Sediments of Galician Continental Shelf: elemental composition and accumulation rates

A geochemical study was carried out in sediment cores along the Galician Continental Shelf and at the mouth of the Vigo and Pontevedra Rias. Multielemental analysis was performed by Instrumental Neutron Activation Analysis and Energy Dispersive X-ray Fluorescence Spectrometry. Sedimentological and geochemical characterisation (lithogenic, biogenic and anthropogenic elements) indicates different supplies for the fine-grained, deposited sediments. No significant heavy metal enrichment could be detected in coastal sediments despite the current sediment contamination in adjacent Rias. Variations in the accumulation rates suggest different factors controlling the sedimentation processes including sediment sources and geomorphology.     

Enhancing discretized formulations: the knapsack reformulation and the star reformulation

Discretized formulations have proved to be useful for modeling combinatorial optimizations. The main focus of this work is on how to strengthen the linear programming relaxation of a given discretized formulation. More precisely, we will study and strengthen subproblems that arise in these formulations. In one case we will focus on the so-called knapsack reformulation which is based on viewing these models as the intersection of two polyhedra, one of them being a specialized knapsack problem. We will show that strong inequalities used in previous works are a special case of inequalities implied by the knapsack formulation. In the second case we will analyze a star-like subproblem and will provide an extended formulation for this problem as well as a set of inequalities on the original space, implied by the inequalities of the extended formulation. We will use a generalization of the Degree Constrained Spanning Tree problem as a setting for this study. In the present work, besides contextualizing these enhancements in terms of discretized models presented in previous works, we also compare and combine together them, for the first time.     

Determination of the REE in environmental samples near a coal power station based on k0-standardized NAA with counting in a LEPD

The Ree contents in lichens and plants determined by k₀-standardized NAA with LEPD and HPGe are compared. We obtain similar values for Sm, Eu, Tb and Yb and better values for Ce and Nd by using LEPD Besides, LEPD allows the determination of Gd, Tm and Lu. The study of REE concentrations in the neighbourhood of two coal power stations show that: i) in the station under construction (C. T. Pego), the index of accumulation of each REE by the lichens and olive tree leaves is identical in two sampling loci where the soils have different REE concentration; and ii) in the station in operation (C. T. Sines), the REE contents in the soils is identical and the differences observed in the lichens and wild terrestrial plants are most probably due to the fly-ashes emmission from the station.     

Natural extensions and profinite completions of algebras

This paper investigates profinite completions of residually finite algebras, drawing on ideas from the theory of natural dualities. Given a class \({\mathcal{A} = \mathbb{ISP}(\mathcal{M})}\), where \({\mathcal{M}}\) is a set, not necessarily finite, of finite algebras, it is shown that each \({{\bf A} \in \mathcal{A}}\) embeds as a topologically dense subalgebra of a topological algebra \({n_{\mathcal{A}}({\bf A})}\) (its natural extension), and that \({n_{\mathcal{A}}({\bf A})}\) is isomorphic, topologically and algebraically, to the profinite completion of A. In addition it is shown how the natural extension may be concretely described as a certain family of relation-preserving maps; in the special case that \({\mathcal{M}}\) is finite and \({\mathcal{A}}\) possesses a single-sorted or multisorted natural duality, the relations to be preserved can be taken to be those belonging to a dualising set. For an algebra belonging to a finitely generated variety of lattice-based algebras, it is known that the profinite completion coincides with the canonical extension. In this situation the natural extension provides a new concrete realisation of the canonical extension, generalising the well-known representation of the canonical extension of a bounded distributive lattice as the lattice of up-sets of the underlying ordered set of its Priestley dual. The paper concludes with a survey of classes of algebras to which the main theorems do, and do not, apply.     

Modeling hop-constrained and diameter-constrained minimum spanning tree problems as Steiner tree problems over layered graphs

The hop-constrained minimum spanning tree problem (HMSTP) is an NP-hard problem arising in the design of centralized telecommunication networks with quality of service constraints. We show that the HMSTP is equivalent to a Steiner tree problem (STP) in an appropriate layered graph. We prove that the directed cut model for the STP defined in the layered graph, dominates the best previously known models for the HMSTP. We also show that the Steiner directed cuts in the extended layered graph space can be viewed as being a stronger version of some previously known HMSTP cuts in the original design space. Moreover, we show that these strengthened cuts can be combined and projected into new families of cuts, including facet defining ones, in the original design space. We also adapt the proposed approach to the diameter-constrained minimum spanning tree problem (DMSTP). Computational results with a branch-and-cut algorithm show that the proposed method is significantly better than previously known methods on both problems.     

Super-efficiency and stability intervals in additive DEA

This study addresses the problem of finding the range of efficiency for each Decision Making Unit (DMU) considering uncertain data. Uncertainty in the DMU coefficients in each factor (input or output) is captured through interval coefficients (ie, these are uncertain but bounded). A two-phase additive Data Envelopment Analysis model for performance evaluation is used, which is adapted to include the concept of super-efficiency to provide a robustness analysis of the DMUs in face of uncertain information, assessing whether each DMU is surely efficient, potentially efficient, or surely inefficient for the uncertainty intervals specified. Another contribution is to present how a maximal stability hyper-rectangle can be computed for each DMU such that its efficiency status does not change when the coefficients vary within that interval.     

A real options approach to labour shifts planning under different service level targets

Firms that experience uncertainty in demand as well as challenging service levels face, among other things, the problem of managing employee shift numbers. Decisions regarding shift numbers often involve significant expansions or reductions in capacity, in response to changes in demand. In this paper, we quantify the impact of treating shifts in workforce expansion as investments, while considering required service level improvements. The decision to increase shifts, whether by employing temporary workers or hiring permanent employees, is one that involves significant risks. Traditional theories typically consider reversible investments, and thus do not capture the idiosyncrasies involved in shift management, in which costs are not fully reversible. In our study, by using real options theory, we quantify managers’ ability to consider this irreversibility, aiming to enable them to make shift decisions under conditions of uncertainty with the maximum level of flexibility. Our model aims to help managers make more accurate decisions with regard to shift expansion under service level targets, and to defer commitment until future uncertainties can be at least partially resolved. Overall, our investigation contributes to studies on the time required to introduce labour shift changes, while keeping the value of service level improvements in mind.     

Superparamagnetic Ni:SiO<Subscript>2</Subscript>–C nanocomposites films synthesized by a polymeric precursor method

In this work, we report on the magnetic properties of nickel nanoparticles (NP) in a SiO₂–C thin film matrix, prepared by a polymeric precursor method, with Ni content x in the 0–10 wt% range. Microstructural analyses of the films showed that the Ni NP are homogenously distributed in the SiO₂–C matrix and have spherical shape with average diameter of ~10 nm. The magnetic properties reveal features of superparamagnetism with blocking temperatures T _B ~ 10 K. The average diameter of the Ni NP, estimated from magnetization measurements, was found to be ~4 nm for the x = 3 wt% Ni sample, in excellent agreement with X-ray diffraction data. M versus H hysteresis loops indicated that the Ni NP are free from a surrounding oxide layer. We have also observed that coercivity (H _C) develops appreciably below T _B, and follows the H _C ∝ [1 – (T/T _B)^0.5] relationship, a feature expected for randomly oriented and non-interacting nanoparticles. The extrapolation of H _C to 0 K indicates that coercivity decreases with increasing x, suggesting that dipolar interactions may be relevant in films with x > 3 wt% Ni.     

Bioadsorption and ion exchange of Cr3+ and Pb2+ solutions with algae

Heavy metals can be removed from effluents and recovered using physico-chemical mechanisms as biosorption processes. In this work “Arribada” seaweed biomass was employed to assess its biosorptive capacity for the chromium (Cr³⁺) and lead (Pb²⁺) cations that usually are present in waste waters of plating industries. Equilibrium and kinetic experiments were conducted in a mixed reactor on a batch basis. Biosorption equilibrium and fluid-solid mass transfer constants data were analyzed through the concept of ion exchange sorption isotherm. The respective equilibrium exchange constants (K _eqCr=173.42, K _eqPb=58.86) and volumetric mass transfer coefficients ((k _mCr a)′=1.13×10⁻³ s⁻¹, (k _mPb a)′=0.89×10⁻³ s⁻¹) were employed for the dynamic analysis of Cr and Pb sorption in a fixed-bed flow-through sorption column. The breakthrough curves obtained for both metals were compared with the predicted values by the heterogeneous model (K _eqCr=171.29, K _eqPb=60.14; k _mCr a=7.81×10⁻² s⁻¹, k _mPb a=2.43×10⁻² s⁻¹), taking into account the mass transfer process. The results suggest that these algae may be employed in a metal removal/recovery process at low cost. An erratum to this article can be found at     

Oil Production Towards Biofuel from Autotrophic Microalgae Semicontinuous Cultivations Monitorized by Flow Cytometry

Two microalgae species (Scenedesmus obliquus and Neochloris oleoabundans) were cultivated in closed sleeve photobioreactors in order to select the best oil producer for further large-scale open raceway pond cultivations, aiming at biofuel production. Scenedesmus obliquus reached a higher maximum biomass concentration (1.41 g l⁻¹) with a lower lipid content (12.8% w/w), as compared to N. oleoabundans [maximum biomass concentration of 0.92 g l⁻¹ with 16.5% (w/w) lipid content]. Both microalgae showed adequate fatty acid composition and iodine values as substitutes for diesel fuel. Based on these results, N. oleoabundans was selected for further open raceway pond cultivations. Under these conditions, N. oleoabundans reached a maximum biomass concentration of 2.8 g l⁻¹ with 11% (w/w) of lipid content. A high correlation between the Nile Red fluorescence intensity measured by flow cytometry and total lipid content assayed by the traditional gravimetric lipid analysis was found for both microalgae, making this method a suitable and quick technique for the screening of microalgae strains for lipid production and optimization of biofuel production bioprocesses. Medium growth optimization for enhancement of microalgal oil production is now in progress.