Iron-Bound Phosphorus

In section 7.4.2.1 the theoretical significance of phosphate adsorption onto iron oxides was illustrated. Numerous studies on natural sediments suggest that iron oxides control phosphate pore water and solid phase concentrations, as well as the overall sedimentary phosphate cycle (Krom and Berner 1980 Froelich et al. 1982 Sundby et al. 1992 Jensen et al. 1995 Slomp et al. 1996a,b). A generalized representation of the sedimentary phosphorus cycle is shown in Fig. 6.11. Apart from the Fe-bound P, organic P and authigenic carbonate fluorapatite are the principal carriers of solid phase P. HPO/ is the predominant dissolved P species under sea water conditions (Kester and Pytkowicz 1967). 

The following information and simple calculation allows the reader to assess and understand the important role of iron oxyhydroxides and their interactions with phosphorous A maximum of 2.5 - 

mol m of adsorbed phosphate on iron oxides were found (Goldberg and Sposito 1984 Pena and Torrent 1984). In order to approximate a maximum adsorbed phosphate concentration in the sediment one can assume 1 cm of sediment with a porosity of 75 %, a dry weight density of 2.65 g cm 50 j,mol/g3 jj, , Fe bound to iron oxides and an iron oxide specific surface area of 120 m g (see section 7.4.2.1). For the wet sediment we can calculate an iron concentration of 33 pmol cm which is bound to iron oxides. This fraction has a specific surface area of 0.22 m within 1 cm of wet sediment which may then adsorb up to 0.57 

---

Jensen, H.S., and Thamdrup, B. (1993) Iron-bound phosphorus in marine sediments as measured by bicarbonate-dithionite extraction. Hydrobiologia 252, 47-59. 

Slomp, C.P., Epping, E.H., Helden, W., and Raaphorst, W.V. (1996) A key role for iron-bound phosphorus in authigenic apatite formation in North Atlantic continental platform sediments. J. Mar. Res. 54, 1179-1205. 

Analytical Chemical Data for Natural Waters. While elemental compositions of various natural waters usually can be determined with good reliability, analytical methods to distinguish between free and complex-bound species, oxidized and reduced forms, simple and polynuclear metal ion forms, and even between dissolved and colloidal or suspended phases are often lacking. Data on the nature and amounts of the individual substances which make up the total concentrations of organic material found in different natural waters are not yet extensive. These analytical deficiencies relate almost solely to the highly reactive, non-conservative elements—e.g., iron, manganese, phosphorus, carbon, nitrogen, aluminum, and other metal ions. 

For example, some plants can increase root volume and surface area to optimize uptake potential. Alternatively, plant roots and/or associated fungi can produce chelating compounds that solubilize ferric iron and calcium-bound phosphorus, enzymes and/or acids that solubilize phosphate in the root vicinity. Plants also minimize phosphorus loss by resorbing much of their phosphorus prior to litterfall, and by efficient recycling from fallen litter. In extremely unfertile soils (e.g., in tropical rain forests) phosphorus recycling is so efficient that topsoil contains virtually no phosphorus it is all tied up in biomass. 

Iron-bound phosphate Calcium-bound phosphate Acid-soluble organic phosphate Sodium hydroxide-extractable phosphorus Fe(OOH) P CaCOj P ASOP NaOH3,-P 0.02 M Ca-NTA/dithionite, pH 7.8-8.0 0.05 M Na-EDTA, pH about 8.0 0.5 M HCI or 0.25 M HjSO (30 min) 2.0 M NaOH (OO C, 30 min)... 

In soils and sediments, complexation can increase organic phosphorus stabilization, especially with iron (III) and calcium ions and their minerals (Harrison, 1987 House and Denison, 2002). The interaction with iron (III) was reported to transform a large part of the labile and moderately labile organic phosphorus forms supplied with manure to paddy soils into more resistant organic phosphorus, possibly because inositol phosphates initially bound to calcium or magnesium were transformed into iron-bound compounds (Zhang et aL, 1994). In the presence of calcium, myo-inositol hexakisphosphate can form two soluble calcium complexes with one or two calcium ions (Ca - or Ca2-phytate), but when three calcium ions are involved (Cag-phytate), the complex precipitates at all pH values (Graf, 1983). This enhances the interaction of myo-... 

Both lipases and lipoxygenases are present in the bran and the germ. Phytases are nutritionally important as they liberate the phosphorus, of which approximately 70% is in the kernel bound to phytin. Phytin blocks the intestinal absorption of both iron and calcium. Phytase is also present in yeast, which is why leavened bread is nutritionally superior to unleavened bread. There have been concerns about the incidence of rickets among those of South Asian origin who eat chapattis, live in the UK, and have a tendency to keep their skin covered up from the sun. 

---