Insight Into The Redox Properties Of Aged Biochar And The Mechanism Of Promoting Chromium Reduction

Biochar is defined as the carbon-rich,highly aromatic,insoluble and porous solid material produced by low-temperature pyrolysis of biomass under inert gas conditions.When added to soil,it can greatly promote cycle of nutrients and pollutants.Surface of biochar is rich in oxygencontaining functional groups such as carboxyl group,phenolic hydroxyl group,methoxy group,quinone group and ketone group.These groups allow biochar to be used as a solid electron shuttle body to participate in the redox reaction in nature environment.Previous studies have shown that after biochar is added to the soil,its physical and chemical properties will change in natural conditions.Aging in a long time,a large number of oxygen-containing functional groups will generate on the surface of biochar,which will affect its physical and chemical properties.Meanwhile,biochar will release a variety of organic compounds during the aging process（biocharderived organics,BDOM）.Previous studies have proved that aging processes have a great significance in the physical and chemical properties of biochar but rarely reported the influence on redox property.This paper focused on the aging process for releasing soluble biochar and producing aged biochar,and subsequently their redox properties and capacity of mediating pollutant（Cr（Ⅵ））reduction and transformation.The main content and conclusions of the paper are as followed:（1）The release patterns of BDOM from biochar pyrolyzed in different temperatures（300 ℃,400 ℃,500 ℃,600 ℃ and 700 ℃）were studied,and the electron transfer capacity（ETC）of BDOM was characterized.The mechanisms of BDOM as electron donor and electron shuttle mediating Cr（Ⅵ）reduction were elucidated.The ETC of BDOM was measured by the electrochemical characterization method,which showed that the ETC value of BDOM-500 was 0.45 mmol e^-1/ g · C,which was higher than the ETC value of other samples.Physical,chemical and spectral characterizations（three-dimensional fluorescence,FTIR,XPS,UV-vis）revealed that the structure of BDOM was affected by the pyrolysis temperature of biomass.The quinone and phenolic groups of BDOM extracted from biochar pyrolyzed in lower temperature（300 ℃-500 ℃）have a higher content.These redox active functional groups may decompose with the increase of pyrolysis temperature.For the reduction of Cr（Ⅵ）promoted by BDOM,the reduction curve of Cr（Ⅵ）was confirmed to be the first-order kinetic equation.The removal rates of Cr（Ⅵ）by different BDOM samples（300 ℃,400 ℃,500 ℃,600 ℃,700 ℃）were 0.137,0.165,0.169,0.163,and 0.156 h-1,respectively.（2）The effects of different aging processes on the redox activity of biochar and processes of mediating Cr（Ⅵ）reduction were studied.Freeze-thaw,acid and oxidation treatments were applied to simulate the natural aging process of biochar,respectively.Electrochemical characterization revealed that freeze-thaw aging weakened the electron transfer capacity of biochar,while acid and oxidative aging enhanced biochar’s electron transfer capacity.Physical and chemical characterizations（XPS,FTIR,BET and Zeta potential）,revealed that the aging process will change the physical and chemical properties as well as species and contents of surface functional groups of biochar.The correlation between the physical and chemical properties of biochar and electron transfer ability was analyzed by PLS model,revealing that apart from the surface redox active groups,specific surface area had a certain influence on the electron transfer capacity of biochar.In addition,all three aged biochar showed increase in adsorption and reduction of Cr（Ⅵ）.This study mainly confirmed that redox characteristics of BDOM during the aging of biochar have a high correlation with the pyrolysis temperature.At the same time,different aging processes of biochar in simulated conditions were applied to explore the effect of aging on the redox activity of bulk biochar.Correlation between the physical and chemical properties of biochar and the electron transfer capacity was analyzed through partial least squares model.This research can provide theoretical basis and technical support for the application of biochar in environmental remediation.