Biochar–nZVI nanocomposite: optimization of grain size and Fe⁰ loading, application and removal mechanism of anionic metal species from soft water, hard water and groundwater

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., HCrO₄⁻ and CrO₄²⁻. 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.