Solar steam generation performance of corncob biochar cake and its application in water treatment

Abstract: Solar steam generation has been considered a promising strategy of renewable energy for the sustainable clean water supply. The environment-friendly solar utilization technology has a wide application prospects in the fields of sewage treatment, seawater desalination, and distillation separation. Among them, the key component of solar steam generation technology is the photothermal conversion material. But, the existing photothermal conversion materials have seriously restricted the popularization and application in practice, due to the low solar energy utilization, high price, and complex preparation process. Inspired by water transport in corncob, a biochar evaporator was proposed for the water treatment using cheap and easy to prepare corncob. A better performance was also achieved in the excellent light absorption, high efficient solar thermal conversion, water transmission, and the low water evaporation enthalpy. The pyrolysis samples of corncob biochar (CB) were prepared at 250℃, 450℃, and 650℃, marked as CB250, CB450, and CB650, respectively. A systematic investigation was made to explore the relationship between the preparation conditions and the CB properties and structural characteristics. As such, the pyrolysis temperature posed a significant effect on the CB physicochemical properties. Specifically, the solar steam generation rate and apparent energy utilization rate of CB increased with the increase of CB pyrolysis temperature under 1-sun illumination. The CB morphologies were gradually shifted from the macropores to the micropores/mesopores, as the pyrolysis temperature increased, indicating the enhanced thermal stability and light absorption. The hydrophilic pore structure of CB650 was developed as the photon trap to enhance the solar absorption of more than 81.6%, 91.7%, and 83.6% in the ultraviolet, visible, and near-infrared light regions, respectively. At the same time, such structural characteristics of CB650 also provided a strong capillary force for the rapid transmission of water. As a result, the pyrolysis temperature of 650°C was the best preparation temperature under one-sun illumination (1 kW/m), particularly with as high as 4.16 kg/(m2·h) evaporation rate and high apparent photothermal conversion efficiency of 97.8%. The comparative experiment of Li+ solution evaporation showed that the solar steam generation of CB650 mainly occurred in the form of small water clusters. The excellent performance of CB650 solar water evaporation was achieved in the higher light absorption capacity (96.4%), higher photothermal conversion efficiency, appropriate water transmission, and low water evaporation enthalpy. In addition, the CB650 was found to be stable solar steam generation after reusing it multiple times (10 cycles) without any noticeable degradation in the solar steam efficiency, where it still be stable at around 4.16 kg/(m2·h). More importantly, there was a significant decrease in the concentration of Na+, Mg2+, K+, and Ca2+ in the corresponding evaporative condensate after the CB650 purification, which was significantly lower than the World Health Organization drinking water standards. The concentration of Pb2+, Cu2+, Zn2+, and Cd2+ also decreased in the evaporative condensate, fully meeting China's drinking water standards. Furthermore, the removal of dyes by CB650 was mainly through solar water evaporation, rather than the decomposition of dyes by a light source. Therefore, the CB650 can be expected to serve as a high durability, excellent seawater, heavy metal, and dye wastewater evaporation. The findings can provide a new technical way for the reduction and resource utilization of corncob. A necessary theoretical and technical basis can also be offered for the application of solar steam generation technology.