Iron orthophosphate

Endpoint summary

Additional information

Iron (III) orthophosphate (CAS 10045-86-0) is a solid under most environmental conditions and is poorly soluble in water. The 28 d transformation/dissolution test according to the OECD guideline 29 determined a maximum dissolution of 21.062 µg/L iron species after 7 d at a loading of 100 mg/L and pH 6, indicating that soil and sediment are expected to be the primary environmental compartments of relevance for the substance. Furthermore, no concerns from bioaccumulation are expected, since both elements iron and phosphorous are essential elements for life and the releases of the metals from the substance are very low. Since the substance is inorganic the biodegradation concept does not apply.    

Ferric phosphate is a common mineral in soils (Lindsay & De Ment, 1961) with a low solubility product of 3.98 x 10^-27(Scheffer & Schachtschabel, 2010). Geochemical and biological processes are responsible for dissolution, transformation and release of the iron and phosphate ions from ferric phosphate (Kendall et al. 2012, Yadav et al. 2012). Limited releases of the respective elements iron and phosphorous from iron phosphate will enter the complex iron and phosphorous cycles in the environment, and their fate is highly dependent on the environmental conditions (e.g. redox conditions, pH, organic matter, metals available etc.).

Both elements iron and phosphorous are very abundant in the environment. Iron is the second most abundant metal in soils while phosphorous, which is mostly found in form of orthophosphate in soils is widely dispersed in the environment (Salminen et al., 2005). Total phosphorous backgrounds in 51 soil samples (topsoil) were measured within the frame of the FAO/IAEA phosphate project. Total phosphorous (digestion with HF/HNO3) was between 63 and 1893 mg/Kg soil (Montange, D., & Zapata, F., 2002). Iron on the other hand is a major element in soil with a median value of 2.1% soil (Salminen et al., 2005).

Iron is present in the environment mostly in two oxidation states: +2 and +3. Redox potential and pH mainly govern the fate of iron in the environment. Under aerobic conditions and common environmental pH, ionic Fe³⁺species exists mainly in form of poorly water soluble precipitates like oxides, hydroxides or oxihydroxides. These sources of iron are basically of low mobility and bioavailability. However, complexation of iron species with organic matter could make the metal more bio-accessible. Under anaerobic conditions iron (III) can be biotically and abiotically reduced to iron (II), which bears a higher water solubility, higher mobility and therefore also higher bioavailability (Colombo et al., 2013).

Dissolved phosphate species on the other hand are very affine to adsorption on soil particles. Iron (hydr)oxides like goethite (α-FeOOH), hydrous ferric oxide, or ferrihydrite, hematite (α-Fe2O3), and lepidocrocite (γ-FeOOH) can for example be a significant sink for phosphate ions and control its availability (Weng et al., 2012). Furthermore, phosphate can rapidly precipitate in soils depending on the conditions (especially pH) and react with aluminum, iron and calcium and form respective precipitates (Ruttenberg, 2014). Thus, the mobility of phosphate in soil is limited. Phosphate in water is found as PO₄^3-, HPO₄^2-or H₂PO₄^-depending on the pH. Natural phosphate levels in fresh water are typically between 10 and 25 μg/L (Salminen et al., 2005).

Iron (III) orthophosphateis a solid inorganic salt and thus not volatile. An extensive accumulation in air and the subsequent transport to other environmental compartments is not anticipated.

A bioaccumulation concern from the substance does not arise since releases of iron and phosphate by the substance are very low and geogenic sources are expected to be higher. Furthermore iron and phosphate ions are essential elements for life and at concentrations released from the substance are expected to be homeostatically regulated in organisms.

References:

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Colombo C., Palumbo G., He JZ., Pinton R., Cesco S. (2013).Review on iron availability in soil: interaction of Fe minerals, plants, and microbes.J Soils Sediments (2014) 14:538–548.

Kendall B., Anbar A. D., Kappler A., Konhauser K. O. 2012.The global iron cycle. Fundamentals of Geobiology First Edition. Edited by Andrew H. Knoll, Donald E. Canfield and Kurt O. Konhauser.

Montange, D., & Zapata, F. (2002). Standard characterization of soils employed in the FAO/IAEA phosphate project (IAEA-TECDOC--1272/CD). International Atomic Energy Agency (IAEA).

Ruttenberg, K. C. (2014), 10.13 - The Global Phosphorus Cycle, in Treatise on Geochemistry (Second Edition), edited by H. D. H. K. Turekian, pp. 499-558, Elsevier, Oxford.

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Yadav R. S., Meena S. C., Patel S. I., Patel K. I., Akhtar M. S., Yadav B. K., Panwar J. (2012) Bioavailability of Soil P for Plant Nutrition. In: Lichtfouse E. (eds) Farming for Food and Water Security. Sustainable Agriculture Reviews, vol 10. Springer, Dordrecht.

Weng L. Van Riemsdijk W. H., Hiemstra T., 2012. Factors Controlling Phosphate Interaction with Iron Oxides. J. Environ. Qual. 41:628–635.