A study published in Engineering introduces an innovative approach to address the issue of perchlorate (ClO₄⁻) contamination in water. Perchlorate is a harmful oxo-anion found in aquatic environments. It can enter the human body through drinking water and inhibit iodine absorption in the thyroid gland, potentially causing various thyroid-related diseases. Given the strict perchlorate limits in drinking water worldwide, such as 70 μg/L in China and 15 μg/L in the United States, developing efficient methods for its removal is crucial.

The research team, hailing from Hunan University and Shanghai Jiao Tong University, prepared an adsorbent by anchoring N⁺–C–H hydrogen bond donors in hydrophobic cavities. This was achieved through the interaction of cationic surfactants, like cetyltrimethylammonium bromide (CTAB) and cetylpyridinium chloride (CPC), with montmorillonite (MMT).

Batch experiments demonstrated the excellent performance of the cationic surfactant-modified MMT. It showed high selectivity for perchlorate over common competing anions, including SO₄^2–, NO₃^–, PO₄^3–, HCO₃^–, and halide anions. The adsorption kinetics were fast, and the adsorption capacity was relatively high compared to traditional adsorbents such as activated carbon, clay, or zeolite. For instance, the pseudo-second-order adsorption rate constants for CTAB-MMT and CPC-MMT were 0.0306 and 0.0153 per minute respectively, much higher than those of some traditional counterparts.

The researchers delved into the adsorption mechanisms. Electrostatic attraction was ruled out as the main driving force because the adsorbent could effectively remove perchlorate even at pH values higher than its isoelectric point. Instead, density functional theory (DFT) calculations indicated that unconventional CH···O hydrogen bonding was the primary mechanism. The C–H bonds adjacent to the quaternary ammonium in the cationic surfactants could act as hydrogen bond donors, forming strong bonds with perchlorate. The hydrophobic cavities formed by the long-chain tail groups of the cationic surfactants also played a role. They sheltered the C–H bonds, allowing only anions with low hydration energy, like perchlorate, to interact with the hydrogen bond donors.

To assess its practicality, cyclic adsorption experiments and fixed-bed column tests were carried out. The adsorbent exhibited good cyclic stability, maintaining a removal efficiency of over 80% after 20 cycles. In the fixed-bed column test, it could treat a large volume of water with an initial perchlorate concentration of 500 μg/L to below the drinking water limit of 70 μg/L, with an enrichment factor of 10.3.

This new adsorbent provides a practical and potentially cost-effective solution for perchlorate removal from water. It holds promise for applications in drinking water treatment plants, potentially improving the quality of drinking water and safeguarding public health.

The paper “Highly Selective Removal of Perchlorate from Water: Roles of Unconventional Hydrogen Bond and Hydrophobic Cavity,” authored by Jian Ao, Lingjun Bu, Yangtao Wu, Jinming Luo, Shiqing Zhou. Full text of the open access paper: https://doi.org/10.1016/j.eng.2024.12.029.