Effect of pH and stream order on iron and arsenic speciation in boreal catchments

Autor(en)

E. Neubauer, S.J. Köhler, F. Von Der Kammer, H. Laudon, Thilo Hofmann

Abstrakt

Riverine transport of iron (Fe) and arsenic (As) is affected by their associations with natural organic matter (NOM) and suspended iron (oxy)hydroxides. Speciation has a strong influence on element transport from the headwaters to the ocean because NOM may be transported over longer distances compared to iron (oxy)hydroxides. We show that Fe speciation changes along the flow path of a boreal watercourse, as water moves from NOM-rich, acidic first-order streams with pH as low as 3.9 to less acidic higher-order systems (up to pH 6.4). Analysis by Flow Field-Flow Fractionation and chemical equilibrium modeling revealed that Fe from wetland-dominated headwaters was mainly exported as Fe-NOM complexes; in catchments with a stream order >1 and with higher pH, Fe was present in Fe-NOM complexes and precipitated as nanoparticulate iron(oxy)hydroxides which aggregated as the pH increased, with their size eventually exceeding the membrane filters cutoff (0.2 μm). The measured NOM-bound Fe decreased with increasing pH, from 0.38 to 0.16 mmol Fe·gNOM -1. The high concentrations of NOM-bound Fe emphasize the importance of boreal catchments to Fe export to the oceans. Concentrations of As in the <0.2 μm fraction but larger than what is usually considered "truly dissolved" (<1000 g·mol -1), decreased from 75% to 26% with increasing pH. The As in this size range was mainly associated with NOM but at pH >4.5 became associated with iron(oxy)hydroxides, and its transport thus became more coupled to that of the iron(oxy)hydroxides downstream in the circumneutral streams.

Organisation(en)

Externe Organisation(en)

Swedish University of Agricultural Sciences (SLU)

Journal

Environmental Science & Technology

Band

47

Seiten

7120-7128

Anzahl der Seiten

9

ISSN

0013-936X

DOI

https://doi.org/10.1021/es401193j

Publikationsdatum

07-2013

Peer-reviewed

Ja

ÖFOS 2012

105302 Hydrochemie, 104002 Analytische Chemie, 104023 Umweltchemie, 105904 Umweltforschung

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/1b5fa3d6-d357-4012-adc0-fca7291fd1c9