Study On Adsorption Mechanism Of Tetracycline And Cu²⁺ In Water By Strong Alkali-modified Biochar

In the complex polluted water,the coordination between tetracycline and copper can produce a new type of polymer,easily to induce the formation of tetracycline resistance genes,and the ecotoxicity is stronger than single pollution.In this paper,the adsorption theory and adsorption mechanism under single/compound pollutant systems were explored by studying the single and composite adsorption behaviors and mechanism of tetracycline and copper by strong alkali modified biochar,so as to explore interaction mechanism among different pollutants in compound pollutant systems and to provide technical support for removal and toxicity mitigation of tetracycline and copper pollution in water bodies.Alkali modification can enrich the pore structure of the original biochar,increase the specific surface area,and enhance the adsorption effect.In this study,the original biochars（LC,BC,WC）were prepared by pyrolysis of French sycamore leaves,soybean straw,and wheat straw as raw materials,and thenmodified with Na OH to obtain LPC,BPC and WPC.The physicochemical properties of biochar before and after modification were compared by specific surface area analyzer,Fourier transform infrared spectrometer,scanning electron microscope and Zeta potential analyzer.Desorption and regeneration experiments were conducted to study the adsorption capacity of modified biochar to tetracycline and copper,and the adsorption kinetics and adsorption isotherm models were used to explore the mechanism of the adsorption process.The main conclusions of this study include:（1）Effect of strong alkali modification on the physicochemical properties of biocharThe specific surface areas of LC,BC and WC were 42.16,16.47 and 24.68 m²·g^-1,respectively,and the average pore diameters were 4.039,11.112 and 7.542 nm.The specific surface areas of LPC,BPC and WPC increased by 2093.4%,7304.4%and4427.3%,respectively,and the average pore size decreased by 37.0%,78.5%,and 67.0%,respectively.The modified biochar showed more pore damage,indicating the development of the pore structure after strong alkali modification.The mixture of alkali metals and carbonate metals covered the surface of the biochar or entered the pores of the biochar,increasing the surface area and roughness;The aromaticity was lower than that of the original biochar,and the hydrophilicity and polarity are enhanced.The FTIR spectrum showed that the absorption peak between 1500cm^-1and 3000cm^-1has no obvious change before and after modification,indicating that the addition of Na OH did not affect its main functional groups.The negative charge of each biochar increased with increasing p H.（2）Single adsorption of tetracycline and Cu²⁺by strong alkali modified biocharIn a single pollution system,when the addition amount of biochar is 0.25 g·L^-1and p H at 11,the adsorption capacity of LC,BC and WC for tetracycline was 38.7,46.5 and39.3 mg·g^-1.LPC,BPC and WPC had the best adsorption effect on tetracycline at p H=3,and the adsorption capacity increased by 415.8,331.8 and 409.2%,respectively.The cations have little effect on the adsorption of tetracycline by LPC,BPC and WPC.The concentrations of Na⁺,Ca²⁺and Na⁺/Ca²⁺（both present at the same time）are in the range of 0.01 to 1 mol·L^-1,and the removal rates are all above 95%,indicating that strong alkali modified biochar has a good resistance to ion interference.When the desorption time was48 h,compared with the desorption rates of the original biochar（LC:31.8%,BC:34.5%,WC:30.3%）,tetracycline and the modified biochar were firmly combined,and no desorption occurred.After the adsorption-desorption-resorption cycle experiment,there is still a high adsorption capacity,indicating that the biochar modified by strong alkali has good regeneration performance.The adsorption capacity of Cu²⁺by each biochar increased with the increase of p H,and reached the highest at p H=6.The effect of different ion concentrations on the adsorption of Cu²⁺on BPC showed a trend of first inhibition and then promotion,and the effect on the adsorption of LPC and WPC showed a trend of inhibition at low concentrations and promotion at high concentrations.With the time of Cu²⁺desorption prolonged（increased from 24 h to 48 h）,the desorption rates of other biochars increased gradually except for LPC.（3）Co-adsorption of tetracycline and copper by strong alkali-modified biocharIn the composite systems(Cu²⁺=5,10,20 mg·L^-1,TC=50 mg·L^-1),the presence of copper decreased the adsorption rate of tetracycline on LPC,BPC and WPC,respectively,and prolonged the adsorption equilibrium time.In the composite system(Cu²⁺=5,10mg·L^-1,TC=100 mg·L^-1),the presence of Cu²⁺increased the equilibrium adsorption capacity of tetracycline.When the concentration of coexisting Cu²⁺is constant and the concentration of tetracycline increases from 50 mg·L^-1to 100 mg·L^-1,the adsorption capacity of tetracycline is obviously promoted by Cu²⁺.Compared with the adsorption of Cu²⁺in a single system,in the composite system of LPC and WPC(TC=25,50 mg·L^-1,Cu²⁺=10 mg·L^-1;TC=25,50,100 m·L^-1,Cu²⁺=20 mg·L^-1),and the composite system of BPC(TC=25,50,100 mg·L^-1,Cu²⁺=10,20 mg·L^-1),the presence of tetracycline increased the equilibrium adsorption of Cu²⁺,and the effect increased with the increasing of coexisting tetracycline concentration.In the composite system of LPC and WPC(TC=100mg·L^-1,Cu=10 mg·L^-1),the tetracycline concentration continued to increase to 100 mg·L^-1,and the equilibrium adsorption capacity of Cu²⁺did not increase,but decreased.When the concentration of tetracycline remained unchanged,the promoting effect on the adsorption of Cu²⁺would be significantly enhanced with the increase of the initial Cu²⁺concentration.（4）Adsorption mechanism of strong alkali-modified biochar on tetracycline and Cu²⁺The adsorption of tetracycline could be better fitted by pseudo-second-order kinetics and Freundlich isotherm model,indicating that the adsorption belonged to multi-layer chemical adsorption,the adsorption process is easy and fast.The adsorption involved the synergy of multiple mechanisms including electrostatic interactions,hydrogen bonding,surface complexation,and pore filling.The Freundlich isotherm model for LPC and BPC could better fit the process of Cu²⁺adsorption,which implied that the adsorption belonged to the multi-molecular layer adsorption.The fitting results of the Langmuir and Freundlich isotherm models for Cu²⁺by WBC were close,and there may be both monolayer and multi-layer adsorption.The coordination,precipitation,electrostatic interaction of tetracycline and Cu²⁺in the composite system,as well as the bridging effect between them and the adsorbent are the reasons for the increase in the adsorption capacity of tetracycline and Cu²⁺in the co-adsorption process.The competition of adsorption sites,the steric hindrance effect and electrostatic repulsion caused by the increase of the particle size of the complex are the reasons for the decrease of adsorption capacity.