Cobalt molybdate

Toxicity to soil microorganisms

Additional information

No data on toxicity to soil microorganisms are available for cobalt molybdenum oxide. However, there are reliable data available for different structurally analogue substances.

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 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 in the derivation of ecotoxicological and environmental fate endpoints, based on the cobalt ion and molybdenum ion, respectively.

Cobalt

Data regarding the toxicity of cobalt to soil microorganisms are limited. The WHO CICAD (2006) reports on a study on the effects of several metals, including cobalt, at single concentrations on respiration of native soil microflora in soil/litter microcosms. A 1362 mg/L solution of cobalt mixed into the soil and litter in the microcosm resulted in a reduction in respiration of 23% after 20 d.

References: World Health Organization (2006). Concise International Chemical Assessment Document 69. COBALT AND INORGANIC COBALT COMPOUNDS.

Molybdenum

A total of 10 topsoils with contrasting properties that may affect the toxicity of Mo in soil were collected and on each of these soils a series of 3 microbial assays (nitrification, glucose induced respiration and mineralisation of plant residues) were performed after spiking with sodium molybdate. The microbial toxicity assays resulted in 18 individual high quality EC10 selected for the assessment. The EC10 values ranged from 10 to 3840 mg added Mo/kg dry weight soil. Additionally, 3 soils were aged outdoors after spiking with sodium molybdate. After 6 and 11 months, subsamples were collected and the same ecotoxicity tests as above were conducted on these soils. In general, this showed that long-term equilibration of Mo in soils decreases its toxicity to micro-organisms. The results of the toxicity in aged soils are used to determine a Leaching/Ageing factor to correct for the effect of spiking on the toxicity of Mo in soil organisms.

For the microbial assays a total of 18 individual high quality EC10 values (for 3 different microbial assays) are selected. These values range from 10 mg added Mo/kg dw (for the glucose induced respiration assay in soil 3) to 3840 mg added Mo/kg dw (for the substrate induced nitrification assay in soil 10). All data are based on added nominal Mo concentrations in soil. A dose confirmation showed an average recovery of 103% for the spiked soils for the nitrification assay and 98% for both the glucose induced respiration and plant residue mineralisation assays.

The substrate induced nitrification, glucose induced respiration and plant residue mineralisation assays yielded reliable, bounded NOEC values for 8, 6 and 4 soils out of 10, respectively. In all the other soils, no toxic effect was observed at the highest dose tested (10000 mg added Mo/kg dw).

For the 3 aged soils (soil 4,5 and 6), the comparison of Mo in freshly spiked and 11 -month aged soils show that long-term equilibration of Mo in soil generally decreases its toxicity to micro-organisms. No toxicity was observed in 2 of the 3 aged soils at the highest concentrations left in these soils. In one soil (soil 6), toxicity was still observed in the 11 -month aged soil for the glucose induced respiration and plant residue mineralisation assays.

The results of the toxicity in aged soils is used to determine a leaching/ageing factor to correct for the effect of spiking on the toxicity of Mo in soil organisms.

Author, year	Endpoint	Value [mg Mo/kg soil dw]
Smolders and Buekers, 2009	28 d EC10	35 - > 10000
Smolders and Buekers, 2009	24 h EC10	10 - > 10000
Smolders and Buekers, 2009	28 d EC10	164 - > 10000