Triiron bis(orthophosphate)

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Additional information

Triiron bis(orthophosphate) (CAS 14940-41-1) is an inorganic iron salt of phosphoric acid. The substance can be transformed by hydrolysis forming iron and orthophosphate species in sewerage systems, sewage treatment plants and in the environment.

Iron is an essential trace element for nearly all organisms. Iron is the second most abundant metal and fourth most abundant element in the Earth’s crust (Taylor 1964). The iron concentration in water is quite low because of low solubility (Shaked et al. 2004). Iron enters the environment due to anthropogenic influences such as wastewater and storm-water discharges, which is the first source of iron in freshwater (Xing et al. 2006). Natural iron sources are weathered rocks and soil around watersheds, which outcome depends on many factors, especially geological process, soil composition, environmental temperature, precipitation and hydrology (Harris 1992). When iron-bearing minerals were break down by chemical and biological weathering, iron could release into aqueous solution.

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).

Adsorption and uptake processes between microorganism and the aqueous solution play an important role for mobility, reactivity and the bioavailability of iron in aqueous solution (sec. source in González et al. 2014). Transformations of iron by microorganisms are often much faster than the respective chemical reactions. They occur in most soils and sediments, both in freshwater and marine environments (Thamdrup 2000; Straub et al. 2001; Cornell and Schwertmann 2003).

With adsorption of Fe(II) to particle surfaces (complexation with surface hydroxyl groups) the oxygenation rate of Fe(II) grows up in a similar way as hydrolysis in solution (complexation with OH− ions). The dissolution kinetics are monitored by surface processes (and not transport processes). The rate of dissolution is proportional to the surface complexes created on the surface of Fe(III) (hydr)oxides. “Thus, a reductant, such as ascorbate, exchanges electrons with a surface Fe(III) ion subsequent to its inner-sphere coordination to the oxide surface.” The Fe(II), which is formed although, is more easily detachable from the surface. Due to complex formation reactions of Fe(III) and Fe(II) with organic and inorganic ligands solute and solid complexes will be formed. Thus, electron cycling of Fe(III)-Fe(II) transformations are possible due to these complexes, which appear over the entire EH range within the stability of water (EH from −0.5 V to +1.1 V). Therefore, solid and solute Fe(II) complexes with silicates, with hydrous oxides (e.g., Fe3O4), and with sulfides are very efficient reductants (thermodynamic and kinetic) (Stumm and Sulzberger 1992).

In a study from Kouakou (2013), the maximum adsorption capacities of Fe2+calculated by the Langmuir model were respectively 15 and 31 mg/g with synthetic solution at an initial Fe2+concentration of 5 mg /L. The removal of Fe2+from wastewater was around 70% (Kouakou et al. 2013).

The availability of inorganic phosphorus in soils depends on precipitation-dissolution and sorption-desorption processes (Cornforth, 2008). Phosphorus ions are mainly immobilised in soils by adsorption to organic matter or by reaction with aluminium or iron to aluminium- and ironphosphates. Sato et al. (2009) observed that Phosphorus released from calciumphosphate was adsorbed to aluminium and iron-oxyhydroxides.

The air compartment is considered not relevant for Triiron bis(orthophosphate). Due to its physico-chemical properties, Triiron bis(orthophosphate) is not distributed or transported to the atmosphere as the substance is usually not emitted to air.

 

References

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Cornforth, I. S. (2005). The fate of phosphate fertilisers in soil. Department of Soil Science, Lincoln University online: www. nzic. org. nz.

González A.G., Pokrovsky O.S., Jiménez-Villacorta F., Shirokova L.S., Santana-Casiano J.M., González-Dávila M. and Emnova E.E. (2014). Iron adsorption onto soil and aquatic bacteria: XAS structural study. Chemical Geology 372, 32-45

Harris, J.E. (1992) Weathering of rock, corrosion of stone and rusting of iron. Meccanica, 27, 233-250.

Kouakou U., Ello A.S., Yapo J.A. and Trokourey A. (2013). Adsorption of iron and zinc on commercial activated carbon. academicJournals, Vol. 5(6), pp. 168-171

Sato et al. (2009) Biogenic calcium phosphate transformation in soils over millennial time scales. Journal of Soils Sediments (2009) 9:194–205

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

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Taylor, S.R. (1964) Abundance of chemical elements in the continental crust: a new table. Geochimica et Cosmochimica Acta, 28, 1273-1285.

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