Unveiling the mechanistic aspects of methylene blue adsorption onto a novel phosphate-decorated coconut fiber lignin

Abstract

The aim of this work was to evaluate the adsorptive performance of the phosphorylated coconut fiber lignin (PCFL) obtained through an innovative biorefinery process for removing methylene blue (MB). PCFL was obtained using coconut fiber mixed with 85 % wt. H3PO4 at 70 °C for 1 h. Milled wood lignin (MWL) and PCFL were characterized by FTIR, CP-MAS ³¹P NMR, phosphorous and hydroxyl contents, pH_PZC, and BET analyses. The batch adsorption tests evaluated the effects of the biosorbent (0.25 - 4 g L^-1) and adsorbate dosages (2.5 - 7.5 mg L^-1), contact time (0 - 60 min), pH (4 - 8), ionic strength (0.001 - 0.1 mol L^-1) and temperature (298.15 - 318.15 K) on MB adsorption. Kinetic, equilibrium, and thermodynamic modeling were used. The phosphorous content on PCFL was 2.5 times higher than that of MWL. PCFL presented an enhanced adsorptive performance for removing MB, which was spontaneous (ΔG⁰ < 0), endothermic (ΔH⁰ > 0), with affinity between the biosorbent and adsorbate (ΔS⁰ > 0), and driven by physisorption (E_a > 40 kJ mol^-1). The adsorptive performance of PCFL was enhanced due to the grafting of new active sites by using an innovative biorefinery process, showing its potential to be used for textile effluent remediation.

Keywords: Coconut wastes; Lignin valorization; Phosphorylation.