Iron estuarine sediments

Nitrogen pollution has received far more attention than that of phosphorus for two reasons. First, it has been considered as the nutrient-limiting primary production in estuaries and coastal waters. Second, its loading into the coastal zone has been far greater than that of phosphorus (Figure 24.21). It is also more efficiently exported into the ocean due in part to formation of iron phosphate minerals in anoxic estuarine sediments. 

Tugel, J.B. Hines, M.E. Jones, G.E. (1986) Microbial iron reduction by enrichment cultures isolated from estuarine sediments. Appl. Environ. Microbiol. 52 1167-1172... 

Luther III, G.W., Giblin, A., Howarth, R.W., and Ryans, R.A. (1982) Pyrite and oxidized iron mineral phases formed from pyrite oxidation in salt marsh and estuarine sediments. Geochim. Cosmochim. Acta 46, 2665-2669. 

HiS levels in pore waters. In these locations, Fe phosphate minerals may control Fe solubility. The combination of laboratory equilibration studies and pore water solute concentration measurements led Martens et al., for example, to conclude that deep pore waters were in equilibrium with vivianite -F63P04 8H2O - in a coastal sediment. Hyacinthe et al. found that iron was sequestered as an Fe(iii) phosphate in low-salinity, estuarine sediments. This ferric phosphate may have been formed in surface sediments (see below) or in the water column. 

Hyacinthe C and Van Cappellen P (2004) An authi-genic iron phosphate phase in estuarine sediments ... 

Ruttenburg KC (1992) Development of a sequential extraction method for different forms of phosphorus in marine sediments. Limnol Oceanol 37(7) 1460-1482 Rysgaard S, Risgaard-Petersen N, Nielsen LP, Revsbech NP (1993) Nitrification and denitrification in lake and estuarine sediments measured by the N dilution technique and isotope pairing. Appl Environ Microbiol 59(7) 2093-2098 Shaked Y, Erel Y, Sukenik A (2002) Phytoplankton-mediated redox cycle of iron in the epilimnion of Lake Kiimeret. Environ Sci Technol 36(3) 460-467 Shukla SS, Syers JKJ, Williams DH (1971) Sorption of inorganic phosphate by lake sediments. Soil Sci Soc Am Proc 35(2) 244-249 Song JM (1997) Chemistry of Sediment-Seawater Interface of China s Seas. China Ocean Press, Beijing, pp. 1-222 (in Chinese)... 

Hgure 2 Percentage metal extracted, relative to the total concentration, from marine sediments by different reagents, enzymes, and the gut fluid from cod. HCI = 1 mol I HCI HOAc=25% glacial acetic acid NaOH =0.1 mol I NaOH Pepsin = Pepsin A pH 2 Trypsin = Trypsin ll-S pH 7.6 Gut fluid (1) = natural filtered intestinal fluid, pH 7 Gut fluid (2) = natural filtered intestinal fluid, pH 2. (A) Cu and (B) Zn. (Adapted from Turner A and Olsen YS (2000) Chemical versus enzymatic digestion of contaminated estuarine sediment Relative importance of iron and manganese oxides in controlling trace metal bioavailability. Estuarine, Coastal and Shelf Science 51 717-728.)... 

Note that plots, such as Figure 5.1, provide information only on the net outcome of chemical reactions. In the case of iron, a small addition does take place in estuaries as a result of desorption of Fe from the surfaces of riverine particles. As these solids move through the estuarine salinity gradient, the major cation concentrations increase and effectively displace the iron ions from the particle surfeces. Since this release of iron is much smaller than the removal processes, the net effect is a chemical removal of iron. Sedimentation of these iron-enriched particles serves to trap within estuaries most of the riverine transport of reactive iron, thereby preventing its entry into the oceans. 

Some studies have reported conservative behavior during estuarine mixing. In the unpolluted Krka Esmary of Yugoslavia, Seyler and Martin (1991) observed a linear increase in total arsenic with increasing salinity, ranging from 0.13 xgL in freshwaters to 1.8 JLgL offshore. Other studies however, have observed nonconservative behavior in estuaries due to processes such as diffusion from sediment pore waters, co-precipitation with iron oxides, or anthropogenic inputs (M. O. Andreae and T. W. Andreae, 1989 Andreae et al., 1983). The flocculation of iron oxides at the freshwater-saline interface as a result of increase in pH and salinity can lead to major decrease in the arsenic flux to the oceans (Cullen and Reimer, 1989). 

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