pH-driven mechanistic insights into As(V) adsorption on biomass-derived AAVB/NZVI synthesized via chemical reduction

A novel biochar-supported nano zero-valent iron (NZVI) composite was synthesized via liquid-phase chemical reduction using carbonized Acantholimon acerosum var. brachystachyum (AAVB) as a biomass-derived support. The composite was characterized by SEM, TEM, BET, FTIR, pHzpc, and XRD analyses. Results revealed a porous structure decorated with uniformly dispersed Fe0 nanoparticles (40–60 nm). The As(V) adsorption performance was systematically evaluated under varying pH, contact time, adsorbent dosage, temperature, and initial concentration. Adsorption was strongly pH-dependent, and a high removal efficiency of 93.64% was achieved at pH 6. This condition was selected as the optimum operating pH due to its proximity to the natural solution pH and its practical applicability. This favorable performance was attributed to the electrostatic interactions between arsenate species and the positively charged composite surface (pHzpc = 8.75). Equilibrium data were best described by the Freundlich isotherm model (R2 = 0.992), indicating heterogeneous multilayer adsorption. Thermodynamic parameters (ΔG° <0, ΔH° >0) demonstrated that the adsorption process was spontaneous and endothermic, while the positive ΔS° value suggested increased randomness at the solid–solution interface. Overall, the AAVB/NZVI composite exhibits high efficiency and sustainability, highlighting its potential as a low-cost adsorbent for effective As(V) removal from aqueous systems.