156 research outputs found
Fast Simulation of Multifactor Portfolio Credit Risk
No full textThis paper develops rare-event simulation methods for the estimation of portfolio credit risk-the risk of losses to a portfolio resulting from defaults of assets in the portfolio. Portfolio credit risk is measured through probabilities of large losses, which are typically due to defaults of many obligors (sources of credit risk) to which a portfolio is exposed. An essential element of a portfolio view of credit risk is a model of dependence between these sources of credit risk: large losses occur rarely and are most likely to result from systematic risk factors that affect multiple obligors. As a consequence, estimating portfolio credit risk poses a challenge both because of the rare-event property of large losses and the dependence between defaults. To address this problem, we develop an importance sampling technique within the widely used Gaussian copula model of dependence. We focus on difficulties arising in multifactor models-that is, models in which multiple factors may be common to multiple obligors, resulting in complex dependence between defaults. Our importance sampling procedure shifts the mean of the common factor to increase the frequency of large losses. In multifactor models, different combinations of factor outcomes and defaults can produce large losses, so our method combines multiple importance sampling distributions, each associated with a shift in the mean of common factors. We characterize "optimal" mean shifts. Finding these points is both a combinatorial problem and a convex optimization problem, so we address computational aspects of this step as well. We establish asymptotic optimality results for our method, showing that-unlike standard simulation-it remains efficient as the event of interest becomes rarer.The ¯rst two authors dedicate this work to the memory of Perwez Shahabuddin, who died after
the original submission of the paper. They thank the reviewers for their careful reading of the
paper and many constructive comments. The second author thanks Dr. Kyungsik Lee for a
discussion on the subset sum problem. This research was partially supported by National Science
Foundation grant DMI 03-00044.
Calixarenes: A Versatile Source for the Recovery of Reactive Blue-19 Dye from Industrial Wastewater
No full textThe present work demonstrates the comparative extraction efficiency of p-tert-butylcalix[4]arene (1) and its derivatives (2-4) toward a series of some selected reactive dyes such as Reactive Black-5 (RB-5), Reactive Blue-19 (RB-19) and Reactive Red-45 (RR-45). The study elaborates effectiveness of different anchoring groups present on the periphery of calix[4]arene platform and highlights the importance of preorganization concerning the application of supramolecular chemistry in separation science and technology. It has been observed that compound 4 shows good extraction efficiency toward RB-19 among the selected reactive dyes. Enhanced extraction efficiency was observed with the addition of NaCl at pH 7. The proposed extraction mechanism through inclusion complexation was confirmed by log-log plot analysis, which shows 1:1 complexation between 4 and RB-19. The solvatochromic response of 4 in various solvents of different polarities showed good response in methanol. The log K of complex was found as 5.2. The complex formation between 4 and RB-19 has also been confirmed by FT-IR spectroscopy. The recovery of 4 and RB-19 dye was achieved at pH 8 that signifies the reusability of 4 again and again
Removal of Parathion from Aqueous Media Through p-tert-Butylcalix[4]arene Based Modified Silica
No full textThis study explores, adsorption efficiency of p-tert-butylcalix[4]arene based modified silica to remove parathion from aqueous environment. The adsorption parameters, i.e. pH, concentration of pesticide solution, contact time and adsorbent dosage were optimized, as 10, 1 mg L-1, 40 min and 0.04 g, respectively. Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models were used to evaluate the adsorption mechanism. Adsorption constants values of these models suggest that the adsorption of parathion is favorable and were found to be best fit with Freundlich isotherm. From the kinetic study it can be predicted that adsorption of parathion follows Ho and McKay model (pseudo-second order). Thermodynamic parameters, enthalpy (ΔH), entropy (ΔS) and Gibbs free energy (ΔG) have also been evaluated and were found as -132.25, 0.45, -4.14 Jmol-1, respectively
Synthesis and complexation studies of 1,3-dialkylated p-tert-butylcalix[4]arene telomers
No full textA series of cone structured lower rim dialkylated p-tert-butylcalix[4]arene (2a-5a) and their relative telomers (2b-5b) have been synthesized to estimate extraction of alkali and transition metal cations from an aqueous phase to an organic phase (dichloromethane). The complexation studies were made by using a liquid-liquid extraction procedure. It has been deduced from the observations that the monomers 2a, 3a and 5a are selective for Hg2+, whereas 4a is not selective but showed higher affinity towards transition metals than the alkali metal cations. The telomer 2b showed more affinity toward Cu2+, Hg2+ and Pb2+ but the telomer 3b is effective for Hg2+ and Pb2+. The telomers 4b and 5b are not selective but they are efficient extractants for the selected transition metal cations. The results indicate that the ligand groups circularly arranged on the lower rim of calixarene cavity form novel binding sites for transition metal cations
Selective online solid-phase extraction of copper using p-morpholino-methylcalix[4]arene appended silica-based column
No full textThis study deals with investigation of selectivity of p-morpholino-methylcalix[4]arene appended silica from a group of metal ions having similar charge and radii. The values of distribution ratio revealed that modified silica possess higher selectivity for Cu2+. In addition, relative selectivity coefficients of modified silica were found to be 24.1, 13.8, 8.6 and 4.01 for Cu2+/Ni2+, Cu2+/Co2+, Cu2+/Cd2+, Cu2+/Pb2+, respectively. The maximum adsorption capacity was 1.5 mmolg(-1). The reusability data suggested no any loss of adsorption capacity of this material up to 10 cycles. The developed material was also applied for determination of copper in lake water samples with satisfactory results.National Centre of Excellence in Analytical Chemistry, University of Sindh, Jamshoro; Scientific and Technological Research Council of Turkey (TUBITAK) under 2216program in Selcuk University, Faculty of Science, Department of Chemistry, Konya, TurkeyThe authors thank the National Centre of Excellence in Analytical Chemistry, University of Sindh, Jamshoro and Scientific and Technological Research Council of Turkey (TUBITAK) for financial support to Dr. Fakhar-un-Nisa Memon, Post-doc. fellow under 2216program in Selcuk University, Faculty of Science, Department of Chemistry, Konya, Turkey
Synthesis and application of calixarene-based functional material for arsenic removal from water
No full textRough time in paradise: Claims, blames and memory making around some protected areas in Kenya
Get PDFThe author uses historical examples of forced relocation, or alleged relocation, from protected areas in Kenya to challenge some of the points made by Rangarajan and Shahabuddin in their 2006 Conservation and Society article
CALIXARENE BASED SORBENTS FOR THE EXTRACTION OF IONS AND NEUTRAL MOLECULES
No full textSorption of ions as well as organic molecules from aqueous media by calixarene based materials has been a widely developing area in material science and technology since last few decades. Mostly, it is achieved by the immobilization (physically or chemically) of modified calixarenes onto various supports such as polymers, silica, and resins. The calixarene macrocycles due to their bowl-shaped geometry are indeed used as hosts allowing organic and inorganic guests to coordinate/sorb onto their cavity. The possibility of designing versatile organic, coordination and organometallic architectures at the lower (narrow) and upper (wide) rims of the calixarenes are also very appealing for extending the cavity, or to take advantage of the proximity to promote substituent interactions. Thus, novel calixarene derivatives are continue to being synthesized and appended in polymeric materials in order to obtain regenerable resins for the recovery of various elements (metals/metalloids/non-metals) and neutral molecules. The calixarene based sorbents are generally applied in various fields such as catalyst recovery, power plant, agriculture, metals finishing, microelectronics, biotechnology processes, rare earths speciation, and potable water. Besides this, they find applications in the area of selective ion extractions, receptors, catalysis, optical devices, chemical sensor devices, the stationary phase for capillary chromatography, ion transport membranes, biomimetics, and luminescence probes etc. This survey is focused to have an overview of calixarene based sorbents for the extraction of ions and neutral molecules. The article does not, however, attempt to cover all of the different approaches to extraction processes
An Excellent Approach Towards the Designing of a Schiff-Base Type Oligocalix[4]Arene, Selective for the Toxic Metal Ions
No full textA new Schiff-base type oligocalix[4]arene containing two nitrile ligating sites at narrow rim was synthesized via a simple condensation reaction through a wide rim of the calixarene moiety. The extraction ability of the oligomer with selected alkali and transition metals is compared with its monomeric analogous. It has been observed that the oligomer 6 is not selective as compared to its analogae 2 and 3, but shows a remarkable high affinity towards toxic metal cations such as Cu2+, Hg2+ and Pb2+
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