Structural Characteristics And Stability Of Rice Straw-Derived Biochars And Their Carbon Sequestration Mechanisms

Biochar as a porous material with abundant of carbon made from low-temperature pyrolysis of all kinds of feedstocks,has many good physical,chemical as well as biological characteristics.It can improve the nature of soils as increasing of soil carbon content,soil aeration and soil moisture-holding capacity,and acidity reduction.It can be used to adsorb large amount of organic and inorganic pollutants and to reduce their movability and availability to the plants and thus will further prevent their transportation between soil and plant system.Biochar has become a hot spot worldwide due to its great potentiality for soil carbon sequestration,but the studies of its soil turnover and environmental influencing factors are still on the on the initial stage.Therefore,to further investigate its structural characteristics and the compositional factors might affect its structural configuration and all-round assessment of its stability,which it's great significant to fulfill the final goal of soil carbon sequestration.In this paper,rice straw was selected as a typical crop waste to produce biochars under increasing pyrolysis temperatures.And their microstructures and compositions were analyzed with Scanning Electronic Microscope-Energy Dispersive Spectrometer(SEM-EDX).Fourier Transform infrared spectroscopy(FTIR).Brunauer.Emmett and Teller theory(BET)suface area,and X-ary diffraction(XRD).The connections between their stabilities and structures were built by using thermal and chemical oxidation with thermogravimetric analysis,acid potassium chromate and peroxygen hydrogen methods.Spherical Aberration corrected Scanning Transmission Electron Microscope(Cs-corrected STEM),Scanning Transmission X-ray microscopy and Near edge X-ray Absorption Fine Spectroscopy(STXM-NEXAFS)were used to denote their oxidized microstructures and elemental composition,and the coupled stability mechanism of rice-straw-biochar related with its structure.A model was proposed to clarify the molecular stability mechanism of biochar as well as their spacial configuration.Several conclusions and original points have made as follows:(1)It was concluded that rice-straw biochar has its unique structure,which was controlled by the pyrolysis temperature and greatly influenced by the existing silicon.Pyrolysis temperature had great impact on enlarging aromatic condensation,which was shown as the increasing surface area.But at 500?,biochar showed a much small surface area mainly attributed to the existing silicon's melting and blocked the pores and formed a complex of Si&C.With the further increase of pyrolysis temperature,biochar showed a(?)sudden increase of surface area,which was attributed to the enlargement of carbon and silicon crystallines and seperation from the Si&C complex.The rice-straw biochar's micro-structural evolved disorder states to short range and then to long-range order states,which was shown as the increasing patches of aromatic carbon and crystalized silicon.Comparatively,deashing biochars were shown to be having more smooth surface with more meso-and micro-pores.The forms of potassium in biochars turned to be closely related with the silicon morphology's transformation as well as on the structural comfiguration of biochar.For low temperature(?300?)biochar,the silicon was mainly composed of amorphous H-abundant silicon gel,and most of potassium componds were potassium oxide and small amount of potassium chloride,whereas the carbon forms were partly carbonized carbon compounds.Silicon,potassium and carbon components were loosely combined with each other in biochar and less homogeneous structure.As for medium temperature(300-600?)biochar,it was composed of amorphous silicon oxide,amorphous and a mixture samll crystalline carbon compounds,and potassium compounds were mainly consisted of crystalline potassium chloride and the three parts combined with each other tighly with less surface area and the biochar strucutre was a medium order one between low and high temperautre biochars.For the high temperature(?600-700?)biochar.carbon components became highly aromatic,and silicon and potassium started to seperate from carbon structure,shown to be large area and pores and the whole biochar strucutre became highly ordered.(2)The stabilization of biochar was not only determined by the aromatic condensation.but more determined by the integral degree of condensed aromaticity and Si&C complex.Thermal experiments told that the ash in biochar had great impact on its stablization.Under the protection of Si&C,500?-biochar showed the greatest stability.While deashing treatment removed the silicon and small amount of metals and made the carbon "naked" and accelerated biochar's degradation,and its stabilization was mainly controlled by the integral degree of aromaticity.K2Cr2O7/H2SO4 oxidation revealed that the stabilization of biochar was not controlled by the silicon's protection,but also controlled by the integral degree of aromatic condensation.While H2O2 oxidation revealed that the carbon pools of biochar were multi-pools and the stabilization of biochar was mainly controlled by Si&C complex.(3)The stabilization mechanism of biochar was revealed in the scale of from nanometer to molecule.The cage-like structure of RS500 within nano-scale was analyzed by using Cs-corrected STEM to denote the stability of Si&C structure by combining the morphology of silicates as well as the carbon forms.A model proprosed to clarify the relationship of molecular structure with its stabilization.This Si&C structure is not hetergenious in all scales,and the bigger size of particle showed a better stabilization than smaller ones,which means that the particle size of biochar has a determining role on the stabilization of biochar.