Characterization of Sodium Alginate Hydrogels: water uptake, adsorption isotherms and numerical analysis for Water Harvesting Generation

The world's increasing population and the consequent enlargment of irrigated agricolture, as well as the improved well-being of life and change in consumption patterns, are severely affecting the growing scarcity of freshwater and thus becoming a menace to the sustainable development of human society. Atmospheric water is a source of freshwater with an availability of approximately 13000 trillion liters that is distributed sufficiently uniformly in the earth atmosphere and its efficient withdrawal could overcome the long-distance transport of water which represents the main drawback of the filtration technologies such as filtration, reverse osmosis and multistage flash distillation of seawater and wastewater. The Atmospheric Water Harvesting Generator (AWHG), if equipped with a suitable moisture harvester material has the ability to address water scarcity in arid regions and can therefore become a suitable technology for decentralized freshwater generators. After a first introduction to the conventional moisture harvesters, the investigation has been focused on a family of new materials: Sodium Alginate Hydrogels. The features that a reliable moisture harvester is expected to possess are a high water uptake, fast sorption/desorption process, high cyclic stability and a low energy demand for regeneration. In this perspective an informal choice of the percentage composition by mass of the elements constituting the hydrogels is made and the experimental work starts with the practical realization of the samples and the subsequent process of spherification, being the latter crucial for the use in a packed bed configuration. The construction of the water uptake curves was subsequently performed using a series of chambers able to render various ambient conditions. The cyclic behavior of the different samples has been tested with a climatic chamber, an industrial machine able to simultaneously control temperature and relative humidity. Being the AWHG a time-intensive cyclic process, in order to assess the absence of performance decay of the material, the water uptake curves have been once again constructed and compared with the water uptake curves of the pristine material. The work continued on the construction of the adsorption Isotherms using the Climatic Chamber. The Linear Driving Force (LDF) and the Fickian Diffusion (FD) models have been compared with the experimental results on different samples, obtaining a calibrated model suitable for the simulations of the hydrogels under analysis. The theoretical simulation of the water production from the AWHG was performed on the peculiar location of Pantelleria (Sicily) using as moisture harvester the configuration of the hydrogel that has shown to be, for both its adsorption performances and technological considerations, the prime fit among the samples analyzed.