On the issue of the presence of iron in water

The article discusses the possibility of the existence of iron in water in various forms.

It is known that iron in water exists in two and trivalent forms. The trivalent form of iron precipitates, i.e. it passes from the liquid into the solid phase and exits the electromagnetic interaction with water molecules. As long as iron is in water in divalent form, it is dissolved in water as an iron cation and participates in electron transfer, i.e. contributes to the value of the electrical conductivity of water.

In natural water, divalent dissolved iron exists in the form of bicarbonate, chloride and sulfate of ferrous. If we consider only the basic anions. Moreover, a uniformly distributed concentration of each type of iron forms in mg-eq/l will be observed in the water [1]. But the transition of iron into the trivalent state and precipitation will be determined by amount of bicarbonate and the redox potential of water.

Is it possible to write down a complete equation of the chemical reaction of the formation of trivalent iron?

Probably not.

There are different forms of writing in the literature.

For example [2],

4Fe + O₂ + 8HCO₃ + 2H₂O = 4Fe(OH)₃^sludge + 8CO₂^delete (1)

Formally, the equation is correct. But let’s look at the Purbe diagram. The diagram shows the boundaries of the existence of two and trivalent iron in water. In the diagram, the pH value =8.4 is highlighted separately. At this pH value, hydrate ion (OH) begins to appear in the water as a result of hydrolysis of bicarbonates.

HCO₃+H₂O = OH + H₂CO₃ = OH+H₂O+CO₂

Carbon dioxide, which is released during the hydrolysis of bicarbonates, is removed or neutralized with alkali. Thus, hydrate is present in the water, and even in the absence of oxygen in the water (ORP of water has a negative value), iron from the divalent form turns into trivalent and begins to precipitate.

It turns out that equation (1) does not quite correctly reflect the process of formation of trivalent iron hydroxide. At a low pH value, the process (1) is practically impossible.

It can be said that the positive ORP of water oxidizes divalent iron into trivalent, but trivalent iron precipitates only if hydrate (OH) is present in the water. At the same time, if hydrate is immediately present in the water, then an oxidizer is not required for the formation of trivalent iron hydrate. If there is no hydrate in the water, but there is enough oxidizer, then the process of iron oxidation will initiate the process of hydrolysis of bicarbonates with the formation of hydrate and carbon dioxide. Iron hydrate will come out of the liquid phase (precipitates), carbon dioxide will remain and if carbon dioxide is not delete or neutralized, the pH of the water will drop. Therefore, the more oxidizer in the water (gaseous oxygen, atomic oxygen), the lower the pH of the water can be for the process of the hydrolysis of bicarbonates to obtain the hydrate required for trivalent iron. If you look at the Purbe diagram, you can determine that at a pH value of less than 4.5, no trivalent iron hydrate is formed in the water in the form of a precipitate. There is only dissolved iron, both divalent and trivalent. At a pH value of less than 4.5, hydrate cannot be obtained from the hydrolysis of bicarbonates, since bicarbonates completely pass into carbon dioxide, and only strong acids can exist in water, which do not cause hydrate formation during dissociation.

The process of iron oxidation initiates the hydrolysis of bicarbonates. In the presence of hydrate in the water, the presence of an oxidizer in the water is not required for the formation of trivalent iron hydrate. It should be taken into account that in the process of precipitation of trivalent iron, the pH value of water and the value of ORP will decrease and this will require either the distillation of carbon dioxide or its neutralization with alkali, as well as maintaining a sufficient amount of oxidizer. That is, it will require constant maintenance of the required pH value and (or) ORP for the precipitation of iron completely.

Example. A certain amount of iron chloride was added to the deoxygenated and desalinated water and the pH of the water was brought to the value of 8.4 with caustic soda. At the same time, the ORP of such water will be slightly below zero. Let’s assume that the ORP will be constantly near zero. In any case, the transfer of divalent iron into the precipitate will bring the ORP to zero from the negative side. The process of formation of trivalent iron hydrate will begin. The more iron passed into the trivalent form, the lower the pH of the water became. Hydrate precipitated with iron. If we look at the Purbe diagram, we will find that when the ORP value is zero and the pH is 8.4, only the trivalent form in the form of a solid phase should be present in the water. But in the process of iron precipitation, the pH of the water drops and comes to a point of equilibrium. In the diagram, this will be the intersection point of the graph with the abscissa axis. This point will correspond to pH = 7.7 and ORP = 0. After reaching this point, the process of precipitation of iron will stop. The equilibrium of divalent iron with the pH and ORP values will be achieved. At the same time, we will observe both divalent iron and trivalent iron in the water. Of course, trivalent iron will be in the solid phase, but in order to precipitate it from water, certain efforts will be required in the form of filtration through catalytic loadings. This process itself is quite complicated.

If a large amount of iron was contained in the water, then part of it will precipitate in the process of simple settling (due to the formation of a suspension heavier than water). But for small iron amount up to 1-2 mg/l, special filtration will be required.

Therefore, both di and trivalent iron will be present in the water, regardless of what parameters this water has at the current time in pH and ORP.

To remove iron from water, you can use the following recommendations:

1. After complete removal of iron from the source water with an increased amount of iron, the intersection point found by the values of the ORP and pH of the purified water should be in the sediment area (trivalent iron hydrate). The intersection point is not for source water, but for purified water!
2. The further the point of intersection of purified water along the ORP and pH is from the border of the transition of divalent iron into trivalent in the direction of sediment, the less time will be required for complete oxidation of iron.
3. A number of factors hinders oxidation of iron and subsequent precipitation. First of all, the presence of organic matter and silicon in water.

Let’s consider the last condition in a little more detail. The Purbe diagram does not take into account some factors that slow down or completely stop the process of iron oxidation.

In natural surface water, an increased value of oxidizability will be observed all the time due to constant contact with surface drains, oxygen, etc. The oxidability in water is mainly represented by humic acids. Humic acids form fairly strong compounds with dissolved iron ions. Such iron is commonly called iron in organic form. This iron is rather poorly oxidized by air oxygen due to its connection with organic matter. Accordingly, such iron cannot be filtered on traditional materials. It is possible to use activated carbon, but it is expensive and most likely will not lead to the desired result. It will only slightly reduce the amount of organic iron as a result of the removal of organic matter on coal. To oxidize organic iron, it is required to oxidize organic matter to acceptable values. It is believed that when the value of water oxidizability is higher than 4.0 mg/l, organic iron begins to appear in the water. The higher the value of organic matter, the more iron is observed in organic form. In this case, atomic oxygen (chlorine, sodium hypochlorite, ozone) must be used to oxidize organic matter and, accordingly, iron. The oxidation of organic matter and iron will occur more fully with an increase in the pH of the treated water. Coagulation can also be used to remove organic iron. But this technology requires quite serious chemical control, as well as additional reagents if the temperature of the treated water is below 20⁰ C. But with a high organic amount, coagulation will be required in any case. It can be said that for small fully automated water purification units from iron, dosing sodium hypochlorite for the purpose of iron oxidation and possibly a small amount of organic matter is a way to make the water treatment plant more compact and reliable. It is also convenient to use hypochlorite dosing instead of aeration for the purposes of pretreatment of water before reverse osmosis units.

Silicon in water also has a retarding effect on the oxidation of iron. Silicon in water forms various types of silicic acids, which in turn form stable colloidal solutions. Silicic acids are slightly soluble in water. This colloidal formation significantly slows down the process of oxidation and filtration of iron.

From my practice. The source water is artesian. Oxidizability is not more than 1.0 mg/l. The initial iron amount is 1.8 mg/l. When the value of silicon in water is up to 3.0 mg/l, there is actually no difficulty in removing iron from artesian water by aeration and subsequent filtration. A similar case. Artesian water. The initial content of iron is 1.1 mg/l, silicon is 9.8-10.2 mg/l. Air and then potassium permanganate were used to oxidize iron. After oxidation and filtration, it was not possible to obtain the required value of iron in water (less than 0.1 mg/l) even with minimal water consumption. On a “clean” load, it was possible to obtain iron values of 0.15-0.3 mg/l. After several weeks, the value of iron in purified water increased to 0.5 mg/l. This increased value was kept constantly. Inspection of the loading showed its complete contamination with some gel. And this is in the conditions of daily full-fledged flushing of filters. Probably, the gel weakened the catalytic properties of the loading. A satisfactory result on iron was obtained only after the contact time of water and loading was doubled.

It is also worth saying that divalent iron can be removed using the cation exchange process. At stable low pH values of water, either H – or Na – cation exchange process can be used to remove iron. But in this case, the ionic composition of the water will be significantly changed.

Литература

1. Тихонов И.А., Взгляд не математика на теорию электропроводности водных растворов сильных электролитов // Сборник статей по организации водно-химического режима теплоэнергообъектов, Издательские решения, 2022, -с. 241-257.
2. Кострикин Ю.М., Мещерский Н.А., Коровина О.В., Водоподготовка и водный режим энергообъектов низкого и среднего давления. Справочник. Энергоатомиздат, Москва, 1990 г. – 252 с.