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Abstract. The effective specific air-water interfacial area (•i) in a sand-packed column was measured at several water saturations (Sw) using a surface-reactive tracer (sodium dodecylbenzene sulfonate (SDBS)) and a nonreactive tracer (bromide). Miscible displacement experiments were conducted under steady water flow conditions to quantify the retardation of SDBS resulting from its adsorption onto the air-water interface in a sand-packed column. A consistent trend of increased retardation of SDBS compared with the nonreactive tracer, bromide, was observed with decreasing S w. The data for air-water surface tension measured at various SDBS concentrations were interpreted using the Gibbs model to estimate the required adsorption parameters. The retardation factors (Rt) for SDBS breakthrough curves were then used in combination with the estimated SDBS adsorption coefficient to calculate the •i values at different Sw. For the range of experimental conditions employed in this study, the retardation factor for SDBS ranged from R t = 1.07 at Sw = 1.00 (R t > 1 due to SDBS sorption on sand) to R t = 3.44 at Sw = 0.29 (which corresponds to •i = 46 cm2/cm3). These values are in agreement with theoretical predictions and recently published data. Improvements needed to overcome the experimental limitations of the presented method are also discussed.

SUMMARY The Arg/N-end rule pathway targets for degradation proteins that bear specific unacetylated N-terminal residues while the Ac/N-end rule pathway targets proteins through their Nα-terminally acetylated (Nt-acetylated) residues. Here we show that Ubr1, the ubiquitin ligase of the Arg/N-end rule pathway, recognizes unacetylated N-terminal methionine if it is followed by a hydrophobic residue. This capability of Ubr1 expands the range of substrates that can be targeted for degradation by the Arg/N-end rule pathway, because virtually all nascent cellular proteins bear N-terminal methionine. We identified Msn4, Sry1, Arl3, and Pre5 as examples of normal or misfolded proteins that can be destroyed through the recognition of their unacetylated N-terminal methionine. Inasmuch as proteins bearing the Nt-acetylated N-terminal methionine residue are substrates of the Ac/N-end rule pathway, the resulting complementarity of the Arg/N-end rule and Ac/N-end rule pathways enables the elimination of protein substrates regardless of acetylation state of N-terminal methionine in these substrates.

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A series of gaseous miscible displacement experiments were conducted to estimate specific air–water interfacial areas (ai) and water contents in an unsaturated sand column. A straight‐chain hydrocarbon (n‐decane) was used as the gaseous interfacial tracer and methylene chloride and chloroform were used as the water‐partitioning gaseous tracers. A gas chromatographic technique was employed for the tracer experiments conducted at room temperature using nitrogen as the mobile phase and water as the immobile liquid. Tracer experiments covered a water saturation (Sw) range of 1.5 to 56%. The largest ai value (∼1500 cm2 cm−3), measured at the lowest Sw (1.5%), was somewhat smaller than the solid surface area (∼2000 cm2 cm−3) determined using the nitrogen‐sorption technique. As Sw increased, ai values decreased exponentially to ∼80 cm2 cm−3 at Sw of 56%. Within a limited Sw range (0.29 < Sw < 0.55), where both aqueous and gaseous interfacial tracer data were measured, the ai values measured using a gaseous tracer (n‐decane) were 2 to 3 times larger than those measured in a previous study using an aqueous interfacial tracer (sodium dodecylbenzene sulfonate [SDBS]). The velocity of the air–water interface was estimated to be between 23 and 36% of the bulk pore‐water velocity. The water contents measured using water‐partitioning tracers were within ±5% of those based on gravimetric measurements.

Laboratory experiments were conducted employing gas chromatographic techniques to evaluate the gaseous transport of volatile organic chemicals (VOCs) in water-unsaturated soil columns as influenced by interfacial (air-water) adsorption and water partitioning. VOCs [methylene chloride, tetrachloroethene (PCE), 1,1,1-trichloroethane (TCA), ethyl-benzene, p-xylene, chlorobenzene] with different water-partitioning and interfacial adsorption coefficients (air-water) were used to evaluate the theoretical basis of using these coefficients to predict the retardation factors (Rt) observed during gaseous transport. A loamy sand from Dover Air Force Base, DE, and a commercial sand were used as the column packing material to assess the effect of grain size on the air-water interfacial area (ai) and retardation at different water saturations (Sw). The ai were measured using n-alkanes. At low Sw, interfacial adsorption contributed most to the retardation for all VOCs during gaseous transport in the Dover soil which has little sorption capacity for the VOCs. As Sw increased, the fraction of Rt attributed to interfacial adsorption decreased, while that due to water partitioning increased for all of the VOCs used for this study. For the sand, with a more uniform grain-size distribution than the Dover soil, the contribution of air-water interfacial adsorption to the Rt of a VOC (p-xylene) was not as significant as that for the Dover soil due to small ai. The fractions of Rt attributed to interfacial adsorption and water partitioning were quantified. The observed Rt for the VOCs agreed well with those predicted based on the sorption coefficients and the quantities of sorption domains (Sw, ai).

We investigated in laboratory column experiments, the aqueous-phase transport of four n-alcohols (n-hexanol-nnonanol), three chlorinated aromatic compounds (chlorobenzene, o-dichlorobenzene, and o-chlorophenol), and two alkylbenzenes (ethylbenzene and p-xylene) in a waterunsaturated porous medium (sand). The influence of gasphase partitioning and interfacial adsorption on solute retardation during steady unsaturated water flow was evaluated over a range of water contents. Air-water interfacial adsorption was a significant factor for the retardation of n-alcohols. For example, nearly 90% of the measured retardation of n-nonanol could be attributed to interfacial adsorption at a water saturation of 34%. Aromatic compounds used in this study were not significantly affected by adsorption at the air-water interface because of both low air-water interfacial area (0-50 cm 2 /cm 3 ) generated in the unsaturated porous medium and the small interfacial-adsorption coefficients. Instead, gas-phase partitioning was the primary mechanism responsible for the measured retardation of most of the aromatic compounds evaluated in this study. Using the batch-measured interfacial adsorption coefficients for n-octanol and n-nonanol and the column-measured retardation factors, the effective air-water interfacial areas were estimated. These values agreed closely with those we reported earlier, based on displacement experiments with an anionic surfactant as an interfacial tracer.

Because iron-based materials that are used for the permeable reactive barrier systems come in various shapes, sizes, and with various surface properties depending on the manufacturing sources, their reductive powers vary in a wide spectrum. A new experimental procedure to evaluate the reductive power of iron material was developed in this study. Tri-iodide (I3(-)) was used as the representative oxidizing agent that reacts with zero-valent iron (ZVI). Three iron-based materials (two scraps, two powders) and four chlorinated chemicals [perchloroethene (PCE), trichloroethene (TCE), 1,1,1-trichloroethane (TCA), and pentachlorophenol (PCP)] were used in this study. Redox reactions were conducted in glass vials containing aqueous solutions of chlorinated compounds or tri-iodide with known masses of iron material. After a predetermined reaction time each vial was opened and the solution was analyzed for the concentration of reduced compound. The apparent rate contant (k(i)(obs)) of iodine reduction reaction with ZVIs was found to be proportional to that (k(c)(obs)) of chlorinated contaminant. The surface area-normalized reduction rate constants (k(c)(nor)) for contaminants and tri-iodide (k(i)(nor)) were also proportional to each other. The ratio of rate constants, K(nor) (= k(c)(nor)/k(i)(nor)) was estimated for each contaminant; 3.29 × 10(-7), 5.86 × 10(-7), 6.70 × 10(-7), and 7.87 × 10(-10) M, for PCE, TCE, TCA, and PCP, respectively. The results of this study suggest that the reductive power of ZVI materials can be standardized using tri-iodide, and thus, can provide a good reference for the quantitative assessment of the reactivity of metallic reducing agents of environmental interest including ZVIs.