Bacterial diversity in selenium reduction of agricultural drainage water amended with rice straw

Bacterial reduction of the Se oxyanions selenate [Se(VI)] and selenite [Se(IV)] to elemental selenium [Se0] is an important biological process in removing Se from drainage water. This study was conducted to characterize the molecular diversity of bacterial populations involved in Se reduction of drainage water amended with rice (Oryza sativa L.) straw and also to monitor the bacterial community shifts during the course of the study. Selenate was removed in the drainage water by the bacteria 5 to 6 d after addition of rice straw. Six Se(VI)- and 32 Se(IV)-reducing bacteria were isolated from rice straw containing sterilized drainage water. Three Se(VI)- and two Se(IV)-reducing bacteria were also isolated from the drainage water. Identification of Se(VI)- and Se(IV)-reducing bacteria by 16S rDNA sequence analysis showed a broad phylogenetic diversity in Se-reducing assemblages. Three major phyla (Proteobacteria, Actinobacteria, and Firmicutes) of bacterial domain with numerous classes, orders, and families constituted the Se-reducing bacterial community. We documented changes in the composition of bacterial assemblages in the drainage water amended with rice straw using polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE) of 16S rDNA. The Shannon-Weaver index (H') revealed higher bacterial diversity at Day 6 in the sterilized and Day 4 in the non-sterilized drainage water amended with rice straw. The results of this study suggest that rice straw, a good source of carbon and energy, harbors a wide range of bacteria useful in Se reduction and may be used in removing Se from drainage water.     

Removal of selenate in simulated agricultural drainage water by a rice straw bioreactor channel system

Removal of selenium (Se) from agricultural drainage water is important in protecting wetland wildlife. Three flow-through bioreactor channel systems (BCSs), each with three channels filled with rice (Oryza sativa L.) straw, were set in the laboratory to determine removal of selenate [Se(VI)] (1020 microg L(-1)) from drainage water with a salinity of 10.4 dS m(-1), a pH of 8.1, and a nitrate (NO3-) range of 0 to 100 mg L(-1). Results showed that the rice straw effectively reduced Se(VI) during 122 to 165 d of the experiments. Calculation of Se mass in the three BCSs showed that 89.5 to 91.9% of the input Se(VI) was reduced to red elemental Se [Se(0)], where 96.6 to 98.2% was trapped in the BCSs. Losses of each gram of rice straw were almost equal to the removal of 1.66 mg of Se from the drainage water as a form of red Se(0), indicating that rice straw is a very effective organic source for removing Se(VI) from drainage water.     

Fate of colloidal-particulate elemental selenium in aquatic systems

Bacterial reduction of selenate [Se(VI)] to elemental Se [Se(0)] is considered an effective bioremediation technique to remove selenium (Se) from agricultural drainage water. However, the fate of the newly formed Se(0) in aquatic systems is not known when it flows out of the treatment system. A set of laboratory experiments was conducted to determine the fate of the colloidal-particulate Se(0) in a water column and in a water-sediment system. Results showed that the newly formed colloidal-particulate Se(0) followed two removal pathways in aquatic systems: (i) flocculation-sedimentation to the bottom of the water and (ii) oxidation to selenite [Se(IV)] and Se(VI). During 58 d of the experiments, 51% of the added Se(0) was precipitated to the bottom of the water and 47% was oxidized to Se(IV) in the water column. In the water-sediment system, Se(IV) in the water accounted for 21 to 25% of the added Se(0). Adsorption of Se(IV) to the bottom sediment resulted in a relatively low amount of Se(IV) in the water. This study indicates that the newly formed Se(0) may be an available form of Se for uptake by organisms if it flows to aquatic systems from a treatment site. Therefore, an effective bioremediation system for removing Se from drainage water must reduce Se(VI) to Se(0) and remove Se(0) directly from the drainage water.     

Removal of selenate from water by zerovalent iron

Zerovalent iron (ZVI) has been widely used in the removal of environmental contaminants from water. In this study, ZVI was used to remove selenate [Se(VI)] at a level of 1000 microg L(-1) in the presence of varying concentrations of Cl-, SO(2-)4, NO(-)3, HCO(-)3, and PO(3-)4. Results showed that Se(VI) was rapidly removed during the corrosion of ZVI to iron oxyhydroxides (Fe(OH)). During the 16 h of the experiments, 100 and 56% of the added Se(VI) was removed in 10 mM Cl- and SO(2-)4 solutions under a closed contained system, respectively. Under an open condition, 100 and 93% of the added Se(VI) were removed in the Cl- and SO(2-)4 solutions, respectively. Analysis of Se species in ZVI-Fe(OH) revealed that selenite [Se(IV)] and nonextractable Se increased during the first 2 to 4 h of reaction, with a decrease of Se(VI) in the Cl- experiment and no detection of Se(VI) in the SO(2-)4 experiment. Two mechanisms can be attributed to the rapid removal of Se(VI) from the solutions. One is the reduction of Se(VI) to Se(IV), followed by rapid adsorption of Se(IV) to Fe(OH). The other is the adsorption of Se(VI) directly to Fe(OH), followed by its reduction to Se(IV). The results also show that there was little effect on Se(VI) removal in the presence of Cl- (5, 50, and 100 mM), NO(-)3 (1, 5, and 10 mM), SO(2-)4 (5 mM), HCO(-)3 (1 and 5 mM), or PO(3-)4 (1 mM) and only a slight effect in the presence of SO(2-)4 (50 and 100 mM), HCO(-)3 (10 mM), and PO(3-)4 (5 mM) during a 2-d experiment, whereas 10 mM PO(3-)4 significantly inhibited Se(VI) removal. This work suggests that ZVI may be an effective agent to remove Se from Se-contaminated agricultural drainage water.     

Selenium removal and mass balance in a constructed flow-through wetland system

A field study on the removal of Se from agricultural subsurface drainage was conducted from May 1997 to February 2001 in the Tulare Lake Drainage District (TLDD) of San Joaquin Valley, California. A flow-through wetland system was constructed consisting of ten 15- x 76-m unlined cells that were continuously flooded and planted with either a monotype or combination of plants, including sturdy bulrush [Schoenoplectus robustus (Pursh) M.T. Strong], baltic rush (Juncus balticus Willd.), smooth cordgrass (Spartina alterniflora Loisel.), rabbitsfoot grass [Polypogon monspeliensis (L.) Desf.], salt-grass lDistichlis spicata (L.) Greene], cattail (Typha latifolia L.), tule [Schoenoplectus acutus (Muhl. ex Bigelow) A. L?ve & D. L?ve], and widgeon grass (Ruppia maritima L.). One cell had no vegetation planted. The objectives of this research were to evaluate Se removal efficiency of each wetland cell and to carry out a mass balance on Se. The inflow drainage water to the cells had average annual Se concentrations of 19 to 22 microg L(-1) dominated by selenate [Se(VI), 95%]. Average weekly water residence time varied from about 3 to 15 d for Cells 1 through 7 (target 7 d), 19 to 33 d for Cells 8 and 9 (target 21 d), and 13 to 18 d for Cell 10 (target 14 d). Average weekly Se concentration ratios of outflow to inflow ranged from 0.45 to 0.79 and mass ratio (concentration x water volume) from 0.24 to 0.52 for year 2000, that is, 21 to 55% reduction in Se concentration and 48 to 76% Se removal in mass by the wetland, respectively. The nonvegetated cell showed the least Se removal both in concentration and in mass. The global mass balance showed that on the average about 59% of the total inflow Se was retained within the cells and Se outputs were outflow (35%), seepage (4%), and volatilization (2%). Independent measurements of the Se retained in the cells totaled 53% of the total Se inflow: 33% in the surface (0-20 cm) sediment, 18% in the organic detrital layer above the sediment, 2% in the fallen litter, < 1% in the standing plants, and < 1% in the surface water. Thus, about 6% of the total Se inflow was unaccounted for in the internal compartments.     

Evaluation of the macroalga,muskgrass, for the phytoremediation of selenium-contaminated agricultural drainage water by microcosms

Previous field studies suggested that the macroalga, muskgrass (Chara canescens Desv. & Lois), plays an important role in the removal of selenium (Se) from agricultural drainage water. This study evaluated the efficiency of Se removal from drainage water by muskgrass-vegetated wetland microcosms, and determined the extent to which muskgrass removed Se through phytoextraction and biovolatilization. Six flow-through wetland microcosms were continuously supplied with drainage water containing an average Se concentration of 22 microg L(-1) over a 24-d experimental period. The Se mass input and outflow and the rate of Se volatilization were monitored daily for each microcosm. Three microcosms containing muskgrass reduced the daily mass Se input in the inflow drainage water by 72.1%; this compared with a reduction of 50.6% of the mass Se input for three unvegetated control microcosms. Selenium accumulated in muskgrass tissues accounted for 1.9% of the total mass Se input in the microcosm, followed by 0.5% via biological volatilization. The low rates of Se volatilization from selenate-supplied muskgrass, which were 10-fold less than from selenite, were probably due to a major rate limitation in the reduction of selenate to organic forms of Se in muskgrass. This conclusion was derived from X-ray absorption spectroscopy speciation analysis, which showed that muskgrass treated with selenite contained 91% of the total Se in organic forms (selenoethers and diselenides), compared with 47% in muskgrass treated with selenate.     

Fate of dimethyldiselenide in soil

Volatilization of dimethyldiselenide (DMDSe) is one of the most important processes for removing selenium (Se) from Se-contaminated environments. However, the fate of DMDSe in soil is not known. In this study, we monitored the changes of DMDSe in the head space of soil samples spiked with known amounts of DMDSe gas, and fractionated and speciated the resulting Se forms in soil. Dimethyldiselenide was highly dissolved in water in a closed air-water system and was highly sorbed onto soil in a closed air-soil system. Chemical and biological transformations of DMDSe in soil converted a large amount of DMDSe to nonvolatile Se compounds. Elemental Se [Se(0)] and nonvolatile organic Se were the major forms of Se transformed from spiked DMDSe. Microbial conversion of DMDSe to dimethylselenide (DMSe) in soil increased the production of DMSe. Calculation of the mass recovery showed that about 85 to 93% of the added DMDSe was recovered as Se(0), organic Se, organic material Se (OM-Se), Se(IV), and volatile organic Se in the head space in the non-autoclaved soils and 50 to 70% of the added DMDSe was recovered in the autoclaved soils. These results indicate that DMDSe is not a stable form of Se, and it may be one of the important precursors of DMSe in the soil environment.     

Selenium stable isotope ratios in California agricultural drainage water management systems

Selenium stable isotope ratios are known to shift in predictable ways during various microbial, chemical, and biological processes, and can be used to better understand Se cycling in contaminated environments. In this study we used Se stable isotopes to discern the mechanisms controlling the transformation of oxidized, aqueous forms of Se to reduced, insoluble forms in sediments of Se-affected environments. We measured 80Se/76Se in surface waters, shallow ground waters, evaporites, digested plants and sediments, and sequential extracts from several sites where agricultural drainage water is processed in the San Joaquin Valley of California. Selenium isotope analyses of samples obtained from the Tulare Lake Drainage District flow-through wetland reveal small isotopic contrasts (mean difference 0.7%) between surface water and reduced Se species in the underlying sediments. Selenium in aquatic macrophytes was very similar isotopically to the NaOH and Na2SO3 sediment extracts designed to recover soluble organic Se and Se(0), respectively. For the integrated on-farm drainage management sites, evaporite salts were slightly (approximately 0.6%) enriched in the heavier isotope relative to the inferred parent waters, whereas surface soils were slightly (approximately 1.4%) depleted. Bacterial or chemical reduction of Se(VI) or Se(IV) may be occurring at these sites, but the small isotopic contrasts suggest that other, less isotopically fractionating mechanisms are responsible for accumulation of reduced forms in the sediments. These findings provide evidence that Se assimilation by plants and algae followed by deposition and mineralization is the dominant transformation pathway responsible for accumulation of reduced forms of Se in the wetland sediments.     

Low-temperature chromium(VI) biotransformation in soil with varying electron acceptors

Effective and low-cost strategies for remediating chromium (Cr)-contaminated soil are needed. Chromium(VI) leaching from contaminated soil into ground water and surface water threatens water supplies and the environment. This study tested indigenous Cr(VI) microbial transformation in batch systems at 10 degrees C in the presence of various electron acceptors. The effects of carbon addition, spiked Cr(VI), and mixing highly contaminated soil with less contaminated soil were investigated. The results indicated that Cr(VI) can be biotransformed in the presence of different electron acceptors including oxygen, nitrate, sulfate, and iron. Sugar addition had the greatest effect on enhancing Cr(VI) removal. Less dissolved organic carbon (DOC) was consumed per amount of Cr(VI) transformed under anaerobic conditions [0.8-93 mg DOC/mg Cr(VI)] compared with aerobic conditions [1.4-265 mg DOC/mg Cr(VI)]. Toxicity of high concentrations (< 160 mg/L) of spiked Cr(VI) were not evident. At Cr(VI) concentrations > 40 mg/L, aerobic conditions promoted faster Cr(VI) reduction than anaerobic conditions with nitrate or sulfate present. Biotransformation of Cr(VI) in highly contaminated soil (22,000 mg Cr/kg) was facilitated by mixing with less-contaminated soil. The study results provide a framework for evaluating indigenous Cr(VI) microbial transformation and enhance the ability to develop strategies for soil treatment.     

Irrigation of broccoli and canola with boron- and selenium-laden effluent

Selenium (Se), boron (B), and salinity contamination of agricultural drainage water is potentially hazardous for water reuse strategies in central California. To demonstrate the feasibility of using plants to extract Se from drainage water, Se accumulation was determined in canola (Brassica napus L.) and broccoli (Brassica oleracea L.) irrigated with drainage effluent in the San Joaquin Valley, California. In the 2-yr field study, both crops were irrigated with a typical drainage water containing Se (150 microg L(-1)), B (5 mg L(-1)), and a sulfate dominated salinity (EC of 7 dS m(-1)). Total dry matter yields were at least 11 Mg ha(-1) for both canola and broccoli, and plant tissue Se concentrations did not exceed 7 mg kg(-1) DM for either crop. Based on the amount of soluble Se applied to crops with drainage water and the estimated amount of soluble Se remaining in soil to a depth of 90 cm at harvest, both canola and broccoli accumulated at least 40% of the estimated soluble Se lost from the soil for both years. Applied Se not accounted for in plant tissue or as soluble Se in the soil was presumably lost by biological volatilization. This study suggests that irrigating two high value crops such as canola and broccoli with Se-laden effluent helps manage Se-laden effluent requiring treatment, and also produces economically viable Se-enriched crops. Future research should focus on managing residual salt and B in the soil for sustaining long time water reuse strategies.     

A method to quantitatively trap volatilized organoselenides for stable selenium isotope analysis

If volatile organoselenides are to be analyzed for their stable Se isotope composition to elucidate sources and formation processes, organoselenides need to be trapped quantitatively to avoid artificial Se isotope fractionation. We developed an efficient trap of organoselenides to be used in microcosms designed to determine the Se isotope fractionation by microbial transformation of inorganic Se to volatile organoselenides. The recoveries of volatilized dimethyldiselenide (DMDSe) from aqueous standard solutions by activated charcoal and alkaline peroxide solution with subsequent freeze-drying and purification via a cation exchange resin were tested. Microcosm experiments with the Se-methylating fungus in a growth medium were conducted, and tightness of the microcosm was assessed by comparing mass balances of total Se of the fungus, medium, and trapped organoselenides with the supplied Se mass. At the end of the experiment, we calculated δSe values of the whole microcosm and compared them with the δSe value of supplied Se(IV) and Se(VI). Our results demonstrated that activated charcoal cannot be used for quantitative trapping of organoselenides because generally <64% of the outgassed DMDSe were recovered. The mean recovery of Se volatilized from an aqueous DMDSe standard trapped in alkaline peroxide, in contrast, was 96 ± 11% (SD) after 2 h ( = 4). The mass balances of total Se in microcosm experiments with alkaline peroxide traps run for 11 to 15 d were 96 ± 15 and 102 ± 2.4% for Se(IV) and Se(VI) ( = 3), respectively. The mass-weighted mean δSe values for the Se(IV) and Se(VI) batch experiments were -0.31 ± 0.05‰ ( = 3) and -0.76 ± 0.07‰ ( = 3), compared with -0.20 ± 0.10‰ and -0.69 ± 0.10‰ in the supplied Se oxyanions, respectively. We conclude that the alkaline peroxide trap can reliably be used to determine the Se isotope composition of organoselenides.     

Evaluation of hexavalent chromium removal in a continuous biological filter with the use of central composite design (CCD)

Hexavalent chromium is frequently found in industrial effluents as a result of the industrial applications of this compound and its anti-corrosive features. However, hexavalent chromium is extremely toxic, and its discharge in water is regulated, with a maximum limit of 0.1 mg/L in accordance with legislation established by CONAMA-Brazil (no. 397, April 3, 2008). To achieve lower discharge values, it is necessary to reduce from Cr(VI) to Cr(III), which is less toxic, and an economic alternative involves biological removal of this compound. Residence time distributions (RTDs) were measured to evaluate the behavior of actual biofilter operation conditions in a biofilter flow. The medium residence time distributions used were 8 and 24 h (recommended by the legislation). To optimize this process, a central composite design was used, considering the initial chromium concentration and pH as the independent variables and the removal of hexavalent chromium as the response. The boundary curves and surface response showed optimal behavior at 3.94 mg/L [Cr(0)] and a pH of 6.2. The removal process of hexavalent chromium is mathematically described by the Michaelis-Menten kinetic model. This model appropriately represents the variation of chromium concentration along the bioreactor.     

Selenium stable isotope investigation into selenium biogeochemical cycling in a lacustrine environment: Sweitzer Lake, Colorado

We present a comprehensive set of Se concentration and isotope ratio data collected over a 3-yr period from dissolved, sediment-hosted, and organically bound Se in a Se-contaminated lake and littoral wetland. Median isotope ratios of these various pools of Se spanned a narrow isotopic range (delta80/76Se(SRM-3149)) = 1.14-2.40 per thousand). Selenium (VI) reduction in the sediments is an important process in this system, but its isotopic impact is muted by the lack of direct contact between surface waters and reduction sites within sediments. This indicates that using Se isotope data as an indicator of microbial or abiotic Se oxyanion reduction is not effective in this or other similar systems. Isotopic data suggest that most Se(IV) in the lake originates from oxidation of organically bound Se rather than directly through Se(VI) reduction. Mobilization of Se(VI) from bedrock involves only a slight isotopic shift. Temporally constant isotopic differences observed in Se(VI) from two catchment areas suggest the potential for tracing Se(VI) from different source areas. Phytoplankton isotope ratios are close to those of the water, with a small depletion in heavy isotopes (0.56 per thousand). Fish tissues nearly match the phytoplankton, being only slightly depleted in the heavier isotopes. This suggests the potential for Se isotopes as migration indicators. Volatile, presumably methylated Se was isotopically very close to median values for phytoplankton and macrophytes, indicating a lack of isotopic fractionation during methylation.     

TOXIC METALS IN SURFACE WATERS FROM COAL ASH1

ABSTRACT: The concentration of 10 [titanium (Ti), manganese (Mn), copper (Cu), chromium (CR), zinc (Zn), arsenic (As), selenium (Se), cobalt (Co), cadmium (Cd), and mercury (Hg)] toxic elements were measured in the water, benthic sediment, plants, invertebrates, and vertebrates of an ash basin and its drainage system at a coal-fired power plant of the Savannah River Project, Aiken, S.C., over a period of two years. During 12 months of this period the basin was essentially filled and little settling of ash occurred. In the remaining 12 months, dredging had been completed, adequate settling occurred and most of the effluent turbidity was removed. All elements were more concentrated in sediment and biota than in water, and five (Mn, Cu, As, Zn, and Se) were biomagnified by at least one biotic component as compared to concentration in benthic sediment. Plants had high accumulations of Ti, Mn, As, and Hg; invertebrates had high accumulations of Co, Hg, Cu, Cr, Cd, and As; and vertebrates greatly biomagnified Se and Zn. The streamlined biotic community of the system accomplished major removal of Mn, Zn, As, Se, and Cd from the effluent. The magnitude of bioaccumulation of Ti, Mn, Zn, As, Se, Cd, and Hg was increased during the period of adequate settling in the basin.     

Assessing energy efficiencies and greenhouse gas emissions under bioethanol-oriented paddy rice production in northern Japan

To establish energetically and environmentally viable paddy rice-based bioethanol production systems in northern Japan, it is important to implement appropriately selected agronomic practice options during the rice cultivation step. In this context, effects of rice variety (conventional vs. high-yielding) and rice straw management (return to vs. removal from the paddy field) on energy inputs from fuels and consumption of materials, greenhouse gas emissions (fuel and material consumption-derived CO(2) emissions as well as paddy soil CH(4) and N(2)O emissions) and ethanol yields were assessed. The estimated ethanol yield from the high-yielding rice variety, "Kita-aoba" was 2.94 kL ha(-1), a 32% increase from the conventional rice variety, "Kirara 397". Under conventional rice production in northern Japan (conventional rice variety and straw returned to the paddy), raising seedlings, mechanical field operations, transportation of harvested unhulled brown rice and consumption of materials (seeds, fertilizers, biocides and agricultural machinery) amounted to 28.5 GJ ha(-1) in energy inputs. The total energy input was increased by 14% by using the high-yielding variety and straw removal, owing to increased requirements for fuels in harvesting and transporting harvested rice as well as in collecting, loading and transporting rice straw. In terms of energy efficiency, the variation among rice variety and straw management scenarios regarding rice varieties and rice straw management was small (28.5-32.6 GJ ha(-1) or 10.1-14.0 MJ L(-1)). Meanwhile, CO(2)-equivalent greenhouse gas emissions varied considerably from scenario to scenario, as straw management had significant impacts on CH(4) emissions from paddy soils. When rice straw was incorporated into the soil, total CO(2)-equivalent greenhouse gas emissions for "Kirara 397" and "Kita-aoba" were 25.5 and 28.2 Mg CO(2) ha(-1), respectively; however, these emissions were reduced notably for the two varieties when rice straw was removed from the paddy fields in an effort to mitigate CH(4) emissions. Thus, rice straw removal avers itself a key practice with respect to lessening the impacts of greenhouse gas emissions in paddy rice-based ethanol production systems in northern Japan. More crucially, the rice straw removed is available for ethanol production and generation of heat energy with a biomass boiler, all elements required for biomass-to-ethanol transformation steps including saccharification, fermentation and distillation. This indicates opportunities for further improvement in energy efficiency and reductions in greenhouse gas emissions under whole rice plant-based bioethanol production systems.     

Removal of Se(IV) from aqueous solution using sulphuric acid-treated peanut shell

A carbonaceous sorbent was prepared from peanut shell via sulphuric acid treatment. Se(IV) removal from aqueous solution on the sorbent was studied varying time, pH, Se(IV) concentration, temperature and sorbent status (wet and dry). Se(IV) removal was faster using the wet sorbent than the dry sorbent following a pseudo-first-order model. Se(IV) removal increases at low pH values, and decreases as pH increases until pH 7. Sorption was found to fit the Langmuir equation and sorption capacity for the wet sorbent was higher than that for the dry one. Both sorbents showed an increased selenium sorption by rising the temperature. Redox processes between Se(IV) and the carbon sorbent are involved. Analysis by scanning electron microscope and X-ray powder diffraction for the sorbent after the reaction with acidified Se(IV) confirmed the availability of elemental selenium as particles on the sorbent surface as a result of Se(IV) reduction. Physicochemical tests showed an increase in sorbent acidity, cation exchange capacity (CEC) and surface functionality after the reaction with acidified Se(IV), indicating the oxidation processes occurring on the sorbent surface. Due to its reduction properties, the sorbent seems efficient for Se(IV) removal from aqueous solution.     

Bioconversion of selenate in methanogenic anaerobic granular sludge

The capacity of anaerobic granular sludge to remove selenate from contaminated wastewater was investigated. The potential of different types of granular sludge to remove selenate from the liquid phase was compared to that of suspended sludge and contaminated soil and sediment samples. The selenate removal rates ranged from 400 to 1500 microg g VSS(-1) h(-1), depending on the source of biomass, electron donor, and the initial selenate concentration. The granular structure protects the microorganisms when exposed to high selenate concentrations (0.1 to 1 mM). Anaerobic granular sludge "Eerbeek," originating from a UASB reactor treating paper mill wastewater, removed about 90, 50, and 36% of 0.1, 0.5, and 1 mM of Se, respectively, from the liquid phase when incubated with 20 mM lactate at 30 degrees C and pH 7.5. Selenite, elemental Se (Se(o)), and metal selenide precipitates were the conversion products. Enrichments from the anaerobic granular sludge "Eerbeek" were able to convert 90% of the 10-mM selenate to Se(o) at a rate of 1505 microg Se(VI) g cells(-1) h(-1), a specific growth rate of 0.0125 g cells h(-1), and a yield of 0.083 g cells mg Se(-1). Both microbial metabolic processes (e.g dissimilatory reduction) as well as microbially mediated physicochemical mechanisms (adsorption and precipitation) contribute to the removal of selenate from the Se-containing medium.     

Kinetic and equilibrium characterization of uranium(VI) adsorption onto carboxylate-functionalized poly(hydroxyethylmethacrylate)-grafted lignocellulosics

This study investigated the feasibility of using a new adsorbent prepared from coconut coir pith, CP (a coir industry-based lignocellulosic residue), for the removal of uranium [U(VI)] from aqueous solutions. The adsorbent (PGCP-COOH) having a carboxylate functional group at the chain end was synthesized by grafting poly(hydroxyethylmethacrylate) onto CP using potassium peroxydisulphate-sodium thiosulphite as a redox initiator and in the presence of N,N'-methylenebisacrylamide as a crosslinking agent. IR spectroscopy results confirm the graft copolymer formation and carboxylate functionalization. XRD studies confirm the decrease of crystallinity in PGCP-COOH compared to CP, and it favors the protrusion of the functional group into the aqueous medium. The thermal stability of the samples was studied using thermogravimetry (TG). Surface charge density of the samples as a function of pH was determined using potentiometric titration. The ability of PGCP-COOH to remove U(VI) from aqueous solutions was assessed using a batch adsorption technique. The maximum adsorption capacity was observed at the pH range 4.0-6.0. Maximum removal of 99.2% was observed for an initial concentration of 25mg/L at pH 6.0 and an adsorbent dose of 2g/L. Equilibrium was achieved in approximately 3h. The experimental kinetic data were analyzed using a first-order kinetic model. The temperature dependence indicates an endothermic process. U(VI) adsorption was found to decrease with an increase in ionic strength due to the formation of outer-sphere surface complexes on PGCP-COOH. Equilibrium data were best modeled by the Langmuir isotherm. The thermodynamic parameters such as DeltaG(0), DeltaH(0) and DeltaS(0) were derived to predict the nature of adsorption. Adsorption experiments were also conducted using a commercial cation exchanger, Ceralite IRC-50, with carboxylate functionality for comparison. Utility of the adsorbent was tested by removing U(VI) from simulated nuclear industry wastewater. Adsorbed U(VI) ions were desorbed effectively (about 96.2+/-3.3%) by 0.1M HCl. The adsorbent was suitable for repeated use (more than four cycles) without any noticeable loss of capacity.     

Assessing alternatives for mitigating net greenhouse gas emissions and increasing yields from rice production in China over the next twenty years

Assessments of the efficacy of mitigation of greenhouse gas (GHG) emissions from paddy rice systems have typically been analyzed based on field studies. Extrapolation of the mitigation potential of alternative management practices from field studies to a national scale may be enhanced by spatially explicit process models, like the DeNitrification and DeComposition (DNDC) model. Our objective was to analyze the impacts of mitigation alternatives, management of water, fertilizer, and rice straw, on net GHG emissions (carbon dioxide, methane, and nitrous oxide fluxes), yields, and water use. After constructing a GIS database of soil, climate, rice cropping area and systems, and management practices, we ran DNDC with 21-yr alternative management schemes for each of the approximately 2500 counties in China. Results indicate that, despite large-scale adoption of midseason drainage, there is still large potential for additional methane reductions from Chinese rice paddies of 20 to 60% over 2000-2020. However, changes in management for reducing CH4 emissions simultaneously affect soil carbon dynamics as well as N2O emissions and can thereby reorder the ranking of technical mitigation effectiveness. The order of net GHG emissions reduction effectiveness found here is upland rice > shallow flooding > ammonium sulfate > midseason drainage > off-season straw > slow-release fertilizer > continuous flooding. Most of the management alternatives produced yields comparable to the baseline; however, continuous flooding and upland rice significantly reduced yields. Water management strategies appear to be the most technically promising GHG mitigation alternatives, with shallow flooding providing additional benefits of both water conservation and increased yields.     

Effects of plants on the removal of hexavalent chromium in wetland sediments

The effect of two wetland plants, Typha latifolia L. (cattail) and Phragmites australis (Cav.) Trin. ex Steud (common reed), on the fate of Cr(VI) in wetland sediments was investigated using greenhouse bench-scale microcosm experiments. The removal of Cr(VI) was monitored based on the vertical profiles of aqueous Cr(VI) in the sediments. The Cr(VI) removal rates were estimated taking into account plant transpiration, which was found to significantly concentrate dissolved species in the sediments. After correcting for evapotranspiration, the actual Cr(VI) removal rates were significantly higher than would be inferred from uncorrected profiles. On average, the Cr(VI) removal rates were 0.005 to 0.017 mg L(-1) d(-1), 0.0003 to 0.08 mg L(-1) d(-1), and 0.004 to 0.13 mg L(-1) d(-1) for the control, T. latifolia, and P. australis microcosms, respectively. The fate of the removed Cr(VI) was examined by determining the quantity and chemical speciation of the Cr in the sediment and plant materials. Chromium(III) was the dominant form of Cr in both the sediment and plants, and precipitation of Cr(III) in the sediment was the major pathway responsible for the disappearance of aqueous Cr(VI) from the pore water. Incubation results showed that abiotic reduction was the primary mechanism underlying Cr(VI) removal in the microcosm sediments. Organic compounds produced by plants, including root exudates and mineralization products of dead roots, are thought to be the factor that is either directly or indirectly responsible for the gap between Cr(VI) removal efficiencies in the sediments of the vegetated and unvegetated microcosms.