Robert G. Arnold

Factors affecting competition between type I and type II methanotrophs in two-organism, continuous-flow reactors

Competition experiments were performed in a continuous-flow reactor using Methylosinus trichosporium OB3b, a type II methanotroph, and Methylomonas albus BG8, a type I methanotroph. The experiments were designed to establish conditions under which type II methanotrophs, which have significant cometabolic potential, prevail over type I strains. The primary determinants of species selection were dissolved methane, copper, and nitrate concentrations. Dissolved oxygen and methanol concentrations played secondary roles. M. trichosporium OB3b proved dominant under copper and nitratelimited conditions. The ratio of M. trichosporium to M. albus in the reactor increased ten-fold in less than 100 hours following the removal of copper from the reactor feed. Numbers of M. albus declined to levels that were below detection limits (<106/ml) under nitrogen-limited conditions. In the latter experiment, the competitive success of M. trichosporiumdepended on the maintenance of an ambient dissolved oxygen level below about 7.5 × 10−5M, or 30% of saturation with air. The ability of M. trichosporium to express soluble methane monooxygenase under copper limitation and nitrogenase under nitrate limitation was very significant. M. albus predominated under methane-limited conditions, especially when low levels of methanol were simultaneously added with methane to the reactor. The results imply that nitrogen limitation can be used to select for type II strains such as M. trichosporium OB3b.

The Lancet Commission on pollution and health

Philip J Landrigan, Richard Fuller, Nereus J R Acosta, Olusoji Adeyi, Robert Arnold, Niladri (Nil) Basu, Abdoulaye Bibi Baldé, Roberto Bertollini, Stephan Bose-O’Reilly, Jo Ivey Boufford, Patrick N Breysse, Thomas Chiles, Chulabhorn Mahidol, Awa M Coll-Seck, Maureen L Cropper, Julius Fobil, Valentin Fuster, Michael Greenstone, Andy Haines, David Hanrahan, David Hunter, Mukesh Khare, Alan Krupnick, Bruce Lanphear, Bindu Lohani, Keith Martin, Karen V Mathiasen, Maureen A McTeer, Christopher J L Murray, Johanita D Ndahimananjara, Frederica Perera, Janez Potočnik, Alexander S Preker, Jairam Ramesh, Johan Rockström, Carlos Salinas, Leona D Samson, Karti Sandilya, Peter D Sly, Kirk R Smith, Achim Steiner, Richard B Stewart, William A Suk, Onno C P van Schayck, Gautam N Yadama, Kandeh Yumkella, Ma Zhong

Bacterial transport in laboratory columns and filters : influence of ionic strength and pH on collision efficiency

Abstract The influence of ionic strength and pH on the transport of Pseudomonas fluorescens P17 in porous media was investigated using continuous-flow laboratory columns and a rapid screening technique in which radiolabeled cells were applied to large-pore, glass-fiber filters. Colloid-filtration theory was used to interpret P17 transport results in the two systems. Bacterial retention was directly related to the ionic strength of the carrying solution. Decreasing the ionic strength from 10 −1 to 10 −5 M caused the bacterial collision efficiency, α, to decrease nearly 90% (from 0.18 to 0.026 in screening experiments and from 0.12 to 0.015 in column experiments). This change in α is qualitatively consistent with double-layer theory, but suggests that very large changes in ionic strength are needed to influence transport. Bacterial transport was unaffected by changes in pH in the range of 5.5

Effect of Temperature‐Controlled Motility on Transport of Bacteria and Microspheres Through Saturated Sediment

Continuous flow column experiments were used at different temperatures to study the importance of motility on advective transport of bacteria through repacked, but otherwise unaltered, natural aquifer sediment. The bacterium used was A0500, a flagellated, spore-forming rod isolated from the deep subsurface (180 m). At 4°C, A0500 was nonmotile because here was no flagellar metabolism. Bacteria removal was greater at 4°C than at 18°C. Similar experiments with microspheres showed an opposite effect, ith greater removal at 18° than 4°C, which was consistent with colloid filtration theory. The sticking efficiency (α) for nonmotile A0500 (4°C), estimated using a steady state colloid filtration model, was over 3 times that of the motile A0500 (18°C), 0.073 versus 0.022. Analysis of complete breakthrough curves using a nonsteady, kinetically limited, transport model suggested that motile A0500 bacteria traveled twice as far as nonmotile A0500 bacteria before becoming attached to the sediment grains. Once attached, nonmotile bacteria detached on a timescale of 9–17 days versus 4–5 days for the motile bacteria. Bacterial motility facilitates advective transport through sediments by changing the attachment-detachment kinetics to effectively reduce retardation and increase the fraction of time bacteria spend in a detached versus an attached state. Consequently, travel times to deep aquifers from recharge waters could be significantly affected by bacterial motility.

Modeling Bacterial Detachment During Transport Through Porous Media as a Residence‐Time‐Dependent Process

Bacterial transport through porous media was modeled using detachment functions that incorporate the dependence of detachment rate on bacterial residence time on the collector. Model parameters and the relative merit of alternative forms for the detachment function were evaluated on the basis of comparisons between model simulations and experimentally derived bacterial breakthrough and elution curves. Only detachment functions that provided an initial period in which bacteria were rapidly released, followed by slow bacterial detachment, were able to reproduce the elution portion of the breakthrough curves. In optimal simulations, 90% of the bacteria that were captured by the porous medium detached within 1 min of attachment. Experiments involving saturated flow through columns packed with sand indicated that the time to achieve complete breakthrough was inversely related to the influent bacterial concentration. On this basis and because of the relatively slow approach to breakthrough that was typically observed in transport experiments, it was hypothesized that the experimental medium contained a number of preferred attachment sites that must be essentially filled before breakthrough is achieved. Only when such (irreversible) sorption sites were included in the model formulations was it possible to produce transport simulations that matched both the breakthrough and elution portions of the empirically derived curves. It is concluded that both a time-dependent detachment function and a degree of sorption site heterogeneity are required to describe bacterial attachment and detachment during transport as observed in our laboratory.

Fate of organics during soil-aquifer treatment: sustainability of removals in the field.

A 5-year program of study was conducted at the Sweetwater Recharge Facilities (SRF) to assess the performance of surface spreading operations for organics attenuation during field-scale soil-aquifer treatment (SAT) of municipal wastewater. Studies were conducted utilizing both mature (approximately 10 yr old) and new infiltration basins. Removals of dissolved organic carbon (DOC) were robust, averaging >90 percent during percolation through the local 37-m vadose zone. The hydrophilic (most polar) fraction of DOC was preferentially removed during SAT; removals were attributed primarily to biodegradation. Reductions in trihalomethane formation potential (THMFP) averaged 91 percent across the vadose zone profile. The reactivity (specific THMFP) of post-SAT organic residuals with chlorine decreased slightly from pre-SAT levels (60 vs. 72 microg THM per mg DOC, respectively). Variations in the duration of wetting/drying periods did not significantly impact organic removal efficiencies.

Transport of Pseudomonas fluorescens strain P17 through quartz sand columns as a function of water content

Porous media column experiments were used to investigate Pseudomonas fluorescens strain P17 transport as a function of water content and the influences of the solid-liquid and gas-liquid interfaces. Retention of radiolabeled P17 in washed quartz sand was evaluated at 100, 84, and 46% water saturation. At the completion of each experiment, the porous medium was extruded and sampled directly for cell retention on the basis of a radiolabel mass balance. Maximum cell retention occurred in the top centimeter of porous media at all three water contents and decreased . with depth in the column. The total fraction of cells retained R was inversely proportional to t . water content, with nearly twice the cell retention at 46% saturation R s 0.95 compared to t .

Electrolytic oxidation of trichloroethylene using a ceramic anode

Trichloroethylene (TCE) was transformed to CO2, CO, Cl− and ClO3− at the anode of a two-chambered electrolytic cell. The working electrode was constructed from Ebonex®, an electrically conductive ceramic (Ti4O7). Under our experimental conditions (anode potential Ea = 2.5 to 4.3 V vs SSCE), the disappearance of TCE was first order in TCE concentration. The transformation rate was independent of pH in the range 1.6 < pH < 11. TCE oxidation occurred only on the anodic surface and was limited by mass transport at high potentials (Ea > 4.0V). The maximum (transport-limited), surface-area-normalized rate constant was about 0.002 43cms−1. Carbon-containing products included CO2 primarily with traces of CO. At neutral and alkaline pHs, the only chlorine-containing products were Cl− and ClO3−. Hydroxyl radicals were detected in the anodic compartment using a spin trap (4-POBN). A kinetic model was successfully correlated with experimental results.

Kinetics and mechanism of reductive dehalogenation of carbon tetrachloride using zero-valence metals

Abstract Elemental iron and zinc reduced part-per-thousand levels of aqueous-phase carbon tetrachloride to chloroform in a few hours. Free metal ions, chloride ion and hydrogen gas were produced in the reaction; protons were consumed. Process kinetics were dependent on solution pH, surface area of the elemental metal, carbon tetrachloride concentration, buffer selection and solvent composition (volume fraction 2-propanol). Reaction rate was first-order with respect to carbon tetrachloride at concentrations less than 7.5 mM. This class of reactions offers promise as a means for initiating the destruction of heavily halogenated organic compounds.

Measurement of bacterial collision efficiencies in porous media

A new method, utilizing radiolabeled (3H-leucine) cells and 1-cm columns packed with 40-μm borosilicate glass beads was used to estimate bacterial collision efficiency by directly measuring the retention of cells in porous media. At a fractional retention of 0.051 (n = 3), the coefficient of variation was 0.037, permitting meaningful estimation of collision efficiencies as low as 3 × 10−5. Collision efficiency was a function of the ionic strength and pecies identity; α increased from 1.6 × 10−3 to 1.4 × 10−2 for A. paradoxus in 10−5 and 10−3 M NaCl solutions, respectively, and from 8.9 × 10−3 to 6.2 × 10−2 for P. fluorescens in the same solutions. Results were not sensitive to test parameters such as velocity, volume filtered and rinse volume. The new procedure provides a convenient, reliable, accurate method for estimating low-end biocolloid collision efficiencies in porous media. In the range α < 0.01, the method is economical, significantly faster, and much more reliable than other published procedures. Its application may accelerate efforts to establish functional relationships between biocolloid collision efficiency and governing physical-chemical variables.