Ion Exchange


   The technology of ion exchange is often employed for aesthetic purposes in the form of water softeners to remove minerals, which cause water hardness. However, this technology is also useful for the removal of inorganic chemicals, heavy metals and dissolved solids when used as water polishing technology. Fluoride, radium, and to some degree nitrate are all effectively treated with ion exchange resins. The removal of these contaminants is accomplished through adsorption of the contaminant ions onto a resin exchange medium. In this case, one ion is substituted for another ion on the charged surface of the medium, usually a synthetic plastic resin. This resin surface is designed as either cationic (negatively charged) or anionic (positively charged) (Figure 1). The medium is saturated with the exchangeable ion before treatment operations begin. During exchange, contaminant ions replace regenerant ions because they are preferred by the medium. After the medium reaches equilibrium with the contaminant ions, the medium is regenerated with a suitable solution, such as sodium chloride, potassium chloride, caustic soda or acid solution. Because of required “down time,” the shortest economical regeneration cycles are often once per day. Ion exchange waste is highly concentrated and requires careful disposal.                                                                                                                                                                       Advantages of ion exchange are mainly its relative insensitivity to flow rate and its ability to achieve very low contaminant concentrations in finished water. The disadvantage is effluent peaking, which occurs when contaminant ions compete with other ions for exchange medium sites. It is most common with poorly adsorbed contaminants, such as nitrates. Effluent peaking necessitates more frequent regeneration of the exchange medium and consequently the cost and regeneration chemicals disposal requirements are not any more negligible.

Figure 1. Ion Exchange process.






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