmium, lead, and zinc; therefore, the recovery of arsenic from tailings is crucial [
In addition to laboratory-scale studies, treatment of tailings using bioleaching technologies has been performed at the industrial scale [
Whether or not there is biomining or biofouling, the leaching mechanism between microorganisms and minerals is still complex [
Cadmium in tailing soil is mainly bound to organic matter and appears in different mineral phases. The bioavailability sequence of lead and cadmium in soil is
Increasing the size of instruments is also a challenge. The small reaction equipment in laboratories enables reactions to be carried out fully, but in large-scale industrial production, the reaction containers are large, and the pH, oxygen concentration, and microbial distribution of the solution cannot be maintained, which leads to a decrease in bioleaching rates.
Following biological leaching, the filter residue may still contain unleachable heavy metals; therefore, follow-up treatment of the filter residue is also a challenge.
Compared with traditional physical and chemical techniques, biotechnology is characterized by more creative options for metal extraction and processing [
At present, the indigenous microorganisms used in biological leaching experiments provide excellent genetic data. These data can be used to synthesize microorganisms that meet the requirements of industrial-scale biological leaching of heavy metal tailings through gene recombination to improve the leaching efficiency, reduce industrial costs, and make full use of tailing resources [
Additional factors influencing biological leaching of heavy metals from tailings have been found recently, which creates new opportunities to improve metal leaching rates. Most of the factors affect the leaching rate by affecting microbial activity. Therefore, future research can build on this breakthrough to enable microorganisms to have genes such as resistance to high pressure and salinity to obtain a greater leaching rate of heavy metals.
Leaching technologies for rare earth elements have also been developed. Owing to the low content of rare earth elements in tailings, biological leaching alone cannot maximize the exploitation of such secondary resources. Innovative technologies such as high-temperature roasting combined with biological leaching, cyanide leaching combined with biological leaching, and bacterial oxidation combined with biological leaching have thus been developed. In addition, through the development of enzymes for biological leaching reactions, heavy metals can be selectively leached from tailings, which can not only improve the metal leaching rate and economic value but also carry out effective treatment for metal pollution [
The residue from biological leaching of tailings has also been studied further. One study reported that following recovery treatment of an arsenic/nickel/cobalt leaching residue, the leaching rates of copper, cobalt, nickel, zinc, and arsenic reached 96.31%, 97.23%, 98.56%, 98.46%, and 93.84%, respectively, which is conducive to the effective utilization of resources and reduces the waste of mineral resources [
Bioleaching is a sustainable method for metal recovery from tailings and controlling their pollution, which can help save nonrenewable energy consumed in the mining industry and make full use of this secondary resource. In this review, previously published results obtained in the field of bioleaching of tailings were reviewed and presented, including the bioleaching mechanism, the type of influencing factors, and the type of tailings. This review shows that the technologies used for the bioleaching of tailings are mature. In addition to the conventional influencing factors, research on factors such as pressure, microcracks, and salinity has provided new paths for improving the rates of metal leaching. The application of CiteSpace (5.7R2) intuitively visualized that the research hotspots of bioleaching of tailings mainly include biocyanidation, the recovery of manganese, and dissolution kinetics. The discovery of new leaching microorganisms, development of biogenomics, and combination of biocyanidation are new opportunities for the industrial production of heavy metals from bioleaching tailings. The main future development directions identified in this review are the development of the industrial production of heavy metal recovery through biological leaching of mine tailings, while constantly optimizing the process, and to create higher economic and ecological benefits.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.