Chromium (VI) in soil poses a significant threat to the environment and human health. Despite efforts to remediate Cr contaminated soil (Cr-soil), instances of re-yellowing have been observed over time. To understand the causes of re-yellowing as well as the influence of overdosed chemical reductant in remediating Cr-soil, experiments on excess reducing agent interference and soil re-yellowing mechanisms under different extreme conditions were conducted. The results show that the USEPA method 3060A & 7196A combined with K2S2O8 oxidation is an effective approach to eliminate interference from excess FeSO4 reducing agents. The main causes of re-yellowing include the failure of reducing agents, disruption of soil lattice, and interactions between manganese oxides and microorganisms. Under various extreme conditions simulated across the four seasons, high temperature and drought significantly accelerated the failure of reducing agents, resulting in the poorest remediation effectiveness for Cr-soil (91.75 %). Dry-wet cycles promoted the formation of soil aggregates, negatively affecting Cr(VI) removal. While these extreme conditions caused relatively mild re-yellowing (9.46 %-16.79 %) due to minimal soil lattice damage, the potential risk of re-yellowing increases with the failure of reducing agents and the release of Cr(VI) within the lattice. Prolonged exposure to acid rain leaching and freezethaw cycles disrupted soil structure, leading to substantial leaching and reduction of insoluble Cr, resulting in optimal remediation effectiveness (94.37 % -97.73 %). As reducing agents gradually and the involvement of the water medium, significant re -yellowing occurred in the remediated soil (51.52 %). Mn(II) in soil enriched relevant microorganisms, and the Mn(IV)-mediated biological oxidation process was also one of the reasons for soil re -yellowing.

Animal experiments suggest that selenium (Se) may alleviate cadmium (Cd) toxicity in animal liver and kidneys, but its effect on human liver and kidneys remains uncertain. In China, areas with black shale have shown elevated levels of Se and Cd. According to the USEPA (U.S. Environmental Protection Agency) evaluation method, the soil and rice in these areas pose significant risks. In black shale regions such as Enshi and Zhuxi County, residents who long-term consume local rice may surpass safe Se and Cd intake levels. Significantly high median blood Se (B-Se) and urine selenium (U-Se) levels were detected in these areas, measuring 416.977 mu g/L and 352.690 mu g/L and 104.527 mu g/L and 51.820 mu g/L, respectively. Additionally, the median blood Cd (B-Cd) and urine Cd (U-Cd) levels were markedly elevated at 4.821 mu g/L and 3.848 mu g/L and at 7.750 mu g/L and 7.050 mu g/L, respectively, indicating substantial Cd exposure. Nevertheless, sensitive liver and kidney biomarkers in these groups fall within healthy reference ranges, suggesting a potential antagonistic effect of Se on Cd in the human body. Therefore, the USEPA method may not accurately assess Cd risk in exposed black shale areas. However, within the healthy ranges, residents in the Enshi study area had significantly greater median levels of serum creatinine and cystatin C, measuring 67.3 mu mol/L and 0.92 mg/L, respectively, than those in Zhuxi did (53.6 mu mol/L and 0.86 mg/L). In cases of excessive Se and Cd exposure, high Se and Cd levels impact the filtration function of the human kidney to some extent. Se is an essential trace element for humans. However, excessive intake of Se can harm humans. Cd is a carcinogen and a chronic potent nephrotoxin that mostly accumulates in the human liver and kidneys. Animal experiments suggest that Se may alleviate Cd toxicity in animal liver and kidneys, but its effect on human liver and kidneys remains uncertain. In China, areas with black shale exposure have shown elevated levels of Se and Cd. According to the USEPA (U.S. Environmental Protection Agency) evaluation method, the soil and rice in these areas pose significant risks. Our results suggested that the exposed black shale areas are simultaneously enriched with Se and Cd. However, residents in these areas were exposed to excessive Se and Cd long-term without significant damage to liver and kidney functions. Therefore, the USEPA method may not accurately assess Cd risk in exposed black shale areas. The risk assessment of heavy metals in high-Se geological background areas cannot be separated from human health surveys. Our study provides evidence for the antagonistic effects of Se and Cd on the human body. Residents in exposed black shale areas consume excessive Se and Cd through local rice Human liver and kidney functions are not significantly damaged in exposed black shale areas The USEPA method may not accurately assess Cd risk in exposed black shale areas