Chromium (III) hydroxide

Endpoint summary

Additional information

At pH 6.8 – 11.5 Chromium(III)hydroxide is insoluble. Therefore no hydrolysis takes place. At low pH < 6.8 Cr(III)hydroxide is hydrolysed very rapidly to soluble species Cr(OH)2+, CrOH2+ and Cr3+. At pH >11.5 hydroxo complexes are formed. The substance is not subject to biodegradation or bioaccumulation. Even bioconcentration factors for soluble chromium(III) compounds were low, between 86 and 192. Based on laboratory experiments the binding strength of Cr(III) to soil was rated by expert judgement. Binding of Cr(III) to soil above pH 4.5 in well-aerated sandy soils low humus content of < 2% of weak acidity and pH and redox potential is very strong. At lower pH the binding strength subsequently decreases to very weak at pH 2.5. Insoluble Chromium(III)hydroxide is formed in soils at pH >4.7. Formation of Chromium(III)hydroxide is forced, as the reaction consumes H⁺and reduces pH of soils. In soils precipitated Chromium(III)hydroxide is aged with time resulting in more crystalline forms with lowered solubility. There is strong evidence that organic bound Chromium(III) is very low. The exchangeable portion of Cr(III) is also negligible.

Cr(III) is removed very rapidly by precipitation as various Cr(III) species, e.g. as hydroxide and/or adsorption. Within 6 hours there is no Cr(III) detectable. Some of the Cr(III) is then oxidized by Mn(IV)-containing minerals within a few hours. Higher levels of carbon may block mineral sites (eg. MnO2) that oxidize Cr(III). Cr(VI) disappears from pore water at a very slow rate compared to the disappearance of Cr(III). Adsorption coefficients increase with increasing pH.

Adsorption coefficients were determined as follows: Kd = 298 — 14346 L/kg (o.c.: 1.92%, pH 4.70 - 6.69), 19716 — 55918 L/kg soil (o.c.: 0.11%, pH 6.03 - 6.69), and 330 — 23658 (o.c.: 3.75%, pH 3.63 - 6.16) for loam, loess and clay, repectively.