Phosphorylation-programmed cellulose nanofibers enable sustainable pyrite–serpentine separation via interfacial assembly and steric stabilization

Regulating mineral surface interactions is essential for achieving particle separation and sustainable resource recovery. However, serpentine-induced hetero-aggregation in sulfide flotation masks valuable mineral surfaces, weakens bubble–particle attachment, and reduces separation selectivity. Herein, phosphorylated cellulose nanofibers (P-CNFs) with controlled phosphorylation degrees were developed as bio-based depressants for sustainable pyrite–serpentine separation. Micro-flotation established a non-monotonic relationship between phosphate content and separation performance, with moderately phosphorylated P-CNF achieving the highest pyrite recovery of 89.33%, whereas insufficient or excessive phosphorylation led to inferior performance. This behavior was governed by phosphorylation-regulated surface chemistry and interfacial adsorption architecture. Increasing phosphorylation introduced more phosphate groups and shifted serpentine surface charge negatively, thereby weakening electrostatic attraction with pyrite. However, higher phosphorylation degrees also reduced cellulose crystallinity and drove the structural evolution of P-CNFs from extended fibrillar networks to shorter particle-like aggregates. On serpentine surfaces, moderate phosphorylation enabled compact assembly of a dense and continuous fibrous adsorption layer, whereas excessive phosphorylation generated discontinuous deposits with lower surface coverage. Direct atomic force microscopy force measurements demonstrated that the optimized P-CNF layer transformed pyrite–serpentine interactions from attraction-dominated to repulsion-dominated by coupling electrostatic double-layer repulsion with long-range steric stabilization, decreasing the adhesion force from 13.00 ± 0.03 to 0.30 ± 0.05 mN m−1. These findings establish a direct structure–interface–performance relationship, demonstrating that adsorption-layer architecture and polymer-mediated steric forces, beyond phosphate-derived mineral affinity, are decisive factors governing cellulose-based depressant performance. This work provides a sustainable interfacial engineering principle for designing bio-based macromolecular reagents for cleaner mineral beneficiation and broader aqueous separation processes.