Iron speciation

Iron speciation shown as % total Fe present In a particular species as a function of pH. (a) In 0.7m NaCI and (b) for organic-free seawater S = 35, both at 25°C. Source-. From Mlllero, F, and D. Pierrot (2002). Chemistry of Marine Water and Sediment, Springer-Verlag, pp. 193-220. 

Although the details of the equilibrium model are still uncertain, the general trends are likely reliable. As shown in Figme 5.16, most of the Fe(III) in seawater is predicted to be in the form of the FeL complex. The equilibrium model also predicts that this degree of complexation should enhance iron solubility such that 10 to 50% of the iron delivered to the ocean as dust will eventually become dissolved if equilibrimn is attained. If this model is a reasonable representation for iron speciation in seawater, uptake of [Fe(III)]jQjgj by phytoplankton should induce a spontaneous dissolution of additional particulate iron so as to drive the dissolved iron concentrations back toward their equilibrium values. 

Figure 4. Electrolyses of 50 mMNa2S04 adjusted to pH 2 continuously saturated with O2 at areticulated vitreous carbon cathode in a flow-cell. Iron speciation versus electrolysis time ( ) Fe, ( ) Fe. ...

Iron hydrolysis and solubility revisited Observations and comments on iron hydrolyses characterizations. Marine Chem. 70 23—38 Byrne, R.H. Kester, D.R. (1976) Solubility of hydrous ferric oxide and iron speciation in seawater. Marine Chem. 4 255—274 Byrne, R.H. Luo,Y.-R. (2000) Direct observations of nonintegral hydreno ferric oxide solubility products K Sq = [Fe ][H ] Geo-chim. Cosmochim. Acta 64 1873-1877 Cabrera, F. de Arambarri, P. Madrid, L. ... 

Sojo, L.E. and de Haan, H. (1991) Multicomponent kinetic analysis of iron speciation in humic lake Tjeukemeer comparison of fulvic acid from the drainage basin and lake water samples. Environ. Sci. Technol., 25, 935-939. 

Ozturk, M. and N. Bizsel. 2003. Iron speciation and biogeochemistry in different nearshore waters. Mar. Chem. 83 145-156. 

Iron speciation is a major factor in Fenton chemistry. As previously discussed, iron solubility, redox potentials, and concentrations of Fe2+ and Fe3+ are all dependent on the ligands that coordinate iron. In order to produce hydroxyl radical, there must be a readily accessible coordination site for H202 to bind to [9,10]. Very strong iron chelators, therefore, inhibit the formation of hydroxyl radical. Iron ligands can also act as hydroxyl radical scavengers. Because the radical is always formed in close proximity to these ligands, they are more likely to react with hydroxyl radical than pollutants that are not in close proximity to the iron. 

C. F. Harrington, S. Elahi, S. A. Merson, P. Ponnampalavanar, A method for the quantitative analysis of iron speciation in meat by using a combination of spectrophoto-metric methods and high-performance liquid chromatography coupled to sector t>eld inductively coupled plasma mass spectrometry, Anal. Chem., 73 (2001), 4422D4427. 

While a number of deckboard incubations of seawater samples show an increase in Fe(II) on irradiation [98,136,151], only a few studies have examined the change in iron speciation in situ as a function of time of day. Waite and Szymczak [152] measured the concentration of iron in waters overlying a coral reef on One Tree Island on Australia s Great Barrier Reef and observed... 

Byrne, R. H. Jr. "Iron speciation and solubility in seawater," Ph. D. Thesis, Univ. Rhode Island, Kingston, R.I., 1974. 

P.L., Croot, and Johansson, M. (2000). Determination of iron speciation by cathodic stripping voltammetry in seawater using the competing ligand 2-(2-Thiazolylazo)-p-cresol (TAC). Electroanalysis 12(8), 565-576. 

Rose, A. L., and Waite, T. D. (2003). Predicting iron speciation in coastal waters from the kinetics of sunhght-mediated iron redox cycling. Aquat. Sd. 65, 375—383. 

Byrne R. H. and Kester D. R. (1976) Solubility of hydrous ferric oxide and iron speciation in sea water. Mar. Chem. 4, 255-274. 

Figure 5 Summary of iron speciation in fine-grained siliciclastic sediments and sedimentary rocks. Total Fe (Fct) is equal to the sum of all these fractions. DOP increases in oxic sediments through the conversion of Fcex to Fcpy, although the HCl procedure generally overestimates the readily reactive iron available. In euxinic settings, high DOP values (Fcx/Al ratios) result from scavenging of dissolved iron during pyrite formation in the water column. See Section 7.06.3.4.3 and Lyons et aL (2003) for further discussion and background.

Iron extraction values show that iron speciation varies significantly between layers in the cave (Fig. 6A). Values for amorphous, total, and ferrous iron range from 2.4 to 84 pmol/g. Extraction results indicate a significant amount of goethite in the lower layers of the sequence as determined by total minus ammonium-oxalate extractable iron (52 pmol/g in the bottom yellow layer) (Schwertmann and Taylor, 1977). The upper layers have total iron values represented almost entirely by ammonium-oxalate extractable iron (83 pmol/g in the red layer and 59 pmol/g in the top orange layer) suggestive of ferrihydrite (a necessary precursor to hematite formation). The ferrihydrite in the upper layers is indicative of formation by rapid oxidation of ferrous iron (Schwertmann, 1993). The black layer contains the only cave sediment with a significant amount (42 pmol/g) of extracted ferrous iron. 

In addition to ferrous iron, ferric iron can be very abundant in acid drainage solutions, especially where the activity of ferrous iron-oxidizing prokaryotic microorganisms is high (see below for details). In fact, the observed iron speciation may be largely determined by the balance between microbial ferrous iron oxidation rates and the rates at which ferric iron is reduced by oxidation of sulfide, sulfur, and sulfoxy species. 

Figure 8 Arsenic and iron speciation data for groundwater collected from the wells along the flow path designated in Fig. 5. Data collected in Spring 1992 by Titan (1995). A, As(Ill) over total arsenic plotted against distance along the flow path. B, Fe(ll) over total iron plotted against distance along the flow path.

Iron speciation of Fe(II) and Fe(III) is reported more often than any other speciation. The methods are based on cation-exchange, anion-exchange, and ion-pairing chromatography. Only a few of the methods are discussed here. Saitoh and Oikawa... 

R.J. Kieber, K. Williams, J.D. Willey, S. Skrabal, G.B. Avery (2001). Iron speciation in coastal rainwater Concentration and deposition to seawater. Mar. Chem., 73, 83-95. 

F. Albertus, A. Cladera, E. Becerra, V. Cerda, A robust multi-syringe system for process flow analysis. Part 3. Time based injection applied to the spectrophotometric determination of nickel(II) and iron speciation, Analyst 126 (2001) 903. 

RB. Martelli, B.F. Reis, M. Korn, I.A. Rufini, The use of ion exchange resin for reagent immobilization and concentration in flow systems. Determination of nickel in steel alloys and iron speciation in waters, J. Braz. Chem. Soc. 8 (1997) 479. 

Ozturk, M., Steinnes, E. and Sakshaug, E. (2002) Iron speciation in the Trondheim Fjord from the perspective of iron limitation for phytoplankton. Estuarine, Coastal and Shelf Science, 55, 197-212. 

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