Discarded Carbon-Zinc Batteries as Source of an Efficient Heterogeneous Fenton-Like Catalyst Employed to Degrade Organic Molecules in an Aqueous Medium

The present work evaluates the feasibility of using the raw material collected from discarded zinc-carbon batteries as heterogeneous catalyst to degrade the dye Indigo Carmine in an aqueous solution. Besides the evident environmental application, this work also presents an economic alternative for the production of new catalysts used to remediate polluted waters. For this, discarded carbon-zinc batteries were gathered, disassembled and their anodic paste collected. After acidic treatment and calcination at 500˚C, characterization measurements, i.e. flame atomic absorption spectroscopy (FAAS), nitrogen sorption, X-ray diffraction (XRD) and scanning electron microscopy (SEM), revealed that the so-obtained material consisted mainly of ZnMn 2 O 4 . This material acts as a heterogeneous catalyst in a Fenton-like process that degrades the dye Indigo Carmine in water. That is probably due to the presence of Mn(III) (manganese in the +3 oxidation state) in this material that triggers the decomposition of hydrogen peroxide (H 2 O 2 ) to yield hydroxyl radicals (HO • ). Moreover, direct infusion electrospray ionization coupled to high resolution mass spectrometry (ESI-HRMS) was employed to character-ize the main by-products resulting from such degradation process. These initial results thus indicate that raw materials from waste batteries can therefore be potentially employed as efficient Fenton-like catalysts to degrade organic pollutants in an aqueous solution.


Introduction
The use of electronic devices has been growing continuously worldwide, with increasing consumption of primary (non-rechargeable) and secondary (rechargeable) batteries [1] [2]. The annual consumption of batteries was estimated to be 8 billion units per year in the USA and Europe, 6 billion in Japan, and 1 billion in Brazil [3] [4]. Alkaline and zinc-carbon batteries are primary disposable batteries and one of their main usage is the powering of day-to-day gadgets [5].
After their use, most batteries are discarded as waste. An immediate consequence of the incorrect disposal of e-wastes in the environment is the contamination of the environment by heavy metals, mainly lead, mercury, cadmium and nickel [6]. The recycle of spent batteries is therefore essential not only for environmental safety and human health issues but also for an economic point of view [7] [8]. Thus, many works have been carried out aiming at recovering the metals from these residues, obtaining alloys, nanoparticles with magnetic, adsorbents or catalytic applications [2] [7] [9].
The zinc-carbon battery uses zinc as anode, manganese dioxide as cathode, and an electrolyte of ammonium chloride and/or zinc chloride dissolved in water [10]. As the cell is discharged, the zinc is oxidized and the manganese dioxide is reduced according to a simplified overall cell reaction Equation (1)

Materials and Reagents
All chemicals were purchased from Sigma-Aldrich (Milwaukee, WI, USA) and used without further purification. Ultrapure water (18 MΩ•cm −1 , Milli-Q system, Millipore, Burlington, Massachusetts, EUA) was used to prepare the solutions.

Battery Dismantling
Discarded batteries were manually dismantled by removing the metallic external cover to access the internal anodic paste. The raw anodic material, a black petrified solid, was grated and triturated in grail and pestle to obtain fine black powder rich in manganese oxides, carbon, zinc, zinc oxides and other trace substances.

Anodic Material: Acid Leaching and Calcination
The black powder anodic material was leached with a sulfuric acid aqueous solu-

Characterization
Analyses  The specific surface area (SSA) and pore size distribution were assessed by employing the multipoint Brunauer, Emmett and Teller (BET) and the density functional theory (DFT), respectively.

Degradation Experiments
Degradation tests were carried out in order to check out the activity of the CBR

Characterization of the CBR Material
Atomic absorption analyses revealed that the CBR material has high contents of Mn (44% w/w) and Zn (23% w/w). These data are in agreement with results previously reported in the literature [9] [10]. Such metals are present as oxides, as will be discussed later in this paper. Other components at trace level could also be present in this sample.  indicating that CBR is a material with a remarkable crystallinity.
The image by scanning electron microscopy (SEM) shows that the CBR material is quite heterogeneous (Figure 2). Note the presence of grains with varied sizes and shapes. This feature is probably due to the conditions employed in the preparation procedure, which favors the particles coalescence.

Fenton-Like Degradation Experiments
Degradation tests were conducted in order to determine the catalytic efficiency of the CBR material towards the degradation of Indigo Carmine in an aqueous solution. Figure 3 shows the degradation rates produced by the Fenton-like

Analyzes by direct infusion electrospray ionization mass spectrometry (ESI-MS)
was conducted aiming at detecting at least the most abundant by-products resulting from the degradation promoted by the CBR/H 2 O 2 system. ESI-MS is a key technique for the identification of by-products resulting from the degradation of water contaminants, and therefore has played an important role in elucidating possible degradation pathways [20] [21]. This technique gently transfers species from the condensed to the gas phase without inducing undesirable side reactions. Because of that, ESI-MS has been successfully applied to monitor an increasing number of environmental processes [19]. Figure 4 shows the mass spectra of aliquots collected after 0 and 120 min of exposure of an aqueous solution of Indigo Carmine to the CBR/H 2 O 2 system. Note that in the mass spectrum of the initial solution (Figure 4(a)) only the ion of m/z of 209.9829, which corresponds to [Indigo Carmine-2H] 2− (Indigo Carmine in its doubly-deprotonated form), can be detected. After 120 min, however, the mass spectrum (Figure 4(b)) reveals the absence of this ion, which indicates that Indigo Carmine was fully degraded. This mass spectrum (Figure 4(b)) also displays a number of other ions, some of them ascribed to be the deprotonated forms of degradation products possibly formed under these conditions. Molecular formula for each one of these ions were proposed based on the high-resolution mass spectrometry data, which presented small differences between the experimental and theoretical accurate masses (Table 1).
Other ions, besides the ones displayed in Table 1, are also detected in Figure  4(b) (for instance, m/z of 260.8643 and 189.9011). However, reasonable molecular formula with an acceptable error could not be proposed for any of them. These compounds probably leached from the heterogeneous catalyst during the degradation process.
Based on these results as well as on the well-known reactivity of hydroxyl radical towards organic molecules in aqueous medium, a route for the degradation of Indigo Carmine by the CBR/H 2 O 2 system could thus be proposed, as outlined in Figure 5. The formation of by-products 2 and 3 arising from the oxidation and desulfonation of Indigo Carmine, respectively, has been reported in the literature [19] [20] [21]. It is important to state that in both pathways the determinant participation of hydroxyl radicals is noticeable.

Conclusion
This work demonstrates that the ZnMn 2 O 4 catalyst can be obtained from discarded batteries (anodic paste). It acts as an efficient Fenton-like catalyst in the degradation of Indigo Carmine in an aqueous solution. The high efficiency of this catalyst is probably due to the presence of Mn(III), a quite unstable and reactive species. The interaction of Mn(III) with H 2 O 2 generates hydroxyl radicals that are responsible for the high removal of the Indigo Carmine dye from an aqueous medium. It is important to mention that the catalyst was obtained from electronic waste (discarded zinc-carbon batteries). Therefore, in addition to avoiding that such electronic waste becomes a potential source of environmental contamination, this work proposes its use as an efficient remediation agent for water bodies containing organic pollutants. Hence, besides the evident environmental application, this work also presents an economic alternative for the production of new catalysts used in Fenton-like processes. It is noteworthy that this is the first report regarding the attainment of an effective Fenton-like catalyst, i.e. ZnMn 2 O 4 , from battery residues. Finally, other possible remediation processes making use of such a promising material are underway in our laboratory.