Redox-Controlled Changes in Cadmium Solubility and Solid-Phase Speciation in a Paddy Soil As Affected by Reducible Sulfate and Copper

Abstract

The solubility of Cd in contaminated paddy soils controls Cd uptake by rice, which is an important food safety issue. We investigated the solution and solid-phase dynamics of Cd in a paddy soil spiked with ∼20 mg kg(-1) Cd during 40 days of soil reduction followed by 28 days of soil reoxidation as a function of the amounts of sulfate available for microbial reduction and of Cu that competes with Cd for precipitation with biogenic sulfide. At an excess of sulfate over (Cd + Cu), dissolved Cd decreased during sulfate reduction and Cd was transformed into a poorly soluble phase identified as Cd-sulfide using Cd K-edge X-ray absorption spectroscopy (XAS). The extent of Cd-sulfide precipitation decreased with decreasing sulfate and increasing Cu contents, even if sulfate exceeded Cd. When both Cu and Cd exceeded sulfate, dissolved and mobilizable Cd remained elevated after 40 days of soil reduction. During soil reoxidation, Cd-sulfide was readily transformed back into more soluble species. Our data suggest that Cd-sulfide formation in flooded paddy soil may be limited when the amounts of Cd and other chalcophile metals significantly exceed reducible sulfate Therefore, in multimetal contaminated paddy soils with low sulfate contents, Cd may remain labile during soil flooding, which enhances the risk for Cd transfer into rice.

How Does Contamination of Rice Soils with Cd and Zn Cause High Incidence of Human Cd Disease in Subsistence Rice Farmers

“…Levels of Mn 2+ used in testing repression may not have been representative of reduced paddy soils. Further study of the interaction of Mn 2+ with Cd 2+ uptake by rice showed that soil solution levels of Mn 2+ in flooded soils strongly inhibited Cd uptake [55] extending the findings of Fulda et al [37] and Sasaki et al [54]. Further, in contrast with Cd absorption by other plant species, increased Zn 2+ had little effect on Cd absorption by NRAMP5.…”

“…Zn is either strongly adsorbed to soil sorbents or transformed to insoluble ZnS and Cd can precipitate as CdS [28,34], although it has been difficult to document the extent of CdS formation in flooded rice soils even using Extended XRay Absorption Fine Structure (EXAFS) [35][36][37]. Although early researchers who reported the very strong effect of soil reduction on Cd uptake suggested that this could be explained by sulfide formation in flooded soils and formation of CdS [28], EXAFS evidence of CdS was first reported by Khaokew et al [35] with highly contaminated Thai rice soils.…”

“…De Livera et al [36,38] argue that these results did not explain the reduced Cd uptake adequately and suggested that mixed solid Cd-ZnS nanoparticles of sulfides undergoing redox might play a role. Subsequently, Fulda et al [37] clearly showed that CdS was unequivocally formed in flooded Cd-spiked soils and that if sulfate was low compared to sulfide forming metals, or if Cu was high, CdS formation was appreciably inhibited and Cd uptake increased. Perhaps this finding explains the unusually high transfer of Cd to rice grain grown in soils contaminated by Cu mine wastes which caused human Cd disease at lower soil Cd than other locations [5,39].…”

“…Now it is clear that the main reason that rice accumulates Cd during grain filling is the role of NRAMP5 at the same time that soil oxidation with great lowering of soil solution Mn, and soil acidification increases Cd solubility in the rhizosphere. Fulda et al [37] reported that soluble Mn dropped remarkably within 2 days after their flooded soils with CdS were made aerobic, which would strongly reduce competition for Cd uptake on NRAMP5.…”

Sulfate reducing bioreactor dependence on organic substrates for remediation of coal-generated acid mine drainage: Field experiments

“…Previous studies have also reported that the spatial organization of S-cycling is more complicated than originally predicted, with SRB differentially distributed in microenvironments (Wieringa et al, 2000;Baumgartner et al, 2006;Fike et al, 2009). Lower pH values within the bioreactors also increased the mobility of sorbed (Fulda et al, 2013;Caraballo et al, 2013) and precipitated phases (Acero et al, 2006;Borch et al, 2009;Chirit¸ a and Schlegel, 2012;Burgos et al, 2012), thus contributing to decreased remediation efficiency.…”