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.