ORNAMENTALS April-May 1978 J.L. Green NORTHWEST Vol. 2, Issue 2 Extension Ornamentals Specialist ARCHIVES Page 15 Oregon State University

TREATMENTS TO CHANGE THE CHEMICAL QUALITY OF WATER

The two most common treatments to change the salt content of water are "-exchange" where one ion is exchanged for another ion(s) of equal charge [i.e the exchange of two sodium cations (2NA+) for one calcium cation (1 Ca++)], and "deionization" processes whereby are removed from the water and the resultant water has a reduced soluble salt content.

1. ION-EXCHANGE: The largest single use of resins in water treatment is the "softening" of water which utilizes cation-exchange resins in the sodium form, or in the sodium cycle (2). This means that the sodium on the resin initially is exchanged for the hardness ions in the water (calcium and magnesium cations); the calcium and magnesium cations are taken up by the resin, and sodium is released from the resin into the water. The potential injury to the plant and soil structure due to the added sodium in the water after passage through a water softener should be considered. Most water softeners actually increase the total amount of soluble salt in the water! "In many cases, water from a softener does have a harmful effect on plant growth. Hard water is hard because it contains huge amounts of calcium and magnesium. Softeners swap the calcium (which is harmless to plants) for sodium which is harmful" (1).

A second form of ion-exchange is "dealkalization". In dealkalization, carbonate and bicarbonate anions are removed from water by strong- and anion-exchange resins operated in the chloride cycle. In this exchange treatment, carbonates and bicarbonates are removed from the water, and chloride is released from the resin into the irrigation water (2). This will lower the alkalinity of the water thereby producing a more acidic pH, but there are reports of chloride phytotoxicity.

2. DEIONIZATION: The total salt content (total ion concentration) of water may be reduced by application of dual, multiple, or mixed beds of "anion and cation-exchange resins" to demineralize the water or by application of the process called "Reverse osmosis".

Resins: Complete demineralization (removal of all mineral ions) is achieved when a strong- acid cation-exchange resin in the hydrogen cycle is used in conjunction with a strong-base anion-exchange resin in the hydroxide cycle. Application of "Deionization" of water by resins is detailed by Mary Hughes (3).

Reverse-Osmosis: The reverse-osmosis system for treating water is based on the principal of osmosis (the spontaneous passage of a liquid from a diluted to a more concentrated solution across a semi-permeable membrane). The process continues until an equilibrium is achieved: When equilibrium is achieved, the quantity of liquid flowing in either direction is equal and no net transfer occurs. The membrane allows the passage of the solvent (i.e. water) and not the solute (the dissolved salt) or suspended matter. In Reverse-Osmosis, the osmotic process is changed by artificially pressurizing the concentrate solution: The pressure forces the liquid to flow in the direction opposite that normally observed in osmosis. Diffusion of the water through the membrane can remove approximately 90% of the solutes contained in the water and allows only a minimal passage of dissolved solids, bacteria, etc. The reverse-osmosis system produces water containing less suspended matter and bacteria than would a deionization unit: The membrane acts as a nonporous barrier and rejects suspended matter including bacteria (3).

Literature Cited:

1. Abraham, Doc and Katy. 1977. Tough questions from customers. Nursery Business, January 1977, page 9.

2. Dow Chemical Company. 1964. DOWEX: . 80 pages.

3.Hughes, Mary. 1977. Water tailored to fit at Culligan. Florists' Review, September 29, 1977. Pages 74, 163-169.

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