Naturally selenium-rich rice grains avoid geogenic cadmium risk via high levels of soil inorganic sulfur: evidence from six seleniferous regions of China.

Naturally seleniferous paddy fields are threatened by geogenic cadmium (Cd), yet certain Cd-polluted paddy fields continue to yield selenium (Se)-rich rice grains being not polluted with Cd. Identifying the reasons for this situation will provide a feasible entry point for source-level Cd risk control under the high Se-Cd geogenic background. This study conducted a field sampling in six naturally seleniferous regions of China. Due to Se-Cd geogenic co-occurrence in soil, paddy fields exhibited 100% Se enrichment in soil, but 83% were simultaneously Cd-polluted. Among the rice grains, not only 61% of them were Se-enriched, but also 53% did not exhibit Cd pollution. This was related to soil Cd availability being significantly suppressed by high soil inorganic sulfur (I-S) concentrations via two mechanisms: primarily via enhancing Cd fixation by soil Fe oxides, whereas direct Cd precipitation played a minor and unstable role. Thus, when soil I-S concentrations were at a high level (above 60-70 mg/kg), compared to low soil I-S level condition, the daily Se supply from rice grains increased significantly, reaching the recommended range, while the carcinogenic risk and hazard index were significantly reduced by 50%, reaching the same safety level as commercial rice. These findings provided suggestions for the safe utilization of naturally Se-rich paddy fields under a geogenic Cd pollution background and also offered further research directions for risk control of geogenic Cd pollution.