Mussel-inspired fabrication of pH-responsive pomelo peels as “smart” bio-based adsorbents for controllable removal of both cationic and anionic dyes

Abstract

In this work, novel and eco-friendly pH-responsive bio-based adsorbents derived from the pomelo peel (PP) were successfully developed. The fabrication process involved the self-polymerization of dopamine to form polydopamine (PDA) on the surfaces of PP, followed by the immobilization of pH-responsive copolymer (P) onto the PDA decorated PP (PDA@PP) using the dip-coating method. The results demonstrated that the as-obtained bio-based adsorbent (PDA@PP-P) with a single dip-coating, denoted as PDA@PP-P-1, exhibited optimal comprehensive properties. Due to the presence of catechol and tertiary amino moieties, the as-prepared PDA@PP-P-1 displayed highly efficient adsorption for both cationic and anionic dyes across a wide pH range from 3 to 10. The adsorption capacity of the PDA@PP-P-1 toward anionic dyes decreased with an increment of solution pH, whereas the opposite trend was observed for cationic dyes. The increment of ionic strength weakened the adsorption performance of as-obtained PDA@PP-P-1 toward dyes except for anionic dyes at high pH, which was slightly influenced. Generally, the adsorption kinetics obeyed the pseudo-second-order kinetic model, while the adsorption isotherm models for dyes’ adsorption were inconsistent. The maximum experimental adsorption amounts were 1750.6 mg g−1 for methyl orange (MO) and 959.2 mg g−1 for congo red (CR) at low pH, 1081.4 mg g−1 for methylene blue (MEB) and 873.4 mg g−1 for crystal violet (CV) at high pH. The electrostatic interactions, π-π interactions and hydrogen bonding were proposed as adsorption mechanisms. Interestingly, as-obtained PDA@PP-P-1 could achieve pH-dependent separation of mixture dyes, due to the significant differences in adsorption capacity and adsorption rate of cationic and anionic dyes under different pH values. Moreover, it exhibited outstanding recyclability under both acidic and alkaline environments. Most importantly, it also presented excellent antibacterial activity against E. coli and S. aureus, followed by antibacterial efficiency of over 99.9 %. Therefore, it could be considered an advanced bio-based adsorbent with enormous potential for dye wastewater remediation.