The Purpose of This Paper Is to Provide Users of Quantum DMI-65 (The Media) Catalytic Material
Revolutionary Silica–Based Granular Catalytic Media for the Removal of Iron, Manganese and Arsenic from Water
Richard M. Johnson ITOCHU Chemicals America Inc., White Plains, NY and Houston. TX
IWC Paper IWC 12-11
Key Words: ironremoval, manganese removal, arsenic removal, catalytic media
Abstract: The purpose of this paper is to provide designers and end-users of water treating equipment with information about a silica-based catalytic material known as DMI-65 (the media). The media boosts the oxidation processes in aqueous solutions (mainly water). The media is part of the broad category of products deriving their physical and chemical action from interaction of their metal oxide surface with water molecules and ions in solution.
In terms of solid surface interaction with water, we distinguish the term adsorption as attraction – interaction at solid surface level and absorption as transport and retention of target ions through fine porosity inside the media. For this media interaction of water molecules and ions is initiated through adsorption. In other words this is oxidation – precipitation – filtration not ion exchange.
Operating in the presence of sodium hypochlorite the media acts as an oxidation catalyst so that any iron or manganese not initially oxidized will be oxidized in the presence of the media. Unlike other materials the media does not need to be periodically regenerated with potassium permanganate.
The media can be used in a pH range of 5.8 – 8.6. Recommended flow rates are based on the concentration of the iron and/or manganese in the raw water; but a good rule-of-thumb would be 2 – 10 gpm/ft2. The media will require occasional backwash and a free chlorine residual of 0.1 – 0.3 ppm in the effluent water. The media has been tested and certified by the Water Quality Association in accordance with the NSF 61 standards.
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BACKGROUND:
The purpose of this paper is to provide users and potential users of Quantum DMI-65 catalytic material (the media) with qualitative information about its capabilities and limitations and to enable them to apply the media to water treating processes with confidence. In the interest of time, the paper avoids the
detailed complexity of solid surface electrochemical layers and colloidal
science. However, before I get to what it is and how it works, I wanted to explainhow and why the media came to be developed and a brief history of some competing products and processes.
In the early days of water treating, naturally occurring zeolites (such as glauconite greensand) were used to soften and remove the iron and manganese from boiler make-up and process waters. As the demand for higher quality water increased (due in part to higher pressure class boilers) the water treating industry largely moved away from these products for softening to the newly developed synthetic ion exchange resins.
However, in the case of iron and manganese removal this move was much slower and the result was that the use of glauconite greensand (greensand) filtration media continues until the present time. Greensand was and is often used as a pretreatment step prior to ion exchange processes since the iron in a feed water can and does foul the cation resin. Other processes include aeration and oxidation-filtration with standard media filters or proprietary types of media and/or filters
While there have been other iron/manganese removal products and processes developed since greensand was introduced the use of greensand continued even though there were several issues that made it a less than ideal media. It required periodic regeneration with potassium permanganate,could not be used in lower pH waters (<6.2), had a relatively low operating temperature (80oF), and tended to soften through time resulting in pressure drop issues at higher flow rates. Additionally, the supply could occasionally become restricted due to environmental concerns with the processing facilities along the Eastern coast of the United States.
Because of these issues in the 1970s water treating companies and end-users began to express an interest in “something else” to replace the greensand. In response to their requests, scientists and researchers in Japan began to look for ways to combine several known oxidizing agents together and apply these to a silica-sand substrate. It was felt that a commercially produced product could be made more powerful, have better physical properties and be more subject to improvements and/or modifications than any naturally occurring media.
The result is Quantum DMI-65 (the media) a granular catalytic media used to boost the reduction/oxidation (redox) processes in water. The media is part of a broad category of products deriving their physical and chemical action from the interaction of their metal oxide surface with the water molecules and ions in solution. We feel that this product is revolutionary due to a tight particle size distribution, low level of fines, wider pH range and a coating that won’t be abraded or diminished under normal operating conditions.
GENERAL DESCRIPTION:
In terms of solid surface interaction with water I want to distinguish the term adsorption as the weak van der Waal forces that hold a hydrophobic molecule in a rigid core media such as activated carbon and absorption as the weak van der Waal forces that hold a hydrophobic molecule in a swellable matrix (such as benzene) in a polymer of T-butyl styrene or absorption by liquid-liquid extraction. Ion exchange resins utilize absorption processes while interaction of the media with water molecules and ions in solution is initiated through adsorption.
The media does not have a coatingin the traditional sense. The active components are not simply layered onto the surface but are applied with improved adherence through diffusion into the silica substrate. Thereforethe coating is not subject to depletion through physical abrasion during the service and backwash portions of a service run. Tests have shown that when subjected to silt density index (SDI) measurements the effluent from filtersutilizing the media shows no evidence of any of the oxidative layer being deposited on the 0.45 micron Millipore disc used in the SDI testing equipment.
The media is granular in nature and is dark brown to black in color produced by the manganese oxide in the outer layers of the granules. The media acts as an oxidation catalyst in the true meaning of the word and facilitates oxidation – precipitation – filtration. Strictly speaking, the media facilitates chemical reactions and does not explicitly remove anything. Once oxidized, the depth filtration aspect of the media removes the solids that are then periodically backwashed out of the filter vessels.
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Figure 1.
In essence, the oxidants and the media work together to oxidize a dissolved solid into a suspended solid that is then filtered out in the depth of the media bed. If an element can’t be oxidized and precipitated the media can’t remove it. Unlike greensand, the media does not require periodic regeneration or reactivation and once activated does not display a decaying activity to do its catalytic work. It does, however, require a feed of an oxidant into the raw water influent.
BASIC OPERATION:
The processes that take place in a bed of the media involve reduction/oxidation (redox). Redox reactions involve a transfer of electrons between species. Reduction is the gain of electrons or a decrease in the oxidation state of a molecule, atom or ion [1]. Oxidation is the loss of electrons or an increase in the oxidation state of a molecule, atom or ion [1]. Redox reactions occur simultaneously whereby there cannot be a reduction reaction without an oxidation reaction. The media “helps” chemical reactions to occur by interacting with the reaction without being permanently altered. Since an in depth discussion about redox chemistry is a topic better left for another paper I will only deal with how the redox process applies in the removal of iron and manganese using the media. The individual redox equations will be covered in the following iron and manganese removal sections.
In order to begin the process of oxidation of the ions in solution and to ensure that the oxidative layer is not compromised the media is designed to operate in the presence of chlorine or other oxidant. In this process the oxidant removes electrons and is consumed in the process. The operator needs to ensure that there is a 0.1 – 0.3 ppm free chlorine residual in the effluent water.
Chlorine, fed as sodium hypochlorite or bleach (12.5% NaOCl), is the preferred oxidant since it is relatively inexpensive, readily available around the world and it is effective. Other oxidants such as hydrogen peroxide (H2O2), chlorine dioxide (ClO2) or ozone canalso be used so long as a residual can be measured and maintained.
Another function of the NaOCl is that it keeps the media free from bacterial or slime growth. The manganese oxide catalytic surface has to remain clean so that the ions in the water can come in contact with it. At the same time, the NaOCl is a source of oxygen more reactive than molecular oxygen. The following chart indicates safe levels for other water constituents that could interfere with the surface interaction.
Figure 2
Unlike ion exchange resins where higher regenerant dosages will increase the ion exchange capacity, NaOCl residuals or concentrations higher than required to oxidize the Fe and Mn do not increase the oxidative properties of the media. Additionally, since the media is often used to pretreat waters prior to an RO system a higher free chlorine residual would require more extensive post treatment to reduce the residual and protect the membranes from chlorine attack.
The media must be activated prior to being placed into service for the first time. This activation requires a higher dosage of NaOCl than used during normal operation but only has to be performed once during the initial start up. The dosage rate is 0.3 liters of 12.5% NaOCl per cubic foot (ft3) of the media. The activation only requires a soak of an hour but an over night soak is preferred.
Once activated, the vessel(s) must be backwashed to remove the excess NaOCl and any fines. Since manganese oxide is one of the constituents used in the manufacture of the media an extended rinse is required at start up to remove any trace free manganese oxide residual left over from the manufacturing process. Once the Mn level in the backwash water reaches 0.03 ppm and the free chlorine residual is set the filter is ready to be placed into service.
Media replacement due to the decreased physical filtration properties of the silica substrate due to physical abrasion will occur before complete degradation of the catalytic layer takes place. In other words, even after the sand portion of the media ceases to function as an effective filter the media will continue to oxidize the iron and manganese in the water. Under normal operating conditions media life is estimated at 5 – 7 years.
IRON REMOVAL PROCESS:
Iron (Fe) is the fourth most common element found in the earth’s crust and exists in a wide range of oxidation states from -2 to +6 although the most common states are ferrous (+2) and ferric (+3). It is a more common contaminant in most waters than manganese (Mn) and since the dynamic processes for the removal processes of these ions from solution are different I will discuss the Fe removal process first followed by the removal of Mn.
Ferrous salts are readily soluble. Before the ferrous iron, a dissolved solid commonly found as ferrous bicarbonate, can be removed by filtration it must be oxidized, become ferric hydroxide and in neutral ph waters precipitate out in the media bed. The reaction of ferrous bicarbonate and NaOCl acting as a catalyst is almost instantaneous and the ferrous bicarbonate oxidizes (gives up an OH-) to become the insoluble ferric hydroxide which is then removed in the catalytic surface of the media. The following redox reaction equation explains the process.
2Fe(HCO3)2 + NaOCl + H2O 2Fe(OH)2 + 2CO2 + NaCl
Figure 3
The catalytic surface of the media (M in the heavy black rimmed circle) contains manganese oxide or exposes manganese and oxygen sites for adsorption of the ions in the water. Water molecules are an electric dipole and they could be attracted to the ion sites on the catalytic surface. This process is called hydroxylation of metal oxide surface. The oxygen end of the water molecule is first attracted to the metal on the surface of the media. Subsequently, oxygen in the neighboring site attracts the hydrogen and cause splitting of the water molecule into hydroxide OH- and hydrogen ion H+. This is a dynamic process not taking place simultaneously at all sites of the catalyst. The attached matrix of hydroxide and hydrogen ions are then released from adsorption and recombination take place at the same time.
Dissolved ferrous hydroxide is attracted with the Fe end towards the oxygen of themedia. This brings the Fe in the proximity of chemical bonding with the hydroxide ion of a neighboring site and ferrous hydroxide changes into ferric hydroxide. Electric charges are better balanced in the ferric hydroxide and it could move easier away from catalyst surface. Moreover, ferric hydroxide is insoluble and precipitates in crystalline form aggregates 3 nanometer and larger. The aggregates coagulate in larger flocks and are filtered in the media bed.
While ferrous hydroxide is converted into ferric hydroxide, the concentration of ferrous hydroxide at the catalytic surface decreases. In the bulk of the water, away from catalytic surface, the concentration of ferrous hydroxide is higher and ferrous hydroxide diffuses towards the lower concentration according to diffusion law. Diffusion flux is linearly dependent with concentration gradient over distance.
Dissolved oxygen contributes to production of hydroxide ions through direct oxidation of hydrogen in combination with Fe splitting the water molecule and by reacting with the hydrogen at the catalytic surface.
It is important to note is that although a source of oxygen is needed, oxidation and precipitation of Fe is driven by the hydroxide ion. The consequences are that even under relatively acidic conditions hydroxide ions are more easily available for binding to Fe than oxygen. Thus, Fe is not very difficult to oxidize and precipitate around neutral pH conditions. In additionthe concentration of hydroxyl ions increases with pH value exponentially and so does the rate of oxidation and precipitation of Fe.
MANGANESE REMOVAL PROCESS:
The media is a catalytic material specifically tailored to the oxidation and removal of manganese. The catalytic surface of the media (M in the heavy black rimmed circle below) contains manganese oxide available for the chemical bonding of manganese and oxygen atoms from water. However, oxidation and removal of manganese, Mn is vastly different from that of Fe. A major difference is caused by solubility of manganese hydroxide, Mn(OH)2. The following redox reaction equation explains the process.
Mn(HCO3)2 + NaOCl MnO(OH)2 + NaCl + 2CO2
Figure 4
Manganese does not precipitate as oxyhydroxide but as MnO2 and higher oxide valences. Presence and concentration of hydroxide anions does not help much in the precipitation and removal of manganese. To oxidize manganese and minimize chemical usage a strong source of highly reactive oxygen such as ozone or aeration is a possible solution and is often used. Molecular oxygen is not necessarily needed but by having dissolved oxygen available the user can reduce the amount of NaOCl required.
Manganese hydroxide will be attracted with the manganese end to the lattice oxygen. The oxygen molecule has to be available in the proximity for facilitating oxidation through the oxygen from lattice and swapping to the lattice with molecular oxygen. Conditions for this to happen are statistically less probable and reaction is of much slower rate than the oxidation of Fe via hydroxide.
While increase in pH facilitates oxidation and removal of manganese under anoxicconditions (although pH could be alkaline), manganese could dissolve back into the waterand also could be mobilized into water from the catalytic surface. Consequently, regardlessof the target contaminant to be removed, anoxic conditions haveto be avoided toprotect the catalytic layer against leaching. When oxidizing manganese therecommended pH is between 7 and 8.
The presence of iron and its oxidation and removal facilitates the removal of manganeseSimilarity of atomic radius (127 pm (picometers) for Mn and 127 pm for Fe) facilitates co-precipitation of manganese with iron as manganeseion. Backwash sludge formed this way is less stable because the highsensitivity of manganese to move in solution.
OTHER REMOVAL PROCESSES:
The media can also be used to remove arsenic (As). In this case, the media does not actually remove the As but rather relies on the fact that As and Fe readily form a complex and when the media takes out the Fe it takes the As with it. If water has As present but no Fe it will be necessary to add ferric chloride to the water. Since As is usually in the ppb range, it will normally suffice to add 0.25 ppm of ferric chloride. It should be noted that this only applies to inorganic As (As III and V) since not all organic As readily bonds with the Fe.
The media can also be used as a simple and low-cost technology to remove H2S. Once again, at H2S concentrations < 6 ppm and in the presence of NaOCl the media acts as an oxidation catalyst and facilitates the oxidation of any H2S not oxidized by the NaOCl injection. The sulfur is then filtered in the media bed.
DESIGN CONDITIONS:
As shown below, the media operates under a broad range of conditions, however, as with any media, as the conditions near the extremes the performance could be affected. Typical flow rates for Fe removal where the Fe is <5 ppm may be 5 gpm/ft2 and 3 - 4 gpm/ft2 for Mn where the Mn is <2 ppm.