Chemical Speciation And Bioavailability Of Heavy Metals In Soils Of Lead-Zinc Mining Area And Farmland

The rapid development of social economy has resulted in serious soils pollution in China,among which heavy metals are a kind of soil pollutants with wide distribution,high toxicity and should be controlled primarily.Mining,smelting,atmospheric deposition and frequent agricultural chemicals application are main sources of heavy metals in soils.China has formulated the environmental quality standards based on heavy metals total contents for agricultural and construction lands,respectively,to protect their quality and safety.However,the bioavailability of heavy metals is not only related to their total contents,but also to their chemical speciations to a greater extent.At present,the revision of environmental quality standards for soils based on bioavailability is one of the most important research directions of soil environmental quality management in China.The chemical speciations of heavy metals in soils can be distinguished by single and successive chemical reagents with different solubility and extractability.Therefore,selecting the universal bioavailability evaluation methods to characterize different heavy metals speciations on bioavailability is a key step for developing limits of heavy metals based on bioavailability in contaminated soils.Lead-zinc mine is one of the main non-ferrous metal mines in China.Pollution of multiple heavy metals is prominent in soils surrounding lead-zinc mining areas.Copper is the main heavy metal pollutant in copper mining areas,orchard soils,pesticide Bordeaux liquid（copper sulfate）production enterprises and their surrounding soils.In this study,heavy metals chemical speciations,bioavailability and their relationships were studied in As,Cd and Pb co-contaminated soils in lead-zinc mining areas of Hunan province and three farmland copper contaminated soils,based on different biological receptors,exposure pathways and protection targets in soils of different regions.Firstly,the human oral bioaccessibility of As,Cd and Pb in lead-zinc mining areas co-contaminated soils were measured by the UBM（Unified Bioaccessibility Research Group Europe Method）,and then compared to relative bioavailability in the mouse model,with the aim to evaluate the applicability of UBM in co-contaminated soils.The effects of soil physico-chemical properties and heavy metal chemical speciations on human bioavailability were further studied.Then the available Cu in three farmland soils（black soils,meadow cinnamon soils and unsubmerged paddy soil）were extracted by 4 chemical reagents with different extraction capacities,and the relationships between Cu enrichment as well as toxic effects in lettuce and earthworm and chemically extracted Cu were established.Furthermore,the ecotoxicity thresholds of Cu were derived based on chemically extracted Cu.The main results of this study were summarized as follows:（1）Heavy metals total contents,organic carbon,and amorphous Fe and Al oxides were important factors affecting the chemical fractions of As,Cd and Pb in 12co-contaminated soil of lead-zinc mining areas.（2）UBM（Unified Bioaccessibility Research Group Europe Method）gastric phase bioaccessibility was highly correlated with the relative bioavailability of As,Cd and Pb in mice（R²>0.67）,and used to characterize the bioavailability of heavy metals in co-contaminated soil.（3）The bioavailability of As,Cd and Pb in co-contaminated soils of mining areas was affected by physical and chemical properties,such as heavy metals total contents,Fe/Mn/Al oxides and their chemical speciations.The acetic acid extracted fraction,oxidized bound fraction and organic bound fraction of heavy metals were the main bioavailable fractions for human.（4）The extraction effects of chemical reagents with different extraction capacity in the three farmland soils were different.The average extraction efficiency of Cu by acid-soluble（HNO₃）（41.38%）and complex（EDTA-Na₂）（56.81%）reagents was significantly higher than that of the exchangeable（NH₄OAc）（0.12%）and weakly exchangeable（CaCl₂）（8.70%）reagents in the three soils.Cu accumulated and Cu toxic effect in lettuce correlated best with CaCl₂ extracted Cu,while Cu accumulated and mortality in earthworms correlated best with HNO₃ extracted Cu.（5）The EC 20（20%inhibitory concentration）and EC 50（50%inhibitory concentration）based on chemically extracted Cu in three soils were derived by selecting the most sensitive indicator of the species.The range of ecotoxicity thresholds EC 20 for lettuce based on different chemically extracted Cu contents was90.45¹70.10 mg/kg（HNO₃ extraction）,102.78¹95.31 mg/kg（EDTA-Na₂extraction）,3.97²0.06 mg/kg（NH₄OAc extraction）,and 0.21⁸.68 mg/kg（CaCl₂extraction）;the range of EC 50 was 110.48¹87.60 mg/kg（HNO₃ extraction）,118.63²30.49 mg/kg（EDTA-Na₂ extraction）,5.69³2.23 mg/kg（NH₄OAc extraction）and 0.26⁹.62 mg/kg（CaCl₂ extraction）.The range of ecotoxicity thresholds EC 20 for earthworm Eisenia foetida based on different chemically extracted Cu contents was 138.26¹93.16 mg/kg（HNO₃ extraction）,107.80²25.88mg/kg（EDTA-Na₂ extraction）,8.92¹1.58 mg/kg（NH₄OAc extraction）,and0.36¹0.57 mg/kg（CaCl₂ extraction）,respectively;the range of EC 50 was187.07²21.23 mg/kg（HNO₃ extraction）,180.38³31.09 mg/kg（EDTA-Na₂extraction）,13.06¹8.30 mg/kg（NH₄OAc extraction）and 0.54¹3.21 mg/kg（CaCl₂extraction）.（6）The ecotoxicity thresholds values of Cu EC 20 and EC 50 based on chemically extracted Cu were significantly different in different farmland soils.In meadow cinnamon soils,the ecotoxicity thresholds values of EC 20 and EC 50 for lettuce and earthworms were higher than that of in black soils.This may due to the higher amounts of coexisting cations and dissolved organic matter in black soils,which have a certain alleviation effect on the biological toxicity of Cu.This study is of great significance for soil environmental risk management based on chemical heavy metals chemical speciation and bioavailability.