Reduction of Sb(V) by coupled biotic-abiotic processes under sulfidogenic conditions.

Increasing use and mining of antimony (Sb) has resulted in greater concern involving its fate and transport in the environment. Antimony(V) and (III) are the two most environmentally relevant oxidation states, but little is known about the redox transitions between the two in natural systems. To better understand the behavior of antimony in anoxic environments, the redox transformations of Sb(V) were studied in biotic and abiotic reactors. The biotic reactors contained Sb(V) (2 mM as KSb(OH)6), ferrihydrite (50 mM Fe(III)), sulfate (10 mM), and lactate (10 mM), that were inoculated with sediment from a wetland. In the abiotic reactors, The interaction of Sb(V) with green rust, magnetite, siderite, vivianite or mackinawite was examined under abiotic conditions. Changes in the concentrations of Sb, Fe(II), sulfate, and lactate, as well as the microbial community composition were monitored over time. Lactate was rapidly fermented to acetate and propionate in the bioreactors, with the latter serving as the primary electron donor for dissimilatory sulfate reduction (DSR). The reduction of ferrihydrite was primarily abiotic, being driven by biogenic sulfide. Sb and Fe K-edge X-ray absorption near edge structure (XANES) analysis showed reduction of Sb(V) to Sb(III) within 4 weeks, concurrent with DSR and the formation of FeS. Sb K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy analysis indicated that the reduced phase was a mixture of S- and O-coordinated Sb(III). Reduction of Sb(V) was not observed in the presence of magnetite, siderite, or green rust, and limited reduction occurred with vivianite. However, reduction of Sb(V) to amorphous Sb(III) sulfide occurred with mackinawite. These results are consistent with abiotic reduction of Sb(V) by biogenic sulfide and reveal a substantial influence of Fe oxides on the speciation of Sb(III), which illustrates the tight coupling of Sb speciation with the biogeochemical cycling of S and Fe.

Nano-enabled, antimicrobial toothbrushes - How physical and chemical properties relate to antibacterial capabilities.

Over the past two decades, Ag and Zn nanoparticles have been integrated into various consumer products as a biocide. While some nano-enabled consumer products have been shown to have antibacterial properties, their antibacterial efficacy as well as the human and environmental health outcomes are not fully known. In this study, we examine a nanoparticle-enabled product that also serves as a conduit for human exposure to bacteria: toothbrushes. We utilize a combination of chemical analyses, laboratory experiments, and microscopy to characterize the nano-enabled toothbrush bristles. Our analysis showed the majority of measured Ag and Zn particles ranged from approximately 50 to 100 nm in size and were located on the surface and within bristles. During simulated brushing, antimicrobial bristles released both Ag and Zn, the majority of which was released in particulate form. While our results demonstrate that antimicrobial bristles have enhanced bactericidal properties compared to control samples, we also show that the surface topography influences nanoparticle retention, microbial adhesion, and bactericidal activity. We thus conclude that Ag or Zn content alone is insufficient to predict antimicrobial properties, which are further governed by the bioavailability of Ag or Zn at the bristle surface.

Testing the component additivity approach to surface complexation modeling using a novel cadmium-specific fluorescent probe technique.

Over the past 40 years, laboratory experiments involving single metal-single sorbent systems have been conducted in order to determine thermodynamic stability constants for metal-bacteria and metal-mineral surface complexes. The component additivity (CA) approach to surface complexation modeling (SCM) represents one method for using these experimentally-derived stability constants to predict the extent of metal adsorption in complex, multi-sorbent systems. However, quantitative tests of the CA approach are rare due to difficulties in determining the distribution of metals in complex multi-sorbent systems. In this study, we use a novel technique that couples the use of a cadmium(Cd)-specific fluorescent probe with confocal scanning laser microscopy to quantify Cd adsorption to bacteria in fully hydrated multi-sorbent samples that contain different ratios of Bacillus subtilis bacterial cells, the clay mineral kaolinite, and the aqueous chelating ligand EDTA. In this approach, we directly determine the distribution of Cd by measuring the total concentration of adsorbed Cd and the concentration of Cd that is adsorbed to bacterial cells, and by difference we calculate the concentration of Cd that is adsorbed to kaolinite. We compare these experimental measurements to the extent of Cd adsorption that is calculated using a CA approach to predict the distribution of Cd under our experimental conditions. In general, the CA predictions of the distribution of Cd between the aqueous phase and the two sorbents agree within uncertainties with the measured concentrations of Cd in each reservoir in both the EDTA-free and the EDTA-bearing experimental systems. This study demonstrates that the Cd-fluorescent probe technique is a suitable, and relatively simple, option for quantitatively testing CA surface complexation models. Our results suggest that although the CA approach can yield reasonable predictions of the distribution of Cd in mixed sorbent systems, the accuracy of the predictions depends directly on the accuracy of the measurements of stability constants for both the aqueous and surface metal-ligand complexes that occur in a system of interest.