Sorption and desorption of arsenate and arsenite on calcite     

Helle U. Sø  Dieke Postma  Flemming Larsen 《Geochimica et cosmochimica acta》2008,72(24):5871-5884

The adsorption and desorption of arsenate (As(V)) and arsenite (As(III)) on calcite was investigated in a series of batch experiments in calcite-equilibrated solutions. The solutions covered a broad range of pH, alkalinity, calcium concentration and ionic strength. The initial arsenic concentrations were kept low (<33 μM) to avoid surface precipitation. The results show that little or no arsenite sorbs on calcite within 24 h at an initial As concentration of 0.67 μM. In contrast, arsenate sorbs readily and quickly on calcite. Likewise, desorption of arsenate from calcite is fast and complete within hours, indicating that arsenate is not readily incorporated into the calcite crystal lattice. The degree of arsenate sorption depends on the solution chemistry. Sorption increases with decreasing alkalinity, indicating a competition for sorption sites between arsenate and (bi)carbonate. pH also affects the sorption behavior, likely in response to changes in arsenate speciation or protonation/deprotonation of the adsorbing arsenate ion. Finally, sorption is influenced by the ionic strength, possibly due to electrostatic effects. The sorption of arsenate on calcite was modeled successfully using a surface complexation model comprising strong and weak sites. In the model, the adsorbing arsenate species were and . The model was able to correctly predict the adsorption of arsenate in the wide range of calcite-equilibrated solutions used in the batch experiments and to describe the non-linear shape of the sorption isotherms. Extrapolation of the experimental results to calcite bearing aquifers suggests a large variability in the mobility of arsenic. Under reduced conditions, arsenite, which does not sorb on calcite, will dominate and, hence, As will be highly mobile. In contrast, when conditions are oxidizing, arsenate is the predominant species and, because arsenate adsorbs strongly on calcite, As mobility will be significantly retarded. The estimated retardation factors for arsenate in carbonate aquifers range from 25 to 200.  相似文献   

Place,Learning Progressions,and Progress     

Thomas B. Larsen  John A. Harrington Jr. 《The Journal of geography》2018,117(3):133-136

This commentary response reflects on the thoughts offered by Solem, Mohan, Rawling, and Lambert regarding Larsen and Harrington's article titled “Developing a Learning Progression for Place” (2018).  相似文献   

Advanced Marginality as a comparative research strategy in praxis: the Danish “Grey Belt” in conversation with the French “Red Belt”     

Troels Schultz Larsen 《Urban geography》2018,39(8):1131-1151

Urban Outcast and advanced marginality belongs to a theory culture rarely engaged in comparative urban studies. Here the potential of advanced marginality as a comparative research strategy is explored paying special attention to the concepts of epistemic reflexivity, analogical reasoning and homology. These concepts are applied as the foundation of a comparative research design comprising three necessary and interconnected analytical moments linking reflexivity, theory and empirical analysis. Empirically this strategy and design is confronted in the case of four Danish “Grey belt” housing estates. The empirical confrontation demonstrates that both strategy and design are relevant and productive. Nevertheless, there is a need to question, adapt and extent the six original properties of advanced marginality. Secondly the empirical confrontation demonstrates that both strategy and design can contribute to the current conversations on the comparative in urban studies empirically and conceptually, underscoring the curiosity of its eclipse in these conversations.  相似文献   

The Influence of Thermal Effects on the Wind Speed Profile of the Coastal Marine Boundary Layer   总被引：1，自引：3，他引：1  

Bernhard Lange  Søren Larsen  Jørgen Højstrup  Rebecca Barthelmie 《Boundary-Layer Meteorology》2004,112(3):587-617

The wind speed profile in a coastal marine environment is investigated with observations from the measurement program Rødsand, where meteorological data are collected with a 50 m high mast in the Danish Baltic Sea, about 11 km from the coast. When compared with the standard Monin—Obukhov theory the measured wind speed increase between 10 m and 50 m height is found to be systematically larger than predicted for stable and near-neutral conditions. The data indicate that the deviation is smaller for short (10–20 km) distances to the coast than for larger (>30 km) distances. The theory of the planetary boundary layer with an inversion lid offers a qualitative explanation for these findings. When warm air is advected over colder water, a capping inversion typically develops. The air below is constantly cooled by the water and gradually develops into a well-mixed layer with near-neutral stratification. Typical examples as well as scatter plots of the data are consistent with this explanation. The deviation of measured and predicted wind speed profiles is shown to be correlated with the estimated height and strength of the inversion layer.  相似文献   

Effect of cortisol and urea on flavin monooxygenase activity and expression in rainbow trout,Oncorhynchus mykiss     

El-Alfy A  Larsen B  Schlenk D 《Marine environmental research》2002,54(3-5):275-278

Expression of flavin-containing monooxygenase(s) (FMO) correlates with salinity exposure in certain species of euryhaline fish, such as the rainbow trout, Oncorhynchus mykiss. The mechanism(s) by which salinity regulates FMO is unclear. Adult rainbow trout were infused through the dorsal aorta with either cortisol or urea. At 500 ng/ml, cortisol caused a significant increase in FMO-catalyzed thiourea oxidase activity in gill and liver microsomes. FMOI expression, however, was significantly increased by the high cortisol dose only in gill microsomes. The levels of TMAO and urea were not altered by cortisol. In the liver, urea infusion caused an increase in hepatic FMO activity. FMO expression and activity correlated with elevated tissue urea levels, but TMAO concentrations were not related. These results indicate that FMO expression and activity may be partially controlled by the osmoregulatory/stress hormone. cortisol, and concentrations of the organic osmolyte, urea, in the rainbow trout.  相似文献   

Polycyclic aromatic hydrocarbons in gulf of maine sediments: Distributions and mode of transport     

Peter F. Larsen  Donald F. Gadbois  Anne C. Johnson 《Marine environmental research》1986,18(4)

Sediments from 19 stations in the Gulf of Maine were analyzed for 16 priority polycyclic aromatic hydrocarbon (PAH) compounds in 1983. Thirteen of the compounds were distributed widely with total concentrations ranging from 10–512 ppb (dry wt). These values are an order of magnitude lower than those observed in the coastal zone but higher than those on Georges Bank. PAH concentration was directly related to grain size, total organic carbon (TOC) and distance from source. PAHs appear to be accumulating in Wilkinson and Jordan Basins but not in Georges Basin. Observed PAH distributions support the conclusion that the principal transport mechanism is through the atmosphere with localized augmentation by sediment resuspension and transport from coastal embayments.  相似文献   

Biochemical and histological responses in mussel (Mytilus edulis) exposed to North Sea oil and to a mixture of North Sea oil and alkylphenols     

N. Aarab  C. Minier  S. Lemaire  E. Unruh  P. -D. Hansen  B. K. Larsen  O. -K. Andersen  J. -F. Narbonne 《Marine environmental research》2004,58(2-5):437

Several environmental chemicals are suspected to be responsible for adverse health effects on the reproductive system in various organisms. During this work, environmentally relevant concentrations of North Sea oil were used alone or in combination with alkylphenols and additional PAH to study the effect on vitellogenin-like protein expression and gonadal development in mussels. North Sea oil (0.5 ppm) induced the expression of phospho-proteins in both sexes indicating that some compounds are oestrogen-mimics. This induction was not seen in samples dosed with the mixture but signs of toxic effects were observed in the gonads. Indeed, numerous degenerating ovarian follicles in females and foci, similar to vertebrate melanomacrophage centres, were observed in testes.  相似文献   

A New Video and Digital Camera System for Studies of the Dynamics of Microtopographic Features on Tidal Flats     

Lars Chresten Lund-Hansen  Einer Larsen  Kurt Thomas Jensen  Kim N. Mouritsen  Christian Christiansen  Thorbj  rn Joest Andersen  Gunnild V  lund 《Marine Georesources & Geotechnology》2004,22(1):115-122

A newly developed video and camera system for tidal flat microtopographic studies is presented. It consists of a SONY handy cam placed in an underwater housing mounted on a frame about 70 cm above the sediment surface. A rectangular surface area of 30 ×40 cm is imaged by the camera. The camera records video sequences and/or digital images at predetermined time lapses. The total number of images is about 540, and a similar number of 10-second long video sequences can be recorded. The camera is programmed with a PC before deployment, and the total deployment time depends on the time lapse between recordings. The camera is connected to an external power supply (12 volt), and a halogen projector pointing towards the sediment surface ensures that the system is operable on a 24-hours scale. The system has been tested in the Danish Wadden Sea. It has proved to be a very useful tool in studies of topographic effects of erosion and deposition sequences, and for studies of benthic organisms-sediment interactions. The test site was further equipped with sensors for water and seabed measurements, which proved to be indispensable regarding the interpretation of recorded image time-series.  相似文献   

Geography and Planning: The Need for an Applied Interface   总被引：1，自引：1，他引：0  

James D. Harrison  Robert D. Larsen 《The Professional geographer》1977,29(2):139-147

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Late-magmatic immiscibility during batholith formation: assessment of B isotopes and trace elements in tourmaline from the Land’s End granite,SW England   总被引：3，自引：0，他引：3  

Kristian Drivenes  Rune B. Larsen  Axel Müller  Bjørn E. Sørensen  Michael Wiedenbeck  Morten P. Raanes 《Contributions to Mineralogy and Petrology》2015,169(6):1-27

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