C. Finnerty, B. Mi
University of California - Berkeley,
Keywords: solar steam generation, heat localization, desalination, zero liquid discharge, graphene oxide
Summary:As traditional water resources become strained and demand continues to soar, desalination is being considered as a viable option to augment existing water supplies. Although advances in membrane technology have dramatically reduce the energy consumption of desalination, state-of-the-art reverse osmosis (RO) systems are already capable of operating within 25% of the practical minimum energy limit when treating seawater at 50% recovery. To make matters worse, the minimum energy required for treatment increases with feed water salinity, as does the system’s fouling/scaling potential. This makes RO-based desalination challenging and costly when operating in high-salinity applications. Currently zero-liquid discharge (ZLD) is the primary approach for treating high-salinity waters, but these technologies are especially energy-intensive. For example, mechanical vapor compression (MVC)-based brine concentrators and crystallizers used in ZLD consume 20−25 kWh/m3 and 52−66 kWh/m3 respectively. Recently, membrane processes such as forward osmosis and membrane distillation have been the considered for ZLD applications due to their ability to use low-grade heat as an energy source, but it is unclear if they will offer substantial advantages over existing technologies. With high energy consumption remaining a major barrier to ZLD, there is a need for innovative technologies that can separate feedwaters into their fundamental components—water and salt—while drawing the energy required for treatment from sustainable sources. In nature, the mangrove tree has a complex salt management system that enables it to thrive in waters that exceed the salinity of seawater. This system is initiated when stomatal pores in the leaves are opened, allowing for water vapor to escape to the atmosphere. Evaporation in the leaf creates a negative pressure in the xylem of the plant which—in combination with capillary action—prompts water to be drawn from the soil into the plant’s roots. This water passes through filtering structures in the roots, allowing for the selective passage of certain salts that serve as essential nutrients for the tree. However, when excess salt accumulates, the mangrove is able to isolate and excrete the salt so that it crystallizes on the leaf’s surface. Inspired by the mangrove tree, a graphene oxide (GO)-based material to mimic the functionality of the leaf has been developed. Through sunlight harvesting and heat localization, the GO leaf is capable of enhancing bulk water evaporation by 185% under a typical solar radiation intensities of 0.825 W/m2. Corresponding to an evaporation rate of 2.0 liters per m2 per hour (LMH), the GO leaf operates at an energy efficiency of 78%. Furthermore, the evaporation rate increases with increasing light intensity and decreases with increasing salinity. During a long-term evaporation experiment with a 15 wt. % NaCl solution, the GO leaf demonstrated stable performance despite severe accumulation of salt crystals on the leaf surface. The GO leaf was restored to its pristine condition by simply scraping off salt crystals and rinsing with water. Therefore, the robust high performance and relatively low fabrication cost of the GO leaf could unlock a new generation of desalination technology that can be entirely solar-powered while achieving zero liquid discharge.