Re-yellowing of chromium-contaminated soil after reduction-based remediation: Effects and mechanisms of extreme natural conditions

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 freeze-thaw 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.

The risks of warm nights and wet days in the context of climate change: assessing road safety outcomes in Boston, USA and Santo Domingo, Dominican Republic

Background

There remains a dearth of cross-city comparisons on the impact of climate change through extreme temperature and precipitation events on road safety. We examined trends in traffic fatalities, injuries and property damage associated with high temperatures and heavy rains in Boston (USA) and Santo Domingo (Dominican Republic).

Methods

Official publicly available data on daily traffic outcomes and weather conditions during the warm season (May to September) were used for Boston (2002–2015) and Santo Domingo (2013–2017). Daily maximum temperatures and mean precipitations for each city were considered for classifying hot days, warm days, and warm nights, and wet, very wet, and extremely wet days. Time-series analyses were used to assess the relationship between temperature and precipitation and daily traffic outcomes, using a quasi-Poisson regression.

Results

In Santo Domingo, the presence of a warm night increased traffic fatalities with a rate ratio (RR) of 1.31 (95% CI [confidence interval]: 1.00,1.71). In Boston, precipitation factors (particularly, extremely wet days) were associated with increments in traffic injuries (RR 1.25, 95% CI: 1.18, 1.32) and property damages (RR 1.42, 95% CI: 1.33, 1.51).

Conclusion

During the warm season, mixed associations between weather conditions and traffic outcomes were found across Santo Domingo and Boston. In Boston, increases in heavy precipitation events were associated with higher traffic injuries and property damage. As climate change-related heavy precipitation events are projected to increase in the USA, the associations found in this study should be of interest for road safety planning in a rapidly changing environment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40621-021-00342-w.