Sustainable Low-Concentration Arsenite [As(III)] Removal in Single and Multicomponent Systems Using Hybrid Iron Oxide–Biochar Nanocomposite AdsorbentsA Mechanistic Study

Rice and wheat husks were converted to biochars by slow pyrolysis (1 h) at 600 °C. Iron oxide rice husk hybrid biochar (RHIOB) and wheat husk hybrid biochar (WHIOB) were synthesized by copyrolysis of FeCl₃-impregnated rice or wheat husks at 600 °C. These hybrid sorbents were characterized using X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy (SEM), SEM–energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, physical parameter measurement system, and Brunauer–Emmett–Teller (BET) surface area techniques. Fe₃O₄ was the predominant iron oxide present with some Fe₂O₃. RHIOB and WHIOB rapidly chemisorbed As­(III) from water (∼24% removal in first half an hour reaching up to ∼100% removal in 24 h) at surface Fe–OH functions forming monodentate Fe–OAs­(OH)₂ and bidentate (Fe–O)₂AsOH complexes. Optimum removal occurred in the pH 7.5–8.5 range for both RHIOB and WHIOB, but excellent removal occurred from pH 3 to 10. Batch kinetic studies at various initial adsorbate–adsorbent concentrations, temperatures, and contact times gave excellent pseudo-second-order model fits. Equilibrium data were fitted to different sorption isotherm models. Fits to isotherm models (based on R² and χ²) on RHIOB and WHIOB followed the order: Redlich–Peterson > Toth > Sips = Koble–Corrigan > Langmuir > Freundlich = Radke–Prausnitz > Temkin and Sips = Koble–Corrigan > Toth > Redlich–Peterson > Langmuir > Temkin > Freundlich = Radke–Prausnitz, respectively. Maximum adsorption capacities, Q_RHIOB⁰ = 96 μg/g and Q_WHIOB⁰ = 111 μg/g, were obtained. No As­(III) oxidation to As­(V) was detected. Arsenic adsorption was endothermic. Particle diffusion was a rate-determining step at low (≤50 μg/L) concentrations, but film diffusion controls the rate at ≥100–200 μg/L. Binding interactions with RHIOB and WHIOB were established, and the mechanism was carefully discussed. RHIOB and WHIOB can successfully be used for As­(III) removal in single and multicomponent systems with no significant decrease in adsorption capacity in the presence of interfering ions mainly Cl^–, HCO₃^–, NO₃^–, SO₄^2–, PO₄^3–, K⁺, Na⁺, Ca²⁺. Simultaneous As­(III) desorption and regeneration of RHIOB and WHIOB was successfully achieved. A very nominal decrease in As­(III) removal capacity in four consecutive cycles demonstrates the reusability of RHIOB and WHIOB. Furthermore, these sustainable composites had good sorption efficiencies and may be removed magnetically to avoid slow filtration.