Iron and Manganese are the most common metals we deal with as contaminants in the water treatment industry. Their presence can generate objectionable tastes, odors, staining, and coloring of anything they come into contact with. The methods for treating and removing them from water supplies have been around for hundreds of years. The process for removing them can be both easy and difficult, depending on their concentration individually or together. Public water supplies don't typically deal with these constituents because they source their water from surface sources. Private water wells in the Mid-Atlantic and Northeast United States can almost guarantee they'll have one, if not both iron and manganese present at some level. Iron is the 4th most abundant element in the earth's crust and comprises around 5% of it, while manganese is around 0.1%. Iron and manganese bearing bedrock formations are dissolved into groundwater typically from the presence of free CO2. Iron concentrations in groundwater typically range from 0-10.0 mg/L, but 20 mg/L or higher are not uncommon. Manganese is much less common, and typically ranges from 0-2.0 mg/L.
There are no health concerns related to either iron or manganese, except for at high concentrations. The EPA has set secondary drinking water standards for iron at 0.3 mg/L and 0.05 mg/L for manganese. At concentrations equal to or higher than these levels, they can cause aesthetic problems in homes and water systems. Iron and manganese are common in four different forms in water. The first is the ferrous state where water appears clear and the metals are dissolved in solution, often referred to as clear water iron or manganese. Groundwater systems are typically high in dissolved carbon dioxide with low or absent levels of dissolved oxygen resulting in clear water iron or manganese. Dissolved iron and manganese exist as bicarbonate salts, Fe(HCO3)2 and Mn(HCO3)4. The second is the ferric state where the metals have been oxidized and have precipitated out of solution, giving a reddish brown to black coloring of the water. After exposure to oxygen over time, the metals will react to form insoluble ferric states and cause discolored water or staining. The third form is the organic state, where the metals have been absorbed by bacteria or they are part of some organic complex. The fourth state is the colloidal form where the metals are bound up in organic substance like tannins or inorganic silica compounds. Determining what form the metal is in and the concentration will dictate the appropriate course of treatment. Most analyses measure iron as total iron and do not dictate between ferrous and ferric iron. A simple way to determine the concentration of ferric and ferrous iron is pass the water sample through a 10 micron (μm) filter paper and perform an iron analysis on the sample before and after the filter paper. The ferric (precipitated) iron will be trapped by the filter paper while the ferrous (dissolved) iron will pass through the filter paper.
Treating or removing iron and manganese from drinking water is dependent on a number of variables including pH, concentration, and the form that it exists in. The most common approach for iron and manganese removal is precipitation and filtration. Precipitation involves the use of some sort of oxidation process to push the iron and manganese from a ferrous or dissolved state to the ferric or precipitated state. Oxidation occurs when one atom transfers electrons in an oxidation-reduction reaction. The atom that loses electrons (reducing agent) is oxidized and the atom that gains the electrons (oxidizing agent) is reduced. Oxidizing agents include oxygen, ozone, and chlorine. During iron oxidation, ferrous bicarbonate Fe(HCO3)2 is oxidized to form ferric hydroxide Fe(OH)3. Both iron and manganese oxidation are heavily dependent on pH. Below a pH of 7, oxidation processes are very slow and require a long contact time for oxidation to occur. Iron oxidation occurs best within a pH range of 7.5-8, while manganese oxidation occurs best at a pH of 8.0 or higher. Generally speaking, a pH increase of 1 results in about a 100 fold increase in the rate of iron oxidation, so a higher pH results in more rapid oxidation.
The purpose of this video is to demonstrate the effect of pH change on iron oxidation. A 50 mL of 20% iron standard solution was prepared and 0.15 g of Sodium Sequicarbonate (Na2CO3 * NaHCO3 * 2H2O) was added to the solution to illustrate the oxidation of iron with a drastic change in pH. The starting pH was very low at 1.71 and the pH was raised to 8.84 with the addition of sodium sequicarbonate.
TREATMENT METHODS
Typically, iron and manganese removal is a two or three step approach depending on conditions and influent water chemistry. They can be grouped into the following:
- Cation Exchange Softening
- Aeration and Filtration
- Chemical Oxidation and Filtration
- Catalytic Oxidation and Filtration
Cation exchange softening works well if the iron or manganese is in the ferrous state and concentrations are below 5 mg/L and 1 mg/L respectively. A water softener is often used after oxidation and filtration has occurred as a water "polisher". A cation exchange softener should never be treated as a filter, and is generally not effective alone when ferrous iron concentrations are in excess of 5 mg/L. Due to the high affinity for resin to hold onto iron and manganese, a resin cleaner (strong acid) is used in the brine tank, and is recommended whenever iron and/or manganese are present. This will help prevent resin fouling, and extend the life of the resin. Our Master Water Satin Series softeners are an excellent choice for this type of application and can use treated water for regeneration. The Satin series also has the ability to adjust the salt dosage settings for stronger brine strength solutions used during regeneration. To learn more about our Satin Series, follow this link.
Aeration and filtration has been used for iron and manganese removal for a long time. During aeration, the water is exposed to oxygen. The oxygen is used to oxidize the iron, which is then filtered through some sort of filtration media (i.e. a multi-media filter). An even more effective aeration technique is the use of ozone. As discussed previously, the Master Water Fusion series is an excellent choice for iron and manganese. Instead of using potentially contaminated, atmospheric oxygen, an ozone generator is used to create ozone which is fed directly into the filter tank. This disinfects the air used and helps reduce fouling of internal system components. Ozone is a stronger oxidizer than oxygen, a strong disinfectant, and an excellent choice when ferrous, ferric, or organic iron is present. To learn more about our Fusion series follow this link.
Chemical oxidation and filtration involves the addition of strong chemical oxidizers like liquid chlorine into the water. A solution tank containing a diluted bleach and water solution is fed into the influent water. The chlorine oxidizes the iron or manganese, which is then filtered through some sort of filtration media (i.e. a multi-media filter). The chlorine also acts as a disinfectant and is useful when iron is present in an organic form. Although chlorine is a strong oxidizer, it often requires longer contact times than ozone. Chlorine injection often requires the use of an activated carbon filter for residual chlorine removal. This is especially important to consider when chlorine injection is used in conjunction with a water softener, as chlorine can significantly reduce the life span of ion exchange resins.
Catalytic oxidation and filtration typically uses manganese oxide based catalytic medias such as Greensand Plus, Birm, or catalytic carbon. A catalyst is a substance that changes the rate of a chemical reaction without being consumed or chemically changed by the chemical reaction. Catalytic medias require specific operating conditions and are heavily dependent on pH. When iron and manganese come into contact with Greensand Plus, they are quickly oxidized, precipitate out of solution, and are filtered out by the media bed. Greensand Plus filters need to be regenerated with a strong oxidizing agent, such as chlorine, as it acts as a catalyst between said oxidizing agent and contaminants such as iron. Birm and catalytic carbon do not require regeneration with a strong oxidizing agent, as they act as a catalyst between dissolved oxygen in the water and the target contaminants. However, since these medias are reliant on enough dissolved oxygen being present in the water to function properly, aeration is often required as pre-treatment.
