Mechanism Underlying the Bioleaching Process of Licoo2 by Sulfur-Oxidizing and Iron-Oxidizing Bacteria

Journal of Bioscience and Bioengineering VOL. 128 No. 3, 344e354, 2019 www.elsevier.com/locate/jbiosc Mechanism underlying the bioleaching process of LiCoO2 by sulfur-oxidizing and iron-oxidizing bacteria Weijin Wu, Xiaocui Liu, Xu Zhang,* Xiyan Li, Yongqiu Qiu, Minglong Zhu, and Wensong Tan State Key Laboratory of Bioreactor Engineering, East China University of Science and Technology, 200237 Shanghai, China Received 14 July 2018; accepted 11 March 2019 Available online 20 April 2019 Benefiting from lower operational costs and energy requirements than do hydrometallurgical and pyrometallurgical processes in metal recovery, the bioleaching of LiCoO2 through the use of sulfur-oxidizing and iron-oxidizing bacteria has drawn increasing attention. However, the bioleaching mechanism of LiCoO2 has not been clearly elaborated. In the D present study, the effects of the energy source of bacteria, such as Fe2 , pyrite and S0, and the products of bacterial 3D oxidation, such as Fe and sulfuric acid, on the chemical leaching of LiCoO2 were studied. The results indicated that D D lithium was dissolved by acid, and cobalt was released by the reduction of Fe2 and acid dissolution. The recovery of Li D D D and Co2 could be significantly improved by pH adjustment. Finally, optimal recoveries of Li and Co2 were observed in the pyrite group, reaching 91.4% and 94.2%, respectively. By using pyrite as the energy source, the role of bacteria in bioleaching of LiCoO2 was investigated. The results showed that bacteria could produce sulfuric acid by oxidizing pyrite D D to promote the mobilization of Li and Co2 . The recovery of lithium and cobalt could be increased to 100.0% and 99.3% by bacteria. Moreover, extracellular polymeric substances secreted by bacteria were found to be a factor for the D D improvement of Li and Co2 recovery. Ó 2019, The Society for Biotechnology, Japan. All rights reserved. [Key words: Bioleaching; LiCoO2; Pyrite; Sulfur-oxidizing and iron-oxidizing bacteria; Extracellular polymeric substances] Lithium-ion batteries are widely used as power sources for consumption and potential safety risks, forcing people to find electric vehicles because of their excellent electrochemical prop- alternative methods (15). Bioleaching, which can overcome these erties (such as high density, long life, small size, and lightweight, shortcomings, is considered a promising alternative for the recov- among others) (1,2). However, when lithium-ion batteries reach ery of discarded lithium-ion batteries (16,17). their service life, the treatment of the discarded batteries becomes Bioleaching is a promising technology, which utilizes microor- a serious issue for many countries (3,4). It has been estimated that ganisms for metal recovery from low grade ore (18,19), waste more than 0.5 million tons of discarded lithium-ion batteries will printed circuit boards (20), sewage (21), spent catalyst (22) and be produced in China in 2020 (5). Due to the presence of toxic el- discarded lithium-ion batteries (8). Acidophilic sulfur-oxidizing ements and compounds in discarded lithium-ion batteries, their and iron-oxidizing bacteria are commonly used for metal extrac- improper disposal poses a serious threat to the environment and tion, such as Acidithiobacillus ferrooxidans, Acidithiobacillus human health (6). Additionally, various valuable metals (such as thiooxidans, Leptospirillum ferrooxidans, and Sulfobacillus thermo- lithium and cobalt) are present in discarded lithium-ion batteries, sulfidooxidans, among others (23). These bacteria use inorganic 2þ and the recycling of these metals generates remarkable economic compounds (Fe ,FeS2 and reduced S) as an energy source and and social benefits (7). utilize their ability to facilitate metal dissolution through a series of The components of lithium-ion batteries are a cathode, an biooxidation and bioleaching reactions (24). anode, a separator, electrolytes, collectors and a metal protective Although bioleaching has some advantages in metal recovery, shell (8). The cathode is made of lithium mixed metal oxide, such as some shortcomings persist in the bioleaching of electronic wastes, LiCoO2, LiMn2O4, LiNiO2, LiFePO4 and LiNixCoyMnzO2 (9). As the such as a long leaching time and a low pulp density, limiting its cathode is the most valuable component in lithium-ion batteries commercial application. Therefore, an in-depth understanding of (10), many researchers have focused on the recovery of metals from the mechanism of bioleaching of metals could provide important the cathode (7,11). theoretical support for the optimization and development of pro- Hydrometallurgical and pyrometallurgical processes are the cesses. Different mechanisms have been proposed according to most widely used methods to recover lithium and cobalt from different research purposes and bacteria. In the bioleaching of discarded lithium-ion batteries (9,12e14). However, their short- metal sulfides using sulfur-oxidizing and iron-oxidizing bacteria, comings include strict equipment requirements, high energy the indirect mechanism is the most recognized mechanism (25).In the direct mechanism, the bacteria contribute to generating the oxidizing agent, ferric iron. The metal ions and reduced sulfur from þ metal sulfides are then released through the attack of Fe3 . Finally, * þ Corresponding author. Tel./fax: 86 21 64252536. the reduced sulfur is biooxidized to sulfuric acid through the E-mail address: [email protected] (X. Zhang). 1389-1723/$ e see front matter Ó 2019, The Society for Biotechnology, Japan. All rights reserved. https://doi.org/10.1016/j.jbiosc.2019.03.007 VOL. 128, 2019 MECHANISM UNDERLYING BIOLEACHING OF LiCoO2 345 thiosulfate or polysulfide pathway (25). During the bioleaching of was adjusted to 1.20 with 5 mol/L sulfuric acid solution. Then, 15 g/L LiCoO2 was added to the solution. Samples were collected at 1, 2, 3, 5 and 7 h to analyze the pH waste printed circuit boards (PCBs), the metals in PCBs are dis- þ þ þ and concentrations of Li and Co2 . solved by the Fe3 generated by bacterial oxidation (26). During the Chemical leaching of LiCoO with pH adjustment The experiment was bioleaching of NieCd batteries, Ni and Cd are mobilized by the 2 designed similar to the previous description. The difference was that the pH during sulfuric acid produced by bacteria (27). Some researchers the leaching process was maintained at 1.20 using 5 mol/L sulfuric acid solution. e e (15 17,28 30) have studied the bioleaching of lithium-ion batte- Bioleaching of LiCoO2 The design of the bioleaching experiments is shown ries, but there is no consistent conclusion regarding the bioleaching in Table 1. Ninety milliliters of 9 K medium and 10 ml stock culture were prepared in mechanism of lithium-ion batteries. Zeng et al. (30) indicated that groups A, B and F. One hundred milliliters of 9 K medium was prepared in groups C, D and E. Ten grams of pyrite were added in groups AeF, and the initial pH was adjusted during the bioleaching of LiCoO2, LiCoO2 is dissolved by the reaction to 1.20 with 5 mol/L sulfuric acid solution. Group F was sterilized by autoclaving. (Eq. 1), and the presence of copper ions can accelerate the reaction Then, 15 g/L LiCoO2 was added to groups AeF. All groups were cultivated in a rate. However, Xin et al. (31) compared the effects of different en- rotary shaker at 42C at 180 rpm. The pH of groups A, C and F was maintained at þ ergy sources on the bioleaching of LiCoO2 and found that Li was 1.20 with 5 mol/L sulfuric acid solution after the addition of LiCoO2. The same 3þ amount of acid solution as in group A was added to group D. Groups B and E were released due to acid dissolution and Co in LiCoO2 was mobilized 2þ evaluated without pH adjustment. Samples were collected at regular intervals to by the combined effects of Fe reduction and acid dissolution. The þ analyze their pH, oxidation-reduction potential (ORP) and concentrations of Li , þ þ þ reaction is shown in Eq. 2. Therefore, to better understand the Co2 ,Fe2 and Fe3 . bioleaching of lithium-ion batteries, more effort is required to Effect of the electron donor on bioleaching of LiCoO2 The bioleaching reveal the bioleaching mechanism of lithium-ion batteries. experiments were carried out with 90 ml 9K medium, 10 g pyrite,10 ml stock culture and 1.5 g LiCoO2. After 24 h of culture, different amounts of FeSO4$7H2O (0, 1, 3, 5 g/L þ þ 2þ 2þ 4LiCoO2 þ 12H /4Li þ 4Co þ 6H2O þ O2[ (1) Fe ) were added to the leachate. The samples were collected at intervals to analyze þ þ þ þ the concentrations of Li ,Co2 ,Fe2 and Fe3 . 2FeSO þ 2LiCoO þ 4H SO /Fe ðSO Þ þ 2CoSO þ Li SO Effect of electron transfer on the bioleaching of LiCoO2 The experiments 4 2 2 4 2 4 3 4 2 4 were carried out in a 250-ml Erlenmeyer flask with 90 ml 9 K medium, 10 g pyrite, þ 4H2O (2) 10 ml stock culture and 1.5 g LiCoO2. The initial pH was adjusted to 1.20. Then, 0 g/L and 0.01 g/L riboflavin were added to the medium. The samples were collected at þ þ 2þ 3þ Moreover, the effect of different energy sources (Fe2 , pyrite, S intervals to analyze ORP and the concentrations of Li ,Co and Fe . and their combination) on the bioleaching of lithium-ion batteries Extraction and analysis of the biochemical characterization of extracellular polymeric substances A culture with 90 ml 9 K medium, 10 g pyrite and 10 ml has been investigated by several researchers (16,17,31). However, stock culture was carried out for 7 d. Then, extracellular polymeric substances were few studies have addressed whether these energy sources them- extracted from the culture using the EDTA method (32).

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