Methyl orange degradation and hydrogen production simultaneously by a reverse electrodialysis−electrocoagulation coupling system powered by salinity gradient energy

Abstract

To produce hydrogen and treat dye wastewater simultaneously, a salinity gradient energy (SGE) driven reverse electrodialysis（RED）− electrocoagulation (EC) coupling system was designed and investigated experimentally. Energy conversion was achieved by means of the ionic electromigration (across the membranes) and redox reaction (on the electrodes). The iron anode electrode was electro-dissolved and releases Fe2+ (to be oxidized to Fe3+), and OH– was produced at cathode electrode, which were combined to be the flocculent. Additionally, hydrogen was produced at the cathode. Experiments were conducted to investigate the effects of electrodes (surface, material, and structure) and concentration gradients (from 10:1 to 250:1) on dye methyl orange (MO) degradation and hydrogen production. It results that the use of iron electrode as anode is significantly more efficient than the aluminum electrode for MO degradation, but inversely for hydrogen production; as for cathodes, the largest MO degradation rate and hydrogen production are achieved by means of the titanium mesh electrode and titanium plate electrode respectively. When the working current is constant at 0.25 A, the MO degradation rate and hydrogen production are both decreased with the increase of the concentration gradient. When the experiments are carried under the short-circuit current condition, the MO degradation rate reaches 98.1 % in 70 min at the concentration gradient of 50:1, and the maximum hydrogen production is 294.15 ml·h−1 at the concentration gradient of 25:1. Besides, the evolutions of the following parameters have been observed, including the current of RED stacks, pH of solution, and dissolution of electrodes.