Characterization Of Organic Matter In Typical Intertidal Sediments Of Guangdong-Hong Kong-Macao Greater Bay Area And Its Effect On Cd Morphology Transformation

The intertidal zone is a transitional area between terrestrial and marine ecosystems,and is one of the most vibrant multifunctional ecosystems in the world,with significant ecological,hydraulic and social benefits.With the rapid development of economy and urbanization,a large number of human activities have led to the continuous degradation of intertidal environmental quality,especially heavy metals are harmful to the ecological security of intertidal zone.Considering that the intertidal environment changes with the tide and human coastal activities intensify,it is of great significance to investigate the sources,migration and transformation processes of heavy metal pollution in intertidal sediments and their influencing factors.Organic matter is widely found in sediments with complex composition and rich active functional groups,can combine with heavy metals to form organic-metal complexes with different solubility and stability,which can affect the morphology,migration and toxicity of heavy metals.In recent years,there have been more studies on the relationship between organic matter and heavy metals in sediments,but few in-depth studies have been conducted on the mechanism of organic matter on the morphological transformation of heavy metals,especially the mechanism of organic matter on the morphology transformation of heavy metals in intertidal surface sediments is rarely reported.Therefore,in this study,14 typical intertidal zones in the Guangdong-Hong Kong-Macao Greater Bay Area were selected as the research objects,and Cd morphology contents in surface water,pore water,suspended solid and sediment were measured.And the structure and composition characteristics of sediment-extracted organic matter were analyzed by ultraviolet absorption spectrum,three-dimensional fluorescence spectrum and high-resolution mass spectrometry,so as to reveal the relationship between sediment organic matter and Cd morphology in the actual intertidal environment.Then the interaction of sediment organic matter on heavy metal Cd was simulated to explore the ternary interaction mechanism of sediment-organic matter-Cd by combining relevant sediment characterization techniques and the structure and composition information of organic matter.The main results are as follows:（1）The results of ultraviolet absorption spectrum analysis showed that the mean values of DOC,a₃₅₅,SUVA₂₅₄,SUVA₂₆₀,E₂/E₃ and S_275²95 of water-soluble organic matter extracting from sediments（WDOM）were 6.43 mg·L^-1,5.77 m^-1,3.34 L·mg^-1·m^-1,3.10L·mg^-1·m^-1,4.07,and 0.011 nm^-1,respectively,while particle bound organic matter extracted from sediments（PDOM）were 54.70 mg·L^-1,127.93 m^-1,3.38 L·mg^-1·m^-1,2.70 L·mg^-1·m^-1,7.03 and 0.028 nm^-1,respectively.Correlation analysis and linear fitting showed that the aromatic components of WDOM and PDOM were significantly different.The results of three-dimensional fluorescence analysis showed that there were three fluorescence components in WDOM,the WDOM humification index（HIX）ranged from 1.21 to 4.69,with a mean value of 2.24.The WDOM biological index（BIX）ranged from 0 to 1.02,with a mean value of 0.56.The there were two fluorescent components in PDOM,and the range of PDOM-HIX was 0～109.85,with a mean value of 4.30.The range of PDOM-BIX was0.44～3.58,with a mean value of 2.01.Based on parallel factor analysis,the main components of WDOM were tryptophan,tyrosine and microbial-derived humus,and the PDOM components were microbial-derived humus and terrestrial-derived humus.High-resolution mass spectrometry analysis showed that the carbon sources and nutrients available microorganisms accounted for 63～79%and 60～75%of the total contents of WDOM and PDOM.Generally,WDOM had a lower degree of humification and higher relative molecular weight and hydrophobicity,while PDOM had a lower relative molecular weight and higher degree of humification,good photosensitivity and strong activity.（2）The contents of Cd in surface water,pore water,suspended solid and sediments in the intertidal zone were 0.41～15.03μg·L^-1,0.41～27.54μg·L^-1,0.41～4.88 mg·kg^-1 and0.20～5.30 mg·kg^-1,respectively.The concentration of Cd in surface water was slightly lower than that in pore water at the same sampling site,while the content of Cd in suspended solid was comparable to that in sediments.The morphology analysis results showed that Cd was mainly ionic in surface water and pore water,and its mass fraction was 67.23%～97.56%and33.33%～97.16%,respectively.In suspended solid and sediments,Cd was mainly in residue state,with mass fraction of 45.45%～96.36%and 45.80%～97.27%,respectively.Person correlation and single factor linear regression analysis showed that the complex Cd in surface water and pore water was significantly negatively correlated with ORP and p H,while positively correlated with TOC.The ORP was determined to be the key factor for the spatial variation of Cd morphology in the solution phase.The bioavailable Cd in each sediment layer was significantly positively correlated with ORP,SOC and clay,while significantly negatively correlated with p H.The clay content had the most significant effect on the spatial distribution of bioavailable Cd in sediments.（3）Person correlation analysis and RDA analysis were used to analyze the relationship between the spectral characteristic parameters of organic matter extracted from sediments and the occurrence morphology of Cd.The showed that the microbial activity and the protein component contributed to the conversion of Cd morphology to the complex and residual states with low toxicity,while the high concentration and high aromatic organic matter groups will lead to the conversion of Cd morphology to the ionic and bioavailable states,increasing the ecological risk of heavy metal.（4）In the sediment-Cd binary system,the effect of sediment on Cd was mainly physical and chemical adsorption.The sediments had stronger adsorption effect on Cd with larger S_BETand richer void structure.The adsorption effect of four types of sediment on Cd followed the order of raw clay sediment>raw sandy sediment>de-organic clay sediment>de-organic sandy sediment.The addition of organic matter could significantly increase the adsorption and complexation on Cd in sediments,and the promoting effect was PSDOM>PCDOM>WSDOM>WCDOM.（5）The main interaction mechanisms of sediment-organic matter-Cd ternary system included:on the one hand,sediment-organic matter directly formed Cd-OH coordination bonds with Cd through functional groups（-OH）.On the other hand,organic matter combines with sediment to form complex minerals with stronger adsorption capacity,and then carried out chemical adsorption of ionic stated Cd.The protonation of amide and rupture of C=O bonds may also occur during this process,thus effecting the redistribution of bioavailable and residual Cd in sediments.Moreover,salinity reduced the aromaticity of organic matter extracted from sediments by inhibiting microbial activity,thus slowing down the adsorption and complexation of Cd,while light reduced the adsorption and complexation of Cd on sediment-organic matter through photolyzing of aromatic compounds to generate aliphatic compounds with reducing surface functional groups.