Na2SO4·10H2O dehydration in view of thermal storage

Concentrating stabilized human urine using eutectic freeze crystallization for liquid fertilizer production

Resource recovery from source-separated urine can be used to produce fertilizers and provide a more sustainable alternative to mineral fertilizers. Reverse osmosis can be used to remove up to 70% of the water in urine that has been stabilized with Ca(OH)₂ and pre-treated with air bubbling. However, further water removal is limited because of membrane scaling and equipment operating pressure limitations. A novel hybrid eutectic freeze crystallization (EFC) and RO system was investigated as a method to concentrate human urine, whilst simultaneously crystallizing salt and ice under EFC conditions. A thermodynamic model was used to predict the type of salts that would crystallize, their associated eutectic temperatures, and how much additional water removal was required (using freeze crystallization) to reach eutectic conditions. This innovative work showed that at eutectic conditions, Na₂SO₄∙10H₂O crystallizes simultaneously with ice in both real and synthetic urine, thus providing a new method to concentrate human urine for liquid fertilizer production. A theoretical mass balance of a hybrid RO-EFC process, including ice washing and recycle streams, showed that 77% of the urea and 96% of the potassium could be recovered with a 95% water removal. The final liquid fertilizer would have a composition of 11.5% N and 3.5% K, and 3.5 kg of Na₂SO₄∙10H₂O could be recovered from 1000 kg of urine. Over 98% of the phosphorus would be recovered as calcium phosphate during the urine stabilization step. A hybrid RO-EFC process would require 60 kWh m^-3 of energy, which is substantially less than other concentration methods.

Softness of hydrated salt crystals under deliquescence

Deliquescence is a first-order phase transition, happening when a salt absorbs water vapor. This has a major impact on the stability of crystalline powders that are important for example in pharmacology, food science and for our environment and climate. Here we show that during deliquescence, the abundant salt sodium sulfate decahydrate, mirabilite (Na₂SO₄·10H₂O), behaves differently than anhydrous salts. Using various microscopy techniques combined with Raman spectroscopy, we show that mirabilite crystals not only lose their facets but also become soft and deformable. As a result, microcrystals of mirabilite simultaneously behave crystalline-like in the core bulk and liquid-like at the surface. Defects at the surface can heal at a speed much faster than the deliquescence rate by the mechanism of visco-capillary flow over the surface. While magnesium sulfate hexahydrate (MgSO₄⋅6H₂O) behaves similarly during deliquescence, a soft and deformable state is completely absent for the anhydrous salts sodium chloride (NaCl) and sodium sulfate thenardite (Na₂SO₄). The results highlight the effect of crystalline water, and its mobility in the crystalline structure on the observed softness during deliquescence. Controlled hydrated salts have potential applications such as thermal energy storage, where the key parameter is relative humidity rather than temperature.

The Phase Transition and Dehydration in Epsomite under High Temperature and High Pressure

The phase stability of epsomite under a high temperature and high pressure were explored through Raman spectroscopy and electrical conductivity measurements in a diamond anvil cell up to ~623 K and ~12.8 GPa. Our results verified that the epsomite underwent a pressure-induced phase transition at ~5.1 GPa and room temperature, which was well characterized by the change in the pressure dependence of Raman vibrational modes and electrical conductivity. The dehydration process of the epsomite under high pressure was monitored by the variation in the sulfate tetrahedra and hydroxyl modes. At a representative pressure point of ~1.3 GPa, it was found the epsomite (MgSO4·7H2O) started to dehydrate at ~343 K, by forming hexahydrite (MgSO4·6H2O), and then further transformed into magnesium sulfate trihydrate (MgSO4·3H2O) and anhydrous magnesium sulfate (MgSO4) at higher temperatures of 373 and 473 K, respectively. Furthermore, the established P-T phase diagram revealed a positive relationship between the dehydration temperature and the pressure for epsomite.