Effects Mechanism Of Biomass Components On Carbon Fate In Biochar And Its Environmental Application Performance

Biochar from biomass pyrolysis is a functional material with great potential,which is widely used in carbon sequestration,environmental pollution control,soil improvement and energychemical engineering.There are many kinds of biomass as raw materials of biochar,and the difference of biochar properties due to their different sources has seriously affected its application and promotion.Therefore,in this paper,three key biomass components and four lignocellulosic biomass with different compositions were used to produce biochar,to explore the conversion of biomass and the transfer of carbon elements in biochar,and to study the removal of inorganic pollutants(hexavalent chromium,Cr(Ⅵ))and organic pollutants(sulfadimethazine,SMT)by different biochars.In addition,a two-compartment model was used to reveal the kinetic mechanism of Cr(Ⅵ)removal;the surface damage of biochar during the catalytic degradation of SMT was investigated in order to provide scientific basis for the environmental application of biochar.The main results of this paper are as follows:(1)Biochar prepared from lignin,cellulose and hemicellulose had obvious difference in properties.In this study,three biomass components were used to produce carbon by pyrolysis at 700℃.And the internal differences were analyzed by combining multiple characterization methods and SMT adsorption experiments.The results showed that lignin carbon had the highest yield(53.2%)due to the influence of internal ash content and aromatic structure.The surface functional group contents of lignin,cellulose and hemicellulose chars were 0.350,0.188 and 0.427 mmol g-1,respectively.Therefore,hemicellulose char showed the best SMT removal effect under different pH conditions,and the maximum distribution coefficient(Kd)reached 1497.7 L kg-1.In combination with the dissociation constants of different functional groups on the biochars surface and the ionic state analysis of SMT at different pH values,the pH of solution dominated the adsorption of SMT on the biochar.In the pH range of 3-7,the electron-deficient benzene ring structure and π electron interaction formed on the electron-rich surface of biochar,and the hydrogen bond formed on the functional groups formed on the surface of biochar played a dominant role.When pH was about 8,anionic SMT molecules could form anion assisted hydrogen bonds with phenolic hydroxyl or carboxyl groups on carbon surface to enhance adsorption(the Kd reached 1116.4 L kg-1).(2)The pyrolysis behavior of biomass with different components and the fate of carbon in biochar were different.The lignin contents in sunflower stalks,cotton stalks,walnut shells and pine branches measured by acid washing method were 20%,19%,29%and 30%,respectively.The pyrolysis temperatures of walnut shells and pine branches containing more lignin were higher.with rapid weight loss at 360.1 and 352.0℃,respectively.The slow pyrolysis of lignin resulted in the release of CO from walnut shell after 400℃,the total CO content of the product was 42%.which was much higher than 18%of cotton stalk and resulted in more aromatic biochar.Carbon retention of lignin,cellulose and hemicellulose after pyrolysis at 500℃ was 78.7%,41.7%and 40.2%,respectively.The calculated values of carbon retention were 50.1-55.0%,and the relative error was 0.5-7.6%.Therefore,it can be inferred that lignin was the main contributing component of carbon retention during biomass pyrolysis.The R50 values of sunflower stalk and cotton stalk char were 0.526 and 0.517,which were lower than those of walnut shell and pine branch char(0.572 and 0.563).In addition,the K2Cr2O7 oxidation experiments showed higher carbon loss(14.5%and 10.6%)for sunflower stalk and cotton stalk char,indicating a positive correlation between the stability of biochar and lignin content of biomass.Combined with FTIR.Raman and XPS analysis.it was the aliphatic hydrocarbon components derived from cellulose or hemicellulose that caused carbon loss in biochar.(3)The electron transfer ability and adsorption/reduction mechanism of Cr(Ⅵ)were different from different sources of biochar.Biochars were prepared by pyrolysis of walnut shell(WB)and cotton stalk(CB)at 300 and 700℃,and the removal mechanism of Cr(Ⅵ)was investigated.The results showed that the removal of Cr(Ⅵ)by biochar was affected by the functional groups and specific surface area of biochar.The removal of Cr(Ⅵ)by WB700 and CB700 with high specific surface area was better,reaching 8.86 and 10.07 mg g-1.The low temperature char CB300 derived from cotton stem contained abundant surface functional groups(1.22 mmol g-1).and the removal capacity reached 7.78 mg g-1.XPS results showed that 75.1-78.7%of Cr on biochar existed in the form of Cr(Ⅲ),indicating that the process of biochar removal of Cr(Ⅵ)was both adsorption and reduction.Therefore,the fitting effect of pseudo-first-order,second-order kinetics and Park model was not good(corrected correlation coefficient R2adj was 0.73-0.93),and the obtained ideal saturated adsorption capacity qe was 5.8%-17.9%smaller than the experimental value.In this study.the two-chamber model was used to divide the removal process of Cr(Ⅵ)by biochar into two parts:fast and slow reactions.The fitted R2adj reached 0.969-0.989.and the qe deviation was reduced to 0.9%-8.6%.After dismantling and analyzing the two-chamber model,it is found that the rapid reaction contributes more(the coefficient was greater than 0.5)in the reaction of high temperature char;while the coefficient of the reaction of low temperature char was about 0.3.And after 40 h of reaction,the removal contribution of the slow reaction would exceed that of the fast reaction.Elemental and Raman analyses showed that the high electrical conductivity of WB700 was caused by the highly graphitized internal structure.(4)Different biochars had different ability to activate peroxodisulfate(PDS)and damage in the catalytic system.WB700 and CB700 were prepared by pyrolysis at 700℃ to activate PDS for SMT degradation,and the damage of biochar during the catalytic process was investigated.The results showed that CB700 had better catalytic performance due to its stronger adsorption capacity,and the degradation rate constant fitted by the pseudo-first-order kinetic model reached 0.0122 min-1.A total of 6 kinds of degradation products were detected in the degradation system,which were mainly produced by the decomposition of the SMT into sulfonamide and pyrimidine groups and then oxidation or substitution.The quencher could not completely block the degradation reaction,but only reduced the degradation rates of CB700 and WB700 systems from 91%and 41%to 80%and 29%,indicating that the catalytic reaction is caused by the combined action of free radical and non-radical pathways.Recycled CB700 and WB700 were again used for catalysis,and their reaction rates decreased by 83%and 33%,respectively.The electrochemical impedance spectroscopy and linear sweep voltammetry results of CB700 after participating in the catalytic reaction did not change significantly,but the conductivity of WB700 decreased.The N2 specific surface area of the regenerated WB700 was greatly reduced from 179.0 m2 g-1 to 12.6 m2 g-1,but the CO2 physical adsorption results were similar to those before the reaction.Combined with the results of the decrease of C element and the increase of O on the biochar surface,it can be seen that the catalytic process does not significantly damage the pore structure of biochar,but it will reduce the adsorption capacity of the surface of biochar,resulting in the decline of its catalytic performance.