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LAYERED MATERIALS FOR SELECTIVE SEPARATION OF SOME TRANSITION METAL CATIONS FROM WATER
Restricted (Penn State Only)
Materials Science and Engineering
Master of Science
Date of Defense:
October 04, 2017
Sridhar Komarneni, Thesis Advisor
Fred Scott Cannon, Committee Member
Dinesh Kumar Agrawal, Committee Member
There is an increasing need to separate transition metals from water resources for technological applications and remediation purposes. Ion exchange using selective ion exchangers is a possible method for these purposes. Na-n-micas (n=2,3 and 4) and NH4+-SnP with layered structures were prepared using NaCl melting and hydrothermal methods, respectively. X-ray diffraction and other characterization techniques were carried out on the above synthetic materials to make sure about their phase purity and crystallinity. The Na-n-micas (n=2,3 and 4) showed high crystallinity with high and sharp intensity peaks by the XRD results. The novel NH4+-SnP showed 16.57 angstrom peak at low angle, indicating its layered structure. Infrared spectroscopy studies confirmed the Si-Al composition and interlayer hydration of the Na-n-micas (n=2,3 and 4) and the existence of NH4+ in the NH4+-SnP structure along with interlayer water. The 29Si and 27Al NMR spectroscopy of Na-n-micas (n=2, 3 and 4) revealed their increasing layer charge density as follows: Na-4-mica > Na-3-mica > Na-2-mica, as expected. SEM characterization showed plate-like morphology for all micas in the range of 1 to 6 μm while NH4+-SnP showed aggregated lath-like particles. TEM micrograph of NH4+-SnP revealed that the lath-like particles are on the order of 20 to 100 nm showing the layered nanostructure of the material with layer thickness of ~16 angstrom. The above cation exchangers were selected for further studies because they showed selective separation of some transition metal cations (Ni2+, Zn2+, Mn2+ and Cu2+) from simulated solution of sea water. Furthermore, aqueous solutions containing sodium and a metal cation in certain ratios were equilibrated in order to construct ion exchange isotherms so as to determine the selectivity of metal cation over Na. Isotherm studies, Kielland plots and distribution coefficients were used to determine the selectivity of different ion exchangers for different metal cations. The 2Na+→M2+ (M= Ni2+, Zn2+, Mn2+, Cu2+) exchange studies showed that Na-4-mica showed selectivity for Zn2+ at XZn<~0.15. Na-2-mica showed high selectivity for Cu2+ at all concentrations while Na-3-mica and Na-4-mica showed high selectivity for Cu2+ at certain concentrations. Unlike other samples, Cu2+ exchanged Na-2-micas showed positive Kielland coefficient, indicating the increasing Cu2+ selectivity as X(Cu) increased. NH4+-SnP showed high distribution coefficients (374 for Ni2+, 810 for Zn2+, 343 for Mn2+ and 748 for Cu2+) than other ion exchangers in the sea-mimicking water experiments. The difference in the selectivity of Na-n-micas (n=2,3 and 4) for the metal cations apparently resulted from their differences in layer charge density and interlayer expansibility. Two (001) phases of ~12 angstrom and ~14 angstrom were found in the XRD results of M2+ (M= Ni2+, Zn2+, Mn2+ and Cu2+) exchanged Na-n-micas (n=2,3 and 4). The ~14 angstrom peaks resulted from two-layer metal cation hydrates while the ~12 angstrom resulted from one-layer metal cation hydrates. The formation of ~14 angstrom phases were found to be higher from cation exchanged Na-2-mica samples compared to other mica samples, because of lower charge density of the former. This difference in the formation of ~14 angstrom resulted from the difference in the layer charge density of the samples (Na-2-mica < Na-3-mica < Na-4-mica) as revealed by 29Si and 27Al NMR spectroscopy in this thesis. The results from this thesis indicate that Na-n-micas (n=2, 3 and 4) are possible candidates for separation of Cu2+ from aqueous solutions of waste water or sea water. NH4+-SnP is a newly synthesized possible candidate for separation of transition metal ions from water, due to its layered structure with high interlayer spacing and high charge density and in addition, relatively higher stability in water compared with the recently synthesized H+-SnP material of ~15 angstrom.
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