Studies Of Cadmium Distribution Characteristics And Diffusion Accumulation Mechanisms In Constructed Wetlands Based On Water Quality Analysis And Evaluation Throughout Tuojiang River Basin

Cadmium pollution in river is a pressing ecological and environmental issue currently confronting China.It is crucial to study its behavior and management in river to prevent the harmful effects of cadmium pollution.The Tuo River,one of the five major tributaries of the Yangtze River in its upper and middle reaches.Despite research on cadmium pollution in the Tuo River,several issues persist,including the limitations of a single water quality assessment model,unclear diffusion and migration of cadmium in constructed wetlands,and inadequate understanding of cadmium’s bio-toxicity at the molecular scale.In this paper,we addressed these issues by analyzing the current water quality and cadmium pollution status of the Tuo River and conducting a source-tracing study on cadmium pollution using a combination of experimental and simulation calculations.We also investigated the diffusion,transfer rules of cadmium among surface water,sediment,and plants,as well as its migration and toxic effects within wetland plants.Based on the above innovation points,the main conclusions obtained in this paper are as follows:（1）A comprehensive analysis of the water quality of the Tuo River reveals a fluctuating trend between 2012 and 2020,with a general decline in the concentration of cadmium pollutants.Spatially,exceeding levels of 5-day biochemical oxygen demand,ammonia nitrogen,total phosphorus,and several heavy metals were observed starting from the Sanhuang Temple sampling site in the upstream region.Temporally,since 2015,the total phosphorus content has continuously exceeded the standard,and downstream sites such as Shengjia Ditch and Shejia Street have exhibited significant exceedances of mercury and lead standards,with potential risks of cadmium exceedances.Based on the spatiotemporal analysis,the neural network method provides a smaller evaluation error for the actual pollution situation compared to the fuzzy mathematics method and is more suitable for evaluating the Tuo River system.Furthermore,the migration rules of cadmium in constructed wetlands’water environment were analyzed,revealing that water depth,flow velocity,and water p H are the primary factors influencing cadmium distribution within water bodies.（2）The adsorption of cadmium ions in water by sediment minerals was analyzed,revealing that stable chemical adsorption can occur between cadmium ions and the calcite（110）crystal face,as well as the quartz（100）,（110）,and（111）crystal faces.The optimal adsorption site for cadmium on quartz was identified as the silicon atoms of the（100）crystal face,with an adsorption energy of-2.96 e V.General calculations were performed to determine the process of cadmium ion migration from the calcite（110）crystal face to the quartz（100）crystal face,revealing the presence of three transition states with oscillating reciprocal motion of cadmium ions between each state.This motion was attributed to the unbalanced electrostatic attraction of minerals.The energy barrier of this diffusion process was determined to be-3.02e V,which is only 0.06 e V less than the cadmium ion adsorption formation energy of the quartz（100）face.These findings suggest that the spontaneous diffusion of cadmium ions from calcite to quartz can occur.（3）Our study examined the competitive adsorption of cadmium and essential nutrients by plant root exudates.We found that succinic acid provides an adsorption advantage for cadmium ions,while the adsorption of other root exudate organic acids for cadmium,calcium,and iron is not very different.Additionally,we analyzed the transport efficiency and defense mechanisms of cadmium in different physiological parts of plants.Our findings indicate that Zephyranthes candida and Cynodon dactylon are the main plants that accumulate cadmium and have relatively high tolerance to cadmium.However,Pontederia cordata has a larger amount of cadmium adsorption,but its cadmium tolerance is relatively poor.（4）The impact of cadmium on respiration has been analyzed,revealing that cadmium ions can form complexation reactions with glucose and its decomposition products,including pyruvic acid and ATP（triphosadenine）.Additionally,these ions can participate in complexation reactions with the key molecules involved in the tricarboxylic acid cycle.The 4d¹⁰ and 5s empty orbitals of cadmium ions react with organic acids and coenzymes mainly involved in respiration,resulting in their adsorption and complexation on the hydroxyl oxygen atoms of carboxyl groups and carbonyl oxygen atoms.Cadmium ions also react with ring-structured molecules such as ATP through naphthalene rings,leading to the disruption of the energy of their frontier orbitals and the respiration process.（5）After conducting an analysis of the effects of cadmium on photosynthesis,it was observed that the dark reaction stage remains unscathed while the light reaction stage is entirely obliterated,resulting in a significant hindrance to photosynthesis.This phenomenon is attributed to the displacement of magnesium ions in the porphyrin ring of chlorophyll by cadmium ions via a copper displacement reaction.This renders the chlorophyll structure less stable,resulting in a reduced efficacy of light energy conversion.The 3-carbon and 5-carbon compounds in the dark reaction stage are not affected by cadmium ions.（6）The adsorption by sediment and plant enrichment in the constructed wetlands of the upstream area is an important factor in reducing the concentration of cadmium pollution entering the Tuo River,explaining why there is a phosphorus exceedance problem in the Tuo River while the upstream cadmium content does not exceed the standard.These discoveries provides important insights for the management of cadmium pollution in the downstream water systems of the Yangtze River.