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21.
北京官厅水库-永定河水系水体中持久性有机氯农药污染 总被引:45,自引:1,他引:45
报道了官厅水库-永定河水系中持久性有机氯农药的污染水平,研究发现在水体中存在六六六(HCHs), 滴滴涕(DDTs), 七氯等在内的有机氯污染物,在所有13个采样点的水样中,18种有机氯农药的浓度为4.2-96.9 ng/L,其中六六六、滴滴涕的含量分别为0.09-53.5ng/L和nd-46.8 ng/L.对污染来源进行分析,发现除了主要是源于历史上农田中有机氯农药的残留外,近期可能有新的农药污染源进入水环境,如林丹和三氯杀螨醇的使用可能造成了新的有机氯农药污染.和国内主要河流相比,官厅水库-永定河水系中有机氯农药的污染属轻度污染. 相似文献
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目的:基于质量源于设计(QbD)理念对肺复方颗粒醇提工艺进行研究与优化。方法:以固体物质提取量、重楼总皂苷提取量为关键质量属性(CQAs),采用单因素和正交试验筛选潜在关键工艺参数(CPPs),通过Box-Behnken设计建立模型,考察关键工艺参数与关键质量属性之间的交叉作用,并建立肺复方颗粒醇提工艺的设计空间及空间优化。结果:方差分析结果显示所建模型具有统计学意义(P<0.05或P<0.01),各因素与响应值之间可用所建模型进行函数化。通过设计空间优化得到肺复方颗粒最佳醇提工艺为:提取2次,提取时间为2 h/次,加75%乙醇6~6.8倍。结论:基于QbD理念研究和开发的肺复方颗粒醇提工艺稳定、可靠,为其制剂的工艺开发及质量控制提供了参考。 相似文献
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In 1987,a catastrophic fire burned over 1330000 ha in the densely forested area of the Da Hinggan Mountains in the northeastern China.After the fire,intensive management including burned trunk harvesting and coniferous tree planting had been conducted to accelerate forest restoration.To study the long term effect of these activities on forest recovery,we used a simulation modeling approach to study long-term(300 years) forest dynamics under current planting and natural regeneration scenarios.Results indicat... 相似文献
26.
华东南中新生代伸展构造类型及其主要特征 总被引:4,自引:0,他引:4
本文根据华东南地区中新生代伸展构造的发育特点,划分出热隆伸展、裂陷伸展和重力伸展等3种伸展构造类型。其中,热隆伸展构造又可进一步划分为浅层次热隆伸展构造和深层次变质核杂岩构造两种亚类。不同类型伸展构造在构造样式、组合型式、运动方式、同构造期岩浆作用及形成作用特征等方面均存在明显的差异,文中分别对其进行了初步总结。 相似文献
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Brian L. Phillips John G. Thompson Yuehui Xiao R. James Kirkpatrick 《Physics and Chemistry of Minerals》1993,20(5):341-352
Nuclear magnetic resonance spectroscopic data are presented for the cristobalite polymorphs of AlPO4 and SiO2 from RT to 770 K, through their respective α-β transitions. The nuclear magnetic resonance (NMR) data include chemical shifts for 31P, 27Al, and 29Si, 27Al quadrupole coupling parameters, and 31P and 27Al spin-lattice relaxation rates. Also presented are electron diffraction patterns of β-cristobalite AlPO4 that show diffuse scattering similar to that reported previously for SiO2. For the α-phases of both AlPO4 and SiO2, the chemical shifts decrease approximately linearly with increasing temperature from RT to Tc and discontinuously by -2 to -3 ppm from α to β. This result is consistent with a small, continuous increase in the mean T-O-T angle (〈θ〉) of the α-phases with increasing T and an increase of 〈θ〉 by about 4° across the α-β transition for both cristobalite and its AlPO4 analogue. Based on the 29Si chemical shifts, the mean Si-O-Si angle for β-cristobalite is 152.7±1° near Tc. For AlPO4-cristobalite, the 27Al nuclear quadrupole coupling constant (CQ) decreases approximately linearly from 1.2 MHz at RT to 0.94 MHz near Tc (493±10 K). At the α-β transition the 27Al CQ approaches zero, in agreement with the cubic average structure observed by diffraction. The satellite transitions retain a small frequency distribution above the α-β transition from electric field gradients attributed to defects. The short-range cubic symmetry of the Al-site and non-linear Al-O-P angle support a dynamically disordered model of the β-cristobalite structure. Complete averaging of the 27Al quadrupole coupling in the β-phase indicates that the lifetime of any short-range ordered domains must be shorter than about 1 μs. 相似文献
29.
MX-1 tridymite is one of the room-temperature polymorphs of SiO2 tridymite and has an underlying monoclinic structure (Cc) with incommensurate modulations along a * and c * (Hoffmann et al. 1983; Löns and Hoffmann 1987). With increasing temperature up to 500° C, MX-1 is reported to experience at least five structural phase transitions. However, its structures and the relationships to other tridymite polymorphs are unclear. We present here a 29Si MAS NMR study of the room-temperature incommensurate structure of MX-1 and its structural phase transitions up to 540° C. Our results suggest that at room temperature, all the Si sites in MX-1 tridymite are in positions with similiar ∠Si-O-Si of ~150° and are consistent with the presence of two incommensurate modulations proposed by Hoffmann et al. (1983). Simulations of the spectra yield modulation amplitudes of 1.33 and 0.87 ppm, corresponding to 0.009 and 0.006 Å for Si-Si. The maximum atomic displacements along a and b due to the modulations appear to be ~0.01–0.02 Å. The structural phase transitions of MX-1 are significantly different from those of MC tridymite below 220° C. Our high temperature results confirm that MX-1 tridymite transforms to the H5 phase at about 65° C. The most important transition occurs near 110° C, where the H5 phase transforms to a phase yielding a single, narrow NMR peak, indicating the disappearance of the superstructure and possibly the onset of the dynamic averaging. The NMR lineshapes of H5 are consistent with the metrically orthorhombic unit cell and commensurate superstructure of 2a, 2b and 10c proposed by Graetsch and Flörke (1991). The phase present above 110° C is probably similar to the OC phase, but has a mean ∠Si-O-Si of ~152.0° at 113° C, 152.9° at 185° C and 154.1° at 500° C. The transitions at ~160 and 220° C for MX-1 are subtle and probably due to impurity MC. Analysis of the modulations in the OS phase of MC tridymite indicates that their amplitudes are of the order of 0.02 Å, significantly less than the value 0.3 Å proposed by Nukui et al. (1979). 相似文献
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Anthropogenic effect on forest landscape pattern and Cervidae habitats in northeastern China 总被引:1,自引:1,他引:0
Journal of Geographical Sciences - Species abundance and habitat distribution are two important aspects of species conservation studies and both are affected by similar environmental factors.... 相似文献