Sorption And Desorption Of Heavy Metal Cu(Ⅱ) On The Biochar Under Electric Field

Recently,the pollution of heavy metal ions in China has become increasingly serious,among them,heavy metal Cu（Ⅱ）pollution is widespread.If Cu（Ⅱ）exceeds a certain limit in human body,it would lead to liver ascites and liver cirrhosis.What’s more,in severe cases,it would result in liver and kidney necrosis.The traditional methods to remove heavy metal ions include chemical precipitation,ion exchange,membrane separation and adsorption.Among them,adsorption method has become a common method due to its low cost,simple operation and no secondary pollution.However,in the conventional adsorption process,it is difficult to separate the adsorbent from the polluted liquid,regeneration needs a lot of chemicals,for the applicability of the ground water treatment is very limited a.The electrochemical adsorption method combines the advantages of electroadsorption and chemical adsorption.It has the advantages of low energy consumption,low price,high treatment efficiency,no secondary pollution,easy regeneration and the like,and a wide application prospect in the treatment of heavy metal wastewater.In this paper,corn straw was used as raw material to obtain by oxygen-limited pyrolysis biochar at different temperatures（BC300,BC500 and BC700）,and they were used as electrode adsorption materials to remove Cu（Ⅱ）from water.Elemental analysis,BET,FT-IR,Zeta potential and XRD were used to characterize the basic physical and chemical properties of biochar at different pyrolysis temperatures.The effects of voltage,p H,biochar dosage,biochar pyrolysis temperature and ionic strength on the electrochemical adsorption of Cu（Ⅱ）were explored.The mechanism of electrochemical adsorption of Cu（Ⅱ）by biochar was preliminarily analyzed;The effects of electrode material,voltage and electrolyte solution on Cu（Ⅱ）desorption were investigated.What’s more,the main factors affecting Cu（Ⅱ）desorption were explored by free radical quenching experiment and buffer solution system,and the properties of biochar after electrodesorption were studied.The electrodesorption mechanism was preliminarily analyzed.The biochar was modified with amino groups,and the factors affecting the electrochemical adsorption of Cu（Ⅱ）by amino-modified biochar were explored.The main conclusions of this paper are as follows:（1）The physical and chemical properties of biochar were analyzed.The results show that with the increase of pyrolysis temperature,the pore structure of biochar materials is more developed,the specific surface area is larger,the aromaticity is enhanced,the hydrophilicity is reduced,the oxygen-containing functional groups are reduced,the absolute value of Zeta potential is reduced,and he conductivity of biochar is enhanced.（2）The influencing factors and electrochemical adsorption mechanism of Cu（Ⅱ）were explained.The results show that the voltage is 0-1.2 V and the p H value of the solution is 2-5,the adsorption capacity of Cu（Ⅱ）by biochar increases with the increase of voltage,Cu（Ⅱ）solution concentration and p H value.Under the same conditions,BC500 has the best electrochemical adsorption effect on Cu（Ⅱ）.When Cu（Ⅱ）concentration is 100 mg/L,voltage is 1.2 V,p H=5,and biochar mass is 70 mg,adsorption of 5 h,The adsorption capacity of BC500 Cu（Ⅱ）is 22.5 mg/g,respectively,and the adsorption capacity is 2 times that of the case without voltage.The results show that the the electrochemical adsorption method has high adsorption efficiency,Cu（Ⅱ）can be effectively removed from water at a low voltage（1.2 V）.The adsorption kinetics of Cu（Ⅱ）by BC300 and BC500 are more consistent with the quasi-second-order kinetic model,and the adsorption kinetics of Cu（Ⅱ）by BC700 is more consistent with the quasi-first-order kinetic model.The isothermal adsorption of Cu（Ⅱ）by these three biochar is more in line with Langmuir isothermal adsorption model.The electrochemical adsorption mechanism of Cu（Ⅱ）on biochar is as follows:1)electroadsorption;2)cation exchange;3)complexe oxygen-containing functional groups;4)chemical precipitation;5)electrochemical reduction.（3）The influencing factors,desorption mechanism and changes of biochar properties were explored.The results show that the larger the chemisorption ratio of Cu（Ⅱ）on the surface of biochar,the harder it is to desorb it at lower voltage.With 0.1mol/L KNO₃ as the desorption electrolyte,Cu（Ⅱ）on the surface of BC300 needs 5 V voltage to basically completely desorb;The desorption on the surface of BC500 and BC700 is almost complete with only 3 V voltage.The release of H⁺from anode water electrolysis plays a positive role in the desorption of Cu（Ⅱ）.The mechanism of electric desorption includes:1)electric field repulsion;2)electrochemical oxidation;3)ion exchange between H⁺generated by water decomposition and heavy metals;4)decrease of equilibrium concentration of Cu（Ⅱ）on the electrode surface leads to physical desorption.In the process of electrodesorption.Biochar will be directly oxidized by anodic positive potential and indirectly oxidized by hydroxyl radicals,which would increase the hydroxyl and carboxyl content of biochar.（4）In order to further improve the adsorption performance,biochar was modified with amino groups.The results show that under the voltage of 1.2 V,p H 5 and the initial concentration of Cu（Ⅱ）100 mg/L,adsorption of 5 h,the adsorption capacity of Cu（Ⅱ）by BC500-NH₂ is 42 mg/g,which is 1.9 times that of unmodified BC500 and 2.3 times that of the case without voltage.The adsorption kinetics of Cu（Ⅱ）on BC500-NH₂ is more consistent with the quasi-second-order kinetic model,and the isothermal adsorption is more consistent with the Langmuir adsorption model.With 0.1 mol/L KNO₃ as the desorption electrolyte,96.5%Cu（Ⅱ）can be desorbed at a voltage of 5 V for 4h.