From an experimental viewpoint, manganese cannot be directly leached from manganese ore with H2SO4 without a suitable reducing agent. Therefore, the effect of the concentration of glucose present in the banana peels on the leaching efficiency of LGMO was investigated by varying the amount of banana peels (1, 2, 4, and 6 g) at an initial H2SO4 concentration of 5vol% at 80°C for 2 h. Fig. 1 shows the relevant results. The leaching efficiency of manganese gradually increased with an increase in the amount of banana peels. The yield reached a maximum value of ~87% when the banana peel amount was 4 g, following which it decreased.
The XRF results listed in Table 1 point to a manganese decomposition of ~18wt%, probably because of the release of Mn2+ from LGMO into the aqueous solution at a banana peel amount of 4 g ; however, this dramatic increase in the leaching efficiency was not remarkable when the banana peel amount was 6 g, in which case the efficiency decreased abruptly to ~78%. Approximately 6wt% of manganese (Table 1) remained in the leached residue, indicating that manganese could not be recovered completely from the multiphase complex manganese ore, where some phases of manganese are difficult to leach in an acidic solution [18−19]. Similarly, the presence of a high silica content (~42wt%) in the residue implies that the leaching is restricted due to the encapsulation of manganese in the gangue.
Chemical composition of as-mined LGMO before leaching SiO2 MnO CaO Fe2O3 Al2O3 TiO2 NiO V2O5 ZnO SrO ZrO2 31.96 24.17 17.80 15.12 7.191 0.196 0.038 0.033 0.031 0.011 0.008 Chemical composition of LGMO after leaching with banana peel under optimum parameters SiO2 MnO CaO Fe2O3 SO3 TiO2 NiO BaO K2O SrO ZrO2 41.60 6.13 0.39 21.06 29.39 0.29 0.01 0.64 0.33 0.02 0.006
Table 1. XRF chemical analyses of LGMO before and after leaching
Several experiments were performed to determine the optimum temperature for leaching manganese from LGMO in dilute sulfuric acid (5vol%). All the experimental conditions were kept constant except for the temperature, which was varied from 60 to 120°C at 20°C intervals. The concentration of manganese in the pregnant leach solution obtained at varying temperatures was measured using AAS. Fig. 2 shows the results, indicating a high manganese leaching efficiency at a leaching temperature of 120°C. The yield of manganese increases from ~28% to 98% as the temperature is gradually increased from 60 to 120°C. The reduction of MnO2 mainly depends on the hydrolysis of cellulose (polysaccharides) or lactose (disaccharides) into monosaccharaide compounds. For example, glucose is responsible for the reduction process and the hydrolysis process is directly proportional to the temperature, which increases the hydrolysis rate . This can be attributed to the decrease in the solution viscosity with increasing temperature. Further increasing the temperature above 100°C did not show any significant increase in the yield of manganese (it only slightly increased from 97% to 98%); therefore, from an economic perspective, the most favorable temperature is 100°C.
The influence of leaching time on the yield of Mn leaching was studied in the 2–5 h time window. Fig. 3 shows the results in graphical form. The observations made can be attributed to the fact that manganese reduces from its ore due to the available fixed amount of banana peel that hydrolyzed its constituent hemicellulose in sulfuric acid. Since no additional hemicellulos is available, further increasing the time does not influence the manganese leaching efficiency.
Previous studies also reported that at low concentrations of banana peel, all the available hemicellulose in the banana peels (25.5wt%) was utilized for manganese reduction [16,21]. The time–yield plot (Fig. 3) shows an anomalous behavior. The leaching efficiency is the highest at a leaching time of 2 h. The leaching efficiency abruptly decreases up to 3 h, then increases in the 3–4 h window, and ultimately decreases after 5 h. The abrupt decrease in the leaching efficiency at a leaching time of 3 h may be attributed to the fact that all the available hemicellulose in banana peels was utilized for manganese reduction during this time. The decrease in the leaching rate for 3 h may also indicate a low leaching activity in hydrometallurgical processes. The optimum leaching time was, therefore, set to 2 h at which the manganese leaching efficiency was maximum (~73%). The dissolution of manganese from LGMO by plant powders containing glucose is known to occur in at least four steps (in series): (1) hydrolyzation of monosaccharaides, (2) diffuses of the reductant from liquid to Mn-grains via the boundary layer, (3) nucleation and hydration of the products at the active sites, and (4) diffusion into the solution. Thus involved reactions are strictly time-dependent .
The influence of H2SO4 concentration on the dissolution of manganese from LGMO was examined by varying the concentration from 5vol% to 20vol% at intervals of 5vol% while keeping the other parameters constant, i.e., leaching time of 2 h, manganese ore amount of 5 g, banana peel amount of 2 g, temperature of 80°C, and agitation rate of 300 r/min. Fig. 4 shows the results. The leaching efficiency was ~73% at a concentration of 5vol% and increased up to ~98% at 15vol%. This demonstrates that the hydrolyzing degree of polysaccharide and cellulose significantly improved at an H2SO4 concentration of 15vol% and accelerated the leaching of manganese from LGMO by obtaining hydrogen ions more easily . The decomposition rate of MnO2 was higher at 15vol% H2SO4, further verifying the encapsulation of manganese in the gangue minerals, because the presence of more concentrated H2SO4 allowed more dissolution and relieved more manganese ions to the solution. However, a further increase in the H2SO4 concentration up to 20vol% caused an abrupt decrease in the manganese leaching efficiency up to ~72%, demonstrating that the leaching activity of Mn from LGMO in hydrometallurgical processes is relatively low at higher H2SO4 concentrations . Consequently, from an economic perspective, an H2SO4 concentration of 15vol% seems to be adequate for Mn leaching; any additional consumption of H2SO4 should be avoided.
To investigate the hydrometallurgical reductive leaching of Mn from LGMO using banana peels, the phase(s), chemical composition, and microstructural analyses before and after leaching were carried out using XRD, XRF, and SEM−EDS, respectively. The XRD pattern of the LGMO sample revealed that the sample was mainly composed of pyrochroite (MnO2), hausmannite (Mn3O4), spessartine (Mn3Al2(SiO4)3), pyrolusite (Mn(OH)2), quartz (SiO2), hematite (α-Fe2O3), and calcite (CaCO3) . The phase analysis after leaching revealed the disappearance of most of the phases leaving quartz as the residue and a single peak for pyrolusite (Fig. 5), where the inter-planner spacing (d-values) and relative intensities corresponding to the observed XRD peaks for the residue (after leaching) matched those of quartz (SiO2) as the major phase along with a single peak at 2θ = 28.66° indicating the presence of pyrolusite as a secondary minor phase in the residue.
Table 1 lists the results of the chemical analyses of LGMO before and after leaching. It is further confirmed that most of the manganese in the LGMO was leached from the sample and that the manganese content decreased from 24.17wt% to 6.13wt% in the residue, i.e., after leaching.
Figs. 6(a) and 6(b) show the morphological changes in the ore grains/particles before and after hydrometallurgical leaching observed by SEM, respectively. The SEM image, shown in Fig. 6(a), reveals highly compact manganese ore grains of varying sizes (~50 mm) before leaching [24−25]. The surface morphology of the investigated LGMO sample was changed to porous and small, thin needle-like micro-features after leaching (Fig. 6(b)).
The LGMO contains several phases of manganese oxide encapsulated in gangue grains, such as quartz and hematite, as confirmed by XRD and XRF analyses. Furthermore, the manganese particles appear to be dispersed within the host gangue mineral, making the structure even more complex. Moreover, the SEM analysis confirms that the morphology of the LGMO after leaching changes significantly from a smooth surface to a rough one with needle-like features. These results support the appropriateness of using the SCM to describe the kinetics of manganese leaching from LGMO. In general, the classical models provide three basic equations to interpret the surface chemical reaction controlled kinetic model (Eq. (2)), internal diffusion-controlled kinetic model (Eq. (3)), and mixed-controlled kinetic model (Eq. (4)) [26−28]:
where x is the fraction of the manganese reacted; t is the leaching time, min; and k is the apparent reaction rate constant, min−1.
The leaching efficiency of manganese leaching from LGMO in the time range of 10–60 min out of the range of 10–120 min at various temperatures (60–100°C (333–373 K)) describe the measurement of a sensible rate that is appropriate for studying the leaching mechanism (Fig. 7). The kinetic data for manganese extraction were evaluated using various kinetic models (Eqs. (2)–(4)) to judge the reaction mechanism. The results are shown in Figs. 8(a)–8(c), which demonstrates the good fit of the SCM (Eq. (2)) with the highest correlation coefficient values (R2). This indicates that the extraction of Mn is controlled by a surface chemical reaction mechanism.
Figure 7. Effect of reaction temperature on the yield of manganese from LGMO at various time durations.
Figure 8. Plots of various kinetic model equations versus time at different temperatures for leaching manganese from LGMO: (a) 1 − (1 − x)1/3 = kt; (b) 1 – (2/3)x − (1 − x)2/3 = kt; (c) [(1−x)−1/3 − 1] + (1/3)ln(1 − x) = kt.
Table 2 lists the values of the shrinking core rate constant obtained at different temperatures. With the rate constant values, the apparent activation energy (Ea) was determined using the well-known Arrhenius equation:
Temperature / K Rate constant / (10–3 min–1) ΔG≠ / (kJ∙mol–1) Ea / (kJ∙mol–1) ΔH≠ / (kJ∙mol–1) ΔS≠ / (J∙K–1∙mol–1) 333 2 105.152 40.19 37.31 –203.72 343 4 107.189 353 5 109.226 363 7 111.263
Table 2. Kinetic and thermodynamic parameters for manganese leaching from LGMO.
where k is the reaction rate constant, and lnA is the Arrhenius factor, Ea is the apparent activation energy (kJ∙mol−1), R is the gas constant (R = 8.315 J∙K−1∙mol−1), and T is the absolute temperature. Fig. 9 shows that the plot of lnk versus 1/T is linear, demonstrating a good fit of the Arrhenius model with a slope Ea/(RT) and intercept ln A. The desired value of the activation energy (Table 2) was determined from the slope of the linear plot, which agreed well with those reported in literature [15,26].
The thermodynamic parameters, such as the activation, enthalpy change (ΔH≠), and entropy change (ΔS≠), were determined using the well-known Eyring equation (Eq. (6)). The linear form of the Eyring equation is given as
where k and T represent the core diffusion rate constant and absolute temperature, respectively, R is the general gas constant, KB is the Boltzmann constant, and h is the Planck’s constant. Fig. 10 shows the thermodynamic Eyring plot, which exhibits a straight line with the slope and intercept equal to ΔH≠/R and (ln KB)/h + ΔS≠/R, respectively. Eq. (7) was used to calculate the Gibbs free energy of activation (ΔG≠):
Table 2 lists the values of ΔH≠, ΔS≠, and ΔG≠. Clearly, the ΔG≠ value increases with an increase in the temperature, demonstrating the spontaneity and feasibility of the reaction.
The higher positive values of ΔG≠ at all the temperatures indicate that a driving force from an external source is necessary for the formation of an activated complex during the extraction. The positive ΔH≠ values point to an endothermic nature of the leaching process, while the negative values of ΔS≠ suggest that the activated complex exhibits a more ordered structure . The results obtained in the present study regarding the maximum leaching efficiency of manganese under optimum conditions were compared with previously reported values and are summarized in Table 3.
Reductant Ore Conditions Key results Refs. Glucose Mn nodules 2.5 M NH3, 0.37 M NH4Cl, 0.2 g of
glucose per gram of nodules, 85°C, 4 h
100% Cu, 90% Ni, 60% Co recovery  Sawdust (C6H10O5) Low Mn ore Aqueous H2SO4 90%–95% Mn recovery (99.6% pure)  Sucrose (C12H24O11) Pyrolusite 50–90°C, 20 g/L of sucrose, 1 M H2SO4, leaching time of 30 min 94%–95% Mn recovery [20,17] Lactose (C12H22O11) Low Mn ore 100 mesh Mn ore, 2 h, 90°C, 20%
90%–92% Mn recovery  Mn–Ag ore Two stage leaching with H2SO4 and
97% Mn, 98% Ag and Au recovery  Glycerine Mn ore Aqueous H2SO4 Increased Mn recovery  Triethanolamine and thiosulfate MnO2 1 M H2SO4, 200 mesh Mn ore, 2 M thiosulfate Maximum Mn recovery  Oxalic acid (OX) Low Mn ore 30.6 g/L OX, 0.5 M H2SO4, 85°C,
98.4% Mn recovery  Carboxylic acid (15%)
Ferro columbite HF + tartaric (TR), citric, formic,
oxalic (OX) acids
86% Fe, 90% Mn recovery 
Table 3. Reductive leaching of manganese using agricultural/biomass wastes