One-step H3PO4-activated pyrolysis of cocoa (Theobroma cacao) shell waste: insights into product characteristics and adsorption performance

Abstract

The conversion of agricultural residues into high-performance adsorbents offers a sustainable approach to treating dye-contaminated wastewater. Cocoa shell (CS), a major agro-industrial byproduct of Theobroma cacao, contains approximately 21.20% cellulose, 20.12% hemicellulose, and 51.93% lignin, making it a promising precursor for porous carbon material (PCM). However, the high lignin and cellulose contents can promote excessive tar formation during conventional pyrolysis, leading to pore blockage and diminished adsorption performance. In this study, a one-step H3PO4-activated pyrolysis strategy was developed to suppress tar formation and enhance pore development and surface functionality in CS-derived PCM. The precursor was impregnated with H3PO4 at a mass ratio of 3:1 (H3PO4:CS) and pyrolyzed under oxygen-limited conditions at 400 and 600°C. The prepared materials were characterized in terms of their surface morphology and chemical functionalities using scanning electron microscopy (SEM) and FTIR spectroscopy. Pore characteristics were examined using a QuantaChrome Nova 1200 analyzer. The PCM prepared at 400°C exhibited a well-developed porous structure, with a BET surface area of 954 m2 g−1 and a pore diameter of 2.01 nm, resulting in a high malachite green (MG) removal efficiency of 98% at pH 11. The findings indicate that PCM produced from CS possesses considerable potential as an efficient and sustainable adsorbent for the removal of dyes from wastewater.