The Effects And Mechanism Of Cadmium Transport And Transformation In Sediments Driven By Dissolved Organic Matter And Manganese Oxide Minerals

Cadmium（Cd）is a hazardous element with chronic toxicity,carcinogenicity,and global mobility,which is ranked as a global and priority-controlled pollutant.Elevated Cd concentrations in aquatic systems due to various human activities,such as mineral mining,mineral processing,smelting,and chemical industry production,are of global concern,which threatens the safety of drinking water,the sustainable development of water ecosystems,and the social economy.Aquatic sediments are essential in Cd’s transfer,transformation,and immobilization because they contain many inorganic and organic colloids.Among them,dissolved organic matter（DOM）and manganese（Mn）oxide minerals have high reactivity and large binding capacity,which significantly impact the environmental fate of Cd.Therefore,a comprehensive understanding of the properties and rules of Cd transfer and transformation near the water-sediment interface under the influence of Mn oxide minerals and DOM and their interactions is the key to preventing and controlling Cd pollution in water bodies.This study takes Cd and sediment from Dongting Lake as the object,based on laboratory cultivation,spectral titration,and other experimental methods,combined with detection techniques such as microscopic detection,molecular biology and spectroscopy,supplemented by complexation models,response surface optimization method（RSM）,two-dimensional correlation spectral analysis（2D-COS）,and other analytical methods,investigated the response characteristics of the release of Cd in sediments to the change of DOM concentration gradient and the composition difference of different DOM sources,and discussed the immobilization of Cd in sediments by different crystal forms of manganese oxides.Finally,the mechanism of manganese oxide-DOM’s interaction on the mobility of Cd in sediments was deeply analyzed.The main content and results are as follows:（1）Laboratory and simulation studies were performed on the Cd（Ⅱ）adsorption by lake sediment.The results found that the adsorption mechanism of Cd（Ⅱ）on the sediment is mainly the complexation of oxygen-containing functional groups and the co-precipitation of carbonate and sulfide.An RSM model suitable for simulating the adsorption of Cd（Ⅱ）on sediments in natural lakes was constructed based on the central composite design method.The theoretical level of the maximum Cd（Ⅱ）adsorption rate（Qs）predicted by the RSM model is p H=8.84,the mass concentration of fulvic acid（FA）is 4.3 g/kg,the flow rate is 0.1 m/s,and the initial concentration of Cd（Ⅱ）is 21.65μg/L,sediment particle size 0.053–0.15 mm,humic acid（HA）mass concentration 2.9 g/kg,and adsorption time 9 min.The extreme value of Qs can provide a theoretical reference for Cd pollution control for environmental protection departments.Among them,the change in DOM concentration is the most significant factor affecting the adsorption of Cd（Ⅱ）on natural sediments.The critical concentrations of HA and FA to promote Cd（Ⅱ）adsorption were 2.9 g/kg and 4.3 g/kg.In contrast,the concentration of organic matter in the natural environment was generally higher than this critical value.Therefore,the excessive organic matter entering the lake due to artificial discharge has the potential risk of inducing Cd pollution in the water,which needs attention.（2）This mechanism of sediment Cd release driven by exogenous DOM and the role of microorganisms were explored.Four typical sources DOM FA,bovine serum albumin（BSV）,sodium alginate（SA）,and sodium dodecyl benzene sulfonate（SDBS）,all promoted the release of Cd and Mn from abiological and biological sediments.The release of Cd²⁺and Mn²⁺mediated by DOM is larger in the biological sediments.Response characteristics of bacterial communities to DOM with different sources differed.The relative abundance of Proteobacteria significantly increased by 38.16%–74.47%,7.22%–46.84%,3.41%–26.8%,56.41%–73.98%under HA,BSV,SA,and SDBS treatment,respectively,while the relative abundance of Actinobacteriota,Chloroflexi,and Acidobacteriota decreased.Microorganism-derived DOM（BSV and SA）decreased the size of the bacterial ecological network but enhanced the positive interaction between bacterial communities.Human activity-derived DOM（SDBS）increased the network size but decreased the positive interaction between bacteria.Land-derived DOM（HA）simultaneously reduced the size of the bacterial ecological network and the positive interaction between bacterial communities.Four DOM all reduced the redox potential of the sediments significantly,leading to an increase in the total abundance of typical heavy metal-reducing bacteria from 25.8%to 33.2±5.9%–77.1±3.0%.The relationship between the total abundance of metal-reducing bacteria and the dissolved Mn（Ⅳ）amount presents a positive correlation,thereby promoting the release of Cd from sediments.（3）The performance and the underlying mechanism of Cd immobilization in sediments by three Mn oxidesδ-Mn O₂,γ-Mn OOH,and Mn₃O₄ were investigated.We first show that synthesized Mn oxides were beneficial for the reduction of leachability by 43.9%–66.81%,and the reduction of bioavailability of Cd by 45.16%–99.40%.Cd speciation analysis showed that the Mn oxides converted the acid-soluble fraction into the residual fraction of Cd.We also found that Mn oxides decreased the Cd mobility in sediment through indirect and direct interactions.That is,Mn oxides indirectly enhanced the Cd adsorption onto sediment by increasing the p H and Eh of the sediment,whereas direct complexation with O-containing groups,ion exchange as>OCd⁺,and precipitation with carbonate reduced bioavailable Cd.This work demonstrated the potential of Mn oxides for efficient remediation and clarified the mechanisms of Cd immobilization.This process is superior to the traditional process in abundant resources,simplified operation,and rapid repairing process.（4）The effect of sediment DOM fractionation by Mn minerals on the DOM binding properties with Cd（Ⅱ）was analyzed.Results showed a subtle structural variation of Mn oxides in response to DOM reduction,and no phase transformations were observed.Two-dimensional correlation spectroscopy based on synchronous fluorescence spectra and Fourier transform infrared spectroscopy indicated that tryptophan-like substances and the amide（Ⅱ）N–H groups could preferentially interact with Cd（Ⅱ）for original DOM.Nevertheless,preferential bonding of Cd（Ⅱ）to tyrosine-like substances and phenolic OH groups was exhibited after DOM fractionations.Furthermore,based on the Stern-Volmer equation,each DOM fraction’s binding stability and capacity to Cd（Ⅱ）decreased after fractionation.These differences may be attributed to DOM molecules with high aromaticity,hydrophobicity,molecular weight,and amounts of O/N-containing group preferentially removed by Mn oxides.Overall,the environmental hazard of Cd will be more severe after DOM fractionation on Mn minerals.This study facilitates a better understanding of the Cd geochemical cycle in lake sediments under the DOM–mineral interactions and recommends that sediments rich in dissolved protein-like components and Mn minerals need to be careful with outbreaks of aquatic Cd pollution.（5）Changes in Cd（Ⅱ）adsorption performance and the underlying mechanism by resulting Mn oxide–DOM binary composites were studied.We synthesized organo-mineral composites using birnessite（BS）and FA during co-precipitation with BS and adsorption to preformed BS.Two FA mass proportions of 5 wt%and 15 wt%were represented general sediment conditions and those“hotspots”rich in organic carbon（OC）,such as the rhizosphere.Consequently,the presence of FA interactions with BS at environmentally representative（～5 wt%OC）improves the adsorption capacity of Cd（Ⅱ）to the resulting organo-minerals by 15.05–37.39%(q_m=156.5–186.9 mg g^-1),which is attributed to the larger SSA and pore volume of BS-FA,as well as the attachment of oxygen-containing groups with a higher Cd（Ⅱ）binding affinity.Nevertheless,Cd（Ⅱ）adsorption to BS was significantly inhibited at the high level of FA（15wt%OC）.In this way,NOM increases Cd’s mobility.Several explanations for this phenomenon are possible,notably:1)it is observed that FA can decrease the pore volume of BS,which likely decreases the amount Cd（Ⅱ）uptake in the pore diffusion process;2)it is also observed that FA can competitively adsorb to manganese（hydr）oxides via ligand exchange of the carboxylic and phenolic functional groups with Mn–O and Mn–OH,thus reducing the number of active sites available for Cd（Ⅱ）uptake;and 3)the dissolved Mn（Ⅱ）induced by FA reduction coexists with Mn oxides and their reaction fills vacancy sites with Mn（ⅡI）,reducing Cd（Ⅱ）binding capacity.Hence,organo-mineral composites,instead of pure mineral phases,are a more suitable analog when predicting trace metal mobility and fate in the environment.In organic ligand extraction experiments,three low molecular weight organic acids（LMWOAs）,oxalic acid,malic acid,and citric acid,generally enhanced the Cd（Ⅱ）desorption from BS-FA composites under weakly acidic conditions（p H 6.5）,which are common in soils.The exchange Cd content decreased by 5.63–7.93%of 5 wt%FA-BS than pure BS,but increased to 33.13–38.97%at a high level of FA（15 wt%）.LMWOAs-Mn oxide reactions may thus enhance Cd bioavailability but also increase the occurrence of Mn toxicity.Solubilization of Mn oxide-bound metals may also enhance migration downwards through a soil profile.Therefore,natural or anthropogenic processes that increase DOM contents in Mn oxide-bearing systems must be careful with outbreaks of aquatic Cd pollution.In summary,under macroscopic conditions,this study found that DOM and manganese oxide were two key variables controlling the characteristics and rules of Cd（Ⅱ）transfer and transformation near the water-sediment interface.On the level of microscopic mechanism,this study proved that manganese oxides preferentially adsorbed DOM molecules with high oxygen/nitrogen functional groups and aromaticity,thus decreasing the binding stability and capacity of each DOM fraction to Cd（Ⅱ）and changing the binding sequence of functional groups to Cd（Ⅱ）.In addition,the Mn-O group and the oxygen-containing functional group in the BS-FA binary composites are primary binding sites for Cd（Ⅱ）.Our results contribute to a better understanding of the Cd environmental behavior in sediments under the interactions of DOM and manganese oxide minerals.