Outer-sphere Complexes Research Articles

Interaction mechanism between chlorinated polyfluoroalkyl ether potassium sulfonate (F–53B) and chromium on different types of soil surfaces

The coexistence of per- and polyfluoroalkyl substances (PFASs) and heavy metals have been found in soils. However, the interaction between the combined pollutants in soils remains unclear. In this study, the adsorption processes of single and combined Cr(VI) and chlorinated polyfluoroalkyl ether potassium sulfonate (F–53 B) in red, yellow and black soils were simulated. When compared with the single F–53 B and Cr(VI), the adsorption amount of the combined F–53 B and Cr(VI) on soils changed with the types of soils. The interactions between F–53 B and Cr(VI) in soils affected their adsorption behavior. The adsorption of the combined F–53 B and Cr(VI) best fit second-order kinetics and the Freundlich equation. Moreover, aluminum and iron oxides are highly correlated with adsorption of F–53 B and Cr(VI). Both F–53 B and Cr(VI) can form complexes with aluminum and iron oxides through electrostatic interactions, but PFOS could be bridged with iron oxides to form an inner sphere complex and with aluminum oxides to form an outer sphere complex. The coexistence of F–53 B and Cr(VI) could change the fluorescent group of dissolved organic matter (DOM) in soils due to the complexation between F–53 B and DOM. In addition, F–53 B increased the acid-soluble portion of Cr and decreased its residual form, which promoted the environmental risk of Cr in soils.

Prediction and optimization of removal performance for europium onto phosphate decorated zirconium-based metal–organic framework nanocomposites: Structure-activity relationship and mechanism evaluation

• Oriented functionalization of UiO-66-PO 4 was successfully prepared. • The maximal adsorption capacity of Eu(III) by UiO-66-PO 4 was 3.86 × 10 -4 mol·g −1 . • UiO-66-PO 4 revealed excellent stability in the removal of radionuclides. • The surface complexation and electrostatic interaction were primary mechanisms. • Long-term evaluation of Eu(III) performances in aqueous medium were needed. Herein, phosphorylation modified zirconium-based metal–organic frameworks composites (UiO-66-PO 4 ) were synthesized by facile and green approach, which was employed to remove Eu(III) from water solution under various experimental conditions. The macroscopic experiments showed that the removal performance of Eu(III) onto the UiO-66-PO 4 composites was endothermic and spontaneous with the tremendous phosphate groups acting as critical binding sites. The SEM, XRD and FTIR provided beneficial evidence for propitious phosphate group grafted onto UiO-66 surfaces. The removal performance of Eu(III) onto UiO-66-PO 4 composites followed the pseudo-second-order model and the adsorption isotherms were well simulated by Langmuir model. The maximum adsorption capacity of UiO-66-PO 4 composites toward Eu(III) was calculated to 3.86 × 10 -4 mol·g −1 using Langmuir model. The UiO-66-PO 4 hybrids having excellently stable could be easily regenerated with no observed significant reduction in the elimination of Eu(III) even after five consecutive cycles. Furthermore, the XPS spectra results further revealed that those including zirconium, phosphate and oxygen functional groups played significantly role in the restricting of europium onto UiO-66-PO 4 surfaces. The experimental results further illustrated that the interactive mechanisms of Eu(III) onto UiO-66-PO 4 hybrids were primarily attributed to the outer-sphere surface complexation at low pH values and then inner sphere surface complexation at high pH values. Based on the discussed results, the UiO-66-PO 4 composites can be as promising candidates with favorably recycled reutilization for the elimination of europium from wastewater systems.

Understanding Cd2+ Adsorption Mechanism on Montmorillonite Surfaces by Combining DFT and MD

The adsorption mechanism of Cd2+ on different cleavage planes of montmorillonite was investigated using density functional theory (DFT) calculations and molecular dynamics (MD) simulations. The most stable adsorption energies of Cd2+ on the (001) and (010) surfaces were −88.74 kJ/mol and −283.55 kJ/mol, respectively. On the (001) surface, Cd2+ was adsorbed on the centre of the silicon–oxygen ring by electrostatic interactions, whereas on the (010) surface, Cd2+ was adsorbed between two ≡Al–OH groups and formed two covalent bonds with O, which was mainly due to the interaction between the Cd s and O p orbitals. Upon the partial substitution of Na+ by Cd2+, Cd2+ was adsorbed on the (001) surface as inner-sphere surface complexes, with a hydration number of 5.01 and a diffusion coefficient of 0 m2/s. Whereas, when Cd2+ completely replaced Na+, part of the Cd2+ moved from the inner-sphere surface complexes to the outer-sphere surface complexes owing to its competitive adsorption. In this case, its hydration number became 6.05, and the diffusion coefficient increased to 1.83 × 10−10 m2/s. This study provides the theoretical background necessary for the development of montmorillonite-based adsorbents.

Comparing the influence of humic/fulvic acid and tannic acid on Cr(VI) adsorption onto polystyrene microplastics: Evidence for the formation of Cr(OH)3 colloids

Microplastics (MPs) can act as vectors for various contaminants in the aquatic environment. Although some research has investigated the adsorption characteristics and influencing factors of metals/organic molecules on MPs, the effects of dissolved organic matter (DOM) (which are ubiquitous active species in ecosystems) on metal oxyanions such as Cr(VI) capture by MPs are largely unknown. This study explored the adsorption behaviors and mechanisms of Cr(VI) oxyanions onto polystyrene (PS) MPs using batch adsorption experiments and multiple spectroscopic methods. The effects of representative DOM components (i.e., humic acid (HA), fulvic acid (FA) and tannic acid (TA)) on Cr(VI) capture by PS were particularly studied. Results revealed a significantly enhanced adsorption of Cr(VI) on PS in the presence of TA. The Cr(VI) adsorption capacity was increased from 2876 μg g−1 to 4259 μg g−1 and 5135 μg g−1 when the TA concentrations raised from 0 to 10 and 20 mg L−1, respectively. Combined microscopic and spectroscopic investigations revealed that Cr(VI) was reduced to Cr(III) by TA and formed stable Cr(OH)3 colloids on PS surfaces. Contrarily, HA and FA inhibited Cr(VI) adsorption onto PS, especially at pH > 2.0 and higher DOM concentrations, due to site competition and electrostatic repulsion. Increase in pH was found to reduce zeta potentials of MPs, resulting in inhibited Cr(VI) adsorption. The adsorbed Cr(VI) declined with increasing ionic strength, implying that outer-sphere surface complexation affected the adsorption process in the presence of DOM. These new findings improved our fundamental understanding of the fate of Cr(VI) and MPs in DOM-rich environmental matrices.

Tunnel cation and water structures of todorokite: Insights into metal partitioning and isotopic fractionation

Tectomanganate todorokite and phyllomanganate vernadite are the two major manganese minerals in marine ferromanganese sediments that contain large quantities of trace metals. Todorokite has a less clear metal partitioning mechanism than vernadite, mainly because the metal cation and water structures in the tunnel cavities remain poorly understood. Here, we report the composition-dependent tunnel-cation site disorder of todorokite obtained from molecular dynamics (MD) simulations. Our simulations of homocationic Ni2+-, Zn2+-, Mg2+-, Ca2+-, or Na+-todorokite revealed that tunnel cations form inner-sphere (IS) complexes with Mn octahedral surface at low hydration states, but at higher hydration states, outer-sphere (OS) complexes are the dominant cation species located at the center of the tunnel. However, K+ and Cs+ form only IS complexes regardless of the tunnel hydration state. Optimum water contents were determined based on the free energy changes associated with water intercalation into the tunnel cavities calculated as a function of water content for two heterocationic todorokite models: Mg0.5Na0.4Ca0.1Mn6O12·nH2O (Mg/Mn = 0.08, average Mn oxidation state = +3.73) and Mg1.1K0.1Ca0.1Mn6O12·nH2O (Mg/Mn = 0.18, average Mn oxidation state = +3.58). These models emulate the chemical compositions of well-characterized terrestrial and marine todorokite samples, respectively. At each optimum water content, MD simulations revealed that OS complexes are the major Mg2+ species in the former model. However, IS complexes are the major Mg2+ species in the latter model, indicating that the tunnel cation speciation of diagenetic todorokite can be influenced by the sediment pore-fluid chemistry. The atomic tunnel cation and water structures of todorokite provide a molecular basis for understanding of metal partitioning to Mn oxides and interpretation of metal stable isotopes recorded in ferromanganese nodules.

Adsorption of Cs(I) and Sr(II) on Bentonites with Different Compositions at Different pH

This paper deals with adsorption regularities and mechanisms of nonradioactive Cs(I) and Sr(II) analogs on bentonites of different chemical and mineral composition from solutions of Cs and Sr nitrates with pH 3, 7, and 10 units at constant ionic strength. The bentonites were taken from the deposits Taganskoe (T), Dash-Salakhlinskoe (DS), Zyryanskoe (Z), and 10th Khutor (10H). The pH of bentonite aqueous suspensions, T and DS, exceeded 9 units. A less alkaline reaction was observed in bentonite suspensions Z and T with pH 8.94 and 7.70, respectively. Bentonites T and DS contained significant amounts of nonsilicate iron compounds, 1.0 and 0.5%, respectively. The recovery rate of the studied clays from aqueous solutions of Cs(I) and Sr(II) ions in concentrations from 0.25 to 5 mmol/L varied from 50% to 90% and decreased in the following order: “Ta-ganskoe” > “Dash-Salakhlinskoe” > “Zyryanskoe” > “10th Khutor” in the studied pH range. The main mechanism of Cs(I) and Sr(II) sorption in the studied pH range was cation fixation in the form of outer-sphere complexes on planar surfaces resulting from ion exchange. Increasing pH (pH > 6) enhanced pH-dependent positions, which allowed Cs(I) and especially Sr(II) ions to fix on them more firmly as inner-sphere complexes. At pH 9–10, Sr(II) could precipitate in the form of carbonates. The sorption of Cs(I) + and Sr(II) was accompanied by competitive interactions with proton at pH < 6 and Na+, Ca2+, Mg2+, and K+ cations at higher pH values. This competition was more apparent at concentrations of Cs(I)and Sr(II) in initial solutions < 0.5 mmol/L. The ability of bentonite T to sorb Cs(I) and Sr(II) in large amounts compared to the other bentonites was determined by high CEC values and charge of smectite T.

Connecting Thermodynamics of Alkali Ion Exchange on the Quartz (101) Surface with Density Functional Theory Calculations.

Periodic plane-wave density functional theory (DFT) calculations were performed on the α-quartz (SiO2) (101) surface to model exchange of adsorbed Li+ and either Na+, K+, or Rb+ in inner- and outer-sphere adsorbed, and aqueous configurations, which are charge-balanced with 2 Cl-. SiO- or SiOH groups represented the adsorption surface sites. The SiO- models included 58 H2O and 2 H3O+ molecules to approximate an aqueous environment, whereas the SiOH models had 59 H2O and 1 H3O+ molecules. The goal of this work is to calculate the heats of exchange for these alkali ions and to compare the results with those measured by flow microcalorimetry to ascertain the most probable mechanisms for these cations exchanging on the α-quartz (101) surface. Energy minimizations of each alkali ion adsorbed as outer-sphere complexes on SiOH surface sites, and as inner- and outer-sphere complexes on SiO- surface sites, were used to determine the energy of exchange (ΔEex) with Li+ for comparison with experimentally determined ΔHex values. Here, we present a novel method for calculating ΔEex using the difference in energies of geometry-optimized end member models. The aqueous and surface structures produced are similar to those observed experimentally. Although the trend for the calculated ΔEex values is consistent with those from the heats of exchange measured experimentally, the magnitude of our modeled ΔEex results is significantly larger than select experimental data from the literature [Peng, L. Zeta-Potentials and Enthalpy Changes in the Process of Electrostatic Self-Assembly of Cations on Silica Surface. Powder Technol. 2009, 193(1), 46-49]; we discuss the reasons for this discrepancy herein. The relative energy differences of the various configurations modeled have implications for the measurements of the surface charge via potentiometric titrations due to the more active role of alkali cations in quartz surface chemistry that have been previously considered as inert background electrolytes.

Solution NMR investigation of phytic acid adsorption mechanisms at the calcite-water interface

As one of the most abundant organic phosphorus (P) species in soils, phytic acid could serve as a mineralizable P reservoir in soils and sediments. It has been widely acknowledged that the adsorption of phytic acid to soil minerals retards P mineralization in soils. However, the adsorption mechanisms of phytic acid in the minerals are not clearly understood. Using solution 31P NMR and 1H-31P 2D NMR, the adsorption mechanism of phytic acid was investigated at the calcite-water interface at pH 6 and 8. Maximum phytic acid adsorption reached 3.07 mmol/g, 2.60 mmol/g, 2.39 mmol/g at pH 6, 8, and 9.5, respectively. The presence of outer-sphere surface complex was evident by a lack of significant change in zeta-potential of phytic acid reacted calcite. Solution NMR analysis showed a fast exchange process between adsorbed and unreacted phytic acid at the mineral surface on an NMR time scale, also indicating the outer-sphere complexation mechanism at both pH values. Interestingly, a more active role of P5 and P4,6 in binding with calcite surface was observed at pH 6. Adsorbed phytic acid on the calcite surface should be labile and is not limiting P mineralization in the terrestrial environment.

Distinguished Ni(II) Capture with Rapid and Superior Capability Using Biochar: Behavior and Mechanism

Batch experiments were used to study the interaction mechanism of Ni(II) with biochar as a function of solution pH, contact time, initial concentrations, reaction temperature, and the addition of humic compounds. The results indicate that Ni(II) can be interacted with biochar by a compound mechanism under different environmental conditions. It is suggested that outer-sphere surface complexes at low pH are transformed into inner-sphere surface complexes and surface precipitates are formed at pH 8.89 or 10.18. The combined macroscopic and microscopic insights provide additional details regarding the mobility, fate, and risk of Ni(II) in a practical aquatic environment, as well as the natural purification of metals in ecological environments.

Toward a mechanistic understanding of cesium adsorption to todorokite: A molecular dynamics simulation study

A mechanistic understanding of cesium (Cs) adsorption to soil mineral phases is essential for effective mitigation of Cs mobility in the subsurface environment. Todorokite, a common tunnel-structured manganese oxide in soil, exhibits sorption capacity for Cs comparable to the capacities of clay minerals. However, the adsorption sites and molecular species of Cs+ adsorbed to todorokite remain uncertain in comparison with those of clay minerals. In this study, we explored adsorption of Cs+ to hydrated todorokite surfaces via atomistic molecular dynamics (MD) simulations. We performed the first MD simulations based on atomic pair potentials for Mn-oxide edge surfaces interfaced with an aqueous solution. MD simulations predicted that Cs+ forms only inner-sphere (IS) complexes within todorokite tunnels; however, Cs+ forms both IS and outer-sphere (OS) complexes at the external (010) and (100)/(001) external surfaces. On the (010) surface, the positions between IS and OS complexes of Cs+ were interchangeable during MD simulations. Detailed molecular structures of IS and OS Cs+ surface complexes are compared to those of Cs+ in an aqueous solution. The current MD simulation results can be used as an atomistic structural proxy for spectroscopic analysis of adsorbed metal speciation and surface complexation modeling of metal adsorption to Mn oxides.

Effectiveness of the engineered pinecone-derived biochar for the removal of fluoride from water

Drinking fluoride (F−)-contaminated water (>1.5 mg L−1) causes severe dental and skeletal disorders. In the central province of Pakistan, ∼20 times higher levels of F− in the drinking groundwater (compared with the 1.5 mg L−1 permissible limit of the World Health Organization) are triggering bone abnormalities in teenagers. In this study, we demonstrated the potential of pinecone-derived biochar (pristine) impregnated with Fe- and Al-salts (engineered) to defluoridate water. Batch mode adsorption experiments were carried out under variable conditions of solution pH, F− initial concentration, adsorbent dose, and contact time. The engineered biochars resulted in greater adsorption than that of pristine biochar. Specifically, the AlCl3-modified biochar exhibited a maximum adsorption capacity of 14.07 mg g−1 in spiked water and 13.07 mg g−1 in in-situ groundwater. The equilibrium isothermal and kinetic models predicted monolayer, cooperative, and chemisorption types of the adsorption process. The chemical interaction and outer-sphere complexation of F− with Al, Na, and H elements were further confirmed by the post-adsorption analysis of the AlCl3-modified biochar by FTIR and XRD. The AlCl3-modified biochar resulted in 87.13% removal of F− from the in-situ F−-contaminated groundwater, even in the presence of naturally occurring competing ions (such as Cl−, HCO3−, SO42−, and NO3−). We conclude that the AlCl3-modified biochar derived from pinecone could be a promising cost-effective adsorbent for the defluoridation of water.

Separation and extraction of niobium from H2SO4 solution containing titanium and iron impurities

A fluoride-free solvent extraction method for niobium separation in leached sulfuric mother liquor of ilmenorutile was proposed in this paper. The extraction system comprising trioctyl tertiary amine (N235) and 4-methyl-2-pentanone (MIBK) demonstrated good extraction efficiency and selectivity of niobium over titanium and iron impurities in sulfuric acid–oxalate solutions. Details of extraction, scrubbing, and stripping processes were investigated herein, and its practical effectiveness in both batch and multi-stage simulated counter-current modes was verified. Over 73.8% niobium was extracted under optimum conditions, and only 2.8% titanium and 1.7% iron were co-extracted. The recovery of niobium reached 99.1% in the four-stage simulated counter-current extraction tests. In the three-stage simulated counter-current stripping tests, the stripping efficiency of niobium reached 99.7% with 2.0 mol/L HNO3. Qualified niobium oxide products were prepared. The enthalpy change, entropy change, and the standard Gibbs free energy change (ΔHo, ΔSo, and ΔGo, respectively) corroborated the exothermicity, orderliness, and spontaneity of the reaction. The possible composition of the extraction complex was [NbO(C2O4)2(H2O)2]−, as supported by a Fourier transform infrared spectroscopy and ultraviolet–visible spectrophotometry. The extraction mechanism was inferred to be the outer-sphere complexation through physical binding which was the association of R3NH+ in N235 with the [NbO(C2O4)2(H2O)2]−. A preliminary mass balance indicated that 203.92 g Nb2O5 was generated from 1000 g raw material ilmenorutile.

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.

A deep look into the diverse surface speciation of the mono-molybdate/lepidocrocite system by ATR-IR and polarized ATR-IR spectroscopy

The present study is focused on the adsorption of monomeric molybdate anions on the surface of lepidocrocite investigated by several infrared spectroscopy methods. To study the surface speciation of molybdates on lepidocrocite, the behavior of the anion at the interface upon increasing anion concentration (fixed pH) or decreasing pH (fixed concentration) was analyzed in situ. The obtained results expand the knowledge for surface speciation by revealing a second molybdate complex, which is more stable, arising from the interaction with defects on the (010) facets of lepidocrocite. A curve fitting procedure enabled us to confirm what is observed from the pristine spectra, and outer-sphere complexes were probed as well. The evolution of the different bands upon increasing adsorbate concentration and decreasing pH, as obtained by curve fitting, shows a complex evolution for the different surface species with protonation, with the formation of strongly bonded complexes at low pH. The 2D synchronous and asynchronous spectra support the interpretation of the data. Furthermore, insights into the orientation of the complexes with respect to the lepidocrocite surface are obtained by using linearly polarized radiation. The results confirm most of the conclusions ascertained on the basis of the unpolarized spectra. The combination of the different methodologies yields a consistent picture for the complexity of this system and paves the way for subsequent utilization of this approach to study other systems.

Removal of Cadmium(II) by hydrated manganese dioxide: behaviour and mechanism at different pH

ABSTRACT Homogeneous precipitation was proposed to prepare hydrated manganese dioxide (HMO) with KMnO4 as oxidant, NaCl as reductant and HNO3 as reaction auxiliary. HMO was applied to remove Cd(II) and the effect of contact time, initial concentration, adsorbent dose and pH value on adsorption efficiency were investigated. The removal mechanisms at various pH values were analysed in detail. Adsorption thermodynamics parameters were calculated as ΔG < 0, ΔH > 0 and ΔS > 0, which meant that the adsorption process was endothermic. The result of adsorption kinetics indicated the adsorption process conformed to pseudo-second-order kinetics. When adsorbing Cd(II) with initial concentration equaling 100 mg·L−1, the activation energy (Ea) was 62.740 kJ·mol−1. The Langmuir model could describe adsorption behaviour on HMO better than the Freundlich model, indicating that the adsorption sites of HMO were homogeneous and that single-layer adsorption was a dominant way in this process. The maximum adsorption capacity of Cd(II) on MnO2 calculated by the Langmuir model was 267 mg·g−1. The adsorbent HMO could be recycled and reused for several times with a high efficiency above 70% by adding HCl. SEM, EDS, FTIR and XPS were used to analyse the mechanisms of removal of Cd(II) at pH = 3,7 and 10. The mechanisms included electrostatic attraction, ion exchange and chemical precipitation. With pH increasing, the zeta potential decreased and the surface negative charge increased, promoting Cd(II) removal through enhanced electrostatic attraction. Meanwhile, ion exchange mechanisms including inner-sphere complexation and outer-sphere complexation occurred during adsorption process at different pH.

Effects of Calcium on Arsenate Adsorption and Arsenate/Iron Bioreduction of Ferrihydrite in Stimulated Groundwater.

The reduction and transformation of arsenic-bearing ferrihydrite by arsenate-iron reducing bacteria is one of the main sources of arsenic enrichment in groundwater. During this process the coexistence cations may have a considerable effect. However, the ionic radius of calcium is larger than that of iron and shows a low affinity for ferrihydrite, and the effect of coexisting calcium on the migration and release of arsenic in arsenic-bearing ferrihydrite remains unclear. This study mainly explored the influence of adsorbed Ca2+ on strain JH012-1-mediated migration and release of arsenate in a simulated groundwater environment, in which 3 mM ferrihydrite and pH 7.5. Ca2+ were pre-absorbed on As(V)-containing ferrihydrite with a As:Fe ratio of 0.2. Solid samples were analyzed by X-ray diffraction (XRD), scanning electron microscopic (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The results show that calcium and arsenate can synergistically adsorb on ferrihydrite due to the electrostatic interactions, and the adsorbed Ca2+ mainly exists on the surface through the outer-sphere complex. Adsorbed Ca2+ entering the stimulated groundwater was easily disturbed and led to an extra release of 3.5 mg/L arsenic in the early stage. Moreover, adsorbed Ca2+ inhibited biogenic ferrous ions from accumulating on ferrihydrite. As a result, only 12.30% Fe(II) existed in the solid phase, whereas 29.35% existed without Ca2+ adsorption. Thus, the generation of parasymplesite was inhibited, which is not conducive to the immobilization of arsenic in groundwater.

Dual-active-sites oriented mesoporous zirconia nanofibers for enhanced removal of phosphate in the presence of humic acid

Excessive phosphate in water will inevitably cause eutrophication and further trigger major environmental issues. Efficient removal of phosphate is of great significance to inhibit water eutrophication, however, humic acid (HA) in water aggravates the uncertainty of phosphate removal. In this work, mesoporous zirconia nanofiber (MZN) was synthesized and applied as an adsorbent for phosphate removal in the presence of HA. The performance and mechanism of the enhanced removal of phosphate by MZN in the presence of HA was systematically studied. The results show that MZN has higher adsorption capacity, stronger affinity, and faster adsorption rate for phosphate in solution with HA than that without HA. In the presence of HA, HA is adsorbed on MZN through complexation, which introduces carboxyl and hydroxyl groups to MZN surface, increasing phosphate adsorption through hydrogen bonding. The boosted removal mechanism of phosphate can be ascribed to the dual-active-sites effect between the complexation of MZN to phosphate and the hydrogen bonding of the adsorbed HA to phosphate. The adsorption of phosphate by MZN in the presence of HA follows the outer-sphere complexation. Column adsorption experiment showed that MZN is feasible and effective for the treatment of phosphate in water containing HA based on the discharge standard of phosphate, and can be reused repeatedly through regeneration. The results indicate that MZN holds a great promise in efficient removal of phosphate in the presence of HA.

The organic contaminants degradation in Mn-NRGO and peroxymonosulfate system: The significant synergistic effect between Mn nanoparticles and doped nitrogen

A family of graphene-based activators including reduced graphene oxide (RGO), nitrogen doped reduced graphene oxide (NRGO), zero-valent manganese loaded reduced graphene oxide (Mn-RGO) and zero-valent manganese loaded N-doped reduced graphene oxide (Mn-NRGO) were fabricated to activate peroxymonosulfate (PMS) for bisphenol A (BPA) degradation. A significant synergistic effect between loaded manganese nano-particles (Mn-NPs) and doped N was revealed by comparing the apparent reaction rate constants with about 12.4, 7.6 and 10.5-folds enhancement when compared Mn-NRGO with RGO, NRGO, Mn-RGO, respectively. By integrating the results of chemical scavenger experiment, electron paramagnetic resonance test, galvanic oxidation process, PMS stoichiometric efficiency, linear sweep voltammetry and in-situ Raman spectrum, a predominant outer-sphere catalyst-PMS complexes mediated electron transfer pathway was uncovered. Based on kinetic studies, the specific contribution of synergistic effect was quantified to be at least 70%. The calculated Fermi level advanced by 0.03 eV in Mn-NG compared with that in NG, demonstrating that Mn-NG is more liable to donate electrons to PMS. The Eads and lO-O results pointed out that synergistic effect between Mn-NPs and doped N relied on the intimate affinity between Mn-NPs and HSO5– which triggered more uneven distribution of charge on the whole carbon sheets, and more adsorption of HSO5– on carbon framework. These newly created adsorption consequently reinforcing the formation of catalyst-PMS complexes and the elevation of mediated electron transfer pathway. Overall, this study firstly provides a comprehensive and definite insight of the synergistic effect between loaded Mn-NPs and doped-N on carbon materials for promoting PMS activation to degrade organics.

Reduction mechanism of Au(III) species adsorbed on δ-MnO2

Au(III)-chloride complex and its hydrolysis products, [AuCl4−n(OH)n]-, are typical Au species in the hydrosphere, and are known to be adsorbed and then reduced to Au(0) on some metal oxides. However, the process from adsorption to reduction has not been made clear. In this paper, adsorption of [AuCl4−n(OH)n]- was performed using two different metal oxides, Al2O3 and δ-MnO2. The experiments were performed for 72 h at pH 4.0, with Au(III) initial concentration of 0.10 mmol/dm3 and 1.0 g/dm3 of δ-MnO2 or Al2O3. The adsorption behavior of [AuCl4−n(OH)n]- differed greatly between the solids. On Al2O3, Au(III) complex ions formed outer sphere complexes and maintained their coordination structure even after adsorption. On the other hand, Au(III) complex ions formed inner sphere complexes on δ-MnO2 and the adsorption ratio reached 100% within 30 min. And then, the adsorbed Au(III) complex ions spontaneously reduced to Au(0). Detailed analysis of both the solid and liquid phases proved that the Au(III) complex ion, [AuCl1.67(OH)2.33]-, first adsorbed onto δ-MnO2 by exchanging its OH- with surface OH groups. After that, Cl-, which was originally coordinated to Au(III) complex ions, was gradually changed to OH- in water to form pseudo [Au(OH)4]- on the surface, and Au(III) was reduced. Our experimental results suggested that ligand exchange caused by the formation of inner sphere complexes is the key to inducing redox reactions after adsorption.

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.