Preparation And Application Of Hydrophilically Enhanced Corn Straw-based Photothermal Material

With the rapid development of human society and modern industry,the declined freshwater resources have become one of the most serious problems threatening human survival.In recent years,many water purification technologies have been developed to address this issue,among which interfacial solar steam generation（ISSG）technology is considered to be an environmentally friendly and energy-saving strategy.However,many challenges（such as high material cost,complicated manufacturing processes,difficult to recycling,and hard to massive production）remain in it.Owing to the outstanding properties of low cost,easy to obtain,and sustainability,biomass-based photothermal conversion materials have great potential to be promising photothermal materials.In the long desalination process,only relying on the inherent hydrophilicity and porous structure of biomaterials cannot remove the serious salt accumulation on the surface of the evaporator caused by the rapid evaporation of water in the interface layer.As a result,the evaporation rate would be reduced and the evaporator can even be damaged.Therefore,this paper prepared a novel solar evaporator based on hydrophilic modified corn straws.Natural corn straws were first decorated by using the citric acid soaking method to increase their hydrophilicity.Subsequently,35 wt%sulfuric acid was used to carbonize the corn straw with enhanced hydrophilicity（here named CCSH）to increase its light absorption.Next,inspired by plant transpiration,a CCSH-interfacial solar steam generation（CCSH-ISSG）system was constructed.Finally,this device was applied for seawater desalination and heavy metal wastewater treatment,and its evaporation performance and heavy metal adsorption performance were tested.The main research was concluded as follows:（1）Preparation of CCSH-ISSG Photothermal Material and Analysis of Its Evaporation PerformanceA photothermal material with high hydrophilicity and great light absorption was prepared using waste corn straw as substrate,citric acid as a carboxyl group introducing source,and sulfuric acid as a carbonizing agent.The photothermal material was characterized by SEM,FT-IR,XRD,contact angle meter,and UV-VIS-NIR spectroscopy,and its photothermal conversion performance was evaluated by its evaporation performance in pure water.The results show that the corn straw modified by citric acid had good hydrophilicity,with an increase in water absorption from 138 wt%to 337 wt%.In addition,the light absorption was increased by sulfuric acid dehydration and carbonization and was up to 96.5%in the solar spectrum range of 300-1000 nm.Evaporation experiment results showed that the evaporation rate and efficiency of CCSH-ISSG reached 1.422 kg m^-2 h^-1 and 89.3%under 1 sun,respectively,and CCSH also had good stability and cyclability（2）Study on Salt Resistance Mechanism and Seawater Evaporation Performance of CCSH-ISSGDuring the long seawater desalination process,the interfacial solar steam generation method would cause salt accumulation problems on the surface of the evaporator.The salt accumulated can damage the evaporator,reduce the evaporation rate,and increase equipment maintenance costs.Therefore,the feasibility of CCSH-ISSG application in seawater desalination and the mechanism of salt resistance were evaluated by artificially simulating its actual evaporation performance and surface salt condensation in different concentrations of brine.The experimental results showed that due to the hydrophilic improvement by citric acid,the evaporation rate and efficiency of CCSH-ISSG reached 1.301 kg m^-2 h^-1 and 81.9%,respectively,even in high-concentration artificial prepared brine（20 wt%）.As the evaporation time extends,no salt deposits were observed on the surface of the evaporator.At the same time,the results of seawater desalination experiments conducted by outdoor sunlight also demonstrated that the quality of fresh water produced by CCSH-ISSG desalination of seawater in the Bohai Sea is excellent,which fully meets the standards of the World Health Organization for drinking water quality.Besides,under low light(<1 k W m^-2),the evaporation efficiency of CCSH-ISSG remained above 80%,which provides important theoretical value for its long-term and stable seawater desalination application.（3）Research on Interfacial solar steam generation of Heavy Metal Wastewater of CCSH-ISSGIn the treatment of heavy metal wastewater,the evaporation method has been widely used because of its high efficiency,strong adaptability,and the fact that no additional chemical reagents are required.However,in process of high acid wastewater treatment,its applications are limited by high operating costs and serious equipment corrosion.Therefore,the feasibility of CCSH-ISSG for treating heavy metal wastewater was explored with the presence of different concentrations of oxidants,acidity,and other coexisting ions.The experimental results showed that the photothermal conversion ability of CCSH-ISSG was not affected in the solution containing acid or oxidant.Even under strong acid（p H=2）and strong oxidant（3 mol/L HNO₃）,the photothermal conversion efficiency of CCSH-ISSG was only slightly reduced.Moreover,CCSH also had good heavy metal ion removal capacity and durability.The detection of the collected condensate found that the five heavy metal ions added after treatment had all decreased by 3-5 orders of magnitude,with the heavy metal ion retention rate of 99.9%,and the hydrophilicity of CCSH was not destroyed after the treatment of the wastewater.