NaOH-benzoic acid modified biochar for enhanced removal of aromatic VOCs

Abstract

This paper proposes a new alkali-acid-based treatment to enhance the capacity of biochar for the removal of toluene and p-xylene as the most abundant VOCs worldwide (BTEX) with acute toxicity to humans. We present the results of the a series of experiments on NaOH-modified benzoic-acid-activated biochar, pyrolysed from wheat straw and hardwood. Biochar samples were modified using the impregnation method and characterised by elemental analysis, scanning electron microscopy, FTIR spectroscopy, and BET surface area analysis. A bespoke experimental setup equipped with an inline GC-FID was utilised to study the adsorption and desorption processes. The proposed technique improved the structural properties of biochar, increasing specific surface area and pore volume while enhancing surface chemistry. Original samples’ SSAs (WS: 58.38 m2/g, HW: 19.92 m2/g) increased significantly post-treatment (WS: 121.72 m2/g, HW: 62.45 m2/g) due to alkalic digestion and were slightly reduced by benzoic acid modification. The treated biochar exhibited an improved surface chemistry, facilitating the formation of oxygen-containing groups acting as efficient sorbents for toluene and p-xylene molecules. Toluene adsorption increased from 32.5 mg/g (WS) and 27.6 mg/g (HW) to 125.3 mg/g (AWSBA3) and 83.3 mg/g (AHWBA3). P-xylene adsorption, higher due to its greater molar mass, remained elevated in modified samples. The treated biochar displayed promising regeneration potential of 88.4 %–97.6 % over five adsorption–desorption cycles. The enhanced adsorption and reusability of the treated biochar demonstrate the potential of the proposed modification method to effectively remove aromatic VOCs at low costs.