Inner-sphere Complexes Research Articles

Comparison of arsenate and arsenite removal behaviours and mechanisms from water by Fe[sbnd]La binary composite (hydr)oxides

The present study compares the performances and mechanisms of As(V) adsorption with As(III) removal by a series of FeLa binary composite (hydr)oxides. The As(V) and As(III) adsorption properties were studied by batch adsorption experiments, and their adsorption mechanisms were explored by SEM-EDS, FTIR, XRD and XPS. The FeLa binary composite (hydr)oxides exhibited excellent As(V) removal performances, and their adsorption properties of As(V) were obviously enhanced with increasing La contents. However, their As(III) removal performances were extremely poor, and the increase of Fe contents could not significantly affect As(III) adsorption performances. The adsorption processes of As(V) were rapid, but the As(III) adsorption were slower. The maximum adsorption capacities of As(V) on Fe/La 3:1, Fe/La 1:1, and Fe/La 1:3 were 166.69, 271.65 and 360.81 mg/g, respectively, and those of As(III) were 30.71, 26.44 and 28.64 mg/g, respectively. Solution pH and presence of PO43−, and citrate can clearly influence the removal efficiencies of As(V) and As(III). The predominant mechanisms of As(V) were the formation of LaAsO4 through ligand exchange reactions under all experimental conditions and coprecipitation of La(III) and As(V) under acidic and neutral conditions, while the formation of inner-sphere surface complexes through ligand exchange reactions between Fe–OH groups and As(III) contributed to the As(III) removal. This work emphasizes the different adsorption properties between As(V) and As(III) by FeLa composite (hydr)oxides, and indicates great application potential in As(V) water treatment and the need for pretreatment for As(III) removal, such as pre-oxidation of As(III) to As(V).

Designing a dual-functional ultrathin ZIF-L@GO adsorbent for simultaneous removal of phosphate and tetracycline hydrochloride: Adsorption capacity and mechanism

Inorganic phosphate and organic antibiotics, usually co-existing in wastewater, were difficult to simultaneously remove by common mono-functional adsorbents. In this work, by in-situ coating zeolitic imidazolate frameworks (ZIF-L) on graphene oxide (GO), a dual-functional ultrathin ZIF-L@GO-25 adsorbent was successfully prepared for efficient and simultaneous removal of phosphate and representative tetracycline hydrochloride (TC) from wastewater. ZIF-L@GO-25 exhibited maximum adsorption capacity for phosphate and TC respectively reached 116.3 ± 2.1 mg g−1 and 338.6 ± 6.7 mg g−1, benefiting abundant Zn-defect sites and dimethylimidazole rings exposed on its surface. Moreover, thanks to the specific innersphere complexation of Zn-defect/phosphate and π-π interaction between dimethylimidazole rings/TC molecules, > 95% removal efficiencies were achieved when the concentration of five competitive matters (Cl−, NO3−, SO42−, CO32− and humic acid) were 10 times that of phosphate/TC. Even with the binary pollutants co-existing system, only ∼3% removal decreases occurred on both phosphate and TC. Thus, this work proposed a novel preparation method of developing ZIF-based composite adsorbents for the deep treatment of actual phosphate/antibiotics-polluted wastewater.

A multi-scale assessment of the impact of salinity on the desorption of chromate from hematite: Sea level rise implications

The solubility and transport of Cr(VI) is primarily controlled by adsorption-desorption reactions at the surfaces of soil minerals such as iron oxides. Environmental properties such as pH, ionic strength, and ion competition are expected to affect the mobility and fate of Cr(VI). Sea level rise (SLR), and consequent seawater intrusion, is creating a new biogeochemical soil environment at coastal margins, potentially impacting Cr(VI) retention at contaminated sites. We employed in-situ ATR-FTIR spectroscopy and DFT calculations to investigate at the molecular level the adsorption of Cr(VI) on the hematite surface and its desorption by sulfate, as a function of pH and ionic strength. We further used a batch experiment to assess Cr(VI) desorption at varying artificial seawater (ASW) concentrations. IR results demonstrate the complexity of Cr(VI) adsorption, showing a combination of monodentate inner-sphere complexation at high pH and dichromate outer-sphere (∼75%) at low pH. The Cr(VI)-complexes exhibited desorption induced by increasing pH values (58% of desorption) and sulfate competition (∼40% desorption). ASW desorbed ∼20% more Cr(VI), even at just 1% concentration. Our findings provide insight into Cr(VI)-adsorption complexation that controls the retention and remobilization of Cr(VI) on Fe-oxide minerals. The results point to an elevated risk of Cr(VI) mobilization in contaminated soils affected by SLR.

On the formation of inner-sphere solvation complexes in dilute acetonitrile solutions of the Hg(NO3)2 and Hg(TfO)2 salts: a Molecular Dynamics study

The solvation properties of dilute acetonitrile solutions of the Hg(NO3)2 and Hg(TfO)2 salts have been investigated by using Molecular Dynamics simulations. The Hg2+ ion in acetonitrile was found to coordinate both counterions and acetonitrile molecules in its first solvation sphere by forming inner-sphere complexes. However, different geometries of the Hg2+ solvation polyhedra are formed depending on the counterion: in the Hg(NO3)2 salt solution, Hg2+ forms a 8-fold cluster of acetonitrile and nitrate ligands with a square antiprism symmetry, while in the system containing the Hg(TfO)2 salts, the Hg2+ solvation shell is composed of eight ligands arranged in a C2 symmetry.

Pine cone derived polyethersulfone/biochar-Fe3O4 mixed matrix membranes for removal of arsenic from acid mine drainage

In this study novel type of mixed matrix membranes (MMMs) were prepared through phase inversion. This fabricated MMMs were impregnated with biochar (BC) and biochar-Fe3O4 (FBC) composites for selective removal of As(V) ions from acid mine drainage through the adsorptive-filtration processes. Scanning electron microscopy (SEM) images revealed that the BC and FBC adsorbents had heterogeneous surface morphologies with varying particle diameters. Fourier transform infrared spectroscopy (FTIR) analysis confirmed the existence of polar functional groups, which aided the improvements in the adsorptive removal of As(V) ions. BET surface area increased from 8.32 m2/g of pristine BC adsorbent to 19.90 m2/g for the FBC adsorbents. Pure water flux increased from 239.9 L/m2.h (M0) to 359.4 L/m2.h (M6), and the water contact angle decreased from 70.7° (M0) to 51.4° (M6). The maximum adsorption capacity achieved was 34.92 mg/g and 49.54 mg/g for BC and FBC MMMs, respectively, at pH 10. The adsorption isotherms and kinetics were analyzed using the Freundlich model and Pseudo-first-order model, respectively, which provided the best-fit to the experimental data. Consequently, the adsorption isotherm data indicated that the removal of As(V) ions was influenced by electrostatic interaction, hydrogen bonding, diffusion (pore filling), and inner-sphere complexation mechanisms. Furthermore, the leaching rate of FBC from membrane labelled M6 was investigated at various pH levels, showing rates of 2.01 µg/m2.h, 0.09 µg/m2.h, and 0.08 µg/m2.h at pH 2, pH 7 and pH 10, respectively. The decrease in leaching rate observed under neutral (pH 7) and basic (pH 10) conditions is ascribed to the improved stability of Fe3O4 NPs within the membranes. The membrane containing 1.5 wt% of FBC demonstrated enhanced effectiveness in removing toxic metals not only for As (99.9 %) but also Cr (99.8 %). These findings indicate that the membrane has great potential towards the treatment of acid mine drainage.

Highly efficient arsenic adsorption onto Mg/Al/Fe layered double hydroxides: Kinetics, isotherm, XPS and Mössbauer spectroscopies

The adsorption of arsenate onto a Mg/Al/Fe layered double hydroxide (Fe-LDH) was studied at different anion concentration, pH, temperature and stirring rate. Different characterization techniques such as FT-IR, X-ray diffraction, XPS and Mössbauer spectroscopies, and zeta potential measurements were used for determining the mechanism of arsenate adsorption onto the studied solid. Two mechanisms are postulated: a) anion exchange reactions with chloride and hydroxyl ions of the interlayer space, and b) ligand exchange reactions with formation of inner-sphere complexes on the Fe-LDH surfaces, with participation of Fe(III) ions from the structure. Arsenate adsorption on Fe-LDH was dependent on the initial pH and the concentration, but was independent of the stirring rate and temperature. Fe-LDH exhibited excellent As(V) removal capacity, much higher than other adsorbents reported in the literature. A dose of 10 g L−1 Fe-LDH was sufficient to reach a percentage removal higher than 95 % for a concentration of arsenic of 1 × 10−4 M found in certain places in Argentina, obtaining residual arsenate values below the recommended World Health Organization limit (0.01 mg L−1). Although the material dissolves at low pH, its high arsenate removal efficiency, the low cost and the green and sustainable characteristics of the material make it an effective adsorbent for the treatment of arsenic contaminated waters.

Efficiency of Modified Natural Clinoptilolite Tuff for As(III) and As(V) Uptake from Model Polluted Water

Granular and powdery clinoptilolite tuff samples from Beli Plast, Eastern Rhodopes, Bulgaria, were coated with metal hydroxides (Fe, Al, Fe-Mg) by two-step alkalization. The prepared sorbents were tested for As(III) and As(V) uptake from a modeled water solution with an arsenic concentration of 44 μg/L. The granular sorbent was used in the sorption columns and showed a high efficiency of arsenic purification from slightly polluted waters. The calculated maximum sorption capacity for the optimal conditions is 32.9 μg/g. The adsorption proceeds according to the Langmuir model. The mechanism of adsorption is most likely inner-sphere complexation. The sorbents’ complete regeneration was achieved by NaOH treatment. The results of the experiments show good possibilities for obtaining water with an arsenic concentration ranging from below the limit of detection to 3.8 µg/L, which is a significantly lower value than the permitted value of 10 μg/L and close to the national environmental quality standard for shallow waters. The high efficiency shows Fe- or Mg,Fe-coated clinoptilolite sorbents. The sorbents are highly efficient in purifying 400 liters of water with an initial concentration of 44 mg/L of either arsenate or arsenite per 1 kilogram of sorbent at a rate of 2.4–2.0 L/h/kg. The powdered sorbents used in a batch system under “static” conditions showed a much higher adsorption capacity than the granular one. Such sorbents are more suitable for small quantities of water with a high concentration of arsenic.

Fabrication and mechanism of La/Al bimetallic organic frameworks for phosphate removal

Lanthanum (La) is constantly used in phosphorus removal due to its high affinity with phosphate. Herein, a novel La/Al bimetallic organic framework was developed to improve the usage efficiency of La species. The effects of metal ratio, pH value and competitive anions were investigated, through which the adsorption mechanism was further explained. Results showed that when the doped La:Al ratio was 1:3, the absorbent (La1Al3-BTC) achieved the most excellent adsorption capacity of 210.3 mg P·g−1 and usage efficiency (P/metal ratio = 1.09). La1Al3-BTC can adapt to the wide pH range and multitudinous coexisting ions in real water, and its adsorption process was in accordance with Langmuir and pseudo-second-order models. Surface morphology of materials transformed from rod-like crystal to amorphous structure as the increase of Al doping. La replaced some Al atoms and embedded in Al-BTC skeleton. The specific surface area raised from 9.27 m2·g−1 of La-BTC to 257.01 m2·g−1 of La1Al3-BTC. The main mechanism for the adsorption of phosphate was the inner-sphere complexation occurred in the La active sites. Electron migration between different metals enhanced the adsorption process. Density-function theory (DFT) calculation well matched the experimental phenomenon. Overall, the development of La1Al3-BTC presented new potential applications for enhanced phosphorus removal and provided new insights for the design of La-based adsorbents.

Comparative Sb(V) removal efficacy of different iron oxides from textile wastewater: effects of co-existing anions and dye compounds.

Elevated Sb(V) concentration in textile wastewater is a growing environmental concern worldwide and has received wider attention in recent years. Iron oxides possess appealing characteristics as efficient and cost-effective adsorbents in large-scale applications. In the present study, Sb(V) adsorption capacity of α-Fe2O3, γ-Fe2O3, and Fe3O4 was compared under experimental conditions close to the practical textile wastewater treatment. Results demonstrated that α-Fe2O3 performed better under different pH values, reaction times, dye compounds, and co-existing ions as compared to γ-Fe2O3 and Fe3O4, and the adsorption equilibrium was achieved within 8h. Sb(V) adsorption is found to be highly pH dependent, and higher removal was achieved in lower pH, indicating the involvement of electrostatic interactions. The pHpzc value of α-Fe2O3 was 7.15, which favored Sb(V) adsorption in practical wastewater having neutral pH value (pH ~ 7). Pseudo-first- and pseudo-second-order described the data and the simulated values of qe fitted well with the experimental values, indicating that pseudo-second-order model described the adsorption kinetics better with R2 (> 0.95) higher than of pseudo-first-order plots. The Langmuir and Freundlich models both described well the sorption data of all the adsorbents, where the R2 values were > 0.90 with a better fit in the Freundlich model for α-Fe2O3, suggesting that the adsorbent has heterogeneous surface characteristics. Similarly, characterizations revealed that the specific surface area, pore volume, and hydroxyl group content in α-Fe2O3 were higher than others, making it easier for contaminants to bind on to the active sites. Furthermore, the effect of dyes and co-existing anions on Sb(V) adsorption was negligible, except for SO42-, CO32-, and PO43- by the formation of inner-sphere complexes with iron oxides through competitive adsorption with [Sb(OH)6]-. Findings from the present study suggested that α-Fe2O3 effectively reduced Sb(V) in textile wastewater and could be a promising alternative for practical textile wastewater treatment.

Highly effective removal of thallium(I) by nanostructured titanium pyrophosphate from acidic wastewater

The highly efficient removal of thallium(I) from water under acidic conditions remains a challenge. In this work, a nanostructured titanium pyrophosphate (TiP2O7) was synthesized via a simple hydrothermal route. The as-prepared TiP2O7 is amorphous and composed of primary irregular nano-grains, with a specific surface area of 64.4 m2/g. It shows strong adsorption ability towards thallium(I) ions over a wide pH range and is still very stable and effective for Tl(I) adsorption even at pH 2.0. The maximal adsorption capacity of thallium(I) at pH 3.0 and 7.0 is 297.8 and 408.3 mg/g, respectively, which outperforms most adsorbents reported. The TiP2O7 is highly selective for thallium(I) adsorption in the presence of multiple cations such as K+, Ca2+, Mg2+, Zn2+ and Cu2+. It also illustrates good reusability and high performance for thallium(I) removal from practical mining wastewater. The removal of thallium(I) is mainly achieved via electrostatic attraction and exchange with the H+ in hydroxyl group on the surface of TiP2O7, forming inner-sphere surface complexes. Due to its high performance, easy preparation and feasibility over a wide pH range, the TiP2O7 is a promising candidate to be used for thallium(I) removal from water and wastewater.

Novel aluminum/lanthanum-based metal organic frameworks for phosphate removal from water

Excessive phosphate can lead to eutrophication and cause many ecological crises, which need to be effectively removed. Recently, metal-organic frameworks (MOFs) showed great effect on removing pollutants due to their special physicochemical features. In this study, La, Al and fumaric acid, as elements with higher affinity to phosphate and one environmentally friendly linker, respectively, were used to explore the environmentally friendly MOFs with high phosphate removal efficiency. Five F-Al/La MOFs with different mole ratio of Al and La ions were synthesized and characterized. F-Al and F-La has uniformly ordered particle structure and a sheet-like structure, respectively. According to the result of this study, the adsorption capacity of F-Al/La MOFs for phosphate was continuously enhanced by increasing La content. F-La exhibited the highest phosphate capture capacity of 101.80 mg P/g, which was almost twice of F-Al (53.11 mg P/g). Compared to F-Al, F-La showed good phosphate removal efficiency in a relatively wide pH range and natural water. In the further investigation, it indicated that the potential phosphate adsorption mechanisms employed by F-La mainly involved inner-sphere surface complexation and surface precipitation, while F-Al underwent more electrostatic interaction than F-La. This work developed one environmentally friendly F-La MOF material for phosphate removing from eutrophication water.

Molecular-level insights into surface complexation of arsenite, selenium and cadmium on {2 0 1} TiO2

Arsenic (As) and Selenium (Se) contaminations present an urgent environmental concern, whereas its removal remains a significant challenge due to the lack of fundamental knowledge of the adsorption mechanism and effective adsorbents. Herein, {201} TiO2 exhibited the application potential for As(III) and Se(IV) removal, and the antagonistic effect was examined at the molecular level using X-ray absorption spectroscopy and density functional theory calculations. The Langmuir adsorption capacity of As(III) and Se(IV) on {201} TiO2 was 0.32 mmol/g and 0.25 mmol/g, respectively. The rate constant k for Se(IV) adsorption was 5.46 g/(mmol/h), which was 10.2 times higher than that of As(III) (0.48 g/(mmol/h)). EXAFS and CD-MUSIC results demonstrated that both As(III) and Se(IV) formed bidentate binuclear inner-sphere complexes at the bridge-Ti4C of {201} TiO2. The pd hybrid orbital energy of Ti-O bonds in Ti2O2SeO complex (−2.42 eV) was lower than that in Ti2O2AsO- complex (−2.26 eV), indicating the better adsorption stability and stronger affinity of Se(IV) adsorbed on {201} TiO2. The coexisting Cd(II) formed ternary complexes with As(III)/Se(IV) on {201} TiO2, enhancing the Freundlich adsorption of Se(IV) to 0.69 mmol/g and As(III) to 0.43 mmol/g. The introduction of Cd(II) altered the orbital hybridization interaction, and the charge transfer from Ti-3d to Cd-5s orbitals improved the adsorption energies of As(III)/Se(IV) on {201} TiO2. The ICOHP value of the Se-O-Cd bond was higher than that of the As-O-Cd bond, indicating the stronger affinity of Cd(II) and Se-O dangling bond. Gaining insights into the surface complexation modeling at the molecular and electronic level reveals the nature of coexisting ions adsorption behaviors.

Nitrogen-doped carbon aerogel framework with in situ encapsulated ZrO2 nanoparticles as pseudocapacitive anode for phosphorus uptake

Phosphorus reduction is crucial for controlling eutrophication and protecting the aquatic environment. Nonetheless, traditional methods suffer from secondary pollution and high costs. Capacitive deionization (CDI) is a novel electrochemical approach for phosphorus removal, which is low-cost and environmentally friendly. Its performance is highly correlated with the electrode material. Herein, a novel nitrogen-doped carbon aerogel framework encapsulated with ZrO2 nanoparticles (ZrNC) was rationally fabricated and employed as an anode for phosphorus removal. The experimental results revealed that the N site and ZrO2 could enhance the physicochemical properties, electrochemical activity, and additional pseudocapacitive contribution, thus raising the phosphorus removal capacity. The optimal ZrNC-3 electrode demonstrated superior phosphorus capture (18.26 mg P/g), low energy consumption at 1.2 V (0.0256 kWh/m3 and 4.159 kWh/kg), outstanding phosphorus selectivity, and excellent stability. Mechanism investigation suggested phosphorus can form mono- or bi-dentate inner-sphere complexes (Zr–O–P) by ligand exchange with ZrO2 sites under an electric field and combined with unsaturated Zr sites and N species by electrostatic attraction. This study presents a promising conception for designing and applying metal oxide/carbon complexes in selective phosphorus uptake electrodes.

Geochemical dynamics of Cu(II) in contrasting cocoa soils: Kinetics and isotherm modeling

Understanding copper (Cu) dynamics in cocoa soils can help predict the fate, mobility, and toxicity, which is crucial for sustaining soil functionality, as well as the quality and health safety of cocoa products. Herein, the study investigated sorption-desorption characteristics of Cu(II) in six cocoa soils of varying weathering by kinetics, isothermal, and pH effect experiments in a batch system. Equilibrium sorption increased in the order of Fluvisol < Lixisol < Luvisol < Acrisol = Ferralsol = Nitisol. Kinetics data fitted well to pseudo-second-order and Elovich models. Cu(II) sorption was dominated by outer-sphere and inner-sphere complexations at low and high pH, respectively. The sorption data fitted well with Freundlich and Langmuir's models. Sorption maximum increased in the order of Nitisol (0.91 mg g−1) < Ferralsol (1.21 mg g−1) < Acrisol (1.29 mg g−1) < Lixisol (1.61 mg g−1) < Luvisol (1.68 mg g−1) < Fluvisol (2.01 mg g−1). The binding energy values obtained showed the highest interaction between Cu(II) and soil matrix in Fluvisol and the least in Ferralsol and Nitisol. The desorption studies consistent with the hysteresis index indicated high sorption irreversibility and less Cu(II) mobilization in Fluvisol, Lixisol, Luvisol, and Acrisol vis-à-vis Nitisol and Ferralsol. Nitisol and Ferralsol may promote the possible toxicity of Cu(II) to the soil ecosystem. The study contributes to the sensitivity of Cu(II) mobility and toxicity to the degree of soil weathering; thus, management of heavy metals in cocoa soils should be dependent on soil type.

Insights into simultaneous adsorption of orthophosphate (PO43−) and 1-hydroxyethane 1,1-diphosphonic acid (HEDP) by kaolin/lanthanum carbonate composites: Experimental analysis and DFT calculations

Orthophosphate and phosphonate often co-exist in industrial wastewater, and it is a challenge to remove the above inorganic and organic phosphorus simultaneously to meet the stringent total phosphorus (TP) discharge standard. The kaolin-based lanthanum carbonate (LC) composites have been demonstrated to decontaminate TP (PO43− and HEDP) from reverse osmosis (RO) concentrate effectively, while the individual and simultaneous adsorption behavior and mechanisms are still not clear. In this study, the individual and simultaneous PO43− and HEDP adsorption were compared. The adsorption kinetics, isotherms, initial pH and matrix effects were performed. The results showed that PO43− exhibited a nice adsorption performance, and was severely suppressed in the presence of HEDP. The maximum adsorption capacities were obtained at initial pH of 4.0, and the co-existing Ca2+ significantly enhanced both PO43− and HEDP removal, while HCO3−/CO32− markedly interfered with their adsorption process. LaCO3OH was the main composition in the composites for PO43− and HEDP adsorption, and no La-P compounds formed after adsorption. A variety of characterization methods were adopted to explore the adsorption mechanisms, and electrostatic attraction, inner-sphere complexation and hydrogen bond were responsible for PO43− and HEDP adsorption. The density functional theory (DFT) calculations were applied to distinguish the PO43− and HEDP adsorption from the molecular level, and the binuclear bidentate complexation configuration of H2HEDP2− species (H2HEDP-BB) with higher adsorption energy (-165.42 KJ/mol) resulted in the predominant species of HEDP in the competitive adsorption. In the fixed-bed column experiments, both PO43− and HEDP adsorption performance also matched the results of batch experiments and DFT calculations. This study provides insights into the competitive adsorption of orthophosphate and phosphonate by LC-based adsorbent.

One-pot fabrication of functional groups-rich carbonaceous nanosheets with TEDA assisted anchoring well-dispersed La(OH)3 for enhanced phosphate removal

Lanthanum-based absorbents have garnered increasing attention due to their potential in phosphate removal. However, several challenges remain to be addressed, especially concerning aggregation issues. In this study, we proposed a novel one-pot method to fabricate carbonaceous nanosheets functionalized with amino groups (NH2-CNS-La), exhibiting large specific surface area, rich available active sites, and well-dispersed La(OH)3. The NH2-CNS-La have more available functional groups, such as carboxylate, hydroxyl, amino, and La(OH)3, which enhance phosphate absorption efficiency. Batch studies suggested that NH2-CNS-La exhibited a high selectivity toward phosphate within pH of 4.0 – 10.0, even in the presence of competing species (including Ca2+, Mg2+, SO42-, Cl-, and NO3–). The Langmuir model analysis revealed a maximum theoretical adsorption capacity of 103.01 mg P/g, while the thermodynamics data implied that the adsorption process was spontaneous and endothermic. Furthermore, XRD, FTIR, and XPS analysis suggested the underlying mechanisms of phosphate adsorption, involving surface precipitation, electrostatic attraction, hydrogen bonds, and inner-sphere complexation.

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.

Complexation-based selectivity of organic phosphonates adsorption from high-salinity water by neodymium-doped nanocomposite

Organic phosphonates have been widely used in various industries and are ubiquitous in wastewaters, and efficient removal of phosphonates is still a challenge for the conventional processes because of the severe interferences from the complex water constitutions. Herein, an Nd-based nanocomposite (HNdO@PsAX) was fabricated by immobilizing hydrated neodymium oxide (HNdO) nanoparticles inside a polystyrene anion exchanger (PsAX) to remove phosphonates from high-salinity aqueous media. Batch experiments demonstrated that HNdO@PsAX had an excellent adsorption capacity (∼90.5 mg P/g-Nd) towards a typical phosphonate (1-hydrox-yethylidene-1,1-diphosphonic acid, HEDP) from the background of 8 g/L NaCl, whereas negligible HEDP adsorption was achieved by PsAX. Attractively, various coexisting substances (humic acid, phosphate, citrate, EDTA, metal ligands, and anions) exerted negligible effects on the HEDP adsorption by HNdO@PsAX under high salinity. FT-IR and XPS analyses revealed that the inner-sphere complexation between HEDP and the immobilized HNdO nanoparticles is responsible for HEDP adsorption. Fixed-bed experiments further verified that HNdO@PsAX was capable of successively treating more than 4500 bed volumes (BV) of a synthetic high-salinity wastewater (1.0 mg P/L of HEDP), whereas only ∼2 BV of effective treatment capacity was received by PsAX. The exhausted HNdO@PsAX was amenable to a complete regeneration by a binary NaOHNaCl solution without significant loss in capacity. The capability in removing other organic phosphonates and treating a real electroplating wastewater by HNdO@PsAX was further validated. Generally, HNdO@PsAX exhibited a great potential in efficiently removing phosphonates from high-salinity wastewater.

3D Composite U(VI) Adsorbents Based on Alginate Hydrogels and Oxidized Biochar Obtained from Luffa cylindrica.

3D naturally derived composites consisting of calcium alginate hydrogels (CA) and oxidized biochar obtained from Luffa cylindrica (ox-LC) were synthesized and further evaluated as adsorbents for the removal of U(VI) from aqueous media. Batch-type experiments were conducted to investigate the effect of various physicochemical parameters on the adsorption performance of materials. The maximum adsorption capacity (qmax) was 1.7 mol kg-1 (404.6 mg·g-1) at pH 3.0 for the CA/ox-LC with a 10% wt. ox-LC content. FTIR spectroscopy indicated the formation of inner-sphere complexes between U(VI) and the surface-active moieties existing on both CA and ox-LC, while thermodynamic data revealed that the adsorption process was endothermic and entropy-driven. The experimental data obtained from the adsorption experiments were well-fitted by the Langmuir and Freundlich models. Overall, the produced composites exhibited enhanced adsorption efficiency against U(VI), demonstrating their potential use as effective adsorbents for the recovery of uranium ions from industrial effluents and seawater.

A novel double-network hydrogel made from electrolytic manganese slag and polyacrylic acid-polyacrylamide for removal of heavy metals in wastewater

Electrolytic manganese slag (EMS), a bulk waste generated in industrial electrolytic manganese production, can be a cost-effective adsorbent for heavy metals removal after appropriate modification. In this study, EMS was activated by NaOH and then used to make the EMS-based double-network hydrogel (an EMS/PAA hydrogel) via a one-pot method. The results showed that the EMS/PAA hydrogel exhibits a high selective adsorption capacity of 153.85, 113.63 and 54.35 mg·g−1 for Pb (II), Cd (II) and Cu (II), respectively. In addition, Density Functional Theory (DFT) suggests that the adsorption energies (Ead) of Pb, Cd and Cu on SiO2/PAA of the EMS/PAA gels are − 4.15, − 1.96, and − 2.83 eV, respectively, and SiO2/PAA, with a strong affinity to Pb2+, is one of the reasons for the selective adsorption capacity of EMS/PAA gel for Pb2+. The removal efficiency of the EMS/PAA gel for Pb2+, Cd2+, Cu2+ decreased after four adsorption-desorption cycles by 20.00 %, 24.56 % and 46.56 %, respectively. Mechanism studies suggested that the elimination of the heavy metals by EMS/PAA gels mainly involves electrostatic attraction, inner-sphere complexation, and coordination interactions. The EMS/PAA hydrogels not only have high adsorption capacity, but are also easy to prepare and circulate, making them ideal for practical applications.