Removal of selenate in river and drainage waters by Citrobacter braakii enhanced with zero-valent iron

A cost-effective remediation method is needed to remove selenium (Se) from Se-contaminated water. In this study, a selenate [Se(VI)]-reducing bacterium, Citrobacter braakii, that is capable of using molasses as a carbon source to reduce Se(VI) from natural river and drainage waters was isolated. During an 8-day experiment, 87-97% of the added Se(VI) in New River water and White River water, California, was reduced to elemental Se [Se(0)] or transformed to organic Se. In highly saline drainage water, removal of Se(VI) by C. braakii was limited, with 20% Se(VI) removal in a 7-day experiment. Addition of zero-valent iron (ZVI) into these waters along with C. braakii inoculation significantly enhanced the removal of Se(VI) and reduced the formation of organic Se. This study suggests that the combination of a bacterial treatment using inexpensive molasses and ZVI can effectively remove Se from natural river water and agricultural drainage waters.     

Assessing the Removal Efficiency of Zn, Cu, Fe and Pb in A Treatment Wetland Using Selective Sequential Extraction: A Case Study

The accumulation and speciation of Zn, Cu, Fe and Pb in the sediments of an artificial surface-flow wetland used to treat domestic wastewater near Christchurch, New Zealand, were examined. Water metal concentrations and total suspended solid (TSS) content were determined at the inflow and outflow both in winter and summer, and metal concentrations were analysed in shoots and roots of selected plants. Water and sediment data suggest that the wetland is acting as a sink for Zn, Cu and Pb, while Fe uptake is minimal and the wetland appears to be releasing Fe in the winter. Metal concentrations in the most mobile fractions (exchangeable and bound to carbonates) are negligible. Cu is mostly associated with the organic/sulphide phase, whereas Zn and Pb show a strong affinity for hydroxides and organics/sulphides. A large fraction of all metals is also present in the residual phase, and is therefore unlikely to be released into the overlying water. Metal concentrations are below the low trigger values of the ANZECC sediment quality guidelines, except for Pb near the inflow. However, results of sequential extraction suggest that a significant proportion of Pb occurs in the detrital phase and is therefore not bio-available. Metal concentrations were higher in the roots than in the shoots of both Juncus sp. and Lythrum hyssopifolia. This study shows that, although metal concentrations are low, TSS and metals, except Fe, are effectively removed by the wetland system. The low removal rate of Fe is possibly due to the young age of the wetland and low loading rates.     

Selenate reduction in river water by Citerobacter freundii isolated from a selenium-contaminated sediment

Bacterial reduction of selenate [Se(VI)] to insoluble elemental Se [Se(0)] is an important remedial technology to remove selenium (Se) from Se-impacted water. Citerobacter freundii, a Se(VI) reducer, isolated from a Se-contaminated sediment was assessed for its ability to reduce Se(VI) in a mineral culture medium and natural river water in a series of laboratory batch experiments. The results showed that a combination of yeast extract and glucose used in the culture medium was more effective than yeast extract alone, yeast extract plus sodium acetate, and yeast extract plus sodium lactate for reduction of Se(VI) to Se(0) by C. freundii. About 89-96% of the added Se(VI) (500-4500 microg/L) was reduced to Se(0) in the culture medium amended with 500 mg/L each of yeast extract and glucose. C. freundii can also survive in natural river water and reduce Se(VI). During an 8-day experiment in both sterile and nonsterile river water, 63-70 and 21-22% of the added Se(VI) was reduced to Se(0) and Se(-II), respectively. These results suggest that C. freundii has great potential for Se(VI) reduction and may be used for remediating Se-impacted water.     

Fate of selenate metabolized by Enterobacter taylorae isolated from rice straw

Rice straw has been successfully tested as an effective organic source and a carrier of selenate [Se(VI)]-reducing bacteria to remove Se(VI) from agricultural drainage water. In this study, an Se(VI)-reducing bacterium identified as Enterobacter taylorae was isolated from rice straw and used to remove Se(VI) from a 0.5% tryptic soy broth (TSB) and high-salt (15.5 dS m(-)(1)) synthetic agricultural drainage water containing Se(VI) in a range of 500-5000 microg/L. Results showed that E. taylorae reduced 81-94% of the added Se(VI) to elemental Se [Se(0)] in the 0.5% TSB solution during a 5-day experiment. In the high-salt drainage water, Se(VI) reduction was rapid during a 9-day experiment. On the final day of the experiment, Se(0) [75%] and Se(-II) [19%] were the major forms of Se in the drainage water with small amounts of Se(VI), Se(IV), and volatile Se released. The pathway of Se(VI) reduction in the drainage water followed the order Se(VI) --> selenite [Se(IV)] --> Se(0) --> selenide [Se(-II)]. This study suggests that E. taylorae may be used to remediate high-salt Se(VI)-contaminated agricultural drainage water.     

Changes in selenium speciation in wetland sediments induced by laboratory testing

Abstract

Analysis of wet and laboratory‐dried samples of wetland sediment showed that the concentrations of soluble and adsorbed selenium (Se) increased greatly if samples were dried. This effect was more pronounced at higher temperatures. The release of organic Se through rapid decomposition of sediment organic material and Se oxidation to selenate appear to be the major processes resulting in the increase of the concentrations of the soluble and adsorbed Se in dried sediment. This study shows that sediment drying, especially at higher temperatures, could affect the evaluation of Se speciation and availability in wetland sediment.     

Factors affecting reduction of selenate to elemental selenium in agricultural drainage water by Enterobacter taylorae

Microbial reduction of selenate [Se(VI)] to elemental selenium [Se(0)] is a useful technique for removing Se from agricultural drainage water. A series of batch experiments were conducted in the laboratory to determine the effects of yeast extract (50-1000 mg/L), salinity (EC, 5-75 dS/m), and NO(3)(-) (5-100 mg/L) on the removal of Se(VI) (2000 microg/L) from drainage water by Enterobacter taylorae. Results showed that relatively high amounts of yeast extract (500 mg/L) were needed for E. taylorae to effectively reduce Se(VI) to Se(0). During a 7-day experiment, approximately 95% of added Se(VI) was reduced to Se(0) in the low-salinity drainage water (5 dS/m) with NO(3)(-) values of 5-50 mg/L. In the high-salinity drainage water (50-75 dS/m), reduction of Se(VI) to Se(0) was limited. E. taylorae was also capable of reducing Se(VI) to Se(0) in the San Joaquin Valley drainage water, with a reduction of the added Se(VI) to Se(0) (73.8%) and Se(-II) (20%). This study suggests that E. taylorae may be used to treat Se(VI)-contaminated drainage water in the field.     

Application of redox mediator to accelerate selenate reduction to elemental selenium by Enterobacter taylorae

Acceleration of bacterial reduction of selenate [Se(VI)] to insoluble elemental Se [Se(0)] plays an important role in Se bioremediation. Anthraquinone-2,6-disulfonate (AQDS), a redox mediator, was assessed for its ability to enhance the reduction of Se(VI) (2000 microg/L) to Se(0) by Enterobacter taylorae in various media. The results showed that addition of AQDS did not increase Se(VI) reduction in the media containing 50 and 250 mg/L yeast extract, suggesting that E. taylorae cannot directly use anthrahydroquinone-2,6-disulfonate (AHQDS, a reduced form of AQDS) to respire Se(VI). An increase of yeast extract concentration from 50 to 250 mg/L in the medium dramatically enhanced the AQDS function for rapid reduction of selenite [Se(IV)] to Se(0). During an 8-day experiment, 85-91% of the added Se was reduced to Se(0) in the AQDS-amended medium in comparison to formation of 46% of Se(0) in the medium without AQDS. These results show that redox mediators have great application potential in bioremediation of Se in Se-contaminated water.     

Phosphorus Retention and Release Characteristics of Sewage-Impacted Wetland Sediments

Sediment deposited in traps positioned along a sewage-impacted wetland receiving phosphorus (P)-retaining reactants from natural wetland water was fractionated into different particle sizes, and the amount of P retained in these particle sizes was investigated. Subsamples of the sediments collected from different sites along the wetland system were also equilibrated with water at different water:sediment ratios and equilibration periods to investigate the extent of P released from these sediments under aerobic and anaerobic conditions. Results obtained showed that most of P deposited in sediments is in fine fractions (<16 µm), particularly in sediments collected from confluence sites where water inflow from the natural wetland provides P-retaining reactants and from sites immediately below these confluence sites (postconfluence sites). The extent of P release from sediments depended on the aerobic-anaerobic conditions of the sediments, equilibration period, water:sediment ratio and the position of sites within the wetland. The rate of P released from sediments associated with an increase in equilibration period tended to be higher under aerobic than anaerobic conditions. Water:sediment ratio was found to be a more important factor in controlling the release of P from sediments under anaerobic than aerobic conditions. The amount of P released from the confluence and postconfluence sites was higher than that from other sites over a range of equilibration periods and water:sediment ratios under aerobic and anaerobic conditions.     

冻融对东北黑土硒酸盐吸附解吸的影响

为探究冻融过程对东北黑土硒酸盐（Se（VI））吸附、解吸的影响机理,通过室内不同初始含水率及冻融次数模拟冻融循环,随后利用冻融后土壤进行Se（VI）的吸附和解吸试验,分别采用Langumuir和Freundlich方程对Se（VI）吸附过程进行拟合。结果表明:冻融显著（P < 0.05）改变了东北黑土pH值、有机质、球囊霉素相关土壤蛋白及各粒级团聚体含量,冻融后土壤Se（VI）吸附量显著高于未冻融土壤。通过拟合发现东北黑土对Se（VI）的吸附更符合Langmuir模型（R2 > 0.967）,高初始含水率及冻融循环次数均增加了冻融后黑土对Se（VI）的最大吸附量及缓冲容量,同时提高了Se（VI）的解吸率。70%含水率及多次冻融循环提高了黑土对Se（VI）的吸附潜能,促进Se（VI）的解吸,使得冻融后土壤硒的生物有效性增加,有利于作物根系对硒的吸收。     

Wetland Simulation Model for Nitrogen, Phosphorus, and Sediments Retention in Constructed Wetlands

Steamboat Creek, Washoe County, Nevada, is considered the most polluted tributary of the Truckee River, therefore the reduction of nutrients from the creek is an important factor in reducing eutrophication in the lower Truckee River. Restoration of the wetlands along the creek has been proposed as one method to improve water quality by reducing nutrient and sediments from non-point sources. This study was aimed to design a simulation model wetlands water quality model (WWQM) that evaluates nitrogen, phosphorus, and sediments retention from a constructed wetland system. WWQM is divided into four submodels: hydrological, nitrogen, phosphorus, and sediment. WWQM is virtual Visual Basic 6.0 program that calculates hydrologic parameters, nutrients, and sediments based on available data, simple assumptions, knowledge of the wetland system, and literature data. WWQM calibration and performance was evaluated using data sets obtained from the pilot-scale constructed wetland over a period of four and half years. The pilot-scale wetland was constructed to quantify the ability of the proposed wetland system for nutrient and sediment removal. WWQM simulates nutrient and sediments retention reasonably well and agrees with the observed values from the pilot-scale wetland system. The model predicts that wetlands along the creek will remove nitrogen, phosphorus, and sediments by 62, 38, and 84 %, respectively, which would help to reduce eutrophication in the lower Truckee River.     

Uptake,transformation and fixation of Se(VI) by a mixed selenium-tolerant ecosystem

The uptake, transformation and fate of Se, added as selenate to a mixed microbial ecosystem containing Se-tolerant bacteria and cyanobacteria, has been studied in simulated laboratory ponds. There was evidence of concerted activities of the microbial community in the system. Motile bacteria were responsible for transport into a surface mat and the combined actions of the microbial consortium provide chemical conditions under which the Se could be reduced to the elemental form and physically entrained in the mat. Strong qualitative indications of the formation of volatile alkylselenium compounds were also observed. The removal of Se from the water column was rapid and essentially quantitative. The advantages of such biological ecosystems for remediation of Se-contaminated aquatic ecosystems are discussed.     

The effect of nitrate and organic matter upon mobility of selenium in groundwater and in a water treatment process

Kesterson Reservoir in Merced County, California was formerly used as a storage and evaporation facility for Se-contaminated agricultural drain water. Very little Se entered the shallow aquifer below the ponds with percolating pond water. With few localized exceptions, most of the Se was removed from the water and retained in the first decimeter of soil, which was rich in decaying organic matter. Where it was present in groundwater, Se was nearly always associated with nitrate. Nitrate induces mildly oxidizing conditions (Eh ≈ 350 mV) and inhibits microbial fixation of Se. In the absence of nitrate, relatively reducing conditions prevail in the groundwater (Eh ≈ -50 mV), and Se is quickly removed. Selenate follows oxygen and nitrate in the order that soil microbes utilize electron acceptors. The process of Se removal from water by soil was reproduced in the laboratory, and the effect of nitrate was confirmed. This natural process might be applied to water treatment for Se removal. The deep water ecosystem at Kesterson Reservoir was highly productive. Vegetation growing in water treatment ponds should supply enough organic matter to maintain anaerobic conditions in the sediment. This process, including algae culture, was operated in an outdoor 79 cm column for 100 days. At percolation rates of 15 and 30 m yr^?1 Se removal averaged 94%, with an additional 2% converted to volatile compounds.     

Chemical forms and stability of selenium in soil

The chemical forms of selenium (Se) in soils derived from different parent rocks, and the stability of Se species in soils were investigated. Around 40% of the total Se occurred as organic Se on the average. Total selenite content was higher than that of total selenate. The content of organic Se decreased with heating of the soils at high temperatures, and most of it disappeared at heating temperatures above 500°C. However, the results of incubation experiments indicated that Se species were stable in soil, and that their chemical forms did not change appreciably with the variation of soil conditions. In soluble Se, organic Se accounted for about 50%, and in contrast to the total Se, the selenate content was higher than that of selenite.     

Influence of submergence and subsequent drainage on the partitioning and lability of added selenium fertilizers in a sulphur‐containing Fluvisol

We used an isotope dilution method to examine the time‐dependent changes in the partitioning and lability of selenium (Se) in a Sri Lankan rice soil after adding fertilizer with selenite (Se(IV)) and selenate (Se(VI)) (1 mg kg^?1) and incubation under anaerobic (submerged) (30 days) and subsequent aerobic (drained) conditions (7 days) in controlled reaction vessels mimicking rice paddy water management practices. The K_d (the ratio of sorbed ion to that in solution) values for Se(IV) were significantly (P≤ 0.001) larger than those for Se(VI) in all treatments and at all sampling times. The K_d values for Se(IV) and Se(VI) decreased significantly (P≤ 0.001) with time during the anaerobic and subsequent aerobic phases. Applied Se(IV) fertilizer was rapidly removed into non‐labile pools during the anaerobic phase (day 0 = 60% labile and day 14 = no labile Se), with no significant increase in the labile pool following short‐term aeration. The results suggest that the rapid decrease in Se(IV) lability may be caused by the strong non‐reversible (at least for 7 days) sorption of Se (IV). In contrast, applied Se(VI) fertilizer was 90% labile at 0 day and decreased during the anaerobic phase to 30% after 30 days. There was no significant change in the lability of Se(VI) following the short‐term aerobic phase following anaerobic conditions. These results indicate that Se(IV) would not be an effective pre‐planting fertilizer for rice production. Selenate is likely to be more effective, but losses to non‐labile forms during the submerged phase of rice production may mean that efficiency of pre‐planting Se(VI) fertilizer is also compromised.     

紫甘薯对硒的吸收和累积特征

以紫甘薯为试验材料,采用盆栽试验的方法研究了基施硒酸钠[Se(VI)]和亚硒酸钠[Se(IV)]条件下,紫甘薯对外源硒的吸收累积规律,并比较了施用两种不同价态硒的紫甘薯富硒效果。结果表明:两种硒源均可显著提高紫甘薯各器官含硒量,且紫甘薯含硒量均随施硒量的增加而增大。当土壤施硒量为Se 8 mg/kg时,施用硒酸钠、亚硒酸钠收获期薯块的硒含量(干基)分别达到6.69、0.88 mg/kg。紫甘薯生育期40 d时各器官硒含量叶茎薯块,130 d时硒含量叶薯块茎。当硒酸钠施用量为Se 4 mg/kg时,紫甘薯薯块中的硒累积量最高达923.81μg/株,硒在紫甘薯块根中的分配率可达67%~70%,硒酸钠处理下,紫甘薯对硒的吸收利用率远远高于亚硒酸钠处理。综合紫甘薯含硒量和施硒量对生长的影响结果分析,施用硒酸钠和亚硒酸钠均能增加紫甘薯薯块的硒含量,紫甘薯对硒酸钠敏感性高于亚硒酸钠,生产过程中应充分考虑施用硒酸钠对作物造成的毒害。     

Germination-enhancing and zinc-sparing roles for selenium in broccoli

Large areas of China have soils low in both available selenium (Se) and zinc (Zn). In order to investigate whether Se supplied as either selenate or selenite can increase germination and growth compared with low-Se controls we used broccoli, an important vegetable with anticancer effects, especially when biofortified with Se. Broccoli was grown under both Zn adequacy and Zn deficiency to determine whether interactions between these minerals affect plant growth. Selenite and selenate at a wide range of doses increased the speed and extent of germination. Both inorganic Se forms increased early root and shoot growth at low concentrations, with selenite having a stronger effect than selenate. A sand culture trial showed a similar growth increase due to low-dose Se under Zn deficiency but not under Zn adequacy. Conversely, at high Se levels, the results provided evidence from biomass, water use, photosynthesis and gas exchange that broccoli growth was inhibited at high Se levels, with selenite being more toxic than selenate. In this broccoli trial, the two Se forms were equally effective in increasing leaf Se concentration, whereas in most plants selenite is largely converted to organic Se forms and stored in the roots. This study suggests that Se, supplied either as selenate or selenite, may improve germination and growth in broccoli, especially on Zn-deficient soils. Field trials conducted on soils which are very low in both plant-available Se and Zn are needed.     

Use of NH4 HCO3‐DTPA soil test to assess availability and toxicity of selenium to alfalfa plants

Abstract

This study was carried out to determine if ammonium bicarbonate‐DTPA soil test (AB‐DTPA) of Soltanpour and Schwab for simultaneous extraction of P, K, Zn, Fe, Cu and Mn can be used to determine the availability index for Se. Five Mollisols from North Dakota were treated with sodium selenate and were subjected to several wetting and drying cycles. These soils were extracted with hot water and with ammonium bicarbonate‐DTPA (AB‐DTPA) solution for Se analysis. Alfalfa plants were grown in these soils in a growth chamber to determine plant uptake of Se. In addition to the above experiment, coal mine soil and overburden materials from Western Colorado were extracted and analyzed as mentioned above.

It was found that hot water and AB‐DTPA extracted approximately equal amounts of Se from Mollisols. A high degree of correlation (r =0.96) was found between Se uptake by plants and AB‐DTPA extractable Se. Extractable level of Se in treated soils was decreased with time due to change of selenate to less soluble Se forms and plant uptake of Se. An AB‐DTPA extractable Se level of over 100 ppb produced alfalfa plants containing 5 ppm or higher levels of Se that can be considered toxic to animals. Soils with about 2000 ppb of extractable Se were highly toxic to alfalfa plants and resulted in plant concentrations of over 1000 ppm of Se. The high rate of selenate (4ppm Se) was less toxic to alfalfa plants in soils of high organic matter content. This lower toxicity was accompanied with lower extractable levels of Se.

The AB‐DTPA solution extracted on the average about 31% more Se than hot water from the mine and overburden samples and was highly correlated with the latter (r =0.92). The results indicated the presence of bicarbonate‐exchangeable Se in these materials.     

Integration of H2-Based Membrane Biofilm Reactor with RO and NF Membranes for Removal of Chromate and Selenate

A new hybrid process has been proposed and evaluated the feasibility for complete removal of chromate and selenate at high level. The process consists of a H₂-based membrane biofilm reactor (MBfR) and reverse osmosis (RO)/nanofiltration (NF) stages. The essential feature of the process is the recycling of the RO and NF concentrate into the membrane biofilm reactor. First, two different H₂-based denitrifying MBfR initially reduced selenate (Se (VI)) or chromate (Cr (III)) stably to Se° or Cr (III) to limited levels (approximately 70–85% removal for selenate and approximately 40–65% removal for chromate). In order to achieve more stable and lower levels, two different membrane (NF and RO) filtration technologies as sequential process were combined. Two wastewaters produced from two MBfRs having similar amounts of target toxic ions (C _o?=?366 μg-Cr L^?1 and C _o?=?326 μg-Se L^?1), pH, and conductivity were tested to determine the solute rejection and the membrane flux for one RO and one NF membranes at varying recovery conditions (10–90%). The results show that the rejection of solutes decreases with increasing the recovery due to the increase in osmotic pressure. The rejections by the RO membrane were >99–98% for chromate and 99–94% for selenate, while slightly lower rejections (<20%) were observed for the NF membrane at the recovery conditions.     

Liquid chromatographic method for quantitation of glyphosate and metabolite residues in organic and mineral soils, stream sediments, and hardwood foliage

A liquid chromatographic method for determining glyphosate (GLYPH) and its major metabolite aminomethylphosphonic acid (AMPA) in various environmental substrates is described. Ion-exchange column chromatography is coupled with post-column ninhydrin derivatization and absorbance detection at 570 nm. Use of a valve-switching technique allowed quantitation of both analytes in a single chromatographic run and eliminated slow-eluting, coextracted interferences. The method was successfully used to quantitate GLYPH and AMPA in organic and mineral soils, stream sediments, and foliage of 2 hardwood brush species. Mean recovery efficiencies for GLYPH as determined from fortified blank field samples were as follows: bottom sediment 84%, suspended sediment 66%, organic soils 79%, mineral soils 73%, alder leaf litter 81%, salmonberry leaf litter 84%, and artificial deposit collectors 87%. Precision for GLYPH determination was good with less than 14% coefficient of variation on mean recovery for all substrates. Limits of detection were lowest for sediments (0.01 microgram/g dry mass) and highest for foliage substrates (0.10 microgram/g dry mass). Using this system, 6 samples/person/day were routinely analyzed.     

Selenium speciation in soil and rice: influence of water management and Se fertilization

Rice (Oryza sativa) is the staple food for half of the world's population, but the selenium (Se) concentrations in rice grain are low in many rice-growing regions. This study investigated the effects of water management on the Se speciation dynamics in the soil solution and Se uptake and speciation in rice in a pot experiment. A control containing no Se or 0.5 mg kg(-1) of soil of selenite or selenate was added to the soil, and plants were grown under aerobic or flooded conditions. Flooding soil increased soluble Se concentration when no Se or selenite was added to the soil, but decreased it markedly when selenate was added. Selenate was the main species in the +selenate treatment, whereas selenite and selenomethionine selenium oxide were detected in the flooded soil solutions of the control and +selenite treatments. Grain Se concentration was 49% higher in the flooded than in the aerobic treatments without Se addition. In contrast, when selenate or selenite was added, the aerobically grown rice contained 25- and 2-fold, respectively, more Se in grain than the anaerobically grown rice. Analysis of Se in rice grain using enzymatic hydrolysis followed by HPLC-ICP-MS and in situ X-ray absorption near-edge structure (XANES) showed selenomethionine to be the predominant Se species. The study showed that selenate addition to aerobic soil was the most effective way to increase Se concentration in rice grain.