William P. Ball

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.

New modeling paradigms for the sorption of hydrophobic organic chemicals to heterogeneous carbonaceous matter in soils, sediments, and rocks.

Abstract Heterogeneity in naturally occurring carbonaceous materials (CMs) causes sorbed hydrophobic organic compound (HOC) concentrations in soils, sediments, and rocks to occur as a combination of surface adsorption and phase partitioning, with the latter typically more linearly dependent on aqueous concentration. In this manuscript, we describe a model to simulate HOC sorption as the combined effect of adsorption to thermally altered CM and a more linear solvation-driven absorption into gel-like CM (organic matter). We describe different forms of thermally altered CM (such as soots, chars, coals, and kerogen), the manner in which these materials can serve as especially strong adsorbents, and the conditions under which they can control solid–aqueous distribution. Specific examples of model fits to soil, sediment and rock samples with identified thermally altered CM components provide a linkage between sorption components and sorbent material properties. Because both the adsorption and partition components are scalable by compound solubility, it may often be possible to estimate nonlinear isotherms for a wide range of chemicals based on comparatively few experimental measurements. Thermally altered CM is widespread in the environment and can serve as an important sorbent even when present in small quantities (especially at low concentrations of adsorbates). In this context, the sorption modeling refinements described in this work are expected to have wide applicability. Given that solid/water distribution is a central process affecting contaminant fate, such refined models are an essential element for better estimates of risk and improved remediation design.

Adsorption-Partitioning Uptake of Nine Low-Polarity Organic Chemicals on a Natural Sorbent

Sorption of comparatively nonpolar organic chemicals by natural solids not only can be predominated by partitioning with organic matter but also can reflect a substantial contribution from adsorption at low relative concentration. Sorption of nine polycyclic aromatic hydrocarbons (PAHs) and chlorinated benzenes (CBs) was investigated on a subsurface aquitard through batch study, with results interpreted by a composite adsorption-partitioning model. For both PAHs and CBs, the low-concentration adsorption slope and the coefficient for partitioning each correlated well with Kow; however, PAHs consistently sorbed more strongly than CBs at given Kow. For all chemicals, adsorption contributions were only important at low relative concentration and could be successfully modeled by assuming either Langmuir-type or Polanyi-type isotherms. Isotherms for all liquid chemicals fell on a single isotherm when plotted on a Polanyi basis (adsorbed volume per mass of sorbent versus adsorption potential density), providing ...

Influence of Surface Oxides on the Colloidal Stability of Multi-Walled Carbon Nanotubes: A Structure−Property Relationship

As with all nanomaterials, a large fraction of the atoms in carbon nanotubes (CNTs) reside at or near the surface. Consequently, surface chemistry will play a crucial role in determining the fate and transport of CNTs in aquatic environments. Frequently, oxygen-containing functional groups (surface oxides) are deliberately grafted into the CNT surface to promote colloidal stability. To study the influence that both the oxygen concentration and the oxygen functional-group distribution have on the colloidal stability of multiwalled carbon nanotubes (MWCNTs), a suite of oxidized MWCNTs (O-MWCNTs) were created using different oxidizing agents and reaction conditions. Stable colloidal suspensions were prepared by low-power sonication of O-MWCNT powders in Milli-Q water. Results from TEM, AFM, DLS, and XPS measurements revealed that, irrespective of the surface chemistry, the colloidal suspensions were composed of individual nanotubes with comparable length distributions. The critical coagulation concentrations (CCC) of O-MWCNTs that exhibited different surface chemistries were measured with time-resolved dynamic light scattering (TR-DLS) using NaCl as the electrolyte. Over a range of environmentally relevant pH values, linear correlations were found to exist between the CCC, total oxygen concentration, and surface charge of O-MWCNTs. In contrast to surface charge, electrophoretic mobility did not prove to be a useful metric of colloidal stability. Information obtained from chemical derivatization studies, carried out in conjunction with XPS, revealed that the distribution of oxygen-containing functional groups also influences the colloidal stability of O-MWCNTs, with carboxylic acid groups playing the most important role. This study highlights the fact that quantitative relationships can be developed to rationalize the influence of surface chemistry on the behavior of nanomaterials in aquatic environments.

Sorption of aqueous Zn[II] and Cd[II] by multiwall carbon nanotubes: the relative roles of oxygen-containing functional groups and graphenic carbon.

Exposure of multiwalled carbon nanotubes (MWCNTs) to oxidizing acids and other oxidants introduces oxygen-containing functional groups such as hydroxyl, carboxyl, and carbonyl groups onto the surface. This research evaluated how changes in oxygen concentration and distribution of oxygen-containing functional groups influenced the sorption of aqueous zinc and cadmium on MWCNTs. Sorption results with natural char, activated carbon, and a suite of MWCNTs (of varying surface oxygen content) were obtained. Results confirmed that surface oxygen enhances the sorption of both Zn[II] and Cd[II] from aqueous solution. Although Zn[II] sorbed more strongly than Cd[II] for all materials studied, surface oxidation had more effect on the sorption of Cd[II] than of Zn[II]. Additional sorption experiments with Zn[II] and 16 MWCNTs of varying surface oxidation level and functional group distribution revealed the relative contributions of different types of surface sites to sorption. Sorption isotherms were fit using a two-site Langmuir adsorption model that incorporated the independent characterization of functional group distribution. Results showed that carboxyl-carbon sites were over 20 times more energetic for zinc sorption than unoxidized carbon (graphenic-carbon) sites, though both site types are important contributors to sorption.

Characterization of a sandy aquifer material at the grain scale.

Abstract Solute sorption in aquifer systems is significantly affected by processes which occur at the scale of individual solid particles, such that proper physical characterization of the solids is requisite to fully understanding solute transport. Methods for characterizing grains of sandy aquifer material with respect to physical properties relevant to sorption are described and assessed. The properties include particle density, porosity, pore size distribution, specific surface area, and carbon content. Because intraparticle porosity, specific surface area, and organic carbon are quite low for sandy materials, methods routinely used for characterizing solids must be carefully evaluated and adapted for use on aquifer solids. The methods considered here were applied to aquifer material acquired at a site in Borden, Ontario, where numerous transport studies have been conducted. Results with well-characterized model solids are also included, as appropriate, for method evaluation. Results with the Borden solids are reported for eight size fractions and for the homogenized bulk material. Mineralogical characterization of the fractions served as an indispensable complement to the physical characterization methods. Pulverization of samples in a shatterbox was shown to be useful for homogenizing samples and reducing variability. We found a modified dry combustion method to be superior to wet oxidation for determining organic carbon. Surface area measurements were indicative of significant internal porosity, and pore size distributions obtained by gas adsorption and mercury porosimetry were found to be consistent and complementary. For the Borden material—which has an immeasurably low clay mineral content—specific surface area, intraparticle porosity, and organic carbon content were all greatest in the larger size fractions.

Assessing the colloidal properties of engineered nanoparticles in water: case studies from fullerene C60 nanoparticles and carbon nanotubes

Environmental context. The fate and bioavailability of engineered nanoparticles in natural aquatic systems are strongly influenced by their ability to remain dispersed in water. Consequently, understanding the colloidal properties of engineered nanoparticles through rigorous characterisation of physicochemical properties and measurements of particle stability will allow for a more accurate prediction of their environmental, health, and safety effects in aquatic systems. This review highlights some important techniques suitable for the assessment of the colloidal properties of engineered nanoparticles and discusses some recent findings obtained by using these techniques on two popular carbon-based nanoparticles, fullerene C60 and multi-walled carbon nanotubes. Abstract. The colloidal properties of engineered nanoparticles directly affect their use in a wide variety of applications and also control their environmental fate and mobility. The colloidal stability of engineered nanoparticles depends on their physicochemical properties within the given aqueous medium and is ultimately reflected in the particles’ aggregation and deposition behaviour. This review presents some of the key experimental methods that are currently used to probe colloidal properties and quantify engineered nanoparticle stability in water. Case studies from fullerene C60 nanoparticles and multi-walled carbon nanotubes illustrate how the characterisation and measurement methods are used to understand and predict nanoparticle fate in aquatic systems. Consideration of the comparisons between these two classes of carbon-based nanoparticles provides useful insights into some major current knowledge gaps while also revealing clues about needed future developments. Key issues to be resolved relate to the nature of near-range surface forces and the origins of surface charge, particularly for the reportedly unmodified or ‘pure’ carbon-based nanoparticles.

Application of inverse methods to contaminant source identification from aquitard diffusion profiles at Dover AFB, Delaware

This paper presents a refinement and expansion of our previously described efforts to estimate contaminant plume history from observed contaminant concentrations within a low-permeability aquitard at the site of a field-scale groundwater remediation experiment at Dover Air Force Base. At this site, a two-layer aquitard has been contaminated with tetrachloroethene and trichloroethene through diffusive mass transfer from an overlying contaminated aquifer. Measurements of contaminant concentrations in the aquitard are used, together with independently obtained information about the sorption and diffusion properties of the aquitard medium, to estimate the timewise variation of the boundary concentration at the interface between the aquitard and aquifer, thus providing evidence related to the overlying plume history. In our refined analysis, we assume the contaminant source history to be a function of time with unknown form, and we supplement our interpretations with a second coring result at a different location. The results demonstrate how “forensic” interpretations of this kind can provide useful and important information regarding the contaminant release history at sites of groundwater contamination and cleanup; however, the results also show that the forensic problem is a highly nonunique problem associated with potentially large uncertainty. Interpretation of the estimated results therefore requires careful consideration in the context of other available information.

Variability of aquifer sorption properties in a field experiment on groundwater transport of organic solutes: Methods and preliminary results

Abstract Methods to characterize the organic solute sorption distribution coefficient, organic carbon content, and specific surface area of aquifer solids from the site of a field experiment on solute transport in groundwater were refined for application to small subsamples of 10-cm depth increments taken from 5-cm diameter cores. Initial results indicate that the average sorption characteristics of the Borden aquifer do not vary appreciably along the trajectory of the solute plumes. However, the sorption distribution coefficient of tetrachloroethylene varied over nearly an order of magnitude among 10-cm depth increments in one core sample. Preliminary evidence suggests that the sorption distribution coefficients for four halogenated organic solutes vary proportionally among core strata. However, the distribution coefficients for sorption of tetrachloroethylene on various depth increments are not well correlated with either organic carbon content or specific surface area, suggesting that as yet unidentified mineral phases may play a significant role in sorption of such solutes by the sandy aquifer solids.

A diffusion‐based interpretation of tetrachloroethene and trichloroethene concentration profiles in a groundwater aquitard

Analysis of subsurface soil cores from the site of a field-scale groundwater remediation experiment at Dover Air Force Base, Delaware, has revealed that tetrachloroethene (PCE) and trichloroethene (TCE) contamination extends into an aquitard underlying a groundwater aquifer. The site location is well downgradient of the locations of contaminant release, and the aquitard contamination is believed to have begun when contaminated groundwater first arrived in the overlying aquifer. Using independent estimates of sorption and diffusion properties in the aquitard layers, mathematical modeling based on diffusion in laminate slabs has been used to make inferences regarding the historical concentration conditions in the overlying aquifer. The results suggest that plume arrival occurred within the last two decades, with some important differences in the inferred TCE and PCE plume histories. The diffusion model was also applied toward predicting future aquitard concentrations and fluxes under scenarios based on the current condition as a starting point and hypothesized conditions of future groundwater cleanup. The results demonstrate how aquitard sampling and diffusion modeling can provide essential information relevant to forensic analysis, risk assessment, and subsurface cleanup.