Theoretical modeling of molecular fractionation of dissolved organic matter on ferrihydrite and its impact on proton and metal binding properties.

Molecular fractionation of dissolved organic matter (DOM) at the mineral-liquid interfaces in soil changes its molecular composition, thus altering its reactivity, such as proton and metal binding properties. Therefore, a quantitative understanding of compositional change of DOM molecules after adsorptive fractionation by minerals is of great environmental significance for predicting the cycling of organic carbon (C) and metals in the ecosystem. In this study, we conducted adsorption experiments to investigate the adsorption behaviors of DOM molecules on ferrihydrite. The molecular compositions of the original and fractionated DOM samples were analyzed with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). For all DOM molecules, three molecular groups with significantly different chemical properties were identified through Spearman correlation analysis between the relative intensities of DOM molecules and organic C concentrations in solutions after adsorptive fractionation. Three corresponding molecular models for the three molecular groups were constructed based on Vienna Soil-Organic-Matter Modeler and FT-ICR-MS results, which were used as base units to construct molecular models for the original or fractionated DOM samples (model(DOM)). The models well described the chemical properties of the original or fractionated DOM as compared with the experimental data. Furthermore, based on model(DOM), the proton and metal binding constants of DOM molecules were quantified by SPARC chemical reactivity calculations and linear free energy relationships. We found the density of binding sites of the fractionated DOM samples was negatively correlated with the adsorption percentage. Our modeling results suggested that adsorption of DOM on ferrihydrite gradually removed acidic functional groups from the solution, dominated by the adsorption of both carboxyl and phenol groups. This study proposed a new modeling approach to quantify the molecular fractionation processes of DOM on Fe oxides and their impact on proton and metal binding properties, which is expected to be applicable to DOM from different environments.

Unified Modeling Approach for Quantifying the Proton and Metal Binding Ability of Soil Dissolved Organic Matter.

Soil dissolved organic matter (DOM) is composed of a mass of complex organic compounds in soil solutions and significantly affects a range of (bio)geochemical processes in soil environment. However, how the chemical complexity (i.e., heterogeneity and chemodiversity) of soil DOM molecules affects their proton and metal binding ability remains unclear, which limits our ability for predicting the environmental behavior of DOM and metals. In this study, we developed a unified modeling approach for quantifying the proton and metal binding ability of soil DOM based on Cu titration experiments, Fourier transform ion cyclotron resonance mass spectrometry data, and molecular modeling method. Although soil DOM samples from different regions have enormously heterogeneous and diverse properties, we found that the molecules of soil DOM can be divided into three representative groups according to their Cu binding capacity. Based on the molecular models for individual molecular groups and the relative contributions of each group in each soil DOM, we were able to further develop molecular models for all soil DOM to predict their molecular properties and proton and metal binding ability. Our results will help to develop mechanistic models for predicting the reactivity of soil DOM from various sources.

Kinetics of cadmium (Cd), nickel (Ni), and lead (Pb) release from fulvic acid: Role of re-association reactions and quantitative models.

Dissolved organic matter (DOM), a ubiquitous ligand for heavy metals, plays a crucial role in regulating the bioavailability and fate of heavy metals in the environment. However, owing to complex structure and heterogeneity of DOM, it is still challenging to develop kinetics models to predict the rates of heavy metal reactions with DOM. In this study, we investigated the kinetics of Cd, Ni, and Pb release from a typical fulvic acid (FA) under a wide range of experimental conditions using a competing ligand exchange (CLE) method. Among three metals, Cd showed the fastest release from FA while Ni and Pb had slower release rates. Reaction pH also had different impact on the release rates of the three metals, presumably attributed to different proton/metal exchange ratios for the metal ion complexation with FA. We formulated a kinetics model for Cd, Ni, and Pb release from FA by considering metal ions dissociation from FA, re-association of metal ions with FA, and metal ion uptake by the resin in the CLE experiments. The chemical speciation model WHAM 7 was used to predict the local equilibrium status that the kinetic reactions were away from, which help to derive the kinetic parameters based on the equilibrium parameters. For both Cd and Pb, model calculations were sensitive to the re-association rates, especially at high pH, while for Ni, the impact of the re-association rates was less significant. Based on the model parameters obtained in this study, our model simulations have also demonstrated that metal-FA complexes may undergo different rates of dissociation in the environment, affecting the dynamic speciation and transfer of metals to other biological processes. This work has provided a quantitative tool for predicting metal release from DOM, which would be useful for predicting the bioavailability and fate of heavy metals in the environment.

Coupled Sorption and Oxidation of Soil Dissolved Organic Matter on Manganese Oxides: Nano/Sub-nanoscale Distribution and Molecular Transformation.

In soil environments, the sequestration and transformation of organic carbon are closely associated with soil minerals. Birnessite (MnO2) is known to strongly interact with soil dissolved organic matter (DOM), but the microscopic distribution and molecular transformation of soil DOM on birnessite are still poorly understood. In this study, the coupled sorption and oxidation of soil DOM on birnessite were investigated at both the microscopic scale and the molecular level. Spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) results revealed, at the nano- to sub-nanoscale, that DOM was located both on the surfaces and within the interflakes or pore spaces of birnessite, and DOM within the interflakes displayed a higher oxidation state than that on the surfaces. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) results suggested that a portion of phenolic compounds were preferentially sorbed and oxidized, resulting in the formation of compounds with higher oxygen contents and polymeric products. Our Cs-STEM and FT-ICR-MS results highlighted the significance of organo-mineral associations in the microscopic mineral structure for the reactivity of organic carbon and provided the molecular evidence for the transformation of soil DOM by birnessite, which contributed to the understanding of the dynamics of soil dissolved organic carbon.

Phase transformation of Cr(VI)-adsorbed ferrihydrite in the presence of Mn(II): Fate of Mn(II) and Cr(VI).

Ferrihydrite is an important sink for the toxic heavy metal ions, such as Cr(VI). As ferrihydrite is thermodynamically unstable and gradually transforms into hematite and goethite, the stability of Cr(VI)-adsorbed ferrihydrite is environmentally significant. This study investigated the phase transformation of Cr(VI)-adsorbed ferrihydrite at different pH in the presence of aqueous Mn(II), as well as the fate of Mn(II) and Cr(VI) in the transformation process of ferrihydrite. Among the ferrihydrite transformation products, hematite was dominant, and goethite was minor. The pre-adsorbed Cr(VI) inhibited the conversion of ferrihydrite to goethite at initial pH 3.0, whereas little amount of adsorbed Mn(II) favored the formation of goethite at initial pH 7.0. After the aging process, Cr species in solid phase existed primarily as Cr(III) in the presence of Mn(II) at initial pH 7.0 and 11.0. The aqueous Mn concentration was predominantly unchanged at initial pH 3.0, whereas the aqueous Mn(II) was adsorbed onto ferrihydrite or form Mn(OH)2 precipitates at initial pH 7.0 and 11.0, promoting the immobilization of Cr(VI). Moreover, the oxidation of Mn(II) occurred at initial pH 7.0 and 11.0, forming Mn(III/IV) (hydr)oxides.

Coadsorption and subsequent redox conversion behaviors of As(III) and Cr(VI) on Al-containing ferrihydrite.

Naturally occurring ferrihydrite often contains various impurities, and Al is one of the most prominent impurities. However, little is known about how these impurities impact the physical and chemical properties of ferrihydrite with respect to metal(loid) adsorption. In this study, a series of Al-containing ferrihydrites were synthesized and exposed to a mixed solution containing As(III) and Cr(VI). The results showed that the two contaminants can be quickly adsorbed onto the surface of Al-containing ferrihydrite under acidic and neutral conditions. With the increase of Al molar percentage in ferrihydrites from 0 to 30, the adsorption capacity of As(III) decreased, whereas it increased for Cr(VI). On the other hand, with the increase of pH value from 3.0 to 11.0, the decreasing rate of As(III) was accelerated first, then slowed down, whereas the Cr(VI) decreasing rate slowed down dramatically. X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis method, transmission electron microscopy (TEM) analysis, energy dispersive spectroscopy (EDS) mapping, Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), and X-ray photoelectron spectroscopy (XPS) were employed to characterize Al-containing ferrihydrite. Interestingly, it was found that the redox transformation occurred between As(III) and Cr(VI) after the two contaminants were coadsorbed onto the surface of Al-containing ferrihydrite. The oxidation of As(III) to As(V) and reduction of Cr(VI) to Cr(III) would greatly lower the environmental hazard of the As(III) and Cr(VI).

Removal of nitrate from water by acid-washed zero-valent iron/ferrous ion/hydrogen peroxide: influencing factors and reaction mechanism.

In this paper, a system consisting of acid-washed zero-valent iron (ZVI), ferrous ion (Fe2+), and hydrogen peroxide (H2O2) was employed for the removal of nitrate (NO3-) from water, and the reaction mechanism for this is discussed. The effects of acid-washed ZVI, Fe2+, H2O2, and initial NO3- concentration on nitrate removal were investigated. Acid-washed ZVI before and after reaction with nitrate were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Results reveal that the combined system can enhance the corrosion of ZVI and facilitate aqueous nitrate reduction. The products of nitrate reduction are mainly ammonium, with some N2. The ZVI particles after reaction may have a core of ZVI with an oxidation layer mainly consisting of Fe3O4.

Removal mechanism of selenite by Fe3O4-precipitated mesoporous magnetic carbon microspheres.

A mesoporous composite of magnetic carbon microspheres (MCMSs) was synthesized via introducing Fe3O4 nanoscale particles to the surface of carbon microspheres (CMSs) by coprecipitation. Scanning electron microscopy and transmission electron microscopy showed the Fe3O4 nanoscale particles were dispersedly immobilized on the surface of CMSs. The MCMSs demonstrated effective removal of selenite (Se(IV)) from wastewater. MCMSs showed the regular pattern where the lower pH value, the lower residual Se(IV) concentration. The coexisting sulfate, nitrate, chloride, carbonate, and silicate had no significant effect on Se(IV) removal, whereas phosphate hindered the removal of Se(IV) by competing with Se(IV) and formed inner-sphere complexes with Fe3O4 on the surface of MCMSs. Through X-ray photoelectron spectroscopy analysis, Se(IV) can not only form inner-sphere complexes with MCMSs, but also be reduced to insoluble elemental selenium (Se0) by Fe3O4 which was oxidized and formed γ-Fe2O3. Moreover, the superparamagnetic MCMSs can be easily separated from solution by means of an external magnetic field. The high removal efficiency for Se(IV) and rapid separability of MCMSs made them promising materials for the application in the practice.

Facile preparation of magnetic mesoporous MnFe2O4@SiO2-CTAB composites for Cr(VI) adsorption and reduction.

Chromium-contaminated water is regarded as one of the biggest threats to human health. In this study, a novel magnetic mesoporous MnFe2O4@SiO2-CTAB composite was prepared by a facile one-step modification method and applied to remove Cr(VI). X-ray diffraction, scanning electron microscopy, transmission electron microscopy, specific surface area, and vibrating sample magnetometer were used to characterize MnFe2O4@SiO2-CTAB composites. The morphology analysis showed that the composites displayed a core-shell structure. The outer shell was mesoporous silica with CTAB and the core was MnFe2O4 nanoparticles, which ensured the easy separation by an external magnetic field. The performance of MnFe2O4@SiO2-CTAB composites in Cr(VI) removal was far better than that of bare MnFe2O4 nanoparticles. There were two reasons for the effective removal of Cr(VI) by MnFe2O4@SiO2-CTAB composites: (1) mesoporous silica shell with abundant CTA+ significantly enhanced the Cr(VI) adsorption capacity of the composites; (2) a portion of Cr(VI) was reduced to less toxic Cr(III) by MnFe2O4, followed by Cr(III) immobilized on MnFe2O4@SiO2-CTAB composites, which had been demonstrated by X-ray photoelectron spectroscopy results. The adsorption of Cr(VI) onto MnFe2O4@SiO2-CTAB followed the Freundlich isotherm model and pseudo-second-order model. Tests on the regeneration and reuse of the composites were performed. The removal efficiency of Cr(VI) still retained 92.4% in the sixth cycle. MnFe2O4@SiO2-CTAB composites exhibited a great potential for the removal of Cr(VI) from water.

Novel mesoporous FeAl bimetal oxides for As(III) removal: Performance and mechanism.

In this study, novel mesoporous FeAl bimetal oxides were successfully synthesized, characterized, and employed for As(III) removal. Batch experiments were conducted to investigate the effects of Fe/Al molar ratio, dosage, and initial solution pH values on As(III) removal. The results showed that the FeAl bimetal oxide with Fe/Al molar ratio 4:1 (shorten as FeAl-4) can quickly remove As(III) from aqueous solution in a wide pH range. The FeAl-4 before and after reaction with As(III) was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED), Brunauer-Emmett-Teller (BET) surface area measurement, and X-ray photoelectron spectroscopy (XPS). The BET results showed that the original FeAl-4 with a high surface area of 223.9 m2/g was a mesoporous material. XPS analysis indicated that the surface of FeAl-4 possessed a high concentration of M-OH (where M represents Fe and Al), which was beneficial to the immobility of As(III). The excellent performance of FeAl-4 makes it a potentially attractive material for As(III) removal from aqueous solution.