Polyethylene microplastic removal using a heavy-metal-free chitosan–tannic acid–calcium ternary system: an FTIR-based chemometric study

The increasing prevalence of polyethylene (PE) microplastics in aquatic systems necessitates the development of environmentally benign removal strategies that avoid secondary contamination associated with conventional metal-based coagulants. In this study, we systematically develop and mechanistically elucidate a fully heavy-metal-free ternary coagulation system composed of chitosan, tannic acid, and calcium ions (Ca2+) for PE microplastic removal under batch conditions. Unlike previously reported trivalent metal-assisted systems, the present formulation employs divalent calcium as a low-toxicity ionic crosslinker to construct a three-dimensional biopolymer network. Comparative evaluation of single, binary, and ternary systems across pH 5–9 revealed a pronounced synergistic enhancement in the ternary configuration, achieving a maximum removal efficiency of 65 ± 9% at pH 6.59 ± 0.06. ATR-FTIR spectroscopy combined with principal component analysis provided multivariate evidence of progressive interfacial chemical modification, demonstrating that removal is governed by cooperative non-Derjaguin–Landau–Verwey–Overbeek interactions including hydrogen bonding, Ca2+-mediated coordination bridging, polyelectrolyte complexation, and polymer enmeshment rather than simple charge neutralization. Although floc buoyancy limited sedimentation efficiency due to the intrinsic low density of PE microplastics, the results establish a sustainable coagulation pathway that prioritizes environmental safety over maximum removal performance. This study provides mechanistic insight into calcium-mediated biopolymer networks for microplastic destabilization and offers a scalable framework for integration with flotation- or membrane-assisted post-treatment systems.