Layered double hydroxides as the next generation inorganic anion exchangers: Synthetic methods versus applicability.

This work is the first report that critically reviews the properties of layered double hydroxides (LDHs) on the level of speciation in the context of water treatment application and dynamic adsorption conditions, as well as the first report to associate these properties with the synthetic methods used for LDH preparation. Increasingly stronger maximum allowable concentrations (MAC) of various contaminants in drinking water and liquid foodstuffs require regular upgrades of purification technologies, which might also be useful in the extraction of valuable substances for reuse in accordance with modern sustainability strategies. Adsorption is the main separation technology that allows the selective extraction of target substances from multicomponent solutions. Inorganic anion exchangers arrived in the water business relatively recently to achieve the newly approved standards for arsenic levels in drinking water. LDHs (or hydrotalcites, HTs) are theoretically the best anion exchangers due to their potential to host anions in their interlayer space, which increases their anion removal capacity considerably. This potential of the interlayer space to host additional amounts of target aqueous anions makes the LDHs superior to bulk anion exchanger. The other unique advantage of these layered materials is the flexibility of the chemical composition of the metal oxide-based layers and the interlayer anions. However, until now, this group of "classical" anion exchangers has not found its industrial application in adsorption and catalysis at the industrial scale. To accelerate application of LDHs in water treatment on the industrial scale, the authors critically reviewed recent scientific and technological knowledge on the properties and adsorptive removal of LDHs from water on the fundamental science level. This also includes review of the research tools useful to reveal the adsorption mechanism and the material properties beyond the nanoscale. Further, these properties are considered in association with the synthetic methods by which the LDHs were produced. Special attention is paid to the LDH properties that are particularly relevant to water treatment, such as exchangeability ease of the interlayer anions and the LDH stability at the solid-water interface. Notably, the LDH properties (e.g., rich speciation, hydration, and the exchangeability ease of the interlayer anions with aqueous anions) are considered in the synthetic strategy context applied to the material preparation. One such promising synthetic method has been developed by the authors who supported their opinions by the unpublished data in addition to reviewing the literature. The reviewing approach allowed for establishing regularities between the parameters: the LDH synthetic method-structure/surface/interlayer-removal-suitability for water treatment. Specifically, this approach allowed for a conclusion about either the unsuitability or promising potential of some synthetic methods (or the removal approaches) used for the preparation of LDHs for water purification at larger scales. The overall reviewing approach undertaken by the authors in this work mainly complements the other reviews on LDHs (published over the past seven to eight years) and for the first time compares the properties of these materials beyond the nanoscale.

Denoising NMR time-domain signal by singular-value decomposition accelerated by graphics processing units.

We present a post-processing method that decreases the NMR spectrum noise without line shape distortion. As a result the signal-to-noise (S/N) ratio of a spectrum increases. This method is called Cadzow enhancement procedure that is based on the singular-value decomposition of time-domain signal. We also provide software whose execution duration is a few seconds for typical data when it is executed in modern graphic-processing unit. We tested this procedure not only on low sensitive nucleus (29)Si in hybrid materials but also on low gyromagnetic ratio, quadrupole nucleus (87)Sr in reference sample Sr(NO3)2. Improving the spectrum S/N ratio facilitates the determination of T/Q ratio of hybrid materials. It is also applicable to simulated spectrum, resulting shorter simulation duration for powder averaging. An estimation of the number of singular values needed for denoising is also provided.

NMR cogwheel phase cycling determination with web tools: amplitude-modulated z-filter MQMAS sequence.

Our recent method based on the use of web tools (XML and XSLT) for constructing cogwheel phase cycles is extended to the selection of two symmetrical coherence transfer pathways in the case of an amplitude-modulated z-filter MQMAS sequence. For all spins (I=3/2, 5/2, 7/2 and 9/2) and all MQ experiments we compare cogwheel phase cycling with the traditional "nested" phase cycling. The principal difference in the number of phase cycling steps lies in the use of digitizer phase. For nested phase cycling, this number depends on the use of reference receiver phase or digitizer phase, while cogwheel phase cycling does not. As an illustration we consider the case of a+/-3QMAS experiment for spin I=7/2 system applied to cobalt-59 in [Co(NH3)6]Cl3 powder. We also explore the selection of two non-symmetrical coherence transfer pathways in the case of an amplitude-modulated shifted-echo MQMAS sequence.

Determination of NMR cogwheel phase cycle with XML.

The selection of correct coherence transfer pathways is an essential component of an NMR pulse sequence. This article describes a new method based on the use of web tools (eXtensible Markup Language and eXtensible Stylesheet Language Transformation) to generate a cogwheel phase cycle for selecting coherence transfer pathways. We illustrate this method with the three-pulse phase-modulated shifted-echo or split-t(1) MQMAS sequences for triple-quantum spin-3/2 systems. After generalization to the different half-integer quadrupole spins, we use the SIMPSON program to confirm our results. Finally, we apply our method to the case of the z-filter 3QMAS sequence for I=3/2 systems.

Procedures for labeling the high-resolution axis of two-dimensional MQ-MAS NMR spectra of half-integer quadrupole spins.

The increasing development and application of the multiple-quantum MAS NMR for half-integer quadrupole spins has led to various RF pulse sequences for improving the excitation of multiple-quantum coherences and their conversion to single-quantum coherences. As a result, several conventions for labeling the Fl dimension of a 2D MQ-MAS spectrum appear in the literature. The corresponding relations for extracting the isotropic chemical shift, the quadrupole coupling constant, and the asymmetry parameter from experimental data are not always provided. We analyze these various conventions systematically and propose a new one, similar to that introduced by J.-P. Amoureux and C. Fernandez (2000, Solid State NMR 10, 339-343). These various conventions are illustrated with 27Al (I = 5/2) nuclei in aluminum acetylacetonate Al(CH3COCHCOCH3)3. Another experimental problem often met, the aliasing of peaks in the 2D spectrum, is analyzed and illustrated with 27Al (I = 5/2) in NH4Y zeolite and 23Na (I = 3/2) in sodium pyrophosphate Na4P2O7.