 The biogeochemistry of cadmium and its release to the environment has been the subject of review , as has the speciation of cadmium in the environment . 
 The biogeochemistry of cadmium and its release to the environment <a href='uninflect' title='1'><mark>has</mark></a> been the subject of review , as <a href='uninflect' title='2'><mark>has</mark></a> the speciation of cadmium in the environment . 
The biogeochemistry of cadmium and its release to the environment have been the subject of review , as have the speciation of cadmium in the environment .