Transformation Rate Constants Research Articles

Transformation of 2-Line Ferrihydrite to Goethite at Alkaline pH.

The transformation of 2-line ferrihydrite to goethite from supersaturated solutions at alkaline pH ≥ 13.0 was studied using a combination of benchtop and advanced synchrotron techniques such as X-ray diffraction, thermogravimetric analysis, and X-ray absorption spectroscopy. In comparison to the transformation rates at acidic to mildly alkaline environments, the half-life, t1/2, of 2-line ferrihydrite reduces from several months at pH = 2.0, and approximately 15 days at pH = 10.0, to just under 5 h at pH = 14.0. The calculated-first order rate constants of transformation, k, increase exponentially with respect to the pH and follow the progression log10 k = log10 k0 + a·pH3. Simultaneous monitoring of the aqueous Fe(III) concentration via inductively coupled plasma optical emission spectroscopy demonstrates that (i) goethite likely precipitates from solution and (ii) its formation is rate-limited by the comparatively slow redissolution of 2-line ferrihydrite. The analysis presented can be used to estimate the transformation rate of naturally occurring 2-line ferrihydrite in aqueous electrolytes characteristic to mine and radioactive waste tailings as well as the formation of corrosion products in cementitious pore solutions.

Using 224Ra/228Th disequilibrium to quantify carbon transformation and export from intertidal sandy and muddy sediments

The intertidal zone is an important pathway for the transport of dissolved carbon across the land–ocean interface. Quantifying carbon transformation and export from the intertidal zone remains large uncertainties due to the natural heterogeneity and various driving mechanisms. Here, we used the 224Ra/228Th disequilibrium method to quantify porewater exchange and dissolved carbon export from intertidal sandy and muddy sediments, two main intertidal wetland types in the coast. Porewater exchange fluxes from sandy sediments ranged from 1.4 ± 0.1 to 1084 ± 53 L m−2 h−1, which gradually increased in the landward direction. Porewater exchange fluxes from mangrove muddy sediments ranged from 0 to 4.4 ± 0.7 L m−2 h−1. Advective porewater exchange was recognized as the dominant solute transport process in sandy sediments, whereas bioturbation may be the key process in mangrove muddy sediments. Furthermore, marine dissolved organic carbon (DOC) was removed in sandy sediments, indicating that the sandy sediment acted as a sink of DOC, likely via aerobic remineralization. DOC consumption fluxes ranged from 3.7 ± 0.2 to 206 ± 9 mmol C m−2 d−1. The total consumption flux of DOC in the entire sandy beach was one order of magnitude higher than the globally maximum DOC removal flux in the upper ocean. Porewater-derived dissolved inorganic carbon (DIC) export fluxes from sandy sediments ranged from 5.6 ± 0.3 to 883 ± 43 mmol C m−2 d−1. Nearly 50% of DIC export originated from the consumption of DOC. The rate constants for DOC transformation ranged from 0.009 to 0.155 h−1. In comparison, porewater-derived DOC and DIC export fluxes from muddy sediments were 25 ± 4 and 206 ± 30 mmol C m−2 d−1, respectively. Porewater-derived DIC export from sandy sediments was comparable or even higher than those from muddy sediments. These results showed that sandy sediments acted as a fast lane for marine DOC remineralization. Overall, both organic-poor sandy and organic-rich mangrove muddy sediments play important roles in the delivery of dissolved carbon to the ocean.

Exploring the effects of intermittent aeration on the performance of nitrifying membrane-aerated biofilm reactors

Membrane-aerated biofilm reactors (MABRs) are an emerging technology for nutrient removal; however, a trade-off remains between their removal rate and oxygen transfer efficiency. This study compares nitrifying flow-through MABRs operated under continuous and intermittent aeration modes at mainstream wastewater ammonia levels. The intermittently-aerated MABRs maintained maximal nitrification rates, including under conditions allowing the oxygen partial pressure on the gas side of the membrane to considerably drop during the no-aeration period. Nitrous oxide emissions of all reactors were comparable and amounted to approximately 20 % of the converted ammonia. Intermittent aeration increased the transformation rate constant of atenolol, yet did not affect the removal of sulfamethoxazole. Seven additional trace organic chemicals were not biodegraded by any of the reactors. The ammonia-oxidizing bacteria in the intermittently-aerated MABRs were dominated by Nitrosospira, previously shown to be abundant at low oxygen concentrations and provide reactor stability under changing conditions. Our findings indicate that intermittently-aerated flow-through MABRs can achieve high nitrification rates and oxygen transfer efficiencies, highlighting the possible implications of air supply discontinuity on nitrous oxide emissions and trace organic chemical biotransformation.

Impact of chlorination and ozonation of dissolved organic matter on its photo-induced production of long-lived photooxidants and excited triplet states

Recent studies suggested that long-lived photooxidants (LLPO), which are reactive intermediates formed during irradiation of dissolved organic matter (DOM), may consist of phenoxyl radicals derived from phenolic moieties of the DOM. Besides the well-studied excited triplet states of chromophoric DOM (3CDOM*), LLPO presumably are important photooxidants for the transformation of electron-rich contaminants in surface waters. The main objective of this study was to further test the potential role of phenoxyl radical as LLPO. Suwannee River fulvic acid (SRFA) as a model DOM was pre-oxidised using the phenol-reactive oxidants chlorine and ozone, followed by its characterization by the specific UV absorption at 254 nm (SUVA254), the ratio of absorbance at λ = 254 nm and λ = 365 nm (E2:E3), and the electron donating capacity (EDC). Subsequently, the photoreactivity of pre-oxidized SRFA was tested using 3,4-dimethoxyphenol (DMOP) as a LLPO probe compound at two initial concentrations ([DMOP]0 = 0.1 and 5.0 μM). Linear inter-correlations were observed for the relative changes in SUVA254, E2:E3, and EDC for increasing oxidant doses. Pseudo-first-order transformation rate constants normalized to the changing SRFA absorption rate (i.e., k0.1obs/rCDOMabsand k5.0obs/rCDOMabs, for 0.1 and 5.0 µM, respectively) exhibited the following distinct trends: The LLPO-dominated k0.1obs/rCDOMabsratio decreased with increasing oxidant dose and with decreasing SUVA254 and EDC, while the 3CDOM*-dominated k5.0obs/rCDOMabsratio positively correlated with E2:E3. Finally, it was concluded that precursors of 3CDOM* and LLPO are chemically modified differently by pre-oxidation of DOM, and LLPO precursors likely consist of phenolic moieties of DOM, suggesting phenoxyl radicals as LLPO.

Improving the non-radical/radical transformation efficiency of halogenated polycyclic aromatic hydrocarbons by using metal ion-exchanged cryptomelane-type OMS-2 under moisture-unsaturated conditions

Halogenated polycyclic aromatic hydrocarbons (HPAHs) have recently remerged as major soil contaminants. However, limited studies have focused on the transformation and removal of HPAHs in soils. The transformation behaviors of HPAHs on cryptomelane-type manganese oxide (K(I)-OMS-2) and its metal ion-exchanged M-OMS-2 (M = Fe(III), Cu(II), and Zn(II)) under moisture-unsaturated conditions were systematically investigated. The transformation kinetics results showed that the transformation rate constants (k) of HPAHs were 0.077–0.829, 0.071–0.629, 0.068–0.530, and 0.064–0.259 d−1 for Fe(III)-OMS-2, Cu(II)-OMS-2, Zn(II)-OMS-2, and K(I)-OMS-2. The metal ion exchange treatment of K(I)-OMS-2 enhanced the transformation efficiency of HPAHs. The characterization (H2-TPR, CV, ESI, XPS, and EPR) and theoretical calculation (density of states and formation energy of oxygen vacancies) results indicated that the non-radical/radical transformation efficiency of HPAHs on M-OMS-2 was higher than that on K(I)-OMS-2. The transformation rate of HPAHs was positively correlated with the redox potential of metal ions and superoxide radical yield of M-OMS-2. Additionally, this study revealed that HPAHs with lower redox potential and more strongly bound to M-OMS-2 were more preferentially transformed. Finally, the experimental results of the addition of M-OMS-2 to the natural soil revealed that M-OMS-2 aids the transformation of HPAHs in soils. This study reveals the chemical transformation behavior of HPAHs in soils and suggests that M-OMS-2 can a promising agent for soil remediation.

Validation of an Accurate and Expedient Initial Rates Method for Characterizing Mechanophore Reactivity

Understanding structure-mechanochemical reactivity relationships is important for informing the rational design of new stimuli-responsive polymers. To this end, establishing accurate reaction kinetics for mechanophore activation is a key objective. Here, we validate an initial rates method that enables the accurate and rapid determination of rate constants for ultrasound-induced mechanochemical transformations. Experimental reaction profiles are well-aligned with theoretical models, which support that the initial rates method effectively deconvolutes the kinetics of specific mechanophore activation from the competitive process of nonspecific chain scission.

Experimental and theoretical studies on the synthesis of 1,4,5-trisubstituted pyrrolidine-2,3-diones.

Substituted 4-acetyl-3-hydroxy-3-pyrroline-2-ones have been prepared via three-component reactions and the tautomerism of these 3-pyrroline-2-ones is due to the slight difference of energy, and the significantly large rate constant of transformation between two tautomers. 1,4,5-Trisubstituted pyrrolidine-2,3-dione derivatives were prepared from the above mentioned 2-pyrrolidinone derivatives and aliphatic amines, which exist in enamine form and are stabilized by an intramolecular hydrogen bond. A possible reaction mechanism between 3-pyrroline-2-one and aliphatic amine (CH3NH2) was proposed based on computational results and the main product is formed favorably following the PES via the lowest ΔG# pathway in both the gas-phase and an ethanol solvent model. DFT calculations showed that kinetic selectivity is more significant than thermodynamic selectivity for forming main products.

Import, export, and speciation of mercury in Kongsfjorden, Svalbard: Influences of glacier melt and river discharge

The major sources and sinks of total mercury (THg) and methylmercury (MeHg) in Kongsfjorden were estimated based on spreadsheet-based ecological risk assessment for the fate of mercury (SERAFM). SERAFM was parameterized and calibrated to fit Kongsfjorden using the physical properties of the fjord, runoff coefficients of Hg, transformation rate constants of Hg, partition coefficients of Hg, Hg loadings from freshwater, and solid balance parameters. The modeled Hg concentrations in the seawater matched with the measured concentrations, with a mean bias of 12% and a calibration error of 0.035. The mass budget showed that the major THg sources were tidal inflow and glacial runoff, while the major MeHg sources were tidal inflow and in situ methylation in shallow halocline water, which agreed with the distributions of THg and MeHg in seawater. The coupling of observation and fate modeling in Kongsfjorden provides a basic understanding of Hg cycles in the Arctic fjords.

Two Pathways Compete in the Mn(II)-Catalyzed Oxidation of Aminotrismethylene Phosphonate (ATMP).

Mn(II)-catalyzed oxidation by molecular oxygen is considered a relevant process for the environmental fate of aminopolyphosphonate chelating agents such as aminotrismethylene phosphonate (ATMP). However, the potential roles of Mn(III)ATMP-species in the underlying transformation mechanisms are not fully understood. We combined kinetic studies, compound-specific stable carbon isotope analysis, and equilibrium speciation modeling to shed light on the significance of such Mn-ATMP species for the overall ATMP oxidation by molecular oxygen. The fraction of ATMP complexed with Mn(II) inversely correlated with both (i) the Mn(II)-normalized transformation rate constants of ATMP and (ii) the observed carbon isotope enrichment factors (εc-values). These findings provide evidence for two parallel ATMP transformation pathways exhibiting distinctly different reaction kinetics and carbon isotope fractionation: (i) oxidation of ATMP present in Mn(III)ATMP complexes (εc ≈ -10 ‰) and (ii) oxidation of free ATMP by such Mn(III)ATMP species (εc ≈ -1 ‰) in a catalytic cycle. The higher reaction rate of the latter pathway implies that aminopolyphosphonates can be trapped in catalytic Mn-complexes before being transformed and suggests that Mn(III)ATMP might be a potent oxidant also for other reducible solutes in aqueous environments.

Enhanced transformation capability towards benzo(a)pyrene by Fe(III)-modified manganese oxides

Manganese oxides (Mn oxides) are ubiquitous and may coexist with Fe(III) ions in soil environments. In this study, acid birnessite, alkaline birnessite, cryptomelane, pyrolusite, manganite, and their Fe(III)-modified analogues were synthesized and used for benzo(a)pyrene transformation. Fe-modified Mn oxides show a markedly enhanced transformation capability towards benzo(a)pyrene. Specifically, the benzo(a)pyrene transformation rate constants k for Bir-H, Bir-OH, Cry, Pyr, and Man were 0.49, 0.080, 0.0071, 0.0055, and 0.0022 h−1, respectively. After Fe(III) modification, the transformation rate constants were increased to 22, 2.7, 0.25, 0.0072 and 0.0098 h−1, respectively. Fe(III)-modified layered birnessites exhibited better activity than Fe(III)-modified tunnel Mn oxides, which was attributed to their high Fe(III) contents and abundant active free radicals. Fe(III) was found to accept electrons from benzo(a)pyrene, thereby accelerating the benzo(a)pyrene transformation. Moreover, modification with Fe(III) increased the surface adsorbed water and oxygen, and promoted the generation of active free radicals. Finally, the physicochemical and biochemical properties of transformation products showed the environmental benefits of this process. Overall, the results indicate that the occurrence of Fe(III) ions could promote the removal of PAHs in Mn oxides-rich soils, and this study provides a credible understanding of PAH fates in natural soils.

Transformation kinetics and mechanism of gibberellic acid with ferrihydrite: Building a novel adsorption-transformation multi-step kinetic model

Gibberellic acid (GA3), a widely used phytohormone, is easily transformed into more toxic products. The soil and groundwater environment are an important sink for GA3, but its transformation catalyzed by soil minerals has not been studied. In this study, the transformation kinetics and mechanism of GA3 with ferrihydrite (Fh) were examined through kinetic batch experiments, microscopic-spectroscopic investigation and mathematical modeling. The results showed that rapid adsorption of GA3 on Fh occurred in the first 4 h, followed by a catalytic pseudo-first-order transformation of the parent compound and products generation (4 h–30 d). Fh predominantly enhanced the transformation of GA3 into Iso-GA3 which was further hydrolyzed into OH-GA3, in which adsorption was a prerequisite for transformation. The catalytic transformation likely resulted from the surface hydroxy of Fh, which not only stabilized the transformation intermediates by forming surface complexes with the carboxyl group of GA3 and its products, but also served as a powerful nucleophile to attack the γ-lactone of GA3 and Iso-GA3. Based on the catalytic isomerization and hydrolysis mechanism of GA3 with Fh, a novel adsorption-transformation multi-step kinetic conceptual model and mathematical model were developed. This model fitted the measured data well (R2 > 0.97) and the fitted parameters suggested that the transformation rate constants of the transformation of GA3 into Iso-GA3 and the transformation of Iso-GA3 into OH-GA3 were facilitated with Fh by ∼26 and ∼9 times, respectively. The multi-step kinetic model has great potential in simulating GA3 fate in soil and groundwater to assess its environmental health risk.

Comparison of the Spectroscopically Measured Catalyst Transformation and Electrochemical Properties of Grubbs' First- and Second-Generation Catalysts.

According to UV–vis spectroscopy (0.10 mM, CH2Cl2 at 25 °C), the catalyst transformation (which could possibly include ligand dissociation with active catalyst formation, dimer formation, and decomposition) rate constants (kobs) of Grubbs’ first (1) and second (2) generation catalysts are 7.48 × 10–5 and 1.52 × 10–4 s–1, respectively. From 31P NMR (0.1 M, CD2Cl2, at 25 °C), the catalyst transformation was 5.1% for 1 and 16.5% for 2 after 72 h. However, due to the larger concentrations of the NMR samples compared to the UV–vis samples, the extent of transformation did not correspond. The oxidation potential of the RuII/RuIII couple of 2 (E°’ = 27.5 mV at v = 200 mV s–1) was considerably lower than that of 1 (E°’ = 167 mV at v = 200 mV s–1). In the case of 1, a second reduction peak appeared at slow scan rates. This may probably be ascribed to an electrochemically active compound that was formed from the intermediate cation 1•+ and the subsequent reduction of the latter. The oxidation/reduction of 1 proceeds according to an ErCi electrochemical mechanism (Er = electrochemically reversible step, Ci = chemically irreversible step), whereas 2 proceeds according to an ErCr electrochemical mechanism (Er = electrochemically reversible step, Ci = chemically reversible step).

Enzymatic characterization of agmatinase (AGM-1) from the filamentous fungus Neurospora crassa

Agmatinase is a metallohydrolase involved in the hydrolysis of agmatine to produce urea and putrescine. Although its role in organisms is still under study, there are no reports of this family of enzymes in filamentous fungi. Recently, a protein showing agmatinase activity was reported in Neurospora crassa. Therefore, the aim of this work is to determine if the protein (AGM-1) found in the filamentous fungus N. crassa is a true agmatinase. The protein AGM-1was purified directly from N. crassa cultures, and its enzymatic characterization was carried out. The catalytic parameters such as optimum pH, thermostability, transformation kinetics, and activity in the presence of a cofactor were determined. The results show that AGM-1 can use manganese as a cofactor for its enzymatic activity, showing a transformation rate constant (kcat) of 77 s−1 and an affinity constant (KM) of 50.5 mM. The protein loses 50% of its activity when incubated 15 min at 30 °C and reaches maximal enzymatic activity at a pH range of 8–8.5. Our results indicate that the AGM-1 from N. crassa shows similar characteristics to true agmatinases already reported in other organisms. Thus, our findings strongly support that the protein annotated as hypothetical agmatinase in N. crassa is a true agmatinase.

Kinetic model parameter estimation using genetic algorithms of the oxidation of phenol in water catalyzed by the laccase enzyme for the design of a biosensor

Predicting the dynamic response of key processes that take place in industrial biochemical systems such as the measurement of operating parameters by means of biosensors, before the biosensors themselves are prototyped, is of utmost importance. The advantages of performing mathematical modeling of biosensor systems in their development stages include cost reduction, easier response tuning and faster performance optimization. In most cases, the mathematical models for enzyme kinetics depend on multiple parameters. Finding the numerical values of such parameters usually requires carrying out a vast number of experiments, which is time intensive, expensive and involves the usage of specialized laboratory equipment. This work proposes the utilization of genetic algorithms as an alternative methodology for kinetic model parameter estimation of the oxidation of phenol in water, catalyzed by the laccase enzyme. The corresponding kinetics mathematical model of the oxidation reaction is used as a case study, to compare the results obtained using the genetic algorithms approach proposed with those found in the literature. The algorithm estimated the values of several parameters of the model, such as reaction rate constants, rate constant of transformation of oxygen by the electrode and stoichiometric coefficient, among others. The results found in this investigation by means of genetic algorithms show an agreement of 91%–99% with the data available in the literature. This approach also proved to be more accurate than the basic polynomial regression estimation method, which is commonly used and was implemented for comparison purposes. The proposed technique for parameters estimation in enzyme reaction models enabled the design of a phenol biosensor for concentrations ranging from 5 to 30 ppm. This technique has a high potential of application in the biosensor industry because of its cost savings, high speed and good accuracy.

Enhanced transformation of aquatic organic compounds by long-lived photooxidants (LLPO) produced from dissolved organic matter

Dissolved organic matter (DOM) plays a crucial role in the photochemical transformation of organic contaminants in natural aquatic systems. The present study focuses on the characterization of a specific effect previously observed for electron-rich phenols, consisting in an acceleration of the DOM-photosensitized transformation of target compounds at low concentrations (< 1 µM). This effect was hypothesized to be caused by DOM-derived "long-lived" photooxidants (LLPO). Pseudo-first-order rate constants for the transformation of several phenols, anilines, sulfonamide antibiotics and phenylureas photosensitized by Suwannee River fulvic acid were determined under steady−state irradiation using the UVA and visible wavelengths from a medium-pressure mercury lamp. A significant enhancement (by a factor of 2.4 − 16) of the first-order transformation rate constant of various electron-rich target compounds was observed for an initial concentration of 0.1 μM compared to 5 μM . This effect points to a relevant reactivity of these compounds with LLPO. For phenols and anilines the enhancement effect occurred only above certain standard one-electron oxidation potentials. From these data series the standard one-electron reduction potential of LLPO was estimated to be in the range of 1.0 − 1.3 V versus the standard hydrogen electrode. LLPO are proposed to mainly consist of phenoxyl radicals formed by photooxidation of electron-poor phenolic moieties of the DOM. The plausibility of this hypothesis was successfully tested by studying the photosensitized transformation kinetics of 3,4-dimethoxyphenol in aqueous solutions containing a model photosensitizer (2-acetonaphthone) and a model electron-poor phenol (4-cyanophenol) as DOM surrogates.

Transformation of sulfadiazine in humic acid and polystyrene microplastics solution by horseradish peroxidase coupled with 1-hydroxybenzotriazole

Enzyme catalyzed coupling with redox mediators are considered as great interesting and viable technologies to transform antibiotics. This work demonstrated the horseradish peroxidase (HRP) was effective in transforming sulfadiazine (SDZ) transformation coupled with 1-hydroxybenzotriazole (HBT) at varying conditions. The removal of SDZ was independent of Na+ and its ionic strength, but Ca2+ could enhance transformation efficiency by increasing the enzyme activity of HRP. The presence of humic acid (HA) and polystyrene (PS) microplastics showed inhibition on the transformation of SDZ, and the transformation rate constants (k) decreased with the concentration of HA and PS particles increased. These primarily attributed to covalent coupling and electrostatic interaction between SDZ and HA, SDZ and PS, respectively, which reduced the concentration of free SDZ in the reaction solution. The presence of cation recovered the inhibition of SDZ transformation by HA and PS particles, which ascribed to compete between cation and SDZ. The divalent cations (Ca2+) showed more substantial competitiveness than mono (Na+) due to more carried charge. Eight possible transformation products were identified, and potential SDZ transformation pathways were proposed, which include δ-cleavage, γ-cleavage, carbonylation, hydroxylation, SO2 extrusion and SO3 extrusion. In addition, HA and PS particles couldn’t affect the transformation pathways of SDZ. These findings provide novel understandings of the transformation and the fate of antibiotics in the natural environment by HRP coupled with redox mediators.

Validation of a fuel particle dissolution model with samples from the Red Forest within the Chernobyl exclusion zone

The contamination in the near exclusion zone of the Chernobyl nuclear power plant (ChNPP) with 90Sr, 238-240Pu and 241Am is associated with irradiated nuclear fuel particles. Fit for purpose models enabling long term prediction of mobility and bioavailability of particle-associated radionuclides are crucial for radiation protection of humans and the environment, as well as for planning of remediation measures and future land use. In the present work, a dynamic fuel particle dissolution model developed in 1999–2002 is described and validated using data based on sampling in 2015. The model is based on the characterization of the radionuclide source term of the waste in a shallow sub-surface radioactive waste storage, trench #22, in the Chernobyl Pilot Site (CPS) located in the Red Forest, 2.5 km west of the ChNPP, as well as the description of physical and chemical properties of the fuel particles and subsequent radionuclide leaching into the soil solution. Transformation rate constants of the fuel particle dissolution model related to topsoil, radioactive waste trench and submerged materials, and drained cooling pond sediments, should largely control the mobility and bioavailability of radionuclides (e.g., solubility in the soil, migration to groundwater and transfer to plants). The predicting power of the Chernobyl fuel particle dissolution model with respect to radionuclide leaching dynamics was validated using samples from the same experimental site, showing that predicted particle leaching and subsequent mobility and bioavailability were within 46 ± 3% of the observed data. Therefore, linking source- and release-scenario dependent characteristics of radioactive particles to their potential weathering can provide information that can be implemented in impact assessments models for existing contaminated areas as well as for future events.

Oxidative dehydrogenation of ethane over M1 MoVNbTeO catalysts modified by the addition of Nd, Mn, Ga or Ge

Modification by Nd, Mn, Ga, and Ge substantially affect both physicochemical and catalytic properties of MoVNbTeO oxides in the oxidative dehydrogenation of ethane (ODE). The introduction of modifying additives changes the content of the M1 phase, surface acidity, and surface composition. It does not have an influence on the state of Mo, V, Nb, and Te on catalyst surface, but it induces surface enrichment (Ga, Mn) or depletion (Ge, Nd) by the modifying element. All of these additives have positive effects on catalytic properties at the M/Mo (M = modifying element) ratio of 0.0075 ÷ 0.025. They exhibit an increase of activity and prevent ethylene over-oxidation. The relationship between the rate constant of total ethane transformation and the M1 content confirms that this phase is active in ODE. The correlation between ethylene selectivity and the relative number of intermediate acid sites is revealed, indicating the role of surface acidity on ethylene over-oxidation.

Estimation of Rate Constants for Nutrient Transformations

Non-point source pollution of surface water is a major impediment to meet water quality objectives. Managing such pollution sources in a sustainable way is a key success factor in maintaining high water quality and to prevent eutrophication. Mathematical models are widely used to simulate ecological and water quality interactions in surface waters. Simulation errors may arise due to uncertainties of the structure, input data and the model parameters. In this study, an attempt has been made to estimate the rate constants for nutrient transformations in Kabini River located in Southern part of Karnataka state in India. The experimental results demonstrated both ammonia and nitrite oxidation. In the river water, DO concentration was 5.2 mg/L. After addition of pollutants it reduced to 3.9 mg/L. EC changed from 370 to 550 μS/cm. pH remained almost the same. At 320C, the rate constants for phosphate, nitrate, nitrite, potassium and ammonia were found to be 0.165, 0.21, 0.077, 0.0777 and 0.078/hr respectively. The results obtained clearly specify that the rate constants are concentration and temperature dependent.

Contributions of biotic and abiotic pathways to anaerobic trichloroethene transformation in low permeability source zones

Low permeability source zones sustain long-term trichloroethene (TCE) groundwater contamination. In anaerobic environments, TCE is transformed by both biological reductive dechlorination and abiotic reactions with reactive minerals. Little is known about the relative contribution of these two pathways as TCE diffuses from low permeability zones (LPZs) into high permeability zones (HPZs). This study combines a flow cell experiment, batch experiments, and a diffusion-reaction model to evaluate the contributions of biotic and abiotic TCE transformation in LPZs. Natural clay (LPZ) and sand (HPZ) from a former Air Force base were used in all experiments. In batch, the LPZ material transformed TCE and cis-1,2-dichloroethene (cis-DCE) to acetylene with pseudo first-order rate constants of 8.57 × 10−6 day−1 and 1.02 × 10−6 day−1, respectively. Biotic and abiotic pathways were then evaluated together in a bench-scale flow cell (16.5 cm × 2 cm × 16.5 cm) that contained a LPZ layer, with a source of TCE at the base, overlain by a HPZ continuously purged with lactate-amended groundwater. Diffusion controlled mass transfer in the LPZ, while advection controlled migration in the HPZ. The mass discharge rate of TCE and its biotic (cis-DCE and vinyl chloride) and abiotic (acetylene) transformation products were measured over 180 days in the flow cell effluent. Depth profiles of these compounds through the LPZ were determined after terminating the experiment. A one-dimensional diffusion-reaction model was used to interpret the effluent and depth profile data and constrain reaction parameters. Abiotic transformation rate constants for TCE to acetylene, normalized to in situ solids loading, were approximately 13 times greater in batch than in the flow cell. Slower transformation rates in the flow cell indicate elevated TCE concentration and/or further degradation of acetylene to other reduced gas compounds in the flow cell LPZ (thereby partially masking TCE abiotic transformation). Biotic and abiotic parameters used to interpret the flow cell data were then used to simulate a field site with a 300 cm thick LPZ. Abiotic processes contributed to a 2% reduction in TCE flux after 730 days. When abiotic rate constants were changed to that observed in batch, or to rate constants previously reported for a pyrite rich mudstone, the TCE flux reduction was 21% and 53%, respectively, after 730 days. Though biotic processes dominated TCE transformation in the flow cell experiment, the simulations indicate that abiotic processes have potential to significantly contribute to TCE attenuation in electron donor limited environments provided suitable reactive minerals are present.