Pyridinium-furfuryl-modified granular agro-waste adsorbent for orthophosphate recovery

Abstract

In this study, a granular composite comprised of 50% oat hulls (Oh), 10% kaolinite (K), and 40% modified chitosan (Chi) was crosslinked with epichlorohydrin (ECH) and subsequently functionalized with pyridinium. The modified Chi was obtained through synthetic conversion of a furfural-moiety attached via the amine-groups of chitosan (Oh50–Furfural). For comparison, non-modified pellets (Oh50), NaOH-neutralised pellets (Oh50–NaOH) and ECH crosslinked composites (Oh50–ECH) were used to elucidate the effect of variable synthetic modification. An analysis of the ¹³C solids NMR spectral results for the Oh50–Furfural composite revealed the presence of a product mixture, both chitosan with imine-linked furfuryl- and pyridinium moieties. Experimental support by infrared spectroscopy and thermogravimetric analysis corroborate the modified structure of Chi. Although the absolute orthophosphate adsorption capacity was low (5 mg g⁻¹ at pH 4.75 and 1 mg g⁻¹ at pH 8.5), the furfuryl-pyridinium modified adsorbent showed pH independent adsorption, which is in contrast to non-modified chitosan-based composites that often require acidic media. This study demonstrates that composites containing agro-waste and chitosan can be synthetically modified to incorporate pyridinium-moieties (alongside furfuryl-moieties) to enhance the adsorption properties towards orthophosphate, especially at low concentrations and neutral or slightly alkaline pH.