Heavy metal pollution is of great concern. Due to expansion of industrial activities, a large amount of metal is released into the environment, disturbing its fragile balance. Conventional methods of remediation of heavy metal-polluted soil and water are expensive and inefficient. Therefore, new techniques are needed to provide environmentally friendly and highly selective remediation. Streptomycetes, with their unique growth characteristics, ability to form spores and mycelia, and relatively rapid colonization of substrates, act as suitable agents for bioremediation of metals and organic compounds in polluted soil and water. A variety of mechanisms could be involved in reduction of metals in the environment, e.g., sorption to exopolymers, precipitation, biosorption and bioaccumulation. Studies performed on biosorption and bioaccumulation potential of streptomycetes could be used as a basis for further development in this field. Streptomycetes are of interest because of their ability to survive in environments contaminated by metals through the production of a wide range of metal ion chelators, such as siderophores, which provide protection from the negative effects of heavy metals or specific uptake for specialized metabolic processes. Many strains also have the equally important characteristic of resistance to high concentrations of heavy metals.Separations 2018, 5, 54 2 of 14 materials, pesticides, phosphate fertilizers, printing pigments, textiles, sewage, irrigation, smelting, steel and electroplating industries, dyes, and wood preservation. While anthropogenic sources increasingly give rise to permanent pollution, natural sources are usually a seasonal phenomenon influenced by weather, which generally does not generate pollution [4].Many heavy metals are capable of entering the food chain, where they cause serious damage. The toxicity of each metal and their potential harmful effects depends on the amount, route of admission, and duration of exposure [5]. A major source of heavy metals in the soil is mining activities and subsequent ore processing. During these processes, metals are mobilized by biological or chemical leaching, and pass to the soil and nearby water sources. As a result of leaching, heavy metals migrate to the lower soil layers and cause destruction and alteration of the ecosystem, including accumulation of pollutants and a loss of biodiversity. The subsequent recovery could take several decades [6]. Techniques for Heavy Metals RemovalRemoval of pollutants in the form of heavy metals has become an urgent requirement in recent decades. Technologies routinely used for heavy metal removal from the environment include chemical precipitation, reverse osmosis, ion-exchange, ultrafiltration, and electro-dialysis. They are, unfortunately, often inefficient and extremely expensive. Moreover, they can generate toxic compounds, which are seen as unfavorable and uneconomical [7]. There is, therefore, an acute need for the development of cheap, highly efficient and selective alternatives that can allevia...

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Three different biological systems, the consortium of autotrophic bacteria Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans, heterotrophic fungus Aspergillus niger and heterotrophic yeast Rhodotorula mucilaginosa, were investigated for lithium extraction from lepidolite. The bacterial consortium was the most effective, 11 mg l −1 of Li was dissolved in the absence of nutrients within 336 days. Fungal and yeast bioleaching was faster (40 days), however, with lower extraction efficiency. Bioaccumulation represented a main process of Li extraction by R. mucilaginosa and A. niger, with 92 and 77% of total extracted Li accumulated in the biomass, respectively. The X-ray diffraction analysis for bioleaching residue indicated changes caused by microorganisms, however, with differences between bacterial leaching and bioleaching by fungi or yeasts. The final bioleaching yields for bacterial consortium, A. niger and R. mucilaginosa were 8.8%, 0.2% and 1.1%, respectively. two-step bioleaching using heterotrophic organisms followed by autotrophic bioleaching could lead to the increase of the process kinetics and efficiency. Bioaccumulation of Li offers strong advantage in Li extraction from solution.

The role of iron in metal-bearing waste bioleaching was studied. Four various types of waste (printed circuit boards (PCBs), Ni-Cd batteries, alkaline batteries and Li-ion batteries) were treated by bioleaching using the acidophilic bacteria A. ferrooxidans and A. thiooxidans (separately or in mixture). Role of main leaching agents (Fe 3+ ions or sulphuric acid) was simulated in abiotic experiments. Results showed that oxidation abilities of Fe 3+ ions were crucial for recovery of Cu and Zn from PCBs, with the efficiencies of 88% and 100%, respectively. To recover 68% of Ni from PCBs, and 55% and 100% of Ni and Cd, respectively, from Ni-Cd batteries both oxidation action and hydrolysis of Fe 3+ were required. The importance of Fe 2+ ions as a reducing agent was showed in bioleaching of Co from Li-ion batteries and Mn from alkaline batteries. The efficiency of the processes has increased by 70% and 40% in Co and Mn bioleaching, respectively, in the presence of Fe 2+ ions. Based on the results we suggest the integrated biometallurgical model of metal-bearing waste recycling in the effort to develop zero-waste and less energy-dependent technologies.