Formation Of Outer-sphere Complexes Research Articles

Sorption of trivalent europium Eu(III) by Na+-substituted bentonite: Mechanistic insight and stabilization effect

The sorption of Eu(III) by Na+-substituted bentonite (Na-bentonite) was investigated as a function of pH and NaNO3 concentration ([NaNO3]0). At pH < ∼7.5, Eu(III) sorption decreased with the increasing [NaNO3]0, whereas at pH > ∼7.5, it remained nearly complete, independent of [NaNO3]0. Our thermodynamic model indicated that the sorption at pH < ∼7.5 occurred via a combination of cation exchange and surface complexation, with the former diminishing as [NaNO3]0 increased. Meanwhile, the sorption at pH > ∼7.5 was primarily due to surface complexation. By X-ray diffraction, the incorporation of hydrated Eu(III) in the interlayers of montmorillonite increased its lattice spacing and crystallinity along the c-axis, where cation exchange was predominant. Also, Eu LIII-edge X-ray absorption spectroscopy revealed that the sample dominated by cation exchange had an absorption edge energy and coordination structure similar to aqueous Eu(III), indicating outersphere complex formation. In other cases, Eu(III) sorption was characterized by innersphere complexation, as indicated by increased covalency and the presence of more pronounced second coordination shells. Importantly, the analysis of dissolved Si and Al suggested that the increased stability of Na-bentonite was likely due to the surface complexation of Eu(III) with aluminol groups at the edges, especially at higher surface coverages. Given its high sorption capacity for Eu(III) and stabilization effect mediated by sorption, Na-bentonite could be serve as an effective backfill material and migration barrier for containing actinides in nuclear waste repositories.

Role of Organic Fertilizer in the Transfer of Lead to Vegetables Produced in Tropical Mountain Agroecosystems.

Understanding the relationship between the aerobic transformation of organic matter (OM) and the bioavailability of lead to plants may allow the safe application of organic fertilizers (OF) in agriculture. The present study aimed to elucidate the relationship of different OM structures with Pb, revealing the action of OF (poultry litter) on Pb dynamics, presenting the effects of OM transformations on bioavailability and transfer to vegetables produced in tropical mountain agroecosystems (TMA). The association of Pb with hydrophilic structures (CAlk-O and CAlk-di-O) during the aerobic transformation of poultry litter (PL) contributes to the increase in the water-soluble form of this metal (3.17-15.30%). The structural changes promoted by the transformation of OM, in addition to reducing the adsorption capacity of Pb in PL (Kd reduction from 1135.50 to 87.49), favor the formation of outer-sphere complexes. PL that have a more labile structure, i.e., those that are less humified, have greater affinity for Pb. The greater affinity of Pb for labile structures that are preserved in PL during OM transformations contributed to its increase and transport to edible plant parts. Considering the edible parts of vegetables grown in TMA and fertilized with fresh PL, 100% of broccoli, 91.78% of cabbage, 80.00% of tomato, 65.96% of parsley, 49.19% of lettuce, and 32.88% of cauliflower showed Pb contamination that exceeded the permitted level. Therefore, OF contributes to lead contamination of food produced in TMA, representing a risk to human health. Studies are needed to propose additional treatments for this residue before its use.

Aluminum adsorption using different models of hydroxyapatite via Molecular Dynamic simulations

The activities of the anodizing industry, mining, and the inadequate use of coagulants and flocculants for wastewater treatment are some of the causes of an alarming increase in aluminum levels in water bodies. Adsorbents, such as bone char, are used for the removal of metal ions from water due to their easy operation, regeneration, and low cost. Bone char is constituted mainly by hydroxyapatite and in order to understand the mechanism of aluminum adsorption onto hydroxyapatite an adequate analysis of the effect of the surface exposed to the aqueous solution is necessary. Different facets of hydroxyapatite, 001 and 010, were studied, which have been protonated to represent the acidic conditions of the wastewater from the anodizing industry. The results of Molecular Dynamic simulations show that the protonation of the 001 facet leads to an increase in the adsorption of aluminum. The coarse surface of the 010 facet at acidic pH conditions cause that the water molecules adsorb on the sub-layers of the surface. The adsorption of aluminum occurs preferentially on the phosphates of hydroxyapatite and is due to the formation of outer-sphere adsorption complexes.

In the presence of the other: How glyphosate and peptide molecules alter the dynamics of sorption on goethite

The interactions with soil mineral surfaces are among the factors that determine the mobility and bioavailability of organic contaminants and of nutrients present in dissolved organic matter (DOM) in soil and aquatic environments. While most studies focus on high molar mass organic matter fractions (e.g., humic and fulvic acids), very few studies investigate the impact of DOM constituents in competitive sorption. Here we assess the sorption behavior of a heavily used herbicide (i.e., glyphosate) and a component of DOM (i.e., a peptide) at the water/goethite interface, inclusive of potential glyphosate-peptide interactions. We used in-situ ATR-FTIR (attenuated total reflectance Fourier-transform infrared) spectroscopy to study sorption kinetics and mechanisms of interaction as well as conformational changes to the secondary structure of the peptide. NMR (nuclear magnetic resonance) spectroscopy was used to assess the level of interaction between glyphosate and the peptide and changes to the peptide' secondary structure in solution. For the first time, we illustrate competition for sorption sites results in co-sorption of glyphosate and peptide molecules that affects the extent, kinetics, and mechanism of interaction of each with the surface. In the presence of the peptide, the formation of outer-sphere glyphosate-goethite complexes is favored albeit inner-sphere glyphosate-goethite bonds (i.e., POFe) are still formed. The presence of glyphosate induces secondary structural shifts of the sorbed peptide that maximizes the formation of H-bonds with the goethite surface. However, glyphosate and the peptide do not seem to interact with one another in solution nor at the goethite surface upon sorption. The results of this work highlight potential consequences of competition for sorption sites, for example the transport of organic contaminants and nutrient-rich (i.e., nitrogen) DOM components in relevant environmental systems. Predicting the rate and extent with which organic pollutants are removed from solution by a given solid is also one of the most critical factors for the design of effective sorption systems in engineering applications.

Facile synthesis of flower shaped magnesium ferrite (MgFe2O4) impregnated mesoporous ordered silica foam and application for arsenic removal from water

Magnesium ferrite (MF0.33) impregnated flower-shaped mesoporous ordered silica foam (MOSF) was successfully synthesized in present study. MOSF was added with precursor solution of MF0.33 during MF0.33 synthesis which soaked the materials and further chemical changes occurred inside the pore. Therefore, no additional synthesis process was required for magnesium ferrite impregnated mesoporous ordered silica foam (MF0.33-MOSF) synthesis. MF0.33-MOSF showed higher morphological properties compared to other magnesium ferrite modified nanomaterials and adsorbed arsenic III [As(III)] and arsenic V [As(V)] 42.80 and 39.73 mg/g respectively. These were higher than those of other Fe-modified adsorbents at pH 7. As MOSF has no adsorption capacity, MF0.33 played key role to adsorb arsenic by MF0.33-MOSF. Data showed that MF0.33-MOSF contain about 2.5 times lower Fe and Mg than pure MF0.33 which was affected the arsenic adsorption capacity by MF0.33-MOSF. Adsorption results best fitted with Freundlich isotherm model. The possible mechanism of arsenic adsorption might be chemisorption by electrostatic attraction and inner or outer-sphere surface complex formation.

Sustainable treatment of boron industry wastewater with precipitation-adsorption hybrid process and recovery of boron species

Boron removal from wastewater has been investigated by using various processes, including ion exchange resins, membrane processes and adsorption. Each method has various advantages and disadvantages, but most produce excessive waste in addition to high operational costs. Therefore, more sustainable methods are required for wastewater containing high concentrations of boron. In this study, a sustainable treatment process was developed for wastewater containing high concentrations of boron. This study investigated the removal of boron from wastewater by Al(OH)3 sorption. The reuse of adsorbent (Al(OH)3) and the potential recovery of boric acid was the main goal of the study. It was observed that although lower concentrations of boron were obtained at pH 10.5 (980 mg/L and 635 mg/L at pH of 9.0 and 10.5, respectively), the amount of sorbed boron at pH 9 was substantially higher (94.7 mg B/g Al(OH)3 and 27.8 mg B/g Al(OH)3 at pH of 9.0 and 10.5, respectively). This was attributed to the higher initial boron concentrations and the formation of boric acid and polyborate complexes at pH 9.0. Results showed that polyborate species sorption was an outer sphere complex formation, which led to the desorption of boron as pH lowered. Adsorbed boron species to Al(OH)3 could effectively be desorbed at low pH values (pH<5.0); which allows Al(OH)3 to be used in successive adsorption studies. Approximately 55% of boron recovery from pretreated wastewater was possible with the effective reuse of adsorbent. A net profit of 2.85 $/m3 could be obtained based on the amounts of chemical consumptions and boron recovery.

Sorption of Selenium(IV) and Selenium(VI) onto Iron Oxide/Hydroxide-Based Carbon Materials: Activated Carbon and Carbon Foam

Selenium pollution in water is a worldwide issue. Se(IV) and Se(VI) are mainly found in contaminated water due to their high solubility and mobility; their presence poses a serious risk as they can severely harm human health. Although iron oxide and hydroxide nanoparticles can be efficient candidates for the removal of selenium oxyanions due to their high adsorption capacity, the role of each iron species has not been fully elucidated. Furthermore, iron species are often found to be less effective for Se(VI) than Se(IV). The challenge and novelty of this study was to develop a carbon material impregnated with different iron phases, including oxides (magnetite/hematite) and hydroxides (goethite/lepidocrocite) capable of removing both Se(IV) and Se(VI). Since the phase and morphology of the iron nanoparticles play a significant role in Se adsorption, the study evaluated both characteristics by modifying the impregnation method, which is based on an oxidative hydrolysis with FeSO4 7H2O and CH3COONa, and the type of carbonaceous support (activated carbon or sucrose-based carbon foam). Impregnated activated carbons provide better removal efficiencies (70–80%) than carbon foams (&lt;40%), due to their high surface areas and point zero charges. These results show that the adsorption of Se(VI) is more favorable on magnetic oxides (78%) and hydroxides (71%) than in hematite (&lt;40%). In addition, the activated carbon decorated with magnetite showed a high adsorption capacity for both selenium species, even in alkaline conditions, when the sorbent surface is negatively charged. A mechanism based on the adsorption of inner-sphere complexes was suggested for Se(IV) immobilization, whereas Se(VI) removal occurred through the formation of outer-sphere complexes and redox processes.

Highly efficient separation of Nb from Ta using a unique solvent system containing tri-n-octyl phosphine oxide in an ionic liquid

The present investigation demonstrated the mutual separation Nb and Ta from their mixture in hydrochloric acid medium using TOPO in an ionic liquid (C6mim•NTf2). The maximum separation factor achieved was ∼ 4400. In C6mim•NTf2, the extraction proceeded via 'cation exchange' mechanism with MOCl2.TOPOIL+ as the species; while in n-dodecane the extraction proceeded via neutral 'solvation' mechanism with MOCl3.2TOPO as the extracted species (where, M = Nb and Ta). The sluggish extraction in C6mim•NTf2 was attributed to the its high viscosity coefficient compared to n-dodecane. The extraction was found to be spontaneous and exothermic in nature with a reduction in overall entropy indicating the formation of outer-sphere complexes. Dilute acid was found to be effective for the back extraction of Nb and Ta from the loaded n-dodecane phase, while EDTA - guanidine carbonate or DTPA - Guanidine carbonate were effective for back extraction from the ionic liquid phase. It was found that 2–3 contacts were sufficient for the quantitative back extraction of the metal ions. The C6mim•NTf2 based solvent systems exhibited better radiolytic stability compared to that of the n-dodecane based solvent system.

Biosorption of Methylene Blue dye by Ligustrum lucidum fruits biomass: Equilibrium, isotherm, kinetic and thermodynamic studies

&lt;p&gt;.In this work, biomass from Ligustrum lucidum fruits (LF) was used for the first time as a novel adsorbent to remove the Methylene Blue dye (MB) from aqueous solutions. The adsorption equilibrium was attained after 60 min, and the adsorption followed the pseudo-second-order kinetic model. The rise of temperature from 295 K to 313 K resulted in a decline in the adsorption capacity, suggesting an exothermic process. The Langmuir isotherm model was found to fit better the experimental adsorption data than the Freundlich isotherm model. The maximum monolayer adsorption capacity obtained from the Langmuir isotherm model at pH 5.6 was evaluated to be 95.4 mg/g and 91.8 mg/g at 295 K and 313 K, respectively. The thermodynamic studies indicated that the adsorption was a spontaneous and exothermic process with increasing randomness at the solid/solution interface. Based on FTIR and ionic strength studies, the adsorption of MB by LF mainly occurs via electrostatic interactions and the formation of outer-sphere complexes.&lt;/p&gt;&#x0D;

Highly efficient adsorption of arsenite from aqueous by zirconia modified activated carbon

In the present study, activated carbon decorated with zirconia nanoparticle composite (ZrO2/AC) have been prepared using hydrothermal method and then utilized as an adsorbent for the removal of arsenite from aqueous solution. The composite is composed of highly agglomerated ZrO2 particles on AC surface. The high surface area of the synthesized adsorbent results in its high adsorption capacity toward arsenite. Adsorption isotherm and kinetic data of arsenite adsorption process can be well fitted to the Langmuir isotherm with maximum monolayer adsorption capacity of 64.00 mg.g-1. The experimental results suggest that the adsorption process of arsenite onto ZrO2/AC material involves the formation of both outer-sphere and inner-sphere complexes between arsenite and adsorbent. The high adsorption ability toward arsenite as well as the high recyclable nature of this material makes it a potential alternative material for the removal of toxic heavy metals from wastewater.

Fast Removal of Tetracycline from Aqueous Solution by Aluminosilicate Zeolite Nanoparticles with High Adsorption Capacity

Aluminosilicate zeolite nanoparticles were synthesized in a water-in-oil solvent and applied as an adsorbent for tetracycline (TC) removal from aqueous solutions. The large specific surface area at 495.8 m2 g–1 confirms that the zeolite nanoparticles have mesopores. The zeolite nanoparticles show an excellent removal efficiency for TC, achieving over 97% in the pH range of 4.70–7.17. A fast adsorption kinetics was observed, reaching equilibrium in only 20 min following a pseudo-second-order kinetic model. The adsorption isotherm was found to follow the Langmuir model, with a maximum adsorption capacity of TC of 454.55 mg g–1 at pH 6.7 with no salt added. After 6 cycles of reuse, the removal efficiency of TC remained high at 90.3%. It was understood that the adsorption process is spontaneous and exothermic, with an activation energy of 37.94 kJ mol–1. The high observed adsorption affinity is attributed to hydrogen bonding between TC and the hydroxyl groups on the zeolite nanoparticles and the formation of outer-sphere surface complexes. This study shows a new way to synthesize aluminosilicate zeolite nanoparticles that are an efficient and recyclable adsorbent for effective removal of TC from contaminated water.

Noncoordinating Secondary Sphere Ion Modulates Supramolecular Clustering of Lanthanides

The role of counterions in molecular recognition of lanthanides is underexplored, especially when they exhibit weak interactions with the metal cations. Here, we report a complementary and comprehensive investigation integrating theoretical calculations with X-ray absorption fine structure spectroscopy, dynamic light scattering, and small-angle X-ray scattering to reveal atomic-scale structural features beyond the immediate coordination sphere of a system used for rare-earth element separations. Our results indicate the formation of an unusual T-shaped outer-sphere lanthanide complex, containing two ligands and two nitrate ions in the first coordination sphere, whereas the third nitrate is weakly coordinated and resides in the second shell. This unique structural arrangement causes inhomogeneous charge distribution, leading to self-assembly of the complexes into larger nanoclusters through sterically directed electrostatic interactions in the nonpolar medium. Our findings point to the importance of "noncoordinating" anions in defining the degree of supramolecular aggregation and ion cluster assembly.

Removal of tetracycline from aqueous solutions by adsorption on raw Ca-bentonite. Effect of operating conditions and adsorption mechanism

• Bentonite had the highest capacity for adsorbing tetracycline (TC) than other clays. • Bentonite presented a maximum adsorption capacity towards TC of 283.5 mg/g. • Adsorption capacity of bentonite was comparable to those of carbon-based materials. • TC adsorbed on the external surface and in the interlayer space of bentonite. • Adsorption of TC was due to cation exchange and electrostatic attraction. This study aims to investigate the adsorption of tetracycline (TC) from water onto several clays with different structural characteristics such as bentonite, sepiolite, vermiculite, halloysite, phlogopite and kaolinite. Among these, raw bentonite (Bent) presented the highest adsorption capacity due to its swelling characteristic, interlayer cations nature, and TC molecular size. The Bent and Bent equilibrated with TC were characterized by SEM, XRD, FT-IR, TGA, N 2 physisorption and Zeta potential techniques. The importance of the pH, ionic strength and temperature upon Bent adsorption capacity towards TC was studied thoroughly, and the reversibility of TC adsorption onto Bent was also analyzed. The results showed that the TC adsorption occurred on the external surface and in the interlayer space of Bent, and both physical and chemical mechanisms governed the adsorption. The basal space (0 0 1) at pH = 7 increased from 1.50 to 1.62 nm, while raised from 1.50 to 1.89 nm at pH = 3. The TC maximum uptake was 283.5 mg/g at pH = 3, relating to cationic exchange between interlaminar cations of Bent and the cationic TC species (TCH 3 + ), and electrostatic attraction among the negative surface of Bent and TCH 3 + . At pH = 7, there are weak electrostatic attractions between the Bent cationic sites and the positively charged dimethylamino group of the zwitterionic TC species (TCH 2 ± ) and the formation of outer-sphere coordination complexes between the TC negative species (TCH - ) and the internal hydration sphere of the interlayer Ca 2+ cation. Additionally, the TC adsorption on Bent was partially reversible and endothermic.

Facile synthesis of hydrochar-supported catalysts from glucose and its catalytic activity towards the production of functional amines

Since the utilization of abundant biomass to develop advanced materials has become an utmost priority in recent years, we developed two sustainable routes (i.e., the impregnation method and the one-pot synthesis) to prepare the hydrochar-supported catalysts and tested its catalytic performance on the reductive amination. Several techniques, such as TEM, XRD and XPS, were adopted to characterize the structural and catalytic features of samples. Results indicated that the impregnation method favors the formation of outer-sphere surface complexes with porous structure as well as well-distributed metallic nanoparticles, while the one-pot synthesis tends to form the inner-sphere surface complexes with relatively smooth appearance and amorphous metals. This difference explains the better activity of catalysts prepared by the impregnation method which can selectively convert benzaldehyde to benzylamine with an excellent yield of 93.7% under the optimal reaction conditions; in contrast, the catalyst prepared by the one-pot synthesis only exhibits a low selectivity near to zero. Furthermore, the gram-scale test catalyzed by the same catalysts exhibits a similar yield of benzylamine in comparison to its smaller scale, which is comparable to the previously reported heterogeneous noble-based catalysts. More surprisingly, the prepared catalysts can be expediently recycled by a magnetic bar and remain the satisfying catalytic activity after reusing up to five times. In conclusion, these developed catalysts enable the synthesis of functional amines with excellent selectivity and carbon balance, proving cost-effective and sustainable access to the wide application of reductive amination.

Dimensionality effects on multicomponent ionic transport and surface complexation in porous media

Coulombic interactions between charged species in pore water and at surface/solution interfaces are of pivotal importance for multicomponent reactive transport in porous media. In this study, we investigate the impact of domain dimensionality on electrostatically coupled dispersion and surface-solution reactions during transport of acidic plumes and major ions in porous media.Column and quasi two-dimensional flow-through experiments were performed, with identical silica porous media and under the same advection-dominated conditions. Equal mass fluxes of different electrolyte solutions (i.e., HCl - pH ∼ 2.8, NaBr - 100 mM, HCl - pH ∼ 2.8 plus NaBr - 100 mM) were continuously injected in the 1-D and 2-D experiments and breakthrough curves of pH and major ions were measured at the outlet of the domains. The presence of pronounced ionic strength gradients in the transverse direction in the 2-D setup caused distinct retardation and transport behaviors of protons and major ions which were not observed in the one-dimensional column experiments. Furthermore, in the cases of salt electrolytes injection, considerably enhanced release of H+ (>61%) from the quartz surface was observed in the multidimensional system compared to the one-dimensional setup.Reactive transport modeling was performed to reproduce the experimental outcomes and to analyse the coupling between transport processes, based on the Nernst-Planck formulation of diffusive/dispersive fluxes and on surface complexation reactions at the solid-solution interface. Electrostatic interactions between Na+, Br−, and H+, and deprotonation of the quartz surface upon the formation of sodium outer-sphere complexes, are the primary controllers of the spatial and temporal features displayed by the pH and major ions measurements. The reactive transport simulations allowed us to interpret the experimental observations, to visualize the distribution and spatio-temporal evolution of dissolved and solid species, to identify a spatially heterogeneous zonation of Coulombic interactions with distinct behavior at the fringe and core of the injected plumes in the multidimensional setup, and to quantify the different components of the Nerst-Planck fluxes of the charged solutes. This study demonstrates that the domain dimensionality directly affects electrostatic interactions between charged aqueous species in the pore water and surface complexation reactions at the solid-solution interface. The non-trivial effects of dimensionality on multicomponent ionic transport result in a significantly different behavior in 1-D and 2-D systems.

Synthetic Zeolites Modified with Salts of Transition Metals in the Reaction of Chemisorption-catalytic Oxidation of Sulphur Dioxide by Air Oxygen

The effect of the nature and concentration of d-metal salts attached to synthetic zeolites NaA and KA on the kinetic and stoichiometric parameters of the chemisorption-catalytic oxidation of sulphur dioxide with air oxygen at ambient temperature was studied. It was found that the adsorption capacity of NaA zeolite relative to SO2 is 100 times higher than that of KA zeolite; the time of protective action of NaA and KA zeolites increases upon modification with transition metal salts and with an increase of their content in the compositions. It was shown that the formation of inner and outer sphere complexes and the relationship between them is determined by the nature and concentration of metal ions and by the nature of the carrier. It was proven that the chemisorption-catalytic process ends with the oxidation of SO2 to H2SO4.

Single-stage production of miscanthus hydrochar at low severity conditions and application as adsorbent of copper and ammonium ions

In the framework of bio-circular economy, miscanthus biomass was valorized through a single-stage, low severity hydrothermal carbonization process. The produced hydrochars were characterized using elemental and spectroscopic methodologies. It was determined that as the temperature increased so did the C content (47.9 and 68.9% for the samples prepared at 180 and 260 °C, respectively), whereas the O content decreased (from 44.2 to 25.5%, respectively). The adsorption behaviour of the hydrochars was investigated in the adsorption of Cu2+ and NH4+ and MIS-180 was determined as the optimum sample, achieving qmax values of 310 and 71 mg g−1, respectively. Isotherm and kinetic analysis indicated the higher number of O-containing functional groups of MIS-180 as the main reason for its higher adsorption capacities. Furthermore, Cu2+ adsorption followed the 2nd-order kinetic model, whereas NH4+ adsorption followed the 1st-order kinetic model, due to the different mechanisms involved, inner-sphere and outer-sphere complex formation, respectively.

Efficient and Selective Adsorption of Neodymium on Expanded Vermiculite

In this study, expanded vermiculite (EV) was applied to the adsorption of neodymium (Nd) from an aqueous solution with focus on the description of the kinetics and the mechanism of adsorption, determination of the best eluent, and selectivity. The EV was characterized before and after the adsorption process by the morphology, composition, textural properties (specific surface area, density, and porosity), and functional groups. The adsorption experiments showed that under optimal conditions, using 0.7 g of adsorbent and at pH 3.3, the efficiency achieved an enhanced adsorption percentage of 98.7%, which represents an adsorption capacity of 0.205 mmol/g. The external mass and diffusional resistance controlled the mass transfer, and ion exchange occurred mainly with magnesium cations. The proposed mechanism of adsorption was cation exchange in the interlayer region of the vermiculite and the formation of outer-sphere complexes. Calcium chloride was the most suitable eluent, regarding efficiency and low toxicity. The adsorbent preferably adsorbed Nd from a multimetal solution, achieving enhanced values of selectivity. Regarding characterization, the EV presented a well-organized structure, a lamellar morphology, and typical functional groups such as Al–O and Si–O. Thus, the EV adsorbed Nd quickly and efficiently, and the ion-exchange mechanism of adsorption allows the selective adsorption of Nd and safe desorption by the exchange with Ca2+.

Amine-bridged periodic mesoporous organosilica nanomaterial for efficient removal of selenate

Contamination of selenate (Se(VI)), an important oxyanion, has been widely detected in numerous water sources because of its high water solubility and mobility. In this study, a family of amine-bridged periodic mesoporous organosilicas (PMOs) with tunable and high contents of amine functionality were prepared through a facile (co–)condensation approach for adsorptive removal of Se(VI). The composition, structure and properties of amine-bridged PMOs were extensively characterized using various techniques, such as scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction, Fourier transform infrared spectroscopy, surface area and porosity measurement, thermo-gravimetric analysis, zeta potential measurement, and elemental analysis. The optimal amine-bridged PMO had a high amine loading of 4.04 mmol/g. The Se(VI) adsorption behaviors strongly depended on the amine content of the amine-bridged PMOs. The optimal amine-bridged PMO exhibited both fast Se(VI) adsorption kinetics and a high Se(VI) adsorption capacity of ~ 175 mg/g, because of the high amine content, uniformly distributed amine functional groups within the mesoporous skeleton, and large surface area. Furthermore, the optimal amine-bridged PMO showed robust performance for efficient removal of Se(VI) under a wide range of pH and in the presence of various common co-existing anions, and can be conveniently regenerated and reused for multiple cycles. Combined spectroscopic characterization and aqueous adsorption experiments suggested that the primary mechanism for Se(VI) removal may be attributed to the formation of outer-sphere surface complexes between Se(VI) and the surface of amine-bridged PMOs.

Oxygen atom release during selenium oxyanion adsorption on goethite and hematite

Adsorption of selenium oxyanions (selenite and selenate) on soil minerals can mitigate Se ecotoxicity effects by removing Se from water resources. Se oxyanion surface complexation on Fe oxides have been investigated for decades using spectroscopic and surface complex modeling. Here, insights on the Fe-O-Se bonding process on goethite and hematite were gained via integrating our observed batch Se adsorption isotherms, H+ coadsorption with Se, X-ray absorption spectroscopy (XAS)-derived surface complex geometry, surface complexation modeling, and 18O tracing across oxide and aqueous phases Adsorption extents of selenite and selenate on goethite and hematite decreased as pH increased from 3.0 to 7.0 and follows a corresponding decrease in positive zeta potential which offers less electrostatic attraction. Surface complexation geometry was determined with XAS results that indicated the formation of inner-sphere binuclear bidentate selenite complexes and outer-sphere selenate complexes on both oxides within the range of experimental conditions. These surface complexes were sufficient to model Se adsorption isotherms and H+ coadsorption on both oxides at pH 3.0, 5.0, and 7.0 using the Charge Distribution Multi-site Complexation (CD-MUSIC) model. Inner-sphere bidentate selenite complex formation requires release of two O, and by tracing 18O release from 18O-enriched oxides to isotopically normal water, it was found that O release from the oxide surface accounted for 22% and 7% of total adsorption-induced O release for goethite and hematite, respectively, but only at pH 3.0. The remaining O release is assumed to be from adsorbed selenite. Selenate at pH 3.0 also induced a slight increase in 18O enrichment in bulk solution, which is unexpected for selenate's outer-sphere configuration that should produce no O ligand exchange, but instead highlights some spontaneous adsorption-induced oxide O release.