Manganese Hydroxide Research Articles

Enhanced removal of Cd(II) by poly(acrylamide-co-sodium acrylate) water-retaining agent incorporated nano hydrous manganese oxide

In this study, the hydrous manganese oxide (MO) nano-particles impregnated into a poly(acrylamide-co-sodium acrylate) (PP), was synthesized as PPM, and its characterization, adsorption properties and mechanism were investigated. The transmission electron microscopy (TEM) revealed the mostly MO with size of lower than 20nm were dispersed in polymer. The MO greatly improved the Cd(II) uptake for PP in aqueous solution, due to its high affinity. These adsorptive processes are corresponded to the Sips isotherm model. It was revealed that maximum adsorption capacity (qm) of PP loaded with MO (Mn wt%=1.05%, PPM-40) and PP for Cd(II) are 507 and 281mg/g in present of 0.1M NaCl. However, the qm of PPM-40 and PP are 698 and 771mg/g without any NaCl. The preferential adsorption of PPM-40 to Cd(II) was showed when inferenced with salt ion. The XPS exhibited the 67% Cd(II) was captured by MO and 33% was by PP, in present of 0.1M NaCl. The seed germination and leaching experiments demonstrated that the PPM-40 can be taken as a filter for Cd(II) and supplied “clean” water for plant roots in soil. It is a potential in-situ fixation agent for remediation of agricultural soil polluted with Cd(II).

The formation of cobalt-bearing ferromanganese crusts under fluid destruction of silicate matter

The processes of fluid destruction of various silicate rocks under diffusion of flows of compressed gases (mainly carbonaceous) were studied. The gas condensate nature was ascertained for the forming alumoslilicate and ore (cobalt–iron–manganese hydroxide) substances produced under this fluid destruction in the forms of microcrusts and microconcretions. The ore condensates contained in high concentrations the typomorphic elements of oceanic ferromanganese formations (Mn, Co, Ni, Cu, Pb, Ce, and Pt). The elemental composition of the ore oxide substance formed under the destruction of various silicate matrices exhibits a definite degree of endemism with prevalence of the Co–Mn association. The pronounced concentration of barium is related to the substantially carbonaceous composition of the fluid systems. A cerium paradox is revealed: Ce3+ is oxidized into Ce4+ and absorbed by ferromanganese hydrogel and the minimum of cerium appears in rare-earth phosphates.

XPS determination of Mn oxidation states in Mn (hydr)oxides

Hydrous manganese oxides are an important class of minerals that help regulate the geochemical redox cycle in near-surface environments and are also considered to be promising catalysts for energy applications such as the oxidation of water. A complete characterization of these minerals is required to better understand their catalytic and redox activity. In this contribution an empirical methodology using X-ray photoelectron spectroscopy (XPS) is developed to quantify the oxidation state of hydrous multivalent manganese oxides with an emphasis on birnessite, a layered structure that occurs commonly in soils but is also the oxidized endmember in biomimetic water-oxidation catalysts. The Mn2p3/2, Mn3p, and Mn3s lines of near monovalent Mn(II), Mn(III), and Mn(IV) oxides were fit with component peaks; after the best fit was obtained the relative widths, heights and binding energies of the components were fixed. Unknown multivalent samples were fit such that binding energies, intensities, and peak-widths of each oxidation state, composed of a packet of correlated component peaks, were allowed to vary. Peak-widths were constrained to maintain the difference between the standards. Both average and individual mole fraction oxidation states for all three energy levels were strongly correlated, with close agreement between Mn3s and Mn3p analyses, whereas calculations based on the Mn2p3/2 spectra gave systematically more reduced results. Limited stoichiometric analyses were consistent with Mn3p and Mn3s. Further, evidence indicates the shape of the Mn3p line was less sensitive to the bonding environment than that for Mn2p. Consequently, fitting the Mn3p and Mn3s lines yielded robust quantification of oxidation states over a range of Mn (hydr)oxide phases. In contrast, a common method for determining oxidation states that utilizes the multiplet splitting of the Mn3s line was found to be not appropriate for birnessites.

Use of Hydrous Manganese Oxides Nanopowders as a Potential Sorbent for Selective Removal of Nickel Ions from Industrial Waste Water, Kinetics and Isotherm Studies

Hydrous Manganese Oxides Nanopowders as nano adsorbent (MONs) was synthesized and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transform infrared spectroscopy (FT-IR). The adsorption of Ni (II) ions from aqueous solution on the MONs was investigated with variations in contact time, pH, initial Ni (II) concentration and adsorbent dosage. The results showed that the adsorption of Ni (II) onto MONs increased within 25 min and reached equilibrium gradually and removal percentage were 83%, 57%, 42% and 35% for 25 ppm, 50 ppm, 75 ppm and 100 ppm Ni (II), respectively, by using 1g/l MONs for 90 min at pH 6. The adsorption behavior of Ni (II) onto MONs was best described by the pseudo-second-order model and Freundlish isotherm. The results also indicated that the type of adsorption involved in this study is physiosorption (physical sorption) which usually takes place at low temperature. The results also revealed that MONs was a promising adsorbent for removal of Ni ions from industrial wastewater.

Electrodeposition of three-dimensional ZnO@MnO2 core–shell nanocables as high-performance electrode material for supercapacitors

Hybrid nano-architecture for supercapacitors has been designed by growing three-dimensional ZnO@MnO2 core–shell nanocables on Ti/RuO2 + TiO2 substrate (titanium plate covered with RuO2+TiO2 coating). Electrochemical depositions were utilized for constructing this core–shell nanostructure, which involved potentiostastic deposition of ZnO nanorod arrays and potentiodynamic deposition of multivalent and partially hydrous manganese oxide. According to cyclic voltammetry and galvanostatic charge–discharge measurements, ZnO@MnO2 core–shell nanocables electrode exhibits higher specific capacitance and better rate capability than those of pure MnO2 electrode. The specific capacitances of ZnO@MnO2 core–shell nanocables reach 537.8 F g−1 at a scan rate of 5 mV s−1 and 613.5 F g−1 at a current density of 1 A g−1. Electrochemical impedance spectroscopies also confirm that ZnO@MnO2 core–shell nanocables electrode has better electrochemical characteristics. Furthermore, ZnO@MnO2 core–shell nanocables losses 10.2% of the initial capacitance after 5000 charge–discharge cycles, which demonstrates its excellent cycling stability. These results indicate that the electrochemical property of manganese oxide has been greatly enhanced due to the supporting of ZnO nanorod arrays, and ZnO@MnO2 core–shell nanocables is promising electrode material for high-performance supercapacitors.

The geochemistry of manganese conglomerates on the Vityaz’ Rise, Japan Sea

Large conglomerate fragments up to 25 cm across were dredged off the submarine volcanic Vityaz Rise, Japan Sea in the 1600–1350 m depth range. These are the first ever that were recovered in this location. The fragments consist of pebble and gravel material that is cemented by manganese hydroxides. The cement is basal in type; the chemical composition is M2+MnO2, where M2+ = Na2, Mg, K2, Ca, Ba. The Mn/Fe ratio is 802. The upper part of the conglomerates is covered with a layered crust of the same hydroxides. A similar crust can be discerned within the conglomerate closer to the bottom of a sample. Some layers of the crusts are rich in NaCl, CaSO4, F, Co, Cu. The crusts contain inclusions of grains that consist of Ag and intermetallic compounds based on Cu and Ni. The superimposed manganese mineralization is Quaternary. The metals originated, most likely, from gas-rich hydrothermal fluids.

Reductive atmospheric acid leaching of spent alkaline batteries in H2SO4/Na2SO3 solutions

This work studies the optimum reductive leaching process for manganese and zinc recovery from spent alkaline battery paste. The effects of reducing agents, acid concentration, pulp density, reaction temperature, and leaching time on the dissolution of manganese and zinc were investigated in detail. Manganese dissolution by reductive acidic media is an intermediate-controlled process with an activation energy of 12.28 kJ·mol−1. After being leached, manganese and zinc were selectively precipitated with sodium hydroxide. The zinc was entirely converted into zincate (Zn(OH) 4 2− ) ions and thus did not co-precipitate with manganese hydroxide during this treatment (2.0 M NaOH, 90 min, 200 r/min, pH > 13). After the manganese was removed from the solution, the Zn(OH) 4 2− was precipitated as zinc sulfate in the presence of sulfuric acid. The results indicated that this process could be effective in recovering manganese and zinc from alkaline batteries.

Treatment of liquid wastes using composite resins

Composite ion exchange resins were prepared by coating copper ferrocyanide and hydrous manganese oxide powders on polyurethane foam. The binder used was polyvinyl acetate in alcohol/acetone medium. Studies were conducted in pilot scale using 50 L ion exchange column and treated category III radioactive liquid wastes. About 2000 to 2400 bed volumes of liquid wastes containing radioactive 137Cs and 90Sr were treated. Digestion of the resins was carried out in a 25 L column using alkaline KMnO4. The digested liquid was fixed in cement matrix and the matrices were characterized with respect to compressive strength, biological and leach resistance.

Investigation on preparing Mn3O4 nanopowder by hydrothermal reaction method

A process to prepare Mn3O4 in the mixed solvent of water and ethanol under hydrothermal condition was studied, in which manganese sulphate was used as raw material, manganese hydroxide from precipitation with sodium hydroxide as a precursor, potassium permanganate as oxidant. The influences of the factors on Mn mass percentage (Mn wt-%) in manganese oxide product, including potassium permanganate dosage, solvent composition, temperature and time of the hydrothermal reaction as well as drying temperature, were analysed. Potassium permanganate dosage could be a third of the theoretical amount because of the auxiliary oxidation of existent O2 in the process. The results show that the optimal conditions are hydrothermal temperature 433 K and time 2–4 hours, solvent composition (ethanol/(ethanol + water), in volume) 0·33 and drying temperature 393 K, Mn wt-% in the optimal product exceeds 71% and Na content meets the standard of magnetic material.

The behavior of uranium and manganese under the diagenesis of carbonate sediments in small lakes of the Baikal region

This report considers the experimental studies of diagenetic processes involving uranium and manganese in lacustrine sediments, especially to prove the possibility of formation of UO2 + x reduced phases. Comparative research was performed for two lakes of different depths, hydrological conditions, mineralization, and chemical composition of waters. The layerwise successive leaching of sediments was carried out, with subsequent measurement of the contents of the uranium-238 and -234 isotopes in chemogenic fractions of the sediment. By means of ESR and successive leaching, the profiles of the manganese oxide and hydroxide content over the section were obtained. The performed studies confirmed the hypothesis of the formation of UO2 + x reduced phases in sediments. The sediments of the deeper freshwater Alyaty Lake are characterized by an increase in the content of the UO2 + x reduced phases and a decrease of manganese oxides and hydroxides downwards and upwards through the section, respectively. This conforms quite well to the behavior of these elements in oceanic sediments. The shallow saline Tsagan-Tyrm Lake is characterized by the reverse situation: the bulk of the UO2 + x reduced phase occurs in upper layers where, in turn, the oxide phases of manganese are simply absent. This is caused by different reductive conditions in the sediments.

Selective mechanochemical alkaline leaching of zinc from zinc plant residue

A large amount of plant residue containing heavy and hazardous metals is generated in Iranian lead–zinc plant annually. The plant residue is known as hot filtercake (HFC) which contains 8–20% zinc, 6–16% manganese, 5–25% calcium and 0.5–3% cobalt. This paper is devoted to the mechanochemical alkaline leaching (MCAL) of zinc from HFC. The possibility of complete selective zinc leaching from HFC by the MCAL method was investigated through a 2×4×5 full factor design wherein effects of NaOH concentration, leaching time and milling speed were evaluated. In addition, the results were compared with the results of conventional leaching method.The results from the conventional alkaline leaching (CAL) indicated that just after a while from the beginning of the reaction, as confirmed by XRD investigations, an insoluble calcium manganese hydroxide layer forms on active surface of ZnO particles which prevents diffusion of leaching solution. Consequently, leaching process was halted and maximum recovery did not exceed 82.4%. Mechanochemical leaching results revealed that intensive milling in an attritor enhanced leaching performance, selectively. This is attributed to a wide structural change and abrasion of insoluble layer during MCAL. Subsequently, zinc recovery of 99.9% was obtained. The statistical analysis of the results showed that NaOH concentration, milling time and milling speed were statistically significant factors affecting the leaching process at 95% confidence level. In conclusion, approximately complete zinc recovery from HFC using the MCAL method can assist cobalt and manganese extraction process from the plant residue in the subsequent process.

TRANSFORMATIONAL CONVERSIONS OF MANGANESE HYDROXIDES FROM FERROMANGANESE DEPOSITS OF THE WORLD OCEAN UNDER THE INFLUENCE OF TEMPERATURE

The paper is aimed at investigation and characterization as well as at crystal-chemical interpretation of products of thermo conversions of manganese hydroxides from ferromanganese deposits from different areas in the World Ocean. The results of research which are represented in this paper give notions about features of crystal-chemical conversions of manganese minerals with different types of crystal structure, different chemical composition and genesis when they are heated. This allows to generate new diagnostic criteria of these minerals and valuation of such type of mineral resources. Such common minerals, in marine and oceanic nodules, as vernadite, birnessite, buzerit I, mixed-layer asbolan– buzerit, todorokit and pyrolusite, were selected for further research. The samples were taken in certain way to have one mineral phase of manganese hydroxide which is represented by two preparations: one of which is with high ratio of iron and manganese, and the other one with low ratio, and with different quantities of nickel, cobalt and copper. To study changes of mineral composition of ferromanganese deposits the samples were heated in consecutive order in the range of temperatures 20-9000С. The mineral composition of initial samples and the products of annealing was determined by means of x-ray diffraction. Transformations of manganese hydroxides under the influence of high temperatures are as follows: if iron is a dominant component in the initial sample, hematite and yakobsite will be formed; if manganese content will be increased in the initial sample then hematite is absent in the products of annealing and along with yakobsite such mineral as biksbiit appears. Later on, with incensement of manganese content in the initial sample such mineral as gausmanite appears in the products of annealing which corresponds to the area of disruption of miscibility in the system. Along with farther incensement of the ratio between iron and manganese, such mineral as yakobsite cannot be formed anymore and the quantity of gausmanite increases. Initial mineralogical composition of the sample determines the character of conversions of crystal lattice up to 7000C. The composition of the final products of annealing depends first of all on the ratio Mn/Fe in the sample. Thus, the trend of transformation of manganese hydroxides from marine and oceanic ferromanganese deposits under the influence of heating depicts commonality of their structure. Under the influence of temperature on manganese-oxygen octahedrons only the way of connection between layers and bands and their order can be changed. The scheme of transformational conversions of manganese hydroxides is suggested in the paper.

Clogging of water supply wells in alluvial aquifers by mineral incrustations, central Serbia

The formation of incrustations on public water supply well screens reduces their performance considerably. The incrustations increase hydraulic losses, reduce the capacity of the well and screen, affect the quality of the pumped water and increase maintenance costs. In alluvial environments, the most common deposits are iron and manganese hydroxides. However, the rates of formation, compositions and levels of crystallization vary, depending on the geochemical characteristics of the alluvial environment, the microbiological characteristics of the groundwater and the abstraction method. Samples of 15 incrustations were collected from wells that tap shallow alluvial aquifers and were found to be dominated by iron. XRD analyses detected low-crystalline ferrihydrite and manganese hydroxide in the samples collected from the water supply source at Trnovce (Velika Morava alluvial). The incrustations from the Belgrade Groundwater Source revealed the presence of ferrihydrite and a substantial amount of goethite ?-FeOOH. Apart from goethite, greigite (Fe3S4) was detected in three samples, while one sample additionally contained bernalite Fe(OH)3 and monoclinic sulfur S8. Among carbonates, only siderite was detected. Iron oxidizing bacteria generally catalyze deposition processes in wells, while sulfate reducing bacteria (SRB) play a role in the biogenic formation of greigite. Determining the nature of the deposited material allows better selection of rehabilitation chemicals and procedure.

Phosphorus and arsenic distributions in a seasonally stratified, iron- and manganese-rich lake: microbiological and geochemical controls

Environmental context Despite being present at trace concentrations, arsenic and phosphorus are among the most important of freshwater contaminants. This research highlights the biogeochemical coupling of both elements in a New Zealand lake. We find that the mineralisation of organic residues coupled to the dissolution of colloidal iron and manganese hydroxides may be an important driver of the bioavailability of phosphorus and arsenic. Abstract Seasonal stratification in temperate lakes greater than a few metres deep provides conditions amenable to pronounced vertical zonation of redox chemistry. Such changes are particularly evident in eutrophic systems where high phytoplankton biomass often leads to seasonally established anaerobic hypolimnia and profound changes in geochemical conditions. In this study, we investigated the behaviour of trace elements in the water column of a seasonally stratified, eutrophic lake. Two consecutive years of data from Lake Ngapouri, North Island, New Zealand, demonstrate the occurrence of highly correlated profiles of phosphorus, arsenic, iron and manganese, all of which increased in concentration by 1–2 orders of magnitude within the anaerobic hypolimnion. Stoichiometric and mass-balance considerations demonstrate that increases in alkalinity in hypolimnetic waters were consistent with observed changes in sulfate, Fe and Mn concentrations with depth, corresponding to dissimilatory reduction of sulfate, FeIII and MnIV hydroxides. Thermodynamic constraints on Fe, Mn and Al solubility indicate that amorphous FeIII, MnIV hydroxides most probably controlled Fe and Mn in the surface mixed layer (~0 to 8m) whereas AlIII hydroxides were supersaturated throughout the entire system. Surface complexation modelling indicated that iron hydroxides (HFO) potentially dominated As speciation in the lake. It is likely that other colloidal phases such as allophanic clays also limited HPO42– activity, reducing competition for HAsO42– adsorption to iron hydroxides. This research highlights the coupling of P, As, Fe and Mn in Lake Ngapouri, and the apparent role of multiple colloidal phases in affecting P and As activity within overarching microbiological and geochemical processes.

Oxidation of CrIII aminocarboxylate complexes by hydrous manganese oxide: products and time course behaviour

Environmental context Oxidation of CrIII (trivalent chromium) to CrVI (hexavalent chromium) is of environmental concern because CrVI is a known mutagen and carcinogen. Our results show that hydrous manganese oxide (HMO) is capable of oxidising soluble CrIII complexed with iminodiacetic acid and nitrilotriacetic acid to CrVI at appreciable rates. CrVI production from soluble CrIII organic complexes is therefore expected to occur in natural and engineered systems that contain HMO. Abstract MnIII,IV (hydr)oxides are believed to be the principal oxidants of CrIII in the subsurface. In nearly all previous work on this subject, the CrIII reactant was prepared from inorganic salts (e.g. nitrate, chloride, sulfate). In our present work, CrIII complexes with the synthetic chelating agents iminodiacetic acid (IDA) and nitrilotriacetic acid (NTA) were reacted with hydrous manganese oxide (HMO) over a wide pH range to examine rates of reaction and product distribution. Capillary electrophoresis was used to quantify changes in reactant (CrIII–IDA and CrIII–NTA) and product (CrVI, free IDA and free NTA) concentrations as a function of time. In addition, a small number of experiments were performed using solutions prepared from CrIII alum (KCr(SO4)2·12H2O(s)) as the CrIII reactant. CrIII–IDA and CrIII–NTA were oxidised to CrVI, but rates were considerably lower than those obtained using inorganic CrIII. Within the timescales of our experiments, complete conversion of CrIII–NTA occurred at pH >7, but not under moderately acidic conditions, even when there was a large stoichiometric excess of HMO. MnCl2 addition experiments indicated that the observed reaction inhibition was attributable to MnII generation during the reaction. Our previous work has shown that citric acid, IDA, NTA and ethylenediaminetetraacetic acid solubilise CrIII from amorphous Cr(OH)3(s) at appreciable rates. The results of this study show that HMO is capable of oxidising the resulting soluble CrIII complexes, providing a viable mechanism for CrIII oxidation to CrVI over a wide pH range.

Synthesis of high capacity cathodes for lithium-ion batteries by morphology-tailored hydroxide co-precipitation

Nickel manganese hydroxide co-precipitation inside a continuous stirred tank reactor was studied with sodium hydroxide and ammonium hydroxide as the precipitation agents. The ammonium hydroxide concentration had an effect on the primary and secondary particle evolution. The two-step precipitation mechanism proposed earlier was experimentally confirmed. In cell tests, Li- and Mn-rich composite cathode materials based on the hydroxide precursors demonstrated good electrochemical performance in terms of cycle life over a wide range of lithium content.

Synthesis of manganese dioxide nanosheets and charge storage evaluation

Manganese oxide nanostructures were synthesized by chemical–electrochemical process using controlled-current electrochemical methods and heat-treatment. Manganese hydroxide precursor was prepared by electrogeneration of hydroxide (OH−) at electrode surface from nitrate solution. MnO2 hexagonal nanostructures were then achieved by heat annealing of the obtained precursor. A variety of characterization methods have been applied to the samples including Fourier transform infrared spectrometry, energy dispersive X-ray analysis, X-ray diffraction and scanning electron microscopy. The scanning electron micrographs showed that the product consists of nanosized disc-like structures with average diameter of 800nm and thickness of 50nm. The electrochemical properties of MnO2 nanostructures were investigated by Cyclic Voltammetry. The maximum specific capacitance achieved was 204Fg−1 at a scan rate of 1mVs−1.

Effect of particle size on rate capability and cyclic stability of LiNi0.5Mn1.5O4 cathode for high-voltage lithium ion battery

LiNi0.5Mn1.5O4 samples with their particle sizes from micro to nano are synthesized via polyvinylpyrrolidone (PVP)-assisted coprecipitation of nickel and manganese hydroxide. Their morphology, structure, and performance as cathode of high-voltage lithium ion battery are investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge/discharge test. The characterizations from SEM and XRD show that the particle size of the resulting LiNi0.5Mn1.5O4 is tunable from micro to nano by controlling the concentrations of PVP for the formation of nickel and manganese hydroxide precursor. The results from CV, EIS, and charge/discharge test reveal that reducing the particle size of LiNi0.5Mn1.5O4 results in its less interfacial resistance for lithium insertion/desertion process, leading to its improved rate capability. Meanwhile, the cyclic stability of LiNi0.5Mn1.5O4 is also improved when its particle size is changed from micro to nano, but too smaller particle size is not beneficial to its cyclic stability, especially at elevated temperature. When evaluated in LiNi0.5Mn1.5O4/Li half cell, the resulting LiNi0.5Mn1.5O4 samples of 800, 250, and 125 nm, in average, deliver a 20 C rate capacity of 40, 58, and 71 mAh g−1, while they exhibit a capacity retention of 79, 89, and 82 % after 250 cycles with 0.5 C rate at room temperature and 33, 77, and 64 % after 200 cycles with 1 C rate at 55 °C, respectively. This difference in capacity retention becomes more significant in LiNi0.5Mn1.5O4/graphite full cells due to the effect of graphite anode.

Hydrous manganese oxide–polyacrylonitrile (HMO–PAN) composite for the treatment of radioactive laundry wastewater

Hydrous manganese oxide-polyacrylonitrile (HMO–PAN) composite was applied for the removal of Co2+, Sr2+ and Cs+ from radioactive laundry wastewater. Single- and multi-solute competitive sorptions onto HMO–PAN were investigated. The maximum sorption capacity was in the order of Co2+ (0.573) > Cs+ (0.551) > Sr2+ (0.310 mmol g−1). Sorption of the metals occurred via physical adsorption due to weak van der Waals force and ion exchange with Mn2+ in HMO–PAN. Sorption behaviors were not related to the types of the surfactants. Among the tested surfactants, SDBS and SOBS remarkably increased the distribution coefficient of Co2+ and Sr2+, respectively.

As3+ removal by Ca–Mn–Fe3O4 with and without H2O2: Effects of calcium oxide in Ca–Mn–Fe3O4

As3+ removal by Ca–Mn–Fe3O4 composites, which contained various wt% of Ca, are investigated. Immobilization of Ca (i.e. as crystalline forms including CaO2) and Mn (i.e. as an amorphous hydrous manganese oxide) on Fe3O4 were identified, and it was revealed that the co-immobilization of Ca and Mn (i.e. especially the wt% ratio of Ca:Mn:Fe=2:3:1) provided higher Ca wt% with greater surface area. The increasing Ca wt% (i.e. 6, 14, 17, and 19%) gradually increased the reactivity of H2O2 to oxidize As3+ to As5+. Moreover, it is suggested that superoxide anion produced from the catalytic decomposition of H2O2 reduces Mn4+ to Mn2+, which is further released into solution. On the other hand, As3+ adsorption was decreased with the highest Ca wt% in Ca–Mn–Fe3O4. It was concluded that the increasing Ca wt% positively affected As3+ oxidation but an excess Ca wt% negatively affected As3+ adsorption. The higher As3+ adsorption was observed when Ca wt% was 17 (i.e. the wt% ratios of Ca:Mn:Fe=2:3:1). Without H2O2, As3+ was adsorbed and oxidized by Ca–Mn–Fe3O4 itself. It is suspected that As3+ oxidation is due to H2O2 produced from CaO2. Mechanisms for As3+ removal by Ca–Mn–Fe3O4 with and without H2O2 are proposed.