This work for the first time evaluates the efficiency of biochar-supported nanoscale zerovalent iron (nZVI) particles combinedly as a function of biochar grain size and iron loading for the removal of anionic metal species, i.e., HCrO 4 − and CrO 4 2− . Both the factors are crucial and have to be optimized in order to achieve the highest and fastest removal. Not only that, it is also crucial to check the applicability of that composite in complex aqueous solutions. For that, nZVI particles were supported on biochar grains of different size ranges, i.e., < 75 µm (IBC-< 75), 75-150 µm (IBC-75-150), 150-300 µm (IBC-150-300) and 300-600 µm (IBC-300-600) with Fe/BC mass ratio = 1. Further, to check the effect of iron loading, < 75 µm grains were modified with iron at Fe/BC = 0.5 (IBC-0.5) and Fe/BC = 2 (IBC-2). All these composites were evaluated for the removal of anionic Cr(VI) species in distilled water (DW). Obtained results suggest that maximum (Q max = 16.30 mg/g) and fastest (100% within 5 min) removal of 10 mg/L Cr (VI) solution was obtained with IBC-< 75. The applicability of IBC-< 75 was further evaluated for Cr(VI) removal from different waters, i.e., soft water (SW), hard water (HW) and groundwater (GW). Results show that complete removal of 10 mg/L Cr(VI) was faster in DW (within 5 min) followed by SW ≈ GW (10 min) and then in HW (20 min), while the order of Q max was GW (22.49 mg/g) > SW (21.54 mg/g) > HW (17.00 mg/g) > DW (16.30 mg/g). Cr(VI) removal mechanisms were chemisorption, reduction and simultaneous coprecipitation as confirmed through various kinetic and isotherm modeling and through pXRD of reaction precipitates.

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Antioxidant coating is emerging as a new technique to preserve the reactivity of redox-sensitive nanocomposites. Current work extends potential environmental applications of antioxidants in preserving Fe 0 -biochar nanocomposites further and highlights the remediation of multimetal-contaminated river water and groundwater. Nanocomposites were prepared via a liquid-phase iron reduction in the presence of almond shell biochar surfaces, followed by its interaction with antioxidants extracted from almond skin. Antioxidant coating resulted in the generation of an iron-carbonyl shell on the composite surface, preserving the redox state of Fe 0 nanoparticles. Synthesized composites were explored for the removal of both cationic and anionic metallic species from river water and groundwater in batch and continuous column modes. Synthesized nanocomposites have shown efficient contaminant removal capacities (AsO 2 − , 167.9 mg/g; Cd 2+ , 134.1 mg/g; CrO 42− , 118.7 mg/g; and Ni 2+ , 130.2 mg/g) in a monometallic system and ultrahigh total metal sorption capacities, i.e., 695, 802, and 770 mg/g, respectively, in 0.01 M NaNO 3 , river water, and groundwater samples, contaminated with multimetallic species. In addition, continuous column separation of all metals (>1,50,000 L/kg), no secondary release of contaminants, suspension stability in river water, and self-settling possibility after contaminant sorption justify their large-scale environmental applicability.

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