Mineralogical Study On Heavy-metal Contamination In Bed Sediments From Lower Reaches Of The Xiangjiang River,Hunan Province Of China

In this study,based on the geochemistry of heavy-metal contaminated bed sediments from the lower reaches of the Xiangjiang River,the mineralogical compositions of the related sediments were analyzed using the X-ray diffraction?XRD?and scanning electron microscopy-energy dispersive spectrometry?SEM-EDS?,and the mineralogical composition of the sediments were characterized.Complementing with the elemental geochemical data,the possible host mineral phases of different heavy metals in the sediments are investigated,the activity of heavy metals in sediments are analyzed,and the relationship between heavy metal contamination in bed sediments and its mineral composition are expounded,so as to provide a scientific reference for predicting the development trend of heavy metal contamination and preventing the heavy metal contamination at the watershed.The main results of this study are as follows.The mineralogical composition of the sediments is very complicated and generally includes both light minerals and heavy minerals.The light minerals represent the major mineral composition,which include the clastic minerals?e.g.,quartz,microcline and albite?,the clay minerals?e.g.,mica,illite,kaolinite,chlorite and gibbsite?,and the carbonate/sulphate minerals?e.g.,calcite,dolomite and gypsum?,and their overall proportions are usually higher than 95%.The heavy minerals are of minor proportion,which include the Fe-bearing minerals?e.g.,hematite,magnetite,goethite and ilmenite?,the stable minerals?e.g,zircon,rutile,apatite,garnet?,and the unstable minerals?e.g.,amphibole,augite?,and their overall proportions are mostly less than 5%.Compared with the sediments from other sections of the river,sediments from the Zhuzhou section have quite different mineralogical composition,which is characterized by much lower proportion of clastic minerals,but much higher proportions of clay minerals,carbonate minerals,and Fe-bearing minerals.This different mineralogical composition of the sediments from the Zhuzhou section spatially matches very well with that there are much lower contents of SiO₂ and K₂O,but much higher contents of TiO₂,Fe₂O₃,and CaO.The analysis of mineralogical composition and major element indicates that the source of the sediments is related to different types of rocks such as granite,carbonate,and sandstone in the river basin.The evaluation results of geoaccumulation index(I_geo)and comprehensive enrichment factors?EI?suggest that heavy metals Mn,Cu,Zn and Pb have reached higher than moderate degree of contamination,Sc,Cr,Co,Ni,Th and U have mostly reached moderate degree of contamination,while heavy metals Ba and V have reached slight degree of contamination.Among them,heavy metals Cu,Zn,Pb,and Mn reached serious degree of contamination in the sediments from zhuzhou section,which is the most heavy metal contaminated section in the lower reach of the Xiangjiang River.The characteristics of heavy metal contamination are different from other section and are corresponding to the characteristics of different mineralogical composition in the sediments?much lower proportion of clastic minerals,but much higher proportions of clay minerals,carbonate minerals,and Fe-bearing minerals?.It indicates that the heavy-metal contamination in bed sediments from the lower reaches of the Xiangjiang River is closely related to the minerals composition mentioned above.The Pearson correlation analysis on proportions?%?of the major minerals and the geoaccumulation index(I_geo)of heavy metals indicates that the heavy metals are mostly hosted within illite-clay,carbonate/sulphate,and Fe-bearing mineral phases.Complementing with the results of principal component analysis on proportions?%?of the major elements and the geoaccumulation index?Igeo?of heavy metals,it is found that heavy metals V,Th,Cr,and Sc are hosted within the clay mineral phases?e.g.,illite?and metals Ba are hosted within the clastic minerals?e.g.,quartz,and feldspar?.The heavy metals hosted by silicate mineral are generally less mobile and they do not cause obvious and potential harzd to the environment.While heavy metals Cu,Zn,Pb,U,and Co are mostly hosted within the Fe-bearing mineral phases?e.g.,hematite?and carbonate or sulphate minerals?e.g.,calcite?and Mn is hosted within manganese minerals.Fe-Mn oxides mineral and carbonate mineral phases are very unstable?e.g.,chemical decomposition,and dissolution?when the physical-chemical conditions?e.g.,pH value and Eh value decreased?of the water system are varied.Therefore,these metals hosted by Fe-Mn oxides mineral and carbonate/sulphate mineral phases are mobile and may migarate and release into water,and so they have great potential harm to the environment as the changing physical-chemical conditions in water system caused the host minerals to be dissolved.It is an important strategy that more attention should be paid to the heavy metals hosted within the carbonate and Fe-Mn oxides minerals for the river watershed to prevention and control of heavy-metal contaminations in the future.Therefore,heavy-metal contamination is serious in the sediments from lower reaches of the Xiangjiang River,and the severity of heavy-metal contamination is closely related to the composition of elements and minerals.