Chromium

Endpoint summary

Additional information

This part indicates the binding capacity of chromium III to solid surfaces.by defined the environmental partitioning and substance persistence.

In 2005, an “European Union Risk Assessment Report” has been published and mainly based on the assessment on 5 different chromium VI substances behavior in the environment. The Risk Assessment was carried out in accordance with Council Regulation (EEC) 793/931 on the evaluation and control of the risks of “existing” substances. “Existing” substances are chemical substances in use within the European Community before September 1981 and listed in the European Inventory of Existing Commercial Chemical Substances. This document also discussed few useful aspects of Cr III prevalence and behavior in the Environment.

About the repartition of chromium III in environmental compartments this risk assessment report concluded with the enclosed points:

• In general, chromium (III) is more likely to partition to solids in the sediment and soil. For the water column, chromium (VI) and chromium (III) have similar adsorption partition coefficients for suspended solids, which are greater than those found for sediment and soil. 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 (Richard and Bourg 1991).

Chromium (III) Acid conditions Alkaline conditions

Kpsusp = 30,000 l/kg Kpsusp = 300,000 l/kg

Kpsed = 11,000 l/kg Kpsed = 120,000 l/kg

Kpsoil = 800 l/kg Kpsoil = 15,000 l/kg

 

The equivalent values for the dimensionless form of the partition coefficient using the methods given in the Technical Guidance document are:

Chromium (III) Acid conditions Alkaline conditions

Ksusp-water = 7,500 m3/m3 Ksusp-water = 75,000 m3/m3

Ksed-water = 5,500 m3/m3 Ksed-water = 60,000 m3/m3

Ksoil-water = 1,200 m3/m3 Ksoil-water = 22,500 m3/m3

 

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 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”.

 

Several publications, scored Klimisch 2 demonstrated that:

The solubility of chromium is higher at low pH (below 5.5) and minimum in the environmentally relevant pH range of 7-10. The free trivalent chromium ion predominates at very low pH (< pH 3), typically not found in the environment. Trivalent chromium can bind with naturally occurring DOC. Organically bound Cr (III) but not unbound Cr (III) can stay in solution at higher pH and can also sorb to and desorb from the organic fraction of suspended matter and settled sediment particles. Dissolved Cr(III) forms coordinate complexes with many inorganic and organic ligands. The oxidation of trivalent chromium only occurs in the presence of at least 1% manganese oxide, but this reaction is unlikely under most environmental conditions. It can be concluded that organic matter is expected to convert soluble chromate to insoluble chromium oxide.