Aging Mechanism Of Biochar During The Field Process

Biochar plays an important role in change soil physicochemical properties and decrease the bioavailability of pollutants,because of the great specific surface area,complex microporous structure and abundant surface functional groups.Remediation of contaminated soils by biochar has been one of the hot topics in environmental science,but its aging process in the field with the long-term impacts on environment are not well understand.Based on existing biochar field amendment,and the experiments of simulating biochar aging process,the changes of components and structures of biochar in field were investigated with the supporting of modern analysis techniques,such as Fourier Transform Infrared Spectroscopy?FTIR?and X-ray Photoelectron Spectroscopy?XPS?.The influence of aging products on soil organic matters and pollutant migration,with the long-term effects and potential environmental risk for biochar amendment in heavy metals contaminated soil were clarified.The main conclusions are as followings:?1?The acid and alkaline soil environments with H₂SO₄/HNO₃ and NaOH/H₂O₂ solutions to simulate the aging biochar and analyzed the mechanisms of abiotic aging process by changing regularities in functional groups and surface structures.The effects of biochar aging process on adsorption capacity with heavy metal pollutants were also discussed.The results showed that carbon loss and oxygen incorporation run throughout the aging process.Aged biochar was consisted of two-layer structure:the interior and the surface,moreover,surface structure was featured with high C%and high O/C.The two-layer structure indicated that biochar aging was an oxidation process which started from the surface.Improvements in O/C enhanced surface polarity and hydrophily of biochar.Oxygen-containing functional groups were found to be increased on the surface of aged biochar,especially carboxyl groups.SEM and pore size distribution data showed biochar microstructure was serious destroyed and led to a great increase in specific surface area and porosity.Batch adsorption experiments revealed that aged biochar have greater adsorption capacity than the original biochar,because of more oxygen-containing functional groups and larger specific surface area.?2?Microorganisms were extracted from remediated field to simulate biotic aging process by aging biochar in the similar environment conditions.Analysis the composition and structure changing on the surface,meanwhile,the effects of aging products on soil microorganisms and organic matters were also studied.According to the results,biochar has positive influence on soil microbial biomass.With the oxidation of microorganisms,oxygen-containing functional groups increased biochar surface polarity and hydrophily.Combined with abiotic aging and field aging process,nitro-groups on aged biochar surface were believed to be caused by acid oxidation,which probable was a standard when biotic and abiotic aging have to be studied separately.Lignin and humic acids were the main water-soluble products in aging process,but the aromaticity of solution in simulate experiments increased with long term because lignin and humic acids were bio-persistent.The dissolving out of humic acids increased the water-soluble organic carbon concentration in soil,and improved soil solution of the capacity with complexing heavy metals,which may result of secondary pollution in underground water.By the difference of water-soluble organic carbon compared microbial inoculation with none microbial inoculation that almost 2 g kg^-1 water-soluble organic carbon increased due to the biotic aging process in biochar remediated soil.?3?According to the concentration changes and characteristics of both organo-mineral complexes and iron oxides,the long-term effects of biochar on soil fertility were researched.The functions of organo-mineral complexes in heavy metals migration were also studied.From the results,loosely combined humus increased by 30.1%in acidic paddy soil with the density of 40 t ha^-1 biochar application,and by 25.1%in saline-alkali soil.In acidic paddy soil,application of biochar decreased the losing of humus which caused by organic or inorganic acid,and relieved soil podzolic layer phenomenon.Oxygen-containing functional groups were increased in humus,and enhanced humus complexation on heavy metals.From XRD analysis,biochar could convert lead/cadmium ions to mineral fractions and reduced the bio-availability.In saline-alkali soil,biochar increased Ca-bound complexes and Fe/Al-bound complexes in topsoil.Soil moisture content increased by 3%with biochar application.The improvement in soil polarity was caused by increased in oxygen-containing functional groups,enhanced soil complexion on magnesium and aluminium,and resulted of increase in organo-mineral complexes and improvement in soil fertility.