Mechanisms of Zinc and Nickel-modified Biochar for Adsorption and Degradation of Ciprofloxacin

Modified biochars have a large specific surface area, a tunable pore structure, and abundant active functional sites, which enable them to effectively remove antibiotics from the environment. However, systematic research on the contribution of different properties of modified biochars to the adsorption and degradation of antibiotics has not been carefully investigated. To clarify the contribution of adsorption and degradation of ciprofloxacin （CIP） by persistent free radicals （PFRs） and the microporous structure of metal-impregnated modified biochars during the removal process, nickel-modified and zinc-modified biochars were prepared under pyrolysis conditions of 500 ℃ and 800 ℃, respectively. Nickel-modified biochar pyrolyzed at 500 ℃ （NCS500） significantly improved the PFR signal intensity, while the specific surface area of zinc-modified biochar pyrolyzed at 800 ℃ （ZCS800） was significantly increased （1 030.94 m2·g-1）. The maximum CIP adsorption capacity of NCS500 and ZCS800 increased dramatically, reaching 78.68 mg·g-1 and 190.07 mg·g-1, respectively, which was 3.35 times and 24.83 times higher than the corresponding values of the original unmodified biochar. The oxygen-containing functional groups such as carboxylic acid, hydroxyl group, and carbonyl group on the NCS500 were responsible for CIP adsorption due to the ion exchange, electrostatic interaction, surface complexation, and hydrogen bonding provided by carbonate components. The formation of a relatively stable graphite structure on the surface of ZCS800 was conducive to hydrophobic interaction, π-π electron donor-acceptor interaction with aromatic molecules, and enhancing the adsorption of CIP. The acetonitrile extraction experiment could distinguish the adsorption and degradation contributions of the two modified biochars in the process of CIP removal, and the amount of CIP degradation by NCS500 could account for 74.67% of the total removal. The total CIP removal by ZCS800 was the highest, but the amount of CIP degradation accounted for only 52.63% of the total removal. This was because these two metal salt-impregnated modified biochars could produce active oxygen species such as hydroxyl radicals to effectively degrade CIP, while the PFRs signal on the surface of NCS500 could also directly react with CIP to effectively degrade CIP. The Zn-modified biochar was conducive to hydrophobic interactions, π-π interactions, and aromatic molecule interactions, which significantly improved the adsorption of CIP.