Changes in size distribution of fresh water nanoscale colloidal matter and associated elements on mixing with seawater

摘要

Changes in size distribution and elemental composition of 0.5-50 nm fresh water colloids during estuarine mixing have been studied by in-laboratory mixing of natural creek water and synthetic seawater, followed by size fractionation with Asymmetrical Flow Field-Flow Fractionation, and online elemental quantification by High-Resolution ICPMS. At least two types of colloids were present in the studied size region; 0.5-3 nm fluorescent dissolved organic matter (FDOM), and >3 nm colloids that were rich in Fe and colored dissolved organic matter (CDOM). Most trace elements were associated in different proportions to these two populations of colloids. Following mixing with synthetic seawater, the >3 nm. Fe-rich colloids and CDOM were extensively removed from the studied size region by salt induced aggregation. The degree of removal with increasing salinity was greatest below 2.5 parts per thousand. salinity, continued to a lesser degree between 2.5%. and 15 parts per thousand. salinity, above which only very small additional removal could be distinguished. At 25 parts per thousand. salinity, the Fe concentration in the 0.5-50 size region had been reduced down to 15% of its original value in freshwater, while the amount of CDOM had been reduced to 55%. On the contrary, the concentration of the 0.5-3 run FDOM was unchanged by the increased concentration of sea salt. Therefore, colloidally associated Al, P, Co, Cu, Zn, Ce, Lu and Ph were removed from the 0.5-50 nm size region according to their relative distributions between the FDOM and the Fe-rich colloids. Consequently, at 25 parts per thousand. salinity, the 0.5-50 nm concentrations of Al, Mn, P and Pb, (mainly associated with the Fe-rich colloids) had been reduced down to 13-26% of their values in freshwater, while the concentrations of Co and Cu (with higher preferences for FDOM) were less reduced, down to 46% and 57%, respectively. Changes in the elemental composition of the remaining colloidal matter were observed, the most pronounced were increased contents of P, A] and Pb in Fe-rich colloidal matter of medium size (similar to 3-15 nm) and increased Pb content in Fe-rich colloidal matter of larger size (similar to 5-50 nm). (c) 2007 Elsevier Ltd. All rights reserved.