Shulan Ma

Efficient Uranium Capture by Polysulfide/Layered Double Hydroxide Composites

There is a need to develop highly selective and efficient materials for capturing uranium (normally as UO2(2+)) from nuclear waste and from seawater. We demonstrate the promising adsorption performance of S(x)-LDH composites (LDH is Mg/Al layered double hydroxide, [S(x)](2-) is polysulfide with x = 2, 4) for uranyl ions from a variety of aqueous solutions including seawater. We report high removal capacities (q(m) = 330 mg/g), large K(d)(U) values (10(4)-10(6) mL/g at 1-300 ppm U concentration), and high % removals (>95% at 1-100 ppm, or ∼80% for ppb level seawater) for UO2(2+) species. The S(x)-LDHs are exceptionally efficient for selectively and rapidly capturing UO2(2+) both at high (ppm) and trace (ppb) quantities from the U-containing water including seawater. The maximum adsorption coeffcient value K(d)(U) of 3.4 × 10(6) mL/g (using a V/m ratio of 1000 mL/g) observed is among the highest reported for U adsorbents. In the presence of very high concentrations of competitive ions such as Ca(2+)/Na(+), S(x)-LDH exhibits superior selectivity for UO2(2+), over previously reported sorbents. Under low U concentrations, (S4)(2-) coordinates to UO2(2+) forming anionic complexes retaining in the LDH gallery. At high U concentrations, (S4)(2-) binds to UO2(2+) to generate neutral UO2S4 salts outside the gallery, with NO3(-) entering the interlayer to form NO3-LDH. In the presence of high Cl(-) concentration, Cl(-) preferentially replaces [S4](2-) and intercalates into LDH. Detailed comparison of U removal efficiency of S(x)-LDH with various known sorbents is reported. The excellent uranium adsorption ability along with the environmentally safe, low-cost constituents points to the high potential of S(x)-LDH materials for selective uranium capture.

Highly Selective and Efficient Removal of Heavy Metals by Layered Double Hydroxide Intercalated with the MoS42– Ion

The MoS4(2-) ion was intercalated into magnesium-aluminum layered double hydroxide (MgAl-NO3-LDH) to produce a single phase material of Mg0.66Al0.34(OH)2(MoS4)0.17·nH2O (MgAl-MoS4-LDH), which demonstrates highly selective binding and extremely efficient removal of heavy metal ions such as Cu(2+), Pb(2+), Ag(+), and Hg(2+). The MoS4-LDH displays a selectivity order of Co(2+), Ni(2+), Zn(2+) < Cd(2+) ≪ Pb(2+) < Cu(2+) < Hg(2+) < Ag(+) for the metal ions. The enormous capacities for Hg(2+) (∼500 mg/g) and Ag(+) (450 mg/g) and very high distribution coefficients (Kd) of ∼10(7) mL/g place the MoS4-LDH at the top of materials known for such removal. Sorption isotherm for Ag(+) agrees with the Langmuir model suggesting a monolayer adsorption. It can rapidly lower the concentrations of Cu(2+), Pb(2+), Hg(2+), and Ag(+) from ppm levels to trace levels of ≤1 ppb. For the highly toxic Hg(2+) (at ∼30 ppm concentration), the adsorption is exceptionally rapid and highly selective, showing a 97.3% removal within 5 min, 99.7% removal within 30 min, and ∼100% removal within 1 h. The sorption kinetics for Cu(2+), Ag(+), Pb(2+), and Hg(2+) follows a pseudo-second-order model suggesting a chemisorption with the adsorption mechanism via M-S bonding. X-ray diffraction patterns of the samples after adsorption demonstrate the coordination and intercalation structures depending on the metal ions and their concentration. After the capture of heavy metals, the crystallites of the MoS4-LDH material retain the original hexagonal prismatic shape and are stable at pH ≈ 2-10. The MoS4-LDH material is thus promising for the remediation of heavy metal polluted water.

Nanocage structure derived from sulfonated β-cyclodextrin intercalated layered double hydroxides and selective adsorption for phenol compounds.

Nanocage structures derived from decasulfonated β-cyclodextrin (SCD) intercalated ZnAl- and MgAl- layered double hydroxides (LDHs) were prepared through calcination-rehydration reactions. The ZnAl- and MgAl-LDH layers revealed different basal spacings (1.51 nm for SCD-ZnAl-LDH and 1.61 nm for SCD-MgAl-LDH) when contacting SCD, while producing similar monolayer and vertical SCD orientations with cavity axis perpendicular to the LDH layer. The structures of the SCD-LDH and carboxymethyl-β-cyclodextrin (CMCD)-LDH intercalates were fully analyzed and compared, and a structural model for the SCD-LDH was proposed. The thermal stability of SCD after intercalation was remarkably enhanced, with decomposition temperature increased by 230 °C. The adsorption property of the SCD-LDH composites for phenol compounds (the effects of adsorption time and phenol concentration on adsorption) was investigated completely. The monolayer arrangement of the interlayer SCD did not affect the adsorption efficiency toward organic compounds, which verified the highly swelling ability of the layered compounds in solvents. Both composites illustrated preferential adsorptive efficiency for 2,3-dimethylphenol (DMP) in comparison with other two phenols of hydroquinone (HQ) and tert-butyl-phenol (TBP), resulting from appropriate hydrophobicity and steric hindrance of DMP. For the two phenols of HQ and TBP, SCD-MgAl-LDH gave better adsorption capacity compared with SCD-ZnAl-LDH. The double-confinement effect due to the combination of the parent LDH host and intercalated secondary host may impose high selectivity for guests. This kind of nanocage structure may have potential applications as adsorbents, synergistic agents, and storage vessels for particular guests.

Intercalation of azamacrocyclic crown ether into layered rare-earth hydroxide (LRH): secondary host-guest reaction and efficient heavy metal removal.

A carboxyethyl substituted azacrown ether (CSAE) derivative was intercalated as a second host into a parent host of layered gadolinium hydroxides (LGdH) by an anion-exchange reaction. The influence of intercalation temperature and starting material ratios of CSAE/LRH on the structures and compositions of CSAE-LRH nanocomposites were investigated. Higher temperature and larger initial CSAE-LGdH weight ratios favor of higher degree of ion exchange at a certain range, while lower temperature gives good morphology for the composites. The adsorptive properties for transition and heavy metal ions were studied using the 20 °C-reacted composite, which showed higher adsorptivity toward transition and heavy metal ions, accompanied by the introduction of nitrate anions. The adsorptive capacity for transition metal ions was in the sequence of Cu(2+) > Zn(2+)∼Ni(2+)∼Co(2+) with a high selectivity to Cu(2+). For the heavy metal ions Ag(+), Hg(2+), Pb(2+), and Cd(2+), the composite showed markedly high selectivity for Ag(+) and Hg(2+). When putting Cu(2+), Ag(+), Hg(2+), Pb(2+), and Cd(2+) together, Ag(+) and Hg(2+) still have higher adsorptive selectivity over Pb(2+) and Cd(2+), and Cu(2+) has also relatively high selectivity but not as high as Ag(+) and Hg(2+). The nanocomposites with a second host in the interlayer are one promising kind of material because of the synergy of the steric effect of the parent host (LRH layer) and the particular characteristics of the secondary host (interlayer crown ether anions).

Rapid Simultaneous Removal of Toxic Anions [HSeO3]−, [SeO3]2–, and [SeO4]2–, and Metals Hg2+, Cu2+, and Cd2+ by MoS42– Intercalated Layered Double Hydroxide

We demonstrate fast, highly efficient concurrent removal of toxic oxoanions of Se(VI) (SeO42-) and Se(IV) (SeO32-/HSeO3-) and heavy metal ions of Hg2+, Cu2+, and Cd2+ by the MoS42- intercalated Mg/Al layered double hydroxide (MgAl-MoS4-LDH, abbr. MoS4-LDH). Using the MoS4-LDH as a sorbent, we observe that the presence of Hg2+ ions greatly promotes the capture of SeO42-, while the three metal ions (Hg2+, Cu2+, Cd2+) enable a remarkable improvement in the removal of SeO32-/HSeO3-. For the pair Se(VI)+Hg2+, the MoS4-LDH exhibits outstanding removal rates (>99.9%) for both Hg2+ and Se(VI), compared to 81% removal for SeO42- alone. For individual SeO32- (without metal ions), 99.1% Se(IV) removal is achieved, while ≥99.9% removals are reached in the presence of Hg2+, Cu2+, and Cd2+. Simultaneously, the removal rates for these metal ions are also >99.9%, and nearly all concentrations of the elements can be reduced to <10 ppb, a limit acceptable for drinking water. The maximum sorption capacities for individual Se(VI) and Se(IV) are 85 and 294 mg/g, respectively. The 294 mg/g capacity for Se(IV) reaches a record value, placing the MoS4-LDH among the highest-capacity selenite adsorbing materials described to date. More interestingly, the presence of metal ions extremely accelerates the capture of the selenium oxoanions because of the reactions of the metal ions with the interlayer MoS42- anions. The sorptions of Se(VI)+Hg and Se(IV)+M (M = Hg2+, Cu2+, Cd2+) are exceptionally rapid, showing >99.5% removals for Hg2+ within 1 min and ∼99.0% removal for Se(VI) within 30 min, as well as >99.5% removals for pairs Cu2+ and Se(IV) within 10 min, and Cd2+ and Se(IV) within 30 min. During the sorption of SeO32-/HSeO3-, reduction of Se(IV) occurs to Se0 caused by the S2- sites in MoS42-. Sorption kinetics for the oxoanions follows a pseudo-second-order model consistent with chemisorption. The intercalated material of MoS4-LDH is very promising as a highly effective filter for decontamination of water with toxic Se(IV)/Se(VI) oxoanions along with heavy metals such as Hg2+, Cd2+, and Cu2+.

Quaternary Chalcogenide Semiconductors with 2D Structures: Rb2ZnBi2Se5 and Cs6Cd2Bi8Te17

Two new layered compounds Rb2ZnBi2Se5 and Cs6Cd2Bi8Te17 are described. Rb2ZnBi2Se5 crystallizes in the orthorhombic space group Pnma, with lattice parameters of a = 15.6509(17) Å, b = 4.218(8) Å, and c = 18.653(3) Å. Cs6Cd2Bi8Te17 crystallizes in the monoclinic C2/ m space group, with a = 28.646(6) Å, b = 4.4634(9) Å, c = 21.164(4) Å, and β = 107.65(3)°. The two structures are different and composed of anionic layers which are formed by inter connecting of BiQ6 octahedra (Q = Se or Te) and MQ4 (M = Zn or Cd) tetrahedra. The space between the layers hosts alkali metal as counter cations. The rubidium atoms of Rb2ZnBi2Se5 structure can be exchanged with other cations (Cd2+, Pb2+ and Zn2+) in aqueous solutions forming new phases. Rb2ZnBi2Se5 is an n-type semiconductor and exhibits an indirect band gap energy of 1.0 eV. Rb2ZnBi2Se5 is a congruently melting compound (mp ∼644 °C). The thermal conductivity of this semiconductor is very low with 0.38 W·m-1·K-1 at 873 K. Density functional theory (DFT) calculations suggest that the low lattice thermal conductivity of Rb2ZnBi2Se5 is attributed to heavy Bi atom induced slow phonon velocities and large Gruneisen parameters especially in the a and c directions. The thermoelectric properties of Rb2ZnBi2Se5 were characterized with the highest ZT value of ∼0.25 at 839 K.