Dual Function Solar Evaporator For Efficient Water Evaporation And Clean Fresh Water Generation

Since the industrial revolution,society and industry have been developing at a rapid pace,and the ensuing water shortage has become a severe problem that all countries in the world need to face.How to obtain clean fresh water through seawater desalination and wastewater treatment has naturally become the object of people’s attention.To address this challenge,many methods for obtaining clean fresh water have been developed,such as multistage flash evaporation,multi-effect evaporation and electrodialysis.However,these methods often require a large amount of heat energy or electric energy,and a large part of them are wasted due to energy loss,which leads to the waste of a large number of fossil energy.At the same time,the use of fossil fuels inevitably leads to the risk of environmental pollution.In order to obtain clean water,it is obviously not the most reasonable way to pollute the environment.In nature,solar energy is one of the main driving forces of global water cycle,and solar water evaporation plays an extremely important role in the global water cycle.As the most abundant renewable energy on the surface of the earth,solar energy can be regarded as inexhaustible for human beings.It is not only abundant in reserves but also cost-free.The energy radiated to the earth is far higher than the total energy consumption of the world.Solar energy also has many advantages,such as universal existence,clean,harmless and pollution-free,no transportation,etc.,so seawater desalination and sewage purification using solar energy is considered to be a very ideal way to obtain clean water.However,due to the low efficiency of evaporation of seawater or wastewater directly exposed to sunlight,so little clean fresh water is available,researchers need to explore the scientific mechanism behind solar-powered water evaporation,so that it can be modified to solve the serious problem of global water shortage.In recent years,due to the proposed interface evaporation system,people through a variety of structural design and the selection of appropriate substrate materials and photothermal conversion materials,various kinds of solar evaporators are prepared,the absorption of sunlight into heat energy restricted in the evaporation interface for local heating,in order to obtain higher evaporation efficiency.On this basis,we made use of the high absorbance of r GO and the characteristics of polyvinyl alcohol hydrogel that can reduce the enthalpy of water evaporation to prepare the solar evaporator,the details are as follows:1.Graphene oxide was prepared by the improved Hummers method.Potassium permanganate in concentrated sulfuric acid was used for oxidation reaction with graphite powder to obtain graphite flakes containing a large number of oxygen-containing groups on the edges and planes,which could be peeled into GO by ultrasonic crushing and formed a stable single layer of GO dispersion in water.Graphene oxide was then treated with ascorbic acid to produce a reduced graphene oxide that still carried a small number of groups.Although the resulting r GO have many defects that make them inferior to real graphene,the process is relatively simple and provides a way to make r GO.2.Preparation and properties of polyvinyl alcohol/reduced GO composite hydrogel solar evaporator with Fe₃O₄:Magnetic Fe₃O₄ particles were prepared by chemical coprecipitation method.Soluble salt of iron bivalent and iron trivalent were mixed into solution in a certain proportion,and then basic precipitator was added into the mixed solution of iron salt.After a long-time stirring reaction,the magnetic Fe₃O₄ particles were centrifuged and placed in a vacuum drying oven for drying.The GO prepared by the above method was evenly dispersed in the water solution of polyvinyl alcohol and poured into the mold,and then the prepared Fe₃O₄ particles were added.The magnetic Fe₃O₄particles were concentrated in the bottom layer by using a magnet,and PVA/r GO composite hydrogel with Fe₃O₄ was obtained as a solar energy evaporator by repeated freeze-thaw and then reduction.Due to the excellent photothermal conversion ability of r GO and the effect of reducing the enthalpy of water evaporation of polyvinyl alcohol hydrogel,the evaporation of water is greatly promoted.Fe₃O₄ as heterogeneous Fenton catalyst can further strengthen the wastewater treatment capacity of the evaporator,can purify the volume of water and the evaporator itself,and broaden the range of use of the evaporator.Under the irradiation of one sun,the surface temperature of PVA/r GO composite hydrogel with Fe₃O₄ increased from 14℃to the highest30.3℃,and the evaporation rate of the evaporator reached the highest 2.15 kg m^-2 h^-1,and the photothermal conversion efficiency was 84.34%.The evaporator also has a good prospect in desalting and purifying water.The ion removal rate is above 96.86%.The treated water also meets the drinking water standard of the World Health Organization.3.Preparation and performance research of polyvinyl alcohol/copper sulfide/reduced GO composite hydrogel solar evaporator with water evaporation and purification function:Cu S/r GO composite material was prepared by hydrothermal method,with water as solvent,copper chloride as copper source and thiourea as sulfur source,and GO dispersion was added,and the hydrothermal reaction was carried out in the reaction kettle after mixing evenly.Cu S/r GO composites were obtained by centrifugation and drying after the reaction was completed.The PVA/Cu S/r GO composite hydrogel solar evaporators were obtained by evenly dispersing the composite materials into PVA aqueous solution and adhering them to cotton cloth.Due to Cu S/r GO’s excellent photothermal conversion ability,and the effect of PVA hydrogels to reduce the enthalpy of water evaporation,water evaporation is greatly promoted.Cu S,as a semiconductor material,not only has good photothermal conversion ability,but also has certain photocatalytic ability.After being attached to r GO,its active site and electron transfer rate will be improved,so as to further improve its performance.Under the irradiation of one sun,the surface temperature of PVA/Cu S/r GO composite hydrogel increased from 23.4℃to the highest 34.3℃,and the evaporation rate of the evaporator reached the highest 2.34 kg m^-2 h^-1,and the photothermal conversion efficiency was 92.90%.The evaporator also has a broad application prospect in desalting and purifying water.The ion removal rate is above 99.87%,and the treated water is much higher than the World Health Organization drinking water standard.In addition,the evaporator has a good purification effect on methylene blue solution,and the purification rate reaches 72.56%in 2 h under the condition of one sun.