Reversible adsorption and storage of secondary explosives from water using a Tröger's base-functionalised polymer

Abstract

A Tröger's base-functionalised covalent organic polymer (TB-COP) was synthesised and used as an adsorbent for the efficient removal of picric acid (PA) from water through the use of weak and reversible supramolecular interactions such as hydrogen bonding and π–π interactions. TB-COP was readily synthesised in quantitative yield using a one-pot metal-free polymerisation reaction strategy between a semi-flexible aromatic triamine [L; benzene-1,3-5-tricarboxylic acid-tris-(4-amino-phenyl-amide)] and dimethoxymethane. The molecular structure, physicochemical and morphological characteristics of TB-COP were analysed by using various spectroscopic and imaging techniques. Thermogravimetric analysis showed TB-COP to be thermally stable up to 380 °C; while the calculated Brunauer–Emmett–Teller (BET) surface area was found to be 34 m² g⁻¹ at 273 K. The picric acid adsorption study of the activated TB-COP showed an excellent adsorption capacity of ca. 90% within 60 minutes of contact time at 298 K (Langmuir isotherm model: K_L = 0.0541 ± 6 L mg⁻¹, R² = 0.9962); the adsorption efficiency being shown to improve with increasing temperature. The extraction of PA was also clearly visible to the naked eye, where the yellow colored PA solution became transparent upon addition of TB-COP. Other interfering phenolic organic pollutants showed poor to moderate adsorption efficiency. Importantly, TB-COP could be used to store PA over a long period of time in a safe manner, without any leakage or any significant loss in extraction efficiency. Moreover, the polymer could be reused for several adsorption cycles, as PA could be released back into the solution by simply changing the pH of the aqueous media. This makes TB-COP an extremely promising material for the selective and efficient removal of picric acid from water, and TB-COP can be considered as being a ‘fast’ and naked eye colorimetric indicator for such analytes.