Development of Vitamin B6-Mediated Biochar with Nano Zero-Valent Iron Coating for Oxytetracycline Removal through Adsorption and Degradation under Harsh Acidic Conditions

Abstract

Oxytetracycline-containing wastewater, particularly produced by pharmaceutical industries, is too acidic to treat with iron-assisted materials. In order to tackle this issue, vitamin B6-mediated biochar with nano zero-valent iron coating (nZVI/B6@BC) was developed. Oxytetracycline (OTC) removal performance of biochar (BC), vitamin B6-coated biochar (B6@BC), nZVI-coated biochar (nZVI@BC), and vitamin B6-mediated biochar with nano zero-valent iron coating (nZVI/B6@BC) were investigated to analyze contributions and mechanisms of adsorption and degradation. Through modification, the adsorption capacity of B6@BC was slightly increased from 81.38 mg/g of BC to 85.64 mg/g. In the removal test, the 5-min OTC removal efficiencies with nZVI@BC and nZVI/B6@BC were 52.25% and 59.05%, yet the BC and B6@BC were limited to 5.61% and 8.54%. The distinct difference may be attributed to the existence of nZVI on biochar strongly improving the reactivity from adsorption to chemical reaction. Moreover, 98.28% of OTC was removed within 60 min in the nZVI/B6@BC suspension. The adsorption of OTC on BC fitted the Freundlich isotherm, Temkin isotherm, and intramolecular diffusion model, whereas that on B6@BC fitted Langmuir isotherm and pseudo-second-order better. Based on HPLC-MS analyses, there were three pathways proposed for OTC degradation in nZVI/B6@BC suspension. nZVI provided active sites on biochar for OTC degradation through oxidization, de-hydroxylation, ring-opening, reduction, addition, demethylation, and alkylation reactions. B6 as a mediate helped improve the stabilization and distribution of nZVI on biochar, which facilitates the capability of nZVI/B6@BC for OTC removal through adsorption and degradation under acidic conditions. The OTC can not only be captured on biochar but also be metabolized to achieve complete removal from aquatic systems.