Degradation of diphenamid (DPA) was examined by a novel process through sulfite activation by Fe impregnated N-doped TiO2 (FeN-TiO2) under visible LED (Vis LED). The FeN-TiO2 was synthesized using an impregnation method and characterized by various techniques. The mechanism of sulfite activation by FeN-TiO2 under Vis LED was proposed. The reaction mechanisms were further elucidated by investigating the XPS spectra of the catalysts before and after the reaction. Thirty intermediates were identified and twenty-four of them are newly reported. A new pathway was reported for the first time in the DPA studies through the rupture of benzene ring linkage. A higher mineralization degree was achieved using the FeN-TiO2/sulfite/Vis LED process, which is not in accordance with previous reports on sulfite-based processes. The absence of sulfate adducts could provide a rational explanation of the higher mineralization degree during DPA degradation. Based on reusability test, the DPA degradation efficiency increased after successive usage of the catalyst. After the complete degradation of DPA, the leached Fe-ions were found to be negligible and sulfite was completely depleted. Considering several factors such as the cheap source of sulfite (an air pollutant waste from flue-gas desulfurization process), low cost of Fe, negligible leaching of Fe-ions, and high energy efficiency of Vis LED light, the FeN-TiO2/sulfite/Vis LED process could be a practical and green technology for the removal of wastewater contaminants.