Crystal structure control of aluminized clay minerals on the mobility of caesium in contaminated soil environments

How accompanying ions affect soil heavy metal removal by polyepoxysuccinic acid during washing?

Soil washing with green eluent is an efficient technique to remediate heavy metal contaminated farmland. In addition to eluent, less is known about the roles of accompanying ions on heavy metal removal. We investigated the effects of accompanying ions including Fe3+, Mn2+, Ca2+, Mg2+, Al3+, Si4+ and PO43- on the desorption of Pb2+ and Cd2+ in paddy and arid soils using ethylenediaminetetraacetic acid and polyepoxysuccinic acid as eluents. The release rates of target and accompanying ions showed significant fast and slow reaction stages based on corresponding analysis and kinetic models. In fast reaction stage, Pb2+ and Cd2+ performed geochemical analogy with Ca2+, Mg2+ and PO43-. The release curves of these ions were fitted well with Elovich model, indicating that they released from oxysalt surface into solution via ion exchange, and dissolution of Fe/Mn/Al/Si (hydr)oxides through H+- and ligand-promoted dissolution. In slow reaction stage, Pb2+ and Cd2+ were related to Fe3+, Mn2+, Al3+ and Si4+, which were controlled by intraparticle diffusion process. H+ slowly diffused into interlayer of phyllosilicates to displace target and accompanying ions by ion exchange. Therefore, this research filled the gap of accompanying ions driving the release behavior of heavy metal ions during leaching.

Comparing Cs+ binding affinity of Keggin type polyoxometalate and sodium Tetrakis(4-florophenyl)borate in solution and from Cs-doped pure phase vermiculite

We assessed the aptitude of cesium (Cs⁺) binding by Keggin type polyoxometalates (POMs) and compared the results with the Cs⁺ binding by sodium tetrakis(4-fluorophenyl)-borate (Na-TFPB). In this work, we aimed to establish a system to treat radioactive Cs⁺ contaminated soil with POMs economically. We evaluated the effect of initial Cs⁺ concentration (0.1M) and precipitant (POMs and TFPB) concentrations (0.01M) on Cs⁺ precipitation. Our comparison of Cs⁺ precipitation by three different POMs and TFPB was obtained by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). We synthesized POMs molybdovanadophosphoric acid, H₅PMo₁₀V₂O₄₀ (MVPA), and silicotungstic acid, H₄SiW₁₂O₄₀ (STA), and used commercially available phosphotungstic acid, H₃PW₁₂O₄₀ (PTA), and TFPB. Cs-doped pure phase vermiculite was also used to demonstrate the extraction potential of Cs⁺ by TFPB, STA, and PTA. All the POMs and corresponding Cs-bound POMs were characterized by UV-visible spectroscopy, Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray powder diffraction (XRD). In this simulation study, we demonstrated that the Cs⁺ removal by POMs is much more effective than TFPB and could be a promising method for the treatment of radiocesium contaminated soil.

Cs selectivity and adsorption reversibility on Ca-illite and Ca-vermiculite

For understanding and predicting the Cs behavior in soils and groundwater, Cs adsorption properties of illite and vermiculite were investigated under various pH conditions and Cs concentrations. Cs adsorption and desorption experiments have been conducted with Ca-homoionic illite and Ca-vermiculite in CaCl₂ solution with an ionic strength of 0.03 and of 0.06 mol.L^-1 respectively, by focusing on cation exchanges between Cs, proton and calcium at thermodynamic equilibrium. Ca-illite displayed more affinity for Cs than Ca-vermiculite. Cs adsorption was non-linear for both clay minerals and a multi-site exchange model approach was adopted to interpret and model adsorption isotherms. Each mineral reactivity was described by their sorption site properties expressed by their exchange capacity and ionic selectivity. High-selective and low-capacity sites were shown to control Cs uptake at concentrations below 10^-8-10^-7 mol.L^-1 for both Ca-illite and Ca-vermiculite. Three high-capacity sites dominated Cs adsorption at higher concentrations. Cs adsorption reversibility was demonstrated for illite at Ca concentrations ranging from 5 10^-2 to 10^-9 mol.L^-1. The partial irreversibility of Cs adsorption onto vermiculite at Cs concentrations greater than 10^-5 mol.L^-1 was related to interlayer collapse. Reversible adsorption may occur as long as the interlayer space stays open. The irreversible adsorbed fraction was quantified and taken into account in the modeling approach to calculate the selectivity coefficient of each site.

In situ investigation of water on MXene interfaces

A continuum of water populations can exist in nanoscale layered materials, which impacts transport phenomena relevant for separation, adsorption, and charge storage processes. Quantification and direct interrogation of water structure and organization are important in order to design materials with molecular-level control for emerging energy and water applications. Through combining molecular simulations with ambient-pressure X-ray photoelectron spectroscopy, X-ray diffraction, and diffuse reflectance infrared Fourier transform spectroscopy, we directly probe hydration mechanisms at confined and nonconfined regions in nanolayered transition-metal carbide materials. Hydrophobic (K+) cations decrease water mobility within the confined interlayer and accelerate water removal at nonconfined surfaces. Hydrophilic cations (Li+) increase water mobility within the confined interlayer and decrease water-removal rates at nonconfined surfaces. Solutes, rather than the surface terminating groups, are shown to be more impactful on the kinetics of water adsorption and desorption. Calculations from grand canonical molecular dynamics demonstrate that hydrophilic cations (Li+) actively aid in water adsorption at MXene interfaces. In contrast, hydrophobic cations (K+) weakly interact with water, leading to higher degrees of water ordering (orientation) and faster removal at elevated temperatures.

Transport behaviors of Cs+ in granite porous media: Effects of mineral composition, HA, and coexisting cations

The transport of radiocesium (RCs) in granite has attracted great concerns for the consideration of a long-term safety assessment and performance evaluation of the nuclear waste disposal repository. In this study, the transport behaviors of Cs⁺ in granite were addressed and quantified by column experiments, sequential extraction, and a convection-dispersion equation model. The transport of Cs⁺ in granite experienced at least two stages including a rapid increase and a slow increase stages. The retardation of Cs⁺ in granite obviously became higher as biotite content increased. However, a consistent breakthrough plateau and almost overlapped breakthrough curves were observed under different feldspar contents, which suggested that the transport behaviors of Cs⁺ in granite was quite close to feldspar. Compared to Na⁺, K⁺ could effectively inhibit Cs⁺ adsorption and facilitate the mobility of Cs⁺ in granite column. In the presence of Sr²⁺, the transport of Cs⁺ was provoked in the granite column mainly due to the high competition effects. Humic acid (HA) did not obviously change the transport behaviors of Cs⁺ in granite column; however, HA could weakly change the adsorption species of Cs⁺ during Cs⁺ transport in granitic media. Both sequential extraction and two-site non-equilibrium model suggested that feldspar was the main contributor to the weak adsorption sites and biotite was responsible for the strong affinity sites for Cs⁺ in Beishan granite. The findings could provide important insights into RCs transport and fate in granitic media.

Constraining the preservation of organic compounds in Mars analog nontronites after exposure to acid and alkaline fluids

The presence of organic matter in lacustrine mudstone sediments at Gale crater was revealed by the Mars Science Laboratory Curiosity rover, which also identified smectite clay minerals. Analogue experiments on phyllosilicates formed under low temperature aqueous conditons have illustrated that these are excellent reservoirs to host organic compounds against the harsh surface conditions of Mars. Here, we evaluate whether the capacity of smectites to preserve organic compounds can be influenced by a short exposure to different diagenetic fluids. We analyzed the stability of glycine embedded within nontronite samples previously exposed to either acidic or alkaline fluids (hereafter referred to as “treated nontronites”) under Mars-like surface conditions. Analyses performed using multiple techniques showed higher photodegradation of glycine in the acid-treated nontronite, triggered by decarboxylation and deamination processes. In constrast, our experiments showed that glycine molecules were preferably incorporated by ion exchange in the interlayer region of the alkali-treated nontronite, conferring them a better protection against the external conditions. Our results demonstrate that smectite previously exposed to fluids with different pH values influences how glycine is adsorbed into their interlayer regions, affecting their potential for preservation of organic compounds under contemporary Mars surface conditions.

Effects of NH4+, K+, Mg2+, and Ca2+ on the Cesium Adsorption/Desorption in Binding Sites of Vermiculitized Biotite

The reversibility of cesium adsorption in contaminated soil is largely dependent on its interaction with micaceous minerals, which may be greatly influenced by various cations. Herein, we systematically investigated the effects of NH₄⁺, K⁺, Mg²⁺, and Ca²⁺ on the adsorption/desorption of Cs⁺ into different binding sites of vermiculitized biotite (VB). Original VB was initially saturated by NH₄⁺, K⁺, or Mg²⁺; we then evaluated the adsorption of Cs⁺ on three treated VBs, and the desorption by extraction with NH₄⁺, K⁺, Mg²⁺, or Ca²⁺ was further evaluated. Our structural analysis and Cs⁺ extractability determinations showed that NH₄⁺ and K⁺ both collapsed the interlayers of VB, resulting in the dominant adsorption of Cs⁺ to external surface sites on which Cs⁺ was readily extracted by NH₄⁺, K⁺, Mg²⁺, or Ca²⁺ irrespective of their species, whereas Mg²⁺ maintained the VB with expanded interlayers, leading to the overwhelming adsorption of Cs⁺ in collapsed interlayer sites on which the Cs⁺ desorption was difficult and varied significantly by the cations used in extraction. The order of Cs⁺ extraction ability from the collapsed interlayers was K⁺ ≫ Mg²⁺ ≈ Ca²⁺ ≫ NH₄⁺. These results could provide important insights into Cs migration in soil and its decontamination for soil remediation.