Pectin derived from pomelo pith as a superior adsorbent to remove toxic Acid Blue 25 from aqueous solution

Low methoxyl (LM) pectin was extracted from pomelo pith for application as an adsorbent to remove toxic Acid Blue 25 (AB25) from an aqueous solution. The adsorptive removal of AB25 onto pectin was systematically investigated across different parameters, from which the adsorption kinetics, rate-controlling step, isotherm, and thermodynamics were evaluated. The maximum adsorption capacity was 719.4 mg g^–1, much higher compared to those of agricultural waste and activated carbons, due to the unique gel formation of pectin, facilitating hydrogen bonding platforms between AB25 and the galacturonic units of pectin strands. The adsorption kinetics was pseudo-second order, governed by intraparticle and film diffusions, and the equilibrium data were well described by the Langmuir and Freundlich isotherm models, suggesting a combination of homogenous and heterogeneous adsorption binding energies. The adsorption process was spontaneous and endothermic, and increased irregularity. Acid treatment efficiently accelerated AB25 desorption and restored the functional groups of the spent pectin, thus successfully keeping the adsorption capacity of the anionic synthetic dye after a few adsorption-desorption cycles. Overall results in this study indicated that LM pectin isolated from fruit peels has great potential as a highly efficient adsorbent to remove acidic synthetic dyes from industrial wastewater.