Highly efficient and selective pillararene-based organic materials for Hg2+ and CH3Hg+ extraction from aqueous solution

Pillararenes, a rising star in macrocyclic chemistry, are recognized as promising organic materials for metal ion separation. Herein, three novel pillar[5]arenes (1a-c) appended with N,N-dimethyldithiocarbamoyl groups were designed for highly efficient extraction of aqueous mercury. This work represents the first case of using pillararene-based materials for organic mercury removal, and rather low Hg(II) residue (3.45 ppb) was found in the extraction raffinate of mercury spiked river water after being treated with the as-prepared extractants, which is a rare example of solvent extraction systems that meet the waste water discharge standard (< 5 ppb). The extraction performance of the extractant materials was evaluated under different acidities, equilibrium time and sodium nitrate concentrations. High extraction efficiency (> 99%) and selectivity (SFHg/M > 690) were achieved under the optimum condition, and the near quantitative stripping after two contacts enable regeneration of the ligands as well as mercury recovery. The stoichiometric ratio between ligands and Hg2+ was ascertained to be 1:1 from the slope analysis method and mass spectroscopy. Investigation of the extracted species by NMR, IR, EXAFS techniques and DFT simulation reveals that the C=S moieties of the ligands play a crucial role in metal complexation. It is noted that the as-prepared extractants show excellent recovery and removal ability towards mercury using simulated samples. After extraction by 1c, the total Hg(II) in spiked river samples was reduced to lower than 5 ppb, the maximum tolerable concentration set by China for industrial waste water. This study demonstrates the potential application of N,N-dimethyldithiocarbamoyl modified pillararenes as functional materials for aquatic mercury remediation.