Philip M. Gschwend

Comparison of quantification methods to measure fire‐derived (black/elemental) carbon in soils and sediments using reference materials from soil, water, sediment and the atmosphere

Black carbon (BC), the product of incomplete combustion of fossil fuels and biomass (called elemental carbon (EC) in atmospheric sciences), was quantified in 12 different materials by 17 laboratories from different disciplines, using seven different methods. The materials were divided into three classes: (1) potentially interfering materials, (2) laboratory-produced BC-rich materials, and (3) BC-containing environmental matrices (from soil, water, sediment, and atmosphere). This is the first comprehensive intercomparison of this type (multimethod, multilab, and multisample), focusing mainly on methods used for soil and sediment BC studies. Results for the potentially interfering materials (which by definition contained no fire-derived organic carbon) highlighted situations where individual methods may overestimate BC concentrations. Results for the BC-rich materials (one soot and two chars) showed that some of the methods identified most of the carbon in all three materials as BC, whereas other methods identified only soot carbon as BC. The different methods also gave widely different BC contents for the environmental matrices. However, these variations could be understood in the light of the findings for the other two groups of materials, i.e., that some methods incorrectly identify non-BC carbon as BC, and that the detection efficiency of each technique varies across the BC continuum. We found that atmospheric BC quantification methods are not ideal for soil and sediment studies as in their methodology these incorporate the definition of BC as light-absorbing material irrespective of its origin, leading to biases when applied to terrestrial and sedimentary materials. This study shows that any attempt to merge data generated via different methods must consider the different, operationally defined analytical windows of the BC continuum detected by each technique, as well as the limitations and potential biases of each technique. A major goal of this ring trial was to provide a basis on which to choose between the different BC quantification methods in soil and sediment studies. In this paper we summarize the advantages and disadvantages of each method. In future studies, we strongly recommend the evaluation of all methods analyzing for BC in soils and sediments against the set of BC reference materials analyzed here.

Sorption kinetics of hydrophobic organic compounds to natural sediments and soils.

Sorption kinetics of hydrophobic organic chemicals to and from suspended sediment and soil particles is described by a radial diffusive penetration model modified by a retardation factor reflecting microscale partitioning of the sorbate between intraaggregate pore fluids and the solids making up the aggregate grains. In light of this and other sorption kinetics models, a closed-loop-stripping apparatus with a photoionization detector operating in-line was used to examine the effects of sorbate hydrophobicity, sorbent particle size, and system temperature on solid-solution exchange over times of seconds to days. The authors results indicate that a single effective diffusivity parameter, which is predictable from compound solution diffusivity, octanol-water partition coefficient, and sorbent organic content, density, and porosity, can be used to quantify the sorption kinetics.

Fluxes of polycyclic aromatic hydrocarbons to marine and lacustrine sediments in the northeastern United States

Polycyclic aromatic hydrocarbons (PAH) were measured in dated sediment cores from several sites in the northeastern United States (Lake Superior, Isle Royale, Somes Sound, Hadlock Lower Pond. Coburn Mountain Pond, and outer Boston Harbor). Fluxes of ten PAH were measured for each site for the periods roughly corresponding to the present, 1950, and 1900. Remote sites consistently demonstrated present-day deliveries of individual PAH near 1 ng cm−2 yr−1, probably reflecting the atmospheric fallout of these combustion-derived pollutants. Sites located nearer to urban centers showed much greater current inputs (average of 35 ng cm−2 yr−1 for most individual PAH), presumably caused by greater fallout of PAH-laden particles nearer their urban origins, augmented by runoff delivery of PAH-contaminated sediments. Differences in the relative abundances of individual PAH at remote-versus-urban locations support suggestions of different delivery mechanisms. The sedimentary historical records of PAH inputs confirm the previous finding that anthropogenic activities began introducing large quantities of PAH into the environment about 80–100 years ago.

Volatile halogenated organic compounds released to seawater from temperate marine macroalgae.

Volatile halogenated organic compounds synthesized by various industrial processes are troublesome pollutants because they are persistent in terrestrial ecosystems and because they may be present in sufficient quantities to alter the natural atmospheric cycles of the halogens. Certain of these compounds, including polybromomethanes and several previously unobserved alkyl monohalides and dihalides, appear to be natural products of the marine environment. A variety of temperate marine macroalgae (the brown algae Ascophyllum nodosum and Fucus vesiculosis, the green algae Enteromorpha linza and Ulva lacta, and the red alga Gigartina stellata) not only contain volatile halogenated organic compounds but also release them to seawater at rates of nanograms to micrograms of each compound per gram of dry algae per day. The macroalgae may be an important source of bromine-containing material released to the atmosphere.

The abundance, distribution, and configuration of porewater organic colloids in recent sediments

Abstract The characterization and quantitation of aquatic organic colloids isolated from sedimentary porefluids was performed by size exclusion chromatography (SEC) with UV detection and ultrafiltration (UF) with total organic carbon analysis and UV detection. Separation of the samples and several well-characterized humic substances by SEC was found to be highly dependent upon the ionic strength of both the mobile phase and sample matrix. The elution of samples containing indifferent electrolyte concentrations by low ionic strength mobile phases yielded multipeak chromatograms and may be subject to misinterpretation as true molecular weight distributions. A dynamic coiling/uncoiling mechanism is offered as an explanation for the observed phenomenon. For well-characterized humic substances eluted with high ionic strength mobile phases, number- and weight-average molecular weights as well as the polydispersivities were calculated and were found to be in good agreement with other recently published values. Molecular weight distributions for Suwanee fulvate determined by both sizing techniques appeared to be in reasonable agreement. Ultrafiltration was used to separate organic colloids from low molecular weight components in sedimentary porewaters. Vertical profiles in the cores showed depletion of colloids near the sediment/water interface and accumulation of macromolecules with depth. Porewater colloids collected using glass fritted samplers appeared to confirm this trend, although lower values were recovered than when extracted by centrifugation methods.

The Flux of Black Carbon to Surface Sediments on the New England Continental Shelf

Abstract Concentrations and 210 Pb-derived fluxes of Black Carbon (BC) were deduced in recent ocean margin sediments off northeastern USA to investigate the role of this carbon form in the biogeochemical cycle of this element. BC concentrations in contemporary open shelf sediments ranged between 0.11 and 1.7 mg/gdw. Temporal trends of BC in cores, as well as the pattern of chemical markers of combustion sources, were consistent with anthropogenic fossil fuel combustion dominating the BC input to these sediments in recent times. BC fluxes of 1–2 g m −2 yr −1 suggest that the northeastern USA shelf sink is of the same magnitude as estimates of the BC production from fossil fuel and biomass burning in the upwind NE USA source region. Identification of about 10% of total organic carbon as BC has implications for sedimentary carbon preservation, as well as several other aspects of the global biogeochemical cycle of carbon.

Monodisperse ferrous phosphate colloids in an anoxic groundwater plume

Groundwater samples collected near a secondary-sewage infiltration site on Cape Cod, Massachusetts were examined for colloidal materials (10–1000 nm). In two wells the water contained a population of monodisperse 100-nm particles, detected using laser-light scattering and autocorrelation data processing. SEM and SEM-EDAX analysis of these colloidal materials collected on ultrafilters confirmed the laser light scattering result and revealed that these microparticles consisyed of primarily iron and phosphorus in a 1.86 Fe to 1.0 P stoichiometric ratio. Chemical analyses of the water samples, together with equilibrium solubility calculations, strongly suggest that the ion-activity product should exceed the solubility product of a 100-nm diameter predominantly vivianite-type (Fe3(PO4)2 · 8H2O) colloidal phase. In light of our results, we conclude that these microparticles were formed by sewage-derived phosphate combining with ferrous iron released from the aquifer solids, and that these colloids may be moving in the groundwater flow. Such a subsurface transport process could have major implications regarding the movement of particle-reactive pollutants traditionally viewed as non-mobile in groundwater.

Effect of solution chemistry on clay colloid release from an iron oxide-coated aquifer sand

This research compared the influence of (1) dissolution of iron oxides and (2) alteration of electrostatic interactions on the mobilization of colloids in a clay- and iron oxide-coated sand obtained from an Atlantic Coastal Plain aquifer in which colloids have been found suspended only in anoxic groundwater. The sediment was flushed with solutions of varying ionic strength, pH, and reductant and surfactant concentrations, and the steady-state rates of clay colloid release and iron oxyhydroxide dissolution were measured. The clay release rates were directly related to the calculated detachment energies and unrelated to rates of iron(III) oxide dissolution, indicating that electrostatic interactions dominated the binding of colloids to grain coatings. Mobilization of colloids by iron(III) oxide dissolution through reductive dissolution was limited by high ionic strength. Flushing of the sediment by a natural groundwater with high dissolved organic carbon concentration released clay without rapidly dissolving iron oxides. 59 refs., 8 figs., 3 tabs.

Hydrocarbons in cores of northwestern Atlantic coastal and continental margin sediments

Abstract The concentrations and composition of hydrocarbons in sections from a 210 Pb dated core from Buzzards Bay, Massachusetts are reported. Terrigenous n -alkane concentrations remain constant throughout the 62 cm and 195 years sampled. Gas Chromatographic analysis on SCOT columns revealed an unresolved complex mixture of alkanes and cycloalkanes which decreases in concentration with increasing depth. Phenanthrene and C 1 and C 2 substituted phenanthrene concentrations also decreased with increasing depth and the relative abundance of C 1 and C 2 homologs compared to phenanthrene suggest a pyrolytic origin for these aromatic hydrocarbons. The various sources of hydrocarbons in surface sediments and the processes which could govern their distribution are discussed. The most likely source of the phenanthrenes and the unresolved complex mixture of alkanes and cycloalkanes in the upper core sections circa 1900 to the present seems to be urban air fallout. A depth distribution of alkanes and cycloalkanes similar to the Buzzards Bay core was found for a second area of Buzzards Bay and for cores from sediments of the Gulf of Maine and Hudson Canyon. Two C 25 cycloalkenes were identified as major hydrocarbons in the sediments. Their concentrations decrease with increasing depth in the cores. Mass spectra of these compounds and their hydrogenation products are reported.

Early Evaluation of Potential Environmental Impacts of Carbon Nanotube Synthesis by Chemical Vapor Deposition

The carbon nanotube (CNT) industry is expanding rapidly, yet little is known about the potential environmental impacts of CNT manufacture. Here, we evaluate the effluent composition of a representative multiwalled CNT synthesis by catalytic chemical vapor deposition (CVD) in order to provide data needed to design strategies for mitigating any unacceptable emissions. During thermal pretreatment of the reactant gases (ethene and H(2)), we found over 45 side-products were formed, including methane, volatile organic compounds (VOCs), and polycyclic aromatic hydrocarbons (PAHs). This finding suggests several environmental concerns with the existing process, including potential discharges of the potent greenhouse gas, methane (up to 1.7%), and toxic compounds such as benzene and 1,3-butadiene (up to 36000 ppmv). Extrapolating these laboratory-scale data to future industrial CNT production, we estimate that (1) contributions of atmospheric methane will be negligible compared to other existing sources and (2) VOC and PAH emissions may become important on local scales but will be small when compared to national industrial sources. As a first step toward reducing such unwanted emissions, we used continuous in situ measures of CNT length during growth and sought to identify which thermally generated compounds correlated with CNT growth rate. The results suggested that, in future CNT production approaches, key reaction intermediates could be delivered to the catalyst without thermal treatment. This would eliminate the most energetically expensive component of CVD synthesis (heating reactant gases), while reducing the formation of unintended byproducts.