The biogeochemical coupling of Cd and Fe cycles in agricultural soils under varying redox and geochemical conditions

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Cadmium (Cd) is present in agricultural soils through the use of Cd-containing phosphate-fertilizers. Consumption of Cd leads to health issues such as renal failure and prostate, liver and kidney cancer. It is therefore important to understand the biogeochemical behavior of Cd, so that the mobility and fate of Cd in agricultural soils can be better predicted and controlled. One way in which this goal can be achieved is by closely examining the coupling of Cd with the iron (Fe) biogeochemical cycle. Fe is the fourth most abundant element in the Earth’s crust, and forms a variety of high-surface area Fe minerals, sorbing and incorporating heavy metals such as Cd. Fe is redox-active, alternating between the Fe(II) and Fe(III) oxidation states. Changes in oxidation state, via microbial metabolism or geochemical reactions, may induce mineral transformation and dissolution, causing the release of Cd that is sorbed to the mineral surface or incorporated in the lattice structure. Previous work from our group has shown that after microbial Fe(III) reduction and dissolution of Cd-bearing abiogenic ferrihydrite, Cd was released into solution and eventually sequestered by secondary minerals. However, crop cultivation methods often lead to redox cycling in agricultural soils (potentially leading also to Fe(II) oxidation and Fe(III) mineral formation). The biogeochemical reactions that occur during redox cycling undoubtedly influence the interaction of Cd with Fe minerals. The influence of redox cycling conditions on the fate of Cd is unknown. Therefore, in the present work we are proposing to rigorously examine the fate of Cd in agricultural soils in relation to microbial Fe metabolism under a variety of redox and geochemical conditions. In addition to redox cycling, the presence of organic matter and phosphate will be varied. Both of these constituents are expected to compete with Cd for sorption sites on Fe mineral surfaces but also have the possibility to form complexes with Cd either in solution or at the mineral surface. By analyzing the mobility of Cd in agricultural soils under many different biogeochemical and redox conditions, the most favorable conditions for forming and maintaining stable Cd-Fe mineral phases will be identified. Additionally, biogeochemical conditions that cause Cd mobilization can be better understood and controlled at Cd-contaminated sites.