Cadmium Migration And Transformation In Soil Under Sea-land Interaction In The Pearl River Delta Plain

Cadmium,rare metal element,chemical symbol Cd,atomic number 48,is extracted as a by-product from zinc or thio-cadmium ores and is used mostly to protect other metals from corrosion and rust.It is also used to make a bright yellow pigment called cadmium yellow.Cadmium is a carcinogen,which can accumulate in human through the food chain and seriously harm human health.Excessive importation of cadmium into soil has attracted worldwide attention because of its toxicity,persistence and bioenrichment.Soil pH,Eh,organic matter and hydrodynamic conditions under the interaction of land and sea are significantly different from those in the continental sedimentary environment.In order to prevent and reduce the damage of heavy metal Cd in soil under the terrestrial sedimentary soil,it is urgent to study the mechanism of Cd migration and transformation in soil under different sedimentary environments.In this paper,therefore,the soil of continental and marine sedimentary environment in Nansha area of Pearl River Delta is selected as the research object.First of all,based on the comprehensive analysis of existing data and research results,hydrological geology survey,soil/sediment samples collection and determination were conducted to analyze the distribution rule of heavy metal cadmium in soil/sediment and groundwater environment,to evaluate soil cadmium pollution and ecological risk,and to study the factors affecting the total amount and speciation distribution of soil cadmium in the study area.Then,the soil of terrestrial and marine sediments was taken as the research object to study the differences of cadmium adsorption rules in soil of two different sedimentary environments through laboratory static adsorption experiments,and to discusse the effects of the initial concentration of solution,time and pH on the adsorption.What’s more,the adsorption and migration of cadmium in terrestrial and marine sedimentary soils were studied by laboratory soil column experiments.Finally,taking the field site as an example to explore the adsorption characteristics and influencing factors of the sea-land interaction soil on the heavy metal cadmium in the field test site.The main research contents and results of this paper are summarized as follows:1.Based on field investigation and sampling,the characteristics of cadmium pollution in soil/sediments under sea-land interaction in Nansha area were analyzed,and it was revealed that most of the sea-land interaction sediments had high potential environmental risks.A total of 350 surface soil sampling points and 6 borehole sampling points were set in the study area,and 404 soil/sediment samples were obtained,with the deepest soil/sediment sampling depth of 34.2m.Findings:The surface soil in the Nansha core area is mostly acidic soil,with a median pH of 5.44 and a cadmium content of 0.01-2.68mg/kg,with a mean and median value of0.54 mg/kg and 0.57 mg/kg,respectively,exceeding the risk screening value of 0.3mg/kg in "Control Standard for Soil Environmental Quality and Agricultural Land Pollution Risk"(GB15618-2018).According to the risk assessment coding method(RAC),the river sediments in Nansha area are currently weakly alkaline to alkaline,and the ecological risk of cadmium ranges from high to very high.The average Cd content in marine sediments is 0.36mg/kg,while that in terrestrial sediments is significantly lower than that in marine sediments,with an average of 0.10 mg/kg.The significant difference in Cd content between the two sediments reflects that the geographical and geological background of the study area affects the enrichment rule of Cd.However,the content of Cd in marine sediments at different depths also varies greatly,which reflects the limitation of the influence of geographical and geological background.The geological accumulation index(Igeo)in the surface soil of more than 65% of the surveyed areas was within the range of 2~3,which was moderate to heavy pollution,and some areas were severe to extreme pollution,showing a high risk(160≤ Er < 320),and the danger(Er ≥ 320)was more than 36%,mainly distributed in Hengli,Wanqingsha,Xinken and other towns.The average value of extractable acid in the sediments from the drilling profile was 56.3%,and the risk assessment coding method(RAC)showed that most of the sediments had high potential environmental risks.The total amount of Cd has a great influence on the fraction of Cd in soil.The correlation between the basic physical and chemical properties of soil and the total amount of Cd is not significant on the whole,but it has a significant influence on the fraction of Cd.2.Through static adsorption experiments,it was revealed that the soil in marine sedimentary environment had stronger adsorption capacity for Cd2+,but the soil in terrestrial sedimentary environment had higher saturated adsorption for Cd than the soil in marine sedimentary environment.Static adsorption experiments were carried out in the laboratory to study the adsorption behavior of soil cadmium from the two aspects of isothermal adsorption equation and kinetic adsorption equation fitting.Meanwhile,the effects of initial concentration of solution,adsorption time and pH value on the adsorption behavior of cadmium in the two kinds of soil were discussed,and the results showed that:The soil of continental sedimentary environment is silty clay,with 35% clay proportion,the soil is weak acid,soil Cd content is 0.52mg/kg.The soil of marine sedimentary environment is sandy silt,the soil is alkaline,and the soil Cd content is0.48mg/kg.The soil cadmium content in the two sedimentary environments was significantly higher than that in Guangdong Province(0.056mg/kg)and the Pearl River Delta(0.11mg/kg).Henry isotherm model,Langmuir isotherm model and Freundlich isotherm model can be used to describe the adsorption characteristics of cadmium in the two soils,and the Freundlich equation has better fitting.The isothermal adsorption constants of Cd in marine and terrestrial environments are 558.21 and 363.04,respectively.Soil clay content is high in terrestrial and marine sedimentary environments,and the saturated adsorption of Cd is 2419.83mg/kg and 2228.25mg/kg,respectively.The pH value may affect the mechanism of soil cadmium adsorption.When the ratio of soil to water was 5.5:1 and the concentration of Cd2+ was 100mg/L,as the reaction time t approached infinity,the saturated adsorption Sm of the two soils on the whole increased with the increase of the solution pH.3.The soil column experiment revealed that the exogenous Cd was mainly enriched in the soil surface,but the content of Cd in the surface soil was not determined by the soil adsorption capacity.Adsorption experiments of cadmium solutions of different concentrations in saturated soil of sea-land interaction and vertical migration of cadmium in the leaching process of cadmium contaminated soil were carried out in the laboratory to study the migration law of cadmium in the soil of marine and terrestrial sedimentary environments.Findings:The flow rate of water through the soil column is larger at first,and it reaches the state of basically uniform flow when the resistance and dynamic force of the solution through the soil are basically balanced.The final stable seepage velocity of the soil column loaded with silty clay in the continental sedimentary environment is2~3kg/m2/d,and the average stable seepage velocity of the soil column loaded with silty sand in the marine sedimentary environment is 44kg/m2/d.After the adsorption experiment,the exogenous input Cd was mainly concentrated in the upper layer of 0~6cm.With the continuous input of Cd2+ in the leaching solution,the Cd in the soil migrated vertically.Cd content in soil on the surface is not determined by the adsorption capacity.The accumulation process of exogenous pollutants into the soil is also affected and restricted by the geochemical background of the soil environment and the migration and transformation process,such as the input and output of pollutants,adsorption and desorption,fixation and dissolution,accumulation and degradation,and so on.These processes are all in dynamic change.When 60(?)g/g Cd2+ was added to 0-5 cm surface soil,the concentration of Cd2+in soil column leaching solution was generally low after continuous leaching,and the content of Cd leaching each time and cumulative leaching from different soil columns were also very low.When pH was 3.5,Cd was more easily transferred from surface contaminated soil to 5-10 cm soil.After the leaching experiment,Cd was mainly adsorbed on the surface of the soil colloid by electrostatic attraction.The main form of Cd in the soil column was the ion exchange state,the proportion of carbonate bound state was also high,and the content of residual state and strong organic bound state was very small.4.Through the field experiment case,it is verified that the type of soil deposition environment and pH value are the main factors affecting the vertical migration and transformation of Cd elements in soil/sediment,and reveals the migration and transformation rule of Cd elements in the sediments with sea-land interaction.A total of 18 deep soil sampling sites(A total of 90 groups of soil samples)in the study area were selected for analysis to grasp the key controlling factors affecting the migration and transformation of cadmium under the interaction between land and sea in the Pearl River Delta Plain,and to study its cycle rules.Findings:In the surface land sediment environment,88.89% of the soil pH≤7.5 and 88.89%of the soil Cd content ≥0.3mg/kg which were higher than the Cd pollution risk screening value in the Soil Environmental Quality Standard for agricultural land.The soil depth of 90~140cm and 140~200cm is mainly marine sedimentary environment,the soil Cd content is less than 0.6mg/kg,77.78% and 83.33% of soil pH>7.5,respectively.Most of them were lower than the screening value of Cd pollution risk in the Soil Environmental Quality Standard of agricultural land.There was a significant negative correlation between the mean value of water-soluble Cd and the mean value of pH in five layers from shallow to deep in Nansha area(P < 0.01),and the correlation coefficient was-0.93.The mean value of Cd extracted from weak acid was positively correlated with the mean value of pH(P <0.01),and the correlation coefficient was 0.92.That is,with the increase of soil depth,soil pH gradually increased,the soil water-soluble state gradually decreased with the increase of pH value,and the weak acid extraction state gradually increased.The migration of Cd in the soil formed by terrestrial sediments also shows different migration ability with difference of environmental pH.When the environment is neutral and alkaline,Cd2+ precipitates as carbonate minerals,which inhibits the vertical migration of Cd in the soil.If the soil environment is weak acid,the soil adsorption capacity of Cd is limited.Only in acidic environment,Cd2+ can be effectively desorbed.The lower the pH value,the higher the reversibility of the adsorption process,and the more difficult it is for Cd2+ to be adsorbed on the surface of soil particles,which will increase the vertical migration amount and depth of Cd,making it easier for Cd to enter deep soil and pollute the lower phreatic aquifer.