| Biochar is a kind of carbonaceous material produced by thermal conversion of biomass in absence of oxygen or with limited oxygen.Thermal conversion of biomass can contribute to biowaste valorization as well as carbon sequestration.Meanwhile,biochar,that is the solid product,has been widely used in pollution control.Biochar is mainly composed of C,H,O,N and mineral elements.The porous structure and surface functional groups(such as O-and N-containing moieties)of biochar make it contain various physicochemical properties.Redox properties enable biochar to be involved in electron transfer processes in the environment,thus affecting the redox reactions of pollutants and microbial metabolism.Redox properties refer to the ability of biochar to reversibly donate and accept electrons,namely electron exchange capacities(EEC),consisting of electron-donating capacities(EDC)and electron-accepting capacities(EAC).However,the structure and physicochemical properties of biochar are various,and the identification of redox-active moieties(RAMs)in existing studies is limited to single-factor analysis with discrepant explanations.A systematic analysis of multiple factors and the microscopic distribution of RAMs on the surface of biochar particles are insufficient.Besides,biochar will be affected by a variety of physicochemical factors during application,for example,physical disintegration causing biochar to fragment into micro-/nanosized particles,nature organic matters adsorbed onto biochar to form complex,etc.The structure and redox properties of biochar will subsequently change,which will affect the long-term utility of biochar redox properties.Therefore,the structure and microscopic distribution of RAMs still need to be further investigated,and the effects of various physicochemical factors on the structure and redox properties of biochar still need to be elucidated.In this work,the relationship between the overall structural characteristics of biochar(such as specific surface area,carbon structure and functional groups),the microscopic distribution characteristics of RAMs and redox properties were established by integrating the characterization methods of overall structure and properties of biochar and exploring a variety of microscopic analysis methods;the changes of RAMs in different layers of bulk biochar were explored to get the spatial distribution of RAMs in bulk biochar,and the effects of physical disintegration on redox properties of biochar were revealed by mulit-exfoliating submicron particles(colloids)from bulk biochar;the molecular adsorption and dissolution behavior of biochar during the interactions with humic acid(HA)as well as the change of redox properties of solid and liquid phases were identified using liquid chromatography-high resolution mass spectrometry.The obtained conclusions are as follows:1)The biochar was produced from pyrolysis of representative biomass components at different temperatures,and the contribution of representative biomass components to EEC was found that,biochar derived from biomass containing more cellulose at medium and low temperature(300-500 ℃)possessed relatively higher EDC,while biochar pyrolyzed from biomass with higher lignin contents at high pyrolysis temperature(700 ℃)contained higher EAC,and the starch-rich biomass pyrolyzed at 700 ℃ can produce biochar with both higher EDC and EAC.Thus,the findings provided a basis for the targeted preparation of biochar through feedstock screening.The quantitative contributions of various RAMs(such as C=O,C-O,andπ-π*)and structural factors(such as specific surface area)to redox properties were obtained by partial least-squares method,which showed that,in addition to RAMs,BET specific surface area also had a significant effect on the EEC of biochar,indicating that improving the accessibility is an effective way to promote the redox properties of biochar.2)The biochar with differential EEC was selected for investigations.The structure and electrical properties of biochar were observed at the nanoscale by in-situ methods such as conductive atomic force microscopy.Combined with ex-situ characterizations of redox properties and structure,it was found that RAMs were mainly distributed on attached nanoparticles or in the pore region for biochars with higher EEC,suggesting that the distribution of RAMs on smaller particles may promote EEC by increasing the accessibility of RAMs.Moreover,unlike existing studies that mainly identify RAMs from statistical analysis of overall functional groups,a visualized connection between EEC and RAMs was obtained through in-situ observation and ex-situ characterization,which is that heteroatoms(O/N)determined EDC,and amorphous aromatic structures also contributed to EDC,while condensed aromatic structures were indispensable to EAC.3)Taking starch-derived biochar at 700 ℃(st700)with excellent EEC and the common pine-derived biochar at 500 ℃(pi500)for examples,the spatial distribution of RAMs in bulk biochar and effects of physical disintegration on EEC were preliminarily explored by means of multi-exfoliation of biochars.As the exfoliation times increased,the EEC of st700 colloids gradually decreased,with decreasing oxygen-containing functional groups,increasing aromatic structure,decreasing oxidizable C-O-C/C-OH and reducible C=O;while the EEC of pi500 colloids gradually increased,with constant aromatic structure,both increasing C-O-C/C-OH and quinones.The difference in the spatial distribution of RAMs between st700 and pi500 came from the different pyrolysis mechanisms of starch(decomposition and condensation)and wood(unreacted-core-shrinking approximation).Compared with bulk biochar,not all biochar colloids formed by physical disintegration contained more oxygenated functional groups(C-O or C=O)and higher EEC.Therefore,particle size was not the determinant for the excellent redox properties of biochar colloids.Compared with pristine biochar,the EDC of the residue biochar after multi-exfoliation decreased due to the decrease of specific surface area as the oxygenated functional groups did not change significantly,while the increase of the aromatic structures caused EAC to increase.4)Taking st700 and pi500 for examples,the adsorption and dissolution behavior of biochar in HA was identified by molecular analysis using liquid chromatography-high resolution mass spectrometry.It is discovered that the dissolution of biochar was far greater than the adsorption,and HA had a promoting effect on the dissolution of biochar.Biochar selectively adsorbed HA components,which is that pi500 tended to adsorb oxygen-containing small-molecule aromatics,and st700 tended to adsorb lipids with higher saturation and lower oxygen contents.This may be due to the fact that pi500 contained more C-O,so H-bond adsorption was dominant,while st700 contained more aromatics so the adsorption was dominated by hydrophobic interactions and π-π interactions.The oxygenated functional groups(such as C=O,C-O,and -COOH,etc.)on the surface of biochar increased after equilibrium,so the EEC of the biochar increased,and the EEC of HA decreased.In addition,the adsorption and dissolution behavior of pi500 reduced the aromaticity and oxygen contents of the HA solution,while the adsorption and dissolution behavior of st700 had no significant effect on the overall composition of HA. |
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