Bioavailability of cadmium-organic complexes to soil alga--an exception to the free ion model

It is generally considered that cadmium bioavailability shows a considerable dependence on chemical speciation of Cd in solution, correlates best with the activity of free metal ion (Cd2+) in solution, and is largely indifferent to soluble metal complexes. The role of soluble organic matter (DOM) and soluble metal-organic complexes in metal bioavailability and toxicity, however, is not clear. Growth studies with a soil alga (Chlorococcum sp.) were conducted on a growth medium and pore water of Cookes Plain soil (Paleuxeralf), spiked with Cd as Cd(NO3)2. Speciation of the Cd in pore water, and in growth medium with and without citrate, was performed using the MINTEQA2 computer model incorporating updated values of the stability constants of Cd-DOM complexes, as well as using anode stripping voltammetry. Analysis of the toxicity data showed that Cd-citrate, as well as the Cd-DOM complexes, is bioavailable and contributes toward the toxicity to alga. These data contradict the long-held notion that Cd-DOM complexes are not bioavailable to soil biota although they may increase the mobility of Cd.