Insights into the Roles of Surface Functional Groups and Micropores in the Sorption of Ofloxacin on Banana Pseudo-Stem Biochars

Abstract

In the present study, banana pseudo-stem (BS) was pyrolyzed under anaerobic conditions without any physical or chemical modification. Their properties, as well as their sorption affinity to ofloxacin (OFL), were studied. As a result, oxalates and KCl formed at a relatively low temperature of 300 °C, while bicarbonates generally formed at a pyrolysis temperature above 400 °C. Surface functional groups of BS biochars facilitated OFL sorption mainly via specific interactions including electronic attraction (EA), π–π electron donor–acceptor (π–π EDA) interaction, the ordinary hydrogen bond (OHB), and the negative charge-assisted hydrogen bond ((−)CAHB). Except for (−)CAHB, these interactions all decreased with an elevated pH, resulting in overall decreased OFL sorption. Significant OFL sorption by BS biochars produced at 300 °C, observed even at an alkaline condition was attributed to (−)CAHB. Micropores formed in BS biochar prepared at 500 °C, with a specific surface area as high as 390 m2 g−1 after water washing treatment. However, most micropores could not be accessed by OFL molecules due to the size exclusion effect. Additionally, the inherent K-containing salts may hinder OFL sorption by covering the sorption sites or blocking the inner pores of biochars, as well as releasing OH− into the solution. Thus, BS biochar produced at 300 °C is an excellent sorbent for OFL removal due to its high sorption ability and low energy. Our findings indicate that biochar techniques have potential win–win effects in recycling banana waste with low energy and costs, and simultaneously converting them into promising sorbents for the removal of environmental contaminants.