Environmental factors influencing sorption of heterocyclic aromatic compounds to soil

Heterocyclic organic compounds containing nitrogen, sulfur, or oxygen (NSOs) are an important class of groundwater contaminants related to the production and use of manufactured gas, heavy oils, and coal tar. Surprisingly little is known about the processes that control sorption and transport of NSOs in the subsurface. In this study, the effects of various environmental factors including temperature, ionic strength, and dissolved/sorbed ion composition on the sorption of NSOs have been investigated by means of a soil column chromatography approach. For the investigated compounds, increased temperature normally decreases their sorption to soil. The enthalpy change of the sorption process corroborates earlier findings that van der Waals forces dominate the sorption of S- and O-heterocyclic compounds such as thiophene, benzothiophene, benzofuran, and 2-methylbenzofuran. Ionic strength and ion composition (Ca2+ vs K+ at given ionic strength) of the aqueous phase show no significant effects on the sorption of these compounds. Previous studies demonstrated that for N-heterocyclic compounds, cation exchange and surface complex formation rather than partitioning into soil organic matter control their overall sorption. In contrast to S- and O-heterocyclic compounds, increasing ionic strength reduced the sorption of ionizable N-heterocyclic compounds (pyridine, 2-methylpyridine, quinoline, 2-methylquinoline, and isoquinoline), due to increased electrostatic competition by cations. At given ionic strength, an increase of the K+/Ca2+ ratio in the mobile phase enhanced the sorption of N-heterocyclic compounds, consistent with cation exchange of the protonated organic species as the dominating sorption process. Among the investigated N-heterocyclic compounds sorption of benzotriazole showed a peculiar feature in that ternary surface complexation with Ca2+ appears to be the dominant sorption mechanism.     

Dissolved organic carbon enhances the mass transfer of hydrophobic organic compounds from nonaqueous phase liquids (NAPLs) into the aqueous phase

The hypothesis that dissolved organic carbon (DOC) enhances the mass transfer of hydrophobic organic compounds from nonaqueous phase liquids (NAPLs) into the aqueous phase above that attributable to dissolved molecular diffusion alone was tested. In controlled experiments, mass transfer rates of five NAPL-phase PAHs (log K(OW) 4.15-5.39) into the aqueous phase containing different concentrations of DOC were measured. Mass transfer rates were increased by up to a factor of 4 in the presence of DOC, with the greatest enhancement being observed for more hydrophobic compounds and highest DOC concentrations. These increases could not be explained by dissolved molecular diffusion alone, and point to a parallel DOC-mediated diffusive pathway. The nature of the DOC-mediated diffusion pathway as a function of the DOC concentration and PAH sorption behavior to the DOC was investigated using diffusion-based models. The DOC-enhanced mass transfer of NAPL-phase hydrophobic compounds into the aqueous phase has important implications for their bioremediation as well as bioconcentration and toxicity.     

Effects of linear alkylbenzene sulfonate on the sorption of Brij 30 and Brij 35 onto aquifer sand

Surfactant sorption is of considerable importance to environmental applications, including surfactant flushing to mobilize hydrophobic contaminants; effects of surfactants on the transport of dissolved contaminants, microorganisms, and colloids through porous media; and bioremediation of hydrophobic organic compounds, as well as understanding the fate and transport of surfactants as environmental contaminants themselves. Although most sorption studies consider pure surfactants, commercial detergent formulations typically consist of mixtures of nonionic and anionic surfactants. In this study, the effects of varying concentrations of the anionic surfactant linear alkylbenzene sulfonate (LAS) on micelle formation and sorption behavior of the two commonly used nonionic surfactants Brij 30 and Brij 35 onto aquifer sand were examined. A strong linear relationship was observed between the critical micelle concentration (CMC) of the Brij surfactants and the concentration of LAS in the mixture, with the CMC decreasing with increasing concentration of LAS. The relative change in CMC as a function of the LAS concentration was identical forthe two Brij surfactants, indicating that LAS interacted with their common alkyl chains. Sorption isotherms were developed for Brij 30 and Brij 35 present as single surfactants in an aqueous solution as well as when present with LAS. Although LAS had minor effects on the maximum sorption plateaus of the Brij surfactants, Brij sorption at was significantly enhanced as a function of the LAS concentration for Brij aqueous concentrations below the CMC. Application of a multi-interaction isotherm model indicated that the formation of surface aggregates (e.g., hemimicelles) decreased with increasing LAS concentration. Overall, these results provide insight into the complex sorption behavior of surfactant mixtures.     

Modeling the Uptake of Semivolatile Organic Compounds by Passive Air Samplers: Importance of Mass Transfer Processes within the Porous Sampling Media

Air sampling based on diffusion of target molecules from the atmospheric gas phase to passive sampling media (PSMs) is currently modeled using the two-film approach. Originally developed to describe chemical exchange between air and water, it assumes a uniform chemical distribution in the bulk phases on either side of the interfacial films. Although such an assumption may be satisfied when modeling uptake in PSMs in which chemicals have high mobility, its validity is questionable for PSMs such as polyurethane foam disks and XAD-resin packed mesh cylinders. Mass transfer of chemicals through the PSMs may be subject to a large resistance because of the low mass fraction of gas-phase chemicals in the pores, where diffusion occurs. Here we present a model that does not assume that chemicals distribute uniformly in the PSMs. It describes the sequential diffusion of vapors through a stagnant air-side boundary layer and the PSM pores, and the reversible sorption onto the PSM. Sensitivity analyses reveal the potential influence of the latter two processes on passive sampling rates (PSRs) unless the air-side boundary layer is assumed to be extremely thick (i.e., representative of negligible wind speeds). The model also reveals that the temperature dependence of PSRs, differences in PSRs between different compounds, and a two-stage uptake, all observed in field calibrations, can be attributed to those mass transfer processes within the PSM. The kinetics of chemical sorption to the PSM from the gas phase in the macro-pores is a knowledge gap that needs to be addressed before the model can be applied to specific compounds.     

Electrocatalytic hydrodechlorination of 2,4,5-trichlorobiphenyl on a palladium-modified nickel foam cathode

Palladium-modified materials have been found to be effective electrodes for the reductive degradation of chlorinated compounds in aqueous solution. This study investigated the electrocatalytic hydrodechlorination (ECH) of polychlorinated biphenyls (PCBs) in solvent/surfactant-aided solutions in a palladium-modified nickel foam electrode using a divided flow-through cell. The reaction pathways of 2,4,5-PCB hydrodechlorination were proposed due to the analysis of intermediates by GC/MS. The mechanism of electrocatalytic reaction on the Pd/Ni foam cathode was examined by studying the effect of surfactant type, sorption behavior of PCBs on the electrode, and current densities on the ECH efficiency of PCBs. The conversion of PCBs was controlled by the micelle structures of the surfactants instead of the charged species. According to the analysis of hydrogen transformation processes on the electrode surface, we propose that the ECH process was initiated by the transfer of highly active hydrogen atoms [H] from the prior polarized Pd particles to the less polarized Pd particles by spillover on the Pd/Ni foam cathode. Therefore, the total available surface was larger than the originally polarized surface, and [H] could smoothly react with PCBs that were adsorbed on the surface. As a result, a high ECH efficiency can be achieved with the Pd/Ni foam electrode.     

Effect of dissolved organic carbon on sorption of pyrethroids to sediments

Despite their strong hydrophobicity, recent studies showed widespread occurrence of pyrethroid in downstream surface waters bodies. In this work, the effect of dissolved organic carbon (DOC) on the sorption and desorption of pyrethroids in sediment was evaluated to understand the role of DOC in facilitating pyrethroid transport. Presence of DOC from three sources at 38 ± 2 mg L?1 in the aqueous phase decreased pesticide sorption to a sediment by 1.7 to 38.9 times and increased their desorption by 1.2 to 41.4 times. The effect on pyrethroid sorption to the sediment was linear. In addition, interactions between DOC and pyrethroids, when taking place prior to the contact with sediment, decreased sorption of some pyrethroids even further, implying that DOC-pyrethroid complexs were relatively stable in solution. DOC sources with higher contents of carboxylic and phenolic groups were found to have a higher potential to associate with pyrethroids. The DOC-water partition coefficients (K(DOC)) obtained by solid-phase microextraction measurement were significantly correlated (P < 0.01) with K(d) values measured for the sediment. These results provide evidence that DOC increases the distribution of pyrethroids from the sediment to the solution phase and plays an important role in mobilizing pyrethroids in runoff and surface streams.     

Enhanced diffusion of polycyclic aromatic hydrocarbons in artificial and natural aqueous solutions

Uptake of hydrophobic organic compounds into organisms is often limited by the diffusive transport through a thin boundary layer. Therefore, a microscale diffusion technique was applied to determine the diffusive mass transfer of 12 polycyclic aromatic hydrocarbons through water, air, surfactant solutions, humic acid solutions, aqueous soil and horse manure extracts, digestive fluid of a deposit-feeding worm, and root exudates from willow plants. In most cases the diffusive mass transfer of PAHs was much higher through the tested media than through water, and the enhancement factors increased with increasing hydrophobicity of the PAHs. The diffusive flux of benzo[a]pyrene was for instance enhanced 74 times through gut fluid of a deposit-feeding worm when compared to water. These findings demonstrate that a wide variety of dissolved organic carbon (DOC) at environmental levels can enhance diffusive mass transfer in various transport scenarios. The diffusive uptake of PAHs into sediment dwelling organisms is particularly efficient within the gut and at direct contract with the sediment matrix. Bioremediation might be enhanced bythe addition of auxiliary agents that enhance diffusive mass transfer. Enhanced diffusion needs also to be considered in dynamic transport models and for the operation and calibration of passive sampling techniques.     

On the reversibility of sorption to black carbon: distinguishing true hysteresis from artificial hysteresis caused by dilution of a competing adsorbate

Sorption hysteresis in environmental sorbents has important implications for pollutant transport and bioavailability. We examined the reversibility of sorption of benzene, toluene, and nitrobenzene, both singly and in pairs, by wood charcoal. A previous study showed that these compounds compete for the same set of adsorption sites on the char. Single-solute sorption was weakly hysteretic at high concentrations. The finding of comparable irreversibility for these compounds was taken as evidence that hysteresis is true and caused by pore elasticity. Hysteresis in the presence of a competitor was weak at low cosolute concentration but became stronger as the cosolute concentration increased. We attribute the growing hysteresis with cosolute concentration to a thermodynamic "competitor dilution effect"--a heretofore-unrecognized cause of hysteresis in multi-solute systems when the competing solute is simultaneously diluted with the target solute in the desorption step. It arises because the target solute re-equilibrates from a sorption point where competition is relatively high, to a desorption point where competition is relatively low. Simulations based on Ideal Adsorbed Solution Theory, a thermodynamic competition model, support the hypothesis. The cosolute also causes an increase in the linearity of the target solute isotherm, also attributable to competition thermodynamics. The competitive dilution effect can play a role in pollutant behavior in real systems if competing substances, natural or anthropogenic, are diluted or degraded making the target less accessible with time.     

Sediment dilution method to determine sorption coefficients of hydrophobic organic chemicals

Sorption coefficients of hydrophobic organic chemicals (HOC) to sediments and soils can easily be underestimated in traditional batch experiments, especially because analysis of the aqueous concentration often includes compounds sorbed to colloidal organic matter. In this work, a "sediment dilution approach" has been combined with measurements of freely dissolved concentrations to determine sorption coefficients of five chlorobenzenes and two chloroanilines in spiked sediment and of two unknown chemicals in field-contaminated sediment. A range of sediment suspensions with different sediment-water ratios was made. Freely dissolved concentrations in these suspensions were measured by negligible depletion solid-phase microextraction (nd-SPME). Sediment-water sorption coefficients (KD) were derived from the decrease of the freely dissolved concentrations as a function of the "dilution factor" (DF = volume water/mass sediment). The determined sorption coefficients were very similar to literature values. The experimental setup provides sorption coefficients without the need for total extractions, and the negligible depletion SPME technique does not require phase separation. The proposed method might be an alternative for batch equilibrium experiments to determine sorption coefficients.     

Environmental influences on the partitioning and diffusion of hydrophobic organic contaminants in microbial biofilms

A biofilm reactor was used to investigate kinetic and thermodynamic aspects of the sorption of polycyclic aromatic hydrocarbons (PAH) as model compounds for hydrophobic organic contaminants (HOC) to intact microbial biofilms. Effective diffusion coefficients are in the range of 10(-10) cm2 x s(-1) resulting in equilibration times of more than 3 days for a biofilm of 100 microm thickness. Diffusion in the biofilm was strongly temperature-dependent and increased by a factor of 3 (phenanthrene) to 6 (fluoranthene, pyrene) between 5 and 35 degrees C. Drying and rewetting of the biofilm as well as the inclusion of Ca2+ ions and of humic acids all strengthened the biofilm rigidity and slowed down the diffusion of PAH. The later two factors also influenced the thermodynamics of the process as they supported the partitioning of PAH into the biofilm. Humic acid inclusion from solution into the biofilm illustrates that a microbial biofilm can act as a primer allowing for the buildup of a particulate organic phase from dissolved organic matter. PAH metabolites (3-hydroxy-phenanthrene and 1-hydroxy-2-naphthoic acid) showed lower partition coefficients as compared to their parent compounds and 3-hydroxy-phenanthrene also showed a higher diffusion constant, indicating that these transformation products would be easily released into the water phase upon formation during PAH biodegradation in a biofilm. These results allow the quantification of the influence of environmental conditions on a biofilm's function as a sink or as a diffusion barrier for PAH from aqueous solution, and they indicate the importance of kinetic aspects of this partitioning process.     

Solid phase dosing and sampling technique to determine partition coefficients of hydrophobic chemicals in complex matrixes

Determination of polymer-water and dissolved organic carbon (DOC)-water distribution coefficients of very hydrophobic chemicals (log K0w > 6) is not straightforward. Poor water solubility of the test compounds complicates the spiking and analysis of actual freely dissolved concentrations. By dosing a system via a PDMS-fiber and monitoring the depletion in the polymer, spiking and analysis of concentrations in the aqueous phase are avoided, and sorption to the polymer and other hydrophobic phases can be determined easily and accurate. In this publication we report the determination of poly(dimethyl-siloxane) (PDMS)-water, and Aldrich humic acid-water distribution coefficients for six PAHs with log K0w values varying from 4.56 to 6.85. The distribution coefficients to a PDMS fiber llog Kf) and the DOC (log KDOC) range from 3.86 to 5.39 and 4.78 to 7.43, respectively. Even for the most hydrophobic compounds, the distribution coefficients show small standard errors (< or = 0.05 log units). Therefore, this method might be applied to determine sorption coefficients of numerous, even more hydrophobic compounds, to humic acids as well as other dissolved hydrophobic matrixes.     

Immobilizing humic acid in a sol-gel matrix: a new tool to study humic-contaminants sorption interactions

Humic substances originated from aquatic, soil, or sediment environments are mixtures of humic compounds with various characteristics. Sorption interactions with isolated, well defined humic fractions can be studied either in an aqueous phase ("dissolved humic substances"), or in a solid-phase, by coating mineral particles with the humic materials, or simply by working with humic acid particles (powder) at low pH to minimize dissolution. Each attitude, by definition, can be studied by different experimental techniques and has a different meaning for understanding natural environmental processes. In this study, a new tool for studying sorption interactions is presented. Sol-gel was used as an inert matrix to immobilize (entrap) various humic acids (HAs), and then used to study the interactions of several polycyclic aromatic hydrocarbons (PAHs) with the entrapped HA. Linear and nonlinear sorption coefficients were highly correlated with contaminant hydrophobicity. Sorption of pyrene to immobilized HA was in the order of soil HA > Aldrich HA approximately = peat HA. It was concluded that the entrapped HAs retained their original properties in the gel matrix and were accessible to the external contaminant through the pore network. Additionally, binding coefficients of pyreneto dissolved humic substances and to dissolved organic matter (DOM) were determined from the reduction in pyrene sorption to immobilized HA in the presence of dissolved humic material or DOM in solution. Binding coefficients of pyrene were in the order of the following: dissolved Aldrich HA > dissolved peat fulvic acid (FA) > DOM derived from mature compost > DOM derived from fresh compost.     

Sorption of heterocyclic organic compounds to reference soils: column studies for process identification

In this study, the sorption behavior of a wide variety of N-, S-, and O-heterocyclic compounds (NSOs) to reference soils (Eurosoils 1-5) was characterized by a soil column chromatography (SCC) approach. The major goal was to identify the compound specific and environmental factors influencing sorption processes. The sorption of S- and O-heterocyclic compounds (thiophene, benzothiophene, 5-methylbenzo[b]thiophene, benzofuran, 2-methylbenzofuran, and 2,3-dimethylbenzofuran) was generally controlled by nonspecific interactions with soil organic carbon (OC). With regard to non-ionizable N-heterocyclic compounds, pyrrole, 1-methylpyrrole, and pyrimidine were hardly retarded in any soil. The sorption of indole, 2-hydroxyquinoline, and benzotriazole was dominated by specific interaction (e.g., complexation of surface-bound cations) rather than partition to soil OC. The sorption of ionizable N-heterocyclic compounds (quinoline, isoquinoline, quinaldine, 2-methylpyridine, and pyridine) can be described by a conceptual model including partitioning to soil OC, cation exchange, and an additional sorption process (probably surface complexation of the neutral species). Cation exchange was usually the dominant mechanism in the sorption of ionizable compounds if the protonated fraction of the compound exceeded 5%. Otherwise, surface complexation became dominant. Soil pH was the most important factor influencing the sorption of ionizable NSOs. Our study suggests that a fairly precise assessment of sorption in most soils can be expected for N-, S-, and O-heterocyclic compounds if the three sorption mechanisms are taken into accountwhere appropriate. Deviations from this behavior indicated special cases where additional soil specific properties (e.g., accessible surface, CEC, charge density) need to be considered such as for 2-methylpyridine and pyridine sorption to Eurosoil 1.     

Relationship between strength of organic sorbate interactions in NOM and hydration effect on sorption

According to a recent conceptual model for hydration-assisted sorption of organic compounds in natural organic matter (NOM), certain polar moieties of dry NOM are unavailable for compound sorption due to strong intra- and intermolecular NOM interactions. Water molecules solvate these moieties creating new sorption sites at solvated contacts. It is expected that the greater a compound's ability to undergo specific interactions with NOM, the greater will be the hydration-assisted sorption effect, because penetration of compounds into solvated contacts must involve competition with water at the solvated contact. To test this model, we compare the hydration effect on sorption kinetics and equilibrium for 4 compounds with differing abilities to undergo specific interactions with NOM. Sorption measured on Pahokee peat in aqueous systems was fast compared with n-hexadecane (dry) systems. No concentration effect on attainment of sorption equilibrium was observed. m-Nitrophenol exhibited the greatest hydration-assisted sorption effect, benzyl alcohol showed an intermediate effect and acetophenone and nitrobenzene showed no hydration-assisted sorption, on an activity scale. The extent of hydration-assisted sorption effect correlates with compound ability to undergo specific interactions. These results support the conceptual model and demonstrate the importance of polar NOM noncovalent links in organizing the NOM phase and in controlling the hydration effect on sorption of organic compounds.     

Enhanced soil washing of phenanthrene by mixed solutions of TX100 and SDBS

Increased desorption of hydrophobic organic compounds (HOCs) from soils and sediments is a key to the remediation of contaminated soils and groundwater. In this study, phenanthrene desorption from a contaminated soil by mixed solutions of a nonionic surfactant(octylphenol polyethoxylate, TX100) and an anionic surfactant (sodium dodecylbenzenesulfonate, SDBS) was investigated. Phenanthrene desorption depended on not only aqueous surfactant concentrations and phenanthrene solubility enhancement but also the soil-sorbed surfactant amount and the corresponding sorption capacity of sorbed surfactants. The added surfactant critical desorption concentrations (CDCs) for phenanthrene from soil depended on both sorbed concentrations of surfactants and their critical micelle concentrations (CMCs). Phenanthrene desorption by mixed solutions was more efficient than individual surfactants due to the low sorption loss of mixed surfactants to soil. Among the tested surfactant systems, mixed TX100 and SDBS with a 1:9 mass ratio exhibited the highest phenanthrene desorption. Mixed micelle formation, showing negative deviation of CMCs from the ones predicted by the ideal mixing theory, was primarily responsible for the significant reduction of soil-sorbed amounts of TX100 and SDBS in their mixed systems. Therefore, mixed anionic-nonionic surfactants had great potential in the area of enhanced soil and groundwater remediation.     

Effect of aqueous Fe(II) on arsenate sorption on goethite and hematite

Biogeochemical iron cycling often generates systems where aqueous Fe(II) and solid Fe(III) oxides coexist. Reactions between these species result in iron oxide surface and phase transformations, iron isotope fractionation, and redox transformations of many contaminant species. Fe(II)-induced recrystallization of goethite and hematite has recently been shown to cause the repartitioning of Ni(II) at the mineral-water interface, with adsorbed Ni incorporating into the iron oxide structure and preincorporated Ni released back into aqueous solution. However, the effect of Fe(II) on the fate and speciation of redox inactive species incompatible with iron oxide structures is unclear. Arsenate sorption to hematite and goethite in the presence of aqueous Fe(II) was studied to determine whether Fe(II) causes substantial changes in the sorption mechanisms of such incompatible species. Sorption isotherms reveal that Fe(II) minimally alters macroscopic arsenate sorption behavior except at circumneutral pH in the presence of elevated concentrations (10?3 M) of Fe(II) and at high arsenate loadings, where a clear signature of precipitation is observed. Powder X-ray diffraction demonstrates that the ferrous arsenate mineral symplesite precipitates under such conditions. Extended X-ray absorption fine structure spectroscopy shows that outside this precipitation regime arsenate surface complexation mechanisms are unaffected by Fe(II). In addition, arsenate was found to suppress Fe(II) sorption through competitive adsorption processes before the onset of symplesite precipitation. This study demonstrates that the sorption of species incompatible with iron oxide structure is not substantially affected by Fe(II) but that such species may potentially interfere with Fe(II)-iron oxide reactions via competitive adsorption.     

Solubility-normalized combined adsorption-partitioning sorption isotherms for organic pollutants

Equilibrium sorption isotherms were measured for five different low-polarity organic compounds (benzene, trichloroethene, 1,2- and 1,4-dichlorobenzene, and phenanthrene) over a wide concentration range. The investigated sorbents can be grouped into the following three classes: (1) humic soil organic matter, which shows linear sorption isotherms (solely partitioning, as observed in the peat sample); (2) carbon materials, which were thermally altered (due to their natural history or industrial production) and thus contain a high specific surface area and exhibit nonlinear isotherms, and (3) pure engineered microporous materials (e.g., zeolites and activated carbon), where adsorption is solely due to a pore-filling process. Sorption of all compounds was fitted very well by the Polanyi-Dubinin-Manes (PDM) model, which for sorbents containing humic organic matter (e.g., peat) was combined with linear partitioning. Both the partitioning and the Polanyi-Dubinin-Manes model predict unique sorption isotherms of similar compounds if the solubility-normalized aqueous concentration is used. In addition, an inverse linear relationship between the distribution coefficient (Kd) and water solubility, which was very well confirmed by the data, is obtained. This also leads to unit-equivalent Freundlich sorption isotherms and explains the often observed apparent correlation between sorption capacity at a given concentration (e.g., Freundlich coefficient) and sorption nonlinearity (Freundlich exponent).     

Modeling tetracycline antibiotic sorption to clays

Sorption interactions of three high-use tetracycline antibiotics (oxytetracycline, chlortetracycline, tetracycline) with montmorillonite and kaolinite clays were investigated undervaried pH and ionic strength conditions. Sorption edges were best described with a model that included cation exchange plus surface complexation of zwitterion forms of these compounds. Zwitterion sorption was accompanied by proton uptake, was more favorable on acidic clay, and was relatively insensitive to ionic strength effects. Calcium salts promoted oxytetracycline sorption at alkaline pHs likely by a surface-bridging mechanism. Substituent effects among the compounds in the tetracycline class had only minor effects on sorption edges and isotherms under the same solution pH and ionic strength conditions. At low ionic strength, greater sorption to montmorillonite than kaolinite was observed at all pHs tested, even after normalizing for cation exchange capacity. These results indicate that soil and sediment sorption models for tetracyclines, and other pharmaceuticals with similar chemistry, must account for solution speciation and the presence of other competitor ions in soil or sediment pore waters.