Inner-sphere Surface Complexes Research Articles

Zn stable isotope fractionation during adsorption onto todorokite: A molecular perspective from X-ray absorption spectroscopy and density functional theory

Mineral-solution interface reactions control the mobility and fate of trace metals (e.g., Zn) and may drive their associated isotopic fractionation in the natural environment, but understanding the coupling between interfacial reactions and isotopic behavior requires a molecular-level understanding of these processes. In this study, we investigate Zn stable isotope fractionation during adsorption to todorokite as a function of reaction time, pH and Zn concentrations. We show that Zn stable isotope fractionation reaches equilibrium at ∼12 h reaction time at pH 6 and Zn concentrations of 0.05 mM. For Zn concentrations of 0.05 and 0.2 mM, Zn isotopic fractionation between adsorbed and aqueous Zn (Δ66Znadsorbed-aqueous) is approximately −0.1 ± 0.04‰ at pH 3–5, and Δ66Znadsorbed-aqueous gradually increases from −0.1 ± 0.04‰ to 0.05 ± 0.05‰ at higher pH 6–8. Extended X-ray absorption fine structure (EXAFS) spectroscopy shows that Zn adsorbs to the todorokite surface as an outer-sphere octahedral complex with an average ZnO interatomic distance of 2.06 Å at pH 3. In contrast, Zn is predominantly present as a tetrahedral coordinated structure with a shorter average ZnO interatomic distance of ∼2.00–2.05 Å at pH 6 and 8, suggesting the presence of a mixture of octahedral outer-sphere and tetrahedral inner-sphere surface complexes. Density functional theory calculations suggest that outer-sphere surface complexes exist in the center of the structural tunnels of todorokite, yielding a theoretical Zn isotopic fractionation (Δ66Znadsorbed-aqueous) of −0.2‰ to −0.3‰, whereas the tetrahedral inner-sphere surface complex results in a large Δ66Znadsorbed-aqueous of +0.5‰ to +0.8‰. Combined laboratory experiments and theoretical calculations demonstrate that different magnitudes of Zn isotopic fractionation are controlled by structural changes (e.g., coordination and bond distance) in the Zn surface complexes formed on todorokite relative to its aqueous form (i.e., aqua Zn(H2O)62+). These results provide important new constraints for understanding Zn isotope signatures in natural Mn-rich sediments and lead to a more complete understanding of Zn isotopes in the ocean.

Mg-Fe LDH-coated biochars for metal(loid) removal: Surface complexation modeling and structural change investigations

Although an increasing number of studies involving layered double hydroxide/biochar composites (LDH/BC) have been published recently, development of advanced modeling approaches of adsorption onto the composites and/or their testing and use in real soils and waters are rare. In this study, Zn(II) and As(V) sorption onto Mg-Fe LDH/BC composites was quantified using bulk sorption experiments, and the observed adsorption behavior was modeled using a non-electrostatic surface complexation model (NEM). In general, it was found that LDH is the main phase causing the removal of metal(loid)s from solution. In contrast, the contribution of BC was evident only in systems containing the most effective type of BC. Inner-sphere surface complexation was found to be the dominant adsorption mechanism at low Zn(II) and As(V) concentrations. In contrast, precipitation of Zn and formation of amorphous ferric arsenates contributed to the total metal removal at higher metal(loid) loadings. Surprisingly, increasing As(V) concentrations led to enhanced Mg leaching from the LDH/BC composites and to the destruction of the LDH structure. Nevertheless, all composite materials were stable at environmentally relevant metal(loid) concentrations. Our study is the first to use a surface complexation component additivity (CA) modeling approach to account for metal(loid) adsorption behavior onto LDH/BC composites, and we find that although site blocking does occur to some extent, the CA approach provides a reasonable estimate of the adsorption behavior across a wide range of pH conditions. Comprehensive studies such as this one are necessary in order to understand and optimize the behavior of novel composite materials before their subsequent use in environmental technologies.

Mercaptocarboxylic acid intercalated MgAl layered double hydroxide adsorbents for removal of heavy metal ions and recycling of spent adsorbents for photocatalytic degradation of organic dyes

Adsorption is considered aneffectiveway for the removal of heavy metal ions in aqueous solutions; however, recycling of spent adsorbents is a formidable challenge. We synthesized mercaptocarboxylic acid/layered double hydroxide (MPA/LDH) adsorbents for the removal of heavy metal ions from wastewater, and it demonstrated excellent adsorption capacities; the maximum adsorption capacities for Hg2+, Pb2+, and Cu2+ were 250.30, 122.10, and 105.33 mg/g at adsorbent dosage of 0.2 g/L and pH of 5.5. The excellent adsorption performance was attributedtothe inner-sphere surface complexation between the heavy metal ions and the S-S and COO– groups in MPA/LDH. Moreover, host–guest interactions between MPA and the LDH laminate may have enhanced the adsorptionefficiencyand selectivity of MPA/LDH. After the adsorption of Hg2+, Pb2+, and Cu2+, highlydispersed metal sulfide NPs were spontaneously formed in the LDH interlayer via confined base catalysis, providing excellent photocatalytic activity for the degradation of organic dyes. High photocatalytic activity was attributed to the suitable energy band and high dispersity of metal sulfides. MPA/LDH-absorbed Hg2+ showed the best photodegradation performance of 99.2% for methylene blue. Notably, MPA/LDH can be used for the simultaneous removal of heavy metals and organic dyes under visible light irradiation. The findings of this study provide insights for developing an environmentally friendly method for conversion from adsorbent to photocatalyst, demonstrating wide prospects in industrial applications.

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.

Fast cost-effective synthesis of metal ions/biopolymer/silica composites by supramolecular hydrogels crosslink with superior tetracycline sorption performance

In this study, a fast one-pot method was developed for the preparation of Cu/CS/Si ternary composites, which can efficiently remove antibiotic tetracycline from aqueous solutions. Our results demonstrated that the Cu and its content in the composites played a significant role in determining the physical properties and internal morphology of the Cu/CS/Si composites, which subsequently affected the efficiency of the composites for the sorptive removal of tetracycline. Among the studied composites, Cu3–CS2–Si materials had the largest sorption capacity for tetracycline (1076.7 mg/g) with a fast sorption kinetics (>99% in 30 min) under a broad working pH range (5-10). The results from the batch sorption experiments, together with spectroscopic and microscopic analyses, collectively indicated that Cu-tetracycline inner-sphere surface complexation through Cu–O bond was responsible for the tetracycline sorption on Cu3–CS2–Si. In addition, the Cu3–CS2–Si showed an excellent reusability in removing tetracycline. The desired sorption and reuse properties, coupled with the facile and cost-effective synthesis method, indicated that Cu/CS/Si composites have a promising potential for the efficient removal of tetracycline from contaminated solutions.

Green sulfidated iron oxide nanocomposites for efficient removal of Malachite Green and Rhodamine B from aqueous solution.

A green and facile pathway was described using Viburnum odoratissimum leaf extract in the presence of sodium thiosulfate for the synthesis of sulfidated iron oxide nanocomposites (S-Fe NCs) adsorbents. The prepared S-Fe NCs can be used for the efficient removal of Malachite Green (MG) and Rhodamine B (RhB) from aqueous solution. Analytical techniques by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) were applied to understand the morphologies and compositions of S-Fe NCs. The stability of the adsorption capacity on S-Fe NCs was studied. Results from the characterization studies showed that S-Fe NCs were mainly composed of iron oxides, iron sulfides and biomolecules. The S-Fe NCs displayed high adsorption capacity for a wide range of pH values. The Koble-Corrigan isotherm model and Elovich model well described the adsorption process. The maximum adsorption capacity for MG and RhB was 4.31 mmol g-1 and 2.88 mmol g-1 at 303 K, respectively. The adsorption mechanism may be attributed to the electrostatic interaction, the hydrogen bonding, the π-π stacking interactions, the inner-sphere surface complexation or the cation bridging among the S-Fe NCs and dye molecules.

Trivalent ion overcharging on electrified graphene

The structure of the electrical double layer (EDL) formed near graphene in aqueous environments strongly impacts its performance for a plethora of applications, including capacitive deionization. In particular, adsorption and organization of multivalent counterions near the graphene interface can promote nonclassical behaviors of EDL including overcharging followed by co-ion adsorption. In this paper, we characterize the EDL formed near an electrified graphene interface in dilute aqueous YCl3 solution using in situ high resolution x-ray reflectivity (also known as crystal truncation rod) and resonant anomalous x-ray reflectivity (RAXR). These interface-specific techniques reveal the electron density profiles with molecular-scale resolution. We find that yttrium ions (Y3+) readily adsorb to the negatively charged graphene surface to form an extended ion profile. This ion distribution resembles a classical diffuse layer but with a significantly high ion coverage, i.e., 1 Y3+ per 11.4 ± 1.6 Å2, compared to the value calculated from the capacitance measured by cyclic voltammetry (1 Y3+ per ∼240 Å2). Such overcharging can be explained by co-adsorption of chloride that effectively screens the excess positive charge. The adsorbed Y3+ profile also shows a molecular-scale gap (⩾5 Å) from the top graphene surfaces, which is attributed to the presence of intervening water molecules between the adsorbents and adsorbates as well as the lack of inner-sphere surface complexation on chemically inert graphene. We also demonstrate controlled adsorption by varying the applied potential and reveal consistent Y3+ ion position with respect to the surface and increasing cation coverage with increasing the magnitude of the negative potential. This is the first experimental description of a model graphene-aqueous system with controlled potential and provides important insights into the application of graphene-based systems for enhanced and selective ion separations.

Efficient Sorption of Arsenic on Nanostructured Fe-Cu Binary Oxides: Influence of Structure and Crystallinity.

To study the structure-performance relationship, a series of nanostructured Fe-Cu binary oxides (FCBOs) were prepared by varying synthesis conditions. The obtained binary oxides were well characterized using X-ray diffraction (XRD), transmission electron microscope (TEM), Brunner-Emmet-Teller (BET), magnetic and Zeta potential measurement techniques. Both As(V) and As(III) sorption on the FCBOs were evaluated by batch tests. Results show that the surface structure and crystallinity of FCBOs are greatly dependent on preparation conditions. The crystallinity of FCBOs gradually increases as the synthesis pH value increasing from 9.0 to 13.0, from amorphous phase to well-crystalline one. Simultaneously, the morphology change of FCBOs from irregular agglomerate to relatively uniform polyhedron has been observed. The sorption of arsenic is greatly influenced by the crystallinity and structure of FCBOs, decreasing with increasing degree of crystallinity. The amorphous FCBO has higher surface hydroxyl density than well-crystalline one, which might be the reason of higher sorption performance. As(V) is sorbed by the FCBOs via formation of inner-sphere surface complexes and As(III) is sorbed through formation of both inner- and outer-sphere surface complexes. This investigation provides new insights into structure-performance relationship of the FCBO system, which are beneficial to develop new and efficient sorbents.

Simultaneous removal of antimony(III/V) and arsenic(III/V) from aqueous solution by bacteria–mediated kaolin@Fe–Mn binary (hydr)oxides composites

Antimony (Sb) and arsenic (As) pollution in aquatic ecosystems has received great attention owing to their environmental toxicity and health risk. In this work, a green kaolin@Fe–Mn binary (hydr)oxides composites (BKFMs) synthesized by iron oxidizing bacteria (IOB) mediation was developed to simultaneously remove Sb and As. The prepared BKFMs possessed large specific surface area and pore volume, as well as abundant functional groups, which contributed to eliminate Sb and As. Among prepared composites, BKFM (1:0.1) exhibited the best Sb and As removal performance, and the kinetics data of Sb(III/V) and As(III/V) by BKFM (1:0.1) conformed to the pseudo-second-order kinetic and Elovich models. Moreover, the isotherm experimental results indicated that the maximum BKFM (1:0.1) adsorption capacities for Sb(III), Sb(V), As(III) and As(V) were 177.19, 56.26, 62.92 and 42.18 mg/g, respectively (pH = 6.0 ± 0.1, T = 25 °C). In addition, the solution pH and co-existing substances had varying effects on Sb(III/V) and As(III/V) adsorption. Characterization techniques of FTIR and XPS revealed that the inner-sphere complexation, surface complexation, hydrogen bonding and oxidation contributed to the mechanism of adsorption of Sb and As. The results demonstrated that BKFM (1:0.1) has great potential for future application in Sb and As removal from contaminated water.

Highly efficient removal of thallium(I) by facilely fabricated amorphous titanium dioxide from water and wastewater

In this study, amorphous hydrous titanium dioxide was synthesized by a facile precipitation method at room temperature, aiming to effectively remove thallium(I) from water. The titanium dioxide prepared using ammonia as precipitant (TiO2I) is more effective for thallium(I) uptake than the one synthesized with sodium hydroxide (TiO2II). The TiO2 obtained particles are amorphous, aggregates of many nanoparticles and irregular in shape. The thallium(I) uptake increases with the rise of solution pH value. Under neutral pH conditions, the maximal thallium(I) adsorption capacities of TiO2I and TiO2II are 302.6 and 230.3 mg/g, respectively, outperforming most of the reported adsorbents. The amorphous TiO2 has high selectivity towards thallium(I) in the presence of multiple cations such as K+, Ca2+, Mg2+, Zn2+ and Ni2+. Moreover, the TiO2I is efficient in removing thallium(I) from real river water and mining wastewater. Additionally, the spent TiO2I can be regenerated using hydrochloric acid solution and reused. The Tl(I) adsorption is achieved via replacing the H+ in hydroxyl group on the surface of TiO2 and forming inner-sphere surface complexes. Owing to its high efficiency, facile synthesis and environmental friendliness, the TiO2I has the potential to be used as an alternative adsorbent to remove Tl(I) from water.

Investigating phosphate-adsorption behaviour on a real ferrallitic-ferritic soil using a pluralistic approach under non-controlled conditions

Context Ferralsols, which cover approximately 6% of the Earth’s continental surface, have unique phosphorus (P) retention properties.Aims The research investigates P adsorption properties under non-controlled conditions on lateritic soil samples, combining different methodological approaches.Methods Ferralsol samples were analysed using (1) adsorption kinetics and capacities (wet chemical experiment methods), (2) scanning electron microscopy (SEM) and transmission electron microscopy and (3) attenuated transmission reflectance-Fourier transform infrared spectroscopy (ATR-FTIR).Key results Wet chemical experiments accord with previous studies on lateritic soils where chemisorption mechanisms govern P adsorption. Further, P adsorption appears to affect soil particles’ structural stability and release of iron (Fe) species in solution. SEM mapping confirmed the location of P compounds in Fe-rich areas. ATR-FTIR identified two inner-sphere complexes: monodentate (FeO)PO2(OH) and bidentate (FeO)2PO(OH) at wavenumber positions 958±5, 1042±5 and 1095±8cm−1; and 930±5, 983±10, 1005±5 and 1122±9cm−1, respectively. Also, a band centred at 1030±4cm−1 suggested evidence of ternary complexes for P concentrations above 500mgP/L. Combined methods suggested the potential involvement of redox mechanisms and other ionic species in the formation and types of phosphate surface complexes.Conclusions Our approach builds on previous work in this field by showing evidence of complex ionic interactions governing P retention on lateritic soils. Novel insights are evidence of fluctuations in physical and chemical factors with phosphate adsorption and suggestion of inner-sphere and ternary surface complexation mechanisms.Implications Given the wide global distribution of lateritic Ferralsols, our findings have important implications for key emerging challenges relating to P cycling for crop production and environmental impact.

Insights into sorption speciation of uranium on phlogopite: Evidence from TRLFS and DFT calculation

Knowledge of the sorption speciation of uranium at mineral/water interface is essential to construct reliable retention and migration models. In this work, the sorption speciation of U(VI) at the phlogopite/water interface was studied at trace concentrations by combining batch sorption, time-resolved luminescence spectroscopy, and theoretical calculation. Batch experiments showed that the sorption of U(VI) on phlogopite was strongly dependent on pH but weakly affected by ionic strength, implying that the inner-sphere surface complexation was mainly responsible for U(VI) sorption on phlogopite. The diverse luminescence spectral characteristics indicated the formation of multiple inner-sphere surface species at the phlogopite/water interface, whose abundances varied as a function of pH. A portion of U(VI) precipitated as uranyl oxyhydroxides such as metaschoepite and becquerelite at high pH. Density functional theory calculation revealed that the bidentate complex at the edge of phlogopite (≡AlO-MgO-UO2(H2O)3) was the most favorable sorption configuration for U(VI) at acidic condition. The increasing temperature enhanced the sorption of U(VI) on phlogopite without altering the sorption species, and such enhancement in U(VI) sorption was withdrawn once the temperature decreased. These findings are essential for understanding the immobilization mechanism of U(VI) in mica-rich granitic terrains at a molecular scale and building a reliable retention model.

Synthesis of Ce-doped magnetic NaY zeolite for effective Sb removal: Study of its performance and mechanism

A Ce-doped magnetic NaY zeolite adsorbent (NaY@Ce−Fe) was synthesized by co-precipitation method for efficient Sb removal. NaY@Ce−Fe exhibited considerable maximum adsorption capacities of Sb (11.56 ± 0.58 mg/g for Sb(III) and 5.36 ± 0.43 mg/g for Sb(V)) and reusability property and excellent saturated magnetization of 71.1 emu/g. The adsorption data of Sb(III) and Sb(V) were well fitted with the Langmuir isotherm model and Freundlich isotherm model, respectively. Kinetic studies showed that the adsorption processes of Sb(III) and Sb(V) followed the pseudo-second-order kinetic model. The excellent adsorption performance of NaY@Ce−Fe maintained over a wide pH range of 4–9 without the release of Ce and Fe, or in the presence of coexisting anions of Cl−, SO42− and NO3−. The combined results of scanning electron microscope (SEM), energy dispersive X-ray spectrometry (EDS), X-ray Powder Diffraction (XRD), Fourier transform infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS) further confirmed the successful doping of Ce and Fe3O4 on NaY. Based on the results of batch experiments and physiochemical analyses, the mechanisms controlling Sb adsorption on NaY@Ce−Fe involved the electrostatic attraction, hydrogen bonding, ligand exchange and surface complexation. The abundant number of surface hydroxyl groups promoted the formation of the Ce−O−Sb inner-sphere surface complexes, which was the major contribution to the adsorption of Sb. All the results suggested that NaY@Ce−Fe could be applied as a promising adsorbent for Sb pollution reduction.

Fabrication of manganese ferrite (MnFe2O4) microsphere-coated magnetic biochar composite for antimonate sequestration: Characterization, adsorption behavior, and mechanistic understanding

A hierarchically structured manganese ferrite microsphere-coated magnetic biochar (BC) composite (MS-MnFe2O4/MBC) was synthesized by a facile solvothermal process to remove Sb(V) from aqueous solutions. In addition to conventional analytical techniques (e.g., XRD, SEM, TEM, XPS), X-ray absorption spectroscopy (XAS) measurements, including X-ray absorption near edge structure and extended X-ray absorption fine structure, were employed to characterize the synthesized MS-MnFe2O4/MBC and to explore its mechanism of interaction with Sb(V) in aqueous solutions. Hierarchically structured single-phase MnFe2O4 microspheres composed of numerous nanocrystallites (6.5–7 nm) were successfully fabricated on the BC surface. The XAS analysis results indicated that MnFe2O4 microspheres consisted of Mn2+ and Fe3+ atoms and possessed a partially inverse spinel structure with an inversion degree of 0.4. After adsorption, the combined results of X-ray photoelectron spectroscopy and XAS analyses demonstrated that the valence state of the adsorbed Sb species was Sb(V), and that its interaction with MS-MnFe2O4/MBC was attributed to the inner-sphere surface complexation through bidentate mononuclear edge-sharing and bidentate binuclear corner-sharing. The high potential of the adsorbent for Sb(V) removal was demonstrated in two actual water matrices, viz., tap water and river water. Overall, the findings indicate that MS-MnFe2O4/MBC has excellent practical applicability for Sb(V) removal from contaminated water.

Sorption of Se(-II) on illite, MX-80 bentonite, shale, and limestone in Na–Ca–Cl solutions

Selenium has been identified as an element of interest for the safety assessment of a deep geological repository (DGR) for used nuclear fuel. In Canada, groundwaters at DGR depths in sedimentary rocks have been observed to have a high ionic strength. This paper examines the sorption behavior of Se(-II) onto illite, MX-80 bentonite, Queenston shale, and argillaceous limestone in Na–Ca–Cl solutions of varying ionic strength (0.1–6 mol/kgw (m)) and across a pH range of 4–9. Little ionic strength dependence for Se(-II) sorption onto all solids was observed except that sorption at high ionic strength (6 m) was generally slightly lower than sorption at low ionic strength (0.1 m). Illite and MX-80 exhibited the expected results for anion sorption, while shale and limestone exhibited more constant sorption across the pH range tested. A non-electrostatic surface complexation model successfully predicted sorption of Se(-II) onto illite and MX-80 using the formation of an inner-sphere surface complex and an outer-sphere surface complex. Optimized values for the formation reactions of these surface species were proposed.

EXAFS investigation of Ni(Ⅱ) sorption at the palygorskite-solution interface: New insights into surface-induced precipitation phenomena

Sorption reactions at the mineral/water interface control the fate and transport of trace metals in aqueous geochemical environments. While surface complexation theory is well established, understanding of surface induced precipitation at mineral/water interfaces is still limited. In this research, we employed a combination of macroscopic batch sorption studies, transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS), and extended X-ray absorption fine structure (EXAFS) spectroscopy to elucidate the sorption mechanisms of nickel (Ni) at palygorskite/solution interfaces. Compared to extensive studies focused on oxides and layer-structured clay minerals, research on chain-type clay minerals (e.g., palygorskite and sepiolite) is sparse. Thus, novel investigations using chain-type clay minerals can provide new insights into the metal sequestration mechanisms and improve equilibrium modeling. In this work, temperature-dependent kinetic experiments demonstrated that the activation reaction energy (Ea) for Ni sorption on palygorskite at pH 6.0 is 35.1 kJ mol−1, significantly lower than that of pH 7.5 (Ea = 102.1 kJ mol−1). These values indicated an adsorption reaction at low pH and a precipitation reaction at high pH. EXAFS analysis confirmed that below pH 6, Ni sorption was dominated by outer-sphere surface complexes at low ionic strength (I = 0.01 M) and inner-sphere surface complexation at high ionic strength (I = 0.1 M). Above pH 7.5, EXAFS data suggested the formation of Ni phyllosilicate precipitates, in agreement with its linear sorption isotherm. An important finding is that the precipitates can form at very low Ni concentrations (∼0.07 mM), with the sorption density of Γ = 0.09 μmol m−2, which corresponds to a 0.3% monolayer coverage for palygorskite. In contrast, at this low level of Ni concentration, surface precipitates did not form on γ-Al2O3. We propose that Ni can be directly induced to precipitate on the palygorskite surface in a process we term “continuous nucleation”. This is mechanistically different from the traditional adsorption-to-polymerization process on the γ-Al2O3 surface, which we term “staged nucleation”. Further testing revealed that a linear-shaped isotherm was also observed for Ni sorption on sepiolite, a clay mineral with similar structure and chemical composition to palygorskite, indicating a similar reactivity for both chain-structure phyllosilicates. This is distinct from the Langmuir-type sorption isotherm observed for Ni reaction with Al oxides and montmorillonite. The findings presented in this study improve not only the current understanding of metal sequestration and surface precipitation at mineral/water interfaces but also shed light on the large differences in the surface reactivities of clay minerals with chain structure versus layered structure.

Effective removal of water-soluble methylated arsenic contaminants with phosphorene oxide nanoflakes: A DFT study

This work explores by density functional theory the ability of oxidized phosphorene nanoflakes (PhosO) to simultaneously remove toxic water-soluble methylarsenicals from contaminated waters. We focused on the microscopic understanding of the PhosO-Methylarsenic complexes in solution, i.e., structure, stability, and intermolecular driving forces. In this way, adsorption energies and conformations, binding and energy decomposition analyses (ALMO-EDA), implicitly/explicitly solvated structures, and competitive adsorption with coexisting molecules in solution afford deep insights into the selectivity adsorption and interaction mechanisms. The PhosO nanoflakes form inner-sphere surface complexes with methylarsenicals in solution, even with enhanced adsorption stability compared to intrinsic phosphorene and without the competition of water molecules for adsorption sites. The inner-sphere surface adsorption of trivalent methylarsenicals is driven by electrostatic and charge-transfer (orbital) intermolecular forces. Surface complexation of pentavalent methylarsenicals occurs by a balanced contribution of orbital and long-range intermolecular forces, while anionic contaminants also show a high stabilization via extra polarization effects. PhosO nanoflakes turn convenient to recycle via simple treatment with alkaline eluents because of the increased affinity with hydroxide anions in solution compared with intrinsic phosphorene. Conceptually understanding the adsorption properties of phosphorene oxide towards hazardous methylarsenicals provides a valuable framework for new developments in future water and wastewater treatment technologies with adsorbents of large surface area, high adsorption capacity/selectivity, and easy recovery.

Removal of water-soluble inorganic arsenicals with phosphorene oxide nanoadsorbents: A first-principles study

A complete picture of the phosphorene oxide (PhosO) sorption properties for the simultaneous removal of inorganic As(III) and As(V) pollutants from water has been developed using first-principles calculations. Calculated adsorption energies, competitive adsorption with co-existing species, energy decomposition analyses (ALMO-EDA), implicitly/explicitly solvated geometries, and adsorption free energies provide deep insights into the adsorption mechanism and origin of the strong sorption selectivity/ability. The PhosO nanoadsorbents establish inner-sphere surface complexes with arsenicals even under competition with water molecules. These proposed structures also show a strong affinity with the highly mobile As(III), where energy saving is achieved by avoiding the pre-oxidation process to convert As(III) into As(V) as requested in related materials. Results show that electrostatic driving forces govern the adsorption of neutral arsenicals, while the interplay between electrostatic and polarization phenomena drives the uptake of anionic arsenicals. By computing the adsorption strength as a function of the oxidation degree, the optimum adsorption efficiency is reached with 25% in the content of oxidizing groups. In this oxidation degree, the strong repulsive surface charge at high pH turns the PhosO nanoadsorbents convenient to recycle via simple treatment with alkaline eluents. Finally, the adsorption ability remains thermodynamically allowed in a wide range of ambient temperatures (enthalpically governed reaction). Conceptually understanding the sorption properties of phosphorene-oxide-based materials towards arsenic pollutants provides a useful framework for future water treatment technologies.

Mechanisms of selenium removal by partially oxidized magnetite nanoparticles for wastewater remediation

Magnetite nanoparticles are a promising cost-effective material for the remediation of polluted wastewaters. Due to their magnetic properties and their high adsorption and reduction potential, they are particularly suitable for the decontamination of oxyanion-forming contaminants, including the highly mobile selenium oxyanions selenite and selenate. However, little is known how the remediation efficiency of magnetite nanoparticles in field applications is affected by partial oxidation and the formation of magnetite/maghemite phases. Here we characterize the retention mechanisms and capacity of partially oxidized nanoparticulate magnetite for selenite and selenate in an oxic system at different pH conditions and ionic strengths. Data from adsorption experiments showed that retention of selenate is extremely limited except for acidic conditions and strongly influenced by competing chloride anions, indicating outer-sphere adsorption. By contrast, although selenite adsorption capacity of oxidized magnetite is also adversely affected by increasing pH, considerable selenite quantities are retained even at alkaline conditions. Using spectroscopic analyses (XPS, XAFS), both mononuclear edge-sharing (2E) and binuclear corner-sharing (2C) inner-sphere selenite surface complexes were detected, while reduction to Se(0) or Se(–II) species could be excluded. Under favourable adsorption conditions, up to ~pH 8, the affinity of selenite to form 2C surface complexes is higher, whereas at alkaline pH values and less favourable adsorption conditions 2E complexes become more dominant. Our results demonstrate that magnetite can be used as a suitable reactant for the immobilization of selenite in remediation applications, even under (sub)oxic conditions and without the involvement of reduction processes.

Nanocellulose aerogel for highly efficient adsorption of uranium (VI) from aqueous solution

Cellulose nanofibers (CNFs) aerogel was prepared via simple covalent crosslinking and freeze-drying method. The porous cellulose aerogel possessed high specific surface area and high metal-chelating capacity, which showed fast adsorption kinetics and high adsorption capacity (440.60 mg g−1) in static uranium adsorption process. In the dynamic filtration system, the maximum adsorption capacity reached 194 mg g−1 with the initial concentration of 10 mg L−1. In addition, the CNFs aerogel possessed excellent selectivity and good regeneration ability for uranium adsorption. The integrated analyses of attenuated total reflection Fourier transform infrared (ATR-FTIR), X-Ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) suggested that the predominant UO22+ species formed inner-sphere surface complexes with two active carboxyl groups in the coordination model. This strategy may provide a sustainable route for development of efficient biomass-based adsorbents for selective uranium removal from aqueous solution.