Exploring the thermal degradation of pine nut shells: a study on biochar production and its efficacy in cationic dye adsorption from water

The thermal degradation kinetics of pine nut shells (PS) (Araucaria angustifolia) was investigated at heating rates of 3, 5, 15, 20, and 25 °C.min−1 using thermogravimetry (TGA) and differential thermogravimetry (DTG) in the temperature range from 30 to 700 °C. The pyrolysis products were characterized via FTIR, SEM/EDS, and calorific value analysis. The isoconversional methods of Ozawa–Flynn–Wall (OFW), Kissinger–Akahira–Sunose (KAS), and Friedman (FR) provided the activation energy (Eα) values which vary from 71.9 to 171.7 kJ.mol−1, 68.4 to 164.0 kJ.mol−1, and 61.5 to 167.9 kJ.mol−1, respectively. The calorific value of five biochars produced in the temperature range of 221 to 488 °C ranged from 18.79 to 26.66 MJ.kg−1, respectively, in line with the values reported in the literature. DTG analysis revealed that low molecular weight compounds exhibited the initial degradation peaks, followed by hemicellulose, cellulose, and lignin biochars prepared at temperatures exceeding 309 °C exhibit promising potential as fuel materials for furnaces and boilers, boasting high carbon content, low ash content, excellent thermal stability, higher combustion temperatures, and a relatively high average energy content (≥ 24 MJ.kg−1). Additionally, our investigation into the adsorption capacity of the five biochars for methylene blue dye revealed significant potential, with the highest observed quantity reaching 53.9 mg.g−1 for biochar BC221. The results indicated that it is feasible to prepare biochars with adjustable properties from pine nut shells through a pyrolysis process, adding commercial value and reducing the environmental impact caused by improper disposal of toxic organic contaminants.