Assessment of the phytotoxicity of chromium in soils using the selective ion exchange resin extraction method

Chromium in soils is present in the form of Cr(VI) oxyanions or Cr(III) cations. The toxicity and mobility of Cr(VI) are higher than those of Cr(III), thus it is essential that the availability of Cr(VI) in soils be accurately estimated in order to assess the phytotoxicity of Cr and its resultant health hazards to animals and humans. In this study, the Cu-saturated selective ion exchange resin (DOWEX M4195) was used as an infinite sink to test the feasibility of using the resin for extracting available Cr(VI) from soil. In the experiments, the results show that the resin had a high affinity for Cr(VI) and that Cr(VI) adsorbed by resins could be desorbed by using 10% NaCl (pH 4). In addition, the adsorption and desorption of Cr(VI) were not affected by pH levels, the forms of Cr(VI) or the presence of major anions in the soil solution. The above results indicate that the Cu-saturated resin can selectively adsorb Cr(VI) from solution. In the soil extraction experiments, three Cr(VI)-spiked soils were processed using the Cu-saturated resin extraction method. The results show that amounts of soil Cr extractable by the resin had a significant negative correlation to the height of wheat seedlings grown in the Neubauer test. Comparing this to the commonly used extractant, 0.1 M HCl, the amount of soil Cr, extractable by the resin, had a higher correlation to plant height. The results suggest that the selective ion exchange resin method developed in this study is useful in evaluating the quantities of plant-available Cr(VI) in soil and can, therefore, assess the phytotoxicity of Cr in soil.     

Using the Selective Ion Exchange Resin Extraction and XANES Methods to Evaluate the Effect of Compost Amendments on Soil Chromium(VI) Phytotoxicity

The Cu-saturated selective ion exchange resin (DOWEX M4195) extraction method was used to investigate the effects of two amendments, 5 and 15% organic matter in the form of hog-dung compost (HC) or cattle-dung compost (CC), on Cr(VI) bioavailability in three soils spiked with various levels of Cr(VI). The results showed that addition of composts could decrease the amounts of resin-extractable Cr(VI) in Cr(VI)-spiked soils, and the CC amendment decreased resin-extractable Cr(VI) more than the HC amendment. The X-ray Absorption Near-edge Structure spectroscopy (XANES) method was used to examine the distribution of Cr(III) and Cr(VI) species in Cr(VI)-spiked soils that were affected by compost amendments, and to elucidate the mechanisms for the decrease of resin-extractable Cr(VI) due to the application of composts. The XANES results suggested that the decrease in the amounts of resin-extractable Cr(VI) after compost addition was mainly due to the reduction of Cr(VI) to Cr(III). The amounts of soil resin-extractable Cr(VI) were also correlated with wheat seedling growth in order to evaluate the effect of compost amendments on decreasing the phytotoxicity of soil Cr(VI). The results showed that there was a sigmoidal relationship between soil resin-extractable Cr(VI) and the plant height of wheat seedlings and the obtained effective concentrations of resin-extractable Cr(VI) resulting in 10 and 50% growth inhibition (EC₁₀ and EC₅₀) were 76 and 191 mg kg⁻¹ respectively. The above results suggested that the resin extraction method was a useful tool for assessing Cr(VI) phytotoxicity and that addition of composts would enhance Cr(VI) reduction to Cr(III) in soils and thus relieve Cr(VI) phytotoxicity.     

Isolation of hexavalent chromium-reducing anaerobes from hexavalent-chromium-contaminated and noncontaminated environments

Hexavalent chromium [Cr(VI)], is a toxic, water-soluble contaminant present in many soils and industrial effluents. Bacteria from various soils were examined for Cr(VI) resistance and reducing potential. Microbes selected from both Cr(VI)-contaminated and-noncontaminated soils and sediments were capable of catalyzing the reduction of Cr(VI) to Cr(III) a less toxic, less water-soluble form of Cr, demonstrating the utility of using a selection strategy for indigenous Cr(VI)-reducing bacteria in a bioprocess. As a result, indigenous Cr(VI)- reducing microbes from contaminated sites should provide the means for developing a bioprocess to reduce Cr(VI) to Cr(III) in nonsterile effluents such as those from soil washes. This approach also avoids the contamination problems associated with pure cultures of allochthonous microorganisms. In addition the apparent ubiquity of Cr(VI)-reducing bacteria in soil and sediments indicates potential for in situ bioremediation of Cr(VI)-contaminated soils and ground water.     

Spatial and temporal heterogeneity in soil nutrient supply measured usingin situ ion-exchange resin bags

Summary Data are presented which illustrate the range of ion values obtained from soil solutions eluted fromin situ ion exchange resin bags in grazed and ungrazed grassland soils sampled in the summer and early autumn. Overall, higher levels of cations were being supplied in both the grazed and ungrazed plots in the autumn compared with during the summer. Variation in ion levels reflected spatial heterogeneity in ion supply in these soils. This variation was correlated with the distribution and abundance of the dominant plants and soil surface microtopography. The use ofin situ ion exchange resin bags allow an understanding of short-term temporal and spatial heterogeneity in ion supply.     

Enhanced Immobilization of Cr(VI) in Soils by the Amendment of Rice Straw Char

This study investigated the effect of rice straw char (RSC) on the immobilization of Cr(VI) in soils. The Cr(VI) sorption experiments on the RSC and RSC-amended soils were conducted using the batch method. RSC exhibited Cr(VI) reduction capacity due to its black carbon content. The addition of RSC to the soils enhanced the overall Cr(VI) immobilization of the soils, which is primarily attributed to the Cr(VI) reduction capacity of RSC. The effects of RSC amendment on the Cr(VI) sorption of the soils increased with increasing RSC content in the soils and decreased with increasing pH or anion contents in the soil solutions. After Cr(VI) was sorbed by the soils, a portion of the Cr(VI) was converted to Cr(III) and the remainder was sorbed onto the soils. The presence of RSC in the soils decreased the portion of sorbed Cr(VI) in the soils and therefore lowered the potential remobilization of Cr(VI) from the soils. The results suggested that RSC amendment can be applied to develop a cost-effective method for immobilizing Cr(VI) in polluted soils, thus lowering the environmental risk from Cr(VI) toxicity.     

Effects of Contaminant Concentration,Aging, and Soil Properties on the Bioaccessibility of Cr(III) and Cr(VI) in Soil

Contaminated soils at numerous U.S. Department of Defense, Department of Energy, and other industrial facilities often contain huge inventories of toxic metals such as chromium. Ingestion of soil by children is often the primary risk factor that drives the need for remediation. Site assessments are typically based solely on total soil-metal concentrations and do not consider the potential for decreased bioaccessibility due to metal sequestration by soil. The objectives of this research are to investigate the effect of soil properties on the bioaccessibility of Cr(III) and Cr(VI) as a function of contaminant concentration and aging. The A and upper B horizons of two well-characterized soils, representative of Cr-contaminated soils in the southeastern United States, were treated with varying concentration of Cr(III) and Cr(VI) and allowed to age. The bioaccessibility of the contaminated soils was measured over a 200-d time period using a physiologically based extraction test (PBET) that was designed to simulate the digestive process of the stomach. The sorption of Cr(III) and Cr(VI) varied significantly as a function of soil type and horizon, and the oxidation state of the contaminant. Solid phase concentrations with Cr(III) were significantly greater than Cr(VI) for any given initial Cr concentration. This is consistent with the mechanisms of Cr(III) vs. Cr(VI) sequestration by the soils, where the formation of Cr(III)-hydroxides can result in the accumulation of large mass fractions of contaminant on mineral surfaces. Overall, Cr bioaccessibility decreased with duration of exposure for all soils and at all solid phase concentrations, with aging effects being more pronounced for Cr(III). The decrease in Cr bioaccessibility was rapid for the first 50 d and then slowed dramatically between 50 and 200 d. In general, the effects of Cr solid phase concentration on bioaccessibility was small, with Cr(III) showing the most pronounced effect; higher solid phase concentrations resulted in a decrease in bioaccessibility. Chemical extraction methods and X-ray Adsorption Spectroscopy analyses suggested that the bioaccessibility of Cr(VI) was significantly influenced by reduction processes catalyzed by soil organic carbon. Soils with sufficient organic carbon had lower Cr bioaccessibility values (～10 to 20%) due to an enhanced reduction of Cr(VI) to Cr(III). In soils where organic carbon was limited and reduction processes were minimal, the bioaccessibility of Cr(VI) dramatically increased (～60 to 70%).     

Mycorrhizal fungi enhance accumulation and tolerance of chromium in sunflower (Helianthus annuus)

Chromium (Cr) is a heavy metal risk to human health, and a contaminant found in agricultural soils and industrial sites. Phytoremediation, which relies on phytoextraction of Cr with biological organisms, is an important alternative to costly physical and chemical methods of treating contaminated sites. The ability of the arbuscular mycorrhizal fungus (AM),Glomus intraradices, to enhance Cr uptake and plant tolerance was tested on the growth and gas exchange of sunflower (Helianthus annuus L.). Mycorrhizal-colonized (AM) and non-inoculated (Non-AM) sunflower plants were subjected to two Cr species [trivalent cation (Cr³⁺) Cr(III) , and divalent dichromate anion (Cr₂O₇⁻) Cr(VI) ]. Both Cr species depressed plant growth, decreased net photosynthesis (A) and increased the vapor pressure difference; however, Cr(VI) was more toxic. Chromium accumulation was greatest in roots, intermediate in stems and leaves, and lowest in flowers. Greater Cr accumulation occurred with Cr(VI) than Cr(III). AM enhanced the ability of sunflower plants to tolerate and hyperaccumulate Cr. At higher Cr levels greater mycorrhizal dependency occurred, as indicated by proportionally greater growth, higherA and reduced visual symptoms of stress, compared to Non-AM plants. AM plants had greater Cr-accumulating ability than Non-AM plants at the highest concentrations of Cr(III) and Cr(VI), as indicated by the greater Cr phytoextraction coefficient. Mycorrhizal colonization (arbuscule, vesicle, and hyphae formation) was more adversely affected by Cr(VI) than Cr(III), however high levels of colonization still occurred at even the most toxic levels. Arbuscules, which play an important role in mineral ion exchange in root cortical cells, had the greatest sensitivity to Cr toxicity. Higher levels of both Cr species reduced leaf tissue phosphorus (P). While tissue P was higher in AM plants at the highest Cr(III) level, tissue P did not account for mycorrhizal benefits observed with Cr(VI) plants.     

Enhancement of cr(lll) phytoaccumulation

Chromium (III) accumulation in high biomass agricultural crops, sunflower (Helianthus annum) and Indian mustard (Brassica juncea) was studied using four soils (pH 4.6 to 7.6) contaminated with different rates of CrCl₃.6H₂O in the presence of synthetic chelate and organic acids. Chromium is essential for normal glucose metabolism in humans and animals, but its contamination and recovery from soils is of environmental concern. Adding ethylenediaminetetraacetic acid (EDTA), citric acid, or oxalic acid to Cr(III)‐contaminated soils significantly increased Cr concentration in plant shoots and roots. Adding Cr(III) complexes of EDTA, citric acid, and oxalic acid to soils dramatically increased (>200‐fold) Cr concentration in shoots and roots. Plant growth was severely decreased but was dependent on soil type, chelate rate, form, and time of chelate application. Chelates and organic acids enhanced Cr(III) accumulation, but its toxic effects were not avoided. Chromium(III) complexes were as toxic to plants as Cr(VI). The phytoaccumulation and recovery of Cr(III) from soils were limited and depended on soil type.     

Cr isotopic insights into ca. 1.9 Ga oxidative weathering of the continents using the Beaverlodge Lake paleosol,Northwest Territories,Canada

The ca. 1.9 Ga Beaverlodge Lake paleosol was studied using redox‐sensitive Cr isotopes in order to determine the isotopic response to paleoweathering of a rhyodacite parent rock 500 million years after the Great Oxidation Event. Redox reactions occurring in modern weathering environments produce Cr(VI) that is enriched in heavy Cr isotopes compared to the igneous inventory. Cr(VI) species are soluble and easily leached from soils into streams and rivers, thus, leaving particle‐reactive and isotopically light Cr(III) species to build up in soils. The Beaverlodge Lake paleosol and two other published weathering profiles of similar age, the Flin Flon and Schreiber Beach paleosols, are not as isotopically light as modern soils, indicating that rivers were not as isotopically heavy at that time. Considering that the global average δ⁵³Cr value for the oxidative weathering flux of Cr to the oceans today is just 0.27 ± 0.30‰ (1σ) based on a steady‐state analysis of the modern ocean Cr cycle, the oxidative weathering flux of Cr to the oceans at ca. 1.9 Ga would have likely been shifted to lower δ⁵³Cr values, and possibly lower than the igneous inventory (–0.12 ± 0.10‰, 2σ). Mn oxides are the main oxidant of Cr(III) in modern soils, but there is no evidence that they formed in the studied paleosols. Cr(VI) may have formed by direct oxidation of Cr(III) using molecular oxygen or H₂O₂, but neither pathway is as efficient as Mn oxides for producing Cr(VI). The picture that emerges from this and other studies of Cr isotope variation in ca. 1.9 Ga paleosols is of atmospheric oxygen concentrations that are high enough to oxidize iron, but too low to oxidize Mn, resulting in low Cr(VI) inventories in Earth surface environments.     

Distribution, transformation and bioavailability of trivalent and hexavalent chromium in contaminated soil

The purpose of this study was to investigate solid-phase distribution, transformation, and bioavailability of Cr in Cr(III) and Cr(VI) contaminated soils. The effects of EDTA treatment on solid-phase distribution of Cr in soils were also examined. The results show that Cr in both initially Cr(III)- and Cr(VI)-contaminated soils was mainly present in the organic matter bound fraction. Chromium had similar solid-phase distribution and similar overall binding intensity in both Cr(III)- and Cr(VI)-contaminated soils after a growing season. Transformation between Cr(III) and Cr(VI) took place in both Cr(III)- and Cr(VI)-treated soils. Chromium in the Cr(III)-contaminated soils was mostly present as Cr(III), while Cr in Cr(VI)-treated soils was mainly transformed into Cr(III). About 2% of Cr in native non-treated soils was found as Cr(VI). EDTA treatment increased Cr in soluble and exchangeable fraction in Cr(III)-treated soils. In both Cr(III)- and Cr(VI)-contaminated soils, Cr in oxide bound and organic matter bound     

The effects of metal ions on the root surface phosphatase activity of grasses differing in tolerance to serpentine soil

Summary Six grass species which differed in ability to grow on serpentine soils were assayed for root surface acid phosphatase activity in solutions of Ca, Mg, Ni, Cr III, Cr VI, and Zn at various concentrations. The root surface phosphatases of relatively serpentine tolerant species (Festuca rubra, Festuca pratensis and Holcus lanatus) were unaffected by the external Ca concentration whereas Ca increased the activity of root surface phosphatase in species relatively intolerant to serpentine soil conditions (Phleum pratense vars Scots and S51 and Lolium perenne). There were no differences shown between a tolerant and an intolerant grass in their responses to Mg, Ni, Cr III, Cr VI or Zn. re]19760611     

Biogeochemical Elimination of Chromium (VI) from Contaminated Water

Ferrous iron [Fe(II)] reductively transforms heavy metals in contaminated groundwater, and the bacterial reduction of indigenous ferric iron [Fe(III)] to Fe(II) has been proposed as a means of establishing redox reactive barriers in the subsurface. The reduction of Fe(III) to Fe(II) can be accomplished by stimulation of indigenous dissimilatory metal-reducing bacteria (DMRB) or injection of DMRB into the subsurface. The microbially produced Fe(II) can chemically react with contaminants such as Cr(VI) to form insoluble Cr(III) precipitates. The DMRB Shewanella algae BrY reduced surface-associated Fe(III) to Fe(II), which in batch and column experiments chemically reduced highly soluble Cr(VI) to insoluble Cr(III). Once the chemical Cr(VI) reduction capacity of the Fe(II)/Fe(III) couple in the experimental systems was exhausted, the addition of S. algae BrY allowed for the repeated reduction of Fe(III) to Fe(II), which again reduced Cr(VI) to Cr(III). The research presented herein indicates that a biological process using DMRB allows the establishment of a biogeochemical cycle that facilitates chromium precipitation. Such a system could provide a means for establishing and maintaining remedial redox reactive zones in Fe(III)-bearing subsurface environments.     

Fe(III), Cr(VI), and Fe(III) mediated Cr(VI) reduction in alkaline media using a <Emphasis Type="Italic">Halomonas</Emphasis> isolate from Soap Lake,Washington

Hexavalent chromium is one of the most widely distributed environmental contaminants. Given the carcinogenic and mutagenic consequences of Cr(VI) exposure, the release of Cr(VI) into the environment has long been a major concern. While many reports of microbial Cr(VI) reduction are in circulation, very few have demonstrated Cr(VI) reduction under alkaline conditions. Since Cr(VI) exhibits higher mobility in alkaline soils relative to pH neutral soils, and since Cr contamination of alkaline soils is associated with a number of industrial activities, microbial Cr(VI) reduction under alkaline conditions requires attention. Soda lakes are the most stable alkaline environments on earth, and contain a wide diversity of alkaliphilic organisms. In this study, a bacterial isolate belonging to the Halomonas genus was obtained from Soap Lake, a chemically stratified alkaline lake located in central Washington State. The ability of this isolate to reduce Cr(VI) and Fe(III) was assessed under alkaline (pH = 9), anoxic, non-growth conditions with acetate as an electron donor. Metal reduction rates were quantified using Monod kinetics. In addition, Cr(VI) reduction experiments were carried out in the presence of Fe(III) to evaluate the possible enhancement of Cr(VI) reduction rates through electron shuttling mechanisms. While Fe(III) reduction rates were slow compared to previously reported rates, Cr(VI) reduction rates fell within range of previously reported rates.     

Oxidation and Mobility of Trivalent Chromium in Manganese-Enriched Clays during Electrokinetic Remediation

This article presents the results of an investigation that assessed the extent and effect of oxidation of Cr(III) in manganese-enriched clays on the electrokinetic remedial efficiency. Because chromium commonly exists along with nickel and cadmium at contaminated sites, the effects of changes in chromium redox chemistry on the migration of the coexisting nickel and cadmium was also studied. Bench-scale electrokinetic experiments were conducted using two different clays: kaolin, a typical low buffering soil, and glacial till, a high buffering soil. Tests were performed with 1000?mg/kg of Cr(III), 500?mg/kg of Ni(II), and 250?mg/kg of Cd(II), both with and without the presence of 1000?mg/kg of manganese. All of these experiments were conducted under a constant voltage gradient of 1.0?VDC/ cm. The experimental results showed that in the presence of manganese, percentages of oxidation of Cr(III) into Cr(VI) ranged from 67% in kaolin to 28% in glacial till even before the application of induced electric potential. The low extent of oxidation of Cr(III) in glacial till may be attributed to the initial precipitation of Cr(III) as Cr(OH)₃ resulting from high soil pH, reducing aqueous Cr(III) concentrations present within the soil. In kaolin, Cr(III), Ni(II), and Cd(II) under electric potential migrated toward cathode and precipitated near the cathode due to high soil pH. When manganese was present in kaolin, Cr(VI) that was formed due to the oxidation of Cr(III) migrated toward anode and adsorbed to the soil surfaces near the anode region due to low soil pH. However, remaining Cr(III) as well as Ni(II), and Cd(II) migrated towards and precipitated near the cathode due to high soil pH. In kaolin, the migration of Ni(II) and Cd(II) was retarded in the presence of manganese due to a larger soil zone of elevated pH near the cathode. In glacial till, the migration of Cr(III), Ni(II) and Cd(II) was insignificant due to precipitation resulting from high soil pH caused by the high buffering capacity of the soil. Cr(VI) that resulted from the partial oxidation of Cr(III) in the presence of manganese, however, migrated toward the anode. Overall, this study demonstrated that the effects of manganese on Cr(III) oxidation in low buffering soils can be significant, which can in turn affect the extent and direction of chromium migration under induced electric potential.     

Isolation of indigenous Staphylococcus sciuri from chromium-contaminated paddy field and its application for reduction of Cr(VI) in rice plants cultivated in pots

Accumulation of Cr(VI) in rice seeds cultivated in Cr-contaminated soil of the Sundarbans (India) is an environmental problem. Cr(VI) concentration in this soil was 6.2 ± 0.3 mg/kg, whereas total chromium was 32.04 ± 1.60 mg/kg. A Cr(VI)-removing bacterium isolated from Cr-contaminated paddy field soil of Sundarbans was identified as Staphylococcus sciuri. Enrichment culture of S. sciuri was applied to pot cultivation of rice in Cr-contaminated soil. After 8 weeks, 71 ± 3% Cr(VI) (final concentration 2.15 ± 0.01 mg/kg) and 65 ± 2% total Cr removal (end concentration 11.3 ± 0.5 mg/kg) were attained in bacterium-treated soils. Growth parameters indicated healthy development of plants cultivated in bacterium-treated soils that was not observed in control plants. Total Cr removal attained in rice seeds of plants cultivated in bacterium-treated soils compared with control rice seeds was 78 ± 4%. Total Cr concentration in test seeds was 0.72 ± 0.05 mg/kg (World Health Organization [WHO] permissible limit: 1.30 mg/kg), whereas the same in control seeds was 3.27 ± 0.16 mg/kg. Cr(VI) reduction achieved in rice seeds cultivated in bacterium-treated soil compared with control rice seeds was 95 ± 5%. Cr(VI) concentration in rice seeds cultivated in treated soil was 0.050 ± 0.003 mg/kg, whereas the same in untreated control was 0.93 ± 0.05 mg/kg. Successful paddy field soil bioremediation by any Staphylococcus species was demonstrated for the first time.     

Pyridine-2,6-bis(thiocarboxylic acid) Produced by <Emphasis Type="BoldItalic">Pseudomonas stutzeri</Emphasis> KC Reduces Chromium(VI) and Precipitates Mercury,Cadmium, Lead and Arsenic

Interactions of the Pseudomonas stutzeri KC siderophore pyridine-2,6-bis(thiocarboxylic acid) (pdtc) with chromium(VI), mercury(II), cadmium(II), lead(II), and arsenic(III) are described. Pdtc was found to reduce Cr(VI) to Cr(III) in both bacterial cultures and in abiotic reactions with chemically synthesized pdtc. Cr(III) subsequently formed complexes with pdtc and pdtc hydrolysis products, and their presence was confirmed using electrospray ionization-mass spectrometry (ESI-MS). Cr(III):pdtc complexes were found to slowly release Cr(III) as chromium sulfide and possibly Cr(III) oxides. Pdtc also formed poorly soluble complexes with Hg, Cd, Pb, and As(III). Hydrolysis of those complexes led to the formation of their respective metal sulfides as confirmed by energy dispersive X-ray spectroscopy (EDS) elemental analysis. The pdtc-producing strain P. stutzeri KC showed higher tolerance to most of these metals as compared to a pdtc-negative mutant. A novel role of pdtc is postulated as its involvement in providing an extracellular pool of thiols that are used for redox processes in detoxification of the bacterial extracellular environment. These redox processes can be mediated by transition metal:pdtc complexes.     

Transport and transformation of hexavalent chromium through soils and into ground water

A detailed characterization of the underlying and adjacent soils of a chrome‐plating shop was performed to provide information on the extent of soil and aquifer contamination at the site and on the potential for off‐site migration and environmental impact. Intact, moist cores were obtained from more than 40 different locations, resulting in more than 200 discrete samples for total metal analysis, selective extraction tests, and adsorption‐reduction experiments, to assess the chemical speciation and distribution of chromium on the contaminated soils and its leaching potential. Surface analytical techniques were also used to determine chemical speciation and to further elucidate mineral fractions responsible for retention of the chromium on the soils and sediments. Adsorption and reduction capacities of the saturated aquifer sediments were variable and low, while the unsaturated soils’ reduction capacities were much greater and were correlated with depth (decreasing capacity with increasing depth). The soils’ adsorption and reduction capacities were eventually overwhelmed, however, and permitted the passage of Cr(VI) into the underlying ground water. Adsorption capacity differences were primarily related to clay content and pH, and less so to the presence of amorphous iron oxide coatings on matrix minerals as operationally defined by the selective extraction methods used in the study. Reduction of Cr(VI) to Cr(III) and subsequent precipitation as (Fe, Cr)(OH)₃ is proposed as the primary attenuation mechanism in the unsaturated soils immediately beneath the shop, based on extraction and surface analyses results.     

Plant Screening for Chromium Phytoremediation

Thirty-six plant species of different agronomic importance, size, dry matter production, and tolerance to heavy metals were evaluated for Cr(III) and Cr(VI) uptake and accumulation as influenced by rate, form, source, and chelate application to a Cr-contaminated soil. There was a significant difference in the degree of tolerance, uptake, and accumulation of Cr among plant species. Sunflower (Helianthus annuus) was the least tolerant to Cr, and Bermudagrass (Cynodon dactylon) and switchgrass (Panicum virgatum) were the most tolerant. Indian mustard (Brassica juncea, cv 426308) and sunflower accumulated more Cr than other agricultural plant species. There was no inhibition of growth and little Cr accumulation in the presence of Cr(III) in soil, but most of the plant species that were treated with Cr(VI) hyperaccumulated Cr and died. EDTA chelate added to soil enhanced Cr(III) accumulation in some plants. The phytoremediation potential of the plant species tested was limited because Cr was accumulated in the plant roots and a high concentration in the shoots was toxic to plants. The difference in behavior between Cr(III) and Cr(VI) and their importance in soil and environment contamination should be the basis for remediation strategies.

     

Application of the knowledge-based approach to strain selection for a bioaugmentation process of phenanthrene- and Cr(VI)-contaminated soil

Aims: The objective of this study was to apply the knowledge‐based approach to the selection of an inoculum to be used in bioaugmentation processes to facilitate phenanthrene degradation in phenanthrene‐ and Cr(VI)‐co‐contaminated soils. Methods and Results: The bacterial community composition of phenanthrene and phenanthrene‐ and Cr(VI)‐co‐contaminated microcosms, determined by denaturing gradient gel electrophoresis analysis, showed that members of the Sphingomonadaceae family were the predominant micro‐organisms. However, the Cr(VI) contamination produced a selective change of predominant Sphingomonas species, and in co‐contaminated soil microcosms, a population closely related to Sphingomonas paucimobilis was naturally selected. The bioaugmentation process was carried out using the phenanthrene‐degrading strain S. paucimobilis 20006FA, isolated and characterized in our laboratory. Although the strain showed a low Cr(VI) resistance (0·250 mmol l^?1); in liquid culture, it was capable of reducing chromate and degrading phenanthrene simultaneously. Conclusion: The inoculation of this strain managed to moderate the effect of the presence of Cr(VI), increasing the biological activity and phenanthrene degradation rate in co‐contaminated microcosm. Significance and Impact of the Study: In this study, we have applied a novel approach to the selection of the adequate inoculum to enhance the phenanthrene degradation in phenanthrene‐ and Cr(VI)‐co‐contaminated soils.     

Changes in biomass and photosynthetic parameters of tomato plants exposed to trivalent and hexavalent chromium

Tomato plants were treated for two weeks with different concentrations of Cr(III) or Cr(VI) compounds to compare their toxic effects. The concentration of total Cr in plant tissues increased linearly with its concentration in the growth medium and Cr accumulated largely in the roots, regardless of the form in which it was supplied to the plant. All measured plant growth parameters were negatively affected by Cr, but Cr(VI) showed much more pronounced toxic effects. Leaf net photosynthetic rate (P_N) was decreased by both Cr forms, and the decrease was also greater for Cr(VI). Cr(III) caused no significant effect on leaf stomatal conductance, whereas Cr(VI) reduced it. Cr(VI) also markedly reduced the variable to maximum chlorophyll a fluorescence ratio, measured in dark-adapted leaves.