XANES study of Cr sorbed by a kitchen waste compost from water

A kitchen waste compost was used to sorb Cr for various times from water containing either Cr(NO3)3 or CrO3 in different concentrations. Scanning electron microscopy (SEM) results show that the composts have been partially oxidized by Cr(VI) during the sorption experiments. X-ray absorption near edge structure (XANES) simulation suggests that about 54.1-61.0% Cr sorbed on the compost is in form of organic Cr(III) through ionic exchange process with the rest being existent as Cr(NO3)3 in the Cr(III) sorption case; no Cr(OH)3 is observed or expected because the solution pH after sorption experiments is or= 5.94. Moreover, organic Cr(III) represents about 51.7-69.0% of the total sorbed Cr, and the rest (6.1-28.5%) is Cr(VI).     

Chromium species behaviour in the activated sludge process

The purpose of this research was to compare trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) removal by activated sludge and to investigate whether Cr(VI) reduction and/or Cr(III) oxidation occurs in a wastewater treatment system. Chromium removal by sludge harvested from sequencing batch reactors, determined by a series of batch experiments, generally followed a Freundlich isotherm model. Almost 90% of Cr(III) was adsorbed on the suspended solids while the rest was precipitated at pH 7.0. On the contrary, removal of Cr(VI) was minor and did not exceed 15% in all experiments under the same conditions. Increase of sludge age reduces Cr(III) removal, possibly because of Cr(III) sorption on slime polymers. Moreover, the decrease of suspended solids concentration and the acclimatization of biomass to Cr(VI) reduced the removal efficiency of Cr(III). Batch experiments showed that Cr(III) cannot be oxidized to Cr(VI) by activated sludge. On the contrary, Cr(VI) reduction is possible and is affected mainly by the initial concentration of organic substrate, which acts as electron donor for Cr(VI) reduction. Initial organic substrate concentration equal to or higher than 1000 mgl(-1) chemical oxygen demand permitted the nearly complete reduction of 5 mgl(-1) Cr(VI) in a 24-h batch experiment. Moreover, higher Cr(VI) reduction rates were obtained with higher Cr(VI) initial concentrations, expressed in mg Cr(VI) g(-1) VSS, while decrease of suspended solids concentration enhanced the specific Cr(VI) reduction rate.     

Species-dependent chromium accumulation, lipid peroxidation, and glutathione levels in germinating kiwifruit pollen under Cr(III) and Cr(VI) stress

The accumulation of chromium by germinating kiwifruit pollen appears to be significantly affected by Cr species, Cr concentration and calcium availability. Cr(III) accumulation always occurred in a linear manner while Cr(VI) uptake followed a logarithmic model. In the absence of exogenous calcium, Cr(III) accumulation was much higher than that of Cr(VI). It was observed that, as the Cr(III) concentration increased, there was a significant decrease in the endogenous calcium content of pollen, ultimately leading to complete calcium depletion after 90 min of incubation at 150 microM Cr(III). This loss of calcium could be responsible for the strong inhibition of tube emergence and growth following exposure of pollen to Cr(III). Indeed, when exogenous calcium was added to the kiwifruit pollen culture medium, significant growth recovery and reduced Cr(III) uptake occurred; the opposite was true in Cr(VI)-treatments. A significant rise in lipid peroxide production occurs in the presence of both Cr species; the effect was more pronounced following Cr(VI) exposure. Finally, glutathione pool dynamics appears to be differentially affected by chromium species and concentrations. In conclusion, results of the present study have provided important information regarding the different activity profiles of Cr(III) and Cr(VI) in relation to kiwifruit pollen performance, and have also demonstrated differences in some biochemical responses of pollen to metal stress.     

Chromium speciation in river sediment pore water contaminated by tannery effluent

Cr(VI) is far more soluble and toxic than Cr(III). Sediment pore water was investigated in a river adjacent to the property of a large former tannery, into which Cr-contaminated effluent was discharged over a 55-year period, and where extremely high Cr concentrations have been found in the sediments. Dialysis cells, or peepers, were used to generate depth profiles of Cr concentration in sediment pore water. Samples were analyzed for total Cr using inductively coupled plasma-mass spectrometry (ICP-MS) and for Cr species using high performance liquid chromatography (HPLC)-ICP-MS. The results show an absence of Cr(VI) in all samples. Furthermore, incomplete recovery of Cr(VI) added to the samples collected at the locations with highest sediment Cr concentrations indicate strong reducing conditions at those locations, which are not conducive to the presence of Cr(VI).     

Studies on hexavalent chromium biosorption by chemically-treated biomass of Ecklonia sp

The biomass of the brown seaweed, Ecklonia sp., is capable of reducing Cr(VI) to Cr(III). However, very little is known about the mechanism of Cr(VI) reduction by the biomass. The aims of the present investigation were to enhance the Cr(VI)-reducing capacity of the biomass using various chemical treatments and to elucidate the mechanisms governing Cr(VI) reduction. Among the various chemical treatments, acid-treatment showed the best performance with regards the improvement of Cr(VI) removal from the aqueous phase, while organic solvent-treatment significantly improved the removal efficiency of total Cr in the equilibrium state. Based on FTIR study, the biomass was subjected to chemical modification of its amino and carboxyl groups, to examine their roles in the Cr(VI) removal from the aqueous phase. Methylation of the amino group significantly decreased the Cr(VI) removal rate, but amination of the carboxyl group significantly increased the Cr(VI) removal rate. Meanwhile, esterification of the carboxyl group and carboxylation of the amino group decreased the Cr(VI) removal rate, but the former showed a more negative effect than the latter. These findings indicated that the amino and carboxyl groups take part in the Cr(VI) removal from the aqueous phase. In conclusion, mechanisms for direct and indirect Cr(VI) removal are proposed, and some aspects for the application of this biomass to Cr(VI) detoxification are discussed.     

Mobility and recalcitrance of organo-chromium(III) complexes

Hexavalent chromium [Cr(VI)] is a major industrial pollutant. Bioremediation of Cr(VI) to Cr(III) is a viable clean-up approach. However, Cr(VI) bioreduction also produces soluble organo–Cr(III) complexes, and little is known about their behavior in the environment. When tested with soil columns, citrate–Cr(III) showed little sorption to soil; malate–Cr(III) had limited partitioning with soil; and histidine–Cr(III) exhibited significant interaction with soil. It appears that the mobility varies depending on the organic ligand. Further, Ralstonia eutropha JMP 134 and Pseudomonas aeruginosa pAO1 readily degraded malate, citrate, and histidine, but not the corresponding organo–Cr(III) complexes. The recalcitrance is not due to toxicity, but the complexes are likely to cause hindrance to enzymes, as malate dehydrogenase and amino acid oxidase could not use malate–Cr(III) and histidine–Cr(III), respectively. The data are in agreement with the reports of soluble organo–Cr(III) complexes in the environment.     

Antioxidant enzyme activities as affected by trivalent and hexavalent chromium species in Fontinalis antipyretica Hedw

The detoxification mechanisms of the aquatic moss, Fontinalis antipyretica Hedw., exposed to Cr was analyzed. In addition, the influence of Cr salts (as Cr nitrate, chloride and potassium bichromate) on these mechanisms has also been studied. The activity of antioxidant enzymes superoxide dismutase (SOD, EC 1.15.1.1.), catalase (EC 1.11.1.6.), ascorbate peroxidase (APX, EC 1.11.1.11.), guaiacol peroxidase (GPX, EC 1.11.1.7.) and glutathione reductase (GR, EC 1.6.4.2.) increased in plants treated with Cr concentrations ranging from 6.25x10(-5) to 6.25mM when given as Cr(NO(3))(3). Antioxidant enzymes responded to the other two salts CrCl(3) and K(2)Cr(2)O(7) only with Cr concentrations higher than 6.25x10(-2)mM. Glutathione level and GSSG/GSH ratio also responded to Cr exposure but no dose-effect relationship could be observed. Moreover, two unknown thiol compounds were observed in mosses exposed to the highest Cr concentrations. Effects on chlorophyll contents and chlorophyll a/b ratios were also shown even at low Cr concentrations. Our results indicated that environmentally realistic concentrations of Cr could lead to impairment of the cellular activity towards F. antipyretica and that Cr(III), when present as a nitrate salt, was as harmful as Cr(VI).     

Chromium transport, oxidation, and adsorption in manganese-coated sand

We examine how the processes of advection, dispersion, oxidation-reduction, and adsorption combine to affect the transport of chromium through columns packed with pyrolusite (beta-MnO2)-coated sand. We find that beta-MnO2 effectively oxidizes Cr(III) to Cr(VI) and that the extent of oxidation is sensitive to changes in pH, pore water velocity, and influent concentrations of Cr(III). Cr(III) oxidation rates, although initially high, decline well before the supply of beta-MnO2 is depleted, suggesting that a reaction product inhibits the conversion of Cr(III) to Cr(VI). Rate-limited reactions govern the weak adsorption of each chromium species, with Cr(III) adsorption varying directly with pH and Cr(VI) adsorption varying inversely with pH. The breakthrough data on chromium transport can be matched closely by calculations of a simple model that accounts for (1) advective-dispersive transport of Cr(III), Cr(VI), and dissolved oxygen, (2) first-order kinetics adsorption of the reduced and oxidized chromium species, and (3) nonlinear rate-limited oxidation of Cr(III) to Cr(VI). Our work supplements the limited database on the transport of redox-sensitive metals in porous media and provides a means for quantifying the coupled processes that contribute to this transport.     

Remediation of Cr(VI) from chromium slag by biocementation

Here we demonstrate a calcifying ureolytic bacterium Bacillus sp. CS8 for the bioremediation of chromate (Cr(VI)) from chromium slag based on microbially induced calcite precipitation (MICP). A consolidated structure like bricks was prepared from chromium slags using bacterial cells, and five stage Cr(VI) sequential extraction was carried out to know their distribution pattern. Cr(VI) mobility was found to significantly be decreased in the exchangeable fraction of Cr slag and subsequently, the Cr(VI) concentration was markedly increased in carbonated fraction after bioremediation. It was found that such Cr slag bricks developed high compressive strength with low permeability. Further, leaching behavior of Cr(VI) in the Cr slag was studied by column tests and remarkable decrease in Cr(VI) concentration was noticed after bioremediation. Cr slags from columns were characterized by SEM–EDS confirming MICP process in bioremediation. The incorporation of Cr(VI) into the calcite surface forms a strong complex that leads to obstruction in Cr(VI) release into the environment. As China is facing chromium slag accidents at the regular time intervals, the technology discussed in the present study promises to provide effective and economical treatment of such sites across the country, however, it can be used globally.     

Effect of chromium species on phytochemical and physiological parameters in Datura innoxia

In soil, chromium can be found in two main valence states: hexavalent Cr(VI) and trivalent Cr(III). In this study, we investigated the impact of Cr on photosynthetic gas exchange, photosystem II (PSII) activity, Cr translocation and accumulation, proline content and alkaloids production, i.e. scopolamine and hyoscyamine, in Datura innoxia. Cr uptake was influenced by its oxidation state and its concentration in growth medium. The plant roots were determined as being the main organ of Cr accumulation. Cr(VI) was more toxic than Cr(III) as indicated by reduction in plant biomass and net photosynthesis. The stomatal conductance showed a similar trend to that of photosynthetic capacity. Cr(III) and Cr(VI) had a different impact on substomatal CO(2) concentration then Cr toxicity was related to its oxidation states. In plants stressed with a Cr(VI) excess, a down regulation of PSII activity was observed with an impairment of photochemical activity. Indeed, the maximum quantum yield of PSII (F(v)/F(m)), the quantum yield of PSII (PhiPSII) and the efficiency of excitation capture by open centers (F'(v)/F'(m)) decreased. Cr(III) had little effects on PSII primary photochemistry, whatever its form induces an increase of scopolamine content without changes in hyoscyamine content in leaves of D. innoxia. These results provide that chromium contamination can change the secondary metabolites composition of leaves, thereby, impacting the quality, safety and efficacy of natural plant products synthesized by D. innoxia plants.     

Trivalent chromium solubility and its influence on quantification of hexavalent chromium in ambient particulate matter using EPA method 6800

Measurement of carcinogenic Cr(VI) in ambient PM is challenging due to potential errors associated with conversion between Cr(VI) (a carcinogen) and Cr(III) (an essential nutrient). Cr(III) conversion is a particular concern due to its >80% atomic abundance in total Cr. U.S. Environmental Protection Agency (EPA) method 6800 that uses water-soluble isotope spikes can be used to correct the interconversion. However, whether the enriched Cr(III) isotope spikes can adequately mimic the Cr(III) species originally in ambient PM is unknown. This study examined the water solubility of Cr(III) in ambient PM and discussed its influence on Cr(VI) measurement. Ambient PM₁₀ samples were collected on Teflon filters at four sites in New Jersey that may have different Cr emission sources. The samples were ultrasonically extracted with 5 mL DI-H₂O (pH 5.7) at room temperature for 40 min, and then analyzed by ion chromatography–inductively coupled plasma mass spectrometry (IC-ICPMS). Cr(III) was below detection limit (0.06 ng/m³) for all samples, suggesting water-soluble Cr(III) species, such as CrCl₃, Cr(NO₃)₃, and amorphous Cr(OH)₃, in the ambient PM were negligible. Therefore, the enriched ⁵⁰Cr(III) isotope spike (in the form of Cr(NO₃)₃) could not mimic the original ambient Cr(III). Only the conversion of ⁵³Cr(VI) (in the form of K₂CrO₄) was taken into account when correcting the interconversion. We then used NaHCO₃-pretreated MCE filters (prespiked with enriched isotope species) to measure Cr(VI) in the ambient PM₁₀. The samples were ultrasonically extracted at 60 C pH 9 solutions for 40 min followed by IC-ICPMS analysis. Due to the correction of Cr(VI) reduction, the Cr(VI) concentrations determined by EPA method 6800, 0.26 ± 0.16 (summer) and 0.16 ± 0.11(winter) ng/m³ (n = 64), were significantly greater than those by the external standard curve, 0.21 ± 0.17 (summer) and 0.10 ± 0.07 (winter) ng/m³ (n = 56) (p < 0.01, Student’s t-test). Our study revealed that appropriate application of EPA method 6800 is important because it only applies to soluble fraction of Cr species in ambient PM.

ImplicationsAccurate measurement of carcinogenic Cr(VI) in ambient PM is challenging due to conversion between Cr(VI) (a human carcinogen) and Cr(III) (a human essential nutrient). The conversion of Cr(III) is of particular concern due to its dominant presence in total Cr (>80%). This study examined the water solubility of Cr(III) in ambient PM that was collected at four locations in New Jersey. Then we discussed the influence of Cr(III) solubility on the application of EPA method 6800, which utilizes enriched isotope spikes to correct the interconversion. Our results suggested that appropriate application of EPA method 6800 is important because it only applies to soluble fraction of Cr species.     

Use of waste iron metal for removal of Cr(VI) from water

Cr(VI) removal from water was evaluated using waste iron particles in batch experimental mode. The reaction rates were inversely proportional to the initial Cr(VI) concentrations, and the reaction rates of Cr(VI) removal with the waste iron metal were faster than those with Peerless iron, a commercial zero-valent iron. The loss in iron reactivity due to the oxidation, from Fe(0) to Fe(II), ultimately to Fe(III), could be recovered by adding iron-reducing consortium (IRC) to the oxidized iron. Bacterial reduction of Cr(VI) also helped to decrease the aqueous concentration of Cr(VI), but the reduction of oxidized iron by IRC and the consequent reduction of Cr(VI) to Cr(III) by the reduced iron was more significant. Thus, reusing waste iron metal for Cr(VI) removal can reduce the cost of reactive media. Furthermore, the addition of IRC to the waste iron metal can accelerate the removal rate of Cr(VI), and can recover the reactivity of irons which were oxidized by Cr(VI).     

Biogeochemical influence on transport of chromium in manganese sediments: experimental and modeling approaches

Hexavalent chromium (Cr(VI)) was reduced to immobile and nontoxic Cr(III) by a dissimilatory metal reducing bacteria, Shewanella alga Simidu (BrY-MT) ATCC 55627. A series of kinetic batch and dynamic column experiments were conducted to provide an understanding of Cr(VI) reduction by the facultative anaerobe BrY-MT. Reduction of Cr(VI) was rapid (within 1 h) in columns packed with quartz sand and bacteria, whereas Cr(VI) reduction by BrY-MT was delayed (57 h) in the presence of beta-MnO2-coated sand. A mathematical model was developed and evaluated against data obtained from column experiments. The model takes into account (1) advective-dispersive transport of Cr(III), Cr(VI), lactate, and protein (mobile and immobile bacteria); (2) first-order kinetic adsorption of Cr(III) and lactate; (3) conversion of solid phase beta-MnO2 to solid phase MnOOH due to oxidation of Cr(III); (4) dual-Monod kinetics, where Cr(VI) is the electron acceptor and lactate is the electron donor. The breakthrough data for Cr(III), Cr(VI), lactate, and protein (mobile and immobile bacteria) were fitted simultaneously. The breakthrough data are well described by the mathematical model that considers the above processes. This result demonstrates the ability of the coupled hydrobiogeochemical model to simulate chromium transport in complex reactive systems.     

Investigation of Cr(VI) reduction in continuous-flow activated sludge systems

The aim of this research was to investigate hexavalent chromium, Cr(VI), reduction by activated sludge and to evaluate the use of continuous-flow activated sludge systems for the treatment of Cr(VI)-containing wastewater. Three series of experiments were conducted using two parallel lab-scale activated sludge systems. During the first experiment, one system was used as a control, while the other received Cr(VI) concentrations equal to 0.5, 1, 3 and 5mg l(-1). For all concentrations added, approximately 40% of the added Cr(VI) was removed during the activated sludge process. Determination of chromium species in the dissolved and particulate phase revealed that the removed Cr(VI) was sorbed by the activated sludge flocs mainly as trivalent chromium, Cr(III), while the residual chromium in the dissolved phase was mainly detected as Cr(VI). Activated sludge ability to reduce Cr(VI) was independent of the acclimatization of biomass to Cr(VI) and it was not affected by the toxic effect of Cr(VI) on autotrophic and heterotrophic microorganisms. During the second experiment, both systems were operated under two different hydraulic residence time (theta equal to 20 and 28h) and three different initial organic substrate concentration (COD equal to 300, 150 and 0mg l(-1)). Cr(VI) reduction was favored by an increase of theta, while it was limited by influent COD concentration. Finally, at the last experiment the effect of anoxic and anaerobic reactors on Cr(VI) reduction was investigated. It was observed that the use of an anoxic zone or an anaerobic-anoxic zone ahead of the aerobic reactor favored Cr(VI) reduction, increasing mean percentage Cr(VI) reduction to almost 80%.     

Kinetics of the reduction of hexavalent chromium with the brown seaweed Ecklonia biomass

The dead biomass of the brown seaweed, Ecklonia sp., is capable of reducing toxic Cr(VI) into less toxic or nontoxic Cr(III). However, little is known about the mechanism of Cr(VI) reduction by the biomass. The objective of this work was to develop a kinetic model for Cr(VI) biosorption, for supporting our mechanism. The reduction rate of Cr(VI) increased with increasing total chromate concentration, [Cr(VI)], and equivalent concentration of organic compounds, [OCs], and decreasing solution pH. It was found that the reduction rate of Cr(VI) was proportional to [Cr(VI)] and [OCs], suggesting the simple kinetic equation -d[Cr(VI)]/dt=k[Cr(VI)][OCs]. When considering the consumption of organic compounds due to the oxidation by Cr(VI), an average rate coefficient of 9.33 (+/-0.65)microM(-1)h(-1) was determined, at pH 2. Although the function of the pH could not be expressed in a mechanistic manner, an empirical model able to describe the pH dependence was obtained. It is expected that the developed rate equation could likely be used for design and performance predictions of biosorption processes for treating chromate wastewaters.     

Reduction of chromate (CrO4(2-)) by an enrichment consortium and an isolate of marine sulfate-reducing bacteria

An enrichment consortium and an isolate (isolate TKW) of sulfate-reducing bacteria (SRB) have been obtained from metal-contaminated marine sediments of Tokwawan, Hong Kong SAR. These bacteria are capable of reducing highly toxic and soluble hexavalent chromium (Cr6+) enzymatically into less toxic and insoluble trivalent chromium (Cr3+) under anaerobic conditions. The enrichment consortium almost completely (98.5%) reduced 0.6 mM Cr6+ in 168 h and the rate of reduction was 0.5 g (Cr6+) g(protein)(-1)h(-1). In comparison, with Cr6+ as the sole electron acceptor (as a surrogate for SO4(2-)), isolate TKW reduced 94.5% of the initially added Cr6+ (0.36 mM) in 288 h, with the rate of 0.26 g (Cr6+) g(protein)(-1)h(-1). Adsorption by these bacteria was not the major mechanism contributing to the transformation or removal of Cr6+. The biomass and Cr3+ in the cultures increased simultaneously with the reduction of Cr6+. These indigenous SRB might have potential application in bioremediation of metal contaminated sediments.     

Comparative study of chromium biosorption by red, green and brown seaweed biomass

Dried biomass of the macroalgae Fucus vesiculosus and Fucus spiralis (brown), Ulva spp. (comprising Ulva linza, Ulva compressa and Ulva intestinalis) and Ulva lactuca (green), Palmaria palmata and Polysiphonia lanosa (red) were studied in terms of their chromium biosorption performance. Metal sorption was highly pH dependent with maximum Cr(III) and Cr(VI) sorption occurring at pH 4.5 and pH 2, respectively. Extended equilibrium times were required for Cr(VI) binding over Cr(III) binding (180 and 120min, respectively) thus indicating possible disparities in binding mechanism between chromium oxidation states. The red seaweed P. palmata revealed the highest removal efficiency for both Cr(III) and Cr(VI) at low initial concentrations. However, at high initial metal concentrations F. vesiculosus had the greatest removal efficiency for Cr(III) and performed almost identically to P. lanosa in terms of Cr(VI) removal. The Langmuir Isotherm mathematically described chromium binding to the seaweeds where F. vesiculosus had the largest q(max) for Cr(III) sorption (1.21mmol g(-1)) and P. lanosa had the largest Cr(VI) uptake (0.88mmol g(-1)). P. palmata had the highest affinity for both Cr(III) and Cr(VI) binding with b values of 4.94mM(-1) and 8.64mM(-1), respectively. Fourier transform infrared analysis revealed interactions of amino, carboxyl, sulphonate and hydroxyl groups in chromium binding to Ulva spp. The remaining seaweeds showed involvement of these groups to varying degrees as well as ether group participation in the brown seaweeds and for Cr(VI) binding to the red seaweeds.     

Removal of Cr(VI) and humic acid by using TiO2 photocatalysis

The removal efficiencies of Cr(VI) and HA, using a TiO(2)-mediated photocatalytic process, were investigated with variations in the pH, TiO(2) dosage and Cr(VI)/HA ratio. During the photocatalytic reaction, the total removal of Cr(VI) occurred through adsorption onto TiO(2), as well as its reduction to Cr(III). However, oxidation and adsorption were identified as important removal processes for the treatment of HA. Due to the anionic type adsorption onto TiO(2) and its acid-catalyzed photocatalytic reduction, the removal of Cr(VI) decreased with increasing pH, while that of HA increased with increasing pH. The TiO(2) dosage was also an important parameter for the removal of Cr(VI). As the TiO(2) dosage was increased to 2.5 g l(-1), the removal of Cr(VI) was continuously enhanced, but decreased at dosages above 3 g l(-1) due to the increased blockage of the incident UV light used for the photocatalytic reaction. The removal of Cr(VI) was greatly enhanced when the system contained both HA and Cr(VI) compared to Cr(VI) alone. Also, the removal of HA was greatly enhanced when the system contained both HA and Cr(VI) compared to HA alone. The removal of Cr(VI) was continuously enhanced as the HA concentration gradually increased; however, no further increase was observed above 20 mg l(-1) HA due to the increased absorption of the UV light. This result supports that the photocatalytic reaction, with illuminated TiO(2), could be applied to more effectively treat wastewater containing both Cr(VI) and HA than that containing a single species only.     

Transportation and distribution of chromium in the anaerobic sludge treating the chromium-containing wastewater

The chromium distribution and transportation in the anaerobic activated sludge was investigated using a sequential extraction method. The results showed that Cr(VI) in aqueous solution was reduced by the metabolic product of SRB and form indissoluble Cr(OH)₃ in a solid phase. More than 99% of Cr(III) in the sludge was in a stable residual form (RES) of bio-reduction. The Cr(VI) was mainly present in RES and organic form (OR). With increasing Cr(VI) concentration, Cr(VI) was transferred from RES into OR and even exchangeable form (EXCH). Meanwhile, sulphate and co-existing metal ions affected the occurrence form of Cr(VI).     

Chromium accumulation by the hyperaccumulator plant Leersia hexandra Swartz

Leersia hexandra Swartz (Gramineae), which occurs in Southern China, has been found to be a new chromium hyperaccumulator by means of field survey and pot-culture experiment. The field survey showed that this species had an extraordinary accumulation capacity for chromium. The maximum Cr concentration in the dry leaf matter was 2978 mg kg(-1) on the side of a pond near an electroplating factory. The average concentration of chromium in the leaves was 18.86 times as that in the pond sediment, and 297.41 times as that in the pond water. Under conditions of the nutrient solution culture, it was found that L. hexandra had a high tolerance and accumulation capacity to Cr(III) and Cr(VI). Under 60 mg l(-1) Cr(III) and 10 mg l(-1) Cr(VI) treatment, there was no significant decrease of biomass in the leaves of L. hexandra (p>0.05). The highest bioaccumulation coefficients of the leaves for Cr(III) and Cr(VI) were 486.8 and 72.1, respectively. However, L. hexandra had a higher accumulation capacity for Cr(III) than for Cr(VI). At the Cr(III) concentration of 10 mg l(-1) in the culture solution, the concentration of chromium in leaves was 4868 mg kg(-1), while at the same Cr(VI) concentration, the concentration of chromium in leaves was only 597 mg kg(-1). These results confirmed that L. hexandra is a chromium hyperaccumulator which grows rapidly with a great tolerance to Cr and broad ecological amplitude. This species could provide a new plant resource that explores the mechanism of Cr hyperaccumulation, and has potential for usage in the phytoremediation of Cr-contaminated soil and water.