Cadmium cycle



The cadmium cycle is a biogeochemical cycle of dispersion and deposition of cadmium through the atmosphere, biosphere, pedosphere, and hydrosphere. Cadmium typically exists in the environment with an oxidation state of +2 but can be found with an oxidation state of +1 (though quite uncommon).

Terrestrial cycling
The majority of cadmium deposition to soils and freshwater is due to anthropogenic atmospheric emissions, contaminants in biosolids, and contaminants in fertilizers. Dry deposition accounts for 30-70% of terrestrial inputs. Cadmium is highly mobile in soils and becomes mineral-associated over time. Higher pH and temperature favor cadmium incorporation into minerals, while lower pH and temperature makes cadmium more soluble. Dissolved cadmium circulates through freshwater systems before introduction to larger bodies of water. In rivers, dissolved cadmium ranges from nanomolar to micromolar concentrations.

Oceanic cycling
The vast majority of marine cadmium (80-90%) comes from wet deposition. Cadmium behaves similarly to nutrients such as phosphate and zinc: dissolved concentrations depend heavily on uptake, assimilation, and deposition by phytoplankton and diatoms. Dissolved cadmium concentrations are sub-nanomolar in the surface ocean and increase with depth, with a maximum in the thermocline. Like other nutrients, cadmium is lowest in the North Atlantic (~0.3 nM). Higher concentrations (up to 1 nM) occur in the deep Indian, Southern, and Pacific oceans due to water mass aging during thermohaline circulation. Coastal waters range from 0.2 to 0.9 nM, denoting a significant terrestrial input.