Ion Exchange Resins for Water Treatment - version 2025

1.01 Introduction/Historic – Part I

Welcome to the ion exchange resins course, and let's start with a historical introduction.
The use of natural materials with ion exchange power for water treatment dates back to biblical times.
In 320 BC, Aristotle used an earthenware vessel containing a material with ion exchange properties to produce fresh water from seawater.
Long before synthetic ion exchange resins derived from polymers, the first ion exchangers were equipped with zeolites, which constitute a large group of minerals that contain a porous structure.
Zeolite is a word of Greek origin that means “boiling stone”, a phenomenon observed by the Swede Axell Cronstendt in 1756, who observed that after the rapid heating of a mineral, the stones began to jump as the water evaporated. Zeolites are minerals that have a porous structure, such as hydrated aluminium silicate, and that have an open structure that can accommodate a wide variety of positive ions, such as sodium, potassium, calcium, magnesium, and others.
In 1818, German chemist Johan Nepomuk von Fuchs discovered sodium zeolite by combining aluminium and sodium silicate.
The first studies involving cation exchange date back to 1850, by J. Thomas Way. In 1905, German chemist Robert Ganz discovered that zeolites could be used to soften hard water.
The first commercial zeolite was obtained by combining kaolin or kaolinite, sand, and ash. In 1934, with Adams & Holmes, there was a great advance in the area in which it was discovered that cation exchangers could be regenerated not only with salt but also with acid. This was the first step in the modern ion exchange process.
Later work led to the development of sulfonated styrene resins and the polymerization of styrene and divinylbenzene.

In 1950, the definitive milestone was reached: based on the research of Gaetano Frank D'Lelio, in 1945, Kunin & Myers developed and patented the process involving sulfonation, crosslinking, and polystyrene.
These advances were followed by McBurney, who around 1952 developed the styrene divinyl benzene copolymer, which was the basis for the definitive establishment of ion exchange technology.
There is academic interest in whether the ion exchange process is an absorption or an adsorption process. "AD" is a process that occurs at an interface or surface involving the interaction of ions. "AB" absorption involves the incorporation of a substance in one state that attracts another in another state, such as solids attracting liquids throughout their entire structure. So the phenomenon of ion exchange can be placed in both categories and can therefore be more safely considered as an sorption process, a term that avoids controversy among those most devoted to the prefixes "AD" and "AB".
Finally, what are ion exchange resins, and what is this process for? The ion exchange technology used for water treatment is a process in which a granular filtering medium composed of microspheres varying between 300 and 1200 microns, composed of polymers and electrically charged, aims to attract certain types of ions to itself, depending on the constitution of the end of the polymer chain.

The process normally takes place inside a vessel, usually cylindrical, and as the water passes through the resin, ion exchange takes place when certain undesirable ions are removed and replaced by more desirable ones.

1.02 – Introduction - Part II

The ions that are attracted by the resin and those that are released by it depend on the type of resin and the type of regenerant. The regeneration involves a chemical support solution that, when applied, allows the resin to be exhausted by the exchange process. Now no longer able to attract new ions, it returns to its initial condition to restart a new exchange cycle, which means that this process is reversible.
Well, how do these ionic exchanges and interactions between resin and filter medium ions occur?
One of the theories is the "Crystalline Network" theory proposed by Paulling and Bragg. A sodium chloride crystal contains sodium ions and chloride ions. Each ion in the crystal is surrounded by a fixed number of oppositely charged ions and subject to certain "Coulombic" attraction forces, whereby channels on the crystal surface experience less attractive force than crystals further below the surface.

When they are in contact with a polar medium, such as water, the forces of attraction around the crystal decrease to such an extent that an exchange of this ion for another becomes possible.
The ease with which surface ions will be replaced by other ions will depend on the nature of the Coulombic forces around the crystal, the concentration of the ion to be exchanged, the electrical charge of the exchange ion, the size of the ions, the accessibility of the lattice formed by the ions and the effects of solubility.

The ion exchange mechanism in resins is analogous to the ion exchange in crystals. Resins can be considered high-molecular-weight and insoluble polymeric electrolytes. The ion exchange capacity of the resins is due to the functional groups at the end of the polymeric chain, such as sulfonic, carboxylic, and phenolic.
Another theory that explains ion interactions is the double layer theory, which was proposed by Helmholtz in 1879, revised by Gouy-Chapman in 1910, and also realized by Stern in 1924.
This theory argues that those present in a diffuse layer of colloids do not behave as if there were a defined division between the layers, but rather the existence of a diffuse layer that remains in equilibrium and that is dependent on concentration and pH.
Although there are other theories that explain ion interactions, they are all similar in one respect: that the exchange of ions must satisfy the law of electroneutrality.
The law or principle of neutrality was proposed by Linus Pauling in 1948 and later revised. The principle states that the electrons in a molecule are distributed in such a way that the charges on the atoms should converge to zero as much as possible.
The differences between the theories are related to the position and origin of the exchange sites. In all cases, the exchange sites are formed essentially by a non-diffuse ionic cluster formed by an electrostatic bond, whose ion can be replaced depending on the binding strength.
The theories of bonds involving ion exchange can become complex, but deepening these theories is of more interest to students, notoriously postgraduate and master's students.
For engineers and technicians more focused on designing ion exchange, softeners, and demineralization systems, it is enough to know that there is an exchange interaction between oppositely charged ions and that this phenomenon provides what we want, that is, the removal of undesirable ions by other less undesirable ones.
This is the essence of ion exchange.

1.03 Resins physical aspects

Okay, talking a little more now about how the resins are presented and how the resin is shaped. The ion exchange resins are microspheres and have a diameter varying between 300 and 1200 microns in the times part; this diameter is a variation for non-uniform resins. For unformed resins, the evaluation is much lower and is in the range of 600 microns, with some variation for more or less. The nomenclature of the resins is designated by three letters, which will be explained later and refers to the international nomenclature. Here are some examples of resin types:

strongly acidic and uniform resin

- then here is another type of strongly acidic resin; you can see the uniformity of the diameter of the particles.

Strongly acidic resin with high crosslinking, we will explain what crosslinking is in the following classes:

Strongly acidic resin does not shape; you can see smaller and larger spheres distributed.

uniform, strongly basic resin, with a lighter and more silvery color,

strong basic resin with high crosslinking, which is basically a better-quality resin,

Strongly basic resin common grade; you notice the difference in particle diameter,

high-quality weakly basic resin of the uniform type, suitable for softening water with a high organic load,

Another example of a weakly basic resin,

Here is an example of mixed resin, which is a mixture of strongly acidic and strongly basic resins.

Another example of mixed resins: let's explain later what mixed resins are for.

Microscope image of the surface of the weakly acidic resin, and here is a scanning electron microscope image of the surface of a strongly acidic resin.

In this image, the resin is already accommodated inside the tank; the view from above looks like this: a strongly acidic resin with a bright golden amber color.

This table shows the variation in the diameter of the resins, starting with the smallest particle at 250 microns and ranging up to 2,000 microns. In practice, however, commercially, the values vary between 300 and 1200 microns, this being the micron for non-uniform resins.

For uniform ones, the granulometry is concentrated in the range between 500 and 700 microns, with the vast majority of them around 600 microns with 50 microns more or less.

The most common commercial presentation of resins is in 25-liter bags or 141.5-liter barrels.

Suppliers usually quote per liter of resin, with prices often quoted in dollars.

With regard to suppliers, a specific class on the market will be given later on, when the main brands of resin available on the market will be presented.

Below, we will present images of the resins that were obtained through observation under a microscope. The device used here was a Nikon Labophot with 40x magnification. Firstly, as a highly acidic, non-uniform resin, you can see the large variation in the diameter of the spheres;

Another image:

It's a new resin, so the spheres are clean.

Here an uniform cationic resin, it is noticed that there is practically no variation in diameter;

Here is footage of this resin; you can see some variation in diameter, but in general, the resin maintains a diameter of around 600 microns on average.

Here is another type of resin from another brand, a new brand on the market. it has a color that is not yellow, let's say a more silvery color, despite being a strong cationic resin. It looks like a more faded yellow, pulling with silver, which is characteristic of this brand.

In this image, it is a strong cationic resin; its manufacturer promises to remove large amounts of iron. It can be seen in the conformation of the internal structure, which is different from that of common resins. This resin model is not classified as a uniform resin because the diameter variations in its spheres are clearly perceived. But it is a resin considered to be of high quality and has the specific function of removing metals, according to the manufacturer.

We will comment on alternatives for removing metals from water in later classes.

We now move on to examples of used resins that were removed from softeners that had already been in use for some years, and the dirt and contamination of this material can be seen.

Here in this image of a uniform resin, you can see the accumulation of material already occurring; there is a little fouling, and this is an aspect of the resin after a few years of use.

In this way, the ion exchange capacity is progressively lost, so a microscope analysis also allows for assessing the condition of the resin and whether or not it is time to change it. These images then show that the resins have exhausted their ion exchange power and that it is necessary to replace them with new resins.

Microscopy of ion exchange resins will be covered in more detail in a specific class later on.

1.04 Chemical structure

Let's talk now about the chemical formula of resin. The chemical formula is basically a polymer of benzene rings and a radical, which can vary but is basically a chain of benzene rings interconnected to the polymer.

Another image of a carbon chain, benzene rings, and a spike represents the exchange site.

The tip can have several conformations and is the part that gives the characteristics of the type of resin.

Here is the most common form of cationic resin: it has an SO3H or SO3Na group, which is even more common. This is where the exchange takes place. The hydrogen, which could also be sodium, will leave calcium, magnesium, or another ion that you want to remove.

The production of resins is basically carried out in two ways: one through the reaction of methacrylic acid with divinylbenzene.

The other way is by combining styrene with divinylbenzene, followed by sulfonation. Here, in the case of the production of the cationic resin, we have the carbon chain, a benzene ring, and the SO3H group. So this grouping here is a common find. So we saw in a generic way how the resin is produced.

Let's now separate the types.

The strongly acidic SAC resin in which the reaction is carried out with hot sulfuric acid, after which the resin is washed to remove excess acid, is a process that has to be conducted with total control because this addition of acid causes the grains to swell in the resin and, if not controlled, can damage the resin.

Then it is produced in the H+ cycle, carrying the acid cycle, so if you want to produce a resin for softening, another step must be added to convert the H+ cycle to the Na+ cycle. We then move on to strongly basic SBA resins. Its production is more complex, more time-consuming, and requires more expensive chemicals. Activation begins with the polymer undergoing a reaction with chlorine methyl ether, which then produces the polymer with the final cluster having CH2Cl. So the first step is chlorine methylation. Next, the chloride group at the end of the chain is bonded to carbon, but it is not ionized, so until this step, there is no charge to be exchanged. This is where the second stage comes in, in which it is reacted with trimethylamine, producing the SBA type 1 resin, or else with dimethyl ethanol amine, producing the SBA type 2 resin. Type 1 SBA resin is strongly basic, and Type 2 SBA resin is intermediate between strongly basic and weakly basic. We then moved on to the weakly basic WBA resin. The same radical is used, which is chlorine methyl styrene, but instead of reacting with dimethyl ethanol amine, it will react with diethyl amine, thus producing the WBA resin, which has one carbon less at the end of the chain. We then arrive at WAC, or weakly acidic resins, which are formed from the polymerization of acrylonitrile or methyl acrylate, compounds that are hydrolyzed. In the case of acrylonitrile, it is hydrolyzed with sulfuric acid, while the acrylate polymer is hydrolyzed with soda. The end product of these two compounds is a carboxylic acid; the WAC resin cluster is a carboxylic acid.

The details of the reactions and more information about chemical reactions to produce resins are a little more complicated because the companies that manufacture them have, in fact, an industrial secret, so it is more complicated to scrutinize further technical information about the reactions, for it is a process well kept under lock and key by the multinational companies that produce the resins and own this technology, in addition to which they compete a lot with each other. But at least we can get a basic idea of the reactions.

1.05 Milliequivalent-gram concept

One of the most used concepts in calculations and sizing of ion exchange resin systems is the concept of milli-equivalent gram or equivalent gram, which defines the capacity of the resin for design calculations of softeners.
Here in the datasheet of a resin, among other information, the theoretical exchange capacity appears, in this case, 2 equivalents per liter.
But what is the milli-equivalent? You can express milliequivalent per ml or equivalent per liter, which is the same thing.
And how to calculate the milli-equivalent?
First, I have to calculate the atomic weight. For example, I want to calculate the gram equivalent or milli-equivalent of calcium.
Calcium has an atomic number of 20 and an atomic mass of 40. As it is bivalent, that is, calcium +2, then I have to divide 40, which is the atomic mass, by 2, which is its valence, resulting in 20. So the equivalent of calcium is 20 g; the milli-equivalent is obtained by dividing the result by 1000, so I will have 0.02 g, which is 20 mg.
One mile-equivalent of calcium is 20 mg.
To better establish the concept, we have a water analysis that shows 100 mg per liter of calcium in the CaCO3 format, which is very common in laboratory analysis. It is difficult to always see total hardness in calcium analysis.
The molecular weight of calcium carbonate is 100 grams; the molecular weight of calcium is 40 grams, so it gives a ratio of 2.5, which then represents that my calcium hardness is 40 mg/l of calcium in calcium format; if we have 1 meq of calcium, it is 20 mg , we saw before, so if we have 40 mg, we have 2 meq/l of calcium.
This unit is important to get used to using in meq or milli-equivalent per liter because it avoids this kind of confusion:
Is it in CaCO3?
Is it calcium?
Is it in mg/l?
The advantage of the meq unit is that it is standard and avoids this type of error.
Therefore, it is recommended to get used to ion exchange from now on, converting everything to milli-equivalents per liter (meq).
Some calculation and sizing software already has various formats, and you can feed the data in any format.
So, there will be no problem with that, but it is recommended to use a meq format and let the program do the necessary conversions.
The milli-equivalent concept also helps explain the issue of valences and how this affects ion exchange.
It is not difficult to understand that calcium, which has a +2 valence, will occupy twice as many sites in the resin when it passes through it, which contains sodium as the species at the end of the chain, and will therefore displace 2 sodiums.
Therefore, meq avoids calculation errors precisely because it already considers the valence of the structure, and in ionic exchange, valency is a very important concept.
A trivalent ion will occupy three times the resin site as a monovalent one, and this will drastically affect the exchange capacity.
The concept of valency will be detailed later.

1.06 Water hardness

One of the most common applications of ion exchange resins is the removal of calcium and magnesium from water, also known as hardness.
Water hardness is a property related to the concentration of ions of certain minerals dissolved in it; more specifically, hard water contains mainly divalent salts and relatively high concentrations.
Water hardness is predominantly caused by the presence of calcium and magnesium salts so these are the main ions taken into account.
Eventually, we will also have zinc, strontium, iron, manganese, and aluminum, among other metals that may be taken into account when measuring hardness.
Concepts of temporary and permanent hardness:
Temporary hardness is caused by calcium and magnesium salts bound to carbonate and bicarbonate ions.
These metals are, by definition, alkaline earth metals, and when they combine with bicarbonate and carbonate ions, they form calcium bicarbonate, calcium carbonate, magnesium bicarbonate, and magnesium carbonate. These substances can be removed by boiling.
Permanent hardness is the same as cations but linked to strong anions, such as sulfate and chloride.
So we have calcium sulfate, calcium chloride, magnesium sulfate, and magnesium chloride.
Therefore, this hardness is what causes the accumulation of material deposited in pipes (incrustation), and it is a very common problem not only in industrial equipment but also in water pipes in condominiums and residences.
Calcium and magnesium removal is one of the most common applications involving ion exchange technology, commonly referred to as softening.
A question that arises and will become evident later on when we study the companies' ion exchange sizing software, concerns the interpretation of the water analysis for the hardness parameter.
The software does not have the option of entering total hardness or total alkalinity. You actually enter the calcium and magnesium readings separately.
For alkalinity, you will enter with bicarbonate and carbonate.
But what can be done in this case, and how does this detail influence the calculation?
So let's start with hardness. Here, the ideal is that, for any situation, you ask the analysis laboratory to measure calcium and magnesium separately. Still, having only the total hardness reading is possible to make the calculation, and let's see how this is possible.
You can estimate if you only have the total hardness measurement.
Considering as an example: 100 ppm of total hardness, considering 80 ppm of calcium and 20 ppm of magnesium, is an estimate and a very rounded calculation, but a good approximation and is usually an average of the most common water analysis.
But what will this influence in the calculation? Very little indeed. Note the selectivity and separation factors, a subject that will be dealt with in detail later: if the selectivity of calcium is 1.90 and magnesium is 1.67, you can say that these two substances are more or less attracted by the same intensity or very little difference in intensity by the sites of the cationic resin.
Testing in practice with sizing software did not result in any difference. If you add 2.0 meq of calcium and 0.0 meq of magnesium, or 1.5 meq of calcium and 0.5 meq of magnesium, or even 1.0 meq of calcium and 1.0 meq of magnesium, always totaling 2.0, you can see that for the calculation it will not influence anything.
So it's not something to worry about, but if you want to do it, ask the laboratory to detail these cations. It is possible that some software could result in some small difference, but it is more of a calculation precision.

1.07 Water Alkalinity

We will now move on to the concept of alkalinity.
Alkalinity for sanitation is not a substance that presents a risk to health and, therefore, has less health interest.
It is important as a buffering effect in effluent treatment, particularly within the anaerobic reactor.
It plays a fundamental role in flocculation reactions since the PAC, or aluminum salt, needs alkalinity to form aluminum hydroxide.

In this process, if the alkalinity is low, it will have to be added, and this explains why in flocculation, the flocculant plus the alkalizing agent are dosed at the same time.
And it is also of some importance in swimming pools, where it is recommended to keep its content between 80 and 120 ppm, since below 80 ppm the pH can suffer great variations and the pool becomes unstable, and above 120 ppm, or when it is very high, can cause incrustation or change the turbidity of the water.
Alkalinity is easier to determine than hardness.
Here is a graph that relates the pH to the two most common substances of alkalinity: carbonate and bicarbonate, remembering only that we have the types of alkalinity: if the pH is greater than 9.4 we have a predominance of hydroxide and carbonate; If the pH is between 8.3 and 9.4, we have a predominance of carbonate and bicarbonate; if the pH is between 4.4 and 8.4, we have a predominance of bicarbonate.

So back to the graph.
The graph shows us two curves, one representing the amount of bicarbonate (HCO3) and another representing the amount of carbonate (CO3).
Observing the pH range between 6.5 and 9.5, the predominance of bicarbonates is noted.
At a more acidic pH, we have a predominance of carbonates and a decrease in the amount of bicarbonates.
At a higher pH, above 10, the condition observed at a more acidic pH is repeated: the carbonate again predominates.
Showing a practical example: if you have a pH of 7.5, look at the graph, you will have a bicarbonate concentration of 0.87. Let's say you have 100 ppm of alkalinity; 87 ppm will correspond to bicarbonate and 13 ppm of carbonate.
In another example, if you have a pH of 8.3, it will correspond to 100% bicarbonate.
It means that from the graph, you can estimate these chemical species because they depend on the pH of the water.
Remembering that it is always recommended to have a more exact calculation, ask the analysis laboratory to perform bicarbonate and carbonate alkalinities.
This example is given just in case you don't have the information, i.e., if the water analysis only specifies total alkalinity.
Finally, the hardness and alkalinity ratio:
- When the total hardness is greater than the total alkalinity, the water contains carbonate and bicarbonate hardness.

When the total hardness is equal to the total alkalinity, the water contains only carbonate hardness; when the total hardness is less than the total alkalinity, the water contains only carbonate hardness, and the excess alkalinity corresponds to sodium, potassium, carbonate, and bicarbonate.

1.08 What to analyze in the water?

The first step for a project involving ion exchange resin, such as softening, removal of specific ions, or demineralization, is the analysis of the water to be treated.
But what should I analyze in the water?
First, it is necessary to define the objective of the treatment. What does your project consist of? Is it a softening project, specific anion removal such as NO3-, silica removal, or demineralization? Is it an anion removal project?
In the case of softening, the main parameters are: calcium, magnesium, sodium, iron, and manganese.
Rarely a water analysis shows calcium and magnesium separately, so you can estimate these contaminants by reading the total hardness (as seen earlier).
Secondary parameters would be potassium, strontium, barium, and the ammonium ion.
For the removal of anions, the main parameters are nitrate, chloride, and sulfate. Normally, when it is desired to remove anions, such as nitrate, the sulfate ion becomes an important contaminant and prevails due to its selectivity, being removed before and with priority to the nitrate. Therefore, when you want to remove nitrate, you must also analyze sulfate.
The secondary parameters for anions would be bromine, fluorine, bicarbonate, and phosphate. All of them are in the companies' software, so with these parameters, the software will be fed with all the data, thus producing a more correct calculation, but we'll see that later.
In the case of silica, it is a little more complicated because silica in water is usually in the form of a colloid (colloidal silica), and, being in colloidal form, the resin does not have the ability to attract this silica format because it has no electrical charge.
Therefore, for silica, the recommended technology is membranes; even so, we will show a topic about silica later on.
For demineralization, all the main ions (anions and cations) must be analyzed, and it is highly recommended that all secondary ions are also analyzed, even more so if you want to have ultra-pure water with very low electrical conductivity.
Last but not least is the analysis of the organic matter parameter, which is rarely included in water analysis but is one of the most important. This is the "organic matter per consumed oxygen" parameter, which practically all the more structured analysis laboratories use, and it is extremely important even to assess the eventual need for pre-treatment in the water since all the water that enters a vase containing ion exchange resin must be previously treated with filtration and, depending on the conditions of the raw water, undergo other pre-treatments.
Finally, if you are going to do a softening project, you don't have to worry about the anions because the sizing software itself will perform a load balance based on the cation data feed. However, if you intend to carry out demineralization or alkalinity removal, which we will see later in a video on alkalinity removal with resin WAC, then it would be interesting to request an alkalinity analysis as well. In this case, you can have only the alkalinity analysis and make the estimate of carbonate and bicarbonate (already explained earlier) or do the analysis of alkalinity to bicarbonate and alkalinity to carbonate.

In “The Nalco Water Handbook,” several tables of water analysis parameters are presented, considering each process:

WAC + SAC with degasser (dealkalization with caustic adjustment);

SAC + SBA (chloride dealkalizer);

mixed bed for dealkalization;

Demineralization: twin bed with degasser

SAC for softening—this will be more detailed.

According to Nalco, cations should be calcium, magnesium, sodium, and potassium. For anions: bicarbonate, chloride, sulfate, and nitrate.

Other parameters are alkalinity, carbon dioxide, silica reactive (not colloidal), conductivity, and pH.

Note that in the case of softening, there is an increase in the amount of sodium in the treated water, which can become a problem depending on the type of use the water will have.
Especially if the water already contains sodium in an appreciable amount before it undergoes softening.
This will be the subject of an extra class, which will detail the limitations of the softening of water intended for human consumption.

The more information and parameters you have, the better, so also take into account the cost of water analysis in the project.

1.09 Resin types

The types of resins can be divided into four groups. It is important to get used to the international nomenclature where resin types are designated by a 3-letter abbreviation:
SAC, which means Strong Acid Cation, or strongly acidic;

SBA, which means Strong Basic Anion, or strongly basic;

WAC, which means Weak Acid Cation, or weakly acidic;

WBA, which stands for Weak Basic Anion, or weakly basic.

Here is a chart showing the four main types of resins and their properties and characteristics. Let's comment on some of these features.
The SAC is perhaps the most used and most popular type among the four types; its chemical structure is a polyvinyl, therefore a polymer, and it has the two main radicals of the sulfonic acid type:
SO3H (acid regeneration cycle) and SO3Na (sodic cycle or regeneration salt cycle)
The acidic cycle is mostly used in demineralization projects, and the sodium or saline cycle is more used for removing calcium and magnesium hardness in softeners.
SBA is the same polymer, but at the end of the chain it has a quaternary ammonia-type grouping and may have an OH- as a kind of exchange at the end of the chain (when regeneration is with soda—demineralization) or else a Cl- (when regeneration is with salt—removal of nitrates, sulfates, and chlorates).
WAC has as a chemical structure the derivative of carboxylic acid or carboxylate as a radical, and its application is the removal of calcium and magnesium hardness in waters that contain a lot of alkalinity.
The WBA has a grouping similar to the previous one, but here the radical, instead of having three methyls, only has two. Application: removal of chloride, sulfate, and nitrate from substances derived from strong acids.

Here is another table listing the types of resin, already showing some important information, such as the ion exchange capacity, which will be discussed in detail later on.
It is noticed that the strongly acidic resin has a capacity of around 1.7 to 2.1 milliequivalents per milliliter of resin, or meq/ml.
We saw before that what was already the definition of resin capacity, in which 1 ml of resin has the capacity to remove from 1.7 to 2.1 meq of the contaminant that is intended to be removed from the water.
Comparing the strongly acidic with the strongly basic, we realize that the strongly basic has a much lower capacity, which is normal. It is much more difficult to remove anions than cations, and this is a characteristic of ion exchange technology.

To better fix the concept of radicals in the four types of resin:
The strongly basic has a quaternary ammonium (Q)-type radical.
The weakly basic has a dimethyl-amino-ethyl, or DEAE-type, radical.
The strongly acidic has a sulfopropyl (SP)-type radical.
The weakly acidic has a radical derived from carboxylic acid, carboxymethyl (CM).
Here is another author who presents a slightly different, but very similar, nomenclature.

Strongly acidic has the sulfonate group. The removed ions are: calcium, magnesium, radium, barium, and lead. The regenerant can be either acid or salt. The pH range is very wide.
The weakly acidic has the carboxylate radical; here, the removed species are practically the same as the SAC but more indicated in water with a high alkalinity content or else as one of the demineralization steps of more sophisticated processes. The recommended pH here is above 7.
The strongly basic resin's radical is quaternary ammonium. The species involved here are nitrate, sulfate, chlorate, arsenite, and selenite.
Regeneration can be done either with soda or with salt. It can also be used in a high-pH range.
The weakly basic have the tertiary ammonia radical (one less methyl). The ions involved are practically the same; however, the regeneration here is with soda or calcium hydroxide.
Application pH: below 6.
So those are the four main types of ion exchange resins.

1.10 Total ion exchange capacity

The total or theoretical ion exchange capacity is determined by the resin manufacturer; as an example here of the SAC Amberlite IR122Na resin (Dow/Dupont), it can be seen in the technical specification that the total exchange capacity appears. The value reported by the manufacturer is 2.1 meq/ml. However, we will comment and detail later that, in practice, this value is lower, but this is a value of the exchange capacity of the resin.

This value can only be obtained in very specific circumstances, which in practice can never be achieved. We would have to have a perfectly distributed bed, hydraulic distributors very well constructed and in large numbers to promote the perfect distribution of water throughout the resin bed, avoiding preferential paths (channeling), contact should be slow and without deviations and across the entire surface of the resin, among other very specific process conditions that can only be obtained in the laboratory. The capacity reported by the manufacturer, therefore, in addition to being theoretical, can be considered the maximum amount of ion exchange that the resin can provide.

Here is the resin model SAC C100 from Purolite, which is equivalent to this one. It has also been informed by the manufacturer that it has a capacity of 2 equivalents per liter. It is what is expected of a strong cationic resin—something around this value, approximately 2.0 meq/ml.
Here, in the evaluation of another author, in this case the famous book "Chemical Engineer's Manual" by Perry, the strong cationic resin appears, and how much the ion exchange capacity varies: From 0.4 to practically 2.3, this is informed by the manufacturer, as Total or Theoretical capacity, so the strongly basic resin also ranges from 0.4 to 2.5 meq/ml, remembering once again that this is the total (theoretical) exchange capacity.
For SBA resins, the ion exchange power is lower, ranging from 0.4 to 1.5 meq/ml.
In later chapters, we will go into greater detail about the issue of ion exchange capacity and the ratio between total or theoretical capacities (informed by the manufacturers) and operational, or real capacities.

The ratio between the total capacity and the operational capacity of an ion exchange resin can be seen in this graph.
The red line represents the water hardness behavior throughout the exchange cycle.
In the first stage, the hardness drops drastically, an effect provided by the resin at the beginning of the cycle and, therefore, with all its exchange sites available.
In the second stage, the hardness remains low because most of the resin exchange sites are still available.
Throughout the third stage, the amount of hardness progressively increases; this is when the exchange sites are becoming exhausted (mass transfer zone). At the end of this stage, the hardness practically returns to the levels prior to the beginning of the cycle; it is time to regenerate.
The area in red represents the amount of hardness removed considering the total capacity of the resin.
In practice, however, what is observed is that this value is much lower, somewhere between 55 and 65% of the total capacity, which is configured as the operational or real capacity and is represented in the graph by the square area in green color.
The ratio between these capacities depends on numerous factors and the type of resin involved; we will see more about ion exchange capacity in later classes.

1.11 Resin datasheet interpretation – Part I

Now let's talk about the resin manufacturers' data sheets to better interpret this document that details the technical specifications of ion exchange resins.
As a practical example, let's start with Purolite's C100E model, which is a strongly acidic resin. Here we have several technical details, and we will detail each one of them:

Polymeric structure: reticulated polystyrene gel with divinylbenzene is the main chain of the polymer from which the resin is manufactured;

Appearance: spheres, which is a standard for all resins;

Functional group: sulfonic acid, as it is a strong cationic acid; if it were another type of resin, another functional group would replace the sulfonic group;

Ionic form: it is a resin that is regenerated with salt, so it is called the sodium cycle, and the functional group is Na+ at the end of the chain;

Total Capacity: 1.9 equivalent per litre (already commented but will be better detailed soon);

Moisture retention: 46 to 50%: the resin already comes with this characteristic of internal humidity, that is, it is a standard, and the range is something around this percentage for all resins;

Particle Size Range: from 300 to 1200 microns; we already saw this at the beginning of the training; this resin is not uniform; it is a more common type, and therefore the variation is great in the size of the spheres, with the small ones around 300 microns to the largest ones above 1,000 microns, which is typical of common resins;

Still, in relation to size, the number of spheres smaller than 300 microns is a maximum of 1%. Values smaller than 300 microns can cause these grains to escape through vessel nozzles;
- Coefficient of uniformity 1.7: for this coefficient, the ideal situation is that the closer to 1, the better, which is a characteristic of high-grade or uniform resins; the best-quality Purolite PPC100 uniform resin has a particle size range of 650 microns with plus or minus 50 micron tolerance; these types of resin usually have a coefficient of uniformity ranging between 1.0 and 1.2;

Reversible swelling: a slightly more technical and less relevant concept; when the resin is exchanging ions, that is, when it is in service, it retains moisture, but after regeneration it returns to normal, returning this level of moisture retention; it can then be said that the resin “swells” when retaining water, and this swelling is around 10%;
Other, less relevant information may appear, depending on the brand and type of product.

Continuing, let's now analyze a resin from the Dupont brand. This is Amberlite IR 120 Na+, strongly acidic type.
The Datasheet presents a lot of information and we can start with;
- Physical properties: “amber” which means “golden color” and form of microspheres;
- Matrix or main exchange radical: styrene divinylbenzene copolymer (standard for this type of resin);
- Functional group: sulfonic acid, as it is a strongly acidic resin, already seen in a previous class;
- Ionic form: uses salt as a regenerant because it is sodium cycle;
- Total exchange capacity: 2.0 equivalent per liter; this value is standard for all manufacturers for this strong cationic resin grade;
- Water retention capacity: 45 to 50%

1.12 Resin datasheet interpretation – Part II

Coefficient of uniformity: 1.9; it is noteworthy that the manufacturer informs 1.9 because at the same time we noticed that the size varies between 600 and 800 microns, which suggests an inconsistency between these data; however, it is known that this resin grade is the most common type, so it can be considered that, in practice, the variation in the size of the microspheres can be greater;
Maximum moisture retention or swelling: < 11%; that is, the difference in moisture retained by the resin in the process in relation to the regeneration stage with salt.
Maximum operating temperature: 135 degrees Celsius or 275 °F;
Minimum bed depth: 700 mm. This is interesting data because it already provides information that can be used in the design of the exchanger vessel. In other words, a very wide and low vase is not recommended, but a narrower and taller vase is preferred. In practice, this is already met when fibreglass-reinforced polyester resin (PRFV) tanks are used, which are becoming more and more common.
The recommended service rate is 5 to 40 BV/h. Later on, the concept of service flow rate will be detailed.
Regeneration: We won't go into details yet about regeneration because there's a whole block of training dedicated to it. But we can quickly show you here that with salt, the recommended level is 80 to 250g; we will see later that the index of 150 grams of salt for each liter of resin contained inside the vessel is used a lot (usual concentration), and the recommended concentration is 10%; all this will be shown and calculated later with details and examples;
When sulfuric acid is used, the recommended level is from 60 to 240 grams per liter of resin, in a concentration of 0.7 to 6%.
When hydrochloric acid is used, the level is from 50 to 150 grams per litre of resin, in a concentration of 1 to 5%.
- The minimum time contact between the regenerant and the resin is 30 minutes, and therefore one more parameter that can be used later in the project;
Other less relevant information, which will also be detailed in the regeneration block.

Finally, let's quickly comment on some hydraulic characteristics.
First, the backwash: usually, in the vast majority of systems, we work with linear velocity rates around 15 m/h because we work with a flow rate in the range of 3 to 10 m3/h and a cross-sectional area of the vessel between 0.3 and 0.6 m2. At this rate, we will have a bed expansion of approximately 50% (for an operating temperature of around 20°C), hence the care when designing the vessel.
Secondly, pressure drop: it is important to note that pressure drop does not depend only on the resin bed, which in this case represents around 0.2 bar/m of bed, considering the same linear velocity.
It also depends on the inlet and outlet piping of the vessel (bends and gauge), the shape and size of nozzles, and encrustations.
In practice and after observing the behaviour of the vessels in operation, it is possible to estimate a head loss of around ½ bar (or 5 mWC) for the entire vessel; this is an approximation that can be used in projects involving both filters and softeners.