| Biochar is recommended as an effective soil amendment to remediate heavy metal-contaminated soil due to its well-developed pore structure,high specific surface area and unique surface chemistry.However,in recent years,researchers have found that biochar may release dissolved organic matter(DOM)with high reactivity and significant mobility during the application process.The environmental behaviors of biochar-derived dissolved organic matter(BCDOM),which are obviously different from that of biochar,may promote the migration of heavy met als.The beneficial functions of biochar,such as contaminant sequestration,is possible to be overestimated.Therefore,a comprehensive understanding of the structural characteristics of BCDOM and its interaction with heavy metals is key to deeply understanding the environmental effects of biochar and reasonably assessing the environmental risks of biochar.However,the extreme diversity,complexity,and heterogeneity of the characteristics of BCDOM have prevented further studies.Based on spectral titration and culture experiments and combined with scanning electron microscope,spectroscopy,mass spectrometry,synchrotron radiation,parallel factor(PARAFAC),complexation model,two-dimensional correlation spectroscopy(2DCOS),differential absorption spectroscopy(DAS),and Gaussian fitting,this paper studied the relationship between the changes in the structure and composition of BCDOM and influence factors including pyrolysis temperature,p H,and microbial effects.Furthermore,the influence mechanism of different heavy metals and changes in the characteristics of biochar-derived DOM on the binding process of biochar-derived DOM with heavy metals was discussed.Finally,the microstructure binding mechanism between BCDOM and heavy metals were analyzed in depth.The main research contents and conclusions of this thesis wer e as follows:(1)This part studied the variations of apparent morphology,structural characteristics,and molecular composition of BCDOM induced by pyrolysis temperature(300,400,and 500?).The results showed that the dissolved organic carbon(DOC)concentration,morphological characteristics,mineral composition,organic structure,and molecular composition of wheat straw biochar and corn straw biochar-derived DOM had similar response characteristics to pyrolysis tempe rature.With pyrolysis temperature increased,the DOC concentration of BCDOM decreased significantly(p<0.05).For each 100?increasein temperature,the DOC concentration decreased by about 2 g kg-1.The results of scanning electron microscope found that the surface of BCDOM was aggregated by smooth irregular structures under the condition of low pyrolysis temperature.However,with pyrolysis temperature increased,the surface of BCDOM was roughened,the polymer structure was blurred,and the surface minerals increased.According to X-ray diffraction analysis,the minerals on the surface of BCDOM might be dominated by potassium salts.Furthermore,the contents of some certain minerals might increase as the pyrolysis temperature increased.In addition,the results of solid nuclear magnetic resonance and Fourier transform ion cyclotron resonance mass spectrometry analysis found that the major molecular composition of BCDOM was CHO compounds.Increase in pyrolysis temperature could promote the release of a large amount of condensed aromaticity and oxygen-containing functional groups from biochar into the dissolved phase,while this could also reduce the content of lignin in BCDOM.(2)This part analyzed the response characteristics of the molecular structure,functional group,and molecular conformation of BCDOM to the changes of p H and pyrolysis temperature.By combining excitation-emission matrix(EEM)spectroscopy with PARAFAC,the research found that the response characteristics of the three fluorescent components in BCDOM to p H were very similar under different pyrolysis temperature conditions,however,the fluorescent components have different variation trends with p H changes.The fluorescence intensity of the humic-like component C1showed an overall upward trend within the studied p H range(3.0–10.0);the fluorescence intensity of humic-like component C2 appeared a peak at p H 4 and showed an upward trend within the range of p H 9.0–10.0;the changing trend of fluorescence intensity of protein-like component C3 was opposite to that of component C2.Affected by the pyrolysis temperature,the DAS of different BCDOM showed different trends along the observed wavelength,and the peak positions and peak numbers were different.With the change of p H,the characteristic peak position of DAS of BCDOM remained unchanged,whereas the peak intensity and peak width changed obviously.The results of Gaussian fitting showed that the characteristic peaks of DAS were closely related to phenolic groups and carboxyl groups,the internal reactions in chromophores,and the changes in the conformation of the molecular structure,indicating that these effects are important in the response of BCDOM to p H changes.(3)This part explored the biotransformation process of BCDOM and its difference in binding with different heavy metals.Based on ultraviolet-visible spectrophotometry and EEM-PARAFAC,the result showed that high carbonization,aromaticity,and molecular weight components enriched in BCDOM,and their content gradually enhanced during the cultivation process.These characteristics may help improve the environmental stability of BCDOM and its ability to bind with heavy metals.The results of synchronous fluorescence spectrum and Fourier transform infrared spectroscopy(FTIR)combined with 2DCOS found that Cd(II)binding to biochar-derived DOM first occurred in the protein-and fulvic-like fraction while protein-and humic-like substances had a stronger affinity for Cu(II).Furthermore,both phenolic and carboxyl groups firstly participated in the binding process of Cd(II)with biochar-derived DOM,while polysaccharide presented the fastest response to Cu(II)binding.These results clearly demonstrated the differences in specific heavy metal binding features of individual fluorescent substances and functional groups in biochar-derived DOM,which may contribute to improving the application effect of biochar in a multi-heavy metal polluted soil system.(4)This part investigated the binding capacities and binding order of specific components in the DOM released from biochar produced at di fferent pyrolysis temperatures with heavy metals.The results showed that with the increase of pyrolysis temperature,the change regularity of fluorescent components in DOM and its response characteristics to heavy metals were similar between wheat straw a nd corn straw biochar.As the pyrolysis temperature increased,the relative distribution of humic-like components increased while proteins decreased.In addition,the binding stability constant log KM of the fluorescent component of BCDOM with Cu(II)increased by approximately one order of magnitude for each 100°C increase in pyrolysis temperature.Through 2D-FTIR-COS analysis,this work found that the binding sites of heavy metals in BCDOM had heterogeneous distribution characteristics and the binding sequence was highly complicated.The amide groups and phenolic groups only participated in the binding process of Cu(II)with low(300and 400?)and high(500?)temperature biochar-derived DOM,respectively,whereas the carboxyl and polysaccharides were involved in the binding process of Cu(II)with all biochar-derived DOM.These results provide a new perspective for the reasonable assessment of the application potential of biochar in polluted environments.(5)This part explored the microstructure mechanism of the interaction of BCDOM with Cu(II).This study found that the DAS graph only showed obvious characteristic peaks around 250 nm,and did not change significantly within the wavelength range above 300 nm,indicating that the participation of carboxy chromophore is the main binding mechanism.Additionally,the internal reaction in molecules and the effect of molecular structure ha d little effect.A further analysis by X-ray absorption spectroscopy revealed that bidentate carboxylic-Cu complexes appear to be the predominant binding pattern for Cu to biochar-derived DOM.In summary,this study found that the composition and structural characteristics of BCDOM are highly dependent on the pyroly sis temperature,p H condition,and microbial activity.This article clarifies the influence of metal characteristics and pyrolysis temperature on the binding behavior of BCDOM with heavy metals,and the microstructural mechanism of the binding between BCDO M and heavy metals.Finally,the correlations among pyrolysis temperature,BCDOM properties and BCDOM-heavy metal binding characteristics were established.These findings help us have a better understand on the environmental fate of biochar-derived DOM and more accurately assess the application potential of biochar before its extensive utilization in environment. |
PDF Full Text Request