Improved isotopic model based on 15 N tracing and Rayleigh-type isotope fractionation for simulating differential sources of N2 O emissions in a clay grassland soil.

RATIONALE: Isotopic signatures of N2 O can help distinguish between two sources (fertiliser N or endogenous soil N) of N2 O emissions. The contribution of each source to N2 O emissions after N-application is difficult to determine. Here, isotopologue signatures of emitted N2 O are used in an improved isotopic model based on Rayleigh-type equations. METHODS: The effects of a partial (33% of surface area, treatment 1c) or total (100% of surface area, treatment 3c) dispersal of N and C on gaseous emissions from denitrification were measured in a laboratory incubation system (DENIS) allowing simultaneous measurements of NO, N2 O, N2 and CO2 over a 12-day incubation period. To determine the source of N2 O emissions those results were combined with both the isotope ratio mass spectrometry analysis of the isotopocules of emitted N2 O and those from the 15 N-tracing technique. RESULTS: The spatial dispersal of N and C significantly affected the quantity, but not the timing, of gas fluxes. Cumulative emissions are larger for treatment 3c than treatment 1c. The 15 N-enrichment analysis shows that initially ~70% of the emitted N2 O derived from the applied amendment followed by a constant decrease. The decrease in contribution of the fertiliser N-pool after an initial increase is sooner and larger for treatment 1c. The Rayleigh-type model applied to N2 O isotopocules data (δ15 Nbulk -N2 O values) shows poor agreement with the measurements for the original one-pool model for treatment 1c; the two-pool models gives better results when using a third-order polynomial equation. In contrast, in treatment 3c little difference is observed between the two modelling approaches. CONCLUSIONS: The importance of N2 O emissions from different N-pools in soil for the interpretation of N2 O isotopocules data was demonstrated using a Rayleigh-type model. Earlier statements concerning exponential increase in native soil nitrate pool activity highlighted in previous studies should be replaced with a polynomial increase with dependency on both N-pool sizes.

Isotope fractionation factors controlling isotopocule signatures of soil-emitted N2O produced by denitrification processes of various rates.

RATIONALE: This study aimed (i) to determine the isotopic fractionation factors associated with N2O production and reduction during soil denitrification and (ii) to help specify the factors controlling the magnitude of the isotope effects. For the first time the isotope effects of denitrification were determined in an experiment under oxic atmosphere and using a novel approach where N2O production and reduction occurred simultaneously. METHODS: Soil incubations were performed under a He/O2 atmosphere and the denitrification product ratio [N2O/(N2 + N2O)] was determined by direct measurement of N2 and N2O fluxes. N2O isotopocules were analyzed by mass spectrometry to determine δ(18)O, δ(15)N and (15)N site preference within the linear N2O molecule (SP). An isotopic model was applied for the simultaneous determination of net isotope effects (η) of both N2O production and reduction, taking into account emissions from two distinct soil pools. RESULTS: A clear relationship was observed between (15)N and (18)O isotope effects during N2O production and denitrification rates. For N2O reduction, diverse isotope effects were observed for the two distinct soil pools characterized by different product ratios. For moderate product ratios (from 0.1 to 1.0) the range of isotope effects given by previous studies was confirmed and refined, whereas for very low product ratios (below 0.1) the net isotope effects were much smaller. CONCLUSIONS: The fractionation factors associated with denitrification, determined under oxic incubation, are similar to the factors previously determined under anoxic conditions, hence potentially applicable for field studies. However, it was shown that the η(18)O/η(15)N ratios, previously accepted as typical for N2O reduction processes (i.e., higher than 2), are not valid for all conditions.

Adsorption of surfactant-rich stickies onto mineral surfaces.

"Stickies" are tacky species, present in recycled paper and coated broke, derived from coating formulations, adhesives etc. They impact negatively on paper quality and cause web runnability problems by deposit build-up. To sustain recycling, stickies are controlled by adsorbing them onto minerals added to the recycled stock. We report isotherms for a fatty acid ester defoamer and an acrylic acid ester copolymer adsorbing from colloidal suspension onto various talcs and modified calcium carbonates. We used commercial preparations of the fatty acid ester defoamer and acrylic acid ester copolymer to provide a simple analogue to the industrial process. The modified calcium carbonates are hydrophilic with anionic and cationic sites present. Adsorption isotherms for low surface area modified calcium carbonate conform to the Langmuir model, while those for high surface area modified calcium carbonate reflect a two stage process involving the formation of a monolayer over the mineral surface and subsequent partial aggregation. Talc platelets display hydrophilic edges and hydrophobic surfaces. Adsorption onto them appears to involve three stages; specifically, a hydrophilic interaction between hydrophilic groups on the molecules and the talc edges, followed by hydrophobic interactions between the molecules and the talc surfaces, and finally by formation of multilayers.

Use of a void network model to correlate porosity, mercury porosimetry, thin section, absolute permeability, and NMR relaxation time data for sandstone rocks.

The Pore-Cor void network model is used to construct stochastic realizations of the void structures of five sandstone samples of varying lithography. A close match was achieved to experimental porosity and mercury intrusion curves. The samples were resin impregnated and the fragments of voids revealed in thin sections photographed by backscatter electron microscopy at two magnifications. The sizes of these pore fragments matched those derived from a simulated microtoming of the network model much more closely than the sizes derived from the traditional capillary bundle approximation. Absolute permeabilities of the network were calculated by finding the flow capacity of the entire flow network, based on parametrized Navier Stokes equations with Klinkenberg correction, applied to each pore-throat-pore arc. A match to the experimental trend was obtained, although the network model considerably underestimated the experimental values. The results were also compared with the semiempirical equations of Thomson et al. and Kozeny and Carmen modified to accept thin section image analysis. Finally, the simulated pore and throat size distributions were compared to proton NMR transverse (T2) spin-echo relaxation times. Although the shapes of the distributions differed markedly, the mean values trended together. The capillary bundle approximation, however, gave a poor match to the NMR data.

Influence of anisotropy on the dynamic wetting and permeation of paper coatings.

A void network model, named Pore-Cor, has been used to study the permeation of an ink solvent into paper coating formulations coated onto a synthetic substrate. The network model generated anisotropic void networks of rectangular cross-sectional pores connected by elliptical cross-sectional throats. These structures had porosities and mercury intrusion properties which closely matched those of the experimental samples. The permeation of hexadecane, used as an analogue for the experimental test oil, was then simulated through these void structures. The simulations were compared to measurements of the permeation of mineral oil into four types of paper coating formulation. The simulations showed that the inertia of the fluid as it enters void features causes a considerable change in wetting over a few milliseconds, a timescale relevant to printing in a modern press. They also showed that in the more anisotropic samples, fast advance wetting occurred through narrow void features. It was found that the match between experimental and simulated wetting could be improved by correcting the simulation for the number of surface throats. The simulations showed a more realistic experimental trend, and much greater preferential flow, than the traditional Lucas-Washburn and effective hydraulic radius approaches.

The Effects of Void Geometry and Contact Angle on the Absorption of Liquids into Porous Calcium Carbonate Structures.

The absorption (permeation) of alcohols into porous blocks of calcium carbonate has been studied experimentally and with a computer model. The experimental measurement was of change in apparent weight of a block with time after contact with liquid. The modeling used the previously developed 'Pore-Cor' model, based on unit cells of 1000 cubic pores connected by cylindrical throats. To gain some insight into absorption into voids of complex geometry, and to provide a representation of heterogeneities in surface interaction energy, the cylindrical throats were converted to double cones. Relative to cylinders, such geometries caused hold-ups of the percolation of nonwetting fluids with respect to increasing applied pressure, and a change in the rate of absorption of wetting fluids. Both the measured absorption of the alcohols and the simulated absorption of the alcohols and of water showed significant deviations from that predicted by an effective hydraulic radius approximation. The simulation demonstrated the development of a highly heterogeneous wetting front, and of preferred wetting pathways that were perturbed by inertial retardation. The findings are useful in the design of high-performance, low-waste pigments for paper coatings, and environmentally friendly printing inks, as well as in wider industrial, environmental, and geological contexts. Copyright 2001 Academic Press.