The Process And Mechanism Of Facet Dependent Light Induced Oxalate Reductive Dissolution Of Hematite

Light induced oxalate reductive dissolution of hematite to release Fe(II)is the main source of bioavailable iron in nature,which significantly affects the migration of pollutants and the elemental geochemical cycle.The process of light induced oxalate reductive dissolution of hematite is facet dependent,but the current research fails to reveal the specific crystal facet dependent mechanism of this reductive dissolution process from the molecular level.In this study,three exposed(001),(012)and(104)crystal facets hematite were synthesized and the effect of crystal facets on light induced oxalate reductive dissolution of hematite included its microscopic controlled mechanism were discussed,which provide theoretical basis for insight understanding the facet dependent hematite mediated biogeochemical cycle of iron element.The main conclusions are as follows:(1)Three kinds of hematite with regular morphology,uniform size and different crystal facets were synthesized.Hematite nanoplates(HNPs)with main exposed crystal facet(001)have a regular hexagonal sheet structure,with the mean width and the thickness of?108and?11.5 nm,respectively.The specific surface area is 22.1 m² g^–1.Hematite nanocubes(HNCs)with main exposed crystal facet(012)are pseudo-cubic structures with length,width,and height of?28 nm.The specific surface area is 25.4 m² g^–1.Hematite rhombohedrals(HNRBs)with main exposed crystal facet(104)have a rhombic regular hexahedral structure with each side length of?120 nm.The specific surface area is 9.65 m²g^–1.(2)The light induced oxalate reductive dissolution of hematite is significantly affected by exposed crystal facet and p H.The order of reductive dissolution efficiency of hematite is HNPs(001)>HNCs(012)>HNRBs(104)under the same p H conditions.For example,the reductive dissolution efficiency of HNPs(001)within 11 h at p H 4.0 is 84.2%,while those of HNCs(012)and HNRBs(104)are 67.2%and 21.2%,respectively.As the p H increases,the reductive dissolution efficiency of hematite gradually decreases.When p H4.0 is increased to 5.5,the reductive dissolution efficiency of HNPs(001),HNCs(012)and HNRBs(104)have decreased by 95.4%,95.1%and 99.5%,respectively.(3)The reductive dissolution of hematite is mainly divided into“induction period”and“autocatalysis period”two stages,and the reductive dissolution rate of hematite is facet dependent.The reductive dissolution rate of hematite in the“induction period”is slower than that in the“autocatalysis period”,at this time,HNCs(012)showed the highest reductive dissolution rate.The reductive dissolution rate of the three crystal facets hematite are HNCs(0.026 m M m^–2 h^–1)>HNPs(0.024 m M m^–2 h^–1)>HNRBs(0.010 m M m^–2 h^–1).In the“autocatalysis period”,the reductive dissolution rate of HNPs(001)is the highest.The reductive dissolution rate of the three crystal facets hematite are HNPs(0.169 m M m^–2h^–1)>HNCs(0.120 m M m^–2 h^–1)>HNRBs(0.074 m M m^–2 h^–1).(4)The adsorption behavior of oxalate on hematite significantly affects the reductive dissolution rate of hematite in the“induction period”.The difference in the adsorption capacity and configuration of oxalate on different exposed crystal facet of hematite leads to the diversity of the reductive dissolution rate.The adsorption capacity of oxalate adsorbed on hematite is HNCs(1.40 m M m^–2)>HNPs(0.850 m M m^–2)>HNRBs(0.600 m M m^–2),which is consistent with the order of the reductive dissolution rate of hematite.Moreover,the bidentate mononuclear five-membered ring complexes formed on the crystal facet of HNCs(012)are more easily to promote the reductive dissolution of hematite than the monodentate mononuclear complexes formed on the crystal facet of HNPs(001)and HNRBs(104).(5)The reductive dissolution rate of hematite in the“autocatalysis period”is affected by the Fe(II)catalysis process,and the hematite(?)Fe(II)site dissolution is the rate-determining step.Due to the difference in crystal facets of hematite,the surface energy of specific crystal facet of hematite from low to high is HNPs(0.80 J m^–2)<HNCs(1.10 J m^–2)<HNRBs(1.45 J m^–2).Therefore,the order of the dissolution capacity of hematite(?)Fe(II)site is HNPs(001)>HNCs(012)>HNRBs(104),resulting in Fe(II)catalyzing different crystal facet hematite reductive dissolution rate is HNPs(0.169 m M m^–2 h^–1)>HNCs(0.120 m M m^–2 h^–1)>HNRBs(0.074 m M m^–2 h^–1).