Characteristics And Mechanisms Of Interaction Between Iron Oxides And Selenomethionine

Selenomethionine（Se Met）is the most common form of organic selenium in the environment,with high bioavailability,low toxicity,and more conducive to the utilization of organisms,while the performance of soil organic selenium has not been reported in relevant studies.Therefore,studying the interaction between iron oxide and Se Met is helpful to understand the effect of soil iron oxides on the bioavailability of organic selenium,which can enrich the biogeochemical cycle process of organic selenium.The adsorption/desorption,redox and other characteristics of iron oxide and Se Met of three kinds of ferrihydrite（Fer）,goethite（Goe）and hematite（Hem）were studied,and the changes of its structure and basic properties after iron oxide adsorption of Se Met were studied,and the interaction mechanism between iron oxide and Se Met was revealed.The main results are as follows:（1）The initial p H of the reaction system had a significant effect on the adsorption of Se Met by the three iron oxides tested.The adsorption capacity of Se Met by all three iron oxides was observed relatively high at p H 5.7 within the p H range of 4-8.5.At p H 5.7 and an initial Se Met concentration of 200 mg/L,the adsorption of Se Met by all three iron oxides did not reach saturation.The Langmuir and Freundlich models were used to fit the equilibrium adsorption data,and the maximum adsorption capacities of Fer,Goe,and Hem were found to be 1065.93mg/g,371.99 mg/g,and 413.92 mg/g,respectively.The Freundlich model provided a better fit to the adsorption data（R²=0.9746-0.9928）.Kinetic adsorption data showed that the adsorption process reached a rapid adsorption stage within the first 120 min,and then gradually approached saturation after 480 min.The kinetic model showed that the adsorption of Se Met by the iron oxides was mainly controlled by particle diffusion and liquid film diffusion,and the pseudo-second-order kinetic equation provided an excellent fit to the kinetic adsorption data（R²=0.9999）,indicating that chemical adsorption was the main mechanism of Se Met adsorption by the three iron oxides.（2）The amount of Se Met desorbed using KH₂PO₄as the desorption agent was not dependent on the desorption agent concentration.The desorption rate decreased with increasing Se Met adsorbed by the iron oxides,and the maximum desorption rates of Se Met were observed for Goe>Hem>Fer.In 0.1 mol/L KH₂PO₄of the p H5.7,trace amounts of Se Met（<1 mg/g）on the surface of the iron oxides promoted the adsorption of Pi.As the amount of Se Met adsorbed by the iron oxides increased,the adsorption of Pi gradually decreased.Se Met solutions were easily oxidized by air,but addition of the iron oxides inhibited the oxidation of Se Met,with Goe showing the best inhibition effect.The affinity of the three iron oxides towards Se Met was much higher than that for Cl^-,NO₃^-,and SO₄^2-.Addition of Cl^-and NO₃^-had no significant effect on the adsorption of Se Met by the iron oxides,while addition of SO₄^2-slightly decreased the adsorption of Se Met by the iron oxides.（3）XRD analysis showed that the crystal structure of the iron oxides did not change after Se Met adsorption.HR-TEM images showed that the particles in the samples after Se Met adsorption were more aggregated,and the surfaces of the crystals were eroded.N₂adsorption/desorption and particle size analysis showed that the specific surface area of the samples decreased,and the total pore volume decreased,while the average particle size increased after adsorption.These results indicated that the basic structure of the iron oxides did not change after interaction with Se Met,and Se Met only bound to the surface of the iron oxides.（4）The p H of the samples continuously increased after Se Met adsorption by the iron oxides at an initial p H of 5.7.Zeta potential analysis showed that the zero point of the Zeta potential of the samples after Se Met adsorption by the iron oxides decreased,with reductions of 15.0,11.8,and 18.9 m V for Fer,Goe,and Hem,respectively,at p H 5.7.HR-XPS analysis showed that Fe2p binding energy（B.E）and N1s B.E both decreased by 0.23-0.40 e V after interaction between the iron oxides and Se Met,indicating a reduction in the oxidation state of Fe and N.The Se3p level exhibited three fitted peaks at-2,0,and+4 oxidation states.ATR-FTIR spectra showed characteristic vibration peaks of Se Met in all three iron oxide-Se Met composites.Compared with Se Met,the symmetric stretching vibration of COO^-in the three composites decreased by 6-23 cm^-1,the deformation vibration of NH₃⁺increased by about 17 cm^-1,the stretching vibration of OH increased by 5-23 cm^-1,and the deformation vibration of C-H increased by 6-14 cm^-1.These results indicate that electrostatic attraction,anion exchange,surface coordination,hydrogen bonding,etc.were important mechanisms for Se Met adsorption by the iron oxides.The carboxyl group in Se Met formed a coordination bond with the surface O of the iron oxides,and the amino group formed an N-H…O hydrogen bond with the surface O of the iron oxides.Additionally,there was a slow redox reaction between the iron oxides and SeMet.