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.

Keywords

• oxytetracycline
• biochar
• nZVI
• adsorption
• degradation
• acidic condition