Ferruginous Manganese Ore Research Articles

The thermal decomposition kinetics of carbonaceous and ferruginous manganese ores in atmospheric conditions

The thermal decomposition of carbonate minerals as pre-treatment before smelting reduces the energy requirement for smelting. It can also make the combustion of fossil fuels for heating unnecassary. Thermal decomposition may become important in reducing greenhouse gas emissions when producing ferromanganese alloys while simultaneously reducing electrical energy demand during smelting. A kinetic reaction rate model for the thermal decomposition of manganese ores is presented, based on published reaction rate kinetics for the decomposition of manganese oxides and calcium carbonate. The model was validated against thermogravimetric data for two carbonaceous manganese ore samples and one ferruginous manganese ore sample. The reaction rate model shows that carbonate minerals in the manganese ores are decomposed at temperatures above 900 °C while pyrolusite is decomposed at temperatures from 450 °C to 500 °C. Mn2O3 decomposes rapidly at 550 °C. Braunite decomposition at temperatures below 1000 °C was negligible. The presence of organic carbon in the samples led to further reduction of the samples during thermal treatment.

Development of a novel magnetic separator for segregation of minerals of dissimilar electromagnetic properties

This study has been carried out to develop a novel magnetic separator which uses alternating magnetic field to exploit dissimilarities in electromagnetic properties of minerals. The alternating magnetic field imparts rotational movement in a free-falling hematite particle to enhance particle deflocculation and it also produces repulsive force on semiconductive pyrolusite and diamagnetic quartz particles to improve separation efficiency. A mathematical model has been developed using first principal modelling and C++ programming language to simulate trajectories of different mineral (hematite & pyrolusite) particles of diverse sizes (0.5, 1, 3 mm) under variable magnetic field intensities (5–7 K gauss) and at different rotation speed (15, 25, 30 rpm) of magnetic roll. A magnetic separator of 10 kg/h has been designed and fabricated using the modelling results and setup has been tested using different size steel balls to validate the idea. Real time applicability of the system is tested using ferruginous manganese ores (25.6–43% Mn & 1.1–1.6 Mn/Fe) and results are compared with conventional magnetic separators. Experimental studies have been carried out for different feed rates and slurry densities to maximize the effect of particle rotation and repulsion to improve mineral selectivity and separation. Studies revealed that particle shape is an important parameter for axial rotation of particles and magnetic coagulation takes place to segregate the magnetic and nonmagnetic particles. The results revealed that new design rotating magnetic field separator has achieved 5 to 10.2% increment in Mn % & 0.4-to-0.7-unit increment in Mn/Fe ratio to produce a concentrate (nonmagnetic product) of 35.8–48% Mn & 1.6–3.3 Mn/Fe. It was found that developed equipment can attain higher selectivity index compared to conventional magnetic separators. These studies have shown that the developed magnetic separator has a potential to improve the efficiency of existing magnetic separation process as well as its application can be explored for other multi-magnetic mineral materials such as electronic waste, ilmenite, chromite, etc.

Facile microwave-assisted synthesis of magnetic ferrite: Rapid interfacial reaction underlying intensifying mechanism

Microwave-assisted (MW-assisted) heating is clean and intensively used to synthesize materials owing to the high-efficiency volumetric and selective heating modes. In the current paper, a design and facile synthesis of magnetic ferrite ceramics by microwave-assisted heating of MnO2 and Fe2O3 was conducted to reduce the synthetic temperature of conventional heating. The microwave absorptivity and temperature rise characteristics of the MnO2–Fe2O3 system were first assessed for the material design of MnFe2O4, and then the phase transformation, microstructure evolution, and magnetism change of the MnO2–Fe2O3 system heated under conventional and MW-assisted heating were compared. Investigations on electromagnetic properties and heating behaviors of MnO2–Fe2O3 mixture suggested that a suitable cylinder sample size for microwave heating was designed to be 10 mm in diameter and 10 mm in length based on the dielectric loss and magnetic hysteresis loss of microwave penetration at different depths. The intensifying mechanism for microwave heating was associated with the rapid interfacial reaction between MnO2 and Fe2O3. Under microwave irradiation at 935 °C, some perfectly crystallized MnFe2O4 was preferentially formed at the interface of MnO2 and Fe2O3, and these MnFe2O4 were evenly distributed in the cylinder samples. Similar to the “self-propagating” effect, the in-situ generation of MnFe2O4 with good energy conversion ability evenly distributed everywhere of the cylinder would facilitate the reaction of MnO2 and Fe2O3 under microwave irradiation based on the distinctive selective and volumetric heating. Compared with conventional heating at 1200–1250 °C, ferrite synthetic temperatures via MW-assisted heating decreased to 900–1000 °C, and the low-temperature synthesis of ferrite also had a good electromagnetic performance. Eventually, the clean and value-added utilization of low-grade ferruginous manganese ores via MW-assisted heating was validated for the preparation of manganese ferrite from minerals under reduced roasting temperatures.

Processing of low grade manganese ore in submerged arc furnace

ABSTRACT This paper presents an innovative technology that has been developed for the smelting of Egyptian low-grade manganese ore to produce low-manganese pig iron and high-manganese slag. This produced slag has a high Mn/Fe ratio, which, therefore, can be used in a second smelting process in the production of manganese alloys. In this developed technology, selective reduction of iron oxides from a ferruginous manganese ore (with Mn/Fe ratio equals 0.68) has been carried out via carbo-thermic reduction, in a 5 kg bench scale submerged arc furnace. The highest metallic yield, iron recovery in the pig iron, and MnO recovery in the manganese-rich slag, equal to 98%, 95.82%, and 94.59% respectively, were achieved at the optimum conditions of 4.5 kVA for pre-reduction power, 15 kVA for smelting power, 0.8 for coke ratio and 0.26 for CaO/SiO2 ratio. The reduction process was applied successfully on a 100 kVA pilot scale submerged arc furnace

Simultaneous removal of NO and elemental mercury from coal-fired flue gas using natural ferruginous manganese ore at low temperature

Natural ferruginous manganese ore (FMO) was developed as a cost-effective NH3-SCR catalyst for the simultaneous removal of NO and elemental mercury (Hg0) from coal-fired flue gas. The chemical composition, pore structure, crystal structure and surface chemistry of FMO were characterized by different methods. The results showed that FMO was constructed with sheet-like structures. There were massive slit-shaped pores in the FMO. Manganese and iron oxides existed in an amorphous form. After calcination, the NO removal performance of FMO was obviously decreased due to the decomposition of MnO2. A high NH3/NO ratio was conductive to NO removal but excessive NH3 was unnecessary for NO removal. NO could promote Hg0 removal whereas Hg0 had little effect on NO removal. The effects of different SCR atmosphere (inlet, middle layer, and outlet), reaction temperature (100 °C–400 °C), and flue gas composition (O2 and SO2) on NO/Hg0 removal was also studied. At the optimal temperature (200 °C), the NO removal efficiency was greater than 80% and the Hg0 removal efficiency was close to 100%. To some extent, FMO presented preferable SO2 poisoning resistance probably because of the presence of iron oxide. The mechanism of NO/Hg0 removal over FMO was further described. Both fast SCR and conventional SCR reactions happened over FMO during NO removal. Mn4+, Fe3+, and lattice oxygen were first consumed and then replenished by gaseous O2. FMO served as a catalyst for Hg0 oxidation during a long period of denitration process.

Green and Efficient Utilization of Ferruginous Gibbsite Ore and Ferruginous Manganese Ore by Synergetic Carbothermic Co-Reduction–Magnetic Separation Process

The synergetic utilization of ferruginous gibbsite ores (Al-Fe ores) and ferruginous manganese ores (Mn-Fe ores) by the carbothermic co-reduction roasting–magnetic separation process was proposed as an innovative and green process for the separation and recovery of the valuable metal elements of Mn, Fe and Al from these ores. In this paper, a ferromanganese crude alloy with 72.47% Fe and 10.19% Mn and a high recovery of 85.89% Fe was prepared, which produces an acceptable feed to produce manganese steels with an electric arc furnace. The synergistic co-reduction of the two kinds of complex and refractory minerals was favored to separate Fe, Mn and Al from these ores. The influence of the operating variables on the recovery and separation of valuable metals from Mn-Fe ores and Al-Fe ores is initially studied. Then, the stepwise reduction behaviors of a composite oxide Mn1-xFexO (0 ≤ x ≤ 1) and hercynite (Mn1−yFeyAl2O4, 0 ≤ y ≤ 1) were investigated to clarify that Mn-Fe ores have a positive impact on the reduction of fayalite and hercynite in Al-Fe ores. This study reported a simple green route, the carbothermic co-reduction–magnetic separation process, to economically and effectively treat Al-Fe ores and Mn-Fe ores.

Hydrometallurgical Recovery of Manganese from Ferruginous Manganese Ore by Reductive-Acid Leaching with Sodium Dithionite

Hydrometallurgical Recovery of Manganese from Ferruginous Manganese Ore by Reductive-Acid Leaching with Sodium Dithionite

Potential of natural ferruginous manganese (NFM) ore as a natural adsorbent for As(III) removal at low concentration

ABSTRACT The findings of the current work pertain to the assessment of potential of natural ferruginous manganese (NFM) as a green, effective and economic adsorbent for decontamination of As(III) from aqueous media. Several techniques such as SEM coupled with EDX, BET, FTIR and XRD were used to characterise the NFM adsorbent. The adsorption parameters such as pH, contact time, temperature and NFM dose were considered and investigated in detail for the As(III) removal. The adsorption of As(III) was observed to increase with increasing temperature, concentration, adsorbent dosage and contact time of the system. The maximum As(III) adsorption of 1.9 mg g−1 over NFM revealed it to be an effective adsorbent for removing As(III) in the range of 0.45–2.5 mg/L. Langmuir, Freundlich and Dubinin Radushkevich (D-R) adsorption models were used to analyse the adsorption data and the Langmuir model provided the best fit with high regression coefficient (R 2). The time profile studies demonstrated that As(III) uptake followed a pseudo-second-order kinetics while the maximum As(III) removal of 96% was obtained within the first 50 min of contact time within a pH range of 4–6. Thermodynamics results showed that As(III) adsorption by NFM was spontaneous and endothermic in nature. Based on the results, it is claimed that natural NFM can effectively serve as green, economic and potential natural adsorbent for As(III) removal and likewise from the aqueous solutions.

Characterizations on phase reconstruction, microstructure evolution and separation of magnetic ferrite ceramics from low-grade manganese ores by novel uphill reaction diffusion and magnetic separation

Traditional utilization of ferruginous manganese ore has mainly concentrated on the separation of Mn and Fe. However, an alternative process of novel uphill reaction diffusion followed by magnetic separation was developed for the synchronous recovery of manganese and iron as the form of ferrite ceramics from the ferruginous manganese ore in this work. The phase reconstruction, interfacial uphill reaction diffusion, ferrite size quantification and separation behavior were characterized via various means of XRD, XPS, TEM, VSM, SEM-EDS, optical microscopy, microhardness and thermodynamic analyses. In the phase reconstruction course, manganese and iron oxides reacted to form ferrites, whereas the impurities were enriched in liquid silicate phases due to the uphill reaction diffusion. Meanwhile, the ferrite particles grew and the liquid silicates aggregated owing to the intensified reaction diffusion by CaO addition. During the water quenching process, microcracks at the cleavage plane between the silicate and ferrites are preferentially generated due to fast cooling shrinkage of liquid silicates. Huge differences in hardness, magnetism, fusibility and cooling shrinkage of ferrites and silicates provided excellent mineralogical conditions for the successful grinding and magnetic separation of ferrites. Eventually, a soft magnetic manganese ferrite precursor with recoveries of 89.2 wt% iron and 85.6 wt% manganese was obtained. The interfacial reaction mechanisms for the formation of ferrites and silicates were also discussed in this work.

Enrichment Of Low Grade Ferruginous Manganese Ore By Pilot Scale Wet High Intensity Magnetic Separation (WHIMS)

Enrichment Of Low Grade Ferruginous Manganese Ore By Pilot Scale Wet High Intensity Magnetic Separation (WHIMS)

Influence of Mineralogy on the Dry Magnetic Separation of Ferruginous Manganese Ore—A Comparative Study

Magnetic separation is often considered pertinent for manganese ore beneficiation when the ore is abundant with siliceous rich gangue mineral phases. However, the process is deemed to be inapposite for the ferruginous type of ore, and remains a grey area of research. In the present investigation, two different types of manganese ore were studied in detail to understand the influence of mineralogy on their magnetic separation performance. Detailed experiments were performed by varying the critical variables of the dry magnetic separator, and the separation features were studied. The ore samples were thoroughly characterized by various techniques, including an automated advanced mineralogical tool. The mineralogical results revealed that primary manganese bearing minerals in both the ores are rich in cryptomelene, pyrolusite, psilomelane, and bixybyite. Similarly, the major gangue minerals were alumina-bearing minerals and iron-bearing phases (hematite and goethite). The optimum grade that could be obtained from single-stage dry magnetic separation was 35.52% Mn, and with a Mn:Fe ratio of 1.77, and 44% Mn recovery in the case of sample 1; whereas, a 33.75% Mn grade, with a Mn:Fe ratio of 1.66 at Mn recovery of 44% was reported for Sample 2. It was observed that both samples had a similar input chemistry (~28% Mn, ~1 Mn: Fe ratio) however, they had distinctive mineralogical assemblages. Furthermore, it was observed that the liberation of manganese mineral was in a course size range, i.e., 300 to 450 µm, while the association of iron and manganese bearing phases was lower in sample 1 when compared to sample 2.

New understanding on reduction mechanism and alloying process of rich manganese slag: Phase formation and morphological evolution

A new understanding of the reduction mechanisms of ferruginous manganese ores for ferromanganese alloy smelting was reported, and this new understanding was used to produce excellent ferromanganese alloys. To fulfill this objective, the effects of the operating variables on smelting ferromanganese alloys were initially investigated. The formation mechanisms and reduction reaction behaviors of olivine (CaxFeyMn2-x-y)SiO4, spinel (CaxFeyMn1-y-x)Al2O4, gehlenite (CaxMn2-x)SiAl2O7 and wüstite MnxFeyCa1-x-yO systems were systematically discussed to understand the phase transformation and reconstruction of manganese ore at high temperatures. The spinel (CaxFeyMn1-y-x)Al2O4 that originated from (FeyMn1-y)Al2O4 was converted to CaAl2O4 and then further reduced to ferromanganese alloys. A similar expression was applied to describe how the reduction mechanism of olivine (CaxFeyMn2-x-y)SiO4 that originated from (FeyMn2-y)SiO4 was converted to Ca2SiO4. In addition, the importance of the formation of gehlenite (CaxMn2-x)SiAl2O7 and wüstite MnxFeyCa1-x-yO phases in smelting ferromanganese alloys was discussed and clarified in detail. Based on the findings, recommendations are provided for the theoretical basis and technical support of smelting ferromanganese alloys.

A sustainable reduction roasting technology to upgrade the ferruginous manganese ores

Reduction roasting of manganese ores is a known process; however, it is not yet commercialized mainly due to various techno-economic issues. The current research work presents results of a large-scale plant trial performed at a commercial Direct Reduced Iron making plant to upgrade low-grade ferruginous manganese ores (Mn: 25–32%, Mn/Fe: ∼1). Trials were carried out using a lot of 2000 of tons low grades ferruginous manganese ores of three different types. Experiments were designed to test 3 levels of two different variables for each type of ore. Raw material feed rate (3, 4.5, and 6tph) and feed coal proportion (25, 29,33% of ore) were the prime process variables. Process settings were identified and fixed to limit the occurrence of undesired chemical reactions to restrict the formation of complex MnxFe3-xO4 phases and dilution of reducing atmosphere. Ores were crushed into 0–15 mm size, reduce-roasted at 950 °C in a rotary kiln, crushed into <3 mm size, and passed through a magnetic separator. The technology can enhance the Mn content of the ores by > 15 units (Mn: 30%→46%) and can bring down the Fe content by > 15 units (Fe: 27%→12%) in the produced ore concentrate. The developed process could achieve product yield ∼50% and Mn Recovery>70%. The Mn-rich concentrate was successfully converted into extrusion briquettes and consumed in a submerged arc furnace for ferroalloy making. The iron-rich by-product was successfully used as a sinter mix for steelmaking process. Techno-economic evaluation and sensitivity analysis have also been carried out. Studies indicated that roasting of ferruginous manganese ores at a commercial Direct Reduced Iron plant is an adaptable and sustainable solution to upgrade the low-grade Mn resources in the dynamic global Mn ore market.

Natural ferruginous manganese ore for efficient immobilization of elemental mercury from coal combustion flue gas

Mercury emission from industrial activities poses a serious threat to the ecosystem. The development of cost-effective sorbents for mercury removal is a feasible method to alleviate this problem. In this study, natural ferruginous manganese ore (NFM) was adopted as a cost-effective sorbent for elemental mercury (Hg0) removal from flue gas. Samples characterization indicated that the pore stricture of NFM was deteriorated after calcination procedure. The secondary recrystallization of manganese oxides and decomposition of MnO2 into Mn2O3 also happened during calcination process. Raw NFM exhibited high Hg0 removal efficiency (ET > 80%) over a broad temperature range (100 °C–200 °C), whereas calcination procedure had negative effects on Hg0 removal performance. The effects of calcination temperature (450 °C–650 °C) and reaction temperature (100 °C–400 °C) on Hg0 removal performance were also explored. Under the simulated flue gas condition, both equilibrium mercury adsorption capacity and adsorption rate were superior to those of a commercial activated carbon specifically used for mercury emission reduction. The Hg0 removal mechanism was revealed by combining with Hg0 adsorption/oxidization behavior and XPS analysis. It was demonstrated that chemisorption dominated Hg0 removal process, where MnO2 could serve as active components for Hg0 capture. The results were affirmed by the mercury temperature programmed desorption experiments.

Development of a Smelting Reduction Process for Low-Grade Ferruginous Manganese Ores to Produce Valuable Synthetic Manganese Ore and Pig Iron

Manganese alloys are important raw materials for steelmaking. However, selective mining of high-grade manganese ore resources has resulted in a scarcity of these high-grade Mn ores (Mn: >42; Mn/Fe: >5). The present research work is carried out to explore a new smelting reduction process to produce high-Mn/Fe synthetic Mn ore from low-grade ferruginous manganese ores. Representative samples of low-grade ores (Mn: 25–30%; Mn/Fe: ~1) were collected and smelting reduction studies were carried out using variable smelting power (20–30 kWh), carbon rate (10–20% of ore) and slag basicity (0.1–0.3) to produce high-MnO slag and manganese pig iron. Studies have shown that different-grade high-MnO slags (MnO: 38.57–58.24%; Mn/Fe: >15) and manganese pigs (Mn: 5.16–24%) can be produced at optimum process conditions. The high-MnO slag can be used as synthetic ore for the ferroalloy production process. The developed process is capable to effectively segregate the iron and manganese in the metal and slag, respectively. The optimum Mn and Fe separation (>95%) was achieved by smelting of 10 kg Mn ore batch in a 50-kVA electric arc furnace using 30 kWh smelting power and 0.16 kg of coke. The slag basicity was maintained at 0.3 and the optimum slag SiO2 was ~30%. The studies indicated that the upgrading of low-grade ferruginous manganese ores is a promising solution for achieving high-Mn/Fe-ratio synthetic ores for ferroalloy production. It can help the manganese alloy industry to overcome challenges associated with the scarcity of high-Mn/Fe-ratio Mn ores and efficient utilization of low-grade resources.

Recovery of manganese from ferruginous manganese ore using ascorbic acid as reducing agent

Being a green and mild acid along with remarkably high solubility for several base metals, ascorbic acid has been used as both leaching agent and reducing agent for the recovery of metals from primary and secondary sources. In the present investigation, ascorbic acid was tested as reducing agent in the presence of sulfuric acid for the leaching of manganese from low grade ferruginous manganese ore. The optimization of the leaching conditions has been comprehensively investigated with respect to H2SO4 concentration, temperature, pulp density, ascorbic acid dosage and agitation speed. Quantitative leaching of manganese was observed under the following optimized conditions: 1 M H2SO4 concentration, 0.070 M ascorbic acid concentration, 70 g/L pulp density, 60 °C leaching temperature, 800 rpm stirring rate and 300 min of reaction time. The leaching kinetic of manganese was evaluated based on the shrinking core model and further validated by SEM and XRD analysis of manganese ores before and after leaching. After further purification of the leach solution, a high purity of 99.95% α-Mn2O3 have been synthesized. The present process therefore deals with achieving the effective recovery of value added product from low grade ferruginous manganese ore.

Synergetic utilization of copper slag and ferruginous manganese ore via co-reduction followed by magnetic separation process

Attempts to economically and effectively treat copper slag (CS) and ferruginous manganese ore(FMO) using the traditional direct reduction–magnetic separation process have faced many challenges, such as low levels of metal recovery, poor cost-effectiveness and the need for a high proportion of additives. This paper proposes an eco-friendly process to synchronously utilise copper slag and ferruginous manganese ore by co-reduction followed by a magnetic-separation process to solve those problems. In the process, MnO2 within FMO acted as a catalyst to promote the reduction of fayalite and copper sulphide contained in CS, and thus improve the metallisation rate of Fe and Cu. Meanwhile, CS, possessing the low melting point, can enables more liquid phase generation and improve the growth of alloy particle, which is beneficial for the full liberation of Fe–Cu alloy. Under the optimum conditions, A crude Fe–Cu alloy powder containing 88.83% Fe and 1.07% Cu was prepared, which can be used in an electronic furnace to produce Cu-bearing steels. A high recovery of 85.89% Fe and 87.74% Cu can be obtained using this process, thereby confirming its efficiency. Additionally, Mn contained in the ferromanganese ore was enriched into the magnetic tailing in the form of low-valence manganese (II) oxides, which can be easily extracted by acid leaching. Moreover, the mechanism involved in the co-reduction of CS and FMO was investigated by XRD, SHTT and SEM-EDS techniques, which further confirmed the complementary advantages about an organic integration of CS and FMO can realise the efficient beneficiation of the valuable metals from them.

Natural ferruginous manganese ore as a potential low-cost adsorbent for Congo red dye removal from aqueous solution

In the present work adsorption characteristics of natural ferruginous manganese ore (FMO) was investigated to assess its potential as an efficient and economic adsorbent for Congo red (CR) dye removal from the aqueous solutions. The characterization of FMO was initially performed using different analytical techniques of XRD, XRF, SEM-EDS, FTIR and pH PZC. The effects of different adsorption parameters, such as, pH, adsorbent dosage, initial dye concentration, contact time and temperature were further investigated in detail. The equilibrium time required to reach maximum uptake was noted at 90 min with 83% removal efficiency. The mean free adsorption energy (E) estimated using D-R model was 1.12 kJ.mol−1 indicating the physical adsorption of CR on FMO. The adsorption data showed a good fitting to the Langmuir isotherm model and pseudo-second order kinetic model with higher regression-coefficient (R2). The adsorption process thermodynamics parameters (∆H, ∆S and ∆G) revealed endothermic and spontaneous nature of reaction accompanied by entropical changes occurred at solid-solution interface. The maximum adsorption capacity of FMO was observed to be 76.9 mg g−1 which is well comparable to the values reported in literature. It is necessary to mention that we are reporting first time the potential of FMO as adsorbent for CR removal and results would be of great importance for the application of FMO for environmental decontamination.

A study on separation of Mn and Fe from high-alumina ferruginous manganese ores by the carbothermal roasting reduction process

Carbothermal reduction followed by magnetic separation has been recognized as an effective recovery strategy to achieve separation of Mn and Fe from ferruginous manganese ores (Fe-Mn ores). However, it is difficult to improve the recovery of manganese more than 85% by changing carbothermic roasting reduction and magnetic separation. This work clarified the underlying reason for the poor separation with the magnetic separation process and proposed a promising process for the recovery of Fe and Mn from Fe-Mn ore. The effect of operating variables on recovery and separation of Mn and Fe from Fe-Mn ore is initially investigated during the carbothermic reduction roasting process followed by magnetic separation to reveal the underlying reason for the poor separation from Fe-Mn ores. Then, the stepwise reduction behaviors of the MnO-Fe2O3 and MnO-Fe2O3-Al2O3 systems were investigated to clarify that the formation of composite oxide Mn1−xFexO (0 < x < 0.497) and hercynite (FeO·Al2O3) directly caused the poor separation of Fe and Mn from high-alumina Fe-Mn ores. Then, the stepwise reduction roasting behaviors of Mn1−xFexO and FeO·Al2O3 were investigated by XRD, SEM-EDS and XPS analysis. After adjusting the experimental parameters, iron-rich products with a Fe recovery of 94.31% and a grade of 85.21% and manganese-rich product with a Mn recovery of 90.05% and a grade of 54.35% are obtained. The mass ratio of Mn/Fe increased to 8.5 in the manganese-rich product.

Arsenite simultaneous sorption and oxidation by natural ferruginous manganese ores with various ratios of Mn/Fe

Arsenite [As(III)], is more toxic and difficult to remove from aqueous systems compared to arsenate [As(V)], and as such poses a significant health risk to humans and the environment. In order to make it more amenable to efficient removal, an oxidation pre-treatment of As(III) is particularly important. Compared to synthetic adsorbents, the low cost and abundance of natural ferruginous manganese ore (NFMO), make it a potentially attractive adsorbent for application in large-scale treatments. Here, we investigated the simultaneous oxidation and adsorption behaviour of As(III) with three NFMOs, each consisting of different Mn/Fe molar ratios. Results demonstrated that the NFMO with a high Mn:Fe ratio had a stronger oxidation capacity for As(III), while As(III) adsorption efficiency of NFMOs increased with increasing Fe content. The sorbent dosage was an important factor for As(III) oxidation with NFMOs having higher Mn contents, but not for ores with low contents. An pH increasing from 6 to 7.9 enhanced the As(III) oxidation by NFMOs. Furthermore, the As(III) oxidation rate of NFMO increased by 68% with the addition of pyrophosphate (PP), and increasing PP concentrations led to higher oxidation rates. During the process, PP formed complexes with Mn(III), thus accelerating the conversion of Mn(IV) to Mn(III). Later, the dissolution of these complexes led to a generation of additional oxidation adsorption sites, favouring As(III) oxidation. FTIR and XPS analyses further confirmed that reduction of Mn(IV) and Mn(III) species played a vital role in the oxidation of As(III) to As(V). As(III) removal by NFMO was attributed to the joint effect of sorption and oxidation processes, where Manganese oxide was responsible for As(III) oxidation, while Fe oxide played a primary role in the arsenic sorption.