Potassium hydroxyoctaoxodizincatedichromate(1-)

Adsorption / desorption

Description of key information

Adsorption of Chromium (III) is stronger in alkaline conditions.  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 avaiolable in the EU RAR for a number of soil and sediment types.

In general, Chromium (III) is more strongly absorbed than Chromium (VI). Adsorption is pH dependant. Adsorption of Chromium (III) is stronger in alkaline conditions.

Additional information

Discussion of adsorption coefficients for chromium

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.

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

Chromium (III) appears to be strongly adsorbed to soils and sediments. 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.

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