Enhanced photocatalytic degradation of oily wastewater using a sustainable ZnO-biochar nanocomposite fabricated from Leucaena leucocephala seed pods biomass

Oily wastewater represents a major environmental challenge due to its complex composition and resistance to conventional treatment methods. In this study, biochar was synthesized using sustainable Leucaena leucocephala seed pods (LLSP) as a biomass source. Then, the LLSP/ZnO nanocomposite was fabricated through a co-precipitation followed by a pyrolysis approach to evaluate its effectiveness in eliminating oily wastewater via photocatalytic degradation. Characterization techniques, including X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Energy-Dispersive X-ray Spectroscopy (EDX), Brunauer–Emmett–Teller (BET), UV –Visible Spectroscopy (UV-Vis), and Fourier Transform Infrared Spectroscopy (FTIR), were used to examine the integration of ZnO into the biochar matrix. Response Surface Methodology (RSM) with a Central Composite Design (CCD) was employed to optimize the operating parameters represented by LLSP/ZnO dose, pH of OWW, and the initial concentration of OWW for maximizing oil and COD removal efficiencies. The results revealed that LLSP/ZnO demonstrated outstanding degradation efficiency under both UV and solar irradiation within 90 min, leading to a substantial reduction in oil content and COD in the wastewater samples. Based on RSM results, the maximum removal efficiency of oil was 98% at optimal operating conditions of pH: 2, catalyst dose: 0.09 g, and initial oil concentration: 50 mg/L. In addition, the maximum removal efficiency of COD was 97.45% at optimal operating conditions of pH: 2, catalyst dose: 0.13 g, and initial COD concentration: 1765 mg/L, under UV irradiation. A slight increase in efficiencies (97.5% for oil and 99.66% for COD) was observed under sunlight irradiation. The degradation of oil and COD followed pseudo-first-order reaction kinetics, with rate constants of 0.0252 min−1 and 0.022 min−1, respectively. The photocatalyst can be reused for up to five cycles while retaining an efficiency above 60%. The findings highlight the potential of employing a low-cost, biomass-derived photocatalyst for the treatment of oily wastewater via photocatalytic degradation.