Dissolved Fe(II) in a river-estuary system rich in dissolved organic matter

doi:10.1016/j.ecss.2014.09.015

Estuarine, Coastal and Shelf Science

Volume 151, 5 December 2014, Pages 1-9

https://doi.org/10.1016/j.ecss.2014.09.015 Get rights and content

Abstract

Reduced iron, Fe(II), accounts for a significant fraction of dissolved Fe across many natural surface waters despite its rapid oxidation under oxic conditions. Here we investigate the temporal and spatial variation in dissolved Fe redox state in a high dissolved organic matter (DOM) estuarine system, the River Beaulieu. We couple manual sample collection with the deployment of an autonomous in situ analyser, designed to simultaneously measure dissolved Fe(II) and total dissolved Fe, in order to investigate processes operating on the diurnal timescale and to evaluate the performance of the analyser in a high DOM environment.

Concentrations of dissolved Fe available to the ligand ferrozine are elevated throughout the estuary (up to 21 μM in freshwater) and notably higher than those previously reported likely due to seasonal variation. Fe(II) is observed to account for a large, varying fraction of the dissolved Fe available to ferrozine (25.5 ± 12.5%) and this fraction decreases with increasing salinity. We demonstrate that the very high DOM concentration in this environment and association of this DOM with dissolved Fe, prevents the accurate measurement of dissolved Fe concentrations in situ using a sensor reliant on rapid competitive ligand exchange.

Introduction

Fe(II) has been reported to account for a large fraction of dissolved Fe in a range of both fresh and marine waters (Hong and Kester, 1985, Landing and Westerlund, 1988, Sarthou et al., 2011). The dominant source of Fe(II) in surface waters is normally photochemical (Collienne, 1983, Croot et al., 2008, Roy et al., 2008). Despite its presence in marine and fresh surface waters, and its greater solubility than Fe(III) (Shaked and Lis, 2012), little information is available on the distribution and speciation of dissolved Fe(II). As formation of Fe(II) is generally represented as a poorly characterised solubilisation process (Emmenegger et al., 2001), this mechanistic gap in our knowledge may impede the development of better global ocean models (Tagliabue and Voelker, 2011).

During estuarine mixing, dissolved Fe is known to be strongly non-conservative (Boyle et al., 1977). However, few studies have distinguished between the estuarine mixing of dissolved Fe(II) and total dissolved Fe. Hong and Kester (1985) demonstrated that oxidation of Fe(II) is not a significant contributor to dissolved Fe removal at low salinities. Given the importance of humic materials in initiating flocculation and the significant difference in the metal-ligand binding preferences of the two redox states of Fe (Sholkovitz et al., 1978), the question is therefore raised as to whether the ratio of dissolved Fe(II):Fe(III) present in riverine waters is maintained during estuarine mixing and delivery to the ocean. If so, estuaries would be a source of Fe(II) to coastal waters.

Measuring the concentration of Fe(II) during estuarine mixing in a high DOM estuary will establish whether or not Fe(II) is removed similarly to Fe(III) in the presence of high concentrations of natural organic compounds. Coastal waters are known to contain organic material that can stabilise higher Fe(II) concentrations than would be possible inorganically under oxic conditions (Zhuang et al., 1995, Rose and Waite, 2003a, Okada et al., 2005). If estuaries are a source of such material we would expect to see a relatively high fraction of dissolved Fe present as Fe(II) throughout estuarine waters. The main objective here was therefore to use the Beaulieu as a model of a high DOM river to investigate the forms of dissolved Fe present and to see how Fe(II) concentration changes during estuarine mixing.

Section snippets

The River Beaulieu

The River Beaulieu drains the New Forest (Hampshire, UK) and thus contains water primarily from exposed heathland and some mixed coniferous/deciduous forest. The river water has a near neutral pH (6.57.8), high dissolved organic carbon (DOC, 250–1800 μM) and high total dissolved (<0.45 μm) Fe (18 μM) content (Holliday and Liss, 1976, Moore et al., 1979, Jones, 1993, Fang, 1995, Smith, 1995).¹

Sample collection

All sampling apparatus and

Temporal and spatial variation in riverine water

Dissolved oxygen in 10 °C river water averaged 370 ± 20 μM (calculated saturation at 10.0 °C is 352 μM). Diurnal changes in irradiance, DFe and Fe(II) concentrations (17 December 2012), as determined manually and using an in situ analyser, are shown in Fig. 4. DFe and Fe(II) concentrations determined manually throughout the day had mean values (and standard deviations) of 20.1 ± 0.40 μM and 3.96 ± 0.87 μM respectively. Over the sampling period, pH increased from 6.4 to 6.6 and water level fell

Temporal and spatial variation at King's Hat

Two features of Fig. 4 are particularly striking. First, there is a very large difference between reported dissolved Fe(II) and DFe using the two sampling and analysis methods. Second, there is an apparent absence of a diurnal sinuous trend in Fe(II). Such a trend has been reported in both fresh and marine surface waters (Johnson et al., 1994, Emmenegger et al., 2001).

Although the trend (Fig. 4) observed throughout the day by both measurement methods is similar, manually collected samples

Conclusions

Five transects measuring dissolved Fe are now reported for the Beaulieu (15 January 2013; 5 February 1974 (Holliday and Liss, 1976); 17 March, 13 May and 22 September 1994 (Fang, 1995)) providing snapshot images of an estuarine Fe distribution that varies hourly and only the transect reported here distinguishes between Fe(II) and DFe. We demonstrate that Fe(II) decreases as a fraction of DFe with increasing salinity. A range of DFe removal factors are reported for the Beaulieu estuary (60–90%),

Acknowledgements

The authors wish to thank the Gillings Foundation for studentship support (MJH), the crew of RV Bill Conway, Dr Gary Fones (University of Portsmouth) for ICP-MS data and Mr Mario Esposito (University of Southampton) for analysis of DOC samples. Two anonymous reviewers are thanked for comments that improved the manuscript.

References (57)

C.J. Borman et al.
Is iron redox cycling in a high altitude watershed photochemically or thermally driven?
Chem. Geol.
(2010)
J.D. Box
Observations on the use of iron(II) complexing agents to fractionate the total filterable iron in natural-water samples
Water Res.
(1984)
E.A. Boyle et al.
Mechanism of iron removal in estuaries
Geochim. Cosmochim. Acta
(1977)
R.E. Cowart et al.
A comparison of bathophenanthrolinedisulfonic acid and ferrozine as chelators of iron(II) in reduction reactions
Anal. Biochem.
(1993)
P.L. Croot et al.
Determination of Fe(II) and total iron in natural waters with 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazin (PDT)
Anal. Chim. Acta
(2000)
W. Davison et al.
The kinetics of the oxidation of ferrous iron in synthetic and natural-waters
Geochim. Cosmochim. Acta
(1983)
G. Figueres et al.
Iron behavior in the Zaire estuary
Neth. J. Sea Res.
(1978)
L.M. Holliday et al.
Behavior of dissolved iron, manganese and zinc in Beaulieu estuary, S England
Estuar. Coast. Mar. Sci.
(1976)
H.S. Hong et al.
Chemical forms of iron in the Connecticut River estuary
Estuar. Coast. Shelf Sci.
(1985)
R.J.M. Hudson et al.
Investigations of iron coordination and redox reactions in seawater using Fe-59 radiometry and ion-pair solvent-extraction of amphiphilic iron complexes
Mar. Chem.
(1992)

K.S. Johnson et al.

Iron photochemistry in seawater from the equatorial Pacific

Mar. Chem.

(1994)

W.M. Landing et al.

The solution chemistry of iron(II) in Framvaren Fjord

Mar. Chem.

(1988)

G.W. Luther et al.

Dissolved organic Fe(III) and Fe(II) complexes in salt marsh porewaters

Geochim. Cosmochim. Acta

(1996)

L.M. Mayer

Aggregation of colloidal iron during estuarine mixing – kinetics, mechanism, and seasonality

Geochim. Cosmochim. Acta

(1982)

R.M. Moore et al.

Behavior of dissolved organic material, iron and manganese in estuarine mixing

Geochim. Cosmochim. Acta

(1979)

J.W. Murray et al.

Geochemistry of iron in puget sound

Geochim. Cosmochim. Acta

(1978)

M.J. Pullin et al.

Colorimetric flow-injection analysis of dissolved iron in high DOC waters

Water Res.

(2001)

M.J. Pullin et al.

The effects of pH, ionic strength, and iron-fulvic acid interactions on the kinetics of nonphotochemical iron transformations. II. The kinetics of thermal reduction

Geochim. Cosmochim. Acta

(2003)

R. Raiswell

Towards a global highly reactive iron cycle

J. Geochem. Explor.

(2006)

A.L. Rose et al.

Kinetics of iron complexation by dissolved natural organic matter in coastal waters

Mar. Chem.

(2003)

P.M. Sarradin et al.

Fe analysis by the ferrozine method: adaptation to FIA towards in situ analysis in hydrothermal environment

Talanta

(2005)

A.M. Shiller et al.

Trace-elements in the Mississippi River-delta outflow region – behavior at high discharge

Geochim. Cosmochim. Acta

(1991)

E.R. Sholkovitz et al.

The removal of dissolved humic acids and iron during estuarine mixing

Earth Planet. Sci. Lett.

(1978)

P.J. Statham

Nutrients in estuaries – an overview and the potential impacts of climate change

Sci. Total Environ.

(2012)

P.J. Statham et al.

The measurement of organically complexed Fe-II in natural waters using competitive ligand reverse titration

Anal. Chim. Acta

(2012)

Y. Suzuki et al.

Existence of stable Fe(II) complex in oxic river water and its determination

Water Res.

(1992)

R. Vuillemin et al.

CHEMINI: a new in situ CHEmical MINIaturized analyzer

Deep Sea Res. Part I Oceanogr. Res. Pap.

(2009)

R.D. Waterbury et al.

Long pathlength absorbance spectroscopy: trace analysis of Fe(II) using a 4.5 m liquid core waveguide

Anal. Chim. Acta

(1997)

Cited by (18)

Dissolved and particulate iron redox speciation during the LOHAFEX fertilization experiment
2022, Marine Pollution Bulletin
Citation Excerpt :
The commonly accepted paradigm is that, since many Fe(III) reduction reactions require the effect of solar radiation, DFe(II) concentrations show a daily period that peaks around midday and reaches a minimum at nighttime (Bowie et al., 2002; Rijkenberg et al., 2005; Waite et al., 1995), notwithstanding the fact that Fe(II) oxidation is accelerated during daytime by (photo)formation of ROS. However, many previous studies have also shown, against the referred paradigm, steady surface DFe(II) concentrations throughout a day/night cycle (Croot et al., 2001; Hansard et al., 2009; Hopwood et al., 2014; Sarthou et al., 2011). Appropriate interpretation of Fe(II) concentrations determined during field surveys must, therefore, consider that iron redox speciation is a very dynamic process with strong spatial and temporal variations.
The redox speciation of iron was determined during the iron fertilization LOHAFEX and for the first time, the chemiluminescence assay of filtered and unfiltered samples was systematically compared. We hypothesize that higher chemiluminescence in unfiltered samples was caused by Fe(II) adsorbed onto biological particles. Dissolved and particulate Fe(II) increased in the mixed layer steadily 6-fold during the first two weeks and decreased back to initial levels by the end of LOHAFEX. Both Fe(II) forms did not show diel cycles downplaying the role of photoreduction. The chemiluminescence of unfiltered samples across the patch boundaries showed strong gradients, correlated significantly to biomass and the photosynthetic efficiency and were higher at night, indicative of a biological control. At 150 m deep, a secondary maximum of dissolved Fe(II) was associated with maxima of nitrite and ammonium despite high oxygen concentrations. We hypothesize that during LOHAFEX, iron redox speciation was mostly regulated by trophic interactions.
The contrasting estuarine geochemistry of rare earth elements between ice-covered and ice-free conditions
2022, Geochimica et Cosmochimica Acta
Rare Earth Elements, including the lanthanide series and Y (REYs), are important tracers and paleo-proxies of biogeochemical processes, water mass transport and oceanic mixing. At the interface between the continents and oceans, the geochemical behaviour of REYs in estuarine environments is generally described as being non-conservative, with large-scale removal by particle scavenging. This conventional interpretation stems from observations carried out in tropical or sub-tropical estuaries. However, major river systems in the mid- and high latitude regions are subjected to winter conditions when a frozen watershed and an ice cover may affect the continental input of particles and the REY geochemistry. Here, we investigate the geochemical behaviour of REYs in the Estuary and Gulf of St. Lawrence (EGSL) in spring 2003 as well as winter and summer 2020. In contrast to the ice-free seasons, REYs and Fe behave conservatively in the estuary during the winter. In addition, we observed a higher REY affinity toward particle surfaces and fractionation in REE patterns in the hypoxic deep waters of the estuary. The latter observation may reflect enhanced REY sorption to mineral-carrier surfaces exposed upon the remineralization of organic coatings on settling particles and to manganese oxides of benthic origin. Computed partition coefficient (K_d) values and strong correlations between Y/Ho, Er/Nd and Ce anomalies with dissolved oxygen concentrations support this hypothesis. The estimated annual dissolved load of REYs from the EGSL to the ocean ranges from 0.4 to 75 tons per year, thus contributing significantly to the global marine budget. The global river loading of dissolved Nd to the ocean was also revised to 4000 tons per year based on data available for 21 river systems.
Behaviour of chromium and chromium isotopes during estuarine mixing in the Beaulieu Estuary, UK
2020, Earth and Planetary Science Letters
Rivers are the principal source of chromium (Cr) to seawater and the Cr isotopic signatures of ancient marine sediments are widely considered to provide a record of the presence or absence of oxidative weathering processes on land. This assumes, however, that the $δ^{53}$ Cr value of river water is faithfully transferred to the oceans and is not modified in the estuarine mixing zone. To test this assumption we have determined the concentration and $δ^{53}$ Cr values of inorganic Cr (Cr(III) + Cr(VI)), and also Cr speciation for water samples collected within the estuarine mixing zone of the Beaulieu River, UK. The $δ^{53}$ Cr values of dissolved inorganic Cr ranged from −0.59 to 1.68‰, Cr(VI) concentrations from 0.39 to 1.83 nmol kg⁻¹ and Cr(III) concentrations from 0.11 to 3.21 nmol kg⁻¹. Both Cr(VI) concentrations and $δ^{53}$ Cr values increased linearly as a function of salinity, while Cr(III) concentrations decreased linearly with salinity. Thus $δ^{53}$ Cr, Cr(III) and Cr(VI) all showed conservative behaviour in the estuarine mixing zone, and the $δ^{53}$ Cr signature of Beaulieu River water was modified only by mixing between the river and seawater endmembers. The calculated average $δ^{53}$ Cr value of the river water endmember (−0.39 ± 0.08‰) was, however, lower than the range that has been observed in other rivers, which we attribute to input of organically-bound Cr(III) released by anoxic weathering processes. This is supported by the fact that Cr recovered by UV irradiation was found to have low $δ^{53}$ Cr values (−0.11 to −0.75‰). While input of Cr from anoxic weathering processes is unlikely to be an important source of Cr to the oceans today, this suggests that processes other than oxidative weathering may have an influence on the $δ^{53}$ Cr values of estuarine and coastal waters on the local scale. The $δ^{53}$ Cr value of the coastal seawater endmember (1.6 ± 0.4‰) was also higher than the range observed in the deep open ocean, due to in situ biogeochemical cycling of Cr. These factors need to be considered in the interpretation of marine sedimentary $δ^{53}$ Cr records.
In-situ fabrication of reduced graphene oxide/leucomethylene blue/platinum nanoparticles modified electrode for voltammetric determination of trace Fe(II) in seawater
2019, Microchemical Journal
In this work, the modified electrode based on composites of reduced graphene oxide/leucomethylene blue/platinum nanoparticles (rGO/LMB/PtNPs) was fabricated in-situ via an electron relay effect. Cationic dye of methylene blue (MB) was absorbed on the surface of graphene oxide (GO) with through π-π bond interactions. Then PtCl₆²⁻ ions were self-assembled on the surface of the composites of GO/MB through Coulomb interactions. Using NaBH₄ as the reducing agent, PtNPs were formed and GO was reduced to rGO. With the aid of PtNPs, MB was catalytically degraded to LMB and a rGO/LMB/PtNPs modified electrode was successfully prepared in situ. Due to the excellent electrochemical properties of rGO and the well-established assistant reductant of MB, as well as the catalytic amplifying effect of PtNPs, this as prepared rGO/LMB/PtNPs modified electrode showed excellent properties for the voltammetric determination of Fe(II) with the linear range of 0.01 to 2 μM and a detection limit of 3 nM. This modified electrode was successfully applied to detect the content of Fe(II) in seawater.
Porewater inputs drive Fe redox cycling in the water column of a temperate mangrove wetland
2018, Estuarine, Coastal and Shelf Science
Citation Excerpt :
cm−1 (Stookey, 1970) to determine the Fe(II) concentrations. Measurements of absorbance were taken every 2 min for 118 min to examine the possibility of ongoing Fe(III) reduction induced by FZ (Bowie et al., 2002; Hopwood et al., 2014; Pullin and Cabaniss, 2003). Duplicates of each sample were measured in each microplate and replicate runs were performed on consecutive days.
Iron is a vital micronutrient within coastal marine ecosystems, playing an integral role in the scale and dynamics of primary production and carbon cycling in the world's oceans. We investigated the relative importance of in situ Fe(II) production from photochemical, microbial and thermal Fe reduction in the surface water column as well as advective porewater inputs in a temperate saline wetland in Australia containing mangrove and saltmarsh vegetation. The diel average concentration of Fe(II) (0.63 ± 0.21 μM, accounting for >70% of the total dissolved Fe present in surface water) was much higher than commonly reported in oxygenated marine waters despite high dissolved oxygen concentrations (81–112% saturation), pH (7.7–7.8) and salinity (33–36) that favor Fe oxidation. In situ production of Fe(II) in the surface water column was primarily driven by microbial processes rather than photochemical and thermal reduction, with a maximum production rate of 4.9 × 10⁻³ nM s⁻¹. Advective porewater Fe(II) inputs to the wetland averaged over a diel cycle (3.0 × 10⁻¹ nM s⁻¹) were an order of magnitude greater than the combined Fe(II) production rate from autochthonous water column processes (1.0 × 10⁻² nM s⁻¹). A bottom up model based on the estimated individual fluxes was used to explain the high Fe(II) concentrations measured during a 24 h time series experiment. Combined, different lines of evidence suggest that advective porewater exchange provides significant quantities of Fe(II) to the estuarine wetland.
Measuring total dissolved Fe concentrations in phytoplankton cultures in the presence of synthetic and organic ligands using a modified ferrozine method
2018, Marine Chemistry
Citation Excerpt :
The presence of NOM in samples can also retard Fe(II)-FZ complexation, resulting in development of an asymptotic absorbance over time (Box, 1984; Rose and Waite, 2003). Some workers have described a possible underestimation of Fe(II) concentrations as a result of inadequate mixing and equilibration time for FZ in the presence of NOM (Croot and Hunter, 2000; Hopwood et al., 2014). Jeitner (2014) also reported an impact of pH and temperature on equilibration duration of Fe(FZ)3 complexes.
Low concentrations and complex speciation can present major challenges for measuring Fe in natural waters easily and accurately. This study describes an optimized ferrozine method for measuring total dissolved Fe in the nanomolar range in seawater samples containing various concentrations of organic matter, as well as the commercially available Fe ligands ethylenediaminetetraacetic acid (EDTA) and desferrioxamine B (DFB), which are widely used in phytoplankton cultures. The method involves sample acidification to liberate Fe from strong complexes and/or colloids, followed by reduction of Fe(III) to Fe(II) using sulfite, and finally the measurement of Fe(II)-ferrozine complexes using long optical path spectrophotometry. The performance of each step was improved in such a way as to achieve the most efficient dissociation rate of Fe from the given ligands in the least possible time, regardless of their type or concentration. Storage of samples at pH 1 for 15 days enabled full recovery of Fe in the presence of 50 μmol/L DFB, while the duration required for full Fe recovery in the presence of the same concentration of EDTA was only about 1 h. Addition of ferrozine after a brief (about 15 min) reduction step with subsequent incubation for 24 h resulted in stable color development in the samples over time. The approach was successfully applied to determine total dissolved Fe in samples from coastal waters containing 10.5 μmol/L of particulate organic carbon (POC), and also in samples containing 2 μmol/L EDTA and 800 μmol/L of POC collected from laboratory cultures of the marine cyanobacterium, Trichodesmium erythraeum.

View all citing articles on Scopus

View full text

Dissolved Fe(II) in a river-estuary system rich in dissolved organic matter

Abstract

Introduction

Section snippets

The River Beaulieu

Sample collection

Temporal and spatial variation in riverine water

Temporal and spatial variation at King's Hat

Conclusions

Acknowledgements

Chem. Geol.

Water Res.

Geochim. Cosmochim. Acta

Anal. Biochem.

Anal. Chim. Acta

Geochim. Cosmochim. Acta

Neth. J. Sea Res.

Estuar. Coast. Mar. Sci.

Estuar. Coast. Shelf Sci.

Mar. Chem.

Mar. Chem.

Mar. Chem.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Water Res.

Geochim. Cosmochim. Acta

J. Geochem. Explor.

Mar. Chem.

Talanta

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Sci. Total Environ.

Anal. Chim. Acta

Water Res.

Deep Sea Res. Part I Oceanogr. Res. Pap.

Anal. Chim. Acta