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.

High temperature behaviour of Ag-exchanged Y zeolites used for PFAS sequestration from water.

Per- and polyfluorinated alkyl substances (PFAS) are anthropogenic compounds which have recently drawn great attention due to their high biological, chemical and physical stability and lipid/water repelling properties. The present work aims to provide for the first time insights on the thermal behaviour of Ag-exchanged Y zeolite loaded with perfluorooctanoic acid (PFOA, C8HF15O2) and perfluorooctane sulfonate (PFOS, C8HF17O3S) emphasizing the close link between crystal structure and desorption/dehydration processes. Elemental and isotopic abundance of carbon analysis, thermal analysis, and in situ high-temperature synchrotron X-ray powder diffraction were used to evaluate critically if the thermal regeneration affects the initial zeolites structural features. Rietveld refinements revealed that PFAS sites are emptied in the 550-650 °C temperature range, when the thermal degradation of PFOA and PFOS are reached. The crystallinity of the samples is not affected by the adsorption/desorption processes. Upon heating, the removal of both PFAS and coadsorbed water molecules induced a cation migration of the silver ions and changes of initial geometry of the framework. The dimensions of the channels remain comparable to those of the pristine materials thus suggesting the potential re-use of the samples in other adsorption PFAS cycles. Additionally, once regenerated and reloaded Ag-exchanged Y can re-adsorb PFAS in amounts comparable to that adsorbed in the first cycle with clear benefits on the costs of the whole water treatment process.