The removal of uranium(VI) from aqueous solutions onto activated carbon: Kinetic and thermodynamic investigations

Removal of metals and assimilable organic carbon by activated carbon and reverse osmosis point-of-use water filtration systems

Activated carbon (AC) systems and reverse osmosis (RO) systems are commonly used point-of-use (POU) water filtration systems for removing trace-level contaminants in tap water to protect human health. However, limited research has been done to evaluate their effectiveness in removing heavy metals like manganese (Mn) and uranium (U), or to assess the potential for undesired microbial growth within POU systems, which can reduce their treatment efficiency. This study aimed to systematically evaluate the removal of metals and assimilable organic carbon (AOC) in POU systems. AC systems were operated to 200% of their designed treatment capacities and RO systems were run for three weeks. The results showed that AC systems were generally ineffective at removing metals from drinking water, while RO systems effectively removed them. Both Mn and U were poorly removed by AC systems. Calcium (Ca) and magnesium (Mg) were poorly removed by AC systems, with efficiencies of less than 1%. Iron (Fe) removal by AC systems varied between 61% and 84%. Copper (Fe), likely due to its low influent concentration (<30 μg L-1), was effectively removed by AC systems with efficiencies over 95%. In contrast, RO systems consistently removed all metals effectively. Mn and U removal in RO systems exceeded 95%, while Ca, Mn, Fe, and Cu were all removed with efficiencies greater than 98%. AOC was effectively removed from all AC and RO systems, but with high variability in removal efficiency, which is likely attributed to the heterogeneity of biofilm and microbial growth within the POU systems. The new knowledge generated from this study can improve our understanding of chemical contaminant removal in POU systems and inform the development of better strategies for designing and operating POU systems to remove chemical contaminants in drinking water and mitigate their associated health risks.

Adsorption of uranium (VI) complexes with polymer-based spherical activated carbon

Adsorption processes with carbon-based adsorbents have received substantial attention as a solution to remove uranium from drinking water. This study investigated uranium adsorption by a polymer-based spherical activated carbon (PBSAC) characterised by a uniformly smooth exterior and an extended surface of internal cavities accessible via mesopores. The static adsorption of uranium was investigated applying varying PBSAC properties and relevant solution chemistry. Spatial time-of-flight secondary ion mass spectrometry (ToF-SIMS) was employed to visualise the distribution of the different uranium species in the PBSAC. The isotherms and thermodynamics calculations revealed monolayer adsorption capacities of 28-667 mg/g and physical adsorption energies of 13-21 kJ/mol. Increasing the surface oxygen content of the PBSAC to 10 % enhanced the adsorption and reduced the equilibrium time to 2 h, while the WHO drinking water guideline of 30 µgU/L could be achieved for an initial concentration of 250 µgU/L. Uranium adsorption with PBSAC was favourable at the pH 6-8. At this pH range, uranyl carbonate complexes (UO2CO3(aq), UO2(CO3)22-, (UO2)2CO3(OH)3-) predominated in the solution, and the ToF-SIMS analysis revealed that the adsorption of these complexes occurred on the surface and inside the PBSAC due to intra-particle diffusion. For the uranyl cations (UO22+, UO2OH+) at pH 2-4, only shallow adsorption in the outermost PBSAC layers was observed. The work demonstrated the effective removal of uranium from contaminated natural water (67 µgU/L) and meeting both German (10 µgU/L) and WHO guideline concentrations. These findings also open opportunities to consider PBSAC in hybrid treatment technologies for uranium removal, for instance, from high-level radioactive waste.

Prediction of Uranium Adsorption Capacity in Radioactive Wastewater Treatment with Biochar

Recently, Japan's discharge of wastewater from the Fukushima nuclear disaster into the ocean has attracted widespread attention. To effectively address the challenge of separating uranium, the focus is on finding a healthy and environmentally friendly way to adsorb uranium using biochar. In this paper, a BP neural network is combined with each of the four meta-heuristic algorithms, namely Particle Swarm Optimization (PSO), Differential Evolution (DE), Cheetah Optimization (CO) and Fick's Law Algorithm (FLA), to construct four prediction models for the uranium adsorption capacity in the treatment of radioactive wastewater with biochar: PSO-BP, DE-BP, CO-BP, FLA-BP. The coefficient of certainty (R2), error rate and CEC test set are used to judge the accuracy of the model based on the BP neural network. The results show that the Fick's Law Algorithm (FLA) has a better search ability and convergence speed than the other algorithms. The importance of the input parameters is quantitatively assessed and ranked using XGBoost in order to analyze which parameters have a greater impact on the predictions of the model, which indicates that the parameters with the greatest impact are the initial concentration of uranium (C0, mg/L) and the mass percentage of total carbon (C, %). To sum up, four prediction models can be applied to study the adsorption of uranium by biochar materials during actual experiments, and the advantage of Fick's Law Algorithm (FLA) is more obvious. The method of model prediction can significantly reduce the radiation risk caused by uranium to human health during the actual experiment and provide some reference for the efficient treatment of uranium wastewater by biochar.

Zeolitic Imidazole Framework (ZIF)-Sponge Composite for Highly Efficient U(VI) Elimination

Herein, a zeolitic imidazole framework (ZIF-67) composite was prepared by a rapid, simple and inexpensive situ hybridization technique applying polyurethane sponge (PU) as support, which was designated as ZIF-67-PU. The ZIF-67 nanoparticle was successfully supported on the surface of sponge skeletons mainly through electrostatic attraction as well as probable π-π stacking interactions with PAM modification of the sponge. The resultant ZIF-67-PU exhibited a remarkably enhanced U(VI) elimination capacity of 150.86 mg∙g-1 on the basis of the Langmuir isotherm model, in comparison to pristine sponge. Additionally, the mechanism for U(VI) elimination was mainly achieved through the complex reaction between C-N(H)/-OH groups in ZIF-67 and U(VI), based on XPS investigations. ZIF-67-PU represents a simple, feasible and low-cost disposal option for preparing ZIF-coated sponges of any shape that can enhance the U(VI) elimination capacity. Furthermore, this approach can be widely applied to the preparation of various kinds of MOF-sponges through this situ hybridization technique.

Super-oxidized "activated graphene" as 3D analogue of defect graphene oxide: Oxidation degree vs U(VI) sorption

Porous carbons are not favorable for sorption of heavy metals and radionuclides due to absence of suitable binding sites. In this study we explored the limits for surface oxidation of "activated graphene" (AG), porous carbon material with the specific surface area of ∼2700 m²/g produced by activation of reduced graphene oxide (GO). Set of "Super-Oxidized Activated Graphene" (SOAG) materials with high abundance of carboxylic groups on the surface were produced using "soft" oxidation. High degree of oxidation comparable to standard GO (C/O=2.3) was achieved while keeping 3D porous structure with specific surface area of ∼700-800 m²/. The decrease in surface area is related to the oxidation-driven collapse of mesopores while micropores showed higher stability. The increase in the oxidation degree of SOAG is found to result in progressively higher sorption of U(VI), mostly related to the increase in abundance of carboxylic groups. The SOAG demonstrated extraordinarily high sorption of U(VI) with the maximal capacity up to 5400 μmol/g, that is 8.4 - fold increase compared to non-oxidized precursor AG, ∼50 -fold increase compared to standard graphene oxide and twice higher than extremely defect-rich graphene oxide. The trends revealed here show a way to further increase sorption if similar oxidation degree is achieved with smaller sacrifice of surface area.

Reduction and adsorption of uranium(VI) from aqueous solutions using nanoscale zero-valent manganese

Nano zero-valent manganese (nZVMn) is theoretically expected to exhibit high reducibility and adsorption capacity, yet its feasibility, performance, and mechanism for reducing and adsorbing hexavalent uranium (U(VI)) from wastewater remain unclear. In this study, nZVMn was prepared via borohydride reduction, and its behaviors about reduction and adsorption of U(VI), as well as the underlying mechanism, were investigated. Results indicated that nZVMn exhibited a maximum U(VI) adsorption capacity of 625.3 mg/g at a pH of 6 and an adsorbent dosage of 1 g/L, and the co-existing ions (K⁺, Na⁺, Mg²⁺, Cd²⁺, Pb²⁺, Tl⁺, Cl^-) at studied range had little interference on U(VI) adsorption. Furthermore, nZVMn effectively removed U(VI) from rare-earth ore leachate at a dosage of 1.5 g/L, resulting in a U(VI) concentration of lower than 0.017 mg/L in the effluent. Comparative tests demonstrated the superiority of nZVMn over other manganese oxides (Mn₂O₃ and Mn₃O₄). Characterization analyses, including X-ray diffraction and depth profiling X-ray photoelectron spectroscopy, combined with density functional theory calculation revealed that the reaction mechanism of U(VI) using nZVMn involved reduction, surface complexation, hydrolysis precipitation, and electrostatic attraction. This study provides a new alternative for efficient removal of U(VI) from wastewater and improves the understanding of the interaction between nZVMn and U(VI).

Ion-Imprinted Polymeric Materials for Selective Adsorption of Heavy Metal Ions from Aqueous Solution

The introduction of selective recognition sites toward certain heavy metal ions (HMIs) is a great challenge, which has a major role when the separation of species with similar physicochemical features is considered. In this context, ion-imprinted polymers (IIPs) developed based on the principle of molecular imprinting methodology, have emerged as an innovative solution. Recent advances in IIPs have shown that they exhibit higher selectivity coefficients than non-imprinted ones, which could support a large range of environmental applications starting from extraction and monitoring of HMIs to their detection and quantification. This review will emphasize the application of IIPs for selective removal of transition metal ions (including HMIs, precious metal ions, radionuclides, and rare earth metal ions) from aqueous solution by critically analyzing the most relevant literature studies from the last decade. In the first part of this review, the chemical components of IIPs, the main ion-imprinting technologies as well as the characterization methods used to evaluate the binding properties are briefly presented. In the second part, synthesis parameters, adsorption performance, and a descriptive analysis of solid phase extraction of heavy metal ions by various IIPs are provided.

Humic acid attachment on chitosan-modified silica gel as an economical, efficient, and selective adsorbent for thorium and uranium removal

A novel, low-cost adsorbent material was prepared by the immobilization of humic acid on a silica gel surface coated with cross-linked chitosan (SiChiHA). The adsorbent was developed to remove selectively of Th(IV) and U(VI) from aqueous solution, including their pre-concentration and separation from lanthanides and high salinity conditions. A simple waste-less humic acid immobilization method was shown to be successful based on FT-IR, SEM-EDS, and zeta potential characterization results. The adsorbent was found to be stable over a wide pH range, with the highest capacities obtained at pH 3.5 (Th(IV)) and pH 5 (U(VI)). Langmuir model calculations yielded a maximum capacity of 30.6 mg g^-1 and 75.4 mg g^-1 for Th(IV) and U(VI). The adsorption process was found to be rapid (half concentration was removed within 10 min) and best described by a pseudo-second order rate equation. Increasing NaCl concentration up to 2 mol L^-1 or lanthanide concentration up to 100 times did not significantly affect the removal efficiency for either Th(IV) of U(VI). Both elements could be sequentially separated by elution with ammonium citrate and nitric acid, respectively. The adsorption-desorption experiment showed that the adsorbent could be used for at least five cycles without significant capacity loss. This study provides insight into the development of low-cost adsorbent with practical functionality, including separation and regeneration ability, the advantageous properties scarcely reported in low-cost adsorbent literature.

A Critical Review of the Removal of Radionuclides from Wastewater Employing Activated Carbon as an Adsorbent

Radionuclide-contaminated water is carcinogenic and poses numerous severe health risks and environmental dangers. The activated carbon (AC)-based adsorption technique has great potential for treating radionuclide-contaminated water due to its simple design, high efficiency, wide pH range, quickness, low cost and environmental friendliness. This critical review first provides a brief overview of the concerned radionuclides with their associated health hazards as well as different removal techniques and their efficacy of removing them. Following this overview, this study summarizes the surface characteristics and adsorption capabilities of AC derived from different biomass precursors. It compares the adsorption performance of AC to other adsorbents, such as zeolite, graphene, carbon nano-tubes and metal-organic frameworks. Furthermore, this study highlights the different factors that influence the physical characteristics of AC and adsorption capacity, including contact time, solution pH, initial concentration of radionuclides, the initial dosage of the adsorbent, and adsorption temperature. The theoretical models of adsorption isotherm and kinetics, along with their fitting parameter values for AC/radionuclide pairs, are also reviewed. Finally, the modification procedures of pristine AC, factors determining AC characteristics and the impact of modifying agents on the adsorption ability of AC are elucidated in this study; therefore, further research and development can be promoted for designing a highly efficient and practical adsorption-based radionuclide removal system.

Preparation of new modified silica gel terminated with phenylphosphonic acid-amide moieties for adsorption of uranium(VI) from aqueous solutions

PAMAM dendrimers modified silica gel terminated phenylphosphonic acid-amide moieties (Si-6G PAMAM-PPAAM) was prepared for uranium(VI) adsorption from aqueous solutions by batch and fixed-bed column methods. The experimental results showed that the maximum capacity was 434.78 mg g−1. Equilibrium isotherm data obeyed Langmuir isotherm model. Kinetic adsorption followed pseudo-second order model and thermodynamic parameters implied the adsorption was spontaneous, endothermic. The adsorption performance of the new adsorbent toward uranium using fixed-bed column method was also investigated. The investigated adsorbent (Si-6G PAMAM-PPAAM) was successfully used to extract uranium from leach liquor of granitic rock sample.

The removal of uranium (VI) from aqueous solution by the anaerobically digested sewage sludge with hydrothermal pretreatment

Anaerobic digestion (AD) with hydrothermal (HT) pretreatment (sequential HT-AD treatment) is a novel technology for sludge management. HT-AD sludge is rich in functional groups and its applications as pollutant sorbents might be a win-win strategy. This study investigated the removal of uranium (VI) from water using HT-AD sludge as affected by solution pH, temperature, and ion strength. The reusability and heavy metal risk of HT-AD sludge were also assessed. The batch sorption experiments demonstrated that even at an acidic initial pH of 3.2, the maximum adsorption of HT-AD sludge for uranium (VI) reached 117.13 mg/g, higher than that of most carbon-based materials. The inner-sphere and out-sphere complexation between uranium (VI) and the HT-AD sludge dominated the adsorption when pH was in the range of 2-6 and 6-11, respectively. The FTIR and XPS analysis indicated that the primary mechanisms of uranium (VI) adsorption by the HT-AD sludge were the surface complexation and the electric attraction between uranium (VI) and the functional groups (e.g. -COO-) on HT-AD sludge. The removal rate of uranium (VI) by HT-AD sludge only decreased by ∼7% after 3 consecutive adsorption cycles. Leaching experiment showed that less than 5% of the total heavy metal were released from HT-AD sludge. Our research proved that HT-AD sludge can be used as an efficient uranium (VI) adsorbent with good reusability and environmental safety.

Reductive and adsorptive elimination of U(VI) ions in aqueous solution by SFeS@Biochar composites

The novel biochar supported by starch and nanoscale iron sulfide (SFeS@Biochar) composites were successfully prepared through coupling of biochar derived from peanut shell with nanoscale ferrous sulfide and starch under nitrogen atmosphere. It had the advantages of biochar, starch, and nanoscale ferrous sulfide. Therefore, it could overcome some shortcomings. The nanoscale ferrous sulfide particles and starch were thought to be loaded successfully on the surface of the biochar by SEM, EDS, BET, XRD, FT-IR, and XPS techniques. High uptake capacity of U(VI) by SFeS@Biochar could be attributed to reactive reaction of FeS nanoparticles and adsorptive of a lot of functional groups. The proposed reaction mechanisms of the U(VI) uptake by SFeS@Biochar were electrostatic attraction, surface complexation, precipitation, and reductive reaction. It also might be an improved environmentally friendly material for U(VI) removal.

Study on Adsorption Behavior of Nickel Ions Using Silica-Based Sandwich Layered Zirconium-Titanium Phosphate Prepared by Layer-by-Layer Grafting Method

In this study, the composite of silica-based sandwich-layered zirconium-titanium phosphate was prepared by a layer-by-layer grafting method and its adsorption properties in a diluted solution of Ni ions were specifically researched by the bath experiment method. The field-emission scanning electron microscope (FESEM) results presented the smooth surface morphology of the pristine adsorbent and a rough surface morphology of the adsorbed adsorbent and the energy dispersive analysis (EDS) results ensured the presence of the original metal element (Si, O, Ti, P, Zr) and the captured nickel element on the adsorbent. The Fourier transformed infrared spectroscopy (FTIR) revealed the new band formation of -Si-Ti-O-, -Si-Ti-O-P-, and -Si-Ti-O-P-Zr-O-, which ensured the successful modification of the silica substrate by zirconium-titanium phosphate. The specific surface area and pore size distribution analysis indicated that the pore structure was changed from type-Ⅳ to H2-type and the specific surface area (BET) of the modified composite was 337.881 m²/g. In the bath experiment, the optimal pH for adsorbing Ni ions on the composite was ~8 with the equilibrium time 30 min at room temperature and the maximum sorption amount was 50.1 mg/g. The adsorption kinetics of the sorption process were corresponded to the pseudo-second-order kinetic equation and the isothermal adsorption data were fitted well to the Redlich-Peterson Model. Thermodynamic simulation results revealed the species of Ni ions and provided a reasonable pH scope for better removal of the Ni element in wastewater.

Carbon materials for extraction of uranium from seawater

With the rapid growth of population and industrialization, the energy crisis and environmental pollution as two main difficulties urgently need to be solved nowadays. The development and utilization of nuclear energy is of great significance for solving energy support, national security and environmental protection. As the raw material of nuclear energy, a lot of uranium in seawater provide a guarantee for the sustainable and green development of nuclear power plants. Recently, various new carbon-based materials (e.g., carbon nanofibers, multiwalled carbon nanotube, graphene) have been attracted widely intense interest in extraction of uranium from seawater due to large specific surface area, excellent acid-base resistance, high adsorption performance, environmental friendly and low cost. Thus, the systematic reviews concerning the extraction of uranium from seawater on various carbon-based materials were highly desirable. In this review, the extraction methods of uranium from seawater, including electrochemical, photocatalytic and adsorption methods are briefly introduced. Then the application and mechanism of four generation carbon-based materials on the extraction of uranium from seawater are systematically reviewed in details. Finally, the current challenges and future trends of uranium extraction from seawaters are proposed. This review provides the guideline for designing carbon-based materials with high adsorption capacity and exceptional selectivity for U(VI) extraction from seawater.

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).

Enhanced uranium removal from acidic wastewater by phosphonate-functionalized ordered mesoporous silica: Surface chemistry matters the most

The removal of uranium species from aqueous phases using non-hazardous chemicals is still an open challenge, and remediation by adsorption is a prosperous strategy. Among the most crucial concerns regarding the design of an efficient material as adsorbent are, except the cost and the green character, the feasibility to be stable and effective under acidic pH, and to selectively adsorb the desired metal ion (e.g. uranium). Herein, we present a phosphonate functionalized ordered mesoporous silica (OMS-P), prepared by a one-step co-condensation synthesis. The physicochemical features of the material were determined by HR-TEM, XPS, EDX, N₂ sorption, and solid NMR, while the surface zeta potential was also measured. The removal efficiency was evaluated at two different temperatures (20 and 50 °C) in acidic environment to avoid interferences like solid phase formation or carbonate complexation and the adsorption isotherms, including data fitting with Langmuir and Freundlich models and thermodynamic parameters are presented and discussed. The high and homogeneous dispersion of the phosphonate groups within the entire silica's structure led to the greatest reported up-todays capacity (345 mg/g) at pH = 4, which was achieved in less than 10 min. Additionally, OMS-P showed that the co-presence of other polyvalent cation like Eu(III) did not affect the efficiency of adsorption, which occurs via inner-sphere complex formation. The comparison to the non-functionalized silica (OMS) revealed that the key feature towards an efficient, stable, and selective removal of the U(VI) species is the specific surface chemistry rather than the textural and structural features. Based on all the results and spectroscopic validations of surface adsorbed U(VI), the main interactions responsible for the elevated uranium removal were proposed.

Ultrafast high-capacity capture and release of uranium by a light-switchable nanotextured surface

Nuclear power is growing in demand as a promising sustainable energy source, its most prevalent source being uranium salts. The resulting processing and transportation of uranium raise concerns regarding the environmental impact and risks for human health. Close proximity to uranium mines puts populations at higher risk for exposure due to elevated uranium concentrations. As the main form of uranium in aqueous solutions, uranyl (UO₂ ²⁺) has been the focus of many methods of uranium sieving; most fall short by being time-consuming or lacking a retrieval mechanism for the captured uranium. Here, we demonstrate the ultrafast and selective uranyl-capturing properties of aptamer-modified branched silicon nanopillar (BSiNP) arrays. Our nanostructured surfaces demonstrate an ultrahigh surface area modified with a uranyl-specific DNA aptamer, allowing for high uranyl-capturing capacity, reaching up to 550 mg g^-1. Uranyl capture is followed by the activation of a covalently bonded photoacid, causing a light-triggerable, ultrafast release. This capture-and-release cycle results in the collection of over 70% of the uranium found in the original samples within less than one hour.

Investigation of shielding properties of impregnated activated carbon for gamma-rays

Carbon related structures are extraordinarily attractive materials for researchers and especially for organic chemistry, besides being the most important basic materials of daily life. The richness of various optical and electronic behaviors has made them one of the rapidly rising materials on the horizon of material science and condensed matter physics. Having the sheets of atoms that are stacked in a disorganized manner makes activated carbon being different from other forms of carbon -graphite- structures. The researches about the shielding properties of activated carbon atoms for gamma-rays are very rare and required to be improved. Since the use of radioactive sources in different fields (nuclear industry, shielding material, radiation biophysics and space research application, etc.) has been increasing expeditiously, the photon interactions with matter has gained more importance in the world of material science technology. In this article, we review the basics of the impregnated activated carbons (AC1 and AC2), as well as the link between the structural behaviors and the gamma shielding properties in terms of both quality and efficiency. Both XCom software and EGSnrc simulation code were used to obtain the theoretical calculations that are significantly important to be able to understand the shielding properties of impregnated activated carbons (AC1 and AC2) for gamma-rays. At the end, the mass attenuation coefficients (μ_m), the total atomic and the electronic cross-sections (σ_t,a and σ_t,e), the effective atomic number and the effective electron density (Z_eff and N_eff), the half value layer (HVL), the tenth value layer (TVL), and the mean free path (MFP) values of such materials were calculated and then compared with those of some other known shielding materials like lead, borosilicate, concrete and vermiculite. The calculated data showed that impregnated activated carbons (AC1 and AC2) are very appropriate and consistent to be one of the candidates for shielding materials of gamma-rays.

Uranium removal from aqueous solution using macauba endocarp-derived biochar: Effect of physical activation

The main aim of this study was to evaluate options for addressing two pressing challenges related to environmental quality and circular economy stemming from wastage or underutilization of abundant biomass residue resources and contamination of water by industrial effluents. In this study we focused on residues (endocarp) from Macaúba palm (Acrocomia aculeata) used for oil production, its conversion to activated biochar, and its potential use in uranium (U) removal from aqueous solutions. Batch adsorption experiments showed a much higher uranyl ions (U(VI)) removal efficiency of activated biochar compared to untreated biochar. As a result of activation, an increase in removal efficiency from 80.5% (untreated biochar) to 99.2% (after activation) was observed for a 5 mg L^-1 initial U(VI) concentration solution adjusted to pH 3 using a 10 g L^-1 adsorbent dosage. The BET surface area increased from 0.83 to 643 m² g^-1 with activation. Surface topography of the activated biochar showed a very characteristic morphology with high porosity. Activation significantly affected chemical surface of the biochar. FTIR analysis indicated that U(VI) was removed by physisorption from the aqueous solution. The adsorbed U(VI) was detected by micro X-ray fluorescence technique. Adsorption isotherms were employed to represent the results of the U adsorption onto the activated biochar. An estimation of the best fit was performed by calculating different deviation equations, also called error functions. The Redlich-Peterson isotherm model was the most appropriate for fitting the experimental data, suggesting heterogeneity of adsorption sites with different affinities for uranium setting up as a hybrid adsorption. These results demonstrated that physical activation significantly increases the adsorption capacity of macauba endocarp-derived biochar for uranium in aqueous solutions, and therefore open up a potential new application for this type of waste-derived biochar.

Efficient cesium encapsulation from contaminated water by cellulosic biomass based activated wood charcoal

In this study, cost-effective cellulosic biomass based activated wood charcoal was developed from Japanese Sugi tree (Cryptomeria japonica) by concentrated nitric acid modification for adsorption of Cs from contaminated water. The physicochemical properties of specimens were investigated using N₂ adsorption-desorption isotherms (BET method), FESEM, FTIR, and XPS spectra analysis. The experimental results revealed that the surface area of the raw wood charcoal was significantly decreased after boiling nitric acid modification. However, several oxygen-containing acidic function groups (-COOH, -CO) were introduced on the surface. The adsorption study confirmed that the equilibrium contact time was 1 h, the optimum adsorption pH was neutral to alkaline and the suitable adsorbent dose was 1:100 (solid: liquid). The maximum Cs was removed when the concentration of Na and K were lower (5.0 mM) with Cs in solution. The Cs adsorption processes well approved by the Langmuir isotherm and pseudo-second-order kinetic models and the maximum adsorption capacity was 35.46 mgg^-1. The Cs adsorption mechanism was clearly described and it was assumed that the adsorption was strongly followed by chemisorptions mechanism based on the adsorbent surface properties, kinetic model and Langmuir isotherm model. Most importantly, about 98% of volume reduction was obtained by burning (500 °C) the Cs adsorbed charcoal, which ensured safe storage and disposal of radioactive waste. Therefore, this study can offer a guideline to produce a functional adsorbent for effective Cs removal and safe radioactive waste disposal.

Efficient Removal Of U(VI) Ions from Aqueous Solutions by Tannic Acid/Graphene Oxide Composites

Tannic acid/graphene oxide (TA/GO) composites were prepared in the present research, and their properties and sorption performance were evaluated by corresponding characterization methods and bath sorption experiments, respectively. The applications of TA/GO to remove U(VI) from aqueous solution were investigated with the maximum adsorption capacity of 87.8 mg·g−1 at low pH (pH = 3.6 ± 0.03). The sorption of U(VI) ions on TA/GO followed the Langmuir model because of the complexation of oxygen-containing functional groups on the surface of TA/GO composites and uranium ions. TA/GO manifested excellent selective adsorption toward uranium ions with other metal ions (Cs+, Sr2+, Co2+). Furthermore, TA/GO as an effective adsorbent was reused to remove a large amount of U(VI) ions from aqueous solution. Therefore, TA/GO is an ideal material to remove highly toxic U(VI) ions from wastewater.

Enhanced Sorption of Radionuclides by Defect-Rich Graphene Oxide

Extremely defect graphene oxide (dGO) is proposed as an advanced sorbent for treatment of radioactive waste and contaminated natural waters. dGO prepared using a modified Hummers oxidation procedure, starting from reduced graphene oxide (rGO) as a precursor, shows significantly higher sorption of U(VI), Am(III), and Eu(III) than standard graphene oxides (GOs). Earlier studies revealed the mechanism of radionuclide sorption related to defects in GO sheets. Therefore, explosive thermal exfoliation of graphite oxide was used to prepare rGO with a large number of defects and holes. Defects and holes are additionally introduced by Hummers oxidation of rGO, thus providing an extremely defect-rich material. Analysis of characterization by XPS, TGA, and FTIR shows that dGO oxygen functionalization is predominantly related to defects, such as flake edges and edge atoms of holes, whereas standard GO exhibits oxygen functional groups mostly on the planar surface. The high abundance of defects in dGO results in a 15-fold increase in sorption capacity of U(VI) compared to that in standard Hummers GO. The improved sorption capacity of dGO is related to abundant carboxylic group attached hole edge atoms of GO flakes as revealed by synchrotron-based extended X-ray absorption fine structure (EXAFS) and high-energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) spectroscopy.

Efficient removal of U(VI) from aqueous solutions using the magnetic biochar derived from the biomass of a bloom-forming cyanobacterium (Microcystis aeruginosa)

The utilization of Microcystis biomass is an urgent issue in the mitigation of cyanobacterial bloom. In this study, Microcystis-derived biochar (MB) and Fe₃O₄-modified biochar (Fe₃O₄/MB) were fabricated for the U(VI) elimination. The results showed that U(VI) sorption process by either MB or Fe₃O₄/MB was pH-dependent and ionic strength-independent. The maximum sorption capacity of MB was higher than that of Fe₃O₄/MB. According to the analysis of X-ray photoelectron spectroscopy, U(VI) sorption on both MB and Fe₃O₄/MB was mainly ascribed to the surface complexation between U(VI) and oxygen-containing functional groups on the surface of MB. Fe₃O₄ particles on the surface of MB didn't provide extra active sites for the sorption of U(VI), but it enabled the adsorbent to be magnetically separated. Five consecutive sorption/desorption cycles verified the good reusability of Fe₃O₄/MB in this study. Therefore, the investigation is not only meaningful for the utilization of nuisance biomass from cyanobacterial blooms, but also provides novel adsorbents for the U(VI) removal from aqueous solutions.

Removal of radionuclides from acidic solution by activated carbon impregnated with methyl- and carboxy-benzotriazoles

The removal efficiencies of metals commonly used to model the fate and transport of aqueous uranium and radioactive its daughter products, were observed on activated carbons impregnated with different benzotriazole derivatives. Acidic solutions containing U(VI), Sr(II), Eu(III), and Ce(III) were used to determine the immobilization potential of carboxybenzotriazole (CBT) and methylbenzotriazole (MeBT), where these derivatives were sorbed to different types of granular activated carbon (GAC). This sorption behavior can be predicted by Redlich–Peterson model. Flow-through column tests showed that the immobilization of uranium and some of its daughter products, significantly improves in response to oxidized GACs saturated with carboxybenzotrzole (CBT), which reached a maximum elimination for U(VI) at 260 BV, Eu(III) at 114 BV, Ce(III) at 126 BV, and Sr(II) at 100. MeBT significantly desorbed from GAC under acidic conditions. Trace amounts of CBT were observed in some column effluents, but this did not appear to alter the effectiveness of metal removal, regardless of the model radionuclide studied. These results suggest that enhanced immobilization of selected metals on GAC, can be achieved by impregnating oxidized activated carbon with carboxylated benzotriazoles, and that metal removal efficiency on this media, is related to their valence and ionic radius in acidic environments.

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.

Anti-Biofouling and Water-Stable Balanced Charged Metal Organic Framework-Based Polyelectrolyte Hydrogels for Extracting Uranium from Seawater

Metal-organic frameworks (MOFs) are diffusely defined as a promising class of porous material for uranium extraction from seawater, but there are still challenges in their stability and anti-biofouling performance. Herein, a water-stable and anti-biofouling ZIF-67/SAP_0.45 composite hydrogel was reported by the sequential processes of electrostatic interactions between the oppositely charged polymer, ionic gelation, and template growth of ZIF-67 crystals. Entanglement of positively charged polyethyleneimine (PEI) and negatively charged sodium alginate (SA) polymer chains provided external porosities, anti-biofouling properties, and mechanical support for the hydrogels and further reduced the possibility of ZIF-67 aggregation. The neutral composite hydrogel possessed the least Nitzschia on the surface after 7 days contact, which endows the adsorbent with a high uranium uptake capacity of 2107.87 ± 41.64 μg g^-1 at 1 mg L^-1 uranium-containing seawater with 8.6 × 10⁵ mL^-1 Nitzschia. Additionally, this adsorbent showed water stability with an uranium uptake capacity of 232.88 ± 8.02 mg g^-1 even after five adsorption-desorption cycles because of the excellent preparation method. Benefitting from the distinctive hierarchical structure and large accessible surface area, the resultant adsorbent achieved a high uranium capacity of 6.99 ± 0.26 mg g^-1 in real seawater. This flexible and scalable approach made the MOF/SAP composite hydrogel a highly desirable uranium adsorbent.

Reduced Graphene Oxide/ZIF-67 Aerogel Composite Material for Uranium Adsorption in Aqueous Solutions

Safe and sustainable development of the nuclear industry has become the focus of attention, so it is important to manage byproducts of radioactive elements, such as uranium, which is inevitably discharged into water bodies. In this work, an adsorbent was fabricated by the in-site assembly of zeolitic imidazolate framework-67 (ZIF-67) on reduced graphene oxide (rGO) hydrogel. The adsorption property of the rGO/ZIF-67 aerogel toward U(VI) was studied via batch adsorption experiment. According to kinetic fitting tests, the adsorption property was in accord well with the pseudo-second-order model, revealing that the adsorption process was chemisorption; the results of the isothermal model conform to the Langmuir model, which exhibited an excellent adsorption capacity of 1888.55 mg/g. The thermodynamic parameter (ΔH° = 11.7 kJ/mol) obtained from the experimental data demonstrated that temperature rise is favorable for the adsorption. Based on the characterization of the material and results of the adsorption, the adsorption mechanism for U(VI) may be explained by surface complexation and electrostatic attraction. In general, all these results and characteristics of the adsorbent show that the rGO/ZIF-67 aerogel provides an alternative way to fabricate novel uranium adsorbent.