Chromium

Endpoint summary

Description of key information

Chromium is highly insoluble as demonstrate in the T/D test as a consequence: chromium (III) produces soluble (bio)available ionic and other chromium-bearing species in environmental media is limited. The insolubility of chromium (III) is expected to determine its behavior, its fate in the environment, and subsequently its ecotoxicity potency.

Several reports (WHO, 2009 ; EU RA on chromates (ECB, 2005) ; Voluntary Risk Assessment of Metallic Chromium and Trivalent Chromium Compounds (ICDA, 2010)) all demonstrate that:

- “Chromium in soil is present mainly as insoluble oxide and is not very mobile. Chromium (III) is expected to be rapidly and strongly adsorbed onto soil, particularly by iron and manganese oxides, clay minerals, and sand.

- “The sorption of chromium to soil depends primarily on the clay content of the soil and, to a lesser extent, on iron oxide and the organic content of soil.

- “Chromium that is irreversibly sorbed onto soil, will not be bioavailable to plants or animals under any conditions (Calder, 1988; Hassan & Garrison, 1996). Chromium (III) appears to be much more strongly adsorbed to soils than is chromium (VI) (Hassan & Garrison, 1996).” (WHO, 2009)

- “Chromium is largely immobile in plants: chromium is strongly bound to soil, mainly retains in plant roots and is not/poorly translocated to plant foliage. This mechanism limits the concentration of chromium in edible plants part and prevent the secondary poisoning mechanism.

 

The chromium partitioning behavior can finally be summarized as follows:

“chromium (III) appears to be much more strongly adsorbed to soils and sediments than chromium (VI). The adsorption of chromium (III) onto soil follows the pattern typical of cationic metals and increases with increasing pH (lowering pH results in increased protonation of the adsorbent leading to fewer adsorption sites for the cationic metal) and the organic matter content of the soil and decreases when other competing (metal) cations are present. Certain dissolved organic ligands may also reduce the adsorption of chromium (III) to the solid phase by forming complexes which enhance the solubility of chromium (III) in the aqueous phase”.

 

If this unlikely behavior of chromium (III) to pass from the soil to plant upper part is largely describes in the literature and assessment reports, several supportive publications, all Klimisch 2 quoted are described here.

Data assessed and summarized in “European Union Risk Assessment Report” (2005) indicates that chromium (III) species have a strong preference for the solid phase of suspended matter, sediment and soil.

Indeed, this EU RAR on chromates Report (ECB, 2005) develops the chromium behavior as follows:

“The adsorption of chromium (III) onto soil follows the pattern typical of cationic metals and increases with increasing pH (lowering pH results in increased protonation of the adsorbent leading to fewer adsorption sites for the cationic metal) and the organic matter content of the soil and decreases when other competing (metal) cations are present. Certain dissolved organic ligands may also reduce the adsorption of chromium (III) to the solid phase by forming complexes which enhance the solubility of chromium (III) in the aqueous phase”. Partition coefficients of 30,000 L/kg (log Kp = 4.47); 11,000 L/kg (log Kp = 4.04) and 800 L/kg (log Kp = 2.90) for acid conditions and 300,000 L/kg (log Kp = 5.47); 120,000 L/kg (log Kp = 5.08) and 15,000 L/kg (log Kp = 4.17) for alkaline conditions were derived for chromium (III) in the EU RA on chromates (ECB, 2005) for suspended matter, sediments and soil, respectively. The partitioning coefficients are confirmed in several reliable studies available on soluble chromium (III) substances and in reviews by U.S. EPA (2005).”

The absence of biomagnification has also been assessed across the terrestrial food chain by WHO (2009) in the “Concise International Chemical Assessment Document 76 INORGANIC CHROMIUM(III) COMPOUNDS” report and summarized as follows:

“Although higher concentrations of chromium have been reported in plants growing in high chromium containing

soils (e.g. soil near ore deposits or chromium emitting industries and soil fertilized by sewage sludge) compared with plants growing in normal soils, most of the increased uptake in plants is retained in roots, and only a small fraction is translocated to the aboveground part of edible plants (Cary, 1982; IPCS, 1988). Therefore, bioaccumulation of chromium from soil to aboveground parts of plants is unlikely (Petruzzelli et al., 1987).

Van Gestel et al. (1993) reported low bioconcentration factors for chromium (III) in earthworms (Eisenia andrei). Chromium nitrate was added to artificial soil; after 3 weeks, bioconcentration factors of 0.03–0.05 were determined at exposure concentrations ranging from 10 to 100 mg chromium (III)/kg dry soil. The elimination half-life for total chromium was estimated to be 51–109 days.

There is no indication of biomagnification of chromium along the terrestrial food-chain (Cary, 1982).”

 

Applying a weight-of-evidence approach, the conclusion of WHO (2009) "no effects were seen with chromium (III) in the form of the insoluble chromium oxide” according to the CICAD for trivalent chromium substances, even though soluble chromium (III) substances may have some potential for toxicity to soil organisms.

 

Taking into account all reported effect concentrations of soluble chromium (III) substances, the insolubility of chromium (III), bioavailability, fate, the toxicity to soil organisms is irrelevant.