Highly Selective Removal of Perchlorate from Water: Roles of Unconventional Hydrogen Bond and Hydrophobic Cavity

Abstract

Selective perchlorate (ClO₄⁻) removal from surface water is a pressing need due to the stringent perchlorate drinking water limits around the world. Herein, we anchored N⁺–C–H hydrogen bond donors in hydrophobic cavities via interactions of cationic surfactants with montmorillonite to prioritize perchlorate bonding. The prepared adsorbent exhibited high selectivity over commonly occurring competing anions, including SO₄²⁻, NO₃⁻, PO₄³⁻, HCO₃⁻, and halide anions. High adsorption capacity, fast adsorption kinetics, and excellent regeneration ability (removal efficiency ≥ 80% after 20 cycles) were confirmed via batch experiments. Unconventional CH···O hydrogen bonding was verified as the primary driving force for perchlorate adsorption, which relies on the higher bond energy (∼80 kcal·mol⁻¹) than conventional bonding. The removal efficiency of anions followed the order of the Hofmeister Series, demonstrating the importance of hydrophobic cavities formed by the tail groups of cationic surfactants. The hydrophobic cavities sheltered the C–H bonds from interacting with anions of low hydration energy (e.g., perchlorate). Furthermore, a fixed-bed column test demonstrated that about 2900 bed volumes of the feeding streams (∼500 μg·L⁻¹) can be treated to ≤ 70 μg·L⁻¹, with an enrichment factor of 10.3. Overall, on the basis of the hydrophobicity-induced hydrogen bonding mechanism, a series of low-cost adsorbents can be synthesized and applied for specific perchlorate removal.