Preparation and performance analysis of solar interfacial evaporator from modified corncob

Solar-driven seawater desalination has been one of the most promising approaches to alleviate energy pressure, freshwater resource shortage and environmental crisis. Solar interfacial evaporation can be confined the heat into the evaporation surface, in order to avoid the bulk water heating for the energy saving. In this study, the dopamine was used to modify the natural corncobs, in order to reduce the degradation of natural biomass for the high evaporation efficiency in seawater desalination. A solar interfacial evaporator was prepared to integrate the photothermal conversion, water transportation, and thermal insulation. The substrate was selected as the carbon black-cellulose film coating on the modified corncob. The light absorptance of natural corncobs with/without the carbon black-cellulose film were measured using a UV-visible spectrophotometer. The corncobs with carbon black-cellulose film was significantly improved the absorption performance, with an average absorption rate of 94% in the entire solar radiation band. The contact angle of water droplets on the modified corncobs was about 29.9°, which was reduced significantly. The hydrophilicity of the modified corncobs was enhanced for the transfer of water in the corncobs. The water transportation along the corncob was visually displayed using infrared images. Once the bottom of the corncob came into the contact with water, no water transport was observed within the unmodified corncob in 60 min. By contrast, the water was observed at a lower temperature along the dopamine modified corncob, indicating the better performance of water transport than before. The evaporation mass of modified and unmodified corncobs was measured under the same conditions. Both the corncobs were covered with the carbon black-cellulose film. The evaporation rates were 1.38 and 1.25 kg/(m²·h) in the modified and unmodified corncob evaporator respectively. There was the increase by about 10.4% in the modified corncob, compared with unmodified ones. Another indicator was the stability of evaporators in seawater environment under cyclic operation for a long time, in order to evaluate the performance of evaporators. There was no salt crystallization on the surface after continuous irradiation of the modified corncob evaporator with a light intensity of 1 kW/m² for 8 hours. 30 cycle experiments were conducted on the modified corncob evaporator. The evaporation rate was 1.374 kg/(m²·h) in the first test, and the second after 30 cycles, the evaporation rate was 1.36 kg/(m²·h). The fluctuation of evaporation rate was with 1% over the 30 cycle experiments, indicating the stable evaporation rate. Moreover, the adhesion of dopamine was remained intact without salt deposition in the multiple cyclic experiments. There was no softening or material degradation in the modified corncob under seawater environments, indicating the long-term stability. Seawater distillation experiments was conducted in outdoor environments, in order to analyze the water production and ion removal of corncob evaporators. The daily water production reached 10.171 kg/m². The various ions in seawater and condensate were measured using the Varian 720-ES inductively coupled plasma emission spectrometer (ICP-OES). The concentrations of the main ions Na⁺, K⁺, Mg²⁺, and Ca²⁺ in condensate significantly decreased by about four orders of magnitude, reaching the limits of the drinking standards released by the World Health Organization (WHO). The evaporating seawater through corncobs can obtain the fresh water to fully meet drinking standards. The modified corncob evaporator was simple preparation, cost-effective and stable evaporation performance. The finding can provide an effective way to expand the application of biomass materials in the field of seawater desalination.