Hydrophobic Organic Contaminants In Soils Research Articles

Use of a Dual-Labeled Bioaccumulation Method to Quantify Microplastic Vector Effects for Hydrophobic Organic Contaminants in Soil.

Although in vitro simulation and in vivo feeding experiments are commonly used to evaluate the carrier role of microplastics in the bioaccumulation of toxic chemicals, there is no direct method for quantitatively determining their vector effect. In this study, we propose a dual-labeled method based on spiking unlabeled hydrophobic organic contaminants (HOCs) into soils and spiking their respective isotope-labeled reference compounds into microplastic particles. The bioaccumulation of the unlabeled and isotope-labeled HOCs in Eisenia fetida earthworms was compared. Earthworms can assimilate both unlabeled and isotope-labeled HOCs via three routes: dermal uptake, soil ingestion, and microplastic ingestion. After 28 days of exposure, the relative fractions of bioaccumulated isotope-labeled HOCs in the soil treated with 1% microplastics ranged from 15.5 to 55.8%, which were 2.9-47.6 times higher than those in the soils treated with 0.1% microplastics. Polyethylene microplastics were observed to have higher relative fractions of bioaccumulated isotope-labeled HOCs, potentially because of their surface hydrophobicity and amorphous rubbery state. The general linear models suggested that the vector effects were mainly due to the microplastic concentration, followed by polymer properties and HOC hydrophobicity. This proposed method and the derived empirical formula contribute to a more comprehensive understanding of the vector effects of microplastics for HOC bioaccumulation.

Coupling polydimethylsiloxane vials with a physiologically based extraction test to predict bioavailability of hydrophobic organic contaminants in soils

As alternatives to in vivo assays, physiologically based in vitro methods have been developed to measure bioaccessibility of hydrophobic organic contaminants (HOCs) in soils. However, bioaccessibility can usually be underestimated since in vitro tests fail to provide sufficient affinity for HOCs. Sorption sink was therefore included to simulate intestinal cell absorption and to promote the mobilization of HOCs from soils. In this study, polydimethylsiloxane (PDMS) vials, widely used as passive dosing, were introduced as a sorption sink to improve the performance of physiologically based extraction test (PBET). The bioaccessibility of PCBs (representatives of HOCs) in 13 lab-spiked soils measured by PBET coupled with PDMS vials ranged from 56.5 ± 2.7% to 109.3 ± 1.5%. Correlation was conducted between the bioaccessibility and relative bioavailability (RBA) of PCBs assessed using an in vivo mouse model. A significant correlation (p < 0.001, R2 = 0.72, slope = 0.85 ± 0.16) was observed between in vitro and in vivo data, indicating that the proposed method here can be a robust in vitro method to predict PCB RBA in soils. The accuracy of this novel method was further shown by extracting one field contaminated soil with environmental relevant levels of PCBs. The relative standard deviation of bioaccessibility measured by PBET with PDMS vials was 1.2–9.8%, and much lower than those by PBET alone with values of 17.1–63.6%. In addition, the PDMS vials can be reusable as sorption sink, and no significant variation (p = 0.44) in PCB bioaccessibility was observed among 5 cycles of extracting soils with PBET coupled with PDMS vials. Due to the high sorption capacity of PDMS and flexibility of PDMS mass used for vials, the novel method here is expected to be applicable in soils with a wide range of contamination levels.

Contribution of phenanthrene in different binding sites to its biodegradation in biochar-amended soils.

Biochars can strongly sorb hydrophobic organic contaminants in soils. However, contribution of contaminants in different binding sites to their biodegradation in biochar-amended soils is not clear. In this work, wheat straw biochars were prepared at pyrolysis temperatures of 400°C (BC400) and 700°C (BC700). During a 42-day experiment, degradation rate constant of phenanthrene in soils was in the order of treatment without biochar (1.64×10-2 d-1)>treatment with BC700 (0.96×10-2 d-1)>treatment with BC400 (0.30×10-2 d-1). At the beginning, amendment of BC400 and BC700 reduced the rapidly desorbing fraction of phenanthrene in soils by 44.8% and 92.5%, respectively. At the end, both phenanthrene and microbial biomass highly concentrated on the biochar separated from soils. The results of a coupled model of desorption and biodegradation revealed that only phenanthrene in rapidly desorbing sites was degraded in BC400-amended soils, whereas degradation of phenanthrene in both rapidly and slowly desorbing sites occurred in BC700-amended soils, contributing 24.4% and 75.6% of the degradation, respectively. High fraction (>95%) of biodegradable phenanthrene in slowly desorbing sites was the key reason for higher biodegradation rate of phenanthrene in soils with BC700 than in soils with BC400.

In-situ and ex-situ measurement of hydrophobic organic contaminants in soil air based on passive sampling: PAH exchange kinetics, non-equilibrium correction and comparison with traditional estimations

It is a great challenge to accurately estimate chemical activity of hydrophobic organic contaminants in field soils. Ex-situ and in-situ determinations were developed for this purpose based on low-density polyethylene (LDPE) passive sampling and non-equilibrium correction by release of performance reference compounds (PRCs) previously spiked to the samplers. This work investigated kinetic processes of target contaminants’ uptake into and PRCs’ release from the sampler in an ex-situ soil suspension incubated for 100 days. A close agreement of kinetic parameters for pyrene’s (target) uptake into and deuterated pyrene’s (PRC) release from LDPE indicated their similar exchange kinetics. Three kinetic models were developed to correct uptake of target compounds in non-equilibrium conditions via release processes of PRCs. The second-order kinetic model was recommended for ex-situ measurements. The PRC-based non-equilibrium corrections were further applied to in-situ static passive sampling from several weeks to months in a PAH-contaminated field site. Two-weeks’ deployments were sufficient for quantifying lighter PAHs (logKOA < 8.0), but not recommended to accurately estimate heavier PAHs (logKOA > 9.0), even if over four months. Concentration estimates from the in-situ and ex-situ passive samplings were comparable in order of magnitude with traditional estimation from equilibrium partitioning models considering both organic and black carbon fractions.

Application and validation of isotope dilution method (IDM) for predicting bioavailability of hydrophobic organic contaminants in soil.

Risk assessment of hydrophobic organic contaminants (HOCs) using the total chemical concentration following exhaustive extraction may overestimate the actual availability of HOCs to non-target organisms. Existing methods for estimating HOC bioavailability in soil have various operational limitations. In this study, we explored the application of isotope dilution method (IDM) to quantify the accessible fraction (E) of DDTs and PCBs in both historically-contaminated and freshly-spiked soils. After addition of 13C or deuterated analogues to a soil sample, the phase distribution of isotope-labeled and native chemicals reached an apparent equilibrium within 48 h of mixing. The derived E values in the three soils ranged from 0.19 to 0.82, depending on the soil properties and also the contact time of HOCs (i.e., aging). The isotope dilution method consistently predicted greater accumulation into earthworm (Eisenia fetida) than that by polyethylene (PE) or solid phase microextraction (SPME) sampler, likely because desorption in the gut enhanced bioavailability of soil-borne HOCs. A highly significant linear regression (R2 = 0.91) was found between IDM and 24-h Tenax desorption, with a slope statistically identical to 1. The IDM-derived accessible concentration (Ce) was further shown to accurately predict tissue residues in earthworm exposed in the same soils. Given the relatively short duration and simple steps, IDM has the potential to be readily adopted for measuring HOC bioaccessibility in soil and for improving risk assessment and evaluation of remediation efficiency.

Nanoparticle-supported lipid bilayers as an in situ remediation strategy for hydrophobic organic contaminants in soils.

Polycyclic aromatic hydrocarbons (PAHs) are persistent environmental organic contaminants due to their low water solubility and strong sorption onto organic/mineral surfaces. Here, nanoparticle-supported lipid bilayers (NP-SLBs) made of 100-nm SiO2 nanoparticles and the zwitterionic lipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) are investigated as constructs for removing PAHs from contaminated sites, using benzo[a]pyrene (BaP) as an example. DMPC in the form of small unilamellar vesicles (SUVs) or DMPC-NP-SLBs with excess DMPC-SUVs to support colloidal stability, when added to saturated BaP solutions, sorb BaP in ratios of up to 10/1 to 5/1 lipid/BaP, over a 2-week period at 33 °C. This rate increases with temperature. The presence of humic acid (HA), as an analog of soil organic matter, does not affect the BaP uptake rate by DMPC-NP-SLBs and DMPC-SUVs, indicating preferential BaP sorption into the hydrophobic lipids. HA increases the zeta potential of these nanosystems, but does not disrupt their morphology, and enhances their colloidal stability. Studies with the common soil bacteria Pseudomonas aeruginosa demonstrate viability and growth using DMPC-NP-SLBs and DMPC-SUVs, with and without BaP, as their sole carbon source. Thus, NP-SLBs may be an effective method for remediation of PAHs, where the lipids provide both the method of extraction and stability for transport to the contaminant site.

Uptake kinetics of five hydrophobic organic pollutants in the earthworm Eisenia fetida in six different soils

Hydrophobic organic contaminants in soils may pose toxicity or transfer to food chains after their uptake to soil biota. However, uptake data for earthworms are usually limited, as: (a) only fixed exposure times are studied instead of whole uptake kinetics and (b) studies including compounds with different environmental properties and more than two soils of different properties are quite rare. In our study, five persistent organic pollutants (phenanthrene, pyrene, lindane, p,p’-dichlorodiphenyltrichloroethane (p,p’-DDT), and polychlorinated biphenyl congener No. 153 (PCB 153)) were added to six soils of a wide range of soil properties. Detailed kinetics of uptake to earthworms Eisenia fetida were measured. Results indicated that an equilibrium of concentrations for p,p’-DDT and PCB 153 was reached after 11 days of exposure in all soils. Uptake of phenanthrene, pyrene, and lindane was strongly influenced by the decrease in concentrations in the soils, resulting in peak-shaped accumulation curves. Only in soils with the highest total organic carbon content (7.9 and 20.2%), the equilibrium of lindane concentrations was achieved (after 17 and 5 days of exposure, respectively). We recommend calculating bioaccumulation factors as a ratio of the uptake and elimination rate constants to precise the risk assessment.

Impact of Deashing Treatment on Biochar Structural Properties and Potential Sorption Mechanisms of Phenanthrene

Knowledge of the mineral effects of biochars on their sorption of hydrophobic organic contaminants (HOCs) is limited. Sorption of phenanthrene (PHE) by plant-residue derived biochars (PLABs) and animal waste-derived biochars (ANIBs) obtained at two heating treatment temperatures (HTTs) (450 and 600 °C) and their corresponding deashed biochars was investigated. The decreased surface polarity and increased bulk polarity of biochars after deashing treatment indicated that abundant minerals of biochars benefit external exposure of polar groups associated organic matter (OM). Organic carbon (OC)-normalized distribution coefficients (K(oc)) of PHE by biochars generally increased after deashing, likely due to enhancement of favorable and hydrophobic sorption sites caused by mineral removal. Positive correlation between PHE log K(oc) by PLABs and bulk polarity combined with negative correlation between PHE log K(oc) values by ANIBs and surface polarity suggested PLABs and ANIBs have different sorption mechanisms, probably attributed to their large variation of ash content because minerals influenced OM spatial arrangement within biochars. Results of this work could help us better understand the impact of minerals, bulk/surface polarity, and sorption domain arrangement of biochars on their HOCs sorption and predict the fate of HOCs in soils after biochar application.

Impact of soil organic matter on the distribution of polycyclic aromatic hydrocarbons (PAHs) in soils

The knowledge on the distribution of hydrophobic organic contaminants in soils can provide better understanding for their fate in the environment. In the present study, the n-butanol extraction and humic fractionation were applied to investigate the impact of SOM on the distribution of polycyclic aromatic hydrocarbons (PAHs). The results indicated that 80.5%–94.8% of the target PAHs could be extracted by n-butanol and 63.1%–94.6% of PAHs were associated with fulvic acid (FA). Concentrations of un-extracted PAHs increased significantly with the increasing soil organic matter (SOM), however, such an association was absent for the extractable fractions. The results suggested that the sequestration played a critical role in the accumulation of PAHs in soils. SOM also retarded the diffusion of PAHs into the humin fractions. It implied that sequestration in SOM was critical for PAH distribution in soils, while the properties of PAH compounds also had great influences.

Mobilization of hydrophobic contaminants from soils by enzymatic depolymerization of soil organic matter

The effect of hydrolytic exoenzymes on the release of hydrophobic organic contaminants (HOC) from two different surface soils was studied in laboratory batch experiments. Incubation of the soils with cellulase with an activity fivefold above the inherent soil activity enhanced the release of hydrophobic contaminants (polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB) and hydroxylated PCB) by 40–200%. Xylanase and invertase did not show measurable effects at comparable relative activity levels. This suggests that cellulose substructures are important for the retention of HOC in soil organic matter (SOM). Hydrolytic exoenzymes, and the microorganisms that release them, contribute to the mobilization of HOC from soil, by shifting the sorption equilibrium in the course of SOM transformation into dissolved organic matter or by facilitating HOC diffusion as a consequence of reduced rigidity of SOM. We conclude that not only biodegradation but also sorption and desorption of HOC in soil can be influenced by (micro-) biology and the factors that determine its activity.

Comparison of Earthworm Bioaccumulation between Readily Desorbable and Desorption-Resistant Naphthalene: Implications for Biouptake Routes

The bioaccumulation of readily desorbable naphthalene and that of desorption-resistant naphthalene in earthworms were compared to examine the effect of desorption resistance on bioavailability of hydrophobic organic contaminants in soil. A series of naphthalene-contaminated soil samples representing different degrees of desorption resistance were prepared using a batch sorption-repetitive desorption approach, and bioaccumulation of these samples was evaluated using earthworms (Eisenia fetida). Soil samples representing high-degree of desorption resistance exhibited considerably lower bioavailability, as indicated by the lower body burden (naphthalene concentration in worm tissue) at a given sorbed-phase naphthalene concentration. Moreover, the body burden of the highly desorption-resistant samples exhibited a significantly different pore-water dependency than that of the readily desorbable samples, indicating that different biouptake mechanisms are likely controlling readily desorbable contaminants and desorption-resistant contaminants. We propose that for readily desorbable contaminants, the primary biouptake route is the pore-water uptake, but for desorption-resistant contaminants enhanced uptake from ingested soil particles can also be important. The surfactant-like substances in worm gut fluids and physical stress such as abrasion are the likely causes for the enhanced release of desorption-resistant contaminants in worm guts. The difference in bioavailability between readily desorbable and desorption-resistant contaminants needs to be taken into account in risk assessment practices.

Effect of Activated Carbon on Microbial Bioavailability of Phenanthrene in Soils

Bioavailability is a governing factor that controls the rate of biological degradation of hydrophobic organic contaminants in soil. Among the solid phases that can adsorb hydrophobic organic contaminants in soil, black carbon (BC) exerts a particularly significant effect on phase distribution. However, knowledge on the effect of BC on the microbial availability of polycyclic aromatic hydrocarbons in soil is still limited. In the present study, the effect of a coal-derived activated carbon on the bioavailability of phenanthrene (PHE) during its degradation by Mycobacterium vanbaalenii PYR-I was measured in three soils. The freely dissolved concentration of PHE was concurrently determined in soil solutions using disposable polydimethylsiloxane fibers. The results showed that PHE mineralization was significantly inhibited after addition of activated carbon in all test soils. After 216 h, only 5.20, 5.83, and 6.85% of PHE was degraded in the 0.5% BC-amended soils initially containing organic carbon at 0.23, 2.1, and 7.1%, respectively. Significant correlation was found between PHE degradability and freely dissolved concentration, suggesting that BC affected PHE bioavailability by decreasing chemical activity. The effect of activated carbon in the amended soils was attributed to its enhancement of soil surface areas and pore volumes. Results from the present study clearly highlighted the importance of BC for influencing the microbial availability of polycyclic aromatic hydrocarbons in soils.

Characterization of Condensed Organic Matter in Soils and Sediments

Condensed organic matter (OM) such as nonhydrolyzable C (NHC) is important in C cycling and the sorption and fate of hydrophobic organic contaminants in soils and sediments, but its structural components are rarely known. Nonhydrolyzable C in three contaminated soils and 22 bulk and size‐fractionated sediments from the Pearl River delta and estuary in China were measured following treatment with an HCl–HF–trifluoroacetic acid method. It was characterized using elemental analysis, radiocarbon accelerated mass spectroscopy, and Raman microspectrometry. The results show that NHC is an important or even dominating component of the organic C (OC) in the investigated soils and sediments. The NHC contents in this study are highly correlated with the OC contents, with a slope of 0.647 for the 25 soil and sediment samples, which is higher than previously reported NHC–OC correlations in the soils. The radiocarbon analysis demonstrates the importance of ancient OM in the NHC samples. The NHC fractions are chemically and structurally different from the biopolymer and humic substances in the soils and sediments, and originated from different thermally matured bitumen or kerogens, polymethylene C (algaenan, cutan, cutin, cuticle, and lipid), aged terrigenous OM (lignin and humin), and black carbon.

Humic Acid Fractionation upon Sequential Adsorption onto Goethite

Mineral-humic complexes are commonly distributed in natural environments and are important in regulating the transport and retention of hydrophobic organic contaminants in soils and sediments. This study investigated the structural and conformational changes of humic acid (HA) and mineral-HA complexes after sequential HA adsorption by goethite, using UV-visible spectroscopy, high performance size exclusion chromatography (HPSEC), Fourier transform infrared (FT-IR) spectroscopy, and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. The HA remaining in the solution after adsorption showed low polarity index values ((N+O)/C), which indicates that polar functional moieties are likely to adsorb on the goethite surface. In addition, we observed decreased E4/E6 and E2/E3 ratios of unbound HA with increasing number of coatings, implying that aliphatic rich HA fractions with polar functional moieties readily adsorb to the goethite surface. According to IR spectra, carbohydrate carbon would be the important fractions associated with goethite. NMR spectra provided evidence for HA fractionation during adsorption onto the mineral surface; that is, aliphatic fractions were preferentially adsorbed by goethite while aromatic fractions were left in solution. Relatively small molecular weight (MW) HA fractions had a greater affinity for the goethite surface based on analyses of the HPSEC chromatograms, which differs from the results reported in the literature. Finally, our results suggest that the polar aliphatic fractions of HA were mainly adsorbed to goethite via electrostatic attraction and/or ligand exchange reactions.

Effects of Microbially Mediated Redox Conditions on PAH−Soil Interactions

The impacts of microbially mediated redox conditions on the bioavailability of persistent polycyclic aromatic hydrocarbons (PAHs) in soils and sediments have received little study, despite the fact that most water-saturated soils and sediments spend a significant portion of the time under reduced conditions. To address this need an uncontaminated surface soil was incubated under various redox conditions (aerobic, nitrate-reducing, sulfate-reducing, and methanogenic). Depending on redox conditions, different quantities of fulvic and humic acids were liberated as dissolved organic matter (DOM) from the soil during incubation. The DOM released under highly reduced conditions was more nonpolar, aromatic, and polydisperse, of higher molecular weight, and had a higher sorption capacity for pyrene compared to that obtained from relatively oxic incubations. The soil-phase organic matter incubated under reduced conditions also became relatively more aromatic, containing nonpolar organic molecules of lower oxygen contents and exhibiting higher capacity and more nonlinear and hysteric sorption/desorption behavior for pyrene. These observations support the hypothesis that reduced environments established by indigenous soil microbes alter soil organic matter in a matter similar to diagenetic processes. Such humification-like alteration occurred principally in relatively more labile fractions of soil organic matter. These findings are important for assessing the ultimate fate and exposure risk of hydrophobic organic contaminants in soils and sediments where living microorganisms play a significant role in formation and evolution of soil/sediment organic matter.

Development of thermal programmed desorption mass spectrometry methods for environmental applications

The physical availability of hydrophobic organic contaminants (HOCs) bound to soils and sediments often controls their environmental toxicity. Currently, complicated and time extensive procedures are necessary to determine physical availability. The development of thermal programmed desorption mass spectrometry (TPD-MS) techniques for environmental samples may make it possible to evaluate the physical availability of HOCs in soils and sediments and also calculate relevant release energy values for bound contaminants. This work focused on developing the analytical protocols and data processing requirements for studying the desorption of HOCs from various simple geosorbents using TPD-MS. The work seeks to document the use of the TPD-MS method as an environmental assessment tool and provide the reader with a working knowledge of the entire process.

Effect of cyclodextrin and transformer oil amendments on the chemical extractability of aged [14C]polychlorinated biphenyl and [14C]polycyclic aromatic hydrocarbon residues in soil

Sequestration of hydrophobic organic contaminants (HOCs) in soils limits chemical and biological availability. Concerns exist regarding the long-term stability of sequestered contaminants in the environment, and stability needs to be demonstrated if bioavailability considerations are to be adopted into the risk assessment and remediation of contaminated land. The aim of the present study was to test the short-term influence of two organic amendments on the chemical extractability of HOC residues that had been present in soils for more than 12 years. The amendments investigated were cyclodextrin and transformer oil (a light, nonaqueous phase liquid [LNAPL]). The contaminants investigated were fluoranthene and benzo[a]pyrene in one soil and the polychlorinated biphenyls (PCBs) 28 and 52 in a second soil. The addition of cyclodextrin to the soils did not result in a significant increase in chemical extractability of the residues after a 36-d contact time. The addition of transformer oil resulted in an increase in chemical extractability of the PCBs after a 14-d soil-LNAPL contact time and a further increase after a 36-d contact time. The present study demonstrates that the chemical availability of aged HOCs in soil may be influenced by the presence of other chemicals and has implications for the long-term management of contaminated land.

Structural and Sorption Characteristics of Adsorbed Humic Acid on Clay Minerals

Clay-humic complexes are commonly distributed in natural environments. They play very important roles in regulating the transport and retention of hydrophobic organic contaminants in soils and sediments. This study examined the structural changes of humic acid (HA) after adsorption by clay minerals and determined phenanthrene sorption by clay-humic complexes. Solid- and liquid-state 13C nuclear magnetic resonance (NMR), for the first time, provided direct evidence for HA fractionation during adsorption on mineral surfaces, that is, aliphatic fractions were preferentially adsorbed by clay minerals while aromatic fractions were left in the solution. The ratio of UV absorbance of HA at 465 and 665 nm (E4 to E6 ratio), which is related to aromaticity, corroborated with the NMR results. For both montmorillonite and kaolinite, adsorbed HA fractions had higher sorption linearity (N) and affinity (K(oc)) than the source HA. The K(oc) of adsorbed HA for the clay-humic complexes could be up to several times higher than that of the source HA. This large increase may be contributed by the low polarity of the bound HA. Moreover, for each mineral, the N values of adsorbed HA increased with increasing HA loading. It is believed that HA may develop a more condensed structure on mineral surface at lower HA loading level due to the stronger interactions between HA and mineral surface as a result of close contacts.

Bioavailability of hydrophobic organic contaminants in soils: fundamental concepts and techniques for analysis

SummarySoils represent a major sink for organic xenobiotic contaminants in the environment. The degree to which organic chemicals are retained within the soil is controlled by soil properties, such as organic matter, and the physico‐chemical properties of the contaminant. Chemicals which display hydrophobic and lipophilic characteristics, as well as a recalcitrant chemical structure, will be retained within the soil, and depending on the ‘strength’ of the association may persist for long periods of time. This review describes the behaviour of hydrophobic organic contaminants in soils, focusing on the mechanisms controlling interactions between soil and contaminants. The bioavailability of contaminants in soil is also discussed, particularly in relation to contact time with the soil. It considers the degradation of organic contaminants in soil and the mechanisms microbes use to access contaminants. Finally, the review discusses the ‘pros’ and ‘cons’ of chemical and biological techniques available for assessing bioavailability of hydrophobic organic chemicals in soils, highlighting the need to quantify bioavailability by chemical techniques. It concludes by highlighting the need for understanding the interactions between the soil, contaminants and biota which is crucial to understanding the bioavailability of contaminants in soils.

Binding of Hydrophobic Organic Contaminants to Humalite-Derived Aqueous Humic Products, with Implications for Remediation

Abstract In order to assess the potential of commercial humic products in environmental remediation, their binding to heptachlor, hexachlorobenzene, hexachloroethane, 1,2,4-trichlorobenzene and n-hexane was measured. The binding was similar in tests with two colloidal-phase humic products prepared from humalite (naturally occurring, oxidized organic deposits found adjacent to coal) from Alberta, Canada (Luscar Ltd.). There were minor or negligible changes in binding strength for these two products over the full range of aqueous concentrations tested (1.67 to 33.4 g organic C/L), and for Aldrich humic acid. Aldrich humic acid was a stronger binder of the hydrophobic organic compounds than the two Luscar humic products, by factors of approximately 3 to 7. The binding of hydrophobic organics to the two Luscar humic products was similar in strength to that predicted for humic substances that occur in natural aquatic environments. Our binding data suggest that the use of concentrated Luscar humic products as colloidal-phase flushing agents would increase the aqueous concentrations of some hydrophobic organic contaminants in soils or aquifers by up to several hundred fold.