Synthesis of Black-TiO₂ and manganese-doped TiO₂ nanoparticles and their comparative performance evaluation for photocatalytic removal of phenolic compounds from agro-industrial effluent

In this paper, the synthesis, characterization, and comparative performance of Black-TiO₂ and manganese-doped TiO₂ (Mn-TiO₂) nanoparticles for photocatalytic removal of phenolic compounds from agro-industrial effluent known as treated palm oil mill effluent (TPOME) are reported. The Black-TiO₂ nanoparticles were synthesized via a green synthesis approach using a renewable chemical, glycerol as a reducing agent. The Mn-TiO₂ nanoparticles were synthesized via the wet impregnation method using potassium permanganate (KMnO₄) as a precursor. Experimental results revealed that both types of TiO₂ nanoparticles, Black-TiO₂ and Mn-TiO₂ nanoparticles were anatase phase with a particle size range from 30–80 nm, improved visible light absorption and narrow bandgap of 2.96 and 2.12 eV, respectively. The improved visible light absorption was ascribed to the presence of Ti³⁺ defect states in Black-TiO₂ nanoparticles and the substation of Mn into TiO₂ matrix in Mn-TiO₂ nanoparticles. The improved visible light absorption led to the enhanced photocatalytic performance of Black-TiO₂ and Mn-TiO₂ nanoparticles. The former was able to remove 48.17% whereas the latter removed 39.11% of 224.85 mg/L of phenolic compounds from TPOME under 180 min of visible light irradiation. The Black-TiO₂ and Mn-TiO₂ nanoparticles showed 2.2-fold and 1.7-fold higher performance, respectively, than the Pure-TiO₂ nanoparticles. The Black-TiO₂ nanoparticles exhibited superior photocatalytic performance, and the highest reaction rate constant (K_app = 0.31127) was achieved which is two-fold higher than the one obtained by Pure-TiO₂ nanoparticles (K_app = 0.14733). The recyclability test showed that Mn-TiO₂ nanoparticles were more stable indicated by their negligible loss (1.55%) of photoactivity after five repeated cycles.