Study of methylene blue removal and photocatalytic degradation on zirconia thin films modified with Mn-Anderson polyoxometalates

Abstract

Recalcitrant pollutants are challenging to degrade during water treatment processes. Methylene blue (MB), a cationic dye, is particularly resistant to degradation and is environmentally persistent. Heterogeneous photocatalysis has emerged as a suitable strategy for removing such pollutants from water. In this work, ZrO₂ thin films were modified with Anderson-type Mn-polyoxometalate (MnPOM) ((NH₄)₃[MnMo₆O₂₄H₆]), and the efficiency of MB removal from water was studied. ZrO₂ was synthesized by a sol–gel method, with thin films deposited using the doctor blade method, and ZrO₂ thin films were modified using chemisorption method. The synthesized materials were characterized using SEM, EDX, UV-Vis diffuse reflectance spectroscopy and FTIR. The adsorption kinetics and isotherms for MB were studied for both bare ZrO₂ and ZrO₂/MnPOM composites. Optical characterization showed a band gap energy of 4.02 eV for bare ZrO₂, while the ZrO₂/MnPOM composite exhibited a band gap of 3.7 eV. Furthermore, ZrO₂ showed lower MB removal capacity (∼8%) than ZrO₂/MnPOM thin films (∼29%). The isothermal adsorption studies indicated that MB adsorption onto both bare ZrO₂ and ZrO₂/MnPOM followed the Langmuir adsorption model (q_m = 20.6 mg g⁻¹ for ZrO₂ and q_m = 62.9 mg g⁻¹ for ZrO₂/MnPOM). Furthermore, the adsorption kinetics of MB were well described by a pseudo-second-order model. Photocatalytic testing under UV irradiation showed an apparent rate constant (k_ap) of 2 × 10⁻³ min⁻¹ for bare ZrO₂ and a value of k_ap 5.4 × 10⁻³ min⁻¹ for ZrO₂/MnPOM after 100 minutes. TD-DFT calculations revealed an LMCT interaction between the ZrO₂ nanoparticle and the MnPOM, which likely contributes to the enhanced photocatalytic activity of the ZrO₂/MnPOM composite.