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Vol. 8. Issue 3.
Pages 2481-3388 (May - June 2019)
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Vol. 8. Issue 3.
Pages 2481-3388 (May - June 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.04.020
Open Access
New photocatalytic materials obtained from the recycling of alkaline and Zn/C spent batteries
Lorena Alcaraza, Irene García-Díaza, Laura Gonzálezb, María Eugenia Rabanalc, Ana Urbietad, Paloma Fernándezd, Félix A. Lópeza,
Corresponding author

Corresponding author.
a Centro Nacional de Investigaciones Metalúrgicas (CENIM-CSIC), Avda. Gregorio del Amo 8, E-28040 Madrid, Spain
b IMDEA-Nanociencia, Campus Universitario de Cantoblanco, Madrid 28049, Spain
c Universidad Carlos III de Madrid & IAAB, Dpto. de Ciencia e Ingeniería Química, Avda. de la Universidad 30, 28911 Leganés, Madrid, Spain
d Universidad Complutense de Madrid, Dpto. de Física de Materiales, Facultad de Ciencias Físicas, Ciudad Universitaria s/n, 28040 Madrid, Spain

  • Several types of Zn/Mn oxide were produced from the spent alkaline batteries.

  • Photocatalysis experiments are carried out.

  • Degradation of methylene blue and Rhodamine B is evaluated under UV radiation.

  • All the compounds studied show photocatalytic properties.

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Figures (9)
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Tables (2)
Table 1. Kinetics constants and correlation coefficients from the different models used for both MB and RhB.
Table 2. Degradation percentage and Egap values obtained to the different analyzed samples.
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Several phases with variable stoichiometry ZnxMn3−xO4 (with x=0.25, 0.85 and 1) and ZnO have been obtained from the black mass, a widely generated residue of wasted alkaline batteries. The obtained samples have been characterized by X-ray diffraction (XRD) and Raman spectroscopy showing results consistent with the stoichiometry obtained from chemical analysis. The study of the degradation of methylene blue (MB) and rhodamine B (RhB) under UV radiation demonstrates the photocatalytic behavior in all samples obtained, reaching degradation percentages higher than 70% and 50%, respectively.

Spent batteries
Photocatalytic materials
Zn and Mn oxides
Methylene blue
Rhodamine B
Full Text

The great development and continuous growth of the population have contributed to a considerable increase in global environmental pollution. For example, industries dump to wastewater between 300 and 400 million tons each year of different pollutants such as heavy metals, solvent and toxic sludge [1]. These residues cause significant environmental problems, so their elimination is of particular importance due to its potentially harmful effect [2].

The legal regulations impose ever stricter criteria to validate the quality of residual waters (maximum values of contaminant to be considered clean or decontaminated), hence the growing interest generated in the search for new methods for the efficient elimination of pollutants [3].

Traditionally the technologies used for cleaning water are mainly based on adsorption processes using activated carbon, or desorption in the air [4], however, these processes only transfer the pollutants from the aqueous solution to another phase, still contaminated, then the problem persists. This scenario has pushed the development of new technologies based on photocatalytic oxidation and consequently, the search for chemicals with a high oxidation power that could lead to an efficient degradation of the pollutants. In these processes, the radicals generated in the oxidation, react with the pollutant, rendering products environmentally harmless [4]. The variety of contaminants that can be eliminated from water by catalytic oxidation assisted by solar radiation goes from organic substances like dyes or pesticides [5–7] to heavy metals [1,3].

The photocatalytic reaction is initiated when a photo-excited electron of a photocatalyst semiconductor is promoted from the valence band filled with a semiconductor photocatalyst to the empty conduction band being the energy of the absorbed photons () is equal to or greater than the energy of the band gap of the photocatalyst [8]. The absorption of photons causes the excitation and transfer of electrons (e) from the valence band (VB) to the conduction band (eCB), leading to the generation of holes (h+) in the valence band (h+VB). Next, the migration of the charge carriers (e and h+) to the surface of the photocatalyst occurs. Highly reactive electrons and holes tend to carry out reduction and oxidation reactions to produce hydroxyl radicals (OH) and superoxide anion radicals (O2•−), respectively, which subsequently react by degrading the contaminant [2], as shown in the scheme of Fig. 1.

Fig. 1.

Scheme of the photocatalytic process.


Different semiconductor materials, in particular oxides, have been used such as titanium dioxide (TiO2), zinc oxide (ZnO), tungsten oxide (WO3) or hematite (α-Fe2O3), proving to be effective photocatalyst [5]. Most of these semiconductors have wide band gaps (above 3.2eV), showing photocatalytic activity after illumination with UV radiation [7]. However, this bandwidth limits its response under direct sunlight, that would be desirable, since the UV fraction is only around 3–5% of the whole spectrum. Another limiting factor is the rapid recombination of photogenerated electron-hole pairs (e–h+), the catalytic photodegradation requires the generation of reactive oxygen species (ROS) such as hydroxyl (OH) or superoxide (O2•−) radicals and H2O2, that cannot occur if the e–h+ recombination is too fast [6]. Different routes such as semiconductor coupling [9] or doping [10] are currently being studied in order to overcome these limitations and improve the photoresponse of these materials.

Doping the semiconductor materials with metals, in particular transition metals (TM), increases the number of surface defects [11] and therefore affects the optical and electronic properties [12]. The incorporation of a TM in the oxide does not, in principle modify its band gap, but introduces defect levels in the mid-gap that enable transitions at energies below the band gap, then extending the usable fraction of solar spectrum [2,13,14].

In the present work, the photocatalytic activity of Zn and binary Zn/Mn oxides obtained from the black mass of spent alkaline batteries has been studied. The recovery process has been described elsewhere [14–16]. Doping with manganese is very promising due to different oxidation states are possible for this ion (Mn2+, Mn3+ and Mn4+) and the consequent ability to act as electron or hole trap [17–19].

2Materials and methods2.1Sample preparation2.1.1Obtaining of phases of stoichiometry ZnxMn3−xO4 (with x=0.25, 0.85 and 1)

The black starting mass for obtaining the different binary oxides was provided by Envirobat España, S.A. (Guadalajara, Spain). This black mass comes from the dismantling of alkaline and Zn–C batteries.

To obtain the different oxides, 100, 200 or 300g of black mass were dissolved in 1L of a solution of: milliQ water (500mL), 7.5M HCl (250mL) and 0.7M H2O2 (250mL). The mixtures were homogenized and, after 1h at room temperature (RT), were filtered using a Millipore Holder filter at 7bar pressure. The liquids collected (pH0) were treated adding 6M NaOH until reaching a pH value between 12 and 14, and the solids obtained were discarded. Finally, the mixtures are filtered obtaining the corresponding binary oxides of stoichiometries ZnxMn3−xO4 (with x=0.25, 0.85 and 1) for the solid/liquid ratios of 100, 200 and 300g/L, respectively.

2.1.2Obtaining of the phases of stoichiometry ZnO

ZnO samples were obtained by two different routes: precipitation (ZnOp) and thermal decomposition (ZnOc). In the first case, after the alkaline precipitation described above, HCl is added to the collected liquids, until pH decreases from the initial value (12–14) to a value of 9.5. From this step, a white precipitate (ZnOp), that will be separated by filtration and subsequently dried, is obtained. In the second route (ZnOc), the precursor Zn5(CO3)2(OH)6 obtained after the leaching of the black mass with a solution of (NH4)2CO3/NH3 is calcined at 800°C for 5h under constant air flow [14].


The structural characterization was carried out by means of X-ray diffraction (XRD) using a Siemens D5000 diffractometer equipped with a Cu anode (Cu Kα radiation) and a LiF monochromator. Rietveld method was applied for the calculation of structural parameters from XRD patterns. We have used the version 4.2 of the Rietveld analysis program TOPAS (Bruker ASX) and crystallographic information of the different phases obtained from Pearson's crystal structure database for inorganic compounds release [20].

Micro-Raman spectra were obtained using a confocal Horiba Jovin-Ybon Lab RAM HR800 system. The samples were excited by a 633nm He–Ne laser on an Olympus BX 41 confocal microscope with a 10× objective. A charge coupled device detector was used to collect the scattered light dispersed by 600linesmm−1 grating (micro-Raman). The spectral resolution of the system used was 1.5cm−1 for the measurements.

All samples were characterized by UV–visible diffuse reflectance spectra (DRS) measuring in the range of 350–500nm and 450–650 for methylene blue (MB) and rhodamine B (RhB) at RT using a UV–visible Spectrophotometer (Varian Cary 100 with DRA-CA-30I Diffuse Reflectance Accessory).

The photocatalysis experiments were carried out in a Pyrex glass reactor at RT. 5mg of solid catalyst was dispersed in 600mL of MB and RhB solutions of concentration 2.5mg/L, and the mixture was magnetically stirred during 20min without illumination, to obtain a homogeneous suspension and reach the adsorption equilibrium. The photocatalytic degradation was carried out for until equilibrium is reached, in continuous stirring under UV-light irradiation (365nm), in a dark room with 125W high-pressure mercury vapor lamp (Jinfei Company, Shanghai). Aliquots (3mL) of the solution extracted every 10min (every 15min for times larger than 75min) are studied by UV–Vis absorption in a Lambda 14P UV–visible Spectrophotometer to monitor the degradation of both methylene blue and rhodamine B solutions.

3Results and discussion3.1Ray diffraction

The structure and phase distribution of the obtained samples were determined by XRD. Fig. 2 shows the analysis of X-ray diffraction patterns after application of the Rietveld method for the samples investigated.

Fig. 2.

Experimental and Rietveld-refined XRD patterns of ZnxMn3−xO4 and ZnO samples.


In the case of mixed binary oxides (Zn/Mn) the majority of the more intense reflexions observed can be indexed based on a tetragonal symmetry of space group I41amd compatible with a spinel type structure of Zn and Mn, and with the stoichiometry indicated in each plot. In all cases purities of approximately 95% were obtained in all samples.

In the case of pure zinc oxide samples, the diffraction patterns show reflections that can be indexed to a hexagonal symmetry of a P63mc space group with a wurtzite structure.

From the X-ray patterns, the average crystallite size (ϕ) was estimated using the Scherrer equation [21] (Eq. (1)):

where ϕ is the average crystallite size, 0.89 is the shape factor assuming spherical particles, λ is the X-ray wavelength, β is the full-width at half-maximum (FWHM) of the experimental diffractions and θ is the Bragg's angle.

The calculated average size for the ZnxMn3−xO4 samples were 55, 43 and 38nm for x=0.25, 0.85 and 1, respectively, as well as 72 and 85nm for the ZnOp and ZnOc samples. In binary oxides the size decreases with the increase in the Zn content in the stoichiometry of the samples obtained according to previous studies carried out in similar phases [22]. In the case of the ZnO phases, the ZnOc sample has a larger particle size compared to ZnOp, what is expected due to the preparation method followed to synthesize [23].

3.2Raman spectroscopy

Fig. 3 shows the normalized Raman spectra recorded for the different samples obtained. The oxides of stoichiometry ZnxMn3−xO4 with spinel-like crystalline structure and spatial group I41/amd could present, according to group theory, 10 Raman active modes in Raman: Γ=2A1g+3B1g+B2g+4Eg[24], however only a few of them are usually observed. The vibration modes with frequencies above 600cm−1 correspond to the movement of the oxygen atoms in the tetrahedral groups AO4 while the low frequency modes are characteristic of the octahedral sites (BO6) [24].

Fig. 3.

Normalized Raman spectra obtained from the samples with the different stoichiometries: (a) ZnxMn3−xO4 and (b) ZnO.


In our case, all spectra corresponding to the binary oxides present a maximum around 670cm−1, typical of the symmetry A1g related to the movement of oxygen (bonding and repulsion effect) of the tetrahedral AO4 groups [25,26]. It can be seen how, as the manganese content increases, the maxima move toward lower values of Raman shift. In addition, as the Mn content decreases a progressive broadening of the Raman bands is observed showing a loss of crystallinity in the samples (Fig. 3a).

Regarding non-doped ZnO samples, a wurtzite structure, space group C46v with two unit formulas per primitive cell is observed, where all the atoms occupy the C3v sites. In this case, we have four Raman active modes (A1+E1+2E2) are possible. A1 and E1 modes are polar phonons, and therefore have different frequencies for the transverse optical (TO) and longitudinal optical (LO) modes of vibration because the LO phonons are associated with the macroscopic electric field. On the other hand, the non-polar modes with E2 symmetry have two different vibration frequencies, associated with the oxygen atoms (E2high) and the Zn sublattice (E2low) [22,27]. These modes are observed at frequencies of 437cm−1 for E2high; 379cm−1 and 410cm−1 for A1 (TO) and E1 (TO) respectively; and 541cm−1, 577cm−1 and 592cm−1 for A1 (LA), A1 (LO) and E1 (LO) respectively [22,28,29].

In the present work, the maxima of the corresponding Raman bands (Fig. 3b) appear close to the reported values, although not all the active modes are observed. As shown in Fig. 3b, the Raman peaks appear at 438cm−1 (E2high), 380 and 411cm−1 (A1 (TO) and E1 (TO) respectively). Additional peak at 331cm−1 attributable to E2(high)-E2(low) mode is also observed [30]. These results are in agreement with previous studies performed in samples of ZnO obtained by ceramic method and also with those found for thin films [29,30]. On the other hand, the peak broadening observed in ZnOp respect to ZnOc is consequent with lower crystallinity degree observed from the XRD patterns.

3.3Optical band gap measurements

Optical band gap has been obtained from UV–Vis absorption experiments in DRS mode. In order to obtain the energy of the band gap for each of the samples, the Kubelka–Munk and Tauc approaches were used [31,32].

The Kubelka–Munk equation (Eq. (2)) is first used to obtain absorption data from the diffuse reflectance spectrum:

where the term R represents the reflectance of an infinite film and the function F(R) is equivalent to the absorption coefficient, α. Then the Tauc plot can be carried out to represent (αhν)2 versus , and obtain the optical band gap (Egap) by extrapolation of the linear part of this curve, as shown in Fig. 4. The estimated band gap values obtained decrease when the Mn content increase. The values are similar to those previously reported by other authors in the bibliography, both for the ZnO [33–35], and for Mn doped ZnO samples [36–38].

Fig. 4.

Tauc's plots obtained from diffuse reflectance spectra. Absorption coefficients have been obtained by Kubelka–Munk approach.


The photocatalytic activity of the samples obtained (both the binary oxides and the zinc oxides) were realized using a UV light lamp and MB and RhB solutions to mimic the pollutants [39]. To monitor the dye degradation, absorption spectra of aliquot fractions of the solutions are recorded every 10–15min, and the degradation fraction (δ) and the apparent reaction constants (k) were estimated, according to equations:

where C0 is the initial MB or RhB solution concentration and C is the MB or RhB solution concentration at the time (t).

The optical absorbance spectra of all samples are presented in Figs. 5 and 6. Two absorption maxima are observed in both cases, centered around 610 and 660nm and 510 and 550nm, characteristic of the absorption spectra of methylene blue [40] and rhodamine B [41]. It can be seen how an increase in the photodegradation reaction time leads to a decrease in absorption bands. As an example, Fig. 7 shows the pictures of the ZnOp sample remaining solution of MB and RhB every 30min and 60min, respectively, until reaching the photocatalytic equilibrium.

Fig. 5.

Absorption spectra obtained at different illumination times for MB.

Fig. 6.

Absorption spectra obtained at different illumination times for RhB.

Fig. 7.

Pictures of the corresponding MB and RhB solution of ZnOp sample.


The study of the kinetics of both pollutants degradation under UV excitation with the different materials was carried out using the pseudo-zero-order linear models (Eq. (5)), pseudo-first-order (Eq. (6)) and pseudo-second-order (Eq. (7)) [42]. Table 1 summarized the obtained results.

where [C]t is the concentration of MB or RhB at the time t; [C]0 is initial MB or RhB solution concentration and kobs,0, kobs,1 and kobs,2 are the kinetics constants to the pseudo-zero, pseudo-first and pseudo-second order, respectively.

Table 1.

Kinetics constants and correlation coefficients from the different models used for both MB and RhB.

Model  MB degradation
  kobs  R2  kobs  R2  kobs  R2  kobs  R2  kobs  R2 
Pseudo-zero-order    0.864    0.874    0.895    0.728    0.693 
Pseudo-first-order  0.015  0.932  0.018  0.899  0.019  0.980  0.032  0.977  0.029  0.910 
Pseudo-second-order  0.017  0.971  0.238  0.965  0.253  0.987  0.280  0.991  0.391  0.933 
Model  RhB degradation
Pseudo-zero-order    0.899    0.849    0.923    0.936    0.550 
Pseudo-first-order  0.064  0.959  0.005  0.916  0.004  0.957  0.009  0.937  0.011  0.951 
Pseudo-second-order  0.014  0.992  0.017  0.966  0.019  0.993  0.025  0.966  0.090  0.975 

The correlation coefficients R2 show that reaction kinetics fit better to the pseudo-second-order model. With this model, the experimental kinetic constant can be obtained from the slope of the straight line when 1/[C]t is plotted against the reaction time. The results summarized in Table 1 show that kobs increases with the Zn content, which indicates that the degradation occurs more easily.

Fig. 8 shows the degradation rate of MB and RhB. The maximum degradation is obtained after 60 and 195min of illumination for MB and RhB, respectively. Thereafter, the degradation percentage remains practically constant. The photocatalytic degradation of RhB is lower than MB similarly to previously reported by others authors to TiO2[43] and ZnO [44] samples. Also, the percentage of degradation increases with the increase in the Zn content of the samples (see Table 2). Previous studies carried out on samples of ZnO doped with Mn [2] have shown an increase in photocatalytic activity with the decrease in particle size, agglomeration and the increase in the surface area of the photocatalyst. The results obtained are in good agreement with these studies [2], where the samples with a smaller average size of the crystalline domain obtained from the XRD data present a higher percentage of degradation.

Fig. 8.

Degradation percentage versus photocatalytic reaction time of (a) MB and (b) RhB.

Table 2.

Degradation percentage and Egap values obtained to the different analyzed samples.

Stoichiometry  % MB degradationa  % RhB degradationb  Egap (eV) 
x=0.25  72  56  1.32 
x=0.85  87  62  1.33 
x=89  70  1.38 
ZnOp  93  90  3.20 
ZnOc  90  89  3.10 

Degradation percentage at 120min reaction time.


Degradation percentage at 240min reaction time.


From the residue obtained of wasted batteries, Mn doped ZnO and ZnO phases have been obtained. The characterization carried out by XRD and Raman spectroscopy show phases consistent with stoichiometries ZnxMn3−xO4 (with x=0.25, 0.85 and 1) and ZnO for the samples. In all cases, the samples show a photocatalytic behavior due to the decrease in MB and RhB absorbance in the recorded spectra. The kinetics of the photodegradation reaction are fitted to a pseudo-second-order model. The percentages of degradation for MB and RhB are greater than 70% and 50%, respectively, increasing with the decrease in particle size which leads to an increase in the surface area of the photocatalyst. The band gap values obtained for the analyzed samples are in the range of 1.32–1.38eV for the samples ZnxMn3−xO4 and between 3.10 and 3.20eV for the ZnO samples. In the Mn doped samples, these values increase as the Zn content of the samples increases. The study of the degradation of both MB and RhB under UV radiation was carried out, showing a photocatalytic activity for the possible degradation of pollutants under UV radiation in all the samples investigated.

Conflicts of interest

The authors declare no conflicts of interest.

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