Ultrastrong Anion Affinity of Anionic Clay Induced by Its Inherent Nanoconfinement.

Layered double hydroxide (LDH), the only anionic clay in the environment, plays a key role in natural ion transportation. The ion retention effect of LDHs was traditionally attributed to ion exchange with low affinity. Here, we demonstrated an ultrastrong interaction between anions and LDHs induced by their inherent nanoconfinement using chromium ore processing residue (COPR) that contained several Cr(VI)-bonded LDHs as a probe. Hydrocalumite (Ca/Al-Cl LDH) was verified as the primary phase for Cr(VI) retention through two types of interactions such as ion exchange and Cr-Ca coordination. More significantly, the confined spacing between two layers of hydrocalumite provided spatial restriction and shielding effects to the intercalated Cr(VI), which enhanced Cr-Ca coordination by shortening the bonding distance and modulating the binding angle to achieve the lowest bonding energy. Such enhancement boosted Cr(VI) affinity up to 3.2 × 105 mL/g, which was 1-3 orders of magnitudes higher than ion exchange. The universality of this mechanism was verified using another Mg/Al-Cl LDH and various anions. This study broke the traditional awareness of low ion affinities of LDHs limited by single ion exchange and disclosed an essential mechanism for unexpected ion retention effects of anionic clays in nature.

Effective capture of aqueous uranium from saline lake with magnesium-based binary and ternary layered double hydroxides.

Uranium in saline lake brine is a nuclear resource that attracts worldwide attention. Relatively low concentrations (about 0.2 mg L-1 to 30 mg L-1) require high affinity for the capture materials. In this paper, magnesium binary layered double hydroxides (MgAl-LDH) and its Fe-induced ternary LDH (MgAlFe-LDH) were synthesized for the extraction of simulated concentrations of U(VI) in the saline lake brine system. Batch experiments have shown that both LDHs have strong affinity towards uranium. MgAl-LDH yielded of stronger affinity in lower U(VI) concentrations (0.2 mg L-1 to 5 mg L-1), while MgAlFe-LDH was at higher U(VI) concentrations (5 mg L-1 to 30 mg L-1). For current uranium extraction, the affinities of MgAl-LDH and MgAlFe-LDH are more than twice the maximum affinity of other LDHs and LDHs-based materials. Therefore, these two LDHs are suitable for U(VI) extraction with different concentration levels in saline lakes. The capture process followed the pseudo-second-order kinetics with fast adsorption speed, and the coexisting cations have little effect on the extraction rate. Research through X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) showed the main adsorption mechanisms are surface complexation and the interlayer carbonate coprecipitation. This work provides a potential method for U(VI) extraction while reusing the waste magnesium resources in saline lake.

Enhanced adsorption of arsenate by spinel zinc ferrite nano particles: Effect of zinc content and site occupation.

In this work, zinc ferrite spinel with different zinc contents (ZnxFe3-xO4) was synthesized by a hydrothermal method and used for removing As(V) in aqueous solution. X-ray diffraction (XRD) results indicated that in the crystal structure of ZnxFe3-xO4, the zinc atoms tended to occupy the octahedral sites for x < 0.6 and diffused into the tetrahedral sites gradually with x > 0.6. The size of ZnxFe3-xO4 crystallites increased with the increasing zinc content. Batch adsorption experiments showed that the adsorption isotherms could be well described by the Langmuir model, while the adsorption kinetics followed the pseudo-second-order kinetic model. Zinc ferrite exhibited the highest adsorption capacity towards As(V) when x = 0.6. Study of the mechanism indicated that doping with zinc increased the number of surface hydroxyl groups on ferrite spinel, and thus enhanced the adsorption capacity when x = 0.6. This work revealed the effects of doping site and content of metal atoms on the adsorption ability of ferrite spinel towards As(V).

Removal of Heavy Metals and Metalloids by Amino-Modified Biochar Supporting Nanoscale Zero-Valent Iron.

Nanoscale zero-valent iron (nZVI), an environmentally benign material, has been used to remove heavy metals and metalloids from the aqueous phase because of its high reactivity and abundant reactive sites. To improve the stability of nZVI, nanoscale zero-valent iron supported by amino-modified biochar (ZVIA-BC) was prepared and characterized. Its ability to remove heavy metals and metalloid was investigated. Fourier transform infrared spectroscopy analyses showed that the amino group was chemically bound to the functional groups of biochar. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction revealed that zero-valent iron was loaded on the biochar surface. High-resolution transmission electron microscope images showed that the particle size of iron was ∼50 nm and the particles consisted of roughly spherical cores covered with a shell that was uniformly 2- to 3-nm thick. Furthermore, measuring the zeta potentials at various pH values indicated that the iso-electric points occurred within the pH range of 7.50 to 7.56. Additionally, heavy metals and metalloids, including Cd(II), Ni(II), Cu(II), Cr(VI) and As(V) adsorption isotherms, on ZVIA-BC were significantly nonlinear, and ZVIA-BC exhibited a superior ability to remove these heavy metals and metalloids, especially for Cr(VI) and As(V). Characterization with high-resolution XPS revealed that reduction of heavy metals and metalloids occurred on the surface of ZVIA-BC. The main mechanisms for removal were reduction, complexation, co-precipitation, and electrostatic interaction.

Defective magnesium ferrite nano-platelets for the adsorption of As(V): The role of surface hydroxyl groups.

In this work, magnesium ferrite (MgFe2O4) nano-platelets with rich defects and abundant surface hydroxyl groups were synthesized, and used for the removal of low concentration As(V) in aqueous solution. Results from scanning electron microscopy (SEM) showed that the as-synthesized MgFe2O4 nano-platelets were consisted of many individual nanospheres. Rietveld refinement of X-ray diffraction (XRD) data indicated that the Mg2+ ions substituted the Fe3+ ions at both the octahedral and the tetrahedral sites of the crystal structure. Batch adsorption experiment showed that the equilibrium concentration of As(V) could be reduced down to 4.9 µg·L-1 when the initial concentration of As(V) is 1 mg·L-1, which complied with the drinking water standard of WHO (10 µg·L-1). The adsorption capacity of synthesized MgFe2O4 towards As(V) was higher than commonly used iron oxide adsorbents (Fe3O4, γ-Fe2O3 and α-Fe2O3). Mechanistic studies proved that the superior adsorption capacity was attributed to: (1) increased amount of surface hydroxyl groups that resulted from the surface defects. (2) formation of tridentate hexanuclear surface complexes instead of bidentate binuclear complexes, and (3) formation of excess Mg-OH surface hydroxyl groups and As-Mg monodentate mononuclear surface complexes. This work disclosed the correlation of the superior As(V) adsorption ability with the surface hydroxyl groups in defective MgFe2O4, and propose MgFe2O4 as a potential candidate for the remediation of As-contaminated water.