Constructing a Solar Evaporator with Salt-Collecting Paper by Stacking Hydrophilic Sponges for Freshwater Production and Salt Collection

Solar water evaporation is universally considered as an effective method to alleviate the freshwater shortage worldwide. A series of three-dimensional, salt-resistant evaporators have been brought out to improve the evaporation performance. However, little attention was paid to the collection of salt from seawater and high salinity brine. In this work, a carbon black polyvinyl alcohol (PVA) sponge was prepared to harvest light using a physical adsorption method to load carbon black on the PVA sponge. The experimental results showed that optimal light absorption (97.8%) and evaporation rate (1.60 kg m^–2 h^–1) were achieved when the carbon black PVA sponge was produced at a carbon black concentration of 1 g L^–1. Moreover, we stacked PVA sponges beneath the carbon black PVA sponge to construct a non-photothermal evaporation area. It was seen that the evaporator with an eight-layer PVA sponge had an excellent evaporation rate as high as 2.35 kg m^–2 h^–1. In addition, the salt-collecting paper, all-black printed by a laser printer, was inserted between the stacked PVA sponges for salt collection, and the eight-layer PVA sponge evaporator with the salt-collecting paper at the fourth floor was tested in a 2 h desalination experiment. The outcome indicated that a square meter of this evaporator can produce about 7.03 L of freshwater and 206 g of salt per day from 10 wt % NaCl solution. As a result, the solar evaporator developed in this work is capable of collecting salt and maintaining a high evaporation rate, which is of great competence in the fields of freshwater production and salt collection.