Adsorption Efficiency Of Selenium-enriched Straw Biochar On Heavy Metal Cadmiu

China is an agricultural nation with a sizable population that annually produces a sizable amount of recyclable agricultural and forestry straw.The ecology has been severely harmed by the long-standing arbitrary disposal or careless burning of agricultural and forestry waste.The use of straw waste has become a hot topic of widespread concern in today’s society due to the advancement of environmental science,agricultural research,and energy science.In recent decades,China’s economic,industrial,and urbanization levels have all increased.The rapid expansion of industrial activity has resulted in a spike in water pollution that is jeopardizing both human life and the environment,drawing a lot of attention.Due to the mobility,high toxicity,and ease of bioconcentration of heavy metals,industrial discharge of heavy metal wastewater can result in growing levels of heavy metal pollution in soil and water bodies,posing major concerns to the environment and public health.Chemical precipitation,filtration,chemical and ion exchange,electrochemical treatment,adsorption,and evaporation are conventional procedures for cleaning up water pollution and are employed in a number of ways to immobilize metal ions in aqueous solutions and soils.The use of biochar adsorption to fix or remove heavy metals from the environment is a straightforward,affordable,and efficient method;however,the adsorption capacity of biochar prepared by conventional methods is constrained and cannot satisfy the needs of environmental treatment,such as that of soils and water bodies.It is crucial to modify biochar to increase its capability for adsorbing heavy metals.In this study,the use of selenium-rich straw waste from agroforestry is investigated,along with the creation of modified biochar for heavy metal adsorption.It is being investigated as a new modification method that may effectively increase the pore size of biochar and offer a better solution for the heavy metal cadmium adsorption by biochar that combines alkali modification with ball milling.As the base material for the modified biochar,rice straw high in selenium was used,and the preparation parameters were optimized through the adsorption of the heavy metal cadmium in water bodies.The adsorption mechanism was investigated through material characterization,and the experimental findings were as follows:（1）Calcium chloride modified biochar was prepared by using selenium-rich straw as raw material and using the method of impregnation followed by pyrolysis.The best parameters for the adsorption of heavy metal cadmium were selected by studying the factors affecting the properties of biochar such as pyrolysis time,pyrolysis temperature,particle size,calcium chloride concentration and yield.The results of adsorption experiments showed that the biochar with the best parameters of pyrolysis temperature of 700℃and 600℃,calcium chloride concentration of 1.5 mol·L^-1,straw particle size of 100 mesh,soaking time of 24 h and pyrolysis time of 1 h had the strongest adsorption capacity of cadmium,and the adsorption rates were 100%.（2）The results of the preparation of selenium-rich straw biochar by calcium chloride modification showed that biochar produced by pyrolysis at 700°C（BC700）had the best adsorption effect than biochar produced by pyrolysis at 600°C（BC600）at two pyrolysis temperatures.The adsorption results indicated that the optimum solution p H for heavy metal Cd adsorption by biochar was 5.Kinetic and isothermal adsorption models were used to predict the adsorption of Cd²⁺by calcium chloride modified biochar,and the fitting results showed that the kinetic equations were consistent with pseudo-secondary kinetics and the isothermal adsorption model was consistent with the Langmuir adsorption model.The analysis of batch adsorption experiments and characterization results showed that electrostatic adsorption,ion exchange and complexation of oxygen-containing functional groups（OFG）were proved to be the main adsorption mechanisms.（3）The selenium-rich straw water-impregnated biochar,KOH-modified biochar and KOH-modified combined ball-mill-treated biochar,named QS700,KBC and KBC-QM,were prepared at a pyrolysis temperature of 700℃with a pyrolysis time of 1 h.The results showed that the ball-mill biochar possessed faster adsorption kinetics（5 min）and had a significant adsorption capacity for Cd²⁺(maximum capacity of 395.5 mg·g^-1).The adsorption process was well described by the Freundlich model and the pseudo-secondary model.The adsorption process includes fast and slow adsorption,and the adsorption rate of Cd²⁺is controlled by the chemical reaction.The adsorption mechanism mainly includes functional group complexation,Cd²⁺-πinteraction and precipitation.The ball-mill modified biochar has great potential for remediation of Cd²⁺wastewater and is important for improving the application of biochar in Cd²⁺ environmental remediation.