Electrode performance in hybrid electrocoagulation and biochar adsorption systems for olive mill wastewater treatment

Olive mill wastewater (OMW) is a major environmental concern in olive-producing regions because of its high organic load and phytotoxic phenolic content, which limit its safe discharge or reuse. This study aimed to develop and evaluate an integrated electrocoagulation-adsorption (ECAD) treatment using olive pomace (OP) biochar and to determine how electrode material influences the performance and cost of the combined process. Specifically, the study characterized OP biochar produced by pyrolysis and evaluated its adsorption behavior toward total phenols (TPh) and soluble chemical oxygen demand (CODsoluble), compared Al- and Fe-based EC and their integrated ECAD performance, and theoretically determined biochar dosages and estimated the operational cost of both configurations. The produced biochar exhibited a BET surface area of 135.23 m2/g, a mesoporous structure with an average pore width of 21.95 Å, a negative zeta potential of -29.89 mV, and a carbon content of 67.21%, confirming the formation of a carbon-rich and porous adsorbent. Isotherm analysis showed that TPh adsorption was best described by the Langmuir model, with a maximum adsorption capacity of 84.74 mg/g, whereas CODsoluble was best fitted by the Dubinin-Radushkevich (D-R) model. Standalone EC achieved comparable TPh and CODsoluble removal using Al and Fe electrodes (about 52-55%). However, the integrated systems showed a strong electrode-dependent response. The Al-ECAD system achieved maximum removals of 70.69% TPh and 71.39% CODsoluble, while the Fe-ECAD system reached 67.40% and 66.33%, respectively, at the lowest biochar dosage, with removal declining at higher dosages. The estimated operational costs were 4.25 and 5.30 US$/m3 for Al- and Fe-based systems, respectively. The novelty of this work lies in demonstrating that the effectiveness of biochar-assisted EC is strongly dependent on electrode-biochar compatibility, identifying electrode selection as a key design factor for ECAD treatment of OMW.