ABSTRACT
During the 7-9th century, the Tibetan Empire constituted a superpower between the Tang Empire and Abbasid Caliphate: one that played significant roles in geopolitics in Asia during the Early Medieval Period. The factors which led to the rise and rapid decline of this powerful Empire, the only united historical regime on the Tibetan Plateau (TP), remain unclear. Sub-annual scale precipitation and decadal-scale temperature records of the central TP are presented, indicating that the height of this Empire coincided with a two-century long interval of uncharacteristically warm and humid climate. The ameliorated climate enabled the expansion of arable land and increased agricultural production. The close relationship between the precipitation records and historical events implied that the Empire implemented flexible strategies to tackle the effects of climate changes. This has implications for agricultural production in alpine regions including the TP, in the context of current global warming.
Subject(s)
Climate Change , Global Warming , Tibet , Temperature , AgricultureABSTRACT
Porous membranes with uniform nanostructures and selective adsorption can realize molecular filtration with high flux and have gained great attention because of their wide application in water treatment and industrial separation. Herein, a novel hyperbranched poly(ether amine)@poly(vinylidene fluoride) (hPEA@PVDF) porous membrane with oriented nanostructures and selective adsorption of guest molecules was fabricated by applying the combined crystallization and diffusion method for the functionalization of the PVDF membrane. The resulting hPEA@PVDF porous membranes were fully characterized by scanning electron microscopy and X-ray photoelectron spectra. The results indicated that the hPEA@PVDF membrane exhibited oriented open channel structure and high water flux up to 2116 L m-2 h-1, in which the PVDF skeleton was covered by the amphiphilic hPEA layer. The adsorption behavior of hPEA@PVDF porous membranes to 12 hydrophilic dyes including batch adsorption and molecular filtration was systematically investigated. The results revealed that the hPEA@PVDF membrane possessed high adsorption capacity toward erythrosin B (577 µmol g-1) and eosin B (511 µmol g-1), while low adsorption capacity toward calcein (76 µmol g-1) and methylene blue (hardly adsorbed), indicating the selective adsorption behavior toward dyes in aqueous solution. On the basis of this selective property, the hPEA@PVDF could be used to separate the dye mixtures very efficiently through molecular filtration. In addition, the separation efficiency remained 100% after five adsorption-desorption cycles, indicating that it had great potential in practical applications.
ABSTRACT
Polymer membranes with well-controlled and vertically oriented pores are of great importance in the applications for water treatment and tissue engineering. On the basis of two-dimensional solvent freezing, we report environmentally friendly facile fabrication of such membranes from a broad spectrum of polymer resources including poly(vinylidene fluoride), poly(l-lactic acid), polyacrylonitrile, polystyrene, polysulfone and polypropylene. Dimethyl sulfone, diphenyl sulfone, and arachidic acid are selected as green solvents crystallized in the polymer matrices under two-dimensional temperature gradients induced by water at ambient temperature. Parallel Monte Carlo simulations of the lattice polymers demonstrate that the directional process is feasible for each polymer holding suitable interaction with a corresponding solvent. As a typical example of this approach, poly(vinylidene fluoride) membranes exhibit excellent tensile strength, high optical transparence, and outstanding separation performance for the mixtures of yeasts and lactobacilli.
