Insights into mass transfer mechanism and micro-interaction of melanoidin adsorption on polyethyleneimine-functionalised pomelo-peel-derived aerogel

Abstract

Melanoidins are recalcitrant colouring and polluting polymers present in the effluent streams of molasses-based distilleries. Adsorption is the most prospective technology with ease of operation and high efficiency. Herein, the removal of melanoidins from spent wash was investigated by synthesising a polyethyleneimine (PEI) functionalised pomelo-peel (PP) derived aerogel (PEI-PPA). The PEI-PPA is distinguished by the porous network structure and abundant accessible functional groups (mainly protonated primary amine) that are conducive to the elimination of colouring contaminants (melanoidins). The incorporation of PEI into supporting matrices (i.e., PP–derived aerogel) effectively improved the melanoidin adsorption capacity (from a previous-best research of 230 mg/g to the current 615.70 mg/g), while the melanoidin removal rate reached 92.36 %. The independent gradient model and Hirshfeld surface analysis were used to investigated the possible electroweak interaction relationship between PEI-PPA and melanoidin. By analysing the number of melanoidin molecules bonded to each active site using a novel double-layer adsorption model, a mixed inclination (parallel and non-parallel orientation) of melanoidin onto the PEI-PPA surface was found. For the two mass transfer models fitted results, the adsorption onto the active sites (AOAS) model better described the behaviour of melanoidin adsorption by PEI-PPA than the external diffusion resistance–internal diffusion resistance (EDR–IDR) mixed model under the same conditions, revealing the AOAS was rate-controlling during the mass transfer process. AOAS both contained chemisorption and physisorption. The calculated rates of chemisorption were far higher than those of physisorption before equilibration, indicating chemisorption was main the rate-determining step. Altogether, this work proposed an effective conversion for PP into bio-adsorbent and provided a detailed analysis of mass transfer mechanism.