Electro-biocatalytic conversion of carbon dioxide to alcohols using gas diffusion electrode.

Impact of gas diffusion electrodes (GDEs) was evaluated in enhancing the CO2 bio-availability for its transformation to C4-organics, especially to alcohols using selective mixed culture. Observed current density was more stable (9-11 A/m2) than submerged experiments reported and significantly varied with pH and respective CO2 solubility. Uncontrolled operating pH (starting with 8.0) showed its impact on shifting/triggering alternate metabolic pathways to increase the carbon length (butyric acid) as well as producing more reduced end products, i.e. alcohols. During the experiments, CO2 was transformed initially to a mixture of volatile fatty acids dominated with formic and acetic acids, and upon their accumulation, ethanol and butanol production was triggered. Overall, 21 g/l of alcohols and 13 g/l of organic acids were accumulated in 90 days with a coulombic efficiency (CE) of 49%. Ethanol and butanol occupied respectively about 45% and 16% of total products, indicating larger potential of this technology.

Significance of acetogenic H2 consumption in dark fermentation and effectiveness of pH.

Two identical thermophilic H(2) fermenters (R1 and R2) were operated at different pH levels between 4.7 and 5.7. In R1, several unexpected and severe drops in H(2) yield inversely proportional to increase in acetate production were experienced at pH 5.5 and 5.7. In contrast, R2 operated at pH 5 and 4.7 performed more stable H(2) production mainly through butyrate fermentation. Although the H(2) partial pressure (>50 kPa) was far above the favorable values, acetate was produced as well as butyrate in all pH levels tested. To determine whether some portion of the acetate is produced through another pathway such as autotrophic synthesis via H(2) dependent reduction of CO(2) or not, batch dissolved H(2) consumption rate tests were performed at pH 5.0, 5.5 and 6. The specific H(2) consumption rate was 488(+/-49) micromol/gVSS.hr at pH 6 and slightly higher than at pH 5 and 5.5. The results of continuous and batch experiments revealed that acetogenic H(2) consumption is more favorable at pH levels above 5.5 and is one of the reasons of instabilities in dark fermentative H(2) production.

Microbial processes as key drivers for metal (im)mobilization along a redox gradient in the saturated zone.

Two sites representing different aquifer types, i.e., Dommel (sandy) and Flémalle (gravelly loam) along the Meuse River, have been selected to conduct microcosm experiments. Various conditions ranging from aerobic over nitrate- to sulphate reducing were imposed. For the sandy aquifer, nitrate reducing conditions predominated, which specifically in the presence of a carbon source led to pH increases and enhanced Zn removal. For the calcareous gravelly loam, sulphate reduction was dominant resulting in immobilization of both Zn and Cd. For both aquifer types and almost all redox conditions, higher arsenic concentrations were measured in the groundwater. Analyses of different specific microbial populations by polymerase chain reaction (PCR) revealed the dominance of denitrifiers for the Dommel site, while sulfate reducing bacteria (SRB) were the prevailing population for all redox conditions in the Flémalle samples.

Potential use of the plant antioxidant network for environmental exposure assessment of heavy metals in soils.

In recent years, awareness has risen that the total soil content of pollutants by itself does not suffice to fully assess the potential ecotoxicological risks involved. Chemical analysis will require to be complemented with biological assays in a multidisciplinary approach towards site specific ecological risk assessment (SS-ERA). This paper evaluates the potential use of the plants' antioxidant response to metal-induced oxidative stress to provide a sensitive biological assay in SS-ERA. To this end, plants of Phaseolus vulgaris were grown for two weeks on 15 soils varying in contamination level. Morphological parameters and enzymatic plant responses were measured upon harvest. Foliar concentrations of the (heavy) metals Al, Cu, Cd, Cr, Fe, Mn, Ni, Pb, Zn were also determined. Metal mobility in the soil was further assessed by determining soil solution and NH4OAc extractable levels. In general more significant correlations were observed between plant responses and foliar metal concentrations or exchangeable/soluble levels than between plant responses and the total soil content. The study demonstrates the potential use of the plants' antioxidant defence mechanisms to assess substrate phytotoxicity for application in SS-ERA protocols. However, the system, based on calculation of a soil Phytotoxicity Index (PI), will require adaptation and fine-tuning to meet the specific needs for this type of environmental monitoring. Large variation was observed in phytotoxicity classification based on the various test parameters. The thresholds for classification of the various morphological and enzymatic response parameters may require adaptation according to parameter stress sensitivity in order to decrease the observed variation. The use of partial PI's (leaves and roots separately) may in addition increase the sensitivity of the system since some metals show specific effects in one of both organs only. Loss of biological functionality of enzymes, as was observed for ICDH in one of the more strongly contaminated soils, may also be recognized as an additional stress symptom when assigning phytotoxicity classification, whereas the current system only considers increasing enzymatic capacities. Other easily distinguishable parameters, which could be added to the current indexation are: failure to germinate and the incapacity to develop roots in the toxic substrate. Additional research will be required to determine the possible application range of soil properties for this biological assay and to further improve its performance in SS-ERA.