Effective Removal of Calcium and Magnesium Ions from Water by a Novel Alginate–Citrate Composite Aerogel

In this work, a novel alginate/citrate composite aerogel (CA-SC) was synthesized by chemical grafting technology combined with vacuum freeze-drying method, and CA-SC was used for removing calcium (Ca²⁺) and magnesium (Mg²⁺) ions from water. The experimental results indicate that the as-prepared CA-SC has a high affinity for Ca²⁺ and Mg²⁺ and can remove 96.5% of Ca²⁺ (or 96.8% of Mg²⁺) from the corresponding solution. The maximum adsorption capacities of CA-SC for Ca²⁺ and Mg²⁺ are 62.38 and 36.23 mg/g, respectively. These values are higher than those of the most reported Ca²⁺-sorbents and Mg²⁺-sorbents. The CA-SC adsorbent can be regenerated through a simple pickling step, and its adsorption performance keeps stable after repeated use. Analysis of the adsorption mechanism shows that the CA-SC combines Ca²⁺ and Mg²⁺ in water mainly through coordination effect.