Research On Adsorption And Electrocatalytic Performance Of Lignite Humic Acid-based Activated Carbon In Wastewater Treatment

Lignite is a low-grade fuel with the characteristics of low grade,low calorific value,high water content,high volatile content and low thermal stability.Combustion could cause environmental pollution and energy loss,which greatly limits the utilization of lignite.Humic acid has a high content in lignite,has high biochemical activity,and is a high value-added product of lignite.The extraction of humic acid from lignite can increase the use of lignite and improve its utilization efficiency.Coal-based humic acid is a natural aromatic polymer composed of oxygen-containing functional groups cross-linked alkyl or aromatic skeleton units.Meanwhile,humic acid has the characteristics of abundant resources,low-cost and high carbon content,and is a new carbon precursor for the preparation of activated carbon.Humic acid-based activated carbon has a large specific surface area,tunable pore structure,abundant oxygen-containing functional groups,abundant defects,and excellent electrical conductivity.In addition to good capacitance characteristics,its adsorption and catalytic properties need to be further studied to realize resource utilization and high-value utilization of lignite.In this paper,humic acid was used as a new carbon precursor to prepare humic acid-based activated carbon,and its adsorption and electrocatalytic properties in heavy metal wastewater and dye wastewater were explored.The adsorption performance and mechanism of humic acid-based activated carbon on heavy metal ions were analyzed.The humic acid-based activated carbon was applied to the modification of electrodes,and the structure-activity relationship between the surface chemical properties and pore structure properties of humic acid-based activated carbon and the sensing performance of heavy metal ions was elucidated.The adsorption performance of humic acid-based activated carbon on methylene blue was investigated,and the electrocatalytic performance of humic acid-based activated carbon was also evaluated,and the degradation and decolorization of methylene blue was realized.The main research results are as follows:（1）Humic acid-based activated carbon had good adsorption properties for methylene blue and heavy metal ion Cd（Ⅱ）.Increasing the p H of the solution,increasing the amount of adsorbent and increasing the temperature was all beneficial to the adsorption of methylene blue and Cd（Ⅱ）by activated carbon.The increase of ionic strength was beneficial to the adsorption of methylene blue,but it was unfavorable for the adsorption of Cd（Ⅱ）.The Langmuir isotherm model was more suitable for describing the isotherm adsorption process of methylene blue and Cd（II）.The pseudo-second-order kinetic model was more suitable for describing the adsorption kinetics of methylene blue and Cd（II）on humic acid-based activated carbon,indicating that the adsorption process was controlled by both external diffusion and intraparticle diffusion.The adsorption of methylene blue and Cd（II）on humic acid-based activated carbon was a spontaneous and endothermic process.The adsorption of methylene blue by activated carbon was mainly through the adsorption process dominated by chemisorption driven by electrostatic interaction,π-πinteraction,hydrogen bonding and pore filling.The adsorption of Cd（II）was mainly a chemical adsorption process dominated by electrostatic attraction,complexation with oxygen-containing functional groups,bonding withπelectrons and pore filling.After the humic acid-based activated carbon was recycled for 5 times,it still maintained good adsorption performance for methylene blue and Cd（Ⅱ）respectively,and had multiple reusability.（2）The humic acid-based activated carbon-doped carbon paste electrode showed good electrochemical detection performance for trace heavy metal ions Cd（II）.The doping of activated carbon on the carbon paste electrode increased the active surface area of the electrode,decreased the charge transfer resistance,and enhanced the response of Cd（II）on the electrode.Compared with the undoped carbon paste electrode,the doping of activated carbon improved the sensitivity of the electrode,expanded the linear detection range,and reduced the detection limit.Compared with the activated carbon doped carbon paste electrodes prepared under different conditions,the AC-0.5-800 doped carbon paste electrode had better Cd（II）detection performance,and the linearity was obtained in the range of 2～120μg/L Cd（II）concentration.The sensitivity of the relationship was 1.90μA/（μg·L）,and its detection limit was 0.35μg/L.When the antimony-bismuth alloy electrode was used to replace the antimony film electrode,the detection range of Cd（Ⅱ）was expanded,the sensitivity of the electrode was significantly improved,and the detection limit was reduced.When activated carbon was used to chemically modify carbon paste electrodes,the surface oxygen-containing functional groups and pore structure of activated carbon would affect the voltammetric response of Cd（Ⅱ）on the electrode.Under acidic conditions,the oxygen-containing functional groups on the surface of activated carbon mainly complexed with Cd（Ⅱ）.In addition,the larger pore size of the activated carbon results significantly increased in the diffusion coefficient,which provided efficient diffusion and mass transfer for the analytes,resulting in an effective increase in the diffusion rate,and higher Cd（II）response could be obtained at the same preconcentration time.The humic acid-based activated carbon-doped carbon paste electrode had strong anti-interference ability,good robustness,repeatability and long-term stability.The humic acid-based activated carbon-doped carbon paste electrode was successfully applied to the detection of Cd（II）in real samples.（3）The humic acid-based activated carbon had potential catalytic active sites such as defects,oxygen-containing functional groups,and abundant micropores,resulting in good electrocatalytic oxygen reduction activity and H₂O₂ production performance.When the mass ratio of the activator KOH and humic acid was 1,the electrocatalytic oxygen reduction reaction of the catalysts at different activation temperatures was mainly carried out by the two-electron pathway,and the H₂O₂ selectivity of the catalysts reached more than 60%.With increasing the amount of activator,the electrocatalytic oxygen reduction reaction of activated carbon was carried out through a two-electron and four-electron mixed pathway.When the activated carbon loading was 4 mg,increasing the air flow,reducing the p H of the solution,and negatively shifting the potential were beneficial to increase the concentration of H₂O₂ produced by electrocatalytic oxygen reduction.AC-1-700 had high H₂O₂ selectivity,while AC-2-800 had high H₂O₂ production rate.Due to their hydrophobicity and strong adsorption of oxygen molecules,ultra-micropores（<0.7 nm）were as the catalytically active sites for electrocatalytic oxygen reduction reaction,which was beneficial to electrocatalytic oxygen reduction to produce H₂O₂.Besides,defects and oxygen-containing functional groups affected the performance of oxygen reduction to H₂O₂ as catalytic active sites.（4）An electro-Fenton system was constructed with activated carbon AC-2-800 catalyst as the cathode,which can effectively degrade and decolorize methylene blue.The decolorization rate can reach 99%at 20 min,and the removal rate of TOC can reach 80%at 180 min.In the electro-Fenton degradation methylene blue system,the cathode had almost no adsorption effect on methylene blue.Methylene blue decolorization can be attributed to a variety of effects,including electro-Fenton oxidation,anodization and the oxidation of free radicals generated by activated carbon and H₂O₂,etc..In the electro-Fenton system,·OH played a leading role in the decolorization of methylene blue.The humic acid-based activated carbon had good reusability and stability in the electro-Fenton degradation and decolorization of methylene blue.In summary,activated carbon prepared from lignite-extracted humic acid had large specific surface area,well-developed pore structure and abundant oxygen-containing functional groups.Humic acid-based activated carbon performed well in the adsorption of methylene blue and Cd（II）,the detection of trace Cd（II）on the doped carbon paste electrode,the production of H₂O₂ by electrocatalytic oxygen reduction,and the degradation of methylene blue by electro-Fenton,realizing the efficient and stable application of the coal-based carbon material prepared from humic acid in the field of environmental pollution detection and control.