99TcO4- remediation by a cationic polymeric network

Anion-exchanged and quaternary ammonium functionalized MIL-101-Cr metal-organic framework (MOF) for ReO4-/TcO4- sequestration from groundwater

There are few effective technologies for the sequestration of highly water-soluble pertechnetate (TcO₄^-) from contaminated water despite the urgency of environmental and public health concerns. In this work, anion exchanged and cetyltrimethylammonium bromide (CTAB) functionalized MIL-101-Cr-NO₃ were investigated for perrhenate (ReO₄^-), a surrogate of TcO₄^-, sequestration from artificial groundwater. Cl^-, I^-, and CF₃SO₃^- exchanged MIL-101-Cr proved more effective at ReO₄^- removal than the parent MIL-101-Cr-F. Compared to the parent framework, CTAB functionalized MIL-101-Cr-NO₃ increased ReO₄^- removal capacity from 39 to 139 mg/g, improved the reaction kinetics from ~30 to <10 min to reach full adsorption capacity and the selectivity for ReO₄^- over competing NO₃^-, CO₃^2-, SO₄^2-, and Cl^-. Spectroscopic data indicated that the chemical speciation of Re in the exchanged MIL-101-Cr remained ReO₄^-, indicating synergistic sequestration through both anion exchange and non-ion exchange binding with the positively charged ligand of CTAB. These studies foreshadow potential applications of MOFs for the remediation of ⁹⁹TcO₄^- from contaminated environments.

Nanotrap Grafted Anion Exchangeable Hybrid Materials for Efficient Removal of Toxic Oxoanions from Water

Water pollution has attracted worldwide significant attention ever since the finding of its harmful effects on the whole ecosystem, including human health. Although several materials are known for selective removal of specific contaminants, designing a single material that can adsorb a variety of water contaminants is still a very challenging task due to a lack of proper design strategies. Herein, we have rationally designed a new class of anion exchangeable hybrid material where the nanosized cationic metal-organic polyhedra (MOP) are embedded inside a porous covalent organic framework (COF) with specific binding sites for toxic oxoanions. The resulting hybrid material exhibits very fast and selective sequestration of high as well as trace amount of a wide range of toxic oxoanions (HAsO₄ ^2-, SeO₄ ^2-, CrO₄ ^2-, ReO₄ ^-, and MnO₄ ^-) from the mixture of excessive (∼1000-fold) other interfering anions to well below the permissible drinking water limit. Moreover, the hybrid cationic nanotrap material can reduce the As(V) level from a highly contaminated groundwater sample to below the WHO permitted level.

A Water-Stable Cationic Metal-Organic Framework with Hydrophobic Pore Surfaces as an Efficient Scavenger of Oxo-Anion Pollutants from Water

Water contamination due to heavy metal-based toxic oxo-anions (such as CrO₄^2- and TcO₄^-) is a critical environmental concern that demands immediate mitigation. Herein, we present an effort to counter this issue by a novel chemically stable cationic metal-organic framework (iMOF-2C) with strategic utilization of a ligand with hydrophobic core, known to facilitate such oxo-anion capture process. Moreover, the compound exhibited very fast sieving kinetics for such oxo-anions and a very high uptake capacity for CrO₄^2- (476.3 mg g^-1) and ReO₄^- (691 mg g^-1), while the latter being employed as a surrogate analogue for radioactive TcO₄^- anions. Notably, the compound showed excellent selectivity even in the presence of other competing anions such as NO₃^-, Cl^-, SO₄^2-, ClO₄^-. etc.. Furthermore, the compound possesses excellent reusability (up to 10 cycles) and is also employed to a stationary phase ion column to decontaminate the aforementioned oxo-anions from water.

Selective Reductive Removal of Silver Ions from Acidic Solutions by Redox-Active Covalent Organic Frameworks

The selective removal and recovery of silver ions from an aqueous solution is necessary, owing to the toxicity, persistency, and recoverable value. Herein, we first reported that silver ions could be selectively removed from an acidic solution by utilizing redox-active covalent organic framework (COF) materials as an adsorbent, resulting in the loading of Ag nanoparticles (NPs) with a narrow size distribution onto the framework simultaneously. The redox-active COF not only showed promising performance in adsorbing silver ions but also had a high selectivity at a low pH value. Subsequently, it was found that the N sites of amine groups within the framework took responsibility for the Ag NP generation after the systematic investigation on the redox adsorption mechanism. Furthermore, the recycled Ag@COF materials could be further used as new adsorbents to remove Hg(II) ions from water via NPs as a "bridge", exhibiting ultrahigh atomic utilization (>100%). Accordingly, this work not only provides a novel insight for the use of redox-active COF in the removal of metal ions but also opens a new field for designing of functionalized COF for their potential application in diverse areas.

Superhydrophilic and highly elastic monolithic sponge for efficient solar-driven radioactive wastewater treatment under one sun

As an effective way to obtain solar energy and separate the soluble contaminants from water, solar-driven interfacial evaporation is used in desalination, wastewater treatment, electricity generation, and domestic water heating system. Herein, we demonstrate a monolithic sponge with three-dimensional porous structure as the solar-energy evaporator, which is composed of hydrophilic polymer (Konjac Glucomannan, KGM) and solar absorbent (reduced graphene oxide, rGO). Under one sun irradiation, the sponge achieves a rapid evaporation rate (1.60 kg m^-2 h^-1) and high interfacial water evaporation efficiency (92 %) due to its good absorption, photothermal, thermal insulation, and fast water transport properties. Meanwhile, the concentrations of radioactive elements (strontium, cesium, and uranium) in wastewater dropped from grams to micrograms after purification, even under radiation and acidic conditions. Additionally, the durability and repeatability of the sponge also have been verified. The results showed that solar-driven interfacial evaporation can effectively treat radioactive wastewater and enrich various radionuclides in a more energy-saving manner.

Technetium immobilization by materials through sorption and redox-driven processes: A literature review

The objective of this review is to evaluate materials for use as a barrier or other deployed technology to treat technetium-99 (Tc) in the subsurface. To achieve this, Tc interactions with different materials are considered within the context of remediation strategies. Several naturally occurring materials are considered for Tc immobilization, including iron oxides and low solubility sulfide phases. Synthetic materials are also considered, and include tin-based materials, sorbents (resins, activated carbon, modified clays), layered double hydroxides, metal organic frameworks, cationic polymeric networks and aerogels. All of the materials were evaluated for their potential in-situ and ex-situ performance with respect to long-term Tc uptake and immobilization, environmental impacts and deployability. Other factors such as the technology maturity, cost and availability were also considered. Given the difficulty of evaluating materials under different experimental conditions (e.g., solution chemistry, redox conditions, solution to solid ratio, Tc concentration etc.), a subset of these materials will be selected, on the basis of this review, for subsequent standardized batch loading tests.

Evaluation of materials for iodine and technetium immobilization through sorption and redox-driven processes

Radioactive iodine-129 (¹²⁹I) and technetium-99 (⁹⁹Tc) pose a risk to groundwater due to their long half-lives, toxicity, and high environmental mobility. Based on literature reviewed in Moore et al. (2019) and Pearce et al. (2019), natural and engineered materials, including iron oxides, low-solubility sulfides, tin-based materials, bismuth-based materials, organoclays, and metal organic frameworks, were tested for potential use as a deployed technology for the treatment of ¹²⁹I and ⁹⁹Tc to reduce environmental mobility. Materials were evaluated with metrics including capacity for IO₃^- and TcO₄^- uptake, selectivity and long-term immobilization potential. Batch testing was used to determine IO₃^- and TcO₄^- sorption under aerobic conditions for each material in synthetic groundwater at different solution to solid ratios. Material association with IO₃^- and TcO₄^- was spatially resolved using scanning electron microscopy and X-ray microprobe mapping. The potential for redox reactions was assessed using X-ray absorption near edge structure spectroscopy. Of the materials tested, bismuth oxy(hydroxide) and ferrihydrite performed the best for IO₃^-. The commercial Purolite A530E anion-exchange resin outperformed all materials in its sorption capacity for TcO₄^-. Tin-based materials had high capacity for TcO₄^-, but immobilized TcO₄^- via reductive precipitation. Bismuth-based materials had high capacity for TcO₄^-, though slightly lower than the tin-based materials, but did not immobilize TcO₄^- by a redox-drive process, mitigating potential negative re-oxidation effects over longer time periods under oxic conditions. Cationic metal organic frameworks and polymer networks had high Tc removal capacity, with TcO₄^- trapped within the framework of the sorbent material. Although organoclays did not have the highest capacity for IO₃^- and TcO₄^- removal in batch experiments, they are available commercially in large quantities, are relatively low cost and have low environmental impact, so were investigated in column experiments, demonstrating scale-up and removal of IO₃^- and TcO₄^- via sorption, and reductive immobilization with iron- and sulfur-based species.

Tailored Porous Organic Polymers for Task-Specific Water Purification

The Industrial Revolution has resulted in social and economic improvements, but unfortunately, with the development of manufacturing and mining, water sources have been pervaded with contaminants, putting Earth's freshwater supply in peril. Therefore, the segregation of pollutants-such as radionuclides, heavy metals, and oil spills-from water streams, has become a pertinent problem. Attempts have been made to extract these pollutants through chemical precipitation, sorbents, and membranes. The limitations of the current remediation methods, including the generation of a considerable volume of chemical sludge as well as low uptake capacity and/or selectivity, actuate the need for materials innovation. These insufficiencies have provoked our interest in the exploration of porous organic polymers (POPs) for water treatment. This category of porous material has been at the forefront of materials research due to its modular nature, i.e., its tunable functionality and tailorable porosity. Compared to other materials, the practicality of POPs comes from their purely organic composition, which lends to their stability and ease of synthesis. The potential of using POPs as a design platform for solid extractors is closely associated with the ease with which their pore space can be functionalized with high densities of strong adsorption sites, resulting in a material that retains its robustness while providing specified interactions depending on the contaminant of choice.POPs raise opportunities to improve current or enable new technologies to achieve safer water. In this Account, we describe some of our efforts toward the exploitation of the unique properties of POPs for improving water purification by answering key questions and proposing research opportunities. The design strategies and principles involved for functionalizing POPs include the following: increasing the density and flexibility of the chelator to enhance their cooperation, introducing the secondary sphere modifiers to reinforce the primary binding, and enforcing the orientation of the ligands in the pore channel to increase the accessibility and cooperation of the functionalities. For each strategy, we first describe its chemical basis, followed by presenting examples that convey the underlying concepts, giving rise to functional materials that are beyond the traditional ones, as demonstrated by radionuclide sequestration, heavy metal decontamination, and oil-spill cleanup. Our endeavors to explore the applicability of POPs to deal with these high-priority contaminants are expected to impact personal consumer water purifiers, industrial wastewater management systems, and nuclear waste management. In our view, more exciting will be new applications and new examples of the functionalization strategies made by creatively merging the strategies mentioned above, enabling increasingly selective binding and efficiency and ultimately promoting POPs for practical applications to enhance water security.

Synthesis of ZnO nanoparticle-anchored biochar composites for the selective removal of perrhenate, a surrogate for pertechnetate, from radioactive effluents

Pertechnetate (TcO₄-) is a component of low-activity waste (LAW) fractions of legacy nuclear waste, and the adsorption removal of TcO₄- from LAW effluents would greatly benefit the site remediation process. However, available adsorbent materials lack the desired combination of low cost, radiolytic stability, and high selectivity. In this study, a ZnO nanoparticle-anchored biochar composite (ZBC) was fabricated and applied to potentially separate TcO₄- from radioactive effluents. The as-synthesized material exhibited γ radiation resistance and superhydrophobicity, with a strong sorption capacity of 25,916 mg/kg for perrhenate (ReO₄-), which was used in this study as a surrogate for radioactive pertechnetate (TcO₄-). Additionally, the selectivity for ReO₄- exceeded that for the competing ions I-, NO₂-, NO₃-, SO₄^2-, PO₄^3-, Cu²⁺, Fe³⁺, Al³⁺, and UO₂²⁺. These unique features show that ZBC is capable of selectively removing ReO₄- from Hanford LAW melter off-gas scrubber simulant effluent. This selectivity stems from the synergistic effects of both the superhydrophobic surface of the sorbent and the inherent nature of sorbates. Furthermore, density functional theory (DFT) calculations indicated that ReO₄- can form stable complexes on both the (100) and (002) planes of ZnO, of which, the (002) complexes have greater stability. Electron transfer from ReO₄- on (002) was greater than that on (100). These phenomena may be because (002) has a lower surface energy than (100). Partial density of state (PDOS) analysis further confirms that ReO₄- is chemisorbed on ZBC, which agrees with the findings of the Elovich kinetic model. This work provides a feasible pathway for scale-up to produce high-efficiency and cost-effective biosorbents for the removal of radionuclides.

A Porous Aromatic Framework Functionalized with Luminescent Iridium(III) Organometallic Complexes for Turn-On Sensing of 99TcO4. ACS APPLIED MATERIALS & INTERFACES 2020;12:15288-15297. [PMID: 32131587 DOI: 10.1021/acsami.0c01929]

Contamination of ⁹⁹TcO₄^-, a problematic radioactive anion in the nuclear fuel cycle, in groundwater has been observed in a series of legacy nuclear sites, representing a notable radiation hazard and environmental concern. The development of convenient, rapid, and sensitive detection methods is therefore critical for radioactivity control and remediation tasks. Traditional detection methods suffer from clear demerits of either the presence of large interference from coexisting radioactive species (e.g., radioactivity counting methods) or the requirement of extensive instrumentation and analysis procedure (e.g., mass spectrometry). Here, we constructed a luminescent iridium(III) organometallic complex (Ir(ppy)₂(bpy)⁺; ppy = 2-phenylpyridine, bpy = 2,2^'-bipyridine)-grafted porous aromatic framework (Ir-PAF) for the first time, which can be utilized for efficient, facile, and selective detection of trace ReO₄^-/TcO₄^- in aqueous solutions. Importantly, the luminescence intensity of Ir-PAF is greatly enhanced in the presence of ReO₄^-/TcO₄^-, giving rise to a distinct turn-on sensor with the detection limit of 556.9 μg/L. Such a superior detection capability originates from the highly selective and strong interaction between ReO₄^-/TcO₄^- and Ir(ppy)₂(bpy)⁺, leading to an efficient pre-enrichment of ReO₄^-/TcO₄^- during analysis and subsequently a much weaker nonradiative decay of the luminescence of Ir(ppy)₂(bpy)⁺, as illustrated by density functional theory (DFT) calculation as well as quantum yield and fluorescence lifetime measurements. Successful quantification of trace ReO₄^- in simulated Hanford low-activity waste (LAW) solution containing large excess of Cl^-, NO₃^-, and NO₂^- was demonstrated, highlighting the bright future of luminescent PAFs in the area of chemical sensing.

A Highly Efficient Coordination Polymer for Selective Trapping and Sensing of Perrhenate/Pertechnetate

A porous cationic Ag(I) coordination polymer, [Ag(1,2,4,5-p4b)](SbF₆) (TJNU-302) with the ligand 1,2,4,5-p4b (1,2,4,5-tetra(pyridin-4-yl)benzene), is reported that shows high sorption capacity (211 mg g^-1) and distribution coefficient K_d (5.8 × 10⁵ mL g^-1) as well as outstanding selectivity in 500 times excess of CO₃^2- or PO₄^3- anion for perrhenate removal. TJNU-302 can act as a crystalline turn-off sensor for perrhenate upon UV radiation. In this way, a test paper strip for sensing ReO₄^- could be produced. In water solution, TJNU-302 shows an efficient fluorescence quenching response to ReO₄^- ion, with the highest quenching percentage (86%) among all reported ReO₄^- sensors. These results could be elucidated by the bonding properties of single-crystal structures of TJNU-302 before and after perrhenate sorption, as well as density functional theory (DFT) calculations.

Uranium biosorption mechanism model of protonated Saccharomyces cerevisiae

Further understanding absorption uranium mechanism of the regenerational biosorbent is very interesting in application of the biosorbent. The regeneration adsorbent of Saccharomyces cerevisiae biomass was made by hydrochloric acid. Using it to absorb uranium at low constant pH(2.50), accompanied with proton releasing the ratio almost 1:2 which is to be analyzed in this paper. The type and amount of functional groups in the biomass such as carboxyl, amino, phosphoryl were determined by Potentiometric titrations and FTIR analysis. Chemical modification showed that the contribution of functional groups to uranium adsorption was carboxyl, phosphoryl and amino in turn. Analysis of SEM-EDX and staining microscopy showed that uranium on the surface of cells did not exist in the form of precipitation at lower pH 2.98, but at higher pH 4.52. The effects of phosphorus release and pH on uranium species was analyzed by MINTEQ software 3.0. Based on the above boundary conditions of the model construction, a multi-site of functional groups model equation of ion exchange absorption mechanism was built in which the final uranium ion concentration and pH as functions. It could well describe the exchange equilibrium of proton with uranium ion at pH2.50 to pH4.00.

Evaluation of an Imidazolium-Based Porous Organic Polymer as Radioactive Waste Scavenger

⁹⁹TcO₄^- is highly radioactive and hazardous to both the environment and public health, meanwhile, it is quite challenging to have it efficiently removed. Herein an imidazolium-based cationic porous polymer (ImPOP-1) is evaluated for removal of TcO₄^-, with nonradioactive ReO₄^- as the surrogate for experimental operation. It is demonstrated that ImPOP-1 is a rare example that can integrate high adsorption capacity (610 mg g^-1), fast kinetics (93.3% in 30 s), and high selectivity (72.9% in 1000 times excess of SO₄^2- ions) in one material. The distribution coefficient K_d is among the top up to 3.2 × 10⁵ mL g^-1. ImPOP-1 also displays high adsorption performance over a wide range of pH values, and removal efficiency up to 64.3% in a highly alkaline solution (3 M NaOH). Recyclability experiments demonstrate that ImPOP-1 can be reused at least four times. The ImPOP-1 also retains a consistent adsorption capacity up to 609 ± 6.1 mg g^-1 between three different batches of samples. In addition, a real-scenario experiment shows that ImPOP-1 can remove 97.4% of ReO₄^- in a simulated Hanford LAW stream.

Two-in-one: construction of hydroxyl and imidazolium-bifunctionalized ionic networks in one-pot toward synergistic catalytic CO2 fixation

Two-in-one hydroxyl-incorporated imidazolium ionic network was constructed in one-pot quaternization for enhancing synergistic catalytic conversion of CO₂ under mild conditions.

A cationic Zr-based metal organic framework with enhanced acidic resistance for selective and efficient removal of CrO42−

A Zr-based cationic metal organic framework shows high acidic resistance and selectivity to the anion contaminant, CrO₄²⁻.

Dual-Functionalized Fluorescent Cationic Organic Network: Highly Efficient Detection and Removal of Dichromate from Water

Dichromate is a widespread contaminant in wastewater, threatening the health of humans and other organisms. Therefore, effective detection and removal of dichromate from water is of great significance. Herein, a tetraphenylethylene functionalized cationic organic network (CON-LDU2) was constructed via a facile quaternization reaction. CON-LDU2 was successfully integrated with both detection and removal functionalities toward dichromate. On the one hand, benefiting from the strong fluorescence, CON-LDU2 was employed as a chemosensor, it could efficiently and selectively probe Cr₂O₇^2- in water with "turn-off" fluorescent response. On the other hand, the cationic skeleton and free anions inside framework make CON-LDU2 an excellent adsorbent for Cr₂O₇^2-, it could capture Cr₂O₇^2- from water with rapid kinetics and high capacity. The kinetic constant for adsorption of Cr₂O₇^2- can reach up to 1.784 g mg^-1 min^-1, while the capacity is determined as 325 mg g^-1. Furthermore, CON-LDU2 displayed good recyclability and can be reused for at least 5 cycles. Therefore, CON-LDU2 can serve as an ideal candidate not only in detection but also in removal of Cr₂O₇^2- in water medium.

Two-Dimensional Cationic Networks and Their Spherical Curvature with Tunable Opening-Closing

Despite many cationic nanomaterials that have been developed for efficient adsorption of anionic pollutants, tailoring a stable shape with denser cations on the surface for advanced removal capability remains challenging. Here, a new strategy is presented for fabricating two-dimensional (2D) cationic laminas and their curvature based on cross-linking of 2D supramolecular networks from hydrogen-bonded trimesic amide derivatives. Owing to the distribution of most cations on the surface, two cationic nanostructures from cross-linking of supramolecular networks show fast sorption kinetics for anionic pollutants. Notably, the removal capacity of the capsule-like curvature adsorbent is more than twice that of lamina  adsorbent for sufficient space around cationic sites in hollow aperture. Moreover, the capsule-like adsorbent is triggered to open and spontaneously release the adsorbed pollutants upon the addition of halogen anions, which can be recovered by subsequent dialysis. Strategy of a capsule-like pocket with tunable opening-closing will provide a new insight for storage and adsorption.

Mechanism-Property Correlation in Coordination Polymer Crystals toward Design of a Superior Sorbent

A methodology was developed to design superior sorbents of oxoanions. To integrate the high efficiency of chemisorption, selectivity, and recyclability into one sorbent, understanding the nature of oxoanions-sorbent interactions and the structural evolution of the sorbents is essential. Three cationic Ag(I) coordination polymers (CPs) are synthesized for dichromate (Cr₂O₇^2-) removal, and three distinct oxoanion-exchange mechanisms are identified, namely, the replacement, breath, and reconstruction processes, depending on the degree of framework distortion induced by the dichromate-CP interactions. The single crystal to single crystal transformation during the oxoanion exchange has been investigated by using single-crystal X-ray diffraction and energy-dispersive X-ray microanalysis. The replacement process, due to a weak chemisorption, shows excellent recyclability at the cost of reduction of efficiency and selectivity of adsorption. The reconstruction process may achieve a high efficiency and selectivity, but it loses recyclability. Due to the formation of a Ag-O(dichromate) bond and the breathing effect of the framework, the sorbent with the breath mechanism shows both superior efficiency and high recyclability in dichromate removal. The study of perrhenate (ReO₄^-) removal using the same CPs demonstrates that one CP performing the reconstruction process during dichromate removal turns to the breath process in removal of perrhenate anions. These results of mechanism-property correlation provide an insight into improvement of the methodology to fabricate a superior CP sorbent for oxoanion removal.

Controllable Synthesis of Porous Cu-BTC@polymer Composite Beads for Iodine Capture

The efficient and safe capture of radioactive iodine (¹²⁹I or ¹³¹I) is of great significance in nuclear waste disposal. Here, we report millimeter-scale poly(ether sulfone) composite beads loaded with porous Cu-BTC [Cu₃(BTC)₂, BTC = 1,3,5-benzenetricarboxylate] (Cu-BTC@PES), prepared by a phase inversion method for the removal of volatile iodine. Three kinds of Cu-BTC@PES composite beads were obtained with different Cu-BTC contents of 48.6, 60.2, and 71.9%, respectively. While maintaining crystallinity, the composite beads exhibited higher I₂ vapor adsorption capacity (639 mg/g) in the form of iodine molecules. The iodine absorption up to 260 mg/g and the adsorption was followed Langmuir isotherm and pseudo-second-order kinetic model. Furthermore, the composite beads can still absorb more than 85% of iodine after 3 cycles of regeneration with excellent recyclability. The resulting Cu-BTC@PES composite beads show great potential for the sustainable removal of radioactive iodine.

Efficient Photocatalytic Reduction of Aqueous Perrhenate and Pertechnetate

The pertechnetate anion (⁹⁹TcO₄^-) is a long-lived radioactive species that is soluble in aqueous solution, in contrast to sparingly soluble ⁹⁹TcO₂. Results are reported for photocatalytic reduction and removal of perrhenate (ReO₄^-), a nonradioactive surrogate for ⁹⁹TcO₄^-, using a TiO₂ (P25) nanoparticle suspension in formic acid under UV-visible irradiation. Re(VII) removal up to 98% was achieved at pH = 3 under air or N₂. The proposed mechanism is Re(VII)/Re(IV) reduction mediated by reducing radicals (^·CO₂^-) from oxidation of formic acid, not direct reduction by photogenerated electrons of TiO₂. Recycling results indicate that photocatalytic reduction of ReO₄^- exhibits excellent regeneration and high activity with >95% removal even after five cycles. ⁹⁹Tc(VII) is more easily reduced than Re(VII) in the presence of NO₃^- with very slow redissolution of reduced ⁹⁹Tc. This study presents a novel method for the removal of ReO₄^-/⁹⁹TcO₄^- from aqueous solution, with potential application for deep geological disposal.

A simple method for preparing ultra-light graphene aerogel for rapid removal of U(VI) from aqueous solution

In this study, graphene aerogel (GA) was successfully prepared through a simple hydrothermal method. The resulting GA exhibited a porous network structure with a large specific surface area (350.8 m²/g), ultra-light mass and easy separation from water. The pH_IEP value of the GA was estimated to be 3.5. The adsorption process and the factors that affect adsorption capacity were studied. The adsorption could be conducted in a wide pH range from 2.0 to 7.0. The maximum adsorption capacity of GA towards U(VI) at pH 4.0 and T = 298 K was 238.67 mg/g calculated from the Langmuir model. The GA had greatly rapid adsorption property for the removal of U(VI) at pH 4.0. Kinetic data showed good correlation with pseudo-second-order equation. Fourier transform infrared spectroscopy and X-ray photoelectron spectrometry characterizations showed that GA adsorbed U(VI) through chemical interaction by oxygen-containing and nitrogen-containing groups functional groups. The results show that GA has excellent application potential as an adsorbent material for removing U(VI) from aqueous solution.

A Cationic Silver Pyrazine Coordination Polymer with High Capacity Anion Uptake from Water

We report the first example of linker modification for an N-donor Ag-based cationic material while maintaining and in some cases increasing the anion exchange capacity. Cationic silver(I) pyrazine nitrate selectively traps harmful oxo-anions from water such as permanganate, perrhenate and a variety of α,ω-alkanedicarboxylates. We chose these anions as initial examples of exchange for potential water purification. The host-guest interaction between the cationic layers of π-stacked silver pyrazine polymers and the incoming/outgoing interlamellar anions allows for the exchange. The exchange capacity over 24 h reached 435 and 818 mg/g for permanganate and perrhenate, respectively, a record for a crystalline metal-organic material and over five times the exchange capacity compared to commercial resin. The material also undergoes organic exchange as an analog for pharmaceutical waste, some of which have a carboxylate functionality at the neutral pH range typical of natural water sources. Both the as-synthesized and exchanged materials are characterized by a variety of analytical techniques.

Cationic Covalent Organic Nanosheets for Rapid and Selective Capture of Perrhenate: An Analogue of Radioactive Pertechnetate from Aqueous Solution

Capture of radioactive TcO₄^- from nuclear wastes is extremely desirable for waste disposal and environmental restoration. Here, we report the synthesis of hydrolytically stable cationic covalent organic nanosheets (iCON) for efficient uptake of ReO₄^-, a nonradioactive surrogate of TcO₄^-. The iCON combines cationic guanidine-based knots with hydroxyl anchored neutral edge units and chloride ions loosely bonded in the pores, rendering extremely fast exchange kinetics toward ReO₄^- with high uptake capacity of 437 mg g^-1 and prominent distribution coefficient of 5.0 × 10⁵. The removal efficiency remains stable over a pH range of 3-12 and allows selective capture of ReO₄^- in the presence of excessive competing anions such as NO₃^-, CO₃^2-, PO₄^3- and SO₄^2- with good removal efficiency for ReO₄^- in a simulated Hanford LAW Melter Recycle Stream. Anion exchange between the ReO₄^- in solution and the chloride ion in iCON plays dominant role in the adsorption of ReO₄^-. The iCON shows promise for effective removal of radioactive ⁹⁹Tc from nuclear waste.

Functionalized Porous Aromatic Frameworks as High-Performance Adsorbents for the Rapid Removal of Boric Acid from Water

This study demonstrates that functionalized, highly porous polymers are promising for the adsorptive capture of boric acid, a neutral contaminant that is difficult to remove from seawater using conventional reverse osmosis membranes. Appending N-methyl-d-glucamine (NMDG) to the pore walls of high-surface-area porous aromatic frameworks (PAFs) yields the adsorbents PAF-1-NMDG and P2-NMDG in a simple two-step synthesis. The boron-selective PAFs demonstrate adsorption capacities that are up to 70% higher than those of a commercial boron-selective resin, Amberlite IRA743, and markedly faster adsorption rates, owing to their higher NMDG loadings and greater porosities relative to the resin. Remarkably, PAF-1-NMDG is able to reduce the boron concentration in synthetic seawater from 2.91 to <0.5 ppm in less than 3 min at an adsorbent loading of only 0.3 mg mL^-1 . The boron adsorption rate constants of both frameworks, determined via a pseudo-second-order rate model, represent the highest values reported in the literature-in most cases orders of magnitude higher than those of other boron-selective adsorbents. The frameworks can also be readily regenerated via mild acid/base treatment and maintain constant boron adsorption capacities for at least 10 regeneration cycles. These results highlight the numerous advantages of PAFs over traditional porous polymers in water treatment applications.

A novel anion exchange method based on in situ selectively reductive desorption of Cr(VI) for its separation from V(V): Toward the comprehensive use of hazardous wastewater

In China, the wastewater produced after vanadate precipitation (AVP wastewater) from industrial vanadium extraction contains toxic V(V) and carcinogenic Cr(VI). When considering environmental protection and wastewater use, V(V) and Cr(VI) must be extracted and separated from the hazardous AVP wastewater. However, separating V(V) and Cr(VI) is difficult because of their highly similar physicochemical properties. Herein, we propose a novel anion exchange method based on the in situ selectively reductive desorption of Cr(VI) to separate and extract V(V) and Cr(VI) using a weak organic reductant (ethanol) to selectively reduce Cr(VI) anions and transform them into Cr³⁺ cations, while maintaining V(V) in a H₂V₁₀O₂₈^4- anion form. We indicate that the efficient separation of Cr(VI) from V(V) can be attributed to selective Cr(VI) anion reduction via ethanol. We applied this anion exchange method to separate and recover Cr(VI) and V(V) in AVP wastewater with a Cr(VI) recovery of 95.59% and a V(V) recovery of 94.54%. The final Cr₂O₃ and V₂O₅ products had a purity of 98.03% and 96.82%, respectively. This study provides novel insights into the simultaneous separation and extraction of analog transition metals and a comprehensive method to use hazardous wastewater.

Optimizing radionuclide sequestration in anion nanotraps with record pertechnetate sorption

The elimination of specific contaminants from competitors poses a significant challenge. Rather than relying on a single direct interaction, the cooperation of multiple functionalities is an emerging strategy for adsorbents design to achieve the required affinity. Here, we describe that the interaction with the target species can be altered by modifying the local environment of the direct contact site, as demonstrated by manipulating the affinity of pyridinium-based anion nanotraps toward pertechnetate. Systematic control of the substituent effect allows the resulting anion nanotraps to combine multiple features, overcoming the long-term challenge of TcO₄^- segregation under extreme conditions of super acidity and basicity, strong irradiation field, and high ionic strength. The top material exhibits the highest sorption capacity together with record-high extraction efficiencies after a single treatment from conditions relevant to the used nuclear fuel (Hanford tank wastes, 95%) and legacy nuclear wastes (Savannah River Sites, 80%) among materials reported thus far.

A cationic porous organic polymer for high-capacity, fast, and selective capture of anionic pollutants

The emerging ionic porous organic materials have achieved various applications in different fields, however, there is limited study on using them to capture ionic pollutants from water. Here we demonstrate a facile method to prepare a cationic porous organic polymer via catalyst-free Schiff base reaction. The imidazolium-based polymer (ImPOP-1) was constructed through copolymerizing cationic molecules with low-cost benzidine. The as-prepared ImPOP-1 exhibits high capacity (e.g., 476.2 mg g^-1 for Pd (II) and 578.5 mg g^-1 for AO7^-), excellent selectivity (e.g., more than 99% removal efficiency for Pd (II) in the presence of 100 times excess of SO₄^2-), and fast kinetics (e.g., 98.6% removal efficiency within 5 min for Pd (II) ions) to the anionic pollutants including organic dyes and heavy metal ions. The excellent performance on scavenging anionic pollutants from water suggests that ImPOP-1 holds promising potential as an ion exchange material for water remediation.

Anion-adaptive crystalline cationic material for 99TcO4- trapping

Efficient anion recognition is of great significance for radioactive 99TcO4- decontamination, but it remains a challenge for traditional sorbents. Herein, we put forward a tactic using soft crystalline cationic material with anion-adaptive dynamics for 99TcO4- sequestration. A cucurbit[8]uril-based supramolecular metal-organic material is produced through a multi-component assembly strategy and used as a sorbent for effective trapping of TcO4-. Excellent separation of TcO4-/ReO4- is demonstrated by fast removal kinetics, good sorption capacity and high distribution coefficient. Remarkably, the most superior selectivity among metal-organic materials reported so far, together with good hydrolytic stability, indicates potential for efficient TcO4- removal. The structure incorporating ReO4- reveals that the supramolecular framework undergoes adaptive reconstruction facilitating the effective accommodation of TcO4-/ReO4-. The results highlight opportunities for development of soft anion-adaptive sorbents for highly selective anion decontamination.

Effective Removal of Anionic Re(VII) by Surface-Modified Ti2CT x MXene Nanocomposites: Implications for Tc(VII) Sequestration

Environmental contamination by ⁹⁹Tc(VII) from radioactive wastewater streams is of particular concern due to the long half-life of ⁹⁹Tc and high mobility of pertechnetate. Herein, we report a novel MXene-polyelectrolyte nanocomposite with three-dimensional networks for enhanced removal of perrhenate, which is pertechnetate simulant. The introduction of poly(diallyldimethylammonium chloride) (PDDA) regulates the surface charge and improves the stability of Ti₂CT _x nanosheet, resulting in Re(VII) removal capacity of up to 363 mg g^-1, and fast sorption kinetics. The Ti₂CT _x/PDDA nanocomposite furthermore exhibits good selectivity for ReO₄^- when competing anions (such as Cl^- and SO₄^2-) coexist at a concentration of 1800 times. The immobilization mechanism was confirmed as a sorption-reduction process by batch sorption experiments and X-ray photoelectron spectroscopy. The pH-dependent reducing activity of Ti₂CT _x/PDDA nanocomposite toward Re(VII) was clarified by X-ray absorption spectroscopy. As the pH increases, the local environment gradually changes from octahedral-coordinated Re(IV) to tetrahedral-coordinated Re(VII). The overall results suggest that Ti₂CT _x/PDDA nanocomposite may be a promising candidate for efficient elimination of Tc contamination. The reported surface modification strategy might result in applications of MXene-based materials in environmental remediation of other oxidized anion pollutants.

Titanium Silicates Precipitated on the Rice Husk Biochar as Adsorbents for the Extraction of Cesium and Strontium Radioisotope Ions

The aim of the work was the development of cheap and effective adsorbents based on titanium silicates deposited on the products of thermochemical processing of rice husk to extract cesium and strontium radioisotopes from aqueous media. Synthesis of adsorbents was carried out using the cheapest and widely used titanium water-soluble reagent, titanium sulfate (an intermediate product of white rutile pigment production), as feedstock. After treatment with titanium sulfate and neutralization, hydrothermal treatment was carried out in various ways. The traditional method of processing in an autoclave was used, as well as the blowing at different temperatures by steam. The distribution coefficients and the adsorption capacity for cesium and strontium ions on these sorbents were studied. Along with the chemical composition of adsorbents obtained by those ways, the type and the temperature of hydrothermal treatment also affected the adsorption properties. It was found that the adsorbent obtained by hydrothermal treatment in an autoclave has the highest degree of cesium ions extraction (Kd = 27,500). The highest degree of strontium ions extraction (Kd = 2,095,000) has an adsorbent obtained by hydrothermal treatment with water vapor blowing.

Successful Decontamination of 99 TcO4 - in Groundwater at Legacy Nuclear Sites by a Cationic Metal-Organic Framework with Hydrophobic Pockets

⁹⁹ Tc contamination at legacy nuclear sites is a serious and unsolved environmental issue. The selective remediation of ⁹⁹ TcO₄ ^- in the presence of a large excess of NO₃ ^- and SO₄ ^2- from natural waste systems represents a significant scientific and technical challenge, since anions with a higher charge density are often preferentially sorbed by traditional anion-exchange materials. We present a solution to this challenge based on a stable cationic metal-organic framework, SCU-102 (Ni₂ (tipm)₃ (NO₃ )₄ ), which exhibits fast sorption kinetics, a large capacity (291 mg g^-1 ), a high distribution coefficient, and, most importantly, a record-high TcO₄ ^- uptake selectivity. This material can almost quantitatively remove TcO₄ ^- in the presence of a large excess of NO₃ ^- and SO₄ ^2- . Decontamination experiments confirm that SCU-102 represents the optimal Tc scavenger with the highest reported clean-up efficiency, while first-principle simulations reveal that the origin of the selectivity is the recognition of TcO₄ ^- by the hydrophobic pockets of the structure.

Organic Compounds of Actinyls: Systematic Computational Assessment of Structural and Topological Properties in [AnO2(C2O4) n](2 n-2)- (An = U, Np, Pu, Am; n = 1-3) Complexes

Exploring the bonding features between organics and actinide elements is a fundamental topic in nuclear waste separation. In this work, [AnO₂(C₂O₄) _n]^{(2 n-2)-} (An = U, Np, Pu, and Am; n = 1-3) complexes have been characterized by density functional theory. The actinyl oxalate complexes are found to exhibit the typical An-O_yl, An-O_eq bonds and O_yl-An-O_yl angles. Interatomic interaction analyzed by electron density difference, charge decomposition analysis, charges population, bond order, electron localization function, and quantum theory of atom in molecules indicates that An-O_eq bonds are ionic (closed-shell) bonding interaction with a small degree of covalent character. The similarities and differences between isomers have been discussed in the actinide coordination chemistry, and the orbital interactions also have been investigated through total, partial, and overlap population density of state diagrams. Besides, the electrostatic potential was used to predict the adsorption sites on the molecular vdW surface.

Mechanism unravelling for ultrafast and selective 99TcO4 - uptake by a radiation-resistant cationic covalent organic framework: a combined radiological experiment and molecular dynamics simulation study

Separation of TcO₄^– by a cationic covalent organic framework is achieved for the first time, showing advantages of extremely fast sorption kinetics, ultrahigh uptake capacity, good anion-exchange selectivity, and excellent radiation resistance.

⁹⁹Tc is one of the most problematic fission products in the nuclear fuel cycle owing to its large inventory in used nuclear fuel, long half-life, potential radiation hazard, high environmental mobility of its major species ⁹⁹TcO₄^–, and its redox-active nature. Ideally, ⁹⁹TcO₄^– should be removed at the first stage, when the used fuel rods are dissolved in highly concentrated nitric acid solution, which can substantially reduce its interference with the solvent extraction process through catalytic redox reactions with the key actinides and diminish the chance of discharge into the environment as the volatile species during the waste vitrification process. However, this task cannot be achieved by any of the reported anion-scavenging materials including traditional polymeric anion-exchange resins, inorganic cationic framework materials, and recently developed cationic metal–organic framework materials, because they either are not stable under the extreme conditions of the combined high acidity and strong radiation field or do not possess the required uptake selectivity towards ⁹⁹TcO₄^– in the presence of a huge excess of competing anions such as NO₃^– and SO₄^2–. Herein, we present the first study of ⁹⁹TcO₄^– removal under extreme conditions by a two-dimensional conjugated cationic covalent organic framework material, SCU-COF-1. This material exhibits ultrahigh acid stability, great resistance towards both large-dose β and γ irradiation and unprecedented ⁹⁹TcO₄^– uptake capabilities including extremely fast sorption kinetics (sorption equilibrium can be reached within 1 min), ultrahigh uptake capacity (702.4 mg g^–1 for the surrogate ReO₄^– at a slightly elevated temperature), and good anion-exchange selectivity towards ⁹⁹TcO₄^–. These excellent features endow SCU-COF-1 with the practical capabilities of separating ⁹⁹TcO₄^– from both simulant highly acidic fuel reprocessing solutions (3 M nitric acid) and low-activity waste streams at the US legacy nuclear site. The anion-exchange mechanism and the ⁹⁹TcO₄^– uptake selectivity are further demonstrated and clearly visualized by the molecular dynamics simulation investigations.

Design Strategy for the Controlled Generation of Cationic Frameworks and Ensuing Anion-Exchange Capabilities

Cationic frameworks are an emerging class of exceptional solid adsorbents capable of encapsulating highly toxic and persistent anionic pollutants. The controlled generation of cationic frameworks, however, lags behind the abundant design strategies devised to control the structures and topologies of neutral frameworks. In this regard, we report a rational approach that allows the conversion of the synthetic approach toward constructing a neutral framework into one allowing for the synthesis of a cationic one without incurring any changes to the overall topology or the selected metal ion. We demonstrate that the replacement of a functional group on an organic linker that promotes a similar coordination mode, but bearing one less negative charge, can yield the systematic generation of cationic frameworks. Moreover, we confirm the cationic nature of the metal-organic frameworks through preliminary anion-exchange experiments and propose a method to retain permanent porosity in cationic frameworks through the use of strongly binding anions. Altogether, these results show great promise for the construction of tunable nanoporous frameworks capable of carrying out anion-exchange processes.

Construction of a cationic organic network for highly efficient removal of anionic contaminants from water

A new cationic organic network is constructed by a facile method from commercially available precursors, and exhibits high efficiency for removal of water contaminants.

Ultra-highly selective trapping of perrhenate/pertechnetate by a flexible cationic coordination framework

This work presents a 3D cationic coordination framework, showing ultra-highly selective trapping of perrhenate/pertechnetate in the presence of sulfate 20 000 times in excess.