Binding of calcium and magnesium by modified onion skins

The use of modified onion skins for binding of Ca²⁺ and Mg²⁺ from solutions has been investigated. The effect of time of equilibration, temperature, and pH on the sorption of the metal ions have been studied. Batch and column experiments have been performed and the adsorption isotherms have been plotted. The capacities with respect to Ca²⁺ and Mg²⁺ were found to be 4 and 16 meq, respectively, per gram of the substrate when separate column experiments were conducted using 1 L of solution containing 1000 meq of the respective metal ions at pH6. With a solution containing 10 meq each of Ca²⁺ and Mg²⁺ together, however, the substrate seems to exhibit greater preference for Ca²⁺ than Mg²⁺. The sorbed metal ions from the substrate can be leached into solution with a decinormal solution of HCl and the washed bed can be reused. In view of the complex organic nature of the onion skin and its considerable capacity to bind Ca²⁺ and Mg²⁺, the possibility of its use for preventing scale formation in boilers is indicated.