Effect of low temperature on abiotic and biotic nitrate reduction by zero-valent Iron.

The effect of low temperatures on abiotic and biotic nitrate (NO3-) reduction by zero-valent iron (ZVI) were examined at temperatures below 25 °C. The extent and rate of nitrate removal in batch ZVI reactors were determined in the presence and absence of microorganisms at 3.5, 10, 17, and 25 °C. Under anoxic conditions, NO3- reduction rates in both ZVI-only and ZVI-cell reactors declined as temperature decreased. In ZVI-only reactor, 62% and 17% of initial nitrate concentration were reduced in 6 days at 25 and 3.5 °C, respectively. The reduced nitrate was completely recovered as ammonium ions (NH4+) at both temperatures. The temperature-dependent abiotic reduction rates enabled us to calculate the activation energy (Ea) using the Arrhenius relationship, which was 50 kJ/mol. Nitrate in ZVI-cell reactors was completely removed within 1-2 days at 25 and 10 °C, and 67% of reduction was achieved at 3.5 °C. Only 18-25% of the reduced nitrate was recovered as NH4+ in the ZVI-cell reactors. Soluble iron concentrations (Fe2+ and Fe3+) in the ZVI reactors were also measured as the indicators of anaerobic corrosion. In the ZVI-cell reactors, soluble iron concentrations were 1.7 times higher than that in ZVI-only reactors at 25 °C, suggesting that the enhanced nitrate reduction in the ZVI-cell reactors may be partly due to increased redox activity (i.e., corrosion) on iron surfaces. Anaerobic corrosion of ZVI was also temperature-dependent as substantially lower concentrations of corrosion product were detected at lower incubation temperatures; however, microbially induced corrosion (MIC) of ZVI was much less impacted at lower temperatures than abiotic ZVI corrosion. This study demonstrated that ZVI-supported microbial denitrification is not only more sustainable at lower temperatures, but it becomes more dominant reaction for nitrate removal in microbial-ZVI systems at low temperatures.

Different bioavailability of phenanthrene to two bacterial species and effects of trehalose lipids on the bioavailability.

Effects of trehalose lipids produced from Rhodococcus erythropolis ATCC 4277 on phenanthrene (PHE) mineralization by two soil microorganisms were investigated. Biodegradation experiments were conducted, with and without the biosurfactant, in three batch systems: water, soil, and soil-water slurry. PHE sorption to the soil did not limit the mineralization by the test microorganisms, Pseudomonas strain R (PR) and Sphingomonas sp. strain P5-2 (SP5-2). Both microorganisms, however, demonstrated significant difference in the PHE mineralization capability in the systems. While SP5-2 mineralized PHE faster than PR in liquid culture, PR having more hydrophobic surface greatly exceeded SP5-2 in ability to access soil-sorbed PHE. While the addition of the biosurfactant little affected the apparent cell hydrophobicity of SP5-2, it substantially improved PHE mineralization by this strain in all systems tested. Contrary to SP5-2, the apparent cell hydrophobicity was significantly stimulated with increasing concentration of the biosurfactant for PR. However, the biosurfactant had no significant effect on PHE mineralization by this microorganism. The results demonstrated that the addition of the biosurfactant may have great potential for remediation of sites contaminated with polycyclic aromatic hydrocarbons but its effects and benefits may be dependent on characteristics of microorganisms involved and environmental conditions.

Drying of fecal sludge in 3D laminate enclosures for urban waste management.

Laminated hydrophobic membranes have been proposed as liners for container-based sanitation systems in developing countries. The laminate allows drying of fecal sludge, which might significantly reduce the frequency of container emptying, while containing liquids and solids. While previous laboratory tests demonstrated rapid drying of fecal sludge or water retained in laminates, experiments did not assess the effects of system dimension or scale on performance. In this study fecal sludge drying and water evaporation were evaluated in 3D laminate boxes (decimeter scale) or 3D laminate-lined 40 L and 55 gallon drums (meter scale) that are prototypes of toilet containers for field application. A stagnant film model described fecal sludge drying and water evaporation in the laminate boxes and laminate-lined drums well. The effective diffusion length (λ) for the laminate was fitted in all systems and increased with system dimension and scale: λ increased by a factor of 1.4 from 1D decimeter-scale envelopes to 3D decimeter-scale boxes, and by a factor of 1.3-1.7 from 3D decimeter-scale boxes to 3D meter-scale drums. The longer λ with increasing dimension and scale is likely due to nonuniform temperature and relative humidity in the air outside the laminate and nonuniform temperature within the laminate. Using best-fit λ for the laminate-lined 40 L and 55 gallon drum experiments conducted in a controlled laboratory, drying was predicted for an 11-day field experiment. Although the air temperature and relative humidity varied significantly in the field tests from -1 °C to 26 °C and 35% to 97%, respectively, the stagnant film model predicted drying over the 11-day period reasonably well with total error ≤ 13% using 24-h average air temperature and relative humidity. Drying of fecal sludge in laminate-lined drums in the field might be adequately described with a stagnant film model using daily-average weather conditions, if wind speeds are low.

A pilot-scale, bi-layer bioretention system with biochar and zero-valent iron for enhanced nitrate removal from stormwater.

Nitrogen (N) removal in conventional bioretention systems is highly variable owing to the low nitrate (NO3-) elimination efficiency. We hypothesized that amending bioretention cells with biochar and zero-valent iron (ZVI) could improve the NO3- removal performance. A well-instrumented, bi-layer pilot-scale bioretention cell was developed to test the hypothesis by investigating its hydrologic performance and NO3- removal efficacy as affected by biochar and ZVI amendments. The cell containing 18% (v/v) wood biochar in the vadose zone and 10% (v/v) ZVI in the saturation zone was monitored for 18 months of field infiltration tests using synthetic stormwater amended with bromide (tracer) and NO3-. Compared to the Control cell without amendments, the Biochar/ZVI cell increased water retention by 11-27% and mean residence time by 0.7-3.8 h. The vadose zone of the Biochar/ZVI cell removed 30.6-95.7% (0.6-12.7 g) of NO3-N from the influent, as compared with -6.1-89.6% (-0.1-2.9 g) by that of the Control cell. While the performance varied with synthetic storm events and seasons, in all cases the Biochar/ZVI cell resulted in greater NO3- removal than the Control cell. This improvement was presumably due to biochar's ability to improve water retention, facilitate anoxic conditions, increase residence time, and provide electrons for microbial denitrification. The saturation zone with ZVI amendment further promoted NO3- removal: removal was 1.8 times greater relative to the control in the first infiltration test, but was minimal in following tests. The reduction in performance of the ZVI amendment in subsequent tests might be due to diminished NO3-N input to the saturation zone after treatment by the biochar-amended vadose zone. The redox potential and dissolved oxygen content at the vadose/saturation zone interface also indicated more favorable denitrification conditions in the Biochar/ZVI cell. Biochar amendment demonstrated significant promise for increasing nitrate removal in bioretention systems.

Transesterification of Waste Activated Sludge for Biosolids Reduction and Biodiesel Production.

Transesterification of waste activated sludge (WAS) was evaluated as a cost-effective technique to reduce excess biosolids and recover biodiesel feedstock from activated sludge treatment processes. A laboratory-scale sequencing batch reactor (SBR) was operated with recycling transesterification-treated WAS back to the aeration basin. Seventy percent recycling of WAS resulted in a 48% reduction of excess biosolids in comparison with a conventional SBR, which was operated in parallel as the control SBR. Biodiesel recovery of 8.0% (dried weight basis) was achieved at an optimum transesterification condition using acidic methanol and xylene as cosolvent. Average effluent soluble chemical oxygen demand (COD) and total suspended solids (TSS) concentrations from the test SBR and control SBR were comparable, indicating that the recycling of transesterification-treated WAS did not have detrimental effect on the effluent quality. This study demonstrated that transesterification and recycling of WAS may be a feasible technique for reducing excess biosolids, while producing valuable biodiesel feedstock from the activated sludge process.

Microbial reduction of nitrate in the presence of zero-valent iron and biochar.

The denitrification of nitrate (NO3(-)) by mixed cultures in the presence of zero-valent iron [Fe(0)] and biochar was investigated through a series of batch experiments. It was hypothesized that biochar may provide microbes with additional electrons to enhance the anaerobic biotransformation of nitrate in the presence of Fe(0) by facilitating electron transfer. When compared to the anaerobic transformation of nitrate by microbes in the presence of Fe(0) alone, the presence of biochar significantly enhanced anaerobic denitrification by microbes with Fe(0). Graphite also promoted the anaerobic microbial transformation of nitrate with Fe(0), and it was speculated that electron-conducting graphene moieties were responsible for the improvement. The results obtained in this work suggest that nitrate can be effectively denitrified by microbes with Fe(0) and biochar in natural and engineered systems.

The short-term toxic effects of TiO2nanoparticles toward bacteria through viability, cellular respiration, and lipid peroxidation.

To better understand the potential impacts of metal oxide nanoparticles (NPs) on Gram(+) Bacillus subtilis and Gram(-) Escherichia coli (K12) bacteria, eight different nanosized titanium dioxide (TiO2) suspensions with five different concentrations were used. Water quality parameters (pH, temperature, and ionic strength), light sources, and light intensities were also changed to achieve different environmental conditions. The photosensitive TiO2 NPs were found to be harmful to varying degrees under ambient conditions, with antibacterial activity increasing with primary particle sizes from 16 to 20 nm. The presence of light was a significant factor under most conditions tested, presumably due to its role in promoting generation of reactive oxygen species (ROS). However, bacterial growth inhibition was also observed under dark conditions and different water quality parameters, indicating that undetermined mechanisms additional to photocatalytic ROS production were responsible for toxicity. The results also indicated that nano-TiO2 particles in the absence and the presence of photoactivation induced lipid peroxidation and cellular respiration disruption.

Effects of biosurfactant-producing bacteria on biodegradation and transport of phenanthrene in subsurface soil.

This study investigated the effects of surfactant-producing microorganism, Pseudomonas aeruginosa ATCC 9027, on phenanthrene (PHE) biodegradation by two different PHE-degrading bacteria (Isolate P5-2 and Pseudomonas strain R) in soil. Phenanthrene mineralization experiments were conducted with soils inoculated with one of PHE-degraders and/or the surfactant-producer. Influence of co-inoculation with the surfactant-producing bacteria on phenanthrene transport and biodegradation was also examined in soil columns. P. strain R mineralized phenanthrene faster and to a greater extent than Isolate P5-2 in the test soil. Co-inoculation with the surfactant-producing bacteria significantly enhanced phenanthrene biodegradation by P. strain R but it did not affect the biodegradation by Isolate P5-2 in both batch and column systems. Production of biosurfactants by P. aeruginosa ATCC 9027 was negligible under the given conditions. This study demonstrated that bioaugmentation with surfactant-producing bacteria could enhance in situ bioremediation of soils contaminated with polycyclic aromatic hydrocarbons (PAHs) and the beneficial effect of the bioaugmentation depended on types of PAH-degrading microorganisms present.

Simultaneous removal of perchlorate and energetic compounds in munitions wastewater by zero-valent iron and perchlorate-respiring bacteria.

Ammonium perchlorate is one of the main constituents in Army's insensitive melt-pour explosive, PAX-21 in addition to RDX and 2,4-dinitroanisole (DNAN). The objective of this study is to develop an innovative treatment process to remove both perchlorate and energetic compounds simultaneously from PAX-21 production wastewater. It was hypothesized that the pretreatment of PAX-21 wastewater with zero-valent iron (ZVI) would convert energetic compounds to products that are more amenable for biological oxidation and that these products serve as electron donors for perchlorate-reducing bacteria. Results of batch ZVI reduction experiments showed that DNAN was completely reduced to 2,4-diaminoanisole and RDX was completely reduced to formaldehyde. Anaerobic batch biodegradation experiments showed that perchlorate (30 mg L(-1)) in ZVI-treated PAX-21 wastewater was decreased to an undetectable level after 5 days. Batch biodegradation experiments also confirmed that formaldehyde in ZVI-treated wastewater was the primary electron donor for perchlorate-respiring bacteria. The integrated iron-anaerobic bioreactor system was effective in completely removing energetic compounds and perchlorate from the PAX-21 wastewater without adding an exogenous electron donor. This study demonstrated that ZVI pretreatment not only removed energetic compounds, but also transformed energetic compounds to products that can serve as the source of electrons for perchlorate-respiring bacteria.

The removal of 1,4-dioxane from polyester manufacturing process wastewater using an up-flow Biological Aerated Filter (UBAF) packed with tire chips.

1,4-Dioxane is one of the by-products from the polyester manufacturing process, which has been carelessly discharged into water bodies and is a weak human carcinogen. In this study, a laboratory-scale, up-flow biological aerated filter (UBAF), packed with tire chips, was investigated for the treatment of 1,4-dioxane. The UBAF was fed with effluent, containing an average of 31 mg/L of 1,4-dioxane, discharged from an anaerobic treatment unit at H Co. in the Gumi Industrial Complex, South Korea. In the batch, a maximum of 99.5 % 1,4-dioxane was removed from an influent containing 25.6 mg/L. In the continuous mode, the optimal empty bed contact time (EBCT) and air to liquid flow rate (A:L) were 8.5 hours and 30:1, respectively. It was also found that the removal efficiency of 1,4-dioxane increased with increasing loading rate within the range 0.04 to 0.31 kg 1,4-dioxane/m(3)·day. However, as the COD:1,4-dioxane ratio was increased within the range 3 to 46 (mg/L COD)/(mg/L 1,4-dioxane), the removal efficiency unexpectedly decreased.

Reduction of perchlorate by salt tolerant bacterial consortia.

Two perchlorate-reducing bacterial consortia (PRBC) were obtained by enrichment cultures from polluted marine sediments. Non-salt-tolerant PRBC (N-PRBC) was enriched without the addition of NaCl, and salt tolerant-PRBC (ST-PRBC) was enriched with 30 g-NaCl L(-1). Although the perchlorate reduction rates decreased with increasing NaCl concentration, ST-PRBC (resp., N-PRBC) could reduce perchlorate until 75 g-NaCl L(-1) (resp., 30 g-NaCl L(-1)). The reduction yield (1.34±0.05 mg-perchlorate per mg-acetate) and maximum perchlorate reduction rate (86 mg-perchlorateL(-1) h(-1)) of ST-PRBC was higher than those (1.16±0.03 mg-perchlorate per mg-acetate and 48 mg-perchlorate L(-1) h(-1)) of N-PRBC. Kinetic analysis showed that NaCl acted as an uncompetitive inhibitor against both PRBCs. The inhibition constants were 25 and 41 mg-NaCl L(-1) for N-PRBC and ST-PRBC, respectively.

Detoxification of PAX-21 ammunitions wastewater by zero-valent iron for microbial reduction of perchlorate.

US Army and the Department of Defense (DoD) facilities generate perchlorate (ClO(4)(-)) from munitions manufacturing and demilitarization processes. Ammonium perchlorate is one of the main constituents in Army's new main charge melt-pour energetic, PAX-21. In addition to ammonium perchlorate, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and 2,4-dinitroanisole (DNAN) are the major constituents of PAX-21. In order to evaluate microbial perchlorate reduction as a practical option for the treatment of perchlorate in PAX-21 wastewater, we conducted biodegradation experiments using glucose as the primary sources of electrons and carbon. Batch experiments showed that negligible perchlorate was removed in microbial reactors containing PAX-21 wastewater while control bottles containing seed bacteria and glucose rapidly and completely removed perchlorate. These results suggested that the constituents in PAX-21 wastewater may be toxic to perchlorate reducing bacteria. A series of batch toxicity test was conducted to identify the toxic constituents in PAX-21 and DNAN was identified as the primary toxicant responsible for inhibiting the activity of perchlorate reducing bacteria. It was hypothesized that pretreatment of PAX-21 by zero-valent iron granules will transform toxic constituents in PAX-21 wastewater to non-toxic products. We observed complete reduction of DNAN to 2,4-diaminoanisole (DAAN) and RDX to formaldehyde in abiotic iron reduction study. After a 3-day acclimation period, perchlorate in iron-treated PAX-21 wastewater was rapidly decreased to an undetectable level in 2 days. This result demonstrated that iron treatment not only removed energetic compounds but also eliminated the toxic constituents that inhibited the subsequent microbial process.

Microbial community analysis of perchlorate-reducing cultures growing on zero-valent iron.

Anaerobic microbial mixed cultures demonstrated its ability to completely remove perchlorate in the presence of zero-valent iron. In order to understand the major microbial reaction in the iron-supported culture, community analysis comprising of microbial fatty acids and polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) techniques was performed for perchlorate reducing cultures. Analysis of fatty acid methyl esters (FAMEs) and subsequent principal component analysis (PCA) showed clear distinctions not only between iron-supported perchlorate reducing culture and seed bacteria, but also among perchlorate-reducing cultures receiving different electron donors. The DGGE pattern targeting the chlorite dismutase (cld) gene showed that iron-supported perchlorate reducing culture is similar to hydrogen-fed cultures as compared to acetate-fed culture. The phylogenetic tree suggested that the dominant microbial reaction may be a combination of the autotrophic and heterotrophic reduction of perchlorate. Both molecular and chemotaxonomic experimental results support further understanding in the function of zero-valent iron as an adequate electron source for enhancing the microbial perchlorate reduction in natural and engineered systems.

Microbial treatment of high-strength perchlorate wastewater.

To treat wastewater containing high concentrations of perchlorate, a perchlorate reducing-bacterial consortium was obtained by enrichment culture grown on high-strength perchlorate (1200 mg L(-1)) feed medium, and was characterized in a sequence batch reactor (SBR) over a long-time operation. The consortium removed perchlorate in the SBR with high reduction rates (35-90 mg L(-1)h(-1)) and stable removal efficiency over 200-day operations. The maximum specific perchlorate reduction rate (qmax), half saturation constant (Ks), and optimal pH range were 0.67 mg-perchlorate mg-dry cell weight(-1) h(-1), 193.8 mg-perchlorate L(-1), and pH 7-9, respectively. The perchlorate reduction yield was 0.48 mol-perchloratemol-acetate(-1). A clone library prepared using the amplicons of cld gene encoding chlorate dismutase showed that the dominant (per)chlorate reducing bacteria in the consortium were Dechlorosoma sp. (53%), Ideonella sp. (28%), and Dechloromonas sp. (19%).

Thermophilic biofiltration of H2S and isolation of a thermophilic and heterotrophic H2S-degrading bacterium, Bacillus sp. TSO3.

Thermophilic biofiltration of H(2)S-containing gas was studied at 60 degrees C using polyurethane (PU) cubes and as a packing material and compost as a source of thermophilic microorganisms. The performance of biofilter was enhanced by pH control and addition of yeast extract (YE). With YE supplement and pH control, H(2)S removal efficiency remained above 95% up to an inlet concentration of 950 ppmv at a space velocity (SV) of 50h(-1) (residence time=1.2 min). H(2)S removal efficiency strongly correlated with the inverse of H(2)S inlet concentrations and gas flow rates. Thermophilic, sulfur-oxidizing bacteria, TSO3, were isolated from the biofilter and identified as Bacillus sp., which had high similarity value (99%) with Bacillus thermoleovorans. The isolate TSO3 was able to degrade H(2)S without a lag period at 60 degrees C in liquid cultures as well as in the biofilter. High H(2)S removal efficiencies were sustained with a periodic addition of YE. This study demonstrated that an application of thermophilic microorganism for a treatment of hot gases may be an economically attractive option since expensive pre-cooling of gases to accommodate mesophilic processes is not required.

Reductive transformation of 2,4,6-trinitrotoluene, hexahydro-1,3,5-trinitro-1,3,5-triazine, and nitroglycerin by pyrite and magnetite.

Reductive transformation of 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and nitroglycerin (NG) by pyrite (FeS(2)) and magnetite (Fe(3)O(4)) was investigated to determine the role of Fe(II)-bearing minerals on the fate of toxic explosives in Fe/S-rich natural environment. Results from batch experiments showed that 65% of TNT and 45% of RDX were transformed from solution in the presence of pyrite under pH 7.4 buffered conditions within 32 days. Without a buffered solution, transformation of TNT and RDX decreased. NG was continuously and rapidly transformed by pyrite under both conditions. Complete removal of NG was achieved in 32 days under buffered conditions. NH(4)(+) was identified as a reduction product for RDX and NG in the pyrite-water system. Reductive transformation of RDX and NG by magnetite was slower than that by pyrite. The results suggest that abiotic transformation of the explosives by pyrite and magnetite may be considered when determining the fate of explosives in Fe/S-rich subsurface environments.

Microbial reduction of nitrate in the presence of nanoscale zero-valent iron.

Microbial reduction of nitrate in the presence of nanoscale zero-valent iron (NZVI) was evaluated to assess the feasibility of employing NZVI in the biological nitrate treatment. Nitrate was completely reduced within 3d in a nanoscale Fe(0)-cell reactor, while only 50% of the nitrate was abiotically reduced over 7d at 25 degrees C. The removal rate of nitrate in the integrated NZVI-cell system was unaffected by the presence of high amounts of sulfate. Efficient removal of nitrate by Fe(II)-supported anaerobic culture in 14 d indicated that Fe(II), which is produced during anaerobic iron corrosion in the Fe(0)-cell system, might act as an electron donor for nitrate. Unlike abiotic reduction, microbial reduction of nitrate was not significantly affected by low temperature conditions. This study demonstrated the potential applicability of employing NZVI iron as a source of electrons for biological nitrate reduction. Use of NZVI for microbial nitrate reduction can obviate the disadvantages associated with traditional biological denitrification, that relies on the use of organic substrates or explosive hydrogen gas, and maintain the advantages offered by nano-particle technology such as higher surface reactivity and functionality in suspensions.

Enhanced reduction of nitrate by zero-valent iron at elevated temperatures.

Kinetics of nitrate reduction by zero-valent iron at elevated temperatures was studied through batch and column experiments. It was hypothesized that under increased solution temperatures, the zero-valent iron may accelerate the reduction of nitrate by overcoming the activation energy barrier to nitrate reduction. The results of the batch experiment showed the synergistic effects of elevated temperature (75 degrees C) and a buffered condition (pH 7.4 with 0.1 M HEPES) to enhance the rate of nitrate reduction by zero-valent iron from 0.072+/-0.006 h(-1) ((0.35+/-0.03) x 10(-4) L m(-2) h(-1)) at room temperature to 1.39+/-0.23 h(-1) ((1.03+/-0.07) x 10(-3) L m(-2) h(-1)). Complete nitrate removal was obtained in a Fe(0) column after 30 min under both buffered and unbuffered conditions at 75 degrees C. These results indicate that a temperature increase could overcome the energy barrier. We suggest that an iron reduction process at moderately elevated temperature (50-75 degrees C) may be a suitable method for removing nitrate from industrial discharges.

Microbial reduction of perchlorate with zero-valent iron.

Microbial reduction of perchlorate in the presence of zero-valent iron was examined in both batch and column reactors to assess the potential of iron as the electron donor for biological perchlorate reduction process. Iron-supported mixed cultures completely removed 65 mg/L of perchlorate in batch reactors in 8 days. The removal rate was similar to that observed with hydrogen gas (5%) and acetate (173 mg/L) as electron donors. Repeated spiking of perchlorate to batch reactors containing iron granules and microorganisms showed that complete perchlorate reduction by the iron-supported culture was sustained over a long period. Complete removal of perchlorate by iron-supported anaerobic culture was also achieved in a bench-scale iron column with a hydraulic residence time of 2 days. This study demonstrated the potential applicability of zero-valent iron as a source of electrons for biological perchlorate reduction. Use of zero-valent iron may eliminate the need to continually supply electron donors such as organic substrates or explosive hydrogen gas. In addition, iron is inexpensive, safe to handle, and does not leave organic residuals in the treated water.

Reduction of acrolein by elemental iron: kinetics, pH effect, and detoxification.

Acrolein is a highly toxic alpha,beta-unsaturated aldehyde that is widely used as a biocide, a cross-linking agent, and an intermediate in the chemical industry, among other applications. In this study we investigated the reductive transformation of acrolein by elemental iron and evaluated the feasibility of using iron to detoxify acrolein. At acidic and neutral pH, acrolein was transformed by iron through reduction of the C=C double bond to propionaldehyde. The reduction appeared to involve the chemisorption of acrolein to the iron surface followed by reduction of adsorbed acrolein. Both the adsorption and reduction rate constants decreased with increasing pH. Between pH 7.0 and 7.4, the acrolein adsorption rate constant decreased precipitously, resulting in a sharp decline in its removal rate. At higher pH, acrolein disappeared rapidly in control without iron, presumably due to reversible, base-catalyzed hydration. At equilibrium, approximately 93% of acrolein was hydrated, corresponding to an equilibrium constant of 13. Acrolein at 25 mg/L completely inhibited aerobic respiration; in contrast, its reduction product propionaldehyde was biodegradable. This suggests that elemental iron may be used to pretreat acrolein-containing wastes prior to aerobic biodegradation. To our knowledge, this is the first report of reduction and detoxification of an alpha,beta-unsaturated aldehyde by elemental iron.