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.     

High dispersions of nano zero valent iron supported on biochar by one-step carbothermal synthesis and its application in chromate removal

A one-step carbothermal synthesis and characterization of biochar-supported nanoscale zero-valent iron (nZVI/BC) was performed for the removal of hexavalent chromium (Cr(vi)) from aqueous solution. High dispersions of nanoscale zero-valent iron supported on biochar were successfully synthesized by the pyrolysis of an iron-impregnated biomass (corn stover) as the carbon and iron source under nitrogen atmosphere. The effects of the pyrolytic temperature on the Fe mineralogies formed on the biochar are discussed. In general, the effects of the treatment time, initial solution pH, and nZVI/BC dosage on the Cr(vi) removal are presented. The results showed high crystallinity and purity, and nZVI/BC was obtained at a pyrolytic temperature of 800 °C. The batch experimental results determined that the adsorption capacity of Cr(vi) decreases with the increase in the initial pH value from 4.0 to 10.0. The Cr(vi) adsorption kinetics data effectively followed a pseudo-second-order kinetics with a calculated rate constant of 0.0.3396 g mg⁻¹ min⁻¹. The calculated thermodynamic parameters, such as ΔG°, ΔH°, and ΔS°, were evaluated, and the results indicated that the Cr(vi) reduction on nZVI/BC was a spontaneous and endothermic process. The adsorption mechanism of Cr(vi) was investigated by XRD and XPS analyses and the results demonstrated that Cr(vi) was reduced to Cr(iii) and the oxidation of nZVI occurred during the reaction process. These results prove that nZVI/BC synthesized by a one-step carbothermal method can be considered as a potential candidate for the removal of Cr(vi) from aqueous solutions.

A one-step carbothermal synthesis and characterization of biochar-supported nanoscale zero-valent iron (nZVI/BC) was performed for the removal of hexavalent chromium (Cr(vi)) from aqueous solution.     

Biosorption of Cr(vi) from aqueous solution using dormant spores of Aspergillus niger

Spores of Aspergillus niger (denoted as A. niger) were used as a novel biosorbent to remove hexavalent chromium from aqueous solution. The effects of biosorbent dosage, pH, contact time, temperature and initial concentration of Cr(vi) on its adsorption removal were examined in batch mode. The Cr(vi) uptake capacity increased with an increase in Cr(vi) concentration until saturation, which was found to be about 97.1 mg g⁻¹ at pH 2.0, temperature of 40 °C, adsorbent dose of 2.0 g L⁻¹ and initial concentration of 300 mg L⁻¹. Scanning electron microscopy, energy dispersive X-ray spectroscopy, field-emission transmission electron microscopy (FETEM), XPS and Fourier-transform infrared spectroscopy were applied to study the microstructure, composition and chemical bonding states of the biomass adsorbent before and after spore adsorption. The mechanisms of chromate anion removal from aqueous solution by the spores of A. niger were proposed, which included adsorption of Cr(vi) onto the spores followed by its reduction to Cr(iii). The reduced Cr(iii) was rebound to the biomass mainly through complexation mechanisms, redox reaction and electrostatic attraction. The removal of Cr(vi) by spores of A. niger followed pseudo-second-order adsorption kinetics. Monolayer adsorption of Cr(vi) was revealed by the better fitting of the Langmuir model isotherm rather than multilayer adsorption for the Freundlich model. The results indicated that A. niger spores can be used as a highly efficient biosorbent to remove Cr(vi) from contaminated water.

Spores of Aspergillus niger (denoted as A. niger) were used as a novel biosorbent to remove hexavalent chromium from aqueous solution.     

Removal of Pb(ii) and Cr(vi) from aqueous solutions using the prepared porous adsorbent-supported Fe/Ni nanoparticles

In this study, Fe/Ni nanoparticles supported by a novel fly ash-based porous adsorbent (FBA-Fe/Ni) for Cr(vi) and Pb(ii) removal were investigated. In order to enhance the reactivity of zero-valent iron (ZVI), ZVI particles were deposited on the surface or in the inner pores of FBA as a support material and Ni nanoparticles were introduced. FBA was prepared with the solid waste such as Enteromorpha prolifera, bentonite and fly ash. FBA-Fe/Ni was characterized via Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction and the Brunauer–Emmett–Teller model and energy-dispersive spectrometry. The effects of various parameters on Cr(vi) and Pb(ii) removal by FBA-Fe/Ni, such as FBA-Fe/Ni dosage, pH of the solution, reaction temperature, Cr(vi) and Pb(ii) concentrations, co-existing ions and ionic strength were discussed. The possible removal mechanisms were proposed and the results indicated that there was a three-step reaction including the adsorption of Cr(vi) and Pb(ii) on the surface of FBA-Fe/Ni, the subsequent reduction and precipitation. The removal capacity of Cr(vi) and Pb(ii) by FBA-Fe/Ni was 25.07 mg g⁻¹ and 164.19 mg g⁻¹ at 303 K with an initial concentration of 1000 mg L⁻¹ and FBA-Fe/Ni dosage of 0.20 g. In conclusion, this work demonstrated that FBA-Fe/Ni was a promising alternative material for Cr(vi) and Pb(ii) removal.

In this study, Fe/Ni nanoparticles supported by a novel fly ash-based porous adsorbent (FBA-Fe/Ni) for Cr(vi) and Pb(ii) removal were investigated.     

Characterization of CMC–LDH beads and their application in the removal of Cr(vi) from aqueous solution

In this study, CMC–LDH beads were prepared and characterized using SEM, FTIR and TG analysis. The beads were applied for the removal of Cr(vi) from aqueous solution. The effects of adsorbent dosage, initial pH and initial concentration of Cr(vi) solution on Cr(vi) uptake were investigated in detail. Moreover, adsorption isotherms and adsorption kinetic models were employed to analyze the adsorption process, and a preliminary study of the reusability of the adsorbent was performed. The experimental results showed that the CMC–LDH beads could remove Cr(vi) from aqueous solution efficiently. When the initial concentration of the Cr(vi) solution was 100 mg L⁻¹ and the adsorbent dosage was 12 g L⁻¹, the removal efficiency of Cr(vi) reached 96.2%. After the CMC–LDH beads were reused 10 times, the removal efficiency of Cr(vi) still remained at 89.6%.

CMC–LDH beads were prepared, characterized and applied for the removal of heavy metal ions in this study.     

Positive effects of concomitant heavy metals and their reduzates on hexavalent chromium removal in microbial fuel cells

Cr(vi) laden wastewaters generally comprise a range of multiple heavy metals such as Au(iii) and Cu(ii) with great toxicity. In the present study, cooperative cathode modification by biogenic Au nanoparticles (BioAu) reduced from aqueous Au(iii) and in situ Cu(ii) co-reduction were investigated for the first time to enhance Cr(vi) removal in microbial fuel cells (MFCs). With the co-existence of Cu(ii) in the catholyte, the MFC with carbon cloth modified with nanocomposites of multi-walled carbon nanotubes blended with BioAu (BioAu/MWCNT) obtained the highest Cr(vi) removal rate (4.07 ± 0.01 mg L⁻¹ h⁻¹) and power density (309.34 ± 17.65 mW m⁻²), which were 2.73 and 3.30 times as high as those for the control, respectively. The enhancements were caused by BioAu/MWCNT composites and deposited reduzates of Cu(ii) on the cathode surface, which increased the adsorption capacity, electronic conductivity and electrocatalytic activity of the cathode. This study provides an alternative approach for efficiently remediating co-contamination of multiple heavy metals and simultaneous bioenergy recovery.

The cooperative cathode modification by BioAu from Au(iii) and in situ Cu(ii) co-reduction enhanced Cr(vi) removal and bioelectricity generation in MFCs.     

Facile preparation of hybrid porous polyanilines for highly efficient Cr(vi) removal

In the present work, leucoemeraldine-based hybrid porous polyanilines (LHPPs) have been synthesized by the Friedel–Crafts reaction of leucoemeraldine and octavinylsilsesquioxane (OVS) for Cr(vi) removal. The resulting LHPPs were characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and N₂ adsorption–desorption. The findings indiated that the LHPPs were amorphous, with apparent surface areas (S_BET) in the range of 147 to 388 m² g⁻¹ and total volumes in the range of 0.13 to 0.44 cm³ g⁻¹. Cr(vi) removal experiments displayed that the LHPPs exhibited highly efficient Cr(vi) removal performance. The maximum Cr(vi) removal capacity of LHPP-1 was 990.1 mg g⁻¹ at 308 K and pH 1, which is higher than those of other reported polyaniline-based adsorbents. The adsorption process was a spontaneous, endothermic and chemical adsorption process. The adsorption behavior agreed well with Langmuir models and pseudo second-order equations. X-ray photoelectron spectroscopy and Fourier transformed infrared (FTIR) spectroscopy analysis revealed that the highly efficient Cr(vi) removal performance can be mainly attributed to the existence of numerous amine and imine groups on the surface of the LHPPs; these can function as adsorption active sites for Cr(vi) removal through electrostatic adsorption and reduction to Cr(iii) under acidic conditions. Moreover, the LHPPs exhibited excellent adsorption selectivity for Cr(vi) despite the presence of other metal ions (K⁺, Cu²⁺, Mn²⁺) and anions (NO₃⁻, SO₄²⁻). Therefore, the LHPPs have potential applications for Cr(vi) removal in industrial wastewater.

In the present work, leucoemeraldine-based hybrid porous polyanilines (LHPPs) have been synthesized by the Friedel–Crafts reaction of leucoemeraldine and octavinylsilsesquioxane (OVS) for Cr(vi) removal.     

Degradation of tetrabromobisphenol A by a ferrate(vi)–ozone combination process: advantages,optimization, and mechanistic analysis

This study systematically investigated the ferrate(vi)–ozone combination process for TBBPA degradation. Firstly, the advantages of a ferrate(vi)–ozone combination process were assessed as compared with a sole ozone and ferrate(vi) oxidation process. Then, the performance of the ferrate(vi)–ozone combination process was investigated under different experimental conditions, including the dosing orders of oxidants, dosing concentrations of oxidants, and the initial solution pH. At the same time, toxicity control (including the acute and chronic toxicity) and mineralization were analyzed after optimization. Finally, a mechanism was proposed about the synergetic effects of the ferrate(vi)–ozone combination process for decontamination. The ferrate(vi)–ozone combination process proved to be an efficient and promising technology for removing TBBPA from water. After being pre-oxidized by ferrate(vi) for 3 min and then co-oxidized by the two oxidants, TBBPA of 1.84 μmol L⁻¹ could be completely degraded by dosing only 0.51 μmol L⁻¹ of ferrate(vi) and 10.42 μmol L⁻¹ of ozone within 10 min in wide ranges of pH (5.0–11.0). Up to 91.3% of debromination rate and 80.5% of mineralization rate were obtained, respectively. In addition, no bromate was detected and the acute and chronic toxicity were effectively controlled. The analysis of the proposed mechanism showed that there might exist a superposition effect of the oxidation pathways. In addition, the interactions between the two oxidants were beneficial for the oxidation efficiency of ferrate(vi) and ozone, including the catalytic effect of ferrate(vi) intermediates on ozone and the oxidation of low-valent iron compounds by ozone and the generated ·OH radical.

This study systematically investigated the ferrate(vi)–ozone combination process for TBBPA degradation.     

The enhanced visible-light-induced photocatalytic activities of bimetallic Mn–Fe MOFs for the highly efficient reductive removal of Cr(vi)

The photocatalytic efficiencies of bimetallic MOFs, namely STA-12-Mn–Fe, for the reductive removal of Cr(vi) were explored. The best effective variable values were obtained and correlation between the response and influential variables was optimized via experimental design methodology. Complete Cr(vi) removal was achieved under natural sunlight and fluorescent 40 W lamp radiation at pH 2, with an initial Cr(vi) concentration of 20 mg L⁻¹, and 10 mg of photocatalyst within 30 min. A pseudo-first-order rate constant of 0.132 min⁻¹ at T = 298 K was obtained for the Cr(vi) reduction reaction. The title catalysts revealed high performance in the visible region based on photoefficiency measurements, while improved activity was observed compared to the corresponding single-metal MOFs under natural sunlight, highlighting the synergistic effect between the two metal ions. Trapping experiment results proved that direct electron transfer is the main pathway during the photocatalytic Cr(vi) reduction process.

The photocatalytic efficiencies of bimetallic MOFs for the reductive removal of Cr(vi) were explored. The catalysts revealed higher performance compared to the corresponding single-metal MOFs, highlighting the synergistic effect between the two metal ions.     

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.     

Highly dispersed and stable nano zero-valent iron doped electrospun carbon nanofiber composite for aqueous hexavalent chromium removal

In this work, a nZVI doped electrospun carbon nanofiber (nZVI-CNF) composite was prepared and applied for aqueous hexavalent chromium (Cr(vi)) removal. Firstly, FeCl₃/PAN nanofibers were prepared by a simple electrospinning method; Then, nZVI-CNFs were obtained by carbonization of FeCl₃/PAN nanofibers at 800 °C. The surface morphology and internal structure of nZVI-CNFs were characterized by SEM and TEM, showing that the uniformly dispersed nZVI particles were well integrated into the carbon layer structure. The Cr(vi) removal efficiency of nZVI-CNFs was 91.5% with a Cr(vi) concentration of 10 mg L⁻¹ and the mechanism was further studied by XRD and XPS. Meanwhile, the nZVI-CNFs exhibited good stability over a wide range of pH values from 4–8 and a long time placement stability. Furthermore, nZVI-CNFs can be used as a filter membrane for continuous treatment of wastewater, suggesting great potential for practical application.

Improving the dispersion and stability of nano zero-valent iron (nZVI) is very important for its practical application.     

Mechanism and modeling of hexavalent chromium interaction with a typical black soil: the importance of the relationship between adsorption and reduction

Black soils have a significant retention effect on the migration of Cr(vi) towards groundwater, and Cr(vi) adsorption and reduction are both involved in this process. However, the adsorption and reduction of Cr(vi) were always investigated separately in previous studies resulting in an unclear relationship between them. In this study, the adsorption and reduction kinetic processes of Cr(vi) by a typical black soil were separately investigated under different initial Cr(vi) concentrations (40–400 mg L⁻¹) and pH conditions (3.5–7.0) by the means of desorption treatment, and the equilibrium relationship between aqueous and adsorbed Cr(vi) was innovatively established based on the kinetic data. It was found that under pH 5.7 the adsorbed Cr(vi) content on soil particles was linearly correlated with the remaining Cr(vi) concentration in solution with time (R² = 0.98), and the reduction rate of Cr(vi) in the reaction system was linearly correlated with the adsorbed Cr(vi) content on soil particles with time (R² = 0.99). With pH decreasing from 7.0 to 3.5, the partition of Cr(vi) between solid and aqueous phases turned out to be of a non-linear nature, which can be fitted better by the Freundlich model. The retention of Cr(vi) by black soil was determined to follow the “adsorption–reduction” mechanism, where the Cr(vi) was first rapidly adsorbed onto the soil particles by a reversible adsorption reaction, and then the adsorbed Cr(vi) was gradually reduced into Cr(iii). A two-step kinetic model was developed accordingly, and the experimental data were fitted much better by the two-step adsorption–reduction kinetic model (R² = 0.89 on average) compared with the traditional first-order and second-order kinetic models (R² = 0.66 and 0.76 on average respectively). This paper highlights the novel two step kinetic model developed based on the proposed “adsorption–reduction” mechanism of Cr(vi) retention by a typical black soil.

A novel two-step kinetic model was developed based on the proposed “adsorption–reduction” mechanism of Cr(vi) retention by a typical black soil.     

Three-dimensional honeycomb-like porous carbon derived from corncob for the removal of heavy metals from water by capacitive deionization

In this study, porous carbon (3DHPC) with a 3D honeycomb-like structure was synthesized from waste biomass corncob via hydrothermal carbonization coupled with KOH activation and investigated as a capacitive deionization (CDI) electrode material. The obtained 3DHPC possesses a hierarchal macroporous and mesoporous structure, and a large accessible specific surface area (952 m² g⁻¹). Electrochemical tests showed that the 3DHPC electrode exhibited a specific capacitance of 452 F g⁻¹ and good electric conductivity. Moreover, the feasibility of electrosorptive removal of chromium(vi) from an aqueous solution using the 3DHPC electrode was demonstrated. When 1.0 V was applied to a solution containing 30 mg L⁻¹ chromium(vi), the 3DHPC electrode exhibited a higher removal efficiency of 91.58% compared with that in the open circuit condition. This enhanced adsorption results from the improved affinity between chromium(vi) and the electrode under electrochemical assistance involving a non-faradic process. Consequently, the 3DHPC electrode with typical double-layer capacitor behavior is demonstrated to be a favorable electrode material for capacitive deionization.

A porous carbon electrode with a 3D honeycomb-like structure demonstrates a high removal efficiency for the removal of chromium(vi) from water.     

Effective removal of Cr(vi) from aqueous solution by biochar supported manganese sulfide

In order to remove hexavalent chromium (Cr(vi)) efficiently and simplify the adsorbent preparation process, we employed a single step method to prepare a new biochar supported manganese sulfide material. The nanoscale MnS particles were highly soldered on the biochar support surface, and this adsorbent displayed the effective removal of Cr(vi) (98.15 mg L⁻¹) via synergistic effect between adsorption and reduction/precipitation under weak acid conditions (pH = 5.0–6.0). The adsorption kinetic data were described well by the pseudo second-order kinetic model, suggesting that the reaction process was a chemisorption process. The adsorption isotherm data were described well by the Redlich–Peterson model, further suggesting that this reaction was a hybrid chemical reaction-sorption process. In addition, the Dubinin–Radushkevich isotherm model with 8.28, 8.57, and 12.91 kJ mol⁻¹ adsorption energy also suggests that it was a chemisorption process. The simple and eco-friendly preparation process, low-cost, and the high removal efficiency could make it a promising material for the purification of Cr(vi)-contaminated wastewater.

In order to remove hexavalent chromium (Cr(vi)) efficiently and simplify the adsorbent preparation process, we employed a single step method to prepare a new biochar supported manganese sulfide material.     

A novel resource utilization method using wet magnesia flue gas desulfurization residue for simultaneous removal of ammonium nitrogen and heavy metal pollutants from vanadium containing industrial wastewater

In the present study, a novel resource utilization method using wet magnesia flue gas desulfurization (FGD) residue for the simultaneous removal of ammonium nitrogen (NH₄–N) and heavy metal pollutants from vanadium (V) industrial wastewater was proven to be viable and effective. In this process, the wet magnesia FGD residue could not only act as a reductant of hexavalent chromium [Cr(vi)] and pentavalent vanadium [V(v)], but also offered plenty of low cost magnesium ions to remove NH₄–N using struvite crystallization. The optimum experimental conditions for Cr(vi) and V(v) reduction are as follows: the reduction pH = 2.5, the wet magnesia FGD residue dose is 42.5 g L⁻¹, t = 15.0 min. The optimum experimental conditions for NH₄–N and heavy metal pollutants removal are as follows: the precipitate pH = 9.5, the n(Mg²⁺) : n(NH₄⁺) : n(PO₄³⁻) = 0.3 : 1.0 : 1.0, t = 20.0 min. Finally the NH₄–N, V and Cr were separated from the vanadium containing industrial wastewater by forming the difficult to obtain, soluble coprecipitate containing struvite and heavy metal hydroxides. The residual pollutant concentrations in the wastewater were as follows: Cr(vi) was 0.047 mg L⁻¹, total Cr was 0.1 mg L⁻¹, V was 0.14 mg L⁻¹, NH₄–N was 176.2 mg L⁻¹ (removal efficiency was about 94.5%) and phosphorus was 14.7 mg L⁻¹.

A novel resource utilization method using wet magnesia flue gas desulfurization residue for the simultaneous removal of ammonium nitrogen and heavy metal pollutants from vanadium industrial wastewater was proven to be viable and effective.     

Post synthetically modified IRMOF-3 for efficient recovery and selective sensing of U(vi) from aqueous medium

A simple and efficient route to develop various novel functionalized MOF materials for rapid and excellent recovery of U(vi) from aqueous medium, along with selective sensing has been demonstrated in the present study. In this connection, a set of four distinct post synthetically modified (PSM) iso-reticular metal organic frameworks were synthesized from IRMOF-3 namely, IRMOF-PC (2-pyridine carboxaldehyde), IRMOF-GA (glutaric anhydride), IRMOF-SMA (sulfamic acid), and IRMOF-DPC (diphenylphosphonic chloride) for the recovery and sensing of U(vi) from aqueous medium. The MOFs were characterized by Fourier transform infrared spectroscopy (FTIR), powder XRD, BET surface area analysis, thermogravimetric analysis (TGA), NMR (¹³C, ¹H and ³¹P), Scanning Electron Microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). Among all MOFs, post synthetically modified IRMOF-SMA showed enhanced thermal stability of about 420 °C. The MOFs were investigated for U(vi) sorption studies using a batch technique. All the MOFs exhibit excellent sorption capacity towards U(vi) (>90%) and maximum uptake was observed at pH 6. Sorption capacity of MOFs have the following order; IRMOF-3-DPC (300 mg U g⁻¹) > IRMOF-SMA (292 mg U g⁻¹) > IRMOF-PC (289 mg U g⁻¹) > IRMOF-GA (280 mg U g⁻¹) > IRMOF-3 (273 mg U g⁻¹). IRMOF-DPC shows rapid sorption of uranium within 5 min with excellent uptake of U(vi) (>99%). The desorption of U(vi) was examined with different eluents and 0.01 M HNO₃ was found to be most effective. The fluorescence sensing studies of U(vi) via IRMOF-3 and its PSM MOFs revealed high sensitivity and selectivity towards U(vi) over other competing rare earth metal ions (La³⁺, Ce⁴⁺, Sm³⁺, Nd³⁺, Gd³⁺, and Eu³⁺), wherein IRMOF-GA displayed an impressive detection limit of 0.36 mg L⁻¹ for U(vi).

Four IRMOFs following PSM strategy were prepared. The MOFs were characterized by different techniques and were investigated for U(vi) sorption. PSM MOFs displayed impressive fluorescent sensing and selectivity of U(vi) over competing metal ions.     

Adsorption and reduction of hexavalent chromium on magnetic greigite (Fe3S4)-CTAB: leading role of Fe(ii) and S(−ii)

In this study, a facile one-step route was used to synthesize a novel magnetic mesoporous greigite (Fe₃S₄)-CTAB composite, which was utilized to remove hexavalent chromium (Cr(vi)). The optimized Fe₃S₄-CTAB_0.75 composite with a CTAB dosage of 0.75 g possessed the maximum specific surface, showing the highest Cr(vi) adsorption capacity of 330.03 mg g⁻¹. The mechanism analysis revealed that Fe(ii) and S(−ii) were critical for the reduction of Cr(vi). CTAB can promote the removal of Cr(vi) by Fe₃S₄-CTAB composites, possibly due to increased S(−ii) concentration, better dispersion of nanoparticles, and greater zeta potential. Besides, there is mild effect of Fe⁰ on Cr(vi) removal, which is confirmed by the disappearance of the Fe⁰ peak from the XPS analysis. The pseudo-second-order kinetic model could explain the Cr(vi) removal processes well. The adsorption of Cr(vi) at different initial concentrations was more consistent with a Langmuir isotherm. The existence of H⁺ was beneficial for Cr(vi) removal by Fe₃S₄-CTAB_0.75. Our work confirmed that the obtained Fe₃S₄-CTAB_0.75 composites exhibit considerable potential for Cr(vi) removal from aqueous solution.

The presence of CTAB can promote the removal of hexavalent chromium from the Fe₃S₄-CTAB surface.     

The development of a ternary nanocomposite for the removal of Cr(vi) ions from aqueous solutions

The aim of this study is to develop a ternary nanocomposite (NC) of polyaniline (PANI)/2-acrylamido-2-methylpropanesulfonic acid (AMPSA)-capped silver nanoparticles (NPs)/graphene oxide quantum dots (PANI/Ag (AMPSA)/GO QDs) as an efficient adsorbent for the removal of the highly toxic hexavalent chromium (Cr(vi)) from polluted water. PANI/Ag (AMPSA)/GO QDs NC was synthesized via in situ oxidative polymerization. The effects of pH, adsorbent dose, initial concentration, temperature, contact time, ionic strength and co-existing ions on the removal of Cr(vi) by PANI/Ag (AMPSA)/GO QDs were investigated. The PANI/Ag (AMPSA)/GO QDs NC (25.0 mg) removed 99.9% of Cr(vi) from an aqueous solution containing 60 mg L⁻¹ Cr(vi) ions at pH 2. Energy dispersive X-ray (EDX) and inductively coupled plasma spectrometry (ICP) studies confirmed the adsorption of Cr(vi) and that some of the adsorbed Cr(vi) was reduced to Cr(iii). Cr(vi) removal by the PANI/Ag (AMPSA)/GO QDs NC followed the pseudo-second order kinetic model, and the removal was highly selective for Cr(vi) in the presence of other co-existing ions. In summary, the PANI/Ag (AMPSA)/GO QDs NC has potential as a novel adsorbent for Cr(vi).

The aim is to develop a ternary nanocomposite of polyaniline/2-acrylamido-2-methylpropanesulfonic acid-capped silver nanoparticles/graphene oxide quantum dots as an efficient adsorbent for the removal of the highly toxic hexavalent chromium (Cr(vi)) from polluted water.     

Self-assembly preparation of lignin–graphene oxide composite nanospheres for highly efficient Cr(vi) removal

Recently, research interest in the application of lignin is growing, especially as adsorbent material. However, single lignin shows unsatisfactory adsorption performance, and thus, construction of lignin-based nanocomposites is worth considering. Herein, we introduced graphene oxide (GO) into lignin to form lignin/GO (LGNs) composite nanospheres by a self-assembly method. FTIR and ¹H NMR spectroscopy illustrated that lignin and GO are tightly connected by hydrogen bonds. The LGNs as an environmental friendly material, also exhibit excellent performance for Cr(vi) removal. The maximum sorption capacity of LGNs is 368.78 mg g⁻¹, and the sorption efficiency is 1.5 times than that of lignin nanospheres (LNs). The removal process of Cr(vi) via LGNs mainly relies on electrostatic interaction, and it also involves the reduction of Cr(vi) to Cr(iii). Moreover, LGNs still have high adsorption performance after repeating five times with the sorption capacity of 150.4 mg g⁻¹ in 200 mg g⁻¹ Cr(vi) solution. Therefore, the prepared lignin–GO composite nanospheres have enormous potential as a low-cost, high-absorbent and recyclable adsorbent, and can be used in wastewater treatment.

Lignin/GO (LGNs) composite nanospheres were prepared by self-assembly method, which showed excellent adsorption performance for Cr(vi) removal.