Construction of high-content α-iron on zero-valent iron @ biochar composite for the ultra-efficient removal of oxytetracycline hydrochloride: A key step of ammonium bicarbonate pretreatment

Pyrolytic reduction as an effective method is generally employed for synthesizing zero-valent iron @ biochar composite (ZVI@BC). However, it suffers from intractable obstacles, such as poor zero-valent iron (ZVI) dispersibility and low α-iron (α-Fe0) content. Herein, we reported a possible way to target the dilemma for the first time, i.e., adding ammonium bicarbonate (NH4HCO3) pretreatment step before pyrolytic reduction. In detail, the special NH4HCO3 not only served as a precipitant to uniformly disperse ZVI over biochar by gradually releasing OH–, but also as a reductant to facilitate γ-Fe0 transforming to α-Fe0 in ZVI (∼71 %) via providing sufficient NH3. Besides, NH-ZVI@BC3/1/5 exhibited outstanding oxytetracycline hydrochloride (OTC) removal ability (∼2869.1 mg/g), with the removal efficiency of 99.0 % at the dosage of 0.05 g/L. The removal mechanism was investigated which involved adsorption dominated by complexation, π-π electron donor acceptor interaction, hydrogen bonds, and electrostatic interactions. As well, OTC degradation was achieved by [H] and ·OH resulting from Fe-C micro-electrolysis and Fenton-like reaction during the corrosion of ZVI in ZVI@BC, respectively. In addition, four possible degradation pathways were proposed, and the contribution role in OTC removal followed the sequence: adsorption > degradation involved with [H] > degradation involved with ·OH. Moreover, N-ZVI@Biochar3/1/5 exhibited easy separability, excellent reusability, and well practical applicability. Overall, this work not only opens a new avenue for the fabrication of ZVI@BC with high ZVI dispersibility and content of α-Fe0, but also shed light on the environmental remediation of antibiotics.