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.
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.
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).
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.
- 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
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