Calcium alginate hydrogel for high-yield adsorption-based desalination driven by ultralow-grade heat

Adsorbent-based desalination stands out as a promising solution to produce fresh water leveraging on ultralow-grade heat. We investigate the use of a bio-derived calcium alginate (CaAlg) hydrogel, an alternative to other common sorbents. A batch of CaAlg is synthesized and characterized in a gravimetric sorption analyzer. The resulting type II isotherm at 30 degrees C shows an exceptional water uptake equilibrium value of 1.28 g/g at a relative humidity of 70%, realizing nearly a 4-fold increase compared to standard silica gels under the same conditions. We test CaAlg under vacuum in a water desalination unit. Our material shows an excellent medium-term cyclability, with a stable uptake over 40 cycles of operation. We achieve water production even at a temperature as low as 45 degrees Celsius, with a specific daily water production (SDWP) of 6 m3/day/ton using a hot-source temperature of 60 degrees Celsius, realizing the most performant small-scale system in the context of ultralow-grade waste heat valorization.