Poly amidoxime functionalized carbon nanotube as an efficient adsorbent for removal of uranium from aqueous solution

Synthesis of pH responsive malononitrile functionalized metal organic framework MIL-100(Fe) for efficient adsorption of uranium U(VI) from real-life alkaline leach liquor

The porous framework of MIL-100(Fe) was functionalized using malononitrile (MN), through an in-situ Knoevenagel condensation reaction to introduce abundant -CN groups on the surface of the developed adsorbent. The resultant MN-functionalized MIL-100(Fe) exhibited excellent Uranium (U(VI)) removal capacity (i.e., 270 mg/g) at highly alkaline pH (⁓ 10). Different coexisting cations and anions show negligible influence on the U-removal and it was 92.1-99.7 % in presence of different co-ions, with the concentration from 10 to 50 mg/L. Moreover, MIL-100(Fe)_MN showed extremely selective U removal from the actual alkaline leach liquor (⁓ 97 %), without any pH adjustment and leaching of the constituent Fe. The surface-grafted -CN groups were predominantly active towards the coordinative interactions with the U(VI) ionic moieties, as evident from the XPS and FTIR analysis. The MIL-100(Fe)_MN adsorbent was also subjected to five consecutive adsorption-desorption cycles, with >90 % U removal after 5th cycle. Moreover, the regenerated MIL-100(Fe)_MN was structurally and functionally resilient, as observed from the morphological and crystallographic analysis. A convection-pore diffusion based transport model was used to analyze the optimized mass transfer parameters. Overall, the present study highlights the simple design and development of malononitrile-functionalized MIL-100(Fe) as an efficient and selective adsorbent for U(VI) removal from U-rich alkaline leach liquor.

Adsorption and catalytic degradation of bisphenol A and p-chlorophenol by magnetic carbon nanotubes

Phenolic compounds are common industrial pollutants that seriously endangers water ecology and human health. Therefore, the development of efficient and recyclable adsorbents is of importance for wastewater treatment. In this research, HCNTs/Fe3O4 composites were constructed using co-precipitation way by loading magnetic Fe3O4 particles onto hydroxylated multi-walled carbon nanotubes (MWCNTs), showing excellent adsorption capacity for Bisphenol A (BPA) and p-chlorophenol (p-CP), and excellent catalytic ability of activating potassium persulphate (KPS) for degradation of BPA and p-CP. The adsorption capacity and catalytic degradation potential were evaluated for the removal of BPA and p-CP from solutions. The results showed that the adsorption took only 1 h to reach equilibrium and HCNTs/Fe3O4 had maximum adsorption capacities of 113 mg g-1 for BPA and 41.6 mg g-1 for p-CP at 303 K, respectively. The adsorption of BPA fitted well using the Langmuir, Temkin and Freundlich models while the adsorption of p-CP fitted well using the Freundlich and Temkin models. BPA adsorption on HCNTs/Fe3O4 was dominated by π-π stacking and hydrogen bonding forces. The adsorption included both the mono-molecular layer adsorption on the adsorbent surface and the multi-molecular layer adsorption on the non-uniform surface. The adsorption of p-CP on HCNTs/Fe3O4 was a multi-molecular layer adsorption on a dissimilar surface. The adsorption was controlled by forces such as π-π stacking, hydrogen bonding, partition effect and molecular sieve effect. Moreover, KPS was added to the adsorption system to initiate a heterogeneous Fenton-like catalytic degradation. Over a wide pH range (4-10), 90% of the aqueous BPA solution and 88% of the p-CP solution were degraded in 3 and 2 h, respectively. After three adsorption-regeneration or degradation cycles, the removal of BPA and p-CP remained up to 88% and 66%, indicating that HCNTs/Fe3O4 composite is cost-effective, stable and highly efficient to remove BPA and p-CP from solution.

Remediation of environmental toxicants using carbonaceous materials: opportunity and challenges

Adsorption and photocatalytic properties of carbonaceous materials, viz., carbon nanotubes (CNTs), fullerene, graphene, graphene oxide, carbon nanofiber nanospheres, and activated carbon, are the legitimate weapons for the remediation of emerging and persistent inorganic/organic contaminants, heavy metals, and radionucleotides from the environment. High surface area, low or non-toxic nature, ease of synthesis, regeneration, and chemical modification of carbonaceous material make them ideal for the removal of toxicants. The research techniques investigated during the last decade for the elimination of environmental toxicants using carbonaceous materials are reviewed to offer comprehensive insight into the mechanism, efficiency, applications, advantages, and shortcomings. Opportunities and challenges associated with carbon materials have been discussed to suggest future perspectives in the remediation of environmental toxicants.

Intercalation of salicylaldoxime into layered double hydroxide: ultrafast and highly selective uptake of uranium from different water systems via versatile binding modes

We report the first example of MgAl layered double hydroxide intercalated with salicylaldoxime (SA-LDH) which exhibits excellent uranium (U(VI)) capture performance. In U(VI) aqueous solutions, the SA-LDH shows a tremendous maximum U(VI) sorption capacity (q_m^U) of 502 mg·g^-1, surpassing most known sorbents. For the aqueous solution with an initial U(VI) concentration (C₀^U) of ∼ 10 ppm, ≥99.99 % uptake is achieved in a wide pH range of 3-10. At C₀^U ∼ 20 ppm, >99 % uptake is reached within only 5 min, and pseudo-second-order kinetics rate constant (k₂) of 44.9 g·mg^-1·min^-1 reaches the record value, placing the SA-LDH amongst the fastest U adsorbing materials reported to date. In contaminated seawater with 35 ppm of U while highly concentrated metal ions of Na⁺, Mg²⁺, Ca²⁺, and K⁺, the SA-LDH still displays exceptionally high selectivity and ultrafast extraction for UO₂²⁺, giving >95 % uptake of U(VI) within 5 min, and the k₂ value of 0.308 g·mg^-1·min^-1 for seawater surpasses most reported values for aqueous solutions. Versatile binding modes toward U by SA-LDH, including complexation (UO₂²⁺ with SA^- and/or CO₃^2-), ion exchange and precipitation, contribute to the preferable uptake of U at different concentrations. X-ray absorption fine structure (XAFS) analyses demonstrate that one uranyl ion (UO₂²⁺) binds to two SA^- anions and two H₂O molecules forming 8-coordinated configuration. The U coordinates with O atom of the phenolic hydroxyl group and N atom of the -CN-O^- group of SA^-, forming a stable six-membered ring motif, which endows the fast and robust capture of U. The wonderful uranium trapping ability makes the SA-LDH among the best adsorbent used for uranium extraction from various solution systems including seawater.

In situ synthesis of magnesium-doped hydroxyapatite aerogel for highly efficient U(VI) separation with ultra high adsorption capacity and excellent recyclability

Mg-doped HAP aerogel (MHAPA) was firstly in situ prepared via freeze-drying-calcination technology to capture U(VI). The U(VI) removal capacity by MHAPA even arrived 2685.6 mg g^-1, which was about 2 times over purchased HAP, illustrating that the incorporation of Mg ions could greatly enhance the U(VI) removal capacity. Compared with HAP, MHAPA also showed better anti-ion interference ability and dynamic removal performances. In comparison with other HAP-based adsorbents, MHAPA possessed good recyclability and its desorption rate was up to 93.4% in the first cycle. The excellent U(VI) removal performances of MHAPA might be owing to its low crystallinity and grain size, fast ion exchange rate and partial ionization under acidic conditions, which would accelerate the process of electrostatic attraction, ion-exchange, and complexation to immobilize U(VI). To sum up, the prepared MHAPA was expected to be an environmentally friendly, recyclable and effective adsorbent to immobilize U(VI) in actual wastewater.

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.

A critical review of various adsorbents for selective removal of nitrate from water: Structure, performance and mechanism

Nitrate is ubiquitous pollutant due to its high water solubility, usually contributing to eutrophication, and posing a threat to aquatic ecosystem and human health. Adsorption approach has been widely used for nitrate removal because of the simplicity, easy operation, and low cost. Adsorbent plays a key role in the adsorptive removal of nitrate. The adsorption performance and adsorption mechanism are determined by the structural feature of adsorbent that is dependent on the preparation method. In this review, various types of adsorbents for nitrate removal were systematically summarized, their preparation, characterization, and adsorption performance were evaluated; the factors influencing the nitrate adsorption performance were discussed; the adsorption isotherm models, kinetic models and thermodynamic parameters were examined; and the possible adsorption mechanisms responsible for nitrate adsorption were categorized; the possible correlation of adsorbent structure to adsorption performance and adsorption mechanism were explained; the potential applications of adsorbents were discussed; finally, the strategies for improving adsorption capacity and selectivity towards nitrate, the challenges and future perspectives for developing novel adsorbent were also proposed. This review will deepen the understanding of nitrate removal by adsorption process and help the development of high-performance adsorbents for selective nitrate removal from water and wastewater.

Efficient uranium adsorbent with antimicrobial function constructed by grafting amidoxime groups on ZIF-90 via malononitrile intermediate

Herein, a dual-function Zeolitic Imidazole Frameworks (ZIFs) ZIF-90 grafted with malononitrile by Knoevenagel reaction and following with an amidoximation reaction to form an efficient U (VI) adsorbent (ZIF-90-AO). The strong chelation power of amidoxime groups (AO) with uranium and ZIF-90's mesoporous structure afforded ZIF-90-AO high maximum uranium adsorption capacity of 468.3 mg/g (pH = 5). In addition, the factors affecting uranium adsorption process were investigated by a batch of adsorption tests under different adsorption conditions. ZIF-90-AO displayed good selectivity to UO₂²⁺ in the solution containing multiple co-existing ions and good regeneration property. More importantly, ZIF-90-AO showed excellent antimicrobial property against both E. coli and S. aureus. Therefore, ZIF-90-AO is a U-adsorbent with great application value for removing U (VI) from wastewater due to the high U (VI) adsorption capacity in weak acid condition and good anti-biofouling properties.

Efficient removal and recovery of uranium from industrial radioactive wastewaters using functionalized activated carbon powder derived from zirconium carbide process waste

Development of efficient sorbents for selective removing and recovery of uranium from radioactive wastewaters is highly important in nuclear fuel industries from the standpoint of resource sustainability and environmental safety issues. In this study, carbon powder waste was modified by various chemical activating agents under atmosphere of nitrogen gas at 725 °C to prepare an efficient sorbent for removal and recovery of uranium ions from radioactive wastewaters of nuclear fuel conversion facility. Activation of the carbon powder with KOH, among different activators, provided maximum porosity and surface area. The activated samples were modified by reacting with ammonium persulfate in sulfuric acid solution to generate surface functional groups. The synthetized sorbents were characterized with FT-IR, XRD, BET, and SEM-EDS techniques. The effects of solution pH, contact time, initial uranium concentration, and temperature on the sorption capacity of the sorbent with respect to U(VI) from wastewater were investigated by batch method, followed by optimizing the effect of influential parameters by experimental design using central composite design. The sorption of UO₂²⁺ ions on the sorbents follows the Langmuir isotherm and pseudo-second-order kinetic models. Maximum sorption capacity for U(VI) was 192.31 mg g^-1 of the modified sorbent at 35 °C. Thermodynamic data showed that sorption of U(VI) on the sorbent was through endothermic and spontaneous processes. The sorption studies on radioactive effluents of the nuclear industry demonstrated that the modified sorbent had a favorable selectivity for uranium removal in the presence of several other metal ions.