Preparation And Study On The Application Of The Modifie Corn Straw

The molecular chain of lignocellulose of straw contains a large number of active groups such as hydroxyl,carboxyl and amino group,which can be used to purify contaminated wastewater through ion exchange and complexation.However,adsorption process of original straw suffers from low adsorption capacity and slow adsorption rate.In order to improve its adsorption performance,many modification methods have been explored,among which the most representative is to conervert it into biochar.The current widely used biochar production technology is the pyrolysis of biomass to form carbon-rich solid products at temperature above 400℃under oxygen-deficient condition.However,the amount of oxygen-containing functional groups of the product is lower than that of original straw.Many studies have shown that the pollutant removal of biochar mainly follows the physical adsorption mechanism.It is expected that the chemical adsorption of the straw can be enhanced by increasing the amount of the oxygen-containing functional groups.To this end,this thesis focuses on the influence of thermal oxidation on adsorption performance of modified straw,as well as the mechanism of adsorbing heavy metals in soil and water,cationic dyes in water and ammonia in livestock and poultry manure,as follows:（1）Using corn straw as raw material,the modified straw was prepared by low temperature thermal oxidation method.The effects of temperature,atmosphere composition and Pt/Ti O₂catalyst on the adsorption performance of the product were discussed.The results show that with the increase of the modification temperature,the yield under nitrogen atmosphere is always higher than that under oxygen atmosphere;The adding of Pt/Ti O₂catalyst slightly accelerated the decreasing trend of the yield under oxygen atmosphere with the increase of the modification temperature.Boehm titration and p H test showed that the surface acidic and alkaline functional groups of the modified straw increased with the increase of the modification temperature in the range of 80-220℃.When modified temperature is higher than 140℃,the oxygen atmosphere favors the generaton of acidic functional groups in comparison with nitrogen atmosphere.The addition of Pt/Ti O₂catalyst is beneficial to the formation of lactonic groups under the oxygen atmosphere.The results of SEM and BET analysis showed that the modified straw skeleton structure did not change significantly,and the specific surface area was basically unchanged.IR and X-ray diffraction profiles indicate that with the increase of modification temperature,lignocellulose in the straw begins to decompose,and the crystallinity of the modified straw decreases;The intensity of C-H peak at2919 cm^-1decreases,and polar groups such as phenolic groups at 1239 cm^-1or alcohol hydroxyl peak at 1159 cm^-1increase;Oxygen atmosphere is more favorable to form more oxygen-containing groups than nitrogen atmosphere;At a modification temperature higher than 180℃,not only can the esterification reactions occur,but also a significant decarboxylation reaction,resulting in increased p H.Aromatic carbon structural substances are formed,evidenced by the C=C peak enhancement at 1602 cm^-1.The addition of Pt/Ti O₂catalyst promoted the occurrence of oxidative degradation reaction under oxygen atmosphere.Lower modification temperature leads to a higher dissolved organic carbon（DOC）content in modified straw due to partial decomposition of straw,but higher modification temperature induces a higher DOC content,which may be due to the occurrence of aromatization reaction and the volatilization or mineralization of small molecular organics.（2）The modified straw after characterization was applied.Firstly,the mechanism of immobilizing Cd（II）in water and soil was explored.Taking the 220℃products with high functional group content as an example,we obtained the XPS spectrum before and after the adsorption of Cd（II）from water.It is confirmed that Cd（II）are mainly absorbed on hydroxylic gruops,carboxylic gruops and aromatic structures through complexation and coordination.With the solution of 160.2 mg·L^-1Cd（II）chloride containing Cd（II）as simulated wastewater,the adsorption experiment of modified straw with dose amount of 4 g·L^-1showed that the adsorption capacity of modified straw to Cd（II）gradually increased with the increase of the modification temperature.However,the p H of the product is not within the range of Cd（II）precipitation,which rules out the possibility of Cd（II）precipitation with high p H,and there is no correlation between the specific surface area and the adsorption properties.However,there is a strong linear relationship between Cd（II）adsorption and total functional groups（acidity+basicity）.It is concluded that the adsorption of Cd（II）is dominated by chemical adsorption.We select the modified straw prepared by 220℃whose mass ratio to soil was 1:20,and the mass concentration of cadmium in soil was 4 mg·kg^-1.The results of soil culture experiments further support that the modified straw adsorption Cd（II）was mainly chemical adsorption.Taking into consideration of surface functional group concentration and the product yield after modification,and the performance of remediation of Cd（II）from water and soil,modified straw prepared by 200℃under oxygen atmosphere conditions is the best choice.（3）Since both Cd（II）and Cu（II）are the primary pollutants in water,the mechanism of removing of Cu（II）from water was explored.In the adsorption experiment,copper sulfate solution containing 1.63 mmol·L^-1Cu（II）was used as simulated wastewater and the dose of modified straw was 4 g·L^-1.With the increase of modified temperature,the adsorption capacity of Cu（II）gradually increased;The BET results indicate that there is no correlation between the specific surface area and the adsorption performance;XPS analysis confirmed that the Cu（II）were mainly absorbed through complexation and cation-πcoordination on carboxylic groups and aromatic structures.Based on the linear fitting between the functional group and Cu（II）adsorption amount of the modified straw,there is a strong correlation between the total functional groups and Cu（II）adsorption.It is concluded that the interaction between Cu（II）and modified straw is mainly chemical adsorption.Taking into consideration of surface functional group concentration,the product yield after modification,and the performance of remediation of Cu（II）from water,modified straw prepared at 220℃under oxygen atmosphere conditions is the best choice.（4）In addition to heavy metal ions,dye is an important pollutant in water,so the mechanism of removing cationic dyes from water by modified straw was explored.The adsorption experiments used 40 mg·L^-1crystal violet（CV）and 30 mg·L^-1methylene blue（MB）solutions as simulated wastewater,and the dosages of modified straw were 0.3 g·L^-1and 0.4g·L^-1,respectively.It showed that the maximum adsorption capacities of modified straw for MB and CV adsorption was achieved at 180℃and 200℃,respectively.The p H and BET results show that the adsorption of modified straw has little correlation with p H,and the specific surface area is not the main factor affecting the adsorption of cationic dyes.FTIR analysis confirmed that the hydroxylic groups,carboxylic groups and aromatic structures interact with cationic dyes mainly throughπ-πandπ-receptor pathways.According to the linear fitting between the functional group and the adsorption amount of the modified straw,it is concluded that the linear fit between the CV adsorption amount and the functional group is low or even negative,and the correlation between MB adsorption amount and functional groups from high to low order is:phenolic groups>lactonic groups>carboxylic groups.This phenomenon is likely attributed to the distinct interactions exhibited by different functional groups,as well as steric effects.Taking into consideration of surface functional group concentration and the product yield after modification,and the performance of remediation of methylene blue and crystal violet from water,modified straw prepared by 180℃and 100-180℃under oxygen atmosphere conditions are the best choice.（5）The mechanism of NH₃removal from modified straw was explored.In the adsorption experiment,the mass ratio of modified straw and NH₃·H₂O is 1:10 and the contact time is 60minutes.It showed that with the increase of modified temperature,the adsorption capacity of modified straw on NH₃gradually increased.The BET results indicate that pore structure is not the main factor of NH₃adsorption in modified straw;the XPS analysis confirms that the carboxylic and hydroxylic groups in the modified straw provide the main adsorption sites for NH₃adsorption.Through the linear fitting between the functional groups on modified straw and NH₃gas adsorption,it is found that there is a strong linear relationship between NH₃gas adsorption and the total functional groups（acidity+alkalinity）,and the dependence of functional groups from high to low is:phenolic groups>lactonic groups.The modified straw prepared by 220℃was selected with mass ratio of modified straw to chicken manure of 1:10.The highest ammonia emission reduction rate is 48.8%,which further supported that the adsorption of NH₃by modified straw was mainly chemical adsorption.Taking into consideration of surface functional group concentration and the product yield after modification,and the performance of remediation of NH₃from livestock manure,modified straw prepared by 220℃under oxygen atmosphere conditions is the best choice.