Page 25 - Opportunities in the Treatment of Water and Other Wast Streams

Application:
Why: Exchange problematic ions for less obtrusive ions as a polishing step for treated water.
Where: Ion exchange is used with low total dissolved solids water to remove hardness, or other hazardous contaminants that remain in the treated water in low concentrations.
Target Contaminants:
Fluoride, nitrate, arsenate, selenate, chromate, or anionic uranium complexes.
Benefits:
• Low energy • Small footprint • Modular and mobile systems Limitations:
• High chemical demand for regeneration • Limited TDS concentrations
Ion exchange processes are reversible chemical reactions to remove dissolved ions from solution, replacing them with less troublesome ions of the same charge. For example, each calcium ion is exchanged for two sodium ions in a salt form cation exchange resin, or two hydrogens in the acid form. Ion exchange resin beads have surface charges (i.e., negative for cation exchange resin beads and positive for anion exchange resin beads) and affinities for certain ions over others depending on concentration and pH. Strong base anion exchange resins have the following relative affinity order (DeSilva, 2005):
The resin must be re-charged periodically when the target ions begin to pass through the system. Concentrated (20 percent by weight) sodium chloride solution is used to recharge anion or cation resin in the salt phase. In the acid-base phase, hydrochloric acid can be used to recharge the cation resin and sodium hydroxide to recharge the anion resin. The is greater with acid and base regeneration because of the higher materials cost of equipment for corrosion resistance, more expensive chemical cost, and disposal cost of the spent regeneration solution, however the resin is more
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