Antimony toxicity upon microorganisms from aerobic and anaerobic environments.

Antimony (Sb) is a toxic and carcinogenic metalloid that can be present in contaminated water generated by mining operations and other industrial activities. The toxicity of Sb (III) and Sb (V) to aerobic microorganisms remains limited and unexplored for anaerobic microorganisms involved in hydrogen (H2) and methane (CH4) production. This study aimed to evaluate the toxicity of Sb (III) and Sb (V) upon aerobic and anaerobic microorganisms important in biological wastewater treatment systems. Sb (III) was more toxic than Sb (V) independently of the test and environment evaluated. Under aerobic conditions maintained in the Microtox assay, Sb (V) was not toxic to Allivibrio fischeri at concentrations as high as 500 mg/L, whereas Sb (III) caused just over 50% inhibition at concentration of 250 mg/L after 5 min of exposure. In the respirometry test, for the specific oxygen uptake rate, the concentrations of Sb (III) and Sb (V) displaying 50% inhibition were 0.09 and 56.2 mg/L, respectively. Under anaerobic conditions, exposure to Sb (III) and Sb (V) led to a decrease in microorganisms activity of fermentative and methanogenic processes. The results confirm that the microbial toxicity of Sb depends on its speciation and Sb (III) displays a significantly higher inhibitory potential than Sb (V) in both aerobic and anaerobic environments.

Iron(II) monosulfide (FeS) minerals reductively transform the insensitive munitions compounds 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO).

As military applications of the insensitive munitions compounds (IMCs) 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO) increase, there is a growing need to understand their environmental fate and to develop remediation strategies to mitigate their impacts. Iron (II) monosulfide (FeS) minerals are abundant in freshwater and marine sediments, marshes, and hydrothermal environments. This study shows that FeS solids can reduce DNAN and NTO to their corresponding amines under anoxic ambient conditions. The reactions between IMCs and the FeS minerals were surface-mediated since they did not occur when only dissolved Fe2+(aq) and S2-(aq) were present. Mackinawite, a tetragonal FeS with a layered structure, reduced DNAN mainly to 2-methoxy-5-nitroaniline (MENA), which in turn was partially reduced to 2-4-diaminoanisole (DAAN). The layered structure of mackinawite provided intercalation sites likely responsible for partial adsorption of MENA and DAAN. Mackinawite entirely reduced NTO to 3-amino-1,2,4-triazol-5-one (ATO). The reduction of IMCs showed concurrent oxidation of mackinawite to goethite and elemental sulfur. A commercial FeS product, composed mainly of pyrrhotite and troilite, reduced DNAN to DAAN and NTO to ATO. At pH 6.5, DNAN and NTO transformation rates were 667 and 912 µmol h-1 m-2, respectively, on the mackinawite surface and 417 and 1344 µmol h-1 m-2, respectively, on the commercial FeS surface. This is the first report of the reduction of a nitro-heterocyclic compound (NTO) by FeS minerals. The evidence indicates that DNAN and NTO can be rapidly transformed to their succeeding amines in anoxic subsurface environments and aquatic sediments rich in FeS minerals.

Covalent binding with model quinone compounds unveils the environmental fate of the insensitive munitions reduced product 2,4-diaminoanisole (DAAN) under anoxic conditions.

2,4-Dinitroanisole (DNAN) is an insensitive munitions compound expected to replace 2,4,6-trinitrotoluene (TNT). The product of DNAN's reduction in the environment is 2,4-diaminoanisole (DAAN), a toxic and carcinogenic aromatic amine. DAAN is known to become irreversibly incorporated into soil natural organic matter (NOM) after DNAN's reduction. Herein, we investigate the reactions between DAAN and NOM under anoxic conditions, using 1,4-benzoquinone (BQ) and methoxybenzoquinone (MBQ) as model humic moieties of NOM. A new method stopped the fast reactions between DAAN and quinones, capturing the fleeting intermediates. We observed that DAAN incorporation into NOM (represented by BQ and MBQ models) is quinone-dependent and occurs via Michael addition, imine (Schiff-base) formation, and azo bond formation. After dimers are formed, incorporation reactions continue, resulting in trimers and tetramers. After 20 days, 56.4% of dissolved organic carbon from a mixture of DAAN (1 mM) and MBQ (3 mM) had precipitated, indicating an extensive polymerization, with DAAN becoming incorporated into high-molecular-weight humic-like compounds. The present work suggests a new approach for DNAN environmental remediation, in which DNAN anaerobic transformation can be coupled to the formation of non-extractable bound DAAN residues in soil organic matter. This process does not require aerobic conditions nor a specific catalyst.

Microbial toxicity and biodegradability of perfluorooctane sulfonate (PFOS) and shorter chain perfluoroalkyl and polyfluoroalkyl substances (PFASs).

Perfluorooctane sulfonate (PFOS) and related perfluoroalkyl and polyfluoroalkyl substances (PFASs) are emerging contaminants that have been widely applied in consumer and industrial applications for decades. However, PFOS has raised public concern due to its high bioaccumulative character, environmental persistence, and toxicity. Shorter PFASs such as perfluorobutane sulfonate (PFBS) and polyfluoroalkyl compounds have been proposed as alternatives to PFOS but it is unclear whether these fluorinated substances pose a risk for public health and the environment. The objective of this research was to investigate the microbial toxicity and the susceptibility to microbial degradation of PFOS and several related fluorinated compounds, i.e., short-chain perfluoroalkyl and polyfluoroalkyl sulfonic and carboxylic acids. None of the compounds tested were toxic to the methanogenic activity of anaerobic wastewater sludge even at very high concentrations (up to 500 mg L-1). All PFASs evaluated were highly resistant to microbial degradation. PFOS was not reductively dehalogenated by the anaerobic microbial consortium even after very long periods of incubation (3.4 years). Similarly, the tested short chain perfluoroalkyl substances (i.e., PFBS and trifluoroacetic acid) and a polyfluoroalkyl PFOS analogue, 6 : 2 fluorotelomer sulfonic acid (FTSA) were also resistant to anaerobic biodegradation. Likewise, no conclusive evidence of microbial degradation was observed under aerobic conditions for any of the short-chain perfluoroalkyl and polyfluoroalkyl carboxylic acids tested after 32 weeks of incubation. Collectively, these results indicate that PFOS and its alternatives such as short chain perfluoroalkyl sulfonates and carboxylates and their polyfluorinated homologues are highly resistant to microbial degradation.

Biogenic sulphide plays a major role on the riboflavin-mediated decolourisation of azo dyes under sulphate-reducing conditions.

The effect of high concentrations of sulphate on the reductive decolourisation of different azo dyes by anaerobic sludge was studied in batch cultures. Sludge cultures were pre-incubated under sulphate-reducing conditions prior addition of dyes. Little or no effects of sulphate (5-10 g sulphate l(-1)) on the rate of decolourisation of Reactive Orange 14 (RO14), Direct Blue 53 (DB53) and Direct Blue 71 (DB71) were observed when no external redox mediator was provided. However, an increase in sulphate concentration, in the presence of riboflavin (20 microM), enhanced the decolourisation of all dyes. The first-rate constant of decolourisation (k) was increased up to 2-, 3.6- and 2-fold for RO14, DB53 and DB71, respectively, by supplying high sulphate concentrations, compared to the controls lacking sulphate, in the presence of the redox mediator. Sulphate reduction did not take place during the course of azo reductions, but was only evident before dye addition and after complete decolourisation, suggesting azo dyes reduction out-competed sulphate reduction for the available reducing equivalents. The experimental data suggest that reduction of azo dyes by riboflavin, which had been reduced by biogenic sulphide, was the major mechanism implicated during decolourisations, which was corroborated by abiotic incubations. Riboflavin greatly accelerated the abiotic reduction of RO14, so that the k value was increased up to 44-fold compared to the control lacking riboflavin.