Mechanistic Evidence for Hg Removal from Wastewater by Biologically Produced Sulfur

A significant quantity of biologically produced sulfur (BPS) is generated as a by-product of chemical and biological desulfurization processes applied to landfill gas treatment. The beneficial upcycling of BPS has seen limited use in the environmental context. The effectiveness and underlying mechanism of BPS as an adsorbent for removing Hg2+ from both solution and wastewater were elucidated based on experiments encompassing surface characterization, adsorption isotherms, kinetics, and thermodynamics. The BPS exhibited remarkable efficacy in removing Hg2+ from solution, with the Langmuir model accurately describing the adsorption process and showing a maximum adsorption capacity of 244 mg g-1. Surface analysis through X-ray photoelectron spectroscopy and scanning electron microscopy revealed that Hg2+ complexed with sulfide on BPS surfaces, forming stable HgS. The adsorbed Hg was strongly retained in BPS, with less than 0.2% of the adsorbed Hg desorbed by strong acids. Adsorption kinetics followed the double-exponential first-order model, showing an initial rapid adsorption phase wherein 75% of the initial Hg2+ was removed within 5 min, followed by a slower adsorption rate. The thermodynamic parameters suggested that adsorption of Hg2+ by BPS was a spontaneous and endothermic process. Additionally, BPS effectively removed Hg2+ from wastewater, showing preference for Hg over other co-existing metals. These findings underscore the potential of BPS as an effective adsorbent for Hg2+ removal from wastewater.

Charge Disproportionation Triggers Bipolar Doping in FeSb2- xSn xSe4 Ferromagnetic Semiconductors, Enabling a Temperature-Induced Lifshitz Transition

Ferromagnetic semiconductors (FMSs) featuring a high Curie transition temperature ( T_c) and a strong correlation between itinerant carriers and localized magnetic moments are of tremendous importance for the development of practical spintronic devices. The realization of such materials hinges on the ability to generate and manipulate a high density of itinerant spin-polarized carriers and the understanding of their responses to external stimuli. In this study, we demonstrate the ability to tune magnetic ordering in the p-type FMS FeSb_{2- x}Sn _xSe₄ (0 ≤ x ≤ 0.20) through carrier density engineering. We found that the substitution of Sb by Sn FeSb_{2- x}Sn _xSe₄ increases the ordering of metal atoms within the selenium crystal lattice, leading to a large separation between magnetic centers. This results in a decrease in the T_c from 450 K for samples with x ≤ 0.05 to 325 K for samples with 0.05 < x ≤ 0.2. In addition, charge disproportionation arising from the substitution of Sb³⁺ by Sn²⁺ triggers the partial oxidation of Sb³⁺ to Sb⁵⁺, which is accompanied by the generation of both electrons and holes. This leads to a drastic decrease in the electrical resistivity and thermopower simultaneously with a large increase in the magnetic susceptibility and saturation magnetization upon increasing Sn content. The observed bipolar doping induces a very interesting temperature-induced quantum electronic transition (Lifshitz transition), which is manifested by the presence of an anomalous peak in the resistivity curve simultaneously with a reversal of the sign of a majority of the charge carriers from hole-like to electron-like at the temperature of maximum resistivity. This study suggests that while there is a strong correlation between the overall magnetic moment and free carrier spin in FeSb_{2- x}Sn _xSe₄ FMSs, the magnitude of the Curie temperature strongly depends on the spatial separation between localized magnetic centers rather than the concentration of magnetic atoms or the density of itinerant carriers.