Effects of pH, ionic strength, temperature, and humic acid on Eu(III) sorption onto iron oxides

Adsorption of U(VI) on the natural soil around a very low-level waste repository

The behaviors of U(VI) in environmental media around radioactive waste disposal site are important for safety assessment of geological repositories. However, the estimation of environmental behaviors of U(VI) in natural media was insufficient. This work aimed to determine the adsorption of U(VI) on natural soil surrounding a candidate very low-level radioactive waste (VLLW) disposal site in southwest China. Results showed that the adsorption process of U(VI) on soils could be well supported by pseudo-second-order kinetic and Freundlich model. The adsorption of U(VI) was pH-dependent but temperature-independent. High ionic strength (NaCl) strongly affected the adsorption process at low pH (2.0-5.5). CO₃^2- remarkably inhibited the U(VI) adsorption, while the adsorption of U(VI) was promoted by PO₄^3- and SO₄^2-. Naturally occurred soil organic matters (SOMs) showed high affinity for U(VI), while the presence of additional humic acid (HA) strongly inhibited U(VI) adsorption. The occurrence of ferrous iron could result in the reduction of U(VI) at low pH values (pH < 4), leading to the promotion of immobilization of U(VI). These findings would provide some guidance for the safety assessments of the VLLW disposal as well as the remediation of contaminated soil.

Plausible Emergence of Biochemistry in Enceladus Based on Chemobrionics

Saturn's satellite Enceladus is proposed to have a soda-type subsurface ocean with temperature able to support life and an iron ore-based core. Here, it was demonstrated that ocean chemistry related to Enceladus can support the development of Fe-based hydrothermal vents, one of the places suggested to be the cradle of life. The Fe-based chemical gardens were characterized with Fourier-transform (FT)IR spectroscopy and XRD. The developed chemobrionic structures catalyzed the condensation polymerization of simple organic prebiotic molecules to kerogens. Further, they could passively catalyze the condensation of the prebiotic molecule formamide to larger polymers, suggesting that elementary biochemical precursors could have emerged in Enceladus.

Magnetite nanoparticles with aminomethylenephosphonic groups: synthesis, characterization and uptake of europium(III) ions from aqueous media

Two adsorbents with covalently bound aminomethylenephosphonic acid functions (and referred to as MNPs/AMPA and MNPs/SiO₂-AMPA) were synthesized from two types of amino-functionalized magnetic nanoparticles (MNPs) via Moedritzer-Irani reaction. The sorbents with anchored dopamine ligand (MNPs/dopa) or aminopropyl groups (MNPs/SiO₂-NH₂), and the MNPs/AMPA were characterized by X-ray diffraction, FTIR, transmission electron microscopy and vibrating sample magnetometry. Surface modification does not adversely impact the physical properties of the starting magnetite. Compared to the size of the unmodified Fe₃O₄ (magnetite) nanoparticles (7-12 nm), the average size of functionalized nanoparticles is increased to 10-16 nm. Similarly, the magnetic saturation decreased from 67.5 emu g^-1 to 42.0 emu g^-1, and the surface area is increased up to 205 m² g^-1 for MNPs/SiO₂-AMPA. The kinetics of the adsorption of Eu(III) on the sorbent is ultra-fast, and equilibria are attained within 5-10 min at room temperature. The adsorption kinetics can be described by a pseudo-second-order model. Adsorption and desorption conditions were tested with respect to the removal of Eu(III) ions from water solution. The adsorption capacities for Eu(III) at pH 7.0 are 77 mg g^-1 and 69 mg g^-1 for MNPs/AMPA and MNPs/SiO2-AMPA nanoparticles, respectively. Eu(III) was quantified by ICP-MS. The limit of detection (LOD) for Eu(III) is 0.05 ng L^-1 (based on the 3σ criterion), with an enrichment factor of 150. The selectivity over ions such as Tb(III), Fe(III), Zn(II), Cu(II), and Ca(II) ions was studied. Under optimal condition the distribution coefficient for Eu(III) relative to these ions is near 10⁵ mL g^-1. The sorbents can be easily retrieved from even large volumes of aqueous solutions by magnetic separations. The method was tested for spiked water samples (with recoveries from 96.6-102.5%) and for rock minerals. Graphical abstract A schematic showing the regeneration of magnetite nanoparticles (MNPs), core-shell (MNPs/SiO₂), and the structures with covalently bonded aminomethylenephosphonic acid (AMPA) after preconcentration of Eu(III) from largewater sample volumes onto a small specimen.

Sorption of Eu(III) on Fe–montmorillonite relevant to geological disposal of HLW

Montmorillonite (Mt) is the major clay mineral of bentonite, which is the candidate buffer material in the engineered barrier system for geological disposal of high level waste (HLW). The alteration of Mt due to its interaction with carbon steel (overpack) can produce Fe–Mt. In order to understand the basic properties of Fe–Mt, the sorption studies using Eu(III) are reported here. For this, Fe(III)–Mt was prepared by conventional cation exchange method using FeCl3 with Na–Mt. The obtained Fe(III)–Mt was then reduced to Fe(II)–Mt using ascorbic acid. Both the samples were characterized based on their X-ray diffraction, Fourier transform infrared spectra, cation exchange capacity and specific surface area. The batch sorption studies of Eu(III) were conducted for both Fe(III)–Mt and Fe(II)–Mt as a function of pH (3–10), ionic strength (0.001 M–1 M) and Eu(III) concentration (10−8–10−3 M). The distribution coefficient (Kd) was found to be higher for Fe(III)–Mt compared to Fe(II)–Mt and Na–Mt. The sudden increase in sorption in the pH range 4.5–6 and remaining constant beyond it indicates ion exchange mechanism at pH<4.5, with surface complexation mechanism dominating the sorption at pH>4.5. This is further corroborated by ionic strength dependent sorption data which shows decrease in sorption capacity of Fe–Mt with increasing ionic strength at low pH, but remaining more or less unchanged at higher pH. Eu(III) adsorption isotherm on Fe–Mt increased linearly with [Eu(III)] reaching saturation at 10−5 M and 10−4 M for Fe(III)–Mt and Fe(II)–Mt, respectively. The amount of iron released from Fe–Mt and Fe(II)/Fetotal during sorption were estimated to understand the effect on Eu(III) sorption behaviour by release of interlayer iron in Fe–Mt.

Sorption of Eu(III) at feldspar/water interface: effects of pH, organic matter, counter ions, and temperature

The sorption of Eu(III) on potassium feldspar (K-feldspar) was studied under various physicochemical conditions such as pH, temperature, counter ions and organic matter. The results showed that the sorption of Eu(III) on K-feldspar significantly increased with the increase of pH, and high Eu(III) concentration can inhibit such immobility to some extent. The presence of humic acid (HA) can increase the sorption of Eu(III) on K-feldspar in low pH range; while inhibit to a large extent under alkaline conditions. It is very interesting that at pH ~6.5, high ionic strength can promote the sorption of Eu(III) on K-feldspar in the presence of HA. In contrast, Eu(III) sorption was restricted obviously by NaCl in the absence of HA. The sorption procedure was involved with ion exchange and/or outer-sphere complexation as well as inner-sphere complexation. The presence of F− and PO4 3− dramatically enhanced Eu(III) sorption on K-feldspar, whereas both SO4 2− and CO3 2− had negative effects on Eu(III) sorption. X-ray photoelectron spectroscopy analysis indicated that Eu(III) tended to form hydrolysates at high initial concentration (3×10−4 mol/L) and high temperature (338 K).

Removal of U(VI) from aqueous solution using TiO2 modified β-zeolite

β-Zeolite was synthesized and modified with TiO2. The synthesized materials were characterized and used for removal of U(VI) from aqueous solutions. The influences of pH, contact time and temperature on U(VI) adsorption onto modified β-zeolite by TiO2 were studied by batch technique, and XPS was employed to analysed the experimental data. The dynamic process showed that the adsorption of U(VI) onto TiO2/β-zeolite matched the pseudo-second-order kinetics model, and the adsorption of U(VI) were significantly dependent on pH values. Through simulating the adsorption isotherms by Langmuir, Freundlich and Dubini–Radushkevich (D–R) models, it could be seen, respectively that the adsorption patterns of U(VI) onto TiO2/β-zeolite were mainly controlled by surface complexation, and the adsorption processes were endothermic and spontaneous. The modification of β-zeolite by TiO2 it shows a novel material for the removing of U(VI) from water environment for industrialized application.

Kinetic study for copper adsorption onto soil minerals in the absence and presence of humic acid

Equilibrium and kinetics of Cu(2+) adsorption onto soil minerals (kaolinite and hematite) in the absence and presence of humic acid have been investigated under various conditions. The influences of ionic strength, pH and solution cations on the rate of the adsorption have been studied. The rate and the amount of adsorbed Cu(2+) onto soil minerals in the absence or the presence of humic acid increased with decreasing ionic strength, increasing pH and in the presence of the background electrolyte K(+) rather than Ca(2+). Humic acid enhanced the rate and the amount of adsorbed Cu(2+) onto soil minerals. The adsorption equilibrium data showed that adsorption behavior of Cu(2+) could be described more reasonably by Langmiur adsorption isotherm than Freundlich isotherm in the absence or presence of humic acid. Pseudo first and pseudo second order models were used to evaluate the kinetic data and the rate constants. The results indicated that the adsorption of Cu(2+) onto hematite and kaolinite in the absence and presence of humic acid is more conforming to pseudo second order kinetics.

The immobilization of U(vi) on iron oxyhydroxides under various physicochemical conditions

The immobilization of U(vi) at the solid-water interface is an important process affecting its transportation and migration in the environment, and is predominantly controlled by the sorption behavior of U(vi). In this study, U(vi) sorption on Fe(ii) and Fe(iii) oxyhydroxides prepared by a coprecipitation method was studied under a range of physicochemical conditions, including pH, ionic strength, presence of humic acid (HA) and temperature. The results showed that the sorption of U(vi) on iron oxyhydroxides is chemical, and that the principal rate limitation is due to intraparticle diffusion. The sorption of U(vi) on iron oxyhydroxides is strongly dependent on pH, but only weakly dependent on ionic strength through the entire pH range studied. Under acidic conditions, the presence of HA increases U(vi) sorption to a large degree, but an inhibiting effect on the sorption of U(vi) can be observed under alkaline conditions, due to the formation of soluble U(vi)-HA complexes. The sorption of U(vi) on iron oxyhydroxides is an endothermic process and favors high temperatures. The surface complexation model suggests three dominant monodentate inner-sphere complexes of [triple bond, length as m-dash]Fe(s)OUO2(+) (log K = 1.65), [triple bond, length as m-dash]Fe(w)OUO2OH(0) (log K = -8.00), and [triple bond, length as m-dash]Fe(w)OUO2(CO3)2(3-) (log K = 17.50), contributing to U(vi) sorption on iron oxyhydroxides over the entire observed pH range.