Sulfate is naturally occurring elements that can be categorized as salts of sulfuric acid. Sulfate is a sulfur oxoanion that is obtained through the deprotonation of OH groups in sulfuric acid. The deprotonation of OH groups is a chemical reaction that removes the H+ proton from the molecule. This is the reason why sulfate is called oxoanions.
- For information, oxoanion is defined as an anion formed due to the loss of a hydrogen cation at the expense of an oxygen atom.
Its basic composition consists of oxygen atoms and sulfur atoms SO4-2). Due to the presence of one sulfur atom (+6) and 4 oxygen atoms (-2), sulfate ions have an ionic charge of -2. It is thanks to this negative ionic charge that sulfate has the ability to bind to certain elements called cationic such as potassium, sodium or magnesium. When bonds are formed, we obtain compounds that are very common in our lives, such as magnesium sulfate.
There is a second group of sulfate which, unlike the first group which consists of sulfuric acid salts, is composed of carbon. In this second group, the hydrogen atoms of the sulfuric acid have been exchanged by ester compounds, which are combined groups containing carbon such as ethyl.
Il existe un deuxième groupe de sulfate qui, au contraire du premier qui se compose de sel d’acide sulfurique, est composé de carbone. Dans ce deuxième groupe, les atomes d’hydrogène de l’acide sulfurique ont été échangés par des composés d’ester, soit des groupes combinés contenant du carbone tel l’éthyle.
Sulphide, Sulfite or Sulfate
Before going any further, in order to fully understand and differentiate sulfate, we need to look at sulphide and sulfite. Although all three are derivatives of sulfur, these three elements are very different and react in completely divergent ways.
Without going into detail, sulphide is a sulfur ion that has gained two electrons. Sulphite and sulfate are polyatomic atoms constructed of sulphide and oxygen atoms. Sulfite (SO3-2) consists of one sulphide atom and three oxygen atoms. Sulfate (SO4-2), on the other hand, consists of one sulphide atom and four oxygen atoms.
Typically, sulfites are chemical compounds with antioxidant and antibacterial properties, which makes them an excellent solution for the preservation of certain everyday products such as wines.
Where do they come from?
Before discussing the sources of sulfate that can influence the composition of water, it is important to note that the different forms of sulfate mean that it can be found in the air or in water. Moreover, the impact of its presence can vary according to the type of sulfate and its state.
For practical purposes, it is important to note that not all sulfate is soluble. For example, sulfate of calcium, barium and various heavy metals are not soluble. Sodium, potassium and magnesium sulfate are.
Natural Sources
Because sulfate is a naturally occurring compound and is widely distributed on earth, one of the reasons for its presence in water is soil erosion. Indeed, when water flows and erodes minerals with traces of sulfur or sulfate, they can accumulate in the water. This is why it is common to find traces of sulfate in crystalline groundwater. Furthermore, it is known that the deeper the water is collected, the higher the sulfate concentrations.
In fact, there are dozens of minerals with sulfate (SO4-2) in their chemical composition. We can think of the following examples:
- Alum
- Gypsum
- Kainite
It is therefore in the form of dissolved matter that sulfate is found in water most of the time.
How do materials dissolve in water?
The dissolution of any material (solute) takes place due to the interaction and attractive forces that are present between the solute and the solution. This means that when the force of attraction between the solute and the solution is stronger than the force of tension between the ions forming the solute, they break up and join the solution. Take for example the addition of NaCl to a sample of H2O to form salt water. In short, the covalent bonds between Na & Cl being less strong than the force of attraction between Na & H2O, these covalent bonds will be broken and the NaCl will go to bind ionically to the water molecules. It is due to these reactions that various materials can dissolve in water.
- As an aside, water is recognized as a universal solvent since its chemical composition allows it to associate with a wide variety of molecules.
This is typically how sulfate is found in water naturally.
Human activities as a source of sulfate
Since the production of synthetic sulfate is relatively simple, its use is widespread in many sectors. There are two methods to produce synthetic sulfate.
- By oxidation of sulfites and metal sulphides.
- By treatment of metal oxide, metal hydroxide and pure metal with sulfuric acid
Sulfate and their derivatives are used extensively in many industries. For example, significant traces of sulfate salt are found in the food sector. These salts are most concentrated in food supplements, bread or canned fruits and vegetables.
In addition to the food sector, sulphate derivatives are used in many ways. Here are some common examples:
- In the water treatment sector, copper sulfate is used as an algicide and barium sulfate.
- Various construction or art activities often use hydrated calcium sulfate for the manufacture of plaster.
- Toothpastes, soaps, shampoos and many other products of this type very often have sulfate compounds.
- Finally, sulfate is often added to cleaning products to improve them since sulfate are strong surfactants.
Obviously, there are other uses for sulfate, but this list gives you a taste of the diversity of these chemical compounds. Their ubiquity makes one wonder about their impact on health and on the various equipment they come into contact with.
Impact of Their Presence
On human health, sulfate is not super dangerous. Typically, when they come into contact with the skin, they can promote the dryness of the skin. On the other hand, when consumed in excessive amounts, sulfate can cause reduced lung capacity or worsening of asthma-related symptoms.
Unlike some contaminants such as silica or hardness that can clog filtration membranes or promote scale formation, the presence of sulfate does not present a risk to water treatment equipment.
Even if the risks associated with the presence of sulphate are relatively low, their presence is still legislated and controlled by the various government authorities. Indeed, since it is recognized that water with a concentration exceeding 500 mg/l gives a bitter taste to the water, can have laxative properties, and can dry out the surrounding fauna and flora, the governmental authorities have issued limits relative to the concentrations of sulphate present in the discharges and in the drinking water.
For example, the Guidelines for Canadian Drinking Water Quality suggest that sulphate levels be kept below 500 mg/l.
Sulfate extraction techniques
As discussed above, sulfate can be found in water in dissolved forms when they are water soluble, or they can be suspended matter when they are not water soluble. Regardless of the case, there are three technologies that can be used for sulfate extraction.
Ion Exchanger
You know them, the ion exchangers are very used and efficient systems. The most widespread of these is without a doubt the water softener. All working on the same principle, ion exchangers allow the permutation of contaminating ions in the water by acid or basic radicals found on the resin.
Typically, for the extraction of sulfate and its derivatives, it is thanks to acid radicals found in the molecular structure of the resin that the exchange between the sulfate molecules and these acid radicals can take place. This ion exchange works because of the ionic charges of the different molecules in the water and in the resin. To simplify things, we can summarize this by saying: "opposites attract". Briefly, negatively charged ions will be attracted to positively charged resin beads and vice versa.
In order not to dwell on the subject, to learn more about the ion exchange process, here are some interesting articles:
Generally speaking, ion exchangers are the preferred choice when high sulfate levels are present. They are found especially in municipal water treatment plants since ion exchangers are very efficient and relatively affordable.
Note that depending on the contaminants you are dealing with, the type of ion exchanger can be adapted. In fact, there are several families of ion exchangers, the most common being mixed bed ion exchangers and separate bed ion exchangers.
Reverse Osmosis
Being the water treatment systems par excellence, reverse osmosis systems are obviously very effective in removing sulfate. As you may already know, reverse osmosis systems use so-called osmotic membranes to treat water. These membranes are made of semipermeable materials, usually polyamide, with pore size of approximately 0.0001 microns.
Simply put, water flows through the osmosis system where a semipermeable membrane is installed to block contaminants. The water that passes through the membrane is called the permeate and is the product of the water treatment. On the other side is the concentrate, which is a concentrated solution of contaminants.
To learn more about reverse osmosis, here are some interesting articles:
- Reverse osmosis explained.
- How to reduce water waste from a reverse osmosis system
- What is osmotic pressure
Reverse osmosis systems are very versatile and can be used for various purposes such as the production of drinking water or even ultra-pure water. It is important to note, however, that depending on the quality of the feed water, pretreatment steps may be necessary to improve performance and reduce the operational costs of reverse osmosis. It is partially for this reason that we often see reverse osmosis systems working in conjunction with other technologies such as microfiltration, ultrafiltration, or even different types of ion exchangers.
Distillation
Although not as popular as the other two techniques, distillation is a recognized method for extracting sulfate. In fact, distillation is a water treatment technique that allows the extraction of dissolved materials. To do this, water is brought to a boil and the vapours are recovered. In order to ensure that the vapour is not contaminated by the environment, it is necessary to make sure that the distillation is done in a sterile place.
Although this technique is effective, it can be complicated, and problems can occur if not done properly. Also, if other contaminants such as silica are present, they can accumulate in the distilled water.
Sulphate and Water
As we have seen, even if their presence does not present a major risk to the environment, equipment or health, sulphate removal may be necessary in your situation. Whether it is for your water use or due to discharge standards, sulfate removal may be necessary. Choosing an extraction technique that represents your needs is an important step in optimizing your financial investment.
It is important to remember that the choice of a water treatment system is based on your needs and your situation. As we have seen here, sulphate is not a concern for different types of systems, but this is not always the case. Some contaminants require special treatment to avoid throwing money down the drain.