Mechanism of\nSulfate Radical Formation on Activation\nof Persulfate Using Doped Metal Oxide and Its Role in Degradation\nof Tartrazine Dye in an Aqueous Solution

Degradation of tartrazine dye (TZD) was performed in\nthis study\nusing sulfate radicals (SO<sub>4</sub><sup>•–</sup>)\ngenerated from the activated sodium persulfate (SPS) using Fe<sub>3</sub>O<sub>4</sub>@PDA nanoparticles (NPs). The NPs were characterized\nby Fourier transform infrared (FTIR), vibrating sample magnetometer\n(VSM), X-ray diffraction (XRD), high-resolution scanning electron\nmicroscopy (HR-SEM), X-ray photoelectron spectroscopy (XPS), high\nresolution transmission electron microscopy (HRTEM), and energy-dispersive\nX-ray spectroscopy (EDX). The average particle size of the NPs was\n17.49 nm from XRD analysis. The presence of the C–N group at\n1129 cm<sup>–1</sup> in FTIR and 2.54% of the nitrogen element\nidentified from the EDX plot was evidence of successful doping of\npolydopamine (PDA). Superparamagnetic nature with a decrease in the <i>M</i><sub>s</sub> value to 42.015 emu/g after doping was determined.\nDoping was further confirmed by XPS analysis with binding energies\nat 399.68 and 400.99 eV. The average particle size from HRTEM analysis\nwas 21.47 nm with a lattice spacing of 0.30 nm. Turnover number (TON)\nand turnover frequency (TOF) values for Fe<sub>3</sub>O<sub>4</sub>@PDA were determined to be 3.72 and 0.0248 min<sup>–1</sup> with respect to different systems, respectively. Optimum conditions\nfor the Fe<sub>3</sub>O<sub>4</sub>@PDA/SPS system were 50 ppm TZD,\n0.9 g/L catalyst, 12 mM SPS, and pH 4 with 94.68% efficiency in 150\nmin. The inhibition effect of ions in TZD degradation followed the\norder humic acid 3<sup>–</sup> < Cl<sup>–</sup> < CO<sub>3</sub><sup>2</sup>. The kinetic model\nfitting the Fe<sub>3</sub>O<sub>4</sub>@PDA/SPS system has pseudo-second-order\nkinetics. Excess formation of SO<sub>4</sub><sup>•–</sup> produced hydroxyl radicals (<sup>•</sup>OH) that were identified\nby a scavenging test. Reusability of the NPs was performed for five\ncycles and showed acceptable degradation performance up to 82.47%.\nThe intermediates identified from gas chromatography–mass spectrometry\n(GCMS) proved the cleavage of azo and sulfonate groups of TZD with\nthe formation of nontoxic intermediates. Finally, the phytotoxicity\nassessment was studied using Vigna radiata (Green grams). The % decrease of the root, shoot, and seedling length\nfor the degraded dye solution was negligible. Hence, this study proved\nthat the Fe<sub>3</sub>O<sub>4</sub>@PDA/SPS system could effectively\ndegrade TZD with nontoxic intermediate formation.