Adsorption of the hydrophobic organic pollutant hexachlorobenzene to phyllosilicate minerals.

Hexachlorobenzene (HCB), a representative of hydrophobic organic chemicals (HOC), belongs to the group of persistent organic pollutants (POPs) that can have harmful effects on humans and other biota. Sorption processes in soils and sediments largely determine the fate of HCB and the risks arising from the compound in the environment. In this context, especially HOC-organic matter interactions are intensively studied, whereas knowledge of HOC adsorption to mineral phases (e.g., clay minerals) is comparatively limited. In this work, we performed batch adsorption experiments of HCB on a set of twelve phyllosilicate mineral sorbents that comprised several smectites, kaolinite, hectorite, chlorite, vermiculite, and illite. The effect of charge and size of exchangeable cations on HCB adsorption was studied using the source clay montmorillonite STx-1b after treatment with nine types of alkali (M+: Li, K, Na, Rb, Cs) and alkaline earth metal cations (M2+: Mg, Ca, Sr, Ba). Molecular modeling simulations based on density functional theory (DFT) calculations to reveal the effect of different cations on the adsorption energy in a selected HCB-clay mineral system accompanied this study. Results for HCB adsorption to minerals showed a large variation of solid-liquid adsorption constants Kd over four orders of magnitude (log Kd 0.9-3.3). Experiments with cation-modified montmorillonite resulted in increasing HCB adsorption with decreasing hydrated radii of exchangeable cations (log Kd 1.3-3.8 for M+ and 1.3-1.4 for M2+). DFT calculations predicted (gas phase) adsorption energies (- 76 to - 24 kJ mol-1 for M+ and - 96 to - 71 kJ mol-1 for M2+) showing a good correlation with Kd values for M2+-modified montmorillonite, whereas a discrepancy was observed for M+-modified montmorillonite. Supported by further calculations, this indicated that the solvent effect plays a relevant role in the adsorption process. Our results provide insight into the influence of minerals on HOC adsorption using HCB as an example and support the relevance of minerals for the environmental fate of HOCs such as for long-term source/sink phenomena in soils and sediments.

Sphere Encapsulated Monte Carlo: Obtaining Minimum Energy Configurations of Large Aromatic Systems.

We introduce a simple global optimization approach that is able to find minimum energy configurations of clusters containing aromatic molecules. The translational and rotational perturbations required in Monte Carlo-based methods often lead to unrealistic configurations within which two or more molecular rings intersect, causing many of the computational steps to be rejected and the optimization process to be inefficient. Here we develop a modification of the basin-hopping global optimization procedure tailored to tackle problems with intersecting molecular rings. Termed the Sphere Encapsulated Monte Carlo (SEMC) method, this method introduces sphere-based rearrangement and minimization steps at each iteration, and its performance is shown through the exploration of potential energy landscapes of polycyclic aromatic hydrocarbon (PAH) clusters, systems of interest in combustion and astrophysics research. The SEMC method provides clusters that are accurate to 5% mean difference of the minimum energy at a 10-fold speed up compared to previous work using advanced molecular dynamics simulations. Importantly, the SEMC method captures key structural characteristics and molecular size partitioning trends as measured by the molecular radial distances and coordination numbers. The advantages of the SEMC method are further highlighted in its application to previously unstudied heterogeneous PAH clusters.

Swarming behavior of gradient-responsive colloids with chemical signaling.

The article describes swarm dynamics of a system composed of colloidal particles that release chemical signals to navigate their peers toward the location of a static point target in two dimensions. The time evolution of the system is calculated by employing a combination of Brownian dynamics method for the particle motion and the diffusion problem for spatial transport of chemical signals, coupled via diffusiophoresis. A parametric study is performed with respect to crucial model parameters that control the diffusivity of the particles and the chemical signals. This includes the initial concentration of chemical signals carried by the particles, the chemical signal release rate, the diffusion coefficient of the chemical signals, the diffusiophoretic mobility of the particles, and the topological complexity of the surrounding environment. Three measures are used to evaluate the performance of the system: the target arrival time, the target localization success rate, and the target residence time. Since the particle motion is determined by the local concentration gradients of the chemical signals, parameter values that result in steep and durable concentration gradients lead to the best performance in navigating a swarm of colloidal particles toward the target. However, the results show that a trade-off principle exists, as it is not always possible to improve all the performance criteria simultaneously. For a topologically complex environment, the particles often become trapped in areas where the chemical signals accumulate, leading to a significant decrease in the localization success rate.

Swarming behavior of gradient-responsive Brownian particles in a porous medium.

Active targeting by Brownian particles in a fluid-filled porous environment is investigated by computer simulation. The random motion of the particles is enhanced by diffusiophoresis with respect to concentration gradients of chemical signals released by the particles in the proximity of a target. The mathematical model, based on a combination of the Brownian dynamics method and a diffusion problem is formulated in terms of key parameters that include the particle diffusiophoretic mobility and the signaling threshold (the distance from the target at which the particles release their chemical signals). The results demonstrate that even a relatively simple chemical signaling scheme can lead to a complex collective behavior of the particles and can be a very efficient way of guiding a swarm of Brownian particles towards a target, similarly to the way colonies of living cells communicate via secondary messengers.

Active targeting in a random porous medium by chemical swarm robots with secondary chemical signaling.

The multibody dynamics of a system of chemical swarm robots in a porous environment is investigated. The chemical swarm robots are modeled as brownian particles capable of delivering an encapsulated chemical payload toward a given target location and releasing it in response to an external stimulus. The presence of chemical signals (chemo-attractant) in the system plays a crucial role in coordinating the collective movement of the particles via chemotaxis. For a number of applications, such as distributed chemical processing and targeted drug delivery, the understanding of factors that govern the collective behavior of the particles, especially their ability to localize a given target, is of immense importance. A hybrid modeling methodology based on the combination of the brownian dynamics method and diffusion problem coupled through the chemotaxis phenomena is used to analyze the impact of a varying signaling threshold and the strength of chemotaxis on the ability of the chemical robots to fulfill their target localization mission. The results demonstrate that the selected performance criteria (the localization half time and the success rate) can be improved when an appropriate signaling process is chosen. Furthermore, for an optimum target localization strategy, the topological complexity of the porous environment needs to be reflected.

Regular moderate exercise reduces advanced glycation and ameliorates early diabetic nephropathy in obese Zucker rats.

Advanced glycation end products (AGEs) play a key role in the pathogenesis of diabetes and its complications, including the diabetic nephropathy. The renoprotective effects of exercise are well known; however, the mechanisms remain elusive. Here we examined whether a regular moderate exercise in obese Zucker rats (OZR), a model of diabetes- and obesity-associated nephropathy, will affect the development of early renal injury in OZR possibly via alteration of AGEs formation. The OZR were left without exercise (sedentary) or subjected to 10 weeks intermittent treadmill running of moderate intensity. Compared with sedentary OZR, kidneys of running OZR had significantly less glomerular mesangial expansion and tubulointerstitial fibrosis. Running OZR had significantly lower plasma AGEs-associated fluorescence and N(epsilon)-carboxymethyllysine. Correspondingly, renal AGEs and N(epsilon)-carboxymethyllysine content were lower in running OZR. Systemically, exercise increased aerobic metabolism, as apparent from urinary metabolite profiling. No differences in plasma glucose, insulin, or lipid profile were found between the 2 groups. Apart from lower advanced oxidation protein products (a marker of myeloperoxidase activity), no other marker of inflammation was altered by exercise, either systemically or locally in kidneys. No indication of changed oxidative status was revealed between the groups. Exercise in OZR decreased advanced glycation. This might represent the early event of exercise-induced renoprotection in diabetic nephropathy in OZR. If confirmed in clinical studies, regular moderate exercise could represent an easy and effective nonpharmacologic approach to reduce advanced glycation.

A novel way of liver preservation improves rat liver viability upon reperfusion.

BACKGROUND/AIM: Currently, the liver is cold-preserved at 0 approximately 4 degrees C for experimental and clinical purposes. Here, we investigated whether milder hypothermia during the initial phase of the preservation period was beneficial for liver viability upon reperfusion. METHODS: In the first set of experiments, rat livers were preserved either conventionally in clinically used histidine-trypthopan-ketoglutarate (HTK) solution (Group A: 45 min and Group B: 24 h) or by slow cooling HTK solution (from 13 degrees C to 3 degrees C) during the initial 45 min of preservation (Group C: 24 h). In the second set of experiments, additional groups of livers were evaluated: Group BB--preservation according to Group B and Group CC--preservation according to Group C. Further, some livers were preserved at 13 degrees C for 24 h. Livers were then reperfused using a blood-free perfusion model. RESULTS: Bile production was approximately 2-fold greater in Group C compared to Group B. Alanine transaminase (ALT) and aspartate transaminase (AST) release into perfusate were 2 approximately 3-fold higher in Group B compared to Group C. No significant differences were found in ALT and AST release between Group C and Group A. Livers in Group CC compared to Group BB exhibited significantly lower portal resistance, greater oxygen consumption and bromosulfophthalein excretion into bile and lower lactate dehydrogenase (LDH) release into perfusate. Histological evaluation of tissue sections in Group BB showed parenchymal dystrophy of hepatocytes, while dystrophy of hepatocytes was absent in Group CC. Livers preserved at 13 degrees C for 24 h exhibited severe ischemic injury. CONCLUSION: These results suggest that the conventional way of liver preservation is not suitable at least for rat livers and that slow cooling of HTK solution during the initial phase of cold storage can improve liver viability during reperfusion.