Innovative and sustainable approach for phytoremediation of mine tailings: a review
- 166 Downloads
A review is designed to innovate a sustainable solution for the treatment of mine tailings using bioremediation and phytoremediation. It emphasizes on achieving the geochemical stability of tailings through the establishment of microbes or plants. It highlights the gaps in achieving the geochemical stability of tailings. Lack of nutrients and low pH discourage the survival of microorganisms and the growth of plants on tailings. Treatment of tailings with agricultural waste (wheat and mustard stalks) would nourish tailings which promote the growth of microorganism and plants. Application of agricultural waste in remediation process is cost-effective. The role of microorganisms such as cyanobacteria, fungi, and algae are well known for mineralogical transformation. Microorganism converts unavailable fraction of nutrient into available form and important initiative to drive tailings towards natural soil. It would support the growth of plants on tailings to achieve successful phytoremediation. Biofuel generation from tailings through high lipid and protein producing plants is important for sustainable development. Phytoremediation will reduce the environmental impact caused by mine tailings. For phytoremediation, use of biofuel producing plants, i.e., Jatropha curcus and Brassica juncea, is recommended.
KeywordsMine tailings Mineral transformation Geochemical stability Phytoremediation
Mine tailings is a waste left after the beneficiation of minerals from ore. Major environmental problems related to tailings are leaching of heavy metals , dispersal of tailing particles with wind , tailing dam failure, and seepage of metals . With increasing demand of minerals, the low-quality ore are being extracted which results in generation of huge quantity of tailings . Implementation of low-cost remediation method seems the probable solution to mitigate the environmental impacts of tailings. Bioremediation and phytoremediation are probable solution for the treatment of tailings. In bioremediation, living organisms are being used for accumulation and removal of heavy metals, and in phytoremediation, plants are being used for the same.
Dispersal of tailings with winds contaminates surrounding soil  and water resources . Most of studies focus on the implementation of remediation technologies at the contamination sites (heavy-metal contaminated soil) rather than at the source of contamination (heavy-metal enriched tailings). Organic or inorganic amendments, microorganism, or plants are frequently being used for the implementation of sustainable and effective remediation technologies [7, 8, 9]. Biological amendments are used as conditioner or to improve nutrient values of soil .
Generation of tailings from mines is increasing continuously which overloads the available resources for their safe disposal. Physical and chemical methods for remediation of tailings are costly and require large quantity of chemicals . It also increases the overall production cost of metal or mineral. The cost-effective way is phytoremediation of tailings. Thus, present review discusses the possible sustainable bioremediation and phytoremediation methods to achieve geochemical stability. Objective(s) of review are to (1) understand the possible methods to achieve sustainable bioremediation and phytoremediation of mine tailings, and (2) to highlight the potential gaps for future studies.
Tailings are combination of crushed rocks and waste-containing processing fluids from concentrators or mills which remain after the extraction of minerals, economic metals, or mineral fuels . The resources are depleting at faster rate, but demand is increasing continuously. It increases pressure on mining industries to extract low-grade ore which further result in more tailings . The ratio of tailings to concentrate is 200:1 which is very high .
Methodology to achieve sustainable bioremediation of tailings
Amendments for facilitating bioremediation
Oxidation of tailings leads to leaching of heavy metals. Adding lime to tailings increases its pH and decreases the leaching of metals. It is temporary solution as addition of new tailings to dam will generate more acidity. It requires huge amount of chemicals which will add monetary value to production cost. The probable solution could be the use of waste rocks from limestone and carbon mines. Waste rocks from these mines have acid neutralization capacity. Additionally, chemicals such as oxalate and citrate are most potent biological weathering agents in the soil [21, 22]. Hence, application of these acids might increase the biological activity in tailings. Tailings are rich in silicate minerals such as feldspars, micas, hornblend, and pyroxene. It is observed that silicate minerals as a source of Ca, Mg, and K and apatite main primary mineral source of P .
Agricultural waste contains functional groups such as carboxyl and hydroxyl groups which are suitable sites for the adsorption of metals . Agricultural waste mainly refers to crop stalks and animal manure . Additionally, application of organic materials like woodchips, pulp waste, and beef manure on tailings give positive results for the treatment of acid mine drainage [18, 26]. Along with low cost and renewable nature, they increase microbial quantity and activity . It is found that application of class A biosolids on the mine tailings promotes bacterial growth . Keeping the agricultural waste on tailings for long duration with continuous application of water leads to its degradation or decomposition. Decomposed waste is more effective compared to raw agricultural waste for microbial growth. Composite alters microbial population by changing pH and solubility of metals which increases allochthonous microbial biomass and nutrients [29, 30].
Decomposition of organic matter (wheat and mustard stalks) would increase the available content of nutrients on substrate (tailings). Increase in available fraction of nutrients on tailings would promote and support the better implementation of plants. Mustard stalk acts as a substrate for saccharification . Wheat stalk is converted to organic acid mixtures  by microbial activities and also an energy source for biological nitrogen fixation . Microbial establishment generates organic acids  and converts unavailable nutrient of tailings into available fraction. Additionally, both wheat and mustard stalks are also used for the removal of heavy metals from solution as they adsorb excessive elements [35, 36]. Application of amendments also increases moisture content which possibly enhances the microbial activity and weathering.
Selection of microorganism for bioremediation
Selection of microorganism is very important criteria for the implementation of bioremediation technology. Single and multiple organelle organisms are successfully grown on heavily contaminated soils. The isolated microbial strains from highly contaminated sites are found highly resistant to heavy-metal toxicity . Isolated bacterial and fungal strains show high resistance and removal capacity for heavy metal.
Bacteria possess adaptive capacity to survive in high abundance of metal concentration environment despite the toxic effects of heavy metals. Bacteria also have the capacity to fix atmospheric nitrogen, solubilize inorganic phosphate, as well as plant growth regulators like siderophores, 1-aminocyclopropane-1-carboxylic acid, indole acetic acid (IAA) gibberellic acid (GA), and cytokinin, which allow greater metal accumulation by host plants [38, 39]. Application of bacterial mixture enhances the efficiency of bioremediation by increasing urease activity, calcite production, and resistance to high concentration of heavy metals .
Existence of growth-promoting bacteria such as Arthrobacter sp., Microbacterium sp., and Pseudomonas chlororaphis is observed on tailings  which indicates the possible use of bacterial strains in bioremediation. Additionally, iron- and sulfur-oxidizing bacterial genera such as Ferroplasma, Leptospirillum, and Acidithiobacillus are also observed on tailings . Researchers isolate metal (Cu, Cr, Pb, Cd, Sb, and Ni) resistant bacterial strains (Bacillus beringensis, Bacillus sp., Bacillus megaterium, Pseudomonas putida, Acidothiobacillus sp., and Kocuria sp.) from acidic Cu mine tailings of north western China and suggested their possibility for being used for bioremediation . Inoculation of bacterial strains (Pseudomonas aeruginosa, Alcaligenes feacalis, and Bacillus subtilis) increases the biomass production and accumulation of heavy metals in the roots of Brassica juncea .
Bacteria (B. subtilis, P. putida, Kocuria flava, Acidithiobacillus ferrooxidans, and Sphingomonas sp.) isolated and cultured from copper mine tailings are found potential for the treatment . Cyanobacteria have the capability to grow in low nutrient environment and shows growth on barren copper mine tailings [19, 44]. Cyanobacteria and microalgae are sustainable source for bioenergy  and future research is needed.
Nowadays, fungi is catching widespread attention in the field of bioremediation because of its high efficiency for heavy-metal sequestration [46, 47]. Aspergillus sp, Penicillium sp, Botrytis sp, Trichoderma sp, Saprolegnia sp, and Neurospora sp are successfully tested for the removal of heavy metals from soil [48, 49]. Fungi have high percentage of cell wall material which results in increase in number of functional groups present on the cell surface for metal binding. It observed involvement of amide, carboxylic acid, and hydroxyl and isocyanate groups of cell surface in the adsorption of lead . Isolated fungal genera (Aspergillus, Fusarium, and Hypocrea) from acidic Cu mine tailings are found potential and effective in bioremediation .
Tailings are solid and dry surface which does not support the growth of algae. Existence of algae is observed in acid drainage from tailings which is also known as acid mine drainage (AMD). AMD is rich in heavy-metal enriched sulfides. Algae (Spirulina sp., Chlorella, Scenedesmus, Cladophora, Oscillatoria, Anabaena, Phaeodactylum, tricornutum) from AMD show high accumulation capacity for heavy metals . In addition, they increase pH of tailings which is essential for precipitation of heavy metals during treatment process . Hence, algae can be successfully used for the bioremediation of AMD generated from tailings. Removal of heavy metals by algae depends on the type of metal, taxon  and season . It is observed that promising features regarding the growth and distribution of Lepocinclis sp. and Klebsormidium sp. for AMD treatment .
Mechanism of microbial remediation
Capacity of microorganisms to degrade pollutants depends on environmental conditions, i.e., temperature, pH, and moisture [56, 57]. pH affects enzymatic activities of microorganisms following two processes: first, by changing the rate of microbial metabolism  and second, by changing the surface charge of the microorganism . Both processes affect the adsorption capacity of microorganisms for heavy metals. Additionally, optimum temperature for different microorganisms is different and mainly varies from 25 to 35 °C .
Biofuel producing plants
Natural oil is one of the main polluters of environment. Production and use of economical viable biofuel are needed for the sustainable development. Major concern nowadays is identification of economical and environmentally friend producer of biofuel. Cyanobacteria , algae , and plants  are found effective producers of biofuel. Biofuel from the mine waste reduces burden of tailings on environment and it will also add economic benefits from the waste. It is well known that the microbial growth depends on the availability of nutrient source. In tailings, the absence of nutrients is major drawback. Plant growth-promoting (PGP) bacteria are suggested as a possible solution for the establishment of plants on tailings [73, 74]. Microbial growth before implanting the biofuel producing plants would transform nutrients in available form on tailings which supports the growth of plants.
The presence of inorganic nutrients in tailing depends on geology or rock type  and climatic conditions . Studies have reported that major limiting factor for the growth of microorganism and plant on mine waste is the absence of N2 . Cyanobacteria are natural N2 fixer. Cyanobacteria are photosynthetic prokaryotes with high biomass and can grow on limited nutrients. Bioremediation and biofuel production capacity of microorganism is positively correlated with their biomass quantity. Cyanobacteria also have high heavy-metal sequestration capacity  and is considered as a metabolic power house for biofuel production . The cultivation of cyanobacteria on tailings will be a sustainable solution for bioremediation and biofuel production. Additionally, optimize production of renewable energy, i.e., biodiesel from microalgae and bioethanol from cyanobacteria is challenging . Research is needed as microalgae and cyanobacteria require light for energy and inorganic carbon (CO2 or bicarbonate) as carbon source. This makes it as a strong candidate for the bioremediation of tailings as tailings are nutrient deprive.
Similarly, growth and feasibility of biofuel producing plants on tailings are important to study for sustainable development. Studies have reported the phytoremediation of Cu mine tailings [20, 80, 81] using different types of plants. Ricinus communis L. is observed grown naturally on mine tailings and also found a potential source for phytoremediation and produces good quantity of oil . Similarly, good content of linoleic acid in oil produced from R. communis grown on mine tailings which enriches its fuel properties . Phytoextracting plants such as Sesamum indicum and Cyamopsis tetragonoloba are used for bioremediation of Cu-contaminated soil . These plants could be used for the bioremediation of Cu tailings. The successful establishment of economic profitable plant on the tailings would be economical viable for bioremediation. The implantation of local plants should be promoted and needs more research.
Microorganism plays an important role in soil genesis and transforms minerals into bioavailable form [84, 85]. Microorganism changes surface chemistry of minerals . Protein content is reported high in bacteria after their interaction with quartz . Type and quantity of polysaccharides and protein secreted by bacteria depend on mineral substrate such as oxides of Al, Fe, and calcium. Microorganism controls metal speciation and mobility  which affects the rate of metal leaching from tailings. Thus, growth of microorganism on tailings shifts tailings towards geochemical stability with least content of metals.
Bioweathering is a process of rocks/mineral dissolution by microorganism/plants through physical/chemical processes . Bioweathering is a significant process in soil genesis and mineral transformation [89, 90]. Significant growth of microorganism on tailings would trigger soil formation which will be geochemical stable state. Microorganism is capable of using inorganic nutrients and seems only possible or feasible solution to mitigate the toxicity of tailings. Capability of mycorrhizal fungi to remove organic and inorganic nutrients from soil minerals  suggests their suitability to be grown on tailings. Changes in geochemistry and mineral composition will drive the tailings towards the geochemical stability. Geochemical stability leads to decrease in the toxicity of tailing as it stops the leaching of toxic elements. Geochemical stability of tailings will further promote the significant growth of plants and microorganism.
Conclusions and recommendations
In current scenario, disposal of tailings is huge environmental problem due to lack of land. Conventional and chemical methods are costly and needs trained manpower. Bioremediation and phytoremediation of tailings seem as a sustainable solution of problem. Future research should emphasize on achieving the geochemical stable state of tailings. Geochemical stable state triggers soil genesis from tailings which promotes plant growth and stops elemental leaching. The major problem in achieving the geochemical state is lack of nutrients and low pH of tailings which discourages the growth of plants and survival of microorganisms.
Treatment of tailings with agricultural waste (wheat and mustard stalks) would possibly nourish tailings and promote the growth of microorganism. Bioremediation using agricultural waste is cost. Establishment of microorganism on tailing promotes and supports the plants. Role of microorganisms such as cyanobacteria, fungi, and algae are well known for initiating the mineralogical transformation. Mineral transformation and geochemical stability lead towards soil genesis. Microorganisms converts unavailable fraction of nutrient into available form. It would support growth of plants on tailings to achieve successful phytoremediation. Future research should focus for identification of microbial strains for sustainable bioremediation of tailings. Biofuel generation from tailings by planting the high lipid and protein producing plants is feasible. Bioremediation reduces the environmental impact caused due to tailings. Phytoremediation of tailings for biofuel production using Jatropha curcus and B. juncea should be studied.
Future scope for research
To minimize environmental impacts, innovation of sustainable bioremediation method for tailings is important. Interdisciplinary studies covering the geochemistry and microbiology are needed to fill the gaps in implementation of sustainable bioremediation technologies. As per my knowledge, no study has been carried out to assess the geochemical and mineralogical transformation of tailings toward soil using microorganisms. Research gaps are there, microbiologist recommends organism those can be grown on tailings, and geochemist discusses only the elemental mobility. More research should be done focusing the geomicrobiology of tailings to find out the sustainable solution for the remediation of tailings. Implementation of bioremediation techniques would be more effective at the source of contamination than at contamination sites.
Studies should focus on the identification of suitable cyanobacterial strains from tailings for bioremediation. Efficiency of bioremediation method depends on the selection of microbial strain. Role of microorganism in geochemical and mineralogical transformation of tailings is important. In most of studies, it is neglected. Growth of microorganism on tailings transforms minerals and changes elemental mobility in tailings. Studies should be carried out to assess the mobility of nutrients and elements in mine tailings before and after the bioremediation. More focus should be to evaluate biofuel production potential combined with phytoremediation of mine tailings.
No funding was received to carry out the research work.
- 1.Wang P, Sun Z, Hu Y, et al. Leaching of heavy metals from abandoned mine tailings brought by precipitation and the associated environmental impact. Sci Total Environ. 2019;695:33893.Google Scholar
- 13.Lottermoser B. Mine wastes: characterization, treatment and environmental impacts. Berlin: Springer; 2007.Google Scholar
- 16.Kazakou E, Dimitrakopoulos P, Baker A, et al. Hypotheses, mechanisms and trade-offs of tolerance and adaptation to serpentine soils: from species to ecosystem level. Biol Rev. 2008;83:495–508.Google Scholar
- 19.Liu M, Zhao X, Zhan J, et al. Cyanobacterial diversity in biological soil crusts on wastelands of copper mine tailings. Shengtai Xuebao Acta Ecol Sin. 2011;31:6886–95.Google Scholar
- 24.Huang YY. Research progress of wastewater treatment by agricultural wastes as biological adsorbent. Appl Chem Ind. 2017;46(2):372–638.Google Scholar
- 25.Liu K. The practical significance and countermeasures of agricultural waste utilization. Environ Dev. 2017;5:254–6.Google Scholar
- 33.Roper MM. Wheat straw: an energy source for biological nitrogen fixation. In: Veeger C, Newton WE, editors. Advances in nitrogen fixation research. Advances in agricultural biotechnology. Dordrecht: Springer; 1984. p. 4.Google Scholar
- 51.Bwapwa JK, Jaiyeola AT, Chetty R. Bioremediation of acid mine drainage using algae strains: a review. S Afr J Chem Eng. 2017;24:62–70.Google Scholar
- 61.Jain S, Arnepalli D. Biominerlisation as a remediation technique: a critical review. In: Proceedings of the Indian Geotechnical Conference (IGC2016), Chennai, India, pp 15–17 December 2016.Google Scholar
- 62.Danis U, Nuhoglu A, Demirbas A. Ferrous ion-oxidizing in Thiobacillus ferrooxidans batch cultures: influence of pH, temperature and initial concentration of Fe2+. Fresenius Environ Bull. 2008;17:371–7.Google Scholar
- 75.Lottermoser BG. Mine wastes. Characterization, treatment and environmental impacts, 3rd edn, Springer, Berlin, Heidelberg; 2010.Google Scholar