Softening (Conditioning)

Softening (Conditioning): “Sodium cycle cation exchange” is the most common use of ion exchange, in which “hardness” (mostly ions of calcium and magnesium) is exchanged for sodium ions to produce “soft” or “conditioned” water. It is made possible by the availability of an ion exchange resin with millions of “active sites” made of the chemical group called “sulfonate,” which can be represented as R-SO₃^- [M⁺]. The R represents the connection to the polymer backbone, and the M⁺ is the balancing cation (called a counter -ion, needed to make an electrically neutral product that doesn’t give a shock when it is touched) at each of the millions of “active sites” in each of the millions of little resin beads. Since the sulfonate is an anion, it can exchange only cations, and it happens that this particular chemical arrangement produces a strong affinity for Ca⁺² and Mg⁺², and a weak affinity for Na⁺. That means that, given a choice, the resin would “prefer” to capture Ca⁺² and Mg⁺² rather than Na⁺, (There is no teal preference or choice. It’s just a matter of stability and what lasts. Just as dissolved molecules are involved in a million-billion-trillion or 10²⁷ collisions per second, attached contaminant molecules or ions are grabbed onto and let go (sorbed and desorbed) millions or billions of times each second, If there is some affinity between the two, then the contaminant will stay attached for a greater percentage of the time, and we can say it has been adsorbed or exchanged, and the fraction of time spent attached describes the strength of the adsorption or exchange.) However, if there is no choice, or if the Na⁺ concentration is much higher than anything else, the resin will “load up” with Na⁺ ion on all of the millions of active sites. That is exactly what is done. The resin is initially treated with about 5% (50,000 ppm) Na⁺ + CI^- salt brine, and the 50,000 ppm Na+ completely overwhelms the effect of any other cations that might be in the water, so the resin loads up with Na• ion. In “service,” this fully regenerated resin is then exposed to water containing Ca⁺² and Mg⁺² ions. The resin exercises its preference and exchanges two Na⁺ ions for each Ca⁺² or Mg⁺² it encounters, as long as there are any Na⁺ ions left attached to the resin. When they’re all gone, the resin is said to be exhausted and ready for a new regeneration with a fresh dosage of 50,000 ppm of salt brine. The resin still “prefers” to have Ca⁺⁺ and Mg⁺⁺ attached rather than Na+, so the strong brine solution needs to flow through the resin bed for 15-30 minutes, continuously. This assures that all of the calcium and magnesium ions on the resin have the opportunity to be displaced by the overwhelming presence of sodium ion. Such timing and the necessary initial back-washing and final rinse of the bed are done automatically by the water conditioner equipment package, so there is no need to go into detail here.

It is sometimes said that softening does not change the overall TDS (Total dissolved Solids) of the water. That is not exactly true. The change is small, but there will be a slight increase due to the atomic weights of the elements that are involved, depending on the water supply. The A.W. of calcium is 40; that of magnesium is 24; and sodium, 23. Thus, when one calcium is exchanged for two sodium ions, their contribution to the TOS is 46, which is 15% more than the 40 they replaced. If it is magnesium that is exchanged for two sodium ions, the change would be 48 versus 40, or 20% more. Most water supplies have considerably more calcium than magnesium, with the average composition being probably about 2/3 calcium and 1/3 magnesium. That would produce an “average atomic weight” of about 35,’ compared with 46 for the two sodiums. Thus, the TDS of a hard water might increase as much as 30%, from, say, 350 mg/L to 450 mg/L after being softened.

Softener resin’s special affinity for Ca⁺⁺ and Mg⁺⁺ is actually for “divalent cations,” which means that any other member of their chemical family (the rest of column 2 of the Periodic Table of the Elements: strontium, barium, and radium), plus a great many other metal ions with two charges, will also be efficiently removed by water conditioning. Removal of radium and its radioactivity is a very important ability, as is removal of moderate concentrations of manganese and iron. Softeners also remove lead, but that contaminant comes mostly from plumbing materials that come after the softener.