4.1 Ion Exchange Reactions

Ion exchange reactions occur when ions in water exchange with those electrostatically bound to the solid phase. They occur commonly in the presence of iron oxides, organic matters, and clay minerals with large surface area. Ion exchange reactions are important in determining natural composition in surface waters (e.g., rivers, lakes) and ground water. They can alter water composition and trigger other reactions including mineral dissolution and precipitation. For example, in the coastal freshwater aquifer, Na⁺ in seawater displaces presorbed Ca²⁺ from solid phase (Figure 1). In this reaction, Ca²⁺ is replaced by Na⁺ and the water changes from Na-rich to Ca-rich (Appelo and Willemsen, 1987; G., 2008; Slomp and Van Cappellen, 2004) while the solid surface change from Ca-rich to Na-rich. In northeastern United States, the use of road salt as deicer increases the salinity of fresh water and mobilize metals through ion exchange reactions (Kaushal et al., 2005).

Applications of ion exchange reactions in industry include drinking water softening (Figure 2), desalination, ultra-pure water production (Rodrigues, 1986), and chromatography. In petroleum refining processes, ion exchange reactions are often used to purify, separate, and dry natural gases (Marinsky and Marcus, 1995). Natural gas extraction in the Marcellus shale has led to the production of large quantity of wastewater with high metal concentrations. Ion exchange is commonly used to remove metals such as Ba²⁺ and Sr²⁺ (Gregory et al., 2011).

Figure 1. Intrusion of seawater into the coastal aquifer. Divalent Ca is partially displaced by monovalent Na from seawater through ion exchange reactions. The water changes from Na-rich to Ca-rich, while the solid surface changes from Ca-rich to Na-rich.

Credit: Figure modified from St. Johns River Water Management District (2008)

Figure 2. Process of water softening. When hard water with high concentration of Ca and Mg flows through the softener resin tank, Ca and Mg exchange with Na, resulting in Ca and Mg concentration decrease in water (soft water).