An extinction-survival-type phase transition in the probabilistic cellular automaton p182-q200

We investigate the critical behaviour of a probabilistic mixture of cellular automata (CA) rules 182 and 200 (in Wolfram`s enumeration scheme) by mean-field analysis and Monte Carlo simulations. We found that as we switch off one CA and switch on the other by the variation of the single parameter of the model, the probabilistic CA (PCA) goes through an extinction-survival-type phase transition, and the numerical data indicate that it belongs to the directed percolation universality class of critical behaviour. The PCA displays a characteristic stationary density profile and a slow, diffusive dynamics close to the pure CA 200 point that we discuss briefly. Remarks on an interesting related stochastic lattice gas are addressed in the conclusions.

A geochemical and lead isotopic record from a small pond in a remote equatorial island, Fernando de Noronha, Brazil

A 72 cm long core was collected from Lagoa da Viracao (LV), a small poind in the Fernando de Noronha island, northern Brazil. Sediments from the lower section of the core (20-72 cm depth) contain essentially mineral matter, while in the upper section (0-20 cm depth) mineral matter is mixed with organic matter. Lithogenic conservative elements - Si, Al, Fe, Ti, Co, Cr, Cu, Ba, Ga, Hf, Nb, Ni, Y, V, Zn, Zr and REE - exhibit remarkably constant values throughout the core, with concentrations similar to those of the bedrock. The vertical distribution of soluble elements - Ca, Mg, Na, K, P, Mn and Sr - is also homogeneous, but these elements are systematically depleted in relation to the bedrock. LOI, TOC, Br, Se, Hg and Pb, although showing nearly constant values in the lower section of the core, are significantly enriched in the upper section. The concentration profiles of Br and Se suggest that they may be accounted for by natural processes, related to the slight affinity of these elements for organic matter. On the other hand, the elevated levels of Hg and Pb in recent sediments may be explained by their long-range atmospheric transport and deposition. Furthermore, the isotopic composition of Pb clearly indicates that anthropogenic sources contributed to the Pb burden in the uppermost pond sediments.

Surfactant degradation by a catechol-driven Fenton reaction

The addition of 0.5 mM catechol is shown to accelerate the degradation and mineralization of the anionic surfactant DOWFaX (TM) 2A1 (sodium dodecyldiphenyloxide disulfonate) under conventional Fenton reaction conditions (Fe(II) plus H(2)O(2) at pH 3). The catalytic effect causes a 3-fold increase in the initial rate (up to ca. 20 min) of conversion of the surfactant to oxidation products (apparent first-order rate constants of 0.021 and 0.061 min(-1) in the absence and presence of catechol, respectively). Although this catalytic rate increase persists for a certain amount of time after complete disappearance of catechol itself (ca. 8 min), the reaction rate begins to decline slowly after the initial 20 min towards that observed in the absence of added catechol. Total organic carbon (TOC) measurements of net mineralization and cyclic voltammetric and high performance liquid chromatographic (HPLC) measurements of the initial rate of reaction of catechol and the surfactant provide insight into the role of catechol in promoting the degradation of the surfactant and of degradation products as the eventual inhibitors of the Fenton reaction. (C) 2010 Elsevier B.V. All rights reserved.