ZnCl2 activated mesoporous carbon from rice straw: optimization of its synthetic process and its application as a highly efficient adsorbent for amoxicillin

Abstract

Optimized and efficient zinc chloride (ZnCl₂)-based chemical activation of rice straw yielded highly mesoporous carbons with an exceptional ability to adsorb the antibiotic amoxicillin (AMX). The maximum AMX adsorption capacity was found to be as high as 1308 mg g⁻¹. Greater understanding of the pyrolysis process was gained through TGA-IR, demonstrating that ZnCl₂ activation could reduce carbonization temperature, inhibit tar formation, and lead to the extensive release of oxygen-containing compounds during the dehydration processes in pyrolysis. In addition, a 2-step strategy for rice straw carbonization activated by ZnCl₂ is proposed, involving biomass (cellulose, hemicellulose, and lignin) decomposition at low temperature and subsequent dehydration at a higher temperature to obtain more graphitic mesoporous carbon. The optimum ratio of rice straw to ZnCl₂ was 1 : 2 (ZAC1:2); X-ray diffraction and X-ray photoelectron spectroscopic analysis confirmed the occurrence of graphitic carbon and revealed the existence of ZnO within the carbon structure. This, in combination with a significant surface area of 941 m² g⁻¹, large pore volume, and 100% mesoporosity with a narrow pore size distribution of 2–6 nm, significantly enhanced AMX adsorption. The Langmuir adsorption isotherm model revealed homogeneous adsorption, while kinetic studies revealed a fit to the pseudo-second order kinetic model. These highlight the significant potential of mesoporous ZnCl₂-activated rice straw carbon for application in wastewater treatment and in the remediation of emerging pollutants such as antibiotics.