Effect of simulated solar light on the autocatalytic degradation of nitrobenzene using Fe3+ and hydrogen peroxide

The effect of simulated solar light on nitrobenzene degradation in Fe³⁺/H₂O₂ solutions was investigated under different experimental conditions. Consumption profiles of NBE and H₂O₂ display an autocatalytic kinetic behavior for both dark and photo-assisted degradation experiments. The rates of the initial slow phase that precedes the catalytic phase are significantly enhanced by irradiation, although the effect of simulated solar light on the rates of the fast phase is negligible. The absolute rates of the slow phase increase with the concentrations of Fe³⁺ and H₂O₂, whereas the initial rate of the degree of conversion increase decreases with organic matter loading. The reaction progress was characterized by HPLC, GC–MS, IC, TOC (total organic carbon) and toxicity analyses. The main products detected were 4-nitrophenol, 3-nitrophenol, 2-nitrophenol, 1,3-dinitrobenzene, phenol, oxalic acid, formic acid, NO₂^? and NO₃^?. Product distribution profiles are discussed in connection with TOC and toxicity measurements. The results show that dark treatment is neither capable of lowering the organic content nor capable of reducing the effluent toxicity to acceptable levels. On the other hand, photo-assisted processes induced by simulated solar light can significantly enhance both mineralization and detoxification efficiencies.     

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A nonlinear strategy to reveal seismic precursory signatures in earthquake-related self-potential signals

The time fluctuations of self-potential data, recorded at the monitoring station Acapulco (Mexico) during 1994-1996 in the seismic area of Guerrero-Oaxaca, are analyzed by means of the Fisher Information Measure (FIM), a nonlinear powerful method to investigate complex dynamics in time series. The time evolution of the FIM shows a clear correlation with the largest earthquakes that occurred in the monitored area during the observation period. Seismic precursory patterns in the FIM evolution are also revealed.     

Estimating VAR models for the term structure of interest rates

In this paper we follow the work of Evans and Marshall and propose new approaches for modelling the joint development of macro variables and the returns of government bond yields of several maturities. The models are estimated and compared with other forecasting schemes previously proposed in the literature, especially those relying on univariate, VAR and error correction methods. The models are then used to judge the hypothesis that the information content of macro variables and the term structure of interest rates as a whole help improving forecasting performance. Our main conclusion is quite simple: if one is interested in computing short-term forecasts, then there is no significant improvement in incorporating information other than the one already present in past observations of the yield at hand; however, if one worries about long-term forecasts (which is frequently the case with pension insurance companies), then the information content of macro variables and the term structure can improve forecasting performance.     

Instability conditions due to structural nonlinearities in regenerative chatter

Chatter is an instability condition in machining processes characterized by nonlinear behavior, such as the presence of limit cycles, jump phenomenon, subcritical Hopf and period doubling bifurcations. Although the use of nonlinear techniques has provided a better understanding of chatter, neither a unifying model nor an exact solution has yet been developed due to the intricacy of the problem. This work proposes a weakly nonlinear model with square and cubic terms in both structural stiffness and regenerative terms, to represent self-excited vibrations in machining. An approximate solution is derived by using the method of multiple scales. In addition, a qualitative analysis of the effect of the nonlinear parameters on the stability of the system is performed. The structural cubic term gives a better representation of the nonlinear behavior, whereas the square term represents a distant attractor in the stability chart. Instability due to subcritical Hopf bifurcations is established in terms of the eigenvalues of the model in normal form. An important contribution of this analysis is the representation of hysteresis in terms of new lobes within the conventional stability limits, useful in restoring stability. This analysis leads to a further understanding of the nonlinear behavior of regenerative chatter.     

How people react to a deadline: time distribution of conference registrations and fee payments

In this paper we present two simple mathematical models to describe human behavior in reaction to deadlines. When a real commitment (e.g. money) is involved, as in the case of a payment deadline, the expected reaction is to postpone it as close as possible to the deadline to minimize the risk of loosing the value. For low risk commitments this tendency is still present but expected to be looser. In order to test these predictions in a quantitative way, we performed data analysis for the total number of registrations and fee payments vs. time for the recent scientific conference “Statphys 23”, comparing it with the data of another conference in order to recover universal features. Two related models respectively for registrations (weak engagement) and fee payment (strong engagement) are then introduced which are able to explain in a simple way both behaviors, and which show an excellent agreement with real data.      

Characterization and Structural Insights of the Reaction Products by Direct Leaching of the Noble Metals Au,Pd and Cu with N,N′-Dimethyl-piperazine-2,3-dithione/I2 Mixtures

In the context of new efficient and safe leaching agents for noble metals, this paper describes the capability of the Me₂pipdt/I₂ mixture (where Me₂pipdt = N,N′-dimethyl-piperazine-2,3-dithione) in organic solutions to quantitatively dissolve Au, Pd, and Cu metal powders in mild conditions (room temperature and pressure) and short times (within 1 h in the reported conditions). A focus on the structural insights of the obtained coordination compounds is shown, namely [AuI₂(Me₂pipdt)]I₃ (1), [Pd(Me₂pipdt)₂]I₂ (2a) and [Cu(Me₂pipdt)₂]I₃ (3), where the metals are found, respectively, in 3+, 2+ and 1+ oxidation states, and of [Cu(Me₂pipdt)₂]BF₄ (4) and [Cu(Me₂dazdt)₂]I₃ (5) (Me₂dazdt = N,N′-dimethyl-perhydrodizepine-2,3-dithione) compared with 3. Au(III) and Pd(II) (d⁸ configuration) form square–planar complexes, whereas Cu(I) (d¹⁰) forms tetrahedral complexes. Density functional theory calculations performed on the cationic species of 1–5 help to highlight the nature of the bonding in the different complexes. Finally, the valorization of the noble metals-rich leachates is assessed. Specifically, gold metal is quantitatively recovered from the solution besides the ligands, showing the potential of these systems to promote metal recycling processes.     

Performance of encryption schemes in chaotic optical communication: A multifractal approach

From the estimation of the Hurst exponent and the multifractality degree we discriminate the security levels of two typical encoding schemes usually applied in chaos-based communication systems. We also analyze the effects that the sampling period and the message amplitude have on the goodness of these techniques. We compare our results with those obtained by considering an information theory approach [O.A. Rosso, R. Vicente, C.R. Mirasso, Phys. Lett. A 372 (2007) 1018]. The Hurst exponent seems to be a sensitive and powerful tool for discriminating the presence of a message embedded in a chaotic carrier.