Triiron bis(orthophosphate)

Hydrolysis

Additional information

An experimental study on hydrolysis as a function of pH is not available for Triiron bis(orthophosphate) (CAS 14940-41-1). 

The iron transport and distribution depend on pH, Eh (redox) and the presence or absence of other dissolved constituents which form with FE(II) or FE(III) dissolved complexes, colloids or poorly soluble mineral phases (Boyd and Ellwood 2010; Konhauser et al. 2011a; Radic et al. 2011; Raiswell 2011). With increasing Eh and pH the amount if iron dissolved in groundwaters, rivers and seawater decreases (see attachment Fig. 2) (Kendall 2012).

Basically Triiron bis(orthophosphate) is subject to hydrolysis when the substance is released to water. During hydrolysis, Triiron bis(orthophosphate) decomposes into iron and orthophosphate ion, whereas Ferric iron (Fe3+) is the stable form in oxygenated waters, which forms at neutral pH highly insoluble oxides and hydroxides (Wang 1998; Simpson 2002; Zhang 1999). In anoxic waters ferrous iron (Fe2+) is stable. As dissolved ion it occurs usually in many freshwater systems. Insoluble salts will be formed in the presences of high carbonate, sulphide and orthophosphate levels (Stumm, W. and Morgan, J. J. 1981).

The 2+ and 3+ oxidation states of iron are stable over broad regions of potentials and pH. Ferric ion can be reduced by hydrogen, while ferrous ions are slowly oxidized by air. The hydrolysis of ferric ions in aqueous solutions is a complicated time-dependent system. It can be defined as hydrolysis-polymerization-precipitation. A simple mechanism describes the process into several steps: (a) primary hydrolysis giving rise to low-molecular-weight complexes (mono- and dimer), i.e., Fe(OH)2+, Fe(OH)2+, Fe2(OH)24+; (b) formation and aging of polynuclear polymers, i.e., Fen(OH)m(H2O)x(3n-m)+ or FenOm(OH)x(3n-2m-x)+; (c) precipitation of ferric oxides and hydroxides, i.e., Fe(OH)3, FeOOH, and Fe2O3. The whole process from hydrolysis to precipitation can take several years. At pH < 7, the dominant species of ferric solution are Fe(OH)2+, Fe(OH)2+, Fe2(OH)24+, Fe(OH)3, FeO(OH) and FeCl2+ (Martin, 1998).

The ferric species in the aqueous solution are theoretically dominated by the following reactions, the strengths of which vary with pH: 

[Fe(H2O)n(OH)(m−1)]4−m + H2O ⇄ [Fe(H2O)n−1(OH)m]3−m + H3O+

The inorganic phosphates are normally found in different forms (see attachment). In dilute aqueous solution, phosphate exists in four forms. In strongly-basic conditions, the phosphate ion (PO43−) predominates, whereas in weakly-basic conditions, the hydrogen phosphate ion (HPO42−) is prevalent. In weakly-acid conditions, the dihydrogen phosphate ion (H2PO4−) is most common. In strongly-acid conditions, aqueous phosphoric acid (H3PO4) is the main form.

References

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Kendall B., Anbar A.D., Kappler A. and Konhauser K.O. (2012).The global iron cycle. In book: Fundamentals of Geobiology, Blackwell Publishing Ltd., chapter 6, 65-92

Konhauser KO, Kappler A, Roden EE (2011) Iron in microbial metabolisms. Elements 7, 89–93.

Martin, R. L., Hay, J. P., Pratt L.R. (2008). Hydrolysis of Ferric Ion in Water and Conformational Equilibrium, J. Chem. Phys., A, 1998, 102, 3565-3573

Radic A, Lacan F, Murray JW (2011) Iron isotopes in the seawater of the equatorial Pacific Ocean: new constraints for the oceanic iron cycle. Earth and Planetary Science Letters 306, 1–10.

Raiswell R (2011) Iron transport from the continents to the open ocean: the aging-rejuvenation cycle. Elements 7, 101–106.

Simpson, S.L., Rochford, L. and Birch, G.F. (2002) Geochemical influences on metal partitioning in contaminated estuarine sediments. Marine and Freshwater Research, 53, 9-17 (cited in: Xing W. and Liu G. (2011))

Stumm, W. and Morgan, J. J. (1981). Aquatic Chemistry. Wiley: New York

Wang, S.M. and Dou, H.S. (1998). Chinese Lake Notes. Science, Press: Beijing. (In Chinese) (cited in: Xing W. and Liu G. (2011))

Xing W. and Liu G. (2011) IRON BIOGEOCHEMISTRY AND IST ENVIRONMENTAL IMPACTS IN FRESHWATER LAKES. Fresenius Environmental Bulletin, Vol 20, No. 6, 1339-1345.

Zhang, X.H. (1999) Iron cycle and transformation in drinking water source. Water and wastewater, 25, 18-22. (In Chinese) (cited in: Xing W. and Liu G. (2011))