Capillary-driven flow combined with electric field and Fenton reaction to remove ionic dyes from water or concentrated NaCl solution: Mechanism and application

A strategy to integrate capillary flow, electric field and Fenton reaction was developed to remove ionic dyes from water or concentrated NaCl solution for the first time. Tissue paper was used to construct capillary flow. The flow rates of water and crystal violet (CV) through it were measured, and then fitted with Lucas-Washburn model, Bosanquet model, and the fixed-bed adsorption model, respectively. Subsequently, an external electric field along the paper channels was introduced to prevent CV from flowing through the paper by electrostatic repulsion. Meanwhile, energy consumption of the integrated process was very low. For a concentrated NaCl (above 1 wt%) and dye solution, HClO formed by the electrolysis of Cl^- ions further degraded CV. The relationship between voltage and NaCl concentration in the separation process was also investigated. Moreover, Fenton reaction was introduced to regenerate MnO₂-coated paper and to enable the reutilization of the polluted device. Finally, removal of three ionic dyes (CV, methyl blue, and methylene blue) in water or 5 wt% NaCl solution were tested with a large paper device, in which the permeate was automatically collected by gravity. All the experiments showed 100% dye rejection with a water flow rate of 10–18 mL/h (at a cross-section area of 25 × 10⁻⁶ m²) and an energy consumption of 0.5–0.8 kwhm⁻³ to treat 100 ppm dye solution, and flow rate of 9–11 mL/h, energy consumption of 4.8–5.2 kwhm⁻³ to treat a solution of 100 ppm dye and 5 wt% NaCl. Overall, the results demonstrate the significant potentials of this new strategy for dye removal application.