Dichromium tris(chromate)

Adsorption / desorption

Description of key information

Adsorption for both Chromium (III) and Chromium (VI) is pH dependant.  Adsorption of Chromium (III) is stronger in alkaline conditions whereas conversely adsorption of Chromium (VI) is stronger in acidic conditions.

Additional information

There are no proprietary studies investigating the adsorption/desorption of the substance. The estimation methods given in the main Technical Guidance document for determining adsorption coefficients for soil, sediment and suspended sediment are not applicable to chromium compounds. Measured values are available in the EU RAR for a number of soil and sediment types. The following data and discussion are taken from the EU RAR (2005).

 

The table below summarises the published values for Kpwater-solids for both freshwater and marine environments. The values are reported according to a variety of methods and may not be directly comparable; however, they do give a general indication of the partitioning of chromium (VI) in the environment relative to that of total chromium and/or chromium (III) for several environmental compartments. 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 (VI) exists mainly as highly soluble oxoanions in the environment and is expected to be mobile in soils and sediments. The adsorption of chromium (VI) is pH dependent. Under alkaline conditions, chromium (VI) is not readily sorbed and remains highly mobile. In acidic oxidised sediments with a high content of iron and manganese oxides or clay minerals, chromium (VI) should be adsorbed more strongly onto the sediment as the higher net positive charge present in acidic sediment should provide more or stronger sites for adsorption of the chromium (VI) anions. The adsorption is thought to occur with the mineral fraction, especially those with exposed hydroxyl groups on their surface such as iron and aluminium oxides and montmorillonite. Decreasing pH results in increasing protonation of the mineral surface and hence increasing adsorption of the chromium (VI)-containing anions. However, other anions present in natural systems such as SO₄^2-can also compete with the adsorption of chromium (VI), resulting in lower adsorption of chromium (VI) than might be expected. In the environment, iron oxides are the primary site of adsorption for chromium (VI) in acidic to neutral soils, with some contribution also from minerals with aluminium-OH groups.

 

Summary of measured partition coefficients (Kp) for chromium

Phase	Kp (l/kg)			Comments	Reference
	Total Cr	Cr(VI)	Cr(III)
Suspended sediment partition coefficients
Kpsusp		250-50,000	25,000-800,000	Freshwater and saltwater  Netherlands estimate	Braunschweiler et al. (1996)
	126,000-786,000 median 290,000			Freshwater, based on routine water quality data from The Netherlands	Van Der Kooij et al.(1991)
	30,100-1,059,600; mean 322,400			Freshwater River suspended sediments, United States - based on monitoring data	Young et al. (1987)
	324,000			Saltwater (0.1-0.5‰), based on routine water quality data from The Netherlands	Van Der Kooij et al.(1991)
	306,000-320,000			Saltwater (1-5‰), based on routine water quality data from The Netherlands
	228,000			Saltwater (>10‰), based on routine water quality data from The Netherlands
	140,000-570,000			Freshwater (Rhine-Meuse delta)	Golimowski et al. (1990)
	29,200-200,000			Saltwater (Tyrrhenian Sea), silty-clay sediment, organic carbon content 1.6%, salinity 3.8‰, pH 8.2-8.3.	Ciceri et al. (1992)
Sediment-water partition coefficients
Kpsed		940a	34,000	Saltwater, organic matter content ~2%, pH 7.8-8.0	Wang et al. (1997)
		2,300a		Saltwater, organic matter content ~10%, pH 7.8-8.0
			25,600-32,800	Freshwater, pH 8.3, organic carbon content 2.65%	Young et al. (1987)
	60-44,800; mean 7,100			Freshwater River sediments, United States -based on monitoring data
			11,000	Freshwater, pH=4.5	Young et al. (1992)
			120,000	Freshwater, pH >6
Soil-water partition coefficients
Kpsoil	524-24,217			Dutch field soils, pH~3.8-7.9; 2-21.8% organic matter	Janssen et al. (1997)
		13-40	298-788	Loam; pH 6; 1.92% organic carbon	Hassan and Garrison (1996)
		1-6.1	2,823-15,382	Loam; pH >6; 1.92% organic carbon
		45.4-52.3		Loess; pH6; 0.11% organic carbon
		1.5-12.1	19,716-55,918	Loess; pH>6; 0.11% organic carbon
		17-44.6	330-27,151	Clay; pH7; 3.75% organic carbon
		1.4-2.0		Clay; pH>7; 3.75% organic carbon
		0.35-17.4b	12-27	Sand, pH 4-8	Pérez et al. (1988)
			21-197	Sandy soil, pH 4-8, 0.77% organic matter
		6.6-18.4b	116-608	Sandy loam, pH 4-8, 1.62% organic matter

Note: a) reduction to chromium

III) occurred in these sediments

b) variation with pH not determined

Overall, chromium (VI) anions can be considered to be mobile in sediments in the environment, except possibly under highly acidic conditions.

 

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.

Based on the available measured values for the adsorption coefficients the values indicated below will be used in the risk assessment. These values are not taken directly from specific tests, but have been chosen by the Rapporteur to be representative for acidic-neutral and neutral-alkaline environments. The values do not correspond to any specific individual test results, nor are they derived statistically from the available data (since these are insufficient to allow meaningful values to be derived). Instead they were selected by inspection of the data to reflect the available information under the two sets of conditions and to reflect the differences between these. Acid-neutral environments are considered to be those at pH 5 and below; neutral-alkaline environments are taken to be those at pH 6 and above. For chromium (VI), the choice of a reliable value, particularly for suspended sediment and sediment, is difficult as reduction to chromium (III) (resulting in enhanced adsorption) cannot be ruled out in most of the available data. The values chosen for Kpsuspand Kpsedare therefore the best estimate that can be made assuming that the adsorption of chromium (VI) is substantially less than that seen for chromium (III) and that the adsorption is higher under acidic conditions than alkaline conditions. There are much better data available for the values of Kpsoilfor chromium (VI), allowing more reliable values to be chosen:

 

 

Chromium (VI)                     Acid conditions                         Alkaline conditions

                                              Kpsusp= 2,000 l/kg                   Kpsusp= 200 l/kg

                                              Kpsed= 1,000 l/kg                    Kpsed= 100 l/kg

                                              Kpsoil= 50 l/kg                         Kpsoil= 2 l/kg

 

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 (VI)                     Acid conditions                         Alkaline conditions

                                              Ksusp-water= 500 m³/m³        Ksusp-water= 50 m³/m³

                                              Ksed-water= 500 m³/m³          Ksed-water= 50 m³/m³

                                              Ksoil-water= 75 m³/m³            Ksoil-water= 3.2 m³/m³

 

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

 

In general, Chromium (III) is more strongly absorbed than Chromium (VI). Adsorption for both Chromium (III) and Chromium (VI) is pH dependant. Adsorption of Chromium (III) is stronger in alkaline conditions whereas conversely adsorption of Chromium (VI) is stronger in acidic conditions.