Study on Characteristics of Different Biochar and Mechanisms for Removing Heavy Metal Ions in Solution

Author:Wang Yan

Supervisor:liu rong hou

Database:Doctor

Degree Year:2018

Download:46

Pages:143

Size:7458K

Keyword:

With the development of economy and population growth,environmental pollution by heavy metals is getting worse.China is a large agricultural country,rich in biomass resources,but most of the biomass is wasted.If the biomass was converted into biochar which was applied to environmental pollution control,it would be an effective way to utilize biomass resources and improve environmental pollution.However,the characteristics of biochar are affected by the raw materials and production conditions greatly,and most of the raw biochar need to be modified to become effective adsorbents.Therefore,this paper mainly discusses the influence of raw materials and technological conditions on the characteristics of biochar,and analyses the potential of biochar as active carbon by comparing the effects of different raw materials,pyrolysis conditions and modification methods on the biochar characteristics and heavy metal ions removal capacity of biochar.The details are as follows:The biochar derived from different raw materials were prepared by slow pyrolysis,fast pyrolysis and gasification.In order to explore the effects of raw materials and pyrolysis conditions on biochar characteristics and the pollutants removal capacity,the biochar characteristics were analyzed by scanning electron microscopy(SEM),elemental analysis,Fourier transform infrared spectroscopy(FTIR)and specific surface area measurement.Inductively Coupled Plasma Atomic Emission Spectrometry(ICP-AES)and ultraviolet-visible spectrophotometer(UV-Vis)were used to determine the pollutants removal efficiency by different biochar.The soil and biochar co-culture test was carried out to investigate the effect of fast pyrolysis biochar on the contents of available nutrients and Pb in the acid soil.The results showed that with the temperature increasing,the biochar yield(38.15%-43.45%),oxygen content(6.50%-24.20%),hydrogen content,O/C ratio and H/C ratio of biochar decreased,while ash content(2.87%-52.24%),carbon content(36.43%-83.80%)and pH value(7.51-11.46)increased.And the linear correlation coefficient between the ash content and the carbon content of the different biochar produced at550 oC was higher than that of the biochar produced at 350oC(P<0.05).The linear correlation coefficient between the ash contents and the carbon contents of the different biochar produced at 550 oC was 0.994.Biochar produced at low temperature were rich in oxygen-containing functional groups,but the specific surface areas were small.The effect of pyrolysis temperature on the pH value was greater than that of the raw material types.Generally,the pH value of biochar increases with the temperature increasing.The removal efficiency of heavy metals and MB by biochar produce at 350 oC was higher than that by biochar produced at 550 oC.The pollutants removal efficiency by fast pyrolysis biochar was higher than that by slow pyrolysis biochar derived from the same raw material at550 oC.Five kinds of biochar were chosen for heavy metal ions and MB removal,including SP350-CTB,SP350-YMB,FP-RHB,GF-WHB and GF-POB.The results of soil experiment showed that adding biochar could invrease the soil pH value and nutrients availabiliy reduce the contents of leaching lead and aluminum in acidic tea garden soil.The removal mechanism of heavy metal ions by biochar was investigated by comparing the effects of organic/inorganic components of biochar on the removal efficiency of heavy metals and the changes of ions in solution before and after adsorption of heavy metal ions by biochar.The results showed that the contribution of each mechanism to the removal of heavy metal ions by different biochar was different.The inorganic matter played an important role in metal ions removal by slow pyrolysis biochar.The changes of X-ray diffraction(XRD)spectrum of biochar after adsorption of heavy metals and the effect of organic matter removal of biochar on removal rate of heavy metal were detected.The results showed that the process of heavy metal removal involved precipitation and complexation.The precipitation was the main mechanism for removing heavy metals by SP350-YMB.The changes of biochar FTIR after adsorption of heavy metals indicated that the oxygen-containing functional group took part in the process of heavy metal ions removal.The concentration changes of K+,Ca2+,Na+,Mg2+and P in the solution after adsorption of heavy metal ions by biochar was demonstrated.The results showed that phosphorus was involved in the forming of precipitate with heavy metal ions.The exchange between K+and heavy metal ions was one of the mechanisms for heavy metal ions removal by GF-WHB and FP-RHB.The exchange between Ca2+/K+and heavy metal ions was one of the mechanisms for heavy metal ions removal by SP350-CTB.The ion exchange had little contribution to the heavy metal removal by GF-POB.To enhance the capacity of heavy metal removal by biochar,KOH and H2O2 were used to modify the above five kinds of biochar that chosen for pollutants removal.The specific surface area of modified biochar was determined by N2 adsorption method.The removal efficiencies of heavy metal ions by modified biochar were determined by ICP-AES.The characteristics of modified biochar with the highest heavy metal removal ability were analyzed before and after adsorption of heavy metal ions.The adsorption isotherm of single-heavy metal and multi-heavy metal by H2O2 modified yak manure biochar(AC-YMB)were also analyzed.And the adsorption kinetics experiment of Pb2+onto AC-YMB was also carried out.The results showed that KOH modification increased the specific surface areas of GF-WHB,GF-POB,SP350-CTB,FP-RHB and SP350-YMB from 0.69-119.2 m2 g-1 to 1181.36-1588.26 m2 g-1.The specific surface areas of slow pyrolysis biochar modified by KOH were larger than other KOH modified biochar.However,KOH modification did not increase the removal efficiency of multi-heavy metal ions by biochar,and even decrease the removal efficiency.H2O2 modification also increased the specific surface area of biochar,but the increases were smaller than KOH modified biochar.H2O2 modification increased the oxygen content and oxygen-containing functional groups of SP350-YMB,and enhanced the heavy metal ions removal in solutions.Precipitation of between phosphate/carbonate and heavy metal ions was the main mechanism of heavy metal ions removal by SP350-YMB.The complexing between acidic oxygen-containing functional groups and heavy metal ions was the main mechanism for metal removal by AC-YMB.The Pb2+removal efficiency by AC-YMB was higher than that of other heavy metal ions(Cu2+,Cd2+and Zn2+)in a single-heavy metal and multi-heavy metal ions solution.The isothermal of SP350-YMB and AC-YMB adsorption of heavy metal ions fitted Langmuir-Freundlich model the best.The maximum adsorption capacity of Pb2+by AC-YMB in a single-heavy metal ion solution was 169.57 mg g-1,which was higher than that by SP350-YMB(76.41 mg g-1).The maximum adsorption capacity of Cu2+and Cd2+by AC-YMB were 1.55 and 1.72 times by SP350-YMB,respectively.In order to compare the effects of modification methods on the removal rates of Pb2+,Cd2+and MB by peach tree branches biochar(FP-PEB)from fast pyrolysis,KOH(alkali),H3PO4(acid)and FeCl3(salt)were used to modify the charicteristics of FP-PEB.FTIR,XRD,elemental analysis and SEM-EDS were used to determine the characteristics of different modified biochar.The adsorption experiments were carried out in a shaker.The mechanism of heavy metal ions and MB removal by modified biochar was explored.The results showed that the specific surface areas and pore volumes of KOH modified biochar were the highest.The specific surface area and pore volume of H3PO4 modified biochar under air atmosphere(H3PO4-Air-PEB)were less than those of the H3PO4 modified biochar under N2 atmosphere(H3PO4-Air-PEB).The O/C ratio of H3PO4-Air-PEB was the largest.-OH and C=C/C=O functional groups were detected in all the biochar,while P=O/P-O and C-O/Si-O functional groups were detected only in H3PO4 modified biochar.H3PO4-Air-PEB contained more oxygen-containing functional groups than H3PO4-N2-PEB.The removal rates of heavy metal ions and MB by KOH-PEB and H3PO4-Air-PEB were higher than that by other modified biochar.The adsorption isotherm of Pb2+and MB onto KOH-PEB and H3PO4-Air-PEB fitted Langmuir-Freundlich model the best.The maximum adsorption capacities of pollutants by KOH-PEB and H3PO4-Air-PEB were estimated by Langmuir-Freundlich model(189.697mg g-1 and 114.244 mg g-1 for Pb2+,180.080 mg g-1 and 585.464 mg g-1for MB,respectively).The kinetics of Pb2+and MB adsorption onto KOH-PEB and H3PO4-Air-PEB fitted Pseudo-second-order model(PSO)the best.In summary,the characteristics of biochar can be affected by raw materials and pyrolysis conditions.Ion exchange,chemical precipitation and functional groups complexation were mainly involved in the process of heavy metal ions removal by biochar.AC-YMB and H3PO4-Air-PEB could remove Pb2+effectively due to the abundant oxygen-containing functional groups.KOH-PEB could remove MB effectively due to the large surface area of biochar.