Pollution Control And Remediation Of Eutrophic Water Bodies By Biomass Carbon-based Capacitive Deionization Technology: Research On Functional Regulation Mechanism And Selectivity Mechanism

Water eutrophication is a water pollution problem that countries all over the world are facing.Nitrate,a nutrient pollutant,is the main culprit in causing water eutrophication.The traditional nitrate remediation technology has a series of problems such as low treatment efficiency,high disposal cost and complex supporting facilities.Capacitive deionization（CDI）and its branch technologies are environmentally friendly pollution remediation technologies with high efficiency,low energy consumption,low cost,no pollution,and simple facilities.CDI technology does not require excessive input of external energy,does not produce toxic and harmful substances,and can recycle and reuse nitrate.Specific nitrate selectivity is necessary to reduce polluted water remediation costs and recover valuable nitrogen resources.However,the research mechanism of selective adsorption on CDI electrodes reported in the past is insufficient,and the electrode materials face problems such as high raw material price,complicated preparation process,and poor remediation performance,which cannot provide a theoretical research basis for the actual eutrophic water pollution remediation.Therefore,biomass carbon with low cost and abundant reserves in nature was used as the raw material to prepare biomass carbon,and the transcapacitive deionization and extended voltage capacitance deionization systems were constructed by introducing special functional groups.It not only realizes the CDI performance of large adsorption capacity and fast adsorption rate,but also realizes the selective separation and recovery of nutrient pollutant nitrate,which achieves the goals of water resources protection and water pollution remediation.The main work contents are as follows:（1）In order to reduce the high preparation cost of traditional CDI electrodes due to expensive raw materials and complicated processes,using chitosan biomass as raw material and potassium hydroxide as activated pore expander,through a simple mixed carbonization method,under different carbonization temperatures Nitrogen-doped porous carbon electrodes（CTS electrodes）with different surface structures and electrochemical properties were synthesized.The CTS-850 electrode has an ultra-high adsorption capacity of 47 mg g^-1 and a fast adsorption rate of 6.7 mg g^-1 min^-1,showing excellent performance in the remediation of polluted water.The experimental results show that the mesopore size has a faster adsorption rate and adsorption capacity than the micropore size,and the pore volume is the key to determine the adsorption capacity;but CTS electrodes still lack the selective adsorption capacity of nitrate.（2）In order to give biomass carbon the ability to selectively adsorb NO₃^-,enhance the removal efficiency of CDI electrode for NO₃^-and the recovery of nitrogen sources,an extended voltage capacitance deionization system loaded with functionalized groups was designed and prepared.（QRAC/CRAC cell）.The anode is loaded with C-O-Si covalently bonded quaternary ammonium groups through silanization reaction,and the cathode is loaded with carboxyl groups through concentrated acid etching.The experimental results show that due to the different adsorption binding energies of quaternary ammonium functional groups to Cl^-and NO₃^-,the QRAC/CRAC cell undergoes an ion exchange adsorption reaction during the adsorption process.The selectivity of QRAC/CRAC cell for NO₃^-at 0.4 V is 2.7,and its adsorption capacity is 140%higher than that of AC/AC cell.The adsorption behavior of QRAC/CRAC cell for NO₃^-mainly includes two aspects:physical adsorption under the external electric field and ion exchange adsorption that occurs on an internal electric field（quaternary ammonium groups）.（3）In order to explore the effect of quaternary ammonium groups with different carbon chain lengths on the selective adsorption of NO₃^-on functionalized coconut shell-based porous carbon electrodes,functionalized coconut shells with adjustable polarity were prepared by adjusting the length of the alkyl chains on the quaternary ammonium groups.Based porous carbon materials,and their assembly was designed as a invert capacitive deionization cell（i-CDI）.The i-CDI experimental results show that the quaternary ammonium functionalized microporous carbon electrode carrying tetradecyl chain has the best adsorption performance,and the electrosorption capacity in Cl^-/NO₃^-mixed solution is 238μmol g^-1,and the selectivity coefficient reaches at3.2.In addition,the slower solution flow rate and higher nitrate concentration will promote the ion exchange reaction and improve the selectivity coefficient of the electrode to NO₃^-.The selectivity mechanism of the quaternary ammonium group to NO₃^-is that the quaternary nitrogen atom has a strong positive charge,and anions have electrostatic adsorption.While long alkyl chains have hydrophobic properties and repel ions with high hydration energy,so that quaternary ammonium groups have different affinities for NO₃^-and other ions.（4）In order to explore the changes in the physical and chemical properties of carbon precursors with different surface structures after loading functional groups,chitosan-based macropores-mesoporous hierarchical carbon network is prepared and constructed of extended voltage capacitive deionization cell（HPC-Si O₃N/HPC-COOH cell）.Experiments show that the HPC-Si O₃N/HPC-COOH cell achieves an adsorption capacity of 530μmol g^-1(～41 mg g^-1)and a selectivity coefficient of 2.8 in Cl^-/NO₃^-mixed solution.The selectivity coefficient continues to rise to 5.4 at 0 V.The functionalized continuous ordered carbon network framework has faster and stronger adsorption characteristics than the functionalized discontinuous irregular carbon particles.OH^-in alkaline solution will compete for adsorption sites and interfere with the ion exchange reaction of NO₃^-,while H⁺in acidic solution enhances the solution conductivity and thus increases the conductivity.In this study,the problem of nitrate pollution in eutrophic water was effectively controlled by preparing functionalized biomass carbon-based capacitive deionization electrodes,which provided a new idea for the selective adsorption mechanism of capacitive deionization technology,and provided an efficient treatment for eutrophic water.