Mechanisms Of Lead Adsorption On Aluminum-substituted Iron Oxides And CD-MUSIC Modeling

Iron oxide is an important active component in soil due to its commonly occurrence,environmentally friendly and abundant active sites.Thus,the environmental effects of iron oxides become the key scientific issue in the scope of environment and geochemistry.The mobility,chemical species and bioavailability of environmental contaminants are strongly affected by the interfacial reactions of iron oxide-water interface.Prediction of their interfacial process thus plays critical roles in agricultural production and environmental protection.Surface complexation models(SCMs)have been demonstrated to be a superior tool for describing and predicting the adsorption reactions at mineral-water interface.The charge distribution(CD)-multisite complexation(MUSIC)model can successfully describe the adsorption behaviors of various ions on the surface of iron oxides.A good CD-MUSIC model results depended on the accuracy description of the model parameters on the mineral surface structures and interfacial reactions.However,the physicochemical properties and surface reactivity of iron oxides are affected by several factors including(1)Al-substitution in iron oxides is ubiquitous in nature and can alter the crystal structure,particle size,crystallinity,growth orientation,and morphology of iron oxides.(2)The surface of iron oxides is composed of multiple crystal facets,while the types,amounts,and affinities of active sites varied with different facets.(3)Natural iron oxides commonly encounter defect structures,which strongly affects the composition and arrangement of surface sites.Given that the tiny changes in the structure of iron oxide may lead to different local coordination and further influence the predictive result of the CD-MUSIC model.Thus,batch adsorption experiment,thermogravimetric(TG)measurement,X-ray photoelectron spectroscopy(XPS)analysis,high resolution electron transmission microscopy(HRTEM)and high angle annular dark-field scanning transmission electron microscopy(HAADF-STEM)were performed to investigate the surface heterogeneous structure of iron oxides,the adsorption mechanisms of Pb on iron oxides was also clarified.These studies provide a deep understanding for the structure-activity relationship of iron oxides at atomic/molecular scale,and developed the applicability of the CD-MUSIC model to more complex interfaces.Core results are shown below:(1)Al-substitution in ferrihydrite increased the surface charge density and decreased the Pb²⁺adsorption capacity of ferrihydrite.With increasing content of Al-substitution from 0%to 30%,the ferrihydrite shows irregular agglomeration morphologies with a similar site density.SSA of all the Al-substituted ferrihydrite samples was close to 300m²/g,and p H of point of zero charge(PZC)was at 8.7?9.0.Al-substitution increased the proton affinity(log K_H)and inner-Stern layer capacitance(C₁)of ferrihydrite,leading to the increase of surface charge density and decrease of log K_Pb.At p H 5.0,the adsorption capacity of ferrihydrite for Pb²⁺decreased from 1.19?mol/m²to 0.67?mol/m²with increasing Al content.The abundance(%)of Pb complexes on ferrihydrite was also strongly affected by p H.Al-substitution contributed limited to affect the coordination structure and speciation distribution of Pb²⁺on ferrihydrite,and the predominate species on the surface of ferrihydrite were bidentate Pb complex.(2)Al-substitution reduced the length-to-width ratio of goethite,leading to the increase of surface charge density and decrease Pb²⁺adsorption capacity of goethite.With Al-substitution content increased from 0%to 15%,the total site density of goethite slightly increased from 6.21 sites/nm²to 6.33 sites/nm²,while the SSA decreased from 65m²/g to 47 m²/g.The PZC of all Al-substituted goethite samples was at 9.2?9.3.Al-substitution led to the higher log K_Hof?FeOH^-0.5site,larger C₁,higher surface charge,smaller log K_Pband lower Pb adsorption capacity of goethite.At p H 5.0,the Pb²⁺adsorption capacity of goethite decreased from 0.95?mol/m²to 0.50?mol/m².Al-substitution had a minor impact on the abundance of Pb²⁺complexes on the surface of goethite with bidentate complexes as the dominant species on goethite.(3)Al-substitution-induced defect sites enhance surface site density,charge density and Pb²⁺adsorption capacity of hematite.With Al-substitution content increased from 0%to10%,an obvious morphology conversion was observed with rhombohedron change to disk-shaped plate.The proportion of(001)facets on hematite increased and the SSA was about 25 m²/g after Al-substitution.HAADF-STEM images indicated that Al-substitution induced more vacancies of Fe atoms on hematite(001)facets.CD-MUSIC modeling results revealed that Al-substitution increased the log K_Hof?FeOH^-0.5site,C₁,surface hydroxyl sites,surface charge density and the log K_Pbof Pb²⁺on hematite.The adsorption capacity of hematite for Pb²⁺at p H 5.0 increased from 0.85?mol/m²to 3.51?mol/m²with increasing Al content.The dominant species of Pb²⁺transfer from tridentate complexes in pure hematite to bidentate complexes in Al-substituted hematite.(4)The active site density on the surface of hematite was strongly influenced by the crystal facet contributions(CFCs).The total site density of hematite nanoplates(HNP)and nanocubes(HNC)was 14.0 and 14.6,while the active site density was 4.4 and 13.4,respectively.CD-MUSIC modeling results revealed that the log K_Hof?FeOH^-0.5and?Fe₃O^-0.5was 7.8 and 10.8 on(012)facets of hematite,and the value changed to 7.7 and11.7 on(110)facets,respectively.The ridges and valleys on(012)facets led to larger surface charge and higher adsorption performance for Pb.in dditional,our modeling further indicated that(012)facets bind Pb via both bidentate and tridentate complexes,while(110)facets bind Pb only through bidentate complexes at p H 3.0?6.5.The model parameters obtained in our study were successfully applied to fit the Pb²⁺adsorption behavior of hematite with different morphologies reported in the literature.(5)Vacant Fe³⁺sites in defective hematite led to a considerable increase in site density as well as heavy metal removal performance.For the first time,the hydroxyl site density on defective hematite was quantified by harmonizing the crystallography,microscopic analysis,and X-ray diffraction Rietveld structure refinement.A considerable increase in site density as well as positive charge was confirmed.The total arsenate-active site density(5.7 sites/nm²)on defective hematite was 1.2 sites/nm²of original sites and 4.5sites/nm²of Fe vacancy-induced sites.The adsorption capacity of defective hematite and defect-free hematite was 2.59?mol/m²and 1.33?mol/m²,respectively.CD-MUSIC modeling results revealed that Fe vacancy increased the C₁and log K_Asof hematite,while decreased the log K_Pbof hematite.The present study revealed that Al-substitution has different effects on the crystal structure,site density and surface reactivity of different types of iron oxides.The results clarified the reaction mechanisms at iron oxide-water interface.The obtained parameters expand the applicability of the CD-MUSIC model to more complex interfaces and the use of surface complexation models in natural environments.