Manganese Oxide, Mn2O3: A Star Performer In Water Purification, Industrial Treatment And Contaminant Chemistry

Far from being an obvious choice for modern industry, manganese oxide - specifically Mn₂O₃ - finds a wealth of uses from water purification to removal of radionuclides. Coupling low toxicity with broad applications, Mn₂O₃ is a highly valued commodity.

Manganese (iii) oxide, manganese oxide, manganese sesquioxide and Mn₂O₃ are used interchangeably.

The Material

There are many oxides of manganese. In fact, manganese can take more oxidation states than any other transition metal. With uses ranging from as a highly potent chemical reagent for oxidation reactions in the laboratory (potassium permanganate) to as a dye in the ceramics and building industries (manganese umber), the applications of the oxides of manganese are as varied as the number of compounds themselves.

In this review, the oxide of manganese discussed is manganese (iii) oxide, Mn₂O₃ - also known as manganese sesquioxide. Unlike many other oxides of manganese, the sesquioxide does not adopt the typical corundum crystal structure, rather possessing two crystal states. ɑ-Mn₂O₃ (a bixbyite-type structure, often stabilised by iron(iii) inclusions) and ɣ-Mn₂O₃ (with a structure similar to the mixed oxide Mn₃O₄). Manganite ore contains in excess of 89% manganese sesquioxide by weight - and this is the primary source of the oxide. It can be synthetically prepared by the oxidation of MnO₂ followed by dehydration of the resultant manganese hydroxide.

It also exists as the hydrate, Mn₂O₃.H₂O which is much more reactive

Water Purification Applications

Water purification is one of the most common uses for metal oxides - in which manganese sesquioxide is no exception. Relying on a combination of porosity and interesting redox chemistry, non-toxic manganese sesquioxide can be highly effective as part of a water treatment regime.

Manganese sesquioxide bears manganese, as mentioned, in the +3 oxidation state. Therefore, it may be used to remove manganese (ii) ions from solution via a redox pathway(1), providing the sesquioxide has been sufficiently supported, for example on sand or anthracite. Such multi modal filtration can be effective at removing broad spectrum contaminants from water. When employed alongside iron oxide, manganese and iron co-filters have been shown as effectively removing ammonia (and related compounds) from water(2). Biological compounds are not beyond the remit of sesquioxide either - ethinyl estradiol is readily immobilised by a supported sesquioxide on sand or anthracite filter(3). Ethinyl estradiol has known effects on the endocrine system, and is found in birth control medication.

Porosity has also been described as a key factor, in addition to other surface science phenomena when manganese (iii) oxide has been shown to be effective at the removal of arsenic (iii) and arsenic (v) from water. Demonstrating absorbance performance at low to neutral pH values, manganese (iii) oxide provided almost complete (in excess of 95%) removal - though there was precedence for arsenic (iii) over (v). The porosity of the manganese sesquioxide is key to its strong performance in this area(4).

Applications of this have been demonstrated in a portable system to provide potable water from water rich in arsenic, alongside other oxides of manganese(5) - with the system being able to remove arsenic ions to below World Health Organization mandated levels. A further study has shown that arsenic uptake by manganese and iron oxide species is improved when some arsenic has already been adsorbed onto the surface - ostensibly due to more sites being created that are suitable for arsenic species adsorption(6). Further studies corroborate that the iron oxide and manganese (iii) oxide combination is especially effective for arsenic removal, at a broad range of pH values(7).

Although less toxic than arsenic or other heavy metal compounds, the removal of dyes is important - particularly from industrial runoffs. Again, based on porosity and surface chemistry, manganese sesquioxide effectively absorbs a range of dyes such as Congo Red, which is water soluble and a carcinogen(8). Removal of such a dye is critical. The mechanism of action is through sorption into mesopores in highly porous Mn₂O₃ ‘cube’ materials. Researchers suggest that adsorption capacity exceeds 125 mg per g.

Part of the reason manganese is utilised in pollutant-contaminant treatment and remediation is that the mechanism of action echoes that which is observed in nature. Manganese oxides act as terminal electron acceptors in microorganism mediated reductive dissolution reactions - via the actions of oxalate and pyruvate(9), with sorption of a broad array of materials modified by manganese present(10). Research has shown that in biological systems, manganese sorption is favoured at moderately lower pH values(11).

Manganese (Iii) Oxide As A Catalyst

Transition metal and transition metal catalysis has become the darling of modern organic chemistry in the laboratory. Avoiding the use of exotic and/or heavier metals is an important part of today’s chemistry. Much of Mn₂O₃’s uses in the laboratory are as a heterogeneous catalyst, i.e. one that does not go into solution. Chemists value compounds like manganese sesquioxide because of its shelf stability, ease of use and broad range of uses.

One of the most notable uses for manganese sesquioxide is in carbon monoxide oxidation. Carbon monoxide is toxic and can be a detriment to certain processes in industry and the lab - and oxidation of CO is critical in hydrogen purification. Mn₂O₃ is a superior oxide for this process, converting CO to CO₂, outperforming other manganese oxides MnO and MnO₂(12). Here, the sesquioxide acts as a broad base oxidant. It can, however, be completely selective for carbon monoxide when used alongside cobalt oxide. Cobalt oxide requires stabilisation, which is provided by the sesquioxide(13).

Such mixed catalysts have been used for the selective catalytic hydrogenation of carbon monoxide. A traditional nickel and alumina catalyst is typically used alone, but when combined with manganese sesquioxide, a promoting effect is observed which favours the conversion of CO and CO₂ to methane and other light hydrocarbons(14). By stabilising the dissociation of the CO, the manganese ensures that the breaking of the carbon - oxygen bond is thermodynamically favourable under mild conditions. CO hydrogenation has long been studied as a potential method for ‘carbon capture’ and energy storage. This can be thought of as a Fischer-Tropsch-like reaction.

In a less industrial setting, the annulation of gamma lactones is (partially) regioselectively catalysed by manganese sesquioxide. The addition of a gamma lactone onto an alkane is an important transformation in organic chemistry. Authors claim that the use of manganese sesquioxide as a source of manganese(iii) is effective, whilst being a fairly mild reagent(15). Regioselectivity is the preference of a chemical bond to break in a particular direction - such transformations can lead to different products depending on where bonds break. Additionally, the authors suggested an amount - but far from complete - stereoselectivity across a range of lactone and alkane combinations. Chemical synthesis does, however, tend to prefer manganese (iii) acetate as a source of manganese(iii). A broader review on porous manganese compounds and their applications in catalysis can be found(16).

Manganese (Iii) Oxide Combined With Other Materials

In some cases, research has found that it may be beneficial to combine manganese sesquioxide with another material. In one study, sand was coated with manganese (iii) oxide and aluminium oxide - in order to remove hexavalent chromium species from solution(17). The coated sand was used in a conventional flow through filter system, resulting in near complete chromium removal. Hexavalent chromium species are highly toxic to humans.

Water filters. Concept of three glasses on a white blue background. Household filtration system.

Removing The Manganese Oxide

Whilst manganese oxide is an excellent material for adsorbing contaminants from water sources, it may not always be easy to separate from the water if particle sizes are especially small. Naturally, larger particles can be readily removed via size exclusion filtration. For other scenarios, the ‘heavy’ partly dissolved manganese oxides can be removed by passing through a blanket clarifier where the local pH is in the 8.5 range and an iron (iii) oxide filter is present(18).

Radionuclide Adsorption

Whilst radionuclides have myriad uses in their own right, their presence in places like water courses must be avoided. Research has shown that trace amounts of various radionuclides (including caesium-137, strontium-89, ytterbium-90 and cobalt-57) may be removed from solution via adsorption onto manganese (iii) oxide(19). Combining the naturally porous nature of the oxide with its appropriate redox chemistry at the surface, effective removal was guaranteed in a short space of time. Furthermore, it has been demonstrated that radionuclide mobility in soils is inhibited by the presence of manganese (iii) oxide. That is to say, the addition of manganese sesquioxide to solis can aid in their remediation, by binding to, and therefore immobilising, radionuclides such as cobalt-60(20). Perennial contamination can be remediated via the use of manganese sesquioxide as an ongoing soil treatment agent, especially when employed in its hydrated form, Mn₂O₃.H₂O. Again, radionuclides such as caesium-137 and strontium-89 are well tolerated across a broad pH range(21). Broad pH tolerance is important owing to the requirement of certain plants to have a soil of certain acidic or basic nature.

Summary

Manganese oxides are useful for the removal of myriad contaminants from water, including heavy metals, biomolecules and arsenic - and even other oxides of manganese
Manganese oxide can be used for the oxidation of carbon monoxide, dye adsorption and other applications where high levels of porosity are useful or required, such as the removal of toxic dyes from solution - useful in both the laboratory and industry
Radionuclides can be absorbed by manganese sesquioxide - immobilising them and making them easier to remove from bulk materials, making soils safer
The broad use cases for manganese sesquioxide coupled with its low toxicity and relative abundance mean that it is an invaluable material across many sectors

References

1 J. K. Piispanen and J. T. Sallanko, J. Environ. Sci. Health, Part A, 2010, 45, 1732

2 Y. Cheng et al., Int. J. Environ. Res. Public Health, 2018, 15, 1822

3 J. de Rutter et al., Water Res., 2004, 38, 184

4 K. Babaeivelni and A. P. Khodadoust, J. Environ. Sci. Health, Part A, 2016, 51, 277

5 D. E. Mitskevich et al., Rus. J. Appl. Chem., 2010, 83, 414

6 D. Ociński et al., Chem. Eng. J., 2016, 294, 210

7 T. Huang et al., Water Env. Res., 2019, 91, 536

8 Y. Shao et al., J. Hazardous Mater., 2017, 333, 222

9 A. T. Stone, Geochim. Cosmochim. Acta, 1987, 51, 919

10 S. Grangeon et al., Geochim. Cosmochim. Acta, 2012, 85, 302

11 A. J. Francis and C. J. Dodge, Appl. Environ. Microbiol., 2021, 54, 1009

12 Y.-F. Han et al., Catal. Today, 2008, 131, 477

13 M. Barreau et al., Appl. Catal. B: Environ., 2021, 297, 120397

14 M. Stockenhuber et al., ACS Catal., 2020, 10, 1535

15 W. E. Fristad and J. R. Peterson, J. Org. Chem., 1985, 50, 10

16 B. B. Snider, Chem. Rev., 1996, 96, 339

17 A. Lloyd et al., Water Res., 1983, 17, 1517

18 A. P. Khodadoust et al., J. Environ. Sci. Health, Part A, 2021, 56, 334

19 A. Dyer et al., J. Mater. Chem., 2000, 10, 1867

20 J. L. Means et al., Nature, 1978, 274, 44

21 O. V. Singh and S. N. Tandon, Int. J. Appl. Radiation and Isotop., 1977, 28, 701

Manganese Oxide, Mn2O3: A Star Performer In Water Purification, Industrial Treatment And Contaminant Chemistry

The Material

Water Purification Applications

Manganese (Iii) Oxide As A Catalyst

Manganese (Iii) Oxide Combined With Other Materials

Removing The Manganese Oxide

Radionuclide Adsorption

Summary

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Manganese Oxide, Mn2O3: A Star Performer In Water Purification, Industrial Treatment And Contaminant Chemistry

The Material

Water Purification Applications

Manganese (Iii) Oxide As A Catalyst

Manganese (Iii) Oxide Combined With Other Materials

Removing The Manganese Oxide

Radionuclide Adsorption

Summary

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Quick Contact Details