Bioleaching for Recovery of Metals from Spent Batteries – A Review

ABSTRACT This review addresses the recovery of metals from all the main types of spent batteries (including Li-based, Zn-based, and Ni-based batteries) using bioleaching processes. Traditional processes are based on pyrometallurgical and hydrometallurgical methods and can be costly and, in some cases, cannot effectively recover the metals used in battery manufacture. Biobased hydrometallurgy may circumvent some of these drawbacks. This review presents a classification of batteries based on chemical composition from a recycling point of view. The bacterial and fungal leaching for each type of battery is reviewed. The research outputs are compared in each section, and chemical and biological mechanisms are presented. In addition, the catalytic effect of metals on the bioleaching process is reviewed for each battery. The results indicated that the main bioleaching mechanisms are the reactions of metals with metabolites, acid dissolution, and their combinations with oxidation-reduction reactions. The reactions presented in the literature are assessed and modified thermodynamically. The advantages and disadvantages of batteries bioleaching are discussed, and a future perspective is drawn. Although battery bioleaching has high efficiency and is an environmentally friendly method, with no gas emission and low energy consumption, slow kinetics and low pulp densities are among the major limiting factors hindering bioprocessing development from pilot to commercial scale.


Introduction
With the increasing use of mobile and cordless electronics, the disposal of spent batteries has also risen considerably.Batteries contain toxic metals such as Li, Co, Ni, Cd, Mn, Zn, and organic components (e.g.ethylene carbonate, dimethyl carbonate, diethyl carbonate) (Bal, Ghosh, and Das 2019;Li et al. 2010;Lopez, Akizu-Gardoki and Lizundia 2021).Thus, spent batteries pose a disposal hazard if discarded in the environment or landfills.Some batteries contain critical metals that are not abundantly available in the global market.The limited availability of these critical metals justifies the development of efficient metal recycling processes for spent batteries.
Two main approaches to metal recovery from batteries have been proposed: pyrometallurgical and hydrometallurgical methods.In the pyrometallurgical approach, hightemperature processes are used to recover metals from battery waste.This method can be applied to recover Co, Ni, Cu, and Fe, but not Li (Chandra et al. 2022;Huang et al. 2018;Liu et al. 2019;Meshram and Pandey, 2019).Hydrometallurgical techniques involve metal dissolution in inorganic or organic acids, followed by solvent extraction, precipitation, or electrochemical deposition (Asadi Dalini et al. 2021;Le and Lee 2021;Meng et al. 2021;Rezaei et al. 2022;Zheng et al. 2018).It was documented that bioleaching is cost-effective compared to the hydro/pyrometallurgical process.It was indicated although biological routes have similar total capital investment costs to chemical routes, the total operational costs of chemical and typical hybrid routes are substantially higher than bioprocess (Borja et al. 2016;Esmaeili et al. 2020;Moazzam et al. 2021;Pollmann et al. 2016).Technology transfer makes it possible to adopt these bioprocesses for recovering metals from spent batteries, an emergent field of bioprocessing.This review addresses microorganisms, thermodynamic and kinetic analysis of reaction mechanisms, and the advantages and disadvantages of these bioprocesses.

Classification of batteries
Batteries are classified as non-rechargeable (primary disposable batteries) and rechargeable (secondary batteries).Based on their metal content, batteries are grouped as Li-ion batteries, Zn-Mn dry and alkaline batteries, Pb-acid batteries, Ni-Cd batteries, and nickel-metal hydride (NiMH) batteries (Bernardes, Espinosa, and Tenório 2004;Provazi et al. 2011;Rogulski and Czerwiński 2006;Zeng, Li and Singh 2014).Disposable batteries are recyclable and classified into three groups: Li-based, Zn-based, and Ni-based batteries (Figure 1).There is no research on the bioprocessing of Pb-based and Hgbatteries to date.
Equations ( 1) and ( 2) show the half-cell reactions at the anode and cathode of a common Li-ion battery, respectively.During discharge, Li released from lithiated graphite (LiC 6 ) moves from the anode to the cathode through electrolyte, while the electrons reach the cathode in an external route to form LiCoO 2 .Thus Co 4+ is reduced to Co 3+ .Based on the cohesive and bond free energy energies and the Gibbs free energy of formation, Li in the cathode is stable as LiCoO 2 (reactions 1 and 2), and the reaction is not reversible to LiC 6 .However, during battery charging, the reverse reactions transfer Li to the thermodynamically less favorable LiC 6 (Klaus 2018).
Different processing routes in pyrometallurgy, hydrometallurgy, and their combinations are employed to recycle Libatteries.In the pyrometallurgical method, spent batteries are fed into a shaft furnace with three different temperature zones: evaporation of electrolyte (~300° C), plastic pyrolysis to incinerate plastic components (700° C), and smelting, which produces an alloy containing Cu, Co, Ni, and Fe, as well as waste slag.The slag also contains Al, Si, Ca, and Fe (King and Boxall 2019).However, the process is suitable for recycling other metals (Co especially) but not Li.
Hydrometallurgical processes have been developed and implemented to recover Li from spent batteries.These processes require mechanical pretreatments such as shredding, milling, and screening to reduce and separate organic from inorganic materials.Metals are dissolved from sorted Libattery electrodes in organic or inorganic acid solutions.Oxidants or reductants may be used to increase the metal recovery and leaching rates, especially for Co. Liu et al. (2019) showed inorganic acid solutions outperformed organic acids in the leaching cathodes for Co and Li recovery.The process's cost, energy consumption, and CO2 emissions also favored the dissolution of inorganic acids.Dissolved metals can be recovered by various combinations of precipitation, solvent extraction, ion exchange, and electrowinning, all established in conventional hydrometallurgy.

Zn-based batteries
Zn-based batteries, including alkaline Zn-Mn and Zn-C batteries, usually supply power for small, low-energy devices such as radios, remote controls, toys, and watches.Zn-Mn batteries are not rechargeable and run out of power in a relatively short period.They contain Hg, Cu, Zn, and Mn (Calin et al. 2021), and the 12-28% Zn and 26-45% Mn increase their recycling potential (Sayilgan et al. 2009).
A typical Zn-Mn cathode contains 79-90% MnO 2 , acting as an oxidizing component, and 2-10% graphite for electronic conductivity.The electrolyte is 7-10% aqueous KOH.The main composition of the anode includes 55-70% Zn, 25-35% aqueous KOH (in the form of a solution with 35-52% potassium hydroxide), and 0.4-2% gelling agent.Reaction 3 shows that MnO 2 is reduced to MnOOH when discharge occurs.The reaction is not reversible, and Zn batteries are not rechargeable.Pyrometallurgical processes include selective metal volatilization and condensation.They are widely used to recycle Zn, Mn, and steel alloys (Belardi et al. 2012;Mocellin et al. 2017;Sun et al. 2016).Evaporation and reduction at 750-1500°C of Zn-Mn battery scrap are also practiced in the battery recycling industry (Hu et al. 2021).
Comminution and separation of steel and plastic are necessary to minimize impurities (Sayilgan et al. 2009).Sulfuric acid leaching can be used to dissolve Zn, Cu, Ni, Cd, and Hg, followed by solvent extraction for Zn recovery and selective ion exchange for other metals.In principle, this can be used to process spent Zn-C, alkaline Mn, and Ni-Cd batteries (Lannoo et al. 2019).Electrochemical techniques are also available to recover metals from solvents.Other products from the sulfuric acid process are MnCO 3 obtained by precipitation and a mixture of manganese oxide and carbon black (Fröhlich and Sewing 1995).

Ni-based batteries
The Ni-Cd batteries are rechargeable and used in a wide range of applications, including portable equipment for wireless communication and computing devices (Bajpai and Dash 2012;Mahandra, Singh, and Gupta 2018).The main component of the cathode is Ni-oxyhydroxide (NiOOH), which is reduced to nickel hydroxide (Ni(OH) 2 ) during the discharge (Equation ( 4)).Graphite in the cathode improves conductivity, and Co (and other metals) in metallic and hydroxide forms increases the cathode life and capacity.Metallic Cd in the anode becomes cadmium hydroxide during discharge (Equation ( 3)).Ni is used in the anode to improve conductivity and prevent crystal growth and agglomeration during battery charge (Dewulf et al. 2010;Morioka 2001).KOH solution has been used as an electrolyte to wet internal cell components (Chakankar, Jadhav, and Hocheng 2017).The overall reaction of discharge and charge is presented in Equation ( 4): Ni-Cd batteries cathode contain 47% Ni and 48% Cd (Cerruti, Curutchet, and Donati 1998).NiMH batteries are comprised of approximately ~40% Ni and Co (~3%) and rare earth metals (8-10% of La, Ce, Pr, and Nd) (Müller and Friedrich 2006).
Dumping and incineration of Ni-Cd batteries pose environmental health risks (Shi et al. 2019;Soria et al. 2001).Pyrometallurgical processes have been developed for Ni-Cd batteries based on Cd distillation in a closed furnace to yield metallic Cd powder, and Ni can be reduced and smelted in an electric furnace to produce metallic nickel (Espinosa et al. 2004).Hydrometallurgical methods can process mixtures of spent Ni-Cd, Zn-C, alkaline Mn, and Li batteries.Physical separation is first employed for size reduction, iron scrap separation, and separation of organic matter from metals.Sulfuric acid selectively dissolves non-Mn metals and noncarbon materials.Zn is recovered by solvent extraction and Cu, Ni, and Cd by ion exchange.Zn, Cu, Ni, and Cd are then recovered by electrolysis and Mn by precipitation (Tanong et al. 2017).

Bioleaching of batteries
Numerous acidophilic iron-and sulfur-oxidizing microorganisms have been identified and characterized from acidic mine water samples and acid bioleaching processes (Chen et al. 2016;Ghassa et al. 2015).Fe 3+ acts as a chemical oxidant, and is reduced to Fe 2+ in the reaction with a sulfide mineral.Microbial reoxidation of Fe 2+ to Fe 3+ ensures continuing Fe 3+ -mediated oxidation and leaching capacity.The oxidation of sulfur compounds produces sulfuric acid and helps maintain acid conditions.Under ambient conditions, the chemical oxidation of Fe 2+ to Fe 3+ at low pH is too slow, but acidophilic microorganisms increase rates by 10 5 -10 6 times (Ghassa et al. 2017).Redox reactions of metals and sulfur are central in microbial bioleaching (Equations ( 5)-( 7)).
Heterotrophs, mostly Aspergillus and Penicillium spp., have also been tested in bioleaching processes (Asghari et al. 2013;Pathak et al. 2021;Shah et al. 2020).Dissolution of metals from minerals by fungi is attributed to three reactions: (i) acidolysis of the mineral, (ii) metal sequestration and complex formation, and (iii) metal redox reactions.In acidolysis, metals dissolve due to a proton attack on the metal.In sequestration, organic acid metabolites (especially carboxylic acids) dissolve metals by forming stable complexes (Equation ( 8)).Metal complexes are formed on the surfaces by ligand exchange, polarizing critical bonds (Brandl, Bosshard, and Wegmann 2001).In the redox reactions, the solubility of metals increases due to electron exchange between metals (Equation ( 9)).

M = metal
The target metals vary with the battery through the bioleaching of waste batteries.The solubilities vary with the target metals and their redox states.Bioleaching conditions are acidic to increase the solubilities, and in some cases, the metal dissolution is based on an H+ attack.Thus, the process may promote microorganisms that oxidize S 0 to sulfuric acid.Spent batteries and battery waste exhibit acid demand, which should be satisfied for an acid bioleaching process.When the battery waste contains Co and Mn, these metals must be first reduced (Co 3+ + e-→ Co 2 + , Mn 4+ +2e-→ Mn 2+ ) to soluble redox states.In comparison, Ni is directly oxidized to Ni 2+ by proton attack.This oxidation requires a reductant, which may be an intermediate in the pathway of sulfur oxidation or Fe 2+ which is oxidized to Fe 3+ in reaction.High concentrations of Fe 3+ provide an oxidative environment.Fe 2+ is oxidized to Fe 3+ by iron-oxidizing acidophiles, considered fundamental in the bioleaching of sulfide minerals.Mishra et al. (2008) used Acidithiobacillus ferrooxidans (A.ferrooxidans) with S 0 and Fe 2+ as the energy sources to leach LiCoO 2 cathodes.At pH 2.5 and 0.5% pulp density, the metals recoveries were 65% Co and 10% Li in 25 d.Increasing the pulp density to 1% decreased the Co leachability to 56%.The mode of the bacterial action in the process was not clear in this study.Iron oxidation clearly enhanced Li dissolution (Figure 2).The dissolution of Co required a reduction of Co 3+ to Co 2+ .

Bacterial leaching
For cobalt leaching from LiCoO 2 cathodes, Zeng et al. ( 2012) used a Cu-catalyzed bioleaching process with A. ferrooxidans and Fe 3+ .The bacteria were first subcultured to adapt them separately to Cu 2+ (10 g/L) and 1% spent LiCoO 2 cathodes.99.9% of Co was leached with 750 mg/L Cu 2+ in 6 d.In the absence of Cu 2+ , 43% of Co leached in 10 d.Dissolution of Li was not specifically determined, but the residual was <1% of the original weight.The Cu catalyst effect of copper was explained by the exchange of Cu for Li in LiCoO 2 , forming a putative intermediate CuCo 2 O 4 , in which Fe 3+ reduced the trivalent cobalt to the soluble Co 2+ form (Equations ( 10)-( 12)) However, it is not clear whether the Cuintermediate is stable because it has not been confirmed analytically.In addition, in the suggested reaction (Equation ( 11)), both Fe 3+ and Co 3+ are reduced, which means that this reaction cannot be correct.The correct reactions for different ions will be suggested in section 3.4.4,based on thermodynamic modeling.13)-( 14)) (Zeng et al. 2013).A polytype of AgCoO 2 is an efficient catalyst of Co oxidation (Dey et al. 2018), and several modifications of physical and chemical properties of this complex with delafossite (copper iron oxide mineral) formula have been described (Iwase et al. 2011;Muguerra et al. 2008).Like Equations ( 11)and ( 14) also cannot be correct.Again, correct reactions will be suggested in section 3.4.4.
Although the catalytic role of Cu+ and Ag+ in the Fe 3+mediated dissolution of LiO 2 is yet to be elucidated, these catalysts seem to enhance the release of Li+ from the Co complex, resulting in reductive leaching of Co, with ironoxidizing microorganism maintaining a Fe 3+ surplus.
Mixtures of A. ferrooxidans and Acidithiobacillus thiooxidans have also been tested for Li, Co, and Ni dissolution from spent Li-ion batteries.Initially, the cathodic and anodic layers from spent batteries, ground to −200 mesh (<75 µm), were inhibitory to the mixed culture, but this was alleviated through successive subcultures at increasing pulp densities of up to 40 g/L (Heydarian et al., 2018).In general, inhibition may be caused by multiple factors such as (i) toxic metals or organics affecting the oxidation activity; (ii) the pulp decreasing the transfer rate of oxygen in culture, affecting the aerobic activity and CO 2 fixation negatively; (iii) collision of particles with cells, causing shearing cell membranes.At 4% pulp density and pH of 1.5, recoveries were 99% Li, 50% Co, and 89% Ni after 16 d in the mixed adapted culture.Co and Ni dissolved through reductive leaching involving the Fe redox couple, whereas Li dissolved through a proton attack.Boxall et al. (2018) used A. ferrooxidans and A. thiooxidans to produce Fe2(SO4)3 and H2SO4 solutions used to leach Liion battery waste, separately and in combination, at 25°C Experiments lasted to 2 h.82% Mn, 74% Cu, 60% Li, 49% Ni, and 53% Co leached at optimum conditions.The initial pH in the range of 1.1-2.5 increased by 2 to 4 pH during the leaching.Xin et al. (2012) used a mixed culture of iron-and sulfuroxidizing microorganisms to leach cathode and anode mixtures of Li-ion batteries.They used S, FeS 2 , or a combination as substrates.The leaching of Li was enhanced with S becoming of H 2 SO 4 .Co dissolution was improved when the culture received both S and FeS 2 as the substrates, thus indicating the role of Fe as a redox carrier for the leaching of Co.   2008) evaluated a two-step biological leaching: one reactor with S or FeSO 4 to produce the bioleaching solution, another to leach anodic and cathodic materials obtained from spent Ni-Cd batteries.The consuming acid (dissolution of oxides and hydroxides) was occurred by acid produced by microorganisms.Ni and Cd dissolved more slowly in the second phase than in the first.In general, Cd-containing anodes dissolved faster than Ni-containing cathodes.Individual sources and reactions are difficult to determine because the leach solution was municipal sludge, and the batteries were processed without comminution or disassembly.Xin et al. (2016) examined the dissolution of Li, Co, Mn, and Ni from cathodes of spent batteries types LiFePO 4 , LiMn 2 O 4 , and LiNi x Co y Mn 1-x-y O 2 .Li leached with acid, and Co, Mn, and Ni leached with Fe 3+ (Fig. S1, please refer to the supplementary materials file).Xin et al. (2009) suggested that contact was required between cathodes and microorganisms for leaching Co, Ni, and Mn, but not for leaching Li. 92% Li, 93% Mn, 43% Co, and 38% Ni were dissolved in a mixed culture of A.
Extracellular polymeric substances (EPS) of iron-oxidizing microorganisms have also been associated with bioleaching processes (Wang et al. 2018;Wu et al. 2019).EPS facilitates cellular attachment and biofilm formation.Wu et al. (2019) purified EPS from A. A. ferrooxidans, and L. ferriphilum to evaluate it for bioleaching of spent battery type LiNi x Co y Mn 1-x-y O 2 .Fe 3+ /Fe 2+ cycle was formed in the EPS resulting in Li, Co, Ni, and Mn dissolution (Wu et al. 2019).Ghassa et al. (2020aGhassa et al. ( , 2020b) tested a consortium of moderate thermophiles, including Acidithiobacillus caldus, L. ferriphilum, Sulfobacillus spp., and Ferroplasma spp.for bioleaching of Li-ion batteries.The consortium received S, Fe 2+ , and Fe scrap.Bioleaching of Co and Ni was two days because of fast Fe and S oxidation in thermophilic bioleaching.Li dissolution was not affected by these substrates.The addition of ferrous iron could be replaced with iron scrap, decreasing the leaching kinetic.

Fungal leaching
Organic acids, such as carboxylic acids, are biodegradable and less corrosive compared to inorganic acids (Asadi et al. 2018;Bahaloo-Horeh and Mousavi 2017).Based on their pKa values, dilute solutions of carboxylic acids have moderately acidic pH values in the range of 3-5.Organic acids are available through chemical synthesis (Karaffa, Sándor, and Fekete 2001) and as metabolites from central metabolic pathways.Fungi can grow over a wide pH range and tolerate toxic metals (Santhiya and Ting 2005).
Carboxylic acids were produced by Aspergillus niger cultures grown with sucrose and tested for bioleaching of Li-ion batteries (Bahaloo-Horeh et al. 2017).Spent media contained citric, oxalic, malic, gluconic, and oxalic acid.Oxalic acid had the highest concentration in A. niger cultures.100% Cu, 95% Li, 45% Co, 38% Ni, 70% Mn, and 65% Al was leached in 8 d.Citric acid efficiently sequestered metals.Oxalic acid forms soluble complexes with Li and Al, whereas Ni-and Co-oxalates are practically insoluble.Subcultures of A. niger at increasing pulp density up to 1%, increased the metal tolerance.Acclimation increased biomass yields, organic acid production, and metal dissolution.The tolerance of A. niger culture was limited to 1% pulp density (Bahaloo-Horeh and Mousavi 2017;Bahaloo-Horeh, Mousavi, and Shojaosadati 2016), and at higher pulp densities spent medium method should be employed.

Bacterial leaching
Zn-based batteries contain metals with relatively low economic values as compared to Li-and Ni-based batteries.The anodic and cathodic formulations of these batteries generally contain ZnO and soluble salts such as Zn 5 (OH) 8 Cl 2 •H 2 O (zinc chloride hydroxide monohydrate) and Zn(NH 3 ) 2 Cl 2 (zinc ammonium chloride), and Mn insoluble phases such as MnO 2 (manganese dioxide) and tetragonal spinel ZnMn 2 O 4 (Lobos, 2017;Mishra et al. 2008).Xin et al. (2012) investigated the bioleaching of Zn and Mn from spent Zn-Mn batteries with a mixed culture of Alicyclobacillus spp.and Sulfobacillus spp.The culture was inhibited at increasing pulp density.By increasing the pulp density from 1 to 8%, the bioleaching of Zn and Mn decreased from 100 to 30%, and from 94 to 2.5%, respectively.The recovery loss was attributed to the uncontrolled pH, which increased with pulp density.Sulfur oxidation or sulfuric acid addition could control pH and thus promote Zn and Mn dissolution, and additional Fe 2+ could augment the dissolution of MnO 2 to Mn 2+ .Niu et al. (2016) evaluated the effect of pulp density on the bioleaching of spent Zn-Mn spent batteries using a mixed culture.Sulfuric acid addiction instead of biogenic acidity via sulfur oxidation was the most efficient and fastest method for pH control necessary for Zn and Mn recoveries at 8-9.7% pulp densities.Recoveries were >50% for both Zn and Mn at 8.0-9.7%pulp densities after 9 d.Niu et al. (2016) proposed the following reaction for ZnMn 2 O 4 dissolution: The cathodic and anodic half-reactions will be presented in section 3.4.4.Niu et al. (2015) examined metal ion catalysis to enhance the leaching of Zn-Mn batteries in the Alicyclobacillus-Sulfobacillus mixed culture (Figure 3).Cu 2+ had the most catalyst effect in bioleaching compared to Co 2+ , Ni 2+ , and Ag +.The dissolution of Zn increased from 48% to 62% and Mn from 31% to 62% with 0.8 g Cu 2+ /l in the culture.They proposed a sequence of reactions for the dissolution (Equations ( 16)and ( 17)).The effect of silver was particularly low, but this was expected because AgCl precipitation; the solution phase contained 5.1% Cl-from the dissolution of Zn 5 (OH) 8 Cl 2 .H 2 O and Zn(NH 3 ) 2 Cl 2 ) (Figure 4).However, these reactions' stoichiometry and redox shuttling are unconfirmed (Niu et al. 2015).Xin et al. (2012) evaluated mixed and single cultures (Alicyclobacillus and Sulfobacillus spp.), energy source (elemental sulfur, pyrite, elemental sulfur + pyrite), and initial pH values for contact or non-contact leaching of Zn-Mn spent alkaline and Zn-C-batteries (Xin et al. 2012).In the nonmode with a dialysis membrane separation, 96% Zn was leached in 1 d with acid regardless of test culture.Mn II O dissolves at low pH, whereas dissolution of Mn III 2 O 3 and Mn III,IV  3 O 4 requires a reductant (Sayilgan et al. 2009).Up to 97% Mn was leached reductively with Fe 2+ as the reductant by the mixed culture in 13 d.Reductive dissolution of Mn could be coupled with microbial oxidation of pyrite as the   (Niu et al. 2015;Zeng et al. 2012Zeng et al. , 2013)).
source of sulfuric acid and dissolved iron.Dialysis membrane experiments demonstrated that bioleaching of Mn required microbial contact (×in et al. 2012b).
A flowsheet for Zn-Mn-ferrite production from spent Zn-Mn batteries was proposed by Song et al. (2015).The proposed process was comprised of bioleaching, co-precipitation, and boiling reflux.A mixed culture with S and FeS 2 was used for the bioleaching at 4% pulp density, with 100% and 89% Mn dissolution in 13 d.For synthesizing Zn-Mn ferrite, various molar ratios of Mn: Zn:Fe (6:4:20, 5:5:20, and 4:6:20) and alkalis (NaOH, NaHCO 3, and NH 4 HCO 3 ) were tested.The high saturation magnetization of 102 emu/g and a uniform particle size of ferrite was obtained using 2 M each of Mn 2+ , Zn 2+ , and Fe 2+ , followed by coprecipitation with NaOH and reflux at 100°C (Song et al. 2015).Kim et al. (2016) employed six Aspergillus strains with various substrates sources to dissolve metals from spent Zn-Mn batteries.The strains produced predominantly citric and oxalic acid when sucrose and malt extract were used as substrates, respectively.Zn and Mn were leached with citric acid.Oxalic acid precipitates Zn 2+ as Zn-oxalate (Sayer and Gadd 1997;Sayilgan et al. 2010;Xin et al. 2012), suggesting that oxalic acid concentrations should be minimized or citric acid should be separated from spent culture medium for use as a lixiviant.Cerruti, Curutchet and Donati (1998) reported two-step bioleaching with A. ferrooxidans for spent Ni-Cd batteries.H 2 SO 4 production from S oxidation reached 80 mmol H+ kg −1 d −1 at pH 1.0 after 3 d. 1 kg of S 0 produced sufficient acidity to maintain the pH at 1.0 for leaching ~25 g of spent batteries.After 93 d, nine successive steps of fresh leach solution replacement and various hydraulic retention times yielded 100% Cd, 96% Ni, and 95% Fe dissolution from spent Ni-Cd batteries (Figure 5).The process involved Ni oxidation to Ni 2+ followed by acid dissolution, and reduction of Ni-oxyhydroxide to Ni 2+ and further acid dissolution.For the reductive step, intermediate sulfur compounds formed by S 0 oxidation were proposed as electron donors.

Bacterial leaching
Sewage sludge has been evaluated as a substrate for bioleaching Ni-Cd batteries (Zhao et al. 2007(Zhao et al. , 2008a;;Zhu et al. 2003).Zhao et al. (2008b) showed that Cd, Co, and Ni bioleaching was enhanced if Fe 2+ instead of S 0 was used.Ni and Cd bioleached rapidly with Fe 2+ and sulfur at pH 1.2.Increasing the sewage sludge residence time from 1 to 4 d increased H 2 SO 4 production to pH 1.9 and dissolution of Ni and Cd.Zhao et al. (2008a) reported 100% dissolution of Ni, Cd, and Co, at the ~25-40 d at pulp densities of 0.2, 0.4, and 0.8%, respectively (Figure 5).Acid attack was the main dissolution mechanism, followed by microbial contact for oxidation (Zhao, Yang and Zhu 2008).Dissolution of Cd and Co was faster than Ni with sewage sludge because Cd and Co dissolutions occur at pH ~ 3-4.5, compared to pH 2.5-3.5 for Ni.
Response surface methodology was used to optimize the bioleaching of metals from Ni-Cd and NiMH batteries (Ijadi Bajestani, Mousavi, and Shojaosadati 2014a).Cd dissolution was independent of the initial Fe 3+ concentration, enhanced at the pH range of 1.4 to 2.6.Cd formed a surface complex FeOCdOH 2 + with hydrous ferric oxide (Dzombak and Morel 1986), which decreased Cd dissolution and precipitated ferric iron as jarosite (KFe₃(SO₄)₂(OH)₆) in sulfate-rich solutions.Co dissolution was dependent on the Fe 3+ concentration and pH.Ni dissolution was enhanced at high redox potential and low pH values.Thus, the conditions of Ni and Cd dissolution were not comparable.Recoveries were 87% Ni, 67% Cd, and 94% Co at the 15 d (Ijadi Bajestani, Mousavi, and Shojaosadati 2014b).Velgosová et al. (2013Velgosová et al. ( , 2014) ) reported that Cd completely dissolved from Cd-Ni batteries in acidic Fe 3 (SO 4 ) 2 solutions.Leaching was faster in the presence of iron-oxidizing

Thermodynamic assessments
Assessing thermodynamic conditions in the bioleaching of spent batteries gives insight into the process.
Increasing the difference in standard potential between the catalyst metal and the more noble metal led to faster kinetics and higher recovery (Ballester et al. 1992;Escudero et al. 1993;Guo et al. 2011).Figure 6 shows the Gibbs free energy for potential catalysts and target metals.Table S-1 (Supplementary Materials) is a list of the data presented in Figure 6.A low Gibbs free energy shows better conditions for substituting the catalyst for the noble metal.Therefore, in the case of Li-ion batteries, the decreasing order of efficacy of catalysts are Bi 3+ , Ru 3+ , Hg 2+ , Cu 2+ , Sn 2+ , Ni 2+ , Co 2+ , and Ag +.For Zn-Mn batteries, the order decreased from Bi 3+ , Ru 3+ , Hg 2+ , Ag+, Cu 2+ , Sn 2+ , to Ni 2+ .There is consistency between the data in Figure 6 and the bioleaching data reported by Ballester et al. (1990).However, catalyst recycling and price, environmental and health effects, and toxicity are factors that may overrule the usefulness of catalysts on a commercial scale.

Potential reactions
3.4.2.1.Li-ion batteries.The Gibbs free energy of formation for CuCo 2 O 4 and AgCoO 2 (reactions 10 and 13) is required to assess the thermodynamics of Co dissolution.
And, there is no data for the Gibbs free energy of formation for the CuCo 2 O 4 and AgCoO 2 compounds based on the literature.Since reactions 9 and 12 proceed to completion, we could assume that the lowest Gibbs free energy of reaction could be 0 to find an approximation for the Gibbs free energy of other species.The Gibbs free energy was calculated to be −578 and −245 kJ/mol for CuCo 2 O 4 and AgCoO 2 , respectively.The Gibbs free energy of reactions 11 and 14 is + 90 and +45 kJ/mol, calculated using .Equation ( 18) indicating that these reactions are not thermodynamically favorable.Alternatives for reactions 10 and 13 are proposed.
Based on the Gibbs free energies of alternative reactions (−265 and −133 kJ/mol), reactions 23 and 24 could describe Co dissolution in metal-catalyzed bioleaching, casting doubt on the reactions proposed by Zeng et al. (2013).Co is trivalent in both intermediate complexes, and it is plausible that Fe 2+ is the reductant for Co 3+ to Co 2+ reaction.The ORP increased over time, consistent with formation of Fe 3+ (Zeng et al. 2012(Zeng et al. , 2013)).Thus, the Fe redox cycle involved the microbial oxidation of Fe 2+ as, well as the coupling between Fe 2+ and Co 3+ (Fe 2+ → Fe 3+ + e-and Co 3+ + e → Co 2+ ).

Zn-Mn batteries.
The leaching of Mn 3+ and Mn 5+ requires reductive dissolution as Mn 2+ .The reaction by Niu et al. (2016Niu et al. ( , 2015) ) cannot describe dissolution.The correct reactions and calculated Gibbs free energy for Cucatalyst bioleaching of Zn-Mn batteries can be suggested as follow:

Perspectives on the bioleaching of batteries
Research indicates that bioleaching can be used for recovering metals from spent batteries, but low recovery, slow kinetics, and lack of optimization remain.Continuous H 2 SO 4 and Fe 2 + /Fe 3+ cycle may prove an efficient approach, but batch testing is indispensable for testing process parameters.The very low pulp densities are a major obstacle to piloting.Fungi produce carboxylic acids are tolerant to metals, and some can grow over a wide pH range.Many acidophilic bacteria and archaea are noted for extreme pH and resistance to metals.They produce acid from pyrite, other Fe-sulfides, or S 0 .In contrast, fungi require organic substrates.Mixed bacterial cultures may be the best.Extreme thermophiles have not been investigated.They have faster metabolism compared to mesophiles and moderate thermophiles, but fast reactions may not offset the energy required to maintain temperature.Fungal culture conditions for bioleaching are incompatible with bacteria and archaea.Culture control, exclusion of contaminants, and aseptic and sterile conditions are not feasible.
Research to date is still exploratory, and some proposed reaction sequences are putative at best.Battery bioleaching has yet to test reactor configuration and develop design and control of upstream and downstream processes, scale-up, operating expenses, capital expenditures, and product recovery.

Figure 1 .
Figure 1.Three important types of batteries and their main elements.
Zeng et al. (2013) investigated the Ag-catalyzed bioleaching of LiCoO 2 cathodes with A. ferrooxidans cultures.With 20 mg/ L Ag+, cobalt leached to 98% in 7 d at 1% pulp density, compared to 43% without Ag.Ag+ was proposed to form an intermediate AgCoO 2 on LiCoO 2 surfaces, with subsequent decay-causing a release of Ag+ and dissolution of Co 2+ (Equations (

Figure 2 .
Figure 2. The negative effect of mixed pulp density on the extraction of Co and Li from spent Li-ion batteries.The box-and-whisker plots show the median concentration, the 25th, and 75th quartiles.

Figure 3 .
Figure 3.Effect of copper as a catalyst on Zn-Mn batteries bioleaching.

Figure 4 .
Figure 4. Effect of metallic ions as a catalyst (Copper and Silver) on the bioleaching of Co, Mn, and Zn from spent Li-ion and Zn-Mn batteries.The box-and-whisker plots show the median concentration, the 25th and 75th quartiles(Niu et al. 2015;Zeng et al. 2012Zeng et al. , 2013)).

Figure 5 .
Figure 5.Effect of HRT on the extraction of Co, Cd, and Ni from spent Ni-Cd batteries.The box-and-whisker plots show the median concentration, the 25th, and 75th quartiles.

Figure 6 .
Figure 6.Gibbs free energy (kJ/mol) of the reaction of substituting metal ions with noble metals.
microorganisms.Ni dissolution was 45% by bioleaching and 56% by the chemical leaching with Fe 3 (SO 4 ) 2 .Cd dissolution was dependent on H 2 SO 4 and Fe 3+ .A process flowsheet for the bioleaching Ni-Cd battery waste is shown in Fig.S2, comprising (i) an acidifying step to meet the acid demand and (ii) an aerated leaching reactor, Both steps need recycling settling basins as well as dewatering before disposal of