Experimental and Theoretical Investigation of Olive Mill Solid Waste Biochar for Vanillic Acid Adsorption Using DFT/B3LYP Analysis

This study discusses the preparation of biochar (BC) from Olive Mill Solid Waste (OMSW) on a semi-industrial scale, which can be utilized as an adsorbent to recover or remove phenolic compounds (vanillic acid). The preparation was carried out under optimized laboratory conditions (carbonization temperature, residence time, heating rate, atmosphere…). The obtained material was characterized using various techniques (BET surface area, Elementary analyses, proximate analyses, Organic Carbon). Different models were used to suit the adsorption isotherms (Langmuir, Freunlich, and Temkin…) and kinetics (pseudo-first-order, pseudo-second-order, and intra-particle diffusion…) of single phenolic acid. The experimental results correlate well with the pseudo-second-order and Langmuir models, with a maximum adsorption capacity (q_m = 57.47 mg.g⁻¹) and a correlation coefficient of R² = 0.999. To elucidate the activated sites on vanillic acid (VA) and the adsorption mechanism, Density Functional Theory (DFT) employing the B3LYP/6-311G basis set was utilized. Lower ΔE_gap values are linked to greater inhibition efficiencies. In this context, the calculations indicate that vanillic acid, with its significantly higher E_HOMO value of -1.3662, possesses a superior ability to donate electrons to the BC surface. Additionally, there is a concurrence between the experimental outcomes and the findings of this computational modeling.