Water Conditioning & Purification Magazine

Elements, Ions and Selectivity

By C.F. ‘Chubb’ Michaud, CWS-VI

Let’s get organized

The mid-1800s was a great age of discovery for the scientific community. With newer means of chemical analysis, chemists discovered many new elements and realized there were many more yet be found. Now they needed some sort of organizational charting to organize the elements and their own thought process…but none existed. Russian Chemist, Dmitri Mendeleev (1804-1907) is credited with the forerunner to the modern Periodic Table of the Elements that he created in 1869. Mendeleev arranged the elements according to atomic weight (AW) and noted that there were similarities in properties that often repeated. He arranged the elements in rows (called periods) while those with similar properties were aligned one above the other in families or groups. There were discreprencies.  It was the atomic number (AN), not the atomic weight that determined the similarities.  Once corrected, everything fell in place including blanks containing descriptions of yet to be discovered elements.  Today, we list 92 elements on The Periodic Table up to the element uranium (AN=92). Most elements listed from AN=93 on up to AN=118 are synthesized by bombarding uranium with various other elements. Some of it sticks to form heavier elements. These elements may have existed in the early history of the planet but they are all so radioactive they have long since disappeared. Over time, the elements above uranium convert to the element lead (Pb, AN=82) through radioactive decay.

Mendeleev himself put his handiwork to good use noting that there were gaps in his table of elements. Number 43, Technecium (Tc), was missing. Being radioactive, most isotopes of Tc had long since vanished. It is produced today as a byproduct of uranium fission in nuclear reactors and extracted from spent fuel rods for use in nuclear medicine. Also missing was element #61, Promethium. Its properties were predicted as early as 1902 (by its absence) but its actual discovery did not take place until 1924. Most of the elements above uranium (AN = 92) have such brief half-lives and have been synthesized in such minute quantities that they have no real commercial value (other than nuclear fuel and weapons studies). It is unlikely you will ever be asked to remove any of these from someone’s well water.

How can water treatment professionals utilize The Periodic Table?

The Periodic Table of the Elements tells the water treatment professional much of what he needs to know about water chemistry…providing he knows how to use it. The modern Table (shown in Figure 1) contains all known elements, with a few unknowns that are simply numbered. Many of these have been discovered from the leftover remnants of nuclear reactions (ie bombs) and there appears to be no real limit on what can be fused together in this type of reaction. Some of them have half lives measured in nanoseconds and they rapidly decay into other elements of lesser AN.

Figure 1. The Periodic Table of the Elements

About the elements

The smallest amount of substance that would have all of the properties of that element is called an atom. For our purposes, all atoms are made up from three particles: the electron (negative charge), the proton (positive charge) and the neutron (no charge). Since all elements are neutral, they must contain the same number of electrons and protons. The number of protons contained in the nucleus of the atom determines it atomic number (AN). Thus, the hydrogen atom (H) which has only one proton (and only one electron) and no neutrons is given an AN = 1. Helium (He) has two electrons and two protons (AN = 2). Since the protons have the same charge and would repel one another, they need something to hold them in place. This nuclear glue is called the neutron and the helium atom has two in its nucleus.

Electrons are very tiny and weigh next to nothing. They are only 1/1836th the mass of the proton and 1/1839th the mass of a neutron. Thus, the atomic weight (AW) of an atom (or element) is determined by the number of protons plus neutrons in its nucleus. In the case of hydrogen, the AW = 1. In the case of helium, the AW is 4. The number of neutrons is not always equal to the number of protons. The next element, lithium (AN=3) has an AW of 7. That’s because it has 3 electrons, 3 protons and 4 neutrons. The next is beryllium (AN=4, AW=9), which has 4, 4 and 5, respectively.

A single atom is very tiny and invisible even under the most powerful microscope. The measurements that would describe their particle size are also hard to fathom. To put this into perspective, we start with the standard length of the meter (39.36 inches –a little over three feet) and we divide that into 1,000 parts. Each part is a millimeter (1/1000th of a meter or about 1/25th of an inch). There are 25.4 mm in an inch.

We then divide the millimeter into 1,000 parts and each part is now a micron (or micrometer) equal to 1/1,000,000th of a meter. Anything below 25 microns is invisible to the unaided naked eye. To filter out cysts, you need a filter that takes you down to about 1 micron and bacteria are even smaller. If we divide that micron into 1,000 parts, we have a nanometer—smaller than bacteria and viruses and smaller than a half wavelength of visible light (which is why we can’t see molecules with a light sourced microscope). Now divide that nanometer by 10 and you have a unit called the Angstrom (Å). This is approximately the size of the hydrogen atom. Even the largest element’s single atom is still measured in Angstroms.

If we picture a single atom of hydrogen as a tiny sphere, it would be comprised a single proton and a single orbiting energy field we call the electron. The total volume actually occupied by the proton and the electron would be less than 0.001 percent of the total sphere. The rest is void space. If you removed the void space from the Empire State Building, it would be reduced in volume to the size of a rice grain (while retaining its total mass). Most of the universe and everything in it (including the planets) is void space.

The real origin of the species

Each of the 118 elements listed in The Periodic Table is unique and has identifiable properties. Yet, each is made up of the same three particles: electrons, protons and neutrons. All matter in the universe is made up the same way (if we disregard the theories on anti-matter and subatomic particles still on the drawing board). There are 67 of these elements that have been identified in the analysis of the solar spectrum from the photosphere of our Sun.1 Others have been discovered in the spectrum of other stars, particularly younger ones. Since our Sun is made up of over 71 percent hydrogen (which produces helium through nuclear fusion) and 27 percent helium; this leaves less than two percent of the total mass of the Sun for all of the other elements. Nonetheless, the remaining percentage of the Sun’s mass, although small, is 5,600 times the total mass of the Earth. All of the elements and all of the energy we have on Earth is believed to have originated from the stars and the supernovas that used to be stars.

Did you know that the Sun fuses 620 million metric tons of hydrogen each second producing more energy than we will collectively use on Earth over the next billion years!

Earlier, I mentioned that The Periodic Table arranged the elements in families or groups with similar properties. What might the similarities between hydrogen (a gas) and lithium (a metal solid) be?  The answer is that both have one electron free to react. We call that its valence.  By reacting and giving up that outer electron (which is a negative charge), those atoms would now have more protons than electrons so we assign the valence for metals a plus sign.  Both hydrogen and lithium have a valence = +1. Then if we look down the whole family under hydrogen, all of the elements have a valence = +1.

If sodium (AN=11) has 11 electrons, how come only one is free to react?  The explanation is that since electrons have the same charge, they also tend to repel one another and there is no neutron equivalent to hold them together. So they separate into different levels that we call orbital rings or shells. After two electrons snuggle up quite close to the nucleus of the atom, the next few are more distant. More distant means more room. So there are eight electrons in the next ring and, thus, forcing the remaining electron into a third ring. This lone electron in the outermost orbit reacts in the same fashion with most substances such as acids and salts which describe the similarities observed by Mendeleev. Beryllium (AN=4) has two electrons in its outer orbit and therefore has a valence = +2. So do all of the other members of the family. Element 21, scandium, would have 2, 8, 8 and 3. Therefore, it has a valence of +3 (as do all members of the family). Element 22, titanium, has a valence = +4. Vanadium, Element 23 has a valence = +5 and chromium, Element 24 is +6. Then things get more complicated.

Only the metals tend to easily give up electrons. Non-metals such as carbon, nitrogen and oxygen tend to pick them up. There is a stepwise dark line on The Periodic Table just to the left of boron that zigs and zags all the way down the table. All of the elements to the right of that line are non–metals. All to the left (except hydrogen) are metals.

Element 8, oxygen, has two electrons in its inner orbit and 6 in the outer. Rather than give up these 6, it prefers to pick up two. Since the two it picks up give it a negative charge, we give oxygen a valence of -2. Element 17 (chorine), having electron orbits of 2, 8 and 7, picks up one electron for a valence of -1.

There are two good examples of families with obvious similarities that exist for the non-metals. Family 17, which includes fluorine, chlorine, bromine, iodine and astatine (and known as the halogen family), can all be used as disinfectants and all form acids with hydrogen. They all react with metals to form salts. The other family, known as the noble gasses or inert gasses, is 18 and includes helium, neon, argon, krypton, xenon and radon. They do not do not react. They have fully satisfied electron orbits with no strays and with rare exception, can be given a valence = 0.

Although the simple electron orbits work well in the lower AW part of The Table, things get a little more complex with higher AW elements. The inner electron orbits start to fill up with more than the 8 electrons. Note in Family 18 (the inert gases), there is a difference in the AN of 8 between Ar, Ne and He but a difference of 18 between Xe, Kr and Ar. This jump first occurs between aluminum (13) and gallium (31). This is due to the huge spread of properties and lack of similarities between magnesium (12) and aluminum (#13). The next period down contains elements in these spaces, which run up the numbers.

When elements react, they do so in proportion to their valences. A +1 and a -1 react one for one such as sodium and chlorine to form sodium chloride (NaCl). Calcium (+2) on the same note, would form CaCl2. It takes two chlorines (-1) to satisfy one calcium (+2). Three oxygens (-2) will satisfy two aluminums (+3) to form aluminum oxide (Al2O3).

Some things never change  

We always know that hydrogen is a +1 and oxygen is a -2. Those are givens, however; some elements can go either way. Carbon (AN=6) has 4 electrons to deal away and can have a +4 valence as in carbon dioxide CO2 or it can pick up 4 electrons and become a -4 as in methane (CH4). Many elements can do this, and the multiple valences listed on The Periodic Table are the valences listed in order of their most common occurrence in nature.

Elements and salts

When two or more elements combine, they form compounds. Water (di-hydrogen monoxide) or H2O is a compound. So is any salt or any metal oxide such as NaCl or Fe2O3. Elements react in proportion to their atomic weight (AW).  Example: How much sodium does it take to fully react with chlorine?  Both are monovalent (+1 and -1). Sodium has an AW = 23 and chlorine = 35.5 (chlorine exists with an extra neutron about 50% of the time so its AW is averaged between Cl35 and Cl36). So it takes 23 grams of sodium (or ounces, or tons) to fully react with 35.5 grams (or ounces or tons) of chlorine and together they will form 58.5 grams (or ounces or tons) of NaCl. This relationship forms the basis of the science known as stoichiometry. NaCl is therefore 23/58.5 or 39.3 percent Na and 60.7 percent Cl. On the other hand, KCl (potassium chloride) is 39/74.5 or 52.3 percent K and 47.7 percent Cl. This is something to think about if you are trying to reduce chloride discharge.

Salts and ions

When we dissolve a salt such as NaCl in water, the elements making up the salt dissociate. This is illustrated in Figure 2. They do not reform into their original elemental compositions, however. The electrons with the give-and-take to satisfy the valences remain with the new partner. Consider this as molecular alimony. The electron donated by sodium is retained by the chloride, forming charged particles called ions. Since the Na0 (the element is represented by the 0 valence indicating it is neutral or elemental) gave up a negative, it becomes positive, Na+. We call this a cation. Since the Cl picked up a negative, it becomes negative Cl- and is called an anion. We represent the ionic forms by including the valence numbers with the chemical symbol: Na+1 and  Cl-1. Another acceptable convention for describing the ion is to use the plus and minus signs for the strength of the charge. Examples are Ca++ for Ca+2, Al+++ for Al+3. Sometimes the Roman numeral is used (i.e., CrIII for Cr+3 or Cr+++ and CrVI for Cr-6 but rarely Cr++++++).

Figure 2. Atoms, salts and ions

What’s in your water?

The elements that end up in our water supply are pretty much the same ones that make up most of the Earth’s crust. Since the planet was covered with water at some point early on and those seas contained salts, those salts are present as well. Combined with CO2 and N and O from the air (to give bicarbonates and nitrates), the typical water analysis will contain: calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, nitrate and silica. Almost anything else will be present in ppb levels.

By definition, a salt is the product of neutralization of an acid and a base. Common salt, sodium chloride (NaCl) is only one of the many compounds we call salt. Potassium chloride (KCl) is also a salt. Calcium sulfate (CaSO4) is as well.  A water softener that regenerates with KCl is, therefore, not saltless. It merely uses a salt other than common salt.

Putting chemistry to use

While there is no substitute for experience, there are some short cuts to be had by making use of The Periodic Table. The next time someone hands you a water analysis, take a good look at it. Can you separate the cations from the anions?  For the most part, the cations are the ones ending in -ium (i.e., lithium, sodium, potassium, barium, magnesium). They are the metals. They give up electrons and take on a positive charge. The -ides, -ites and -ates are the anions (i.e., chloride, sulfate, sulfite, carbonate, sulfide, nitrate). They take on electrons or form complexes with oxygen to take on a negative charge. Can an -ium become an anion? Yes. When chromium combines with oxygen, it becomes chromate or hex chrome (Cr+6) and is an anion (CrO4)-2. Plain old chrome or chromium, known as trivalent chrome (Cr+3) is a cation. Selenium forms selenate (SeO4)-2, molybdenum forms molybdate (MoO4)-2.  Good to know that softeners (cation exchangers) do not remove hex chrome. If someone claims to have a chrome problem, ask to see the analysis. The two ionic species are removed by different systems.

Ionic strength and selectivity

The ionic strength of an ion exchange resin is based on its charge density, which is controlled by the moisture content, which is controlled by its crosslink level. The more highly cross-linked resin will have lower moisture and therefore, the plastic portion of the bead structure will be proportionately greater (less water = more plastic) and the number of charges on the resin will be closer together. This resin is said to have a higher charge density and, therefore, will exhibit higher total capacity and attract ions from solution more strongly. Is there a drawback?  Yes. Resins that exhibit higher attraction for ions do a better job at cleaning up water but are in themselves more difficult to clean up (regenerate).

The diffusion of water and ions through the denser matrix of the higher cross-linked resin is slower, especially in cold water. This can actually decrease the operating capacity of the system and make it necessary to use higher amounts of regenerant to reduce leakage. The cross-linker (divinyl-benzene or DVB) is also much more expensive than the backbone (usually styrene), so the premium resins also come with a premium in price.

Size matters

How does the position of an element on The Periodic Table impact the ease of removal by ion exchange?  In general, within the family, the farther down the column, the bigger the molecule and the stronger its ionic charge (relative to the member directly above it). Potassium (K) with an AN of 19 is more strongly attracted to a cation exchanger than is sodium (Na) with an AN of 11, which is still stronger than lithium (Li) with an AN of 3. Iodine (AN = 53) is more strongly attracted to an anion exchanger than is bromine (Br AN = 35) which is stronger than chlorine (Cl  AN=17) which is stronger than fluorine (F  AN = 9). Divalent ions such as Ca+2 and Mg+2 are generally more selective than monovalent ions such as Na+1 or K+1. Trivalent ions such as Al+3 or Cr+3 are preferred over divalent ions. In general, the farther we move down the family (higher AN) and the farther we move to the right (higher valence), the higher the selectivity. Just make sure you have properly identified the cations and anions.

Resin selection

The higher the crosslinking level of the resin, the more it differentiates between the ion selectivity value.2 In other words, a 10 percent cation exchanger will have a higher selectivity for sodium than does an 8 percent or a 6 percent resin.3 However, it will have an even higher selectivity for calcium than does the 8 percent or 6 percent (relative to sodium). The proper resin choice is a fine balance of the degree of removal needed and economics that is further impacted by the water analysis. The approach to recovering gold from solution will be different from removing fluoride. Likewise, removing lead will be a different approach than removing calcium. In some instances, it is common practice to choose the resin with the absolute highest capacity and/or the most complete removal and then change out the resin (because the contaminant is so well stuck it is impossible to remove economically).

Conclusion

The Periodic Table of the Elements can be a useful tool in determining the approach to an ion exchange removal method. Metal ions tend to form cations while non-metals lean towards forming anions. The table is also a good predictor of the ease of which different ions can be removed from solution. A typical cation selectivity sequence of Pb>Ca>Mg>Na>H tells us that Na+ will displace H+ on the resin (as in a deionizer) and Mg will displace Na (as in a softener). It also tells us that Ca++ will bump Mg++ at breakthrough and Pb (lead) will be readily removed by a softener—even after it has exhausted on hardness. A useful chart on ion selectivity can be found in past articles published by WC&P (wcponline.com).3

References

  1. http://hyperphysics.phy-astr.gsu.edu/hbase/tables/suncomp.html
  2. Michaud, C.F., The Role of Crosslinking in Ion Exchange Resins, WC&P, June, 2011
  3. Michaud, C.F., Oxidation of Ion Exchange Resins, WC&P, August, 2000

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 aor 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 cmichaud@systematixUSA.com.

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