Synthesis of zeolites from volcanic ash (Tajogaite, Spain) for the remediation of waters contaminated by fluoride.

In the eruptive event of Tajogaite (2021) in La Palma, Canary Islands, large quantities of volcanic ash were accumulated, affecting the local environment and urban areas. In this study, volcanic ash sampled from urban areas (catalogued as municipal waste (20 03 03) by the European Wastes Catalogue) were converted into zeolites by hydrothermal synthesis at 100 °C with previous alkaline fusion at 550 °C with distilled water. During this process, new phases of zeolite principally type X and sodalite have been identified by XRD at 2 h of incubation. These zeolites, with the course of incubation time, present competitive processes where the transformation into sodalite develops after 24 h as the predominant phase. The synthesized zeolitic material presents a high concentration as impurities in Fe2O3 (13.70 wt%), Na2O (12.70 wt%), CaO (11.65 wt%), and TiO2 (3.89 wt%) coming from the volcanic ash and NaOH introduced in the synthesis methodology. These impurities impart different physicochemical capabilities to the zeolitic material. The application of zeolites obtained in a preliminary fluoride adsorption experiment with volcanic leachate water rich in fluoride has been tested in a novel way. Removal efficiencies of 41.4% at acidic pH (5.77) have been obtained with 2 g L-1 adsorbent zeolitic material doses. A value-added material is obtained and applied in a preliminary way to solve a problem generated by the volcanic ash itself, allowing the End of Waste status and meeting different objectives of the sustainable development goals of the UN Agenda 2030.

Sialolithiasis: mineralogical composition, crystalline structure, calculus site, and epidemiological features.

The purpose of this paper was to describe the characteristics of salivary calculi and their relationship to epidemiological factors, through a cross-sectional study. We analysed 100 calculi obtained in 2017-2021. Patient data including age, time since onset of symptoms, gland involved, and site of location in the salivary system were studied. The calculi were studied to determine their morphological features using scanning electron microscopy and energy dispersive plain radiographic analysis. Most of the calculi had formed in the submandibular gland (SG) (82%). The mean age of patients at onset was 45.83 years; patients presenting parotid gland (PG) stones were somewhat older (p = 0.031). The mean time since the onset of symptoms was longer in PG calculi (p = 0.038). The most common lithiasis site was the main duct (74%), followed by the hilum (22%). Hilar stones were the largest (p < 0.05) and heaviest (p = 0.028). Octacalcium phosphate (OCP) was the most common crystalline phase (Cp) founded, followed by hydroxyapatite (HA) and whitlockite (WH). Specifically, OCP had a higher presence in PG calculi (p = 0.029) and WH was the most common phase in SG calculi (p = 0.017). The most prevalent site of lithiasis was the main duct, and the largest and heaviest calculi were found in the SG. PG stones were associated with a longer history of symptoms and older age. OCP was the most frequent Cp of the calculi studied, and the main Cp in PG stones. WH was the predominant Cp in SG stones. The Cp of the calculi was not influenced by location, patient age, or time of symptoms.

The application of thermal solar energy to high temperature processes: case study of the synthesis of alumina from boehmite.

The aim of this paper is to evaluate the feasibility of obtaining alumina from boehmite using a free, clean, and unlimited power source as the solar energy. Boehmite was obtained by hydrothermal treatment of a hazardous waste coming from aluminum slag milling. The waste is considered as a hazardous substance because of it releasing toxic gases (hydrogen, ammonia, methane, and hydrogen sulfide) in the presence of water. The as-obtained boehmite was transformed into alumina, in air atmosphere, using a solar energy concentrator (Fresnel lens). The solar installation provides a power density of 260 W · cm(-2) which allows reaching temperatures upper than 1000 °C at few minutes of exposure. Tests were performed at different periods of time that ranged between 5 and 90 min. The percentage of transformation of boehmite into alumina was followed by the water content of samples after solar radiation exposure. Samples were characterized by X-ray diffraction, infrared spectroscopy, and thermogravimetry. Metastable aluminas started to appear at 5 min and the crystalline and stable phase corundum at 10 min of solar radiation exposure.

Recycling of hazardous waste from tertiary aluminium industry in a value-added material.

The recent European Directive on waste, 2008/98/EC seeks to reduce the exploitation of natural resources through the use of secondary resource management. Thus the main objective of this study was to explore how a waste could cease to be considered as waste and could be utilized for a specific purpose. In this way, a hazardous waste from the tertiary aluminium industry was studied for its use as a raw material in the synthesis of an added-value product, boehmite. This waste is classified as a hazardous residue, principally because in the presence of water or humidity, it releases toxic gases such as hydrogen, ammonia, methane and hydrogen sulfide. The low temperature hydrothermal method developed permits the recovery of 90% of the aluminium content in the residue in the form of a high purity (96%) AlOOH (boehmite). The method of synthesis consists of an initial HCl digestion followed by a gel precipitation. In the first stage a 10% HCl solution is used to yield a 12.63 g L(-1) Al( 3+) solution. In the second stage boehmite is precipitated in the form of a gel by increasing the pH of the acid Al(3+) solution by adding 1 mol L(-1) NaOH solution. Several pH values were tested and boehmite was obtained as the only crystalline phase at pH 8. Boehmite was completely characterized by X-ray diffraction, Fourier transform infrared and scanning electron microscopy. A study of its thermal behaviour was also carried out by thermogravimetric/differential thermal analysis.