Iron sinter

Water solubility

Description of key information

The aqueous solubility measurements on iron sinters (Jackson et al, 2010) demonstrates that the material is slightly soluble, principally owing to the dissolution of calcium to a concentration of 1.54 to 1.94 mg/L. However, the dissolved calcium only accounts for ca. 2% of its total concentration within the sinter sample. Other metallic elements including iron are technically insoluble since their solubilities are below the instrumental limits of detection (<0.1 mg/L). The measured concentration of iron was <50 ng/L.

FACTSAGE mathematical modelling (Thompson, 2010) emphasises the high insolubility of sinter. Chemical equilibrium modelling (ETAP, 2010) was carried out to calculate a solubility limit for iron using iron (III). Solubility limits of 0.015, 0.0024 and 0.00001 mg/L were calculated at pH 6, 7, and 8, respectively.

Key value for chemical safety assessment

Water solubility:

0.05 µg/L

at the temperature of:

25 °C

Additional information

Iron sinter is a heterogeneous material that is composed principally of di-iron trioxide (Fe2O3), tri-iron tetraoxide (Fe3O4) and calcium ferrites together with small amounts of silicates and glassy phases. In simple terms the structure of iron sinter may be regarded as consisting of iron oxide grains bonded together into the form of an agglomerate by the calcium ferrites and silicate phases. The overall solubility of iron sinter is therefore determined by the solubilities of the constituent phases present in the agglomerate. Thermodynamic calculations (Thompson, 2010) and experimental measurements (Currenta, 2009; Klawonn, 2018) show that iron oxides are extremely insoluble with < 2 μg/L of iron dissolved for both di-iron trioxide and tri-iron tetraoxide. These data are confirmed by the study of Jackson et al on the solubility of iron sinter, in which the solubility of iron was determined to be < 0.05 μg/L. The only element in iron sinter that exhibited significant solubility in the study by Jackson et al was calcium, which afforded a concentration of < 2 mg/L. The solubilities of all other main elements present in iron sinter, i.e. aluminium, magnesium and manganese, were all < 0.1 mg/L. The dissolved calcium represented the dissolution of approximately 2 % of the total calcium present in iron sinter. The soluble calcium derives from the presence of the soluble phase dicalcium silicate, which is typically present at concentrations of < 5% m/m in iron sinter. On the basis of the partial solubility of calcium, iron sinter is classified overall as slightly soluble but the iron compounds present in sinter are completely insoluble. The dissolution of calcium in the work of Jackson et al represents a worst-case scenario since the solubility measurements were performed on finely ground iron sinter with a particle size < 200 μm, whereas iron sinter is produced with a particle size > 5 mm. In solubility tests on material ground to < 200 μm the available surface area in contact with the liquid is approximately 250-fold greater than it is for an identical mass of sinter with particle size > 5 mm. Therefore, the actual amount of calcium dissolved in practice will be reduced by a similar degree. The results demonstrate that the aqueous solubility of iron sinter is so low that it does not become bioavailable to humans or ecosystems.