Impact of co-existing ions on arsenic remediation using green nano zero-valent iron supported on nanocellulose: A fractional factorial design study

This study focused on developing a novel adsorbent material of green nano zero-valent iron supported by nanocellulose (G-NZVI/NCC) from agricultural waste for the effective removal of arsenic. The adsorption isotherm for arsenite (As3+) and arsenate (As5+) on G-NZVI/NCC showed maximum capacities of 4.2123 and 4.9579 mg·g‒1, respectively. The model indicated that As3+ forms a multilayer on a heterogeneous surface, while As5+ forms a monolayer on a homogeneous surface. The Gibbs free energy for the adsorption of As3+ and As5+ on G-NZVI/NCC from water demonstrates a non-spontaneous process at higher temperatures for As3+, but it occurs spontaneously for As5+. Furthermore, most previous studies have largely overlooked the influence of co-existing ions in water, or have investigated them individually. Consequently, this study aimed to simulate real-world conditions by examining the simultaneous effects of all relevant ions. This comprehensive approach is critical for the practical implementation of the system, and statistical principles are employed for data analysis and interpretation. However, surface water contains competing ions such as calcium (Ca2+), phosphate (PO43‒), bicarbonate (HCO3‒), chloride (Cl‒), and sulfate (SO42‒) that can interfere with arsenic adsorption. A 25‒2 fractional factorial design was used to systematically evaluate individual factors, significant parameters, and their interactions on arsenic removal efficiency. The results revealed that Ca2+, PO43‒, and HCO3‒ play crucial roles in arsenic removal, with Ca2+ and PO43‒ enhancing removal efficiency, while HCO3‒ exhibits inhibitory effects. Notably, high PO43‒ concentrations unexpectedly enhanced arsenic removal compared to previous studies, attributed to the synergistic physical and chemical adsorption properties of iron oxide and hydroxide phases. Mechanistic analysis revealed that As5+ can substitute PO43‒ in vivianite formation, creating a vivianite-symplesite solid solution (Fe3(PO4)2.7(AsO4)0.3·8H2O), providing new insights into arsenic immobilization mechanisms under realistic water chemistry conditions.