Heavy metal pollution in soils threatens food safety and human health. Calcium sulfate and ferric oxide are commonly used to immobilize heavy metals in soils. However, the spatial and temporal variations of the heavy metals’ bioavailability in soils regulated by a combined material of calcium sulfate and ferric oxide (CSF) remain unclear. In this work, two soil column experiments were conducted to investigate the spatial and temporal variations of CSF immobilized Cd, Pb, and As. In the horizontal soil column, the results showed that CSF’s immobilization range for Cd increased over time, and adding CSF in the center of the soil column decreased the concentrations of bioavailable Cd significantly, up to 8 cm away by day 100. The CSF immobilization effect on Pb and As only existed in the center of the soil column. The CSF’s immobilization depths for Cd and Pb in the vertical soil column increased over time and extended to 20 cm deep by day 100. However, the CSF’s immobilization depths for As only extended to between 5 and 10 cm deep after 100 days of incubation. Overall, the results from this study can serve as a guide to determine the CSF application frequency and spacing distance for the in-situ immobilization of heavy metals in soils.

Mitigating acid mine drainage (AMD) at its source, specifically within rocks containing pyrite in underwater environments, poses a significant environmental challenge worldwide. Existing passivation techniques are primarily designed for open-air conditions, involving direct contact with coating materials at a solid–liquid interface, making them ineffective beneath a water barrier. In this study, we introduce a novel passivation method inspired by the design of underwater bio-adhesives. Tannic acid (TA) combined with polyethylene glycol (PEG) was employed to form a hydrophobic film directly on the pyrite surface, overcoming water resistance and addressing the limitations of current techniques. Electrochemical experiments and chemical leaching experiments were conducted to evaluate the oxidation resistance of the passivating films. TA–PEG-coated pyrite exhibited a lower oxidation rate and a higher static contact angle of 126.2°, achieving suppression efficiencies of 71.6% for total Fe release and 68.1% for total S release. A comprehensive characterization approach, including scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), was employed to investigate the passivation mechanism. The results of this study may provide new insights into the preparation of simpler and greener passivating agents to suppress pyrite oxidation at its source in underwater environments.

Manganese (Mn) oxides exhibit significant potential to either stabilize or destabilize soil organic carbon (SOC) through the polymerization and/or oxidation of organic molecules via organo-mineral interactions. Birnessite (MnO₂) is known to strongly interact with soil dissolved organic matter (DOM), which is DOM composition-dependent. Humic acid (HA) and fulvic acid (FA) are commonly used as organic fertilizers in soils. In this study, the contrasting reaction of DOM with birnessite in flooded paddy soil with HA and FA amendment was investigated at a molecular level. The results demonstrated that HA amendment enhanced the reaction of phenolic compounds in soil DOM with birnessite, leading to the formation of condensed aromatic compounds and polymeric products (PP) with higher molecular weights and aromaticity. This suggests that HA amendment enhances the birnessite-induced polymerization of soil DOM. In contrast, FA facilitated the birnessite-induced oxidation of soil DOM, yielding dicarboxylic acids (DA), monocarboxylic acids (MA), and quinones products (QP). These findings demonstrate that the reactivity of soil DOM with birnessite is significantly influenced by the composition of DOM exogenously added. This study provides comprehensive understandings of the interactions among Mn and C and helps to predict behaviors of DOM molecules in flooded paddy soil, which is critical for optimizing sustainable soil management.