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Triiron bis(orthophosphate) (CAS 14940-41-1) is an inorganic iron salt of phosphoric acid. The substance does not undergo biological degradation. Triiron bis(orthophosphate) will be removed from the water column by hydrolytic transformation or chemical precipitation. The precipitates of iron and phosphorus will be further transformed in soil and sediment systems by mineralisation. 

Iron and phosphorus are both natural elements, which are present in all environmental compartments.

Iron is an essential trace element for nearly all organisms. It is well known, iron is essential for biological requirements of phytoplankton and aquatic plants, e.g. photosynthesis is influenced by iron as well as chlorophyll pigment biosynthesis (Xing 2011). 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 of most iron compounds (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.

The environmental fate and transport of iron salts is dominated by three processes: the oxidative conversion between Iron (II) and Iron (III), the formation of insoluble oxides and hydroxides, and adsorption of iron salts with other soil components. Iron is redox-active, and therefore, it is involved in many redox processes in the environment. “Redox transformation of iron, leading to either dissolution or precipitation, and thus mobilization and redistribution of iron, are caused by chemical and to a significant extent by microbial processes (see attachment Fig. 1).”

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).

In the environment different Fe(II), Fe(III) and mixed Fe(II)-Fe(III) minerals are found. Microbial activities play an important role when many of them are used, produced or transformed (see attachment Table 1).

The iron transport and distribution depend on pH, redox potential (Eh) 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).

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).

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 (Geesey 1977; Harvey 1982; Mahmood 1993; Corapcioglu 1995).  In a study from Kouakou (2013), the maximum adsorption capacities of Fe2+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).

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 is 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).

The phosphorus and phosphate anions are ubiquitous in natural waters and essential micronutrient for many organisms. Orthophosphates are also formed by natural hydrolysis of human urine and faeces, animal wastes, food and organic wastes, mineral fertilisers, bacterial recycling of organic materials in ecosystems, etc. Phosphates are bio-assimilated by the bacterial populations and the aquatic plants and algae found in these different compartments and are an essential nutrient (food element) for plants, and stimulate the growth of water plants (macrophytes) and/or algae (phytoplankton) if they represent the growth-limiting factor.

Bioaccumulation and secondary poisoning are not considered significant for Triiron bis(orthophosphate) (CAS 14940-41-1). Triiron bis(orthophosphate) is not expected to bioaccumulate in organisms and food chains. In a GLP guideline study from the comparable substance Iron II sulphate heptahydrate (CAS 7782-63-0) following OECD 305 a BCF ≤ 20 was determined. An accumulation of phosphate in organisms is unlikely to pose a hazard potential, as the phosphate anion is an essential micronutrient for many organisms and the internal concentration is regulated biologically.

The availability of inorganic phosphorus in soils depends on precipitation-dissolution and sorption-desorption processes (Cornforth, 2005). 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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