Solar-driven advanced oxidation processes for full mineralisation of azo dyes in wastewater

Mineralisation of synthetic azo dyes in industrial wastewater is an energy-intensive process in treatment technology. The Solar Thermal Electrochemical Process for advanced oxidation processes (STEP-AOPs) utilises solar energy and electricity for the activation and electrooxidation of organic pollutants to harmless, small and non-toxic molecules with no other energy consumption. Based on molecular structure and chemistry, the STEP-AOPs for the treatment of azo dyes in wastewater, as exemplified with a typical azo dye, methyl orange, is reported for the first time. Thermodynamic calculations of the temperature-dependent potentials of methyl orange demonstrate that Gibbs free energy decreased by 161kJmol(-1) and the potential decreased by 0.019V with an increase of temperature from 20 to 80 degrees C, which indicates that the drop in both energy and potential specifically fits the STEP-AOPs technique. Experimental results showed that the STEP-AOPs achieved a total organic carbon (TOC) removal of 95.6% for methyl orange. The TOC removal rate improved by 39.8% and the unit TOC electricity consumption decreased by 53.8% at 80 degrees C compared with conventional methods (20 degrees C). The mineralisation mechanism for methyl orange was a gradual shortening of the molecular chain through cleavage of the azo bond, breakdown of the benzene ring and formation of inorganic small molecules susceptible to be oxidised to non-toxic small molecules, and carbon dioxide via STEP-AOPs. The evidence shows that the STEP-AOPs is capable of mineralising azo compounds completely.