Cobalt(2+) hydrogen citrate

Bioaccumulation: aquatic / sediment

Description of key information

BCF/BAF values in the range of 7.4 to 3110 L/kg were reported for cobalt (mean 878, median 720). Further data demonstrates that cobalt, like other essential elements, shows homeostatic control by organisms.

Additional information

No data for aquatic bioaccumulation are available for cobalt hydrogen citrate. However, there are reliable data available for different analogue substances and for cobalt measured as element in field investigations, respectively.

The environmental fate pathways and ecotoxicity effects assessments for cobalt metal and cobalt compounds is based on the observation that adverse effects to aquatic, soil- and sediment-dwelling organisms are a consequence of exposure to the bioavailable cobalt ion , released by the parent compound. The result of this assumption is that the ecotoxicology will be similar for all soluble cobalt substances used in the ecotoxicity tests. Therefore, data from soluble cobalt substances are used for the derivation of ecotoxicological and environmental fate endpoints, based on the cobalt ion. With respect to these considerations, data collected on elemental cobalt (e.g. environmental concentrations for Co2+) can also be taken into account.

Information taken from Environment Canada (2011):

Considering all aquatic data, 31 acceptable bioaccumulation factors were reported for various species of algae, invertebrates, fish, and zooplankton. These values ranged from 7.4 to 3110 L/kg, with a mean value of 878 L/kg and a median value of 720 L/kg. Five biota-to-sediment accumulation factors (BSAF-sed.) were reported.BSAF-sed values ranged from 0.091 to 0.645, with a mean value of 0.232 and a median value of 0.138 (Environment Canada, 2011).

If marine and freshwater data are pooled, then for aquatic invertebrates, 16 BCF and BAF values were obtained, ranging from 21.8 to 2280 L/kg with an average value of 724 L/kg and a median value of 441 L/kg (wet weight). In comparison, values for fish (n=11) ranged from 7.4 to 3110 L/kg, with an average value of 1010 L/kg and a median value of 849 L/kg. Many studies have noted that homeostatic mechanisms likely exist to regulate cobalt accumulation, due to the fact that it is an essential element (Environment Canada, 2011).

One study, done by Norwood et al. (2006), was unique in its use of a mechanistically-based saturation model for the bioaccumulation of cobalt. The test organism was the freshwater amphipod Hyalella azteca. The wet-weight BCF was calculated according to the equation: BCF = (max)(DW-1)1000K-1, where max is the maximum above-background accumulation of the metal in the organism, measured in nmol/g, DW-1 is the mean dry-to-wet weight ratio for the organism, and K is the half saturation constant (i.e. the metal concentration in the water at which the concentration in the organism is halfway between the maximum and the background accumulations), measured in nmol/L. So, it is seen that this model estimates a BCF based on background-corrected metal accumulation at low aqueous concentrations; thus, unlike with other approaches, background contaminant concentrations will not dictate the BCF values observed. In this case, the wet-weight BCF for Hyalella azteca was found to be 515 L/kg.

In the study carried out by El-Shenawy (2004), metal concentrations were measured in the bivalve Ruditapes decussatus, and in surrounding waters at two different contaminated sites for the calculation of BAFs ranging from 227.1 to 365.7. A lower BAF was observed at the site with a higher ambient cobalt concentration. This inverse relationship between BAF and ambient cobalt concentration provides evidence for the existence of regulation mechanisms in this invertebrate, as previously explained.

Several of the studies used field observations to calculate relevant values. While these data are environmentally realistic, the presence of multiple contaminants, especially other metals, likely influenced the BAF values observed for cobalt. Along these lines, one laboratory experiment by Fraysse et al. (2002) investigated the effect of the presence of cadmium and/or zinc on cobalt accumulation. Two species of freshwater bivalves (Dreissena polymorpha and Corbicula fluminea) were exposed to either cobalt alone, cobalt plus cadmium, cobalt plus zinc, or cobalt plus cadmium and zinc. ForD. polymorpha, a BCF of 1100 was determined for whole body wet weight (17 for whole soft body), while for C. fluminea, a BCF of 530 was reported for whole body wet weight (10 for whole soft body). In the end, maximum concentration factors were observed when organisms were exposed to cobalt alone; and, the addition of zinc alone had the greatest inhibitory effect on cobalt uptake (though cadmium and cadmium plus zinc treatments also had an inhibitory affect). Thus, it is important to consider both polymetallic field exposures and controlled laboratory exposures when evaluating cobalt accumulation data.

Biomagnification

Additionally, a study done by Baudin and Fritsch (1989) is referenced. Here, when the carp Cyprinus carpio received cobalt from contaminated food (the mollusc Lymnea stagnalis), the biomagnification factor was reported to have been in the order of 10-2 (though the actual value was not reported). Additionally, it was concluded that water is the dominant pathway for cobalt uptake, and that accumulation from food and water is additive.

Ikemoto et al. (2008) considered the freshwater food web of the Mekong Delta in South Vietnam, examining phytoplankton, snails, five species of crustaceans, and fifteen species of fish. A TMF (trophic magnification factor) for cobalt of 0.95 resulted, but again there was no statistical significance (r2=0.013, p=0.506). Thus the results showed no biomagnification or biodilution of cobalt through the food chain.

There are several lines of evidence to suggest that the bioaccumulation potential of cobalt in natural ecosystems is relatively low. First of all, low BAFs have been reported in eight laboratory (steady state) studies and four field studies; five BSAF-sediment values have been found to be well below 1; and, four (out of four) average BSAF-soil values have been reported to be well below 1. In addition, results from six field investigations plus two laboratory studies indicate the absence of biomagnification of cobalt in natural food webs. Finally, cobalt is an essential micro-nutrient, the uptake of which is expected to be regulated to some extent by many organisms (Environment Canada, 2011).

References:

Environment Canada. Health Canada (2011). Screening Assessment for the Challenge. Cobalt, cobalt chloride, cobalt sulfate.