Adsorption of Phenol Using Eucalyptus saligna Biochar Activated with NiCl₂

The treatment of produced water (formation water) still represents one of the greatest challenges in the petroleum industry. Among the various pollutants found in produced water, phenol is of particular concern, due to its high toxicity. The aim of this work was to synthesize a Eucalyptus saligna sawdust biochar activated with NiCl₂, followed by evaluation of its capacity for the adsorption of phenol. The results confirmed that the chemically activated adsorbent showed higher phenol removal efficiency compared to a non-activated one, possibly due to the interactions involving new and/or intensified functional groups. The adsorbents were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD), Brunauer–Emmett–Teller (BET), and Barrett, Joyner, and Halenda (BJH). Excellent adsorption capacity of the NiCl₂-activated biochar was achieved at different pH (2 to 9) of the phenolic solution, with the ideal dosage being 1 g L⁻¹. The adsorption kinetics could be fitted using the Elovich model, indicating that the adsorbent surface was heterogeneous. The adsorption isotherms indicated that multiple layers of the adsorbent were involved in phenol removal, in agreement with the Redlich-Peterson model with β_RP trending toward 0. The maximum adsorption capacity of the NiCl₂-activated biochar was 76.65 mg g⁻¹. Thermodynamic evaluation indicated that the adsorption was exothermic and physical. Additionally, study of the effect of ionic strength conducted with NaCl, showed no substantial changes for phenol adsorption. Finally, thermal treatment in an inert atmosphere (pyrolysis) enabled effective desorption, with regeneration efficiency maintained at around 70% after three cycles.