9.1 Introduction

Biohydrometallurgy is one of many different processes for metal recovery. As a highly interdisciplinary field, biohydrometallurgy combines microorganisms and their metabolites (−bio) in a mainly aquatic environment (−hydro) and the treatment of metal-containing materials or solutions (−metallurgy) for metal production and treatment. It is applied to many different metal-rich materials from primary mineral sources, secondary mining products, and numerous manufactured resources (Watling 2015). Biohydrometallurgy uses biological tools for the processing of primary ores for many years – especially in case of bioleaching. Besides that, special biological tools can enhance the metal recovery from manufactured resources such as technical waste products, processing wastes, industrial waste waters, and other secondary sources (Pollmann et al. 2018). In nature, multiple processes exist that influence biogeochemical cycles of elements. These microorganism-driven processes contribute to bioaccumulation, bioweathering, biomineralization, and precipitation or microbial reduction. Using these bio-inspired processes promotes biological recycling strategies as well as several clean industrial processes, bio-based materials, and bioremediation. Modern bio-based approaches that are currently being developed for the recycling of valuable elements found in technical products contribute to a “green” circular economy. Main processes in biohydrometallurgy are bioleaching, biosorption, bioflotation, and bioreduction.

9.2 Bioleaching: Metal Winning with Microbes

Bioleaching is defined as the extraction of metals by the metabolic activity of bacteria (direct bioleaching) or metabolic compounds (indirect bioleaching). It is applicable to metal extraction from low-grade ores, beneficiation of ores or coal, removal of toxic metals, and recovery of metals from waste materials.

Many bioleaching studies concentrate on the metal extraction from ores, but the processes can be applied principally also to other sources such as industrial residues and waste materials that can be considered as an artificial ore. During the last years, numerous studies were published that describe the application of bioleaching approaches for the recovery of metals from various technical and industrial waste products, e.g. fly ash from municipal waste incineration, electronic scrap such as printed circuit boards from computers and mobile phones, spent catalysts and batteries, and others. Bioleaching approaches on industrial residues and waste materials are based on the use of chemolithoautotrophic bacteria, heterotrophic bacteria, yeasts and fungi, and cyanogenic bacteria. Most studies with chemolithoautotrophic bacteria used the acidophilic organisms Acidithiobacillus ferrooxidans or A. thiooxidans (e.g. Brombacher et al. 1998; Gholami et al. 2011; Karwowska et al. 2014; Mishra et al. 2008). These approaches obtained relatively high leaching efficiencies (in many cases of >80% of metals) but required the addition of sulfur and acidification of the cultivation media to maintain bacterial growth and solubilization process. Studies describing approaches with heterotrophic microorganisms used bacteria, fungi, or yeasts that produce diverse organic acids such as citric acid, gluconic acid, or acetic acid as lixiviant for the respective metals. Especially the fungus Aspergillus niger was used for a broad range of materials (Bosshard et al. 1996; Brandl et al. 2001; Qu et al. 2015). Depending on growth conditions, this organism produces huge amounts of diverse organic acids (e.g. citric acid, gluconic acid, oxalic acid). The organism is applied commercially for organic acid production and cultivation as well as its metabolism has been studied in detail. In some cases, more than 90% of metals could be mobilized by application of this organism (Brandl et al. 2001). Biogenic cyanide produced by Chromobacter violaceum was used for the mobilization of gold and other noble metals, copper, and nickel from shredded printed circuit boards and automotive catalysts by using direct or indirect leaching approaches (Campbell et al. 2001; Chi et al. 2011; Faramarzi et al. 2004; Shin et al. 2015b). In indirect bioleaching processes, more than 90% of metals were mobilized. These results were comparable with the usage of commercial sodium cyanide (NaCN) demonstrating the principal suitability of such approaches for commercial applications (Shin et al. 2015b).

Most studies aimed at the recovery of valuable metals such as Cu, Ni, Au, or other noble metals. These elements can be found in high concentrations in diverse electronic wastes or in residues from mineral processing, e.g. smelter dust, fly ash, or incineration slag (Auerbach et al. 2019; Bosshard et al. 1996; Brandl et al. 2001; Brandl et al. 2008; Brombacher et al. 1998; Klink et al. 2016; Oliazadeh et al. 2006; Ramanathan and Ting 2016). However, starting with the resource crises in 2009, an increasing number of studies have been published that concentrate on other valuable elements. Several research groups investigated the mobilization of rare-earth elements (REE) that are an essential component in most modern technologies, from different solid materials. Most of these studies used a variety of heterotrophic bacteria for the extraction of REE from REE-bearing minerals such as monazite (Hassanien et al. 2014; Shin et al. 2015a). However, some recent studies investigated bioleaching approaches for different REE-bearing secondary resources such as red mud from alumina production (Cizkova et al. 2019; Qu and Lian 2013a; Qu et al. 2013b) or electronic waste material, e.g. from waste phosphors (Hopfe et al. 2017, 2018; Reed et al. 2016). In most cases, REE are mobilized either by different organic acids or by enzymatic activity of phosphate solubilizing microorganisms.

As consequence of increasing electromobility, lithium and cobalt as components of energy storage devices moved into the center of attention. Some studies investigated the extraction for Li and Co from waste lithium ion batteries either by a combination of chemical treatment with citric acid and bioactivity (Dolker and Pant 2019) or by A. niger (Horeh et al. 2016) or A. ferrooxidans (Mishra et al. 2008).

Maneesuwannarat et al. (2016) used a strain of Cellolosemicrobium funkei, which was isolated from cadmium- and arsenic-contaminated soil for bioleaching of GaAs (Maneesuwannarat et al. 2016). It was supposed that proteins are involved in Ga mobilization, indicating a new mechanism for metal dissolution (Maneesuwannarat et al. 2019).

These studies demonstrate the great potential that microorganisms offer for the transformation of materials that can be used for new recycling routes. It can be expected that the ongoing growing demand of other elements, and development and growth of new technologies such as renewable energies will promote further studies.

Although all these studies gave a proof of principle for the application of microorganisms for metal mobilization from various waste materials, only few of these approaches have been implemented in industrial processes just yet. Major challenges are selectivity, efficiency, and economy of bioprocesses. More recent studies developed stepwise approaches by combining different chemical or biological leaching steps. For example, Pourhossein and Mousavi obtained high leaching rates of >80% for different elements (Cu, Ni, Ga) from waste light-emitting diodes (WLEDs) by applying a stepwise indirect bioleaching approach using a biogenic ferric agent (Pourhossein and Mousavi 2019). Rizki et al. (2019) combined chemical leaching using thiourea with bioleaching by a thiourea-tolerant Fe-oxidizing microorganism to extract gold from electronic waste (Rizki et al. 2019). Huang et al. (2019) reported on a bio-electro-hydrometallurgical process that combines bioleaching by using sulfur-oxidizing bacteria with an electrokinetic recovery process (Huang et al. 2019). It can be expected that such approaches overcome current barriers in using biological approaches.

9.3 Biosorption: Selective Metal Recovery from Waste Waters

Biosorption is defined as the property of biomass or certain biomolecules to bind and concentrate selected ions or other molecules from aqueous solutions (Volesky 2007). It is a passive process and independent from metabolic activities. Therefore, nutrients are not required, and processes can be performed in environments with high toxicity. Biosorption has been mainly applied for the removal of toxic metals from polluted waters, such as arsenic, chromate, cadmium, or uranium (Volesky and Holan 1995).

Another attractive application is the recovery of valuable metals such as gold, platinum, palladium, or others from solutions (Das 2010; Pollmann et al. 2006b). Conventional pyrometallurgical or hydrometallurgical methods (e.g. adsorption by ion-exchange resin, activated carbon, or minerals, solvent extraction, chemical precipitation) require high amounts of energy and addition of chemical agents, thus generating secondary wastes, or are inefficient especially for highly diluted solutions (Das 2010). Biosorption is an environment-friendly alternative to these methods because it uses biodegradable compounds that can be easily produced in high amounts. Further, biomass is considered as carbon-neutral and petrochemical-independent process as it does not emit extra carbon dioxide when burned (Maruyama et al. 2007; Ritter 2004). Various types of biomass have been reported to bind and concentrate metal ions from industrial effluents and aqueous solutions.

Metal-containing solutions such as industrial waste waters, leachates, and mining waters are often acidic with pH < 3, have a complex composition containing different competing elements, and contain toxic chemicals or organic compounds that influence biosorptive properties. Therefore, major challenges of biosorptive approaches are the stability of materials, selectivity, effectivity, and cost efficiency. Several approaches address these challenges. Most studies concentrated on the use of bacterial cells, fungi, yeast (Volesky and Holan 1995), algae (reviewed by He and Chen (2014)), seaweed biomass (Figueira et al. 2000), or biocomponents such as crab shells (Daubert and Brennan 2007), plant fibers (Salamun et al. 2015), etc. as biosorptive components that can be easily produced or are waste materials (e.g. in case of crab shells). Especially biopolymers such as cellulose, chitin, or chitosan materials are chemically resistant. However, these materials possess no selectivity and bind a broad range of different elements. This is a drawback for their application in metal recovery processes because these applications are intended only the concentration of metals from highly diluted solutions but also the selective recovery of metals of interest.

A different approach was propagated by Bonificio and Clarke (2016). These authors described a selective recovery of REE by biosorption on immobilized bacterial biomass followed by a selective desorption as a function of pH. This approach enabled the separation of the three heaviest lanthanides Tm, Lu, and Yb from a mixture of different lanthanides.

Other recent developments concentrate on the use of defined proteins from biomass or the direct engineering of improved microbes and enzymes. Maruyama et al. (2007) tested different model peptides, proteins, and protein-rich biomass regarding their capability to selectively bind different precious metals from model solutions, metal-refining solutions, and industrial wastes at acidic conditions (Maruyama et al. 2007). All tested biomasses as well as proteins selectively adsorb Pd and Au ions in the presence of transition elements. Further, it was possible to remove Au, Pd, and Pt from plating wastes using protein-rich chicken egg-shell membrane.

Other approaches use the metal-binding motifs of natural proteins, e.g. metallothioneins, as biosorptive component. Metallothioneins (MT) are cysteine-rich proteins that bind different metals such as Cd, Hg, Cu, and Pb. MTs from different natural sources have been expressed in Escherichia coli and Pseudomonas putida and used as biosorbent, mainly for removal of heavy metals (reviewed by Chen et al. (1999)) and Mejare and Bulow (2001). However, these proteins are also attractive for the recovery of valuable metals. Terashima et al. (2002) produced a fusion protein composed of the maltose-binding protein and human MT and immobilized it on Chitopearl resins (Terashima et al. 2002). These materials were used for binding of Cd and Ga in a concentration range of 0.2–1.0 mM. Further, the biosorbents could be used several times without loss of binding activity.

CadR, which is a Cd-binding protein first isolated from rhizobacterium P. putida, has been expressed on the surface of E. coli cells. These engineered cells show a high Cd²⁺ adsorption capacity of 19.5 μmol Cd(II) g⁻¹ cells (Liu et al. 2015).

Phytochelatins (PCs) are naturally occurring metal-binding peptides, which contain multiple repeats of the γGlu-Cys moiety terminated by a Gly residue. Various researchers have expressed different synthetic PCs onto the surface of bacterial cells to improve metal uptake and biosorption. For example, recently, Tan et al. (2019) displayed the synthetic phytochelatin EC20 onto the surface of E. coli. The obtained constructs showed an increased biosorption of Pt(IV) accompanied by the formation of platinum nanoparticles (Tan et al. 2019).

Proteinaceous bacterial surface layers that envelope many bacterial cells are other interesting biomolecules that have been used for the binding of different elements such as U, Pd, Au, or Cu (Allievi et al. 2011; Merroun et al. 2005; Pollmann et al. 2006b). The binding of U, Pd, and Au has been investigated in more detail in case of the S-layers from Lysinibacillus sphaericus JG-A12 and NCTC 9602 (Fahmy et al. 2006; Jankowski et al. 2010; Merroun et al. 2005). These elements were coordinated by phosphate and carboxyl groups (Fahmy et al. 2006; Merroun et al. 2005); in case of Au(III), it was assumed that amine groups were involved in complexation (Jankowski et al. 2010). Due to their self-assembling properties that enable the formation of nanostructured protein arrays on various technical surfaces (Sleytr et al. 2014; Toca-Herrera et al. 2005; Weinert et al. 2015), S-layer proteins are attractive biomolecules for the construction of biosorptive composites (Suhr et al. 2014). For example, so-called biocers were produced by entrapping S-layer carrying cells or S-layers in porous ceramics using sol–gel technology and used for the removal of U from contaminated waters (Soltmann et al. 2002). Pollmann et al. (Pollmann and Matys 2007) constructed modified His-tagged S-layer proteins that exhibited enhanced Ni-binding capacities while self-assembling to a nanoporous protein meshwork (Pollmann and Matys 2007).

Peptides are other less complex and easily synthesizable biomolecules that have been used for the design of various biosorbents. Stair et al. (Stair and Holcombe 2005) synthesized and immobilized various peptides of different lengths composed of Gly, Asp, and Cys residues on commercial Tentagel resins and used it as biosorbents for the binding of Ni²⁺, Cd²⁺, Co²⁺, and Mg²⁺ (Stair and Holcombe 2005).

All the previously discussed biomolecules are able to interact with a specific number of ions. A selectivity to the target ion is not given in the above shown approaches.

9.4 Bioflotation: Separation of Particles with Biological Means

Microbial cells, cell components, metabolites, or other biomolecules can interact with solid substrates and modify surface properties, e.g. by introducing hydrophobic properties by adhesion to the surfaces (Das et al. 1999; Patra and Natarajan 2006). These properties can be used for mineral beneficiation. For example such biocompounds have been reported as environment-friendly collectors or depressants and were applied as flotation reagents in selective mineral separation (reviewed by Behera and Mulaba-Bafubiandi (2017)). Most of these approaches concentrate on the use of bacterial cells or their products that have been described to specifically interact with minerals such as Acidithiobacillus or Leptospirillum ferrooxidans or Rhodococcus opacus or on model organisms (Behera and Mulaba-Bafubiandi 2017). However, newer investigations demonstrate the applicability of a much broader range of microorganisms beyond the classical bioleaching bacteria or model organisms. Luque Consuegra et al. investigated the influence of different marine bacteria on bioflotation of pyrite and chalcopyrite and identified strains of Halobacillus sp. and Marinococcus sp. to depress pyrite in artificial sea water conditions while improving the flotation of chalcopyrite (Consuegra et al. Consuegra et al. 2019). These studies prove the high potential of the application of various bacteria in diverse environments for particle separation, thus opening up not only new perspectives in mineral separation technologies but also in recycling technologies and other industrial applications.

Biopolymers and so-called extracellular polymeric substances (EPS) mediate the attachment of bacterial cells to surfaces and biofilm formation (Gehrke et al. 1998; Kinzler et al. 2003; Vu et al. 2009). They form the structure and architecture of the biofilm matrix. The EPS are composed of an undefined complex mixture of biopolymers primarily consisting not only of polysaccharides, but also lipids, proteins, humic acids, and nucleic acids (reviewed by Vu et al. (2009)). Their composition depends on type of microorganisms, age of biofilm, and environmental conditions, including surface properties (Donlan 2002). Their affinity to surfaces makes them interesting for their application in flotation processes. Consequently, several studies investigated the effect of EPS as bioreagent in mineral separation (Figure 9.1).

Besides EPS, other biomolecules have been investigated regarding their application in mineral separation. Especially biosurfactants are interesting compounds that have been applied as frothers in many flotation experiments. Biosurfactants are surface-active organic molecules that are produced by many microorganisms. They have been attributed to lowering the surface tension at the interfaces of solid, liquid, and gases. In contrast to common commercial chemical surfactants, they are less toxic, biodegradable, and effective under extreme conditions. Consequently, there are numerous potential fields of application, including pharmaceutical industry, environmental remediation, and petroleum industry (reviewed by Saha and Rao (2017)). Amphiphilic siderophores are another class of surface-active compounds produced by microorganisms. The amphiphilic siderophore marinobactin is composed of a hydrophilic chelating head group and a hydrophobic fatty acid tail of different lengths and interacts with iron minerals. These properties make the molecules attractive as flotation agents for mineral separation in froth flotation as propagated by Schrader et al. (2017) (Schrader et al. 2017).

Hacha et al. (2018) combined innovative electroflotation processes that reduces bubble sizes, with R. opacus cells as bioreagent (Hacha et al. 2018). By this, it was possible to separate fine hematite particles from a mixture. The combination of different methodologies from different fields enables innovations in classical flotation procedures, thus overcoming current limitations.

All these approaches include living cells or natural biocompounds that were isolated from living cells. Principally, these developments could be transferred to the separation of fine particles that are released during recycling processes and cannot be targeted by existing processes. Lederer and coauthors described the selection of phages displaying LaPO₄:Ce³⁺, Tb³⁺ (LAP), and CeMgAl₁₁O₁₉:Tb³⁺ (CAT)-specific peptides on their surface (Lederer et al. 2017; Braun et al. 2018). Moreover, the researchers introduced modifications and reached a > 5000 fold higher binding strength to LAP in comparison to the wild type. The directed modification of individual amino acids was proven to increase or decrease the binding specificity and affinity of a peptide to the target material drastically. These phages bind to several components of compact fluorescent lamps (LAP, CAT), but not or only weak to Y₂O₃:Eu³⁺ (YOX), LaPO₄, SiO₂, and BaMgAl₁₀O₁₇:Eu²⁺ (BAM). This proof of principle shows that the researchers are able to identify perfectly fitting biomolecules for target particles by using the improved phage surface display techniques. The authors proposed an application as collectors in bioflotation processes for the separation and recycling of fluorescence phosphor components from electronic scrap (Lederer et al. 2019).

Other approaches anchored ZnO-, Au-, or TiO₂-binding peptides or organic molecules to magnetic particles and separated the respective particles from colloidal mixtures (Essinger-Hileman et al. 2013; Shen et al. 2017; Vreuls et al. 2011). Given the high number of peptides that have been described to selectively interact with various inorganic surfaces and that were mostly applied for material syntheses or sensory applications (reviewed by Care et al. (2015); Seker and Demir 2011)), one can assume that these separation technologies can be easily transferred to other materials.

9.5 Bioreduction and Bioaccumulation: Nanomaterials from Waste

Bioaccumulation and bioreduction are often accompanied by the formation of nanoparticles. These properties make the processes attractive for the creation of novel nanomaterials. Bioaccumulation describes the accumulation and enrichment of metals in the cells, relative to the environment. This mechanism can be applied for the recovery and concentration of valuable elements from diluted solutions. For example, different microorganisms have been described to accumulate Ga and were used for Ga removal (Gascoyne et al. 1991b). In this case, the accumulation is mediated by siderophores that are complexing with Ga.

The transformation of metal ions into nanoparticles is one strategy to overcome the toxic effects of the metals. Nanoparticles are formed either via bio-precipitation or biologically catalyzed metal reduction (bioreduction). Among the various described biologically produced inorganic nanoparticles, precious metals are the most interesting metals for many applications.

Precious metals of the platinum group metals such as platinum, palladium, rhodium, and ruthenium are widely used in medicine, electronics, for optical devices, and catalysis (Yong et al. 2002, 2003). Especially it is commonly used as catalyst, e.g. in automotive catalytic converters or as catalyst in chemical syntheses. Consequently, significant amounts of Pd are released during production processes, consumption and recycling processes. For minimizing loss of Pd and enabling a circular economy, Pd-efficient recycling processes while avoiding secondary waste streams of toxic chemicals as well as an efficient recovery of Pd from industrial waste waters are mandatory. The application of Pd(II)-reducing microorganisms is an attractive approach that combines the removal of Pd from waste streams, thus minimizing the loss of Pd, with the synthesis of nanocatalysts via bio-reduction and deposition of Pd-nanoparticles on biomass using nontoxic biological means (De Corte, De Corte et al. 2012). The catalysts themselves can be used for the degradation of different recalcitrant pollutants or chemical syntheses.

Different microorganisms have been described that mediate the reduction of Pd(II). In case of the extensively studied anaerobic sulfur-reducing bacteria Desulfovibrio desulfuricans and Geobacter sulfurreducens and the facultative anaerobic iron-reducing Shewanella oneidensis, it has been proposed that hydrogenases and cytochrome c3 are involved in bioreduction of Pd(II) and nanoparticle deposition (De Corte et al. 2012; De Windt et al. 2005; Lloyd et al. 1998; Pat-Espadas et al. 2013; Yates et al. 2013). These reactions require H₂ or formate as electron donor. In other cases (e.g. different cyanobacteria, E. coli), it was assumed that other enzyme systems such as nitrogenase enzyme or molybdenum-containing enzyme systems are responsible for Pd(II) reduction and deposition of Pd(0) in the medium (Foulkes et al. 2016). The majority of the formed Pd(0) particles were formed outside the cell. In other cases, Pd(0) formation is not based on enzyme activities and it was assumed that organic functional groups of the cell wall are responsible for the bioreductive process. For example, Pd(0) nanoparticles could be deposited on S-layer carrying Gram–positive bacteria as well as on the S-layer proteins following the array structure (Pollmann et al. 2006a, b; Wahl et al. 2001). Experiments with native and dead cells of E. coli, S. oneidensis, and P. putida and artificial systems demonstrated that the presence of amine groups mediates the reduction of Pd(II) bound to cell surfaces suggesting the use of amine-rich biomaterials rather than native cells for Pd-recovery (Rotaru et al. 2012). Consequently, De Corte et al. (2013) replaced the bacteria and used amine-functionalized surfaces as target for the synthesis of Pd(0) nanocatalysts (De Corte et al. 2012, 2013).

The application of metal-reducing bacteria for the removal of precious metals from industrial waste streams is a quite attractive alternative to conventional methods, because it requires less toxic chemicals and new products (nano-catalysts) are formed as “byproducts.” A wide range of natural or genetically engineered metal-reducing bacteria were successfully applied by several authors for recovery of precious metals from synthetic solutions or scrap leachates (Creamer et al. 2006; Ito et al. 2016; Konishi et al. 2006, 2007a, b; Mabbett et al. 2006; Maes et al. 2016, 2017; Martins et al. 2013; Pat-Espadas et al. 2013). Metal reduction was accompanied by nanoparticle formation. In these approaches, recovery rates of up to 99% were obtained.

In all cases, the formed bio-Pd was catalytically active. It was especially applied to transform a wide range of pollutants, mainly by reduction (Cr(VI), ClO⁴⁻) (Tuo et al. 2013; Mabbett et al. 2006; Humphries et al. 2007) or dehalogenation (e.g. printed circuit boards (PCBs), trichloroethylene, pharmaceuticals) (Baxter-Plant et al. 2004; De Windt et al. 2005; Hennebel et al. 2009a, b, 2010). Further, bio-Pd was used as a catalyst for diverse chemical syntheses, e.g. the hydrogenation of organic molecules or for coupling reactions in synthetic organic chemistry (Creamer et al. 2007). The doping of bio-Pd with other metals, e.g. Au, thus producing bimetallic catalysts, significantly enhanced catalytic activity. This relatively new approach will extend the applicability of metallic biocatalysts.

Besides catalytic applications, bio-Pd has been applied in microbial fuel cells, e.g. proton-exchange membrane fuel cells, for the generation of energy. In these approaches, biologically produced Pd(0) particles were deposited onto the anode, e.g. carbon papers, of the fuel cells and used for energy generation (Yong et al. 2009, 2010; Quan et al. 2015a). The now-modified anode possessed both electrooxidation and biodegradation capability (Quan et al. 2015a, b).

Most studies concentrated on precious metals. However, some newer publications studied the reduction and recovery of other valuable metals or used bioreduction for removal of toxic elements from industrial waste waters. For example, Lv et al. (2018) synthesized copper nanoparticles via bioreduction by a Shewanella loihica strain and discussed their use as antibacterial material (Lv et al. 2018). Maleke et al. (2019) described the reduction and intracellular accumulation of the REE europium by a Clostridium strain, probably mediated by active transport and intracellular precipitation (Maleke et al. 2019). The authors suggested an application for REE recovery from waste materials. Moreno-Benavides et al. (2019) used a Bacillus cereus strain for reduction of toxic Cr(VI) from electroplating wastewater (Moreno-Benavides et al. 2019).

9.6 Conclusion

In conclusion, many efforts have been done to recover metals from solutions by biological means ranging from the application of different biomasses, construct biosorptive composites, engineering of chelators, and use of different metabolic microbial processes. These approaches were used especially for the removal of toxic elements from waters. There are some reports on the removal of precious metals but only few studies describing the recovery of other valuable metals, e.g. REE, Ga, In. It can be assumed that many technologies developed for heavy metal removal can be transferred to other elements. Most current approaches concentrate on the usage of well-studied microorganisms such as chemolithoautotrophic sulfur-oxidizing bacteria like A. ferrooxidans, bioreducing bacteria such as Shewanella strains, or heterotrophic citric acid–producing fungi such as A. niger. However, there are some reports exploring the potential of novel microorganisms from diverse environments. It can be expected that especially extreme habitats such as salt lakes, volcanoes, deep sea, etc. bear many microorganisms with new metabolic properties that can be used for metal recovery also from e-wastes. Another approach is the smart design of new biomolecules, e.g. of new metal-chelating agents. For example, peptides can be designed interacting with numerous inorganic target materials. Such bioreagents can be integrated into various resource technologies such as metal extraction, fine particle flotation, and metal complexing. First results in these fields are highly promising. The combination of diverse biotechnological methods with classical resource technologies leads to new opportunities to find more environment-friendly and efficient solutions for metal extraction. Thus, a high potential for future applications also in recycling technologies can be expected. Opening up to these new multidisciplinary ideas offers new chances for a green economy.

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