Degradation Process And Damage Mechanism Of Salix Psammophila Sand Barriers In Atmospheric Exposure Section

Salix psammophila sand barriers are widely used in the field of desertification control,as a type of engineering sand control technology measure with good effect of wind prevention and sand fixation to set the sand barrier on mobile sand dunes,offering a solid foundation for desertification control.At present,the aging problem can easily weaken or eliminate the windbreak and sand-fixing benefits,which has become one of the main factors restricting the application of S.psammophila sand barriers.While studying the ecological protection benefits,we should also focus on the sustainable utilization of S.psammophila sand barrier resources.Therefore,it is important to understand the degradation process and performance degradation mechanism of S.psammophila sand barrier in long-term field environments.We studied the aging problems of S.psammophila sand barriers in a desert atmosphere exposed section.The atmospheric exposed sections of a S.psammophila sand barrier and fresh S.psammophila,which were set for 1 to 9 years in Northwest China,were selected as the research objects.The aging status of the S.psammophila sand barrier was explored,and its aging causes were analyzed through structural observations,as well as physical,mechanical,and chemical property measurements.At the same time,aiming at several typical environmental factors of S.psammophila sand barrier setting for a field environment for a long time period,the artificial environment was designed to accelerate the simulation of aging phenomena of S.psammophila sand barriers in sun and rain environments in the desert,and provide basic research on the physical and mechanical properties and chemical change mechanism of S.psammophila sand barriers.This provided a theoretical basis for the application of S.psammophila sand barrier pretreatment and improving resource utilization in production practice.The main conclusions as follows:（1）The degradation process of the S.psammophila sand barrier in the exposed atmosphere was mainly affected by water and ultraviolet radiation,desert atmospheric conditions have shown a remarkable impact on the protective performance of S.psammophila sand barriers,and a series of irreversible changes and loss of physical and mechanical properties,as well as main chemical components,will occur.In terms of the exposure time gradient,an exposure time of 5 to 7 years was an important time node for measuring the deterioration degree of S.psammophila sand barriers in the atmospheric exposed section.After setting for 7 years,the structure of the exposed atmospheric section of the S.psammophila sand barrier was seriously damaged,and obvious deterioration occurred in the structure.The phloem was completely decomposed,the xylem was basically completely cracked,and cracks appeared in the form of ring cracks,which extended to the adjacent side and fractured,and multiple through cracks appeared.After 7years of exposure,the mass loss was 48.19%,and the basic density,volume shrinkage percentage and MOR physical and mechanical indices were reduced by 17.38%,34.03%,and 18.08%,respectively.The content of cellulose,hemicellulose and lignin decreased by21.19%,22.99%,and 23.43%,respectively.The crystallinity decreased obviously,and the crystalline area in the cellulose decreased,while the mechanical strength of the S.psammophila sand barrier was affected.The absorption peaks of the chemical groups in the sand barrier of the S.psammophila sand barrier vibrated and shrank obviously,and the degradation of chemical components was very obvious.The content of C elements in the S.psammophila sand barrier decreased by 12.18%after 7 years,while the content of O elements and the ratio of oxygen to carbon（O/C）increased by 28.36%and 36.15%,respectively.（2）The alternating 20 treatments served as an obvious inflection point for the whole alternated desorption-absorption process.The alternating desorption-absorption accelerated aging treatment showed a tendency to remarkably weaken the physical and mechanical properties of the S.psammophila sand barrier,and the dimensional stability changed obviously.The deformation was the smallest in the middle,the deformation became larger as it developed toward the two ends,and the contraction was particularly obvious in the chordal direction.After 40 alternating desorption-absorption treatments,the mass loss percentage was about 8.49%,the density decreased by 28.28%,and the MOR decreased by 16.99%.The decrease in cellulose content,hemicellulose content,and lignin content were 26.32%,31.03%,and 3.51%,respectively.Hemicellulose and cellulose containing polysaccharides more easily degraded in the degradation process of the S.psammophila sand barrier resulting from alternating desorption-absorption,and only a small amount of lignin was degraded.With increasing alternating desorption-absorption times,the number of cracks in the S.psammophila sand barrier increased and continued to extend to both ends.We inferred that the shrinkage and cracking of the S.psammophila sand barrier occurred during the process of humidity environmental change,and the dry shrinkage crack was the main reason leading to the decline of its physical and mechanical properties.The depolymerization of carbohydrates and crystal reduction contributed to the change in the protective performance of the S.psammophila sand barrier.（3）UV radiation times of 96 and 144 days were important time nodes to measure the degree of optical aging of the S.psammophila sand barrier,lightness could be used to simply measure the color changes of the S.psammophila sand barrier.The photochemical degradation and photochromism of the S.psammophila sand barrier were mainly caused by the changes in lignin content.The polysaccharides（cellulose and hemicellulose）of the S.psammophila sand barrier were less affected by UV radiation.The decreases in lignin content,hemicellulose content,and cellulose content were 23.12%,14.30%,and 6.96%,respectively.At the same time,the carbon binding form changed,C₁content decreased significantly,C₂and C₃content increased,and the carbon oxidation state and binding energy increased gradually.At 1610,1510,1460,1425,and 1242 cm^-1,the absorption peak intensity of the aromatic ring structure of lignin obviously weakened by the wave number,indicating that a photooxidation reaction occurred.At the same time,the UV radiation of the S.psammophila sand barrier was a long process,and the single factor of UV radiation had less influence on the physical and mechanical properties of the S.psammophila sand barrier than alternating desorption-absorption.（4）Considering UV radiation,the addition of water treatment for 288 h was an important time point to measure the process of UV radiation and water interactions.The damage degree of the performance index by combined treatment of UV radiation and water was two to three times that of pure UV radiation treatment.The increase in water transport after water treatment accelerated the deformation and cracking phenomenon of the S.psammophila sand barrier,increasing the contact area of the S.psammophila sand barrier structure with UV radiation and water,allowing it to receive UV rays to a greater extent,and forming more free radicals after fully absorbing the UV rays.Under the combined action of oxygen and water,the free radicals and hydrogen peroxide triggered stronger chemical reactions,which accelerated the decomposition of the lignin polymer under UV radiation and destroyed the lignin hemicellulose matrix in the cell wall;thus,degrading the lignin,cellulose,and hemicellulose in the S.psammophila sand barrier more quickly.In the combined treatment process of UV radiation and water mist,elemental C was constantly in contact with the oxygen and water in the air.After the oxidation reaction,the chemical bonds became more stable and bonds with more stable properties were generated.With extended combined treatment time,the contents of C₂and C₃showed an increasing trend,which meant that the increase in water treatment intensified the changes in the C1s components of the S.psammophila sand barrier samples.Compared to the single factor effect of alternating desorption-absorption and UV radiation,the combined action of UV radiation and water resulted in more serious damage,which accelerated the damage and deterioration process of the S.psammophila sand barrier.