Goethite Formed in the Periplasmic Space of Pseudomonas sp. JM-7 during Fe Cycling Enhances Its Denitrification in Water.

Denitrification-driven Fe(II) oxidation is an important microbial metabolism that connects iron and nitrogen cycling in the environment. The formation of Fe(III) minerals in the periplasmic space has a significant effect on microbial metabolism and electron transfer, but direct evidence of iron ions entering the periplasm and resulting in periplasmic mineral precipitation and electron conduction properties has yet to be conclusively determined. Here, we investigated the pathways and amounts of iron, with different valence states and morphologies, entering the periplasmic space of the denitrifier Pseudomonas sp. JM-7 (P. JM-7), and the possible effects on the electron transfer and the denitrifying ability. When consistently provided with Fe(II) ions (from siderite (FeCO3)), the dissolved Fe(II) ions entered the periplasmic space and were oxidized to Fe(III), leading to the formation of a 25 nm thick crystalline goethite crust, which functioned as a semiconductor, accelerating the transfer of electrons from the intracellular to the extracellular matrix. This consequently doubled the denitrification rate and increased the electron transport capacity by 4-30 times (0.015-0.04 µA). However, as the Fe(II) concentration further increased to above 4 mM, the Fe(II) ions tended to preferentially nucleate, oxidize, and crystallize on the outer surface of P. JM-7, leading to the formation of a densely crystallized goethite layer, which significantly slowed down the metabolism of P. JM-7. In contrast to the Fe(II) conditions, regardless of the initial concentration of Fe(III), it was challenging for Fe(III) ions to form goethite in the periplasmic space. This work has shed light on the likely effects of iron on environmental microorganisms, improved our understanding of globally significant iron and nitrogen geochemical cycles in water, and expanded our ability to study and control these important processes.

Transformation of siderite to goethite by humic acid in the natural environment.

Humic acid (HA) is particularly important in iron-bearing mineral transformations and erosion at the water-mineral boundary zone of the Earth. In this study, three stages of the possible pathway by which HA causes mineral transformation from siderite to goethite are identified. Firstly, a Fe(II)-HA complex is formed by chelation, which accelerates the dissolution and oxidation of Fe(II) from the surface of siderite. As the Fe(II)-HA complex retains Fe atoms in close proximity of each other, ferrihydrite is formed by the agglomeration and crystallization. Finally, the ferrihydrite structurally rearranges upon attachment to the surface of goethite crystals and merges with its structure. The influence of low concentrations of HA (0-2 mg/L) on phosphate adsorption is found to be beneficial by the inducing of new mineral phases. We believe that these results provide a greater understanding of the impact of HA in the biogeochemical cycle of phosphate, mineral transformation.

Light-activated inorganic CsPbBr2I perovskite for room-temperature self-powered chemical sensing.

Halide perovskite materials are excellent light harvesters that have generated enormous interest for photovoltaic technology and an increasing number of other optoelectronic applications. Very recently, their use for miniaturized chemical sensors has shown a promising room-temperature response. Here, we present some insights on the use of CsPbBr2I (CPBI) perovskites for self-powered room-temperature sensing of several environmentally and medically relevant compounds demonstrating rapid detection of down to concentrations of 1 ppm. Notably, the photocurrent of these self-powered CPBI-based devices increases under exposure to both reducing (e.g. acetone, propane) and oxidizing (e.g. NO2, O2) gas molecules and decreases rapidly upon reverting to an inert atmosphere. In situ photoluminescence (PL) analysis of the CPBI during exposure to oxidizing molecules reveals a strongly increased PL intensity and longer lifetime indicating a prevalent role of CPBI trap states in the sensing mechanism. These findings provide new insights for the engineering of perovskite-based materials for their future chemical sensing applications.