Biochar Physicochemical Properties And Sorption Characteristics After Chemical Oxidation

Biochar,the incomplete combustion product of organic materials,was considered stable in soils and potentially beneficial in different areas,which ws not only soil fertility improvement,but carbon sequestration.Biochars were proven to be strong sorbates to both heavy metals and organic pollutants,and thus could effectively retain these pollutants.Biochars in soils system could decrease the mobility as well as plant uptake of pollutants.However,after being applied in the environment,biochars may be chemically,physically or biologically altered.These processes will significantly change biochar surface properties including oxygen-containing functional groups,particular size and pore size distribution,which consequently alters the environmental functions of biochars.Therefore,the initial properties of biochar cannot be used to forecast its environmental impacts,including its effects on pollutant fate and risk.This work is thus specifically designed to study the effect of biochar aging on biochar properties and pollutant sorption.Biochars were made from four types of parent biomasses,including corn straw,pine wood sawdust,cellulose and lignin.These parent biomasses were pyrolyzed at 200-500℃.Chemical oxidation of biochars by NaClO was applied to simulate biochar chemical aging in the environment.Biochar properties were characterized and compared by elemental analysis,BET surface analysis,Fourier transform infrared spectroscopy and molecular biomarkers information analysis before and after oxidation.Three model pollutents,namely bisphenol A(BPA),sulfamethoxazole(SMX),and phenanthrene(PHE)were chosen to study biochar sorption characteristics before and after oxidation.The results showed that:1)With the increasing pyrolysis temperature,C content increased while O and H content decreased for all the investigated biochars.H/C,O/C and(O+N)/C atomic ratios were also decrease.After the biochars were treated with NaCIO,their O contents slightly increased and C contents decreased,suggesting the formation of polar functional groups on biochar surfaces,including carboxyl,carbonyl and phenolic functional groups.2)The specific surface area(SA)of biochars increased with pyrolysis temperature.When the biochars were treated with NaC10,the SAs significantly decreased for biochars produced using corn straw,pine wood sawdust and cellulose.Relatively,lignin derived biochars did not show obviously change of surface areas after oxidation.The reason for this phenomenon can be understood by the higher lignin content,which composed by many benzene rings,formed condensed aromatic structures during pyrolysis,and resisted to NaClO oxidation.3)The total Benzene polycarboxylic acids(BPCAs)content for all biochars increased with pyrolysis temperature.The proportions of less condensed structure represented by B3CA,B4CA and B5CA decreased,whereas the highly condensed structure represented by B6CA increased with increasing pyrolysis temperature for all the investigated biochars.After the biochars were treated with NaClO,a slight decrease was observed for the BPCA contents,but the distribution patterns of BPCAs were hardly changed.4)When we compared the quantity of all the biochars and B6CA contents,a significant correlation was observed and could be described using anequation:y = 2627.4 x-0.885)Among the three organic contaminants investigated in this work,PHE generally showed higher sorption,suggesting the important role played by hydrophobic effect.BPA and SMX have comparable water solubilities,but BPA showed higher sorption than SMX.The electrostatic repulsion between biochars and SMX molecules may have decreased SMX sorption.A significantly decline of PHE sorption after oxidation with NaClO.The reason can be attributed to the newly created polar functional groups greatly hinder sorption sites of hydrophobic molecules on the surfaces of the biochars.For BPA,the Kd values decreased of two feedstocks derived biochars on 200℃ and 500℃ while increased on 300℃ and 400℃ after oxidation with NaCIO.This may be unstable components of loss of 200℃ biochar,the significantly decrease SA of 500℃ biochar and the oxygen-containing functional groups increased lead to π-π EDA interactions between BPA and 300℃ and 400℃biochars.An greatly increased of Kd between SMX and all biochars after oxidation with NaClO except PWO500.This may be attribute to the increasing of surface oxygen-containing functional groups after oxidation and lead to a litte pH,which will reduce electrostatic repulsion.Generally,chemical oxidation decreased the surface area,but increased the O content of biochars.The biomarker molecule,specifically B6CA,could well quantify biochar amount before and after the simulated chemical aging.This result suggested a potentially useful technique to biochar characterization after their application in the environment.