A concentration-dependent multi-term linear free energy relationship for sorption of organic compounds to soils based on the hexadecane dilute-solution reference state

A LFER of the type in the title is applied to sorption of numerous compounds to polyethylene and three soils for which sorption to natural organic matter (NOM) is presumed dominant. It provides fractional contributions to the Gibbs free energy of sorption corresponding to hydrophobic effects, dipolar/polarizability (D/P) effects in excess of the reference state, and the sum of possible specific forces such as H-bonding and pi-pi electron donor-acceptor (pi-pi EDA) interactions in excess of the reference state. Minimal inputs are the isotherm, the n-hexadecane-water partition coefficient and the Abraham pi parameter representing D/P effects. Sorption of all compounds to polyethylene can be described by considering only hydrophobic effects. Sorption of a calibration set of apolar compounds (aromatic and aliphatic hydrocarbons and chlorinated hydrocarbons) to the natural sorbents is well-described by a combination of hydrophobic and D/P effects. For the apolar set, D/P contributes approximately 15-40% (2-8% for cyclohexane) of sorption free energy. D/P effects increase with the degree of chlorination for aliphatic compounds. For aromatic compounds D/P effects increase with fused ring size but do not vary with degree of chlorination and chlorine substitution pattern. H-bonding contributes substantially to sorption of alcohols, and similarly for 2-nonanol and 2,4-dichlorophenol (33-44%). pi-pi EDA forces contribute to phenanthrene sorption in one case. The effects of concentration, sorbent aromaticity (literature NMR), and sorbent polarity [(O + N)/C] on hydrophobic and D/P contributions for all compounds indicate that (a) molecules fill sites of progressively greater hydrophilic character; (b) the energy penalty for cavity formation in the solid decreases with concentration due to plasticization and greater intermolecular contact; (c) sorbent aromatic content more than sorbent polarity controls D/P interactions. Basing free energy on an inert electrostatic chemical environment afforded by n-hexadecane permits evaluation of direct electrostatic forces in NOM that contribute to sorption.     

Hydration of natural organic matter: effect on sorption of organic compounds by humin and humic acid fractions vs original peat material

Natural organic matter (NOM) hydration is found to change activity-based sorption of test organic compounds by as much as 2-3 orders of magnitude, depending on the compound and the specific NOM sorbent. This is demonstrated for sorption on humin, humic acid, and the NOM source material. Hydration assistance in organic compound sorption correlates with the ability of the sorbate to interact strongly with hydrated sorbents, demonstrating the important role of noncovalent polar links in organizing the sorbent structure. Differences in hydration effect between the sorbents are caused mainly by differences in compound-sorbent interactions in the dry state. For a given compound, hydration of the sorbent tends to equalize the sorption capability of the three sorbents. No correlation was found between the strength of sorbate-sorbent interactions or the type of sorbate functional groups and the extent of sorption nonlinearity. Sorption nonlinearity compared over the same sorbed concentration range is greater on the original NOM than on either of the two extracted fractions. In elucidating sorption mechanisms on hydrated NOM, it is important to explicitly consider the participation of water molecules in organic compound interactions in the NOM phase.     

Effects of competitor and natural organic matter characteristics on the equilibrium sorption of 1,2-dichlorobenzene in soil and shale

The competitive sorption behaviors of 1,2-DCB in binary solute systems in four natural sorbents having natural organic matter (NOM) matrixes of different physicochemical characters were investigated in batch reactors. Specifically, the study focused on investigating how the extent of 1,2-DCB competitive sorption depends on (i) the rigidity of NOM matrixes as assessed by the efficiency of chemical oxidation and (ii) the closeness of competitor structure to that of the primary solute. The chemical oxidation and elemental composition results suggest that the shale NOM is the most reduced and condensed, the peat was the most oxidized and amorphous, and two surface soils had intermediate NOM structures. Four chlorinated benzenes and phenanthrene were used as competing solutes. All five chemicals exhibited competition against 1,2-DCB in all sorbents, including the peat, but the extent of competition varied significantly. Little difference in the extent of competition with 1,2-DCB was observed for the various chlorinated benzenes even though some were liquids and some were solids at the experimental temperature. All of the chlorobenzenes were more effective competitors than phenanthrene. The shale showed markedly different competition features from the other sorbents, with a much smaller competitive effect at a given sorbed volume of competitor. However, normalizing sorbed competitor volumes by the capacity of the adsorption domain in the Polanyi-Manes single-solute partition-adsorption model (V0) produced qualitatively similar competitive behavior for each solute; displacement of 1,2-DCB increased with increasing sorbed competitor volumes up to V0, and little additional competition occurred beyond that point. The extent of competition was positively correlated with the maximum adsorption capacity and the fraction of "hard" and "soot" carbon contents as assessed by chemical and thermal oxidation methods. These findings indicate that competition is associated with voids in the NOM structure, that these voids are likely present within the condensed ("hard" plus "soot") carbon domain, and therefore that diagenetic alteration of NOM plays a central role in determining competitive sorption characteristics for hydrophobic contaminants.     

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.     

Elucidating differences in the sorption properties of 10 humic and fulvic acids for polar and nonpolar organic chemicals

In this work we present a dataset of more than 1000 natural organic matter (NOM)/air partition coefficients covering polar and nonpolar organic compounds measured in 10 different humic and fulvic acids (HAs/FAs) from terrestrial and aquatic origins. Differences of more than 1 order of magnitude in the sorption coefficients of a given compound measured in HAs and FAs from different origins were found. The terrestrial HAs exhibited substantially higher sorption coefficients compared to aquatic HAs and FAs. The difference between any two types of NOM is mainly reflected by a constant shift in the partition coefficients that applies to all compounds in the same way. This indicates that it is the number of available sorption sites per mass of sorbent rather than the types of intermolecular interactions between the sorbate and the sorbent that governs the major differences between the sorption properties of various types of NOM. The experimental partition coefficients measured in all HAs and FAs were successfully described by polyparameter linear free energy relationships (pp-LFERs) that explicitly account for van der Waals as well as H-donor/acceptor interactions between the sorbate and the sorbent. These pp-LFER equations provide for the first time a tool that allows including the variability of the sorption properties of NOM in environmental fate models.     

Oil is a sedimentary supersorbent for polychlorinated biphenyls

The often-observed enhanced sorption of hydrophobic organic chemicals (HOCs) to sediments is frequently attributed to the presence of soot and soot-like materials. However, sediments may contain other hydrophobic phases, such as weathered oil residues. Previous experiments have shown that these residues can be efficient sorbents for certain PAHs. In this study we investigated sorption of PCBs to sediments contaminated with different concentrations and types of oils, and from that derived oil-water distribution coefficients (Koil). Sorption of PCBs to both fresh and weathered oils was proportional to sorbate hydrophobicity, and no effects of PCB planarity were observed. Furthermore, the experiments demonstrated that different oils sorbed PCBs similarly and extensively (Koil up to 108.3 for PCB 169), and that weathering caused an almost 2-fold increase in sorption of the lower chlorinated PCBs. Koil values indicated that at the PCB equilibrium concentrations tested (pg-ng/L range), for many congeners weathered oil is a stronger sorbent than pure soot and soot-like materials. Due to attenuation of adsorption to the latter materials in sediments (caused by competitive adsorption with organic matter), sedimentary weathered oil will therefore, if present as a separate phase, defeat sedimentary soot, coal, and charcoal as PCB sorbent in most cases. Consequently, weathered oil probably is the ultimate sedimentary sorbent for PCBs and should be included in HOC fate models.     

Characterization of disinfection byproduct precursors based on hydrophobicity and molecular size

Natural organic matter (NOM) from five water sources was fractionated using XAD resins and ultrafiltration membranes into different groups based on hydrophobicity and molecular weight (MW), respectively. The disinfection byproduct formation from each fraction during chlorination and chloramination was studied. In tests using chlorination, hydrophobic and high MW (e.g., >0.5 kDa) precursors produced more unknown total organic halogen (UTOX) than corresponding hydrophilic and low MW (e.g., <0.5 kDa) precursors. Trihaloacetic acid (THAA) precursors were more hydrophobic than trihalomethane (THM) precursors. The formation of THM and THAA was similar among different fractions for a water with low humic content. Hydrophilic and low MW (<0.5 kDa) NOM fractions gave the highest dihaloacetic acid (DHAA) yields. No significant difference was found for DHAA formation among different NOM fractions during chloramination. Increasing pH from 6 to 9 led to lower TOX formation for hydrophobic and high MW NOM fractions but had little impact on TOX yields from hydrophilic and low MW fractions. Bromine and iodine were more reactive with hydrophilic and low MW precursors as measured by THM or HAA formation than their corresponding hydrophobic and high MW precursors. However, hydrophobic and high MW precursors produced more UTOX when reacting with bromine and iodine.     

Characterization of the polarity of natural organic matter under ambient conditions by the polarity rapid assessment method (PRAM)

The polarity rapid assessment method (PRAM) characterizes the polarity of aqueous natural organic matter (NOM) by quantifying the amount of material adsorbed onto different solid-phase extraction (SPE) sorbents. The analysis is performed under ambient conditions resulting in the elimination of pretreatment steps that may alterthe chemical characteristics of the NOM, allowing an accurate representation of its polarity as it exists in the environment. Additionally, analysis only requires 200 mL of sample and can be performed in 2 h. In this paper, the underlying theory of the method is presented, followed by its optimization, with emphasis on the development of conditions for the analysis of NOM in natural waters. A series of organic probe compounds showed that the most important physicochemical property describing the interaction between the NOM and the SPE sorbents was the hydrophobic surface area, allowing for the estimation of the hydrophobic character under ambient conditions. Evaluation of the effects of chemical concentration, pH, and ionic strength show that (1) concentration did not have an effect on PRAM characterization as long as the pH and ionic strength remained constant; (2) changes in pH and ionic strength resulted in considerable changes in PRAM characterization, as a result of the changes in configuration of the NOM; and (3) PRAM characterization of NOM can be completed in the concentration range of < 10 mg C/L, although this range could be expanded by evaluating the effect of concentration on a site-specific basis. Results indicate that measurement of both ultraviolet absorption and dissolved organic carbon show complementary results as they measure different aspects of NOM.     

Development of engineered natural organic sorbents for environmental applications. 2. Sorption characteristics and capacities with respect to phenanthrene

The effects of superheated water processing on enhancement of phenanthrene sorption by various source materials of natural organic matter (NOM) were systematically examined. Sorption capacities and subsequent phenanthrene retention characteristics of all organic materials tested were markedly increased by superheated water processing. Temperature effects on enhancement of the sorption behaviors of the test materials were greater than those of processing time, moisture content, and the presence of mineral catalysts. Greatest enhancement was observed for processing at 250 degrees C for 5 h with a moisture content of 50%. Strong correlations of sorption capacities and isotherm nonlinearities with processing temperature, and with the atomic ratios of oxygen to carbon (O/C), aromaticity, and hydrophobicity of the processed materials were observed. The sorption/desorption hysteresis indices of the processed materials also increased linearly with processing temperatures and O/C ratios. This is consistent with our observations of increased condensation and aromatization of NOM during superheated water processing presented in the first paper of this series. The relationships described provide direct experimental evidence that the sorption-desorption properties of NOM geosorbents are closely related to their degree of aromatization and condensation, and the work suggests strong potential for production of efficient and cost-effective engineered natural organic sorbents for environmental applications.     

Assessing the effect of humic acid redox state on organic pollutant sorption by combined electrochemical reduction and sorption experiments

Natural Organic Matter (NOM) is a major sorbent for organic pollutants in soils and sediments. While sorption under oxic conditions has been well investigated, possible changes in the sorption capacity of a given NOM induced by reduction have not yet been studied. Reduction of quinones to hydroquinones, the major redox active moieties in NOM, increases the number of H-donor moieties and thus may affect sorption. This work compares the sorption of four nonionic organic pollutants of different polarities (naphthalene, acetophenone, quinoline, and 2-naphthol), and of the organocation paraquat to unreduced and electrochemically reduced Leonardite Humic Acid (LHA). The redox states of reduced and unreduced LHA in all sorption experiments were stable, as demonstrated by a spectrophotometric 2,6-dichlorophenol indophenol reduction assay. The sorption isotherms of the nonionic pollutants were highly linear, while paraquat sorption was strongly concentration dependent. LHA reduction did not result in significant changes in the sorption of all tested compounds, not even of the cationic paraquat at pH 7, 9, and 11. This work provides the first evidence that changes in NOM redox state do not largely affect organic pollutant sorption, suggesting that current sorption models are applicable both to unreduced and to reduced soil and sediment NOM.     

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.     

Sorption of the veterinary antimicrobial sulfathiazole to organic materials of different origin

Sulfonamides (SA), ionizable, polar antimicrobial compounds, may reach the environment in substantial amounts by the spreading of manure. The environmental behavior of SA is still difficult to predict. We investigated the influence of the main factors supposed to control SA sorption to organic materials: composition of sorbent, solute chemistry, and contact time. For that purpose, sulfathiazole (STA) sorption to compost, manure, and humic acid after 1 and 14 d was studied under sterile conditions. The experiments demonstrated that sorption was most strongly affected by contact time and pH. Irrespective of sorbent and pH, sorption continued substantially after the fast initial sorption within 1 d. For all sorbents and both contact times, STA sorption exhibited a pronounced pH dependence. Species-specific Koc values decreased in the order KoccatiOn> Kocneutral > Kocanion. Differences in sorbent composition influenced STA sorption weaker. Forthe neutral STA species, NMR chemical shift regions assignable to ketonic, carboxylic, and phenolic C as well as aromatic C-H and methoxyl/N-alkyl C seemed to control sorption. Forthe cations, sorption followed the cation exchange capacities of the sorbents. STA sorption to manure and humic acid increased with higher ionic strength (0.31 M compared to 0.06 M) at pH 7.5.     

Analysis of SO2 sorption capacity of rice husk ash (RHA)/CaO/NaOH sorbents using response surface methodology (RSM): untreated and pretreated RHA

The SO2 sorption capacity (SSC) of sorbents prepared from rice husk ash (RHA) with NaOH as additive was studied in a fixed-bed reactor. The sorbents were prepared using a water hydration method by slurrying RHA, CaO, and NaOH. Response surface methodology (RSM) based on four-variable central composite face centered design (CCFCD) was employed in the synthesis of the sorbents. The correlation between the sorbent SSC (as response) with four independent sorbent preparation variables, i.e. hydration period, RHA/CaO ratio, NaOH amount, and drying temperature, were presented as empirical mathematical models. Among all the variables studied, the amount of NaOH used was found to be the most significant variable affecting the SSC of the sorbents prepared. The SSC for sorbent prepared with the addition of NaOH was found to be significantly higher than sorbents prepared without NaOH. This is probably because NaOH is a deliquescent material, and its existence increases the amount of water collected on the surface of the sorbent, a condition required for sorbent-SO2 reaction to occur at low temperature. The effect of further treatment of RHA at 600 degrees C was also investigated. Although pretreated RHA sorbents demonstrated higher SSC as compared to untreated RHA sorbents, nevertheless, at optimum conditions, sorbents prepared from untreated RHA was found to be more favorable due to practical and economic concerns.     

Exploring molecular sieve capabilities of activated carbon fibers to reduce the impact of NOM preloading on trichloroethylene adsorption

Adsorption of trichloroethylene (TCE) by two activated carbon fibers (ACFs) and two granular activated carbons (GACs) preloaded with hydrophobic and transphilic fractions of natural organic matter (NOM) was examined. ACF10, the most microporous activated carbon used in this study, had over 90% of its pore volume in pores smaller than 10 A. It also had the highest volume in pores 5-8 A, which is the optimum pore size region for TCE adsorption, among the four activated carbons. Adsorption of NOM fractions by ACF10 was, in general, negligible. Therefore, ACF10, functioning as a molecular sieve during preloading, exhibited the least NOM uptake for each fraction, and subsequently the highest TCE adsorption. The other three sorbents had wider pore size distributions, including high volumes in pores larger than 10 A, where NOM molecules can adsorb. As a result, they showed a higher degree of uptake for all NOM fractions, and subsequently lower adsorption capacities for TCE, as compared to ACF10. The results obtained in this study showed that understanding the interplay between the optimum pore size region for the adsorption of target synthetic organic contaminant (SOC) and the pore size region for the adsorption of NOM molecules is important for controlling NOM-SOC competitions. Experiments with different NOM fractions indicated that the degree of NOM loading is important in terms of preloading effects; however the waythatthe carbon pores are filled and loaded by different NOM fractions can be different and may create an additional negative impact on TCE adsorption.     

Influence of rhamnolipids and triton X-100 on the desorption of pesticides from soils

A rhamnolipid biosurfactant mixture produced by P. aeruginosa UG2 and the surfactant Triton X-100 were tested for their effectiveness of enhancing the desorption of trifluralin, atrazine, and coumaphos from soils. Sorption of both surfactants by the soils was significant and adequately described by the Langmuir-type isotherm. Values of maximum sorption capacity (Qmax) and Langmuir constant (Klang) did not correlate with the amount of soil organic matter. Our results indicate that clay surfaces play an important role in the sorption of surfactants. When surfactant dosages were high enough to reach soil saturation and maintain an aqueous micellar phase, pesticide desorption was only enhanced. At dosages below soil saturation, surfactants sorbed onto soil, increasing its hydrophobicity and enhancing the sorption of the pesticides by a factor of 2. Similar values of water-soil partition coefficients (Ksol*) for aged and fresh added pesticides to soils indicate that the aging process used did not significantly after the capability of either surfactant to desorb the pesticides. A model able to estimate equilibrium distributions of organic compounds in soil-aqueous-micellar systems was tested against experimental results. The determined organic carbon partition coefficients, Koc values, indicate that, on a carbon normalized basis, sorbed Rh-mix is a much better sorbent of pesticides than TX-100 or soil organic matter. These results have significant implications on determining the effectiveness of surfactants to aid soil remediation technologies.     

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.     

Fabrication of CaO-based sorbents for CO₂ capture by a mixing method

Three types of sorbent were fabricated using various calcium and support precursors via a simple mixing method, in order to develop highly effective, durable, and cheap CaO-based sorbents suitable for CO(2) capture. The sorption performance and morphology of the sorbents were measured in a thermogravimetric analyzer and a scanning electron microscopy, respectively. The experimental results indicate that cement is a promising low-cost support precursor for contributing to the enhancement of cyclic CO(2) sorption capacity, especially when organometallic calcium precursors were used. A sorbent (with 75% CaO content) made from calcium l-lactate hydrate and cement showed the highest CO(2) sorption capacity of 0.36 g of CO(2)/g of sorbent and its capacity decreased only slightly after 70 cycles of carbonation and calcination.     

Fractionation of natural organic matter in drinking water and characterization by 13C cross-polarization magic-angle spinning NMR spectroscopy and size exclusion chromatography

Natural organic matter from drinking water sources was fractionated, and the fractions were characterized by NMR and SEC with the aim of relating NOM structure to treatability. Organic matter was isolated from two Australian surface waters (Horsham, Moorabool) by reverse osmosis and from a groundwater (Wanneroo) by anion exchange. The isolates were fractionated according to polarity and charge by resin adsorption. 13C NMR spectra of the freeze-dried fractions showed the most hydrophobic fraction to be high in aliphatic and aromatic carbon while slightly hydrophobic organics have more carbonyl and alkoxyl carbon. The Horsham and Wanneroo hydrophilic fractions show strong alkoxyl signals attributed to carbohydrate. Moorabool hydrophilics contain aromatic (phenolic) carbon; the apparent absence of carbohydrate appears to be an artifact. Size-exclusion chromatograms were recorded on the original and fractionated organics with both UV and dissolved organic carbon detection. The Horsham and Moorabool organics have similar molecular size distributions while Wanneroo is dominated by strongly absorbing species having large hydrodynamic radii. The hydrophobic and charged hydrophilic fractions also have high apparent MW, while the neutral fraction is higher in low-MW compounds of relatively low specific absorbance, suggestive of carbohydrates.     

Evaluation of shale and organoclays as sorbent additives for low-permeability soil containment barriers

A natural shale and four synthetic organoclays were compared as potential sorbent additives to containment barriers at hazardous waste sites. Trimethylphenyl ammonium bentonite (TMPA-bent) was shown in batch experiments to have the greatest sorption capacities for 1,2,4-trichlorobenzene, trichloroethylene, and methyl isobutyl ketone, followed by the shale and a commercial organoclay. Sorption capacities were lowest for hexadecyltrimethyl ammonium bentonite (HDTMA-bent) and hexadecyl pyridinium bentonite (HDP-bent). Operative sorption mechanisms for the organoclays depended on the size of the organic modifier, i.e., uptake by the TMPA-bent occurred via adsorption onto mineral surfaces, while that for the HDTMA-bent and HDP-bent took place by absorption into organic phases formed by their long hydrocarbon tails. The shale was found to be by far the most cost-effective sorbent, an important factor for large scale applications. Solids concentration effects (i.e., higher apparent sorption capacities at lower experimental sorbent concentrations) were exhibited by HDTMA-bent and HDP-bent. This can be attributed to aggregation of sorbent particles as a result of interactions among their hydrocarbon chains. Solids effects were observed to decline and eventually disappear as sorbent concentrations were increased. Such effects must be considered in applying batch sorption results to flow-through systems.