Boosting sensitivity of organic vapor detection with silicone block polyimide polymers

We demonstrate that silicone block polyimide polymers have an unusually high sensitivity to nonpolar organic vapors, including chlorinated organic solvent vapors. When 0.18-5.34-microm-thick films of silicone block polyimide polymers were deposited onto 10-MHz thickness shear mode (TSM) oscillators, these films were implemented to detect parts-per-billion concentrations of trichloroethylene (TCE) with a detection sensitivity of 0.5-23.5 Hz per 500 ppb of vapor. With a film thickness of 3.4 microm (91.5-kHz frequency shift upon film deposition), optimized for the minimal sensor noise of 0.04 Hz, the calculated detection limit of sensor response (S/N = 3) was 3 ppb of TCE. Detection limits for other chlorinated organic solvent vapors, such as perchloroethylene (PCE), cis-1,2-dichloroethylene (DCE), trans-1,2-DCE, 1,1-DCE, and vinyl chloride (VC) were 0.6, 6, 6, 11, and 13 ppb, respectively. Assuming only the mass-loading response when deposited onto the TSM devices, silicone block polyimide polymers have partition coefficients of over 200 000 to parts-per-billion concentrations of TCE that make them at least 100 times more sensitive than other known polymers for TCE detection. We observed that unlike conventional polyimides, water sensitivity of the new hybrid polyimides is suppressed because of the silicone soft block. Water sensitivity is comparable with the sensor response to nonpolar organic vapors. The high sensitivity and long-term stability of these sensor materials make them attractive for ultrasensitive practical sensors.     

Effects of pH on dechlorination of trichloroethylene by zero-valent iron

The surface normalized reaction rate constants (k(sa)) of trichloroethylene (TCE) and zero-valent iron (ZVI) were quantified in batch reactors at pH values between 1.7 and 10. The k(sa) of TCE linearly decreased from 0.044 to 0.009l/hm(2) between pH 3.8 and 8.0, whereas the k(sa) at pH 1.7 was more than an order higher than that at pH 3.8. The degradation of TCE was not observed at pH values of 9 and 10. The k(sa) of iron corrosion linearly decreased from 0.092 to 0.018l/hm(2) between pH 4.9 and 9.8, whereas it is significantly higher at pH 1.7 and 3.8. The k(sa) of TCE was 30-300 times higher than those reported in literature. The difference can be attributed to the pH effects and precipitation of iron hydroxide. The k(sa) of TCE degradation and iron corrosion at a head space of 6 and 10ml were about twice of those at zero head space. The effect was attributed to the formation of hydrogen bubbles on ZVI, which hindered the transport the TCE between the solution and reaction sites on ZVI. The optimal TCE degradation rate was achieved at a pH of 4.9. This suggests that lowering solution pH might not expedite the degradation rate of TCE by ZVI as it also caused faster disappearance of ZVI, and hence decreased the ZVI surface concentration.     

Effects of aquifer heterogeneity and reaction mechanism uncertainty on a reactive barrier.

This paper addresses impacts of aquifer heterogeneity and reaction mechanism uncertainty on permeable reactive barrier (PRB) performance and describes modeling tools and preliminary guidelines for risk-based design of reactive barriers at heterogeneous sites. A braided stream aquifer was generated stochastically, using a fixed correlation structure and four levels of variability in the hydraulic conductivity field. A vertical, homogeneous barrier was placed in the aquifer. Based on a deterministic design, the size of the PRB for uniform conditions was considered conservative (factor of safety=3.3). Monte Carlo simulation was used to model cis 1,2-DCE reduction by iron metal with uncertainty in the reaction mechanism rate constants. These results were combined with flow and particle tracking results to predict the spatial distribution and flow-averaged concentrations of cis 1,2-DCE and vinyl chloride at the exit face of the PRB. Evaluated on a risk basis, the deterministic design method was found to be unconservative for more heterogeneous aquifers. Uncertainty in the reaction mechanism accentuated the negative effects of aquifer heterogeneity. Several compensating factors that may reduce the vulnerability of reactive barriers to aquifer heterogeneity are discussed.     

Laboratory study on sequenced permeable reactive barrier remediation for landfill leachate-contaminated groundwater

Permeable reactive barrier (PRB) was a promising technology for groundwater remediation. Landfill leachate-polluted groundwater riches in various hazardous contaminants. Two lab-scale reactors (reactors A and B) were designed for studying the feasibility of PRB to remedy the landfill leachate-polluted groundwater. Zero valent iron (ZVI) and the mixture of ZVI and zeolites constitute the first section of the reactors A and B, respectively; the second section of two reactors consists of oxygen releasing compounds (ORCs). Experimental results indicated that BOD5/COD increased from initial 0.32 up to average 0.61 and 0.6 through reactors A and B, respectively. Removal efficiency of mixed media for pollutants was higher than that of single media (ZVI only). Zeolites exhibited selective removal of Zn, Mn, Mg, Cd, Sr, and NH4+, and removal efficiency was 97.2%, 99.6%, 95.9%, 90.5% and 97.4%, respectively. The maximum DO concentration of reactors A and B were 7.64 and 6.78mg/L, respectively, while the water flowed through the ORC. Therefore, sequenced PRB system was effective and was proposed as an alternative method to remedy polluted groundwater by landfill leachate.     

Effects of ferrous ions on the reductive dechlorination of trichloroethylene by zero-valent iron

The surface characteristics of zero-valent iron (ZVI) and the efficiency of reductive dechlorination of trichloroethylene (TCE) in the presence of ferrous ions were studied. The experimental results indicated that the acid-washing of a metallic iron sample enhanced the efficiency of TCE degradation by ZVI. This occurred because acid-washing changed the conformation of oxides on the surface of iron from maghemite (gamma-Fe(2)O(3)) to the more hydrated goethite (alpha-FeOOH), as was confirmed by XPS analysis. However, when ferrous ions were simultaneous with TCE in water, the TCE degradation rate decreased as the concentration of ferrous ion increased. This was due to the formation of passive precipitates of ferrous hydroxide, including maghemite and magnetite (Fe(3)O(4)), that coated on the surface of acid-washed ZVI, which as a result inhibited the electron transfer and catalytic hydrogenation mechanisms. On the other hand, in an Fe(0)-TCE system without the acid-washing pretreatment of ZVI, ferrous ions were adsorbed into the maghemite lattice which was then converted to semiconductive magnetite. Thus, the electrons were transferred from the iron surface and passed through the precipitates, allowing for the reductive dechlorination of TCE.     

Semicontinuous microcosm study of aerobic cometabolism of trichloroethylene using toluene

A semicontinuous slurry-microcosm method was applied to mimic trichloroethylene (TCE) cometabolic biodegradation field results at the Que-Jen in-situ pilot study. The microcosm study confirmed the process of aerobic cometabolism of TCE using toluene as the primary substrate. Based on the nucleotide sequence of 16S rRNA genes, the toluene-oxidizing bacteria in microcosms were identified, i.e. Ralstonia sp. P-10 and Pseudomonasputida. The first-order constant of TCE-degradation rate was 0.5 day(-1) for both Ralstonia sp. P-10 and P.putida. The TCE cometabolic-biodegradation efficiency measured from the slurry microcosms was 46%, which appeared pessimistic compared to over 90% observed from the in-situ pilot study. The difference in the TCE cometabolic-biodegradation efficiency was likely due to the reactor configurations and the effective time duration of toluene presence in laboratory microcosms (1 days) versus in-situ pilot study (3 days). The results of microcosm experiments using different toluene-injection schedules supported the hypothesis. With a given amount of toluene injection, it is recommended to maximize the effective time duration of toluene presence in reactor design for TCE cometabolic degradation.     

Long-term aerobic cometabolism of a chlorinated solvent mixture by vinyl chloride-, methane- and propane-utilizing biomasses

The aerobic cometabolic biodegradation of a mixture of chlorinated aliphatic hydrocarbons (CAHs) including vinyl chloride (VC), cis- and trans-1,2-dichloroethylene (cis-DCE, trans-DCE), trichloroethylene (TCE), 1,1,2-trichloroethane (1,1,2-TCA) and 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA) was investigated at both 25 and 17 degrees C by means of bioaugmented and non-bioaugmented sediment-groundwater slurry microcosm tests. The goals of the study were (i) to study the long-term aerobic biodegradation of a CAH mixture including a high-chlorinated solvent (1,1,2,2-TeCA) generally considered non-biodegradable in aerobic conditions; (ii) to investigate the efficacy of bioaugmentation with two types of internal inocula obtained from the indigenous biomass of the studied site; (iii) to identify the CAH-degrading bacteria. VC, methane and propane were utilized as growth substrates. The non-bioaugmented microcosms were characterized, at 25 degrees C, by an average 18-day lag-time for the direct metabolism of VC (accompanied by the cometabolism of cis- and trans-DCE) and by long lag-times (36-264 days) for the onset of methane or propane utilization (associated with the cometabolism of the remaining CAHs). In the inoculated microcosms the lag-phases for the onset of growth substrate utilization and CAH cometabolism were significantly shorter (0-15 days at 25 degrees C). Biodegradation of the 6-CAH mixture was successfully continued for up to 410 days. The low-chlorinated solvents were characterized by higher depletion rates. The composition of the microbial consortium of a propane-utilizing microcosm was determined by 16s rDNA sequencing and phylotype analysis. To the best of our knowledge, this is the first study that documents the long-term aerobic biodegradation of 1,1,2,2-TeCA.     

Zero-valent iron pretreatment for detoxifying iodine in liquid crystal display (LCD) manufacturing wastewater

This study investigated reductive transformation of iodine by zero-valent iron (ZVI), and the subsequent detoxification of iodine-laden wastewater. ZVI completely reduced aqueous iodine to non-toxic iodide. Respirometric bioassay illustrated that the presence of iodine increase the lag phase before the onset of oxygen consumption. The length of lag phase was proportional to increasing iodine dosage. The reduction products of iodine by ZVI did not exhibit any inhibitory effect on the biodegradation. The cumulative biological oxidation associated with iodine toxicity was closely fitted to Gompertz model. When iodine-laden wastewater was continuously fed to a bench-scale activated sludge unit, chemical oxygen demand (COD) removal efficiencies decreased from above 90% to below 80% along with a marked decrease in biomass concentration. On the other hand, the COD removal efficiency and biomass concentration remained constant in the integrated ZVI-activated sludge system. Respirometric bioassay with real iodine-laden LCD manufacturing wastewater demonstrated that ZVI was effective for detoxifying iodine and consequently enhancing biodegradability of wastewater. This result suggested that ZVI pretreatment may be a feasible option for the removal of iodine in LCD processing wastewater, instead of more costly processes such as adsorption and chemical oxidation, which are commonly in the iodine-laden LCD wastewater treatment facility.     

Intrinsic bioremediation of trichloroethylene and chlorobenzene: field and laboratory studies.

Activities at a former fire training area at Robins Air Force Base in Georgia, USA resulted in contamination of groundwater with a mixture of trichloroethylene (TCE) and chlorobenzene (CB). Results from the field investigation suggest that intrinsic bioremediation process is occurring, which caused the decrease in TCE and CB concentrations, and increase in TCE degradation byproducts [e.g., dichloroethylene isomers (DCEs), vinyl chloride (VC)] concentrations. Contaminated groundwater samples collected from this site were used to conduct microbial enumeration tests, and used as the inocula for microcosm establishment. Results from the microbial enumeration study indicate that methanogenesis was the dominant biodegradation pattern within the source and mid-plume areas, and the aerobic biodegradation process dominated the downgradient area. Laboratory microcosm experiments were conducted to evaluate the feasibility of using CB as the primary substrate to enhance the intrinsic biodegradation of TCE. Microcosm results suggest that CB can serve as the primary substrate (electron donor), and enhance TCE biodegradation to less-chlorinated compounds under both aerobic cometabolism and reductive dechlorination conditions.     

Advanced treatment of coking wastewater by coagulation and zero-valent iron processes

Advanced treatment of coking wastewater was investigated experimentally with coagulation and zero-valent iron (ZVI) processes. Particular attention was paid to the effect of dosage and pH on the removal of chemical oxygen demand (COD) in the two processes. The results showed that ZVI was more effective than coagulation for advanced treatment of coking wastewater. The jar tests revealed that maximal COD removal efficiency of 27.5-31.8% could be achieved under the optimal condition of coagulation, i.e. 400mg/L of Fe(2)(SO(4))3 as coagulant at pH 3.0-5.0. On the other hand, the COD removal efficiency could be up to 43.6% under the idealized condition of ZVI upon 10 g/L active carbon and 30 g/L iron being dosed at pH 4.0. The mechanisms for COD removal in ZVI were dominated by coagulation, precipitation and oxidation-reduction. ZVI would also enhance the biodegradability of effluent by increasing BOD5/COD from 0.07 to 0.53. Moreover, some ester compounds could be produced in the reaction. Although ZVI was found more efficient than coagulation in eliminating low molecular weight (<2000 Da) compounds in the wastewater, there were still a few residual contaminants which could hardly be eliminated by either of the process.     

Synthesis of granular activated carbon/zero valent iron composites for simultaneous adsorption/dechlorination of trichloroethylene

The coupling adsorption and degradation of trichloroethylene (TCE) through dechlorination using synthetic granular activated carbon and zerovalent iron (GAC-ZVI) composites was studied. The GAC-ZVI composites were prepared from aqueous Fe²⁺ solutions by impregnation with and without the use of a PEG dispersant and then heated at 105 °C or 700 °C under a stream of N₂. Pseudo-first-order rate constant data on the removal of TCE demonstrates that the adsorption kinetics of GAC is similar to those of GAC-ZVI composites. However, the usage of GAC-ZVI composites liberated a greater amount of Cl than when ZVI was used alone. The highest degree of reductive dechlorination of TCE was achieved using a GAC-ZVI700P composite (synthesized using PEG under 700 °C). A modified Langmuir-Hinshelwood rate law was employed to depict the behavior of Cl liberation. As a result, a zero-order Cl liberation reaction was observed and the desorption limited TCE degradation rate constant decreased as the composite dosage was increased. The GAC-ZVI composites can be employed as a reactive GAC that is not subject to the limitations of using GAC and ZVI separately.     

The removal of nitrate by nanoscale iron particles produced using the sodium borohydride method

This study was conducted to investigate removal of nitrate by nanoscale zero-valent iron (ZVI) particles in aqueous solution. ZVI particles was produced from wasted acid that is by-products of a pickling line at a steel work. The reaction activity of ZVI particles was evaluated through decomposition experiments of NO3-N aqueous solution. Addition of a larger amount of ZVI particles resulted in a higher decomposition rate. ZVI particles showed higher decomposition efficiencies than commercially purchased ZVI particles at all pH values. Both ZVIs showed a higher decomposition rate at a lower pH. Virtually no decomposition reaction was observed at pH of 4 or higher for purchased ZVI. The ZVI particles produced directly from wasted acid by the sodium borohydride method were not easy to handle because they were very small (10-200 nm) and were oxidized easily in the air.     

Reactivity of Fe(II)/cement systems in dechlorinating chlorinated ethylenes

Ferrous iron (Fe(II)) in combination with Portland cement is effective in reductively dechlorinating chlorinated organics and can be used to achieve immobilization and degradation of contaminants simultaneously. Reactivities of chlorinated ethylenes (perchloroethylene (PCE), trichloroethylene (TCE), 1,1-dichloroethylene (1,1-DCE), vinyl chloride (VC)) in Fe(II)/cement systems were characterized using batch slurry reactors. Reduction kinetics of the chlorinated ethylenes were sufficiently fast to be utilized for the proposed treatment scheme, and were described by a pseudo-first-order rate law. The order of reactivity of the chlorinated ethylenes was TCE>1,1-DCE>PCE>VC. Reduction of TCE and PCE mainly yielded acetylene, implying that the transformation of the two compounds occurred principally via reductive beta-elimination pathways. Transformation of 1,1-DCE and VC gave rise to primarily ethylene, implying that major degradation pathways were a reductive alpha-elimination for the former and a hydrogenolysis for the latter. The reactivity of the Fe(II)/cement systems in dechlorinating TCE was proportional to Fe(II) dose when the Fe(II)/cement mass ratio varied between 5.6 and 22.3%. The Fe(II)/cement systems with a higher Fe(II) loading were less extensively affected by pH in reductive reactions for TCE than in the previous experiments with PCE or chlorinated methanes. Amendment of Fe(II)/cement systems with Fe(III) addition was found effective in increasing the reactivity in the previous study, but the current findings indicated that the extent to which the reaction rate increased by the amendment might be dependent on the source of the cement and/or the compounds tested.     

Evaluation of natural attenuation rate at a gasoline spill site

Contamination of groundwater by gasoline and other petroleum-derived hydrocarbons released from underground storage tanks (USTs) is a serious and widespread environmental problem. Natural attenuation is a passive remedial approach that depends upon natural processes to degrade and dissipate contaminants in soil and groundwater. Currently, in situ column technique, microcosm, and computer modeling have been applied for the natural attenuation rate calculation. However, the subsurface heterogeneity reduces the applicability of these techniques. In this study, a mass flux approach was used to calculate the contaminant mass reduction and field-scale decay rate at a gasoline spill site. The mass flux technique is a simplified mass balance procedure, which is accomplished using the differences in total contaminant mass flux across two cross-sections of the contaminant plume. The mass flux calculation shows that up to 87% of the dissolved total benzene, toluene, ethylbenzene, and xylene (BTEX) isomers removal was observed via natural attenuation at this site. The efficiency of natural biodegradation was evaluated by the in situ tracer method, and the first-order decay model was applied for the natural attenuation/biodegradation rate calculation. Results reveal that natural biodegradation was the major cause of the BTEX mass reduction among the natural attenuation processes, and approximately 88% of the BTEX removal was due to the natural biodegradation process. The calculated total BTEX first-order attenuation and biodegradation rates were 0.036 and 0.025% per day, respectively. Results suggest that the natural attenuation mechanisms can effectively contain the plume, and the mass flux method is useful in assessing the occurrence and efficiency of the natural attenuation process.     

In situ reductive dechlorination of chlorinated ethenes in high nitrate groundwater

In situ bioremediation using carbohydrate was evaluated as an in situ treatment alternative for trichloroethene (TCE) and cis-1,2-dichloroethene (cDCE) in groundwater containing high nitrate concentrations. Upon addition of carbohydrate to groundwater, sequential reduction of electron acceptors was observed, where nitrate was reduced early in the pilot test, followed by sulfate and TCE. Reduction of cDCE to vinyl chloride and ethene occurred in conjunction with increased iron and manganese, and increased methane concentrations, approximately 7 months into the evaluation period, following depletion of nitrate and sulfate. TCE, cDCE, and vinyl chloride concentrations decreased from approximately 500 to >10 microg/L within 21 months of operation.     

Effect of spatial distribution and aging of ZVI on the reactivity of resin–ZVI composites for arsenite removal

A zero-valent iron (ZVI)–resin composite (D201–ZVI) has been proven as an effective arsenic removal material. Here, the effect of ZVI distribution and aging on the reactivity of the hybrid composite was investigated by comparing the As(III) removal performances of freshly synthesized and aged D201–ZVI composites. The ZVI distribution and structures of these composites were characterized using X-ray diffraction, X-ray photoelectron spectrometry, and scanning electron microscope equipped with an energy-dispersive X-ray analyzer (SEM-EDX). After aging in aerated water for 96 h, the ZVI distribution in the aged composites did not change significantly, which was confirmed by SEM-EDX. However, the Fe⁰ content decreased as the aging time increased. Among the as-prepared composites with variant ZVI distributions, the hybrids with more uniform ZVI distribution exhibited higher removal efficiency and faster reaction rate. Experimental results show that the D201–ZVI gradually lost reactivity with an increase in aging time from 24 to 96 h. The effects of aging time on the speciation of As suggest that the reduced As(III) removal efficiency was attributable to the decrease of the Fe⁰ content. Furthermore, the importance of the ZVI distribution is proposed to explain the aging effect.     

Reduction of chromate from electroplating wastewater from pH 1 to 2 using fluidized zero valent iron process

Fluidized zero valent iron (ZVI) process was conducted to reduce hexavalent chromium (chromate, CrO(4)(2-)) to trivalent chromium (Cr(3+)) from electroplating wastewater due to the following reasons: (1) Extremely low pH (1-2) for the electroplating wastewater favoring the ZVI reaction. (2) The ferric ion, produced from the reaction of Cr(VI) and ZVI, can act as a coagulant to assist the precipitation of Cr(OH)(3(s)) to save the coagulant cost. (3) Higher ZVI utilization for fluidized process due to abrasive motion of the ZVI. For influent chromate concentration of 418 mg/L as Cr(6+), pH 2 and ZVI dosage of 3g (41 g/L), chromate removal was only 29% with hydraulic detention time (HRT) of 1.2 min, but was increased to 99.9% by either increasing HRT to 5.6 min or adjusting pH to 1.5. For iron species at pH 2 and HRT of 1.2 min, Fe(3+) was more thermodynamically stable since oxidizing agent chromate was present. However, if pH was adjusted to 1.5 or 1, where chromate was completely removed, high Fe(2+) but very low Fe(3+) was present. It can be explained that ZVI reacted with chromate to produce Fe(2+) first and the presence of chromate would keep converting Fe(2+) to Fe(3+). Therefore, Fe(2+) is an indicator for complete reduction from Cr(VI) to Cr(III). X-ray diffraction (XRD) was conducted to exam the remained species at pH 2. ZVI, iron oxide and iron sulfide were observed, indicating the formation of iron oxide or iron sulfide could stop the chromate reduction reaction.     

Substrate interactions during anaerobic biodegradation of BTEX by the mixed cultures under nitrate reducing conditions

The enriched BTEX-degrading bacteria were used to investigate the substrate interactions during anaerobic biodegradation of all the possible BTEX binary combinations. Beneficial and detrimental substrate interactions were observed in comprehensive mixtures of benzene, toluene, ethylbenzene, o-xylene, m-xylene and p-xylene. The amendment of toluene or ethylbenzene could stimulate benzene degradation. Lower concentrations of m-xylene would enhance the degradation of benzene, whereas degradation of benzene was inhibited with higher concentrations of m-xylene. The simultaneous presence of toluene and ethylbenzene could stimulate the degradation of each other. The addition of toluene stimulated o-xylene degradation, whereas the amendment of ethylbenzene inhibited the degradation of o-xylene. Lower concentrations of toluene or ethylbenzene would enhance the degradation of m-xylene and p-xylene, whereas higher concentrations of toluene or ethylbenzene had a slight inhibitory effect on m-xylene and p-xylene degradation. The amendment of benzene, m-xylene or p-xylene would inhibit the degradation of other BTEX compounds. When the concentration of BTEX mixtures was over 150mg/l, the degradation of benzene, o-xylene, m-xylene and p-xylene was severely inhibited.     

The effect of salinity conditions on kinetics of trichloroethylene biodegradation by toluene-oxidizing cultures

This study investigates the effect of salt (NaCl) conditions on the biodegradations of trichloroethylene (TCE) by mixed cultures enriched on toluene. Two cultures were separately cultivated in this investigation, involving culture LHTO4, cultivated with freshwater and culture HHTO4, cultivated with 3.5% (w/v) NaCl solution. Batch tests were conducted to elucidate the degradations of toluene, TCE and a mixture of toluene and TCE by cultures LHTO4 at salinities of 0, 2 and 3.5% and by HHTO4 at salinity of 3.5%. The measurements were analyzed with microbial kinetics. The results show that for culture LHTO4 in the resting cells, when the transient salinities increased from 0 to 3.5%, the maximum specific rate of TCE degradation, k(TCE), declined from 2.28 to 1.45 d(-1), and the observed TCE transformation capacity, T(c,obs), decreased from 0.060 to 0.036 mgTCE/mgVSS. In the presence of toluene, TCE degradation was more inhibited by toluene (inhibition coefficients, K(I,TOL) were 0.8, 2.2, and 0.96 mg/L for salinity 0, 2, and 3.5%, respectively) than toluene degradation was by TCE (K(I,TCE) were 14, 5.8, and 1000 mg/L for salinity 0, 2, and 3.5%, respectively). Under long-term salinity stress, the culture HHTO4 maintained its capacity to utilize toluene but lost its effectiveness in the cometabolic transformation of TCE: k(TCE) fell to 0.25 d(-1) and T(c,obs) dropped to 0.024 mgTCE/mgVSS. This work reveals that the degradation of TCE by toluene-oxidizing cultures under saline conditions can be best described by the chosen kinetic equations and experimentally estimated constants, which can thus be used to lay a foundation for the rational design of biological processes to remove TCE from saline solutions.