"What's this doing?"
Kana stared at the column. Inside the transparent tube, resin was packed.
Toma answered. "Ion exchange. When you pass water through, ions get replaced."
"Replaced?"
Rei explained. "Charged groups fixed on the resin surface capture ions in solution."
Kana pointed at the resin. "This captures them?"
"More precisely, exchanges them. Ions on the resin swap with ions in solution."
Toma demonstrated. "For example, this cation exchange resin. Has Na ions attached."
"Sodium?"
"If water contains Ca ions, Na and Ca are exchanged."
Rei supplemented. "It's an equilibrium reaction. Ions with higher affinity preferentially bind to the resin."
Kana wrote in her notebook. "What determines affinity?"
"Charge, size, degree of hydration. Multivalent ions tend to bind strongly."
Toma started flowing solution. "Look, the water coming out changes."
"How does it change?"
"Hardness decreases. Ca and Mg are removed, replaced with Na."
Kana asked curiously. "But when the resin gets full?"
"Regenerate it," Rei answered. "Flow high-concentration salt solution to swap the ions back."
"It's reversible?"
"Yes. So it can be reused."
Milia entered the room. "The same thing happens in living organisms."
"In cells?" Kana was surprised.
"Ion channels, ion pumps. Cell membranes are the site of ion exchange."
Rei continued. "Sodium pump transports Na out of cells and K inside."
"Why?"
"To maintain concentration gradients. This is the foundation of nerve transmission."
Milia drew a diagram. "Different from resin—much higher selectivity."
"Selectivity?"
"Only pass specific ions. Strictly distinguish by size and charge."
Toma asked. "Stricter than resin?"
"Far more. Na and K are chemically similar, but channels distinguish them precisely."
Kana was impressed. "How?"
Rei explained. "Channel pore size, arrangement of coordinating atoms. High precision in molecular recognition."
Milia added. "The process of shedding hydration shells is also important. Naked ion size is the key."
"Hydration shells?"
"Water molecules stick around ions. They're removed before passing through channels."
Toma looked at the resin. "So this considers hydration too?"
"Of course," Rei answered. "Competition between hydration energy and ion exchange energy."
Kana organized. "So ion exchange is an equilibrium problem?"
"Exactly. Free energy change is the driving force."
Milia proposed an experiment. "Try changing pH."
Toma added acid. "Oh, the amount exchanged changed."
"Proton concentration affects it. Competing ion effect," Rei explained.
Kana asked. "How is it controlled in living organisms?"
"Active transport using ATP. Consuming energy to work against concentration gradients."
"So resin is passive?"
"Yes. Just follows concentration gradients. Living systems are more active."
Milia said quietly. "But the principle is the same. Utilizing chemical properties of ions."
Kana stared at the column. "If the resin spoke, what would it say?"
Toma laughed. "'Lots of Ca ions today' maybe?"
"'See you later, Na ions,'" Rei continued.
Milia smiled. "'Maintaining equilibrium is my job.'"
Kana wrote in her notebook. "Ion exchange—the stage of chemical equilibrium."
The four gazed at the resin's work. Invisible exchanges support water, and life.