Oxyanion Removal from Impaired Water by Donnan Dialysis Plug Flow Contactors

In the last twenty-five years, extensive work has been done on ion exchange membrane bioreactors (IEMB) combining Donnan dialysis and anaerobic reduction to remove trace oxyanions (e.g., perchlorate, nitrate, chlorate, arsenate) from contaminated water sources. Most studies used Donnan dialysis contactors with high recirculation rates on the feed side, so under continuous operation, the effective concentration on the feed side of the membrane is the same as the exit concentration (CSTR mode). We have built, characterized, and modelled a plug flow Donnan dialysis contactor (PFR) that maximizes concentration on the feed side and operated it on feed solutions spiked with perchlorate and nitrate ion using ACS and PCA-100 anion exchange membranes. At identical feed inlet concentrations with the ACS membrane, membrane area loading rates are three-fold greater, and fluxes are more than double in the PFR contactor than in the CSTR contactor. A model based on the nonlinear adsorption of perchlorate in ACS membrane correctly predicted the trace ion concentration as a function of space-time in experiments with ACS. For PCA membrane, a linear flux dependence on feed concentration correctly described trace ion feed concentration as a function of space-time. Anion permeability for PCA-100 was high enough that the overall mass transfer was affected by the film boundary layer resistance. These results provide a basis for efficiently scaling up Donnan dialysis contactors and incorporating them in full-scale IEMB setups.

Electrodialysis ion-exchange membrane bioreactor (EDIMB) to remove nitrate from water: Optimization of operating conditions and kinetics analysis

Nitrate pollution has become a worldwide problem. In this study, we remove nitrate from water by electrodialysis ion-exchange membrane bioreactor (EDIMB) and enabling simultaneous nitrate enrichment and denitrification. In this reactor, nitrate migrated from the water chamber to the biological chamber via electrodialysis and was degraded by microorganisms. The effects of voltage and biomass concentration on the reactor performance were examined, and the kinetics data of the water chamber and biological chamber were fitted. The experimental results showed that the migration of nitrate in the water chamber conformed to the first-order model, and the constructed zero-Michaelis-Menten model described changes in nitrate concentration in the biological chamber. Furthermore, when the inflow nitrate concentration was 40 mg N/L, 5 V was the best voltage, and 3.00 g VSS/L was the best biomass concentration. The nitrate removal rate in the water chamber was 98.94%, and there was no accumulation of nitrate or nitrite in the biological chamber. Compared with traditional ED processes, the nitrate removal efficiency was 8.86% higher, and the current efficiency was 22.14% higher. The total organic carbon (TOC) of the water chamber was only 1.43 mg C/L, which proves that the structure of the EDIMB confined the denitrifying bacteria and organic carbon donors in the biological chamber and avoided secondary pollution in the water chamber. Microbial community analysis showed that Thauera (66.06%) was the dominant bacterium in the EDIMB system, and Azoarcus (9.81%) was a minor denitrifying genus.

Inhibition of perchlorate biodegradation by ferric and ferrous iron

Previous observations from in-situ biological treatments in the subsurface of a perchlorate-contaminated site revealed multiple reduction processes occurring parallel to perchlorate degradation. Iron reduction was accelerated and correlated with a decline in the efficiency of the in-situ perchlorate reduction. In the current study, we examined the influence of iron forms on perchlorate reduction. A series of kinetic laboratory experiments were conducted, using an indigenous mixed perchlorate-reducing culture, enriched from the polluted soil that was undergoing bioremediation. The results show that ferrous iron was a non-competitive inhibitor with a 41% decrease in µ_max for perchlorate reduction. Moreover, chlorate was accumulated in all samples treated with ferrous iron, indicating a disruption to the chlorate reduction step. Ferric iron, however, had less impact on perchlorate degradation with non-competitive inhibition reaching a 23% decrease in µ_max. Scanning electron microscopy (SEM) revealed that the presence of ferrous iron in the perchlorate degradation enrichment culture initiated cell encrustation. We propose that during perchlorate reduction and the emission of oxygen from chlorite dismutation, the chemical oxidation of ferrous iron occurred near the bacteria's surface where the enzyme is located, forming an oxidized iron crust layer that can directly affect the perchlorate reduction enzymatic system.

DMSO Reductase Family: Phylogenetics and Applications of Extremophiles

Dimethyl sulfoxide reductases (DMSO) are molybdoenzymes widespread in all domains of life. They catalyse not only redox reactions, but also hydroxylation/hydration and oxygen transfer processes. Although literature on DMSO is abundant, the biological significance of these enzymes in anaerobic respiration and the molecular mechanisms beyond the expression of genes coding for them are still scarce. In this review, a deep revision of the literature reported on DMSO as well as the use of bioinformatics tools and free software has been developed in order to highlight the relevance of DMSO reductases on anaerobic processes connected to different biogeochemical cycles. Special emphasis has been addressed to DMSO from extremophilic organisms and their role in nitrogen cycle. Besides, an updated overview of phylogeny of DMSOs as well as potential applications of some DMSO reductases on bioremediation approaches are also described.

Biological perchlorate reduction: which electron donor we can choose?

Biological reduction is an effective method for removal of perchlorate (ClO₄^-), where perchlorate is transformed into chloride by perchlorate-reducing bacteria (PRB). An external electron donor is required for autotrophic and heterotrophic reduction of perchlorate. Therefore, plenty of suitable electron donors including organic (e.g., acetate, ethanol, carbohydrate, glycerol, methane) and inorganic (e.g., hydrogen, zero-valent iron, element sulfur, anthrahydroquinone) as well as the cathode have been used in biological reduction of perchlorate. This paper reviews the application of various electron donors in biological perchlorate reduction and their influences on treatment efficiency of perchlorate and biological activity of PRB. We discussed the criteria for selection of appropriate electron donor to provide a flexible strategy of electron donor choice for the bioremediation of perchlorate-contaminated water.

Electrochemical nitrate reduction by using a novel Co3O4/Ti cathode

Co₃O₄ film coated on Ti substrate is prepared using sol-gel method and applied as cathode material for electrochemical denitrification in this research. Preparation conditions including precursor coating times and calcination temperature are optimized based on NO₃^--N removal, NO₂^--N generation, NH₄⁺-N generation and total nitrogen (TN) removal efficiencies. The influences of electrolysis parameters such as current density and NO₃^--N initial concentration are also investigated. In comparison with other common researched cathodes (Ti, Cu and Fe₂O₃/Ti), Co₃O₄/Ti exhibits better NO₃^--N removal and NH₄⁺-N generation efficiencies. In order to remove NO₃^--N completely from water, Cl^- is added to help further oxidize NH₄⁺-N to N₂. TN removal after 3 h treatment increases from 65% to 80%, 90% and 96% with the increase of Cl^- from 0 mg L^-1 to 500, 1000 and 1500 mg L^-1, respectively. The mechanisms of NO₃^--N reduction on cathode and NH₄⁺-N oxidation on anode in the absence and presence of Cl^- are investigated in a double-cell reactor. Actual textile wastewater containing both NO₃^- and Cl^- is also treated and the Co₃O₄/Ti cathode exhibits excellent stability and reliability. It is interesting to find out that FeCl₂-H₂O₂ Fenton pretreatment is needed to remove extra COD and provide more Cl^- to help oxidize NH₄⁺-N to N₂ at the same time.

Using the natural biodegradation potential of shallow soils for in-situ remediation of deep vadose zone and groundwater

In this study, we examined the ability of top soil to degrade perchlorate from infiltrating polluted groundwater under unsaturated conditions. Column experiments designed to simulate typical remediation operation of daily wetting and draining cycles of contaminated water amended with an electron donor. Covering the infiltration area with bentonite ensured anaerobic conditions. The soil remained unsaturated, and redox potential dropped to less than -200mV. Perchlorate was reduced continuously from ∼1150mg/L at the inlet to ∼300mg/L at the outlet in daily cycles. Removal efficiency was between 60 and 84%. No signs of bioclogging were observed during three operation months although occasional iron reduction observed due to excess electron donor. Changes in perchlorate reducing bacteria numbers were inferred from an increased in pcrA gene abundances from ∼10⁵ to 10⁷ copied per gram at the end of the experiment indicating the growth of perchlorate-reducing bacteria. We proposed that the topsoil may serve as a bioreactor to treat high concentrations of perchlorate from the contaminated groundwater. The treated water that infiltrates from the topsoil through the vadose zone could be used to flush perchlorate from the deep vadose zone into the groundwater where it is retrieved again for treatment in the topsoil.