Migration and transformation of Sb are affected by Mn(III/IV) associated with lepidocrocite originating from Fe(II) oxidation.

Antimony (Sb) is a recognized priority pollutant with toxicity that is influenced by its migration and transformation processes. Oxidation of Fe(II) to Fe(III) oxides, which is a common phenomenon in the environment, is often accompanied by the formation of Mn(III/IV) and might affect the fate of Sb. In this study, incorporated Mn(III) and sorbed/precipitated Mn(III/IV) associated with lepidocrocite were prepared by adding Mn(II) during and after Fe(II) oxidation, respectively, and the effects of these Mn species on Sb fate were investigated. Our results indicated that the association of these Mn species with lepidocrocite obviously enhanced Sb(III) oxidation to Sb(V), while concomitantly inhibiting Sb sorption due to the lower sorption capacity of lepidocrocite for Sb(V) than Sb(III). Additionally, Mn oxide equivalents increased in the presence of Sb, indicating that Sb oxidation by Mn(III/IV) associated with lepidocrocite was a continuous recycling process in which Mn(II) released from Mn(III/IV) reduction by Sb(III) could be oxidized to Mn(III/IV) again. This recycling process was favorable for effective Sb(III) oxidation. Moreover, Sb(V) generated from Sb(III) oxidation by Mn(III/IV) enhanced Mn(II) sorption at the beginning of the process, and thus favored Mn(III/IV) formation, which could further promote Sb(III) oxidation to Sb(V). Overall, this study elucidated the effects of Mn(III/IV) associated with lepidocrocite arisen from Fe(II) oxidation on Sb migration and transformation and revealed the underlying reaction mechanisms, contributing to a better understanding of the geochemical dynamics of Sb.

Effects of fermented organic fertilizer application on soil N2O emission under the vegetable rotation in polyhouse.

Vegetable field is one of the main sources of soil nitrous oxide (N2O) emission, yet soil N2O emission from vegetable rotation with combined application of fermented organic fertilizer with inorganic fertilizer in polyhouse is not well evaluated. In this study, we investigated the soil N2O emission in cabbage-tomato rotation management system under different treatments of fertilizer nitrogen (N) sources, including: 100% inorganic fertilizer (IF), 75% IF+25% fermented organic fertilizer (OF), 50% IF+50% OF, 75% IF+25% OF, 100% OF, and no fertilizer (CK). The fertilization amount of N was 180 kg ha-1 to cabbage and 200 kg ha-1 to tomato. Results showed that soil N2O emission flux was in a high level during 1-3 days after basal fertilization for cabbage, and decreased as the proportions of OF increased. During the whole cabbage-tomato rotated cultivation, N2O emission flux was positively related to soil NO3--N and NH4+-N contents, with correlation coefficients of 0.72 and 0.90, respectively. A higher proportion of OF increased the soil total carbon (C), organic C and C/N ratio, but decreased the soil nitrifiers and denitrifiers. The fertilizer N loss caused by N2O emission under different OF treatments was 1.23-2.77%, significantly (p < 0.05) lower than under 100% IF treatment (3.58%), and the loss decreased with the increase of OF proportion. Our study quantitatively revealed the N2O emission under vegetable rotation systems with different fertilizations in polyhouses, and the overall results suggested that the higher soil pH, the lower soil mineral NO3--N and NH4+-N as well as lower soil nitrifiers and denitrifiers contributed to less N2O emission for the OF treatments.

Bioretention for removal of nitrogen: processes, operational conditions, and strategies for improvement.

As one of the low-impact development measures, bioretention plays an important role in reducing the runoff peak flow and minimizing runoff pollutants, such as heavy metals, suspended solids, and nutrients. However, the efficiency of nitrogen removal in the bioretention system is unstable, owing to the different chemical properties of various forms of nitrogen and the limitations of current bioretention system for nitrogen transformation. This review article summarizes the recent advances in bioretention system in treatment of urban stormwater and agricultural runoff for nitrogen removal. The microbial characteristics and main processes of nitrogen transformation in bioretention are reviewed. The operational conditions affecting nitrogen removal, including climatic conditions, pH, wet-dry alternation, influent loads and nitrogen concentration, and hydraulic residence time are discussed. Finally, measures or strategies for increasing nitrogen removal efficiency are proposed from the perspectives of structural improvement of the bioretention system, optimization of medium composition, and enhancement of the nitrogen removal reaction processes.

Solid-phase denitrification for water remediation: processes, limitations, and new aspects.

Nitrate pollution in water environments is a ubiquitous problem. Solid-phase denitrification (SPD) is a technology that has attracted in recent years increasing attention due to its significant advantage suitability over the aqueous-based denitrification for in situ water remediation. This paper provides a view of new aspects of the application of SPD for water remediation. The processes and mechanisms for nitrogen transformation in SPD, for example, direct denitrification, dissimilatory nitrate reduction to ammonium, and anammox are presented. The main processes of the transformation of the carbon substrate in SPD are also discussed. The major limitations of SPD, including low carbon availability, NO2 - and N2O accumulation, dissolved organic carbon release, and NH4 + production, are summarized and the factors associated with such limitations are also discussed. In addition, some novel measures to mitigate these limitations, such as applying a biodegradable polymer substrate and heterotrophic-autotrophic denitrification (HAD) process, are described. Finally, simultaneous removal of nitrate and some typical concurrent contaminants for expanded application of SPD are discussed. This review attempts to advance our understanding of engineered denitrification processes for wastewater treatment or water remediation.