Isotope Tracer Study On The Migration Of Cu And Cd In Typical Surface Environment

Heavy metal pollution is one of the most widespread and harmful environmental problems in today’s environment.Analyzing the sources of heavy metal pollution in the environment is of great significance for identifying the sources of heavy metal pollution and conducting targeted pollution prevention and control.Studying the migration mechanism of heavy metals in the surface environment can provide theoretical supports for identifying the sources of heavy metal pollution.Traditional methods such as principal component analysis and cluster analysis require a large number of databases and complex statistical analysis,making it difficult to evaluate the sources of pollution.Moreover,heavy metal migration models are greatly affected by the actual environment,making model studies different.With the rapid development of Multiple-collector inductively coupled plasma-mass spectrometer(MC-ICP-MS),metal isotope data can further provide fingerprint information about sources,providing strong supports for describing the migration of heavy metals in the environment.In surface environments,biogeochemical processes such as bioaccumulation,mineral weathering,and adsorption/co-precipitation can cause isotopic fractionation of metal elements.Cu is redox sensitive element,the changes of redox conditions in the environment can cause fractionation of Cu isotopes.Cd element is one of the toxic heavy metal elements.Bioaccumulation can easily generate large Cd isotope fractionation.Furthermore,Cd isotope fractionation can indicate the form and mechanism of bioaccumulation of Cd.In this thesis,we studied the Cu isotopes in a typical Suzhou paddy profile,and Cd isotopes in one sediment core of the western Chaohu Lake,hoping to provide theoretical supports for the migration of Cu and Cd in the surface environment.We also studied the Cd isotopes of Tetrahymena under Cd stress,with the aim of providing theoretical support for Tetrahymena to control Cd pollution in water.Paddy soils provides essential mineral nutrient elements for rice growth,among which Cu is an essential nutrient element for photosynthesis and respiration.However,excessive Cu in paddy soils can generate Cu pollution.In order to effectively control Cu pollution in paddy soil,it is crucial to understand the migration of Cu in paddy soil.We studied the contents and isotopic compositions of Cu in paddy soil profile in Suzhou of the Yangtze River Delta to reveal the migration of Cu in paddy soil.The results show that the variation of Cu contents and δ65Cu values of the profile is not coupled.Due to the alternate irrigation of paddy soil,the Cu(Ⅰ)enriched in light Cu isotopes in the upper layer of the profile migrates downward with the soil solution,resulting in the enrichment of heavy Cu isotopes in the Ap and Br1 layers.In the Br2 layer,the δ65Cu values decrease significantly with the increase of Cu contents,which is mainly due to the re-adsorption of light Cu isotopes by clay minerals.In the Br3 oxide layer,Cu(Ⅰ)is oxidized to Cu(Ⅱ),and there is an obvious accumulation of Fe oxides in this layer.The previous studies show that either Fe oxide adsorption or coprecipitation of Cu will generate obvious isotope fractionation.However,the homogeneous δ65Cu values in Br3-BCrg layers indicate that the accumulated Fe oxides have little effect on the fractionation of Cu isotopes in this profile.The homogeneous δ65Cu values may be attributed to the equilibrium adsorption of Cu by clay minerals.The decreased Cu contents and δ65Cu values in the lower layer are mainly due to the removal of heavy Cu isotopes by groundwater.The Cu isotopes in paddy soil can effectively identify the migration mechanism of Cu in paddy profile,which provides theoretical basis for the study of Cu circulation in paddy soil.Chaohu Lake is one of the five freshwater lakes in our country,and its sediments can record the accumulation of heavy metals during historical periods.The high contents of heavy metals in the sediments of Chaohu Lake have potential re-migration capacity and ecological security risks.To control the pollution of heavy metals in sediments,it is crucial to study the migration of heavy metals in sediments.We studied the contents and isotopes of Cd in the sediment core of the western Chaohu Lake.The results show that the Cd contamination in the sediment core reach a moderate to strong degree,and the degree of pollution in shallow layer reaches strong pollution.The Cd isotopic compositions of shallow sediments are obviously heavy,which may be affected by smelting and coal burning activities around Chaohu Lake.At the depth of 6 cm,theδ114/110Cd value of sediment is significantly lower than that of shallow layers,indicating that 1960s agricultural fertilization may aggravate the Cd pollution in Chaohu Lake.Meanwhile,the heavy Cd isotopes in the shallow layer migrates downward with the pore water of the sediments,resulting in the heavy Cd isotope composition and the increase of Cd contents in the lower layer.At the depth of 18～24 cm,the Cd isotopic compositions of sediments are obviously lighter,which may be attributed to the enrichment of light Cd isotopes caused by the increase of Fe/Mn(hydrogen)oxides and organic matter(TOC).The loss of light Cd isotopes and heavy Cd isotope adsorption may result in the increase of δ114/110Cd values at the bottom of the sediment core.The homogenes Cd contents in the sediment core may be the result of combined processes of pore water leaching,Fe/Mn(hydrogen)oxides adsorption and TOC complexation or co-precipitation.Cd is a toxic heavy metal that can cause bioaccumulation along the food chain.Cd pollution is a serious threat to water safety and human health,which has aroused serious public concern.Tetrahymena is a typical protozona that can be amplificated rapidly under experimental conditions and is friendly to the environment.Tetrahymena possesses the potential to remediate Cd contaminated water given the rapid generation of thiols under environmental Cd stress.However,the form and pathway of Cd accumulation in Tetrahymena has not been clear,which hinders its application in environmental remediation.We analyzed Cd contents and isotopes of Tetrahymena exposed to different Cd concentrations though batch culture expreiment.The results show that Tetrahymean preferentially absorbs light Cd isotopes(Δ114/110CdTetrahymenasolution=0.20±0.02‰～-0.29±0.02‰),which implies that the intracellular Cd is probably in the form of Cd-S.The fractionation generated by Cd complexation with thiols is constant(Δ114/110CdTetrahymena-remaining solution～-0.28±0.02‰),which is not affected by the concentrations of Cd in intracellular and culture medium,nor by the physiological changes in cells.Furthermore,the detoxification process of Tetrahymena results in an increase in cellular Cd accumulation from 11.69%to 23.29%with the elevated Cd concentrations in batch Cd stress culture experiments.Cd isotope can clearly indicate the accumulation pathway of Cd in Tetrahymena,which provides theoretical basis for the application of protozoa to control environmental Cd pollution.In summary,Cu and Cd isotopes can effectively identify the geochemical migration mechanism of heavy metals in surface environment,providing important scientific basis for the management and remediation of heavy metal pollution in modern environment.In addition,Cd isotopes can identify the form and pathway of Cd accumulation in protozoa,which provides reliable theoretical supports for the application of protozoa in the remediation of environmental heavy metal pollution.