Study On 3D Solar Evaporator Based On Tannic Acid Preparation And Its Treatment Of High Concentration Brine

Solar-powered water desalination technology using light-to-heat conversion materials has many advantages,such as being free from fossil fuel consumption,low carbon emissions,and minimal pollution.As a result,this technology has garnered significant attention from researchers as a sustainable,cost-effective,and environmentally friendly means of obtaining drinking water from seawater.However,dealing with high-concentration saltwater（≥10 wt%）poses a challenge as salt crystals tend to form on the evaporator surface,which significantly reduces the light absorption performance of the light-to-heat material.Salt crystals can also block water transport channels and steam vent holes,leading to a significant reduction or even failure of the evaporator’s performance.In recent years,various strategies have been developed to suppress or transfer salt crystallization,but achieving high evaporation efficiency and excellent anti-salt pollution performance when dealing with high-concentration saltwater simultaneously has proven difficult.This study used a low-cost commercial porous melamine sponge as the substrate and modified it with a tannic acid-based light-to-heat coating.By adjusting the surface wettability of the light-to-heat coating and the evaporator’s assembly method,the bottleneck of the solar evaporator’s difficulty in achieving high evaporation rate and excellent anti-salt pollution performance when dealing with high-concentration saltwater was overcome.The main research contents of this paper are outlined below:Firstly,a three-dimensional solar evaporator was developed from the perspective of photothermal material design and evaporator structure design,consisting of photothermal superhydrophilic/superhydrophobic sponges and hydrophilic wires wound on the sides.Various characterization methods,including scanning electron microscopy（SEM）,Fourier transform infrared attenuated total reflection spectroscopy（ATR-FTIR）,X-ray photoelectron spectroscopy（XPS）,contact angle analysis（CA）,and ultraviolet-visible-infrared absorption spectroscopy,were used to investigate the physical and chemical properties of the obtained photothermal sponges.Subsequently,sponges with opposite water wettability were arranged in different combinations and assembled into a solar 3D evaporator using ordinary nylon ropes fixed and tied from the periphery.It was found that the solar evaporator obtained by alternating photothermal superhydrophilic and superhydrophobic sponges had the optimal photothermal water evaporation rate.This was because the addition of superhydrophobic photothermal sponges in this evaporator significantly increased the evaporating area of water,which was conducive to the rapid overflow of steam.The heat generated by the superhydrophobic photothermal sponge was also absorbed by the adjacent superhydrophilic sponge,which further promoted evaporation.The study also demonstrated that this 3D evaporator had excellent oriented salt crystallization function when dealing with highly concentrated saltwater.This was achieved through the unique design of the 3D evaporator:the nylon ropes at different positions on the3D evaporator had different temperatures and water transport rates.Saltwater was easily saturated and precipitated salt crystals on the ropes in contact with the superhydrophobic sponge,while ropes in contact with the superhydrophilic sponge were difficult to reach saturation and therefore did not have salt crystals precipitating.As a result,salt crystals only formed on the ropes in contact with the superhydrophobic photothermal sponge,which is called oriented salt crystallization.Since the salt crystals were formed on the side of the superhydrophobic sponge,they did not affect the absorption of solar radiation by the evaporator or hinder the overflow of water vapor.Therefore,even when dealing with highly concentrated saltwater,the 3D solar evaporator could operate continuously and stably for a long time.Under simulated sunlight,the 3D evaporator showed a high and stable indoor evaporation rate(2.3 kg m^-2h^-1)for concentrated seawater（20 wt%）.This evaporation process could also be achieved in outdoor fieldwork,with a clean water production rate of 1.82 kg m^-2h^-1from concentrated seawater（20 wt%）.This design solved the bottleneck problem of solar evaporators,which was the difficulty in balancing high evaporation rate and excellent resistance to salt pollution when dealing with highly concentrated saltwater.This will further promote the application of photothermal interface water evaporation technology in the treatment of highly concentrated saltwater.In nature,the trajectory of the sun’s movement is from east to west.The first developed 3D solar evaporator system had only two sides with a Janus structure,alternating between superhydrophilic and superhydrophobic light-thermal sponges,while the other two sides were both superhydrophilic light-thermal sponges.Under real outdoor lighting conditions,only one side of the Janus structure on the south-facing side is exposed to sunlight,while the Janus structure on the north-facing side is difficult to illuminate.This results in the concentration of salt precipitation mainly on the south-facing Janus surface when processing high-concentration saltwater,which significantly reduces the rate of salt precipitation under real outdoor lighting conditions,and is not particularly advantageous for certain scenarios that require salt collection.Additionally,this can also lead to a reduction in the rate of water evaporation under real outdoor conditions.To address these issues,a further design was developed,where all four sides（front,back,left,and right）of the 3D solar evaporator have a Janus structure.This new 3D evaporator is formed by embedding multiple superhydrophobic sponge strips into the superhydrophilic light-thermal sponge,forming a Janus structure composed of hydrophilic and hydrophobic light-thermal sponges on all four sides.The new design was tested under real outdoor lighting conditions,and the results showed that compared with the previous system where only two sides had a Janus structure,the new design with a Janus structure on all four sides,including the front,left,and right sides exposed to sunlight,had higher evaporation efficiency.This is because the increased number of Janus structures not only increases the evaporation area and gas escape channels but also provides more salt precipitation sites,enabling more efficient utilization of sunlight for high-efficiency water evaporation and efficient directional salt precipitation.When tested indoors using a xenon lamp to simulate solar radiation,the new 3D evaporator achieved a high and stable desalination rate(3.3 kg m^-2h^-1)for concentrated seawater（20 wt%）.Under outdoor solar radiation,the evaporation rate reached 2.08 kg m^-2h^-1.