Kx[Bi4–xMnxS6], Design of a Highly Selective Ion Exchange Material and Direct Gap 2D Semiconductor

Leaching of structural Ca2+ ions from a chalcogenide adsorbent by H+ lifts Cs(I) uptake

Acidic wastewater containing radioactive ¹³⁷Cs is difficult to treat by selective adsorption. Abundant H⁺ under acidic conditions damages the structure of adsorbents and competes with Cs⁺ for adsorption sites. Herein, we designed a novel layered calcium thiostannate (KCaSnS) that contains Ca²⁺ as a dopant. The dopant Ca²⁺ ion is metastable and larger than the ions attempted before. The pristine KCaSnS demonstrated a high Cs⁺ adsorption capacity of 620 mg/g at 8250 mg/L Cs⁺ solution and pH 2, which is 68% higher than that at pH 5.5 (370 mg/g), a trend opposite to all previous studies. The neutral condition allowed the release of Ca²⁺ present only in the interlayer (∼20%); whereas the high acidity facilitated the leaching of Ca²⁺ from the backbone structure (∼80%). The complete structural Ca²⁺ leaching was made possible only by a synergistic interaction of highly concentrated H⁺ and Cs⁺. Doping a large enough ion, such as Ca²⁺, to accommodate Cs⁺ into the Sn-S matrix upon its liberation opens a new way of designing high-performance adsorbents.

Amino-modified chitosan/gold tailings composite for selective and highly efficient removal of lead and cadmium from wastewater

In this work, a novel amino-modified chitosan/tailings composite (CS-PEI-nGT) was successfully synthesized from gold tailings particle treated by ball milling (nGT), chitosan (CS) and polyethyleneimine (PEI) as raw materials, for Lead (Pb(Ⅱ)) and Cadmium (Cd(Ⅱ)) removal from aqueous solutions. The CS-PEI-nGT was characterized by using FTIR, XRD, SEM, BET, TGA and XPS techniques. The results showed that CS-PEI-nGT had maximum adsorption capacity of 192.78 mg·g^-1 and 99.46 mg·g^-1 for Pb(Ⅱ) and Cd(Ⅱ) respectively at pH 5. The adsorption kinetics was described well by pseudo-second-order kinetic adsorption model, and suggested that chemisorption as the rate-controlling step for adsorption of Pb(Ⅱ) and Cd(Ⅱ). The isotherm data was accurately explained by Langmuir model with higher correlation coefficient (R²) of 0.9911 and 0.9642 for Pb(Ⅱ) and Cd(Ⅱ) respectively. In addition, CS-PEI-nGT retained its selective adsorption capacity for Pb(Ⅱ) and Cd(Ⅱ), compared to other metals such as Zn(Ⅱ), Mn(Ⅱ), Mg(Ⅱ) and Al(Ⅲ). The mechanism of the adsorption was investigated and the results revealed that amino (-NH₂), silicon oxide groups (Si-O) and hydroxyl (-OH) functional groups on composite surface were accountable for metals adsorption, suggesting surface complexation, electrostatic interactions and ion exchange. Our work presents a promising strategy for tailings recycling and highly efficient removal of toxic metals ions from wastewater.

Highly selective cesium removal under acidic and alkaline conditions using a novel potassium aluminum thiostannate

The pH values of nuclear wastewater are extremely low or high, which make the efficient removal of ¹³⁷Cs a major concern among the issues for safety management and environmental remediation. Existing metal sulfides for Cs⁺ adsorption have shown poor performance at acidic and alkaline conditions, and the reason has not been revealed yet. Herein, a novel potassium aluminum thiostannate (KAlSnS-3) adsorbent was designed and its Cs⁺ adsorption mechanism over a wide pH range was investigated. We hypothesized that Al³⁺ dopant on Sn⁴⁺ sites would allow stable adsorption for Cs⁺ upon its partial release at acidic and alkaline conditions. As a result, KAlSnS-3 demonstrated excellent adsorption performance across a broad pH range (1-13), and high selectivity toward Cs⁺, even under high salinity conditions (in tap water K_d = 3.12 × 10⁴ mL/g; and in artificial seawater K_d = 3.42 × 10³ mL/g). KAlSnS-3 also exhibited rapid adsorption kinetics (R = 97.6% in the first minute), a remarkable adsorption capacity (259.31 mg/g), and a high distribution coefficient (2.09 × 10⁵ mL/g) toward Cs⁺. In addition, the high reusability of KAlSnS-3 was observed, suggesting its potential for real-world applications. The mechanism for enhancing performance at low and high pH values was discussed with the evidence of crystallinity, elemental concentrations, and binding energy of electrons based on the concept of electrostatic interactions and chemical affinity. In summary, this work provides insights into the mechanism of Cs⁺ removal under a wide pH range, and the impressive Cs⁺ adsorption performance indicates the application potential of KAlSnS-3 in wastewater treatment.

Removal of Cd2+ from water containing Ca2+ and Mg2+ using titanate nanotubes modified by carbon

Calcium and magnesium ions usually exist in natural water. When Cd²⁺ is removed from water by adsorption, it will be inhibited by these two ions. Titanate nanotubes (TNTs) have an effective adsorption capacity for Cd²⁺ due to extraordinary ion exchange property. However, TNTs also adsorb Ca²⁺ and Mg²⁺ in water. In this study, carbon-modified TNT (TNT/C) and TNT/C further treated with acid (TNT/HC) were synthesized by hydrothermal synthesis. The transmission electron microscope (TEM) images show that TNT/C or TNT/HC still keep nanotube morphology. The experimental results show the order of adsorption amount to Cd²⁺ is TNT (171.56 mg/g) > TNT/C (166 mg/g) > TNT/HC (159.88 mg/g) when there is no Ca²⁺ or Mg²⁺. But when there is 0.1 M Ca²⁺ or Mg²⁺ in the water, the order of Cd²⁺ adsorption capacity becomes TNT/HC (44.28, 49.04 mg/g) > TNT/C (58.84, 69.32 mg/g) > TNT (65.52, 70.6 mg/g). It indicates that the surface carbon modification can alleviate the hindrance of Ca²⁺ or Mg²⁺ to Cd²⁺ removal. This is because the carbon on the surface of TNT captured part of Ca²⁺ or Mg²⁺; it made more Cd²⁺ be successfully absorbed by TNT through ion exchange. This mechanism was confirmed by the X-ray photoelectron spectroscopy (XPS) spectra analysis. The results of this paper can provide ideas for the adsorption and removal of Cd²⁺ in water in the presence of Ca²⁺ or Mg²⁺.

MoSe2/CdSe Heterojunction Destruction by Cation Exchange for Photoelectrochemical Immunoassays with a Controlled-Release Strategy

Herein, a split-type immunoassay strategy instigated by cation exchange (CE) and changing the capacity of an electron donor in an electrolyte solution is optimized, namely, for differentiating the biological-specific binding assay and photoelectrochemical (PEC) analysis. MoSe₂/CdSe, a Z-scheme heterojunction with efficient visible light absorption and a low recombination of carriers, is used as a photoelectrode substrate. Silver ions (Ag⁺) as the initiator of CE are generated by the acidolysis of evenly loaded silver nanoparticles on mesoporous silica nanospheres (MSNs). The theoretical calculation and experimental results confirm that Ag⁺ replaces Cd²⁺ in CdSe and retains the crystal structure of MoSe₂. However, this behavior destroys the perfectly matched heterojunction structure and introduces defects, which led to the reduction of the photocurrent response. In addition, ascorbate oxidase in combination with MSNs can be used as a consumptive agent of the electron donor, which further improves the sensitivity and reliability of the sensor. As a proof of principle, neuron-specific enolase was applied to elucidate the potential application of the PEC immunoassay in clinical diagnosis, and the obtained linear range of the sensor was from 0.0001 to 100 ng/mL with a detection limit of 28 fg/mL (S/N = 3).

Fe-Mn Oxides Based Multifunctional Adsorptive/Electrosensing Nanoplatforms: Dynamic Site Rearrangement for Metal Ion Selectivity

Achieving structural requirements for the exclusive selectivity of adsorbent to a specific metal remains challenging, as certain metal ions show similar adsorptive behaviors and preference toward a given site. We reported the morphology and oxidation state-dependent selectivity manipulating of layered oxides by controlling the dynamic evolution of different adsorptive sites. The computational investigation predicted the site-specific partitioning trends of metal ions at two sites of manganese oxide (MnO₂) layers: the lateral edge sites (LESs) and octahedral vacancy sites (OVSs). In contrast to the predominant occupation of the OVSs for other metal ions, the binding of lead (Pb) ions was energetically favored at both the sites. We assembled ultrathin MnO₂ nanosheets on the magnetic iron oxides to first enhance the accessibility of the LESs. A sequential ligand-promoted partial reduction of the atomic MnO₂ layers induced the edge-to-interlayer migration of Mn atoms to block the nonspecific OVSs and activate the LESs, enabling a superior selectivity to Pb. In addition, the iron oxides helped construct a multifunctional adsorptive/electrosensing platform for Pb regarding their facile magnetic separation and electrochemical activity. Simultaneous selective adsorption and on-site monitoring of Pb(II) were achieved on this nanoplatform, owing to its satisfactory stability and sensitivity without an obvious matrix effect.

Rapid and selective removal of Cs+ from water by layered potassium antimony thiostannate

¹³⁷Cs is radioactive and highly hazardous to human health and the environment and its efficient removal from water is still challenging. In this study, potassium antimony tin sulfide (KATS-2) was synthesized using a hydrothermal method and utilized for the first time for cesium removal from water. KATS-2 showed a high maximum ion exchange capacity (358 mg g^-1) and distribution coefficient (1.59 × 10⁵ mL g^-1) toward Cs⁺. In particular, KATS-2 showed rapid ion exchange kinetics and reached the adsorption equilibrium within 5 min with 99% removal efficiency. The adsorption was good at a wide active pH range (1-12) even in extreme alkaline conditions (K_d = 3.26 × 10⁴ mL g^-1 at pH 12). The effect of coexisting ions was also investigated, and a high selectivity toward Cs⁺ was maintained even in artificial seawater (K_d = 3.28 × 10³ mL g^-1). Powder X-ray diffraction and thermogravimetric analysis demonstrated that KATS-2 was chemically and thermally stable. The results showed that owing to its excellent adsorption performance as well as chemical and thermal stability, KATS-2 is a promising adsorbent for Cs⁺ removal from contaminated water.

Enhanced removal of zinc and cadmium from water using carboxymethyl cellulose-bridged chlorapatite nanoparticles

Zinc (Zn²⁺) and cadmium (Cd²⁺) in water pose serious threats to human health and the environment. In search for a more effective treatment technology, we prepared a type of carboxymethyl cellulose (CMC) bridged chlorapatite (CMC-CAP) nanoparticles and tested the material for removal of Zn²⁺ and Cd²⁺ from water. CMC macromolecules were attached to CAP by bidentate bridging and hydrogen bonding, preserving the high adsorption capacity of CAP nanoparticles while allowing for easy gravity-separation of the nanoparticles. CMC-CAP showed rapid adsorption kinetics and 22.8% and 11.2% higher equilibrium uptake for Zn²⁺ and Cd²⁺, respectively, than pristine CAP. An extended dual-mode isotherm model, which takes into account both sorption and chemical precipitation, provided the best fits to the sorption isotherms, giving a maximum Langmuir sorption capacity of 141.1 mg g^-1 for Zn²⁺ and 150.2 mg g^-1 for Cd²⁺ by CMC-CAP. Na⁺ at up to 5 mM showed modest effects on the uptake of the heavy metals, while 2-5 mM of Ca²⁺ exerted notable inhibitive effects. Dissolved organic matter (up to 5 mg L^-1 as TOC) inhibited the Zn²⁺ uptake by 16.5% but enhanced the Cd²⁺ removal by 8.6%. Material characterizations and surface binding analyses revealed that ion exchange, surface precipitation, and surface complexation were the removal mechanisms for the heavy metals. This study demonstrates stabilizer bridging may serve as a convenient strategy to facilitate water treatment uses of nanoparticles.

Influence of the NiII/MnII ratio on the physical properties of heterometallic Ni2xMn(2−2x)P2S6 phases and potassium intercalates K0.8Ni2xMn(1.6−2x)P2S6·2H2O

Physical properties of bimetallic Ni^II/Mn^II phases obtained by microwave assisted reaction.

The solvothermal synthesis and characterization of quaternary arsenic chalcogenides CsTMAsQ3 (TM = Hg, Cd; Q = S, Se) using Cs+ as a structure directing agent: from 1D anionic chains to 2D anionic layers

Four new quaternary transition metal arsenic chalcogenide Cs-TM-As-Q compounds (TM = Hg, Cd; Q = S, Se) were synthesized using different mixed solvents. A 1,4-diaminobutane(1,4-dab)/water mixed solvent system was used to solvothermally synthesize the selenoarsenates CsTMAsSe₃ (TM = Hg (1), Cd (2)). In 1, the eight-membered ring anion chain consists of trigonal-planar [HgSe₃] and trigonal-pyramid [AsSe₃]. Compound 2 is similar to the eight-membered ring chain anion of compound 1, and [CdAsSe₃]^- further joins, through μ₃-Se and Cd, to form the layer anions [CdAsSe₃]^-. A 2-diaminopropane (1,2-dap) and water mixed solvent system was used to synthesize two thioarsenates, Cs₂TM₂As₂S₆ (TM = Cd (3), Hg (4)). Compounds 3, 4 and 2 are isostructural; the Cd and Hg atoms are four coordinated. Compounds 1-4 utilize Cs⁺ cations as a structural directing agent. Finally, the structural and optical properties of the novel series of arsenic chalcogenides were characterized.

Renewable P-type zeolite for superior absorption of heavy metals: Isotherms, kinetics, and mechanism

Zeolite is a characteristic material for removing heavy metals exhibiting by low tolerance quantities. It is particularly desirable although challenging to cultivate an unmodified and reusable zeolite for eradicating heavy metals with great capacity. Herein, we sought out and firstly synthesized the uniform octahedral zeolite Na₆Al₆Si₁₀O₃₂·12H₂O for heavy metal ions trap, proven extraordinarily effective decontamination of M²⁺ (Pb²⁺, Cd²⁺, Cu²⁺, and Zn²⁺). The maximum capacities of Pb²⁺, Cd²⁺, Cu²⁺, and Zn²⁺ were 649, 210, 90 and 88 mg/g, and the distribution coefficients (K_d) was ~10⁸ mL/g for Pb²⁺ which emphasized the superior effectiveness of Na₆Al₆Si₁₀O₃₂·12H₂O contrasted with other zeolites. Rapid adsorption was observed that Pb²⁺ concentration (7.5 ppm) was reduced to 0.6 ppb in 2 min. The removal mechanism was ascribed to the ion exchange and hydroxyl groups thereby affording high adsorption capacity. We also investigated the heavy metal removal of zeolite 13X and 4A for comparison and concluded the determining factor affecting absorption capacity. The removal rate of Pb remained at 97% even after five regeneration recycles. The zeolite was therefore promising for practical water purification and industrialization.