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531.
532.
Fei Xie Bowei Zhao Ying Cui Xiao Ma Xiao Zhang Xiuping Yue 《Frontiers of Environmental Science & Engineering》2021,15(6):121
533.
Level and distribution of DDT in surface soils from Tianjin, China 总被引:28,自引:0,他引:28
Gong ZM Tao S Xu FL Dawson R Liu WX Cui YH Cao J Wang XJ Shen WR Zhang WJ Qing BP Sun R 《Chemosphere》2004,54(8):1247-1253
One hundred and eighty eight surface soil samples were collected from the Tianjin area to study the contamination of DDT and its metabolites. Measurements were taken for p,p'-DDE, p,p'-DDD, p,p'-DDT, o,p'-DDE, o,p'-DDD and o,p'-DDT for all samples. The results indicated that p,p'-DDT and p,p'-DDE were the predominant contaminant compounds in the surface soil samples, with mean concentrations of 27.5 and 18.8 ng g(-1) respectively. No significant differences in DDT concentrations were found between the soils from wastewater treated irrigated areas and other areas, suggesting that wastewater irrigation is not an important source of DDT in the area. However, the spatial distribution of soil DDTs levels in the area did correlate well with early direct application rates of pesticides. In addition, both pH level and organic carbon content are also known factors affecting the level of DDT and its metabolites. Although it was assumed that the use of these chemicals was banned in the early 1980s, the current concentration levels appear to be too high to be mere residuals after 20 years degradation. 相似文献
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535.
为了研究大直径变截面钢管复合桩竖向承载性能,选取鱼山大桥37#桩开展了自平衡试桩试验,并利用数值模拟软件ABAQUS对实际工况进行模拟,在验证了数值模型正确性的基础上对模型施加桩顶竖向荷载,分析了其桩身轴力与钢管应力传递规律,并通过改变变截面位置参数建立不同工况模型,分析了变截面位置对桩基竖向承载性能的影响。研究结果表明:自平衡试桩试验Q-s曲线没有突变较为平缓,经计算该桩单桩承载力为120 056kN,为单桩设计桩顶最大竖向荷载38 212 kN的3.14倍,桩基足够安全;数值模拟结果与实测较为吻合,数值模拟方法以及参数选取得当;变截面位置以上桩身轴力减小较缓,变截面处桩身轴力存在突变,变截面以下桩身轴力减小较快,桩端有端阻力存在,钢管处于弹性变形阶段并未屈服;变截面位置以上桩段越长桩基抵抗竖向变形能力越强,但生产实际中应考虑材料用量以达到最优。 相似文献
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A membrane aerated biofilm reactor is a promising technology for wastewater treatment. In this study, a carbon-membrane aerated biofilm reactor (CMABR) has been developed, to remove carbon organics and nitrogen simultaneously from one reactor. The results showed that CMABR has a high chemical oxygen demand (COD) and nitrogen removal efficiency, as it is operated with a hydraulic retention time (HRT) of 20 h, and it also showed a perfect performance, even if the HRT was shortened to 12 h. In this period, the removal efficiencies of COD, ammonia nitrogen (NH4^+-N), and total nitrogen (TN) reached 86%, 94%, and 84%, respectively. However, the removal efficiencies of NH4^+-N and TN declined rapidly as the HRT was shortened to 8 h. This is because of the excessive growth of biomass on the nonwoven fiber and very high organic loading rate. The fluorescence in situ hybridization (FISH) analysis indicated that the ammonia oxidizing bacteria (AOB) were mainly distributed in the inner layer of the biofilm. The coexistence of AOB and eubacteria in one biofilm can enhance the simultaneous removal of COD and nitrogen. 相似文献
538.
L. P. Zhao W. Cui X. Bai Z. B. Wang J. M. Cheng P. Sun F. R. Zhao 《Russian Journal of Ecology》2017,48(6):551-558
To understand the effects of different disturbance regimes on the bud bank demography in perennial grasslands, we conducted a demographic field investigation of the bud bank in a typical steppe on the Loess Plateau. Grazing significantly decreased and fire significantly increased bud bank density compared to fenced grasslands. For different bud bank types, grazing significantly decreased tiller bud bank density, and fire significantly increased the density of root crown buds and rhizomatous buds in relation to fenced grasslands. Tiller buds accounted for the majority of the total bud banks independently of the disturbance regimes. Our study suggests that the grasslands with a large reserve bud bank may be the most resistant to disturbance, and fencing is an effective grassland management measure to keep a large bud bank in typical steppe on the Loess Plateau. 相似文献
539.
前混合式磨料射流及其切割特性 总被引:3,自引:2,他引:1
本文讨论了前混合式磨料射流发生装置的工作特性,给出了一种新型供料系统,能实现磨料可靠均匀地供给,并可以方便、准确地调节磨料浓度。在此基础上研究了压力,喷嘴直径、靶距等多种参数对前混合式磨料射流切割特性的影响,给出了这些参数的选取范围及评价方法。试验中使用的切割材料有石灰岩、钢材、铝材以及五种强度的混凝土试块。 相似文献
540.
Sincetheinternationalgeophysicalyear(IGY,1957—1958),Antarcticahasbeentheobjectofintensivescientificinvestigationbyteamsofscientists... 相似文献