What is Iron?
Iron is a chemical element found in the group 8 metals of the periodic table. It is the most widely used and least expensive metal on earth. In the earth's crust, iron is generally found in the form of ground iron, which means that it is alloyed with a small percentage of nickel.
Iron comes mainly from minerals found around the globe. Many types of rocks are composed mostly of iron. These rock types vary depending on the soil and geographic location. In short, the most common iron-based minerals are magnetite, hematite, goethite and siderite.
Typically, there are two types of iron that can contaminate water:
- Ferrous iron
- Ferric iron
Ferrous Iron or Ferric Iron?
These two types of iron come from the oxidation of iron. When the iron ion loses two electrons, it is classified as ferrous iron (Fe2+). When three electrons are lost, the iron ion becomes ferric iron (Fe3+). It is therefore understood that iron oxides, whether we think of Fe2+ or Fe3+, are cations.
In addition to the number of electrons, the difference between ferrous oxide and ferric oxide lies in its solubility. Indeed, ferrous oxide dissolves completely in water. This makes it a dissolved material. Ferric oxide does not dissolve in water. However, it can oxidize further, which can cause the water to turn a rusty (orange/red) color. In other words, ferric oxide is an integral part of suspended solids.
In addition to these two iron oxides, iron can be amalgamated with other molecules to form different contaminants. Although they can both be categorized as Fe2+ or Fe3+, the subcategories have their own characteristics.
Whether called ferrobacteria, iron bacteria or siderobacteria, they are a group of bacteria found in soils and water that, when in an anaerobic or anoxic environment, have the ability to oxidize iron. When a high concentration of ferrobacteria is found in a water sample and they come into contact with oxygen, the oxidation of iron and the death of the bacteria promote the formation of a gelatinous mass known as iron ochre.
Organically bound iron
This type of iron is caused by the combination of Fe and acids or tannins. Since the acids and tannins responsible for forming the bonds are only found in soils, this type of iron is only found in groundwater, for example, when the water supply comes from a well.
How does iron get into water?
Going back to the principle that iron is generally found in the various minerals that make up the different types of soils, erosion is one of the causes of the spread of iron in water. In other words, since water is a universal solvent, when it comes into contact with an iron ore, it erodes and is found in water. Depending on the type of oxide found, the iron is found in the water in dissolved form or as suspended matter.
This phenomenon can be caused by soil erosion due to water runoff or by erosion as water moves through the soil towards the water table.
The first example occurs naturally, but iron can also be from human sources. It is mainly due to the erosion of iron-bearing infrastructure that iron can end up in water through human interaction. For example, imagine a plumbing system built with galvanized steel pipes. Over time, these pipes corrode and release iron particles into the flowing water.
As we have seen, the iron in water comes largely from soil erosion. This is why iron concentrations vary so much depending on the source of the water. For example, iron concentrations in the Atlantic and Pacific oceans range from 1-3 ppb (parts per billion). For rivers and surface water bodies, concentrations of 0.5-1 PPM (parts per million) are typical. Finally, although we have implied it above, we feel it is important to point out that it is mainly in groundwater that iron concentrations are highest. Indeed, groundwater and wells can have iron concentrations exceeding 100 PPM.
When considering maximum concentrations for drinking water, although standards vary, maximum iron concentrations for drinking water are generally around 0.2 PPM. However, it should be noted that iron, being a necessary mineral for the human body, is not harmful to health. This means that drinking water with a higher concentration of iron is not dangerous. On the other hand, the higher the concentration of iron, the less attractive the water is, since odors, tastes and a particular color may be formed.
- As an aside, although it is typically not dangerous to consume water with a high iron concentration, there are obviously exceptions. Without going into too much detail, overconsumption of iron can cause hemochromatosis.
The Risks Associated with High Iron in Water
Even if the risks associated with its consumption are minimal, this does not make iron a harmless contaminant. Obviously, the risks associated with iron are not comparable to silica, but it is still important to deal with it.
In short, some inconveniences can result from a high presence of iron in water. These include unpleasant odors, unattractive color, or the formation of "rust" spots on your equipment. For some uses, this inconvenience does not affect operations. However, other problems may occur.
In addition to the inconveniences mentioned above, iron in water can affect the efficiency of your equipment by promoting the formation of accumulation on the internal walls of the systems. Pressure problems can occur when the water is heavily contaminated with iron. In fact, the two main reasons that can cause a pressure differential in your equipment are
- The formation of residual clusters that reduce the internal dimension of the pipe;
- The pressure drop caused by a higher water viscosity and the friction point inside your systems.
- Pressure drop is a principle of fluid mechanics that demonstrates the impact that friction points have with a liquid. Learn more here.
Finally, although it is virtually harmless to the integrity of systems, the presence of iron oxide, whether Fe2+ or Fe3+, tends to foul membranes and ionic resins.
How to extract iron?
Now that we have a better understanding of where iron comes from, how it accumulates in water and the impact it can have on equipment, we need to ask ourselves how to remove it?
To begin with, let's remember that there are several types of iron that can accumulate in water. It is important to mention this because depending on the type of iron found, it can be categorized as suspended or dissolved matter.
In the event of the presence of ferric iron (Fe3+), which does not dissolve, these forms of suspended matter that you must extract. To do this, several techniques can be used. Oxidation can be done by adding oxygen to the water (aeration). Other types of oxidants such as chlorine (Cl2) or potassium permanganate (KMnO4) can be added to the water.
- A small note, aeration has the advantage of restoring the pH through the removal of carbon dioxide. This makes the oxidation rate of Fe2+ to Fe3+ faster.
In short, once hydrolysis is complete, Fe2+ is now Fe3+, so it is no longer dissolved in water, but suspended.
Once in this state, ferrous oxide is part of the suspended solids family. With an approximate size of 0.5 micron, their extraction is not super complicated. In fact, Fe3+ can be extracted using standard technology such as a membrane filtration system. Generally, for this type of extraction, one can think of microfiltration, ultrafiltration, nanofiltration or reverse osmosis. It all depends on the desired result.
In addition to the types of filtration, some ion exchange technologies can also be used to extract iron. One can think of electrodeionization or certain ion exchange systems with cationic resin.
Note that iron concentrations can have an impact on the system that is appropriate for your situation. In fact, whether you opt for filtration or ion exchange systems, if they are confronted with too high a concentration of iron, the quality of the treatment may be affected.
In the case of membranes, iron tends to clog the membrane. This increases the pressure required to allow water to pass through and, as a result, forces some contaminants to pass through the membrane due to the higher than normal pressure. This increases the cost of operation since the energy required to treat the water is higher and the membranes must be changed more frequently.
When too much iron is fed into an ion exchange system, the resins can become clogged. Due to the fact that the resin is overloaded, tunnels are formed. From the top of the resin bed to the collector, these tunnels allow water to pass through without being treated by the resin.
In order to avoid problems related to a high concentration of iron in your water, tests should be performed before selecting a water treatment system. It is difficult to define the exact thresholds to be respected since many factors can influence the treatment (flow rate, types of contaminants, quantities, type of treatment, types of membranes, type of resins, etc.).
Following the tests, refer to experts who can help you choose a system that meets your needs and is optimal for your situation.
In short, even if the harmful effects associated with the presence of iron in water are minimal, it is no less important to extract it. Not to mention that removing it is relatively simple and can save you from future complications.
Whether by ion exchange or a membrane system, if iron concentrations are too high, pre-filtration steps may be necessary.
Whether your goal is to produce water free of problematic contaminants such as iron, hardness, or any other, or whether you are aiming to produce ultra pure water for the pharmaceutical industry, the right solution starts with a thorough understanding of your needs and situation. This way, you ensure that the equipment you choose meets your needs and that its use is optimized for the situation you face.
In closing, we hope this article has enlightened you on iron, its derivatives and its relationship with water. If you have any questions or comments, please do not hesitate to contact us.