The Effect Of Soil Typical Active Components On The Speciation And Transformation Of Adsorbed Se(Ⅳ)

Selenium（Se）is an essential micronutrient element for humans and animals,and soil Se is the main source for human Se intake.The bioavailability of Se is largely attributed to the Se speciation in soils other than the soil total Se content.Iron（oxyhydr）oxides（for short,iron oxides）and low-molecular-weight organic acids（LMWOAs）are widely present in soil and water-body sediments,and are important inorganic and organic components in soil.Both two components have a significant impact on the chemical speciation and transformation of soil Se,thereby directly or indirectly affecting or controlling the bioavailability of soil Se.With extensive efforts,the interaction among Se-iron oxides-LMWOAs has been widely reported,however,those studies usually focused on the adsorption performance of Se on iron oxides or on the competition effects between Se and LMWOAs.It is still unclear about the relationship between the adsorption speciation of Se on the iron oxide surface and its desorption speciation.In addition,the coupling effect of LMWOAs on the soil Se speciation due to the reduction characteristics of LMWOAs and its contribution to the dissolution of iron oxides has not yet been studied extensively in literature,which limits the understanding of the environmental fate of Se and the regulation of Se availability.In present study,the effect of several soil typical active components（iron oxides and LMWOAs）on the chemical speciation and speciation transformation of Se（Ⅳ）was explored by batch adsorption-desorption experiments,with the help of attenuated total reflectance Fourier transform infrared spectroscopy（ATR-FTIR）,X-ray photoelectron spectroscopy（XPS）,high-resolution electron microscopy（HRTEM）,automatic potentiometric titration,and chemical speciation model（CD-MUSIC model）.Mineral-type-specific binding information of surface Se（Ⅳ）and the intrinsic response mechanism between speciation transformation of Se（Ⅳ）and LMWOAs were obtained to depict a molecular-level understanding of Se speciation and its availability.The main results are as follows:（1）The adsorption of Se（Ⅳ）on iron oxides was mainly through ligand exchange,and the adsorption capacities of Se（Ⅳ）on iron oxides varied with mineral types.At p H 5.0,the adsorption of Se（Ⅳ）on ferrihydrite（Fh）（0.592 mmol/g）was higher that of goethite（Goe）（0.147 mmol/g）and hematite（Hem）（0.130 mmol/g）,and respectively 4.0 and 4.6 times greater than that of goethite and hematite.The adsorption isotherms of Se（Ⅳ）on Fh,Goe and Hem were fitted well by Langmuir model,and the adsorption affinity constants（ln K_C）were in the following order:Fh（15.8）>Goe（15.3）>Hem（13.7）,and the free energy of adsorption of Se（Ⅳ）was-33.9–-39.1 k J/mol.The absorption kinetics of Se（Ⅳ）on Fh,Goe and Hem appeared to follow pseudo-second-order model,and the adsorption rate constants were Goe（1.848）>>Hem（0.221）>Fh（0.170）.The adsorption of Se（Ⅳ）on Fh,Goe and Hem decreased to different degrees as the p H increased from 3.0 to 11.0,with the adsorption capacities of Se（Ⅳ）decreased respectively by 65.2%,65.3%and 65.9%.（2）The species of adsorbed Se（Ⅳ）on the surface of iron oxides changed remarkedly with mineral types,and the proportion of ligand-exchangeable Se（EX-Se）to Se（Ⅳ）bound to iron oxides was associated with the structure of iron oxides,and affected by surface coverage,p H and adsorption time.Bidentate binuclear complex was the main chemical form of adsorbed Se（Ⅳ）on Fh and Hem.At low surface coverage（<20%）,Se（Ⅳ）adsorbed preferentially on the high-energy sites of Fh and Hem with high adsorption energy,resulting in a significant decrease of the proportion of EX-Se as well as a lower availability of surface Se（Ⅳ）;As the surface coverage increased,the proportion of surface EX-Se of Fh and Hem increased respectively from 19.3%and 33.7%to 50.5%and 44.7%.The dominant chemical form of adsorbed Se（Ⅳ）on Goe was monodentate mononuclear complex with lower adsorption energy.The proportion of EX-Se on Goe（96.7%）was higher than that of Fh and Hem and was not affected by the surface coverage,and thus leading to a higher availability of surface Se（Ⅳ）.The chemical shifts of Se 3p binding energy of adsorbed Se（Ⅳ）on Fh and Hem decreased gradually with the increase of p H,and the species of adsorbed Se（Ⅳ）transformed partly from bidentate binuclear complexes to monodentate mononuclear complexes.However,the chemical shifts of Se 3p binding energy of adsorbed Se（Ⅳ）on Goe did not change with p H and much less than that of Fh and Hem,the species of adsorbed Se（Ⅳ）on Goe was not affected by p H.Under the strong acidic condition（p H<4.0）,the adsorbed Se（Ⅳ）of Fh and Goe transformed partly into amorphous Fe₂（Se O₃）₃precipitation,resulting in a decrease in availability of surface Se（Ⅳ）.As the increase in reaction time,the speciation of adsorbed Se（Ⅳ）on Goe had no obvious change.Whereas,it transformed gradually from bidentate binuclear complexes to monodentate mononuclear complexes,and thus the proportion of EX-Se increasedand the availability of surface Se（Ⅳ）got higher.In addition,the species of adsorbed Se（Ⅳ）on Hem transformed from outer-sphere complex into inner-sphere complex and the proportion of EX-Se decrease from 57.3%to 45.5%and therefore the availability of surface Se（Ⅳ）decreased.（3）The surface properties and adsorption speciation of Se（Ⅳ）on Hem and its desorption speciation were strongly influenced by particle size.As particle size dramatically decreased（from 120 nm to 7 nm）,the crystallinity and morphology regularity of hematite decreased,the edge sites increased,and the specific surface area increased significantly.At p H 5.0,the charge density per unit mass of hematite with particle size of 7nm（Hem-7）was 2.1 times beyond that of hematite with particle size of 120 nm（Hem-120）,and thus the number of surface-active sites increased significantly,which derived from the increase in oxygen vacancy defects（OVs）on the hematite surface.In comparison to Hem-120,the adsorption capacity of Se（Ⅳ）on Hem-7（0.349 mmol/g）was significantly increased by 2.9 times,with the initial adsorption rate（h）increased remarkably by 2.2 times but the rate constant（k₂）decreased.The adsorption of Se（Ⅳ）on Hem-7 and Hem-120decreased with increasing p H,which is in accordance with the general law of anion adsorption.The combination of ATR-FTIR and CD-MUSIC modeling indicated that three types of Se（Ⅳ）complexes were formed on Hem-7 and Hem-120,including monodentate mononuclear monoprotonated complex,monodentate mononuclear nonprotonated complex and bidentate binuclear complex.Under acidic conditions,bidentate binuclear complex was the dominant species of adsorbed Se（Ⅳ）on hematite.With increasing p H,monodentate mononuclear nonprotonated complex became increasingly important.The species of Se（Ⅳ）adsorbed on Hem was significantly affected by particle size.In comparison to Hem-7,monodentate mononuclear monoprotonated complex was more important for Hem-120 at low p H（<5.0）.The proportion of surface OVs-OH on Hem-7（37.2%）was significantly higher than that of Hem-120（19.4%）,which resulted in increased adsorption strength of Se（Ⅳ）,and thus remarkably decreased the proportion of EX-Se as well as the availability of surface Se（Ⅳ）.The proportion of EX-Se on Hem-7 and Hem-120 decreased respectively from 48.7%and 55.8%to 16.8%and 32.2%as the p H increased,therefore,the availability of Se（Ⅳ）adsorbed on hematite was higher under the acidic condition.（4）The surface site heterogeneity and solubility of Goe was significantly enhanced and the immobilization of Se（Ⅳ）on Goe was promoted by low-molecular-weight organic acids（LMWOAs）in the following order:oxalic acid（Oxa）>succinic acid（Suc）>citric acid（Cit）.The adsorption experiments indicated that LMWOAs exhibited a competitive-synergistic coupling effect on Se（IV）adsorption to Goe,The Sips model provided a better fit to the adsorption isotherms of Se（Ⅳ）in both the Goe-Se（Ⅳ）binary and Goe-LMWOA-Se（Ⅳ）ternary systems,exhibiting higher regression coefficient values（R²>0.986）.The maximum adsorption capacity of Se（Ⅳ）was 0.217,0.210 and 0.178 mmol g^-1 and increased by 39.1%,34.6%and 14.1%in the presence of Oxa,Suc and Cit,respectively.The inhibition effect of Se（Ⅳ）on adsorption of LMWOAs was remarkably enhanced as the equilibrium concentration of Se（Ⅳ）increased and the amount of released Fe³⁺decreased dramatically in the presence of LMWOAs.The correlation analysis indicated significant positive correlations between the amount of adsorbed Cit and Suc and the release of Fe³⁺（Pearson’s r=0.77 and 0.78,respectively;P<0.05）.However,the correlation was not significant for Oxa（Pearson’s r=0.35;P>0.05）.In comparison to the results obtained without LMWOAs,the adsorption rate of Se（IV）onto goethite was decreased,and the k₂ values for Se（IV）adsorption decreased by 8.0（Cit）,13.3（Suc）and14.1（Oxa）times.At pH 5.0,LMWOAs promoted the speciation transformation of adsorbed Se（IV）on Goe from EX-Se into residual species（RE-Se）.The proportion of RE-Se increased respectively by 19.5%,16.5%,and 20.0%,resulting in a lower availability of surface Se（IV）.The ATR-FTIR and high-resolution XPS analyses demonstrated that the formation of≡Fe O（Se O）O―CO surface metastable complex and dissolved Fe³⁺played a crucial role in the promoting effect of LMWOAs in the speciation transformation process.The≡Fe O（Se O）O―CO surface metastable complex has an important function of increasing adsorption sites and redox mediator.In addition,the predominant mechanism for the speciation transformation of adsorbed Se（IV）is LMWOA specific,in which amorphous Fe₂（Se O₃）₃ precipitation was dominant for Suc（12.1%）and Cit（10.9%）and reduction was dominant for Oxa（18.5%）.This study systematically ascertained the effect and mechanisms of iron oxide structure,particle size and LMWOAs on speciation distribution and transformation of Se（Ⅳ）,which may facilitate mechanistically understanding the environmental behavior of Se and promote effective management of Se bioavailability.It is also of great theoretical and practical significance to elucidate the geochemical cycle of Se.