Chromium trioxide

Adsorption / desorption

Link to relevant study record(s)

Reference

Endpoint:

adsorption / desorption, other

Remarks:

measurement

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

Please refer to Read across statement section 13

Reason / purpose for cross-reference:

read-across source

Sample No.:

#1

Type:

Kd

Value:

831 L/kg

Matrix:

loam

% Org. carbon:

1.92

Sample No.:

#2

Type:

Kd

Value:

2 625 L/kg

Matrix:

Sild

% Org. carbon:

0.11

Sample No.:

#3

Type:

Kd

Value:

5 283 L/kg

Matrix:

clay

% Org. carbon:

3.75

Transformation products:

yes

No.:

#1

Effect of equilibration time on Cr(III) adsorption and oxidation to Cr(VI)

Effect of pH on the adsorption profile of Cr(lll)

Effect of Eh on the adsorption of Cr(III),

Effect of ionic strength on the adsorption of Cr(Ill)

Effect of TOC - adsorption isotherms

please refer to the attached publication for detailed description

Conclusions

(1) Cr(III) reacts rapidly with soil solids. Within 6 hours of the addition of about 0.5 mg to a soil-water slurry (0.15 g of soil), no Cr(III) was detectable in any of the three EPA soil porewaters. It is removed by precipitation as various Cr(III) species or by sorption to various soil components.

(2) The lag time for detection of hexavalent chromium in porewaters formed by oxidation of Cr(III) depends on the organic carbon content of the soil. At low TOC, immediate formation of Cr(VI) was observed. Soils of medium to high TOC began to show Cr(VI) in their porewaters within 2 hours of addition. This indicates that higher levels of carbon may block mineral sites that oxidize Cr(III).

(3) Cr(III) solutions in water without soil showed no Cr(VI) formation, even after shaking for 168 hours. This excludes the possibility of direct oxidation by atmospheric oxygen. In addition, because Cr(III) adsorbs rapidly to the solid phase, the adsorbed Cr(III) forms, not the soluble forms, must be the species oxidized to Cr(VI), and the oxidants must be part of the soil solids.

( 4) The concentration of soluble Mn(II) ions in certain soil porewaters is elevated after addition of Cr(III), which indicates that Mn(VI) in soil minerals is responsible for oxidation of Cr(III) after its adsorption. The porewater Eh of soils containing Cr(III) gradually decreases due to reduction of those strong oxidants present in the soil solids. The pH of soil porewaters increases between 1 and 2 units in 1 week's time after the addition of Cr(III).

(5) The yield of Cr(VI) formed by oxidation of added Cr(III) can reach significant levels - after 6 hours about 2% of the added Cr appeared as Cr(VI).

(6) Different equilibration times and initial Cr(III) concentrations result in different total chromium Kd values. At early stages of mixing Cr(III) with the soil porewater slurries, adsorption reactions are not at

equilibrium; low Kd values are observed. Later, the Kd reaches a maximum value. For example, EPA no. 2 soil (#1) exhibited a maximum Kd after 6 hours of equilibration, whereas the Kd of EPA no. 9 (#2) reached a maximum after 24 hours.

(7) Cr(III) concentrations in soil porewaters increase as pH decreases, whereas Cr(VI) concentrations increase as pH increases; this is in accord with the cationic and anionic nature, respectively, of these species and the charge on the soil organic carbon, which is a function of pH. The pH influences the electrostatic interactions that serve to remove Cr from solution.

(8) The sorption of Cr(III) decreases with increased porewater concentration of cations, which compete for the adsorption sites on the soils. Increased porewater concentration of anions, however, has very slight effect on the sorption of Cr(VI) anions.

(9) The magnitude of the Kd values measured at a constant equilibration time and under the natural conditions of the soil-water slurries are much lower for Cr(VI), ranging from less than 1 to about 50 L/kg for the 3 soils studied. For Cr(III), the Kd values range from about 850 to 5,600 L/kg.The TOC concentration of the soil has an important effect on the Kd values of both Cr(III) and Cr(VI).

Validity criteria fulfilled:

not specified

Conclusions:

Conclusions
(1) Cr(III) reacts rapidly with soil solids. Within 6 hours of the addition of about 0.5 mg to a soil-water slurry (0.15 g of soil), no Cr(III) was detectable in any of the three EPA soil porewaters. It is removed by precipitation as various Cr(III) species or by sorption to various soil components.
(2) The lag time for detection of hexavalent chromium in porewaters formed by oxidation of Cr(III) depends on the organic carbon content of the soil. At low TOC, immediate formation of Cr(VI) was observed. Soils of medium to high TOC began to show Cr(VI) in their porewaters within 2 hours of addition. This indicates that higher levels of carbon may block mineral sites that oxidize Cr(III).
(3) Cr(III) solutions in water without soil showed no Cr(VI) formation, even after shaking for 168 hours. This excludes the possibility of direct oxidation by atmospheric oxygen. In addition, because Cr(III) adsorbs rapidly to the solid phase, the adsorbed Cr(III) forms, not the soluble forms, must be the species oxidized to Cr(VI), and the oxidants must be part of the soil solids.
( 4) The concentration of soluble Mn(II) ions in certain soil porewaters is elevated after addition of Cr(III), which indicates that Mn(VI) in soil minerals is responsible for oxidation of Cr(III) after its adsorption. The porewater Eh of soils containing Cr(III) gradually decreases due to reduction of those strong oxidants present in the soil solids. The pH of soil porewaters increases between 1 and 2 units in 1 week's time after the addition of Cr(III).
(5) The yield of Cr(VI) formed by oxidation of added Cr(III) can reach significant levels - after 6 hours about 2% of the added Cr appeared as Cr(VI).
(6) Different equilibration times and initial Cr(III) concentrations result in different total chromium Kd values. At early stages of mixing Cr(III) with the soil porewater slurries, adsorption reactions are not at
equilibrium; low Kd values are observed. Later, the Kd reaches a maximum value. For example, EPA no. 2 soil (#1) exhibited a maximum Kd after 6 hours of equilibration, whereas the Kd of EPA no. 9 (#2) reached a maximum after 24 hours.
(7) Cr(III) concentrations in soil porewaters increase as pH decreases, whereas Cr(VI) concentrations increase as pH increases; this is in accord with the cationic and anionic nature, respectively, of these species and the charge on the soil organic carbon, which is a function of pH. The pH influences the electrostatic interactions that serve to remove Cr from solution.
(8) The sorption of Cr(III) decreases with increased porewater concentration of cations, which compete for the adsorption sites on the soils. Increased porewater concentration of anions, however, has very slight effect on the sorption of Cr(VI) anions.
(9) The magnitude of the Kd values measured at a constant equilibration time and under the natural conditions of the soil-water slurries are much lower for Cr(VI), ranging from less than 1 to about 50 L/kg for the 3 soils studied. For Cr(III), the Kd values range from about 850 to 5,600 L/kg.The TOC concentration of the soil has an important effect on the Kd values of both Cr(III) and Cr(VI).

Executive summary:

The distribution of chromium species between soil and porewater was assessed by Hassan and Garrison in 1996 and the study was published in Chemical Specification and Bioavailability.

Batch experiments were carried out using three soils representing different geologic parts of the continental USA to identify processes influencing partitioning and interconversion of chromium species in soil porewaters. The partition coefficient (Kd) of each of the two main species, Cr(lll) and Cr(VI), was calculated for each soil using adsorption isotherms measured after an equilibration time of 6 hours for the former species and 48 hours for the latter at the natural conditions of the soils. Cr(VI) in porewater was measured directly by ion chromatography, while total chromium was measured by inductively coupled plasma- atomic emission spectrophotometry (ICP-AES). Total chromium in the soil solids was measured by ICP-AES after microwave assisted digestion. The magnitude of the Kd values are much lower for Cr(VI), ranging from less than 1 to about 50 L/kg. For Cr(III), the Kd values range from about 850 to 5,600 L/kg. The effects of the initial porewater concentration of the chromium species, the equilibration time, and the pH, Eh and ion concentration of the porewater on the Kd values were investigated; the results are discussed relative to the differences in physical and chemical properties of the soils. Cr(III) concentration in porewater increases as pH decreases, but Cr(VI) increases as pH increases. The sorption of Cr(III) decreases with increased porewater concentration of cations, but increased concentration of common anions has

slight effect on the sorption of Cr(VI) anions. The Kd values of both Cr(III) and Cr(VI) depend on equilibration time because of the kinetics of interconversion of species upon reaction with the soil and the resultant change in total chromium in the porewater with time. Cr(III) added to porewater reacts rapidly with the soil through adsorption and/or precipitation; some is then oxidized by Mn(IV)-containing minerals within a few hours, liberating both Cr(VI) and Mn(ll) into the porewater. The yield of Cr(VI) can reach significant levels relative to the 100 µg/L drinking water standard, and the Cr(VI) disappears from porewater at a very slow rate compared to the disappearance of Cr(III).

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.

The distribution of chromium species between soil and porewater was assessed by Hassan and Garrison in 1996 and the study was published in chemical Specification and Bioavailability.

Batch experiments were carried out using three soils representing different geologic parts of the continental USA to identify processes influencing partitioning and interconversion of chromium species in soil porewaters. The partition coefficient (Kd) of each of the two main species, Cr(III) and Cr(VI), was calculated for each soil using adsorption isotherms measured after an equilibration time of 6 hours for the former species and 48 hours for the latter at the natural condition of the soils. cr/VI) in porewater was measured directly by ion chromatography, while total chromium was measured by inductively coupled plasma-atomic emission spectrophotometry (ICP-AES). Total chromium in the soil solids was measured by ICP-AES after microwave assisted digestion. The magnitude of the Kd values are much lower for Cr(VI), ranging from less than 1 to about 50 L/kg. For Cr(III), the Kd values range from about 850 to 5,600 L/kg. The effects of the initial porewater concentration of the chromium species, the equilibration time and the pH, Eh and ion concentration of the porewater on the Kd values were investgated; the results are discussed relative to the differences in physical and chemical properties of the soils. Cr(III) concentration in porewater increases as pH decreases, but Cr(VI) increases as pH increases. The sorption of Cr(III) decreases with increased porewater concentration of cations, but increased concentration of common anions has slight effect on the sorption of Cr(VI) anions. The Kd values of both Cr(III) and Cr(VI) depend on equilibration time because of the kinetics of interconversion of species upon reaction with the soil and the result change in total chromium in the porewater with time. Cr(III) added to porewater reacts rapidly with the soil through adsorption and/or precipitation; some is then oxidized by Mn(IV)-containing minerals within a few hours, liberating both Cr(VI) and Mn(II) into the porewater. The yield of Cr(VI) can reach significant levels relative to the 100 µg/L drinking water standard, and the Cr(VI) disappears from porewater at a very slow rate compared to the disappearance of Cr(III).

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. These values are presented below.

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 Kpsusp and Kpsed are 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 Kpsoil for 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 Chromum (VI) is stronger in acidic conditions.