One-step removal of free and complexed heavy metals using a biomass-derived amphiphilic network flocculant

Conventional flocculation technologies are ineffective at removing dissolved heavy metal ions and their stable organic complexes from wastewater, often requiring costly multi-step downstream processes. Herein, a fully biomass-derived amphiphilic network flocculant (BNF) was developed using kraft lignin (hydrophobic segment) and tannic acid (hydrophilic segment) as building blocks, crosslinked by thiosemicarbazide and formaldehyde. This rationally designed network integrates abundant chelating sites and tunable amphiphilicity, enabling efficient capture of both free and complexed heavy metals. Jar tests demonstrated that BNF achieved >90% removal of various heavy metal ions (Cu²⁺, Cr³⁺, Ni²⁺) and >95% removal of their organic complexes (except TCCr³⁺). When applied to real cold-rolling acid-washing steel wastewater, BNF exhibited superior heavy metal removal efficiencies (59.2- 97.6% removal, far exceeding commercial flocculants). At the same time, it maintained comparable performance in conventional water quality parameters (SS, COD, TN). Mechanistic investigations revealed that chelation accompanied by partial reduction dominates free metal removal, while hydrogen bonding, π-π stacking, and hydrophobic interactions govern the capture of metal-organic complexes. BNF offers a sustainable, cost-effective, and scalable strategy to upgrade conventional flocculation technology and achieve high-value utilization of biomass resources for advanced heavy metal wastewater treatment.