Adsorption performance of antimony by modified iron powder

Antimony pollution resulting from industrial production is a great threat to the environment, ecology and the human body. Zero-valent iron powder is low-cost and easy to obtain. Nevertheless, the lower adsorption capacity limits its application when it is used as an adsorbent. In the present study, ball-milling and acid modification were developed to change its surface characteristics and γ-Fe₂O₃, γ-FeOOH and Fe₃O₄ were obtained after treatment, which promoted its adsorption capacity. Compared with the raw iron powder, the adsorption capacities for Sb(iii) and Sb(v) using the modified material were increased from 12.93 mg g⁻¹ and 5.47 mg g⁻¹ to 17.96 mg g⁻¹ and 10.58 mg g⁻¹, respectively. The study showed that the experimental data fitted the Langmuir model and the pseudo-first-order kinetic model better; the adsorption process was monolayer and chemically controlled at pH 5.0 ± 0.2. XPS and FT-IR analysis showed that Fe–O–Sb bonds arose during the adsorption process. The effect of pH on the adsorption capacity was also studied and the pH affected the adsorption of Sb(v) more than the adsorption of Sb(iii). In addition, the modified iron powder presented better efficiency when applied to the removal of low levels of antimony in drinking water. Based on the increase of adsorption capacity and low cost, iron powder should be a promising adsorbent for aqueous antimony removal.

Modified iron powders were obtained by ball-milling and acid modification and the adsorption behavior and mechanism for Sb(iii) and Sb(v) analyzed. The improved adsorption capacity and low cost show the application potential of the modified material.     

Optimization of the adsorption and removal of Sb(iii) by MIL-53(Fe)/GO using response surface methodology

In this study, a graphene oxide metal–organic framework (MIL-53(Fe)/GO) composite adsorbent was successfully synthesized using a simple method at room temperature. The specific surface area of the synthesized MIL-53(Fe)/GO nanoparticles was 268.43 m² g⁻¹, with an average pore size of 2.52 nm. The Box–Behnken response surface method was applied to optimize the adsorption time, dosage, pH, temperature, and initial concentration of Sb(iii) in the MIL-53(Fe)/GO adsorption treatment employed for synthetic wastewater containing Sb(iii). We determined the optimal adsorption conditions and explored the isotherm model, adsorption kinetic model, and adsorption mechanism during the adsorption process. For an optimal adsorption of Sb(iii) by MIL-53(Fe)/GO, the adsorption time, dosage, pH, temperature, and initial Sb(iii) concentration should be set to 4.86 h, 85.79 mg L⁻¹, 10.00, 39.29 °C, and 10.09 mg L⁻¹, respectively. Under these optimal conditions, the removal rate of Sb(iii) will be as high as 97.97%. The adsorption of Sb(iii) by MIL-53(Fe)/GO conformed to the Freundlich isotherm adsorption model, and its maximum adsorption capacity was 69.014 mg g⁻¹. The adsorption kinetics process, which is a nonhomogeneous reaction, could be fitted using a quasi-first-order kinetic model. A Fourier transform infrared spectroscopy analysis showed that MIL-53(Fe)/GO hydroxyl and amine groups play a vital role in the adsorption process. MIL-53(Fe)/GO did not exhibit any changes in its adsorption efficiency in the presence of its anion and showed high specificity to Sb(iii). XPS characterization showed that Sb successfully adsorbed onto the adsorbent and that no oxidation–reduction reaction occurred during the adsorption process. The adsorption efficiency remained high even after four cycles of use. MIL-53(Fe)/GO is highly recyclable with significant application potential for treating wastewater containing Sb(iii).

In this study, a graphene oxide metal–organic framework (MIL-53(Fe)/GO) composite adsorbent was successfully synthesized using a simple method at room temperature.     

Submicron fibers as a morphological improvement of amorphous zirconium oxide particles and their utilization in antimonate (Sb(v)) removal

Mesoporous and large surface area zirconium oxide aggregate granules with good adsorption properties were synthesized using a simple precipitation method. Since utilization of these small and fragile particles is considered rather difficult in larger scale column operation, the product was formed into a fibrous form to improve its usability. The submicron fibers were obtained from an optimized electroblowing synthesis that resulted in elastic and uniform fibers with a tetragonal structure and high length-to-diameter ratio. In antimonate (Sb(v)) adsorption experiments, the higher calcination temperature (350 °C) of the fibers did not seem to decrease the Sb(v) adsorption capacity excessively since the high theoretical adsorption capacities were 113 mg g⁻¹ and 58 mg g⁻¹ for the aggregate and fibers, respectively. Both materials had fast kinetics, fibers being faster in the beginning of the reaction. Moreover, both materials offered efficient Sb(v) removal in the studied pH range from 1 to 11 by reaching over 99.9% adsorption in the optimal pH range. X-ray absorption near edge spectroscopy (XANES) revealed that Sb(v) stays as pentavalent antimony after being adsorbed by these materials and based on the isoelectric point shifts in the zeta potential measurement, adsorption occurs mainly by an inner-sphere complexation reaction. Finally, our study showed that pressure buildup in a flow-through column packed with zirconium oxide fibers was significantly lower than in a column packed with aggregates. Thus, zirconium oxide aggregates can be formed into submicron fibers with enhanced column operation properties without a too large compromise in the adsorption properties.

Zirconium oxide was formed into submicron fibers to improve the Sb(v) separation performance compared to a conventional aggregate material.     

Nanoscale iron (oxyhydr)oxide-modified carbon nanotube filter for rapid and effective Sb(iii) removal

Herein, nanoscale iron (oxyhydr)oxide-coated carbon nanotube (CNT) filters were rationally designed for rapid and effective removal of Sb(iii) from water. These iron (oxyhydr)oxide particles (<5 nm) were uniformly coated onto the CNT sidewalls. The as-fabricated hybrid filter demonstrated improved sorption kinetics and capacity compared with the conventional batch system. At a flow rate of 6 mL min⁻¹, a Sb(iii) pseudo-first-order adsorption rate constant of 0.051 and a removal efficiency of >99% was obtained when operated in the recirculation mode. The improved Sb(iii) sorption performance can be ascribed to the synergistic effects of convection-enhanced mass transport, limited pore size, and more exposed active sorption sites of the filters. The presence of 1–10 mmol L⁻¹ of carbonate, sulfate, and chloride inhibits Sb(iii) removal negligibly. Exhausted hybrid filters can be effectively regenerated by an electrical field-assisted chemical washing method. STEM characterization confirmed that Sb was mainly sequestered by iron (oxyhydr)oxides. XPS, AFS and XAFS results suggest that a certain amount of Sb(iii) was converted to Sb(v) during filtration. DFT calculations further indicate that the bonding energy for Sb(iii) onto the iron (oxyhydr)oxides was 2.27–2.30 eV, and the adsorbed Sb(iii) tends to be oxidized.

Herein, nanoscale iron (oxyhydr)oxide-coated carbon nanotube (CNT) filters were rationally designed for rapid and effective removal of Sb(iii) from water.     

An efficient mercapto-functionalized organic–inorganic hybrid sorbent for selective separation and preconcentration of antimony(iii) in water samples

This work reported on the application of mercapto-functionalized silica-supported organic–inorganic hybrid sorbent as a solid phase extraction (SPE) extractant for effective separation and preconcentration of Sb(iii) species in real water samples. The influences of pH, sorbent amounts, flow rates and the concentration of eluent on the adsorption and desorption of Sb(iii) species had been evaluated. The recovery of Sb(iii) species at pH 5 with 100 mg mercapto-functionalized hybrid sorbent at the flow rate of 5.0 mL min⁻¹ was greater than 95% without interference from all of metal ions tested. The trapped Sb(iii) species by extractant was then eluted with 5% HCl solution at the flow rate of 5.0 mL min⁻¹. The proposed procedure permitted large enrichment factors of about 200 and higher for 10 μg L⁻¹ of Sb(iii) species. The merits of analytical figures for the determination of Sb(iii) species were as follows: detection limit (3σ, n = 11), 2 ng L⁻¹; precision, 1.6% (n = 11) for 10 μg L⁻¹ of Sb(iii) species; the linear calibration curve presented in the concentration range of 1.0–200.0 μg L⁻¹. The validity of the proposed procedure was checked by the analysis of standard reference materials. Excellent agreement between the analytical results and the certified values (t-test at 95% confidence level) was found. The mercapto-functionalized hybrid sorbent as a SPE extractant was applied to the determination of Sb(iii) species in various water samples with satisfactory results.

This work reported on the application of mercapto-functionalized silica-supported organic–inorganic hybrid sorbent as a solid phase extraction (SPE) extractant for effective separation and preconcentration of Sb(iii) species in real water samples.     

Arsenic(iii) removal from aqueous solution using TiO2-loaded biochar prepared by waste Chinese traditional medicine dregs

Oxidation of As(iii) to As(v) is an effective way to improve the performance of most arsenic removal technologies. In this study, a new alternative biosorbent, TiO₂-loaded biochar prepared by waste Chinese traditional medicine dregs (TBC) was applied in remediation for As(iii) from aqueous solution. Compared with unmodified biochar, the specific surface areas and total pore volumes of TBC increased while the average aperture decreased due to the loading of nano-TiO₂. The X-ray diffraction (XRD) of TBC confirmed that the precipitated titanium oxide was primarily anatase. pH did not have a significant effect on the adsorption capacity at 10 mg L⁻¹ As(iii) in suspension with a pH ranging from 2 to 10. Adsorption kinetics data were best fitted by the pseudo-second-order model (R² > 0.999). The Sips maximum adsorption capacity was 58.456 mg g⁻¹ at 25 °C, which is comparable with other adsorbents reported in previous literature. The Gibbs free energy (ΔG) of As(iii) adsorption was negative, indicating the spontaneous nature of adsorption. The results of free radical scavenging and N₂ purging experiments indicated that O₂ acted as an electron accepter and O₂˙⁻ dominated the oxidation of As(iii). The oxidation of As(iii) obviously affected the adsorption capacity for As(iii) by TBC. X-ray photoelectron spectroscopy (XPS) studies showed that As(iii) and As(v) existed on the surface of TBC, suggesting that the oxidation of As(iii) occurred. TBC played multiple roles for As(iii), including direct adsorption and photocatalytic oxidation adsorption. Regeneration and stability experiments showed that TBC was an environment-friendly and efficient adsorbent for As(iii) removal.

TiO₂-loaded biochar prepared by waste Chinese traditional medicine dregs (TBC) was applied in remediation for As(iii) from aqueous solution.     

Removal of arsenic(v) from aqueous solutions using sulfur-doped Fe3O4 nanoparticles

In this study, magnetic sulfur-doped Fe₃O₄ nanoparticles (Fe₃O₄:S NPs) were applied as adsorbents for the removal of As(v). Fe₃O₄:S NPs were fabricated by a two-step route, which included low-temperature mixing and high-temperature sintering. The as-prepared Fe₃O₄:S NPs could effectively remove As(v) under a wide pH range of 2–10 and presented a high As(v) adsorption capacity of 58.38 mg g⁻¹, which was much better than undoped Fe₃O₄ nanoparticles (20.24 mg g⁻¹). Adsorption experiments exhibited a pseudo-second-order model of adsorption kinetics and a Langmuir isotherm model of adsorption isotherms. Additionally, the coexisting ions such as NO₃⁻, SO₄²⁻, and CO₃²⁻ had no significant effect on As(v) adsorption and the adsorbent worked well in actual smelting wastewater. XPS and FTIR spectra of Fe₃O₄:S NPs before and after As(v) adsorption showed that Fe–OH groups played a significant role in the adsorption mechanisms. Moreover, the magnetic Fe₃O₄:S NPs adsorbents after adsorption could be rapidly separated from wastewater with an external magnetic field. Therefore, Fe₃O₄:S NPs could be an ideal candidate for the removal of As(v) from water.

Magnetic Fe₃O₄:S NPs presented a much better As(v) adsorption performance than undoped Fe₃O₄ NPs due to sulfur doping.     

Simultaneous removal of Sb(iii) and Cd(ii) in water by adsorption onto a MnFe2O4–biochar nanocomposite

In this study, a jacobsite–biochar nanocomposite (MnFe₂O₄–BC) was fabricated and used to simultaneously remove Sb(iii) and Cd(ii) from water via adsorption. The MnFe₂O₄–BC nanocomposite was prepared via a co-precipitation method and analyzed using various techniques. The results confirm the successful decoration of the biochar surface with MnFe₂O₄ nanoparticles. The maximum Sb(iii) removal efficiency was found to be higher from bi-solute solutions containing Cd(ii) than from single-solute systems, suggesting that the presence of Cd(ii) enhances the removal of Sb(iii). The Langmuir isotherm model describes well Sb(iii) and Cd(ii) removal via adsorption onto the MnFe₂O₄–BC nanocomposite. The maximum adsorption capacities are 237.53 and 181.49 mg g⁻¹ for Sb(iii) and Cd(ii), respectively, in a bi-solute system. Thus, the prepared MnFe₂O₄–BC nanocomposite is demonstrated to be a potential adsorbent for simultaneously removing Sb(iii) and Cd(ii) ions from aqueous solutions.

In this study, a jacobsite–biochar nanocomposite (MnFe₂O₄–BC) was fabricated and used to simultaneously remove Sb(iii) and Cd(ii) from water via adsorption.     

Development,modification, and application of low-cost and available biochar derived from corn straw for the removal of vanadium(v) from aqueous solution and real contaminated groundwater

In this work, a low-cost and available material for use as a permeable reactive barrier (PRB) to prevent vanadium in groundwater from leaking into river water was developed. Three modified biochars were prepared from available corn straw pretreated with CsCl, Zn(ii), and Zr(iv) to enhance ion exchange capacity (IEC) and specific surface area, and were designated as Cs-BC, Zn-BC, and Zr-BC, respectively. These materials were characterized via IEC, N₂ adsorption–desorption, Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) analyses. The Langmuir isotherm model could be applied for the best fit for the adsorption data of Cs-BC and Zr-BC, indicating that vanadium(v) sorption occurred in a monolayer. The vanadium(v) adsorption capacities of Cs-BC, Zn-BC, and Zr-BC were 41.07, 28.46, and 23.84 mg g⁻¹, respectively, which were 3.22–5.55 times higher than that of commercial activated carbon (AC) (7.40 mg g⁻¹), probably because of their higher IECs and specific surface areas after modification. In addition, no heavy metal leaching was found from the modified biochars during the adsorption processes when pH > 2. According to the FTIR and XRD patterns, the adsorption mechanism of Cs-BC and Zr-BC was ion exchange, whereas for Zn-BC, it was mainly surface precipitation and electrostatic attraction. The adsorption of vanadium(v) onto the modified biochars was independent of pH in the range of 4.0 to 8.0. Furthermore, the removal efficiency of the vanadium(v) in real contaminated groundwater from the catchment of the Chaobei River by Zn-BC reached 100% at a dose of 4 g L⁻¹. Hence, modified biochars are promising PRB filling materials for removing vanadium(v) from contaminated groundwater.

In this work, a low-cost and available material for use as a permeable reactive barrier (PRB) to prevent vanadium in groundwater from leaking into river water was developed.     

Lead and uranium sorptive removal from aqueous solution using magnetic and nonmagnetic fast pyrolysis rice husk biochars

This paper discusses the sorption characteristics of Pb(ii) and U(vi) on magnetic and nonmagnetic rice husk biochars. The porosity, specific surface area, hydrophobility, and reusability of biochar were effectively improved (1–2 times) after magnetic modification. The optimum adsorption conditions were as follows: biochar loading was 0.4 g L⁻¹, pH value was 7.0, and anion strength of NO₃⁻ and PO₄³⁻ were 0.01 mol L⁻¹ for Pb(ii) and 0.04 mol L⁻¹ for U(vi) respectively. Compared with U(vi), Pb(ii) had the faster adsorption rate and higher adsorption capacity on magnetic biochar (MBC). The adsorption experimental data were well fitted by pseudo-second-order kinetic and Langmuir isotherm models. The maximum adsorption capacity of Pb(ii) and U(vi) on MBC was 129 and 118 mg g⁻¹ at 328 K respectively, which was significantly higher than that of other sources biochars. Pb(ii) was mainly bonded to biochar by physisorption but the adsorption of U(vi) on biochar was mostly chemisorption. Fe oxides in MBC noticeably improved the ion exchange and complexation action between biochar and metal ion especially for U(vi). The experimental results confirmed MBC material can be used as a cost-effective adsorbent for the removal of Pb(ii) and U(vi) and can be separated easily from aqueous solution when application.

This paper discusses the sorption characteristics of Pb(ii) and U(vi) on magnetic and nonmagnetic rice husk biochars.     

Three-dimensional mesoporous calcium carbonate–silica frameworks thermally activated from porous fossil bryophyte: adsorption studies for heavy metal uptake

In this paper, three-dimensional mesoporous calcium carbonate–silica frameworks have been created from the straw tufa (ST) originating from porous fossil bryophyte by a thermal activation technique. A batch of adsorption kinetic and thermodynamic experiments were used to investigate the adsorption capacity of Cd(ii) onto the samples. The ST after thermal activation has shown a significant ability for the uptake of heavy metals. It exhibited maximum adsorption capacities of 12.76 mg g⁻¹, 14.09 mg g⁻¹, 17.00 mg g⁻¹, and 33.81 mg g⁻¹ for Cd(ii) at the activation temperature of 300, 450, 600 and 750 °C, respectively. Through competitive adsorption for Cd(ii)and Pb(ii), the ST thermally activated at 750 °C exhibited maximum equilibrium adsorption capacities of 24.65 mg g⁻¹, 25.91 mg g⁻¹, and 30.94 mg g⁻¹ for Cd(ii) uptake at 298.1 K, 308.1 K and 318.1 K, respectively, whereas it exhibited values of 91.59 mg g⁻¹, 101.32 mg g⁻¹, and 112.19 mg g⁻¹ for Pb(ii) removal. The adsorption capacities of Cd(ii) and Pb(ii) both decrease with the addition of the other heavy metal cations, indicating that the adsorption is hindered by the competitive adsorption and the adsorption active sites on the mineral surface are readily exchangeable. The adsorption of Cd(ii) and Pb(ii) followed the pseudo-second order kinetics model well. In addition, the Langmuir model could accurately describe the adsorption isotherms. Based on the results of characterization with TEM, XRD and XPS, the adsorption mechanisms could be primarily explained as formation of Cd(OH)₂ and CdCO₃ as well as Cd(HCO₃)₂ precipitation on the surface of ST. These characteristics of ion-exchange and the adsorptive property for ST modified allow it to be widely used in artificial wetland landfill and environmental protection.

Three-dimensional mesoporous calcium carbonate–silica frameworks have been created and have shown excellent adsorption capacities for Cd(ii) and Pb(ii).     

Synthesis of alginate–polycation capsules of different composition: characterization and their adsorption for [As(iii)] and [As(v)] from aqueous solutions

The uptake of arsenite [As(iii)] and arsenate [As(v)] by functionalized calcium alginate (Ca-Alg) beads from aqueous solutions was investigated. Ca-Alg beads were protonated with poly-l-lysine (PLL) or polyethyleneimine (PEI) using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide/N-hydroxysuccinimide (EDC/NHS) or glutaraldehyde (GA) as crosslinking agents. Four types of protonated beads were prepared: Ca-Alg-EDC/NHS (PLL or PEI) and Ca-Alg-GA (PLL or PEI). Fourier transform infrared spectroscopy in total attenuated reflection mode (FTIR-ATR), analysis showed presence and increased intensity of bands corresponding to OH, NH, CH₂ and CH₃ groups in modifications with both polycations. In addition, thermogravimetric analysis and atomic force microscopy of all modified capsules showed an increase in thermal stability and uniformity of the capsules, respectively. Ca-Alg-EDC/NHS-PLL beads had the maximum adsorption capacity of [As(v)] (312.9 ± 4.7 μg g⁻¹ of the alginate) at pH 7.0 and 15 minute exposure, while Ca-Alg-EDC/NHS-PEI beads had the maximum adsorption capacity of [As(iii)] (1052.1 ± 4.6 μg g⁻¹ of alginate). However, all these EDC containing beads were degraded in the presence of citrate. Ca-Alg-GA-PEI beads removed 252.8 ± 9.7 μg of [As(v)] μg g⁻¹ of alginate and 524.7 ± 5.3 de [As(iii)] μg g⁻¹ of alginate, resulting the most stable capsules and suitable for As removal.

A simple protonation of alginate beads allows the absorption of arsenate and arsenite.     

Facile transmetallation of [SbIII(DOTA)]− renders it unsuitable for medical applications

The antimony(iii) complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (DOTA) has been prepared and its exceptionally low stability observed. The Sb(iii) ion in Na[Sb(DOTA)]·4H₂O shows an approximately square antiprismatic coordination geometry that is close to superimposable to the Bi(iii) geometry in [Bi(DOTA)]⁻ in two phases containing this anion, Na[Bi(DOTA)]·4H₂O, [H₃O][Bi(DOTA)]·H₂O for which structures are also described. Interestingly, DOTA itself in [(H₆DOTA)]Cl₂·4H₂O·DMSO shows the same orientation of the N₄O₄ metal binding cavity reflecting the limited flexibility of DOTA in an octadentate coordination mode. In 8-coordinate complexes it can however accommodate M(iii) ions with r_ion spanning a relatively wide range from 87 pm (Sc(iii)) to 117 pm (Bi(iii)). The larger Bi³⁺ ion appears to be the best metal–ligand size match since [Bi(DOTA)]⁻ is associated with greater complex stability. In the solution state, [Sb(DOTA)]⁻ is extremely susceptible to transmetallation by trivalent ions (Sc(iii), Y(iii), Bi(iii)) and, significantly, even by biologically important divalent metal ions (Mg(ii), Ca(ii), Zn(ii)). In all cases just one equivalent is enough to displace most of the Sb(iii). [Sb(DOTA)]⁻ is resistant to hydrolysis; however, since biologically more abundant metal ions easily substitute the antimony, DOTA complexes will not be suitable for deployment for the delivery of the, so far unexploited, theranostic isotope pair ¹¹⁹Sb and ¹¹⁷Sb.

The antimony(iii) complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (DOTA) has been prepared and its exceptionally low stability observed.     

Mechanistic insights into dioxygen activation by a manganese corrole complex: a broken-symmetry DFT study

The Mn–oxygen species have been implicated as key intermediates in various Mn-mediated oxidation reactions. However, artificial oxidants were often used for the synthesis of the Mn–oxygen intermediates. Remarkably, the Mn(v)–oxo and Mn(iv)–peroxo species have been observed in the activation of O₂ by Mn(iii) corroles in the presence of base (OH⁻) and hydrogen donors. In this work, density functional theory methods were used to get insight into the mechanism of dioxygen activation and formation of Mn(v)–oxo. The results demonstrated that the dioxygen cannot bind to Mn without the axial OH⁻ ligand. Upon the addition of the axial OH⁻ ligand, the dioxygen can bind to Mn in an end-on fashion to give the Mn(iv)–superoxo species. The hydrogen atom transfer from the hydrogen donor (substrate) to the Mn(iv)–superoxo species is the rate-limiting step, having a high reaction barrier and a large endothermicity. Subsequently, the O–C bond formation is concerted with an electron transfer from the substrate radical to the Mn and a proton transfer from the hydroperoxo moiety to the nearby N atom of the corrole ring, generating an alkylperoxo Mn(iii) complex. The alkylperoxo O–O bond cleavage affords a Mn(v)–oxo complex and a hydroxylated substrate. This novel mechanism for the Mn(v)–oxo formation via an alkylperoxo Mn(iii) intermediate gives insight into the O–O bond activation by manganese complexes.

DFT calculations revealed a novel mechanism for the formation of Mn(v)–oxo in the dioxygen activation by a Mn(iii) corrole complex involving a Mn(iii)–alkylperoxo intermediate.     

Efficient removal of chromate ions from aqueous solution using a highly cost-effective ferric coordinated [3-(2-aminoethylamino)propyl]trimethoxysilane–MCM-41 adsorbent

The present investigation involves synthesis and characterization of MCM-41–AEAPTMS–Fe(iii)Cl using coordinated Fe(iii) on MCM-41–AEAPTMS for efficient removal of hazardous Cr(vi) ions from aqueous solution. The adsorbent MCM-41–AEAPTMS–Fe(iii)Cl was characterized using small-angle X-ray diffraction (SAX), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Fourier-transform infrared (FT-IR) and Brunauer–Emmett–Teller (BET) surface analyzer techniques. The BET surface area was found to be 87.598 m² g⁻¹. The MCM-41–AEAPTMS–Fe(iii)Cl effectively adsorbs Cr(vi) with an adsorption capacity acquiring the maximum value of 84.9 mg g⁻¹ at pH 3 at 298 K. The data followed pseudo-second-order kinetics and obeyed the Langmuir isotherm model. The thermodynamic data proved the exothermic and spontaneous nature of Cr(vi) ion adsorption on MCM-41–AEAPTMS–Fe(iii). Further, the higher value of ΔH° (−64.339 kJ mol⁻¹) indicated that the adsorption was chemisorption in nature.

The present investigation involves synthesis and characterization of MCM-41–AEAPTMS–Fe(iii)Cl using coordinated Fe(iii) on MCM-41–AEAPTMS for efficient removal of hazardous Cr(vi) ions from aqueous solution.     

Hydrogels derived from galactoglucomannan hemicellulose with inorganic contaminant removal properties

The adsorption of Cu(ii), Cd(ii), and Pb(ii) ions onto hydrogels derived from modified galactoglucomannan (GGM) hemicellulose was studied. GGM hemicellulose was modified with methacrylate groups (GGM-MA) to incorporate vinyl groups into the polymeric structure, which reacted later with synthetic monomers such as 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS). The results show that all the synthesized hydrogels were capable of adsorbing contaminating ions with high adsorption efficiency during short periods of time. Furthermore, an increase in the content of GGM-MA generated a hydrogel (H3) with a similar ion adsorption property to the other hydrogels but with a lesser degree of swelling. The H3 hydrogel had an adsorption capacity of 60.0 mg g⁻¹ Cd(ii), 78.9 mg g⁻¹ Cu(ii), and 174.9 mg g⁻¹ Pb(ii) at 25 °C. This result shows that modified GGM hemicelluloses can be employed as renewable adsorbents to remove Cu(ii), Cd(ii), and Pb(ii) ions from aqueous solutions.

The adsorption of Cu(ii), Cd(ii), and Pb(ii) ions onto hydrogels derived from modified galactoglucomannan (GGM) hemicellulose was studied.     

Efficient recovery of Cr(vi) from electroplating wastewater by iron-modified sludge-based hollow-structured porous carbon: coexistence effects and competition for adsorption

In the present work, porous carbon was made from sewage sludge and hybrid liriodendron leaves, and modified with iron ions (Fe@LS-BC) carried out on Cr(vi) in aqueous solution from a single-component system and in competitive biosorption with methyl orange (MO) from a binary-component system. The iron ion-modified porous carbon (Fe@LS-BC) showed higher efficiency in the removal of Cr(vi) compared to porous carbon prepared by the co-pyrolysis of sludge and hybrid liriodendron leaves. The incorporation of the Fe element improved the ability of the material to redox Cr(vi), while imparting magnetic characteristics to the porous carbon and improving the reusability of the porous carbon. On the other hand, Fe@LS-BC exhibited a better pore volume, facilitating the contact of the material with Cr(vi) ions. The highest adsorption capacity was 0.33 mmol g⁻¹, and the adsorption experimental results for the single-component and binary-component systems of Cr(vi) matched well with the Langmuir–Freundlich models. When the concentration of MO was 0.2 and 0.8 mmol L⁻¹, respectively, the highest adsorption capacity of Cr(vi) was 0.35 and 0.46 mmol g⁻¹ in the binary system. The positively charged N–CH₃⁺ on the MO molecule promoted the electrostatic adsorption between HCrO₄⁻, CrO₄²⁻, and Fe@LS-BC, and increased the adsorption potential of Cr(vi).

Mechanism for the adsorption of hexavalent chromium and methyl orange in a binary system.     

Extraction of polyoxotantalate by Mg–Fe layered double hydroxides: elucidation of sorption mechanisms

The extraction of Ta(v) as polyoxometallate species (H_xTa₆O₁₉^(8−x)−) using Mg–Fe based Layered Double Hydroxide (LDH) was evaluated using pristine material or after different pre-treatments. Thus, the uptake increased from 100 ± 5 mg g⁻¹ to 604 ± 30 mg g⁻¹, for respectively the carbonated LDH and after calcination at 400 °C. The uptake with calcined solid after its reconstruction with Cl⁻ or NO₃⁻ anions has also been studied. However, the expected exchange mechanism was not found by X-ray Diffraction analysis. On the contrary, an adsorption mechanism of Ta(v) on LDH was consistent with measurements of zeta potential, characterized by very negative values for a wide pH range. Moreover, another mechanism was identified as the main contributor to the uptake by calcinated LDH, even after its reconstruction with Cl⁻ or NO₃⁻: the precipitation of Ta(v) with magnesium cations released from MgO formed by calcination of the LDH. This latter reaction has been confirmed by the comparison of the uptake of Ta(v) in dedicated experiments with solids characterized by a higher magnesium solubility (MgO and MgCl₂). The obtained precipitate has been analyzed by X-ray diffraction (XRD) and would correspond to a magnesium (polyoxo)tantalate phase not yet referenced in the powder diffraction databases.

Reaction of polyoxotantalate ions and MgFe Layered Double Hydroxide leads to magnesium polyoxotantalate precipitate.     

Selective preconcentration separation of Hg(ii) and Cd(ii) from water,fish muscles,and cucumber samples using recycled aluminum adsorbents

Modified aluminum scrap waste was used in the selective extraction of Hg(ii), and Cd(ii) ions. The aluminum scraps were modified with dibenzoylmethane, or isatoic anhydride, or 5-(2-chloroacetamide)-2-hydroxybenzoic acid. The modified aluminum sorbents were characterized by FT-IR, SEM, XRD, XPS, TGA, and elemental analysis. Modes of chelation between adsorbents and target metal ions were deduced via DFT. The highest adsorption capacity was observed for benzo-amino aluminum (BAA) toward Hg(ii), which reached 234.56 mg g⁻¹, while other modified sorbents ranged from 135.28 mg g⁻¹ to 229.3 mg g⁻¹. Under the optimized conditions, the BAA adsorbent showed a lower limit of detection (1.1 mg L⁻¹) and limit of quantification (3.66 mg L⁻¹) for mercury ions than other sorbents. The prepared aluminum adsorbents also exhibited significant selectivities for Hg(ii) and Cd(ii) ions in the presence of competing metal ions.

Modified aluminum scrap waste was used in the selective extraction of Hg(ii), and Cd(ii) ions.