Adsorption behavior and surface complexation modeling of oxygen anion Sb(V) adsorption on goethite

A critical review on arsenic and antimony adsorption and transformation on mineral facets

Arsenic (As) and antimony (Sb), with analogy structure, belong to VA group in the periodic table and pose a great public concern due to their potential carcinogenicity. The speciation distribution, migration and transformation, enrichment and retention, as well as bioavailability and toxicity of As and Sb are influenced by several environmental processes on mineral surfaces, including adsorption/desorption, coordination/precipitation, and oxidation/reduction. These interfacial reactions are influenced by the crystal facet of minerals with different atomic and electronic structures. This review starts with facets and examines As and Sb adsorption and transformation on mineral facets such hematite, titanium dioxide, and manganese dioxide. The main focus lies on three pressing issues that limit the understanding of the environmental fate of As and Sb: the facet-dependent intricacies of adsorption and transformation, the mechanisms underlying facet-dependent phenomena, and the impact of co-existing chemicals. We first discussed As and Sb adsorption behaviors, structures, and bonding chemistry on diverse mineral facets. Subsequently, the reactivity of various mineral facets was examined, with particular emphasis placed on their significance in the context of environmental catalysis for the oxidation of As(III) and Sb(III). Finally, the impact of co-existing cation, anion, or organic substances on the processes of adsorption and transport of As and Sb was reviewed. This comprehensive review enhances our understanding of the facet-dependent phenomena governing adsorption, transformation, and fate of contaminants. It underscores the critical role of mineral facets in dictating environmental reactions and paves the way for future research in this intriguing field.

Tungsten Adsorption on Goethite: Insights from First-Principles Molecular Dynamics Simulations

The environmental fate of tungsten (W) has received particular attention due to its increasing utilization and potential health hazards. Adsorption on minerals is considered as a major factor in governing tungsten's mobility and bioavailability. Goethite, a highly stable iron oxide in soils and sediments, is pivotal in determining tungsten's environmental behavior. In this study, the sorption mechanisms of tungsten on the primary (110) surface of goethite were investigated by using systematic first-principles molecular dynamics (FPMD) simulations. First, we computed the bidentate corner-sharing complexation structures of tungsten in all protonation states (i.e., WO42-, HWO4-, and H2WO40) on the goethite surface. Tungsten exhibits a fivefold coordination in the WO42- and HWO4- systems, whereas it transforms into a sixfold coordination in the H2WO40 system. By using the vertical energy gap method for pKa calculations, it is revealed that the adsorbed WO4(H2O)2- species is predominant at pH > 2.0, which is different from WO42- in aqueous solutions (pH > 4.9). The desorption free energy of WO4(H2O)2- species suggest that the bidentate corner-sharing form of WO4(H2O)2- is highly stable with a binding energy of 19.8 kcal/mol. This study fills a critical gap in the atomic-scale knowledge of tungsten behavior and stability in natural environments, providing a theoretical foundation for managing tungsten mobilization in both natural and industrial settings.

Compositional Differences of Various Sorption States of Cu and Cd on Goethite in the Presence of Oxyanions: A Quantitative Study

The anionic species of antimony(V) and phosphate(V) are commonly found in the contaminated soil of mining areas, exerting a significant influence on the sorption of heavy metals and thus affecting their migration. This study quantitatively discussed the sorption mechanism of Sb and P in promoting the sorption of Cd or Cu on goethite through a series of extraction methods. In the single sorption system, the majority of Cu (87-98%) is adsorbed on goethite in the form of EDTA-extractable Cu (EF Cu, possibly inner-sphere complexes) under pH conditions of 3.5-6.5. Cadmium is primarily adsorbed in the form of EDTA-extractable Cd (49-71%), with a considerable amount of Mg(NO3)2 extractable Cd (MF Cd, possibly outer-sphere complexes) also present (25-51%), within the pH range of 4.5-7.5. The presence of either Sb or P greatly enhances the sorption of Cd and Cu on goethite, although the mechanisms differ significantly. Sb enhances the sorption of Cu mainly by increasing the amount of EF Cu (61.7-68.1% of total Cu enhancement), with less significant effects on MF Cu. Furthermore, Sb shows similar enhancing effects on both MF Cd and EF Cd. As the pH increases, the enhancing effects of Sb on various forms of Cu and Cd decrease. Phosphate mainly promotes the formation of MF Cu and MF Cd, accounting for 53.9- 80.8% (Cu) and 78.0-94.9% (Cd) of total enhancement at different pH levels. ΔMF and ΔEF Cu increase with increasing pH when P is present, while ΔMF/ΔEF Cd remains essentially constant. Based on the extracted results and characterization analysis, the main mechanism of synergistic sorption between elements was discussed, and the connection modes of elements at the goethite interface were preliminarily speculated. The results indicate that the promotion of oxyanions on the fixation of heavy metal cations is more complex than expected, making it difficult to describe using only one mechanism.

A new insight into the straw decomposition associated with minerals: Promoting straw humification and Cd immobilization

Organic matter (OM) derived from the decomposition of crop residues plays a key role as a sorbent for cadmium (Cd) immobilization. Few studies have explored the straw decomposition processes with the presence of minerals, and the effect of newly generated organo-mineral complexes on heavy metal adsorption. In this study, we investigated the variations in structure and composition during the rice straw decomposition with or without minerals (goethite and kaolinite), as well as the adsorption behavior and mechanisms by which straw decomposition affects Cd immobilization. The degree of humification of extracted straw organic matter was assessed using excitation-emission matrix (EEM) fluorescence and Ultraviolet-visible spectroscopy (UV-vis), while employing FTIR spectroscopy and XPS to characterize the adsorption mechanisms. The spectra analysis revealed the enrichment of highly aromatic and hydrophobic components, indicating that the degree of straw decomposition and humification were further intensified during incubation. Additionally, the existence of goethite (SG) accelerated the humification of OM. Sorption experiments revealed that the straw humification increased Cd adsorption capacity. Notably, SG exhibited significantly higher adsorption performance compared to the organic matter without minerals (RS) and the existence of kaolinite (SK). Further analysis using FT-IR spectroscopy and XPS verified that the primary mechanisms involved in Cd immobilization were complexion with -OH and -COOH, as well as the formation of Cd-π binds with aromatic C=C on the surface of solid OMs. These findings will facilitate understanding the interactions of the rice straw decomposing with soil minerals and its remediation effect on Cd-contaminated farmland.

Enhanced Treatment of Antimony Mine Wastewater by Sulfidated Micro Zerovalent Iron (S-mZVI)

Commercial micron zerovalent iron (mZVI) and sulfur were used to prepare sulfidated micro zerovalent iron (S-mZVI) through ball milling. The corrosion potentials of mZVI and S-mZVI were -0.01 and -0.37 V, respectively, indicating S-mZVI possessed a stronger electron-donating ability. The practical antimony mine wastewater (C0(Sb(V)) = 3.8296 mg/L, pH = 8.29) was treated. If meeting the national discharge standard of 5 μg/L, 2.0 g/L mZVI and 1.6 g/L S-mZVI were required within 120 min. Passing N2 or reducing wastewater pH enhanced the treatment of Sb(V) by S-mZVI, in which the wastewater acidification was more effective. Once the wastewater pH was adjusted to 3.00, only 0.7 g/L S-mZVI and 40 min long time were needed to achieve the emission below 5 μg/L. Even S-mZVI underwent four cycles, and the final concentration of Sb(V) was as low as 4.67 μg/L. As the pHzpc value was 4.09 and the corrosion potential was -0.56 V at pH 3.0, the electron-donating ability of S-mZVI as well as the electrostatic attraction between the surface of S-mZVI and Sb(V) increased. Sulfidation of mZVI and then application under the acid condition significantly improved the treatment efficiency of Sb(V).

The effect of goethite aging on Cd adsorption: Constraints of mineral condensation and surface site density

Iron (hydr)oxides, as natural geosorbents, play a crucial role in retaining toxic heavy metals, and their aging process greatly influences heavy metals distributions and migration in soil systems. However, limited attention has been given to the interaction between heavy metals and crystalline-aged goethite. In this study, we investigated the sorption behavior and sorption mechanism of cadmium (Cd) with freshly synthesized or aged goethite. We quantified the total Cd sorption load, as well as the proportion of Cd with different sorption strengths on minerals. It has been found that in different aged goethite samples, approximately 71.3-84.7 % of Cd is strongly bound (bidentate inner-sphere complexes) and 16.0 % to 26.4 % of Cd is weakly bound (electrostatic adsorption and partially through monodentate inner-sphere complexes) by goethite. This observation is consistent with the distribution characteristics of Cd species fitted by the charge distribution and multisite surface complexation model. Additionally, the total Cd load and strongly bound Cd content on goethite aged at pH 7.5 decreased with extended aging time. Upon combining the mineral characterization analysis and surface hydroxyl density calculation, we found that the morphology transformation and the deterioration in sorption ability on goethite results from a condensation process through a surface hydroxyl oxolation reaction on the {110} facet between adjacent goethite crystals during the aging process at pH 7.5. This condensation process causes goethite to lose many hydroxyl sites, which is the dominant reason for the decrease in inner-sphere complexed Cd. The amount of weakly bound Cd decreases slightly with aging, because the decrease in inner-sphere complexed Cd is not conducive to balancing the positively charged mineral surface, resulting in a slight reduction in the amount of Cd adsorbed through electrostatic attractions.

Insights on the Contradiction between the Affinity and Capacity of Ferrihydrite toward As(III) and As(V): Surface Reaction Revisited

Ferrihydrite is omnipresent in nature, and its adsorption of As(III/V) decides the migration of arsenic. Although As(III) is commonly recognized as the more mobile species of inorganic arsenic, it sometimes exhibits less mobility in ferrihydrite systems, which calls for further insights. In this study, we elucidated the adsorption behavior and mechanisms of As(III/V) on ferrihydrite under different loading levels (molar ratio As/Fe = 0-0.38), solution pH (3-10), and coexisting ions [P(V) and Ca(II)] based on batch adsorption experiments, surface complexation modeling, density functional theory calculations, and X-ray photoelectron spectroscopy. Our results show that As(III) exhibits weaker adsorption affinity but a larger capacity compared with that of As(V). On ferrihydrite, As(III) and As(V) are adsorbed mainly as bidentate mononuclear complexes at type-a sites [≡Fe(OH-0.5)2] and bidentate binuclear complexes at type-b sites (2≡FeOH-0.5), respectively. As the dosage increases, As(III) further forms mononuclear monodentate complexes at both surface sites, resulting in a higher site utilization efficiency, while As(V) does not due to repulsive electrostatic interaction. The difference in surface species of As(III/V) also leads to complex responses when coexisting with high concentrations of P(V) and Ca(II). This study helps us to understand environmental behavior of As(III/V) and develop remediation strategy in As(III/V) contaminated systems.

Effect of calcium and phosphorus on ammonium and nitrate nitrogen adsorption onto iron (hydr)oxides surfaces: CD-MUSIC model and DFT computation

Calcium (Ca2+) and phosphorous (PO43-) significantly influence the form and effectiveness of nitrogen (N), however, the precise mechanisms governing the adsorption of ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) are still lacking. This study employed batch adsorption experiments, charge distribution and multi-site complexation (CD-MUSIC) models and density functional theory (DFT) calculations to elucidate the mechanism by which Ca2+ and PO43- affect the adsorption of NH4+-N and NO3--N on the goethite (GT) surface. The results showed that the adsorption of NH4+-N on the GT exhibited an initial increase followed by a decrease as pH increased, peaking at a pH of 8.5. Conversely, the adsorption of NO3--N decreased with rising pH. According to the CD-MUSIC model, Ca2+ minimally affected the NH4+-N adsorption on the GT but enhanced NO3--N adsorption via electrostatic interaction, promoting the adsorption of ≡FeOH-NO3- and ≡Fe3O-NO3- species. Similarly, PO43- inhibited the adsorption of ≡FeOH-NO3- and ≡Fe3O-NO3- species. However, PO43- boosted NH4+-N adsorption by facilitating the formation of ≡Fe3O-NH4+ via electrostatic interaction and site competition. DFT calculations indicates that although bidentate phosphate (BP) was beneficial to stabilize NH4+-N than monodentate phosphate (SP), SP-NH4+ was the main adsorption configuration at pH 5.5-9.5 owing the prevalence of SP on the GT surface under site competition of NH4+-N. The results of CD-MUSIC model and DFT calculation were verified mutually, and provide novel insights into the mechanisms underlying N fixation and migration in soil.

Face-dependent phosphate speciation on goethite: CD-MUSIC modeling and ATR-FTIR/2D-COS study

Understanding the face-dependent phosphate adsorption mechanisms and their variations with environmental conditions is of great significance for revealing phosphate adsorption mechanisms on various goethites and predicting phosphorus speciation in iron-rich soils. In this study, micro- (MicroGoe) and nano-sized goethite (NanoGoe) were synthesized and used to investigate the face-dependent adsorption behaviors of proton and phosphate on goethite by combining the charge distribution-multisite surface complexation (CD-MUSIC) model and attenuated total reflectance Fourier transform infrared (ATR-FTIR). The results demonstrated that MicroGoe had a higher charge density and phosphate adsorption capacity than NanoGoe, which could be attributed to the higher site density of ≡FeOH-0.5 and inner-layer capacitance arising from a higher proportion of capping face and rougher surface of MicroGoe. The logKH of ≡FeOH-0.5 on the main and capping face was 8.2 and 8.9, respectively. Three types of monodentate mononuclear phosphate complexes in different protonated states were identified, along with the non-protonated bidentate complex. Protonated monodentate complexes were formed at relatively low pH and high surface loadings, whereas non-protonated complexes were the predominant species at intermediate to high pH. MicroGoe had a higher percentage of monodentate complexes than NanoGoe, and both goethites had considerably lower phosphate adsorption on the capping face than on the main face. The results provide valuable insights into the interfacial reactivity of goethite prepared with various methods and facilitate further prediction of phosphorus speciation and availability in iron-rich soils.

Transport of polystyrene nanoplastics with different functional groups in goethite-coated saturated porous media: Effects of low molecular weight organic acids and physicochemical properties

The influence of low molecular weight organic acids (LMWOAs) and goethite on the migration of nanoplastics in the soil environment remains poorly understood. To elucidate the mechanism of influence, the study investigated the impact of LMWOAs on the migration ability of functionalized polystyrene nanoplastics (PSNPs-NH2/COOH) in quartz sand (QS) and goethite (α-FeOOH)-coated quartz sand (FOS). We investigated the effect of changes in iron valence induced by LMWOAs on the migration of PSNPs. The results revealed that the migration ability of polystyrene nanoplastics (PSNPs) declined as the ionic strength (IS) increased and the pH decreased, primarily due to the compression of the double layer and protonation reactions. The migration of PSNPs is facilitated by LMWOAs through distinct mechanisms in the two media. Specifically, LMWOAs were adsorbed on the FOS and QS surfaces through complexation and hydrogen bonding, respectively. At pH 4.0, LMWOAs exhibit redox activity, resulting in the generation of additional Fe(III). This redox process enhances the electrostatic attraction between the media and PSNPs, thereby reducing the competition at specific points and spatial resistance associated with LMWOAs. In contrast to FOS, LMWOAs at pH 4.0 reduced the migration ability of PSNPs in QS, following the trend of MA > TA > CA. This difference was attributed to the pKa of LMWOAs and the weak hydrogen bonding on the QS surface. The relevant mathematical models effectively validate the migration results. The above conclusions suggest that LMWOAs can alter the valence state of iron on the surface of goethite, thereby influencing the migration of plastic particles in environmental media.

Efficient Removal of Antimony(V) from Antimony Mine Wastewater by Micrometer Zero-Valent Iron

This paper investigates the effectiveness of two commercial micron zero-valent irons (mZVIs) in removing Sb(V) from antimony mine wastewater. The wastewater contains a range of complex components and heavy metal ions, including As(V), which can impact the removal efficiency of mZVI. The study aims to provide insights into actual working conditions and focuses on influencing factors and standard conditions. The results demonstrate that mZVI can reduce Sb(V) concentration in the mine wastewater from 3875.7 μg/L to below the drinking water standard of 5 μg/L within 2 h. Adding a small amount of mZVI every 30 min helps to maintain a high removal rate. The study confirms the existence of a reduction reaction by changing the atmospheric conditions of the reaction, and the addition of 1,10-phenanthroline highlights the important role of active Fe(II) in the adsorption and removal of Sb(V) by mZVI. Additionally, the paper presents an innovative experimental method of acid treatment followed by alkali treatment, which proves the interfacial reaction between mZVI and Sb(V). Overall, the study demonstrates that the removal of Sb(V) by mZVI entails a dual function of reduction and adsorption, highlighting the potential of mZVI in repairing Sb(V) in antimony mine wastewater.

Recovery of rare earth elements from mine wastewater using biosynthesized reduced graphene oxide

Recycling rare earth elements (REEs) from sources of secondary waste such as REEs mine wastewater has emerged as a sustainable approach with both waste reuse and wastewater processing. In this study, green reduced graphene oxide (G-rGO) was prepared utilizing green tea extract with the advantages of being environmentally friendly, sustainable, and low cost. To understand how G-rGO functions, it was compared to commercial reduced graphene oxide (rGO), and the efficiencies in adsorbing Y(III) were 91.6% and 11.9%, respectively. This indicated there is a synergistic adsorption between the capping layer of G-rGO and rGO alone. G-rGO and rGO were characterized before and after exposure to Y(III). This comparison indicated that Y(III) was adsorbed on the surface of G-rGO through complexation and electrostatic interaction. The adsorption kinetics best fit the pseudo-second-order model and the Langmuir model isotherm model, with adsorption capacities of 24.54 mg g^-1. A probable adsorption mechanism of Y(III) by G-rGO was proposed, involving electronic complexation, electrostatic adsorption and ion exchange. Furthermore, the adsorption efficiencies of G-rGO for Y(III), Ce(III) and Zn(II) in mine wastewater were 22.1%, 89.1% and 14.6%, respectively. These results demonstrate that G-rGO has great potential in the recovery of REEs from mine wastewater.

The heteroaggregation behavior of nanoplastics on goethite: Effects of surface functionalization and solution chemistry

Nanoplastics have attracted extensive attention in recent years. However, little is known about the heteroaggregation behavior of nanoplastics on goethite (FeOOH), especially the contribution of surface functional groups. In this study, the heteroaggregation behavior between polystyrene nanoplastics (PSNPs) and FeOOH was systematically investigated under different reaction conditions. Moreover, the effect of different functional groups (-NH₂, -COOH, and bare) of PSNPs and solution chemistry was evaluated. The results showed that PSNPs could heteroaggregate with FeOOH, and the heteroaggregation rate of PSNPs with surface functionalization was significantly faster. The removal of suspended PSNPs was enhanced with increasing NaCl or CaCl₂ concentration. However, heteroaggregation was significantly inhibited with the increase of solution pH. The zeta potentials analysis, time-resolved dynamic light scattering (DLS) and heteroaggregation experiments suggested that the electrostatic force affected the heteroaggregation process significantly. Fourier transform infrared (FTIR) spectra proved that the adsorption affinity between PSNPs and FeOOH was stronger after surface functionalization, especially for CH, O-C=O, and -CH₂- groups, indicating that chemical bonding also made a contribution during the heteroaggregation process. This work is expected to provide a theoretical basis for predicting the environmental behavior between PSNPs and FeOOH.

A molecular level understanding of antimony immobilization mechanism on goethite by the combination of X-ray absorption spectroscopy and density functional theory calculations

A molecular level understanding of antimony (Sb) immobilization mechanism on Fe oxides is required to clarify the fate of Sb in the soil. In this study, macroscopic sorption experiments, combined with extended X-ray absorption fine structure (EXAFS) spectroscopy and density functional theory (DFT), were utilized to explore the interaction between Sb and goethite. The ion strength has no effect on Sb sorption on goethite, indicating the inner-sphere complex Sb formed on goethite. Goethite has the higher sorption potential to Sb(III) than Sb(V), consistent with the higher thermodynamic stability of the geometry for Sb(III) formed on goethite than Sb(V) revealed by DFT calculations. By comparing the Sb-Fe distances obtained by EXAFS spectroscopy and DFT, eight kinds of Sb(III) surface complexes and nine kinds of Sb(V) surface complexes were considered to be the possible geometries Sb formed on different crystal planes of goethite, including monodentate mononuclear, bidentate mononuclear, bidentate binuclear, tridentate mononuclear, tridentate binuclear, tridentate four-nuclear complexes. The structural and energetic details of these filtered geometries provide comprehensive information on Sb immobilization mechanism on goethite, helpful in clarifying the fate of Sb in soils.