The Mechanisms For Degradation Of Refractory Organics In The Hematite/Sulfite System Under Photo Irradiation

Main ways to generate SO₄^·-contributing predominantly to contaminants removal are the activation of peroxydisulfate（PDS）or peroxymonosulfate（PMS）.Although satisfactory performance in pollutants removal has been achieved,the inherent drawbacks cannot be ignored.For example,the productions of PDS/PMS cost much;the peroxide ions are stable and can reside in waters,which induces unexpected reactions and poses threat to ecosystems;the materials utilized to as activators for PMS/PDS usually have environmental toxicity.All these drawbacks limit their wide industrial applications.Recently,employing sulfite to generate SO₄^·-has also drawn extensive attention.Compared with PDS and PMS,sulfite is cheap,eco-friendly because it can be oxidized to sulfate via the dissolved oxygen in the long term.Also,it is the byproduct of flue-gas desulfurization process in industry.All these make the research into sulfite activationto meaningful.Herein,a naturally abundant Fe oxide mineral named hematite was used to activate sulfite under photo irriadation,and various influencing factors were investigated;the reaction products were analyzed by high-performance liquid chromatography（HPLC）and high-performance liquid chromatography coupled with mass spectrometry（HPLC-MS）.Electron spin resonance（EPR）,quenching experiments and X-ray photoelectron spectroscopy（XPS）were employed to reveal underlying mechanisms.The main conclusions are listed as follows:1.In the hematite/sulfite/photo system,the main active species accounting for the pollutant degradation is different at different reaction pH.Combination of EPR,HPLC-MS techniques verifies the active oxygen species to be SO₄^·﹣with initial pH at 4-7.However,at alkaline conditions（pH 8-13）,the high valence iron（Fe（Ⅳ））is the intermediates,which is supported by the identification of the oxidation producs being methyl phenyl sulfone when using the methyl phenyl sulfoxide as the probe compound.2.The different coordination mechanisms of sulfite anions on the hematite surfaces are mainly responsible for the production of different active species.The sulfite group can be coordinated to a Fe center via the S or O atom,depending on the reaction pH.In alkaline conditions（pH>9）,surfur-bonded metal-sulfite complexes are formed,which is beneficial to the reaction of h⁺with Fe（Ⅲ）.In the acidic or the neutral media,e.g.pH=4-7,O-bonded metal-sulfite complexes were formed,and the reaction between h⁺and sulfite is preferable,thereby resulting in the generation of SO₄^·﹣to degrade organic pollutants.3.Innovatively,replacement of sodium sulfite with calcium sulfite greatly enhances the removal ability towards the organic pollutants.The low solubility of calcium sulfite makes the concentration of sulfite dissolved in the solution lower than that of system containing sodium sulfite,effectively avoiding the reaction between sulfite and ROS.Thereby the reaction rate between reactive species and targeted contaminant is increased.4.The hematite/sulfite/photo system demonstrates fascinating removal capacity towards a series of refractory organics even if after 5-6 cycles.