Carbon source/sink function of a subtropical, eutrophic lake determined from an overall mass balance and a gas exchange and carbon burial balance

Although studies on carbon burial in lake sediments have shown that lakes are disproportionately important carbon sinks, many studies on gaseous carbon exchange across the water-air interface have demonstrated that lakes are supersaturated with CO2 and CH4 causing a net release of CO2 and CH4 to the atmosphere. In order to more accurately estimate the net carbon source/sink function of lake ecosystems, a more comprehensive carbon budget is needed, especially for gaseous carbon exchange across the water-air interface. Using two methods, overall mass balance and gas exchange and carbon burial balance, we assessed the carbon source/sink function of Lake Donghu, a subtropical, eutrophic take, from April 2003 to March 2004. With the overall mass balance calculations, total carbon input was 14 905 t, total carbon output was 4950 1, and net carbon budget was +9955 t, suggesting that Lake Donghu was a great carbon sink. For the gas exchange and carbon burial balance, gaseous carbon (CO2 and CH4) emission across the water-air interface totaled 752 t while carbon burial in the lake sediment was 9477 t. The ratio of carbon emission into the atmosphere to carbon burial into the sediment was only 0.08. This low ratio indicates that Lake Donghu is a great carbon sink. Results showed good agreement between the two methods with both showing Lake Donghu to be a great carbon sink. This results from the high primary production of Lake Donghu, substantive allochthonous carbon inputs and intensive anthropogenic activity. Gaseous carbon emission accounted for about 15% of the total carbon output, indicating that the total output would be underestimated without including gaseous carbon exchange. (C) 2007 Elsevier Ltd. All rights reserved.

An improved method for the extraction of low molecular weight organic acids in variable charge soils

Due to specific adsorption to variable charge soils, low molecular weight organic acids (LMWOAs) have not been sufficiently extracted, even if common extractants, such as water and 0.1 M sodium hydroxide (NaOH), were employed. In this work, the method for extracting LMWOAs in soils with 0.1 M NaOH was improved for variable charge soils; e.g. 1.0 M potassium fluoride (KF) with pH 4.0 was applied as an extractant jointed with 0.1 M NaOH based on its stronger ability to change the electrochemical properties of variable charge soils by specific adsorption. With the proposed method, the recoveries of oxalic, tartaric, malic, citric and fumaric acids were increased from 83 4, 93 1, 22 2, 63 +/- 5 and 84 +/- 3% to 98 +/- 2, 100 +/- 2, 85 +/- 2, 90 +/- 2 and 89 +/- 2%, respectively, compared with NaOH alone. Simultaneously, the LMWOAs in Agri-Udic Ferrosol with field moisture were measured with a satisfactory result.

Hydrodynamics in an expanded bed of large size ion-exchange resin, and natural product adsorption

Successful applications of expanded bed adsorption (EBA) technology have been widely reported in the literature for protein purification. Little has been reported on the recovery of natural products and active components of Chinese herbal preparations using EBA technology. In this study, the hydrodynamic behavior in an expanded bed of cation resin, 001 x 7 Styrene-DVB, was investigated. Ephedrine hydrochloride (EH) was used as a model natural product to test the dynamic binding capacity (DBC) in the expanded bed. EBA of EH directly from a feedstock containing powdered herbs has also been investigated. These particles are different from commercially available expanded bed adsorbents by virtue of their large size (20S to 1030 gm). When the adsorbent bed is expanded to approximately 1.3 to 1.5 times its settled bed height, the axial liquid-phase dispersion coefficient was found to be of the order 10(-5) m(2) s(-1), which falls into the range 1.0 x 10(-6) to 1.0 X 10(-5) m(2) s(-1) observed previously in protein purification. Because of the favorable column efficiency (low axial dispersion coefficient), the recovery yield and purification factor values of EH directly from a feedstock reached 86.5% and 18, respectively. The results suggest that EBA technology holds promise for the recovery of natural products and active components of Chinese herbal preparations.

First-principles study of UC2 and U2C3

The electronic structure and mechanical properties Of UC2 and U2C3 have been systematically investigated using first-principles calculations by the projector-augmented-wave (PAW) method. Furthermore, in order to describe precisely the strong on-site Coulomb repulsion among the localized U 5f electrons, we adopt the generalized gradient approximation +U formalisms for the exchange-correlation term. We show that our calculated structural parameters and electronic properties for UC2 and U2C3 are in good agreement with the experimental data by choosing an appropriate Hubbard U = 3 eV. As for the chemical bonding nature, the contour plot of charge density and total density of states suggest that UC2 and U2C3 are metallic mainly contributed by the 5f electrons, mixed with significant covalent component resulted from the strong C-C bonds. The present results also illustrate that the metal-carbon (U-C) bonding and the carbon-carbon covalent bonding in U2C3 are somewhat weaker than those in UC2, leading to the weaker thermodynamic stability at high temperature as observed by experiments.

Theoretical analysis and experimental study of the space-charge-screening effect in uni-traveling-carrier photodiode

A new optimized structure of an UTC (uni-traveling-carrier) photodiode is developed and epitaxied by metal-organic chemical vapor deposition. We fabricated a UTC photodiode of 30 mu m in diameter. Theoretical simulation based on drift-diffusion model was used to analyze the space-charge-screening effect in UTC photodiode primarily in two aspects: the carrier concentrations and the space electric field. The simulation results were generally in agreement with the experimental data.

Electrical control of dynamic spin splitting induced by exchange interaction as revealed by time-resolved Kerr rotation in a degenerate spin-polarized electron gas

Manipulation of the spin degree of freedom has been demonstrated in a spin-polarized electron plasma in a heterostructure by using exchange-interaction-induced dynamic spin splitting rather than the Rashba and Dresselhaus types, as revealed by time-resolved Kerr rotation. The measured spin splitting increases from 0.256 meV to 0.559 meV as the bias varies from -0.3 V to -0.6 V. Both the sign switch of the Kerr signal and the phase reversal of Larmor precessions have been observed with biases, which all fit into the framework of exchange-interaction-induced spin splitting. The electrical control of it may provide a new effective scheme for manipulating spin-selected transport in spin FET-like devices. Copyright (C) EPLA, 2008.

Anisotropic exchange splitting of excitons in (001)GaAs/Al0.3Ga0.7As superlattice studied by reflectance difference spectroscopy

Anisotropic exchange splitting (AES) is induced by the joint effects of the electron-hole exchange interaction and the symmetry reduction in quantum wells and quantum dots. A model has been developed to quantitatively obtain the electron-hole exchange energy and the hole-mixing energy of quantum wells and superlattices. In this model, the AES and the degree of polarization can both be obtained from the reflectance difference spectroscopy. Thus the electron-hole exchange energy and the hole-mixing energy can be completely separated and quantitatively deduced. By using this model, a (001)5 nm GaAs/7 nm Al0.3Ga0.7As superlattice sample subjected to [110] uniaxial strains has been investigated in detail. The n=1 heavy-hole (1H1E) exciton can be analyzed by this model. We find that the AES of quantum wells can be linearly tuned by the [110] uniaxial strains. The small uniaxial strains can only influence the hole-mixing interaction of quantum wells, but have almost no contribution to the electron-hole exchange interaction. (c) 2008 American Institute of Physics.

Electric-field tuning s-d exchange interaction in quantum dots

We investigate theoretically the electron-hole pair states in CdTe quantum dot (QD) containing a single Mn2+ ion by the magneto-optical spectrum tuned by the electric field. It is shown that the electric field does not only tune the spin splitting via the sp-d exchange interaction but also affect significantly the anticrossing behavior in the photoluminescence spectrum. This anticrossing is caused by the s-d exchange interaction and/or the hole mixing effect, which depends sensitively on the shape of the QD. (C) 2008 American Institute of Physics.

Spin states and persistent currents in a quantum ring with an embedded magnetic impurity

Spin states and persistent currents are investigated theoretically in a quantum ring with an embedded magnetic ion under a uniform magnetic field including the spin-orbit interactions. The magnetic impurity acts as a spin-dependent delta-potential for electrons and results in gaps in the energy spectrum, consequently suppressing the oscillation of the persistent currents. The competition between the Zeeman splittings and the s-d exchange interaction leads to a transition of the electron ground state in the ring. The interplay between the periodic potential induced by the Rashba and Dresselhaus spin-orbit interactions and the delta-potential induced by the magnetic impurity leads to significant variation in the energy spectrum, charge density distribution, and persistent currents of electrons in the ring.

Reduced dynamics with renormalization in solid-state charge qubit measurement

Quantum measurement will inevitably cause backaction on the measured system, resulting in the well-known dephasing and relaxation. In this paper, in the context of solid-state qubit measurement by a mesoscopic detector, we show that an alternative backaction known as renormalization is important under some circumstances. This effect is largely overlooked in the theory of quantum measurement.