Continuous stirred-tank degradation of persistent dyes via recycled iron/periodate oxidation system: process optimization

This work introduces a sustainable and efficient continuous-flow process for the degradation of persistent dyes, employing a continuous stirred tank reactor (CSTR) integrated with a recycled iron rod (rIR) that simultaneously serves as a mechanical agitator and a catalyst source. The system relies on Fe(II)-mediated activation of periodate (PI) to drive oxidative degradation of persistent textile dyes without the need for external iron dosing. Process performance was systematically evaluated under varying conditions, including PI flow rate (40–300 µL/s), submerged rod length (1–8 cm), rotation speed (0–500 rpm), dye concentration (5–40 mg/L), pH (3–6), and dye flow rate (1.20-1.83) mL/s. High conversion efficiency (up to 100%) was achieved under moderate PI flow, extended rIR immersion, increased rotation speed, acidic conditions, and low inlet dye concentrations (5–10 mg/L). pH values superior than 3 significantly hindered Fe(II) release and diminished degradation efficiency. Water matrix effects revealed minimal interference in mineral water but marked inhibition in river and seawater due to competing action. Comparison with external Fe(II)/PI systems showed that rIR releases Fe(II) in the range of 10–40 µM, depending on oxidant availability and hydrodynamic conditions. The system likely operates through a non-radical mechanism involving high-valent Fe(IV) = O intermediates. Overall, the rIR/PI process presents a low-cost and environmentally friendly strategy for continuous dye removal from lightly polluted water sources.