| Spatial distribution, contamination conditions and potential ecological risk of heavy metals (Cr, Pb, Cd, Cu, Zn, Ni, As and Hg) and geochemical characteristics of rare earth element (REEs) including La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu in filtered water and sediments from Gansu province, Ningxia and Inner Mongolia Autonomous Regions of the Yellow River were studied by high resolution inductively coupled plasma-mass spectrometry (HR-ICP-MS) in dry and normal seasons in 2014, respectively. Compared with the results obtained in the wet season in the year of 2011 and 2012, the results indicated that:The maximum values of heavy metal concentrations in filtered water in normal season were higher than that of dry and wet seasons except Cr. Single factor pollution index (Ii) indicated that the Ii values of Cr in wet seasons and Hg in normal season were between 1-2 (slight pollution), and the Ii values of the others (except Ni) in all seasons were below 1, indicating they were relatively safer to the surrounding environments. Besides, the Ii values of As, Cd, Cu, Hg, Pb and Zn in normal season were higher than that of dry and wet seasons, especially for Hg (1.300-1.400) and Pb (0.009-0.786). Results of Integrated Nemerow pollution index (I) demonstrated that except dry season, the heavy metals showing enrichment in all sample sites in wet season in 2012 and part of sites in wet season in 2011 and normal season in 2014.Concentrations of Cr and Zn in sediment samples were relatively higher than other metals in all seasons. Geo-accumulation index (Igeo) showed that IgeoHs and Igeo Cu in wet seasons were much higher than that of dry and normal seasons, while the/geo values of Pb, Zn, Ni and As in study regions were below 0, suggesting these metals were safer to the environments. Furthermore, the Igeo values of Hg, Cu, Cd and Cr were relatively higher at in Sanhuhekou (D11), Yellow River bridge, Bayannur (S8) and Lamawan (P6), which implied that the water quality in these sites of the Gansu, Ningxia and Inner Mongolia sections of the Yellow River wasn’t an ideal source for drinking and using in aquaculture any more.Results of chemical fractions of heavy metals in sediment samples suggested that Cd and Cr likely mainly derived from anthropogenic sources and it could potentially pose the greatest hazards to human being and gave adverse impacts to the surrounding environments since they had the highest ability and susceptibility to be released from the sediments by the simple ion exchanged mechanism. On the contrary, the metals including Pb, Cu, Zn and Ni showed that they likely to be controlled by lithology, and thus had a lower transfer ability and relatively safer to surrounding environments.Correlation analysis (CA) and principal component analysis (PCA) revealed that the significant correlation were found among Pb, Cu, Zn and Ni, which suggested these metals may have the similar geochemical behaviors and sources. Moreover, the high toxic Cr and Cd in investigated regions especially for Inner Mongolia section of the Yellow River may be contributed to anthropogenic sources, such as the wastes emissions from steel, rare earth and chemical industrial of the local regions.The distribution of the rare earth elements (REEs) concentrations in filtered water in Gansu, Ningxia and Inner Mongolia sections of the Yellow River showed the larger difference at sampling sites in normal season with higher values than other seasons, especially for Ce, La and Nd. Besides, the concentrations values of light rare earth elements (LREEs) were higher than heavy rare earth elements (HREEs) in all seasons.The distribution of the REEs concentrations in sediments in Gansu, Ningxia and Inner Mongolia sections of the Yellow River suggested that except Ho, Er, Tm, Yb and Lu in normal season, the others in all seasons showing the similar distribution trends. In addition, the maximum values of all REEs concentrations were found at sediments in wet season in 2012, with the exception of Eu and Yb.Except Yingugongluqiao (D7), the (La/Yb)sN ratios of all sediment samples were above 1 in wet season in 2011, indicating that LREEs were enriched in respect to HREEs, and the stronger REEs fractionations were more evident in the wet season in 2012. On the contrary, the (La/Yb)sN ratios in sediments in dry season were below 1, which display more LREE-depleted patterns. Moreover, the weakly negative Ce anomaly in sediments in all seasons would mark human-induced modifications of the oxygenated environments in the study regions. The positive Eu anomaly of sediments may suggest an anthropogenic source of the elements, which may be derived from the use of phosphate fertilizer in densely populated and highly industrialized countries. Plots of shale-normalized REEs data of sediments showed slight concave subparallel patterns with a little depletion in HREEs relative to LREEs in all seasons.pH is one of the important influence factors which contributed to the bioavailability and variation in REEs concentrations. Low pH favored conversion of REEs from precipitated fractions into soluble fractions. The sediment samples in study regions in all seasons showed alkali pH, which could favour complex more REEs in sediments than filtered water. Organic ligands exerted strong controls on REEs behaviour in the natural environment. The higher total organic carbon (TOC) value was observed as 2.27% at Maqu (D1) in wet season in 2011 in present study.The study of heavy metals distribution, geochemical behavior of REEs, ecological risk and sources of pollution in the aquatic environment could lead to better understanding of their behavior, the nature of their biogeochemical processes. The results can provide reliable experimental data for effective protecting environment, and are of great significance to implement targeted prevention and control measures for sustainable development in Gansu province, Ningxia and Inner Mongolia Autonomous Regions of the Yellow River.
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