Formation Of Inner-sphere Complexes Research Articles

Mechanism study about the adsorption of Pb(II) and Cd(II) with iron-trimesic metal-organic frameworks

This study examined the effects of two kinds of iron-trimesic metal-organic frameworks (MOFs) [MIL-100(Fe) and Fe-BTC] on adsorption of Pb(II) and Cd(II). For both materials, adsorption of Pb and Cd nearly approached equilibriums in the initial 2 min with high pseudo-second-order kinetics rate constants (0.895 g·mg−1·min−1 for Pb and 1.416 g·mg−1·min−1 for Cd). Both materials showed better adsorption performance with the increase of solution pH (from 2 to 7). Adsorption processes were endothermic, entropy-increased and spontaneous. The higher enthalpy change (ΔH0) and entropy change (ΔS0) values for Cd adsorption reflected the more difficult adsorption of Cd compared with Pb, which is related with more negative hydration enthalpy (ΔHhydΘ) and larger ionic radius of Cd. Multilayer adsorption phenomena appeared with no maximum adsorptive capacity observed. Both Langmuir-Freundlich and Freundlich-BET models could well describe isotherms. Multilayer adsorption may be attributed to the formation of inner-sphere complexation and outer-sphere complexation. Bockris-Devanathan-Muller (BDM) electrical double layer theory was used for illustrating adsorption mechanism. The fixed-bed adsorption experiments with Fe-BTC showed outstanding elimination performance in initial operating period (>99.8% for Pb and >99.6% for Cd), with effective filtration volume of 980 mL for Pb and 300 mL for Cd. Recyclability test showed that Fe-BTC could be well regenerated by ethylenediamine tetraacetic acid disodium salt (EDTA-2Na). This study showed that MIL-100(Fe) and Fe-BTC are potential for practical heavy metal removal applications.

Synthesis and characterization of a novel Fe3O4-loaded oxidized biochar from pine needles and its application for uranium removal. Kinetic, thermodynamic, and mechanistic analysis

This work investigates the fabrication of magnetic biochar (pncm) and Fe3O4-loaded oxidized biochar (pncom) obtained from pine needles for uranium removal. Adsorbent properties were characterized by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) techniques. Using batch-type experiments the effect of the uranium concentration, solution pH, contact time, temperature and ionic strength on the uranium adsorption was investigated. The results showed better adsorptive properties for pncom, particularly in the acidic pH range. The experimental adsorption data were found to be well fitted with the Langmuir isotherm and the pseudo-second order kinetic model. For pncom, the maximum adsorption capacity obtained applying the Langmuir isotherm model was found to amount 2.6 mol/kg at pH 6 and 25 °C. Spectroscopic data indicated that the U(VI) adsorption was associated with the formation of inner-sphere complexes. Regeneration and reusability studies were performed with 0.1 M Na2CO3. After four cycles, the % relative adsorption and the desorption for pncom decreased from 99.5% to 87.2% and 99.6%–62.6%, respectively. The present results show that magnetization of oxidized pine needle biochar improves significantly the adsorption characteristics regarding the uranium removal from aqueous solutions.

Highly selective adsorption of vanadium (V) by nano-hydrous zirconium oxide-modified anion exchange resin

Adsorption is widely used in removal of toxic vanadium (V) [V(V)] from water streams, and a fit-for-purpose adsorbent plays a vital role in this process. Herein HZrO@D201, an adsorbent with decoration of nanosized hydrous zirconium oxide (HZrO) on anion exchange resin D201, is fabricated for efficient V(V) removal. Compared to pristine D201, HZrO@D201 excelled in V(V) removal with a maximum adsorption capacity of 118.1 mg/g, due to potential formation of inner sphere complexation between V(V) and HZrO. HZrO@D201 could also functioned well in a wide pH range (3.00 to 9.00) and exhibited outstanding selective V(V) adsorption under the presence of competing anions (chloride, nitrate, sulfate, and phosphate). The adsorption thermodynamics was in accordance with the Langmuir model, while adsorption kinetics followed the Pseudo-Second-Order model. When treating actual vanadium contaminated groundwater from Panzhihua region (China), HZrO@D201 indicated a satisfactory lifespan in the column experiment for V(V) removal (2.41 times longer than D201), and the treated groundwater could meet the vanadium standard of drinking water source in China (less than 50 μg/L). Regeneration of HZrO@D201 was easily achievable with negligible capacity loss. Results from this work suggests a promising application potential of HZrO@D201 in vanadium pollution control.

Recyclable high-affinity arsenate sorbents based on porous Fe2O3/La2O2CO3 composites derived from Fe-La-C frameworks

Efficient removal of aqueous arsenic is still a great challenge in particular for producing clean drinking water and for waste water treatment. To this end, a novel magnetic porous Fe-La composite, comprising La2O2CO3 and Fe2O3, was fabricated via a self-sacrificing template method based on bimetallic (Fe, La)-MOFs calcined at 550 °C. Batch adsorption results demonstrated that Fe-LaXY (X and Y represent the feeding mole ratio of Fe and La precursors) exhibited a maximum adsorption capacity of As(V) (241, 251 and 410 mg g−1 for Fe-La51, Fe-La21 and Fe-La11, respectively), and they also showed satisfactory adsorption kinetics, which can be ascribed to a strong coordination between La and arsenate species. Coexisting sorbates including carbonate, silicate, sulfate, and humic acid had a slight influence on adsorption performance of arsenate, while phosphate isostructural to arsenate gave severe interference. More importantly, the Fe-La11 could be easily separated from water due to its magnetism (7.0 emu g−1), and it also exhibited excellent recyclability (above 80% of removal efficiency at the fifth cycle) as well as stability in terms of release of Fe and La ions (< 0.5 mg L−1 at pH 4.0–10.0). Based on results from X-ray photoelectron spectroscopy (XPS), Powder X-ray diffraction (PXRD) and Raman spectroscopy, it is demonstrated that ligand exchange of surface hydroxyl groups and the formation of inner-sphere surface complexes are main responsible for the As(V) removal mechanism. Combining the magnetic properties, the absorption properties and the recyclability make Fe-LaXY derived from bimetallic MOFs promising sorbents for arsenate removal.

Magnetic polymer-supported adsorbent with two functional adsorption sites for phosphate removal.

In this paper, a new magnetic polymer-supported phosphate adsorbent MPVC-EDA-Ce was prepared by loading cerium (hydr)oxides onto ethylenediamine-functionalized polyvinyl chloride for the first time. MPVC-EDA-Ce showed excellent adsorption performances towards phosphate and easy recovery. The adsorption isotherm and kinetics of MPVC-EDA-Ce followed Langmuir monolayer model and the pseudo-second-order model, respectively. The pH results demonstrated that the MPVC-EDA-Ce could effectively remove phosphate in a wide range of pH with insignificant cerium leaching. Furthermore, analyses on adsorption mechanism and effect of competing anions demonstrated the formation of strong inner-sphere complexation between cerium (hydr)oxides and phosphate, which was a selective adsorption process, while positively charged quaternary ammonium groups adsorbed phosphate via relatively weak electrostatic attraction which was a non-selective adsorption process. The study provided a good reference to design novel phosphate adsorbents with two even more functional adsorption sites and a deep insight to investigate the adsorption mechanism towards phosphate.

Adsorption of U(VI) on montmorillonite in the presence of ethylenediaminetetraacetic acid

The adsorption, diffusion and migration behaviors of uranium may be highly influenced by organics in natural environments. Ethylenediaminetetraacetic acid (EDTA) is one of the most important small organics in groundwaters, especially at the radioactive contaminated sites. In this work, we studied U(VI) adsorption on montmorillonite in the presence of EDTA with batch method. Our results indicated that U(VI) adsorption was significantly different in the presence and absence of EDTA. The equimolar EDTA inhibited the U(VI) adsorption at a wide pH range while only enhanced the adsorption at pH ˜7.0. The pseudo-second-order model showed the best fit for describing the adsorption kinetics in comparison with three other popular models. The influence of different addition sequences on adsorption of EDTA and U(VI) on montmorillonite was also investigated, and the results indicated that EDTA played the role as a bridge between montmorillonite surface sites and U(VI). The X-ray photoelectron spectroscopy and attenuated total reflectance Fournier-transform infrared spectroscopy also suggested the formation of EDTA-bridge inner-sphere surface complexes on montmorillonite. This work provides the valuable information of U(VI) adsorption on montmorillonite in the presence of EDTA, and helps us understand the diffusion and migration behaviors of uranium in natural environments.

Removal of antimony from water by iron-coated cork granulates

This study demonstrated the viability of the application of iron-coated cork granulates, previously developed for As removal, in the adsorption of Sb from water. Batch adsorption assays were carried out both for high Sb initial concentrations and typical environmental values. The ionic strength was varied in environmentally relevant scenarios by changing the concentration of NaCl as a background electrolyte.Adsorption kinetics were best described by the Elovich model at high Sb initial concentrations and the pseudo-second-order model at lower levels. Adsorption of Sb(III) was faster than Sb(V) and an increase in ionic strength decreased the rates of adsorption. Equilibrium data was modelled by the Freundlich and Langmuir isotherms and the maximum adsorption capacities were estimated as 5.8 ± 0.5 mg g−1 Sb(III) at pH 6 and 12 ± 2 mg g−1 Sb(V) at pH 3.Sb(III) adsorption was unaffected by pH and positively or neutrally affected by ionic strength, indicating that the adsorption mechanism was the formation of inner-sphere complexes. Sb(V) adsorption, however, was higher at acidic values and decreased with increasing pH, and was negatively affected by ionic strength. The surface charge of the adsorbent was also influenced by the presence of Sb(V), suggesting that Sb(V) is adsorbed by electrostatic interactions and outer-sphere complexation. Speciation studies were carried out with analysis of Sb(III) and no evidence was found of oxidation-reduction reactions occurring parallel to Sb uptake.Iron-coated cork granulates showed good potential to be used as adsorbents of both Sb(III) and Sb(V), with possible economical and environmental benefits.

An electrokinetic perspective into the mechanism of divalent and trivalent cation sorption by extracellular polymeric substances of Pseudomonas fluorescens

Extracellular polymeric substances (EPS) contain a vast number of functional groups which can provide sorption sites for heavy metal cations in solution, however, the mechanisms for the interaction of EPS with various metal cations were not well understood. In this study, the sorption potential of EPS from Pseudomonas fluorescens for different cations was investigated. The changes of electrokinetic properties that occurred on the surface of EPS once they adsorbed these cations were also studied using zeta potential measurements as a function of pH and cation concentration. The adsorption data fitted Freundlich isotherm better than Langmuir and D–R isotherms. The interactions of the cations with EPS were favourable with the separation factor Kr < 1. Under different pH conditions, the zeta potential of EPS in the different cation solution followed the order: Fe(III) (at pH ≤ 5.0) > Al(III) > Cu(II) > Mn(II) > Ni(II)≈Cd(II) > Ca(II) > EPS, while with respect to the initial cation concentration, the zeta potential of EPS was in the order: Fe(III) > Al(III) > Cu(II) > Cd(II) > Ni(II)≈Mn(II)≈Ca(II). The effect of cation sorption on the surface charge of EPS increased with pH as well as cation concentration. The thermodynamic analysis demonstrated that besides the sorption of Fe which was exothermic, all the other cations were adsorbed through an endothermic process. The ΔSads revealed that most of the cations interacted with EPS through the formation of inner-sphere complexes. The ATR-FTIR analyses confirmed that complexation occurred between the cations and functional groups on the surface of EPS. The zeta potential of EPS shifted to positive value direction due to sorption of cations on EPS, indicating that the specific interactions were involved in the sorption process. This study enhances our understanding of EPS aggregation and heavy metal bio-sorption through the electrokinetic mechanism. The results will provide useful references for immobilization of heavy metals and alleviation of Al toxicity in acidic soils.

Cu(II) adsorption on 2-thiouracil-modified Luffa cylindrica biochar fibres from artificial and real samples, and competition reactions with U(VI)

The adsorption of Cu(II) ions by biochar fibres prior and after modification with 2-thiouracil on real and artificial samples has been studied by batch-type adsorption experiments, FTIR and XPS spectroscopy and competition reactions using U(VI) ions as competitor cations. The experimental data of the artificial samples clearly show that the modified material presents extraordinary higher affinity for Cu(II) ions even in the acidic pH range, the spectroscopic data indicate the formation of inner-sphere complexes and the competition reactions significantly higher selectivity of the 2-thiouracil modified biochar fibres for Cu(II). The 2-thiouracil-modified biochar fibres have been successfully applied to acid mine drainage (AMD) samples regarding the selective separation of Cu(II) ions from “real” samples. Regarding the desorption of copper from the biochar surface, although 100% copper recovery was achieved by eluting the metal ion using 1 M HNO3, the deterioration of the modified biochar fibers due to extensive 2-thiouracil release from the biochar surface limits the applicability of the present adsorbent in routine and large-scale applications.

Layered double hydroxide–loaded biochar as a sorbent for the removal of aquatic phosphorus: behavior and mechanism insights

This study was aimed to enhance the phosphorus (P) sorption capability of biochar through embedding of Mg-Fe layered double hydroxide (LDH) particles within its matrix. The structure, morphology, and surface chemistry of the prepared LDH/biochar composite were investigated via X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Fourier transform infrared (FTIR) analysis. The P sorption behavior of LDH/biochar composite was assessed in comparison with the raw biochar under batch conditions. The effects of initial P concentrations, pH levels, and contact times on P sorption were examined. Results showed that the P sorption on LDH/biochar composite was pH-dependent, and the maximum P sorption was found at the pH range of 2–4. The sorption isotherm and kinetic data were best fitted with the Langmuir and pseudo-second-order models, respectively. The maximum P sorption capacity (Qmax) improved from 1.39 mg g−1 for raw biochar to 17.46 mg g−1 for LDH/biochar composite, respectively. Also, the equilibrium contact time decreased from 8 h for raw biochar to 1 h for LDH/biochar composite, respectively. The sorption process followed electrostatic attraction, ligand exchange, and surface inner-sphere complex formation mechanisms. Overall, the results of the present study revealed that the synthesized LDH/biochar composite can be potentially used as a carbon-based sorbent for the removal of phosphorus from aqueous solutions.

Resolving the kinetics of individual aqueous reaction steps of actinyl (AnO2 + and AnO2 2+; An=U, Np, and Pu) tricarbonate complexes with ferrous iron and hydrogen sulfide from first principles

Abstract The solubility and mobility of actinides (An), like uranium, neptunium, and plutonium, in the environment largely depends on their oxidation states. Actinyls (AnV,VIO2 +/2+ (aq)) form strong complexes with available ligands, like carbonate (CO3 2−), which may inhibit reduction to relatively insoluble AnIVO2(s). Here we use quantum-mechanical calculations to explore the kinetics of aqueous homogeneous reaction paths of actinyl tricarbonate complexes ([AnO2(CO3)3]5−/4−) with two different reductants, [Fe(OH)2(H2O)4]0 and [H2S(H2O)6]0. Energetically-favorable outer-sphere complexes (OSC) are found to form rapidly, on the order of milliseconds to seconds over a wide actinyl concentration range (pM to mM). The systems then encounter energy barriers (E a), some of which are prohibitively high (&gt;100 kJ/mol for some neptunyl and plutonyl reactions with Fe2+ and H2S), that define the transition from outer- to inner-sphere complex (ISC; for example, calculated E a of ISC formation between UO2 + and UO2 2+ with Fe2+ are 35 and 74 kJ/mol, respectively). In some reactions, multiple OSCs are observed that represent different hydrogen bonding networks between solvent molecules and carbonate. Even when forming ISCs, electron transfer to reduce An6+ and An5+ is not observed (no change in atomic spin values or lengthening of An–Oax bond distances). Proton transfer from bicarbonate and water to actinyl O was tested as a mechanism for electron transfer from Fe2+ to U6+ and Pu6+. Not all proton transfer reactions yielded reduction of An6+ to An5+ and only a few pathways were energetically-favorable (e. g. H+ transfer from H2O to drive Pu6+ reduction to Pu5+ with ΔE = −5 kJ/mol). The results suggest that the tricarbonate complex serves as an effective shield against actinide reduction in the tested reactions and will maintain actinyl solubility at elevated pH conditions. The results highlight reaction steps, such as inner-sphere complex formation and electron transfer, which may be rate-limiting. Thus, this study may serve as the basis for future research on how they can be catalyzed by a mineral surface in a heterogeneous process.

Application of fly ash-based geopolymer for removal of cesium, strontium and arsenate from aqueous solutions: kinetic, equilibrium and mechanism analysis.

Geopolymerization is a developing reaction process for the utilization of solid wastes. In the present study, fly ash-based geopolymer and its derivative (Fe(II)-modified geopolymer) were synthesized and characterized using XRD, SEM, FTIR, BET, UV-Vis DRS as well as TG-DTA, and adopted as adsorbents for removal of Cs+ and Sr2+, and AsO3- 4 from solutions. Each sorption kinetic was well fitted to the pseudo-second-order model. The sorption of Cs+ and Sr2+ onto original geopolymer were better fitted to the Langmuir model. However, the Freundlich model is more befitting for sorption of AsO3- 4 onto Fe(II)-modified geopolymer. The free energies calculated from the D-R isotherm indicated that the sorption for Cs+ and Sr2+ were dominantly ion exchanges. Ring size plays a decisive role in ion exchanges for both Cs+ and Sr2+. Furthermore, the arrangement of SiO4 and AlO4 tetrahedrons has significant impacts on the ion exchange of Sr2+. XPS results indicated that a part of Fe2+ in Fe (II)-modified geopolymer had been oxidized to Fe3+ after sorption. Precipitation of FeAsO4 could partially contribute to the arsenate removal from solution. AsO3- 4sorption has also occurred through the formation of inner-sphere complexes via ion exchange reaction, which could be predominantly attached by bidentate linkages.

Uranium adsorption by polyvinylpyrrolidone/chitosan blended nanofibers

PVP/chitosan blended nanofibers have been prepared and investigated as adsorbent material for the removal of hexavalent uranium (U(VI)) from aqueous solutions. The nanofibers have been characterized prior and after U(VI) adsorption by SEM and FTIR measurements, and the effect of various parameters such as metal-ion concentration temperature and contact time on the adsorption efficiency has been investigated by batch-type experiments. The material presents increased sorption capacity (qmax= (167 ± 25) g kg−1 at pH 6.0) and increased chemical affinity for U(VI), which is attributed to the fibrous structure of the material and the presence of polar groups (e.g. carbonyl groups) on the blended nanofibers. FTIR spectroscopic measurements indicate the formation of inner sphere complexes between U(VI) and the surface moieties, and thermodynamic and kinetic data reveal a relatively fast (k1 = 0.01 min−1), entropy-driven process (ΔHo = 56.3 kJ mol−1 and ΔSo = 293.7 J K−1 mol−1). Recycling experiments have shown that the material can be used up to four times with less than 10% efficiency loss.

Lead binding to wild metal-resistant bacteria analyzed by ITC and XAFS spectroscopy

Metal-resistant bacteria can survive exposure to high metal concentrations without any negative impact on their growth. Biosorption is considered to be one of the more effective detoxification mechanisms acting in most bacteria. However, molecular-scale characterization of metal biosorption by wild metal-resistant bacteria has been limited. In this study, the Pb(II) biosorption behavior of Serratia Se1998 isolated from Pb-contaminated soil was investigated through macroscopic and microscopic techniques. A four discrete site non-electrostatic model fit the potentiometric titration data best, suggesting a distribution of phosphodiester, carboxyl, phosphoryl, and amino or hydroxyl groups on the cell surface. The presence of these functional groups was verified by the attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, which also indicated that carboxyl and phosphoryl sites participated in Pb(II) binding simultaneously. The negative enthalpy (−9.11 kJ mol−1) and large positive entropy (81.52 J mol−1 K−1) of Pb(II) binding with the bacteria suggested the formation of inner-sphere complexes by an exothermic process. X-ray absorption fine structure (XAFS) analysis further indicated monodentate inner-sphere binding of Pb(II) through formation of C−O−Pb and P−O−Pb bonds. We inferred that C−O−Pb bonds formed on the flagellar surfaces, establishing a self-protective barrier against exterior metal stressors. This study has important implications for an improved understanding of metal-resistance mechanisms in wild bacteria and provides guidance for the construction of genetically engineered bacteria for remediation purposes.

U(VI) adsorption by biochar fiber–MnO2 composites

The removal of U(VI) by biochar fibers from aqueous solutions has been investigated prior and after MnO2 surface-deposition. The removal efficiency has been studied as a function of pH, U(VI) concentration, ionic strength, temperature and contact time. The fibers morphology and surface complexes were analyzed by SEM–EDX and FTIR, respectively. Evaluation of the experimental data indicates that the composite presents extraordinary adsorption capacity (qmax = 3.8 mmol g−1, 904 mg g−1), which is attributed to the formation of inner-sphere surface complexes, and that the adsorption reaction is a relatively fast, endothermic and entropy-driven process.

A facile one-pot hydrothermal synthesis of hydroxyapatite/biochar nanocomposites: Adsorption behavior and mechanisms for the removal of copper(II) from aqueous media

In this study, hydroxyapatite/biochar nanocomposites (HAP/BC-NCs) were synthesized through a simple one-pot hydrothermal process and utilized as an adsorbent for the removal of copper(II) from aqueous media. Characterization results revealed that rod-shaped HAP nanoparticles were successfully incorporated on the surfaces of synthesized HAP/BC-NCs. A set of systematically designed batch experiments were carried out to determine the influences of adsorbent dosage, solution pH, ionic strength, and temperature on the adsorption behavior of the HAP/BC-NCs. Overall findings from batch experiments and extended X-ray absorption fine structure analysis demonstrated that the potential mechanisms responsible for the removal of Cu(II) from aqueous media are cation exchange between Cu2+ in solution and Ca2+ in the HAP on the surfaces of the as-synthesized nanocomposites and the formation of inner-sphere surface complexes on the surfaces of the HAP/BC-NCs. Kinetic studies revealed that the adsorption process follows the pseudo-second-order model and that the overall adsorption rate is controlled by film diffusion as the dominant mechanism and intraparticle diffusion as a secondary mechanism. Adsorption isotherms were accurately represented by a Langmuir isotherm model and the maximum adsorption capacity was determined to be 99.01 mg/g at 298 K, which represents a higher efficiency for Cu(II) adsorption compared to previously reported composite materials. Thermodynamic analysis indicated that the process is thermodynamically spontaneous and endothermic process. Overall, the findings presented in this paper suggest that HAP/BC-NCs have promising applicability for the removal of heavy metals from aqueous media as an alternative, low-cost, and eco-friendly adsorbent for environmental remediation.

Fabrication of carboxyl and amino functionalized carbonaceous microspheres and their enhanced adsorption behaviors of U(VI)

Design and fabrication of materials with abundant functional groups represent an important and promising direction in the remediation of aqueous contaminants. In recent years, carbon-based materials have drawn widespread concern on account of their low cost, large specific surface area, good chemical stability as well as controllable porosity. The U(VI) binding onto raw carbon-based materials is still confronted with the problem of limited adsorption capacity, which severely hinders the practical applications of materials. Herein, amino- and carboxyl-functionalized carbonaceous spheres (denoted as ACSs and CCSs) were fabricated by in-situ polymerization and simply annealing methods, respectively. The highest adsorption amounts of ACSs and CCSs towards U(VI) were calculated to be 73.83 and 401.61 mg·g−1 from Langmuir model at pH = 4.0 and 298 K, which were higher to that of raw carbonaceous material (19.31 mg·g−1). Interestingly, the Freundlich model could well simulate U(VI) sorption isotherms on ACSs at these temperatures (T = 298, 313 and 328 K), while U(VI) sorption isotherms on CCSs were able to significantly conform to the Langmuir model. According to FT-IR and XPS analysis, U(VI) was enriched on both materials on account of the ample functional groups on ACSs (e.g. CNOH/CNH2) and CCSs (e.g. OH/COOH). In addition, effect of ionic strength manifested that U(VI) elimination on ACSs and CCSs were greatly caused by the formation of outer-sphere and inner-sphere surface complexes, respectively. This work promises to provide two facile approaches for engineering diverse functionalized materials towards a highly rapid and efficient sequestering U(VI) from contaminated water.

Fabrication of porous carbon and application of Eu(III) removal from aqueous solutions

An efficient and easy-made material for elimination of radioactive pollution from environment is highly desirable while remains challenging. Herein, porous carbon material (PC) was fabricated by hydrothermal carbonization method for radionuclide adsorption. The characterization revealed that PC had a large amount of micropores with an average size of 100–200 nm. The adsorption mechanism of Eu(III) on PC were explored by batch and spectroscopic techniques. Batch experiments showed that Eu(III) adsorption onto PC was independent of ionic strength, indicating the formation of inner-sphere surface complexes. The maximum adsorption capacity of PC calculated from Langmuir model were 28.6 mg·g−1 for Eu(III) at pH 6.0. FT-IR and XPS analysis revealed that the interaction of Eu(III) onto PC was mainly ascribed to the surface complexation and/or electrostatic interaction of Eu(III) and hydroxyl groups. This work indicated that PC can be regarded as a promising adsorbent for rapid and efficient sequestration of Eu(III) from aqueous solution.

Phosphorus removal and recovery from water with macroporous bead adsorbent constituted of alginate-Zr4+ and PNIPAM-interpenetrated networks

Currently, there is a growing trend in employing natural biomaterials (e.g., alginate) to prepare a novel bead adsorbent for phosphorus (P) elimination. However, the utilization of alginate beads to remove and recover P from effluents possesses limitations associated with its physical characteristics such as a dense gel layer, poor mechanical strength and low stability. To overcome the limitations and improve the adsorption performances, we synthesized a novel alginate-derived bead constituted of PNIPAM network interpenetrated in alginate-Zr4+ network (PNIPAM/SA-Zr) decorated with polyethylene glycol as a pore-forming agent, and then investigated its ability to remove and recover P from effluents. The morphology, functional groups, surface area, and mechanical strength of the beads were evaluated by SEM, FTIR, BET, and swelling analysis. The adsorption of P was investigated by varying various factors. The adsorption kinetics, isotherms, and thermodynamics were studied. Particularly, the P-loaded beads exhibited a faster desorption rate under thermal stimulus, and remained good desorption efficiency and reusability within five consecutive cycles. Zeta-potential and XPS results revealed that the adsorption mechanisms were related to electrostatic interactions, ligand exchange, and the formation of inner-sphere complexes. The beads possessed favorable fixed-bed column operation performances for P removal and recovery from real wastewater.

The role of anatase impurity in the leaching of Cd(II) from contaminated kaolinite

This study was performed to enhance the understanding of the Cd(II) leaching mechanism from contaminated kaolinite with a focus on the role of anatase impurities. Experimental data obtained from titration and batch Cd(II) adsorption experiments were analyzed using a surface complexation model coupled with ion exchange reaction. Anatase showed significantly higher Cd(II) adsorption capability than those of kaolinite and goethite (α-FeOOH), with inner-sphere complex formation as the dominant adsorption mechanism. Under acidic and near-neutral conditions, kaolinite absorbs Cd(II) mainly via outer-sphere complex formation, while both outer-sphere and inner-sphere complex formation are important at higher pH. Binding constants obtained from modeling analyses were then used to predict the leaching extent of Cd(II) from contaminated kaolinite with different contents of anatase impurity at pH 4.0–7.0. The experimental data and model predictions consistently showed that the leaching of Cd(II) decreased with increasing anatase content and this trend was more significant under acidic conditions. These results showed that the content of anatase impurity is a factor controlling the leaching extent of Cd(II) from kaolinite when exposed to acid rain.