Hydrodynamic cavitation coupled with zero-valent iron produces radical sulfate radicals by sulfite activation to degrade direct red 83.

In the present research, hydrodynamic cavitation (HC) and zero-valent iron (ZVI) were used to generate sulfate radicals through sulfite activation as a new source of sulfate for the efficient degradation of Direct Red 83 (DR83). A systematic analysis was carried out to examine the effects of operational parameters, including the pH of the solution, the doses of ZVI and sulfite salts, and the composition of the mixed media. Based on the results, the degradation efficiency of HC/ZVI/sulfite is highly dependent upon the pH of the solution and the dosage of both ZVI and sulfite. Degradation efficiency decreased significantly with increasing solution pH due to a lower corrosion rate for ZVI at high pH. The corrosion rate of ZVI can be accelerated by releasing Fe ions in an acid medium, reducing the concentration of radicals generated even though ZVI is solid/originally non-soluble in water. The degradation efficiency of the HC/ZVI/sulfite process (95.54 % + 2.87%) was found to be significantly higher under optimal conditions than either of the individual processes (<6% for ZVI and sulfite and 68.21±3.41% for HC). Based on the first-order kinetic model, the HC/ZVI/sulfite process has the highest degradation constant of 0.035±0.002 min. The contribution of radicals to the degradation of DR83 by the HC/ZVI/sulfite process was 78.92%, while the contribution of SO and OH radicals was 51.57% and 48.43%, respectively. In the presence of HCO and CO ions, DR83 degradation is retarded, whereas SO and Cl ions promote degradation. To summarise, the HC/ZVI/sulfite treatment can be viewed as an innovative and promising method of treating recalcitrant textile wastewater.