Effect Of Low Molecular Weight Organic Acids On The Transformation And Stability Of Iron Oxides In Soil

Iron oxides with various species and extra specific surface area,are important components of soil.Their structure and properties are susceptible to external factors.Low molecular weight organic acids?LMWOA?are widely distributed in the soil,especially in rhizosphere.LMWOA influence the flocculation,dispersion,transport,adsorption,dissolution and crystallization processes of iron oxides via the change in their surface chemical properties,which plays important roles in controlling the concentration and speciation of nutrient,pollutant elements as well as their geochemical behavior and bioavailability.Thus,the interactions between iron oxides and LMWOA affect the surface reactivity of soil,determining the formation and transformation of iron oxides.However,up to now,little is known about how LMWOA influences the formation,transformation,and stabilization processes of iron oxides.In this paper,ferrihydrite and goethite were selected as the research subject.LMWOA was regarded as the key variable.The molecular-level insights into the impacts of LMWOA on the formation,transformation,and stabilization processes of iron oxides were obtained by means of batch-dissolution and column experiments,form which the samples were characterized by the modern analytical techniques.These different methods of characterization include dynamic light scattering?DLS?,high resolution transmission electron microscopy?HRTEM?,X-ray diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?,and Fourier transform infrared spectroscopy?ATR-FTIR?.Meanwhile,DFT molecular simulation and mathematical modeling of the dissolution kinetics based on Noyes-Whitney equation were also carried out.The main conclusions in this paper are as follows:1.Making clear that the effect of oxalate amount on generation and stabilization processes of colloidal ferrihydrite particles.According to the HRTEM image,the sizes of ferrihydrite nanoparticles were about 2.5–5 nm,and the particles could be considered to have grain-to-grain contact in the aggregates.At a relatively small addition of oxalate?Oxalate/Fh=0.01-0.2?,the chemical analysis and DLS results show that the large ferrihydrite aggregates were disaggregated into colloidal particles with hydrodynamic diameters of 174–116 nm.Modelling of dissolution kinetics revealed that colloid stabilization was most pronounced at Oxalate/Fh=0.01-0.2.With increasing addition of oxalate?Oxalate/Fh=0.5?,the aggregates or larger colloidal particles were further disaggregated into smaller colloidal particles with hydrodynamic diameters of 64–35 nm,accompanying the increasing the amount of dissolved Fe ions.Modelling of dissolution kinetics revealed that higher ratios led to enhanced dissolution of both colloidal and larger aggregated fractions.2.Column flow-through experiments were carried out to further investigate the ferrihydrite dynamic dissolution.Fe was measurable by eluting with a 1 mmol/L oxalate solution at 6 pore volumes?PVs?and then gradually reached a plateau at approximately 8 PV.A similar trend was observed for dissolved Fe.The colloidal Fe was calculated from the difference between the total Fe and the dissolved Fe,which was approximately 60%of the total Fe,indicating that the Fe released from the media column was primarily colloidal.Fe was measurable by eluting with a 5 mmol/L oxalate solution at 8 pore volumes?PVs?.The data showed a smaller difference between the total and dissolved Fe,suggesting the Fe released from the media was primarily dissolved at a higher concentration of oxalate.In the other flow-through experiments,the column was continuously flushed by different concentrations of the oxalate solution.When the column was eluted with a 1 mmol/L oxalate solution,the total Fe remained at a stable concentration and the colloidal Fe was approximately60%of the total Fe.When the concentration of oxalate was increased,the total Fe rapidly increased to approximately 5%of the colloidal fraction.When the column was eluted with a 1 mmol/L oxalate solution again,the total Fe decreased and the colloidal Fe increased correspondingly.These results suggest that a chemical perturbation resulting from the change in oxalate amount,could lead to different environmental behavior for iron hydroxides.3.Determing that the roles of oxalate surface complex species in the dissolution of iron hydroxides and the generation of colloidal particles.ATR-FTIR results show that at least four different species were present at or near the ferrihydrite surface in the process of ferrihydrite aggregate dissolution.?1?inner-sphere species with binuclear bidentate binding geometry(1434 cm^-1)at the oxalate/Fh ratio of 0.01–0.1;?2?inner-sphere species with mononuclear bidentate binding geometry(1390 cm^-1)at the oxalate/Fh ratio of 0.2–0.5;?3?outer-sphere species(1309 cm^-1)at the oxalate/Fh ratio of 0.1–0.5;?4?aqueous oxalate species in the diffuse swarm of ions(1570 cm^-1).The increasing oxalate amount or the extension of reaction itime results in oxalate molecules were bound to ferrihydrite surface,in this order:aqueous oxalate species?outer-sphere species?binuclear bidentate inner-sphere species?mononuclear bidentate inner-sphere species.For these oxalate species,binuclear bidentate complexes mainly caused electrostatic repulsion between particles,resulting in the disaggregation of large ferrihydrite aggregates into colloidal particles;while the mononuclear bidentate oxalate complexes promoted the dissolution of ferrihydrite colloids into dissolved Fe–oxalate ions.These results provide on the molecular scale insight into impacts of LMWOA on the formation,transformation,and stabilization processes of iron oxides.4.Quantum chemistry calculations based on density functional theory?DFT?and universal force-field were carried out to reveal that the dissolution characteristics of goethite?110?face.Assuming slab models represent ideal-goethite?110?face,and cluster models represent defect-goethite?110?face.The simulation results show that for ideal-goethite?110?face,no oxalate-enhanced dissolution was observed via ligand–exchange reaction;weak oxalate-enhanced dissolution was observed in the presence of Fe?II?.The strong oxalate-enhanced dissolution was observed in corner sites with low lattice coordination of defect-goethite?110?face in the presence of Fe?II?.However,taking into consideration of Bader charge and molecular structure of dissolution sites,the synergistic effects of ligand-controlled and reductive dissolution is not responsible for the enhancement of dissolution.In the process,the adsorbed Fe?II?may play a role of catalyst,which causes electron transfer to surface defect sites.Compared the results of ideal-and defect-goethite?110?face,Fe?II?-catalysis capability could be strongly related with the inherent mineralogical properties of iron?hydr?oxides.5.The effect of citrate on the species and levels of Al impurities within ferrihydrite was investigated by chemical analysis,TEM,XRD,XPS and ATR-FTIR.The results show that in the ternary system of Fe?III?-Al?III?-citrate,citrate promoted the formation of stable colloidal ferrihydrite particles,but only at high citrate molar ratio citrate?50%?nearly complete stability occurred due to citrate adsorption.Meanwhile,citrate induced accumulation of Al in the surface layer of ferrihydrite for citrate ratios of 25 and 50 mol%.With increasing citrate content,the surface sites of ferrihydrite became occupied with citrate adsorption;partially this inhibited the substitution of Fe by Al,but it increased the adsorption of Al through Al-citrate surface complexation Thus,with increasing citrate content,the mechanism of Al binding by ferrihydrite changes from isomorphous substitution in the ferrihydrite core to adsorption of Al?III?-citrate on the ferrihydrite surface.These results suggest that citrate causes the heterogeneous reactivity of Al-bearing ferrihydrite.The properties of ferrihydrite aggregates could be close to Al-substitution ferrihydrite,while that of colloidal ferrihydrite particles could be similar to pure ferrihydrite with the passivating surface sites.