Environmental Behavior And Mechanism Of Seaweed And Manure Organic Fertilizers-derived DOM Adsorption On Pb(Ⅱ)

Organic fertilizers play an important role in agricultural production in China,with an annual output of 16300 kt.As an important component and active substance in organic fertilizers,dissolved organic matter（DOM）can form complexes with heavy metals through its rich functional groups,which plays an important role in the transformation and fate of heavy metals in soil.There are great differences in the composition and properties（fluorescence composition,group composition and physicochemical properties）of DOM from different sources,which determines the different binding ability and mechanism of DOM with heavy metals in the environment.The previous study on the one-year in-situ degradation process of organic fertilizer showed that the mineralization rate,material composition,adsorption characteristics and properties of heavy metals of organic fertilizer from different sources changed significantly.The study on the changes of DOM composition and lead adsorption mechanism of organic fertilizers from different sources during decomposition can reveal the lead adsorption mechanism of dissolved DOM of organic fertilizers from three aspects:composition mechanism,process mechanism and driving mechanism,which is helpful to understand and understand the environmental behavior of DOM in the decomposition process of organic fertilizers.In this study,seaweed and chicken manure organic fertilizers（SWOF and CMOF）were selected as the research objects,and the nylon bag method was used to simulate the in-situ decomposition experiment of organic fertilizers.（1）Modern analysis techniques such as TOC analyzer,ultraviolet–visible（UV-Vis）absorption spectrum,synchronous fluorescence spectrum（SF）,excitation and emission spectrum（EEM）and Fourier transform infrared spectrum（FTIR）,combined with parallel factor analysis（PARAFAC）and binary correlation analysis（2D-COS）,were used to explore the characteristics of DOM composition and properties during organic fertilizer decomposition at the molecular level.（2）Quenching titration test was used to explore the binding mechanism of DOM and lead by the combination of spectral technology and stoichiometric analysis.（3）Through the experiment of microbial degradation of DOM,the law of composition transformation and the characteristics of group changes in the process of DOM decomposition were clarified,and the influence of the change of DOM composition properties on lead adsorption was explored at the molecular level.The main results of this study are as follows:（1）（1）There were significant differences in the content of dissolved DOM in different sources of organic fertilizer.The content of DOM in seaweed organic fertilizer was 32.8 times higher than that of chicken manure organic fertilizer,and the content of aromatic and hydrophobic components was higher than that of CMOF-DOM.With the decomposition of organic fertilizer for one year,the SWOF-DOM content decreased by63.1%and the CMOF-DOM content decreased by 32.3%.In addition,the decrease of SWOF-DOM content mainly occurred in the last 6 months,while the decrease of CMOF-DOM content mainly occurred in the first 6 months.Furthermore,the aromaticity and hydrophobicity of DOM from the two kinds of organic fertilizers increased significantly,indicating that the aromatic and hydrophobic substances were retained with the decomposition.（2）EEM-PARAFAC showed that the SWOF-DOM contained two humic-like substances（C1 and C2）and one fulvic-like substance（C3）,The proportion of C1 and C3 increased by 11%and 7%,and the proportion of C2decreased by 18%;The C1、C4 and C2 in the CMOF-DOM are two humic-like and one fulvic-like substances,similar to the corresponding three components in the SWOF DOM.In addition,The CMOF component is C3 fulvic-like with lower molecular weight,which is related to the content of phenolic groups.The percentage of C1 and C2 in the CMOF-DOM increased dramatically by 44%and 14%,The percentage of C3 and C4 decreased by 39%and 18%,respectively.（3）The humic-like of SWOF-DOM in the decomposition of organic fertilizer is more obvious than fulvic-like.The order of fluorescence composition of CMOF-DOM is humic acid→fulvic acid→protein.（4）2D-FTIR-COS indicate that the order of response of functional groups in SWOF-DOM to time changes is phenolic（O-H）→polysaccharide（C-O）→aromatic（C=C）→carboxyl groups;CMOF-DOM response to time changes in order carboxyl groups→polysaccharides（C-O）→aromatic groups（C=C）.（2）（1）The fulvic-like of the DOM of the two kinds of organic fertilizers were preferentially combined with Pb（Ⅱ）than humic-and protein-like when the organic manure was not decomposed.On the 180th day after the decomposition of organic fertilizer,the binding order of SWOF-DOM to lead（Ⅱ）was humic-like→fulvic-like→protein-like.The binding order of CMOF-DOM with lead（Ⅱ）was fulvic-like→humic-like→protein-like.But the binding order of CMOF-DOM and SWOF-DOM with lead（Ⅱ）was the same,both of them were humic-like→fulvic-like→protein-like on the360th day.（2）Days 0 and 180 of decomposition of SWOF,the binding order of SWOF-DOM functional groups to lead（Ⅱ）is polysaccharide（C-O）→aromatic（C=C）,phenolic（O-H）,But on day 360,The order of binding to lead（II）was aromatic（C=C）→phenolic（O-H）and polysaccharide（C-O）;When CMOF is not decomposed,the binding order of CMOF-DOM functional groups to lead（Ⅱ）is aromatic（C=C）→carboxyl（C-O）,Day 180 and day 360,the binding order of lead（Ⅱ）was polysaccharide（C-O）→phenolic（O-H）→aryl（C=C）.（3）（1）Degradation experiments showed that SWOF-DOM and CMOF-DOM content decreased by 62.0%and 29.4%,respectively.Compared with CMOF-DOM,SWOF-DOM degrades more significantly,and the degradation is accompanied by enhanced aromaticity.（2）EEM-PARAFAC showed that the content of C1 and C3components of SWOF-DOM and CMOF-DOM both decrease,the content of C2component of SWOF-DOM remains unchanged,while the content of C2 component of CMOF-DOM increases.The reason for the low degradation rate of CMOF-DOM may be the conversion of organic matter.（3）The adsorption capacity of Pb（Ⅱ）by SWOF-DOM and CMOF-DOM is 19.31μmg^-1 and 2.97μmol mg^-1,respectively.With the progress of degradation,the content of bound lead increased,and SWOF-DOM had a higher adsorption capacity of Pb（Ⅱ）.（4）The correlation analysis showed that the content of ionic lead was negatively correlated with SUVA₂₅₄ and SUVA₂₆₀（p<0.01）.The adsorption of Pb（Ⅱ）by),DOM was affected by the aromaticity and hydrophobicity of DOM.In this study,from the results of EEM-PARAFAC and 2D-SF/FTIR-COS,we deeply understood the binding ability,binding sites and binding sequence of heavy metals to DOM in the environment,and clarified the differences in the fluorescence composition of DOM from different sources and the binding characteristics of functional groups with Pb（Ⅱ）.SWOF-DOM has a higher adsorption capacity of Pb（Ⅱ）,which indicates that the effects of more aromatic DOM on the mobility,bioavailability and toxicity of Pb（Ⅱ）in polluted farmland cannot be ignored.These results provide a more detailed theoretical basis for the interaction mechanism between heavy metals and DOM at the molecular level,and are also the key to the rational application of commercial organic fertilizers and effective control of the migration and transformation of heavy metals in farmland soils.