Photochemical transformation of dissolved organic matter (DOM) has been studied for more than two decades. Usually, laboratory or "in-situ" experiments are used to determine photodegradation ...variables. A common problem with these experiments is that the photodegradation experiments are done at higher than ambient temperature. Five laboratory experiments were done to determine the effect of temperature on photochemical degradation of DOM. Experimental results showed strong dependence of photodegradation on temperature. Mathematical modeling of processes revealed that two different pathways engaged in photochemical transformation of DOM to dissolved inorganic carbon (DIC) strongly depend on temperature. Direct oxidation of DOM to DIC dominated at low temperatures while conversion of DOM to intermediate particulate organic carbon (POC) prior to oxidation to DIC dominated at high temperatures. It is necessary to consider this strong dependence when the results of laboratory experiments are interpreted in regard to natural processes. Photodegradation experiments done at higher than ambient temperature will necessitate correction of rate constants.
•The breakthrough curves of the NPs exhibited a bimodal shape with increasing solution ionic strength.•Deposition dynamics of the NPs was simulated using a two-site kinetic model.•NP deposition is ...controlled by the coupled effects of flow velocity, solution chemistry, and particle size.•NP interactions with the collector tend to strengthen with increasing contact time.
This study aims to systematically explore the coupled effects of hydrodynamic and solution chemistry conditions on the long-term transport and deposition kinetics of nanoparticles (NPs) in saturated porous media. Column transport experiments were carried out at various solution ionic strengths (IS), ionic composition, and flow velocities utilizing negatively charged carboxyl-modified latex NPs of two different sizes (50 and 100nm). These experiments were designed to obtain the long-term breakthrough curves (BTCs) in order to unambiguously determine the full deposition kinetics and the fraction of the solid surface area (Sf) that was available for NP deposition. The BTCs exhibited a bimodal shape with increasing solution IS; i.e., BTCs were initially delayed, next they rapidly increased, and then they slowly approached the influent particle concentration. NP deposition was much more pronounced in the presence of Ca2+ than Na+ at any given solution IS. Deposition kinetic of NPs was successfully simulated using a two-site kinetic model that accounted for irreversible deposition and blocking on each site, i.e., a decreasing deposition rate as the site filled. Results showed that Sf values were controlled by the coupled effects of flow velocity, solution chemistry, and particle size. Data analyses further demonstrated that only a small fraction of sand surface area contributed in NP deposition even at the highest IS (60mM) and lowest flow velocity (1m/day) tested. Consistent with previous studies, our results imply that NP deposition is controlled by physicochemical interactions between the NPs and nanoscale physical and/or chemical heterogeneities on the sand surfaces that produce localized nanoscale favorable sites for deposition. Furthermore, our results suggest that the NP interactions with the collector surfaces tended to strengthen with increasing contact time.
Nitrous oxide (N2O) is a potent greenhouse gas produced during nitrogen cycling. Global nitrogen enrichment has resulted in increased atmospheric N2O concentrations due in large part to increased ...soil emissions. There is also a potentially important flux from streams, rivers and estuaries; although measurements of these emissions are sparse, and role of aquatic ecosystems in global N2O budgets remains highly uncertain. Using the longest‐term measurements of N2O fluxes from streams to date, we found annual fluxes from 14 sites in five streams of south‐central Ontario, Canada varied widely–from net uptake of 3.2 ± 0.2 (standard deviation)μmol N2O m−2 d−1 to net release of 776 ± 61 μmol N2O m−2 d−1. N2O consumption was associated with very low nitrate concentrations (<2.7 μM). Mean annual (log‐transformed) N2O emissions from our study streams (across sites and years) were positively related to nitrate concentrations (r2= 0.59).This nitrate‐N2O relationship can be generalized across all 20 streams (in Canada, Japan, Mexico, and the midwestern United States) for which published data now exist and could provide a new basis for the IPCC to calculate agricultural emissions from streams. In addition to predicting annual emissions, we present the first measurements of N2O concentrations under ice in streams. Nitrate was a strong predictor of N2O % saturation during periods of ice cover (r2 = 0.89), when gas exchange is negligible. Given the small surface area of streams within a catchment and the fact our measured areal fluxes are comparable to reported fluxes from agricultural soils, this suggests streams are a small regional N2O source.
Key Points
Nitrate is a predictor of annual nitrous oxide emissions across 20 streams globally
Nitrous oxide consumption occurs below a threshold nitrate concentration
Nitrous oxide concentrations are related to nitrate concentrations under ice
Reframing healthcare delivery in terms of the principles of complex adaptive systems has practical implications for addressing the challenges in improving surgical care. In an Integrated Practice ...Unit (IPU) - such as a surgical service line, a surgical in-patient floor, or an acute care unit - a diverse group of caregivers must interact in a highly interdependent fashion in an environment characterized by ambiguity, uncertainty, and time constraints. Understanding of the concept of teaming and the tenets of relational coordination are crucial to the promotion of a successful patient-centric approach to surgical care.
Background
Improved mortality rates following pancreaticoduodenectomy by high-volume surgeons and hospitals have been well documented, but less is known about the impact of such volumes on length of ...stay and cost. This study uses data from the Healthcare Cost and Utilization Project (HCUP) National Inpatient Sample (NIS) to examine the effect of surgeon and hospital volume on mortality, length of stay, and cost following pancreaticoduodenectomy while controlling for patient-specific factors.
Methods
Data included 3,137 pancreaticoduodenectomies from the NIS performed between 2004 and 2008. Using logistic regression, the relationship between surgeon volume, hospital volume, and postoperative mortality, length of stay, and cost was estimated while accounting for patient factors.
Results
After controlling for patient characteristics, patients of high-volume surgeons at high-volume hospitals had a significantly lower risk of mortality compared to low-volume surgeons at low-volume hospitals (OR 0.32,
p
< 0.001). Patients of high-volume surgeons at high-volume hospitals also had a five day shorter length of stay (
p
< 0.001), as well as significantly lower costs (US$12,275,
p
< 0.001).
Conclusions
The results of this study, which simultaneously accounted for surgeon volume, hospital volume, and potential confounding patient characteristics, suggest that both surgeon and hospital volume have a significant effect on outcomes following pancreaticoduodenectomy, affecting not only mortality rates but also lengths of stay and costs.
Using measured rates of bubble release and diffusive gas transport (also termed surface aeration), we address the role of these transport mechanisms in emissions of nitrous oxide and methane from ...four streams. While ebullition in streams and rivers has received little study, we found that ebullition was an important mode of methane emissions, contributing 20%–67% of methane emissions (among streams). Nitrous oxide emissions via ebullition were negligible (<0.1% of diffusive emissions). Total methane emissions (ebullition + diffusive transport) were over ten times greater than N2O emissions in terms of CO2 equivalents. Rates of bubble release were highly variable, ranging from 20 mL m−2 d−1 to 170 mL m−2 d−1 (seasonal average among streams, with volumes reflecting ambient temperature and pressure). Methane was the most abundant of the bubble gases that were measured (26% by volume on average among streams), followed by carbon dioxide (1% on average) then nitrous oxide. Average bubble nitrous oxide concentrations were below atmospheric mixing ratios for the majority of streams; however, one stream showed concentrations as high as 3600 ppbv. Sediment characteristics were strong predictors of bubble composition. Concentrations of methane and nitrous oxide were positively related to the proportion of fine sediments. High methane concentrations in bubbles were related to high sediment organic carbon.
Key Points
Ebullition is an important mode of methane emissions from streams
Bubble gas concentrations and emissions are related to sediment characteristics
Methane emissions significantly exceed nitrous oxide emissions in terms of GWP
Many engineering applications, particularly in extreme environments, require components with properties that vary with location in the part. Functionally graded materials (FGMs), which possess ...gradients in properties such as hardness or density, are a potential solution to address these requirements. The laser-based additive manufacturing process of directed energy deposition (DED) can be used to fabricate metallic parts with a gradient in composition by adjusting the volume fraction of metallic powders delivered to the melt pool as a function of position. As this is a fusion process, secondary phases may develop in the gradient zone during solidification that can result in undesirable properties in the part. This work describes experimental and thermodynamic studies of a component built from 304L stainless steel incrementally graded to Inconel 625. The microstructure, chemistry, phase composition, and microhardness as a function of position were characterized by microscopy, energy dispersive spectroscopy, X-ray diffraction, and microindentation. Particles of secondary phases were found in small amounts within cracks in the gradient zone. These were ascertained to consist of transition metal carbides by experimental results and thermodynamic calculations. The study provides a combined experimental and thermodynamic computational modeling approach toward the fabrication and evaluation of a functionally graded material made by DED additive manufacturing.
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We explore the role of lakes in carbon cycling and global climate, examine the mechanisms influencing carbon pools and transformations in lakes, and discuss how the metabolism of carbon in the inland ...waters is likely to change in response to climate. Furthermore, we project changes as global climate change in the abundance and spatial distribution of lakes in the biosphere, and we revise the estimate for the global extent of carbon transformation in inland waters. This synthesis demonstrates that the global annual emissions of carbon dioxide from inland waters to the atmosphere are similar in magnitude to the carbon dioxide uptake by the oceans and that the global burial of organic carbon in inland water sediments exceeds organic carbon sequestration on the ocean floor. The role of inland waters in global carbon cycling and climate forcing may be changed by human activities, including construction of impoundments, which accumulate large amounts of carbon in sediments and emit large amounts of methane to the atmosphere. Methane emissions are also expected from lakes on melting permafrost. The synthesis presented here indicates that (1) inland waters constitute a significant component of the global carbon cycle, (2) their contribution to this cycle has significantly changed as a result of human activities, and (3) they will continue to change in response to future climate change causing decreased as well as increased abundance of lakes as well as increases in the number of aquatic impoundments.
Banking water in aquifers during wet years for long-term storage then recovering it in drought is an application of managed aquifer recharge (MAR) that minimises evaporation losses. This requires a ...suitable aquifer for long-term storage of banked water and occasional periods when entitlements to surface water are available and affordable. This has been widely practised in Arizona and California but thus far not in Australia, in spite of severe impacts on agriculture, society, and the environment during recent droughts in the Murray–Darling Basin. This preliminary study based on a simple area exclusion analysis using six variables, some on a 90 m grid, over the 1 million km2 basin produced a first estimate of the order of 2–4 × 109 m3 of additional aquifer storage potential in surficial aquifers close to rivers. For 6 of the 23 catchments evaluated, banking capacity exceeded an average water depth of 0.3 m for the irrigated area. At one prospective site in the Macquarie River catchment in New South Wales, water banking operations at various scales were simulated using 55 years of historical monthly hydrologic data, with recharge and recovery triggered by dam storage levels. This showed that the estimated 300 × 106 m3 additional local aquifer capacity could be fully utilised with a recharge and recovery capacity of 6 × 106 m3/month, and recharge occurred in 67% of months and recovery in 7% of months. A novel simulation of water banking with recharge and recovery triggered by water trading prices using 11 years of data gave a benefit cost ratio of ≈ 2. Data showed that water availability for recharge was a tighter constraint on water banking than aquifer storage capacity at this location. The analysis reveals that water banking merits further consideration in the Murray–Darling Basin. Firstly, management across hydrologically connected systems requires accounting for surface water and groundwater entitlements and allocations at the appropriate scale, as well as developing equitable economic and regulatory arrangements. Of course, site-specific assessment of water availability and hydrogeological suitability would be needed prior to construction of demonstration projects to support full-scale implementation.
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