Simultaneous adsorption and reduction of U(VI) on reduced graphene oxide-supported nanoscale zerovalent iron

Comparative Study of Cationic Dye Adsorption Using Industrial Latex Sludge with Sulfonate and Pyrolysis Treatment

Industrial latex sludge as raw material was made into sulfonated latex sludge (SLS) and latex sludge active carbon (LSC) adsorbents by sulfonate and pyrolysis treatment to remove textile dye cationic blue X-GRRL from aqueous solution. The adsorption properties of SLS and LSC for X-GRRL were studied and compared by investigating the experimental parameters such as adsorbents dosage, pH, contact time and initial concentration. The kinetics of adsorption on SLS and LSC followed the pseudo-second-order kinetic model well. The adsorption isotherm and thermodynamic studies were further used to evaluate and compare the adsorption process of X-GRRL on SLS and LSC. The maximum adsorption capacities were 1219.6 mg/g for SLS and 476.2 mg/g for LSC according to the Langmuir model, respectively. These findings not only provide a sustainable strategy to turn industrial solid waste latex sludge into useful material for environment remediation, but also develop an efficient adsorbent for the treatment of dye wastewater.

Sorption behavior of dimethyl phthalate in biochar-soil composites: Implications for the transport of phthalate esters in long-term biochar amended soils

The characteristics and content of organo-mineral complex were confirmed to be changed in agriculture soils under the biochar application with long-term, but the resulting environmental effects in the retention and lasting of agrochemicals and xenobiotic pollutants is far from clear. In this study, biochar-soil composites were prepared by one-step dry ball-milling method, and a sorption case study was proceed to investigate the biochar incorporated affection in soils on the transport of dimethyl phthalate (DMP). More surface oxygen-containing functional groups on ball-milled biochar enhanced its complexation with soil minerals. Sorption isotherms of DMP onto the biochar-soil composites were well described by the Freundlich model, both heterogeneous surface and multilayer interactions occurred simultaneously. The kinetics of sorption could be simulated with the pseudo-second-order model (R² > 0.98), while the average sorption energy (E_a) calculated from Dubinin-Radushkevich isotherms were found in the range of 3.83-5.60 kJ mol^-1, which revealed that the sorption processes coexist of chemisorption and physisorption, and π-π electron donor-acceptor interaction, pore-filling and hydrophobic interactions could be identified as the main sorption mechanisms. Desorption of absorbed DMP appeared obvious nonlinear characteristics and lag effect, the calculated hysteresis index (HI) increased with the application of biochar into soil. Considering the phenomenon of biochar aging and soil complexation, it is important to verify how the transport and natural attenuation of contaminant will be influenced by biochar addition, especially the long-term effect in soil ecosystem.

Black liquor as biomass feedstock to prepare zero-valent iron embedded biochar with red mud for Cr(VI) removal: Mechanisms insights and engineering practicality

Black liquor (BL) is an agro-industrial residue with high number of lignocellulosic components which could be recognized as a biomass feedstock. In this work, BL coupled with red mud (RM), were applied to prepare cost-effective zero-valent iron (ZVI) embedded in biochar. The oligomers in BL acted as reductants for RM to generate ZVI, while the organic components could be converted into biochar during pyrolysis. The RM/BL demonstrated excellent performance in the removal of Cr(VI) (349.5 mg/g), as the mechanisms were reduction and adsorption. The fixed-bed column study was conducted and 1.7 L simulated wastewater could be treated by 1.0 g RM/BL. After reaction, 95.5% ± 0.8% and 82.5%±3.2% Cr-loaded adsorbents could be recovered by an external magnet for batch and fixed-bed experiments, respectively. All these results shed light on valorizing these two widespread agro-industrial byproducts, and bridged the knowledge gap between magnetic bio-adsorbent preparation and its industrial practicality on wastewater purification.

Remediation of heavy metals polluted environment using Fe-based nanoparticles: Mechanisms, influencing factors, and environmental implications

Environmental pollution by heavy metals (HMs) has raised considerable attention due to their toxic impacts on plants, animals and human beings. Thus, the environmental cleanup of these toxic (HMs) is extremely urgent both from the environmental and biological point of view. To remediate HMs-polluted environment, several nanoparticles (NPs) such as metals and its oxides, carbon materials, zeolites, and bimetallic NPs have been documented. Among these, Fe-based NPs have emerged as an effective choice for remediating environmental contamination, due to infinite size, high reactivity, and adsorption properties. This review summarizes the utilization of various Fe-based NPs such as nano zero-valent iron (NZVI), modified-NZVI, supported-NZVI, doped-NZVI, and Fe oxides and hydroxides in remediating the HMs-polluted environment. It presents a comprehensive elaboration on the possible reaction mechanisms between the Fe-based NPs and heavy metals, including adsorption, oxidation/reduction, and precipitation. Subsequently, the environmental factors (e.g., pH, organic matter, and redox) affecting the reactivity of the Fe-based NPs with heavy metals are also highlighted in the current study. Research shows that Fe-based NPs can be toxic to living organisms. In this context, this review points out the environmental hazards associated with the application of Fe-based NPs and proposes future recommendations for the utilization of these NPs.

Environmental application of emerging zero-valent iron-based materials on removal of radionuclides from the wastewater: A review

Owing to high surface energy, strong chemical reactivity and large surface area, nanoscale zero-valent iron (nZVI) as a novel emerging material has been extensively utilized in environmental cleanup. Although a lot of reviews regarding the removal of organic contaminants and heavy metals on nZVI are summarized in recent years, the advanced progress concerning the removal of radionuclides on nZVI is still scarce. In this review, we summarized the removal of technetium (Tc), uranium (U), selenium (Se) and other radionuclides on nZVI and nZVI-based composites, then their interaction mechanisms were reviewed in details. This review is crucial for the environmental chemist and material engineer to exploit the actual application of nZVI-based composites as the emerging materials of permeable reactive barrier on the removal of radionuclides from aqueous solutions.

Effect and mechanism of graphene structured palladized zero-valent iron nanocomposite (nZVI-Pd/NG) for water denitration

Nitrate reduction by zero-valent iron-based materials has been extensively studied. However, the aggregation of nanoparticles and the preference for unfavored ammonia products limit the application of this technology. To overcome this issue, this study introduced a novel synthesized nanoscale palladized zero-valent iron graphene composite (nZVI-Pd/NG) and explored its nitrate reduction efficiency. A nitrate removal rate of 97.0% was achieved after 120 min of reaction for an initial nitrate concentration of 100 mg N/L. The nitrogen gas selectivity was enhanced from 0.4% to 15.6% at the end point compared to nanoscale zero-valent iron (nZVI) particles under the same conditions. Further analyses revealed that zero-valent metal nanoparticles spread uniformly on the graphene surface, with a thin layer of iron (hydr)oxides dominated by magnetite. The nZVI-Pd/NG exhibited good catalytic activity with the associated activation energy of 17.6 kJ/mol being significantly lower than that with nZVI (42.8 kJ/mol). The acidic condition promoted a higher nZVI utilization rate, with the excess dosage of nZVI-Pd/NG ensuring a high nitrate removal rate for a wide pH range. This study demonstrates an improvement in nitrate reduction efficiency in a nZVI system by combining the exceptional properties of graphene and palladium.

Enhanced removal of vanadium(V) from groundwater by layered double hydroxide-supported nanoscale zerovalent iron

To reduce the toxicity of vanadium(V) [V(V)] and inhibit the desorption of adsorbed vanadium in groundwater, we synthesized nanoscale zerovalent iron (nZVI) dispersed on layered double hydroxide (LDH) composites (nZVI@LDH) to remove V(V) from simulated groundwater. We found that nZVI@LDH could reduce high-valence vanadium to low-valence vanadium, then forming vanadium-containing precipitation to reduce the toxicity and inhibiting vanadium from returning to groundwater. SEM and XRD characterizations exhibited the uniform dispersal of nZVI on the surface of LDH. nZVI@LDH with nZVI/LDH at a mass ratio of 1:2 provided the maximum adsorption capacity of 93.7 mg g^-1 at pH 3.0. Coexisting anions and dissolved oxygen in groundwater have little effect on V(V) removal. nZVI@LDH performed well across a wide pH range (3.0-8.0). The surface characterizations and XPS analysis revealed that LDH as supporting materials inhibited the aggregation and passivation of nZVI. The adsorbed V(V) was reduced to V(IV) and V(III) by nZVI and spontaneously transformed into insoluble VO₂ and V₂O₃. The DFT calculations indicated the strong complexation and better stability of the V(IV) and V(III) species with nZVI@LDH than V(V). This work suggests that nZVI@LDH has the potential to serve as an efficient material for the immobilization of V(V) in groundwater.

A robust prediction of U(VI) sorption on Fe3O4/activated carbon composites with surface complexation model

A robust prediction of U(VI) on Fe₃O₄/activated carbon (Fe₃O₄/AC, fabricated by co-precipitation method under N₂ conditions) under different pH was developed via diffuse layer model, in accordance with FI-IR, XRD and XPS analysis. No influence of ionic strength onto U(VI) adsorption by Fe₃O₄/AC under ambient conditions suggested the inner-sphere surface adsorption, which was attributed to abundant surficial functional groups according to FT-IR and XPS analysis. The batch experiments indicated Fe₃O₄/AC with fast adsorption rate (equilibrium within 60 min), high adsorption capacity (56 mg/g at pH 4.0) towards U(VI). The adsorbed U(VI) was partly reduced by Fe²⁺ of Fe₃O₄/AC by XPS analysis. Surface complexation modeling showed that a single set of monodentate and mononuclear species (SOUO₂⁺) cannot predict U(VI) adsorption at high pH, whereas the robust prediction of U(VI) adsorption over wide pH range was observed by adding the other binuclear and tridentate species ((SO)₂UO₂(CO₃)^6-). These findings revealed that magnetic AC as a candidate for immobilization and/or preconcentration of radioactive wastewater in environment management.

Preparation of porous chitosan/carboxylated carbon nanotube composite aerogels for the efficient removal of uranium(VI) from aqueous solution

The porous chitosan/carboxylated carbon nanotubes composite aerogels (CS-CCN) with different CCN contents were prepared for the efficient removal of U(VI) from aqueous solution. The successful formation of CS-CCN aerogels with highly porous structure was confirmed by different characterizations (such as SEM, TEM, XRD, etc.). The sorption capacity of the aerogels depends on CCN content, which has significant impact on the porous structure and the sorption ability of the aerogels. The CS-CCN aerogels were found to be very effective for U(VI) sorption: the maximum mono-layer sorption capacity for CS-CCN2 aerogel reached 307.5 mg/g at pH 5.0 and 298 K. The chemisorption or surface complexation through sharing of O/N lone pair electrons on the active sites (carboxylic and amine groups) was responsible for U(VI) sorption, which is confirmed by the IR and XPS analysis. Meanwhile, the good-fitting of both sorption kinetics by pseudo-second-order model and sorption isotherms by Langmuir model also indicates chemisorption mechanism. The thermodynamic data suggest that U(VI) sorption on CS-CCN aerogel is endothermic and spontaneous. The unique characteristics such as high sorption capacity, fast kinetic, and easy recovery from solution make CS-CCN aerogels be very efficient sorbents for the treatment of radioactive wastewater.

Removal of Arsenic, Chromium and Uranium from Water Sources by Novel Nanostructured Materials Including Graphene-Based Modified Adsorbents: A Mini Review of Recent Developments

Groundwater is commonly used as a drinking water resource all over the world. Therefore, groundwater contamination by toxic metals is an important issue of utmost concern for public health, and several technologies are applied for their effective removal, such as coagulation, ion exchange, adsorption, and membrane applications like reverse osmosis. Adsorption is acknowledged as a simple, effective and economic technology, which has received increased interest recently, despite certain limitations regarding operational applications. The respective scientific efforts have been specifically focused on the development and implementation of novel nano-structured adsorbent materials, which may offer extensive specific surface areas, much higher than the conventional adsorbents, and hence, are expected to present higher removal efficiencies of pollutants. In this paper, the recent developments of nanomaterial applications for arsenic, chromium and uranium removal from groundwaters are critically reviewed. Particularly, the use of novel composite materials, based mainly on hybrid metallic oxide nanoparticles and on composites based on graphene oxide (GO) (i.e., graphene-based hybrids), showed promising evidences to achieve efficient removal of toxic metals from water sources, even in full scale applications.

A novel stabilized carbon-coated nZVI as heterogeneous persulfate catalyst for enhanced degradation of 4-chlorophenol

Nano zero-valent iron (nZVI) and its composite materials have been extensively studied in the field of environmental remediation. However, the oxidation and agglomeration of nZVI limits the large-scale application of nZVI in environmental remediation. This study developed a two-step method to prepare stable carbon-coated nZVI (Fe⁰@C) which combined hydrothermal carbonization and carbothermal reduction methods and used glucose and iron oxide (Fe₃O₄) as precursors. When the carbothermal reduction temperature was 700 °C and the elemental molar ratio of carbon to iron was 22:1, stable Fe⁰@C can be generated. The nZVI particles are encapsulated by mesoporous carbon and embedded in the carbon spheres. The unique structure of carbon coating not only inhibits the agglomeration of nZVI, but also makes nZVI stable in air for more than 120 days. Not only that, the as-synthesized Fe⁰@C exhibited high catalytic activity toward the degradation of 4-chlorophenol (4-CP) by activating persulfate. Different from conventional nZVI catalysts in generation of sulfate radicals, Fe⁰@C selectively induced hydroxyl radicals for 4-CP degradation. Moreover, Fe⁰@C has been shown to efficiently degrade 4-CP by using the dissolved oxygen in water to form hydroxyl radicals. This study not only provides a simple, green method for the preparation of stabilized nZVI, but also provides the possibility of large-scale application of nZVI in the field of environmental remediation.

Removal of U(VI) from aqueous and polluted water solutions using magnetic Arachis hypogaea leaves powder impregnated into chitosan macromolecule

In this study m-AHLPICS (magnetic Arachis hypogaea leaves powder impregnated into chitosan) was prepared and utilized as an adsorbent to remove U(VI) from aqueous and real polluted wastewater samples. m-AHLPICS was characterized by using the BET, XRD, FTIR, SEM with elemental mapping and magnetization measurements. Different experimental effects such as pH, dose, contact time, and temperature were considered broadly. Chitosan modified magnetic leaf powder (m-AHLPICS) exhibits an excellent adsorption capacity (232.4 ± 5.59 mg/g) towards U(VI) ions at pH 5. Different kinetic models such as pseudo-first-order, and pseudo-second-order models were used to know the kinetic data. Langmuir, Freundlich and D-R isotherms were implemented to know the adsorption behavior. Isothermal information fitted well with Langmuir isotherm. Kinetic data followed by the pseudo-second-order kinetics (with high R² values, i.e., 0.9954, 0.9985 and 0.9971) and the thermodynamic data demonstrate that U(VI) removal using m-AHLPICS was feasible, and endothermic in nature.

Sulfur Loading and Speciation Control the Hydrophobicity, Electron Transfer, Reactivity, and Selectivity of Sulfidized Nanoscale Zerovalent Iron

Sulfidized nanoscale zerovalent iron (SNZVI) is a promising material for groundwater remediation. However, the relationships between sulfur content and speciation and the properties of SNZVI materials are unknown, preventing rational design. Here, the effects of sulfur on the crystalline structure, hydrophobicity, sulfur speciation, corrosion potential, and electron transfer resistance are determined. Sulfur incorporation extended the nano-Fe⁰ BCC lattice parameter, reduced the Fe local vacancies, and lowered the resistance to electron transfer. Impacts of the main sulfur species (FeS and FeS₂ ) on hydrophobicity (water contact angles) are consistent with density functional theory calculations for these FeS_x phases. These properties well explain the reactivity and selectivity of SNZVI during the reductive dechlorination of trichloroethylene (TCE), a hydrophobic groundwater contaminant. Controlling the amount and speciation of sulfur in the SNZVI made it highly reactive (up to 0.41 L m^-2 d^-1 ) and selective for TCE degradation over water (up to 240 moles TCE per mole H₂ O), with an electron efficiency of up to 70%, and these values are 54-fold, 98-fold, and 160-fold higher than for NZVI, respectively. These findings can guide the rational design of robust SNZVI with properties tailored for specific application scenarios.

Ultrafast and highly capture of U(VI) by hierarchical mesoporous carbon

In this study, the hierarchical mesoporous carbon (HMC) was synthesized by the hydrothermal method. The batch adsorption experiments showed that HMC exhibited the ultrafast equilibrium fate (80 % U(VI) capture efficiency within 5 min), high UO2 2+ capture capacity (210 mg/g, pH = 4.5) and well recyclability. The investigations of XPS techniques indicated the oxygen-containing functional groups were responsible for high efficient UO2 2+ adsorption. The pH-dependent adsorption was simulated by three surface complexation modellings, revealing that UO2 2+ adsorption on HMC was excellently fitted by triple layer model using two inner-sphere complexes (i. e. SOUO2 + and SOUO2(CO3)3 5− species) compared to constant capacitance model and diffuse layer model. These findings are crucial for expanding actual applications of HMC towards the removal of radionuclides under environmental cleanup.

Influence of Leifsonia sp. on U(VI) removal efficiency and the Fe-U precipitates by zero-valent iron

Zero-valent iron (ZVI) has been widely applied to the remediation of uranium (U)-contaminated water. Notably, indigenous bacteria may possess potential positive or unfavorable influence on the mechanism and stability of Fe-U precipitates. However, the focus of the researches in this field has mainly been on physical and/or chemical aspects. In this study, batch experiments were conducted to explore the effects of an indigenous bacterium (Leifsonia sp.) on Fe-U precipitates and the corresponding removal efficiency by ZVI under different environmental factors. The results showed that the removal rate and capacity of U(VI) was significantly inhibited and decreased by ZVI when the pH increased to near-neutral level (pH = 6~8). However, in the ZVI + Leifsonia sp. coexistence system, the U(VI) removal efficiency were maintained at high levels (over 90%) within the experimental scope (pH = 3~8). This revealed that Leifsonia sp. had a synergistic effect on U(VI) remove by ZVI. According to scanning electron microscope and energy dispersive X-ray detector (SEM-EDX) analysis, dense scaly uranium-phosphate precipitation was observed on ZVI + Leifsonia sp. surface. The X-photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analysis indicated that Leifsonia sp. facilitated the generation of U(VI)-phosphates precipitates. The X-ray diffraction (XRD) analyses further revealed that new substances, such as (Fe(II)Fe(III)₂(PO₄)₂(OH)₂), Fe(II)(UO₂)₂(PO₄)₂·8H₂O, Fe(II)Fe(III)₅(PO₄)₄(OH)₂·4H₂O, etc., were produced in the coexisting system of ZVI and Leifsonia sp. This study provides new insights on the feasibility and validity of site application of ZVI to U(VI)-contaminated subsurface water in situ. Graphical abstract.

Biochars prepared from rabbit manure for the adsorption of rhodamine B and Congo red: characterisation, kinetics, isotherms and thermodynamic studies

Biochar was prepared from rabbit faeces (RFB₅₅₀) at 550 °C through pyrolysis and was characterised using elemental analysis, scanning electron microscopy, Brunauer-Emmett-Teller analysis and Fourier transform infrared spectroscopy (FTIR). The related factors, kinetics, isothermal curves and thermodynamics of the adsorption behaviours were investigated by conducting batch experiments. The results revealed the adsorption equilibrium of rhodamine B (RhB) and Congo red (CR) onto RFB₅₅₀ with initial concentrations of 30 mg · L^-1 at 25 °C and 210 min, and the best adsorption was observed when the pH of the RhB and CR solutions was 3 and 5, respectively. Pseudo-second-order kinetics was the most suitable model for describing the adsorption of RhB and CR onto RFB₅₅₀, indicating that the rate-limiting step was mainly chemical adsorption. The isotherm data were best described by the Freundlich model, and the adsorption process was multi-molecular layer adsorption. Thermodynamic parameters revealed the spontaneous adsorption of RhB and CR onto RFB₅₅₀. According to the results of the FTIR analysis, the oxygen-containing functional groups and aromatic structures on the surface of RFB₅₅₀ provided abundant adsorption sites for RhB and CR, and the adsorption mechanism was potentially related to the hydrogen bonds and π-π bonds.

Synergistic construction of green tea biochar supported nZVI for immobilization of lead in soil: A mechanistic investigation

Biochar-based nanocomposites with functional materials provide an excellent prospect in reactivity and stability. Most biochar reported have no reusability upon aging and offer the risk of release of immobilized components after short-term immobilization. To overcome this, we developed nano zero-valent iron (nZVI) impregnated magnetic green tea biochar (nZVI@GTBC) and studied its performance in immobilizing Pb and long-term effectiveness in the soil. The reactive nZVI units were obtained from iron oxide solution by reducing with polyphenol solution (green tea extract) and were successively stabilized by impregnation onto the remaining green tea waste matrix through co-precipitation technique. Finally, the magnetic biochar was developed from the above nZVI impregnated green tea waste through oven drying and slow pyrolysis technique in different temperature range (150-650 °C). The synthesized nZVI@GTBC biochar was characterized and studied by XRD, FTIR, Raman, UV-Vis, TG/DSC, XPS, SEM, and TEM. The nZVI@GTBC obtained with a particle size of 130 nm and surface charge of +2.8 C/m² at 450 °C. Moreover, colloidal stability and mobility experiments were considered to explain the transport behavior and stability of bare nZVI and magnetic nZVI@GTBC in the soil. The immobilization of Pb by pristine nZVI, GTBC, and nZVI@GTBC was compared and explained under different soil pH conditions. The bioavailability of Pb content before and after immobilization was investigated through leaching experiments. Further, thirty days of soil incubation were carried out to examine different species of Pb according to the Tessier sequential extraction scheme. The study suggested that nZVI@GTBC enhanced the immobilization efficiency by 19.38% in comparison with pristine nZVI and 57.14% in comparison with bare GTBC biochar.

A magnetic hierarchical zero-valent iron nanoflake-decorated graphene nanoplate composite for simultaneous adsorption and reductive degradation of rhodamine B

Fabrication of a hierarchical ZVI nanoflake@graphene nanoplate composite for simultaneous adsorption and reductive degradation of rhodamine B dye.

A hydrolytically stable anionic layered indium-organic framework for the efficient removal of 90Sr from seawater

Efficient removal of radioactive ⁹⁰Sr from nuclear waste solutions and natural water systems is of vital importance due to its radioactive nature and high mobility. We present here an anionic layered compound (NC₄H₁₂)(NC₂H₈)₂[In₃(pydc)₆]·13.1H₂O (SZ-6; pydc = 2,5-pyridinedicarboxylic acid) with the potential remediation ability towards radioactive Sr²⁺ from seawater. This material exhibits excellent β and γ radiation resistance both in air and in aqueous solutions. Besides, this material could maintain its structural integrity in real seawater for 77 days. The adsorption experiment results show that SZ-6 exhibits superior Sr²⁺ removal capability over a wide pH range from 4 to 12 with fast adsorption kinetics and high selectivity. The effective removal of ⁹⁰Sr from real seawater was demonstrated as well. Our results strongly suggest the potential application of SZ-6 for selectively capturing radionuclides in natural water systems.

Resource utilization of a typical vegetable waste as biochars in removing phthalate acid esters from water: A sorption case study

It is very important to utilize associated vegetable products as resources, especially in large-scale vegetable cultivation areas. In this study, pepper straw, a vegetable waste, was pyrolyzed into pepper straw biochars (PBs) to investigate their sorption potential for phthalate acid esters (PAEs). The results showed that PBs have porous structures and abundant surface functional groups. Dibutyl phthalate (DBP) and dimethyl phthalate (DMP) removal by PBs was divided into two stages, fast and slow sorption. The PBs pyrolyzed at 500 °C showed greater DBP and DMP sorption capacity than those pyrolyzed at 400 and 600 °C. Both chemical and physical sorption occurred in the whole sorption process of PAEs to PBs. It is proposed that converting pepper straw into biochars to use as sorbents could be an environmentally friendly way of vegetable waste resource utilization.

Facile synthesis of acid-modified UiO-66 to enhance the removal of Cr(VI) from aqueous solutions

The adsorption behavior and mechanism of Cr(VI) on different acid-modified UiO-66s (Form-UiO-66 and Ac-UiO-66) were systematically investigated for the first time through a series of characterizations, and theoretical calculations of batch experiments. The characterization results demonstrate that acid-modified UiO-66 exhibited a larger specific surface area than did unmodified UiO-66. In addition, since the regulator (formic acid) of Form-UiO-66 was the stronger competition, the specific surface area of Form-UiO-66 (1138 m² g^-1) was larger than that of Ac-UiO-66 (915 m² g^-1). Under optimal experimental conditions, the maximum adsorption capacity of Cr(VI) was 243.9 mg g^-1 on Form-UiO-66, and 151.52 mg g^-1 on Ac-UiO-66, which was far higher than on the reported unmodified UiO-66 (36.4 mg g^-1). The results of pH testing, zeta potential, and X-ray photoelectron spectroscopy analysis indicate that Cr(VI) ions were fixed to adsorbent surfaces via electrostatic adsorption. Acid-modified UiO-66 increased the surface active site via the increase in its specific surface area to enhance adsorption capacity of Cr(VI). These results indicated that both the surface charge and specific surface area of the adsorbent primarily determined the Cr(VI) adsorption capacity. Acid modified UiO-66 exhibited enhanced adsorption capacity, stability, and regeneration, compared to traditional adsorbents, and these results provide new insights into adsorption by MOFs.

Comprehensive evaluation on removal of lead by graphene oxide and metal organic framework

Graphene oxide (GO) and metal-organic framework (MOF) as adsorbents were applied to removal of Pb(II) with comprehensive characterizations and various experimental conditions. Various characterizations were conducted to clarify the physico-chemical properties of adsorbents. The analyses of adsorption experiments included (i) dosage amounts, (ii) isotherm and kinetic studies, and (iii) several factors related to water chemistry (i.e., solution pH, background ions, and humic acid). The maximum equilibrium adsorption capacity (q_e) for Pb(II) using the GO and MOF was 555 and 108 mg g^-1, respectively, as determined in the optimum dosage experiments. Although the surface area of the MOF (629 m² g^-1) was much larger than that of the GO (19.8 m² g^-1), the adsorption capacity of the MOF was five times lower due to electrical repulsion. Thus, the MOF was utilized as the control group for comparison with the GO to evaluate the adsorption mechanisms in the experiments related to surface charge (i.e., under various pH and humic acid conditions). The adsorption isotherms and kinetics model determined using GO followed the Langmuir model (R² > 0.99) and pseudo-second-order model (R² > 0.99), respectively. Additionally, three adsorption-desorption cycles were conducted with the GO adsorbent to evaluate the maintenance of the removal ratio after regeneration and the equilibrium adsorption capacity was determined. Finally, the adsorption of other heavy metals (i.e., Cu(II), Cd(II), and Zn(II)), separately and in mixtures, was also evaluated to determine the selectivity of the adsorbents.

Adsorption mechanism of Pb2+ ions by Fe3O4, SnO2, and TiO2 nanoparticles

Nanosized sorbents for the removal of heavy metal ions are preferred due to high surface area, smaller size, and enhanced reactivity during adsorbate/adsorbent interactions. In the present study, Fe₃O₄, SnO₂, and TiO₂ nanoparticles were prepared by microemulsion-assisted precipitation method. The particles were characterized by BET surface area, X-rays diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, transmittance electron microscopy (TEM), and X-ray photoelectron (XPS) spectroscopy. The respective particle sizes calculated from TEM were 7 nm (± 2), 10 nm (± 2), and 20 nm (± 3) for Fe₃O₄, SnO₂, and TiO₂. The adsorbents were employed for the adsorption of Pb²⁺ ions from the aqueous solutions. The respective maximum adsorption capacity for Fe₃O₄, SnO₂, and TiO₂ nanoparticles was 53.33, 47.21, and 65.65 mg/g at 313 K. Based on the exchange reaction taking place on the surfaces of Fe₃O₄, SnO₂, and TiO₂, it is concluded that Pb²⁺ ions are adsorbed in hydrated form. The X-ray photoelectron spectroscopy (XPS) studies also support the exchange mechanism and confirmed the presence of elements like Fe, Sn, Ti, Pb, and O and their oxidation states. Both Langmuir and Freundlich models in non-linear form were applied, however, based on R_L values, the Langmuir model fits well to the sorption data. Moreover, adsorption parameters were also determined by using non-linear form of the Langmuir model along with statistical approaches to remove error. The q_m and K_b values confirm better adsorption capacity and binding strength for Pb²⁺ ions as compared to the values reported in the literature.

A comparison of the characteristics and atrazine adsorption capacity of co-pyrolysed and mixed biochars generated from corn straw and sawdust

The biochar used in this study was prepared from corn straw and sawdust mixed in a ratio of 1:1 (m/m) at temperatures of 300 °C and 800 °C, which adopted the methods of co-pyrolysis (BC_300A and BC_800A) and mixing of single biochar (BC_300B and BC_800B). The obtained biochar was characterized by SEM, BET and FTIR analysis. Adsorption properties, including the atrazine adsorption kinetics and isotherms of the four kinds of biochar to atrazine, were investigated. The results showed that a pseudo-second order kinetic model was suitable for describing the adsorption of atrazine by BC_800B because its R² value is greater than the pseudo-first order model. The adsorption capacity (q_e) of BC_800B, blended from the two single biochars produced at 800 °C, is 37.2 mg g^-1, which is better than that of the other three species. This value is 4-6 times the q_e values of BC_300A, BC_300B and BC_800A, which are 6.74 mg g^-1, 7.77 mg g^-1 and 5.26 mg g^-1, respectively. At higher pyrolysis temperature, the pore structure of biochar is more developed, the specific surface area is larger, and the species and number of surface functional groups are also significantly different. At the same time, the results also showed that the order of mixing and pyrolysis affected the characteristics and adsorption capacity of biochar. This study reveals the atrazine adsorption mechanism of four kinds of biochar and provides information about the potential of these types of materials for the removal of atrazine in the aquatic environment.

Decontamination of U(VI) on graphene oxide/Al2O3 composites investigated by XRD, FT-IR and XPS techniques

The decontamination of U(VI) on graphene oxide/nano-alumina (GO/Al₂O₃) composites were investigated by batch, XRD, FT-IR and XPS techniques. The characterization results showed that GO/Al₂O₃ composites presented a variety of oxygen-containing functional groups, which provided the more surface reactive sites. The batch experiments indicated that sorption equilibrium of U(VI) on GO/Al₂O₃ composites was achieved within 30 min, and the maximum sorption capacity derived from Langmuir model was 142.8 mg/g at pH 6.5. In addition, the slight decrease of sorption capacity was observed even after fifth recycling times. These results indicated that GO/Al₂O₃ composites displayed the fast sorption rate, high sorption capacity and good regeneration performance. No effect of ionic strength revealed the inner-sphere surface complexation of U(VI) on GO/Al₂O₃ composites. FT-IR and XPS analysis demonstrated that the high adsorption of U(VI) on GO/Al₂O₃ was attributed to the various oxygen-bearing functional groups. In addition, the nano Al₂O₃ was transferred to amorphous AlO(OH) mineral phase by XRD pattern, which provided the additional reactive sorption sites. These observations indicated that GO-based composites can be regarded as a promising adsorbent for immobilization and pre-concentration of U(VI) from aqueous solutions in the environmental remediation.

Graphene oxide-facilitated uranium transport and release in saturated medium: Effect of ionic strength and medium structure

Natural subsurface environment is a complex heterogeneous system. To investigate the effect of ionic strength (IS) and heterogeneity on the transport and remobilization of graphene oxide (GO)-facilitated uranium (U(VI)) in saturated porous media, column experiments were performed by the injection of U(VI) alone and U(VI)+GO mixtures into homogeneous and heterogeneous porous media under low and high ionic strength (1 and 50 mM) conditions, and then the columns were successively flushed with background solution and DI water. Results showed that when U(VI) only was introduced into the columns, IS had little effect on the migration of U(VI) alone in both media and the presence of preferential flow in heterogeneous media slightly enhanced the mobility of U(VI). As U(VI)+GO mixtures were injected into the columns, GO showed strong mobility at low IS and high released peak at high IS. The appearance of GO significantly enhanced U(VI) transport in both media. Under low IS condition, the mobility of U(VI) was significantly enhanced at the injection phase, and the medium heterogeneity further promoted the amount of GO-sorbed U(VI) transport. At high IS, less GO-sorbed U(VI) was observed during injection phase, and a large amount of retained GO-sorbed U(VI) were released with GO remobilization during water flushing phase, and the release showed the longer-tailing phenomenon and the release amount was more pronounced in heterogeneous media. The findings in this study showed that the coupled effect of solution chemistry and media heterogeneity played important roles on GO-facilitated U(VI) transport and release in soil and groundwater system.

Synthesis of magnetic biochar composites for enhanced uranium(VI) adsorption

Magnetic biochar composites were successfully fabricated by pyrolysis of siderite and rice husk under N₂ condition. The results of a variety of characterization implied magnetic biochar displayed porous structures with larger specific surface area. The batch adsorption experiments showed high adsorption properties of magnetic biochar composites toward U(VI) (52.63 mg/g at pH 4.0), whereas U(VI) adsorption was significantly influenced by Na₂CO₃ and HA. U(VI) adsorbed onto magnetic biochar was reduced to U(IV) by Fe₃O₄ according to XPS and XANES analyses. In addition, no significant effect of ionic strength of NaCl and EXAFS results, illustrated the inner-sphere surface complexation of U(VI) on magnetic biochar. Owing to the simple synthesis procedure, low cost, high adsorption efficiency, easy separation and environmental friendly, magnetic biochar can be considered as a potential adsorbent for the purification of U(VI)-bearing wastewater in environmental remediation.

Influence of carbonate on sequestration of U(VI) on perovskite

Cubic perovskite (CaTiO₃) was successfully synthesized by a facile solvothermal method and was utilized to sequestrate U(VI) from aqueous solutions. The batch experiments revealed that carbonate inhibited U(VI) sequestration at pH > 6.0 due to the formation of uranyl-carbonate complexes. The maximum sequestration capacity of U(VI) on perovskite was 119.3 mg/g (pH 5.5). The sequestration mechanism of U(VI) on perovskite were investigated by XPS and EXAFS techniques. According to XPS analysis, the presence of U(IV) and U(VI) oxidation states revealed the photocatalytic reduction of U(VI) by perovskite under UV-vis irradiation. In addition, photocatalytic reduction performance significantly decreased in the presence of carbonate. Based on EXAFS analysis, the occurrence of U-Ti and U-U shells revealed the inner-sphere surface complexation and reductive precipitation of U(VI) on perovskite. These findings herein are crucial for the application of perovskite-based composites in the decontamination of U(VI) in aquatic environmental cleanup.

A comprehensive review of applications of magnetic graphene oxide based nanocomposites for sustainable water purification

With the rapid growth of industrialization, water bodies are polluted with heavy metals and toxic pollutants. In pursuit of removal of toxic pollutants from the aqueous environment, researchers have been developed many techniques. Among these techniques, magnetic separation has caught research attention, as this approach has shown excellent performance in the removal of toxic pollutants from aqueous solutions. However, magnetic graphene oxide based nanocomposites (MGO) possess unique physicochemical properties including excellent magnetic characteristics, high specific surface area, surface active sites, high chemical stability, tunable shape and size, and the ease with which they can be modified or functionalized. As results of their multi-functional properties, affordability, and magnetic separation capability, MGO's have been widely used in the removal of heavy metals, radionuclides and organic dyes from the aqueous environment, and are currently attracting much attention. This paper provides insights into preparation strategies and approaches of MGO's utilization for the removal of pollutants for sustainable water purification. It also reviews the preparation of magnetic graphene oxide nanocomposites and primary characterization instruments required for the evaluation of structural, chemical and physical functionalities of synthesized magnetic graphene oxide nanocomposites. Finally, we summarized some research challenges to accelerate the synthesized MGO's as adsorbents for the treatment of water pollutants such as toxic and radioactive metal ions and organic and agricultural pollutants.

Optimizing the removal of nitrate from aqueous solutions via reduced graphite oxide-supported nZVI: synthesis, characterization, kinetics, and reduction mechanism

Graphene has been considered an ideal absorbent and excellent carrier for nanoparticles. Reduced graphite oxide (rGO)-supported nanoscale zero-valent iron (nZVI@rGO) is an effective material for removing nitrate from water. nZVI@rGO nanocomposites were prepared by a liquid-phase reduction method and then applied for nitrate-nitrogen (NO₃^--N) removal in aqueous solution under anaerobic conditions. The experimental results showed that the stability and activity of the nZVI@rGO nanocomposites were enhanced compared with those of nZVI. The influence of the reaction conditions, including the initial concentration of NO₃^--N, coexisting anions, initial pH of the solution, and water temperature, on NO₃^--N removal was also investigated by batch experiments. In a neutral or slightly alkaline environment, 90% of NO₃^--N at a concentration less than 50 mg/L could be removed within 1 h, and nitrogen production was approximately 15%. The process of NO₃^--N removal by nZVI@rGO fits well with different reaction kinetics. In addition, magnetite was the main oxidation product. RGO-supported nZVI might become a promising filler in the permeable reactive barrier process for groundwater remediation.

Application of Wasted Oolong Tea as a Biosorbent for the Adsorption of Methylene Blue

Tea powder, a biosorbent prepared from wasted oolong tea, was collected as a prospective adsorbent for the adsorption of methylene blue (MB) from aqueous solution. The effect of factors on adsorption efficiency, isotherms, kinetics, and potential mechanism was carried out. Adsorption capacity of MB onto wasted tea powder increased with the MB concentration and contact time, whereas the increase in pH value and ion strength appeared to have a negative effect for the adsorption process. The adsorption efficiency increased rapidly and reached a stable state within 120 min. The optimal tea powder loading weight is suggested to be at 0.1 to 0.2 g, and the highest efficiency of 94.8% is achieved at 333 K. There were no significant changes in adsorption efficiency when the effect of temperature is considered. The Langmuir isotherm model was found to be the best isotherm models to elucidate the adsorption mechanism in this study. The maximum adsorption capacities calculated at different temperatures by the Langmuir model ranging from 312.5 to 333.3 mg·g−1 were much close to the experimental results. From the kinetic analysis, the pseudo--second-order model was found to be the best model to describe the adsorption behavior. The calculated adsorption capacities at different initial MB concentrations by the pseudo-second-order model ranging from 92.34 to 400 mg·g−1 were well close to the experimental data. The fitting results obtained from the intraparticle diffusion model suggested that the intraparticle diffusion was not the only rate-controlling step and some other mechanisms along with the intraparticle diffusion were probably involved. The intraparticle diffusion of MB molecules into pore structures of wasted tea powder is the rate-limiting step for the adsorption process in this study. The repetitive cycle experiments indicated that the wasted oolong tea powder was efficiently regenerated using NaOH and thus be used for many times.

Co-transport of U(VI) and akaganéite colloids in water-saturated porous media: Role of U(VI) concentration, pH and ionic strength

Remediating uranium contamination becomes a worldwide interest because of increasing uranium release from mining activities. Due to ubiquitous presence of pyrite and the application of iron-based technology, colloidal iron oxy-hydroxides such as akaganéite colloid (AKC) extensively exist in uranium polluted water at uranium tailing sites. In this context, we studied individual and co-transport of U(VI) and AKC in water-saturated sand columns at 50 mg/L AKC and environmentally relevant U(VI) concentrations (5.0 × 10^-7 ∼ 5.0 × 10^-5 M). It was found that, in addition to the impact of pH and ionic strength, whether AKC facilitated U(VI) transport depended on U(VI) concentration as well. The presence of AKC facilitated U(VI) transport at relatively low U(VI) concentration (5.0 × 10^-7 ∼ 5.0 × 10^-6 M), which was due to the strong adsorption of U(VI) on AKC and faster transport of AKC than that U(VI) as observed in their individual transport experiments. At relatively high U(VI) concentrations (5.0 × 10^-5 M), however, AKC impeded U(VI) transport because U(VI) of high concentration decreased AKC colloidal stability and increased AKC aggregation and attachment. Thus, U(VI) and AKC co-transport was even blocked completely at relatively high pH and ionic strength. The mechanisms behind the co-transport of U(VI) and AKC were also confirmed by assessing the evolutions of aqueous pH and AKC zeta potential and particle size distribution in the column effluents. A two-site non-equilibrium model and a two-site kinetic attachment/detachment model well-described the breakthrough curves of U(VI) and AKC, respectively. Knowledge generated from this study provides a thorough understanding of uranium transport in the absence/presence of AKC, and brings new insights into the influence of contaminant concentration on co-transport in the presence of colloids.

Adsorption of U(VI) ions from aqueous solution using nanogoethite powder

Goethite is a stable and widespread mineral present in soil with many uses, and it affects the transportation and immobilization of heavy metals in solution. Nanogoethite was synthesized by a chemical precipitation method and used to batch adsorb U(VI) in solution. Adsorption experiments were used to understand the role of nanogoethite in controlling the U(VI) adsorption behavior in soil. The morphology and the crystallinity of nanogoethite were characterized by scanning electron microscopy and wide-angle X-ray powder diffractometry, respectively. The results showed that the crystallinity of nanogoethite after the adsorption of uranium did not change, but small particles appeared on the surface of the scales. The surface area was determined from N2 adsorption–desorption experiments using the Brunauer–Emmett–Teller to be 81.86 m2/g. The effects of factors such as the contact time, pH, adsorbent dosage, and the initial concentration of uranium on the adsorption of U(VI) were investigated. The experimental results showed that nanogoethite removed over 85% of the U(VI) in an aqueous 5.0 mg/L U(VI) solution at pH 4.0 and at 298 K. The pseudo-second-order model was used to simulate the adsorption process. The results show that chemisorption plays a major role in the adsorption process. The results of this study suggest that nanogoethite may play a significant role in controlling the migration and transfer of U(VI) in the soil, thus controlling the presence of U(VI) in soil.