Iron orthophosphate

Endpoint summary

Additional information

One publication evaluating the short-term toxicity of iron (III) orthophosphate according to the OECD guideline 207 towards earthworms is available (Edwards et al. 2009). In the study Eisenia fetida was exposed to pure iron phosphate and iron phosphate with a chelating or solubilizing agent (ethyldiaminetetraceticacid (EDTA) and ethylenediaminesuccinicacid (EDDS)) to concentrations up to 10 000 mg/kg. No toxicity was observed in the treatments with only iron phosphate revealing that no short-term toxicity by the substance is expected towards soil macroorganisms.

Iron (III) orthophosphate is authorized for use as the active substance in molluscicide preparations, specifically targeting snails and slugs. Such preparations however contain a solubilizing agent such as EDTA. The addition of a solubilizing agent makes the preparation more toxic by making the metal ion from iron orthophosphate bioavailable, leading to iron poisoning in the target species. Pure iron phosphate is not considered to be toxic due to its low bioavailability. This assumption is supported by the above mentioned study by Edwards et al. (2009), who additionally tested the short-term toxicity of iron phosphate towards earthworm, when a chelating/ solubilizing agent (ethyldiaminetetraceticacid (EDTA) and ethylenediaminesuccinicacid (EDDS)) is present. The organisms were exposed to nominal concentrations up to 10 000 mg/kg. While no toxicity was observed in the treatments with pure iron (III) phosphate, toxic effects were observed when iron (III) phosphate was combined with a solubilizing agent. This study reveals that pure iron (III) phosphate is not expected to be toxic to soil macroorganisms.

Iron (III) orthophosphate (CAS 10045-86-0) is poorly water soluble according to the OECD guidance 29, revealing that only little release of its ions Fe³⁺and phosphate can occur in pore water. Consequently, the substance is poorly bioavailable for soil organisms under aerobic conditions and neutral pH.

Ferric phosphates are generally common and stable minerals in the environment (Lindsay & De Ment, 1961). Geochemical and biological processes are responsible for dissolution, transformation and release of the iron and phosphate ions from ferric phosphate in soils (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.).

Iron ions in the trivalent oxidation state are highly reactive, and in case of release will mainly form poorly water soluble precipitates or bind to organic matter and will not stay in the ionic form under aerobic conditions. Under anaerobic conditions iron (III) can be reduced to the more soluble iron (II) species, which is stable only in absence of oxygen (Kappler et al., 2015).

Phosphate on the other hand is very affine to adsorption on for example iron (hydr)oxides, which can be a significant sink for phosphate ions and control its availability (Weng et al., 2012). Furthermore, phosphate can rapidly react depending on the conditions (especially pH) with aluminum, iron and calcium and form respective precipitates (Ruttenberg, 2014).

Iron and phosphorous are furthermore essential nutrients for all terrestrial organisms and will be homeostatically regulated in organisms at concentrations released from the substance (Ruttenberg, 2014, Kendall et al., 2012).

Even if not always easily bio-accessible, both elements are very abundant in the environment and exposure of soil organisms to iron and phosphate of geogenic origin is expected to be higher compared to the concentrations released form the substance.

Based on these considerations, no concerns arise from Iron (III) orthophosphate towards terrestrial organisms.

References:

Kappler, A., Emerson, D., Gralnick, J. A., Roden, E. E., & Muehe, E. M. (2015). Geomicrobiology of iron. In Ehrlich's Geomicrobiology, Sixth Edition (pp. 343-400). CRC Press. https://doi.org/10.1201/b19121

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.

Lindsay, W. L. and Stephenson, H. F., 1959. Nature of the reactions of monoealeium phosphate monohydrate with solls: IV. Repeated reactions with metastable triplepoillt solution. Soil Sei. See. Am. Proe. 23, 342-345.

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.

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

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.