Influence of Synthesis Method on LTA Time-Dependent Stability.

Time-stability of LTA zeolite formed by hydrothermal method with or without the action of ultrasonic irradiation was investigated by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). The results show that 6 months after the synthesis by hydrothermal process with continuous sonication, LTA evolves into a more stable sodalite, whereas no differences are detected 12 months after LTA synthesis by conventional pre-fused hydrothermal process. These data confirm that using the two approaches, different mechanisms control both zeolite crystallization and time-stability of the newly-formed mineral at solid state. The results are particularly important in the light of the synthetic zeolite application.

Environmental risk induced by TiO2 dispersions in waters and sediments: a case study.

A southern Italian area that is characterized by large outcrops of rocks that are rich in titanium oxide (TiO2) phases were investigated to determine the mineralogical risk induced by the natural dispersion of TiO2 minerals. Rock, sediment and surface water samples were collected to determine the physicochemical and mineralogical factors (i.e., size distribution, morphology and alteration) indicative of potential TiO2 toxicity. X-ray diffraction data suggested that titanium oxides were present as rutile and anatase. Scanning electron microscopy images showed elongated TiO2 morphologies; fibres were found as either isolated or embedded/enclosed in flake-like phyllosilicates. The concentration of fibres in stream water ranged from 1.7 to 4.6 million fibres per litre. The highest fibre amounts in the sediments were in the <8-µm fraction, while single fibres were primarily concentrated in the <2-µm fraction. The results indicate that titanium oxide minerals represent a natural source of environmental risk and that the geomineralogical characterization of rich TiO2 areas is indispensable for understanding their geoavailability, dispersion and distribution.

Removal of Mn from aqueous solution using fly ash and its hydrothermal synthetic zeolite.

A number of water purification processes have been developed in recent years based on the utilisation of low-cost materials with high pollutant removal efficiency. Among these materials, fly ash and zeolite synthesised from fly ash are two examples of high-efficiency adsorbents. Column absorption tests were performed in order to compare the manganese sorption behaviour of an Italian coal fly ash and zeolite synthesised from it. Different masses of both materials (10-60 g) were exposed to solutions containing a total metal concentration of 10 mg/L. Batch adsorption studies were also conducted to determine the effect of time on the removal on Mn sequestration. The results indicate that both materials are effective for the removal of Mn from aqueous solution by precipitation due to the high pH of the solid/liquid mixtures. However, the leaching tests reveal that the amount of Mn removed from the fly ash was greater than that leached from the zeolite, thereby indicating that the metal is partially sequestrated by zeolite.

Red mud treated with KOH: synthesis of sustainable materials from waste for water treatment.

Solid waste resulting from bauxite ore (red mud) was converted into useful products consisting in hydrogarnet together with zeolite. Red mud (RM) transformation from disposal material into new source was carried out using potassium hydroxide as an activator and hydrothermal process (HY) or vapor phase crystallization (VPC) approach. HY process was performed at 60, 90, and 130 °C whereas during the VPC method, red mud was contacted only with vapor from the distilled water heated at 60 and 90 °C. The results indicate the formation of katoite and zeolite L (LTL topology) with both approaches. All the synthetic products display magnetic properties. In addition, a preliminary investigation on arsenic removal from drinking water (from 59 to 86%), makes the synthetic materials appealing for environmental applications. Finally, the synthesis of a large amount of very useful newly-formed phases using vapor molecules confirms the efficiency of the innovative and green VPC process in waste material transformation.

Effect of H2O Activity on Zeolite Formation.

In an effort to understand the effects of H2O activity on zeolite formation, we have synthesized LTA zeolite using a combination of freezing processes and varying drying temperatures. Sodium aluminate and sodium silicate were used to form LTA zeolite, according to the IZA (International Zeolite Association) protocol. The synthesis steps were modified by adding the precursor frozen process by a rapid liquid nitrogen (-196 °C) treatment or slow conventional freezer treatment (-20 °C). The samples were subsequently sonicated and then dried at 80 °C or 40 °C. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were performed on the samples immediately after the drying process as well as after 2 weeks and 1 month of aging the solid products. The results indicated that LTA zeolite does not form. The silica-alumina precursor after both freezing processes and after being dried at 80 °C showed the presence of sodalite displaying stable behavior over time. Both sets of samples dried at 40 °C and did not show the presence of zeolite immediately after the drying process. However, after 2 weeks, the liquid nitrogen-frozen precursor was characterized by the presence of EMT whereas zeolites never formed in the -20 °C samples. These results suggest that freezing processes differently control the H2O activity during the drying and aging processes in the solid state. Thus, although the precursor chemical composition is the same, the type of zeolite formed is different.

Obsidian as a Raw Material for Eco-Friendly Synthesis of Magnetic Zeolites.

A sample of rhyolitic obsidian (OS) was used as raw material for zeolite synthesis by long (4 days) and fast (2 h)-aging hydrothermal processes. Zeolite synthesis was also performed by a fast (2 h) sonication method. The products were analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) both immediately after and 3 years after their formation in order to determine the stability of synthetic materials according to the method used. The results confirm zeolitization of obsidian both by long-aging conventional hydrothermal heating and fast hydrothermal process. However, the data highlight the efficiency of direct ultrasound energy in achieving more stable zeolite crystals over time. These results carried out using a natural source, follow those already obtained using wastes and pure sources as raw materials thus providing a definitive validation of the different mechanisms controlling zeolite formation according to the process used. Moreover, the results confirm the effectiveness of ultrasonic energy in the formation of zeolites that are more stable over time. Due to the chemical composition of the obsidian precursor, all synthetic zeolites show good magnetic properties (i.e., saturation magnetization), in view to potential magnetic separation.

Ultrasonic vs hydrothermal method: Different approaches to convert fly ash into zeolite. How they affect the stability of synthetic products over time?

The action of direct sonication (US) versus conventional hydrothermal method (HY) was investigated to determine the differences in the crystallization mechanism of zeolite formed from fly ash. The results showed that ultrasonic energy is decisive in very fast faujasite and A-type zeolite transformation into more stable sodalite phase. The data display the main presence of sodalite together with a low amount of faujasite and zeolite A after the first 3 h of sonication. The full transformation of the latter two phases into sodalite takes place after 1 h more of treatment. The samples incubated by hydrothermal process for 3 h, instead, are characterized by the main presence of faujasite and A-type zeolites. The progressive synthesis of sodalite at the expense of the other two phases begins only after 4 h of treatment. The conclusion is that the crystallization of zeolites by ultrasonic and hydrothermal method proceeds via two different mechanisms. The data also show that the two approaches affect the stability of the synthetic products in a different way over the years.

Are conventional statistical techniques exhaustive for defining metal background concentrations in harbour sediments? A case study: The Coastal Area of Bari (Southeast Italy).

Sediment contamination by metals poses significant risks to coastal ecosystems and is considered to be problematic for dredging operations. The determination of the background values of metal and metalloid distribution based on site-specific variability is fundamental in assessing pollution levels in harbour sediments. The novelty of the present work consists of addressing the scope and limitation of analysing port sediments through the use of conventional statistical techniques (such as: linear regression analysis, construction of cumulative frequency curves and the iterative 2σ technique), that are commonly employed for assessing Regional Geochemical Background (RGB) values in coastal sediments. This study ascertained that although the tout court use of such techniques in determining the RGB values in harbour sediments seems appropriate (the chemical-physical parameters of port sediments fit well with statistical equations), it should nevertheless be avoided because it may be misleading and can mask key aspects of the study area that can only be revealed by further investigations, such as mineralogical and multivariate statistical analyses.

Ultrasonic waves induce rapid zeolite synthesis in a seawater solution.

The synthesis of zeolites from fly ash was performed through a low-temperature hydrothermal process with seawater. Compared with the results obtained using the same hydrothermal method but in the absence of sonication, the application of an ultrasonic pre-treatment to the conventional hydrothermal process with seawater reduces the crystallization temperature below that observed when hydrothermal synthesis is performed using distilled water.

Effects of ultrasonic treatment on zeolite synthesized from coal fly ash.

The synthesis of zeolites from three samples of fly ash was carried out through a low-temperature (25-60°C) hydrothermal process with a NaOH pre-fusion treatment preceded by sonication. The results were compared with those of conventional hydrothermal syntheses. XRD and SEM investigations demonstrate that the application of ultrasonic treatment facilitates the formation of zeolites at a lower-temperature (25°C) than syntheses not preceded by sonication. No significant difference in type, temperature of crystallization, or amount of zeolites synthesized was noted between the three different samples of fly ash, implying that the chemical composition of fly ash had little influence on the zeolite product within the compositional range of these fly ash precursors. Although there appears to be a correlation between the SiO(2)/Al(2)O(3) ratio of the fly ash and the temperature of zeolite formation by conventional synthesis, no correlation was apparent when ultrasonic pre-treatment was used at low-temperatures.

Synthesis of zeolite from Italian coal fly ash: differences in crystallization temperature using seawater instead of distilled water.

In this study Italian coal fly ash was converted into several types of zeolite in laboratory experiments with temperatures of crystallization ranging from 35 up to 90 degrees C. Distilled and seawater were used during the hydrothermal synthesis process in separate experiments, after a pre-treatment fusion with NaOH. The results indicate that zeolites could be formed from different kind of Italian coal fly ash at low temperature of crystallization using both distilled and seawater. SEM data and the powder patterns of X-ray diffraction analysis show that faujasite, zeolite ZK-5 and sodalite were synthesized when using both distilled and seawater; zeolite A crystallized only using distilled water. In particular the experiments indicate that the synthesis of zeolite X and zeolite ZK-5 takes place at lower temperatures when using seawater (35 and 45 degrees C, respectively). The formation of sodalite is always competitive with zeolite X which shows a metastable behaviour at higher temperatures (70-90 degrees C). The chemical composition of the fly ash source could be responsible of the differences on the starting time of synthesized zeolite with distilled water, in any case our data show that the formation of specific zeolites takes place always at lower temperatures when using seawater.

Immobilization of Ni by synthesising zeolite at low temperatures in a polluted soil.

Over the last few years a great deal of research has been carried out in order to develop remediation methods for reducing environmental risks due to polluting metals. Zeolite formation in contaminated soils mixed with coal fly ash could be a useful method to reduce both the availability and the mobility of metals in contaminated areas. In this study a soil sample--treated with coal fly ash and artificially contaminated with a high concentration of Ni--was used for synthesizing zeolite at low temperatures. The role played by this mineral in the immobilization of heavy metal was investigated. The materials were analysed chemically (sequential extraction) and by XRD and SEM-EDS analyses. The synthesis was carried out both in the laboratory and on a bench-scale for 1 year. Zeolite crystallization readily occurred after a month. The presence of Ni does not exert any influence on zeolite formation. On the other hand newly-formed zeolites reduce the toxicity of the element in the polluted soil. A reduction in heavy metal availability was observed after ammonium acetate extraction. The use of the modified BCR three-step sequential extraction (sequential extraction protocol developed by Community Bureau of Reference of the Commission of the European Communities) suggests that Ni mobilization takes place when zeolite structure collapses after the BCR second step. The Ni thus available was mobilized in the third step.