Hydraulic performance of a proposed in situ photocatalytic reactor for degradation of MTBE in water

Methyl tert-butyl ether (MTBE) groundwater remediation projects often require a combination of technologies resulting in increasing the project costs. A cost-effective in situ photocatalytic reactor design, Honeycomb II, is proposed and tested for its efficiency in MTBE degradation at various flows. This study is an intermediate phase of the research in developing an in situ photocatalytic reactor for groundwater remediation. It examines the effect of the operating variables: air and water flow and double passages through Honeycomb II, on the MTBE removal. MTBE vaporisation is affected by not only temperature, Henry’s law constant and air flow to volume ratio but also reactor geometry. The column reactor achieved more than 84% MTBE removal after 8 h at flows equivalent to horizontal groundwater velocities slower than 21.2 cm d⁻¹. Despite the contrasting properties between a photocatalytic indicator methylene blue and MTBE, the reactor efficiency in degrading both compounds showed similar responses towards flow (equivalent groundwater velocity and hydraulic residence time (HRT)). The critical HRT for both compounds was approximately 1 d, which corresponded to a velocity of 21.2 cm d⁻¹. A double pass through both new and used catalysts achieved more than 95% MTBE removal after two passes in 48 h. It also verified that the removal efficiency can be estimated via the sequential order of the removal efficiency of one pass obtained in the laboratory. This study reinforces the potential of this reactor design for in situ groundwater remediation.     

Intercomparison of the measurements of oxalic acid in aerosols by gas chromatography and ion chromatography

Oxalate, the anion of oxalic acid, is one of the most abundant measurable organic species in atmospheric aerosols. Traditionally, this bifunctional species has been measured by gas chromatography (GC) after derivatization to butyl ester and by ion chromatography (IC) without derivatization. However, there are few published comparisons of the two techniques. Here, we report the results of an intercomparison study for the measurement of oxalic acid in Arctic aerosols (<2.5 μm, n = 82) collected in 1992 using GC and IC. The concentrations of oxalic acid by GC ranged from 6.5–59.1 ng m^?3 (av. 26.0 ng m^?3, median 26.2 ng m^?3) whereas those by IC ranged from 6.6–52.1 ng m^?3 (av. 26.6 ng m^?3, median 25.4 ng m^?3). They showed a good correlation (r = 0.84) with a slope of 0.96. Thus, observations of oxalate obtained by GC employing dibutyl esters are almost equal to those by IC. Because the accuracy of oxalic acid by GC method largely depends on the method used, it is important to strictly examine the recovery in each study.     

The Development of Open Pit Incinerators for Solid Waste Disposal

A discussion is presented of a newly developed incinerator for the disposal of solid wastes. It is distinguished from conventional types by its open top, and features a system of closely spaced nozzles admitting a screen of high-velocity air over the burning zone. Results, with a variety of solid industrial wastes, show high burning rates, leading to complete combustion and high flame temperature.     

The Sensitivity of PM25 Source-Receptor Relationships to Atmospheric Chemistry and Transport in a Three-Dimensional Air Quality Model

ABSTRACT

Air quality model simulations constitute an effective approach to developing source-receptor relationships (so-called transfer coefficients in the risk analysis framework) because a significant fraction of particulate matter (particularly PM_2.5) is secondary (i.e., formed in the atmosphere) and, therefore, depends on the atmospheric chemistry of the airshed. In this study, we have used a comprehensive three-dimensional air quality model for PM_{2 5} (SAQM-AERO) to compare three approaches to generating episodic transfer coefficients for several source regions in the Los Angeles Basin. First, transfer coefficients were developed by conducting PM_2.5 SAQM-AERO simulations with reduced emissions of one of four precursors (i.e., primary PM, sulfur dioxide (SO₂), oxides of nitrogen (NO_x), and volatile organic compounds) from each source region. Next, we calculated transfer coefficients using two other methods: (1) a simplified chemistry for PM_2.5 formation, and (2) simplifying assumptions on transport using information limited to basin-wide emission reductions. Transfer coefficients obtained with the simplified chemistry were similar to those obtained with the comprehensive model for VOC emission changes but differed for NO and SO emission changes. The differences were due to the parameterization of the rates of secondary PM formation in the simplified chemistry. In 90% of the cases, transfer coefficients estimated using only basin-wide information were within a factor of two of those obtained with the explicit source-receptor simulations conducted with the comprehensive model. The best agreement was obtained for VOC emission changes; poor agreement was obtained for primary PM_2.5.     

Identifying potential receptors and routes of contaminant exposure in the traditional territory of the Ouje-Bougoumou Cree: Land use and a geographical information system

Great concern has been raised with respect to the 13 traplines that constitute the traditional territory of the Ouje-Bougoumou Cree located in the James Bay region of northern Quebec, Canada, with respect to mine wastes originating from three local mines. As a result, an “Integrative Risk Assessment” was initiated consisting of three interrelated components: a comprehensive human health study, an assessment of the existing ecological/environmental database, and a land use/potential sites of concern study. In this paper, we document past and present land use in the traditional territory of the Ouje-Bougoumou Cree for 72 heads of households, including 13 tallymen, and use a Geographic Information System (GIS) to layer harvest/hunting and gathering/collecting data over known mining areas and potential sites of concern. In this way, potential receptors of contamination and routes of human exposure were identified. Areas of overlap with respect to land use activity and mining operations were relatively extensive for certain harvesting activities (e.g., beaver, Castor canadensis and various species of game birds), less so for fish harvesting (all species) and water collection, and relatively restrictive for large mammal harvesting and collection of firewood (and other collection activities). Potential receptors of contaminants associated with mining activity (e.g., fish and small mammals) and potential routes of exposure (e.g., ingestion of contaminated game and drinking of contaminated water) were identified.     

Fate and toxicity of endosulfan in Namoi River water and bottom sediment

Endosulfan (6,7,8,9,10,10,-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepine-3-oxide) sorption (standardized to 1% total organic carbon and dry weight) was significantly (P < 0.05) more concentrated on the large (>63 microm) particle fraction compared with smaller size fractions (<5 microm and 5-24 microm) of bottom sediments from the Namoi River, Australia. Following completion of the particle size fractionation (6 to 12 wk) and a sediment toxicity assessment (2 wk), the sediments showed large decreases in concentrations of alpha-endosulfan that coincided with an increase in endosulfan sulfate concentrations and minimal changes in beta-endosulfan concentrations. In the Namoi River, similar patterns were observed in the composition of total endosulfan in monthly measurements of bottom sediments and in passive samplers placed in the water column following runoff from cotton (Gossypium hirsutum L.) fields. The toxicity of endosulfan sulfate in river water indicated by the nymphs of the epibenthic mayfly Jappa kutera, was more persistent than the alpha- and beta-endosulfan parent isomers due to its longer half-life. This suggests that endosulfan sulfate would contribute most to previously observed changes in population densities of aquatic biota. Measured concentrations of total endosulfan in river water of up to 4 microg L(-1) following storm runoff, exceed the range of the 96-h median lethal concentration (LC50) values in river water for both alpha-endosulfan (LC50 = 0.7 microg L(-1); 95% confidence interval [CI] = 0.5 to 1.1) and endosulfan sulfate (LC50 = 1.2 microg L(-1); 95% CI = 0.4 to 3.3). In contrast, the 10-d LC50 value for total endosulfan in the sediment toxicity test (LC50 = 162 microg kg(-1); 95% CI = 120 to 218 microg kg(-1)) was more than threefold higher than the highest measured concentration of total endosulfan in field samples of bottom sediment (48 microg kg(-1)). This suggests that pulse exposures of endosulfan in the water column following storm runoff may be more acutely toxic to riverine biota than in contaminated bottom sediment.     

Insights on the solubilization products after combined alkaline and ultrasonic pre-treatment of sewage sludge

This work provides insights on the solubilization products after a simultaneous combination of alkaline and ultrasonic (ALK + ULS) pre-treatment of sewage sludge. Soluble chemical oxygen demand (SCOD) increased from 1200 to 11,000 mg/L after such treatment. Organics with molecular weight around 5.6 kDa were solubilized because of the synergistic effect of ultrasound and alkali. Organics with molecular weight larger than 300 kDa increased from 7.8% to 60%, 16% and 42.3% after ULS, ALK and ALK + ULS treatment, respectively. Excitation emission matrix fluorescence spectroscopy analysis identified soluble microbial product-like and humic acid-like matters as the main solubilization products. Sludge anaerobic biodegradability was significantly enhanced with the simultaneous application of ALK + ULS pre-treatment. ALK + ULS pre-treatment resulted in 37.8% biodegradability increase compared to the untreated sludge. This value was higher compared to the biodegradability increase induced by individual ALK pre-treatment (5.7%) or individual ULS pre-treatment (20.7%) under the same conditions applied.     

Simulation of solute transport across low-permeability barrier walls

Low-permeability, non-reactive barrier walls are often used to contain contaminants in an aquifer. Rates of solute transport through such barriers are typically many orders of magnitude slower than rates through the aquifer. Nevertheless, the success of remedial actions may be sensitive to these low rates of transport. Two numerical simulation methods for representing low-permeability barriers in a finite-difference groundwater-flow and transport model were tested. In the first method, the hydraulic properties of the barrier were represented directly on grid cells and in the second method, the intercell hydraulic-conductance values were adjusted to approximate the reduction in horizontal flow, allowing use of a coarser and computationally efficient grid. The alternative methods were tested and evaluated on the basis of hypothetical test problems and a field case involving tetrachloroethylene (PCE) contamination at a Superfund site in New Hampshire. For all cases, advective transport across the barrier was negligible, but preexisting numerical approaches to calculate dispersion yielded dispersive fluxes that were greater than expected. A transport model (MODFLOW-GWT) was modified to (1) allow different dispersive and diffusive properties to be assigned to the barrier than the adjacent aquifer and (2) more accurately calculate dispersion from concentration gradients and solute fluxes near barriers. The new approach yields reasonable and accurate concentrations for the test cases.