Gerardo Rodríguez-Fuentes

Natural clinoptilolite as an exchanger of Ni2+ and NH4+ ions under hydrothermal conditions and high ammonia concentration

Natural clinoptilolite from Caimanes deposit (Moa-Cuba) has been used for the nickel removal from aqueous solution by ion exchange. The Ni2+ elution, using ammonia-rich solution, has been also studied in form of a nickel removal–elution ion-exchange cyclic process. The stability of clinoptilolite as an exchanger of Ni2+ and NH4+ cations under hydrothermal conditions and high ammonia concentrations is analyzed. No loss in the clinoptilolite exchange capacity of Ni2+ and NH4+ ions during the Ni2+ removal–elution cycles was found. The treatment of the clinoptilolite with NH4OH concentrated solutions provokes not only the Ni2+ elution by ion exchange, but also a substantial increase of the Ni2+ ions removal capacity of the zeolite. This is explained by OH− anions adsorption (or occlusion) on the zeolitic phase particles and the consequent nickel hydroxide precipitation.

Time evolution of a natural clinoptilolite in aqueous medium: conductivity and pH experiments

The chemical behavior in an aqueous medium of a natural zeolitic material, NZ (mainly consisting in crystals of the zeolites clinoptilolite/heulandite) has been studied through electrochemical techniques, complemented by atomic absorption spectrometry. The time dependence of the conductivity of the aqueous medium in the presence of NZ, showed a steep increase at the beginning followed by a slower increase for longer times until saturation of the system is reached. The release of the ions present in the zeolitic material at different times, was followed by means of atomic adsorption spectroscopy, which indicated that the sodium ion is the main responsible for the conductivity response. The interaction of NZ with the aqueous medium revealed a pH increase in the weakly acid region and a decrease in the basic region. The correlation between conductometric and pH-metric studies suggests that an ion-exchange mechanism takes place within the very first minutes of the interaction of NZ with the aqueous medium, followed by the temperature-dependent dissolution of minority phases up to several days. Finally, a comparative study between NZ and a modified form of this zeolite with sodium carbonate (NZ1), showed a faster pH increase with time for NZ1.

Thermal and cation influence on ir vibrations of modified natural clinoptilolite

Abstract The study of IR spectra in the range of 400 to 1700 cm −1 of cation exchanged natural clinoptilolite-heulandite samples has shown the influence of the characteristics of cations and their content in the frequency and transmittance of vibration modes of zeolite. The 450 cm −1 band assigned to T-O internal bending, 1205 cm −1 band of T-O internal asymmetric stretching, and 1640 cm −1 band of OH bending, were most affected by the cation exchange and thermal history of the sample.

Characterization and neutralizing properties of a natural zeolite/Na2CO3 composite material

Abstract A new zeolitic active principle called NZ* has been obtained, the neutralizing properties of which are better than those of its predecessor, Neutacid N. The new product is the result of hydrothermal transformations applied to purified natural clinoptilolite. NZ* and other related products have been characterized physically and chemically using atomic emission spectroscopy with inductively coupled plasma, X-ray fluorescence analysis, X-ray diffraction, IR spectroscopy, neutralizing capacity in the presence of synthetic gastric juice and UV spectroscopy. It was demonstrated that the structure of the zeolitic raw mineral remained unchanged after hydrothermal transformation, and that NZ* is structurally stable after interaction with synthetic gastric juice. The effect of NZ* dose on the gastric enzyme pepsin was evaluated.

The role of carbonate ions in the ion-exchange Ni2+=2NH4+ in natural clinoptilolite

The removal and recuperation of nickel and other heavy metals, from the ammoniacal-carbonate residual liquors of the Cuban Nickel Industry, by ion exchange have been studied using natural clinoptilolite from Caimanes deposit (Moa, Cuba). The present study was conducted to analyze the influence of carbonate anions during the NH4+ and Ni2+ exchange in clinoptilolite. The main ion-exchange kinetic parameters were determined on the basis of the Barrer theory and other criteria. The analysis of the results obtained by kinetic study, infrared spectrometry and X-ray diffraction led to the conclusion that, in addition to ion exchange, the Ni precipitates in the form of low soluble phases.

Advantages of base exchanged natural clinoptilolite as a catalyst for the Knoevenagel reaction

Abstract The potential use of natural zeolites as base catalysts was ascertained by studying the catalytic behavior of a purified natural clinoptilolite (NZ) and its sodium carbonate modified form (NZC) in the aldol condensation reaction of benzaldehyde with methyl cyanoacetate (Knoevenagel reaction). The natural zeolites activities were compared to that of cesium modified X and Y faujasite type zeolites (CsFAUX and CsFAUY). It was observed that the activity of the NZC zeolite was five times higher than that of NZ. The normalized activity per surface area of the modified clinoptilolites was higher than that shown by synthetic FAUs zeolites. The higher activity of NZC is attributed to the presence of single and double carbonate species, such as Na2CO3 and Na2Ca(CO3)2, occluded in the internal cavities of the clinoptilolite zeolite, as confirmed by XRD and IR diffuse reflectance. The CO2 and NH3 TPD analysis corroborated the presence of stronger basic sites in the natural zeolites in good agreement with the catalytic behavior shown by these solids in the Knoevenagel condensation reaction.

Characterization of Fe2+-containing natural clinoptilolite and its interaction with saccharides

Abstract The Fe 2+ -containing natural clinoptilolite (FZ) was studied by X-ray diffraction, thermal analysis, infrared (diffuse reflectance), dielectric, and Mossbauer spectroscopies. The presence of FeSO 4 occluded in the cavities of the clinoptilolite was confirmed by the appearance of a new vibration band at 3400–3500 cm −1 , the exothermic response around 140°C, the occurrence of two peak losses in the dielectric response of FZ and the detection of a new doublet corresponding to FeSO 4 identified in the Mossbauer spectrum. The study of the Mossbauer spectra of FZ–glucose (FZ–G) system showed the formation of the iron–glucose complex in the clinoptilolite. This complex intervenes, in a small percentage, as one of the adsorption mechanisms of the glucose for FZ. The mechanism of action proposed for the FZ–G interaction is a strong adsorption governed by the reactive characteristics of glucose.

Clinoptilolite–heulandite polymorphism: structural features from computer simulation

Static lattice computer modelling techniques are applied to investigate the framework and extra-framework cation distribution in clinoptilolite/heulandite zeolitic materials. The results emphasise the close coupling between the framework Al distribution and the location of the extra-framework cations. In the majority of the polymorphs, Al is predicted to be preferentially localised at the T2 and T3 crystallographic sites. Extra-framework cation distributions are often controlled by a strong driving force for the location of Ca2+ close to Al-rich regions. Cation–cation repulsion also play a substantial role in controlling the position of extra-framework cations. We find significant differences in both the distribution of Al in the framework and in the location and distribution of the extra-framework cations depending on the exact composition of the heulandite-structured material considered.

Silicon–aluminium distribution in dehydrated calcium heulandite

The distribution of aluminium within the lattice of a model calcium heulandite is considered. Significant segregation of Al to particular sites is noted with T2 being preferentially occupied whereas T4 has a low Al occupancy. T5 is also occupied, in contrast to earlier work on Na-clinoptilolite. The resulting distributions are also significantly different to previous work on Na-clinoptilolite, demonstrating the influence of the Si/Al ratio and cation species on the spatial arrangement of Si and Al in the framework.

Conductivity activation energy and analysis of the sintering process of dielectric ceramics

Abstract The electrical properties of BaTiO 3 ceramics with Bi 4 Ti 3 O 12 added were studied by using AC measurements performed at various temperatures. The activation energies for the conductivity process of both the grain and grain boundary phases were estimated using Arrhenius-type plots. It is found that the poor material densification is accompanied by a significant low activation energy of the grain boundary phase, while the high material densification leads to an increase. On the other hand, the activation energy for the grain phase is nearly not affected by the particularities of each sample sintering.