A synergistic platform enables co-oxidation of halogenated organic pollutants without input of organic primary substrate

While co-oxidation is widely used to biodegrade halogenated organic pollutants (HOPs), a considerable amount of organic primary substrate is required. Adding organic primary substrates increases the operating cost and also leads to extra carbon dioxide release. In this study, we evaluated a two-stage Reduction and Oxidation Synergistic Platform (ROSP), which integrated catalytic reductive dehalogenation with biological co-oxidation for HOPs removal. The ROSP was a combination of an H₂-based membrane catalytic-film reactor (H_2-MCfR) and an O₂-based membrane biofilm reactor (O_2-MBfR). 4-chlorophenol (4-CP) was used as a model HOP to evaluate the performance of ROSP. In the MCfR stage, zero-valent palladium nanoparticles (Pd⁰NPs) catalyzed reductive hydrodechlorination that converted 4-CP to phenol, with a conversion yield over 92%. In the MBfR stage, the phenol was oxidized and used as a primary substrate that supported the co-oxidation of residual 4-CP. Genomic DNA sequencing revealed that phenol produced from 4-CP reduction enriched bacteria having genes for functional enzymes for phenol biodegradation in the biofilm community. In the ROSP, over 99% of 60 mg/L 4-CP was removed and mineralized during continuous operation: Effluent 4-CP and chemical oxygen demand concentrations were below 0.1 and 3 mg/L, respectively. H₂ was the only added electron donor to the ROSP, which means no extra carbon dioxide was produced by primary-substrate oxidation.

Dark transformation from 17β-estradiol to estrone initiated by hydroxyl radical in dissolved organic matter

The occurrence and fate of 17β-estradiol (E₂) in natural water have gained extensive attention owing to its high ecotoxic risk to wildlife. Dissolved organic matter (DOM) is a ubiquitous water constituent and contributes significantly to E₂ removal, although the reaction mechanism is rarely clarified. The present study aims to investigate E₂ transformation in water containing fresh or aged DOM surrogates at environmentally relevant concentrations in the dark. Experiments along with radical probes of benzene and furfuryl alcohol reveal that reactive radicals, particularly hydroxyl radical (·OH), formed non-photochemically at higher concentrations in aged DOM than in fresh DOM. The contribution of ·OH in E₂ removal is indicated by the decreases in the removal of radical probes in the presence of E₂; moreover, E₂ removal is inhibited in the presence of radical scavengers. The dose-dependent inhibitive effect of substrate concentrations, including E₂ and coexistent propylparaben, shows that the radical concentration is a limiting factor for E₂ removal, which could be enhanced by increasing DOM concentration, dissolved oxygen, and light supply. As the main byproduct, estrone (E₁) is persistent in the current DOM water in the dark, but it can be easily photodegraded when exposed to light. Theoretical analysis reveals that the initial step is ·OH-initiated H- abstraction on the hydroxyl group in the cyclopentane ring of E₂. The formed singlet excited state of E₂ undergoes further intramolecular rearrangement and oxidative dehydrogenation to generate E₁ and the hydroperoxy radical (·HO₂). Considering the universal occurrence of E₂ in DOM-rich aquatic matrices, the present findings have special implications for the biogeochemical cycle and risk assessment of this pollutant in natural aquatic environments, particularly those beyond the photic zone.

A review of the biotransformations of priority pharmaceuticals in biological wastewater treatment processes

Wastewater effluent discharges have been considered as one of the main sources of synthetic chemicals entering into the aquatic environment. Even though they occur at low concentrations, pharmaceutically active compounds (PhACs) can have an impact on ecological toxicity that affects aquatic organisms. Moreover, new regulations in development toward preserving water quality reinforces the increasing need to monitor and abate some PhACs in wastewater treatment plants (WWTPs), where they are typically only partially eliminated. Unlike most previous reviews, we have focussed on how the main biological and chemical molecular factors impact the biotransformations of key PhACs in biological WWTP processes. Biotransformations have been found to be an important contributor towards the removal of PhACs from WWTP effluents. This review paper critically assesses these aspects and the recent advances that have been achieved in wastewater treatment processes for biodegradation of 7 PhACs; namely the non-steroidal anti-inflammatory drug (NSAID) diclofenac (DCF); the macrolide antibiotics azithromycin (AZM), erythromycin (ERY) and clarithromycin (CLR); the two natural estrogens estrone (E1) and 17β-estradiol (E2), and the synthetic estrogen 17α-ethinylesradiol (EE2). These represent the micropollutants of the EU Watch list in Decision 2015/495/EU that are most relevant to WWTPs due to their frequent detection. The metabolic pathways, transformation products and impact of relevant factors to biological WWTP processes is addressed in this review. The biokinetics of PhAC biodegradation in different engineered bioprocesses is also discussed. Promising technologies and operational strategies that are likely to have a high impact on controlling PhAC releases are highlighted and future research needs are also proposed.

Aerobic cometabolism of 1,4-dioxane by isobutane-utilizing microorganisms including Rhodococcus rhodochrous strain 21198 in aquifer microcosms: Experimental and modeling study

Aerobic cometabolism of the emerging contaminant 1,4-dioxane (1,4-D) by isobutane-utilizing microorganisms was assessed in pure culture and aquifer microcosm studies. The bacterium Rhodococcus rhodochrous strain ATCC 21198 transformed low, environmentally-relevant concentrations of 1,4-D when grown on isobutane. Microcosms were constructed with aquifer solids from Fort Carson, Colorado, a site contaminated with 1,4-D and trichloroethene (TCE). Multiple additions of isobutane and 1,4-D over 300 days were transformed in microcosms biostimulated with isobutane and microcosms bioaugmented with strain 21198. Results showed that, over time and with sufficient inorganic nutrients, biostimulation of native microorganisms with isobutane was just as effective as bioaugmentation with strain 21198 to achieve 1,4-D transformation in the microcosms. The presence of TCE at 0.2 mg/L did not inhibit 1,4-D transformation, though TCE itself was not readily transformed. An iterative process was used to determine kinetic parameter values to fit Michaelis-Menten/Monod models to experimental data for simultaneous isobutane utilization, biomass growth, and cometabolic transformation of 1,4-D. Parameter optimization resulted in good model fit to the data over multiple transformations of isobutane and 1,4-D in both short- and long-term experiments. Results suggest low concentrations of 1,4-D studied in the microcosms were cometabolically transformed according to a pseudo first-order rate of 0.37 L/mg TSS/day of 21198. Isobutane consumption was modeled with a maximum rate of 2.58 mg/mg TSS/day and a half saturation constant of 0.09 mg/L. 1,4-D transformation was competitively inhibited by the presence of isobutane and transformation rates were significantly reduced when inorganic nutrients were limiting. Simulations of the repeated additions found a first-order microbial endogenous decay coefficient of 0.03 day^-1 fit the alternating periods of active transformation and stagnation between isobutane and 1,4-D additions over approximately one year. The model fitting process highlighted the importance of determining kinetic parameters from data representing low concentrations typically found in the environment.

Roles of ammonia-oxidizing bacteria in improving metabolism and cometabolism of trace organic chemicals in biological wastewater treatment processes: A review

While there has been a significant recent improvement in the removal of pollutants in natural and engineered systems, trace organic chemicals (TrOCs) are posing a major threat to aquatic environments and human health. There is a critical need for developing potential strategies that aim at enhancing metabolism and/or cometabolism of these compounds. Recently, knowledge regarding biodegradation of TrOCs by ammonia-oxidizing bacteria (AOB) has been widely developed. This review aims to delineate an up-to-date version of the ecophysiology of AOB and outline current knowledge related to biodegradation efficiencies of the frequently reported TrOCs by AOB. The paper also provides an insight into biodegradation pathways by AOB and transformation products of these compounds and makes recommendations for future research of AOB. In brief, nitrifying WWTFs (wastewater treatment facilities) were superior in degrading most TrOCs than non-nitrifying WWTFs due to cometabolic biodegradation by the AOB. To fully understand and/or enhance the cometabolic biodegradation of TrOCs by AOB, recent molecular research has focused on numerous crucial factors including availability of the compounds to AOB, presence of growth substrate (NH₄-N), redox potentials, microorganism diversity (AOB and heterotrophs), physicochemical properties and operational parameters of the WWTFs, molecular structure of target TrOCs and membrane-based technologies, may all significantly impact the cometabolic biodegradation of TrOCs. Still, further exploration is required to elucidate the mechanisms involved in biodegradation of TrOCs by AOB and the toxicity levels of formed products.

Aerobic biodegradation kinetics for 1,4-dioxane under metabolic and cometabolic conditions

Biodegradation of 1,4-dioxane has been studied extensively, however, there is insufficient information on the kinetic characteristics of cometabolism by propanotrophs and a lack of systematic comparisons to metabolic biodegradation. To fill in these gaps, experiments were performed with suspended growth cultures to determine 16 Monod kinetic coefficients that describe metabolic consumption of 1,4-dioxane by Pseudonocardia dioxanivorans CB1190 and cometabolism by the propanotrophic mixed culture ENV487 and the propanotroph Rhodococcus ruber ENV425. Maximum specific growth rates were highest for ENV425, followed by ENV487 and CB1190. Half saturation constants for 1,4-dioxane for the propanotrophs were one-half to one-quarter those for CB1190. Propane was preferentially degraded over 1,4-dioxane, but the reverse did not occur. A kinetic model was used to simulate batch biodegradation of 1,4-dioxane. Propanotrophs decreased 1,4-dioxane from 1000 to 1 μg/L in less time than CB1190 when the initial biomass concentration was 0.74 mg COD/L; metabolic biodegradation was favored at higher initial biomass concentrations and higher initial 1,4-dioxane concentrations. 1,4-Dioxane biodegradation was inhibited when oxygen was below 1.5 mg/L. The kinetic model provides a framework for comparing in situ biodegradation of 1,4-dioxane via bioaugmentation with cultures that use the contaminant as a growth substrate to those that achieve biodegradation via cometabolism.

Biotransformation of pharmaceuticals by ammonia oxidizing bacteria in wastewater treatment processes

Pharmaceutical residues could potentially pose detrimental effects on aquatic ecosystems and human health, with wastewater treatment being one of the major pathways for pharmaceuticals to enter into the environment. Enhanced removal of pharmaceuticals by ammonia oxidizing bacteria (AOB) has been widely observed in wastewater treatment processes. This article reviews the current knowledge on the biotransformation of pharmaceuticals by AOB. The relationship between the pharmaceuticals removal and nitrification process was revealed. The important role of AOB-induced cometabolism on the biotransformation of pharmaceuticals as well as their transformation products and pathways was elucidated. Kinetics and mathematical models describing the biotransformation of pharmaceuticals by AOB were also reviewed. The results highlighted the high degradation capabilities of AOB toward some refractory pharmaceuticals, with their degradations being clearly related to the nitrification rate and their transformation products being identified, which may exhibit similar or higher ecotoxicological impacts compared to the parent compound.

Cometabolic degradation of ethyl mercaptan by phenol-utilizing Ralstonia eutropha in suspended growth and gas-recycling trickle-bed reactor

The degradability of ethyl mercaptan (EM), by phenol-utilizing cells of Ralstonia eutropha, in both suspended and immobilized culture systems, was investigated in the present study. Free-cells experiments conducted at EM concentrations ranging from 1.25 to 14.42 mg/l, showed almost complete removal of EM at concentrations below 10.08 mg/l, which is much higher than the maximum biodegradable EM concentration obtained in experiments that did not utilize phenol as the primary substrate, i.e. 2.5 mg/l. The first-order kinetic rate constant (kSKS) for EM biodegradation by the phenol-utilizing cells (1.7 l/g biomass/h) was about 10 times higher than by cells without phenol utilization. Immobilized-cells experiments performed in a gas recycling trickle-bed reactor packed with kissiris particles at EM concentrations ranging from 1.6 to 36.9 mg/l, showed complete removal at all tested concentrations in a much shorter time, compared with free cells. The first-order kinetic rate constant (rmaxKs) for EM utilization was 0.04 l/h for the immobilized system compared to 0.06 for the suspended-growth culture, due to external mass transfer diffusion. Diffusion limitation was decreased by increasing the recycling-liquid flow rate from 25 to 65 ml/min. The removed EM was almost completely mineralized according to TOC and sulfate measurements. Shut down and starvation experiments revealed that the reactor could effectively handle the starving conditions and was reliable for full-scale application.

Evaluating alternate biokinetic models for trace pollutant cometabolism

Mathematical models of cometabolic biodegradation kinetics can improve our understanding of the relevant microbial reactions and allow us to design in situ or in-reactor applications of cometabolic bioremediation. A variety of models are available, but their ability to describe experimental data has not been systematically evaluated for a variety of operational/experimental conditions. Here five different models were considered: first-order; Michaelis-Menten; reductant; competition; and combined models. The models were assessed on their ability to fit data from simulated batch experiments covering a realistic range of experimental conditions. The simulated observations were generated by using the most complex model structure and parameters based on the literature, with added experimental error. Three criteria were used to evaluate model fit: ability to fit the simulated experimental data, identifiability of parameters using a colinearity analysis, and suitability of the model size and complexity using the Bayesian and Akaike Information criteria. Results show that no single model fits data well for a range of experimental conditions. The reductant model achieved best results, but required very different parameter sets to simulate each experiment. Parameter nonuniqueness was likely to be due to the parameter correlation. These results suggest that the cometabolic models must be further developed if they are to reliably simulate experimental and operational data.

Degradation kinetics of chlorinated aliphatic hydrocarbons by methane oxidizers naturally-associated with wetland plant roots

Chlorinated aliphatic hydrocarbons (CAHs) are common groundwater contaminants that can be removed from the environment by natural attenuation processes. CAH biodegradation can occur in wetland environments by reductive dechlorination as well as oxidation pathways. In particular, CAH oxidation may occur in vegetated wetlands, by microorganisms that are naturally associated with the roots of wetland plants. The main objective of this study was to evaluate the cometabolic degradation kinetics of the CAHs, cis-1,2-dichloroethene (cisDCE), trichloroethene (TCE), and 1,1,1-trichloroethane (1,1,1TCA), by methane-oxidizing bacteria associated with the roots of a typical wetland plant in soil-free system. Laboratory microcosms with washed live roots investigated aerobic, cometabolic degradation of CAHs by the root-associated methane-oxidizing bacteria at initial aqueous [CH4] ~1.9mgL(-1), and initial aqueous [CAH] ~150μgL(-1); cisDCE and TCE (in the presence of 1,1,1TCA) degraded significantly, with a removal efficiency of approximately 90% and 46%, respectively. 1,1,1TCA degradation was not observed in the presence of active methane oxidizers. The pseudo first-order degradation rate-constants of TCE and cisDCE were 0.12±0.01 and 0.59±0.07d(-1), respectively, which are comparable to published values. However, their biomass-normalized degradation rate constants obtained in this study were significantly smaller than pure-culture studies, yet they were comparable to values reported for biofilm systems. The study suggests that CAH removal in wetland plant roots may be comparable to processes within biofilms. This has led us to speculate that the active biomass may be on the root surface as a biofilm. The cisDCE and TCE mass losses due to methane oxidizers in this study offer insight into the role of shallow, vegetated wetlands as an environmental sink for such xenobiotic compounds.

Aerobic degradation of trichloroethylene by co-metabolism using phenol and gasoline as growth substrates

Trichloroethylene (TCE) is a common groundwater contaminant of toxic and carcinogenic concern. Aerobic co-metabolic processes are the predominant pathways for TCE complete degradation. In this study, Pseudomonas fluorescens was studied as the active microorganism to degrade TCE under aerobic condition by co-metabolic degradation using phenol and gasoline as growth substrates. Operating conditions influencing TCE degradation efficiency were optimized. TCE co-metabolic degradation rate reached the maximum of 80% under the optimized conditions of degradation time of 3 days, initial OD₆₀₀ of microorganism culture of 0.14 (1.26 × 10⁷ cell/mL), initial phenol concentration of 100 mg/L, initial TCE concentration of 0.1 mg/L, pH of 6.0, and salinity of 0.1%. The modified transformation capacity and transformation yield were 20 μg (TCE)/mg (biomass) and 5.1 μg (TCE)/mg (phenol), respectively. Addition of nutrient broth promoted TCE degradation with phenol as growth substrate. It was revealed that catechol 1,2-dioxygenase played an important role in TCE co-metabolism. The dechlorination of TCE was complete, and less chlorinated products were not detected at the end of the experiment. TCE could also be co-metabolized in the presence of gasoline; however, the degradation rate was not high (28%). When phenol was introduced into the system of TCE and gasoline, TCE and gasoline could be removed at substantial rates (up to 59% and 69%, respectively). This study provides a promising approach for the removal of combined pollution of TCE and gasoline.

Aerobic biotransformation of N-nitrosodimethylamine and N-nitrodimethylamine in methane and benzene amended soil columns

Aerobic biotransformation of N-nitrosodimethylamine (NDMA), an emerging contaminant of concern, and its structural analog N-nitrodimethylamine (DMN), was evaluated in benzene and methane amended groundwater passed through laboratory scale soil columns. Competitive inhibition models were used to model the kinetics for NDMA and DMN cometabolism accounting for the concurrent degradation of the growth and cometabolic substrates. Transformation capacities for NDMA and DMN with benzene (13 and 23μg (mgcells)(-1)) and methane (0.14 and 8.4μg (mgcells)(-1)) grown cultures, respectively are comparable to those presented in the literature, as were first order endogenous decay rates estimated to be 2.1×10(-2)±1.7×10(-3)d(-1) and 6.5×10(-1)±7.1×10(-1)d(-1) for the methane and benzene amended cultures, respectively. These studies highlight possible attenuation mechanisms and rates for NDMA and DMN biotransformation in aerobic aquifers undergoing active remediation, natural attenuation or managed aquifer recharge with treated wastewater (i.e., reclaimed water).

Kinetics of 1,2-dichloroethane and 1,2-dibromoethane biodegradation in anaerobic enrichment cultures

1,2-Dichloroethane (1,2-DCA) and 1,2-dibromoethane (ethylene dibromide [EDB]) contaminate groundwater at many hazardous waste sites. The objectives of this study were to measure yields, maximum specific growth rates (μ), and half-saturation coefficients (K(S)) in enrichment cultures that use 1,2-DCA and EDB as terminal electron acceptors and lactate as the electron donor and to evaluate if the presence of EDB has an effect on the kinetics of 1,2-DCA dehalogenation and vice versa. Biodegradation was evaluated at the high concentrations found at some industrial sites (>10 mg/liter) and at lower concentrations found at former leaded-gasoline sites (1.9 to 3.7 mg/liter). At higher concentrations, the Dehalococcoides yield was 1 order of magnitude higher when bacteria were grown with 1,2-DCA than when they were grown with EDB, while μ's were similar for the two compounds, ranging from 0.19 to 0.52 day(-1) with 1,2-DCA to 0.28 to 0.36 day(-1) for EDB. K(S) was larger for 1,2-DCA (15 to 25 mg/liter) than for EDB (1.8 to 3.7 mg/liter). In treatments that received both compounds, EDB was always consumed first and adversely impacted the kinetics of 1,2-DCA utilization. Furthermore, 1,2-DCA dechlorination was interrupted by the addition of EDB at a concentration 100 times lower than that of the remaining 1,2-DCA; use of 1,2-DCA did not resume until the EDB level decreased close to its maximum contaminant level (MCL). In lower-concentration experiments, the preferential consumption of EDB over 1,2-DCA was confirmed; both compounds were eventually dehalogenated to their respective MCLs (5 μg/liter for 1,2-DCA, 0.05 μg/liter for EDB). The enrichment culture grown with 1,2-DCA has the advantage of a more rapid transition to 1,2-DCA after EDB is consumed.

Efficacy of pentane, toluene, and benzene to support aerobic cometabolism of ethylene dibromide

The ability of pentane, benzene, and toluene to support aerobic cometabolism of ethylene dibromide (1,2-dibromoethane, EDB) was evaluated. A pentane enrichment culture cometabolized EDB, with a transformation capacity of 0.35 μmol EDB/mg biomass (66.2 μg EDB/mg biomass) in the absence of growth substrate. It also cometabolized EDB while actively growing on pentane. However, enrichment cultures grown on benzene or toluene could not cometabolize EDB, with or without their respective growth substrates.

Aerobic cometabolism of trichloroethene and cis-dichloroethene with benzene and chlorinated benzenes as growth substrates

Using inoculum from a microcosm study that exhibited aerobic transformation of cis-1,2-dichloroethene (cDCE) and trichloroethene (TCE) commensurate with biodegradation of monoaromatic compounds, enrichment cultures were developed by providing benzene, chlorobenzene (CB), dichlorobenzene (DCB) isomers and 1,2,4-trichlorobenzene as carbon and energy sources. Isolates that grow on benzene, CB, 1,2-DCB and 1,3-DCB were identified as Rhodococcus, Ralstonia, Variovorax and Ralstonia spp., respectively. Cometabolic transformation of cDCE and TCE by resting cells was demonstrated. Transformation capacities (T(c)=0.47-1.0 μg TCE mg(-1)biomass; 1.3-5.3 μg cDCE mg(-1)biomass), transformation yields (T(y)=0.18-0.27 μg TCE mg(-1)substrate; 0.46-2.1 μg cDCE mg(-1)substrate), and pseudo-first-order cometabolic degradation rate constants (0.00081-0.0031 L mg TCE(-1)d(-1); 0.0012-0.030 L mg cDCE(-1)d(-1)) for resting cells grown on benzene, CB, 1,2-DCB and 1,3-DCB were generally lower in comparison to phenol and toluene-grown isolates. Cometabolic transformation of cDCE and TCE also occurred while the cultures concurrently consumed their growth substrate (T(c)(')=0.15-0.33 μg TCE mg(-1)biomass; 4.9-11 μg cDCE mg(-1)biomass; T(y)(')=0.06-0.11 μg TCE mg(-1)substrate; 1.7-4.6 μg cDCE mg(-1)substrate), a condition that is more likely to be encountered in situ compared to cometabolic transformations by resting cells. This study is the first to report transformation rates, capacities, and yields for cometabolism of cDCE and TCE during aerobic growth on benzene, CB, 1,2-DCB and 1,3-DCB. This type of information is needed to predict the potential for natural attenuation when these compounds occur as co-contaminants.

Cometabolic degradation kinetics of TCE and phenol by Pseudomonas putida

Modeling of cometabolic kinetics is important for better understanding of degradation reaction and in situ application of bio-remediation. In this study, a model incorporated cell growth and decay, loss of transformation activity, competitive inhibition between growth substrate and non-growth substrate and self-inhibition of non-growth substrate was proposed to simulate the degradation kinetics of phenol and trichloroethylene (TCE) by Pseudomonas putida. All the intrinsic parameters employed in this study were measured independently, and were then used for predicting the batch experimental data. The model predictions conformed well to the observed data at different phenol and TCE concentrations. At low TCE concentrations (<2 mg l(-1)), the models with or without self-inhibition of non-growth substrate both simulated the experimental data well. However, at higher TCE concentrations (>6 mg l(-1)), only the model considering self-inhibition can describe the experimental data, suggesting that a self-inhibition of TCE was present in the system. The proposed model was also employed in predicting the experimental data conducted in a repeated batch reactor, and good agreements were observed between model predictions and experimental data. The results also indicated that the biomass loss in the degradation of TCE below 2 mg l(-1) can be totally recovered in the absence of TCE for the next cycle, and it could be used for the next batch experiment for the degradation of phenol and TCE. However, for higher concentration of TCE (>6 mg l(-1)), the recovery of biomass may not be as good as that at lower TCE concentrations.

Electrolytic methanogenic-methanotrophic coupling for tetrachloroethylene bioremediation: proof of concept

Coupling of methanogenic and methanotrophic catabolisms was performed in a single-stage technology equipped with a water electrolysis cell placed in the effluent recirculation loop. The electrolysis-generated hydrogen served as an electron donor for both bicarbonate reduction into CH4 and reductive dechlorination, while the O2 and CH4, supported the cometabolic oxidation of chlorinated intermediates left over by the tetrachloroethylene (PCE) transformation. The electrolytical methanogenic/methanotrophic coupled (eMaMoC) process was tested in a laboratory-scale setup at PCE loads ranging from 5 to 50 micromol/L(rx) x d (inlet concentrations from 4 to 11 mg/L), and at various hydraulic residence times (HRT). Degradation followed essentially a reductive dechlorination pathway from PCE to cis-1,2-dichloroethene (DCE), and an oxidative pathway from DCE to CO2. PCE reductive dechlorination to DCE was consistently over 98% while a maximum oxidative DCE mineralization of 89% was obtained at a load of 4.3 micromol PCE/ L(rx) x d and an HRT of 6 days. Controlling dissolved oxygen concentrations within a relatively low range (2-3 mg/L) seemed instrumental to sustain the overall degradation capacity. Degradation kinetics were further evaluated: the apparent half-saturation constant (K(s)) had to be set relatively high (29 microM) for the simulated data to best fit the experimental ones. In spite of such kinetic limitations, the eMaMoC system, while fueled by water electrolysis, was effective in building and sustaining a functional methanogenic/methanotrophic consortium capable of significant PCE mineralization in a single-stage process. Hence, degradation standards are within reach so long as the methanotrophic DCE-oxidizing potential, including substrate affinity, are optimized and HRT accordingly adjusted.

Degradation of phenol and TCE using suspended and chitosan-bead immobilized Pseudomonas putida

The degradability of phenol and trichloroethene (TCE) by Pseudomonas putida BCRC 14349 in both suspended culture and immobilized culture systems are investigated. Chitosan beads at a size of about 1-2mm were employed to encapsulate the P. putida cells, becoming an immobilized culture system. The phenol concentration was controlled at 100 mg/L, and that of TCE was studied from 0.2 to 20 mg/L. The pH, between 6.7 and 10, did not affect the degradation of either phenol or TCE in the suspended culture system. However, it was found to be an important factor in the immobilized culture system in which the only significant degradation was observed at pH >8. This may be linked to the surface properties of the chitosan beads and its influence on the activity of the bacteria. The transfer yield of TCE on a phenol basis was almost the same for the suspended and immobilized cultures (0.032 mg TCE/mg phenol), except that these yields occurred at different TCE concentrations. The transfer yield at a higher TCE concentration for the immobilized system suggested that the cells immobilized in carriers can be protected from harsh environmental conditions. For kinetic rate interpretation, the Monod equation was employed to describe the degradation rates of phenol, while the Haldane's equation was used for TCE degradation. Based on the kinetic parameters obtained from the two equations, the rate for the immobilized culture systems was only about 1/6 to that of the suspended culture system for phenol degradation, and was about 1/2 for TCE degradation. The slower kinetics observed for the immobilized culture systems was probably due to the slow diffusion of substrate molecules into the beads. However, compared with the suspended cultures, the immobilized cultures may tolerate a higher TCE concentration as much less inhibition was observed and the transfer yield occurred at a higher TCE concentration.

Mixed pollutant degradation by Methylosinus trichosporium OB3b expressing either soluble or particulate methane monooxygenase: can the tortoise beat the hare?

Methanotrophs have been widely investigated for in situ bioremediation due to their ubiquity and their ability to degrade halogenated hydrocarbons through the activity of methane monooxygenase (MMO). It has been speculated that cells expressing the soluble form of MMO (sMMO) are more efficient in cleaning up sites polluted with halogenated hydrocarbons due to its broader substrate range and relatively fast degradation rates compared cells expressing the other form of MMO, the particulate MMO (pMMO). To examine this issue, the biodegradation of mixtures of chlorinated solvents, i.e., trichloroethylene (TCE), trans-dichloroethylene (t-DCE), and vinyl chloride (VC), by Methylosinus trichosporium OB3b in the presence of methane using either form of MMO was investigated over longer time frames than those commonly used, i.e., days instead of hours. Growth of M. trichosporium OB3b along with pollutant degradation were monitored and analyzed using a simple comparative model developed from the Omega model created for analysis of the competitive binding of oxygen and carbon dioxide by ribulose bisphosphate carboxylase. From these findings, it appears that at concentrations of VC, t-DCE, and TCE greater than 10 microM each, methanotrophs expressing pMMO have a competitive advantage over cells expressing sMMO due to higher growth rates. Despite such an apparent growth advantage, pMMO-expressing cells degraded less of these substrates at these concentrations than sMMO-expressing cells during active growth. If the concentrations were increased to 100 muM, however, not only did pMMO-expressing cells grow faster, they degraded more of these pollutants and did so in a shorter amount of time. These findings suggest that the relative rates of growth substrate and pollutant degradation are important factors in determining which form of MMO should be considered for pollutant degradation.

Cometabolism of cis-1,2-dichloroethene by aerobic cultures grown on vinyl chloride as the primary substrate

An aerobic enrichment culture was grown on vinyl chloride (VC) as the sole source of carbon and energy. In the absence of VC, the enrichment culture cometabolized cis-1,2-dichloroethene (cDCE) and, to a lesser extent, trans1,2-dichloroethene (tDCE), beginning with oxidation to the corresponding DCE-epoxides. When provided with VC (1.3 mM) and cDCE (0.2-0.3 mM), the enrichment culture cometabolized repeated additions of cDCE for over 85 days. Cometabolism of repeated additions of tDCE was also demonstrated but at a lower ratio of nongrowth substrate to VC. VC-grown Pseudomonas aeruginosa MF1 (previously isolated from the enrichment culture) also readily cometabolizes cDCE, with an observed transformation capacity (Tc,obs) of 0.82 micromol of cDCE/mg of total suspended solids (TSS). When provided with VC and cDCE, MF1 did not begin cometabolizing cDCE until nearly all of the VC was consumed. The presence of cDCE reduces the maximum specific rate of VC utilization. A kinetic model was developed that describes these phenomena via Monod parameters for substrate and nongrowth substrate, plus inactivation and inhibition coefficients. MF1 did not show any cometabolic activity on tDCE or trichloroethene and very limited activity on 1,1-DCE (Tc,obs = 2 x 10(-5) micromol/mg TSS). Above 40 microM, tDCE and TCE noticeably increased the maximum specific rate of VC utilization, even though neither compound was consumed during or after VC consumption. High concentrations of 1,1-DCE (950 microM) completely inhibited VC biodegradation. As there is currently no evidence for aerobic biodegradation of cDCE as a sole source of carbon and energy, the results of this study provide a potential explanation for in situ disappearance of cDCE when the only other significant substrate available is VC. It is fortuitous that the VC-grown cultures tested exhibit their highest cometabolic activity toward cDCE, because it is the predominant DCE isomer formed during anaerobic reductive dechlorination of trichloroethene and tetrachloroethene.

Transition from cometabolic to growth-linked biodegradation of vinyl chloride by a Pseudomonas sp. isolated on ethene

Pseudomonas aeruginosa strain DL1 was isolated on ethene as a sole carbon and energy source. When ethene-grown DL1 was first exposed to vinyl chloride (VC), the rate of VC consumption was very rapid and then declined sharply, indicative of a cometabolic process. A lack of growth and significant release of soluble products during this interval also indicates that the initial activity on VC was cometabolic. Following the rapid initial rate of VC cometabolism, a slow rate of VC utilization continued. After an extended period of incubation (>40 days), a transition occurred that allowed DL1 to begin using VC as a primary growth substrate, with an observed yield, maximum growth rate, and Monod half saturation coefficient of 0.21 mg of total suspended solids/mg VC, 0.046 d(-1), and 1.17 microM VC, respectively, at 22 degrees C. Acetylene inhibits consumption of ethene and VC by ethene-grown cells, suggesting a monooxygenase is responsible for initiating metabolism of these alkenes. Resting cells grown on ethene cometabolized VC with an observed transformation capacity of 9.1 micromol VC/mg total suspended solids and a transformation yield of 0.22 mol VC/mol ethene. The presence of 40 microM ethene increased the rate and amount of VC cometabolized. However, consumption of higher concentrations of ethene decreased the total amount of VC consumed, and VC inhibited ethene utilization. A kinetic model was developed that describes substrate interactions during batch depletion of ethene and VC for a range of initial concentrations. The results suggest that ethene may stimulate in situ biodegradation of VC either by functioning as a primary substrate to support cometabolism of VC or by selecting for organisms that can utilize VC as a primary substrate.

Modeling the kinetics of vinyl chloride cometabolism by an ethane-grown Pseudomonas sp

Pseudomonas sp strain EA1 was isolated under aerobic conditions using ethane as the sole organic carbon and electron donor source, with an observed yield of 0.99 mg total suspended solids/mg ethane (0.85 mg volatile suspended solids / mg ethane) and a maximum specific growth rate of 0.015 d(-1). When grown on ethane, EA1 cometabolizes vinyl chloride (VC) at a maximum rate of 0.350 micromol/mg volatile suspended solids/d and with a half saturation constant of 0.62 microM VC. The rate of VC use by EA1 is twice as high when ethane is also provided, even though consumption of ethane is almost completely inhibited until VC is consumed. The presence of ethane also reduces the total amount of VC cometabolized. A model was developed that adequately describes the batch kinetics of VC cometabolism in the presence and absence of ethane, as well as ethane metabolism in the presence and absence of VC. Terms are included that increase the initial rate of VC use in the presence of ethane (according to the ratio of initial ethane concentration to the half saturation coefficient) but decrease the total amount of VC cometabolized. Parameter estimates for the model were obtained using a step-wise experimental approach, with varying initial concentrations of VC and ethane. Strain EA1 completely dechlorinates VC in the presence and absence of ethane. Measurements of soluble chemical oxygen demand indicate that approximately 50% of the VC consumed is mineralized, with the balance released as soluble, nonchlorinated products. Ethene is not used as a substrate by EA1 but it does inhibit ethane metabolism and VC cometabolism. In mixtures containing all three compounds, more VC is degraded and at a faster rate compared to VC plus ethene. The results suggest that ethane-enhanced biodegradation of VC may contribute to VC removal at the aerobic fringe of groundwater plumes undergoing reductive dechlorination.

Growth kinetics of Pseudomonas putida in cometabolism of phenol and 4-chlorophenol in the presence of a conventional carbon source

Growth kinetics of Pseudomonas putida (ATCC 49451) in cometabolism of phenol and 4-chlorophenol (4-cp) in the presence of sodium glutamate (SG) were studied. In the ternary substrate mixture, phenol and SG are growth substrates while 4-cp is a nongrowth substrate. Cell growth on phenol was found to follow Andrews kinetics and cells displayed substrate inhibition pattern on sodium glutamate in the range of 0-4 g L(-1) as well. A cell growth model for the ternary substrate system was established based on a simplified cell growth mechanism and subsequently modified by experimental results. Model analysis over a wide range of substrate concentrations shows that the inhibition of SG is much larger than phenol at low phenol concentrations (</=200 mg L(-1)) while phenol exerts dominant inhibition on cell growth at higher phenol concentrations (>/=600 mg L(-1)). The nongrowth substrate, 4-cp, inhibits cell growth mainly through inactivation of cells (cell decay) and competitive inhibition to cell growth on phenol. In the absence of SG, 4-cp retards cell growth severely and cells cannot grow at 250 mg L(-1) 4-cp. Addition of sodium glutamate, however, greatly attenuates the toxicity of 4-cp and supports cell growth at 4-cp concentration higher than 250 mg L(-1). By using the proposed cell growth model, we were able to optimize the amount of SG needed to enhance cell growth rate and validate model predictions against experimental data.