Vanadium-Substituted Polyoxomolybdates for Methylene Blue Adsorption from Aqueous Solutions

The objective of this study is to assess the effectiveness of three vanadium-substituted polyoxomolybdates, [PMo11VO40]4−, [PMo10V2O40]5−, and [PMo9V3O40]6−, for the removal of methylene blue dye (MB) from aqueous solutions. The materials were synthesized and initially characterized by Fourier transform infrared spectroscopy (FTIR) to confirm purity. Then the effect of several experimental factors, such as contact time (0–55 min), adsorbent dose (0.003–0.2 g), initial dye concentration (2–35 mg/L), as well as pH (1–10) and temperature (293–313 K) of the reaction, were investigated in order to identify the optimum conditions for each parameter. Accordingly, MB concentrations were recorded before and after adsorption using an ultraviolet–visible spectrophotometer (UV–Vis) at 664 nm. Hence, the synthesized compounds, which are all negatively charged, showed high removal efficiency towards the cationic dye. Optimized results for 25 mg/L methylene blue exhibited a contact time of only 10 min for [PMo11VO40]4− and [PMo10V2O40]5−while only 5 min for [PMo9V3O40]6−. For [PMo9V3O40]6−, the percentage of removal decreases from 92.70 to 78.20% with increasing pH showing the suitability of acid pH.