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1.
Cold season (winter and thaw) CH 4, CO 2 and N 2O fluxes from freshwater marshes (47°35′N, 133°31′E, Northeast China) were measured, using the static chamber method. The mean CH 4 and CO 2 fluxes from Carex lasiocarpa (Cl) were 0.5 ± 0.19 and 6.23 ± 1.36 mg C m −2 h −1, respectively, and those from Deyeuxia angustifoli (Da) were 0.18 ± 0.15 and 5.22 ± 2.48 mg C m −2 h −1, respectively in winter. There was no significant difference between Cl and Da ( p > 0.05). The contributions of winter CH 4 fluxes were about 5.5% and 3% in the Cl and Da, respectively. Marshes are an important potential N 2O sink in winter season in northeast China. During thaw, the CH 4 and CO 2 emissions rapidly increased, 4.5–6 times of winter emissions. Wetland became a source of N 2O. Cold season gases flux from northern wetlands play an important role in the seasonal gas exchange. 相似文献
2.
Anthropogenic release of greenhouse gases, especially CO 2 and CH 4 has been recognized as one of the main causes of global warming. Several measures under the Kyoto Protocol 1997 have been drawn up to reduce the greenhouse gases emission. One of the measures is Clean Development Mechanisms (CDM) that was created to enable developed countries to cooperate with developing countries in emission reduction activities. In Malaysia, palm oil industry particularly from palm oil mill effluent (POME) anaerobic treatment has been identified as an important source of CH 4. However, there is no study to quantify the actual CH 4 emission from the commercial scale wastewater treatment facility. Hence, this paper shall address the CH 4 emission from the open digesting tanks in Felda Serting Hilir Palm Oil Mill. CH 4 emission pattern was recorded for 52 weeks from 3600 m 3 open digesting tanks. The findings indicated that the CH 4 content was between 13.5% and 49.0% which was lower than the value of 65% reported earlier. The biogas flow rate ranged between 0.8 l min −1 m −2 and 9.8 l min −1 m −2. Total CH 4 emission per open digesting tank was 518.9 kg day −1. Relationships between CH 4 emission and total carbon removal and POME discharged were also discussed. Fluctuation of biogas production was observed throughout the studies as a result of seasonal oil palm cropping, mill activities, variation of POME quality and quantity discharged from the mill. Thus only through long-term field measurement CH 4 emission can be accurately estimated. 相似文献
3.
Methane fluxes were measured from three exposed tundra sites and four snowpack sites on the Fildes Peninsula in the maritime Antarctic in the summertime of 2002. The average fluxes at two normal tundra sites were −15.3 μg m −2 h −1 and −14.3 μg m −2 h −1, respectively. The fluxes from tundra site with fresh penguin dropping addition showed positive values with the average of 36.1 μg m −2 h −1, suggesting that the deposition of fresh droppings greatly enhanced CH 4 emissions from the poor Antarctic tundra during penguin breeding periods. The summertime variation in CH 4 flux was correlated with surface ground temperature and the precipitation. The correlation between the flux and PT 0, which is the product of the precipitation and surface ground temperature, was quite strong. The diurnal cycle of CH 4 flux from the tundra soils was not obtained due to local fluky weather conditions. The fluxes through four snowpack sites were also obtained by the vertical CH 4 concentration gradient and their average fluxes were −46.5 μg m −2 h −1, −28.2 μg m −2 h −1, −46.4 μg m −2 h −1 and −17.9 μg m −2 h −1, respectively, indicating that tundra soils under snowpack also consume atmospheric CH 4 in the maritime Antarctic; therefore these fluxes could constitute an important part of the annual CH 4 budget for Antarctic tundra ecosystem. 相似文献
4.
Measurements of carbonyls and C 2–C 6 non-methane hydrocarbons (NMHCs) were made in ambient air at a rural site at the summit of Whiteface Mountain (WFM) in New York State. Alkanes dominated in the samples, with ethane and propane making up about 55% of the total on a carbon-atom basis. Ethane, the longest-lived of the NMHCs, showed a mixing ratio in the range of 0.86–2.1 ppbv. Photochemical ageing analysis indicated an anthropogenic influence on the NMHC levels. The photochemical reactivity of the hydrocarbons, calculated in terms of propylene-equivalent concentration, was dominated by alkenes (propene and ethene), which accounted for 74% of the total NMHC sum. Air mass back-trajectories have been used to investigate the origin of the observed NMHCs and carbonyls. Higher concentrations were found when air masses arrived from the midwestern US corridor. Acetone was the most abundant species, comprising from 31% to 53% of the total detected carbonyls, followed by MEK (15–53%), HCHO (7–39%), and CH 3CHO (7–19%). Average concentrations were determined to be 1.61 ppbv for CH 3C(O)CH 3, 1.40 ppbv for MEK, 1.16 ppbv for HCHO, and 0.49 ppbv for CH 3CHO. The variations in carbonyl concentrations were observed to follow patterns similar to variations in O 3 concentrations, typical of secondary products. Correlations and statistical analysis of the carbonyls and NMHCs were performed, and showed that most of the compounds derived from mixing and photochemical transformation of long-range transported pollutants from the major source areas. Ranking of the carbonyls with respect to removal of the OH radical showed HCHO to be the most important species, followed by CH 3CHO, MEK, and CH 3C(O)CH 3. 相似文献
5.
Dissolved methane was monitored by means of a diffusion probe in combination with a photoacoustic (PA) detector cell placed in the cavity of a liquid nitrogen-cooled CO laser. The detection limit of the photoacoustic detector was 1 ppbv methane (≈ 2 μM in aqueous solution), the time response was 60 s, the spatial resolution was 1.36 mm. These limits were determined by the acoustic noise and the configuration of the diffusion probe. The combination of PA detector with gas diffusion probes was found to be useful for monitoring gaseous compounds. However, the membrane material of the diffusion probe was critical. Silicone as membrane material was useful only for measurement of CH 4. Goretex as membrane material was applicable to measurement of dimethylsulfide (DMS), but did not give a stable signal for trimethylamine (TMA). Vertical concentration profiles of CH4 in anoxic paddy soil agreed well with earlier results obtained with a gas chromatograph as detector. Methane was produced in anoxic soil layers below 8–10 mm depth and diffused upwards to the surface through a layer of CH4-consuming bacteria situated at about 2 mm depth. In the oxic upper 2 mm soil layer the concentration of CH4 decreased below the detection limit of our system. Methane-containing gas bubbles that were embedded in the soil were detected by a steep increase of the CH4 signal. The combination of PA detector and gas diffusion probe was found to be a useful tool to measure CH4 gradients in submerged soil or sediment with high temporal and spatial resolution, thus allowing the localization and quantification of CH4 production and CH4 oxidation rates within the soil profile. 相似文献
6.
Biogeochemical distinction of methane emissions to the atmosphere may essentially rely on the surface area and morphometry of Amazon hydroreservoirs. Tucuruí (deep) and Samuel (shallow) reservoirs released in average 13.82 ± 22.94 and 71.19 ± 107.4 mg CH 4 m −2 d −1, respectively. δ 13C–CH 4 values from the sediments to the atmosphere indicate that the deep reservoir has extended methanotrophic layer, oxidizing large quantities of light isotope methane coming from the sediments, while sediment-generated methane can easily evade the shallow reservoir. 相似文献
7.
An investigation was conducted to estimate mercury emission to the atmosphere from different environmental surfaces and to assess its contribution to the local mercury budget in Chongqing, China. Mercury flux was measured using dynamic flux chamber (DFC) at six soil sites of three different areas (mercury polluted area, farmland and woodland) and four water surfaces from August 2003 to April 2004. The mercury emission fluxes were 3.5 ± 1.2–8.4 ± 2.5 ng m −2 h −1 for three shaded forest sites, 85.8 ± 32.4 ng m −2 h −1 for farming field, 12.3 ± 9.8–733.8 ± 255 ng m −2 h −1 for grassland sites, and 5.9 ± 12.6–618.6 ± 339 ng m −2 h −1 for water surfaces. Mercury exchange fluxes were generally higher from air/water surfaces than from air/soil surfaces. The mercury negative fluxes were found in tow soil sites at overcast days (mean = −6.4 ± 1.5 ng m −2 h −1). The diurnal and seasonal variations of mercury flux were observed in all sites. The mercury emission responded positively to the solar radiation, but negatively to the relative humidity. The mercury flux from air/soil surfaces was significantly correlated with soil temperature, which was well described by an Arrhenius-type expression with activation energy of 31.1 kcal mol −1. The annual mercury emission to the atmosphere from land surface is about 1.787 t of mercury in Chongqing. 相似文献
8.
This study examines the oxidation of reduced sulfur compounds (RSCs) in urban ambient air. The photochemical conversions of RSC (such as DMS, CS 2, H 2S, DMDS, and CH 3SH) to a further oxidized form (e.g., SO 2, MSA, and H 2SO 4) were assessed using a photochemical box model. For our model simulation of RSC oxidation, measurements were taken at an urban monitoring station in Seoul, Korea (37.6° N, 127° E) during three separate time periods (e.g., Sept. 17–18, Oct. 23, and Oct. 27–28, 2003). The results indicate that DMS and H 2S were the dominant RSCs with concentrations of 370 ± 140 and 110 ± 60 pptv, respectively. The photochemical conversion of DMDS to SO 2 was found to occur more efficiently than other RSCs. The overall results of our study suggest that photochemical conversion of RSCs accounted for less than 15% of the observed SO 2 during the measurement period. The SO 2 production from DMS oxidation (mainly by the reaction with OH) was found to be affected primarily by the abstraction channel due to high NO x levels during the experimental conditions. 相似文献
9.
To elucidate mechanisms of Cr 3+ sorption onto the unaltered solid natural organic matter, the comparative studies of this ion binding from a solution at pH 4.0 onto three selected particle size fractions: 2000–1000 μm, 630–200 μm and 63–20 μm of markedly different HS content and structure, separated by a wet sieving from an overall sample of peat (Brushwood Peat Humus) were carried out. Comparable patterns of COOH groups and CEC t confirmed that for cation exchange capacity were responsible mainly cations connected with COO − functional groups. It was though found that aliphatic acids in the solid state did not take part in Cr 3+ binding, thus the finest studied fraction 63–20 μm of the highest contents of functional groups showed the lowest sorption capacity for Cr 3+, while similar patterns of sorbed Cr 3+, soluble HS content and base CEC 0 indicated that these parameters were directly interrelated. The base ion exchange processes determined by CEC 0 (with Ca 2+ as a predominant exchangeable cation) appeared to be not the major mechanisms responsible for Cr 3+ sorption. For this metal, strong binding to insoluble large molecular weight organic pool two- to threefold prevailed over the ion exchange processes. Very low acid desorption indicated generally low mobility of Cr 3+-organic compounds. 相似文献
10.
A field experiment was conducted on a Calcaric Cambisol soil to study the consequences of the penetration depth and properties of pesticides on the risk of subsequent leaching. Three pesticides with different mobility characteristics and bromide were injected at 30 cm (where soil organic matter (OM) was 2%) and 80 cm (soil OM 0.5%) on irrigated plots without a crop. The migration of injected solutes was assessed for two years by sampling the soil solution using six porous cups installed at 50 and 150 cm depth and by relating solute contents to drainage water flux estimated by the STICS model (Simulateur mulTIdisciplinaire pour les Cultures Standard). Pesticides injected at 30 cm were strongly retained so that no metolachlor or diuron was detected at 50 and 150 cm. The ratio of atrazine peak concentration in the soil solution to concentration in the injected solution ( C/ C0) was 1 × 10 −3 and 0.2 × 10 −3, respectively, at 50 and 150 cm. When injected at 80 cm, ( C/ C0) of atrazine, metolachlor and diuron were 10 × 10 −3, 1 × 10 −3 and 0.3 × 10 −3 at 150 cm, respectively; 1/( C/ C0) was correlated with Koc values reported from databases. The ratio of drainage volume to the amount of water at field capacity in the soil layer between the injection point at 30 cm and the water sampling level ( V/ V0) at 50 and 150 cm was 0.6 and 0.9, respectively, for bromide and 1.6 and 1.0 for atrazine. V/ V0 of the injected solutes at 80 cm was for bromide, atrazine, metolachlor and diuron 0.6, 0.9, 1.2 and 1.7, respectively; pesticide V/ V0 was correlated with Koc. The retardation factor was a good indicator of migration risk, but tended to overestimate retardation of molecules with high Koc. Atrazine desorption represented an additional leaching risk as a source of prolonged low contamination. The large variability in soil solution of bromide and pesticide concentrations in the horizontal plane was attributed to flow paths and clods in the tilled soil layer. This heterogeneity was assumed to channel water fluxes into restricted areas and thereby increase the risk of groundwater contamination. The methodology used in the field proves to provide consistent results. 相似文献
11.
Methane flux from rice varieties grown under two identical soils of Assam were monitored. In the first experiment, variety Jaya and GRT was grown in sandy loam soil of Lower Brahmaputra Valley Zone of Assam and the second experiment was conducted with variety Jyotiprasad and Bishnuprasad in sandy to sandy loam soils of Upper Brahmaputra Valley Zones of Assam. Methane flux recorded from variety Jyotiprasad and GRT was higher compared to variety Bishnuprasad and Jaya. The seasonal integrated flux recorded was 10.76 g m −2, 9.98 g m −2, 9.74 g m −2 and 11.31 g m −2 for variety GRT, Jaya, Bishnuprasad and Jyotiprasad, respectively. All the varieties exhibited two methane peaks one at maximum tillering stage and other at panicle initiation stage of the crop. Crop growth parameters such as leaf number, number of tillers and leaf area index (LAI) showed strong positive relationship with total methane flux. In both the experiments it was calculated that CH 4 emission was substantially influenced by crop phenology and growth. This study emphasise the relationship of different growth parameters with methane emission. 相似文献
12.
The cycle of mercury (Hg) from a gigantic landfill area (area ∼2.72 km 2) was investigated by conducting micrometeorological measurements of its exchange rates across soil–air boundary during the spring season of 2000. Based on this field campaign, we attempted to provide various insights into the Hg exchange processes, especially with respect to the decoupling of the mixed signatures of complex source processes. According to our analysis, the cycle of Hg in the study site appeared to be affected significantly by the vent processes; excessive amount of Hg was expected to be released via ventpipes penetrating up to 60 m depths of the deep landfilled waste layer. The influence of these vent source processes was reflected very sensitively by the windrose pattern. The data collected during the non-easterly winds were representing the typical pattern for a strong source area in which upward emission is predominant in both strength and frequency. On the other hand, the data collected from the easterly winds were characterized by excessive deposition of Hg which we suspect is due mostly to the nearest vent located easterly from our measurement spot. The unique characteristics of each data group, divided by windrose pattern, were consistent from apparent difference in: (1) the absolute magnitude of gradient/flux data sets, (2) frequency of exchange for each of two vertical directions, and (3) E/ D (emission/deposition) ratios for most relevant parameters. The analysis of the short-term variability of exchange patterns over a 24-h scale, also exhibited that the patterns for two different conditions were quite contrasting as a function of time. The magnitude of bidirectional fluxes in the present study is significantly high with values of 254±224 ( N=71 emissions out of 79 fluxes quantified during non-easterly winds) and −1164±1276 ng m −2 h −1 ( N=14 depositions out of 16 fluxes during easterly winds), respectively. If the computed emission rate is extrapolated, we estimate that annual emission of Hg from the study area can amount to approximately 6 kg which is comparable with the estimates for other areas around the globe under strong Hg-pollution. 相似文献
13.
A five-stage sequential leaching procedure was used to fractionate 13 heavy metals (Cd, Cu, Pb, Cr, Zn, Fe, Mn, Al, Ni, Co, As, V, Ba) and sulphur (S) in lime waste from the lime kiln at the causticizing plant of Stora Enso Oyj Veitsiluoto Pulp Mills at Kemi, Northern Finland, into the following fractions: (1) water-soluble fraction (H 2O), (2) exchangeable fraction (CH 3COOH), (3) easily reduced fraction (HONH 3Cl), (4) oxidizable fraction (H 2O 2 + CH 3COONH 4), and (5) residual fraction (HF + HNO 3 + HCl). Although metals were leachable in all fractions, the highest concentrations for most of the metals were observed in the residual fraction (stage 5). It was also notable that the total heavy metal concentrations in lime waste did not exceed the maximal allowable heavy metal concentrations for soil conditioner agents set by the ministry of the Agricultural and Forestry in Finland. The heavy metals concentrations in lime waste were also lower than the maximal allowable heavy metals concentrations of the European Union Directive 86/278/EEC on the protection of environment, and in particular of the soil, when sewage sludge is used in agriculture. The Ca concentration (420 g kg −1; d.w.) was about 262 times higher than the typical value of 1.6 g kg −1 (d.w.) in arable land in Central Finland. However, the concentration Mg (0.2 g kg −1; d.w.) in lime waste was equal to the Mg concentration in arable land in the Central Finland. The lime waste has strongly alkaline pH (12.8) and a neutralizing value (i.e. liming effect) of 47.9% expressed as Ca equivalents (d.w.). This indicates lime waste to be a potential soil conditioner and improvement as well as a pH buffer. 相似文献
14.
Fertilized agricultural soils are a major anthropogenic source of atmospheric N 2O. A credible national inventory of agricultural N 2O emission would benefit its global strength estimate. We compiled a worldwide database of N 2O emissions from fertilized fields that were consecutively measured for more than or close to one year. Both nitrogen input (N) and precipitation (P) were found to be largely responsible for temporal and spatial variabilities in annual N 2O fluxes (N 2O–N). Thus, we established an empirical model (N 2O–N = 1.49 P + 0.0186 P · N), in which both emission factor and background emission for N 2O were rectified by precipitation. In this model, annual N 2O emission consists of a background emission of 1.49 P and a fertilizer-induced emission of 0.0186 P · N. We used this model to develop a spatial inventory at the 10 × 10 km scale of direct N 2O emissions from agriculture in China. N 2O emissions from rice paddies were separately quantified using a cropping-specific emission factor. Annual fertilizer-induced N 2O emissions amounted to 198.89 Gg N 2O–N in 1997, consisting of 18.50 Gg N 2O–N from rice paddies and 180.39 Gg N 2O–N from fertilized uplands. Annual background emissions and total emissions of N 2O from agriculture were estimated to be 92.78 Gg N 2O–N and 291.67 Gg N 2O–N, respectively. The annual direct N 2O emission accounted for 0.92% of the applied N with an uncertainty of 29%. The highest N 2O fluxes occurred in East China as compared with the least fluxes in West China. 相似文献
15.
The temporal variability and bioaccumulation dynamics of C 12–25 n-alkanes, isoprenoids and unresolved aliphatic hydrocarbons (UCM) were studied in a detritivorous fish (Sábalo: Prochilodus lineatus) collected from 1999 to 2005 in the sewage impacted Buenos Aires coastal area. Fish muscles contain huge amounts of n-C 12–25 (165 ± 93, 70 ± 48 or 280 ± 134 μg g −1, dry, fresh and lipid weight, respectively) and UCM (931 ± 560, 399 ± 288 and 1567 ± 802 μg g −1) reflecting the chronic bioaccumulation of fossil fuels from sewage particulates. On a temporal basis, lipid normalized aliphatic concentrations peaked by the end of 2001–2002 during the rainiest period over the last four decades (1750 vs. 1083 ± 4.6 mm in 1999, 2004 and 2005), reflecting an enhanced exposition due to massive anthropogenic fluxes from Metropolitan Buenos Aires in wet years. The hydrocarbon composition in fish muscles is enriched in n-C 15–17 and isoprenoids relative to a fresh crude oil and settling particulates, with fresher signatures during the 2001–2002 maxima. Fish/settling material bioaccumulation factors (BAFs: 0.4–6.4 dry weight or 0.07–0.94 lipid-organic carbon) plotted against Kow showed a parabolic pattern maximizing at n-C 14–18 and isoprenoids. The optimal bioaccumulation window corresponds to highly hydrophobic (log Kow: 7.2–9.9), intermediate-size C 14–18 n-alkanes and C 15–20 isoprenoids (MW: 198–282; length: 17.9 to 25.4 Å) with melting points ranging from −19.8 to 28 °C. The uptake efficiency is inversely correlated to melting points and increased from 75% for n-C 25 to above 90% for n-C 14–15 and isoprenoids. 相似文献
16.
Seven metals (Cd, Co, Cr, Cu, Fe, Ni and Pb) were monitored at the Seine-Aval wastewater treatment plant during 6 sampling campaigns in April 2004. Particulate and dissolved metals have been measured in 24 h composite samples at each treatment stage (primary settling, secondary activated sludge and tertiary flocculation by FeCl 3). In addition, the diffusive gradient in thin film technique (DGT) was used to determine the dissolved inert and labile metal fraction. Although all treatment stages were able to decrease particulate metals concentrations in wastewater, most dissolved metals concentrations were mainly affected during primary settling. This unexpected result was attributed to tertiary sludge filtrate recirculation. Metals added via the FeCl 3 reagent at the tertiary treatment were shown to lower the overall Cr removal from wastewater and to enrich Ni in effluents. The plant operating conditions (recirculation and reagent addition) appear therefore as important as treatment processes for the metals removal. Total metal fluxes were highly decreased by the whole treatment plant for Cd, Cr, Cu and Pb and to a lesser extend for Co and Ni. However, the labile metal fluxes were poorly decreased for Cu (18%), not significantly decreased for Ni and increased for Fe. The labile fraction of Cd, Co and Cr was not detectable at any stage of the plant. Discharged labile fluxes, at least for Ni, were potentially significant compared to the labile metal fluxes in the river measured downstream the plant. Treated urban wastewater discharges should be carefully considered as a possible source of bioavailable trace metals. 相似文献
17.
The synthesis of TiO 2 and Fe–TiO 2 by sol–gel method is demonstrated and characterized. The characterization of TiO 2 and Fe–TiO 2 is performed with instruments, including TGA/DTA, FTIR, UV–Vis, N 2 adsorption and SEM. Dichloromethane is used for the photocatalytic activity test. From the results of dichloromethane photocatalyitc degradation, the calcined temperature of TiO 2 and the presence of water vapor influence the photocatalytic activity. The optimum doping amount of iron ions is 0.005 mol%, and this can enhance the photocatalytic activity, while too great an amount will make the iron ions become recombination centers for the electron–hole pairs and reduce the photocatalytic activity. UV–Vis diffuse reflectance spectra of Fe–TiO 2 show an increase in absorbency in the visible light region with the increase in iron ions doping concentration The intermediate of dichloromethane photodegradation includes CHCl 3, CCl 4, CH 2Cl 2 and COCl 2. The presence of iron ions may reduce the adsorption of Cl element on the surface of the photocatalyst. 相似文献
18.
An investigation involving the supplement of different concentrations of substrates and microorganisms was carried out under anaerobic condition to assess the feasibility of bioremediation of carbon tetrachloride (CCl 4) with the amendment of low concentrations of auxiliary substrate and microorganisms. The concentrations of substrate and microorganisms ranged from 10 to 100 mg/l and from 3.7 × 10 4 to 3.7 × 10 6 cell/ml, respectively. The biotransformation rate of CCl 4 increased progressively with the increase in the concentrations of the substrate and microorganisms. In the low biomass-amended system (3.7 × 10 4cells/ml), 28–71% and 57–96% of CCl 4 removals were exhibited when 10–100 mg/l of acetate or glucose was supplemented, respectively, whereas nearly complete degradation of CCl 4 was observed in the heavily inoculated systems (3.7 × 10 6 cells/ml). An addition of electron donor in the low microbial activity batches enhanced greater efficiency in dechlorination than in the high microbial activity batches. The second-order rate constants ranged from 0.0059 to 0.0092 l/mg/day in high biomass input system, while a two- to four-fold increase in rate constant was obtained in the low microbial activity system. This study indicates that biomass was the more important environmental parameter than substrate affecting the fate of CCl 4. The addition of auxiliary substrates was effective only in low biomass-amended batches (0.56 mg-VSS/l) and diminished inversely with the increase of microbial concentration. 相似文献
19.
Wetland loss along the Louisiana Gulf coast and excessive nitrate loading into the Gulf of Mexico are interrelated environmental problems. Nitrate removal by soil denitrification activity was studied in a ponded freshwater marsh receiving diverted Mississippi River water for the purpose of reversing or slowing wetland loss. Labeled 15N-nitrate was applied at 3.8 g N m −2 into four replicate study plots after removing above ground vegetation. Nitrogen gas (N 2) and nitrous oxide (N 2O) emissions from the plots were determined by isotope ratio mass spectrometry (IRMS). Nitrous oxide emissions were also compared with the results determined by gas chromatograph (GC). Results showed that it took 2 weeks to remove the added nitrate with N 2O emission occurring over a period of 4 d. The apparent denitrification dynamics were assumed to follow the Michaelis–Menten equation. The maximum denitrification rate and Km value were determined as 12.6 mg N m −2 h −1, and 6.5 mg N l −1, respectively. Therefore the maximum capacity for nitrate removal by the marsh soil would be equivalent to 110 g N m −2 yr −1, with more than 30% of nitrogen gas evolved as N 2O. For typical nitrate concentrations in Mississippi River water of about 1 mg N l −1, nitrate would be removed at a rate of 14.7 g N m −2 yr −1 with N 2O emission about 1.5%. A denitrification dynamic model showed that the efficiency of nitrate removal would largely depend on the water discharge rate into the ponded wetland. Higher discharge rate will result in less retention time for the water in the marsh where nitrate is denitrified. 相似文献
20.
Hydrogen titanate (H-titanate) nanowires were prepared via a hydrothermal reaction of TiO 2 powders (P25) in KOH solutions and then calcined at various temperatures. The phase structure, crystallite size, morphology, specific surface area, and pore structures of the calcined H-titanate nanowires at various temperatures were characterized with field emission scanning electron microscope, X-ray diffraction, transmission electron microscopy and nitrogen adsorption–desorption isotherms, and their photocatalytic activities were evaluated by photocatalytic oxidation of acetone in air. With increasing calcination temperature, the specific surface area and porosity of the calcined samples steadily decreased. At a calcination temperature range of 400–600 °C, the calcined H-titanate nanowires showed higher photocatalytic activity than P25 powders for photocatalytic oxidation of acetone. Especially, at 500 °C, the calcined H-titanate nanowires showed the highest photocatalytic activity, which exceeded that of P25 by a factor of about 1.8 times. This can be attributed to the synergetic effect of larger specific surface area, higher pore volume and the presence of brookite TiO 2. With further increase in the calcination temperature (700–900 °C), the photocatalytic activity of the samples decreased obviously owing to the growth of TiO 2 crystallites. 相似文献
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