Contaminant Degradation Rates Research Articles

Novel Fenton-like Catalyst HKUST-1(Cu)/MoS2-3-C with Non-Equilibrium-State Surface for Selective Degradation of Phenolic Contaminants: Synergistic Effects of σ-Cu-Ligand and ≡Mo–OOSO3− Complex

Novel Fenton-like catalyst HKUST-1(Cu)/MoS2-3-C with a non-equilibrium-state surface was constructed for selective degradation of phenolic contaminants. Electron-polarized distribution facilitated the formation of σ-Cu-ligand between electron-poor Cu centre and phenolic compounds, which not only enhanced radicals generation but also accelerated the Cu(I)/Cu(II) redox. Meanwhile, ≡Mo–OOSO3− complexes formed by the electron-rich Mo centre and peroxymonosulfate (PMS), could directly oxidize phenolic contaminants with the generation of SO4•−. The radical quenching experiments and EPR tests indicated that both SO4•− and •OH played a dominant role in the reaction. Additionally, O2 could be reduced to O2•− by OVs and subsequently converted into 1O2 over the Mo centre. DFT calculation, FT-IR, and in situ Raman spectra analysis results demonstrated that phenolic compounds and PMS were respectively adsorbed by electron-poor Cu centre and electron-rich Mo centre, favouring the electrons transfer from phenolic contaminants to Mo centre for PMS activation. With synergistic effects of σ-Cu-ligand and ≡Mo–OOSO3− complexes, HKUST-1(Cu)/MoS2-3-C achieved a high degradation rate of phenolic contaminants and utilization efficiency of PMS.

Caffeine degradation in a plasma-liquid reactor with the lateral liquid flow: Elucidating the effects of mass transport on contaminant removal

This work identifies the phenomena relevant to designing and operating highly efficient plasma reactors for water treatment applications using a simple plasma-liquid system featuring laminar, unidirectional fluid flow in contact with the plasma phase. The reactor performance was established by treating caffeine under various combinations of plasma-liquid contact area and liquid flow velocity.The plasma-liquid contact area was the most significant contributor to caffeine degradation. A complex, nonlinear effect of fluid flow on caffeine degradation was observed for a constant plasma area. The experimental data suggested two caffeine transport modes disambiguated using a transient diffusion–reaction model with interfacial adsorption. Two distinct rate-limiting contaminant transport regimes were identified depending on the solution-plasma contact time: diffusion on the timescale of the plasma discharge period (milliseconds) and diffusion on a liquid flow timescale (seconds). A simple criterion for estimating the optimal contact time for pulsed plasma reactor operation was developed based on the modeling insights.Model-based lower and upper bounds for the maximum contaminant degradation rates were established and compared to the observed performance of plasma reactors reported in the literature. Most reactors with flat plasma-liquid interfaces performed within the predicted bounds, suggesting contaminant mass transport is the rate-limiting process in the overall degradation. Furthermore, the area-normalized degradation rate can be used to determine the rate-limiting process for a given reactor configuration and compare performance between different plasma reactors. These findings have significant implications for the design and scale up of plasma reactors for water treatment applications.

Reviewing the Bioremediation of Contaminants in Groundwater: Investigations over 40 Years Provide Insights into What's Achievable.

Biodegradation and biotransformation of contaminants in groundwater commonly occurs naturally. However, natural biodegradation rates can be slow leading to elongated contaminant plumes and prolonged risks that demand greater remedial intervention. Enhancement of the biodegradation of contaminants in groundwater can be induced by the addition of amendments to change the geochemical conditions to those that are more favorable for indigenous or added biota. Enhancing biodegradation requires collocation of the contaminant of concern with the 'right' microbial communities under the 'right' geochemical conditions, so that the microbiota thrive and bio-transform, degrade or lock up the contaminant of interest. This is most easily achievable at laboratory or bench scale where mixing is easily performed, and mass transfer limitations are minimized. However, inducing such changes at field scale in aquifers is non-trivial - amendments do not easily mix into groundwater because it is a laminar (non-turbulent) and low-energy flow environment. Bioaugmentation of cultured or genetically modified organisms have also been considered to add to groundwater to enhance contaminant degradation rates. Here we provide an overview of research studies over approximately 40 years that highlight the progression of understanding from natural biodegradation of plumes in groundwater to active bioremediation efforts that have been variably successful at field scale. Investigated contaminants providing insights include petroleum hydrocarbons, chlorinated and brominated hydrocarbons, ammonium, metals, munition compounds, atrazine and per- and polyfluorinated alkyl substances. The redox and electron acceptor/donor conditions that are inducive to biodegradation for a range of contaminants are highlighted. Biodegradation is challenged by the availability of electron donors/acceptors in the core of plumes and on plume fringes. Cases for bioaugmentation are identified. A long history of investigations provides examples of the importance of amendment delivery mechanisms, scale-up from laboratory to field, and field-scale demonstration of the effectiveness of groundwater bioremediation technologies. Advantages and disadvantages of remedial approaches are tabulated. The value and contributions of integrative modelling advances are identified. The literature review and example cases provide a deep understanding of what scale of bioremediation might be achievable for groundwater plumes. Limitations to bioremediation strategies outlined here will help direct future efforts. Addressing the sources of groundwater plumes as well as bioremediation of the plume itself will achieve more effective outcomes. Twelve 'lessons learnt' are synthesized from the review.

Reactivity of Dissolved Organic Matter with the Hydrated Electron: Implications for Treatment of Chemical Contaminants in Water with Advanced Reduction Processes.

Advanced reduction processes (ARP) have garnered increasing attention for the treatment of recalcitrant chemical contaminants, most notably per- and polyfluoroalkyl substances (PFAS). However, the impact of dissolved organic matter (DOM) on the availability of the hydrated electron (eaq-), the key reactive species formed in ARP, is not completely understood. Using electron pulse radiolysis and transient absorption spectroscopy, we measured bimolecular reaction rates constant for eaq- reaction with eight aquatic and terrestrial humic substance and natural organic matter isolates ( kDOM,eaq-), with the resulting values ranging from (0.51 ± 0.01) to (2.11 ± 0.04) × 108 MC-1 s-1. kDOM,eaq- measurements at varying temperature, pH, and ionic strength indicate that activation energies for diverse DOM isolates are ≈18 kJ mol-1 and that kDOM,eaq- could be expected to vary by less than a factor of 1.5 between pH 5 and 9 or from an ionic strength of 0.02 to 0.12 M. kDOM,eaq- exhibited a significant, positive correlation to % carbonyl carbon for the isolates studied, but relationships to other DOM physicochemical properties were surprisingly more scattered. A 24 h UV/sulfite experiment employing chloroacetate as an eaq- probe revealed that continued eaq- exposure abates DOM chromophores and eaq- scavenging capacity over a several hour time scale. Overall, these results indicate that DOM is an important eaq- scavenger that will reduce the rate of target contaminant degradation in ARP. These impacts are likely greater in waste streams like membrane concentrates, spent ion exchange resins, or regeneration brines that have elevated DOM concentrations.

Sulfolane degrading bacteria from petrochemical plant: activated sludge enrichment, isolation and characterization

Sulfolane is extensively used as an extractive solvent in sour-gas processing plants in the petrochemical industry. After repeated use, deteriorated sulfolane becomes corrosive and may leak into the environment to threaten aquatic and terrestrial organisms or cause shock loading of wastewater treatment systems. It is therefore important to remove sulfolane from the environment and maintain stable wastewater treatment efficiency. To address this issue, bioaugmentation provides a solution by adding specific microorganisms into the biological treatment system to speed up the contaminant degradation rate. Isolation of microbes capable of degrading target contaminant is key point. Understanding the physiological characteristics of isolated microbes is indispensable for subsequent successful applications. In this study, sulfolane degrading bacteria from the activated sludge of a petrochemical plant were enriched using 200–1000 mg L−1 sulfolane and then isolated. After confirming their sulfolane degrading ability, sulfolane biodegradation was then investigated under various pHs, sulfolane and sulfate concentrations. Enrichment could enhance the sulfolane degrading rate of the mixed culture from 3.9 to 7.1 times. Three strains (strain Y-a, Y-d and Y-f) capable of degrading sulfolane were isolated. The degrading microbes were identified as Cupriavidus sp. using 16S rDNA sequencing. Sulfolane concentration biodegraded by strain Cupriavidus sp. Y-d at pH of 7–9 was 1.40–1.45 times higher than that at pH of 6. The optimal pH for strain Cupriavidus sp. Y-d to degrade sulfolane was 8. When increasing the sulfolane concentration from 500 to 2000 mg L−1, the specific growth rate and specific substrate utilization rate increased from 0.19 to 0.48 d−1 and 0.48 to 0.59 d−1, respectively. Strain Cupriavidus sp. Y-d was capable of degrading 2000 mg L−1 sulfolane. Sulfate concentrations higher than 2511 mg L−1 had a negative effect on sulfolane biodegradation.

Predicting Active Sites in Photocatalytic Degradation Process Using an Interpretable Molecular-Image Combined Convolutional Neural Network

Machine-learning models have great potential to accelerate the design and performance assessment of photocatalysts, leveraging their unique advantages in detecting patterns and making predictions based on data. However, most machine-learning models are “black-box” models due to lack of interpretability. This paper describes the development of an interpretable neural-network model on the performance of photocatalytic degradation of organic contaminants by TiO2. The molecular structures of the organic contaminants are represented by molecular images, which are subsequently encoded by feeding into a special convolutional neural network (CNN), EfficientNet, to extract the critical structural features. The extracted features in addition to five other experimental variables were input to a neural network that was subsequently trained to predict the photodegradation reaction rates of the organic contaminants by TiO2. The results show that this machine-learning (ML) model attains a higher accuracy to predict the photocatalytic degradation rate of organic contaminants than a previously developed machine-learning model that used molecular fingerprint encoding. In addition, the most relevant regions in the molecular image affecting the photocatalytic rates can be extracted with gradient-weighted class activation mapping (Grad-CAM). This interpretable machine-learning model, leveraging the graphic interpretability of CNN model, allows us to highlight regions of the molecular structure serving as the active sites of water contaminants during the photocatalytic degradation process. This provides an important piece of information to understand the influence of molecular structures on the photocatalytic degradation process.

Quantifying Hydrated Electron Transformation Kinetics in UV-Advanced Reduction Processes Using the Re-,UV Method.

Ultraviolet advanced reduction processes (UV-ARP) have garnered significant attention recently for the degradation of several hard to treat contaminants, including recalcitrant per- and polyfluoroalkyl substances (PFAS). The rate of contaminant degradation in UV-ARP is directly related to the available hydrated electron concentration ([eaq-]). However, reports of [eaq-] and other parameters typically used to characterize photochemical systems are not widely reported in the UV-ARP literature. Deploying monochloroacetate as a probe compound, we developed a method (Re-,UV) to quantify the time-based hydrated electron concentration ([eaq]t) available for contaminant degradation relative to inputted UV fluence. Measured [eaq]t was then used to understand the impact of eaq- rate of formation and scavenging capacity on the degradation of two contaminants─nitrate and perfluorooctane sulfonate (PFOS)─in four source waters with varying background water quality. The results show that the long-term treatability of PFOS by UV-ARP is not significantly impacted by the initial eaq- scavenging conditions but rather is influenced by the presence of eaq- scavengers like dissolved organic carbon and bicarbonate. Lastly, using [eaq]t, degradation of nitrate and PFOS was modeled in the source waters. We demonstrate that the Re-,UV method provides an effective tool to assess UV-ARP treatment performance in a variety of source waters.

Efficient sonocatalytic degradation of heavy metal and organic pollutants using CuS/MoS2 nanocomposites

Eco-friendly and highly effective catalysts are receiving considerable attention for the removal of heavy-metal ions and organic pollutants. In this study, we developed CuS/MoS2 nanocomposite sonocatalysts to enhance the degradation rate of environmental contaminants by harnessing ultrasonic irradiation. The successful synthesis of nanocomposite sonocatalysts was confirmed by X-ray diffraction (XRD) analysis, and energy-dispersive X-ray spectroscopy. The incorporation of CuS into MoS2 resulted in a flower-like structure with an increased surface area. Importantly, the sonocatalytic efficiency was enhanced by increasing CuS concentration in the nanocomposites, achieving maximum removal efficiencies of 99% and 88.52% for rhodamine B (RhB) and Cr(VI), respectively. In addition, they showed excellent stability and recyclability over five consecutive cycles, without noticeable changes in the nanocomposite structure. Reactive oxygen species (ROS) used for the degradation were identified using ROS scavengers. We believe that this strategy of exploiting nanocomposite sonocatalysts has a great potential in the field of environmental catalysis.

A semi-analytical solution to organic contaminants transport through composite liners considering a single crack in CCL

Compacted clay liners (CCLs) are extensively used as engineering barriers for groundwater and soil pollution. The existence of cracks/fractures in CCL caused by thermally induced shrinkage is reported to importantly damage the performance of the CCL. An analytical model is developed to study the effects of the cracks/fractures on the migration of organic contaminants through a composite liner system. Laplace transformation and Laplace inversion using the Stehfest method are adopted to derive the analytical solution, which is validated by the experimental data. The existence of crack shows a significant impact on the breakthrough curve and bottom flux of organic contaminants. Increasing the crack width from 1 to 25 mm results in an enhancement of contaminant bottom concentration by a factor of 280. Increasing the adsorption factor and degradation rate of contaminants can effectively improve the performance of the composite liner with cracks. The effects of degradation of contaminants on the breakthrough curve are found to be more significant for the case with a larger retardation factor. This may be due to the fact that increasing the retardation factor can significantly slow down the transport of contaminants, which may indirectly create a longer period for the degradation of contaminants.

Synchronous removal of emulsions and soluble organic contaminants via a microalgae-based membrane system: performance and mechanisms

In this study, we applied a flexible strategy to manufacture a microalgal biochar-based membrane (MBCM). Due to the hierarchical surface topography on a micro-nano scale, the MBCM was found to have both underwater superoleophobic and underoil superhydrophobic properties. Combining an underoil superhydrophobic oil-containing region (OCR) with an underwater superoleophobic water-containing region (WCR) achieved the successive filtration of multiphase emulsions. The MBCM also served as a high-performance carbocatalyst for advanced oxidation processes (AOPs), due to the N functionalities (5.08%) of the graphene-like structure. This was caused by the high-temperature pyrolysis of rich proteins and alkaline salts in the algal residue. As a result, the MBCM/AOPs system achieved greater than 99.5% emulsions separation efficiency in different emulsion mixtures, while also achieving an outstanding degradation rate (99.8%) of soluble organic contaminants (SOCs). This in-depth exploration resulted in a low-cost and green strategy for developing multifunctional membranes to treat complex wastewater. The paper explains the mechanisms used by MBCM to synchronously remove emulsions and SOCs from wastewater.

Impacts of water table fluctuations on actual and perceived natural source zone depletion rates

A viable means of quantifying the rate of natural source zone depletion (NSZD) at hydrocarbon contaminated sites is by the measurement of carbon dioxide (CO2) and methane (CH4) effluxes at the surface. This methodology assumes that gas effluxes are reflective of actual contaminant degradation rates in the subsurface, which is only accurate for quasi-steady state conditions. However, in reality, subsurface systems are highly dynamic, often resulting in fluctuations of the water table. To quantify the effects of water table fluctuations on NSZD rates, a simulated biodiesel spill in a 400 cm long, 100 cm wide and 150 cm tall sandtank was subjected to lowering and raising the water table, while soil-gas chemistry and surface CO2 and CH4 effluxes were measured. Results show that water table fluctuations have both short-term (perceived) and long-term (actual) effects on NSZD rates, interpreted using surface efflux measurements. When the water table was lowered, surface effluxes immediately increased up to 3 and 344 times higher than baseline for CO2 and CH4 effluxes, respectively, due to the liberation of anaerobically produced gas accumulated below the water table. After this immediate release, the system then reached quasi-steady state conditions 1.4 to 1.6 times higher for CO2 than baseline conditions, attributed to increased aerobic degradation in the broadened and exposed smear zone. When the water table was raised, quasi-steady state CO2 and CH4 effluxes declined to values of 0.9 and 0.4 times baseline effluxes, respectively, implying that contaminant degradation rates were reduced due to submergence of the smear zone. The findings of this study show that the dynamic effects of water table fluctuations and redistribution of the contaminants affect surface effluxes as well as short-term (perceived) and long-term (actual) contaminant degradation rates. Therefore, water table fluctuations need to be considered when quantifying NSZD at contaminated sites using sparse temporal rate measurements to estimate NSZD rates for extended periods of time (e.g., annual rates).

The photocatalytic selectivity between molecularly imprinted TiO2 and target contaminants

In this paper, it is proposed that the photocatalysis selectivity exists between the catalysts and the target contaminants. Therefore, the selectivity of the two catalysts, MIP-TiO2 and NIP-TiO2, can be distinguished by using the same test wavelength to measure the absorbance of different pollutants and different test wavelength to measure the absorbance of the pollutants. At first, MIP-TiO2 was obtained in one step to degrade ethyl parahydroxybenzoate wastewater. Then, the effects of test wavelength and test time on the standard curve, reaction rate, selectivity coefficient, and selectivity factor, as well as the effect of the amount of molecular imprinting on photocatalytic activity and selectivity are studied through photocatalytic experiments. XRD, FT-IR, and TEM were used to characterize the catalysts. The results show that the test wavelength and reaction time have impact on the reaction rate, selectivity coefficient, while selectivity factor, and have no effect on the simulation of the standard curve. Furthermore, the selectivity factor does not change its applicability to calculate the selectivity due to the different selectivity coefficients. In addition, the degradation rate of target contaminants in 2 h by MIP-TiO2 is 81%, while NIP-TiO2 is only 74%. Moreover, MIP-TiO2 has a selectivity factor of 2.680, indicating that it has a higher selectivity for target contaminants than NIP-TiO2. This paper provides a further understanding of the photocatalytic selectivity between molecularly imprinted TiO2 and target contaminants.

An old story with new insight into the structural transformation and radical production of micron-scale zero-valent iron on successive reactivities

It is generally recognized that the formation and accumulation of iron oxides on the surface of zero-valent iron (Fe0) resulting in significant decrease of contaminant degradation rates during the long-term reactions. However, in this study, we found that the removal efficiencies of p-nitrophenol (PNP) by micro zero-valent iron (mFe0) could maintain at the satisfactory level in the process of continuous reactions (20 cycles). The removal rate constant (0.1779 min−1) of the 5th cycle was 6.74 times higher than that of the 1st reaction (0.0264 min−1), even the 20th cycle (0.0371 min−1) was higher than that of the 1st reaction. Interestingly, almost no dissolved iron was detected in the solution, and the total iron concentrations decreased dramatically with the process of continuous reactions. The results of scanning electron microscope and energy dispersive spectrometry (SEM-EDS) and X-ray diffraction (XRD) revealed that the structure and composition of corrosion products change from amorphous to highly crystal with the increase of the number of cycles. The corrosion products were mainly magnetite (Fe3O4) and a small part of maghemite (γ-Fe2O3), which were in the form of microspheres on the surface of mFe0. The formation of surface oxidation shell hindered the release of Fe2+. X-ray photoelectron spectroscopy (XPS) results illustrated that partial Fe3O4 could be converted into γ-Fe2O3. Electrochemical analysis proved that the electron transfer rate of mFe0 increased with the formation of the oxides shell. However, the consumption of iron core and thicker oxide film weakened the electron transfer rate. Besides, the quenching experiments indicated that the reaction activity of mFe0 could be enhanced with the addition of scavengers. This study deepened the understanding of the structural transformation and radical production of mFe0 in continuous reactions.

Laboratory tests for aerobic bioremediation of the contaminated sites in the Czech Republic

Laboratory-scale testing methods applicable to evaluation of contaminated subsurface microbial communities are discussed in relation to their potential in supporting effective site bioremediation. Both culture-dependent and culture-independent techniques are considered here with special emphasis on their capacity to contribute to bioremediation system design, in optimal cases by providing information on contaminant degradation rates. In this regard, microbial soil respiration tests seem to be the most useful tool since microbial soil respiration is a sensitive and easily measurable parameter for determination of metabolic activity within the sample and is closely related to other microbial parameters such as microbial biomass.

Hydroxyl radical scavenging by solid mineral surfaces in oxidative treatment systems: Rate constants and implications

Advanced oxidation treatment processes used in various applications to treat contaminated soil, water, and groundwater involve powerful radical intermediates, including hydroxyl radicals (•OH). Inefficiency in •OH-driven treatment systems involves scavenging reactions where •OH react with non-target species in the aqueous and solid phases. Here, •OH were generated in iron (Fe)- and UV-activated hydrogen peroxide (Fe-AHP, UV-AHP) systems where the loss of rhodamine B served as a quantitative metric for •OH activity. Kinetic analysis methods were developed to estimate the specific •OH surface scavenging rate constant (k≡S). In the Fe-AHP system, k≡S for silica (2.85 × 106 1/m2 × s) and alumina (3.92 × 106 1/m2 × s) were similar. In the UV-AHP system, estimates of k≡S for silica (4.50 × 106 1/m2 × s) and alumina (7.45 × 106 1/m2 × s) were higher. k≡S for montmorillonite (MMT) in the UV-AHP system was ≤4.22 × 105 1/m2 × s. Overall, k≡S,silica ∼ k≡S, alumina > k≡S,MMT indicating k≡S is mineral specific. Radical scavenging was dominated by surface scavenging at 10–50 g/L silica, alumina, or MMT, in both Fe-AHP and UV-AHP systems. The experimentally-derived surface •OH scavenging rate constants were extended to in-situ chemical oxidation (ISCO) treatment conditions to contrast •OH reaction rates with contaminant and aqueous phase reactants found in aquifer systems. •OH reaction was dominated by solid surfaces comprised of silica, alumina, and montmorillonite minerals relative to •OH reaction with trichloroethylene, the target compound, and H2O2, a well-documented radical scavenger. These results indicate that solid mineral surfaces play a key role in limiting the degradation rate of contaminants found in soil and groundwater, and the overall treatment efficiency in ISCO systems. The aggressive •OH scavenging measured was partially attributed to the relative abundance of scavenging sites on mineral surfaces.

Iron-based catalysts under solar and visible radiation for contaminants of emerging concern removal

The ubiquitous presence of CBZ, SMX and LRZ in water is a subject of increasing concern. This study represents a new approach in terms of these contaminants’ removal through Fenton process in three different ways, employing a solid catalyst, RM, which, alone, is a source of Fe3＋ just as much is a source of environmental problems. Alongside RM, 100 mg/L of H2O2 were utilized as the initial solution’s pH remained unaltered. It was observed that, at these operating conditions, an appreciable degradation rate of contaminants was achieved. Solar photo-Fenton under visible light radiation led the way by attaining more than 50% removal for all three contaminants, reaching its peak when degrading around 62% of LRZ initially present. Therefore, RM, a residue from the alumina industry, seems to be a promising choice in terms of CECs degradation through Fenton process. Nevertheless, some parameters still need optimization, in order to achieve a better understanding and certainty of this oxidation process’ skilfulness for detoxifying CECs from the world’s water resources.

A flexible and efficient electro-Fenton cathode film with aeration function based on polyphenylene sulfide ultra-fine fiber

A novel cathode film with aeration function was prepared using a polyphenylene sulfide (PPS) ultra-fine fiber and conductive carbon black (CB). The flexible and non-metallic PPS cathode film was applied in the electro-Fenton (e-Fenton) system in order to degrade the contaminants in wastewater. The effects of the current density, contaminant species, initial pH, and aeration rate on the degradation rate of contaminants were systematically studied. The results show that the optimized conditions include a pH of 3 and a current density of 16 A m−2. It was found that the generation of H2O2 could be greatly enhanced, and several contaminants (methyl orange (MO), methylene blue (MB), rhodamine-B (RhB), malachite green (MG), Orange II (OII), and heptanaphthenic acid (HNA) could be efficiently degraded when the PPS cathode film was used in the e-Fenton system. This robust function of the e-Fenton system results from the benefits of the large porosity, good adsorptivity, and more active sites of the fabricated PPS cathode film. Additionally, the aeration function of the PPS cathode film strongly increased the removal rate of the contaminants. The PPS cathode film can be of potential application in the e-Fenton system for cost-efficient degradation of organic contaminants.

Bisulfite triggers fast oxidation of organic pollutants by colloidal MnO2.

Colloidal MnO2 is the most reactive phase of Mn(IV) while HSO3- is a common reductant in water treatment. This study shows that the presence of HSO3- resulted in significant increase in the decomposition rate of organic contaminants by colloidal MnO2. The degradation rate of contaminants in the MnO2/HSO3- process dropped with elevating pH and a proper MnO2/HSO3- molar ratio was critical for efficient decomposition of contaminants. The time-resolved spectroscopy of manganese species, the influence of pyrophosphate on UV absorbance spectra, and the relative rate constants of contaminants oxidation in MnO2/HSO3- process suggested that the synergetic effect of HSO3- and colloidal MnO2 arose from the generation of Mn(III)aq, which could oxidize contaminants rapidly. The presence of pyrophosphate, ethylenediaminetetraacetic acid, and humic acid depressed the degradation of contaminants in MnO2/HSO3- process by complexing with Mn(III)aq, buffering the solution or competing with contaminants for Mn(III)aq, and/or inhibiting the consumption of bisulfite. However, Ca2+ and Mg2+ accelerated the oxidation of contaminants in MnO2/HSO3- process by enhancing the reduction of MnO2 by HSO3-. The good negative correlation of the O/N or H Mulliken charges of organic contaminants with their removal in MnO2/HSO3- process suggested that organic contaminants were oxidized by Mn(III)aq via electrophilic attack.

Aeration and sawdust application effects as structural material in the bioremediation of clayey acid soils contaminated with diesel oil

Soil contamination, due to the leakage of petroleum or its by-products, is a reason for concern nowadays. Technologies, such as bioremediation, have been developed to remove these pollutants from the environment. This paper aims to evaluate the bioremediation of clayey acid soils contaminated with diesel oil, comparing the effect of aeration use and the addition of different sawdust ratios (5% and 10%) as bulking material. This research was developed in three phases, with experiments of (a) natural attenuation (exclusively natural processes), (b) sawdust-amended soil and (c) aeration combined with sawdust addition. The manometric respirometry test was applied to monitor CO2 production during biodegradation experiments and sodium hydroxide (NaOH) was used as an absorbent solution. The contamination of the samples was carried out by the addition of diesel oil (5% w/w (gg−1)) into the soil. The results indicated that natural attenuation presented a slower bioremediation process when compared to the treatments with sawdust addition. The addition of 5% of sawdust was beneficial to the microbial activity of the soil, but a proportion above 5% caused a negative effect on the samples, reducing the microbial activity. Statistical analysis (Kruskal-Wallis test) showed significant oil removals on experiments with the combination of sawdust and aeration. The aeration was considered the main agent in increasing oil biodegradation in the soil. The degradation rate reactions followed a zero-order linear model. The highest contaminant degradation rate was observed in the samples with 5% of sawdust and aeration (24.79 mgO2 kg−1dry soil day−1), which also improved microbial activity and diesel oil removal.

Plasmid-Mediated Bioaugmentation for the Bioremediation of Contaminated Soils

Bioaugmentation, or the inoculation of microorganisms (e.g., bacteria harboring the required catabolic genes) into soil to enhance the rate of contaminant degradation, has great potential for the bioremediation of soils contaminated with organic compounds. Regrettably, cell bioaugmentation frequently turns into an unsuccessful initiative, owing to the rapid decrease of bacterial viability and abundance after inoculation, as well as the limited dispersal of the inoculated bacteria in the soil matrix. Genes that encode the degradation of organic compounds are often located on plasmids and, consequently, they can be spread by horizontal gene transfer into well-established, ecologically competitive, indigenous bacterial populations. Plasmid-mediated bioaugmentation aims to stimulate the spread of contaminant degradation genes among indigenous soil bacteria by the introduction of plasmids, located in donor cells, harboring such genes. But the acquisition of plasmids by recipient cells can affect the host’s fitness, a crucial aspect for the success of plasmid-mediated bioaugmentation. Besides, environmental factors (e.g., soil moisture, temperature, organic matter content) can play important roles for the transfer efficiency of catabolic plasmids, the expression of horizontally acquired genes and, finally, the contaminant degradation activity. For plasmid-mediated bioaugmentation to be reproducible, much more research is needed for a better selection of donor bacterial strains and accompanying plasmids, together with an in-depth understanding of indigenous soil bacterial populations and the environmental conditions that affect plasmid acquisition and the expression and functioning of the catabolic genes of interest.