Cobalt recovery from the stripping solution of spent lithium-ion battery by a three-dimensional microbial fuel cell
Introduction
The industry of lithium ion batteries (LIBs) has been rapidly propelled forwards with a tremendously rising demand from the emerging market (e.g., smart-phones, tablet PCs, and new energy vehicles) in recent years [1], [2]. The spent LIB is commonly categorized to the hazardous solid wastes, majorly consisting of heavy metals (HMs), organic chemicals, and plastics. An inappropriate disposal of spent LIBs can result in serious environmental problems [3], [4], [5]. Currently, the active materials identified as cathode in LIBs include LiCoO2, LiFePO4, LiNiO2, and LiMn2O4, etc. The utility of the layered LiCoO2 has occupied the highest rate in the market [6], [7], [8], [9]. The recycling (or recovery) of Co element from spent LIBs not only benefits the habitat protection also offsets the inadequate supply of Co in the world. The technologies employed for the recycling process of spent LIBs are commonly categorized to pyrometallurgy and hydrometallurgy [10], [11], [12], [13], [14]. The hydrometallurgical methods are more preferably adopted due to the advantages of lower energy-expenditure, easier conduction, and smaller gas emission [15], [16]. However, the high-dependency on some specified chemical regents are seemingly inevitable for some traditional hydrometallurgical techniques, such as solvent extraction, pH-controlling precipitation and leaching, etc. It is quite meaningful to explore some appropriate methods to progressively handle or relieve the issues facing at present [10], [17], [18], [19].
One novel promising method for the setting goal is the employment of microbial fuel cells (MFCs). Microorganisms are applied in MFC to anaerobically decompose (i.e., oxidize) biodegradable organic materials to drive the bio-electrochemical (BE) system [20], [21], [22]. The electrons are bio-catalytically transferred from the organics to anode surface and further to cathode through microbial respiring metabolism and external circuit [23], [24], [25]. The potential differences between the electrodes render both cations and anions electro-migratable [26]. The accumulation of target contaminants in a specified area can be fluently achieved by the electromigration and electrochemical reduction in combination [27]. However, the electrochemical reduction of Co is thermodynamically unfavorable due to its lower potential in contrast to the anodic potential values. Therefore, the removal of Co is reassigned to the traditional chemical precipitation [28], [29], [30], [31]. The pH ascension combining with the enhanced adsorption in catholyte becomes essential in achieving the removal of Co from the stripping liquor in MFC systems.
A three-dimensional (3D) electrochemical process integrates the charged particle electrodes and the two-dimensional (2D) counterpart process, expanding the specific surfaces of the electrodes and shortening the mass-transferring distance [32], [33], [34]. Those polarized particles forming anode and cathode along a unified direction strengthen the conductivity and adsorption of contaminants and buffer the dramatic changes in the acid-alkali environments [35], [36]. As known, the 3D electrochemical system has been extensively studied in the treatments of various wastewaters based on its own advantages [36], [37], [38]. However, the construction of 3D BE system without external energy input (Power source) and its subsequent applications have scarcely been investigated. The definition of 3D BE model have not been mentioned in the environmental domain until now.
In this study, a 3D-MFC system was originally designed aiming at substituting the chemical precipitation regents for the effective recovery of divalent Co cations (i.e., Co(II)) from the stripping solutions of CoSO4. The homemade granular activated carbon (GAC) using as both adsorbent and microelectrode was tightly stacked closing to the cathode in order to enhance the electrostatic adsorption and positively induce the continuous precipitation of Co(II) ions. The pH environments in the catholyte were reasonably controlled through restraining the electromigration of the biologically generated protons in the anode chamber. The effects of carbonization temperatures on the removal and recovery of Co and the electric output performances of BE system were comprehensively investigated based on the electrical results. The optimized combination of the running parameters was determined based on the orthogonal results of indicators. The influence of (NH4)2CO3 concentration on the physical characteristics and experimental indicators was studied in final to consummate the preparation of Co3O4 powders. The reaction mechanisms in the 3D-MFC system for the removal and recovery of Co were specifically discussed and tested by measurements of morphologies and mineral phases of GAC particles before and after experiments. This research quantitively testifies the feasibility of a 3D-MFC platform for the resource recycling and comprehensively explores the operating conditions influencing the running performance of 3D electrochemical system. The experimental results in this study will support the application of 3D-MFC platform constructing for recovery of other precious cations from the spent LIBs.
Section snippets
The preparation of CoSO4 solution and the activated carbon
The cathode materials were obtained from the spent LIBs through manual dismantling and ultrasonic cleaning procedures. The ultrasonic conditions were set at 60 °C, 160 W, and 15 min. The leaching experiments were conducted by stirring cathode materials with 2 M of H2SO4 and 5 vol% of H2O2 (30%) at a ratio of solid to liquid (S-L) of 1:20 mg/L, 75 °C and 250 rpm over 30 min. The metal ions of Al, Fe, and Cu were recovered using NaOH (10%) and Na2S in the form of hydroxide-precipitations with pH
Acclimation of inoculum
The changes in the indicators for the performance of 3D-MFC system during the acclimation of inoculum are shown in Fig. 2a–d. The electrogenic acclimation tests were conducted to guarantee the biofilm formation, the reproduction of the electroactive microorganisms, and the bio-oxidation of acetate before the Co(II)-removal process [43], [44]. The sulfuric acid solution with the initial pH of 4 and AC particles prepared at 300 °C was adopted as the catholyte and the cathodic stacking material,
Conclusions
The maximum power output was obtained at 12th d during the acclimation. However, an unreasonable prolongation in terms of setup would cause potential inhibition to microorganisms and significantly increase the internal resistances and over-potentials. Besides, the carbonization temperatures for the preparation of GAC particles also affected the running performance of the 3D-MFC platform. The increases in the anodic temperatures improved the electrogenic activities of microorganisms but
Acknowledgements
The research is not funded by any organization.
Declaration of interests
The authors have no competing interests to declare.
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