Redox Mechanism Of Biochar And Its Environmental Application

Biochar produced from biomass waste has been extensively used in soil carbon sequestration,soil amendment and pollution restoration.In recent years,studies have reported that after biochar was introduced into soil,there were many interface reactions between biochar and soil pollutants and microorganisms,which were closely related to their redox properties.In this paper,the effect of biomass source and pyrolysis temperature on the redox capacities of biochar were investigated;the effects of redox functional groups and carbon structure on the redox capacity of biochar were probed;the effects of insoluble and soluble components on the redox capacity of biochar were revealed;Finally,biochar was applied to the reduction of ferrihydrite by microorganism and acting as electrode material to catalyze oxygen reduction.The primary research results are as follows:?1?Effects of biomass source and pyrolysis temperature on the electron transfer of biochar.The mediated electrochemical method was applied to determine the electron donating?EDC?and accepting capacities?EAC?of biochar pyrolyzed at the varying temperatures of 350?-800? which were derived from wheat straw,barley grass,pine wood,populus tremula wood?PTW?,pig manure?PM?and sewage sludge?SS?.As a result,all tested biochar had certain redox capacities,their EDC values were in the range of 0.26-1.83 mmol e^-?g biochar?^-1 and EAC values were in the range of 0.20-0.77 mmol e^-?g biochar?^-1.In terms of biomass sources,plant biochar,especially wheat straw and barley straw,had a relatively high EDC values.However,the EDC values of pig manure and sludge biochar were lower,biomass sources had little effect on the redox capacities of biochar.In terms of pyrolysis temperature,EDC dominated the redox capacity of biochar at low pyrolysis temperature?350-650??;EAC dominated the redox capacity of biochar at high pyrolysis temperature?650-800??.The EDC and EAC values showed an n-shaped trend with the increase of pyrolysis temperature.The results obtained from variability analysis indicated that pyrolysis temperature opposed significant changes on the redox capacity of biochar compared with biomass source.?2?Effects of active functional groups on the electron transfer of biochar.The redox functional groups of biochar were qualitatively and quantitatively analyzed by Fourier transform infrared spectrum,X-ray photoelectron spectroscopy,Boehm titration and temperature-programmed desorption mass spectrometry,and then the relationship between the redox capacities of biochar and redox functional groups was revealed.The results showed that the total content of redox functional groups?O/C?were linear dependent with the exchanged electron capacities?EEC??R²=0.952?at the low pyrolysis temperature of 350 to 650?.The phenolic hydroxyl was the main electron donating group with the linear relation of EDC=6.325[Ar-OH]?R²=0.884?;Quinone,carbonyl and carboxyl group were the main electron accepting groups,as the contribution of carboxyl,carbonyl and quinone groups to the EAC of biochar were 4.3%-24.6%,0%-61.3%and32.6%-95.7%,respectively.The linear relation was:EAC=0.0113[carboxylic acid]+0.0792[carbonyl]+0.282[quinone]?R²=0.998?.At higher temperature range?650?-800??,the content of redox functional groups of biochar was extremely low,and no longer dominated its redox capacity,instead,the graphite-like structure began to dominate the redox capacity of biochar?3?Effects of carbon structure on the electron transfer of biochar.The parameters of defects and crystal size of graphite-like structure when biochar was produced more than 650? were determined by Raman spectroscopy and X-ray diffraction?XRD?analysis,which further revealed the relationship between redox capacities of biochar and the graphite-like structure.The results indicated that there was a linear correlation between the redox capacity of biochar and the lateral?L_a?size of graphitic crystals and the defects I_D/I_G?EEC=0.199[L_a]+3.196[I_D/I_G],R²=0.804?,and the defects of graphite-like structure was the main source of its redox capacity.When the pyrolysis temperature was higher than 650?,the graphite-like crystal structure growed rapidly,and impurity atoms?O,N,S,P,etc.?defects increased.Finally,abundant redox active sites formed on the surface of biochar,thus improving the redox capacity of biochar.?4?Effects of composition on the electron transfer of biochar.Biochar consisted a variety of soluble?soluble organic and inorganic?and insoluble biochar,and their redox capacity was analyzed.The results showed that insoluble biochar dominated the redox capacity of biochar with the EDC values in the range of 0.063-0.855 mmol e^-?g biochar?^-1 and EAC values in the range of 0.211-0.871 mmol e^-?g biochar?^-1.Moreover,soluble biochar also had a certain redox capacity and its EDC and EAC values were in the range of 0.005-0.276 mmol e^-?g biochar?^-1 and 0.099-0.566 mmol e^-?g biochar?^-1,respectively.The soluble components of the biochar?pig manure and sludge?involved in this study contained organic and inorganic components,the organic components were mainly humic acid,while the inorganic components contained Fe and Mn.At low pyrolysis temperature?350-500??,the soluble components had high redox capacity that provided by the soluble organic matter and inorganic transition metal.At high pyrolysis temperature?>500??,organic components disappeared,then inorganic components were mainly composed of soluble biochar,thus the redox capacity was provided only by soluble inorganic transition metal,possessing lower redox capacity.The total redox capacity of insoluble and soluble components of sludge biochar was greater than that of the original sludge biochar,indicating that more redox active sites could be exposed during the separation process.?5?Effect of electron transfer of biochar on reduction of iron ore by bacteria?MR-1?.Choosing bareley grass biochar pyrolysis at 400? and 700?,and the electron shuttle mechanism in microbial reduction iron ore was discussed.As a result,bothof biochar can promote the reduction efficiency and rate of ferrihydrite by MR-1.The reduction efficiency increased from 16.0%to 35.9%and 16.0%to 30.2%respectively,and the reduction rate?k value?increased from 3.7×10^-3 to 9.21×10^-3 and 3.7×10^-3 to 6.5×10^-3respectively.Additionally,biochar also promoted the transformation of iron,since biochar inhibits the agglomeration of magnetite,leading to the formation of more amorphous magnetite.?6?Biochar as electrode material for catalyzing reduction of oxygen.The catalytic mechanism and potential application value of sewadge sludge,dairy manure and wood chip biochar produced at 500? to 700? acting as electrode materials for catalyzing oxygen reduction were disscused.The results indicated that all biochar had ability to catalyze oxygen reduction with the current density of 0.25-3.23 mA/cm^-2.The catalytic capacity and electrochemical impedance of biochar electrodes were affected by the preparation temperature and the biomass source.With the increase of pyrolysis temperature from 500? to 700?,the catalytic ability of sewadge sludge,dairy manure and wood chip biochar electrode increased 8.1,3.2,and 7.0 times respectively.The reason was that the aromatization of biochar enhanced and the specific surface area increased at high temperature.For biomass species,sludge biochar had strong catalytic oxygen reduction capacity,small impedance,and high capacitance due to the doped of transition metal and heteroatoms such as nitrogen,and its appropriate mesoporous and macroporous ratios,thus promoting the electron transfer and oxygen diffusion.In summary,compared with biomass sources,pyrolysis temperature was the dominant factor influencing the redox capacity of biochar,and redox functional groups and carbon structure both dominanted its redox capacities.Biochar produced at low temperature had a“battery-like”electron transfer mechanism due to its rich redox functional groups.Therefore,it had the potential to act as an electron shuttle in soil and other environments.Due to graphite-like structure and edge heteroatom defects,high temperature biochar had“conductive”electron transfer mechanism and catalytic active sites,which can be considered as electrode and catalytic material.Both insoluble and soluble components of biochar had redox capacity,and more redox groups were exposed after the separation of the two components.Therefore,when biochar was applied to soil,its redox capacity may be enhanced with the dissolution of its soluble components.Biochar plays an important role in many redox interface reactions and thus affect biogeochemical processes in the environment.