Study On Migration And Transformation Mechanism Of Antimony Mediated By The Redox Of Manganese

Antimony（Sb）is a toxic and carcinogenic element that has been classified as a priority pollutant.Due to mining,smelting and other industrial activities,a large amount of Sb is discharged into the environment,and the Sb pollution in soil is particularly serious.The concentration of Sb of the soil in the most mining areas in China exceeds the maximum concentration of Sb allowed by the World Health Organization（36mg/kg）.The antimony mainly enters the soil in the form of antimony minerals stibnite（Sb₂S₃）and antimony trioxide（Sb₂O₃）,and then processes the dissolution,oxidation,adsorption and precipitation which will increase the concentration of Sb in the environment and cause harmful effect to the surrounding environment and human health.As a redox-sensitive element,Sb exist mainly Sb（Ⅲ）and Sb（Ⅴ）in the environment.Sb（Ⅲ）predominates in the form of neutral Sb（OH）₃ under anoxic conditions,while Sb（Ⅴ）predominates in the form of anion Sb（OH）_?6 under oxic environment.Sb（Ⅴ）as anion is more mobile than neutral Sb（Ⅲ）in the environment.However,Sb（Ⅲ）has a high affinity with red blood cells and is easy to reach the key biological targets.Thus,Its toxicity is 10 times than that of Sb（Ⅴ）.Therefore,the migration,toxicity and bioavailability of Sb are closely related to its redox state.Manganese（Mn）oxide is ubiquitous in soil,and plays an important role in the migration and transformation behavior of Sb in the environment due to its high redox potential and strong surface activity.Mn（Ⅱ）oxidation is an important way to form diverse Mn oxide in the environment.The redox process of Mn profoundly affects the migration and transformation of Sb in the environment,and is easily affected by environmental factors such as redox conditions and co-existing organic matter.Therefore,studying the migration and transformation process of Sb mediated by Mn redox and the influence of environmental limiting factors is helpful to understand the environmental fate of Sb,and lay a foundation for the risk assessment of Sb pollution and the establishment of effective pollution control measures.In this study,the surface soil of Shenxiandong tailing pond in Yunnan Province was selected as the research object to analyze the pollution characteristics of heavy metal and risk assessment.Secondly,by exploring the co-transformation behavior of Mn（Ⅱ）oxidation and the oxidative dissolution of Sb₂O₃,the the mechanism of the oxidative dissolution of Sb₂O₃ mediated by Mn（Ⅱ）oxidation was revealed.Then,the ternary system of“dissolved organic matter-Mn O₂-heavy metal Sb”was established,to explore the the influence of the loaded order of citric acid in the system on immobilizing Sb（Ⅲ）by Mn O₂ and its effect mechanism.Finally,the soil polluted by Sb in tailings pond and exotically added with Sb₂O₃ were selected to periodically change the redox state of the soil and explore the migration behavior of Sb under the fluctuation of redox conditions.The main research results are as follows:（1）The pollution characteristics and risk assessment of 10 heavy metals Sb,Mn,Co,Cu,Cd,Pb,As,Cr,Ni and Zn in the surface soil of Shenxiandong tailings pond show that,there is serious heavy metal pollution in Shenxiandong tailings pond.On the whole,the content of heavy metals in the tailings pond is much higher than that in the soil surrounding the tailings pond,and the average content of heavy metals is mostly far higher than the soil background value in Yunnan Province.Among them,the average content of Sb,As and Pb in tailings pond soil far exceeds the control value of the first type of land use in《Soil environmental quality—Risk control standard for soil contamination of development land（Trial）》（GB-36600-2018）.Sb,As,Pb and Cd also have strong potential ecological risks.And the sources of Sb,Mn,Cd and As are related to the mining activities of lead-zinc ore.There is obvious fractionation of Sb isotope in soil,ranging from-0.28‰to 0.45‰.Moreover,the oxidation process of Sb and/or adsorption and precipitation by Fe/Mn plays an important role in Sb isotope fractionation,leading to the enrichment of heavy Sb isotope in the soil thata gradually away from abandoned plants and tailings ponds.（2）The transformation process of Sb₂O₃ oxidative dissolution mediated by Mn（Ⅱ）oxidation shows that,the oxidative dissolution of Sb₂O₃ is enhanced at higher p H and Mn（Ⅱ）concentration.When Mn（Ⅱ）concentration is low（0.01mmol/L）at p H 9.0,the promotion of the oxidative dissolution of Sb₂O₃ is mainly due to the active Mn（Ⅲ）species produced by Mn（Ⅱ）oxidation.However,at higher Mn（Ⅱ）concentrations,the formation of amorphous Mn（Ⅲ）oxide and intermediate active Mn（Ⅲ）species both promote the oxidative dissolution of Sb₂O₃.Most of the released Sb is fixed by Mn oxides,and Sb（Ⅴ）was dominant in dissolved total Sb and adsorbed total Sb.At the same time,the presence of Sb₂O₃ not only inhibits the removal of Mn（Ⅱ）by reducing Mn（Ⅲ）to Mn（Ⅱ）,but also affects the final product of Mn（Ⅱ）oxidation.The formation of amorphous Mn oxides replaces the formation of crystallised Mn（Ⅲ）oxides such as Mn OOH.In addition,when Mn（Ⅱ）/Sb₂O₃ ratio is high,rhodochromite（Mn CO₃）can be formed.However,Mn CO₃ formation was not observed when Mn（Ⅱ）/Sb₂O₃ ratio was low.This study further deepened the understanding of the effect of Mn oxidation process on Sb migration and transformation in the soil-water system.（3）The adsorption and oxidation behavior of Sb（Ⅲ）onδ-Mn O₂ in the ternary system of citrate-δ-Mn O₂-Sb indicates that,the loading order of citrate not only affects the oxidation and fixation of Sb byδ-Mn O₂,but also affects the degree of Mn（Ⅱ）dissolution,and the process is affected by p H value.At p H 7.0,when citric acid is pre-loaded withδ-Mn O₂,more solid Mn（Ⅲ）is generated and the content of ligand-OH on the surface ofδ-Mn O₂ is increased,which jointly promoting the adsorption and oxidation capacity ofδ-Mn O₂ for Sb and reduces the mobility of Sb.However,when citric acid is added with Sb（Ⅲ）simultaneously,citric acid can directly compete for the adsorption sites and reductively dissolveδ-Mn O₂ to generate more dissolved Mn that can indirectly compete for adsorption with Sb,which leads to the weakening of the adsorption and oxidation capacity ofδ-Mn O₂ for Sb.The adsorption and oxidation capacity of Sb（Ⅲ）and the fixation capacity of Sb（Ⅴ）ofδ-Mn O₂ are weakened under acidic conditions,compared with that unduer neutral and alkaline conditions.In addition,different loading sequences of citric acid lead to the different degrees of reductice dissolution ofδ-Mn O₂.Compared with the system in which citric acid and Sb（Ⅲ）were added simultaneously,both prior and subsequent addition of citric acid would increase the concentration of dissolved Mn and increase the possibility of Mn（Ⅱ）toxicity.This study further deepens the understanding of the migration and transformation behavior of Sb by Mn oxide reduction in the ubiquitous organic environment.（4）The migration behavior of Sb in polluted soil and exogenously added Sb₂O₃under redox fluctuations show that,there is a negative correlation between dissolved Sb and dissolved Mn.Sb can be released by reductive dissolution of Mn oxides under reduction period and can be fixed by Mn oxides during oxidation period.The redox process of Fe seems to have little effect on the migration of Sb.The concentration of colloidal Sb is much higher than that of dissolved Sb,and there is a good positive correlation between colloidal Sb and colloidal Fe and Mn,indicating that the colloidal Fe/Mn plays an important role in transporting Sb in soil.The dissolved Sb is negatively correlated with SO₄^2-.Combing the changes of solid phase of S in soil,it is speculated that the oxidation of solid sulfide with low oxdation state seems to have little effect on the migration of Sb.The immobilization of Sb in soil is related to the formation of Ca（Sb（OH）₆）₂ precipitates.In Sb contaminated soil,Sb is basically retained in the residual fraction,accounting for about 90%of the total Sb.In the soil with the addition of Sb₂O₃,the residual fraction of Sb is also the main form.At the end of the first week of oxidation period,the residual fraction of Sb accounts for about 73%,indicating that the transformation process of Sb in the soil is fast,and it is mainly related to the stubborn components in the soil.This study further revealed the migration behavior of Sb in soils subject to redox fluctuations and deepened the understanding of the environmental fate of Sb.