The Speciations Of Heavy Metals In Soils Around A Mining Area In Southern China And The Transformation Of Heavy Metal Under Reductive Conditions

Soil pollution is becoming a common concern of the world,especially heavy metal pollution.The biotoxicity of heavy metal is not only related to the total concentration but also to the species existed.Heavy metal pollutants in soils exising as various speciations have different toxicity,transport behaviors and bioavailability,and thus lead to different environmental effects.Further,when the environmental conditions change(e.g,soil pH,Eh,the types of organic matter content and soil microorganisms,etc.),the speciations of heavy metals will also be suffered changes.Our previous study has shown that the sequence of binding affinity between heavy metal ions and crystalline iron oxide in the soils around Dabaoshan mining area was Cd>Pb>Zn>Cu.As the soil type of Dabaoshan mining area belongs to the latosolic red soil in dry land and its developed paddy soil,the soil in Hengyang mining area used in this paper belongs to the red soil in dry land and its developed paddy soil.The purpose of this study was to compare the similarities and differences of the heavy metal distribution characteristics in similar soils in the two mining areas and their contents and types of heavy metals associateded with crystalline iron oxide.The speciations of several typical heavy metal pollutants(Pb,Zn,Cu and Cd)in dry land and paddy soils around a nonferrous metal mining area in Hengyang,Hunan province were analyzed.Further,the changes of the heavy metal speciations in contaminated soil in Dabaoshan mining area under reductive conditions were studied.The main results obtained are listed as following:(1)The clay minerals in Hengyang mining area are mainly composed of kaolinite,mica and 1.4 nm minerals.Specifically,in the dabai village,the clay minerals is mainly composed of kaolin,with content of 54.97~73.55%,followed by hydromica with content of 23.98~42.03%,and 1.4 nm mineral with content of 1.81~4.92%.The clay minerals in shuibian village was mainly hydromica(65.34~93.7%),and then kaolinite(4.63~25.43%)and 1.4 nm minerals(1.77~7.7%)respectively.(2)The main types of iron oxide in these soils are goethite,with a small amount of hematite.Except for the samples DB1 and SL12 containing 10.3% and 7.6% hematite respectively,all the iron oxides in the soil samples were goethite.(3)The contents of Cd,Pb,Cu and Zn in the soils of Hengyang non-ferrous metal mining area is significantly higher than the background value.Speciation analysis shows that: Cd is mainly in the residual state(30.49~56.71%),followed by the exchange state(26.65~37.72%);the main speciations of Pb are associated with the easily-reducible Fe-Mn oxides(31.6~38.42%),well crystalline iron oxide associated(22.22~31.63%)and residual states(30~40.79%);the main forms of Cu are organic associated(34~45%)and residue state(39.36~57.1%);Zn is mainly in the residual state(42.39~76.5%),followed by the well crystalline iron oxide associated(10.4~11.9%)and the easily-reducible Fe-Mn oxide associated(8.3~22.3%).The corresponding speciations of heavy metals in colloid are as following: Cd,Pb and Zn are mainly in residue state with proportions of 34.02~62.65%,49.5~70.96% and 50.35~70.82% respectively;while Cu is mainly associated with amorphous iron oxide(37.74~57.96%).(4)Mimic reductive conditions,e.g.with hydroxylamine hydrochloride and ascorbic acid as reducing agent to cultivate the soil,result in the decrease of fractions of cadmium,copper,lead,zinc associated with easily-reduciable Fe-Mn oxides,and increase of the exchangeable heavy metals.With the increase of culture time and reducing agent concentration,some stable state metals(organic associated and those in residual form)gradually transform to exchangeable state.Further addition of reducing agent can slightly increase the amount of heavy metals dissolved.(5)Due to the differences in soil heavy metal contents,contents of iron oxide and environmental management of the mining area between dabaoshan and hengyang mining area,the speciations and distributions of heavy metals pollutants in these soils and the types and contents of heavy metals associated with crystalline iron oxide are different.The proportions of Pb,Cu,Zn and Cd in the colloids associated with well crystalline iron oxide are 14.37 ~ 60.19%,11.44 ~ 18.61%,6.3 ~ 23.96% and 19.66 ~ 38.77% respectively.That is,the association of heavy metals in soils around the hengyang mining area with well crystallized iron oxides decreases in the order Pb > Cd > Cu > Zn.