Geochemistry of Framboids

The stoichiometry of pyrite in framboids is unknown. The trace element content of framboids has been reported since framboids usually constitute the earliest pyrite phase in a sediment and therefore are more likely to pick up trace element variations in contemporary seawater. The trace element ratios in sedimentary framboids are similar to those in the host shales. Analyses of hydrothermal framboids are fewer, and As, Sb, and Tl appear to be enriched in hydrothermal framboids, with As, Sb, Ni, and Co also being enriched in framboids formed during metamorphism. In contrast with trace element distributions, no spatial variations in sulfur isotopic compositions have been reported within individual framboids. Framboids pick up a more accurate measure of the sulfur isotopic composition of the prevailing dissolved sulfide and are likely to retain this over geologic time. Although it is probable that pyrite framboids collect the local environmental trace element variations, interpretations of the results in terms of paleoenvironmental reconstructions are currently complex. The original sequestration of trace elements is likely to be in part determined by the pyrite crystal chemistry, and there may be a limit to how much of any given trace element can be sequestered by pyrite. This is likely to be enhanced during late diagenesis and early metamorphism and it is not altogether clear how individual trace elements behave over geologic time.

Autotrophic denitrification via nitrate as an effective approach for removal of dissolved sulfide in anaerobic reactors

This work assessed the effect of adding different concentrations of nitrate (50–300 mg ·L−1) on the removal of dissolved and gaseous sulfide in an anaerobic reactor treating synthetic effluent containing sulfate (100 mg ·L−1) and organic matter (1 g COD·L−1). Autotrophic denitrification, stimulated by the addition of nitrate, was demonstrated to be a very effective approach for removal of dissolved sulfide even in the presence of a high concentration of organic matter (complete removal with 50 mg mg·L−1). However, it had a minor effect on H2S(g). Sulfide remained partially oxidized to elemental sulfur even with excess nitrate (100–300 mg mg·L−1). Therefore, the competition for this electron acceptor between the autotrophic and heterotrophic denitrification pathways may have prevented the conversion of the generated sulfide into sulfate again. No evidence of inhibition of methanogenesis and sulfidogenesis was found during nitrate supplementation.

The Effect of Chemical Sulfide Oxidation on the Oxygenic Activity of an Alkaliphilic Microalgae Consortium Deployed for Biogas Upgrading

The oxygenic photosynthetic activity (OPA) of an alkaliphilic microalgae consortium was evaluated at different concentrations of dissolved sulfide under room temperature and well-defined conditions of irradiance and pH in a tubular closed photobioreactor. The kinetic assays showed that it was optimal at a sulfide concentration of 3.2 mg/L under an external photosynthetically active radiation of 50 and 120 μE/m2 s together with a pH of 8.5 and 9.2. In contrast, the oxygenic photosynthetic activity was insignificant at 15 μE/m2 s with a pH of 7.3, both in the absence and presence of sulfide. Consecutive pulse additions of dissolved sulfide evidenced that the accumulation rate of dissolved oxygen was decreased by the spontaneous chemical oxidation of sulfide with dissolved oxygen in alkaline culture media, mainly at high sulfide levels. At 3.2 mg/L of sulfide, the oxygenic photosynthetic activity was improved by around 60% compared to the treatment without sulfide at external irradiances of 120 μE/m2 s, 30 °C, and pH of 8.5 and 9.2. Additionally, an even higher OPA enhancement (around 85%) was observed in the same previous conditions but using 16 mg/L of sulfide. Thiosulfate was the major end-product of sulfide by oxic chemical reaction, both in biotic and abiotic assays with yields of 0.80 and 0.68, respectively.

Impact of microaeration bioreactor on dissolved sulfide and methane removal from real UASB effluent for sewage treatment

Two bioreactors were investigated as an alternative for the post-treatment of effluent from an upflow anaerobic sludge blanket (UASB) reactor treating domestic sewage, aiming at dissolved sulfide and methane removal. The bioreactors (R-control and R-air) were operated at different hydraulic retention times (HRT; 6 and 3 h) with or without aeration. Large sulfide and methane removal efficiencies were achieved by the microaerated reactor at HRT of 6 h. At this HRT, sulfide removal efficiencies were equal to 61% and 79%, and methane removal efficiencies were 31% and 55% for R-control and R-air, respectively. At an HRT of 3 h, sulfide removal efficiencies were 22% (R-control) and 33% (R-air) and methane removal did not occur. The complete oxidation of sulfide, with sulfate formation, prevailed in both phases and bioreactors. However, elemental sulfur formation was more predominant at an HRT of 6 h than at an HRT of 3 h. Taken together, the results show that post-treatment improved the anaerobic effluent quality in terms of chemical oxygen demand and solids removal. However, ammoniacal nitrogen was not removed due to either the low concentration of air provided or the absence of microorganisms involved in the nitrogen cycle.

Sedimentary molybdenum and uranium sequestration in a non-euxinic coastal setting: role of the sulfate-methane transition zone

<p>Molybdenum (Mo) and uranium (U) contents in sedimentary records are commonly used to track past changes in seafloor oxygenation. However, inadequate understanding of Mo and U sequestration mechanisms in non-euxinic coastal areas limits their use as redox proxies in these settings. Because large areas of the coastal oceans are currently undergoing partial deoxygenation due to anthropogenic nutrient inputs and increased stratification, it is critical to improve our understanding of these proxies to allow robust assessment of the trajectory of environmental change. Here, we use a comprehensive set of sediment pore water and solid-phase analyses to deconvolve the mechanisms of authigenic Mo and U sequestration in a shallow non-euxinic coastal setting in the northern Baltic Sea. Despite the permanently oxic bottom waters in the area, eutrophication over the past decades has led to establishment of a shallow sulfate-methane transition zone (SMTZ) in the sediment, which is typical for human-impacted coastal settings on a trajectory towards hypoxia. Our results demonstrate remarkably synchronous patterns of Mo and U sequestration, whereby their authigenic uptake is largely predicated upon the depth and intensity of the SMTZ. Based on sequential extraction analyses, the authigenic Mo pool is dominated by refractory Fe-S phases such as pyrite and nanoscale FeMoS<sub>4</sub>, signaling that authigenic Mo uptake largely proceeds through the Fe-sulfide pathway. In addition, we observe a pool of extremely labile Mo deep within the SMTZ, potentially denoting a transient phase in authigenic Mo uptake and/or partial switch in the mode of sequestration to the organic matter pathway at low levels of dissolved iron. Authigenic U is largely hosted by acid-extractable and refractory phases, reflecting sequestration into poorly crystalline monomeric U(IV) and crystalline uraninite, respectively. Analogously to Mo, authigenic sequestration of U proceeds at two distinct fronts within the SMTZ, which are characterized by shifts in dissolved sulfide concentrations, providing strong evidence for a link between sulfide-producing processes and U reduction. Our results imply that both Mo and U have the potential to capture temporal shifts in bottom water oxygenation indirectly, through the connection between oxygenation and the depth of the SMTZ. Of the two elements, Mo appears a more viable redox proxy because of the substantially higher share of the authigenic pool. However, temporal resolution of these proxies is restricted by the relatively deep authigenic uptake within the sediment column and the integrated character of the signal caused by vertical migrations of the SMTZ. These findings set a framework for interpreting sedimentary Mo- and U-based paleoredox archives in other non-euxinic coastal settings.</p>

Dimethylmercury demethylation in the presence of sulfide

<p>In marine systems, the methylated mercury pool is approximately evenly distributed between monomethylmercury (MMHg) and dimethylmercury (DMHg). While MMHg is well-studied due to its direct link with Hg accumulation in aquatic food webs, there is a general lack of knowledge of processes controlling DMHg formation or degradation. By acting as a net sink or source for MMHg, DMHg may exert control over marine MMHg concentrations and subsequent Hg bioaccumulation in fish and seafood in ways currently not understood. </p><p>At present, recognized degradation pathways of DMHg in marine systems include photochemical demethylation (although this pathway has been debated). Degradation through protonolysis of the Hg-C bond by dissolved sulfide has also been suggested and supported by density function theory calculations (Ni et al, J. Phys. Chem. A, 2006). However, experimental support for this pathway is currently missing. Here, we present data from a series of experiments for the stability of DMHg in the presence of dissolved sulfide or sulfide minerals (e.g. FeS (s)). Our results show that degradation rates are dependent on the sulfide phase and DMHg:sulfide ratios. For dissolved sulfide, we observed a non-linear response between DMHg degradation and sulfide concentrations. Our results indicate that DMHg can be demethylated by sulfide at concentration ratios viable under natural marine conditions. As we found MMHg to be the first product of demethylation, this process could also constitute a significant MMHg source in marine systems.</p>