Phosphonium-based ionic liquid immobilised in sodium alginate as a novel adsorbent: Kinetics, thermodynamics, and toxicity study on 2,4-dichlorophenol removal

This study aimed to immobilise a phosphonium-based ionic liquid (IL) (trihexyl(tetradecyl)phosphonium decanoate) in sodium alginate capsules, and its adsorption capacity was evaluated using 2,4-dichlorophenol (2,4DCP) as pollutant target. This novel adsorbent was characterized in terms of equivalent diameter, moisture content, impregnated IL percentage, and by Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA), and Brunauer, Emmett and Teller (BET) Surface Area Analysis. The effect of pH, temperature, contact time, and salt concentration on the adsorption efficiency was examined. In addition, the phytotoxicity and cytotoxicity of the capsules were also determined. Adsorption was hindered in alkaline and strongly acidic environments and favoured at pHs between 6 and 7, as well as at temperatures close to 15 degrees C. The pseudo-second-order kinetic model best suited the data, and equilibrium was reached with a contact time close to 50 minutes. After several batch cycles, the cumulative capacity resulted in 172.01-217.55 mg g-1. Also, the phenol extraction efficiency of immobilised IL on sodium alginate was similar to the free IL used on liquid-liquid extraction. The IL-impregnated capsules proved to be non-toxic for lettuce seeds and onion bulbs. Trihexyl(tetradecyl)phosphonium decanoate could be efficiently impregnated into sodium alginate capsules and potentially used as an adsorbent.