Rupali Datta

Symbiotic role of Glomus mosseae in phytoextraction of lead in vetiver grass [Chrysopogon zizanioides (L.)]

Lead (Pb) has limited solubility in the soil environment owing to complexation with various soil components. Although total soil Pb concentrations may be high at a given site, the fraction of soluble Pb that plants can extract is very small, which is the major limiting factor for Pb phytoremediation. The symbiotic effect of arbuscular mycorrhizal (AM) fungus, Glomus mosseae was examined on growth and phytoextraction of lead (Pb) by vetiver grass [Chrysopogon zizanioides (L.)]. A hydroponic study, Phase I (0, 1, 2, and 4mM Pb) was conducted followed by an incubation pot study, Phase II (0, 400, 800, and 1200 mg kg(-1) Pb) where vetiver plants were colonized with G. mosseae. The results obtained indicate that plants colonized by the AM fungi not only exhibit better growth (increase in plant biomass), but also significantly increase Pb uptake in root and higher translocation to the shoot at all given treatments. Moreover, plants colonized with AM fungi had higher chlorophyll content and reduced levels of low molecular weight thiols, indicating the ability to better tolerate metal-induced stress. Results from this study indicate that vetiver plants in association with AM fungi can be used for improved phytoextraction of Pb from contaminated soil.

Effect of solution chemistry on arsenic sorption by Fe- and Al-based drinking-water treatment residuals

Drinking-water treatment residual (WTR) have been proposed as a low-cost alternative sorbent for arsenic (As) - contaminated aquatic and soil systems. However, limited information exists regarding the effect of solution chemistry on As sorption by WTR. A batch incubation study was carried out to investigate the effect of solution pH (3-9) on As(V) sorption by Al- and Fe-based WTR as a function of solid: solution ratio (SSR) and initial As concentration. The effect of competing ligands (phosphate-P(V) and sulfate), and complexing metal (calcium) on As(V) sorption envelopes at the optimum SSR (200gL(-1)) was also evaluated. At 200gL(-1) SSR, maximum As(V) sorption ( approximately 100%) exhibited by the Fe-WTR was limited at the pH range of 3-7, whereas, the Al-WTR demonstrated approximately 100% As(V) sorption in the entire pH range. The negative pH effect on As(V) sorption became more pronounced with increasing initial As concentrations and decreasing SSR. Sorption of As(V) by surfaces of both WTR decreased in the presence of P(V), exhibiting strong pH dependence. Only for the Fe-WTR, increased dissolved iron concentrations at pH>7 supported a Fe-hydroxide reductive dissolution mechanism to account for the enhanced As sorption at alkaline pH. Addition of sulfate did not influence As(V) sorption by both WTR. A cooperative effect of calcium on As(V) sorption was observed at alkaline pH due to the formation of a calcium-arsenate phase. The constant capacitance model provided reasonable fits to the sorption envelope data for both single ion and binary ion (As and P) systems, but it was unable to explain the enhanced As sorption by the Fe-WTR at pH>7.

Analysis of phytochelatin complexes in the lead tolerant vetiver grass [Vetiveria zizanioides (L.)] using liquid chromatography and mass spectrometry.

Ethylenediamene tetraacetic acid (EDTA) has been used to mobilize soil lead (Pb) and enhance plant uptake for phytoremediation. Chelant bound Pb is considered less toxic compared to free Pb ions and hence might induce less stress on plants. Characterization of possible Pb complexes with phytochelatins (PCn, metal-binding peptides) and EDTA in plant tissues will enhance our understanding of Pb tolerance mechanisms. In a previous study, we showed that vetiver grass (Vetiveria zizanioides L.) can accumulate up to 19,800 and 3350 mg Pb kg(-1) dry weight in root and shoot tissues, respectively; in a hydroponics set-up. Following the basic incubation study, a greenhouse experiment was conducted to elucidate the efficiency of vetiver grass (with or without EDTA) in remediating Pb-contaminated soils from actual residential sites where Pb-based paints were used. The levels of total thiols, PCn, and catalase (an antioxidant enzyme) were measured in vetiver root and shoot following chelant-assisted phytostabilization. In the presence of 15 mM kg (-1) EDTA, vetiver accumulated 4460 and 480 mg Pb kg(-1) dry root and shoot tissue, respectively; that are 15- and 24-fold higher compared to those in untreated controls. Despite higher Pb concentrations in the plant tissues, the amount of total thiols and catalase activity in EDTA treated vetiver tissues was comparable to chelant unamended controls, indicating lowered Pb toxicity by chelation with EDTA. The identification of glutathione (referred as PC1) (m/z 308.2), along with chelated complexes like Pb-EDTA (m/z 498.8) and PC(1)-Pb-EDTA (m/z 805.3) in vetiver root tissue using electrospray tandem mass spectrometry (ES-MS) highlights the possible role of such species towards Pb tolerance in vetiver grass.

Mechanisms of ciprofloxacin removal by nano-sized magnetite.

An understanding of the interaction mechanisms of antibiotics with environmentally relevant sorbents is important to determine the environmental fate of antibiotics and to develop wastewater treatment strategies. Magnetite (Fe(3)O(4)(s)) is ubiquitous in the environment and occurs as a secondary corrosion product of iron nanoparticles that are commonly used as a remediation material. In this study, we aimed to assess the sorption mechanisms of ciprofloxacin (CIP), an important class of fluoroquinolone antibiotics, with magnetite nanoparticles using a combination of wet chemical and in situ ATR-FTIR spectroscopic measurements. Ciprofloxacin sorption was characterized as a function of pH (3.4-8.0), CIP concentration (1-500 μM), ionic strength (0.5, 0.1, and 0.01 M NaCl), and competing anion such as phosphate (0.1mM) to cover a broad range of environmentally relevant geochemical conditions. Results indicated a bell-shaped sorption envelop where sorption of CIP on nano-Fe(3)O(4)(s) increased from 45% to 80% at pH 3.44-5.97; beyond that sorption gradually decreased to a value of 25% at pH 8.39. Phosphate had negligible effect on CIP sorption. In situ ATR-FTIR results indicated inner-sphere coordination of CIP at the magnetite surface mediated by carboxylic acid groups. Results suggest that nano-Fe(3)O(4)(s) has the potential to remove CIP from wastewater effectively.

Antimony sorption at gibbsite-water interface.

Antimony (Sb) is extensively used in flame retardants, lead-acid batteries, solder, cable coverings, ammunition, fireworks, ceramic and porcelain glazes and semiconductors. However, the geochemical fate of antimony (Sb) remained largely unexplored. Among the different Sb species, Sb (V) is the dominant form in the soil environment in a very wide redox range. Although earlier studies have examined the fate of Sb in the presence of iron oxides such as goethite and hematite, few studies till date reported the interaction of Sb (V) with gibbsite, a common soil Al-oxide mineral. The objective of this study was to understand the sorption behavior of Sb (V) on gibbsite as a function of various solution properties such as pH, ionic strength (I), and initial Sb concentrations, and to interpret the sorption-edge data using a surface complexation model. A batch sorption study with 20 g L(-1) gibbsite was conducted using initial Sb concentrations range of 2.03-16.43 μM, pH values between 2 and 10, and ionic strengths (I) between 0.001 and 0.1M. The results suggest that Sb (V) sorbs strongly to the gibbsite surface, possibly via inner-sphere type mechanism with the formation of a binuclear monodentate surface complex. Weak I effect was noticed in sorption-edge data or in the isotherm data at a low surface coverage. Sorption of Sb (V) on gibbsite was highest in the pH range of 2-4, and negligible at pH 10. Our results suggest that gibbsite will likely play an important role in immobilizing Sb (V) in the soil environment.

Synthesis of phytochelatins in vetiver grass upon lead exposure in the presence of phosphorus

In a hydroponic setting, we investigated the possible role of phytochelatins (metal-binding peptides) in the lead (Pb) tolerance of vetiver grass (Vetiveria zizanioides L.). Pb was added to the nutrient medium at concentrations ranging from 0 to 1,200xa0mgxa0L−1. Furthermore, we simulated the effect of soil phosphorus (P) on potentially plant available Pb by culturing vetiver grass in P-rich nutrient media. After 7xa0days of exposure to Pb, we evaluated the Pb uptake by vetiver grass. Results indicate that vetiver can accumulate Pb up to 3,000xa0mgxa0kg−1 dry weight in roots with no toxicity. Formation of lead phosphate inhibited Pb uptake by vetiver, suggesting the need for an environmentally safe chelating agent in conjunction with phytoremediation to clean up soils contaminated with lead-based paint. Unambiguous characterization of phytochelatins (PCn) was possible using high pressure liquid chromatography coupled with electrospray ionization mass spectrometry (LC-ESMS). Vetiver shows qualitative and quantitative differences in PCn synthesis between root and shoot. In root tissue from vetiver exposed to 1,200xa0mg Pb L-1, phytochelatins ranged from PC1 to PC3. Collision-induced dissociation of the parent ion allowed confirmation of each PCn based on the amino acid sequence. Possible Pb-PC1 and Pb2-PC1 complexes were reported in vetiver root at the highest Pb concentration. The data from these experiments show that the most probable mechanism for Pb detoxification in vetiver is by synthesizing PCn and forming Pb–PCn complexes.

Induction of lead-binding phytochelatins in vetiver grass [Vetiveria zizanioides (L.)].

Elevated lead (Pb) concentrations in residential houseyards around house walls painted with Pb-based pigments pose serious human health risks, especially to children. Vetiver grass (Vetiveria zizanioides L.) has shown promise for use in in situ Pb phytoremediation efforts. However, little is known about the biochemical mechanisms responsible for the observed high Pb tolerance by vetiver. We hypothesized that vetiver exposure to Pb induced the synthesis of phytochelatins (PC(n)) and the formation of Pb-PC(n) complexes, alleviating the phytotoxic effects of free Pb ions. Our main objective was to identify PC(n) and Pb-PC(n) complexes in root and shoot compartments of vetiver grass using high-performance liquid chromatography coupled to electrospray mass spectrometry (HPLC-ES-MS). After 7 d of exposure to Pb, vetiver accumulated up to 3000 mg Pb kg(-1) in shoot tissues, but much higher Pb concentrations were measured in root ( approximately 20,000 mg kg(-1)), without phytotoxic symptoms. Scanning electron micrographs showed Pb deposition in the vascular tissues of root and shoot, suggesting Pb translocation to shoot. Collision-induced dissociation analyses in MS/ MS mode during HPLC-ES-MS analysis allowed for the confirmation of four unique PC(n) (n = 1-4) based on their respective amino acid sequence. The high tolerance of vetiver grass to Pb was attributed to the formation of PC(n) and Pb-PC(n) complexes within the plant tissues, using ES-MS and Pb mass isotopic patterns. These data illustrate the mechanism of high Pb tolerance by vetiver grass, suggesting its potential usefulness for the remediation of Pb-contaminated residential sites.

Vetiver grass is capable of removing TNT from soil in the presence of urea

The high affinity of vetiver grass for 2,4,6 trinitrotoluene (TNT) and the catalytic effectiveness of urea in enhancing plant uptake of TNT in hydroponic media we earlier demonstrated were further illustrated in this soil-pot-experiment. Complete removal of TNT in urea-treated soil was accomplished by vetiver at the low initial soil-TNT concentration (40 mg kg(-1)), masking the effect of urea. Doubling the initial TNT concentration (80 mg kg(-1)) significantly (p<0.002) increased TNT removal by vetiver, in the presence of urea. Without vetiver grass, no significant (p=0.475) change in the soil-TNT concentrations was observed over a period of 48 days, suggesting that natural attenuation of soil TNT could not explain the documented TNT disappearance from soil.

Bioavailability and bioaccessibility of arsenic in a soil amended with drinking-water treatment residuals.

Earlier incubation and greenhouse studies in our laboratory confirmed the effectiveness of drinking-water treatment residual (WTR) in decreasing soil arsenic (As) bioaccessibility as determined with in vitro tests, which led us to hypothesize a similar outcome if animal studies were to be conducted. Our objective was to evaluate the potential of WTR in lowering soil As bioavailability by conducting in vivo experiments and compare the in vitro to the in vivo As data. This study was performed using 6-week-old male BALB/c mice that were fed with an As-contaminated soil slurry using the gavage method. Blood and stomach contents were collected at 1 and 24xa0h after feeding. Urine and excreta were collected at time 0 (before feeding) and 24xa0h after feeding. Relative As bioavailability (RBA) values calculated from the blood samples of mice fed with WTR and WTR-amended soil samples ranged from 13% to 24% and from 25% to 29%, respectively; both were significantly (pxa0<xa00.001) lower than that of the unamended (no-WTR) soil (~100% RBA). Absolute As bioavailability (ABA) in the gastric phase was significantly (pxa0<xa00.001) lowered, to 7–16%, in the WTR-amended soil compared with that of the unamended control (26%). A significant (pxa0<xa00.001) linear correlation (rxa0=xa00.94) was observed between the in vitro (stomach-phase) and the in vivo RBA data. Percentage recovery of As obtained from four mice tissue compartments (i.e., blood, stomach, urine, and fecal matter) after oral and intramuscular administrations was 63–80%. Results illustrate the effectiveness of in situ WTR amendment in decreasing in vivo soil As bioavailability, thereby lowering the potential cancer risk via an oral ingestion pathway.

X-ray absorption spectroscopy as a tool investigating arsenic(III) and arsenic(V) sorption by an aluminum-based drinking-water treatment residual

Historic applications of arsenical pesticides to agricultural land have resulted in accumulation of residual arsenic (As) in such soils. In situ immobilization represents a cost-effective and least ecological disrupting treatment technology for soil As. Earlier work in our laboratory showed that drinking-water treatment residuals (WTRs), a low-cost, waste by-product of the drinking-water treatment process exhibit a high affinity for As. Wet chemical experiments (sorption kinetics and desorption) were coupled with X-ray absorption spectroscopy measurements to elucidate the bonding strength and type of As(V) and As(III) sorption by an aluminum-based WTR. A fast (1h), followed by a slower sorption stage resulted in As(V) and As(III) sorption capacities of 96% and 77%, respectively. Arsenic desorption with a 5mM oxalate from the WTR was minimal, being always <4%. X-ray absorption spectroscopy data showed inner-sphere complexation between As and surface hydroxyls. Reaction time (up to 48h) had no effect on the initial As oxidation state for sorbed As(V) and As(III). A combination of inner-sphere bonding types occurred between As and Al on the WTR surface because mixed surface geometries and interatomic distances were observed.