Low-Molecular-Weight Organic Acids Mediated Photochemical Cycling Of Iron: Mechanism And Its Application In Water Treatment

With the rapid development of industrialization, how to control the seriousproblem of water pollution has become an important topic in the environmentalscience and technology. This paper expounds the water treatment by photo-Fentonreaction. Iron is the main reagent in photo-Fenton reaction, and different valencestates and forms of iron would play a very important role in the photo-Fenton reaction.Low-molecular-weight organic acids (LMWOAs) would paticipate in thephotochemical cycle of iron, when iron coexisted. In this work, the roles ofdicarboxylates (DCAs) in iron cycling and photo-Fenton reaction were investigated indetails. The main studies are as followed:(1) The dicarboxylic acids oxalate (Oxal) and malonate (Mal) are frequentlydetected as the final LMWOAs during oxidative degradation of aromatic compounds.Here a distinct effect of Oxal versus Mal on iron-based photocatalytic technologieswas reported by testing the degradation efficiency of the dye rhodamine B (RhB). Therates of dye degradation in irradiated mixture solutions depended on Fe(III/II)speciation, photoreactivities of Fe complexes and reactivities of Fe(II) complexeswith H2O2. Photolysis of the Fe(III)-oxalato complex was favorable due to theformation of O2-, HO2and OH for oxidizing the dye, however, the excess of H2O2could quench the excited state of ferrioxalate, decreasing the RhB degradationefficiency. In contrast, activities of UV/Fe(III) in the presence of Mal weresignificantly diminished because Fe(III)-Mal complexes, with much lower quantumyield of Fe(II) from photoreduction, dominated Fe(III) speciation. The results providedata for an understanding of the mechanism of iron redox (photo)chemistry mediatedby DCAs, which will aid in selecting appropriate Fe ligands, screening photo-Fentonconditions and designing UV/Fe3+treatability.(2) Reduction of carcinogenic hexavalent chromium (Cr6+) by Fe2+to essentiallynontoxic trivalent chromium (Cr3+) is a simple and effective method for Cr6+decontamination. In this study, the photoreduction efficacy of Cr6+in acidic Fe3+/Fe2+solutions was examined in the presence of dicarboxylic acids (i.e. Oxal, Mal, succinic acid (Suc), and glutaric acid (Glu)). Rates of Cr6+reduction under UV irradiationdepend on the complexes photochemistry of Fe3+/Fe2+with DCAs, following theorder of Oxal system> Suc system≈Glu system> Mal system. Fe(III)-oxalatocomplexes are favorable for transforming Cr6+due to the formation of relativeabundance of Fe2+and reductive radicals such as CO2-, O2-after photoreaction,whereas Fe(III)-Mal complexes upon UV irradiation generate the lowest amount ofFe2+among all the Fe-DCAs complexes, and thus have poor capacity for Cr6+reduction. The effects of Suc and Glu on the Cr6+reduction are insignificant since theFe(III)-OH complexes are still the main Fe(III) species due to their weak chelatingabilities with Fe3+. This work has important implications for selecting the favorableligands of Fe3+for light-induced Cr6+transformation and designing the methods fortreating wastewater with LMWOAs and Cr6+.(3) In order to diminish the inhibition of Mal on the photo-Fenton reaction, thisstudy examined its capacity to degrade aqueous organic contaminants and heavymetal mediated by the range of UV light irradiation and the LMWOAs coexisting. Wefound the shorter wavelength UV light could promote the photolysis of Fe(III)-Mal.The effects of pH, O2, and the concentrations of Oxal and Mal on cycling of Fe3+/Fe2+and the removal of Cr6+were examined with different range of wavelength lightirradiation. Meanwhile, The changes in Fe2+concentration, CHCOO-concentration,and the mineralization of organic ligands were also measured in the same system, anda possible pathway of Fe(III)-Mal photolysis by changing the wavelength range of theUV light was proposed. Moreover, RhB photocatalytic degradation in photo-Fentonreaction in the presence of Mal mediated by the other LMWOAs (i.e. Oxal, Mal, citricacid (Cit), tartaric acid (Tar), pyruvic acid (Pyr) and ethylene diamine tetraacetic acid(EDTA))was investigated. The wastewater treatment efficiency was effected by theFe(III) species in the presence of DCA and its photochemistry. RhB degradationUV/Fe3+/Mal could be highly promoted by adding Oxal as Oxal has a higherclathrating ability than Mal and the higher photoactivity of Fe(III)-oxalato complexes.Whereas the addition of Pyr and EDTA has no significant effect on the RhB removal,it was attributed to the poor clathrating ability of Pyr and Fe3+and the photoactivity ofFe(III)-EDTA complexes. In addition, a relative less RhB degradation mediated by Citand Tar.