Transformation kinetics of exogenous nickel in a paddy soil during anoxic-oxic alteration: Roles of organic matter and iron oxides

Distribution, risk evaluation, and source allocation of cesium and strontium in surface soil in a mining city

Radioactive nuclides cesium (Cs) and strontium (Sr) possess long half-lives, with 135Cs at approximately 2.3 million years and 87Sr at about 49 billion years. Their persistent accumulation can result in long-lasting radioactive contamination of soil ecosystems. This study employed geo-accumulation index (Igeo), pollution load index (PLI), potential ecological risk index (PEPI), health risk assessment model (HRA), and Monte Carlo simulation to evaluate the pollution and health risks of Cs and Sr in the surface soil of different functional areas in a typical mining city in China. Positive matrix factorization (PMF) model was used to elucidate the potential sources of Cs and Sr and the respective contribution rates of natural and anthropogenic sources. The findings indicate that soils in the mining area exhibited significantly higher levels of Cs and Sr pollution compared to smelting factory area, agricultural area, and urban residential area. Strontium did not pose a potential ecological risk in any studied functional area. The non-carcinogenic health risk of Sr to the human body in the study area was relatively low. Because of the lack of parameters for Cs, the potential ecological and human health risks of Cs was not calculated. The primary source of Cs in the soil was identified as the parent material from which the soil developed, while Sr mainly originated from associated contamination caused by mining activities. This research provides data for the control of Cs and Sr pollution in the surface soil of mining city.

Transformation and migration of Hg in a polluted alkaline paddy soil during flooding and drainage processes

Mercury (Hg) contamination in paddy soils poses a health risk to rice consumers and the environmental behavior of Hg determines its toxicity. Thus, the variations of Hg speciation are worthy of exploring. In this study, microcosm and pot experiments were conducted to elucidate Hg transformation, methylation, bioaccumulation, and risk coupled with biogeochemical cycling of key elements in a Hg-polluted alkaline paddy soil. In microcosm and pot experiments, organic- and sulfide-bound and residual Hg accounted for more than 98% of total Hg, and total contents of dissolved, exchangeable, specifically adsorbed, and fulvic acid-bound Hg were less than 2% of total Hg, indicating a low mobility and environmental risk of Hg. The decrease of pH aroused from Fe(III), SO42-, and NO3- reduction promoted Hg mobility, whereas the increase of pH caused by Fe(II), S2-, and NH4+ oxidation reduced available Hg contents. Moreover, Fe-bearing minerals reduction and organic matter consumption promoted Hg mobility, whereas the produced HgS and Fe(II) oxidation increased Hg stability. During flooding, a fraction of inorganic Hg (IHg) could be transported into methylmercury (MeHg), and during drainage, MeHg would be converted back into IHg. After planting rice in an alkaline paddy soil, available Hg was below 0.3 mg kg-1. During rice growth, a portion of available Hg transport from paddy soil to rice, promoting Hg accumulation in rice grains. After rice ripening, IHg levels in rice tissues followed the trend: root > leaf > stem > grain, and IHg content in rice grain exceed 0.02 mg kg-1, but MeHg content in rice grain meets daily intake limit (37.45 μg kg-1). These results provide a basis for assessing the environmental risks and developing remediation strategies for Hg-contaminated redox-changing paddy fields as well as guaranteeing the safe production of rice grains.

Biogeochemical cycling in paddy soils controls antimony transformation: Roles of iron (oxyhydr)oxides, organic matter and sulfate

In paddy fields, periodic flooding and drainage phases can significantly affect the availability of antimony (Sb), but the underlying mechanisms remain unclear. In this study, Sb-contaminated paddy soil was incubated under anaerobic (40 day) and subsequently aerobic (40-55 day) conditions. The Sb fractions was investigated and a kinetic model was established to quantitatively evaluate the main processes controlling Sb transformation. Under anaerobic conditions, the reductive dissolution of iron (Fe) (oxyhydr)oxides, the release of soil colloids, and dissolved organic carbon (DOC) could facilitate the release of Sb(V), while newly released Sb(V) were synchronously reduced to Sb(III) that could be incorporated into the solid phase (34.1%, 40 day) or precipitated as Sb2S3 (9.7%, 40 day). After soil aeration, a significant increase in dissolved and extracted Sb(V) (34.7%, 45 day) was observed due to the Sb(III) oxidization by the reactive oxygen species (ROS) generated from Fe(II) oxidization. The dissolved and extracted Sb(V) were simultaneously incorporated into the solid phase as the re-aggregation of soil colloids and DOC, and only contributed to 17.1% of the total Sb content at the end of aerobic phase (55 day). Our results elucidated the mechanisms about how biogeochemical Fe/S/C cycling jointly controlled Sb transformation in paddy systems.

Transformation kinetics of exogenous lead in an acidic soil during anoxic-oxic alteration: Important roles of phosphorus and organic matter

Lead (Pb) can enter soil environment during flooding events such as surface runoff and intensive rainfall. However, the key transformation processes of exogenous Pb during anoxic-oxic alteration remain poorly understood particularly how phosphorus and organic matter contribute to Pb immobilization/release. Here, a kinetic model was established to investigate the Pb transformation in an acidic soil with two levels of Pb contamination under alternating anoxic-oxic conditions, based on the results of seven-step sequential extraction, dissolved organic carbon, sulfate, iron, phosphorus, and surface sites. Results showed that the potentially available Pb, including dissolved, exchangeable, and specifically adsorbed fractions, was gradually transferred to the fulvic complex, Fe-Mn oxides bound, and sulfides bound Pb after 40-day incubation under anoxic conditions, while the fulvic complex Pb further increased after 20-day incubation under oxic conditions. The concentration of phosphorus that was extracted by 0.5 M HCl or 0.03 M NH4F in 0.025 M HCl increased under anoxic conditions and decreased under oxic conditions. When Pb-binding to phosphorus is considered during kinetic modeling, the simulated results of Pb transformation suggest that phosphorus is more important than organic matter for Pb immobilization under anoxic conditions, while the phosphates, Fe-Mn oxides, and sulfides immobilized Pb is slowly released and then complexed by fulvic acids during the re-immobilization of dissolved organic matter in soil under oxic conditions. The model established with low Pb level has been successfully applied to describe the Pb transformation with high Pb level. This study provides a comprehensive understanding of the roles of phosphorus and organic matter in controlling Pb transformation in soil from kinetic modeling.