Keywords

10.1 Introduction

Heavy metals (HMs) have been utilized in many industrial areas such as agriculture, textile, construction, electronics, metal, cosmetics, food processing, etc. In the worldwide, heavy metals are raw materials which have an economic value due to their industrial utilization (Nancharaiah et al. 2015). However, these raw materials are not infinite and they are distributed unevenly. It is also known that the growing world population and consumption as well as changing in the global economy led to increased demand of certain HMs such as lead, copper, and zinc (Hofmann et al. 2018). Hence, certain heavy metals can be included “critical raw materials” group recently (Hofmann et al. 2018). On the other hand, it is known that these valuable metals have toxic and carcinogenic effects to the living organism. After using or during industrial processing, HMs mostly release to the environment and lead to increasing environmental and health problems because of their non-biodegradable properties (Tchounwou et al. 2012; Nagajyoti et al. 2010; Jaishankar et al. 2014). Therefore, certain HMs are acclaimed priority pollutants by the US Environmental Protection Agency (US EPA) (Fairbrother et al. 2007).

Due to environmental concerns and strict regulations, varied treatment methods such as membrane separation, precipitation, adsorption, coagulation/flocculation, and ionic exchange have been used for the removal of HMs from water/wastewater until today (Fu and Wang 2011; Kurniawan et al. 2006; Babel 2003; Al-Rashdi et al. 2013). However, most treatment methods are inadequate, expensive, generating secondary pollution, and need long operational time (Fu and Wang 2011). However, bio-electrochemical systems (BESs) are one of the most innovative technologies used both for the removal of pollutants and simultaneously for energy production from wastewater (Bajracharya et al. 2016). Basically, BES systems have four components: anode, cathode, membranes, and external circuit (Jain and He 2018; Dizge et al. 2019). In the anode, organic substances are oxidized and electrons are formed. Produced electrons are transferred to the cathode by an external circuit and energy is produced. While these reactions happen, the positively charged ions formed in the anode pass through a membrane and migrate to the cathode (Chaudhuri and Lovley 2003).

Recently, BESs have been shown remarkable performance on the removal or recovery of HMs such as chromium (VI), gold (III), lead (II), zinc (II), cadmium (II), mercury (II), and silver (I) (Li et al. 2008; Lu et al. 2015; Nancharaiah et al. 2015). Considering the environmental/economic problems of heavy metals, BESs can be considered as an innovative and one of the best technology to ensure the recovery of heavy metals. In this review chapter, the situation of the latest technologies and applications of HMs removal/recovery in BESs are discussed.

10.2 Definition of Heavy Metals

Generally, heavy metals (HMs) are known as natural elements that have a high density (>5 g cm−3) and atomic mass (>23) (Appenroth 2010; Koller and Saleh 2018). With the lack of authoritative definition, heavy metals are defined as the elements including both metals and semimetals (metalloids) (Duffus 2002). In a specific approach, they are elements that occupy columns 3–16 of the periods from 4 to 6, including the transition metals, post-transition metals, and lanthanides in the periodic table (Duffus 2002; Koller and Saleh 2018). Heavy metals position in the Periodic Table is shown in Fig. 10.1 (Hoodaji et al. 2012). Therefore, HMs contain transition metals such as chromium (Cr), cadmium (Cd), iron (Fe), cobalt (Co), nickel (Ni), silver (Ag), lead (Pb), zinc (Zn) elements, and the platinum group metals such as ruthenium (Ru), palladium (Pd), platinum (Pt), rhodium (Rh), and metalloids like arsenic (As), tellurium (Te), selenium (Se) (Nagajyoti et al. 2010; Nose and Okabe 2014; Gunn 2013).

Fig. 10.1
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Heavy metals position in the periodic table (www.webelements.com)

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Although HMs are found all over the world, they are recently classified as pollutants. It is thought that HMs are originated from two-source; geogenic and anthropogenic (Adriano 1986; Noll 2002). The geogenic source is the earth’s crust that contains at low concentrations (ppb) of HMs and they can be released to the environment due to weathering and erosion. Therefore, HMs originated from geogenic sources generate trace level pollutant (Kabata-Pendias 2010). However, volcanic eruption significantly contributes to high level of HMs pollution (Nagajyoti et al. 2010). Conversely, the anthropogenic source is originated from human activities such as domestic, mining, agricultural, and industrial utilizing of HMs. Therefore, these activities lead to the release of HMs in high concentrations and in bioavailable form that can be easily transported and absorbed by living organisms (Adriano et al. 2004).

It is known that living organisms need low concentrations of HMs such as Ni, Cu, Fe, As, and Zn (Singh et al. 2011). Nevertheless, it is also known that all HMs are toxic at high concentrations (Valko et al. 2016). For instance, Zn has significant role for male reproductive activity and Zn deficiency causes anemia as well as inhibits growth and development. On the other hand, excess Zn can damage the living metabolism and cause the same diseases (Nolan 1983; Leah Harris and Gitlin 1996; Rajaganapathy et al. 2011). Furthermore, HMs are utilized in the industrial areas such as agriculture, medical, and technological manufacturing. Excessive use of HMs has led to widespread distribution in the environment (Njati and Maguta 2019). HMs accumulate in the environment because of their non-biodegradability and stability characterization. They accumulate at increasing concentrations in living organisms through the food chain, causing adverse effects (Kazemipour et al. 2008). At higher permissible limits, heavy metals in plants or crops may cause mutations of genetic materials, functional, and structural membrane disintegration, and may inhibit their growth. HMs can accumulate in fatty tissues/human bones. They are toxic and harmful to humans and other living organisms at higher permissible limits (Carolin et al. 2017; Rai et al. 2019). Hence, in the past decades, HMs and their pollution have been evaluated by the environmental law regulators and agencies in worldwide. Some types of HMs, their industrial usage area, and their permissible limits for humans and plants are demonstrated in Table 10.1.

Table 10.1 Heavy metals, their industrial usage area, and permissible limits for humans and plants
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10.3 Potential Toxicity of Heavy Metals for the Environmental Life

The environment can be defined as a dynamic organism that contains soil, air, water, and living organisms. In recent decades, the environment has witnessed industrial development and unparalleled rapid population growth. Due to these developments, nowadays “the environment” is mentioned along with mostly “pollution.” Until today, many harmful pollutants such as polycyclic aromatic hydrocarbons (PAHs), pesticides, polychlorinated biphenyls, synthetic dyestuff, heavy metals, and many others have been released to the environment (Chen et al. 2015). Among these pollutants, heavy metals are known to threaten environmental sustainability (Liu et al. 2019). HMs can enter the environment and can be deposited in the water, air, and soil because of geogenic and anthropogenic sources (Jacob et al. 2018).

The heavy metal pollutants can release into the environment due to industrial wastewater, sewage irrigation, atmospheric deposition, agricultural activities (usage of fertilizer and pesticide), mining activities, metal production, irregular storage of industrial and municipal solid waste (Wuana and Okieimen 2014). Atmospheric deposition seems to be a small factor, but it is one of the major factors causing HMs pollution in the soil (Yan et al. 2018). Combustion of fossil fuel, using Pb-containing fuel for transportation, the production metallurgy, and construction materials lead to producing emissions of HMs and their aerosol forms enter into the atmosphere (Cheung et al. 2011; Duan and Tan 2013). These aerosol forms of HMs are mostly oxidized and condense as fine HM particles in the atmosphere (Wuana and Okieimen 2014). Due to the effect of the wind, they are distributed and adsorbed by the mineral particles and precipitated in the soil (Manafi et al. 2012). When these particles consumed by the microorganisms, it may cause protein denaturation, dysfunction, the destruction of cell membrane integrity in the microorganisms (Chodak et al. 2013). Furthermore, the enzyme activity of soil microorganisms can be decreased. Due to decreasing enzyme activity, organic matter decomposition, and nutrient cycling processes can be affected adversely (Tang et al. 2019). Consequently, the environment ecosystem deteriorates because of heavy metal pollution.

Additionally, HMs can penetrate the soil with mostly poorly treated industrial, agricultural, and domestic wastewater (Vardhan et al. 2019; Chowdhury et al. 2016). Humus in the soil has a high affinity for HM cations and it absorbs HMs from the water passing through the soil (Jacob et al. 2018). Generally, about 98% of the HMs in the wastewater are absorbed by the soil and the rest of it are absorbed by the plant (Singh et al. 2011). The roots of plants absorb water containing HMs and they are transported to plant tissues (Ahmad et al. 2016). At higher toxic concentrations of HMs, reactive oxygen species (ROS) such as superoxide (O•2−) and hydroxyl radicals (•OH) can be produced and the plant may enter the oxidative stress. As a result, the physiological and genetic structure of the plants change with the carbohydrate and protein content, and HMs inhibit germination or growth (Berni et al. 2019). If these contaminated plants are eaten by animals, heavy metals transfer to other animals and humans. In the case of forest fires, heavy metals return to the atmosphere (Fang et al. 2010).

HMs sorbed by soil mineral particles or plants are not biodegradable by microbial microorganisms. Therefore, they remain permanently in the soil. During this time, HMs can convert into different chemical forms that have different toxicity, bioavailability, and mobility (Wuana and Okieimen 2014). For instance, chromium is mostly available as Cr (VI) and Cr (III) forms in soil. Whereas Cr (III) is a micronutrient and a nontoxic metal for some microbial species, Cr (VI) is highly toxic (Garnier et al. 2006). However, ionic forms of HMs such as As3+, Hg2+, Pb2+, Cd2+, and Ag+ in the soil can be transported to surface water (like the river, lake, and sea) and groundwater reservoir via filtration. It is also possible that atmospheric deposition of HMs can also enter into the aquatic systems by acid rain (Singh et al. 2011). These ionic forms of HMs are mostly bound to particulate matter and settle down to the sediment in aquatic systems (Singh and Kalamdhad 2011). Sediment-bound HMs can be also taken up by aquatic organisms (Peng et al. 2008). For example, fish, as an aquatic organism, can take HMs from food, non-food particles, gills, water consumption, and skin (Singh and Kalamdhad 2011). When HMs (especially Hg) enter into the fish, ROS is produced, which can damage their metabolism (Woo et al. 2009). In addition, HMs can accumulate in fish oils and tissues. The fish containing HMs can be consumed by carnivores and HMs are transported to humans through the food chain (Afshan et al. 2014; Peralta-Videa et al. 2009).

The human body can tolerate trace amounts of heavy metals without serious health problems. But for long-term exposure, HMs may cause to the consuming of essential nutrients in the human body, thereby functional disorder of vital organs such as the brain, heart, kidney, liver, and nervous system (Fig. 10.2) (Sardar et al. 2013; Jacob et al. 2018). Furthermore, some HMs such as Pb, Cd, and Hg have carcinogenic effects (Chowdhury et al. 2016).

Fig. 10.2
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Heavy metal effects on vital organs in the human body

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Due to the adverse effects of HMs on human health, it is emphasized by the regulatory agencies such as WHO and US EPA that safe drinking water is crucial to human life. Furthermore, these agencies, which protect human life and the creation of healthy generations, have proposed the maximum permissible limit values for some HMs in drinking water (Table 10.2).

Table 10.2 Recommended permissible limits of some HMs in drinking water (Kumar et al. 2017)
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Literature survey showed that the main factor for HMs pollutions was the inadequate treatment of industrial, agricultural, and domestic wastewater. It is also considered that the soil and these polluted wastewater act like a distribution system and it is the most important factor in HMs pollutions that threatening living organisms and the environment.

10.4 Conventional Heavy Metal Recovery Technologies

Wastewater produced from industries mentioned above includes a significant amount of heavy metal concentrations which have toxic or harmful effect to surroundings (Carolin et al. 2017). These pollutants can be converted into less toxic substances by sequential conventional heavy metal recovery methods such as adsorption, ion exchange, coagulation/flocculation, chemical precipitation, electrochemical processes, advanced oxidation processes, and membrane filtration systems (Azimi et al. 2017). All these treatment technologies have some advantages and disadvantages compared to each other such as treatment performance, produced water quality, operational, and maintenance (O & M) cost.

Chemical precipitation is an easily applicable process which can be used for the treatment of HMs from inorganic discharges. In chemical precipitation, the dissolved heavy metal ions react with chemicals and are transformed into the insoluble solid phase at high pH conditions (pH 11) (Azimi et al. 2017). Then, the solid phase is separated from the treated water by filtration or sedimentation. Generally, precipitated heavy metal ions are in the form of phosphate, sulfide, carbonate, and hydroxide (Nzihou and Sharrock 2010). The chemical precipitation technique is not proper for the low concentrations and high amount of hazardous sludge formation is another problem that is difficult to manage (Kuan et al. 2010). However, the chemical precipitation technique is easy to implement on a large scale due to its low cost.

Coagulation/flocculation is another method to remove HMs from wastewater. In this process, the net surface charge of the colloids is reduced by electrostatic repulsion in the coagulation mechanism and the size of the stabilized colloids are increased by the addition of polymers in the flocculation mechanism. Then, the flocculated particles are separated from wastewater by filtration or sedimentation. The formation of hazardous sludge, a requirement for the high amounts of chemicals are the main disadvantages similar to the chemical precipitation process (Pang et al. 2011; Johnson et al. 2008).

Ion exchange processes have many advantages such as high removal capacity and efficiency for the treatment of HMs from wastewater. Especially, synthetic resins have high efficiency to remove almost all heavy metals. Especially, both strongly and weakly basic resins are the main ion exchangers in which the metal cations are changed with hydrogen ions in the sulfonic groups or carboxylic groups and the processes can be applied to remove HMs (Fu and Wang 2011). Similarly, the negative charged heavy metals can be replaced by the anions in the synthetic resins such as hydroxyl and chloride ions. Natural resins such as zeolites can be used as an alternative to synthetic resins because of their low cost. The less sludge formation makes the ion exchange process advantageous over other processes such as chemical coagulation/flocculation and precipitation.

The adsorption process is a low-cost alternative method for the removal of HMs which supplies high removal efficiency and low fouling problems. In the adsorption process, heavy metals are adsorbed into the active sites of the adsorbents by physically or chemically interactions (Bilal et al. 2013; Ojedokun and Bello 2016). The presence of various adsorbents, being a reversible technology, the repeated use of adsorbents, and the absence of the formations of toxic pollutants are the main advantages of this process (Carolin et al. 2017).

Membrane filtration is another alternative method to remove HMs from wastewater in which both heavy metal removal/recovery and disinfection takes place together. The separation of the contaminants depends on their charge, molecular size, concentration, solution pH, and applied transmembrane pressure (Basaran et al. 2016). Basically, reverse osmosis, nanofiltration, ultrafiltration, and electrodialysis are the main membrane separation processes used for heavy metal removal. Membrane processes have many advantages, for example, better removal efficiency of contaminants, smaller footprint, easy operation, long filtration media life, and some disadvantages, for example, their high capital and O & M costs (Qdais and Moussa 2004; Nadeem et al. 2019).

Heavy metals can be treated by electrochemical processes such as electrocoagulation, electrodeposition, electroflotation, electrodialysis, electrodeionization, and bio-electrochemical systems (Bazrafshan et al. 2015). The process efficiency depends on the electrode material in the electrochemical reactor, current density, wastewater characterization, etc. Although high removal efficiencies of HMs make this technology advantageous, high capital cost, the short service life of electrodes, and expensive electricity requirement limit its extensive usage (Zhang et al. 2013). However, bio-electrochemical systems (BESs) can overcome these limits for HMs recovery from water/wastewater.

10.5 Definition of Bio-electrochemical Systems

BESs can be called as a microbial electrochemical system (MEC) in which microbes or enzymes are implicated in the at least one of the oxidation or reduction reactions (Kumar et al. 2018). In a microbial fuel cell (MFC), electrical power is obtained by the degradation of organic matter by microorganisms in bioanode. In microbial electrolysis cell (MEC), an exterior electrical power is provided to drive the formation of valuable products. In microbial electrosynthesis (MES), CO2 or organic compounds are reduced cathodically to high value-added products. Besides, there are other BESs, for example, used for the desalination of water called as microbial desalination cells (MDCs) (Fig. 10.3) (Bajracharya et al. 2016; Dizge et al. 2019).

Fig. 10.3
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Schematic diagram of microbial electrochemical technologies. MFC microbial fuel cell, MDC microbial desalination cell, MEC microbial electrolysis cell, BET bio-electrochemical treatment, MES microbial electrosynthesis system (Dizge et al. 2019)

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Bio-electrochemical systems (BESs) have electrochemical cells in which microorganisms act as a catalyst on one or both electrodes (Hamelers et al. 2010). In the BES mechanism, microorganisms catalyze the reactions taking place at the electrodes, and electrons are moved from the oxidized component to the anode for the oxidation reaction or to the cathode for the reduction reaction. Because of the many microorganisms are electrochemically active, BESs have great potential for the formation of energy and chemicals (Sleutels et al. 2012).

In the BES mechanism, a principal electron contributor should be provided to the anode and a final electron acceptor should be supplied to the cathode. The benefit of using two separate compartments is to separate oxidation and reduction products from each other and to facilitate the extraction of valuable products (Hamelers et al. 2010).

The application areas of the BESs have been increased in recent years and they have a high possibility of the usage of various oxidizable components at the bio anode such as municipal wastewater, many industrial wastewater, acetate, starch, etc. Besides, microorganisms could catalyze many reduction reactions at cathode such as oxygen to water, proton to hydrogen, and nitrate to nitrogen gas. The energy efficiency of the BESs relies on the voltage and Coulombic efficiencies of the reactions that occurred at both electrodes. In order to increase the applicability of BESs, advantages of the process (the economic price of the products and treatability of the wastewater) must be greater than capital and operational costs. Generally, an increased current density is a necessity for low capital costs and a high removal efficiency, which lets smaller space demand (Sleutels et al. 2012).

10.6 Applications of Bio-electrochemical Systems in Heavy Metals Recovery

Waste streams containing metals should be considered as a valuable resource for the recovery of worthy and rare elements. Even though conventional biological treatment systems are evaluated as an economical technology for heavy metal-containing wastewater treatment, they remain inadequate for heavy metal recovery. Hence, BESs are considered as an attractive technology for the removal/recovery of HMs from different kinds of wastewater and metallurgical and process wastes recently. The general concept for metal removal using BESs is that organic matters are biodegraded by electroactive biofilms at the anode and produced electrons are transferred to the cathode and heavy metal ions are reduced at the cathode to recover metal(loid)s (Fig. 10.4) (Nancharaiah et al. 2015). Reduced metal(loids) in cathodes can also follow three ways in the cathode chamber to be removed or recovered in BESs: (1) deposited on the cathode, (2) precipitated in the solution, or (3) dissolved in the solution (Lu et al. 2015).

Fig. 10.4
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Redox tower of selected metal ions as electron donors and electron acceptors to remove or recover of metal ions in BESs (Nancharaiah et al. 2015; Dominguez-Benetton et al. 2018)

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In BESs, several metal ions can be a representative for oxygen and serve as an effective terminal electron acceptor. For example, when the cathode chamber was fed with a fly ash leachate, copper was removed and recovered in BES without extra energy input and metal copper was deposited on the cathode of an MFC. It was reported that higher than 97.1% of Cu(II) removal efficiency with an initial Cu(II) concentration of 52.1 mg/L was obtained for 36 h operation period in the leachate. Cu(II) was reduced and recovered mainly as metallic Cu on cathodes (Tao et al. 2014). In addition to the recover of copper, metallic Ag recovery and power generation were also achieved by using cathodic reduction in BESs (Tao et al. 2012). An electron donor (acetate) on the anode and an electron acceptor (both Ag+ ions and Ag(I) thiosulfate complex) on the cathode was used for metallic Ag recovery in dual-chamber BESs. They reported that up to 95% of Ag(I) removal was succeeded and metallic Ag with >91% purity was electrodeposited on the cathode (Tao et al. 2012).

The reduction of copper ions in a cathode with simultaneous electricity generation with glucose as a substrate in MFCs was also proposed by Tao et al. 2011 and metallic copper and cuprous oxide (Cu2O) were recovered. Different concentrations of CuSO4 solution from 50.3 to 6412.5 mg Cu2+/L as the catholyte solution at pH 4.7 and different resistors from 0 to 1000 Ω as external load were examined by using dual-chamber MFCs. High Cu2+ removal efficiency (>99%) with 1.3 mg/L final Cu2+ concentration was obtained at an initial 196.2 mg Cu2+/L concentration with an external resistor of 15 Ω, or without an external resistor. Cu2+ was reduced to cuprous oxide and metallic copper on the cathodes according to X-ray diffraction (XRD) analysis (Tao et al. 2011).

Cobalt, which is a rare metal, was recovered from the aqueous solution after leaching of Co(II) from LiCoO2 using BES (Huang et al. 2014a). LiCoO2 used in Li-ion batteries was the source of Co(II) ions that were reduced to Co(0) on the cathode of a MEC. A sequential MFC–MEC (MFC-Co(III)/Co(II)–MEC-Co(II)/Co(0)) process was suggested for leaching and recovery of cobalt from waste lithium-ion batteries (Huang et al. 2014b). The cobalt leaching and Co(II) reduction were obtained 46 and 7 mg/L/h in MFCs and in MECs, respectively, with an overall system cobalt yield of 0.15 g Co/g Co. The results showed that cobalt was completely recovered and recycled to spent lithium-ion batteries with no external energy consumption using the sequential MFC–MEC system. Other studies on the recovery of HMs using different BESs from solutions or wastewaters are presented in Table 10.3.

Table 10.3 Heavy metal recovery efficiency using different BESs
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Metallic copper and nickel recovery from acid mine drainage (AMD) and simultaneous H2 production on the cathode were carried out by using two-chamber MEC with an externally applied voltage of 1.0 V (Luo et al. 2014). The recovery efficiency of Cu2+ reached 99.2% within 42 h. However, the Ni2+ recovery efficiency reached 97% at the end of batch operation for 62 h. It can be concluded that the MEC was successfully used to separate metals from the AMD, to recover value-added products of metallic copper and nickel, and to produce H2 gas (Luo et al. 2014).

Uranium was also recovered from the groundwater with a pure culture of Geobacter sulfurreducens using BES and graphite electrodes were used as the electron donor (Gregory and Lovley 2005). Under optimized conditions, 80 μM of U(VI) was deposited on the cathode and 87% of the uranium was recovered from the electrode surface.

10.7 Challenges and Perspectives

Developments in BESs are promising technology for the recovery of heavy metal ions in practical applications and commercialization. However, the biggest challenge facing BESs is their inability to treat all metal ions and required external power supply (Ezziat et al. 2019). The strategic factors influencing the BESs performance for recovery of heavy metal ions are biocatalysts, electrodes, electrolytes, and membranes (Jadhav et al. 2017). The development of heavy metals resistance microbial community is essential for efficient recovery. Moreover, adsorption and diffusion of heavy metals from the cathode chamber to the anode chamber should be reduced to obtain a high recovery. Valuable metal catalysts such as Pt should be abolished to decrease the operating costs for real applications. For this purpose, cheaper materials with large surface areas, such as stainless steel wool, activated carbon cloth, and foam may be used as an alternative to Pt catalyst. There are still major deficiencies and difficulties for recovering valuable and rare metals from real waste/wastewaters, although BESs have been performed successfully using different metal ions as electron acceptors (Nancharaiah et al. 2016). Membrane biofouling is another important challenge. This undesired event occurs in MFCs, as biofilm will unsurprisingly grow on and inside chambers during long-term operation (Ezziat et al. 2019). The use of new polymers such as polybenzimidazole in membrane synthesis can be a solution to prevent membrane biofouling because of inhibited the adhesion of bacteria on the membrane surface by this polymer (Angioni et al. 2017). From a broader perspective, BES technologies should be evaluated not only by the economic feasibility but also by the need to meet biotechnological expectations for large scale applications. In addition, there is a necessity to explain the behavior for metal removal under non-ideal conditions from real streams (Pant et al. 2012; Yu 2016).

10.8 Conclusions

Recently, BES technologies which discovered through the search for alternative energy sources, have been remarkably attracted attention due to their functionality and performance. Besides the production of hydrogen and electricity, BESs are a useful stage and they have great potential for recovering heavy metal ions from wastewater, groundwater, aqueous streams, and wastes. The cathodic reduction of metal ions coupled to organic substrate oxidation can be used for the recovery of several heavy metals. Heavy metal ion concentration, heavy metal type, applied voltage, microbial community, membrane type, conductivity, anode or cathode materials, and system configurations will affect heavy metal recovery efficiency. Further researches at the molecular level are needed to understand deeply the mechanisms between heavy metals and biocatalysts.