Maddox Catalyst: High Performance Treatment For Contaminated Water
Introduction to Maddox catalyst
Maddox is a catalyst produced from pure manganese ore that has been treated in a special furnace using proprietary techniques. It is used as a heterogeneous catalyst for the in-line removal and/or reduction of certain contaminants from water. Such contaminants include iron, manganese, aluminium, chlorides and various dissolved and acidic gases. Maddox can act as a transitional metal oxide acting as a catalyst.
How it works
The central concept surrounding the bed-based Maddox catalyst for water purification is that solubilised iron and other metal ions and dissolved gases become oxidised by the highly oxidising Maddox catalyst, which causes some to precipitate or decompose on contact.
Precipitates are then filtered using a suitable filter, which precipitates are removed from the filter medium or media by backwashing.
In the case of iron removal by the Maddox (manganese) catalyst, the ionic equations are as below.
2Fe2+ + MnO2 + 2H2O ⇌ 2Fe3+ + Mn2+ + 4OH-
Mn2+ + MnO2 + 2H2O ⇌ 2Mn3+ + 4OH-
The manganese in MnO2 is in the +4 oxidation state. It can be seen that the often water-soluble iron(ii) ion is oxidised to iron(iii) by the manganese oxide of the Maddox catalyst; iron(iii) compounds are typically less soluble in water than their iron(ii) counterparts. Manganese(ii) compounds/complexes have a tendency to be water soluble, however further oxidation in the second equation to the manganese(iii) ion renders poorer solubility. The balance of solubility-insolubility is crucial in water purification settings, with the fewer solubilised metallic compounds passing through the system the better. Nonetheless, some semi- or poorly soluble iron and manganese salts produced during the oxidation will need to be removed downstream by 1 to 5 μm filtration.
pH is, broadly speaking, the measure of how acidic or basic a substance is. Low pH values are experienced in acidic environments, whereas higher pH values are found when the medium is basic/alkaline. One of pH’s most profound abilities is to modulate the solubility of various substances.
The ability of Maddox to operate over a wide range of pH values results from the medium’s ability to firstly remove dissolved acidic gases from low pH water, thereby causing the dissolved metals or some of it (in the case of Fe, Mn and Al) to naturally precipitate at it’s normal pH precipitation range – this precipitation process is enhanced by the oxidation capacity of the Maddox itself, altogether resulting in very high removal/reduction rates.
In the case of high pH water, i.e. in the absence of dissolved acidic gases being present, the dissolved portion of the substance is then oxidised by the Maddox alone, to a lesser or greater degree of efficiency, depending on overall water quality.
As no hard and fast rules can be applied in many instances, or in the absence of a credible data base or detailed analysis, it is advised to perform laboratory or small scale pilot test work in order to determine more precise operating parameters and process efficiency.
The basic set-up for a Maddox bed is a water inlet and outlet , the Maddox bed itself, pre and/or post treatment filters and some means of backwashing to remove the recovered contaminants.
Maddox bed catalytic processes can be run at continuous rates of 8-12 to cubic metres per hour per square metre of catalyst, depending on particular raw water characteristics. Backwashing requires a substantially higher flow rate of generally between 30 to 50 m3h-1m-2.
The high bulk density (1.8) of Maddox allows for the medium to be used in an up-flow pattern – this causes the Maddox particles to partly fluidise which process optimises more available of active surface area – it further enhances the release of precipitated particles from the medium, thus preventing fouling and allowing the Maddox not to perform as a suspended particle filter.
Regeneration of the Maddox catalyst bed is needed after a certain number of cycles - i.e. after the oxidising power of the Maddox bed has been exhausted. Crucially, this process is to be completed before the complete exhaustion of the Maddox catalyst bed. This facile process is performed by the application of potassium permanganate (KMnO4, 1.75 to 3.5% solution) or calcium hypochlorite - both of these materials are strong oxidants and return the ‘spent’ manganese catalyst back to high oxidation state manganese to be used again. Common regeneration treatments require at least one hour of exposure to the regenerative oxidants, after which a rigorous backwashing process is used to remove excess oxidant and other undesirable materials.
Continuous regeneration has been recently developed and uses low- and medium intensity ultraviolet lamps applied over the bed. This generates oxygen (ca. 3 g per hour) and ozone (ca. 2 g per hour) as oxidants in situ, which provide low level but constant regeneration of the Maddox catalyst bed. Efficiency levels of this in situ treatment are dependent on water flow rate, volume and lamp power.
Physical Properties Of The Catalyst And Catalyst Bed
Maddox beds are typically deployed in the 300 to 800 mm depth range, depending on designed water flow capacity and raw water quality. Crucial is the fact that the Maddox media has suitable levels of under-bedding where required.
As it is a heterogeneous catalyst, Maddox grain size matters as grain/particle size is a proxy for surface area. Surface area is one of the most crucial aspects of a heterogeneous catalyst. Effective grain sizes of Maddox catalyst range from 0.25 to 3.3 mm - i.e. an easily workable size - with corresponding mesh sizes ranging from 0.3 to 3 mm. When used on the multi-kilogram scale, such particle sizes are commensurate with large surface areas.
Attrition is a minor concern with Maddox catalyst beds, with losses in the region of 3 to 5% per year, viable processes can operate for lengthy periods of time before total media replacement is required. Such longevity is yet another string in the bow of Maddox beds as a robust, high performing system.
Pretreatment Of Upstream Liquid Material
Pre-oxidation of upstream liquids as a common strategy employed by process engineers whereby oxidation of upstream incoming raw waters is carried out with chlorine gas, sodium- or calcium hypochlorite; or more directly with ozone, oxygen or potassium permanganate. To an extent, pre-oxidation can remove contaminants that would be removed by Maddox, but the use of these pre-treatments is not as facile a process as allowing the contaminated water to pass across the Maddox bed as designed.
Therefore it can be said that the use of Maddox catalyst negates the need for treatment of the water supply ahead of time using sodium or potassium hypochlorite, or any of the aforementioned upstream pre-treatment oxidants.
Turbidity (the broad measure of suspended solids in a liquid, high turbidity means that there are high levels of suspended matter, therefore less pure/clean liquid) is an important consideration in any process that deals with precipitation and filtration.
Metal salts will be oxidised into suspended solids using a pre-treatment process, which would itself require a filtration process in advance of the Maddox bed to filter out the generated suspended solids so that turbidity is reduced, allowing optimal catalyst function. High levels of turbidity would necessitate frequent backwashing therefore filter size and/or sequential filters are important factors to consider.
The Effect Of Chlorine
Many manganese oxide catalysts are susceptible to poisoning by the presence of elevated levels of chlorine, which causes lower activity levels of the catalyst(1). The calcination that occurs during the initial firing/heating in Maddox catalyst manufacture that likely causes the catalyst to have a high surface area leads to a generally more active catalyst, with more ‘active sites’ and so would require highly elevated levels of chlorine present to sufficiently poison.
Specific Applications In Iron, Other Metals (And Related Compounds) Removal
Owing to the deployment of Maddox catalyst in a ‘bed’ structure, it is most commonly used for the removal of various contaminants from liquids, such hydrogen sulfide, iron and a variety of metal salts. Reducing to trace levels or the complete elimination of contaminants can make the difference between a viable water supply and a dangerous one.
One of the leading applications of Maddox bed catalysts is for the removal of iron compounds from water. Ingestion of iron in levels that meet or exceed 60 mg per kilogram of body weight is associated with toxicity, especially in children(2). Whilst toxicity is a function of dosage and the individual’s gastrointestinal lining, soluble iron(ii) compounds in water should be avoided as far as possible. Crucial to Maddox’s utility is its ability to remove iron across a broad range of pH values, depending on overall water quality. Iron compounds find their way into water sources in many ways including, but not limited to, leaching from waste facilities and enhanced presence in local soils.
Hydrogen sulphide is described as a broad-spectrum poison and is highly soluble in water. There are several methods for its removal from water including direct chlorination (with chlorine gas), nitrate addition to spoiled water sources or by simple aeration (bubbling oxygen through the contaminated water). Such methods are effective at certain concentrations, but the Maddox catalyst is equally as effective. By utilising the oxygen on the catalyst - not the manganese, c.f. metals removal - the sulphide is oxidised to sulphate, which is insoluble, less toxic and can be easily removed by filtration. The slightly increased pH afforded in the earlier mentioned iron-manganese ionic equations, if iron is present, enhances the efficiency of the desulphurisation process(3). Hydrogen sulphide may occur in water courses near wastewater treatment facilities, landfill sites and areas where agricultural waste is poorly managed.
It should be noted that the small molecule manganese compounds formed during the oxidation also need to be removed, lest risking causing manganese poisoning further downstream (4). A suitable multimedia filter akin to the ones used to remove insoluble iron/other metal/sulphide/salt compounds should be used, or its equivalent.
- Maddox catalyst is a manganese oxide species that is formed from the proprietary heating of manganese ore, affording a high surface area catalyst that has many uses
- One of the leading uses for Maddox catalyst is in bed form for the purification and decontamination of water
- It behaves as a heterogeneous catalyst, removing contaminants such as iron, manganese and hydrogen sulphide by a mechanism of oxidation which is followed by subsequent fine filtration
- Operating over a wide temperature, pH and contamination level range, the Maddox bed catalysis system is a resilient, robust and long lasting catalyst that is easily regenerated (regeneration can even be achieved in situ)
1 W. Xu et al., Catalysts, 2019, 9, 726
2 T. Madiwale and E. Liebelt, Curr. Opin. Pediatr., 2006, 18, 174
3 M. L. McFarland and T. L. Provin, Hydrogen Sulphide in Drinking Water, Texas A&M University, College Station, Texas, United States, 1999
4 J. E. Tobiason et al., Curr. Poll. Rep., 2016, 2, 168