Look around: thousands of heavenly bodies in the night sky (comprising a mere fraction of 1% of the known universe), hundreds of cities, millions of houses filled with tens of millions of people. It is somewhat amazing to realize that all of it – every single thing is made up of only three components: electrons, protons and neutrons. Each grouping of these components forms a unique structure we call an “element”. There are barely 100 elements here on Earth or in stellar space and their collective study is called “chemistry”. Each compound (water, air, steel, rubber) has its own chemistry. We can predict the properties of most things by studying its unique make up of its components.
The chemistry of water is basic but, nonetheless, it is still chemistry. Some people shy away from trying to understand this subject because they feel it’s over their heads. Understanding the fundamentals of chemistry, however, is necessary in order to grasp the full breadth of how certain aspects of water filtration work—particularly ion exchange.
Part 1 of this article will point out the basic differences between the elements by way of an Introduction to the Periodic Table. Further, it will explain the differences between elements, salts and ions and show how the process of Ion Exchange can be used to remove those ions, considered contaminants, from water. Part II will provide the detail on converting a water analysis to usable information needed to set up ion exchange as a method of treatment.
The building blocks
In the worlds around us, there are barely 100 elements that occur naturally and, by definition, they are all separate and distinct from one another. Sodium, calcium, sulfur and oxygen are all elements. Elements are made up of a balanced number of positive and negatively charged particles called protons (+) and electrons (-), which, along with neutrons (which are neutral), form an atom of that element. The atom was first theorized by Democritus in the 5th century BC and derives from the Greek word for “uncutable”. It is the smallest particle still identifiable as having the properties of the element. “Modern” science finally accepted this theory but not until the development of nuclear weapons in the 1940s.
Because elements are balanced with the same number of electrons and protons, they are neutral in charge. All elements can—and do—have different numbers of protons with a matching number of electrons. Hydrogen (H) has only one of each whereas Helium (He) has two. Lithium (Li) has three and so on all the way up to Uranium (U), which has 92. Plutonium (Pu), a manmade element that doesn’t exist in nature, has 94 electrons and protons. Scientists have now synthesized elements all the way to Number 118 although not all are named as yet and no real uses are anticipated. Many man made elements exist only for fractions of a second and only under laboratory conditions. All numbers from 1 to 118 are accounted for and each differs by only one proton/electron and each is a totally separate substance with its own unique properties.
The Periodic Table of the Elements
We use the term Atomic Number (AN) to identify each of the elements and this number corresponds to the number of protons of the element. These various elements are conveniently arranged on a chart we refer to as the Periodic Table of Elements (see Figure 1).
Atomic weight (AW) represents the mass of an element and is the total of its protons and neutrons. The two components are assigned an equal weight of 1 while electrons have essentially zero. It is possible to have elements of differing atomic weight, but with the same atomic number because the number of neutrons can vary. We refer to these variations as isotopes. For example, chlorine, which is element 17 (AN), can have 18 or 19 neutrons but always has 17 protons and electrons (neutral). Therefore, it has an atomic weight of 35 or 36 (total of protons plus neutrons). Since these two common isotopes exist in nearly the same percentage, we assign chlorine an atomic weight of 35.5.
When two or more elements react to form a new material (such as hydrogen and oxygen react to form water: 2H + O H2O or carbon and oxygen react to form carbon dioxide: C+ O2 CO2 ), this new material is called a “compound”. Compounds are also unique in that all of the elements making up the compound fit together in a balanced and neutral fashion but they can exist in a variety of forms. Water, for instance, can be a solid (ice), liquid (water) or vapor (steam) but they are all still H2O.
If we put two or more compounds together with limited variation, we call the new substance a “mixture”. Sugar (a compound of hydrogen, carbon and oxygen) and water (a compound of hydrogen and oxygen) form a mixture. Air is a mixture of nitrogen and oxygen. Mixtures can be physically separated (i.e. separating nitrogen from air or sugar from water by crystallization). Compounds have to be torn apart with energy (making hydrogen fuel from water may actually require more energy that the use of the hydrogen will produce as fuel. Compounds can also be combined and reacted to form new compounds (Mother Nature combines water and carbon dioxide to form cellulose and sugar: C12H22O11 and gives off oxygen in the process). If we simply add CO2 to H2O without the magic of photosynthesis, we form carbonic acid (CO2 + H2O H2CO3). Most rainfall is a dilute solution of carbonic acid.
The jagged line drawn through the chart in Figure 1 separates the metals from the non-metals (on the right). This helps you to determine how that substance will react with oxygen and subsequently, how that compound will react with water. You might have noticed that boron (B), carbon (C), nitrogen (N), fluorine (F), silica (Si), phosphorous (P), sulfur (S), chlorine (Cl), arsenic (As), etc., on the non-metal side all seem to end up on the same side of the salt molecule. In other words, they are the acid formers (such as carbonate, fluoride, silicate, phosphate, nitrate, etc) whereas metals such as sodium, potassium, calcium, etc., are the base formers. The base formers become cations and the acid formers become anions.
Formation of acids and bases
When subjected to heat in the presence of oxygen, most metals will form a metal oxide. The most common observation of this is rust, which is iron oxide. Lime is calcium oxide (CaO) and caustic (Na2O) is sodium oxide. If we subscribe to the theory of a fiery creation, we can readily see where the heat came from. When a metal oxide is dissolved into water, a basic, or alkaline solution is created, as can be seen in Reaction 1 in Figure 2. Non-metals, such as sulfur (S) and nitrogen (N) also form oxides, but when dissolved into water, they form acids. (See Reaction 2 in Figure 2.)
When elements combine to form compounds, nature preserves the laws of neutrality. Ammonia (NH3) is a gaseous compound made up of one atom of nitrogen and three atoms of hydrogen. Sodium chloride (NaCl) is a compound that is a salt. Although it is referred to as “common” salt, it is not the only salt. Magnesium sulfate is a salt; potassium citrate is a salt. The names of salts usually have “-ide,” “-ite” and “-ate” endings. When acids and bases combine, they neutralize one another to form a salt and water (Reaction 3 in Figure 2). This is essentially what happens in an ion exchange demineralizer. What determines how many of this will react with how many of that to form so many of those also is fixed by the nature of the element (it’s valence). The Periodic Table tells us the most common valences of each element.
The Importance of Orbits
The electrons contained in each of the elements are arranged in electron orbits around the shell of the atom’s nucleus (center). There is more than one orbit—in fact, there are many. However, each orbit is filled with only a certain number of electrons and that number is more or less the same for all of the elements. Since the number of electrons differs by only one from one element to the next on the periodic chart, only the outermost orbit will contain a different number of electrons. The exceptions are the Lanthanam Series, elements #57 thru #71 and the Ac Series, elements #89 thru #103. Each element in these two series has the same number of electrons in the outermost orbits but differing numbers in an inner orbit. This tiny difference of one pair of electons and protons determines many of the properties of that element and the family to which it belongs. For instance, hydrogen, lithium, sodium and potassium all have only one electron in their outermost orbit. Magnesium, calcium and strontium each have two. Fluorine, chlorine, bromine and iodine—the halogen family—each have seven. On the far right of the Periodic Table, helium, neon, argon, krypton, xenon and radon form the inert gasses (non-reactive). Are we starting to get the picture of just how valuable the periodic table might be?
When electrons react to form compounds, they tend to go to a less reactive state. In other words, they try to imitate the “relaxed” state of the inert gases by filling their outer orbits to completion. The innermost orbit needs only two electrons (or zero). The outermost generally wants eight. We can see from the periodic table that hydrogen, AN=1, has only one electron in its outer orbit. Oxygen with an AN=8 has two in its inner and six in the outer. To be “satisfied,” hydrogen will give up its electron and oxygen will pick it up. However, to satisfy the full demand of the oxygen, it will require two hydrogens to make the supreme sacrifice—thus, forming the basis of water. This is shown in Reaction 4 in Figure 2.
The inner most orbit of an atom can be filled with just 2 electrons. Thus, Helium has a filled outer orbit, is satisfied, and is non-reactive (inert gas). An important lesson learned from history is that although Hydrogen is lighter and more buoyant that Helium, it is also very reactive. The tragedy of the Hindenburg in 1937 lead to the use of non-reactive helium in balloons. To appreciate the reactivity of hydrogen, bear in mind that the rocket fuel used to propel the NASA Space Shuttle into orbit is merely the controlled combustion of hydrogen with oxygen. The “smoke” trail seen as it lifts off is simply steam.
The second orbit is satisfied with 8 electrons. Neon (AN 10) has 10 electrons with two in the inner orbit and 8 in the outer. It is the second inert or noble gas and is also non-reactive. The third orbit, also containing 8 electrons, describes Argon (AN 18), the third of the inert gases. With the next inert gas, Krypton (AN 36), the third orbit actually fills to 18 electrons while the fourth fills with 8. Again, with the next, Xenon (AN 54) fills both its third and fourth with 18 electrons while having only 8 in its outer most or fifth orbit. Radon, the heaviest of the inert gases (AN 86) grows by an atomic number of 32 meaning it has added 32 additional electrons on top of Xenon. None the less, it has only eight in its outermost orbit with the balance added to the interior orbits. (see Figure 3).
Figure 3. Electron orbits
Balancing Electron Orbits
Other than the inert gases, all elements will have from one to seven electrons in their outer orbits. They can either give them up or pick up additional ones to satisfy a full orbit. Sodium, which has one, will give that up to chlorine, which has seven. Thus both the chlorine and the sodium are satisfied with “full” outer orbits and the resulting compound, NaCl, is neutral. Potassium has one and oxygen has six. Therefore, oxygen needs two and the resulting compound of potassium oxide is balanced as K2O.
When we write a chemical formula, we represent the balanced compound. It takes two H to react with one O and two C to react with one O. We write them as H2O and CO2. Although it is a little more complicated, the same holds true for more complex compounds such as H2SO4, sulfuric acid which chemically shares electrons with its neighbors as follows:
Figure 4. Sharing electrons to satisfy orbits
Salts and Water
When salts (which are neutral) dissolved in water, the two components of the salt separate. However, they don’t regain their original electron counts and therefore are no longer neutral. Since they now have either gained or lost electrons (which have a negative charge), they’ll have either a net positive (loss of electrons) or net negative (gain of electrons) charge. We call these charged particles ions. The positive ion is called a cation and a negative ion is called an anion. The number of electrons gained or lost by the element determines the strength of the charge. We call this charge its valence and we denote this by writing the symbol for the element or compound with a corresponding number to signify its ionic charge. Thus, sodium is Na and its ion is Na+. Chlorine is Cl and its ion is Cl–. Table 1 lists some of the more common elements found in tap water, the compound form most likely and its valence.
By way of a quick summary, all matter is made up of elements (which are made up of electrons (-), protons (+) and neutrons (=). When elements combine, they form compounds. When compounds combine, they can form new compounds or mixtures. Acids and bases neutralize each other to form salts (and water). When salts are dissolved in water, they separate into cations (+) and anions (-) which carry charges (and are, therefore, attracted to other charged substances such as ion exchange resins. Water, H2O, does not ionize as H+ and O—. Instead, it becomes H+ and OH– (called hydrogen and hydroxyl). These two ions are the backbone of the ion exchange demineralizer reaction.
Indeed, if we add enough Na2CO3 (soda ash) to CaCl2, we will precipitate CaCO3, (calcium carbonate or calcite) leaving a solution of salt (NaCl). This process has been used for effectively softening water (removing excess hardness) through soda ash precipitation and filtration. In effect, we have done an ion exchange reaction by exchanging the calcium and sodium with their former partners on the basis of selectivity.
Introduction to ion exchange
In the above case, the “unused” part of the exchange reaction remains in the water and raises the total dissolved solids (TDS — that would be the Na and the Cl ions). But, what if we could anchor the reactive ions to a solid matrix so we didn’t have to filter them out and their partners would not go into solution to add to the TDS? That is exactly what ion exchange resin does.
Ion exchange resins are plastic beads with a built in reactive partner and an exchangeable “soluble” partner. While the exchangeable partner is free to jump on and off the bead, the fixed reactive partner remains attached. In the case of a softening exchange resin, the partners are sodium (Na+ free to jump) and sulfonate (HSO3– which is fixed). When a calcium salt is introduced (as hard water), the calcium replaces the sodium on the bead and sodium replaces the calcium in the water (on a one for one equivalent basis) and there is no increase in TDS and no further filtration needed.
The reason this reaction takes place is because the calcium from the hardness has a higher attraction (divalent) to the exchange resin than does the sodium (monovalent). This is known as ion selectivity and is the backbone of the ion exchange process. As shown by Figure 6, certain elements or compounds in water can be made to undergo specific selective reactions and these reactions are predictable to some degree according to the element’s family association in the periodic table. Divalent ions (those with a double positive charge) such as calcium and magnesium, will react with soap and cause “bathtub ring.” They also will react with the carbonate ion to form scale in pipes and heaters. Although we could precipitate these salts with the addition of carbonate ions (see Reaction 5 ), we have no easy way to remove the resulting solid except in an industrial setting with large tankage.
With ion exchange resins, only the exchangeable ion is soluble or free to move. The counter ion, which is the resin bead itself, is not. This makes the separation after the exchange very easy. In the case of a softener, the resin has an exchangeable Na+. The hardness (Ca++ and Mg++) combined with the resin forms a very strong bond. The water, minus the hardness, passes on through because the resin is retained in the exchange column. Sodium replaces the hardness on an equivalent basis. This means that it will take two sodium ions from the exchange bead to replace a single calcium or magnesium ion.
In the above reaction, the resin continues to remove hardness until most of the sodium sites are used up (exhausted). At that point , hardness begins to show up in the effluent. The service run is stopped and the resin is regenerated by passing a strong solution of sodium or potassium chloride over the exhausted resin which drives the hardness off and restores the capacity.
Demineralization by ion exchange
In the case of demineralization, both the cations and the anions must be exchanged. This is done by using two different resins regenerated with acid (H+) and caustic (OH–), respectively. The water passes through the cation exchanger first where the positive ions (cations) are exchanged for hydrogen ions (H+). The acid solution is then passed through an anion exchanger where the acid is neutralized by the exchange of the acid ion (Cl-) for the hydroxyl (OH-) ion. In the end, the cations are replaced by H+ and the anions by OH- and we have H + OH = H2O. ( DI water).
Periodic table of the elements places all elements into families that help us predict properties and determine similarities. We have shown that there is a preferred coupling of certain elements to form reactions (such as CaCO3 precipitation) that lead us to methods of removing those elements from water. This can be done either selectively (such as in softening) or completely (as in demineralization) utilizing ion exchange resins.
About the author
C.F. ‘Chubb’ Michaud is the CEO and Technical Director of Systematix Company, Buena Park, CA, which he founded in 1982. An active member of the Water Quality Association, Michaud has been a member of its Board and of the Board of Governors and past Chair of the Commercial/Industrial Section. He is a Certified Water Specialist Level VI. He serves on the Board of Directors of the Pacific WQA (since 2001) and Chairs its Technical Committee. A founding member of WC&P’s Technical Review Committee, Michaud has authored or presented over 100 technical publications and papers. He can be reached at Systematix, Inc., 6902 Aragon Circle, Buena Park CA 90620; telephone (714) 522-5453 or via email at [email protected].