Adsorption Properties Of ZnCl₂-modified Biochars Prepared From Co-pyrolysis Method

Biochar,as a new type of environment-friendly adsorbent,has an excellent effect on sewage treatment.The co-pyrolysis modification is a method of preparation for biochars which can describe by mixing well with exogenous additive and biomass in advance before process pyrolysis in a temperature-programmed furnace.This method introduces an important way to improve the specific surface area,ameliorate the structure of mesopores or micropores of biochar and render magnetic property to biochar.In order to prepare efficient environmental remediation materials,we develop a co-pyrolysis method of mixing ZnCl2 and biomass to prepare biochar.The removal effect of the pollutants on biochars was exhibited,and its application scope and practical application were studied in this research.The chief research contents are presented as following three aspects:1.Pb(?)adsorption by modified Biochars prepared from co-pyrolysis of ZnCl2 and Fraxinus mandshuricaThe modified biochar adsorbents were prepared by co-pyrolysis of pre-impregnation of ZnCl2 solution and Manchurian ash.The materials were characterized by means of N2 adsorption and desorption isotherms,CV,FSEM,FTIR,XRD,Boehm titration and pH drift experiments.The Pb(?)was treated as the model of heavy metals and the adsorption properties and mechanisms of Pb(?)on biochars were studied,the results show that:(1)The modified biochar adsorbents which obtained by co-pyrolysis of ZnCl2 and Fraxinus mandshurica were composed of amorphous carbon of graphite-like microcrystalline and ZnO crystal of wurtzite.(2)Compared with the original biochar,ZnCl2 modification significantly improved the specific surface area and pore volume of the composite adsorbent.The specific surface area and pore volume were increased with the increasing amount of ZnCl2.The specific surface area,pore volume and pore size reached 1406.53 m2/g,0.66 cm3/g and 0.94 nm when the ratio of ZnCl2 and Fraxinus mandshurica biomass was 1:1.09.(3)The ZnCl2 modified biochar has more carboxyl and phenolic hydroxyl functional groups and stronger cation exchange capacity compared with the original biochar.The content of carboxyl,phenol hydroxyl groups of BZ-4 was 0.20 mmol/g,0.45 mmol/g,and pHpzc was 7.1(as compared,the pHpzc of B was 8.8).(4)Modified biochar had strong adsorption capacity for Pb(II)in the initial pH range of 3-6.The adsorption rates increased with the increasing of initial pH and the dosage of biochars for the same kind of biochars while the dosage of ZnCl2 added into biochars for different kind of biochars.(5)BZ-4 had a good kinetic properties for the adsorption of Pb(II).Under the condition of pH 5.5 and 25?,adsorption kinetics followed the pseudo second order kinetics equation,the rate constant is 4.17×10-4,the equilibrium adsorption capacity calculated by the equation was 50.5 mg/g;By contrast,the rate constant of B is 1.60×10-3,the equilibrium adsorption capacity calculated by the equation was 16.6 mg/g.(6)The adsorption of Pb(?)onto BZ-4 is a spontaneously thermodynamic process.Under the condition of pH 5.5 and 25?,the adsorption isotherm accorded with the Langmuir model well,the adsorption coefficient was 0.31 and the saturated adsorption capacity was 52.4 mg/g;By contrast,the adsorption coefficient of B was 0.10,the saturated adsorption capacity was 13.2 mg/g.The adsorption was an endothermic process,and increasing of temperature was beneficial to the adsorption of Pb(?).(7)The adsorption of Pb(?)onto BZ-4 was mainly through the ion exchange mechanism between its carboxylic functional groups and phenolic hydroxyl functional groups.2.Application of ZnCl2 co-pyrolysis modification method:using corncob and peanut shell as raw materialsThe applicability of ZnCl2 co-pyrolysis modification method to different biomass was investigated by using peanut shell and corncob as raw materials.The synthesized materials were characterized by N2 adsorption and desorption isotherms,FSEM,FTIR,XRD methods.The adsorption properties of biochars on Pb(?)were studied.The results showed that:(1)The modified biochar adsorbents which were obtained by co-pyrolysis of ZnCl2 and peanut shell were composed of amorphous carbon of graphite-like microcrystalline and ZnO crystal of wurtzite(named peanut shell in this research).And the modified biochar of corncob was composed of amorphous carbon of graphite-like microcrystalline(named corncob in this research).(2)The classification of N2 adsorption and desorption isotherms of corncob and peanut shell was I type adsorption isotherm(IUPAC),the specific surface area is 1442.36 and 1376.34 m2/g respectively,and the total pore volume were 0.64 and 0.74 cm3/g,respectively.The average pore size were 0.90 and 1.07 um respectively.The micropores of the peanut shell accounted for 90.7%of the total pore volume,and the corn core was 78.4%.(3)Corncob and peanut shell had strong adsorption capacity for Pb(?)in the initial pH range of 4-6.The adsorption rates increased with the increasing of initial pH and the dosage of biochars for the same kind of biochar while the adsorption capacity of corncob is higher than that of peanut shell by more than 10%.When pH is 5.5,the adsorption rate is over 76%(peanut shell 77%,corncob 87%).(4)Corncob and peanut shell had a good kinetic properties for the adsorption of Pb(?).Under the condition of pH 5.5 and 25?,adsorption kinetics followed the pseudo second order kinetics equation,the rate constant and equilibrium adsorption capacity calculated by the equation of corncob and peanut shell is 2.22×10-3,5.19×10-4 and 47.11,47.50 mg/g respectively.(5)Under the condition of pH 5.5 and 25?,The adsorption process of Pb(?)onto corncob and peanut shell were accorded with the Langmuir model well,the adsorption coefficient and saturated adsorption capacity were 0.75,0.73 and 47.26,40.77 mg/g respectively.3.Application of ZnCl2 co-pyrolysis modification method:molding of adsorbents and its application in eliminating Pb(?)and dyesAccording to the practical application of the ZnCl2 modified biochar made from peanut shell?Fraxinus mandshurica and corncob,We studied the molding method of adsorbents and prepared the modified biochar balls;The physical and chemical properties of the modified biochar materials were characterized by SEM,XRD and static pressure tests.The Pb(?)methylene blue,and Acid Orange 7 was treated as the model of contaminants and the adsorption effects on biochar balls were studied,the results show that:(1)The surface of biochar balls prepared without modification(called original peanut shell,original fraxinus mandshurica and original corncob)showed nonporous structure;Among the three modified biochar balls(called modified peanut shell,modified fraxinus mandshurica and modified corncob),modified fraxinus mandshurica and modified corncob exhibited cross-linked porous structures meanwhile the pore size of modified fraxinus mandshurica is bigger than modified corncob,modified peanut shell showed nonporous structure.(2)The modified fraxinus mandshurica and modified peanut shell were composed of amorphous carbon of graphite-like microcrystalline and ZnO crystal of wurtzite.And the modified corncob was composed of amorphous carbon of graphite-like microcrystalline.(3)ZnCl2 co-pyrolysis modified method can increase the compressive strength of the biochar balls.The maximum static pressure of modified peanut shell,modified corncob and modified fraxinus mandshurica can be 2595 g,1068 g and 769 g respectively,which were 7 times than the unmodified biochar balls.(4)In the process of elimination of pollutants,modified fraxinus mandshurica and modified corncob were floating on the water while the modified peanut shell sinked.The removal of Pb(?),Acid Orange 7 and methylene blue modified biochar balls were significantly higher than that of unmodified biochar balls,and the adsorption process were consistent with the pseudo first order kinetics equation.(5)The modified corncob was selected as the most suitable modified biochar ball for application because of its appropriate adsorption capacity,compressive strength compression and density.The attenuation effect of cyclic adsorption test of modified corncob onto Pb(?),Acid Orange 7 and methylene blue was less than 17%when the modified corncob was reused for three times.