Submitted to: Biometals
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/10/2004
Publication Date: 10/15/2004
Citation: Chaney, R.L., Reeves, P.G., Ryan, J.A., Simmons, R.W., Welch, R.M., Angle, J.S. 2004. An improved understanding of soil cd risk to humans and low cost methods to remediate soil cd risks. Biometals. 17(5):549-553. Interpretive Summary: Food-chain transfer of soil Cd has been shown to comprise risk to subsistence rice farmers who grow their crops on flooded soils contaminated by Zn-Pb mine and smelter wastes, but other cropping patterns have not caused this adverse effect. Over the last decade we have reported research which clarifies why this one cropping system allowed soil Cd to reach excessive levels in consumers. The manuscript summarizes the overall research findings. 1) when rice is grown in flooded soils rich in Zn mine wastes, grain Cd is substantially increased but grain Zn is not increased; other crops transport high levels of Zn into edible crop tissues; 2) diets of subsistence rice farmers may be deficient in Fe, Zn and Ca because polished rice grain is deficient in these nutrients; deficiencies of these elements cause animals to absorb a much higher fraction of dietary Cd than when these nutrients are adequate; we have found that nearly all dietary Cd can be absorbed into the duodenum of animals deficient in Fe, Zn and Ca, and is only slowly excreted or absorbed into the body; this intestinal turnover Cd pool of animals with deficient Fe, Zn and Ca can increase risk of dietary Cd by at least 10-fold compared to individuals with adequate Fe, Zn and Ca. 3) Prevention of Cd disease in subsistence rice farmers may be prevented by providing supplements of Fe, Zn and Ca so that they absorb only normal fractions of dietary Cd; a field test of this hypothesis is needed. Alternatively, phytoextraction of soil Cd using the southern France strains of Thlaspi caerulescens (accumulate up to 5 kg Cd/ha-year, 100-fold greater than other crops proposed for phytoextraction) can remove soil Cd and allow production of rice with lower Cd. Improvement of the density of bioavailable Fe and Zn in rice grain could aid in reducing risks from soil Cd, and improve diets of subsistence rice consumers, perhaps reducing adverse public health effects of these diets.
Technical Abstract: We have described a new paradigm for human risk from soil Cd that reflects many years of agronomic, nutritional and toxicological research. This new model for soil Cd risk reflects the ability of rice to accumulate soil Cd in grain while excluding Fe, Zn and Ca even though the soil contains 100-times more Zn than Cd. Further, polished rice grain for human consumption is deficient in Fe, Zn and Ca for humans, which promotes Cd absorption into duodenal cells. New kinetic studies clarified that dietary Cd was accumulated in the duodenum enterocytes; 109Cd from a single meal remained in the duodenum for up to 16 days; part of the turnover pool 109Cd moved to the liver and kidneys by the end of the 64-day 'chase' period. Thus malnutrition induced by subsistence rice diets caused a higher absorption of dietary Cd and much higher risk from soil Cd than other crops. Because rice-induced Fe-Zn-Ca-malnutrition is so important in soil Cd risk, it seems evident that providing nutritional supplements to populations of exposed subsistence rice farmers could protect them against the Cd during a period of soil remediation. In the long term, high Cd rice soils need to be remediated. The process of removal and replacement of contaminated soil is very expensive (on the order of $3 million/ha); while phytoextraction using the high Cd accumulating ecotypes of the Zn-Cd hyperaccumulator, Thlaspi caerulescens, should provide low cost soil Cd remediation.