Photoreaction characteristics of ferrous oxalate recovered from wastewater

Photoreaction with the Fe(III)-oxalate complex has been reported to play an important role in various photochemical reactions in the natural atmospheric environment, and are applicable to treat various recalcitrant compounds in wastewater. We previously showed that a Fe(II) oxalate (FeOx) crystal can be recovered from the wastewater generated from soil washing, which can then be applied to the photoreaction for degradation of recalcitrant pollutants; however, photoreactions with FeOx compounds have not been fully reported yet. Therefore, this study aims to investigate the photoreaction characteristics of FeOx recovered from wastewater, to demonstrate the feasibility of its application to wastewater treatment. The physical and chemical properties of FeOx were characterized with X-ray diffraction, scanning electron microscopy/energy-dispersive X-ray spectrometry, and X-ray photoelectron spectroscopy analyses. The photoreaction of FeOx showed high methylene blue (MB) removal efficiencies similar to the reaction with TiO₂, indicating that FeOx is applicable to the photoreaction for degradation of pollutants. Furthermore, the photodegradation of MB with FeOx was observed under visible light, as opposed to TiO₂. MB could be removed by the photoreaction of FeOx under both basic and acidic pH conditions. Under basic pH conditions, MB could be removed by FeOx via both photoreaction and surface adsorption. The concentration of FeOx affected light penetration and Fe and oxalate levels in the solution, resulting in different MB removal kinetics. The photoreaction efficiency of FeOx could be affected by both photoreaction of Fe and oxalate in the solution and photoreaction and adsorption reaction of the FeOx surface.