Comparative valorization of olive pomace via ZnCl2 and H3PO4 activation: toward high-performance adsorbents for phenol removal.

This study focuses on the valorization of olive pomace from an olive oil extraction industry in Kabylia (Algeria) for the sustainable production of high performance activated carbons. Olive pomace was chemically activated using ZnCl2 and H3PO4, followed by carbonization at 500 °C, yielding OP-AC(ZC) and OP-AC(PA) respectively. This work presents a novel comparative approach by evaluating two widely used activating agents under identical preparation conditions, allowing a direct assessment of their influence on the physicochemical properties and adsorption performance of the resulting materials. Comprehensive characterization (SEM-EDS, BET, XRD, FTIR) revealed a significant enhancement of textural properties after activation. BET surface areas increased from 48 m2 g-1 (biochar) to 430.9 m2 g-1 for OP-AC(ZC) and 1012.6 m2 g-1 for OP-AC(PA). The point of zero charge (pHpzc) was determined at 6.34 and 3.85, respectively. XRD analysis confirmed the predominantly amorphous nature of the carbons with slight structural differences induced by the activation agents, while FTIR and SEM-EDS results evidenced the presence of oxygen- and phosphorus-/zinc-containing functional groups contributing to surface reactivity. A parametric study on phenol adsorption demonstrated that the natural solution pH offered the highest removal efficiency for both adsorbents. The optimum adsorption performance was achieved at the natural solution pH (≈6.5), using adsorbent doses of 30 mg for OP-AC(ZC) and 20 mg for OP-AC(PA), highlighting the favorable effect of near-neutral conditions and appropriate adsorbent dosage on phenol removal. The adsorption performance was further influenced by adsorbent dose and initial phenol concentration, highlighting the role of surface availability and mass transfer limitations. Increasing the initial phenol concentration enhanced adsorption capacity while reducing removal efficiency. Adsorption modelling using Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherms showed that the Langmuir model best described the adsorption process. Maximum monolayer capacities reached 143.54 mg g-1 for OP-AC(ZC) and 269.03 mg g-1 for OP-AC(PA). Comparative analysis with literature data indicates that the prepared materials exhibit competitive to superior adsorption capacities, particularly for the H3PO4-activated carbon. Overall, the results confirm that olive pomace is an efficient low-cost precursor for producing activated carbon with excellent performance toward phenol removal.