Layered Double Hydroxides Derived from MIL-88A(Fe) as an Efficient Adsorbent for Enhanced Removal of Lead (II) from Water.

The efficient removal of lead (II) from aqueous solution remains a big problem and the development of novel nanomaterials as adsorbents by various technologies to solve this problem is promising. This study contributed a novel nanostructure of MIL-88A-layered double hydroxides (LDHs) as the adsorbent for Pb2+, which was synthesized by a two-step solvothermal method with MIL-88A(Fe) as the precursor. The as-prepared material featured a chestnut-like core-shell structure, and exhibited excellent removal performance towards Pb2+ from water in comparison to MIL-88A(Fe) and LDHs (directly synthesized). The adsorption of Pb2+ by the MIL-88A-LDHs conformed to the pseudo-second-order kinetic model and the Langmuir and Freundlich isotherm models. The maximal adsorption capacity was 526.32, 625.00, and 909.09 mg g-1 at 278, 298, and 318 K, respectively. The thermodynamic parameters suggested that the adsorption was an endothermic, entropy-increasing, and spontaneous reaction. X-ray photoelectron spectroscopy (XPS) analysis indicated that the surface complexation was mostly responsible for Pb2+ elimination. The MIL-88A-LDHs can be readily regenerated and showed good cyclic performance towards Pb2+. Thus, the as-prepared MIL-88A-LDHs may hold promise for the elimination of aqueous heavy metals.

Mesoporous cerium oxide-anchored magnetic polyhedrons derived from MIL-100(Fe) for enhanced removal of arsenite from aqueous solution.

Efficient elimination of As(III) from drinking water and wastewater has been a challenge because of its neutral molecular form. To address this problem, a novel nanocomposite, mesoporous cerium oxide-anchored magnetic polyhedrons derived from MIL-100(Fe) was fabricated via a strategy combining impregnation and calcination. The resultant products (denoted as Fe2O3/CeO2-t) exhibited a unique octahedral nanostructure decorated by mesoporous cerium oxide. Surface modification of CeO2 enhanced As(III) removal in comparison to unmodified Fe2O3. Particularly, Fe2O3/CeO2-4 h can reduce As(III) concentration from 180 to 10 µg/L within 20 min, which was almost 9 times faster than unmodified Fe2O3. The adsorption behavior conformed to the pseudo-second-order kinetic model (R2 = 0.9908) and the Freundlich isotherm model (R2 = 0.9943). The maximum adsorption capacity of As(III) by Fe2O3/CeO2-4 h was 68.25 mg/g, higher than those reported for similar adsorbents. Its enhanced removal mechanism can be attributed mainly to the mesoporous characteristics and oxidization ability of surface ceria. The composite can be separated from water by external magnets and easily regenerated. This study may offer a clue to the design of metal-organic framework-based composites as an alternative adsorbent for arsenite cleanup.

Magnetic zeolitic imidazolate frameworks composite as an efficient adsorbent for arsenic removal from aqueous solution.

In this study, magnetic zeolitic imidazolate frameworks (ZIF-8) was prepared by a one-step method, where its evolution involved the coprecipitation reactions concomitant with the self-assembly reactions. Structural characterizations indicated that magnetic ZIF-8 showed irregular polyhedral morphology with a large specific surface area (696.5 m2/g) and saturation magnetization (4.31 emu/g). The as-prepared magnetic ZIF-8 enhanced the adsorption performance of As(III) and As(V), compared with bare Fe3O4. The pseudo second-order kinetic model (R2 = 0.9627 and 0.9893 for As(III) and As(V), respectively) and the Langmuir model (R2 = 0.9441 for As(III) and 0.9851 for As(V)) can fit the adsorption process well, confirming the nature of single-layer homogeneous chemisorption. The adsorption capacity was 30.87 and 17.51 mg/g, and their corresponding values of PC were 2.664 and 1.286 L/g, for As(III) and As(V), respectively. Solution pH showed an adverse effect on As(V) adsorption whereas no obvious effect on As(III). The ionic strength and coexisting ions had not obvious influence on adsorption of As(III) and As(V). The adsorption mechanism was explored and discussed based on the detailed spectroscopy analysis. This adsorbent can be recovered magnetically after use, which is promising for the practical application.

Adsorptive removal of Sr(II) from aqueous solution by polyvinyl alcohol/graphene oxide aerogel.

In this study, a new adsorbent, polyvinyl alcohol (PVA) and graphene oxide (GO), was prepared, characterized and used for the removal of Sr2+ from aqueous solution. In PVA/GO composite, the inter-lamellar spacing of adjacent GO layers was dramatically enlarged due to the intercalation of PVA molecules, such a unique architecture significantly mitigated the aggregation of GO layers, which facilitated the accessible exposure of active sites and the mass transfer of strontium ions (Sr2+), thus enhancing the adsorption capacity toward Sr2+. The adsorption of Sr2+ by PVA/GO composite conformed to the pseudo second-order kinetic model (R2 = 0.9994), the Langmuir model (R2 = 0.9042), and the Freundlich model (R2 = 0.9598). The complexation interaction between Sr2+ and oxygen atoms/π-electron domain of PVA/GO composite was primarily responsible for the adsorption mechanism, based on the characterization results of X-ray photoelectron spectroscopy (XPS), scanning electron microscope equipped with energy dispersion spectroscopy (SEM-EDS) and powder X-ray diffraction (PXRD).

Porous walnut-like La2O2CO3 derived from metal-organic frameworks for arsenate removal: A study of kinetics, isotherms, and mechanism.

Exploration of renewable materials for efficient elimination of arsenic from water is highly imperative. Herein, one kind of novel porous walnut-like La2O2CO3 composite is reported for the first time, fabricated via direct pyrolysis of La-MOFs at 550 °C under the air atmosphere. The as-synthesized material predominantly consists of La2O2CO3, featuring micrometer-scale walnut-like morphology and an abundant mesoporous structure. Adsorption experiments demonstrated that a pseudo-second-order model with a high correlation coefficient (0.9976-0.9988) can depict this adsorption process in a good manner and indicates chemical adsorption. Analysis of the isotherms further revealed that this adsorption is a monolayer and homogeneous process, with an excellent adsorption capacity (210.1 As mg/g), as calculated from the Langmuir model. Thermodynamic parameters indicated this adsorption process to be a spontaneous and endothermic, with a positive change in entropy. By characterization results, it can be deduced that the anion-exchange interaction (i.e. carbonate is prone to being replaced by arsenate) and inner-sphere complexation were both responsible for arsenate removal. A broad working pH range (3.0-9.0) and a good cyclic performance (removal rate is above 90% for the fourth cycle) as well as an excellent adsorption capacity make this adsorbent a promising arsenic scavenger.