Synergistic strategies for phenol removal from olive mill wastewater (OMWW): A combined experimental and theoretical investigation using Chlorococcum sp.-derived CuO nanoparticles

Metal nanoparticle biosynthesis using micro-organisms has emerged as a clean and eco-friendly option as compared to chemical methods. This study demonstrates eco-friendly CuO nanoparticle synthesis using Chlorococcum sp. microalgal cell lysate supernatant (CLS) as a reductant. Design-Expert software was employed to optimize CuO nanoparticle synthesis, considering CuSO₄·5H₂O:CLS ratio, CuSO₄·5H₂O concentration, and pH. CuO nanoparticles were characterized and used to form sodium alginate (SA)-CuO nanoparticle beads (CuO-SA beads) through a cross-linking step, exhibiting crystalline monoclinic phases with an average size of 22 nm. The best synthesis yield (94%) of CuO nanoparticles was obtained at pH 10, 2 mM CuSO₄·5H₂O and CuSO₄·5H₂O/CLS ratio of 4:1. These beads showed high phenol removal in batch and fixed-bed column adsorption tests, with a capacity of 444.45 mg/g in fixed-bed column tests using olive mill wastewater (OMWW) with a phenol concentration of 4247 mg L⁻¹. Batch and fixed-bed column adsorption of phenol tests were conducted to evaluate the adsorption capacity of CuO-SA beads, and adsorption tests showed high phenol removal capacity, fitting well with pseudo-second-order and Langmuir models. Over five consecutive cycles, regeneration of the CuO-SA beads reduced the removal rate from 50% to 30% at the same phenol concentration. Density functional theory (DFT) analysis revealed chemisorption dominance and hydrogen bonding interactions between phenol and SA-CuO bead surfaces.