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The redox and complexation chemistry of iron within freshwater sources to the ocean: rivers, rain and glaciers

The redox and complexation chemistry of iron within freshwater sources to the ocean: rivers, rain and glaciers
The redox and complexation chemistry of iron within freshwater sources to the ocean: rivers, rain and glaciers
In Fe-limited or Fe-stressed waters Fe(II) cycling is an important feature of Fe biogeochemistry because processes favouring Fe(II) production or stability always expand the pool of Fe available to aquatic microorganisms. However almost nothing is known about Fe(II) organic speciation in natural waters and this limits our understanding of how this metal ion may interact with biological Fe uptake systems. Therefore I investigate how organic material affects Fe(II) biogeochemistry in rainwater, riverwater and in suspensions of aged glacial particulates. We compare Fe(II) concentrations (12-3600 nM) and, by adapting a ferrozine based reverse titration technique, ligand binding constants (logKFe(II) <5.5-11) in 5 temperate and sub-tropical river-estuary systems with varying dissolved organic carbon (DOC 200-1300 ?M), labile dissolved Fe (Fe <0.2?m available to ferrozine after reduction with ascorbic acid 100 nM-20 ?M) and pH (5.5-8.5). In riverwater we identify a natural class of humic Fe(II) ligands with weak Fe(II) binding constants (logKFe(II) <8) and concentrations in excess of Fe(II). Stronger ligands (logKFe(II) 11) were found only in the Itchen (Hampshire, UK) and may be associated with anthropogenic waste inputs. Similarly, the previously identified stabilisation of Fe(II) within North Carolina rainwater may be associated with anthropogenic emissions. Fe(II) concentrations in Wilmington (North Carolina, USA) rainwater have decreased from a high of 52 nM (and a 1:1 ratio of Fe(II):Fe(III)) in summer 2000 to a record low of 3 nM (and a 0.23:1 ratio of Fe(II):Fe(III)) in summer 2013 concurrently with improving air quality.

The weak (logKFe(II) <8), terrestrially derived ligands we report in 4 temperate/sub-tropical river systems are likely to be found in most surface freshwater systems, but their impact in higher salinity coastal seawater will be minimal. Stronger Fe(II) ligands (logKFe(II) 11) associated with effluent emissions however may exert a stabilising effect on Fe(II) concentrations in the natural environment. This may result in localised increases in bioavailable Fe concentrations and have implications for aquatic ecosystems in industrialised areas.
Hopwood, Mark James
5c80a302-b99c-4a9b-a0f1-81610a460822
Hopwood, Mark James
5c80a302-b99c-4a9b-a0f1-81610a460822
Statham, Peter
51458f15-d6e2-4231-8bba-d0567f9e440c

Hopwood, Mark James (2014) The redox and complexation chemistry of iron within freshwater sources to the ocean: rivers, rain and glaciers. University of Southampton, Ocean & Earth Science, Doctoral Thesis, 151pp.

Record type: Thesis (Doctoral)

Abstract

In Fe-limited or Fe-stressed waters Fe(II) cycling is an important feature of Fe biogeochemistry because processes favouring Fe(II) production or stability always expand the pool of Fe available to aquatic microorganisms. However almost nothing is known about Fe(II) organic speciation in natural waters and this limits our understanding of how this metal ion may interact with biological Fe uptake systems. Therefore I investigate how organic material affects Fe(II) biogeochemistry in rainwater, riverwater and in suspensions of aged glacial particulates. We compare Fe(II) concentrations (12-3600 nM) and, by adapting a ferrozine based reverse titration technique, ligand binding constants (logKFe(II) <5.5-11) in 5 temperate and sub-tropical river-estuary systems with varying dissolved organic carbon (DOC 200-1300 ?M), labile dissolved Fe (Fe <0.2?m available to ferrozine after reduction with ascorbic acid 100 nM-20 ?M) and pH (5.5-8.5). In riverwater we identify a natural class of humic Fe(II) ligands with weak Fe(II) binding constants (logKFe(II) <8) and concentrations in excess of Fe(II). Stronger ligands (logKFe(II) 11) were found only in the Itchen (Hampshire, UK) and may be associated with anthropogenic waste inputs. Similarly, the previously identified stabilisation of Fe(II) within North Carolina rainwater may be associated with anthropogenic emissions. Fe(II) concentrations in Wilmington (North Carolina, USA) rainwater have decreased from a high of 52 nM (and a 1:1 ratio of Fe(II):Fe(III)) in summer 2000 to a record low of 3 nM (and a 0.23:1 ratio of Fe(II):Fe(III)) in summer 2013 concurrently with improving air quality.

The weak (logKFe(II) <8), terrestrially derived ligands we report in 4 temperate/sub-tropical river systems are likely to be found in most surface freshwater systems, but their impact in higher salinity coastal seawater will be minimal. Stronger Fe(II) ligands (logKFe(II) 11) associated with effluent emissions however may exert a stabilising effect on Fe(II) concentrations in the natural environment. This may result in localised increases in bioavailable Fe concentrations and have implications for aquatic ecosystems in industrialised areas.

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Published date: December 2014
Organisations: University of Southampton, Ocean and Earth Science

Identifiers

Local EPrints ID: 377299
URI: http://eprints.soton.ac.uk/id/eprint/377299
PURE UUID: 719b7eff-b398-417c-a9e3-8f0dbc4aeace

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Date deposited: 21 May 2015 09:59
Last modified: 17 Jul 2017 21:01

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Contributors

Author: Mark James Hopwood
Thesis advisor: Peter Statham

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