The Micro-Scaled Characterization of Natural Terrestrial Ferromanganese Coatings and Their Semiconducting Properties

: Di ﬀ erent types of ferromanganese coatings were collected from the Chinese mainland to study their mineralogical characteristics and semiconducting properties. Measurements, including by optical microscope, scanning electron microscope, energy dispersive X-ray spectroscopy, micro-Raman spectrometer and transmission electron microscope, were employed to study their morphology, mineral assemblage, element abundance and distribution patterns. Soil Fe coatings are mainly composed of Al-rich hematite and clays. Soil Fe / Mn coatings can be divided into an outer belt rich in birnessite and an inner belt rich in hematite, goethite, ilmenite and magnetite. Goethite is the only component of rock Fe coatings. Rock Fe / Mn coatings mainly consist of birnessite and hematite, and alternating Fe / Mn-rich layers and Fe / Mn-poor layers can be observed. Powders were scraped o ﬀ from the topmost part of ferromanganese coatings to conduct laboratory photochemical experiments. The photocurrent–time behavior indicates that natural coating electrodes exhibit an immediate increase in photocurrent intensity when exposed to light irradiation. Natural coatings can photo-catalytically degrade 14.3%–58.4% of methyl orange in 10 h. Under light irradiation, the photocurrent enhancement and organic degradation e ﬃ ciency of the rock Fe / Mn coating, which has a close intergrowth structure of Fe and Mn components, is most signiﬁcant. This phenomenon is attributed to the formation of semiconductor heterojunctions, which can promote the separation of electrons and holes. Terrestrial ferromanganese coatings are common in natural settings and rich in semiconducting Fe / Mn oxide minerals. Under solar light irradiation, these coatings can catalyze important photochemical processes and will thus have an impact on the surrounding environment.


Introduction
Fe and Mn are the most abundant transition elements on Earth and can form different oxide mineral phases. So far, more than 16 Fe oxide minerals and 30 Mn oxide minerals have been identified in a wide variety of geological settings [1,2]. Among various carriers rich in Fe/Mn oxides, soil cutan and rock varnish are the most representative deposits which have been extensively studied and reported in the literature. Soil cutan covers the surface of large soil blocks and mineral grains. However, the mineralogy of soil cutan has not been totally revealed due to the fine grains, poor crystallinity and low content of Mn oxides. It is reported that Mn oxide minerals exhibit several phases, including birnessite, lithiophorite, todorokite, vernadite, hollandite, etc., among which lithiophorite and brinessite are the most frequently reported ones [3][4][5]. Huang et al. [6,7] systematically studied the elemental distribution patterns and morphology of soil cutan in China and indicated that cutan can be easily Deep and smooth micro-basins on the soil block and rock surface were located for the preparation of powders, because these coatings can be thick and the incorporation of substrate composition can hence be reduced. We scraped off the topmost part of the coatings and prepared fine-grained powders by using a high-purity quartz rod. For each type of coating, 50 mg powders were mixed with anhydrous ethanol (400 µL) and Nafion (10 µL) to make the mineral paste. Then, the mineral paste was evenly smeared on a transparent conductive fluorine-doped tin oxide (FTO) substrate. The coating electrodes were used for photocurrent measurements after drying for 10 h in air.

Micro-Raman Spectroscopy
Most Fe/Mn oxide minerals are poorly crystalized with small sizes. Therefore, they are difficult to accurately identify by traditional methods like XRD. Vibrational spectroscopy, such as Raman spectra, is sensitive to amorphous components and those with a short-range order, which can yield a more complete and reliable result [32,33]. Micro-Raman spectra analysis of sample thin sections can exclude the interference of substrate. The spectra were recorded using a micro-Raman spectrometer (inVia Reflex, Renishaw, UK) with a laser excitation wavenumber of 785 nm to avoid fluorescence signals. Spectra acquisition was performed under a 50× Leica objective lens (NA = 0.75) across the 100-1300 cm −1 wavenumber range, and the spot size was 1 µm. The integration time for individual measurement was 5 s. Accumulation times ranged from 3 to 10, which was based on the spectrum quality and signal to noise ratio. The Raman test had a wavenumber resolution of less than 1 cm −1 .

Scanning Electron Microscope
Representative sample thin sections were chosen and sputtered with Cr for analysis using a Quanta 650 FEG field emission SEM with EDS for chemical analysis (Hillsboro, OR, USA). Morphological observation was carried out by the application of secondary electron and back-scattered electron detectors. The chemical composition of different ferromanganese coatings was investigated with EDS using automated quantitative elemental analysis mode. Element content was given as weight percentage. EDS mapping was conducted to investigate the elemental distribution patterns. The SEM/EDS analysis was carried out at an acceleration voltage of 14 kV.

Transmission Electron Microscope
TEM coupled with EDS equipment and selected area electron diffraction were used to provide crystallographic information of Fe/Mn minerals in coatings. After ultrasonic dispersion, 0.1 mL alcohol, which contained coating powders, were dripped on a mesh copper grid. The alcohol was then absorbed by filter paper from the other side of the grid. The mesh copper grid was placed into the sample holder which was attached to the specimen stage. The experiment was carried out with a JEM-2100F TEM (JEOL, Aichi, Japan), operating at 200 kV. The point resolution was 0.19 nm.

Photoelectrochemical Measurements
Photocurrent measurements were carried out with a set-up consisting of a cylindrical quartz glass reactor. A white LED lamp with a three-electrode configuration was used to simulate sun light irradiation of the samples. A platinum sheet (1 cm × 1 cm) was used as the counter electrode. The reference electrode was an Ag/AgCl (4 M KCl) electrode and working electrode was the natural coating powders covered in FTO substrate. To maintain sufficient solution conductivity, 0.5 mol/L Na 2 SO 4 was chosen as the supporting electrolyte. An area of 2.5 × 3 cm 2 of the natural coating electrodes was irradiated by the lamp from the back side, and the incident intensity was adjusted to 120 mW/cm 2 . The cooling of the reactor was established by means of air flow using an incorporated fan. A potential of 0.8 V (vs. Ag/AgCl) was applied in the photocurrent experiments.
In the MO degradation experiments, 10 mg coating powders were added into the quartz glass reactor, which contained 100 mL MO solution with 4.0 mg/L initial concentration. The pH of the solution was fixed at 7.0 using a Mettler Toledo pH meter. The light source configuration was the Coatings 2020, 10, 666 4 of 16 same as for the photocurrent experiments described above. One milliliter of suspension was collected from the reactor at regular time intervals (30 min) to determine the residual concentration of MO. The concentration of MO was determined by measuring the absorbance at 476 nm with a diode array UV-2102 PC spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA). This maximum absorbance peak was attributed to the azo bond of MO [34]. The assays were repeated twice to contribute to a data error ≤ 2%. The total reaction time was 600 min. The efficiency of MO degradation was estimated by this equation: Here, C 0 (mg/L) and C t (mg/L) stand for the MO concentration at the beginning and at time t, respectively.

Mineralogical Characteristics of Soil Ferromanganese Coatings
The soil Fe coating is red or yellow in color, wrapping large soil blocks and mineral clasts like quartz (Figure 1a,b). The observation of thin sections indicates that the thickness of the coating is 10-50 µm (Figure 1c). In the SEM image, the Fe coating is porous and features holes with sizes less than 1 µm (Figure 1d). The elemental Fe distribution correlates with the Al distribution ( Figure 1e,f), indicating that these two elements may co-exist in a common mineral phase. The Fe content in soil Fe coatings can be enriched to 3.59 wt.%-17.42 wt.% (Table S1). Si is much more abundant in the substrate when compared with its low content in the coating (Figure 1g). The micro-Raman spectra analysis indicates that the Fe oxide mineral in the coating is mainly hematite. Three diagnostic Fe-O symmetric bending vibration modes can be discriminated at 297, 409 and 609 cm −1 (Figure 1h). Hematite belongs to the D 3d 6 crystal space group and seven phonon lines are expected in the Raman spectrum, namely two A 1g modes and five E g modes [35]. Commonly, the 293 and 297 cm −1 bands are only resolved at 100 K or less [36]. The shoulder band at 498 cm −1 in our results is too weak to be detected. The diagnostic Raman bands at 225 and 247 cm −1 in our results are missing due to Al substitution for Fe, as indicated by the EDS mapping. According to the literature [37], the Raman intensity of these two bands decreases sharply with growing Al substitution for Fe in hematite. This variation is caused by a local disorder correlated to the insertion of Al 3+ ions into the FeO 6 octahedron [37].
Coatings 2020, 10, x FOR PEER REVIEW 4 of 17 concentration of MO was determined by measuring the absorbance at 476 nm with a diode array UV-2102 PC spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA). This maximum absorbance peak was attributed to the azo bond of MO [34]. The assays were repeated twice to contribute to a data error ≤ 2%. The total reaction time was 600 min. The efficiency of MO degradation was estimated by this equation: Here, C0 (mg/L) and Ct (mg/L) stand for the MO concentration at the beginning and at time t, respectively.

Mineralogical Characteristics of Soil Ferromanganese Coatings
The soil Fe coating is red or yellow in color, wrapping large soil blocks and mineral clasts like quartz (Figure 1a,b). The observation of thin sections indicates that the thickness of the coating is ~10-50 μm (Figure 1c). In the SEM image, the Fe coating is porous and features holes with sizes less than 1 μm (Figure 1d). The elemental Fe distribution correlates with the Al distribution ( Figure 1e,f), indicating that these two elements may co-exist in a common mineral phase. The Fe content in soil Fe coatings can be enriched to 3.59-17.42 wt.% (Table S1). Si is much more abundant in the substrate when compared with its low content in the coating (Figure 1g). The micro-Raman spectra analysis indicates that the Fe oxide mineral in the coating is mainly hematite. Three diagnostic Fe-O symmetric bending vibration modes can be discriminated at 297, 409 and 609 cm −1 (Figure 1h). Hematite belongs to the D3d 6 crystal space group and seven phonon lines are expected in the Raman spectrum, namely two A1g modes and five Eg modes [35]. Commonly, the 293 and 297 cm −1 bands are only resolved at 100 K or less [36]. The shoulder band at 498 cm −1 in our results is too weak to be detected. The diagnostic Raman bands at 225 and 247 cm −1 in our results are missing due to Al substitution for Fe, as indicated by the EDS mapping. According to the literature [37], the Raman intensity of these two bands decreases sharply with growing Al substitution for Fe in hematite. This variation is caused by a local disorder correlated to the insertion of Al 3+ ions into the FeO6 octahedron [37].   The soil Fe/Mn coating is black in color, covering the surface of soil blocks or mineral clasts ( Figure 2a). The observation of thin sections indicates that the coating is~30-50 µm in thickness ( Figure 2b). The coating consists of different layers. The outer layer is rich in Mn with a thickness of~30 µm, while the inner layer is rich in Fe with a thickness of~20 µm. EDS mapping indicates a distinct boundary between these two layers (Figure 2d,e). Si and Al are not enriched in this coating compared with their high contents in the substrate (Figure 2f,g). The Mn content can be enriched to 9.77 wt.%-11.57 wt.% in the outer layer (Table S2). Micro-Raman spectra analysis reveals that the Mn phase is birnessite. Three diagnostic Mn-O stretching vibration modes at 514, 589 and 649 cm −1 (A g phonon species) and one Mn-O bending vibration mode at 292 cm −1 (B g phonon species) can be observed (Figure 3a). The Fe-rich layer exhibits several different phases, including hematite, goethite and ilmenite (Figure 3b The soil Fe/Mn coating is black in color, covering the surface of soil blocks or mineral clasts ( Figure 2a). The observation of thin sections indicates that the coating is ~30-50 μm in thickness ( Figure 2b). The coating consists of different layers. The outer layer is rich in Mn with a thickness of ~30 μm, while the inner layer is rich in Fe with a thickness of ~20 μm. EDS mapping indicates a distinct boundary between these two layers (Figure 2d,e). Si and Al are not enriched in this coating compared with their high contents in the substrate (Figure 2f,g). The Mn content can be enriched to 9.77-11.57 wt.% in the outer layer (Table S2). Micro-Raman spectra analysis reveals that the Mn phase is birnessite. Three diagnostic Mn-O stretching vibration modes at 514, 589 and 649 cm −1 (Ag phonon species) and one Mn-O bending vibration mode at 292 cm −1 (Bg phonon species) can be observed (Figure 3a). The Fe-rich layer exhibits several different phases, including hematite, goethite and ilmenite (Figure 3b    The soil Fe/Mn coating is black in color, covering the surface of soil blocks or mineral clasts ( Figure 2a). The observation of thin sections indicates that the coating is ~30-50 μm in thickness ( Figure 2b). The coating consists of different layers. The outer layer is rich in Mn with a thickness of ~30 μm, while the inner layer is rich in Fe with a thickness of ~20 μm. EDS mapping indicates a distinct boundary between these two layers (Figure 2d,e). Si and Al are not enriched in this coating compared with their high contents in the substrate (Figure 2f,g). The Mn content can be enriched to 9.77-11.57 wt.% in the outer layer (Table S2)    Under TEM, the EDS mapping was carried out to locate the regions of Fe/Mn-rich particles. The EDS data reveal an area with an Mn content of 23.20% (Figure 4a,b). There are many Si and Al components in this region because the incorporation of clays could not be avoided during sample preparation. Ba is also enriched in this region, mainly because of its good affinity to Mn oxide minerals ( Figure 4b). Mn oxide minerals are commonly very small with poor crystallinity. Some weak and discontinuous lattice fringes can be discriminated in the crystalline domains ( Figure 4c). A region of this lattice photograph is chosen for fast Fourier transform (FFT), inverse FFT and calibration ( Figure 4d). The d spacing value of 0.72 nm can be assigned to the crystal face of (001) of birnessite, and the diffraction pattern shows a clear hexagonal lattice (Figure 4d).
preparation. Ba is also enriched in this region, mainly because of its good affinity to Mn oxide minerals ( Figure 4b). Mn oxide minerals are commonly very small with poor crystallinity. Some weak and discontinuous lattice fringes can be discriminated in the crystalline domains ( Figure 4c). A region of this lattice photograph is chosen for fast Fourier transform (FFT), inverse FFT and calibration ( Figure 4d). The d spacing value of 0.72 nm can be assigned to the crystal face of (001) of birnessite, and the diffraction pattern shows a clear hexagonal lattice (Figure 4d).
Similarly, an area with an Fe content of 51.70% is discovered (Figure 5a,b). This particle is an aggregate composed of different Fe nanocrystals, demonstrating a granular morphology with a diameter of ~70 nm (Figure 5a). A region with clear lattice fringes is selected (Figure 5c), in which a broad d spacing value of 0.42 nm is most obvious (Figure 5c,d) and can be assigned to the lattice face of (110) of goethite. The parallel lattice fringes in Figure 5e,f display a d spacing value of 0.27 nm, which can be attributed to the crystal face of (020) of hematite. Under TEM, we also locate one single crystal of magnetite ( Figure 6a). Lattice fringes in Figure 6b are obvious and continuous with a d spacing value of 0.29 nm, and they can be assigned to the crystal face of (220) of magnetite. The electron diffraction pattern shows a clear cubic lattice (Figure 6c), which is consistent with the crystal structure of magnetite.  Similarly, an area with an Fe content of 51.70% is discovered (Figure 5a,b). This particle is an aggregate composed of different Fe nanocrystals, demonstrating a granular morphology with a diameter of~70 nm (Figure 5a). A region with clear lattice fringes is selected (Figure 5c), in which a broad d spacing value of 0.42 nm is most obvious (Figure 5c,d) and can be assigned to the lattice face of (110) of goethite. The parallel lattice fringes in Figure 5e,f display a d spacing value of 0.27 nm, which can be attributed to the crystal face of (020) of hematite. Under TEM, we also locate one single crystal of magnetite (Figure 6a). Lattice fringes in Figure 6b are obvious and continuous with a d spacing value of 0.29 nm, and they can be assigned to the crystal face of (220) of magnetite. The electron diffraction pattern shows a clear cubic lattice (Figure 6c), which is consistent with the crystal structure of magnetite.

Mineralogical Characteristics of Rock Ferromanganese Coatings
The rock Fe coating is black in color, covering sandstone with a thickness of ~50 μm (Figure 7a). Raman spectra indicate that the Fe oxide mineral in this coating is goethite. Five diagnostic Raman bands of goethite can be observed, including two sharp bands at 299 and 397 cm −1 , and three weak bands at 242, 477 and 553 cm −1 (Figure 7b). EDS mapping is carried out in the rectangular region in Figure 7c and results indicate that the Fe distribution pattern matches the morphology of the Fe coating in the back-scattered image (Figure 7d). The mapping results and EDS data reveal that the Si and Al amount is very low in this coating (Figure 7e,f, Table S3) and the Fe content can be enriched to 58.18-61.61 wt.% (Table S3).

Mineralogical Characteristics of Rock Ferromanganese Coatings
The rock Fe coating is black in color, covering sandstone with a thickness of ~50 μm (Figure 7a). Raman spectra indicate that the Fe oxide mineral in this coating is goethite. Five diagnostic Raman bands of goethite can be observed, including two sharp bands at 299 and 397 cm −1 , and three weak bands at 242, 477 and 553 cm −1 (Figure 7b). EDS mapping is carried out in the rectangular region in Figure 7c and results indicate that the Fe distribution pattern matches the morphology of the Fe coating in the back-scattered image (Figure 7d). The mapping results and EDS data reveal that the Si and Al amount is very low in this coating (Figure 7e,f, Table S3) and the Fe content can be enriched to 58.18-61.61 wt.% (Table S3).

Mineralogical Characteristics of Rock Ferromanganese Coatings
The rock Fe coating is black in color, covering sandstone with a thickness of~50 µm (Figure 7a). Raman spectra indicate that the Fe oxide mineral in this coating is goethite. Five diagnostic Raman bands of goethite can be observed, including two sharp bands at 299 and 397 cm −1 , and three weak bands at 242, 477 and 553 cm −1 (Figure 7b). EDS mapping is carried out in the rectangular region in Figure 7c and results indicate that the Fe distribution pattern matches the morphology of the Fe coating in the back-scattered image (Figure 7d). The mapping results and EDS data reveal that the Si and Al amount is very low in this coating (Figure 7e,f, Table S3) and the Fe content can be enriched to 58.18 wt.%-61.61 wt.% (Table S3). The rock Fe/Mn coating in this research is commonly known as rock varnish, which covers quartzite and granite. The coating is black (Figure 8a,b) and has a thickness of ~30-50 μm (Figure 8c   The rock Fe/Mn coating in this research is commonly known as rock varnish, which covers quartzite and granite. The coating is black (Figure 8a,b) and has a thickness of~30-50 µm (Figure 8c). EDS mapping indicates that Mn and Fe are distributed throughout the whole sample profile (Figure 8e,f), and there is no clear boundary between the Mn and Fe components. Alternating Mn-rich and Mn-poor layers can be discovered in this coating with a thickness of~25 µm, and each layer is~2-4 µm wide. Some common petrogenetic elements, like Si, are not enriched in this coating contrary to their high contents in the substrate (Figure 8g). The Mn content can be enriched to 11.73 wt.%-27.61 wt.% and the Fe content can be enriched to 5.30 wt.%-13.98 wt.% in the rock Fe/Mn coating (Table S4). Micro-Raman spectra analysis reveals that the Mn oxide mineral in the varnish is birnessite. Two diagnostic Mn-O stretching vibration modes at 592 and 658 cm −1 and one Mn-O bending vibration mode at 296 cm −1 can be observed (Figure 9a). According to the literature [38], the Raman band at 145 cm −1 (Figure 9a) can be attributed to the stretching modes of the interlayer cation groups (NaO 6 or KO 6 ) in birnessite. The Fe oxide mineral in the varnish is mainly hematite. Diagnostic Raman bands of hematite are located at 224, 292, 408, 475 and 607 cm −1 (Figure 9b). The rock Fe/Mn coating in this research is commonly known as rock varnish, which covers quartzite and granite. The coating is black (Figure 8a,b) and has a thickness of ~30-50 μm (Figure 8c (Figure 10g,h) and are attributed to the lattice faces of (110) and (104) of hematite, respectively. The parallel lattice fringes in Figure 10k show a broad d spacing value of 0.5 nm, which can be assigned to the crystal face of (020) of goethite (Figure 10l).   (Figure 10g,h) and are attributed to the lattice faces of (110) and (104) of hematite, respectively. The parallel lattice fringes in Figure 10k show a broad d spacing value of 0.5 nm, which can be assigned to the crystal face of (020) of goethite (Figure 10l).  (Figure 10g,h) and are attributed to the lattice faces of (110) and (104) of hematite, respectively. The parallel lattice fringes in Figure 10k show a broad d spacing value of 0.5 nm, which can be assigned to the crystal face of (020) of goethite (Figure 10l).

Semiconducting Properties of Natural Ferromanganese Coatings
The capability of converting light energy to electricity was confirmed by photochemical experiments. The photocurrent-time behavior of natural ferromanganese coating electrodes demonstrates that photocurrent intensity immediately increases when light is turned on and decreases back to the baseline level when light is switched off under a given voltage of 0.8 V (vs. Ag/AgCl) (Figure 11). The dark currents in all cases were negligible, while during the irradiation period, enhanced, sensitive and stable current signals were recorded. This phenomenon indicates that simulated sunlight facilitated the formation of more free electrons from semiconducting Fe/Mn oxide minerals. In particular, the rock Fe/Mn coating and soil Fe/Mn coating produced stable photocurrent densities of 18.5 and 16.5 µA/cm 2 under irradiation, respectively, showing the best photoelectric conversion efficiency compared with other counterparts. The soil Fe coating and rock Fe coating only yield enhanced photocurrent densities of 3.5 and 2.0 µA/cm 2 , respectively.

Semiconducting Properties of Natural Ferromanganese Coatings
The capability of converting light energy to electricity was confirmed by photochemical experiments. The photocurrent-time behavior of natural ferromanganese coating electrodes demonstrates that photocurrent intensity immediately increases when light is turned on and decreases back to the baseline level when light is switched off under a given voltage of 0.8 V (vs. Ag/AgCl) ( Figure 11). The dark currents in all cases were negligible, while during the irradiation period, enhanced, sensitive and stable current signals were recorded. This phenomenon indicates that simulated sunlight facilitated the formation of more free electrons from semiconducting Fe/Mn oxide minerals. In particular, the rock Fe/Mn coating and soil Fe/Mn coating produced stable photocurrent densities of 18.5 and 16.5 μA/cm 2 under irradiation, respectively, showing the best photoelectric conversion efficiency compared with other counterparts. The soil Fe coating and rock Fe coating only yield enhanced photocurrent densities of 3.5 and 2.0 μA/cm 2 , respectively. Methyl orange dye is a model compound to evaluate the activity of photocatalysts. During the photochemical experiments, two main processes could have contributed to the decrease in the MO concentration. The composition of natural ferromanganese coatings is complicated and inhomogeneous, and MO can hence be physically adsorbed on clays, Fe/Mn oxide minerals and even on the experimental device. The second is the photocatalysis process mediated by Fe/Mn oxide minerals in natural coatings. Under irradiation, electrons are excited from the valence band (VB) of semiconducting minerals to the conduction band (CB), thus leaving photo-generated holes in the VB with good oxidizing capability. Consequently, MO can be scavenged by the holes or free radicals [28]. Therefore, control groups were set up during our experiments ( Figure 12). These four types of natural ferromanganese coatings demonstrate similar trends in MO degradation. In the presence of natural coating powders, the concentration of MO decreases with time in both light and dark systems. The degradation efficiency is much better in the light system in comparison with the dark system. This observation indicates that the photocatalytic degradation process is more effective than physical adsorption. After a reaction time of 600 min, the degradation ratios of MO in the soil Fe coating light system and dark system are 34.7% and 6.8%, respectively, which means the degradation ratio of the photocatalytic process is 27.9% (Figure 12a). Similarly, the degradation ratios of the photocatalytic processes in the soil Fe/Mn coating, rock Fe coating and rock Fe/Mn coating systems are 43.3%, 14.3% and 58.4%, respectively ( Figure 12). The rock Fe/Mn coating, which has a close intergrowth structure of Mn and Fe components, exhibits the best performance in MO degradation (Table 1). Methyl orange dye is a model compound to evaluate the activity of photocatalysts. During the photochemical experiments, two main processes could have contributed to the decrease in the MO concentration. The composition of natural ferromanganese coatings is complicated and inhomogeneous, and MO can hence be physically adsorbed on clays, Fe/Mn oxide minerals and even on the experimental device. The second is the photocatalysis process mediated by Fe/Mn oxide minerals in natural coatings. Under irradiation, electrons are excited from the valence band (VB) of semiconducting minerals to the conduction band (CB), thus leaving photo-generated holes in the VB with good oxidizing capability. Consequently, MO can be scavenged by the holes or free radicals [28]. Therefore, control groups were set up during our experiments ( Figure 12). These four types of natural ferromanganese coatings demonstrate similar trends in MO degradation. In the presence of natural coating powders, the concentration of MO decreases with time in both light and dark systems. The degradation efficiency is much better in the light system in comparison with the dark system. This observation indicates that the photocatalytic degradation process is more effective than physical adsorption. After a reaction time of 600 min, the degradation ratios of MO in the soil Fe coating light system and dark system are 34.7% and 6.8%, respectively, which means the degradation ratio of the photocatalytic process is 27.9% (Figure 12a). Similarly, the degradation ratios of the photocatalytic processes in the soil Fe/Mn coating, rock Fe coating and rock Fe/Mn coating systems are 43.3%, 14.3% and 58.4%, respectively ( Figure 12). The rock Fe/Mn coating, which has a close intergrowth structure of Mn and Fe components, exhibits the best performance in MO degradation (Table 1).

Discussion
Chemical reactions catalyzed by minerals have been frequently reported. For example, clay minerals can adsorb organic matters to form RNA, which is important to the origin of life [39] and colloidal (Fe,Ni)S can adsorb α-amino acids and carbon monoxide on its surface to form peptides [40]. Photocatalysis, as one of the most fundamental chemical reactions that happens on mineral surfaces, should have some influence on modern environments as well. In the field of photocatalysis, rutile TiO2 is the most extensively studied due to its excellent performance. However, the wide band gap of rutile (~3.2 eV) leads to a lack of absorption in the visible portion of the solar light spectrum, which limits its application. In order to increase its photocatalytic activity under visible irradiation, efforts were taken to narrow the band gap of TiO2 by doping metals and introducing vacancies [41,42]. Furthermore, new photocatalysts with lower band gap energies were alternatively explored [43,44]. For a long time, natural semiconducting Fe/Mn oxide minerals, which are much more abundant than TiO2, ZnS and WO3 in storage, have been neglected.
Fe/Mn oxides are common semiconducting minerals that occur in various weathering environments. They can accumulate to form different Fe/Mn-rich coatings under certain conditions. Rock coatings are common in dry arid regions where the pH condition is alkalescent. Soil coatings are common in wet subtropical areas where the pH condition is faintly acid. In this paper, we studied four types of ferromanganese coatings and they displayed different elemental distribution patterns and mineral assemblage. However, in either case, hematite, goethite and birnessite are the major mineral components in terrestrial ferromanganese coatings. The band gaps of hematite, goethite and

Discussion
Chemical reactions catalyzed by minerals have been frequently reported. For example, clay minerals can adsorb organic matters to form RNA, which is important to the origin of life [39] and colloidal (Fe,Ni)S can adsorb α-amino acids and carbon monoxide on its surface to form peptides [40]. Photocatalysis, as one of the most fundamental chemical reactions that happens on mineral surfaces, should have some influence on modern environments as well. In the field of photocatalysis, rutile TiO 2 is the most extensively studied due to its excellent performance. However, the wide band gap of rutile (~3.2 eV) leads to a lack of absorption in the visible portion of the solar light spectrum, which limits its application. In order to increase its photocatalytic activity under visible irradiation, efforts were taken to narrow the band gap of TiO 2 by doping metals and introducing vacancies [41,42]. Furthermore, new photocatalysts with lower band gap energies were alternatively explored [43,44]. For a long time, natural semiconducting Fe/Mn oxide minerals, which are much more abundant than TiO 2 , ZnS and WO 3 in storage, have been neglected.
Fe/Mn oxides are common semiconducting minerals that occur in various weathering environments. They can accumulate to form different Fe/Mn-rich coatings under certain conditions. Rock coatings are common in dry arid regions where the pH condition is alkalescent. Soil coatings are common in wet subtropical areas where the pH condition is faintly acid. In this paper, we studied four types of ferromanganese coatings and they displayed different elemental distribution patterns and mineral assemblage. However, in either case, hematite, goethite and birnessite are the major mineral components in terrestrial ferromanganese coatings. The band gaps of hematite, goethite and birnessite are estimated to be 2.2, 2.6 and 1.82 eV, respectively [45,46]. These minerals are visible-light responsive, i.e., the excitation of photoelectrons from their VBs to CBs can occur under the irradiation wavelength ≤478-683 nm. In geological settings, Fe/Mn oxide minerals continuously absorb solar energy and drive photoredox reactions relevant to the evolution of substances. For example, Fe/Mn oxide minerals can catalyze the formation of ROSs, which can later promote the abiotic oxidation of Mn(II) [47,48] and the decomposition of organic matter [28]. In the euphotic zones of marine and freshwater systems, the photocatalytic self-reduction of birnessite, hematite and goethite serves as a key driver for Mn(II) and Fe(II) ion release [49][50][51].
The semiconducting properties of natural ferromanganese coatings were studied by laboratory experiments. The photocurrent-time behavior indicates that natural coating electrodes have a good response to irradiation. The ferromanganese coatings can also promote the degradation of MO through photocatalysis. Considering the inhomogeneous chemistry of natural ferromanganese coatings, it is not feasible for us to quantitatively determine the photochemical performance of Fe/Mn oxide minerals. However, among these four types of coatings, the rock Fe/Mn coating, which has a close intergrowth structure of Fe and Mn compounds as observed by EDS mapping (Figure 8), exhibited the best photochemical performance (Figures 11 and 12; Table 1). This phenomenon may be caused by the enhancement of electron-hole separation at the hematite-birnessite interface expected from their band edge position ( Figure 13). The CB potentials of birnessite and hematite are −4.37 and −4.87 eV, respectively, while the VB potentials of birnessite and hematite are −6.19 and −7.07 eV, respectively [45,46]. Thus, it is thermodynamically feasible for photoelectrons to transfer from the CB of birnessite to that of hematite, and for holes to transfer from the VB of hematite to that of birnessite ( Figure 13). This spatial separation of photo-generated electrons and holes is beneficial to prevent charge recombination, and therefore results in a higher photocatalytic activity [52].
Coatings 2020, 10, x FOR PEER REVIEW 12 of 17 birnessite are estimated to be 2.2, 2.6 and 1.82 eV, respectively [45,46]. These minerals are visible-light responsive, i.e., the excitation of photoelectrons from their VBs to CBs can occur under the irradiation wavelength ≤ 478-683 nm. In geological settings, Fe/Mn oxide minerals continuously absorb solar energy and drive photoredox reactions relevant to the evolution of substances. For example, Fe/Mn oxide minerals can catalyze the formation of ROSs, which can later promote the abiotic oxidation of Mn(II) [47,48] and the decomposition of organic matter [28]. In the euphotic zones of marine and freshwater systems, the photocatalytic self-reduction of birnessite, hematite and goethite serves as a key driver for Mn(II) and Fe(II) ion release [49][50][51].
The semiconducting properties of natural ferromanganese coatings were studied by laboratory experiments. The photocurrent-time behavior indicates that natural coating electrodes have a good response to irradiation. The ferromanganese coatings can also promote the degradation of MO through photocatalysis. Considering the inhomogeneous chemistry of natural ferromanganese coatings, it is not feasible for us to quantitatively determine the photochemical performance of Fe/Mn oxide minerals. However, among these four types of coatings, the rock Fe/Mn coating, which has a close intergrowth structure of Fe and Mn compounds as observed by EDS mapping (Figure 8), exhibited the best photochemical performance (Figures 11 and 12; Table 1). This phenomenon may be caused by the enhancement of electron-hole separation at the hematite-birnessite interface expected from their band edge position ( Figure 13). The CB potentials of birnessite and hematite are −4.37 and −4.87 eV, respectively, while the VB potentials of birnessite and hematite are −6.19 and −7.07 eV, respectively [45,46]. Thus, it is thermodynamically feasible for photoelectrons to transfer from the CB of birnessite to that of hematite, and for holes to transfer from the VB of hematite to that of birnessite ( Figure 13). This spatial separation of photo-generated electrons and holes is beneficial to prevent charge recombination, and therefore results in a higher photocatalytic activity [52]. Band edges are important parameters to evaluate redox reactions and they can be predicted from the electronegativity of the semiconductor [45]. The CB edge of a compound can be expressed in an absolute vacuum scale (AVS, eV) according to the following equation: where arameter X is the absolute electronegativity for this compound, Eg stands for the band gap and pHpzc is the pH condition at which a compound demonstrates zero electrical charge on its surface. Furthermore, the energy position of band edges with respect to the normal hydrogen electrode (NHE) can be converted from the values in the AVS scale using: (3) Band edges are important parameters to evaluate redox reactions and they can be predicted from the electronegativity of the semiconductor [45]. The CB edge of a compound can be expressed in an absolute vacuum scale (AVS, eV) according to the following equation: where arameter X is the absolute electronegativity for this compound, E g stands for the band gap and pH pzc is the pH condition at which a compound demonstrates zero electrical charge on its surface.
Furthermore, the energy position of band edges with respect to the normal hydrogen electrode (NHE) can be converted from the values in the AVS scale using: The specific semiconducting parameters of hematite, goethite and birnessite are listed in Table 2. The curves of the band edge position as a function of pH value are plotted ( Figure 14). Irrespective of the faintly acid or alkalescent pH conditions for red soils or arid deserts, the redox potential of humic acid (HA) is well above the VB position of these semiconducting minerals. Humic acid is abundant in almost all environments and it can be oxidized by VB photoholes and separate CB photoelectrons. These free photoelectrons are scavenged by O 2 in the atmosphere, reduce materials in the environment or provide energy sources for bacterial metabolisms [53,54]. The specific semiconducting parameters of hematite, goethite and birnessite are listed in Table  2. The curves of the band edge position as a function of pH value are plotted ( Figure 14). Irrespective of the faintly acid or alkalescent pH conditions for red soils or arid deserts, the redox potential of humic acid (HA) is well above the VB position of these semiconducting minerals. Humic acid is abundant in almost all environments and it can be oxidized by VB photoholes and separate CB photoelectrons. These free photoelectrons are scavenged by O2 in the atmosphere, reduce materials in the environment or provide energy sources for bacterial metabolisms [53,54]. Table 2. Absolute electronegativity (X), band gap (Eg) and band edge position (Ev and Ec) of hematite, goethite and birnessite in an AVS (absolute vacuum scale) scale at pH 7.

Minerals
Eg ( [55,56] is above the VB (valence band) position of these minerals, which indicates that MO and HA can serve as photohole scavengers to promote the separation of photoelectrons.

Conclusions
Natural terrestrial ferromanganese coatings can develop different mineralogical features. Soil Fe coatings are mainly composed of Al-rich hematite and clays. Soil Fe/Mn coatings can be divided into an outer belt rich in birnessite and an inner belt rich in hematite, goethite, ilmenite and magnetite. Goethite is the only component of rock Fe coatings. Rock Fe/Mn coatings mainly consist of birnessite and hematite, and alternating Fe/Mn-rich and Fe/Mn-poor layers can be discriminated. The photocurrent-time behavior indicates that natural coating electrodes exhibit an immediate increase in photocurrent intensity when exposed to light irradiation. Natural coatings show good performance in the photocatalytic degradation of MO. Under light irradiation, the photocurrent enhancement and organic degradation efficiency of the rock Fe/Mn coating, which has a close intergrowth structure of Fe and Mn components, is most significant. This phenomenon is attributed to the formation of semiconductor heterojunctions, which can promote the separation of electrons and holes. Terrestrial ferromanganese coatings are common in natural settings and have a good  [55,56] is above the VB (valence band) position of these minerals, which indicates that MO and HA can serve as photohole scavengers to promote the separation of photoelectrons.

Conclusions
Natural terrestrial ferromanganese coatings can develop different mineralogical features. Soil Fe coatings are mainly composed of Al-rich hematite and clays. Soil Fe/Mn coatings can be divided into an outer belt rich in birnessite and an inner belt rich in hematite, goethite, ilmenite and magnetite. Goethite is the only component of rock Fe coatings. Rock Fe/Mn coatings mainly consist of birnessite and hematite, and alternating Fe/Mn-rich and Fe/Mn-poor layers can be discriminated. The photocurrent-time behavior indicates that natural coating electrodes exhibit an immediate increase in photocurrent intensity when exposed to light irradiation. Natural coatings show good performance in the photocatalytic degradation of MO. Under light irradiation, the photocurrent enhancement and organic degradation efficiency of the rock Fe/Mn coating, which has a close intergrowth structure of Fe and Mn components, is most significant. This phenomenon is attributed to the formation of semiconductor heterojunctions, which can promote the separation of electrons and holes. Terrestrial ferromanganese coatings are common in natural settings and have a good responding capability to visible light irradiation. Therefore, these coatings can exert great influence on surrounding environments by catalyzing important photochemical reactions.
Supplementary Materials: The following are available online at http://www.mdpi.com/2079-6412/10/7/666/s1, Table S1: Chemical compositions of soil Fe coating. Note: b.d.l. = below the detection limit, Table S2: Chemical compositions of the soil Fe/Mn coating. Spots 1-5 are for the outer Mn-rich layer and spots 6-10 are for the inner Fe-rich layer. Note: b.d.l. = below the detection limit, Table S3: Chemical compositions of the rock Fe coating. Note: b.d.l. = below the detection limit, Table S4: Chemical compositions of the rock Fe/Mn coating. Note: b.d.l. = below the detection limit.