Construction Of Porous Aerogel System And Application In Desalination Of Saline Alkali Water

Saline water is formed by people washing saline soil with large amounts of fresh water through diffuse irrigation,and because it contains large amounts of soluble salt ions that are difficult to remove,random discharge can cause environmental problems.Photothermal desalination using interfacial evaporation materials is an effective method for sustainable removal of soluble salt ions from saline water.It uses clean solar energy as the energy source,achieves photo-thermal conversion through the light absorber,and localizes the energy in the interface region to heat a small amount of water in the material to obtain a high photo-thermal vapor conversion efficiency for effective separation of water and soluble salt ions.The technology is clean,portable,low cost and low energy consumption,which helps desalination and utilization of saline water in remote areas and promotes rapid regional economic development.However,interfacial evaporation materials have poor salt and acid-base resistance,weak cyclic stability,and easy accumulation of salt particles on the surface in practical applications,thus reducing the photothermal conversion efficiency and the rate of water vapor generation.To solve the above problems,this thesis constructs a porous aerogel system with resistance to oil contamination and salt crystallization as well as fast water transport based on the theory of local thermal effect,using hydrophilic polymers and photothermal polymers as the main raw materials,and explores the mechanism of its efficient evaporation in the process of photothermal desalination through surface modification and structural modulation.Its main work is as follows:（1）Aerogels with three-dimensional porous structure were constructed by chemical cross-linking,in-situ polymerization and freeze-drying techniques using polyvinyl alcohol（PVA）,sodium alginate（SA）and aniline monomer as the main raw materials.Due to its rich macroporous structure,relatively rough material surface and the synergistic effect of hydrophilic polymer matrix,the aerogel exhibits good photothermal conversion properties and oil stain resistance.The maximum photothermal conversion efficiency of this 3D porous structure aerogel was 91.5%under the light of 1 k W·m^-2,and the evaporation rate of water was 1.46 kg·m^-2·h^-1,and the evaporation rate was stable in oil-containing wastewater and actual saline water.Even in water containing emulsified oil,aerogels exhibit good resistance to oil and photothermal evaporation.（2）The aerogel with vertically oriented structure was constructed using sodium acetate（Ac Na）salt template method with acrylamide（AM）,sodium carboxymethyl cellulose（CMC-Na）and pyrrole monomer as the main raw materials.This aerogel facilitates rapid water transmission and effective light absorption due to its vertically oriented porous structure,thus realizing surface resistance to salt crystallization and efficient evaporation of aerogel in saline water desalination.The experimental results can be concluded that the maximum photothermal conversion efficiency of this vertical structured aerogel is 90.7%and the evaporation rate of water is 2.42 kg·m^-2·h^-1 under the light of 1 k W·m^-2.Compared with the disordered porous structure,the vertical structure of this aerogel can quickly and effectively transport water and exchange salt ions with the aqueous body below the aerogel,thus achieving salt crystallization resistance on the aerogel surface.（3）Aerogels with layered structures were constructed by a dual-template method using Ac Na and silicon dioxide（Si O₂）with AM,CMC-Na and pyrrole-dopamine monomers as the main raw materials.This aerogel facilitates a more rapid and efficient water transport due to its well-layered structure,thus achieving salt crystallization resistance on the surface of the aerogel during photothermal evaporation.The experimental results yielded that the maximum photothermal conversion efficiency of the aerogel with this layered structure was 88.3%and the evaporation rate of water was 2.46 kg·m^-2·h^-1 under the light of 1k W·m^-2.Meanwhile,the aerogel with layered structure exhibited good mechanical properties with a maximum compressive stress of 79 MPa relative to the disordered porous structure and vertical structure.（4）CMC-Na,AM and pyrrole-dopamine monomers were used as the main raw materials to construct vertically structured aerogels with phase change thermal storage capability by Ac Na salt template method and phase change substance sodium rigidate（SA）modification.The anionic groups on SA molecules can break the hydrogen bonds of water molecules within the micro-nano pore channel,making it difficult for them to form clusters within the pore channel,thus accelerating the rate of water transport.SA is also a phase change energy storage material that enables the cyclic storage and release of thermal energy.The experimental results yielded a maximum photothermal conversion efficiency of 92.3%and an evaporation rate of 3.07 kg·m^-2·h^-1 for this aerogel under 1 k W·m^-2.of light.（5）Laminated fibrous structured aerogels with vertical orientation were constructed by the Ac Na salt template method using CMC-Na,AM,and pyrrole-dopamine monomers as the main raw materials along with the introduction of anionic polymer monomers（AMPS）and cationic polymer monomers（MAPTAC）.The laminar fiber structure of this aerogel facilitates the regionalization of moisture inside the micro-nano pores and allows for effective thermal management during light exposure,thus reducing heat dissipation.The following conclusions were obtained from the experimental results:the maximum photothermal conversion efficiency of this aerogel was 95.3%and the evaporation rate was 3.31 kg·m^-2·h^-1under 1k W·m^-2 of light.