The Effects of Surfactants on Colloidal, Nanoparticulate, and Dissolved Sulfur

poster abstractElemental sulfur is generally insoluble in water unless in the presence of a surfactant. This phenomenon was investigated by Steudel and Holdt in 1988 by filtering mixtures of sulfur, water, and surfactants through a 0.45 micron filter; however, since then sulfur nanoparticles smaller than 0.45 microns have been detected. The smaller than expected particle size suggests that the distribution of elemental sulfur in water with surfactants may be partitioned into colloidal, nanoparticulate, and truly dissolved components. Experiments have been conducted measuring the sulfur solubility in water with several chemical surfactants and varying filter sizes smaller than 0.45 microns. These experiments were conducted under equilibrium conditions with the solubility being measured using HPLC and square wave voltammetry. Kinetic studies detailing the solubility of sulfur with the surfactants over time have also been investigated. Data regarding the size and occurrence of sulfur nanoparticles present in water and the surfactants has been collected as well to give a complete description of the system under examination. Sulfur isotope fractionation of the dissolved sulfur species is also an interesting component of the system that is currently being investigated using stable isotope ratio mass spectrometry of 34S

Cluster Reserves: A Mechanism for Resource Management in Cluster-based Network Servers

In network (e.g., Web) servers, it is often desirable to isolatethe performance of different classes of requests from each other. That is, one seeks to achieve that a certain minimalproportion of server resources are available for a class ofrequests, independent of the load imposed by other requests. Recent work demonstrates how to achieve this performanceisolation in servers consisting of a single, centralized node;however, achieving performance isolation in a distributed, cluster based server remains a problem.This paper introduces a new abstraction, the cluster reserve, which represents a resource principal in a cluster based net-work server. We present a design and evaluate a prototypeimplementation that extends existing techniques for performance isolation on a single node server to cluster basedservers. In our design, the dynamic cluster-wide resource manage-ment problem is formulated as a constrained optimizationproblem, with the resource allocations on individual machines as independent variables, and the desired cluster-wideresource allocations as constraints. Periodically collected re-source usages serve as further inputs to the problem. Experimental results show that cluster reserves are effectivein providing performance isolation in cluster based servers.We demonstrate that, in a number of different scenarios, cluster reserves are effective in ensuring performance isola-tion while enabling high utilization of the server resources

Flash: An Efficient and Portable Web Server

This paper presents the design of a new Web server architecture called the asymmetric multi-process event-driven (AMPED)architecture, and evaluates the performance of an implementation of this architecture, the Flash Web server. The Flash Web server combines the high performance of single process event-driven servers on cached workloads with the performance of multi-process and multi-threaded servers on disk-bound workloads. Further- more, the Flash Web server is easily portable since it achieves these results using facilities available in all modern operating systems. The performance of different Web server architectures is evaluated in the context of a single implementation in order to quantify the impact of a server’s concurrency architecture on its performance. Furthermore, the performance of Flash is compared with two widely-used Web servers, Apache and Zeus. Results indicate that Flash can match or exceed the performance of existing Web servers by up to 50% across a wide range of real workloads. We also present results that show the contribution of various optimizations embedded in Flash

Efficient Support for P-HTTP in Cluster-based Web Servers

This paper studies mechanisms and policies for supporting HTTP/1.1 persistent connections in cluster-based Web servers that employ content-based request distribution. We present two mechanisms for the efficient, content-based distribution of HTTP/1.1 requests among the back-end nodes of a cluster server. A trace-driven simulation shows that these mechanisms, combined with an extension of the locality-aware request distribution (LARD) policy are effective in yielding scalable performance for HTTP/1.1 requests. We implemented the simpler of these two mechanisms, back-end forwarding. Measurements of this mechanism in connection with extended LARD on a prototype cluster, driven with traces from actual Web servers, confirm the simulation results. The throughput of the prototype is up to four times better than that achieved by conventional weighted round-robin request distribution. In addition, throughput with persistent connections is up to 26% better than without

The use of dithiothreitol for the quantitative analysis of elemental sulfur concentrations and isotopes in environmental samples

Determining the concentration and isotopic composition of elemental sulfur in modern and ancient environments is essential to improved interpretation of the mechanisms and pathways of sulfur utilization in biogeochemical cycles. Elemental sulfur can be extracted from sediment or water samples and quantified by converting to hydrogen sulfide. Alternatively, elemental sulfur concentrations can themselves be analyzed using HPLC and other methodologies; however, the preparation and analysis times can be long and these methods are not amenable to stable isotopic analysis. Current reduction methods involve the use of costly and specialized glassware in addition to toxins such as chromium chloride or cyanide to reduce the sulfur to hydrogen sulfide. The novel reduction method presented here uses dithiothreitol (DTT) as a less toxic reducing agent to obtain both elemental sulfur concentrations and isotopic composition from the same sample. The sample is dissolved in an aqueous or organic liquid medium and upon reaction with DTT, the elemental sulfur is volatilized as hydrogen sulfide and collected in a sulfide trap using an inexpensive gas extraction apparatus. The evolved sulfide concentrations can easily be measured for concentration, by absorbance spectrophotometery or voltammetry techniques, and then analyzed for sulfur isotopic composition. The procedure is quantitative at >93% recovery to dissolved elemental sulfur with no observed sulfur isotope fractionation during reduction and recovery. Controlled experiments also demonstrate that DTT is not reactive to sulfate, sulfite, pyrite, or organic sulfur

Sulfur and oxygen isotope insights into sulfur cycling in shallow-sea hydrothermal vents, Milos, Greece

Shallow-sea (5 m depth) hydrothermal venting off Milos Island provides an ideal opportunity to target transitions between igneous abiogenic sulfide inputs and biogenic sulfide production during microbial sulfate reduction. Seafloor vent features include large (>1 m2) white patches containing hydrothermal minerals (elemental sulfur and orange/yellow patches of arsenic-sulfides) and cells of sulfur oxidizing and reducing microorganisms. Sulfide-sensitive film deployed in the vent and non-vent sediments captured strong geochemical spatial patterns that varied from advective to diffusive sulfide transport from the subsurface. Despite clear visual evidence for the close association of vent organisms and hydrothermalism, the sulfur and oxygen isotope composition of pore fluids did not permit delineation of a biotic signal separate from an abiotic signal. Hydrogen sulfide (H2S) in the free gas had uniform δ34S values (2.5 ± 0.28‰, n = 4) that were nearly identical to pore water H2S (2.7 ± 0.36‰, n = 21). In pore water sulfate, there were no paired increases in δ34SSO4 and δ18OSO4 as expected of microbial sulfate reduction. Instead, pore water δ34SSO4 values decreased (from approximately 21‰ to 17‰) as temperature increased (up to 97.4°C) across each hydrothermal feature. We interpret the inverse relationship between temperature and δ34SSO4 as a mixing process between oxic seawater and 34S-depleted hydrothermal inputs that are oxidized during seawater entrainment. An isotope mass balance model suggests secondary sulfate from sulfide oxidation provides at least 15% of the bulk sulfate pool. Coincident with this trend in δ34SSO4, the oxygen isotope composition of sulfate tended to be 18O-enriched in low pH (75°C) pore waters. The shift toward high δ18OSO4 is consistent with equilibrium isotope exchange under acidic and high temperature conditions. The source of H2S contained in hydrothermal fluids could not be determined with the present dataset; however, the end-member δ34S value of H2S discharged to the seafloor is consistent with equilibrium isotope exchange with subsurface anhydrite veins at a temperature of ~300°C. Any biological sulfur cycling within these hydrothermal systems is masked by abiotic chemical reactions driven by mixing between low-sulfate, H2S-rich hydrothermal fluids and oxic, sulfate-rich seawater

Scalable Content-aware Request Distribution in Cluster-based Network Servers

In network (e.g., Web) servers, it is often desirable to isolate the performance of different classes of requests from each other. That is, one seeks to achieve that a certain minimal proportion of server resources are available for a class of requests, independent of the load imposed by other requests. Recent work demonstrates how to achieve this performance isolation in servers consisting of a single, centralized node; however, achieving performance isolation in a distributed, cluster based server remains a problem.This paper introduces a new abstraction, the cluster reserve, which represents a resource principal in a cluster based network server. We present a design and evaluate a prototype implementation that extends existing techniques for performance isolation on a single node server to cluster based servers.In our design, the dynamic cluster-wide resource management problem is formulated as a constrained optimization problem, with the resource allocations on individual machines as independent variables, and the desired cluster-wide resource allocations as constraints. Periodically collected resource usages serve as further inputs to the problem.Experimental results show that cluster reserves are effective in providing performance isolation in cluster based servers. We demonstrate that, in a number of different scenarios, cluster reserves are effective in ensuring performance isolation while enabling high utilization of the server resources.LABOSBest paper awar