Comparative Study of Biocarbon-Supported Iron Nanoparticle Composites (nZVI@BC) Synthesized by Carbothermal Versus Borohydride Reductions for Heavy Metal Removal

Biocarbon (BC) has been widely employed as a support to disperse nanoscale zerovalent iron (nZVI) particles to prevent their aggregation and rapid oxygen passivation. Here, we compare the chemical stability of nanozerovalent iron composites (nZVI@BC) made by liquid-phase reduction (LPR) versus carbothermal reduction (CTR). In the LPR route, Fe³⁺ was impregnated onto demineralized bamboo-BC formed at 600 °C, followed by NaBH₄ reduction under N_2. The CTR method employed aqueous FeCl₂-impregnated bamboo-BC, which was dried and carbonized from 50 to 1000 °C under N₂. nZVI@BC’s chemical stabilities were compared in air, water, and soil. Both routes produced Fe⁰, confirmed by the XRD peak at 2θ = 44.6°. Fresh LPR-nZVI@BC vs. CTR-nZVI@BC exhibited efficient Cu²⁺uptakes of 32 mg/g (212 mg/g Fe⁰) and 40 mg/g (266 mg/g Fe⁰) in 30 min, respectively, via Fe⁰ reduction of Cu²⁺to Cu⁰. Exposing LPR-nZVI@BC samples to water for 4 h led to the complete disappearance of the Fe⁰ XRD peak and the appearance of the Fe₃O₄ peak at 2θ = 35.0°, reducing Cu²⁺ uptake by 98%. In contrast, CTR-nZVI@BC only experienced a 51% drop in capacity due to the presence of a layered graphene sheet shell, preventing Fe⁰ from rapid oxidation. No Fe₃O₄ XRD peaks were observed in CTR-nZVI@BC after 7 days of air and soil exposure, unlike in LPR samples. Resistance to passivation in air, water, and soil makes the CTR a promising synthetic route to nZVI@BC.