Effect of Pyrolysis Temperature and Residence Time on the Physicochemical Properties of Biochar Derived from Bio-Agricultural Olive Mill Solid Wastes

Olive mill solid waste (OMSW) is a major by-product of the agro-industrial sector and can be converted into biochar through pyrolysis, with potential applications in environmental management. This study investigated the effects of pyrolysis temperature (400, 500, and 600 °C) and residence time (1, 3, and 5 h) on the physicochemical, structural, and surface properties of OMSW-derived biochar. As the temperature increased, the biochar yield decreased from 32.08% at 400 °C (1 h) to 25.18% at 500 °C (1 h) and 24.67% at 600 °C (1 h). Surface area and pore development varied considerably with the processing conditions. At 500 °C, the BET surface area increased from 6.99 m2/g at 1 h to 44.19 m2/g at 5 h, accompanied by an increase in total pore volume from 0.006 to 0.026 cm3/g. At 600 °C, the maximum Langmuir surface area (201.6 m2/g) and total pore volume (0.075 cm3/g) were obtained at 1 h, whereas the BET surface area reached 135.23 m2/g at 5 h. These differences indicate that residence time influenced the measured surface area and pore characteristics differently. Moisture content reached 4.18% at 600 °C for 5 h, while ash content reached 10.83% under the same conditions. Moisture content varied with temperature and residence time, reaching 4.18% at 600 °C for 5 h. Ash content also reached its maximum value of 10.83% under these conditions, while electrical conductivity reached 0.077 mS/cm. All biochar samples exhibited negative zeta potentials, ranging from −22.72 to −32.21 mV at 400 and 500 °C, while values at 600 °C ranged from −23.53 to −29.89 mV. Elemental analysis revealed variations in the H/C and O/C atomic ratios, with H/C values ranging from 0.2468 to 0.5984 and O/C values ranging from 0.0369 to 0.296. These variations indicate differences in elemental composition and relative carbonization characteristics among the samples. Calcium and potassium were the predominant mineral elements. Overall, the findings provide a basis for further investigating OMSW-derived biochar for potential adsorption, soil-related, and nutrient-management applications, subject to application-specific experimental validation.