Process optimization of Venturi-based hydrodynamic cavitation for industrial azo-dye removal: Sodium chloride effect on performance, kinetics, and costs

Treatment of textile wastewater is challenging because dyes, salts, and organic matter can strongly affect advanced oxidation processes (AOPs). This study evaluated azo-dye decolorization in a 14 L hydrodynamic cavitation (HC) system previously characterized for nanobubble generation, focusing on sodium chloride (NaCl) effects on performance, kinetics, oxidant demand, and operating cost. Hydrogen peroxide (H2O2) and sodium persulfate (PS) were tested at pH 2–7 and oxidant-to-dye ratios of 50:1–230:1. HC was first assessed for Reactive Red 120 (RR120) and Congo Red (CR) in deionized water, while NaCl effects were investigated for RR120 at 2.5 g/L because CR precipitated under acidified saline conditions. In deionized water, RR120 decolorization reached 95% with both oxidants, whereas CR reached 94% with PS and 69% with H2O2. PS remained effective across the pH range, while H2O2 was most effective at pH 2. Concentration profiles showed an initial rapid decrease followed by one or two slower intervals, potentially reflecting reduced oxidant availability, mass-transfer limitations, competition from transformation products, and changing cavitation conditions. NaCl reduced RR120 decolorization under all investigated conditions, especially at pH 5–7, slowing down the initial and intermediate kinetic stages. Under saline conditions, H2O2 generally outperformed PS at equivalent oxidant-to-dye ratios. Screening-level cost analysis showed that PS-based systems were mainly constrained by oxidant cost, whereas H2O2-based systems were more sensitive to treatment time and energy demand. Overall, NaCl affected removal efficiency, kinetics, oxidant selection, dosage requirements, and treatment economics, identifying salinity as a key design parameter for HC treatment of textile wastewater.