Cobalt molybdate

Endpoint summary

Additional information

No data for bioaccumulation are available for cobalt molybdenum oxide. However, there are reliable data available for different structurally analogue test substances and for cobalt and molybdenum measured as element in field investigations, respectively.

The environmental fate pathways and ecotoxicity effects assessments for cobalt metal and cobalt compounds as well as for molybdenum metal and molybdenum 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 and molybdenum ion, released by the parent compound. The result of this assumption is that the ecotoxicology will be similar for all soluble cobalt and molybdenum substances used in the ecotoxicity tests. Therefore, data from soluble cobalt and molybdenum substances are used for the derivation of ecotoxicological and environmental fate endpoints, based on the cobalt ion and molybdenum ion, respectively. With respect to these considerations, data collected on elemental cobalt (e.g. environmental concentrations for Co2+) and molybdenum (e.g. environmental concentrations for Mo2 +) can also be taken into account.

Cobalt

Information taken from Environment Canada (2011):

Cobalt is essential in small amounts for nitrogen fixation by bacteria, blue-green algae, and symbiotic systems such as those in the root of leguminous plants. It is also an essential micro-nutrient element for animals and is required for the formation of vitamin B12 and for its participation in enzymatic processes (Environment Canada, 2011).

Bioaccumulation of metals - like that of organic substances - is of potential concern because of the possibility of chronic toxicity to the organisms accumulating these substances in their tissues and the possibility of toxicity to predators eating these organisms. Bioaccumulation potential is typically quantified by determining either a bioaccumulation factor (BAF), or a bioconcentration factor (BCF). However, these ratios are currently the object of criticism when applied to metals because they are considered of little usefulness in predicting metal hazards. For example, some metals may naturally be highly accumulated from the surrounding medium because of their nutritional essentiality. Furthermore, both essential and non-essential metals may be regulated within relatively narrow margins by the homeostatic and detoxification mechanisms that many organisms possess. It follows that when ambient concentrations of metals are low, BCFs and BAFs are often elevated. Conversely, when ambient metal concentrations are high, BCFs and BAFs tend to decrease. Thus, inverse relationships may be observed between BCF and BAF values and metal exposure concentrations, and this complicates the interpretation of BCF/BAF values. Natural background concentrations in organisms may contribute to these negative trends. In addition, inverse relationships can occur for non-essential elements as well because there are a finite number of binding sites for these metals within an organism that could become saturated at higher concentrations (Environment Canada, 2011).

 

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.

Molybdenum

Molybdenum is a natural element that is omnipresent in the environment.

Data on aquatic bioaccumulation that were retrieved, suggest that molybdenum bioconcentration through the food chain is negligible: whole body internal concentrations remain below 1 mg/kg at concentration levels up to several mg/L. Reported wholebody BAFs vary more than 2 orders of magnitude, but there is a distinct inverse relationship between exposure concentration and BAF, i. e., decreasing BAFs with increasing Mo levels in the water column. Of all the 27 BCF/BAF reported all below 100 with the exception of one BAF measured for a mollusc exposed to background Mo water concentrations (BAF of 164). The data demonstrates that Mo, like other essential elements, shows homeostatic control of Mo by these organisms. The homeostatic control of Mo is observed to continue to function up to the milligramme range of exposure. Limited information on transfer of Mo through the food chain indicates that molybdenum does not biomagnify in aquatic food chains.

For the terrestrial compartment, Mo concentration ranges in environmental matrices have been compiled. The data includes concentrations of Mo in the environmental compartments, excluding geogenic enriched areas, and at moderate levels below Mo concentrations causing a toxic effect.

This data suggests that Mo is not significantly concentrated from soil to plants, or soil invertebrates with bioconcentration factors (BCF) or bioaccumulation factors (BAF) of < 5, and that there is no further significant increase in concentration from diet to mammals or birds, even including organs such as kidney or liver (diet tissue concentration ratios <10 and even <1 for muscle tissue). This suggests that biomagnification of Mo, if any, is not significant in the terrestrial compartment and foodchain.

Eisler (1989) made a more exhaustive compilation of environmental concentrations of Mo. That review concluded equally that Mo concentrations in plants, mosses and wildlife tissues (liver and kidney included) are well below 10 mg/kg dry weight, excluding legumes (e.g. clover) that contain up to 28 mg/kg dry weight.