The effect of Fe³⁺ ions on the electrochemical behaviour of ocean manganese nodule reduction leaching in sulphuric acid solution

Abstract

The effect of Fe³⁺ ions on the ocean manganese nodule reductive leaching in imitated sulphuric acid solutions was investigated. This work is presented in two courses, including the influence of Fe³⁺ ions on valuable metal extraction and the electrochemical reductive dissolution of manganese nodules. The results show that the beneficial effects of Fe³⁺ ion can be interpreted based on two aspects: the first is the acceleration caused by the active transformation of Fe³⁺/Fe²⁺ pair, and the second is the hydrogen ion buffer action generated by Fe³⁺ ion hydrolysis on the surface. On one side, Fe³⁺ ion could lessen the hydrogen consumption happening at the interface layer of the nodule supported by the leaching test and cyclic voltammetry results. On the other side, Fe³⁺ ions could be converted into Fe²⁺ ions and then preferentially reduce manganese oxide leading to an acceleration of the charge transfer reaction of the manganese nodule based on cyclic voltammetry, polarization, and impedance analysis results. The reduction leaching of manganese nodules in sulphuric acid solution is mainly controlled by the electrochemical interface reduction corresponding to manganese oxide dissolution, and the active conversion of the Fe³⁺/Fe²⁺ couple affects the dissolution of high valence manganese oxide.

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1. Introduction

Ocean manganese nodules, also known as ocean polymetallic nodules and ocean ferromanganese nodules, are complex polymetallic iron–manganese ores that are rich in manganese, nickel, cobalt, copper, iron, and other valuable metals. It has been considered the most economically valuable oceanic mineral resource.^1–3 Generally, a manganese nodule is an aggregate of a shell of iron and manganese oxides wrapping a core of clay minerals. With regard to the shell of the manganese nodule, manganese minerals are usually present in vernadite, todorokite, birnessite, and buserite, and iron mainly forms oxides and hydroxides of ferric iron. It has been reported that manganese oxides and iron oxides present in the form of poorly crystalline states are nearly disseminated associations.^4–7 Moreover, studies have suggested that the extraction of Ni, Co, and Cu from manganese nodules is much related to the destruction of the structure of Fe–Mn oxides as these metals are presenting specialistic and preferential adsorption of ferromanganese oxides.^8–10 In other words, the transformation and separation of the Fe–Mn oxides are a necessary precondition for the recoveries of valuable metals from manganese nodules.

Research on the beneficiation and metallurgy process of manganese nodules are conducted in many countries throughout the world since the early 1950s. Now, there have been reports on some representative processing methods, such as reduction roasting-ammonia leaching,¹¹ cuprous ion catalytic reduction-ammoniacal leaching,¹² reduction acid leaching in H₂SO₄ or HCl system,^13,14 rusting-leaching process,¹⁵ and bio-leaching.¹⁶ A critical review of the literature, especially in terms of energy consumption, carbon emission, and selective extraction, indicates that the reductive leaching of manganese nodules in the sulphuric acid system is a relatively promising and inexpensive approach.

In regard to the metal extraction from manganese nodules using sulphuric acid solution, it has been profusely investigated the applications of reduction technologies and reducing agents. At present, many reductive methods and agents have been reported for their feasibility and potential, such as the application of pyrite, sulfur dioxide, hydrogen sulphide, ferrous ion, organic wastewater, and glucose as reducing solvents for polymetallic nodules in the sulphuric acid system.^17–20 From these methods, the efficient extraction of multiple metals from manganese nodules is largely depended on the reducing leaching of Mn–Fe oxides, that is to say, Mn–Fe oxides of manganese nodule dissolution in the solution is a precondition for the metal extraction from manganese nodule in sulphuric acid solution.

So far, many studies have reported the dissolution performance of Mn–Fe oxides from manganese nodules, and some studies have investigated the interface reaction of Mn-oxides from manganese nodules.^21–23 Fe-oxides, as a basic mineral in manganese nodules, could be dissolved to produce Fe³⁺ ions in the solution, affecting the process of manganese nodule reductive leaching in sulphuric acid solution. However, the performance of Fe³⁺ ions on the reductive leaching of manganese nodules in sulphuric acid solution is considerably less-studied. As is well known, the Mn–Fe oxide reductive dissolution of manganese nodule in sulphuric acid solution is an electrochemical corrosion process, related to the transformation of substances between the ore electrode and dissolved solution. Hence, the electrochemical behaviour function analysis of Fe³⁺ ion-effect on the reduction of Mn–Fe oxides from manganese nodules may be an invaluable topic for investigation.

The aim of this work was to investigate the effect of Fe³⁺ ions on the reduction leaching of manganese nodule in sulphuric acid solution. The influence of Fe³⁺ ions on the extraction of Ni, Co, Cu, Mn, and Fe was evaluated using simulated dissolution tests in sulphuric acid systems. The reductive electrochemical corrosion processes of manganese nodules in the presence and absence of Fe³⁺ ion electrolysis solution, were investigated using multiple electrochemical methods. The effect of Fe³⁺ ions on the dissolution process of manganese nodules was discussed in the context of Fe³⁺ ion-sulphuric acid system.

2. Materials and methods

2.1 Materials and solvent system

The ocean manganese nodules were obtained from the China pioneer area of the eastern Pacific ocean. The nodules with a specific surface area of 8.8 m² g⁻¹ were air-dried, crushed, and ground to 74 μm size before use. The main chemical composition analysis results of nodules are shown in Table 1. Fig. 1 presents the XRD patterns of the nodule. The main metals in nodules include manganese, nickel, copper, cobalt, and iron. Manganese and iron were mainly present in the form of high valence oxides in the ferromanganese nodule, and nickel, copper, and cobalt ions formed characteristic adsorption dispersed in the lattice gape of the Mn–Fe oxides.

Table 1 Main chemical compositions of the ferromanganese nodule (%, mass fraction)

Element	Ni	Cu	Co	SiO2	Na2O	K2O
wt%	1.63	1.37	0.20	22.45	0.32	3.44
Element	Mn	Fe	S	Al2O3	CaO	TiO2
wt%	22.15	11.67	0.31	7.82	3.12	1.03

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Fig. 1 XRD pattern of the manganese nodule.

The solvent systems were prepared using Fe³⁺ ion-free and Fe³⁺ ion-containing diluted sulphuric acid solution. The control group was Fe³⁺ ion-free diluted sulphuric acid solution. A selected mass of ferric sulfate (Fe₂(SO₄)₃) was dissolved in diluted sulphuric acid solution containing Fe³⁺ ion-containing solution. A solution of 10% H₂SO₄ was used to adjust the pH of the solution. Analytical reagents were used in this work.

2.2 Metal extraction performance tests

The metal extraction tests on manganese nodules were performed in a simulated electrolytic leaching system connected to an electrochemical workstation (CHI1140C, CH Instruments, USA). The schematic of the simulated leaching of manganese nodules is shown in Fig. 2. A three-electrode system was used, using a Pt plate (1.0 cm²) as a counter electrode, a standard calomel electrode (SCE) as a reference electrode (0.24 V vs. the standard hydrogen electrode (SHE)), and a loaded ferromanganese nodule powder on a graphite felt as a working electrode. The preparation of ferromanganese nodule electrode involved depositing a thin layer of the nodule ink over a graphite felt (1.0 cm²). The nodule grain ink was prepared by mixing the manganese nodule (90 wt%) and, polytetrafluroethylene (10 wt%) in N-methyl-2-pyrrolidone (the mass of the nodule was fixed at 0.2 g).²⁴ The leaching of metals from manganese nodule was measured by electrolyzing manganese nodules electrode in a simulated dissolution solution. Simulated leaching solutions were prepared as Fe³⁺ ion-free and Fe³⁺ ion-containing dilute sulphuric acid solutions. The input voltage of the manganese nodule working electrode was at −1.0 V, and 200 mL of the dissolution solution was used for each test. The influences of leaching characteristic on the pH value and Fe³⁺ ion concentration were measured from Mn, Ni, Co, Cu, and Fe extraction efficiencies calculated from the raw material content in lixiviums at a stated time. A water bath kettle was used to maintain the leaching simulation temperature at 25 °C, and a magnetic rotor was used for agitating the leaching solution. The pH of the leaching solution was buffered using dilute sulphuric acid and potassium hydroxide solution. The dissolved Mn, Ni, Co, Cu, and Fe concentrations in the obtained solutions were determined from ICP-OES analysis (Agilent 5800 ICP-OES, Agilent, Germany).

Fig. 2 Schematic of the simulated leaching measurement of manganese nodule.

2.3 Electrochemical measurements

Electrochemical measurements were performed in a typical electrochemical cell with three electrodes on a CHI1140C microcomputer-based electrochemical system, the electrodes and electrolytes were prepared in line with the description from Section 2.2. The three electrodes were immersed in the Fe³⁺ ion-free or Fe³⁺ ion-containing leaching solution for 20 min and the scan was started. The cyclic voltammetry evaluations were performed by sweeping from −0.6 V (positive-going potential scan) to 1.2 V (negative-going potential scan) and back to the starting potential with a sweep rate of 25 mV s⁻¹. Potentiodynamic polarization curves were obtained from −0.6 to 1.5 V with a scanning rate of 5 mV s⁻¹. Impedance spectra were generated at the open circuit potential (OPC) by using a sine wave voltage amplitude of 5 mV during the frequency region from 0.01 to 10⁵ Hz. The EIS (electrochemical impedance spectroscopy) data were presented in Nyquist plots and analysis using Zview 2.0 software. The temperature of all electrochemical measurements was kept at 25 °C. All the potentials shown in this work were relative to SCE.

3. Results and discussion

3.1 Effect of Fe³⁺ ion on metal extraction

3.1.1 Metal extraction response of leaching system. The influence of Fe³⁺ ion on the extraction of metals from manganese nodules in the presence and absence of Fe³⁺ ion solutions, as determined from the extraction ratios of Mn, Ni, Cu, Co, and Fe, is presented in Fig. 3. The dissolution efficiencies of Ni, Cu, Co, and Mn positively responded to the presence of Fe³⁺ ions, and the main metals Ni, Cu, and Co showed extractions similar to the leaching tendency of Mn dissolution under the test conditions. Combined with previous studies,^25,26 it is reasonable to adopt Mn extraction efficiency to discuss the effect of Fe³⁺ ions on the reductive leaching of manganese nodules in sulphuric acid solution.

Fig. 3 Effects of Fe³⁺ ions on Mn, Fe, Ni, Co, and Cu extraction ratios from the manganese nodule: (a) the initial Fe³⁺ iron-free solution, pH 1.8; (b) Fe³⁺ ion-containing solution, initial Fe³⁺ ion concentration of 1.0 g L⁻¹, pH 1.8.

3.1.2 Response of pH and Fe³⁺ ion concentration. Fig. 4 shows Mn dissolution from the manganese nodule under different pH values, assisted with or without Fe³⁺ ions in the initial leaching solution. The Mn dissolution of manganese nodule responds positively to Fe³⁺ ion concentration and acidity under the test conditions, and Fe³⁺ ions affect the Mn leaching that correlates with the acidity of the bath solution (Fig. 4).

Fig. 4 Effects of Fe³⁺ ion concentration and pH value on the Mn-extraction rate from manganese nodules: (a) response to Fe³⁺ ion concentration, Fe³⁺ ion-free solution as control, pH 1.8; (b) response to pH, Fe³⁺ iron-free solution as control, initial Fe³⁺ ion concentration of 1.0 g L⁻¹ as the Fe³⁺ ion-containing solution.

A higher initial Fe³⁺ ion concentration leads to a faster leaching rate of Mn, illustrating that Fe³⁺ ion could accelerate the reduction of Mn⁴⁺ to Mn²⁺ as shown in Fig. 4(a). With the usage of 0.1 g L⁻¹ Fe³⁺ ion in the initial solution, Mn leaching rate increased nearly 10% with reference to the control at pH 1.8, and there was about 18% increase in the Mn leaching rate when Fe³⁺ ion concentration was over 1.0 g L⁻¹, indicating that the effect of Fe³⁺ ion on Mn leaching, caused by Fe³⁺ ion concentration increased, was shown to a limited extent.

A high-level of acidity corresponds to a high leaching rate of Mn dissolution in non-Fe³⁺ ion and with Fe³⁺ ion solutions, as the Mn reductive dissolution of manganese nodule is a reaction involving hydrogen consumption (Fig. 4(b)). Hence, the Mn extraction rate is nearly two and a half times at pH 1.5 as compared to pH 3.0 in Fe³⁺ion-containing an initial solution, as well as Fe³⁺ ion-free solution. With acidity decreasing, the effect of the presence of Fe³⁺ ion increased, and Mn leaching rate showed 15.8 mg (L h)⁻¹ increase at pH 3.0 and 2.7 mg (L h)⁻¹ at pH 1.5 relative to the control group, suggesting that Fe³⁺ ion hydrolysis on the surface of manganese nodules, facilitating the conversion of free hydrogen.²⁷

According to the leaching results shown in Fig. 4, Fe³⁺ ions, and acidity should have a synergistic effect on Mn-leaching efficiency. Therefore, it could be concluded that the preferred acidity of pH 1.8 and Fe³⁺ ion concentration of 1.0 g L⁻¹ as an appropriate influence factor for the metal extraction of the manganese nodule and electrochemical measurements.

3.2 The effect of Fe³⁺ ions on the electrochemical behaviour

3.2.1 Cyclic voltammetry analysis. Fig. 5 shows cyclic voltammograms of manganese nodules in different simulated solutions. The dissolution of manganese nodules in sulphuric acid solution principally presents a result of the redox reaction of Mn-and-Fe oxides, as shown in Fig. 5(a)–(c). The pairs of redox currents corresponding to MnO₂/Mn(OH)₂, Fe³⁺/Fe²⁺, and MnO₂/Mn²⁺ emerge between −0.3 and 0.1 V, 0.3 and 0.8 V, and 0.8 and 1.2 V (vs. SCE), respectively, illustrating that the dissolution of the nodule is mainly related to the reductive conversion of high valence manganese and iron oxides in sulphuric acid systems. Acidity and Fe³⁺ ion have significant influence on the Mn–Fe oxides reduction.

Fig. 5 Plots of cyclic voltammogram analysis of the manganese nodule in various solutions: 25 °C, 25 mV s⁻¹, −0.6–1.2 V (vs. SCE), (a)–(c) solutions, (a) pH 2.3, non Fe³⁺ ion as a control, Fe³⁺ ion concentration of 1.0 g L⁻¹ as Fe³⁺ ion-containing; (a) pH 1.8, non Fe³⁺ ion as a control, Fe³⁺ ion concentration of 1.0 g L⁻¹ as Fe³⁺ ion-containing; (c) pH 1.5, non Fe³⁺ ion as a control, Fe³⁺ ion concentration of 1.0 g L⁻¹ as Fe³⁺ ion-containing.

In the Fe³⁺ ion-free solutions, the conversion of MnO₂ to Mn²⁺ shows an evident current response, stronger than other reduction responses, indicating that the dissolution of manganese nodules is mainly concerned with a reduction of high valence Mn-oxides. The current density of the reduction peak of MnO₂/Mn²⁺ is higher at pH 1.5 than at pH 1.8, and the potential positively moves closer to the balance potential of MnO₂/Mn²⁺ with acidity increasing to pH 1.5, suggesting that the increase in acidity leads to a high reducing rate of the nodule electrode. Moreover, an increment in the solution acidity causes a promotion on the reduction of Fe³⁺/Fe²⁺, indicating that the increased hydrogen ion concentration could facilitate the dissolution of the nodule Fe-oxides.

Fe³⁺ ion has a significant effect on the reduction leaching of manganese nodules in sulphuric acid leaching solution based on optimization to the conversions of iron species. The closed area of the cyclic voltammetry curve increased when 1.0 g L⁻¹ Fe³⁺ ion was added to the control solution, suggesting a boost in the transformation between the manganese nodule and solvent solution. With the addition of Fe³⁺ ions, the reduction leaching of manganese nodules lowers the influence of acidity of the solution as the hydrolysis of Fe³⁺ ions generates free hydrogen ions on the surface of the nodule, well coinciding with the leaching test results. When the Fe³⁺ ion-containing solution was of pH value 1.8, the reduction current density response to Fe³⁺/Fe²⁺ had the strongest intensity compared to those at other pH value solutions, while the reductive response of MnO₂/Mn²⁺ was weaker than pH 1.5 solution, suggesting that Fe³⁺ ions, similar to high valence Mn-oxides, as an electron acceptor, could compete with electron from Mn⁴⁺-oxides on the solid–liquid interface, and an electron donor preferred the reductive transformation of MnO₂/Mn²⁺ under an appropriated acidity. The course of the metal extraction from the manganese nodule could be listed as follows:^24,28,29

MnO₂ + 4H⁺ + 2e → Mn²⁺ + 2H₂O E⁰ = 0.99 V (1)

MnO₂ + 2H₂O + 2e → Mn(OH)₂ + 2OH⁻ E⁰ = −0.29 V (2)

Fe³⁺ + e → Fe²⁺ E⁰ = 0.53 V (3)

Me(Ni, Cu, Co)O + 2H⁺ → Me(Ni, Cu, Co)²⁺ + H₂O (4)

3.2.2 Polarization curve analysis. Fig. 6 presents potentiodynamic polarization curves of the manganese nodule in various solutions. With the presence of Fe³⁺ ions in the imitated leaching solution, corrosion current density increased, indicating that Fe³⁺ ion facilitates the dissolution rate of manganese nodule as shown in Fig. 6 and Table 2. There is a 0.07 V negative movement of the corrosion potential appearing at 0.84 V and a 93% increase of corrosion current density if 1.0 g L⁻¹ Fe³⁺ ions are added to the non-iron sulphuric acid solution when using negative-going scan (1.5 to −0.6 V) (Fig. 6(a)). When the curves were measured by positive-going scan (−0.6 to 1.5 V) (vs. SCE) (Fig. 6(b)), the corrosion potential of the manganese appear at 0.61 V in the presence of Fe³⁺ ions, which was positively moved by 0.17 V in relation to the absence of Fe³⁺ ion, and the self-corrosion current density was 3.23 × 10⁻³ mA cm⁻² in Fe³⁺ ion-containing solution, which has about six-fold increase than that of the control solution. Fig. 6(c) shows the cathodic and anodic current densities of the nodule increasing during the polarized region in the Fe³⁺ ion-containing solution relative to the control group. These results illustrate that the contribution of Fe³⁺/Fe²⁺ transformation accelerates the dissolution of Mn–Fe oxides from manganese nodules, and Fe³⁺ ions, on the surface of the manganese nodule, are transformed to Fe²⁺ ions and then oxidized by Mn⁴⁺-oxide leading to a preferential dissolution of Mn⁴⁺-oxide.

Fig. 6 Plots of polarization analysis of manganese nodules in various solutions: 25 °C, 5 mV s⁻¹, −0.6–1.5 V (vs. SCE), (a) negative-going scan, pH 1.8, non Fe³⁺ ions as a control, Fe³⁺ ion concentration of 1.0 g L⁻¹ as Fe³⁺ ion-containing; (b) positive-going scan, pH 1.8, non Fe³⁺ ions as a control, Fe³⁺ ion concentration of 1.0 g L⁻¹ as Fe³⁺ ion-containing; (c) pH 1.8, non Fe³⁺ ions as a control, Fe³⁺ ion concentration of 1.0 g L⁻¹ as Fe³⁺ ion-containing.

Table 2 Analysis results of corrosion potential and corrosion current density for manganese nodule electrodes in various solutions

Scanning direction	Simulated solution	Corrosion potential (V)	Corrosion current density (mA cm^−2)
Negative-going scanning (1.5 to −0.6 V (vs. SCE))	Fe^3+ ion-containing	0.836	3.75 × 10^−3
	Control	0.907	1.94 × 10^−3
Positive-going scanning (−0.6 to 1.5 V (vs. SCE))	Fe^3+ ion-containing	0.610	3.23 × 10^−3
	Control	0.438	0.48 × 10^−3

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3.2.3 Alternating-current impedance analysis. Fig. 7 show Nyquist plots (a), Bode plots (b), and the fitting equivalent circuit for the manganese nodule electrodes in Fe³⁺ ion-free and Fe³⁺ ion-containing solutions. Table 3 shows the fitting results for impedance data used as a simple equivalent circuit of R_sol(R_actCPE₁). R_sol and R_act corresponded to the solution resistance values and the charge transfer resistance, respectively, CPE₁ represent the double-layer capacitance between the electrode and electrode interface, and η reflected the dispersion index of CPE₁.

Fig. 7 Nyquist plots of the manganese nodule in various solutions: (a) Nyquist plots, (b) Bode plots, (c) fitting equivalent circuit, amplitude 5 mV, frequency 0.01–10⁵ Hz, bias voltage OPC, pH 1.8, non Fe³⁺ ion as control, Fe³⁺ ion concentration of 1.0 g L⁻¹ as Fe³⁺ ion-containing.

Table 3 Fitting results using the equivalent circuit (Fig. 7) for manganese nodule electrodes

Simulated solution	Rsol/(Ω cm^2)	η	CPE1 (10^−6 Ω^−1 cm^−2 s^η)	Ract/(Ω cm^2)
Fe^3+ ion-containing	24.3	0.987	0.803	2671
Control	29.4	0.992	0.970	3512

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Fe³⁺ ions accelerate the electron transmission during manganese nodule reductive leaching. Nyquist curves for nodules present as almost semicircles, and Bode curves show one characteristic parameters in Fe³⁺ ion or non Fe³⁺ ion sulphuric acid solutions (Fig. 7), indicating that the reductive dissolution of manganese nodules in the sulphuric acid system is controlled by the interfacial chemical reaction. Using 1.0 g L⁻¹ Fe³⁺ ion in the sulphuric acid solution, it leads to the maximum peak of phase angle moving to high frequency, illustrating that Fe³⁺ ions could shorten the time for electron travel through the interfacial electric double-layer (when frequency: f ≥ 10³ Hz). Furthermore, the addition of 1.0 g L⁻¹ Fe³⁺ ions to the control solution generates an approximately 23.9% decrease in the charge-transfer resistance of manganese nodules.

3.3 Effect of Fe³⁺ ions on the reduction of manganese nodules

The reductive dissolution of ocean ferromanganese nodules in sulphuric acid solution, is a multi-component dissolution process of iron- and manganese-oxides reduction combined with acid dissolution of metal oxides, which remarkably rely on the reductive conversion of Fe–Mn oxides. The reductive leaching of ferromanganese nodule in the sulphuric acid system is primarily concerned with reduction leaching of Fe–Mn oxides and acid dissolution of Me(Ni, Co, Cu) metal, could be expressed as:

Me(Ni, Cu, Co)O + 2H⁺ → Me(Ni, Cu, Co)²⁺ + H₂O (7)

In accordance with the earlier finding, Fe³⁺ ions have positive effects on the reductive leaching of manganese nodules under the test conditions. The leaching performance of manganese nodules and cyclic voltammetry analysis suggests that Fe³⁺ ions could lessen the hydrogen consumption happening at the interfacial layer.³⁰ Fe³⁺ ions could be converted into Fe²⁺ ions, which preferred to reduce high valence manganese oxides on the nodule surface according to the results of cyclic voltammetry and polarization results.³¹ The impedance test results indicate that Fe³⁺ ions accelerate the charge transfer reaction of manganese nodule reduction.³² Combining the results of the electrochemical measurements, it also can be concluded that the reduction leaching of manganese nodules is mainly controlled by the interfacial electrochemical reduction corresponding to manganese oxide, and the active conversion of Fe³⁺/Fe²⁺ couple could affect the transformation of Mn⁴⁺-oxide in the nodule.

As discussed above, Fe³, the fundamental ion affects the reductive leaching of ferromanganese nodule in sulphuric acid solutions, and it can be interpreted based on two courses: the first is the acceleration generated by the transformation of the Fe³⁺/Fe²⁺ pair, and the second is the hydrogen ion buffer action responding to Fe³⁺ ion hydrolysis on the surface. Fig. 8 presents the probable roles of Fe³⁺ ions on the manganese nodule reduction leaching in the sulphuric acid system.

Fig. 8 Effects of Fe³⁺ ions on manganese nodules reductive leaching in sulphuric acid solution.

4. Conclusions

The reductive leaching of ocean manganese nodule in sulphuric acid solution is mainly presented in two courses, an interface electrochemical reduction of Fe–Mn oxides and an acid dissolution of metal oxides, it is much depended on the reductive failure of the crystal structure of Mn–Fe oxides, and it is principally under the control of the reductive dissolution of high valence manganese oxide.

Fe³⁺ ions, as fundamental ions leaching into lixivium, have beneficial effects on the reductive leaching of manganese nodules in the sulphuric acid solution with limits. The simulated leaching performance of the manganese nodule shows that Fe³⁺ ion could facilitate metal extraction efficiencies under different pH values and Fe³⁺ ion concentration conditions. According to the results of the leaching test and electrochemical analysis, Fe³⁺ ion hydrolysis leads to a compensation of free hydrogen ion on the surface, which positively affects high valence manganese oxide reduction. On the other hand, the presence of Fe³⁺ ions promotes the electrochemical transformations of manganese oxide. The results of electrochemical measurements present that Fe³⁺ ions could accelerate the charge transfer of manganese oxide reduction dissolution in the sulphuric acid leaching system, due to the high-activity conversion of the Fe³⁺/Fe²⁺ redox couple and the underlying reduced trends of high valence manganese oxide.

Author contributions

Kang Jin-xing: writing-original draft preparation, conceptualization, investigation, methodology. Wang Ya-yun: investigation; software. Qiu Yun-fei: writing-reviewing and editing, formal analysis, visualization.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by ‘China Ocean Biological Resources Development Program (DY135-B2-15)’.

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