Abstract
Biomass wastes are abundant and common in our daily life, and they are cost-effective, promising, and renewable. Herein, collected willow catkins were used to prepare a hydrophilic biochar composite membrane, which was placed in a tree-like evaporation configuration to simulate a natural transpiration process. The strong light absorption (∼96%) of the biochar layer could harvest light and convert it into thermal energy, which then is used to heat the surrounding water pumped by a porous water channel via capillary action. A hydrophilic light-absorber layer remarkably increased the attachment sites of water molecules, thereby maximizing the use of thermal energy. At the same time, hierarchically porous structure and large specific surface area (∼1380 m2 g–1) supplied more available channels for rapid water vapor diffusion. The as-prepared composite membrane with a low-cost advantage realized a high evaporation rate (1.65 kg m–2 h–1) only under 1 sun illumination (1 kW m–2), which was improved by roughly 27% in comparison with the unmodified hydrophobic composite membrane. The tree-like evaporation configuration with excellent heat localization resulted in the evaporator achieving a high solar-to-vapor conversion efficiency of ∼90.5%. Besides, the composite membrane could remove 99.9% sodium ions from actual seawater and 99.5% heavy metal ions from simulated wastewater, and the long-term stable evaporation performance proved its potential in actual solar desalination. This work not only fabricated an efficient evaporator but also provided a strategy for reusing various natural wastes for water purification.
Highlights
Due to the lack of clean and safe water in the world, how to solve the crisis has become one of the most important focus in scientific research studies.[1−3] Desalination is an important technique for obtaining pure water from seawater
Large specific surface area, and porous structure, biochar derived from the biomass have been utilized in energy storage, energy conversion, and environment remediation.[26−31] In recent years, the carbonized biomass has been considered as an effective light absorber in solar steam generation, and most of the studies have focused on only using the high-temperature carbonization method.[32−35] For example, wood-based evaporators with low tortuosity have a low evaporation rate due to a relatively thick light-absorbing layer via direct carbonization or flame treatment.[32]
They only achieved high evaporation rate and energy conversion efficiency under strong sun illumination, which could increase the cost and energy consumption in the practical application.[33,36−41] Liu’s group directly carbonized lotus seedpods, which exhibited a lower evaporation rate of 1.3 kg m−2 h−1.42 Considering that the capillary action is inversely proportional to the pore size, the reasons of a low evaporation rate might be that the large pore size (>0.8 mm) of the petiole decreased the capillary action and water supply rate
Summary
Due to the lack of clean and safe water in the world, how to solve the crisis has become one of the most important focus in scientific research studies.[1−3] Desalination is an important technique for obtaining pure water from seawater. Solar steam generation is expected to be an efficient method to produce pure water.[7−11] With respect to the conventional nanofluid system, energy cannot be completely utilized to heat the bulk water; the process usually involves considerable heat loss between the bulk water and outer environment.[12−15] As for another reported light absorbers, such as plasmonic nanoparticles,[16,17] graphene oxide, and carbon nanotubes,[18−21] they still face high cost and complicated preparation procedure, which significantly restrict their large-scale application in desalination.[22] it is necessary to develop cost-effective materials with a high energy conversion efficiency.[23−25]. A tree-like evaporation configuration, which has been proved to have excellent heat localization ability, was utilized to simulate the natural transpiration process and collect clean water.[53,54] As a result, the composite membrane exhibited a highly efficient solar steam generation and a stable desalination performance. This work proposed a novel strategy of reusing willow catkins, which could be very promising in solar desalination and water purification
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