John C. Walton

Desalination coupled with salinity-gradient solar ponds

Thermal desalination by salinity-gradient solar ponds (SGSP) is one of the most promising solar desalination technologies. Solar ponds combine solar energy collection with long-term storage and can provide reliable thermal energy at temperature ranges from 50 to 90°C. Solar-pond-powered desalination has been studied since 1987 at the El Paso Solar Pond Project, El Paso, Texas. From 1987 to 1992, the research mainly focused on the technical feasibility of thermal desalination coupled with solar ponds. Since 1999, the research has focused on long-term reliability, improvement of thermodynamic efficiency, and economics. During this period, a small multi-effect, multi-stage flash distillation (MEMS) unit, a membrane distillation unit, and a brine concentration and recovery system (BCRS) were tested over a broad range of operating conditions. The most important variables for the MEMS operation were flash range, concentration level of reject brine, and circulation rate of the first effect. The brine concentration and recovery system is part of the goal of developing a systems approach combining salinity-gradient solar pond technology with multiple process desalination and brine concentration. This systems approach, called zero discharge desalination, proposes concentrating brine reject streams down to near NaCl saturated solutions and using the solution to make additional solar ponds. In addition to presenting the test results on the MEMS and BCRS units, this paper also presents a summary of solar pond operation experiences obtained from the 16-year operation at the El Paso solar pond.

Correlation of in vitro cytokine responses with the chemical composition of soil-derived particulate matter.

We treated human lung epithelial cells, type BEAS-2B, with 10–80 μg/cm2 of dust from soils and road surfaces in the western United States that contained particulate matter (PM) < 2.5 μm aerodynamic diameter. Cell viability and cytokine secretion responses were measured at 24 hr. Each dust sample is a complex mixture containing particles from different minerals mixed with biogenic and anthropogenic materials. We determined the particle chemical composition using methods based on the U.S. Environmental Protection Agency Speciation Trends Network (STN) and the National Park Service Interagency Monitoring of Protected Visual Environments (IMPROVE) network. The functionally defined carbon fractions reported by the ambient monitoring networks have not been widely used for toxicology studies. The soil-derived PM2.5 from different sites showed a wide range of potency for inducing the release of the proinflammatory cytokines interleukin-6 (IL-6) and IL-8 in vitro. Univariate regression and multivariate redundancy analysis were used to test for correlation of viability and cytokine release with the concentrations of 40 elements, 7 ions, and 8 carbon fractions. The particles showed positive correlation between IL-6 release and the elemental and pyrolyzable carbon fractions, and the strongest correlation involving crustal elements was between IL-6 release and the aluminum:silicon ratio. The observed correlations between low-volatility organic components of soil- and road-derived dusts and the cytokine release by BEAS-2B cells are relevant for investigation of mechanisms linking specific air pollution particle types with the initiating events leading to airway inflammation in sensitive populations.

Multivariate Analyses of Water Chemistry: Surface and Ground Water Interactions

Multivariate statistical methods (MSMs) applied to ground water chemistry provide valuable insight into the main hydrochemical species, hydrochemical processes, and water flowpaths important to ground water evolution. The MSMs of principal component factor analysis (FA) and k-means cluster analysis (CA) were sequentially applied to major ion chemistry from 211 different ground water-sampling locations in the Amargosa Desert. The FA reduces the number of variables describing the system and finds relationships between major ions. The CA of the reduced system produced objective hydrochemical facies, which are independent of, but in good agreement with, lithological data. The derived factors and hydrochemical facies are innovatively presented on biplots, revealing composition of hydrochemical processes and facies, and overlaid on a digital elevation model, displaying flowpaths and interactions with geologic and topographic features in the region. In particular, a distinct ground water chemical signature is observed beneath and surrounding the extended flowpath of Fortymile Wash, presenting some contradiction to contemporary water levels along with potential interaction with a fault line. The signature surrounding the ephemeral Fortymile Wash is believed to represent the relic of water that infiltrated during past pluvial periods when the amount of runoff in the wash was significantly larger than during the current drier period. This hypothesis and aforementioned analyses are supported by the examination of available chloride, oxygen-18, hydrogen-2, and carbon-14 data from the region.

Advancements in Salinity Gradient Solar Pond Technology Based on Sixteen Years of Operational Experience

The El Paso salinity gradient solar pond, initiated in 1983, has been in operation since 1985. Through 16 years of research and operation, the El Paso Solar Pond has successfully demonstrated applications including desalination, waste brine management, industrial process heat production, and electricity generation; and has developed and implemented key technical advancements to improve the technical viability and economic feasibility of salinity gradient solar ponds, including: 1) an automated instrumentation monitoring system, 2) a stability analysis strategy and high temperature (60-90°C) gradient maintenance methods, 3) a scanning injection technique for improved salinity gradient construction and maintenance, 4) new liner technology, and 5) an improved heat extraction system.

Modeling the adsorption of Cr(III) from aqueous solution onto Agave lechuguilla biomass: Study of the advective and dispersive transport

The biosorption of Cr(III) onto packed columns of Agave lechuguilla was analyzed using an advective-dispersive (AD) model and its analytical solution. Characteristic parameters such as axial dispersion coefficients, retardation factors, and distribution coefficients were predicted as functions of inlet ion metal concentration, time, flow rate, bed density, cross-sectional column area, and bed length. The root-mean-square-error (RMSE) values 0.122, 0.232, and 0.285 corresponding to the flow rates of 1, 2, and 3 (10(-3))dm3min(-1), respectively, indicated that the AD model provides an excellent approximation of the simulation of lumped breakthrough curves for the adsorption of Cr(III) by lechuguilla biomass. Therefore, the model can be used for design purposes to predict the effect of varying operational conditions.

Fluid flow through fractures in below ground concrete vaults

Abstract Cement and concrete are used extensively to isolate waste materials from the environment and to control groundwater flow rates in mining, waste disposal, and site remediation activities. High quality concrete is a very low permeability material, however it is brittle and subject to cracking. In practice, the permeability of concrete is controlled by the fractures or cracks which form in the structures. Simplified models for flow through and around underground concrete vaults are presented and applied. Results of the applications indicate that flow rates through cracked concrete are a function of size and spacing of cracks. When cracks are small and closely spaced, resistance in passing through the cracks controls flow. However, when cracks are larger and widely spaced, resistance through the adjacent porous medium (i.e., soil) governs flow rates through cracks. If calculations consider both the adjacent porous media and the cracks, maximum predicted flow rates do not occur for the largest or smallest crack sizes and spacings, but somewhere inbetween the extremes. Further calculations illustrate that perched water will frequently be expected on top of vaults justifying use of saturated flow equations. Perched water can occur because the cracked roof of a concrete vault is analogous to capillary barriers used in some engineered cover designs. Although not a critical design concentration, results indicate that keeping the vault roof below saturation to avoid flow into the cracks is difficult and has the best chance of success for smaller, modular vaults, arid locations, and very shallow burial.

The Role of Oxygen Diffusion in the Release of Technetium from Reducing Cementitious Waste Forms

Cementitious materials provide an ideal geochemical environment (e.g., high pH pore fluids and large surface areas for sorption) for immobilizing nuclear waste. The inclusion of reducing agents, such as blast furnace slag (BFS) can immobilize radionuclides by forming of solid sulfide phases. Thermodynamic calculations using the MINTEQ geochemical computer code indicate that elemental sulfur present in BFS reacts with the highly mobile pertechnetate anion (TcO 4 − ) anion to form an insoluble technetium sulfide phase (Tc 2 S 7(8) ). Initially, the waste form very effectively immobilizes technetium. However, as oxygen diffuses into the waste form, an outer zone of oxidized concrete and a shrinking core of reduced intact concrete develops. Oxidation of sulfur in the outer zone results in increased technetium concentrations in the pore fluid because Tc 2 S 7(8) oxidizes to the mobile TcO 4 − anion. The TcO 4 − anion can then diffuse from the waste form into the environment. A mathematical model that accounts for diffusion of oxygen into concrete coupled with oxidation of sulfur and sulfide to sulfate has been developed. This model assumes the existence of an oxidized outer layer of concrete surrounding a shrinking core of reducing intact concrete. A sharp boundary between the two zones moves slowly inward resulting in oxidation of Tc 2 S 7(8) and subsequent release of TcO 4 − via aqueous diffusion in the concrete pore fluids. The model indicates that this mechanism results in a linear dependance of release with the square root of time similar to pure diffusion. In addition, the release of technetium is related to the inverse of the square root of the concentration of BFS, indicating that performance will significantly increases with the addition of approximately 20 percent BFS to the cement mix.

The Effects of Calcite Solid Solution Formation on the Transient Release of Radionuclides from Concrete Barriers

The wealhering of concrete by carbonation causes the formation of calcite solid solutions that can significantly improve concrete as a barrier to the migration of divalent radionuclide cations. Example calculations for 90 Sr and 60 Co indicate that release rates from carbonated concrete are five orders of magnitude lower than for intact, unweathered concrete and that the thin carbonated zone is a significant sink for these radionuclides.

Effects of evaporation and solute concentration on presence and composition of water in and around the waste package at Yucca Mountain

For the proposed repository at Yucca Mountain, the amount, timing, and expected composition of water contacting the waste package are major concerns. In the presence of thermal loading from radioactive decay, a flow system is set up with water leaving as vapor and returning as liquid flowing through the matrix and fractures. A hypothesis is developed suggesting that most water in the vicinity of the waste package evaporates, leaving behind a scale deposit and concentrated solution. The vapor pressure lowering caused by the brine solution maintains a water film on the rock and waste package materials even in the presence of a dry environment with thermal gradients. The chemical environment within which container corrosion and spent fuel alteration occur is anticipated to be an initial period of concentrated brine, followed by solutions of lower ionic strength as the repository cools. The specific environment and timing could vary widely between different waste packages and within individual waste packages subsequent to container failure.

PM source identification at Sunland Park, New Mexico, using a simple heuristic meteorological and chemical analysis

Abstract The causes for evening low-wind PM10 and PM2.5 peaks at Sunland Park, NM, were investigated by using wind sector analysis and by assessing relationships between PM loadings and meteorological parameters through canonical ordination analysis. Both PM10 and PM2.5 concentrations during the evening hours accounted for ∼50% of their respective 24-hr averages, and the PM10 was mainly composed of coarse material (PM10–2.5 amounted to 77% of PM10). A wind sector analysis based on data from three surface meteorological monitoring stations in the region narrowed the potential source region for PM10 and PM2.5 to an area within a few kilometers south of Sunland Park. Canonical ordination analysis confirmed that the peak frequently occurred under stable conditions with weak southerly winds. Chemical analyses of PM showed that elemental and organic carbon (EC and OC, respectively) dominate PM2.5 and inorganic elements dominate PM10–2.5. The combined data for EC/OC, geologic elements, and various trace elements indicate that under low wind and stable conditions, traffic-related PM emissions (motor vehicle exhausts and re-suspended road dust) from the south of the site are the most likely sources for the evening PM10 and PM2.5 peaks.