Remediation of marine dead zones by enhancing microbial sulfide oxidation using electrodes.

Marine dead zones caused by hypoxia have expanded over the last decades and pose a serious threat to coastal marine life. We tested sediment microbial fuel cells (SMFCs) for their potential to reduce the release of sulfide from sediments, in order to potentially protect the marine environment from the formation of such dead zones. Steel electrodes as well as charcoal-amended electrodes and corresponding non-connected controls of a size of together 24 m2 were installed in a marine harbour, and the effects on water quality were monitored for several months. Both pure steel electrodes and charcoal-amended electrodes were able to reduce sulfide concentrations in bottom water (92 % to 98 % reduction, in comparison to disconnected control steel electrodes). Also phosphate concentrations and ammonium were drastically reduced. SMFCs might be used to eliminate hypoxia at sites with high organic matter deposition and should be further investigated for this purpose.

Enrichment of sulfur-oxidizing bacteria using S-doped NiFe2O4 nanosheets as the anode in microbial fuel cell enhances power production and sulfur recovery.

Microbial fuel cells (MFCs) have great promise for power generation by oxidizing organic wastewater, yet the challenge to realize high efficiency in simultaneous energy production and resource recovery remains. In this study, we designed a novel MFC anode by synthesizing S-doped NiFe2O4 nanosheet arrays on carbon cloth (S10-NiFe2O4@CC) to build a three-dimensional (3D) hierarchically porous structure, with the aim to regulate the microbial community of sulfur-cycling microbes in order to enhance power production and elemental sulfur (S0) recovery. The S10-NiFe2O4@CC anode obtained a faster start-up time of 2 d and the highest power density of 4.5 W/m2 in acetate-fed and mixed bacteria-based MFCs. More importantly, sulfide removal efficiency (98.3 %) (initial concentration of 50 mg/L S2-) could be achieved within 3 d and sulfur (S8) could be produced. Microbial community analysis revealed that the S10-NiFe2O4@CC anode markedly enriched sulfur-oxidizing bacteria (SOB) and promoted enrichment of SOB and sulfate-reducing bacteria (SRB) in the bulk solution as well, leading to the enhancement of power generation and S0 recovery. This study shows how carefully designing and optimizing the composition and structure of the anode can lead to the enrichment of a multifunctional microbiota with excellent potential for sulfide removal and resource recovery.

Turnover of lake sediments treated with sediment microbial fuel cells: a long-term study in a eutrophic lake.

Sediment microbial fuel cells (SMFCs) have previously been successfully used to reduce phosphate release from the sediments of eutrophic lakes. In this study, we investigate the risk that SMFCs stimulate sediment decomposition with the unwanted side effect being the release of legacy pollutants stored in sediments. Electrode pairs (16 m2 each) were installed in a eutrophic lake in Denmark and the electricity production was monitored over more than a year at three electrode fields. Equations were derived that allow calculation of the substrate turnover by the SMFCs from the working potential, the open circuit potential, and the external resistance of the SMFCs. The resulting turnover data suggest that the decomposition of the sediment is only slightly expedited by the SMFCs, and that the decomposition process is not significantly stimulated by the type of SMFCs installed in the lake. The measured maximum power density with stainless steel electrodes in the lake sediment was 0.9 mW/m2, which was sufficient to reduce P outflux from sediment. At this power density, the decomposition half-life of the lake sediment (top 5 cm) is calculated to be 277 years, which is only about 10% of natural lake sediment decomposition half-lives. Higher power densities are not necessary for P fixation but inadvertently increase the risk that legacy pollutants buried in the sediment are released.

Novel method to immobilize phosphate in lakes using sediment microbial fuel cells.

Phosphate pollution in lakes poses an intractable remediation challenge. Accumulated stocks of phosphorus in sediments cause high concentrations in the overlying water despite elimination of external sources. We propose to use sediment microbial fuel cells (SMFCs) for lake remediation by sediment phosphorus immobilization. The hypothesis is that SMFCs can increase sediment redox potential at the top layer, and that such changes will allow the sediment to retain phosphorus as immobile species. This study placed an emphasis on scalability, practicality, and use of low-cost materials. Stainless steel net was selected as electrode material, and modifications were tested: (i) chronoamperometric operation with anode poised at +399 mV (versus standard hydrogen potential); (ii) injection of graphite slurry; and (iii) coating with nickel-carbon matrix. Stainless steel electrodes were implemented in laboratory microcosms (1.3 L) and at field scale in a eutrophic freshwater lake. All tests were carried out in untreated sediment and water from Lake Søllerød, Denmark. Phosphate immobilization was shown at lab scale, with 85% decrease in overlying water using steel electrodes. At field scale maximum phosphate decrease of 94% was achieved in the water body above a 16 m2 stainless steel SMFC electrode. Results are promising and warrant further study, including remediation trials at full scale. Added benefits include degradation of sediment organic matter and pollutants, inhibition of methane and sulfide emission and production of electricity.

Microbial Turnover of Glyphosate to Biomass: Utilization as Nutrient Source and Formation of AMPA and Biogenic NER in an OECD 308 Test.

Environmental fate assessment of chemicals involves standardized simulation tests with isotope-labeled molecules to balance transformation, mineralization, and formation of nonextractable residues (NER). Methods to predict microbial turnover and biogenic NER have been developed, having limited use when metabolites accumulate, the chemicals are not the only C source, or provide for other macroelements. To improve predictive capability, we extended a recently developed method for microbial growth yield estimation to account for incomplete degradation and multiple-element assimilation and combined it with a dynamic model for fate description in soils and sediments. We evaluated the results against the unique experimental data of 13C3-15N co-labeled glyphosate turnover with AMPA formation in water-sediment systems (OECD 308). Balancing 13C- and 15N- fluxes to biomass showed a pronounced shift of glyphosate transformation from full mineralization to AMPA formation. This may be explained by various hypotheses, for example, the limited substrate turnover inherent to the batch conditions of the test system causing microbial starvation or inhibition by P release. Modeling results indicate initial N overload due to the lower C/N ratio in glyphosate compared to average cell composition leading to subsequent C demand and accumulation of AMPA.

Prediction of the Formation of Biogenic Nonextractable Residues during Degradation of Environmental Chemicals from Biomass Yields.

Degradation tests with radio or stable isotope labeled compounds enable the detection of the formation of nonextractable residues (NER). In PBT and vPvB assessment, remobilisable NER are considered as a potential risk while biogenic NER from incorporation of labeled carbon into microbial biomass are treated as degradation products. Relationships between yield, released CO2 (as indicator of microbial activity and mineralization) and microbial growth can be used to estimate the formation of biogenic NER. We provide a new approach for calculation of potential substrate transformation to microbial biomass (theoretical yield) based on Gibbs free energy and microbially available electrons. We compare estimated theoretical yields of biotechnological substrates and of chemicals of environmental concern with experimentally determined yields for validation of the presented approach. A five-compartment dynamic model is applied to simulate experiments of 13C-labeled 2,4-D and ibuprofen turnover. The results show that bioNER increases with time, and that most bioNER originates from microbial proteins. Simulations with precalculated input data demonstrate that precalculation of yields reduces the number of fit parameters considerably, increases confidence in fitted kinetic data, and reduces the uncertainty of the simulation results.

The impact of temperature on the transformation of illicit drug biomarkers in wastewater.

Temperature is one of the key factors, influencing the transformation kinetics of organic chemicals. In the context of wastewater-based epidemiology, however, temperature differences among sewer catchments and within the same catchment (due to, e.g., seasonal variations) have been neglected to date as a factor influencing the estimation of illicit drug consumption. In this study, we assessed the influence of temperature on the transformation of biomarkers in wastewater and its ensuing implications on the back-calculation of chemical consumption rate in urban catchments using the example of selected illicit drugs. Literature data, obtained in laboratory-scale experiments, on the stability of drug biomarkers in untreated wastewater at trace levels was systematically reviewed, and transformation rates obtained at different temperatures were collected. Arrhenius-based equations were fitted to empirical data and identified to describe the transformation of selected cocaine and morphine biomarkers at applicability temperature range (from 2-9 °C to 30-31 °C), with estimated exponential Arrhenius coefficients between 1.04 and 1.18. These empirically-derived relationships were used to assess the influence of temperature on the transformation of drug biomarkers during in-sewer transport and its effect on the back-calculation of drug consumption rate in hypothetical urban catchment scenario simulations. Up to 4-fold increase in removal efficiency was estimated when wastewater temperature increased from 15 °C to 25 °C. Findings from this study can help reducing the uncertainty intrinsic to wastewater-based epidemiology studies, and will be beneficial in comparing chemical consumption estimates from different catchments worldwide.

Transformation and Sorption of Illicit Drug Biomarkers in Sewer Biofilms.

In-sewer transformation of drug biomarkers (excreted parent drugs and metabolites) can be influenced by the presence of biomass in suspended form as well as attached to sewer walls (biofilms). Biofilms are likely the most abundant and biologically active biomass fraction in sewers. In this study, 16 drug biomarkers were selected, including the parent forms and the major human metabolites of mephedrone, methadone, cocaine, heroin, codeine, and tetrahydrocannabinol (THC). Transformation and sorption of these substances were assessed in targeted batch experiments using laboratory-scale biofilm reactors operated under aerobic and anaerobic conditions. A one-dimensional model was developed to simulate diffusive transport, abiotic and biotic transformation, and partitioning of drug biomarkers. Model calibration to experimental results allowed estimating biotransformation rate constants in sewer biofilms, which were compared to those obtained for suspended biomass. Our results suggest that sewer biofilms can enhance the biotransformation kinetics of most selected compounds. Through scenario simulations, we demonstrated that the estimation of biotransformation rate constants in biofilm can be significantly biased if the boundary layer thickness is not accurately estimated. This study complements our previous investigation on the transformation and sorption of drug biomarkers in the presence of only suspended biomass in untreated sewage. A better understanding of the role of sewer biofilms-also relative to the in-sewer suspended solids-and improved prediction of associated fate processes can result in more accurate estimation of daily drug consumption in urban areas in wastewater-based epidemiological assessments.