Metals recovery from wastewater by microbial electrochemical technologies

C H A P T E R

13 Metals recovery from wastewater by microbial electrochemical technologies Pau Rodenasa, Colin Wardmana, Abraham Esteve-Nun˜eza,b a

IMDEA Agua, Avenida Punto Com, Parque Cientı´fico Tecnolo´gico de la Universidad de Alcala´, Madrid, Spain b Department of Analytical Chemistry and Chemical Engineering, University of Alcala´, Alcala´ de Henares, Spain

Principles of metal recovery in microbial electrochemical technologies Metals and their impact on society Metals are ubiquitous in nature and in many areas of human life. For example, iron and zinc are essential to proteins like hemoglobin or peroxidase that use metals bound in their protein structure [1]. Also, they are essential to the global economy, to many industries, and to maintain our quality of life [2]. Metals represent almost 80% of the known elements on the periodic table and their properties and applications are as diverse as they are numerous. Metals like sodium, potassium, lithium, calcium, and magnesium are from the alkali and alkaline groups and are representative of the most reactive metals due to the configuration of their electrons. Metals like gold, silver, platinum, or rhodium represent the least reactive metals and are precious due to their scarcity and unique

Wastewater Treatment Residues as Resources for Biorefinery Products and Biofuels https://doi.org/10.1016/B978-0-12-816204-0.00013-8

chemical properties. These metals are as commonly used for jewelry as for catalysts or electronic components [3]. Furthermore, metals like copper, zinc, nickel, or cadmium are considered heavy metals. These, combined with iron and aluminum, produce alloys commonly found in construction and manufacturing. These alloys are integral in batteries, materials for heat dissipation [4], catalysts, pesticides, and fertilizers, and are essential components for the electronics industry. Rare earth metals like niobium, cerium, gadolinium, samarium, and praseodymium are also used as catalysts, magnets, or tracers for medical use [5, 6]. With the ever-expanding role that metals play in our everyday life, great effort has been made to increase their extraction, but the reality of their toxicity to our person and environment cannot be avoided. Throughout human history research on metal extraction and ways to recycle/recover it has

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13. Metals recovery from wastewater by microbial electrochemical technologies

been constant. Gold is an excellent example of a metal that, over the centuries, has been mined, melted, and recycled many times over as jewelry, art, coinage, and ingots. However, many mining and recovery processes, through the discharge of wastewater, have released toxic metals into the environment. Concurrently with the great increase in demand for new consumer electronics and building materials, the market demand for metals has increased, leading mining companies to use lower-grade minerals for metal extraction to meet this demand. As a consequence, companies use extraction and refining processes that are more costly and use an increasing amount of polluting agents [7, 8]. For example, copper is extracted from chalcopyrite, chalcocite, azurite, and malachite by one or a combination of metallurgical processes. A common practice, known as hydrometallurgy, leaches the metal from the ore using an aqueous solution of sulfuric acid. The acidic solution leaches metal sulfates that are later recovered as the refined metal [9]. In the end, the produced wastewater has a high concentration of heavy metals, such as arsenic, zinc, cadmium, and nickel. These remaining metals are very toxic to aquatic environments and, therefore, need to be removed before release [10]. Currently, there are several different technologies on the market to remove metals from wastewater. The most commonly used are chemical precipitation, ion exchange, cementation, and electrowinning [11]. Moving beyond these conventional methods, it is important to reduce energy consumption, toxic chemical usage, and the risk posed to the environment by these processes. Microbial electrochemical technologies (METs) are a novel set of technologies with the ability to remove and recover metals while consuming less energy and chemicals. The following is a review of the state of the art of applying METs to metal recovery from wastewater.

Fundamentals of microbial electrochemical technologies METs utilize bacteria capable of interacting with electrodes (electroactive bacteria) to oxidize or reduce chemicals of interest while producing or consuming electricity, respectively. There are two major types of METs. The first is the reductive or anodic type. In this type, the electroactive bacteria (EAB) colonize the anode of an electrochemical cell where they oxidize an electron donor, respire, and use the anode as the terminal electron acceptor. In this type, the electrode or anode is reduced by the EAB. The other is the oxidative or cathodic type. In this type, the opposite occurs. The EAB colonizes the cathode and receives electrons from the electrode to respire and finally donate to an electron acceptor. In both of these types, there is a reduction event coupled with an oxidation event, one or the other or both being carried out by a microbial intermediary. These electrochemical reactions are defined in thermodynamics by the reduction potentials for each of the reductor-oxidant (redox) couples. The potentials are measured as the voltage difference between the electrode, where an oxidation or reduction reaction is taking place, and a reference electrode. There are exceptions where the oxidation and reduction processes cannot be measured directly, because those reactions do not take place on the surface of an electrode. These electrodeless oxidation or reduction reactions can only be measured by changes in the concentration of the chemical of interest. The anode is the electrode where the oxidation of organic or inorganic substances transfers electrons from these substances to the electrode. In METs the anode potential depends on (a) the substrate to be oxidized, (b) its concentrations, and (c) the pH of the electrolyte. As an example, the potential for acetate oxidation can be calculated with the Nernst equation according to Eq. (13.1),

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Principles of metal recovery in microbial electrochemical technologies

Eanod ¼ E0Ac 

R∙T ½Ac  ∙ ln 8 8∙F ½HCO3  ∙½H + 

! (13.1)

where E0Ac is the standard acetate potential (V), R is the ideal gas constant (8.31 J/Kmol), T is the temperature (K), F is the Faraday constant (96,485 mol/C), [Ac] is the acetate concentration in solution (mol/L), [HCO 3 ] is the bicarbonate concentration in solution (mol/L), and [H+] is the proton concentration (mol/L). On the other electrode, the cathode, the reduction reaction takes place when electrons are given from the electrode to an electron acceptor. The cathode potential depends on the ionic species concentration of the electron acceptors that are targeted for being reduced. Ions being reduced are represented by the following reaction: n+ Mm + + n e! !M

The cathode potential depends on the oxidized free ion concentration ([Mm+]), the reduced free ion concentration ([Mn+]) and on the species in solution that can be involved in the reactions (Eq. 13.2). The cathode potential can be calculated according to  n+  ½M  R∙T (13.2) ∙ ln Ecat ¼ E0Mm + =Mn +  ½Mm +  n∙F where EMm+/Mn+0 is the standard reduction potential of the target ion (V), and [Mn+] and [Mm+] are the concentrations of the ions in solution (mol/L). The concentration of free ions in solution can, for example, be influenced by parasitic complexation reactions. Microbial fuel cell versus microbial electrolysis cell The difference between the cathode (Ecat) and the anode (Ean) potentials defines the cell voltage (EMET) of the device (Eq. 13.3). EMET ¼ Ecat  Ean

(13.3)

The cell potential is defined by the thermodynamics of the system and can be calculated with Eq. 13.3. The cell voltage is a magnitude proportional to the Gibbs free energy (Δ G) (Eq. 13.4) of the reduction and the oxidation reaction combined: EMET ¼ 

ΔGMET nF

(13.4)

where n is the number of the electrons transferred in the overall reaction from electron donor oxidation and electron acceptor (metal ion) reduction, F is the Faraday constant, and EMET is the voltage of the whole cell (V). When the cell potential is positive, the process is exergonic and electricity can be harvested, and the microbial electrochemical system is called a microbial fuel cell (MFC). However, when the cell potential is negative the process is endergonic and an external power supply is needed to drive the reactions, and in this case the microbial electrochemical system is called a microbial electrochemical cell (MEC). Kinetics of microbial electrochemical systems The potential measured during the reaction process differs from the calculated potential due to the reaction evolution. As a consequence, a constant change in the concentration of the chemical species involved takes place. The thermodynamic viability of a reaction is determined by the standard electrode potentials. However, these standard electrode potentials do not provide the reaction rate. The difference between the equilibrium and the measured potential is called overpotential. Overpotentials are closely related to the reaction evolution, in other words, with the reaction’s kinetic behavior. Overpotential is a measure of the energy losses during the reactions at the electrode surface.

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13. Metals recovery from wastewater by microbial electrochemical technologies

The closest relationship between the theoretically calculated potential by the Nernst equation and that measured between the anode and cathode is the voltage at open circuit (Voc). This voltage is measured when there is no current flowing from anode to cathode and the system is close to equilibrium. In most of the cases, Voc is close to the thermodynamically calculated cell potential (EMET). On the other hand, the operational measure of the voltage in the cell (Vop) is given by Eq. (13.5), as adapted from Sleutels et al. and Choi and Cui [12, 13]: Vop ¼ Voc  ηΔpH  ½ηact Cath + ηmt Cath   ½ηact Anod + ηmt Anod   jΣ ρi di  ηmem i

(13.5) where Vop is the operational voltage difference measured between cathode and anode, Voc is the voltage at open circuit, ηact Cath is the cathode activation overpotential, ηmt Cath is the cathode mass transfer overpotential, ηact Anod is the anode activation overpotential, ηmt Anod is the anode mass transfer overpotential, j is the current density, ρi is the resistivity of the solutions, di is the distance between membrane and electrode, and ηmem is the membrane transport overpotential. The internal resistance of the system is a parameter that summarizes the energy losses of the system when current flows from anode to cathode. From Eq. (13.5), the internal resistance can be simplified as: Vop ¼ Voc  i∙Rint

(13.6)

where Vop is the operational voltage, Voc is the voltage at open circuit, i is the operational current that is measured between cathode and anode, and Rint is the internal resistance of the electrochemical device. The internal resistance will depend on the overpotentials across membranes, electrode surface, and/or due to the solution resistivity. Current and overpotentials are codependent variables of each other. Both variables are

affected by charge transfer and mass transfer. There are two primary mechanisms leading the kinetics of the system: • Mass transfer of the ionic species from the bulk of the solution through the surface of the electrode. • The heterogeneous charge transfer from the electrode surface to the surface of the ionic species. The overall reaction rate will be limited by the slowest step. Mass transfer can be explained by three different phenomena: diffusion, convection, and migration. Depending on the operational flow rate of the solution in the cell and the morphology of the cell, the mass transfer can be the limiting factor. The second mechanism, charge transfer on the electrode surface, is related to the activation losses. Considering the Arrhenius and the activated complex theories, the reaction rate on the electrode surface for the anode and the cathode is described in the following equations as a function of the cathode and anode activation overpotentials:  ∗    ΔGred ð1  αÞFηact Cath exp kred ¼ A exp RT  RT ð1  αÞFηact Cath ¼ k°red exp RT (13.7)  ∗    ΔGox αFηact Anod exp kox ¼ A exp RT   RT αFηact Anod ¼ k°ox exp (13.8) RT where A is the Arrhenius constant, Δ G∗red and Δ G∗ox are the Gibbs free energy of the reduction and oxidation processes, respectively, and α is the transfer coefficient, which can range from zero to unity. The current density limited by activation losses is expressed as a function of the overpotentials by the Butler-Volmer equation as a simple model for explaining the kinetics:

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Principles of metal recovery in microbial electrochemical technologies

"

  αFηact Anod i ¼ nFAk° Cox exp RT  # ð1  αÞFηact Cath (13.9) Cred exp RT where n is the number of electrons transferred, F is the Faraday constant, A is the electrode area, and Cox and Cred are the concentrations of the oxidize and reduce species, respectively. Also, the current could be limited by the transport through the membrane as Wilhelm et al. (2001) described in the next equation [14]: i ¼ 2F  Dm

C2s s cfix

(13.10)

where C2s is the square of the solute concentration, Dm is the diffusion coefficient of the species through the membrane, s is the membrane thickness, and cfix is the fixed charge density inside the membrane. A complete understanding of the system starts with electrochemical characterization.

FIG. 13.1

285

The most common techniques are voltage measurements over time using data loggers and the use of a passive element closing the circuit (resistors) [12, 15]. The study of the individual processes occurring in the different compartments of a MET is done using cyclic voltammetry, as Varia et al. showed [16]. Role of metals in microorganisms Metals can be reduced from their ionic species to another ionic species or to their elemental state. Metal ions are well known as good electron acceptors: for example, iron and manganese are essential electron acceptors for many bacteria in the environment [17]. Microorganisms can transfer electrons via their cytochromes from the electron transport chain (ETC) or outside the membrane to metal oxides or dissolved metallic ions present in their media [18]. Furthermore, bacteria can play a major role in the mobilization or immobilization of metals in soils or water due to their abilities to interact with metallic ions as a terminal electron acceptor or by sorbing metals directly [19]. Fig. 13.1 shows

Schematic representation of interactions between metal ions and bacteria. Me, metal.

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13. Metals recovery from wastewater by microbial electrochemical technologies

the different strategies of a microorganism to interact with metal ions in solution to immobilize, precipitate, or reduce them.

Fundamentals of metal recovery Conventional technologies to remove or recover metals from wastewater can be divided into two groups: remediation or recovery. The first group is focused on the removal of the heavy metals from wastewater, ashes, or soils in order to generate a concentrated product for disposal. The technologies from this group are chemical precipitation, ion exchange, absorption, membrane filtration, and flocculation or sedimentation (coagulation techniques). The second group is focused on the extraction and harvesting of metals for use. The technologies in this group center on electrochemical metal precipitation, also known as electrowinning (EW). Table 13.1 summarizes the comparative advantages and disadvantages of the two groups of technologies for metal recovery or removal. EW is a similar technique to METs, but instead of oxidizing organic matter, water is oxidized to oxygen. The evolution of oxygen is not a reaction favored by kinetics or thermodynamics; TABLE 13.1

these nonfavorable conditions imply a high energy cost to recover metals from solution. In contrast, METs can oxidize other species with lower energy requirements. The first step is to know what the standard potentials are of the reactions taking place. Fig. 13.2 shows the relative position at pH 7 of electron donors and electron acceptors in METs for metal recovery. Fig. 13.2 shows an overview of possible reactions in microbial electrochemical systems for metal recovery. On the left side, the oxidation potentials (electron donor potential) for several electron donors like acetate, ethanol, lactate, sulfide, sulfur, and hydrogen are depicted. On the right side is an overview of different possible reduction potentials (standard reduction potentials) for metal recovery. In this figure, the metal ions with a standard reduction potential higher than the electron donor potential have an overall positive cell potential (MFC). By contrast, the metal ion species that have a standard reduction potential lower than the electron donor potential require the input of additional energy and, therefore, a voltage applied across the cell (MEC). Fig. 13.2 also shows that, theoretically, the electron donor potential of hydrogen can reduce metals, like nickel, lead, copper, silver, and mercury, allowing for power production

Comparative advantages and disadvantages of technologies for metal removal [7, 20, 21].

Technology

Advantages

Disadvantages

Chemical precipitation

– High recovery – High volume treated

– High consumption of chemicals

Coagulation

– High recovery – High volume treated

– High consumption of chemicals

Ion exchange/adsorption

– High efficiency

– Low volume treated – High cost of materials

Membrane filtration

– High volume treated

– High cost of materials

Electrowinning

– High purity recovery

– High energy consumption

Cementation

– Low cost

– Low-grade product

Biosorption

– High specificity

– Incompatible with high concentrations and high volumes

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287

FIG. 13.2 Energy diagram with standard reduction potentials vs. normalized hydrogen electrode (NHE). MFC, microbial fuel cell; MEC, microbial electrolysis cell.

using an MFC. In practice, due to the internal resistance of the systems, most of these reduction reactions require an external power input and are run as an MEC. As explained previously, the energy requirement of the electrochemical process depends strongly on the relative position of the redox potentials. These positions can change with temperature, pH, or presence of other ionic species that can react with the electron acceptor or the electron donor. A useful tool for working in favorable conditions for metal deposition is the Pourbaix diagram. The plot diagrams the

relationship between the state of the metal ion in question with the voltage potential and the electrolyte pH. Such plots show whether an ion species is in solution or in a solid phase depending on how the potential changes with pH [22]. The Pourbaix diagrams are used to determine if the metal ion will end up as an oxide or in a precipitate instead of being deposited on the surface of the electrode. Fig. 13.3 shows an example of a Pourbaix diagram with a 1 g/L CuSO4 solution and a copper electrode. The grey area shows when a solid copper phase forms on the electrode surface, while the green

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13. Metals recovery from wastewater by microbial electrochemical technologies

FIG. 13.3 Pourbaix diagram of copper species present at different electrode potentials (E) measured in volts (V) at standard hydrogen electrode (SHE) represented at different pH conditions for a 1 g/L CuSO45H2O solution.

area shows when soluble and nonsoluble compounds are formed.

Categorization of the microbial electrochemical technology-based metal recovery mechanism Dominguez-Benetton et al. categorizes METs for metal recovery by the reaction mechanism that takes place between the oxidation and reduction processes. They classify the technologies into four categories [2]: • Category A are systems with a microbial anode and an abiotic cathode. Metals are reduced by electrons on the cathode surface. This is the case with most of the systems in the literature due to its similarity to the traditional EW process.

• Category B systems use an electrochemical reduction at the cathode to generate a chemical species that reacts with the metal in the solution, either reducing it or reacting with it to form a nonsoluble species. An example is the reduction of O2 to H2O2 to reduce Cr(VI) to Cr(III). • Category C systems use an electrochemical or bioelectrochemical reduction followed by chemical or biochemical oxidation. The aim is to enhance the power output of the system by using a paired redox reduction on the electrode surface followed by oxidation in the solution. Fe3+ can be reduced on the anode surface and later oxidized again in the solution, at high pH, to be recovered as Fe(III) oxide. • Category D systems use an electrochemically active microorganism to reduce the ions on the cathode surface or metals directly in solution.

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Principles of metal recovery in microbial electrochemical technologies

289

Cell configuration and reactor design

Flow rate and volume of water to be treated

A wide variety of MECs have been tested. These include the H-Cell bioreactor at laboratory scale (100 mL), filter press type reactor, and tubular reactors, as depicted in Fig. 13.4. Each of the reactors depicted in Fig. 13.4 has advantages and disadvantages. H-Cell reactors are easy to operate for pure culture experiments, due to their ability to be autoclaved, but the internal solutions contribute to higher internal resistance. On the contrary, filter press reactors, as depicted in Fig. 13.4C, show a lower internal resistance due to internal solutions allowing the production of higher current densities and higher removal rates [25]. Cells and electrochemical reactors should be designed taking into consideration the factors described in the following subsections.

Flow rates and solute concentrations of the influent are the boundary conditions of the process to determine the maximum yields and the efficiency reaction. Aaron et al. demonstrated, by impedance spectroscopy, that the flow rate of the anode affects the resistance of an air cathode MFC. This study showed that an increase in anode flow rate produced an overall decrease in the total resistance of the cell [28]. This shows the importance of flow rate selection for the physical properties of the cell, but the flow rate can greatly affect the biological properties and conditions of the microbial electrochemical system. The shear stress due to the flow pattern in the cell is an important influence on biofilm formation. Franks et al. stated that low shear stress favors the formation of complex and thicker biofilms. They showed that the first 20 μm of the biofilm on an electrode have the highest metabolic activity, while the bacteria on the layer between 30 and 60 μm have lower activity [29]. In contrast, Pham et al. achieved highperformance biofilms with high shear stress. High shear stress benefits the high metabolic biofilm layer of the first micrometers from the electrode surface, due to the high diffusivity of protons and nutrients [30]. High flows can benefit the renewal of nutrients and decrease the stress produced by acidification. Flow rate and flow patterns influence the performance of METs; hence cell design must provide an adequate flow to allow nutrients without removing the biofilm. The volume of the reactor influences the treatment capacity by influencing the flow patterns, electrode contact time, and diffusive and osmotic forces. Not many attempts have been reported in the literature on the scaling up of METs for metal recovery. Tao et al. reported on a tubular reactor of 16 L and Rodenas Motos et al. reported on a 32 L electroplating bath. Those two attempts are the largest volumetric capacity METs built for metal recovery reported in the literature [24, 26] (Fig. 13.4).

Electrode surface area Most of the microbial electrochemical-based metal recovery systems use a biotic anode and an abiotic cathode where metals are deposited. The anode surface, where the biofilm grows, requires a high surface area to maximize the electronic interactions between the microbe and the electrode. The most common materials used for these electrodes are graphite or carbon due to their conductivity and biocompatibility. Felts or brushes are often used to increase the surface area. Carbon or graphite felts allow microbes to grow on the surface of the fibers, while the space between them allows water flow. An example of the advantage of felts can be seen in the cell design by Rodenas et al.; unlike previous work by ter Heijne et al., where a flat graphite electrode was used, graphite felt was able to obtain higher current densities [15, 25]. On the other hand, the abiotic cathode requires less surface area due to the fast kinetics of the reduction of metals, as compared to biological oxidation. The cathode material is usually stainless steel, graphite, copper, or aluminum [15, 25, 27].

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w/o DNA

E. coli

G. sulfurreducens 16S rRNA

PEM Live/dead assay Dead Live

Anode

(B)

(A)

Framework

(a) Membrane

Framework

Current Framework collector Graphite felt Membrane

Cathode

Pressure regulator valve

Air pump (Start-up period only)

(b)

Framework

CC1

SC2

CC2

SC3

CC3

SC4

CC4

SC5

CC5

SC6

CC6

C1 Data acquisition system

C2 C3 C4 C5

External resistance

SC1

Biogas

Separator

Framework

(C)

Copper electrode

Abbreviation: C: Cathode

Spacer Graphite felt

(D)

SA (PBM outlet)

A: Anode SC: Sampling port of cathode SA: Sampling port of anode CC: Cathode chamber AC: Anode chamber

AC

A Peristaltic pump

PBM N2

FIG. 13.4 (A) Example of H-Cell microbial fuel cell (MFC) [23]. (B) Biocassette system design for metal recovery in electroplating baths [24]. (C) Filter press type MFC [25]. (D) Membrane-less tubular reactor [26].

13. Metals recovery from wastewater by microbial electrochemical technologies

G. sulfurreducens

Inlet

180 W

Data acquisition system

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MFC

Outlet

Metal recovery with microbial fuel cells

Reactor materials The materials used for the construction of MET cells/reactors are polymers, graphite, carbon, and different metals as listed in Table 13.2. The polymeric structure is used for the support framework and for the membranes. As mentioned previously, graphite, carbon, or metals are used for electrodes and current collectors, while metals are also used for fasteners and structural support. The selection of proper materials has impact on the capital cost required to develop and implement the technology in the industry. Table 13.2 shows the cost of the different materials used on MFC and MEC reactors. For example, Zhang and Angelidaki reported the capital cost of MFCs and MECs around 5500 €/m3 [37]. This study takes into consideration a reactor design made of polycarbonate (PC) and a cathode with a Pt coating for oxygen catalysis, while metal recovery reactor designs by Rodenas et al. used stainless steel electrodes, decreasing the cost [24]. Operation of the reactor The design of the cell will directly affect the operational cost of the system. The cell design that considers the easy removal of the electrodes, like the one published in Rodenas et al. based on the plating baths, can help to decrease the time for collecting the plated metals [24]. It is

TABLE 13.2 Materials cost list required for microbial electrochemical technology metal recovery reactor design. Material

Cost

Ref.

Polytetrafluoroethylene

8.0 €/kg

[31]

Polycarbonate

2.70 €/kg

[32]

Stainless steel

2

35.7 €/m

[33]

Graphite plates

4.16 €/kg 31.3 €/m2

[34] [35]

Membranes

400 €/m2

Carbon felt/paper electrodes

62.55 €/m

[36] 2

[33]

291

estimated that a total annual cost for operating electroplating baths is around 34,368€ [38]. This operational cost includes the manpower required for the processes of cleaning and maintenance of the EW system. The operational cost of METs will be higher due to the formation of inorganic and biological scaling in the reactor due to bacterial activity.

Metal recovery with microbial fuel cells MFCs are driven by an exergonic process where cathode potential is higher than anode potential. Due to thermodynamics, the number of metals that are suitable for removal or recovery without the use of an external power source is small. MFCs can recover gold, silver, mercury, chromium, and copper. Only the reactions with a standard reduction potential greater than the electron donor potential can be recovered by an MFC. Besides the potential, the overpotential must also be considered, because the overpotential can poise the reduction potential below the electron donor potential. The five elements presented in the following paragraphs have more positive potentials than most of the electron donors reported in the literature. Some other metals, like nickel or lead, are also close thermodynamically (Fig. 13.2), but not favorable due to the overpotentials required for the reactions to take place.

Gold Gold can be found in its native state in nature due to its chemical properties [6]. However, Au(III) is a species that can be found in acidic environments like metal or electronic industry wastewater. Au(I) can also be formed in the presence of chloride or with complexing agents that can stabilize this ionic species [3]. Gold is one of the most valuable metals and one of the oldest to be extracted from mines. In its native state (reduced), it can be found in ore or

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13. Metals recovery from wastewater by microbial electrochemical technologies

sediments [5]. Gold can be leached from ore or sediment by thiourea or cyanide and then recovered in an acidic solution by reduction [39]. The reduction reaction is: Au3 + ðaqÞ + 3e $ AuðsÞ E° ¼ 1:52 V

½AuCl2  ðaqÞ + e $ AuðsÞ + 2Cl ðacÞ E° ¼ 1:15 V

Kalathil et al. recovered gold using an MFC with a current density of 0.198 A/m2, several orders of magnitude lower than the one used for traditional electroplating currents (100 A/m2) [40]. However, this gold recovery process does not require an external power source to recover the metal, being a less energy consuming process than traditional electroplating. Varia et al. also studied gold recovery from the kinetics and thermodynamics points of view, measuring the linear sweep voltammetry for gold deposition using an H-Cell reactor with Shewanella putrefaciens CN32 [16]. Chen et al. reduced Au(III) to form nanoparticles in the biofilm and observed an increase of 40% of anodic current density. Their work shows the capability of Geobacter sulfurreducens to perform a biomineralization process in the anode, increasing its acetate metabolism 2.2 times [41].

Silver Silver is one of the oldest metals mined by humanity. Like gold, silver can also be found in ores and extracted without complex technologies [6]. Silver recovery is attractive due to its extensive use in electronic components; e.g., in mobile phones silver content is approximately 0.54 mg/g. Silver is released to water bodies when electronic equipment is dumped or when silver is used as a biocide. It is possible to recover silver by melting the electronic scrap; however, it is difficult to recover once it is solubilized by acidification or released into the environment. In environments with elevated sulfur concentrations, like wastewater treatment streams, silver reacts and forms Ag2S. The source of the sulfur can vary from gases to organics to metal sulfide

minerals. Due to the stability of Ag2S, silver can strip sulfur from metallic sulfides that include zinc, iron, lead, and copper. The Ag2S has a large effect on the bioavailability and toxicity of silver in aqueous environments. Ag2S is less soluble than Ag+, by about 7-fold; this leads to a decrease in bioavailability and about 5.5-fold decrease in its toxicity [42, 43]. Given the toxicity of silver and its common use as a biocide, aqueous silver recovery is not only important for our aquatic environments but also as a profitable prospect. The most stable ionic species of silver is Ag(I); it is reduced by the following reaction: Ag + ðaqÞ + e $ AgðsÞ E° ¼ 0:799 V Silver reduction takes place on the cathode of an MFC. Several authors have recovered silver using METs, with removal efficiencies of up to 99.9% [44] and current densities from 4.25 to 5.67 A/m2 [45] (Table 13.3). A current density of 5.67 A/m2 is equivalent to a deposition of 22.84 g/m2h. At this current density, recovery rate, and a market price of silver of about 430 €/kg, these MFC could produce 9.82 €/m2h.

Mercury Mercury is a highly toxic heavy metal used in many industrial processes. Its uses can vary from high-temperature industrial processes to artisanal gold mining. Mercury is present in small concentrations in fuel sources, like coal and oil. Through all these varying avenues, mercury generally finds its way into the aquatic environment, whether transported from the atmosphere by rain or by direct contamination TABLE 13.3 Silver removal by microbial fuel cells. Voc

Jmax (A/m2)

Pmax (W/m2)

Removal (%)

Ref.

0.749

5.67

4.25

98.3

[45]

0.95

4.25

1.93

98.2

[45]

0.317

99.9

[44]

0.11

75

[46]

0.6 0.89

0.3

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Metal recovery with microbial fuel cells

or dumping into waterways [47]. Mercury use is banned in many countries, but artisanal and small-scale gold mining still accounts for 37% of the annual anthropogenic mercury released. The areas with the highest emission levels are East and Southeast Asia, sub-Saharan Africa, and South America [48]. Currently, the cheapest removal treatment for mercury is activated carbon adsorption, but it does not allow for mercury recovery and is not feasible for global application. An alternative to carbon adsorption is reducing mercury on the MFC cathode. The reduction reactions are as follows: 2Hg2 + ðaqÞ + 2e $ Hg22 + ðaqÞ E° ¼ 0:911 V Hg22 + ðaqÞ + 2e $ 2HgðlÞ E° ¼ 0:796 V Hg2 + ðaqÞ + 2e $ HgðlÞ E° ¼ 0:851 V Wang et al. were successful in the reduction of mercury at the cathode of an MFC, using acetate, in the form of artificial residential wastewater, as the electron donor and Hg2+ as the electron acceptor. The MFC achieved a maximal current density of 1.44 A/m2 and a power density of 433.1 mW/m2. With varying concentrations of Hg2+ from 25 to 100 mg/L, the system achieved removal efficiencies of 98.22%–99.54%. Mercury was mainly recovered as elemental mercury on the cathode or as a precipitate of Hg2Cl2 in the cathode chamber. This study shows the viability of the MFC, fueled by wastewater, to recovery mercury [49]. This is further confirmed by a 2012 patent awarded to C.S. Choi, for a method of mercury and other heavy metals recovery by electroplating using METs [50].

Chromium Chromium is a metal commonly used to make alloys, metal treatments, and metal plating. Chromium and stainless steel are used throughout the world in construction, industry, medicine, and scientific research. Cr(VI) is known to be a carcinogenic component commonly used in welding, electroplating, and

paints. Normally, chromium is reduced from Cr(VI) to Cr(III). Chromium has many soluble ionic species and each oxidation state has a different color going from purple to red to green [5]. Chromium comes from khr
oma, a Greek word that means color. This is why the reactions for complete reduction are the longest. Cr2 O7 2 ðaqÞ + 14H + + 6e $ 2 Cr3 + ðacÞ + 7H2 O E° ¼ 1:29 V Cr5 + ðaqÞ + e $ Cr4 + ðacÞ E° ¼ 1:34 V Cr4 + ðaqÞ + e $ Cr3 + ðacÞ E° ¼ 2:10 V Cr3 + ðaqÞ + e $ Cr2 + ðacÞ E° ¼ 0:424 V Cr2 + ðaqÞ + 2e $ CrðsÞ E° ¼ 0:790 V All categories of METs have been studied to remove Cr(VI). The large number of studies in the literature for removal and recovery are due to chromium’s environmental and human health risks [51, 52]. Table 13.4 shows a list of studies that removed chromium from water by MET. This table shows how current density has slightly increased from the first experiments in 2008 until today. Gangadharan et al. showed one of the highest current densities achieved for the removal of chromium and one of the highest removal percentages [56]. The 3.4 A/m2 current density achieved is equivalent to a removal of 4.7 g/m2h of dichromate salts from the water. Recently Gupta et al. showed promising current densities of 7.86 A/m2 and a power density of 1.53 W/m2 [59]. TABLE 13.4

Chromium removal by microbial fuel cells.

VOC (V)

Jmax (A/m2)

Pmax (W/m2)

Removal (%)

Ref.

–

2.5

0.53

97

[53]

0.79

0.02

0.0017

73

[54]

0.3

0.14

0.009

79.3

[55]

0.55

3.4

0.776

98

[56]

0.088

–

0.027

86.7

[57]

1.09

1.5

0.453

99

[58]

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13. Metals recovery from wastewater by microbial electrochemical technologies

Copper

TABLE 13.5 Copper removal by microbial fuel cells.

One of the metals of greatest interest is copper. This is due to its abundance, value, and high market demand for all kinds of products, from construction to electronics. Traditionally, once copper has been smelted, it is refined by EW at current densities between 100 and 200 A/m2. These current densities and the use of water as the electron donor indicate that 2.15 kWh/kg of energy is required for its final production. When copper is recycled from scrap, it is easily separated from other metals, due to its characteristic color, and then melted. Copper recycling requires large amounts of energy for smelting and even repetition of the EW process for lower-grade scrap. In contrast, METs can produce copper with low energy consumption, while simultaneously treating wastewater with high COD content. This makes METs an energysaving technology that has additional advantages over the more traditional technologies like EW. Table 13.5 shows that copper can be plated at current densities of 23 A/m2, which is an order of magnitude lower than EW. The efficiency of the metal recovery is 99.97%, showing promising results for this novel technology. Copper reduction reactions are as follows:

VOC (V) Jmax (A/m2)

Cu2 + ðaqÞ + 2e $ CuðsÞ E° ¼ 0:337 V Cu2 + ðaqÞ + e $ Cu + ðacÞ E° ¼ 0:159 V Cu + ðaqÞ + e $ CuðsÞ E° ¼ 0:520 V CuðNH3 Þ4 2 + ðaqÞ + e $ CuðNH3 Þ2 + ðacÞ + 2NH3 E° ¼ 0:10 V Cu2O(aq) + H2O + 2e $ 2 Cu(s) + 2 OH E ° ¼  0.360 V

Metal recovery with microbial electrolysis cells Through electrolysis, all metals can be plated in their elemental form using water as a solvent. More exotic electrolytes can be used to handle

Pmax (W/m2) Removal (%) Ref.

0.63

12 (A/m3)

0.85 (W/m3) 82.8

[60]

0.42

26.9 (A/m3)

6.5 (W/m3)

99.7

[61]

–

0.57

0.45

63.7

[62]

–

0.47

–

[63]

51

[64]

–

141.3 (A/m ) 18.8 (W/m ) 79

[65]

0.32

0.09

0.128 3

3

38(A/m )

0.68

4.8 (W/m ) 3

3

–

0.007 3

3

[66]

0.64

45 (A/m )

4.62 (W/m ) 28

[67]

0.485

23

5.5

–

[25]

0.45

5.5

1.7

87

[68]

0.4

0.34

–

84.3

[69]

0.58

0.95

0.2

96

[70]

0.48

0.143

0.016

96

[71]

-

0.142

0.022

70

[72]

0.595

1.2

0.319

99.97

[73]

0.61

6.2

0.8

99.88

[15]

metals requiring more negative potentials and to avoid the potential for hydrogen evolution. In this section, we present a list of metals studied due to their high toxicity and environmental risk.

Cobalt Cobalt is an essential element for life and is the central element in vitamin B12. In contrast, cobalt can be toxic in doses over 20 mg/m3. Cobalt is also used commercially as a pigment, catalyst, in batteries and alloys, and as a radioisotope. A clear example is the use of cobalt in lithium batteries [74]. Cobalt recovery is of great interest due to its high market price and many uses in multiple industries. The reduction reactions are: Co2 + ðaqÞ + 2e $ CoðsÞ E° ¼ 0:280 V

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Metal recovery with microbial electrolysis cells

TABLE 13.6 Cobalt removal by microbial electrolysis cells.

TABLE 13.7 Cadmium removal by microbial electrolysis cells.

Vin (V) Jmax

Removal (%) Authors

Vin (V) Jmax (A/m2) Pmax (W/m2) Removal (%) Authors

51

[64]

0.5

2.5

1.25

46.6

[79]

0.7 (A/m ) 0.102 (W/m ) 28

[67]

1.173

13.05

10.22

95

[80]

[75]

0.316

11.7

3.7

90

[81]

88.1

[76]

0.6

1.689

1.01

62

[69]

92.7

[77]

0.68 0.64 0.36 0.24 0.25

Pmax

38 (A/m3) 4.8 (W/m3) 2

2

3

3

3

3

26 (A/m ) 1.5 (W/m ) 21 (A/m ) 3.7 (W/m ) 2

2

0.2 (A/m ) 0.05 (W/m )

Nickel As seen in Table 13.6, MECs have a maximum recovery efficiency of 92.7% as shown by Huang et al. [77]. This was achieved with current densities three orders of magnitude lower than conventional cobalt EW [78]. With energy savings of this magnitude over traditional methods, the MEC has great potential once properly scaled up.

Cadmium Cadmium is a toxic metal used in batteries, pigments, and for high-performance automotive, aerospace, and military plating applications. It has been included in the Restriction of Hazardous Substances Directive 2002/95/EC since 2003. Its use is only allowed for printed circuits, bulbs, batteries, glass, paints, pigments, PVC cables, and metal parts. Cadmium telluride is present also in some solar panels and photosensitive systems where cadmium telluride is the semiconductor excited by light. A common source of cadmium pollution is the improper disposal of rechargeable batteries. These NiCad batteries corrode if they are not properly recycled and can release a large amount of cadmium and nickel into the surrounding environment. Cadmium reduction is: Cd2 + ðaqÞ + 2e $ CdðsÞ E° ¼ 0:400 V Table 13.7 shows the state of the art of cadmium recovery with METs. The MECs have achieved recovery efficiencies up to 95% and with current densities over 10 A/m2.

Nickel is a common component in many alloys and batteries. It is mainly used to produce stainless steel and NiCad batteries. Wastewaters from metallurgical industries contain an elevated nickel concentration that is normally removed by precipitation, flotation, or EW. Nickel is a difficult metal to plate due to its electrochemistry. Nickel oxidizes almost immediately in contact with air, forming a green layer. The passivation process of its surface prevents nickel from oxidizing further, as with many other metals. This is a desirable property for use in commercial products, but this passivated surface also increases the overpotential. Its overpotential shifts the electrode potential closer to hydrogen, but by increasing the nickel ion concentration, this will effectively lower the overpotential required for the electroplating. Consequently, nickel recovery from wastewater requires a concentration step before EW. The use of additives to obtain a valuable product for the metal market is also required, due to the easy oxidation of the electrode surface. For that reason, the selection of the electroplating solution is essential. There is an alternative method for nickel recovery with MECs; these systems have current densities between one to two times lower than the EW process. In one case the removal efficiency was only 67%, compared to around 95% removal by other authors; see Table 13.8. Ni2 + ðaqÞ + 2e $ NiðsÞ E° ¼ 0:250 V

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13. Metals recovery from wastewater by microbial electrochemical technologies

Zinc

TABLE 13.8 Nickel removal by microbial electrolysis cells. Vin (V) Jmax

Pmax

Removal (%) Ref.

0.598 (A/m2) 0.184 (W/m2) 94.8

1.2

3

304 (A/m )

1.1

3

54.0 (A/m )

3

334 (W/m )

[82]

67

[83]

95

[84]

Lead Lead is one of the most toxic metals due to its ability to accumulate in living organisms. Though lead is only deadly in large doses, in humans lead accumulation can lead to a disease known as saturnism. This disease has been known since antiquity. It was known to afflict Roman aristocracy due to their exposure while drinking contaminated wine, since lead was used in ceramic covers of wine jars [85, 86]. Many artists, from Michelangelo to van Gogh, showed symptoms of lead poisoning caused by exposure to paints containing lead. Lead pipes have also been used in households due to the malleability of lead [87]. These pipes are still common throughout the developed world. One example of this is in the city of Flint, Michigan (United States), where a change in the water-supplying body and an increase in chlorination caused lead to leach from pipes. This drastically increased lead levels in the drinking water and caused a public health crisis [88]. Like nickel, lead is a difficult metal to electrochemically recover due to its reactivity. Lead also passivates similarly to nickel. To decrease the overpotential, a preconcentration step is required before recovery. Lead reduction reactions are: Pb2 + ðaqÞ + 2e $ PbðsÞ E° ¼ 0:250 V Lead was recovered successfully in 2012 by Modin et al. at a current density of 1.69 A/m2 and a removal efficiency of 47.5% [69].

Zinc is one of the most common metals used for alloys, batteries, and electroplating or galvanization. In both galvanization and refining of zinc by EW, hydrogen is often evolved from the plating electrodes. Hydrogen evolution while plating zinc has its advantages and disadvantages. The main disadvantages are a reduction in plating efficiency and in the quality of the plating by embrittlement, caused by microfractures in the plating surface. The advantage of hydrogen evolution is that it can be recovered as fuel to offset some of the energy costs in the plating process. This evolution could also be an advantage for combining metal recovery efforts with MET. In MFCs hydrogen has been used as an electron donor for copper recovery by Ntagia et al. and Rodenas et al. [89, 90]. Zn2 + ðaqÞ + 2e $ ZnðsÞ E° ¼ 0:762 V Table 13.9 shows a list of studies in the removal and recovery of zinc via a MEC. The most successful was by Teng et al., who achieved current densities of 50 A/m2 with a removal of 99% of the zinc from the solution [92].

Metal recovery by microbial reactions The most extensive process to recover metals is to bind the metallic ions, forming a complex or a precipitate with a counterion or a specific TABLE 13.9 Zinc removal by microbial electrolysis cells. Vin (V)

Jmax (A/m2)

Pmax (W/m2)

Removal (%)

10.8

Ref. [91]

0.7

50

35

99

[92]

0.472

0.786

0.372

56

[93]

0.316

11.7

3.7

97

[81]

0.8

1.689

1.35

44.2

[69]

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Metal recovery by microbial reactions

molecule. These binding molecules can be produced by a microbial reaction either in the waste-stream contaminated with heavy metals or in a separate reactor.

Precipitation Precipitation is one of the most-used techniques to recover metals. The most common precipitating agent is a sulfide. Sulfide is produced by many means, but most commonly it is formed by sulfate-reducing bacteria. The reduction of sulfate to sulfide on the surface of an electrode via a direct inorganic reaction is not feasible. However, bacteria carry out this reaction by consuming an electron donor like hydrogen or acetate [94]. Another common precipitating agent is the CaCO3 produced by the carbonation of media through biological activity [95, 96].

Complexation Metal complexation or chelation is another method for recovery. The concept is to introduce a specific molecule into the media that binds the metal and changes its chemical properties, enabling recovery of the metal by concentration, membrane separation, flotation or coagulation. The most common chelating agents in the metallurgy industry are thiourea, citric acid, tartaric acid, and oxalate. Thiourea is a known biocidal agent, though oxalate and other organic acids are often used. Aspargillus niger is a fungus known to produce oxalate. The cleavage of oxaloacetate produces equimolar quantities of oxalate and acetate. Oxalate is also produced by oxidation of glyoxylic acid. Oxalate has been used successfully to recover zinc, cobalt, and copper from acidic solutions [97, 98]. Other natural chelating agents like tartaric and citric acid have also been tested successfully. These agents are also synthesized by fungi. However, the drawback to these organic acids is their strong antimicrobial activity.

297

Another example of metal recovery is the formation of nanoparticles of gold and silver using Apiin [99]. Apiin is an extract from henna leaves that allows the chelation of the metal and controls the formation of nanoparticles. Other chelating agents are extracted from hibiscus, magnolia, and similar plants [100].

Nanoparticle formation on microbial membranes The accumulation of metals in membranes and cell walls is a defense mechanism of bacteria from the poisoning effect of metals. The detoxification by microbes takes place either by an intracellular bioaccumulation, extracellular biomineralization, biosorption, complexation, or precipitation. Intracellular formation There are many examples of microbes that can produce nanoparticles intracellularly. Table 13.10 shows examples of gold nanoparticles formed by species like Bacillus subtilis 168, Geobacter ferrireducens, Shewanella algae, Plectonema boryanum UTEX485, Escherichia coli DH5α, and Rhodobacter capsulatus. Silver has been recovered using Pseudomonas stutzeri AG259, Bacillus sp., Lactobacillus sp., and Corynebacterium sp. SH09. These nanoparticles accumulate either in the periplasmic space or in membrane vesicles. The bacteria produce liposaccharides and phospholipids for protection from the toxicity. Another mechanism is to reduce the metal ion by using it as an electron acceptor for respiration. Extracellular formation A list of microbes that can recover metals extracellularly using their extracellular polymeric substances (EPS) was provided by Narayanan et al. (see Table 13.11). The location of the nanoparticles depends on where the

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13. Metals recovery from wastewater by microbial electrochemical technologies

TABLE 13.10

List of bacteria that synthesize metal nanoparticles by Narayanan and Sakthivel [101].

Microorganism

Nanoparticle

Localization/morphology

Size

Ref.

Bacillus subtilis 168

Au

Octahedral inside cell wall

5–25 nm

[102]

Sulfate-reducing bacteria

Au

Cell envelope

<10 nm

[103]

Shewanella algae

Au

Periplasmic space, bacterial envelope

10–20 nm

[104]

Bacterium-intracellular

15–200 nm Plectonema boryanum UTEX485

Au

Membrane vesicles/cubic

10 nm a

[105]

Escherichia coli DH5α

Au

Cell surface/spherical

ND

[106]

Rhodobacter capsulatus

Au

Plasma membrane

ND

[107]

Pseudomonas stutzeri AG259

Ag, Ag2S

Periplasmic space

<200 nm

[108]

Corynebacterium sp. SH09

Ag

Cell wall

10–15 nm

[109]

Bacillus sp.

Ag

Periplasmic space

5–15 nm

[110]

Lactobacillus sp.

Au, Ag, AuAg

Hexagonal/contour

20–50 nm

[111]

Pseudomonas aeruginosa SNT1

Se

Spherical/contour

ND

[112]

Shewanella. algae

Pt

ND

5 nm

[113]

Desulfovibrio desulfuricans

Pd

Cell surface

50 nm

[114]

Shewanella. oneidensis MR-1

Pd

Periplasmic space

ND

[115]

Aquaspirillum magnetotacticum

Fe3O4

Octahedral prism

40–50 nm

[116]

Magnetotactic bacterium MV-1

Fe3O4

Cell inside/parallelepiped

40  40  60 nm

[117]

Magnetotactic bacterium

Fe3S4, FeS2

Octahedral/Cubo-octahedral

7.5 nm

[118]

Sulfate-reducing bacteria

FeS

Cell surface

2 nm

[119]

Magnetospirillum magnetotacticum

Fe3O4

Membrane-bound/cubo-octohedrons

47.1 nm

Magnetospirillum. magnetotacticum ( MS-1)

Fe3O4

Inside the cell/cuboctahedral

50 nm

[120]

Magnetospirillum gryphiswaldense

Magnetite

Membrane-enclosed/cubo-octahedral elongated hexagonal prismatic

35–120 nm

[121]

Desulfosporosinus sp.

UO2

Cell surface

1.5–2.5 nm

[122]

Clostridium thermoaceticum

CdS

Cell surface

ND

[123]

Klebsiella pneumoniae

CdS

Cell surface

5–200 nm

[124]

Escherichia coli

CdS

Spherical, elliptical

2–5 nm

[125]

Desulfobacteriaceae

ZnS

Spherical

2–5 nm

[126]

NCIMB 8307

a

ND: not defined.

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Metal recovery by microbial reactions

TABLE 13.11

List of bacteria that synthesize metal nanoparticles by Narayanan and Sakthivel [101].

Microorganism

Nanoparticle

Localization/morphology

Size

Ref.

Au

Spherical

10–20 nm

[127]

Triangular nanoplates

50–400 nm

Spherical nanowires

50–60 nm

[128]

Bacterium-extracellular Rhodopseudomonas capsulata

a

Pseudomonas aeruginosa

Au

ND

15–30 nm

[129]

Bacillus megatherium D01

Au

Spherical

1.9  0.8 nm

[130]

Aeromonas sp. SH10

Ag

ND

6.4 nm

[131]

Enterobacter cloacae, Klebsiella pneumonia, Escherichia coli

Ag

ND

52.5 nm

[132]

Bacillus licheniformis

Ag

ND

50 nm

[133]

Acetobacter xylinum

Ag

Cellulose fibre

ND

[134]

Morganella sp.

Ag

Spherical

20  5 nm

[135]

Sulfurospirillum barnesii, B. selenitireducens, Selenihalanaerobacter shriftii

Se

Nanospheres

300 nm

[136]

Bacillus selenitireducens

Te

Nanorods

10 nm

[137]

Sulfurospirillum barnesii

Te

Irregular Nanospheres

<50 nm

Lactobacillus sp.

Ti

Spherical

40–60 nm

[138]

Physoceras boryanum UTEX 485

Pt

Spherical, Chains, Dendritic

30 nm–0.3 μm

[139]

Geobacter metallireducensGS-15

Magnetite

ND

10–50 nm

[140]

Thermophilic bacteria TOR-39

Magnetite

Octahedral

<12 nm

[141]

Thermoanaerobacter ethanolicus (TOR-39)

Co, Cr, Ni-substituted-

Octahedral

ND

[142]

Actinobacter sp.

Magnetite

Quasispherical

10–40 nm

[143]

b

Shewanella oneidensis MR-1

UO2

Extracellular (UO2-EPS )

1–5 nm

[144]

Klebsiella aerogenes

CdS

Spherical on cell wall

20–200 nm

[145]

Rhodopseudomonas palustris

CdS

Spherical

8.01  0.25 nm

[146]

Gluconoacetobacter xylinus

CdS

Cellulose fiber

30 nm

[147]

Rhodobacter sphaeroides

ZnS

Spherical

8 nm

[148]

Rhodobacter sphaeroides

PbS

Spherical

10.5  0.15 nm

[149]

Brevibacterium casei

Co3O4

ND

5–7 nm

[150]

a b

ND: not defined. EPS: extracellular polymeric substances.

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13. Metals recovery from wastewater by microbial electrochemical technologies

reductive enzymes or the enzymes involved in the reductive process are located [101]. Table 13.11 also shows that bacteria like Rhodopseudomonas capsulata can turn the nanoparticles from the same element into different shapes. At pH 7 the nanoparticles are spherical, while at pH 4 they are triangular. In their work, He et al. discussed that the reduction power relates to pH, which in turn plays an important role in the concentration of AuCl 4 [127]. These factors affect the rate of nanoparticle formation, which defines the final shape. The nanoparticles formed by bacteria are not only applicable to remediating or recovering metals, but bacteria can also make nanoparticles that can be applied in medical treatments or in several technological applications, such as optoelectronic devices (screens or photovoltaic [PV] panels) [151, 152].

Future considerations for metal recovery Benetton et al. has coined the new term microbial electrochemical metallurgy (MEM) as a new field in METs [2]. As they reviewed it, the evolution of MEM shows promising results for metal recovery. Most of the literature reviewed in this chapter and by Benetton et al. showed a prevalence of type A cells, where the microbial activity is separated from the reduction of metals [2]. However, as reported here, many bacterial strains have the capability to reduce metals directly in their environment, either extracellularly, intracellularly, or by generating substances to precipitate them. An example is the UO2 formation by Shewanella oneidensis that is a well-known electroactive microbe [144]. The future of MEM seems to be in the development of Type D cells where a biocathode with reducing bacteria can work simultaneously with a bioanode in the oxidation of an electron donor to minimize the energy requirements of the recovery process and increase metal recovery rates [2, 153, 154].

References [1] A.L. Lehninger, Role of metal ions in enzyme systems, Physiol Rev. 30 (3) (1950) 393–429. PMID:15430152. [2] X. Dominguez-Benetton, J.C. Varia, G. Pozo, O. Modin, A. Ter Heijne, J. Fransaer, et al., Metal recovery by microbial electro-metallurgy, Prog. Mater. Sci. 94 (2018) 435–461, https://doi.org/10.1016/j.pmatsci. 2018.01.007. [3] Y. Chehade, A. Siddique, H. Alayan, N. Sadasivam, S. Nusri, T. Ibrahim, Recovery of gold, silver, palladium , and copper from waste printed circuit boards, in: International Conference on Chemical, Civil and Environment Engineering (ICCEE’2012) March 24–25, Dubai, 2012. [4] R.W. Peters, Y. Ku, Evaluation of recent treatment techniques for removal of heavy metals from industrial wastewaters, in: American Industrial Chemical Engineering Symposium Series, Environmental Engineering Department, School Of Civil Engineering, 1985. [5] F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochman, Advanced Inorganic Chemistry, sixth ed., John Wiley & Sons, Inc, 1999. [6] G. Rayner-Canham, Descriptive Inorganic Chemistry, second ed., W.H. Freeman & Co Ltd, New York, United States, 1999. [7] H.H. Tabak, R. Scharp, J. Burckle, F.K. Kawahara, R. Govind, Advances in biotreatment of acid mine drainage and biorecovery of metals. 1. Metal precipitation for recovery and recycle, Biodegradation 14 (2003) 423–436. [8] P.I. James, M.R. Baker, Profitable copper production from low-grade waste ores, Min. Eng. 67 (4) (2015) 61–67. [9] Z. Wu, W. Yuan, J. Li, X. Wang, L. Liu, J. Wang, et al., A critical review on the recycling of copper and precious metals from waste printed circuit boards using hydrometallurgy, Front. Environ. Sci. Eng. 11 (8) (2017) https://doi.org/10.1007/s11783-017-0995-6. [10] M.E. Schlesinger, M.J. King, K.C. Sole, W.G. Davenport, G. William, Extractive Metallurgy of Copper, Elsevier, 2011. [11] M.A. Barakat, New trends in removing heavy metals from industrial wastewater, Arab. J. Chem. 4 (2011) 361–377, https://doi.org/10.1016/j.arabjc.2010.07.019. [12] T.H.J.A. Sleutels, H.V.M. Hamelers, R.A. Rozendal, C. J.N. Buisman, Ion transport resistance in microbial electrolysis cells with anion and cation exchange membranes, Int. J. Hydrog. Energy 34 (2009) 3612–3620, https://doi.org/10.1016/j.ijhydene.2009.03.004. [13] C. Choi, Y. Cui, Recovery of silver from wastewater coupled with power generation using a microbial fuel cell, Bioresour. Technol. 107 (2012) 522–525.

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