Cr(VI)-Mediated Homogeneous Fenton Oxidation for Decolorization of Methylene Blue Dye: Sludge Free and Pertinent to a Wide pH Range

Abstract

Fe(II)-mediated Fenton process is commonly employed for oxidative degradation of recalcitrant pollutants in wastewater. However, the method suffers from limitations like narrow working pH range and iron sludge formation. The present work deals with the degradation of Methylene Blue (MB) dye using Fenton-like oxidation by replacing Fe(II) with Cr(VI), which eliminates the limitations of classical Fenton oxidation. The Fenton-like oxidation of MB is brought about by HO^• radicals generated by the disproportionation of chromium-coordinated peroxo complexes. It was observed that the working pH range for the Cr(VI)-mediated Fenton oxidation was 3-10, and no sludge formation takes place up to four cycles as the oxidation remains in the pure solution phase. The complete mineralization of dye was confirmed by observing the decay of MB peaks by a spectrophotometer and cyclic voltammetry. The reaction parameters like pH of the solution, temperature, degradation time, concentrations of H₂O₂, Cr(VI), and MB were studied for optimal performance of the Cr(VI) as the catalyst. Kinetic studies revealed that the Cr(VI)-mediated Fenton reaction follows pseudo-first-order reaction kinetics and depends on the concentration of HO^• radicals. The proposed Cr(VI)-mediated Fenton oxidation in the present work is best suited for the degradation of organic dyes by adding H₂O₂ as a precursor in chromate-contaminated wastewaters.

Figure 1

MB degradation in only Cr(VI), only H₂O₂, and Cr(VI)+H₂O₂ system at different time intervals (pH = 6; [H₂O₂] = 19.4 mM; [Cr(VI)] = 3 mM; [MB] = 15.7 μM; and temperature = 298 K). In the plot, A represents the absorbance at any given time “t” and A₀ represents the initial absorbance.

Figure 2

Cyclic voltammogram of MB in the Cr(VI)/H₂O₂ system before and after degradation (pH = 6; [H₂O₂] = 19.4 mM; [Cr(VI)] = 3 mM; [MB] = 15.7 μM; and temperature = 298 K).

Figure 3

Effect of pH on MB degradation in the Cr(VI)/H₂O₂ system ([H₂O₂] = 19.4 mM; [Cr(VI)] = 3 mM; [MB] = 15.7 μM; and temperature = 298 K).

Figure 4

Effect of catalyst [Cr(VI)] concentration on MB degradation in the Cr(VI)/H₂O₂ system (pH = 6; [H₂O₂] = 19.4 mM; [MB] = 15.7 μM; and temperature = 298 K). In the plot, “A” represents the absorbance at any given time “t” and “A₀” represents the initial absorbance.

Figure 5

Effect of H₂O₂ concentration on MB degradation in the Cr(VI)/H₂O₂ system (pH = 6; [Cr(VI)] = 3 mM; [MB] = 15.7 μM; and temperature = 298 K). In the plot, “A” represents the absorbance at any given time “t” and “A₀” represents the initial absorbance.

Figure 6

Effect of catalyst/oxidant ratio [Cr(VI)]/[H₂O₂] on MB degradation in the Cr(VI)/H₂O₂ system (pH = 6; [Cr(VI)] = 3 mM; [MB] = 15.7 μM; and temperature = 298 K).

Figure 7

UV–visible spectrum of MB in the Cr(VI)/H₂O₂ system before and after degradation (pH = 6; [H₂O₂] = 19.4 mM; [Cr(VI)] = 3 mM; and temperature = 298 K).

Figure 8

Effect of temperature on MB degradation in the Cr(VI)/H₂O₂ system (pH = 6; [H₂O₂] = 19.4 mM; [Cr(VI)] = 3 mM; and [MB] = 15.7 μM).

Figure 9

Schematic representation of complex chemistry between chromate and hydrogen peroxide.

Figure 10

UV–visible spectra of MB at different time intervals in the Cr(VI)/H₂O₂ system. Reaction condition: [Cr(VI)] = 3 mM, [H₂O₂] = 19.4 mM, [MB]= 15.7 μM, temperature = 298 K; and pH = 6.

Figure 11

(a–d) UV spectrum of MB before and after degradation in first, second, third, and fourth cycles, respectively. (e) Bar graph of four degradation cycles along with percentage degradation of MB in each cycle (pH = 6; [H₂O₂] = 19.4 mM (total cycles); [Cr(VI)] = 3 mM (total cycles); [MB] = 15.7 μM (per cycle); and temperature = 298 K).