Production of NaOH-Modified Banana Pseudostem Biochars through Controlled Heating for Tetracycline Removal

Abstract Proposing efficient adsorbents from agro-industrial waste for the removal of emerging contaminants, particularly tetracycline (TC), from aqueous systems remains a critical environmental challenge. In this study, we systematically investigate the influence of NaOH pretreatment combined with stepwise pyrolysis on the physicochemical properties and TC adsorption performance of biochars derived from banana pseudostem biomass. The biomass (with or without NaOH pretreatment) was subjected to pyrolysis at a heating rate of 15 °C min–1 under different thermal regimes, comprising either a single isothermal step at 400 °C or a stepwise heating pathway (215–400 °C), yielding unmodified and alkali-modified biochars for each condition. Physicochemical characterization (FTIR, XRD, BET, and pHPZC) revealed that alkaline treatment increased surface basicity, promoted the formation of oxygenated and carbonate phases, and improved pore development. The introduction of intermediate heating plateaus during pyrolysis also favored gradual devolatilization and structural rearrangement, contributing to the development of a more organized porous structure and greater accessibility of adsorption sites. Under unbuffered conditions at an initial pH of 6.0, the alkali-modified biochar produced through the 215–400 °C heating pathway exhibited an apparent adsorption capacity of 177.1 mg g–1. For the NaOH-modified biochars, kinetic data were better represented by the pseudo-second-order model, whereas equilibrium data were adequately described by Freundlich and Sips-type models, indicating heterogeneous adsorption behavior. The enhanced adsorption capacity was associated with the combined effects of surface chemistry, aromatic domains, and pore accessibility rather than with a single adsorption mechanism. These findings demonstrate that NaOH modification combined with controlled heating stages leads to the formation of biochars with enhanced TC adsorption capacity.