Rare earth elements (REE) are known to be emerging contaminants in hydrosphere, but roles of hydrous manganese oxyhydroxides (HMO) in REE transport in groundwater remains unknown. In this study, groundwater was sampled along a flow path in the North China Plain to determine the behavior of REE surface complexation to HMO by a modeling and field study approach. Results show that the proportion of neodymium (Nd) complexed by HMO ranges from 0.2% to 95.8%, and from 0.3% to 99.6% in shallow groundwater and deep groundwater, respectively. The amount of complexed REE increases along the flow path. REE bound to HMO exhibit decreasing trends with increasing atomic number. The process was determined to be independent of pH, HMO content, and metal loading. This finding further demonstrates HMO-REE complexation plays a key role in transport of REE in groundwater through preferential scavenging of light REE (LREE) over heavy REE (HREE). Nevertheless, carbonate ligands appear to be robust competitors in reducing the amount of REE sorbed to HMO when solution pH rises above 8.0. Assuming that 50% of Mn concentration occurs as HMO, the amount of complexed REE was predicted to show a more marked decrease in LREE compared to that of HREE.
As the first span and aqueduct of the middle route of the South-to-North Water Diversion Project, Diaohe Aqueduct has brought good engineering and social benefits since its completion. In order to understand the present water quality of the Diaohe River section of the route, four samplings were conducted at Diaohe Aqueduct in the winter of 2018. The main results of this study are as follows: 62 species (or variants) of phytoplankton belonging to 32 genera, 22 families and 6 phyla were identified during the study period, among which Bacillariophyta are the most dominant species. The identified phytoplankton fall into 8 indicator grades: os; ps, αm; ps, αm, βm; αm; αm, βm; αm, βm, os; βm; and βm, os. A total of 17 functional groups are detected, among which TB is the absolute dominant functional group. Based on the composition characteristics of phytoplankton, the water quality trophic state of Diaohe Aqueduct is mesotrophic. However, other physical indicators show that the water quality of the Middle Route of the South-to-North Water Diversion Project is well. The conclusions drawn using phytoplankton alone as the evaluation index may also be biased. In the future, it is necessary to monitor the water quality changes many aspects.
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