Eco-Friendly Wastewater Treatment Using Biochar Adsorbents: Removal of Tyrosol and Hydroxytyrosol and Life Cycle Assessment

Olive mill wastewater (OMWW), a major by-product of olive oil production, is generated in large volumes throughout the Mediterranean region and contains high concentrations of phenolic compounds, notably tyrosol (TY) and hydroxytyrosol (HT). Owing to their high mobility and toxicity toward microorganisms and aquatic ecosystems, these compounds can disrupt soil processes and pose significant environmental challenges. In this study, olive stone-derived biochar was evaluated as a sustainable and low-cost adsorbent for the removal of TY and HT from aqueous solutions, with performance benchmarked against commercial activated carbon (AC). Comprehensive physicochemical characterization showed that the biochar possesses a high specific surface area (258 m²/g), well-developed porosity, and abundant surface functional groups favorable for phenolic adsorption. Batch adsorption experiments were conducted under controlled conditions (initial phenolic concentration of 50 mg L⁻¹, contact time of 1 h, and neutral pH) to investigate adsorption equilibrium, kinetics, and thermodynamics. Adsorption behavior was described using pseudo-first-order, pseudo-second-order, and Elovich kinetic models, as well as Langmuir and Freundlich isotherms. Regeneration experiments demonstrated the reusability of the biochar over multiple adsorption–desorption cycles. A life cycle assessment (LCA) was performed following a cradle-to-grave approach, encompassing biomass collection, biochar production, adsorption application, and regeneration/recovery stages. The environmental impact was evaluated using the global warming potential over a 100-year time horizon (GWP100, kg CO₂-eq). The environmental performance of olive stone biochar was compared with that of a commercial activated carbon subjected to identical adsorption experiments, enabling a consistent and fair comparison. Under the tested conditions, olive stone biochar achieved removal efficiencies of 87% for TY and 91% for HT, comparable to those obtained with AC. Overall, these findings highlight the strong potential of olive stone-derived biochar as an eco-friendly and high-performance alternative for sustainable OMWW treatment, with potential for phenolic recovery and clear relevance to circular economy strategies.