John F. McCarthy

ADSORPTION AND DESORPTION OF DIFFERENT ORGANIC MATTER FRACTIONS ON IRON OXIDE

Abstract Natural organic matter (NOM) is a complex mixture of different organic components (or fractions), yet few studies have examined the fractional adsorption of NOM on mineral surfaces. In this study, we fractionated NOM into hydrophobic (HbA) and hydrophilic (HL) subcomponents and two size fractions (with nominal molecular weights cut off at 3000 (3 K) dalton in an attempt to elucidate the adsorption and desorption mechanisms of NOM on iron oxide surfaces. Results indicated that, on a C weight basis, larger size HbA fraction was preferentially adsorbed (with a higher adsorption affinity and capacity) over smaller size HL fraction. However, on an O weight basis, less HbA fraction was adsorbed relative to the HL fraction, because HbA contained about 1.34 times more C but 0.82 times less O than the HL. These observations are consistent with results which indicate that only limited adsorption sites are available on the iron oxide surfaces and that the mechanism of HbA and HL adsorption was dominated by surface complexation-ligand exchange. FTIR and NMR spectroscopy and studies with several substituted benzoic acids/phenols further indicated that carboxyl and hydroxyl functional groups of these NOM fractions were actively involved in the reactions, and the steric arrangement of these functional groups may have played an important role in determining the adsorption of NOM fractions. Desorption studies indicated that the adsorbed NOM macromolecules on iron oxide surfaces were strongly bound at a given pH and ionic composition, resulting in a strong adsorption-desorption hysteresis. One possible explanation for the observed hysteresis is that the solution composition and equilibria are not identical between adsorption and desorption phases of the experiment because of preferential or selective adsorption of certain NOM fractions. This study implies that, due to the polydispersity of NOM, the competitive and fractional adsorption-desorption of NOM subcomponents must be considered in order to better predict NOM partitioning between the solution and solid phases and, therefore, the transport behavior of NOM in the subsurface soil environment.

Interactions between polycyclic aromatic hydrocarbons and dissolved humic material: binding and dissociation

Binding of polycyclic aromatic hydrocarbons (PAHs) to dissolved humic material (DHM) was examined by using equilibrium dialysis and fluorescence techniques. There was a direct relationship between the hydrophobicity of the PAH and the affinity for binding to DHM. The binding affinity P/sub a/ for benzo(a)pyrene (BaP), benzanthracene, and anthracene decreased slightly as the concentration of DHM increased. The binding of BaP to DHM was completely reversible and the extent of reversibility was unrelated to the sorption time. The rate of binding of BaP to DHM, measured by the quenching to BaP fluorescence, was very rapid and was completed within 5-10 min. The results suggest that the presence of DHM, or other sorptive components of the dissolved organic pool, may affect binding to sediment or suspended particles and thus alter the fate and transport of organic contaminants in aquatic systems.

The use of bioindicators for assessing the effects of pollutant stress on fish

Abstract The use of bioindicators in environmental pollution studies involves monitoring a suite of selected stress responses at several levels of biological organization in order to (1) assess the effects of sublethal stress on fish, (2) predict future trends (early warning indicators) and (3) obtain insights into causal relationships between stress and effects at the community and ecosystem level. We have successfully applied this approach with redbreast sunfish (Lepomis auritus) in freshwater systems receiving inputs of complex contaminant mixtures containing polychlorinated biphenyls (PCBs), hydrocarbons, heavy metals and chlorine. Indicators such as mixed-function oxidase (MFO) enzymes and DNA damage have provided direct evidence of toxicant exposure, while condition indices and indicators related to lipid biochemistry and histopathology have reflected impaired lipid metabolism, immune and reproductive system dysfunction, and reduced growth potential. At higher levels of organization, stress-mediated effects have included changes in the richness and biotic integrity of fish communities.

Effects of XAD-8 fractions of dissolved organic carbon on the sorption and bioavailability of organic micropollutants

The dissolved organic carbon (DOC) from a stream water near a peat deposit was fractionated into hydrophobic-acid (HbA), hydrophobic-neutral (HbN), and hydrophilic (Hl) subcomponents by XAD-8 chromatography. The capacity of these fractions and the total (unfractionated) water to bind hydrophobic organic contaminants was measured by equilibrium dialysis, and the effect of binding on contaminant bioavailability was measured inDaphnia magna. Model contaminants were the poly cyclic aromatic hydrocarbons, naphthalene (NPH) and benzo(a)pyrene (BaP), and the polychlorinated biphenyl, 2,2′,5,5′-tetrachlorobiphenyl (TCB).Both BaP and TCB exhibited high partition coefficients (Kps) for binding to both the total DOC and the hydrophobic components of the DOC. BaP had a higher affinity for binding to the HbA fraction, while TCB (and 3 other PCBs had a higher affinity for the HbN fraction. The Kps for binding to the Hl fraction were twofold to tenfold lower than for binding to the hydrophobic fractions. The less hydrophobic compound, NPH, had a much lower Kp, and little difference was seen between the fractions. The total water and the different DOC fractions reduced the uptake and accumulation of BaP and TCB byD. magna in proportion to the capacity of the DOC for binding the contaminants. Data were consistent with the hypothesis that a contaminant bound to DOC (total water or any of the fractions) is unavailable for uptake by biota. Uptake of NPH was not substantially affected by the DOC, consistent with its lower Kp.

Effect of dissolved humic material on accumulation of polycyclic aromatic hydrocarbons: Structure-activity relationships

Abstract The effect of binding of polycyclic aromatic hydrocarbons (PAHs) to dissolved humic material (DHM) on the uptake and bioaccumulation of PAHs is examined in the cladoceran Daphnia magna. To the extent that a PAH binds to DHM, which is related to its hydrophobicity, it becomes unavailable for uptake by the organisms. The structure-activity relationship developed here suggests that the high affinity for binding to DHM and the resultant decrease in bioavailability might mitigate the biological impact of those very hydrophobic contaminants having the greatest potential for bioaccumulation and transfer to man via food chains.

Transport and Retention Mechanisms of Colloids in Partially Saturated Porous Media

The transport, retention, and release of hydrophobic and hydrophilic polystyrene latex microsphere colloids were examined in 0.5-cm-thick, 26-cm-long slab chambers filled with either regular (hydrophilic) or weakly water-repellent sand. The water-repellent sand consisted of a mixture of 0.4% strongly water-repellent grains with unmodified regular sand for the remainder. The concentration of colloids in the outflow water was measured at the same time as the pore-scale distribution of colloids was recorded in still and video images. Although the type of sand affected the flow pattern in the top of the chamber, it did not affect the breakthrough for the same type of colloids. More hydrophilic colloids were eluted in the drainage water than hydrophobic colloids. Images showed that there was a greater retention of the hydrophobic colloids due to strongly attractive hydrophobic interaction forces between colloids and subsequent filtering of colloidal aggregates in the narrow passages between grains. Once filtered, these aggregates then served as preferred sites for attachment of other hydrophobic colloids. The hydrophilic colloids were retained primarily in a thin film of water at the edge of the menisci, the air–water–solid (AWS) interface. Centrifugal motion within the pendular rings observed in the videos contributed to movement of the colloids toward the AWS interface, where colloids were retained due to both low laminar flow velocities near the grain surface and straining in the thin water film at the edge of the meniscus. Except near the solid interface, sorption at the air–water (AW) interface was not observed and appeared unimportant to the retention of colloids. The findings form an essential link between colloid retention and transport processes at the interfacial, pore, and Darcy scales.

Competitive adsorption, displacement, and transport of organic matter on iron oxide: II. Displacement and transport

Abstract The competitive interactions between organic matter compounds and mineral surfaces are poorly understood, yet these interactions may play a significant role in the stability and co-transport of mineral colloids and/or environmental contaminants. In this study, the processes of competitive adsorption, displacement, and transport of Suwannee River natural organic matter (SR-NOM) are investigated with several model organic compounds in packed beds of iron oxide-coated quartz columns. Results demonstrated that strongly-binding organic compounds are competitively adsorbed and displace those weakly-bound organic compounds along the flow path. Among the four organic compounds studied, polyacrylic acid (PAA) appeared to be the most competitive, whereas SR-NOM was more competitive than phthalic and salycylic acids. The transport of SR-NOM is found to involve a complex competitive interaction and displacement of different NOM subcomponents. A diffuse adsorption and sharp desorption front (giving an appearance of irreversible adsorption) of the SR-NOM breakthrough curves are explained as being a result of the competitive time-dependent adsorption and displacement processes between different organic components within the SR-NOM. The stability and transport of iron oxide colloids varied as one organic component competitively displaces another. Relatively large quantities of iron oxide colloids are transported when the more strongly-binding PAA competitively displaces the weakly-binding SR-NOM or when SR-NOM competitively displaces phthalic and salicylic acids. Results of this study suggest that the chemical composition and hence the functional behavior of NOM (e.g., in stabilizing mineral colloids and in complexing contaminants) can change along its flow path as a result of the dynamic competitive interactions between heterogeneous NOM subcomponents. Further studies are needed to better define and quantify these NOM components as well as their roles in contaminant partitioning and transport.

Pore-Scale Visualization of Colloid Transport and Retention in Partly Saturated Porous Media

In unsaturated porous media, sorption of colloids at the air–water (AW) interface is accepted as a mechanism for controlling colloid retention and mobilization. However, limited actual pore-scale observations of colloid attachment to the AW interface have been made. To further investigate these processes, a real-time pore-scale visualization method was developed. The method builds on the light transmission technique for fingered flow studies in packed-sand infiltration chambers and combines it with high-resolution, electro-optical hardware and public domain imaging software. Infiltration and drainage of suspensions of hydrophilic negatively charged carboxylated latex microspheres provides compelling visual evidence that colloid retention in sandy porous media occurs via trapping in the thin film of water where the AW interface and the solid interface meet, the air–water–solid (AWS) interface. With this modified theory of trapped colloids at the AWS interface, we are able to explain the apparent discrepancy between previous experimental evidence of hydrophilic colloids seemingly partitioning to the AW interface and more recent findings that suggest this type of colloid does not adsorb at the AW interface.

Using reverse osmosis to obtain organic matter from surface and ground waters

Abstract Reverse osmosis (RO) is a rapid, powerful technique for concentrating dissolved organic matter (DOM) in surface and ground waters. The portable RO system used in our research group can process 150–2001 of water per hour with 90% recovery of organic carbon without exposing DOM to harsh chemical reagents. The system has been tested in blackwater streams, low-DOM streams, and in ground waters from very deep aquifers (> 1000 m). This paper discusses (1) percent recovery of organic carbon from a variety of surface and ground waters, (2) laboratory and modeling studies of the pH dependence of the recovery of low molecular weight organic acids from dilute solutions, and (3) operational factors that potentially affect the performance of the RO system in field experiments.

Mobilization of transuranic radionuclides from disposal trenches by natural organic matter

Abstract Transuranic (TRU) radionuclides in groundwater at the Oak Ridge National Laboratory migrate rapidly and with little retardation of the radionuclides over distances of 80 m. Several interacting hydrogeochemical processes contribute to the observed releases of actinides ( 244 Cm and 241 Am) from the shallow unlined disposal trenches, through the highly weathered, fractured shale (saprolite) and to the surface-water seeps at White Oak Creek. Major releases are promoted when seasonal fluctuations in the water table permit groundwater to contact actinide-contaminated waste. Local recharge of stormwater into the trenches appears to permit minor releases, perhaps due to transient saturation within the trenches but above the local water table. Although the hydrogeology of the site permits contact of the TRU waste with the groundwater, the expected inorganic species of the actinides should strongly adsorb to the layer silicates and mineral oxides of the shale saprolite. Yet the timing of the actinide releases relative to when rising groundwater intercepts the trenches suggests that actinide transport is rapid, and the relative magnitude of peak actinide levels in wells near the trenches and at downgradient seeps suggests that there is very limited retention of the actinides by the formation. Based on anion exchange chromatography of the groundwater and geochemical modeling, the mobilization and transport of the actinides is demonstrated to result from complexation of the actinides by natural organic matter (NOM). Storm events contribute to mobilization by promoting hydrologic links between the TRU waste and groundwater, and by increasing the concentration of NOM in the mobile soil and groundwater. This study demonstrates that even in formations characterized by abundant mineral phases known to strongly adsorb actinides, the actinides can be transported essentially conservatively as NOM complexes.