Research On The Effect Of DOM On The Migration Of Heavy Metal Cadmium In The Water-Level-Fluctuating Zone Of The Xiangxi Bay

As a special land and water transition environmental system with long-term high-intensity dry and wet alternation,the water-level-fluctuating zone（WLFZ）of the Xiangxi Bay（XXB）is an active area for material and energy migration and transformation in the ecological environment.A large area of stable vegetation layer of Cynodon dactylon（L.）as the dominant species growing during spring and summer outcrops（from May to October）in the WLFZ will input dissolved organic matter（DOM）into XXB after submerged decomposition during the flooding period.DOM,as an active substance in the biogeochemical cycle,has an important effect on the migration of heavy metal pollutants in soil in the WLFZ.There is a risk of cadmium（Cd）and phosphogypsum pollution in the Three Gorges Reservoir area.Previous studies on Cd release from flooding in the Three Gorges Reservoir area were limited to the direct interaction between water and soil.Further studies are needed on the effects of DOM released by vegetation and phosphogypsum pollution risk on the environmental behavior of heavy metals in XXB under different environmental conditions during the actual water storage process in the Three Gorges Reservoir Area.Therefore,in this study,the WLFZ of the XXB in the Three Gorges Reservoir area was selected as the research area,and the heavy metal Cd was selected as the research object to explore the DOM released by the Cynodon dactylon（L.）and the influence of phosphogypsum on Cd migration under anaerobic,aerobic,flooding and drying conditions,which will help to further interpret the activity rule and mechanism of Cd during flooding in the WLFZ of the Three Gorges Reservoir,and provide some practical reference for reducing the risk of ecological environment.The research contents and conclusions are as follows:（1）Twelve sampling points were arranged along the WLFZ of the XXB,and the changes of suspended sediment,various physicochemical factors,the content and optical characteristics of DOM in the river were monitored at the initial flooding stage of the WLFZ in October 2021.The results showed that the p H of the water in the XXB was weakly alkaline overall,and moderate eutrophication at the initial flooding stage,with total nitrogen content ranging from2.231-5.874 mg/L and total phosphorus content in the range of 0.020-0.294 mg/L.The content of chlorophyll-a showed the distribution pattern of high in the middle and upper reaches of the XXB and low in the lower reaches.From the main reaches of the Yangtze River to the tributaries of the XXB,there was a phenomenon of river water back-irrigation,and the content,turbidity and total solid suspended solids of the suspended sediment gradually decreased from the intersection to the upper reaches of the XXB.The DOM content of water in the WLFZ of the XXB showed obvious uneven distribution law.In the middle and lower reaches of the WLFZ of the XXB,the content of DOM was abundant.The components were mainly C1-fulvic acid and C2-humus.The proportion of C1 component was relatively high in the middle and upper reaches of XXB,and the content of DOM fatty substances was richer.The DOM in the middle and lower reaches was mainly C2,and the content of polysaccharide was higher.The XXB is located in a complex environment,suspended sediment and nutrient content in water were closely related to DOM content and its optical properties.The increase of total nitrogen content increased the autogenic input of DOM.Suspended sediment could absorb DOM in water and flocculate with DOM humus samples under disturbance,reducing the degree of DOM humus in water.（2）Selecting heavy metal Cd as the research object,the content and optical characteristics of DOM released by Cynodon dactylon（L.）and soil Cd dissolution during flooding in the subsidence zone were carried out to explore the content and optical characteristics of DOM released by Cynodon dactylon（L.）in anaerobic and aerobic environments,as well as the effect on Cd migration in the WLFZ.The results showed that the DOM release process of Cynodon dactylon（L.）included a rapid growth stage in the early 9 days after flooding,when the DOC concentration reached the peak.In the later 14 days,the easily degradable components were consumed,and the DOM content decreased.The DOM released by flooding of Cynodon dactylon（L.）mainly comes from the joint input of endogenous and exogenous,including three components of tyrosine,humus and lignin phenol in the overlying water,and tyrosine and two humus components in the pore water.The aerobic environment was more conducive to the accumulation of lignin phenols.The anaerobic environment was more conducive to the accumulation of protein components tyrosine.In addition,the DOM released by Cynodon dactylon（L.）could cause the dissolution of Cd,Fe and Mn from the soil.The specific mechanism is that the aliphatic part of the DOM forms Cd-DOM complex with Cd,and a part of it competes with Cd for the surface binding site of soil particles,promoting the migration of Cd²⁺upward overlying water.（3）The soil of the XXB was selected as the research material,and the two conditions of flooding and drying in the WLFZ of the XXB were simulated.The change process of Cd,DOM,SO₄^2-and other indexes in soil was monitored to explore the influence of the combined action of DOM and phosphogypsum on soil nutrient changes and soil Cd migration.The results showed that the Cynodon dactylon（L.）and phosphogypsum would reduce p H value in the soil significantly,and the content of available nitrogen,available phosphorus and SO₄^2-in soil were significantly increased（P<0.05）,but the nutrients would be lost after flooding.The addition of Cynodon dactylon（L.）promoted the release of DOM,while the addition of phosphogypsum reduced the DOM content.In the process of flooding,the components of DOM changed from the initial C1-terrestrial humus sample and C2-protein sample to C1-terrestrial humus sample and C3-microbial humus sample.The migration of Cd in the environment was affected by DOM,Fe,Mn and soil nutrient content.Doping of phosphogypsum with Cynodon dactylon（L.）promoted the migration of Cd from soil to water at the initial stage of flooding,which was mainly due to the decrease of environmental p H,and the increase of DOM and Mn content.（4）On the monitoring platform of the XXB,the release risk of heavy metal Cd under the combined action of Cynodon dactylon（L.）and phosphogypsum in the actual environment was studied through the field in situ hanging bottle experiment.The results showed that during the flooding period of the WLFZ of the XXB,the Cynodon dactylon（L.）would consume dissolved oxygen,and phosphogypsum would reduce the p H of the water,increase sulfate content.Under the combined action of Cynodon dactylon（L.）and phosphogypsum,the redox potential value of water decreased,and the reduction characteristics of water were enhanced.Flooding the Cynodon dactylon（L.）to release the DOM was a rapid process that peaked in a week.After one month,the DOM content decreased and equilibrium was reached in the water environment.The combined action of Cynodon dactylon（L.）and phosphogypsum promoted the release of Fe,Mn and Cd in the environment,and enhanced the risk of heavy metal pollution in the XXB.