Transforming olive mill solid waste: innovative biochar production for sustainable organic pollutant removal in olive mill wastewater treatment

Abstract

Olive mill wastewater (OMW) is characterized by a high organic load and complex composition, requiring efficient and sustainable treatment strategies. This study investigates the potential of biochar (BC) derived from olive mill solid waste (OMSW) as an adsorbent for the removal of total phenols (TPh) and chemical oxygen demand (COD) from OMW. OMSW-derived BC was produced by slow pyrolysis at 600°C for 5 h and characterized using Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The BC exhibited a carbon content of 67.21%, a BET surface area of 135.2 m 2 /g, and a total pore volume of 0.0055 cm 3 /g. Batch adsorption experiments were conducted to evaluate the effects of contact time, BC dosage, and temperature. Adsorption kinetics, isotherms, and thermodynamic parameters were evaluated using established models. TPh adsorption followed a pseudo-second-order kinetic model with an excellent fit (R 2 = 1), whereas COD showed a weaker fit (R 2 = 0.76), indicating more complex adsorption behavior. Increasing the BC dosage from 0.05 to 0.5 g resulted in only limited improvements in removal efficiency, with COD removal increasing from 73.79% to 77.45% and TPh removal from 75.87% to 79.30%. Temperature significantly affected adsorption performance. COD removal reached 75.61% at 25°C and 79.46% at 35°C. TPh removal from OMW increased from 0.12% at 25°C to 19.15% at 35°C and reached 24% at 45°C. For the gallic acid (GA) solution, TPh removal reached 72.28% at 35°C before declining at 45°C. Isotherm analysis showed that the Freundlich model provided a better description of TPh adsorption at elevated temperatures, with R 2 = 0.9833 at 45°C. Thermodynamic analysis indicated that TPh adsorption was spontaneous and endothermic (ΔH° = +71.37 kJ/mol), with negative entropy suggesting increased ordering at the BC surface. The results demonstrate that OMSW-derived BC has potential as a sustainable adsorbent for the treatment of OMW, particularly for the removal of COD and phenolic compounds. The conversion of OMSW into BC provides an approach for valorizing an agricultural waste stream while producing an adsorbent with useful physicochemical properties. These findings support the potential application of agricultural waste-derived BC in targeted OMW treatment and contribute to more sustainable wastewater management practices in the olive oil sector.