Megharaj Mallavarapu

Integration of traditional and innovative characterization techniques for flux-based assessment of Dense Non-aqueous Phase Liquid (DNAPL) sites

Key attributes of the source zone and the expanding dissolved plume at a trichloroethene (TCE) site in Australia were evaluated using trends in groundwater monitoring data along with data from on-line volatile organic compound (VOC) samplers and passive flux meters (PFMs) deployed in selected wells. These data indicate that: (1) residual TCE source mass in the saturated zone, estimated using two innovative techniques, is small ( approximately 10 kg), which is also reflected in small source mass discharge ( approximately 3 g/day); (2) the plume is disconnecting, based on TCE concentration contours and TCE fluxes in wells along a longitudinal transect; (3) there is minimal biodegradation, based on TCE mass discharge of approximately 6 g/day at a plume control plane approximately 175 m from source, which is also consistent with aerobic geochemical conditions observed in the plume; and (4) residual TCE in the vadose zone provides episodic inputs of TCE mass to the plume during infiltration/recharge events. TCE flux data also suggest that the small residual TCE source mass is present in the low-permeability zones, thus making source treatment difficult. Our analysis, based on a synthesis of the archived data and new data, suggests that source treatment is unwarranted, and that containment of the large TCE plume (approximately 1.2 km long, approximately 0.3 km wide; 17 m deep; approximately 2000-2500 kg TCE mass) or institutional controls, along with a long-term flux monitoring program, might be necessary. The flux-based site management approach outlined in this paper provides a novel way of looking beyond the complexities of groundwater contamination in heterogeneous domains, to make intelligent and informed site decisions based on strategic measurement of the appropriate metrics.

DDT remediation in contaminated soils: a review of recent studies

Over the past few decades significant progress has been made in research on DDT degradation in the environment. This review is an update of some of the recent studies on the degradation and biodegradation pathways of DDT and its metabolites, particularly in soils. The latest reports on human toxicity shows that DDT intake is still occurring even in countries that banned its use decades ago. Ageing, sequestration and formation of toxic metabolites during the degradation processes pose environmental challenges and result in difficulties in bioremediation of DDT contaminated soils. Degradation enhancement strategies such as the addition of chelators, low molecular organic acids, co-solvent washing and the use of sodium and seaweeds as ameliorant have been studied to accelerate degradation. This review describes and discusses the recent challenges and degradation enhancement strategies for DDT degradation by potentially cost effective procedures based on bioremediation.

A Comprehensive Review of Aliphatic Hydrocarbon Biodegradation by Bacteria

Hydrocarbons are relatively recalcitrant compounds and are classified as high-priority pollutants. However, these compounds are slowly degraded by a large variety of microorganisms. Bacteria are able to degrade aliphatic saturated and unsaturated hydrocarbons via both aerobic and anaerobic pathways. Branched hydrocarbons and cyclic hydrocarbons are also degraded by bacteria. The aerobic bacteria use different types of oxygenases, including monooxygenase, cytochrome-dependent oxygenase and dioxygenase, to insert one or two atoms of oxygen into their targets. Anaerobic bacteria, on the other hand, employ a variety of simple organic and inorganic molecules, including sulphate, nitrate, carbonate and metals, for hydrocarbon oxidation.

Effects of ageing and soil properties on the oral bioavailability of benzo[a]pyrene using a swine model

Oral bioavailability of benzo[a]pyrene (B[a]P) was studied in a swine model using eight spiked soil samples after incubation for 50 and/or 90 days. Silica sand was used as a reference material and the relative bioavailability (RB) of B[a]P in soils was calculated as the quotient of the area under the plasma B[a]P curve (AUC) for soil and AUC for the silica sand. Significantly reduced RB was observed in all study soils after 90 days ageing, ranging from 22.1±0.4% to 62.7±10.1%, except for one very sandy soil (sand content 87.6%) where RB was unchanged (108.1±8.0%). Apart from this, bioavailability decreased during ageing with the decrease (from day 50 to day 90) being only significant for a clayey soil containing expandable clay minerals. Statistical analyses of B[a]P RB at day 90 (eight soils) and soil properties showed no direct correlation between RB and specific soil properties such as total organic carbon (TOC) and clay content which were commonly linked to organic contaminant sequestration. However, strongly significant relationships (p<0.001) were found between RB and the fine particle associated carbon (FPAC) defined as (Silt+Clay)/TOC, and between RB and the soil mesopore (<6nm; p<0.001) fraction, after two samples with high pH and high EC being excluded from the analyses. The bioaccessibility estimated by four in vitro extraction methods: dichloromethane/acetone sonication (DCM/Ace), butanol vortex (BuOH), hydroxypropyl-β-cyclodextrin extraction (HPCD) and Milli Q water leaching methods at different sampling time (1 day, 50 days and 90 days after spiking) also showed a decreasing trend. Significant correlations were found between B[a]P RB and DCM/Ace (R(2)=0.67, p<0.05) extractable fraction and BuOH (R(2)=0.75, p<0.01) extractable fraction.

Effect of ageing on benzo(a)pyrene extractability in contrasting soils

Changes in benzo[a]pyrene (B[a]P) extractability over 160 days ageing in four contrasting soils varying in organic matter content and clay mineralogy were investigated using dichloromethane: acetone 1:1 (DCM/Ace), 60 mM hydroxypropyl-β-cyclodextrin (HPCD) solution, 1-butanol (BuOH) and Milli-Q water. The B[a]P extractability by the four methods decreased with ageing and a first-order exponential model could be used to describe the kinetics of release. Correlation of the kinetic rate constant with major soil properties showed a significant effect of clay and sand contents and pore volume fraction (<6 nm) on sequestration of the desorbable fraction (by HPCD) and the water-extractable fraction. Analysis of (14)C-B[a]P in soils after ageing showed a limited loss of B[a]P via degradation. Fractionation of B[a]P pools associated with the soil matrix was analysed according to extractability of B[a]P by the different extraction methods. A summary of the different fractions is proposed for the illustration of the effect of ageing on different B[a]P-bound fractions in soils. This study provides a better understanding of the B[a]P ageing process associated with different fractions and also emphasises the extraction capacity of the different methods employed.

Bioaccessibility of arsenic and cadmium assessed for in vitro bioaccessibility in spiked soils and their interaction during the Unified BARGE Method (UBM) extraction.

Recent decades have seen a growing popularity of in vitro bioaccessibility being utilised as a screening tool in human health risk assessment. However the existing bioaccessibility studies only focus on single contaminant. Considering human are likely to ingest multi-contaminants, these contaminants could interact within human gastrointestinal tract which may lead to an increase or decrease in bioaccessibility. In this study, seven different types of soil were spiked with arsenic (As) or cadmium (Cd) and aged for one year. The effects of soil properties on the bioaccessibility were examined. Moreover, the interaction between As and Cd in simulated human digestive system was studied by mixing As-spiked soil with Cd-spiked soil of the same type during bioaccessibility test. Results shows the bioaccessibility of As ranged from 40 ± 2.8 to 95 ± 1.3% in the gastric phase and 16 ± 2.0 to 96 ± 0.8% in the intestinal phase whilst a significant difference was observed between Cd gastric bioaccessibility (72 ± 4.3 to 99 ± 0.8%) and intestinal bioaccessibility (6.2 ± 0.3 to 45 ± 2.7%). Organic carbon, iron oxide and aluminium oxide were key parameters influencing the bioaccessibility of As (gastric and intestinal phases) and Cd (intestinal phase). No interactions between As and Cd during bioaccessibility test were observed in any soils, which indicates As and Cd may age independently and did not interact while being solubilised during bioaccessibility test. Thus additive effect may be proposed when estimating the bioaccessibility of mixtures of independently-aged As and Cd in soils.

A pyrosequencing-based analysis of microbial diversity governed by ecological conditions in the Winogradsky column

The Winogradsky column is used as a microcosm to mimic both the microbial diversity and the ecological relationships between the organisms in lake sediments. In this study, a pyrosequencing approach was used to obtain a more complete list of the microbial organisms present in such columns and their ratios in different layers of this microcosm. Overall, 27 different phyla in these columns were detected in these columns, most (20 phyla) belonged to bacteria. Based on this study, Proteobacteria (mostly Sphingomonadales), Cyanobacteria (mostly Oscillatoriales) and Bacteroidetes (mostly Flavobacteriales) were the dominant microorganisms in the water, middle, and bottom layers of this column, respectively. Although the majority of organism in the water layer were photoautotrophic organisms, the ratio of the phototrophic organisms decreased in the lower layers, replaced by chemoheterotrophic bacteria. Furthermore, the proportion of aerobic chemoheterotrophic bacteria was greater in the higher layers of the column in comparison to the bottom. The green and purple sulfur phototrophic bacteria inhabited the bottom and middle of these columns, with none of them found in the water layer. Although the sulfur oxidizing bacteria were the dominant chemolithotrophic bacteria in the water layer, their ratio decreases in lower layers, being replaced with nitrogen oxidizing bacteria in the middle and bottom layers. Overall, the microbial population of these layers changes from a phototrophic and aerobic chemoheterotrophic organisms in the water layer to a mostly anaerobic chemoheterotrophic population of bacteria in the bottom layers.

In silico approach to support that p-nitrophenol monooxygenase from Arthrobacter sp. strain JS443 catalyzes the initial two sequential monooxygenations.

Abstractp-Nitrophenol (PNP), used primarily for manufacturing pesticides and dyes, has been recognized as a priority environmental pollutant. It is therefore important to reduce the input of this toxicant into the environment and to establish approaches for its removal from the contaminated sites. PNP monooxygenase, a novel enzyme from Gram-positive bacteria like Arthrobacter sp. and Bacillus sp., that comprises two components, a flavoprotein reductase and an oxygenase, catalyzes the initial two sequential monooxygenations to convert PNP to trihydroxybenzene. Accurate and reliable prediction of this enzyme–substrate interactions and binding affinity are of vital importance in understanding these catalytic mechanisms of the two sequential reactions. As crystal structure of the enzyme has not yet been published, we built a homology model for PNP monooxygenase using crystallized chlorophenol 4-monooxygenase from Burkholderia cepacia AC1100 (3HWC) as the template. The model was assessed for its reliability using PROCHECK, ERRAT and ProSA. Molecular docking of the physiological substrates, PNP and 4-nitrocatechol (4-NC), was carried out using Glide v5.7 implemented in Maestro v9.2, and the binding energies were calculated to substantiate the prediction. Docking complexes formed by molecular level interactions of PNP monooxygenase-PNP/4-NC without or with the cofactors, FAD and NADH, showed good correlation with the established experimental evidence that the two-component PNP monooxygenase catalyzes both the hydroxylation of PNP and the oxidative release of nitrite from 4-NC in B. sphaericus JS905. Furthermore, molecular dynamics simulations performed for docking complexes using Desmond v3.0 showed stable nature of the interactions as well.

Comparison of oral bioavailability of benzo[a]pyrene in soils using rat and swine and the implications for human health risk assessment.

BACKGROUND There are many uncertainties concerning variations in benzo[a]pyrene (B[a]P) soil guidelines protecting human health based on carcinogenic data obtained in animal studies. Although swine is recognised as being much more representative of the human child in terms of body size, gut physiology and genetic profile the rat/mice model is commonly used in practice. OBJECTIVES We compare B[a]P bioavailability using a rat model to that estimated in a swine model, to investigate the correlation between these two animal models. This may help reduce uncertainty in applying bioavailability to human health risk assessment. METHODS Twelve spiked soil samples and a spiked silica sand (reference material) were dosed to rats in parallel with a swine study. B[a]P bioavailability was estimated by the area under the plasma B[a]P concentration-time curve (AUC) and faecal excretion as well in the rats. Direct comparison between the two animal models was made for: firstly, relative bioavailability (RB) using AUC assay; and secondly, the two assays in the rat model. RESULTS Both AUC and faecal excretion assays showed linear dose-response for the reference material. However, absolute bioavailability was significantly higher when using faecal excretion assay (p<0.001). In aged soils faecal excretion estimated based on solvent extraction was not accurate due to the form of non-extractable fraction through ageing. A significant correlation existed between the two models using RB for soil samples (RBrat=0.26RBswine+17.3, R(2)=0.70, p<0.001), despite the regression slope coefficient revealing that the rat model would underestimate RB by about one quarter compared to using swine. CONCLUSIONS In the comparison employed in this study, an interspecies difference of four in RB using AUC assay was identified between the rat and swine models regarding pharmacokinetic differences, which supported the body weight scaling method recommended by US EPA. Future research should focus on the carcinogenic competency (pharmacodynamics) used in experiment animals and humans.

Application of neural networks with novel independent component analysis methodologies to a Prussian blue modified glassy carbon electrode array

Sodium potassium absorption ratio (SPAR) is an important measure of agricultural water quality, wherein four exchangeable cations (K(+), Na(+), Ca(2+) and Mg(2+)) should be simultaneously determined. An ISE-array is suitable for this application because its simplicity, rapid response characteristics and lower cost. However, cross-interferences caused by the poor selectivity of ISEs need to be overcome using multivariate chemometric methods. In this paper, a solid contact ISE array, based on a Prussian blue modified glassy carbon electrode (PB-GCE), was applied with a novel chemometric strategy. One of the most popular independent component analysis (ICA) methods, the fast fixed-point algorithm for ICA (fastICA), was implemented by the genetic algorithm (geneticICA) to avoid the local maxima problem commonly observed with fastICA. This geneticICA can be implemented as a data preprocessing method to improve the prediction accuracy of the Back-propagation neural network (BPNN). The ISE array system was validated using 20 real irrigation water samples from South Australia, and acceptable prediction accuracies were obtained.