Wet strength and anion removal efficiency of fibrous adsorbent based on clicked, quaternary amine-functionalised kapok fibres

In this study, two types of functionalities (bifunctionality) were introduced in kapok fibres through chemical modifications to improve both wet strength and anion adsorption. Some hydroxy (–OH) groups of the kapok fibres were modified by azide-alkyne click chemistry to improve the wet strength, while other hydroxy groups were modified with quaternary amine group using epichlorohydrin-triethylamine (ECH-TEA) mixture at various modification ratios to improve the adsorption of anions. The successful incorporation of quaternary amine on the surface of the fibres was proven by the existence of bands at 1750 and 3700 cm−1 in Fourier Transform Infra-Red (FTIR) spectra. There were still hydroxy groups left due to the steric hindrance, the crystalline nature of cellulose, and the formation of new covalently bonded quaternary amine groups. The bifunctionality of the kapok fibres was confirmed using FTIR analysis and fluorescent labelling. The wet strength of the newly-formed fibre-based adsorbent was better than those unclicked ones, as a proof of successful clicking. The performance of the bifunctional adsorbent was equivalent to the control adsorbent but with improved wet strength. The Langmuir isotherm suggested it was a monolayer chemisorption. For single anion, the maximum adsorption capacity (qmax) were 4 to 44 times greater than the second highest values reported in the current literature. For mixed anions, these adsorbents could remove up to 85% sulphate (SO42−) ion from aqueous solution, the highest among all the anions tested. The anion adsorption would become less efficient at pH greater than 5.5 due to the high negative charges at the surface of the adsorbents. Overall, the best anion adsorption was achieved with kapok fibre adsorbent that underwent one-day treatment and six hours ball-milling time (TT1, BM6), clicked, with ratio of oven-dried weight of the clicked fibres: ECH-TEA mixture of 1:30. Graphical abstract