The Mechanism Of Oxalate-Promoted Iron Dissolution Of Hematite

Iron is omnipresent in nature.The iron cycle affects the geochemical cycle of redox active elements,and the migration and transformation of pollutants.The iron dissolution from iron(hydrogen)oxide is a key step in the iron cycle and iron bioavailability improvement.Various common organic acids promoted iron dissolution tremendously in nature,and have received widespread attention.Oxalic acid is one of the most common small molecule carboxylic acids in nature,which widely coexists with(hydrogen)iron oxide in water and soil,and promotes the iron dissolution.However,the details of iron dissolution process are still unclear.Moreover,compared to dark conditions,light irradiation increased the rate of oxalic acid-promoted iron dissolution significantly,but the roles of light irradiation in iron dissolution are still controversial.To understand the microscopic mechanism of iron dissolution from iron(hydrogen)oxide induced by oxalic acid and other small carboxylic acids,we investigated the interaction between oxalic acid and iron(hydrogen)oxide,and the effect of photogenerated carrier on the iron dissolution.The specific research contents are as follows:1.Taking the {001} exposed hematite nanoplates as a model,the iron dissolution rate and the adsorption capacity of oxalic acid on hematite surface were determined under various pH and oxalic acid concentration by batch experiments.The results confirmed that the iron dissolution rates of hematite were proportion to the adsorption amount of oxalic acid.Attenuated total reflection Fourier transform infrared(ATR-FTIR)spectroscopy and density functional theory(DFT)simulation proved oxalic acid adsorbed on the hematite surface in a bidentate mononuclear configuration.According to DFT calculation,part of electrons was transferred to surface Fe atoms after oxalic acid adsorbed on hematite surface.The electron transfer process was accompanied with spontaneous proton migration,which was proton removed from adsorbed oxalic acid to the surface hydroxyl groups.The proton migration process protonated the surface hydroxyl groups and further increased the charge density of surface Fe atoms.According to iron dissolution,in-situ electrochemistry,and in-situ ATR-FTIR of H/D exchange experiments,we clarified that the process of oxalic acid adsorption was a typical proton coupled electron transfer process(PCET).The density of states(DOS)analysis and Crystal Orbital Hamiltonian Population analysis(COHP)were carried out to quantify the electron donating ability of oxalic acid and the strength of the surface Fe-O bond.This work revealed the significante influence of proton on oxalic acid on iron dissolution through the PCET process for the first time,and shield light to understand the role of small natural carboxylic acid in promoting iron dissolution and providing biologically effective iron.2.On the basis of the above work,we investigated the iron dissolution way of hematite promoted by oxalic acid under visible light and the role of light irradiation on iron dissolution.In order to avoid the influence of photo-reduction process of iron-oxalate complexes in solution,we designed two experimental schemes:(1)Using EDTA to chelate solution Fe(?)so that Fe(?)can not form iron-oxalate complexes and can not be reduced to Fe(?)under visible light;(2)Changed the Xenon light to monochromatic LED light,controled the wavelength of LED light,and selected the non-photochemically active wavelength of iron-oxalate complexes in solution to investigate the photo-promoted iron dissolution process.The results confirmed that oxalic acid promoted hematite dissolution in a non-reduced way under light irradiation.The migration path of the photogenerated carriers was confirmed by fluorescence spectra,phosphorescence spectra and DFT simulation.It was proved that the Fe(?)reduction and oxalic acid decomposition both occurred in the solution.Molecular orbital simulations further explained the non-reduced iron dissolution in oxalate-hematite system and the roles of light irradiation in iron dissolution process.