Sodium dichromate

Endpoint summary

Description of key information

Hydrolysis and biodegradation are not relevant for inorganic substances, including inorganic chromium (VI) substances.

Regarding the bioaccumulation of chromium (VI), based on a weight-of evidence approach applying studies on fish, crustacea and aquatic plants, the potential for bioaccumulation of Cr(VI) by aquatic and terrestrial organisms seems low. A similar conclusion was made in the EU RAR (ECB, 2005), i.e. the bioaccumulation factor for chromium (VI) in fish is relatively low at around 1 L/kg.

Regarding the partitioning of chromium (VI) in the environment, adsorption to soil seems to be rather low, i.e. Kp values (solids-water in soil) of 2 L/kg and 50 L/kg are applied in the EU RAR on chromates (ECB, 2005) for alkaline and acidic conditions, respectively and supported by available studies.

However, available data on the partitioning of Cr(VI) in sediments and suspended matter are scarce, which may be due to the Cr(VI) reduction observed in sediments. Accordingly, Bryne et al. (2017) analysed contaminated aquatic sediment samples from the metropolitan area of Manchester city (Salford Quays, UK). Speciation analysis of sediment cores confirmed Cr(III) as the dominating species, with the fraction of Cr(VI) species estimated to be <5%.

Values applied in the EU RAR on chromates (ECB, 2005) appear to be the most accurate estimates, assuming that the adsorption of Cr(VI) is substantially less when compared to Cr(III) and taking into account the high pH-dependency of Cr(VI) adsorption as follows:

Acidic conditions:

Kp(solids-water in sediment) = 1000 L/kg

Kp(solids-water in suspended matter) = 2000 L/kg

Alkaline conditions:

Kp(solids-water in sediment) = 100 L/kg

Kp(solids-water in suspended matter) = 200 L/kg

 

In sum, chromium (VI) exists mainly as dissolved oxoanion in the environment and is expected to be mobile in soils and sediments. Important aqueous- and solid-phase parameters controlling partitioning of Cr(VI) to solid phases include pH, redox, and the concentrations of iron, aluminium and manganese oxides and clay minerals and organic matter.

 

Additional information

Speciation of chromium(VI) in the environment

According to the EU risk assessment of hexavalent chromium substances and references therein (EU RAR, 2005), hexavalent chromium is a strong oxidising agent and as a result only exists as oxygenated species in solution. The major dissolved species of chromium (VI) are HCrO₄^-and CrO₄^2-. The relative proportion of these two species depends on the pH of the surrounding medium. In addition, these two species dimerise to form dichromate anions (e.g. HCr₂O₇^-or Cr₂O₇^2-) at higher chromate concentrations (e.g. >0.08 mol/l = 0.4 g Cr/l). Thus, in aqueous solution, chromate and dichromate anions exist in a chemical equilibrium.

The actual species present in solution depend on the pH of the surrounding medium according to the following equilibria:

H₂CrO₄⇌HCrO₄⁻+ H⁺       pKa₁= -0.6 – 0.8

HCrO₄⁻⇌CrO₄²⁻+ H⁺        pKa₂= 5.9 – 6.51

In addition to the above equilibria, chromate ions are in equilibrium with dichromate ions according to the following equilibria:

2 CrO₄²⁻+ 2 H⁺⇌Cr₂O₇²⁻+ H₂O

2 HCrO₄^-        ⇌Cr₂O₇²⁻+ H₂O  

Based on these equilibria, the fully protonated form (H₂CrO₄) and the dimer HCr₂O₇^-exist only at very low pH, i.e. near pH of 0 that is environmentally not relevant. At a pH between 2 and 6-6.5, the dominant chromate species in solutions are HCrO₄^-and Cr₂O₇^2-whereas CrO₄^2-is the main chromate species in solution at a pH ≥ 6-6.5.

In addition, there are also the following base-hydrolysis equilibria:

Cr₂O₇^2-+ OH^-  ⇌HCrO₄^-+ CrO₄^2-

HCrO₄^-+ OH^-   ⇌CrO₄^2- + H₂O

In sum, at a pH <6, HCrO₄^-and Cr₂O₇^2-are present in equilibrium, and at a higher pH (>7) the main species present is CrO₄^2-. At environmentally relevant pH, the species found in solution are a mixture of Cr₂O₇^2-, HCrO₄^-and CrO₄^2-, irrespective of the form in which the chromium (VI) enters the environmental solution.

The chromium (VI) species present in the environment are much more soluble than chromium (III) forms. The presence of barium ions in the environment could reduce the solubility of hexavalent chromium as the relatively insoluble barium salt can be formed. Other insoluble salts can be formed with a variety of cations, such as copper, lead, strontium and zinc. These salts have a wide range of solubilities, and the rate of precipitation/dissolution varies greatly and is pH dependent. Formation of such salts could limit the solubility of chromium (VI) in the environment. A significant proportion of total chromium in aquatic systems is associated with the solid phase. Reduction of chromium (VI) to chromium (III) may also occur to some extent, particularly where oxygen-deficient conditions exist. The most important naturally occurring reducing agents (in order of decreasing reducing strength) are organic substances, hydrogen sulphide, sulphur, iron sulphide, ammonium and nitrite. Other potential reducing agents include aqueous Fe²⁺ions and Fe(II)-containing minerals. Under anaerobic conditions, the reduction of hexavalent chromium in water by sulfide or Fe²⁺ions is fast, with reduction half-lives ranging from instantaneous to a few days. Organic material in sediments and soils, for instance, can reduce Cr(VI), with half lives ranging between 4 to 140 days and depending on type and quantity of organic matter and redox conditions. Chromium (VI) ions can bind with naturally occurring dissolved organic carbon DOC and oxidize organic ligands leading to the formation of esters.