Research On The Adsorption Mechanism Of Modified Biochar For Sulfonamide Antibiotics In Water

Antibiotics are organic substances that are difficult to be biodegraded,and they pose a serious threat to human health when they enter the human body in the water environment.Using biochar to adsorb antibiotics in water is an effective way of pollution control,but the adsorption capacity of biochar is limited,and most of the modification methods to improve the adsorption capacity of biochar have the problem of high cost.Therefore,in order to solve the increasing problem of antibiotic pollution of water bodies and the high cost of biochar modification,biochar was prepared using agricultural and industrial wastes（distillers grain,wood chips,rice straw,corn straw）at different temperatures（300,450,600°C）,as well as phosphogypsum modified biochar from industrial waste phosphogypsum（10%,30%,50%,70%of phosphogypsum mixture）.The biochar was characterized and analyzed,and the adsorption capacity,adsorption mechanism,and dynamic adsorption experiments of sulfadiazine（SD）and sulfadimethoxine（SM2）before and after modification of biochar were investigated through adsorption influence factor experiments（carbonization temperature,phosphogypsum ratio,biochar dosing,initial p H of solution,and ionic strength of solution）,batch adsorption experiments（adsorption kinetics,adsorption isotherms,and adsorption thermodynamics）.The adsorption capacity,mechanism and factors influencing adsorption were investigated.As a result,this paper investigates the feasibility of using phosphogypsum to prepare modified biochar and apply it to the remediation of antibiotic pollution in water bodies.The main results obtained from the study are as follows.（1）Zeta potential analysis indicated that1-HBC-450-70 and 2-HBC-450-70 had zero charge points between p H 2 and 3,and their negative charge increased gradually with the increase of p H,i.e.they had a stronger adsorption capacity for anionic pollutants under acidic conditions.SEM analysis showed that a large number of white particles were successfully attached to the surface of the biochar,i.e.,in the pores,and the content of Ca elements in the surface composition was significantly increased.XRD analysis revealed that these particles were Ca SO₄,indicating that phosphogypsum was successfully attached to the surface of the biochar.These phosphogypsum particles attached to the biochar blocked the pores on the surface of the biochar and changed the pore structure,which led to a decrease in the specific surface area of the modified biochar,i.e.,physical adsorption such as microporous filling did not dominate the adsorption process of this biochar.However,FTIR analysis showed that the peak values of organic metals tensile vibrations and C-O of biochar were significantly increased after modification by phosphogypsum,and these groups were beneficial to promote hydrogen bonding and thus enhance the adsorption capacity.（2）Under the three carbonation temperatures of 300,450 and 600℃,the adsorption capacity of distillers grain prepared at 450℃and wood chip biochar（1-HBC-450 and 2-HBC-450）was the highest,especially 2-HBC-450,whose removal rate of SM2 reached almost 100%.And the adsorption capacity of modified biochar phosphogypsum gradually increased with the increase of its proportion.Modified distillers grain biochar（1-HBC-450-70）and modified wood chips biochar（2-HBC-450-70）prepared at 70%phosphogypsum ratio had the strongest adsorption effect,and the adsorption capacity of SD and SM2 was enhanced by 14-21 times compared with unmodified biochar.At the antibiotic concentration of 10 mg/L,the optimum dosage of 1-HBC-450-70 for SD and SM2 were 0.20 and 0.10 g,respectively,and 0.10 and 0.05 g for 2-HBC-450-70,respectively.At the optimum dosage,the adsorption capacity of both biochar species decreased significantly with the increase of p H,which was attributed to the change of the presence form of antibiotics in the high p H water environment and the inhibition of the original adsorption mechanism.The adsorption mechanisms of 1-HBC-450-70 and 2-HBC-450-70 for antibiotics and for Cl^-and SO₄^2-anions were not consistent,so the adsorption capacity of biochar was less affected by the ion concentration in solution.The main adsorption mechanisms of 1-HBC-450-70 and 2-HBC-450-70on SD and SM2 were hydrogen bonding,π-πdonor-acceptor interaction,electrostatic interaction,and hydrophobic interaction.（3）Among the adsorption kinetics,the Elovich model and the quasi-secondary kinetic model are more suitable to explain the adsorption processes of 1-HBC-450-70 and 2-HBC-450-70 on SD and SM2.The equilibrium adsorption amounts of SD and SM2 by 1-HBC-450-70 were 0.89 and1.78 mg/g,which were 18.0 and 16.2 times higher than those by unmodified biochar 1-BC-450,respectively.The equilibrium adsorption amounts of 2-HBC-450-70 on SD and SM2 were 1.94and 4.13 mg/g,respectively,which were 32.3 and 12.3 times higher than those of unmodified biochar 2-BC-450,respectively;in the adsorption isotherms,the Freundlich model and Langmuir model were more suitable to describe the adsorption of 1-HBC-450-70 and 2-HBC-450-70 on antibiotics,and the whole adsorption process was dominated by chemisorption,and both were heat absorption reactions.From the Langmuir model,the maximum adsorption of SD and SM2 by1-HBC-450-70 was 2.98 and 4.18 mg/g,respectively,which was 8.93 and 11.5 times higher than that of unmodified biochar at a temperature of 308 K,a solution p H of 3,and using biochar at the optimal dosing rate.The maximum adsorption amounts of 2-HBC-450-70 on SD and SM2 were4.40 and 8.91 mg/g,respectively,which were 11.6 and 22.4 times higher than those of unmodified biochar.2-HBC-450-70 showed the highest adsorption capacity for SM2,almost twice as high as1-HBC-450-70.The photodegradation process of SD and SM2 by Ti O₂phosphogypsum modified biochar was verified by photodegradation experiment.The photodegradation efficiency of two antibiotics by Ti O₂phosphogypsum modified biochar is 1.1%～5.2%.（4）Dynamic adsorption experiments using packed columns found that 1-HBC-450-70 and2-HBC-450-70 exhibited good adsorption capacity during the simulated treatment of real contaminated water,with almost complete removal of antibiotics from the effluent solution in the preadsorption period.Both Thomas and Yoon-Nelson models fit this adsorption process well.The50%adsorption penetration times for SD were 164.84 and 215.30 min for 1-HBC-450-70 and2-HBC-450-70,and 180.36 and 236.40 min for SM2,respectively,when the amount of biochar in the packed column was 5.0 g and the flow rate of the solution was 2 m L/min.The adsorption capacity of the phosphogypsum-modified biochar appeared to be significantly enhanced and showed good adsorption capacity in the simulated real treatment process.Based on the above,this study presents an alternative means of investigating the feasibility of using phosphogypsum to prepare functionalised biochar and use it for the remediation of antibiotic pollution in water bodies.