Application methods (i.e., pouring and mixing method) of Microbially Induced Calcite Precipitation (MICP) and its effect on wind erosion were investigated on four soil types (i.e., medium sand, fine sand, loamy fine sand and loam). With mixing method, calcite precipitated evenly throughout the upper part (0 - 5 cm) of all the soils tested, but with pouring method, only medium sand showed even calcite distribution. The reason can be ascribed to the limited permeability of MICP-inducing solution (i.e., calcium, urea and <i>Sporosarcina pasteurii</i>) through loamy fine sand and loam due to their low hydraulic conductivity (i.e., < 10<sup>-5</sup> cm/s). Moreover, bacterial penetrability was also reduced by calcium (i.e., 70 to 20%) in fine sand. Hence, pouring method for medium sand and mixing method for the others were applied with various MICP-inducing solution concentrations (i.e., 0.1 to 1 M of urea and calcium). When exposed to wind of 15 m/s after MICP application, 0.25 M solution in medium and fine sand, and 0.1 M solution in loamy fine sand and loam showed little or no soil loss. The results suggest that a proper application method be chosen considering soil properties that affect even calcite distribution to mitigate soil erosion.
The distinctive optical and electronic properties of two-dimensional (2D) molybdenum disulfide (MoS 2 ) make it a promising photocatalyst and photothermal agent in aqueous applications. In terms of environmental stability, MoS 2 has been considered insoluble, but 2D MoS 2 nanosheets can be susceptible to dissolution, owing to their large surface areas and highly accessible reactive sites, including defects at the basal plane and edge sites. Under light illumination, the dissolution of 2D MoS 2 nanosheets can be further accelerated by their photochemical reactivity. To elucidate MoS 2 reactivity in the environment, here we investigated the thickness-dependent dissolution of MoS 2 under illumination. To synthesize nanoscale thicknesses of MoS 2 , we exfoliated bulk MoS 2 by ultrasonication and controlled the layer thickness by iterative cascade centrifugation, producing MoS 2 nanosheets averaging either ∼18 nm or ∼46 nm thick, depending on the centrifugation rate. Under simulated sunlight, MoS 2 dissolution was accelerated, the Mo 6+ composition increased, and the solution pH decreased compared to those in the dark. These results suggest that light exposure promotes the oxidation of MoS 2 , causing faster dissolution. Importantly, 18 nm thick MoS 2 exhibited faster dissolution than either 46 nm or bulk MoS 2 , driven by the superoxide radical (O 2•− ) generation promoted by its relative thinness. These findings highlight the important role of the thicknessdependent photochemistry of MoS 2 nanosheets in their dissolution, which is directly linked to their environmental behavior and stability.
Nanobubble (NB) generation of reactive oxygen species (ROS), especially hydroxyl radical (·OH), has been controversial. In this work, we extensively characterize NBs in solution, with a focus on ROS generation (as ·OH), through a number of methods including degradation of ·OH-specific target compounds, electron paramagnetic resonance (EPR), and a fluorescence-based indicator. Generated NBs exhibit consistent physical characteristics (size, surface potential, and concentration) when compared with previous studies. For conditions described, which are considered as high O2 NB concentrations, no degradation of benzoic acid (BA), a well-studied ·OH scavenger, was observed in the presence of NBs (over 24 h) and no EPR signal for ·OH was detected. While a positive fluorescence response was measured when using a fluorescence probe for ·OH, aminophenyl fluorescein (APF), we provide an alternate explanation for the result. Gas/liquid interfacial characterization indicates that the surface of a NB is proton-rich and capable of inducing acid-catalyzed hydrolysis of APF, which results in a false (positive) fluorescence response. Given these negative results, we conclude that NB-induced ·OH generation is minimal, if at all, for conditions evaluated.
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