Phosphorylated chitosan nanosorbents from shrimp waste for enhanced dual removal of methylene blue and lead(II)

Shrimp-derived chitosan nanosorbents offer a sustainable and efficient method for removing dyes and metals. Their inherent sorption capacity is substantially enhanced via simple chemical modification. Functionalized chitosan derivatives, phospho-(phenyl)methylated chitosan nanosorbent (FCCs) and their magnetic analogues (MFCCs) were engineered via continuous in-situ reaction with benzaldehyde and H₃PO₃, followed by Fe₃O₄ nanoparticles incorporation. The physicochemical properties were characterized via CHN/P, XRD, BET, ZPC, TEM, XPS, and VSM analyses. Functionalized versus non-functionalized (crosslinked chitosan (CCs) and magnetic-CCs (MCCs)) were compared for methylene blue (MB) and Pb(II) sorption. The maximal sorption capacities increased progressively, aligned with their physicochemical properties: CCs < MCCs < FCCs < MFCCs, with values of 0.367, 0.459, 0.809, 0.887 mmol/g for Pb(II) and 0.269, 0.344, 0.626, 0.896 mmol/g for MB at pH₀ ⁓5.0 and 6.0, respectively. Phosphorylation and Fe₃O₄ dual-functionalization boost sorption kinetics/efficiency. MFCCs/FCCs reached equilibrium in 1/1.5 h (vs. 2–3 h for CCs/MCCs), with rapid half-sorption times. Equilibrium and kinetic data were captured by Sips (implying monolayer heterogeneity) and pseudo-first-order (suggesting diffusion-controlled) models, confirming structure-property relationships and physicochemical sorption mechanisms. XPS analysis confirmed Pb(II) chemisorption via P–O/–OH/–NH covalent/electrostatic interactions, contrasting MB physisorption via electrostatic π-interactions/hydrophobicity. Thermodynamically, sorption was endothermic (except MB on FCCs/MFCCs) and an entropy-driven spontaneous process. Recycling assessments revealed that the sorbents maintained 85.5 % of their capacity after seven cycles, achieving over 88.7 % regeneration efficiency with 0.1 M HCl. Functionalized sorbents show superior Pb(II) selectivity in simulated lead-acid battery wastewater.