A Study On Element Mobility During Weathering Of Black Shales In Central Hunan,China

The black shale is a kind of sedimentary rock with particularly chemical properties,which formed under a marine anoxic condition,and thus was generally enriched in organic matter and sulfide minerals.Thus,black shale is prone to be weathered and decomposed when exposing to the earth's surface where it is frequently affected by the atmosphere and water.The black shales weathering can cause the exhalation of CO₂,the production of the acidic mine drainage,and the release of heavy metals such as Cd,Cu,Pb,Zn,and others.Therefore,the black shales weathering can cause many kinds of environmental issues,among which the heavy metal contamination of soil and water systems is one of the most serious,which can cause heavy metal contamination to environment(soil and water,etc.).Thus,the study on black shales weathering and its environmental impacts has received special attention.The elements mobility during black shale weathering is the key to characterize the weathering characteristics of black shale and clarify its environmental impacts.Therefore,it has always been the core topic of black shale weathering research.This paper explores furtherly the elements mobility of black shales weathering from the perspective of element analysis and sequential extraction analysis.The research results and findings are as follows.The fresh black shales were more enriched in heavy metals such as Cu,V,Ni,Pb,Cd,U,Mo and Ba than weathered black shales.Both of Mn and Co were depleted in fresh and weathered black shales.And it was no significant difference for Zn and Th between fresh and weathered black shales.Meanwhile,High field strength elements or large ionic lithophile elements(except Sr),which did not differ significantly in elemental composition between fresh and weathered rocks.The weathered black shales were deficient in HREE than in fresh black shales.The main occurrence of Mn,Cd and REE during weathering process was iron-manganese oxide bonding fraction,followed by activation fraction,which had strong mobility during weathering process and a great impact on environment.Cu and Zn mainly existed in the iron-manganese oxide fraction and were sensitive to the reduction conditions.The mass transfer coefficient indicated that the Ba?Cd?U?Mo?Ni and Sr were more active and the leaching release increases with the increase of weathering degree.V,Pb and Zn were relatively enriched and leached,with moderate mobility.Co,Cu,Th,Ti,Zr,Hf,Nb,Ta,Ga,Ge,Rb and Cs with weak mobility were not easy to leach out and might be combined with silicate minerals.The mobility of REE was relatively strong while it was weaker for LREE than HREE.The judgement about the mobility of Mn was not inconsistent by two methods such as the sequential extraction,mass transfer coefficient,so we inferred the mobility of Mn was complicated.The evaluation of geo-accumulation index showed there was heavy metals pollution by Mo,Cd,U and Ba,etc.And the specific release of elements such as Ba,Mo,U and Cd per unit mass in black shales weathering was estimated.The results showed that the active elements were a threaten to the environment,especially the impacts of heavy metals on environmental quality.