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

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) function-alised 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 elim-ination 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 orien-tation) 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 phys-isorption 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.