Sridhar Susarla

Phytoremediation: An ecological solution to organic chemical contamination

Abstract Phytoremediation is a promising new technology that uses plants to degrade, assimilate, metabolize, or detoxify metals, hydrocarbons, pesticides, and chlorinated solvents. In this review, in situ, in vivo and in vitro methods of application are described for remediation of these compounds. Phytoaccumulation, phytoextraction, phytostabilization, phytotransformation, phytovolatilization and rhizodegradation are discussed and the role of enzymes in transforming organic chemicals in plants is presented. The advantages and constraints of phytoremediation are provided. Our conclusions is that phytoremediation prescriptions must be site-specific; however, these applications have the potential for providing the most cost-effective and resource-conservative approach for remediating sites contaminated with a variety of hazardous chemicals.

Phytotransformations of Perchlorate Contaminated Waters

Six vascular plant species (sweet gum, black willow, pickleweed, smartweed, water-lily and duckmeat) were used to determine the uptake and transformation of perchlorate at three concentrations (0.2, 2.0, and 20.0 mgl−1) for 10 days under laboratory conditions. Perchlorate uptake followed a first-order reaction, with rate constants ranging between 0.001 to 0.007 h−1. Accumulation of perchlorate was between 0 to 981 mg kg−1 of plant weight for various plants. Based on the results the accumulation of perchlorate was in the order: smartweed > pickleweed > sweet gum > water-lily > willow. Analysis of plant tissues (roots, stems, and leaves) confirmed the presence of transformation products, chlorate, chlorite and chloride. The distribution of perchlorate and its products indicate that the transformation reaction occurred in each tissue independently of the other and accumulation of byproducts was significantly higher in leaves than in roots or stem. No external signs of perchlorate toxicity were observed at these high concentrations in plant tissues suggesting that these plant species show potential for phytoremediation.

Phytotransformation of Perchlorate and Identification of Metabolic Products in Myriophyllum aquaticum

ABSTRACT The uptake and transformation of perchlorate in the presence of Myriophyllum aquaticum (parrot-feather) were examined in sand and aqueous treatments with concentrations between 0.2 to 20 ppm. Controls were included without plants to confirm the uptake of perchlorate by parrot-feather. The kinetic data followed a first-order reaction mechanism with rate constants ranging from 0.004 to 0.090, resulting in half-lives between 7 to 173 days. Uptake rates were five times higher in aqueous treatments than in sand treatments. The intermediates detected in the plant tissue suggest that perchlorate transformed in a step-wise fashion to form chloride. Accumulation of perchlorate in the plant tissues (1.2 g/kg) suggests that parrot-feather has a high capacity for accumulating this contaminant.

Reductive transformations of halogenated aromatics in anaerobic estuarine sediment : Kinetics, products and pathways

Abstract The transformation of different classes of halogenated aromatic compounds in sulfidogenic sediment slurries was examined in laboratory incubation experiments. The results indicated that several processes were shown to occur, leading to the formation of a number of intermediate metabolites. The kinetics of transformation followed a first-order reaction mechanism, with k values in the range of 1.67 and 0.0005 day −1 or half-lives of 0.4 and 1386 days. The order of transformation rates among different classes of compounds was: phenols > anilines > benzenes > pesticides. The pathway of transformation among single-ring aromatic compounds indicated that ortho and para halogen removal was the predominant compared to meta halogen removal.

Reductive Dehalogenation of Chloroanilines in Anaerobic Estuarine Sediment

The reductive dehalogenation of chloroanilines (CAs) was investigated in anaerobic sediment/water systems. The sulfate content of the interstitial and overlaying waters was approximately 20 mmoles l−1 and the sediment was apparently sulfidogenic. The CAs exhibited first-order disappearance kinetics with the rate constants ranging from 0.012–0.005 day−1 or half-lives between 40 and 672 days. Pentachloroaniline (PeCA), 2,3,4,5-tetrachloroaniline (2,3,4,5-TeCA) and 3,4,5-trichloroaniline (3,4,5-TCA) were transformed by the sequential removal of halogen from the para and ortho positions. The disappearance of the parent compound and accumulation of metabolites were simulated using a first-order branched kinetic model.

Redox Potential as a Parameter To Predict the Reductive Dechlorination Pathway of Chloroanilines in Anaerobic Environments

A bstractThe hypothesis that the microbially catalyzed pathway proceeds with a step that would yield the highest energy was examined for the reductive dechlorination of chloroanilines (CAs) under anaerobic conditions. The Gibbs free energy of formation was estimated with Bensons method, then the redox potentials were determined using an H+/H2 couple as the reference system. The observed pathways were compared using the redox potential of the reaction, and the results showed that the redox potential correctly predicts the pathway that yields the highest energy for the dechlorination step.

Reductive dechlorination pathways of chloro organics under anaerobic conditions

The transformation pathways of chloroanilines (CAs), chlorobenzenes (CBs) and chlorophenols (CPs) were examined in anaerobic sediment collected from the Tsurumi river, Japan. The sediment was pre-exposed to various organic chemicals from the surrounding industries and appears to be sulfidogenic. Experiments were carried out for each compound in the sediment slurry, which was spiked at a desired concentration. The transformation of the parent substrate and the appearance of its metabolites were monitored for a year. All the compounds transformed without any lag period. For CBs, the preferential Cl removal was in the order: two Cl atoms on adjacent carbon atoms > one Cl atom on adjacent carbon > no adjacent Cl atom on the carbon. In case of CPs, ortho dechlorination was the preferred pathway, while for CAs, sequential removal of ortho and para positions was observed.

Transformation Kinetics and Pathways of Chlorophenols and Hexachlorobenzene in Fresh Water Lake Sediment Under Anaerobic Conditions

The anaerobic transformation of chlorophenols (CPs) and hexachlorobenzene (HCB) was examined in a fresh water lake sediment collected from Lake Kasumigaura in Japan. Anaerobic experiments were carried out by spiking each compound in a number of test tubes and the change in parent compound concentration was monitored for a year. Lag periods of 1–31 days were observed for the substrates examined. The first-order rate constants determined following the lag period, ranged between 0.0021 and 0.072 day−1 for chlorophenols and 0.011 day−1 for hexachlorobenzene. Two pathways for pentachlorophenol (PCP) in the sediment were observed: in the first one, under sulfidogenic conditions, PCP transformed to 3-chlorophenol (3-CP) as, 2,3,5,6-tetrachlorophenol (2,3,5,6-TeCP), 2,3,5-trichlorophenol (2,3,5-TCP), 3,5-dichlorophenol (3,5-DCP), 3-CP; and in the second pathway, under methanogenic conditions, PCP transformed via 2,3,4,5-tetrachlorophenol (2,3,4,5-TeCP), 3,4,5-trichlorophenol (3,4,5-TCP), 3,4-dichlorophenol (3,4-D...

Uptake and transformation of perchlorate by vascular plants

Phytoremediation is a promising new method that uses green plants to cleanse soil and water contaminated with organic or inorganic pollutants. In this study, the uptake and transformation of sodium perchlorate (NaClO4) using four vascular plant species were examined in batch experiments. The species include two trees, cabbage gum (Eucalyptus amplifolia) and eastern cottonwood (Populus deltoides), a herbaceous wetland plant, perennial glasswort (Salicornia virginica), and a herbaceous aquatic plant, waterweed (Elodea canadensis). Perchlorate was depleted from solution in the presence of all but one species (waterweed). Depletion was calculated as a first‐order kinetics reaction with k values in the range of 0–0.013 per day and accumulation of perchlorate was between 3.2 and 3138 mg/kg. Perchlorate and transformation metabolites (chlorate, chlorite, chloride) were observed in all plant tissues (e.g., roots, stems, leaves) analyzed. Results suggest that significant influences on perchlorate uptake include: (1) plant species present, (2) concentration of perchlorate, (3) sand versus hydroponic treatments, (4) the presence or absence of plant nutrients or competing ions, (5) stage of plant maturity.

Biotransformation of halogenated benzenes in anaerobic sediments

The transformation kinetics of halogen substituted benzenes was examined in estuarine sediment. The sediment was sulfidogenic with sulfate concentration of 20 mmole/l. All compounds transformed without any lag period, with rate constants between 0.0016 and 0.0342 day-1 or half-lives of 20 and 433 days. For the compounds with different halogen substituents on the aromatic ring, the transformation rate of the compound decreased in the order: I s> Br s> Cl s> F.