Increasing Dissolved Organic Matter Concentration Research Articles

The Influence of Dissolved Organic Matter to the Seawater Refractive Index Using Optical Refractometer

Optic refractive index is a good proxy for in situ salinity measurement of seawater according to TEOS-10. Currently, salinity of seawater is estimated by practical salinity (SP), which is calculated by measuring electric conductivity, temperature and pressure using Conductivity-Temperature-Depth (CTD) sensors. However, the electric conductivity method only takes into account the conductive compositions to seawater salinity and ignores the contribution of non-ionic components. However, non-ionic components in seawater affect seawater salinity according to the definition of salinity of seawater, but they could not be measured by conductivity sensor. Optical refractive index is sensitive to all dissolved matters of seawater including ionic and non-ionic components. Here, we take the dissolved organic matter (DOM) extracted from Aoshan Bay seawater as the representative sample to experimentally study the contribution of the non-ionic component to the refractive index of the standard seawater. Seawater refractive indices with different DOM concentrations were measured by a home-made V-shaped groove refractometer. The experimental results show that refractive index of seawater increases linearly with increasing DOM concentration at rate of 1.19 × 10-4/1(g/kg) DOM.

Dark transformation from 17β-estradiol to estrone initiated by hydroxyl radical in dissolved organic matter

The occurrence and fate of 17β-estradiol (E2) in natural water have gained extensive attention owing to its high ecotoxic risk to wildlife. Dissolved organic matter (DOM) is a ubiquitous water constituent and contributes significantly to E2 removal, although the reaction mechanism is rarely clarified. The present study aims to investigate E2 transformation in water containing fresh or aged DOM surrogates at environmentally relevant concentrations in the dark. Experiments along with radical probes of benzene and furfuryl alcohol reveal that reactive radicals, particularly hydroxyl radical (·OH), formed non-photochemically at higher concentrations in aged DOM than in fresh DOM. The contribution of ·OH in E2 removal is indicated by the decreases in the removal of radical probes in the presence of E2; moreover, E2 removal is inhibited in the presence of radical scavengers. The dose-dependent inhibitive effect of substrate concentrations, including E2 and coexistent propylparaben, shows that the radical concentration is a limiting factor for E2 removal, which could be enhanced by increasing DOM concentration, dissolved oxygen, and light supply. As the main byproduct, estrone (E1) is persistent in the current DOM water in the dark, but it can be easily photodegraded when exposed to light. Theoretical analysis reveals that the initial step is ·OH-initiated H- abstraction on the hydroxyl group in the cyclopentane ring of E2. The formed singlet excited state of E2 undergoes further intramolecular rearrangement and oxidative dehydrogenation to generate E1 and the hydroperoxy radical (·HO2). Considering the universal occurrence of E2 in DOM-rich aquatic matrices, the present findings have special implications for the biogeochemical cycle and risk assessment of this pollutant in natural aquatic environments, particularly those beyond the photic zone.

Bentonite binding with mercury(II) ion through promotion of reactive oxygen species derived from manure-based dissolved organic matter.

This study reports the mercury binding by bentonite clay influenced by cattle manure-derived dissolved organic matter (DOM). The DOM (as total organic carbon; TOC) was reacted with bentonite at 5.2 pH to monitor the subsequent uptake of Hg2+ for 5days. The binding kinetics of Hg2+ to the resulting composite was studied (metal = 350µM/L, pH 5.2). Bentonite-DOM bound much more Hg2+ than original bentonite and accredited to the establishment of further binding sites. On the other hand, the presence of DOM was found to decrease the Hg2+ binding on the clay surface, specifically, the percent decrease of metal with increasing DOM concentration. Post to binding of DOM with bentonite resulted in increased particle size diameter (~ 33.37- ~ 87.67nm) by inducing the mineral modification of the pore size distribution, thus increasing the binding sites. The XPS and FTIR results confirm the pronounced physico-chemical features of bentonite-DOM more than that of bentonite. Hydroxyl and oxygen vacancies on the surface were found actively involved in Hg2+ uptake by bentonite-DOM composite. Furthermore, DOM increased the content of Hg2+ binding by ~ 10% (pseudo-second-order qe = 90.9-100.0) through boosting up Fe3+ reduction with the DOM. The quenching experiment revealed that more oxygen functionalities were generated in bentonite-DOM, where hydroxyl was found to be dominant specie for Hg2+ binding. The findings of this study can be used as theoretical reference for mineral metal interaction under inhibitory or facilitating role of DOM, risk assessment, management, and mobilization/immobilization of mercury in organic matter-containing environment.

Binding of waterborne pharmaceutical and personal care products to natural dissolved organic matter

Information on how key environmental conditions such as natural dissolved organic matter (DOM) and water pH alter the possible risks posed by pharmaceuticals (PPCPs) is still scarce. In our previous study, the presence of natural DOM at high pH reduced the toxicity of a mix of waterborne PPCPs to algae. DOM-complexation and pH effect on speciation of the more hydrophobic and neutral compounds of the mix was suggested to be driving this behaviour. However, the study design did not allow the verification of this hypothesis. Here, the DOM- PPCPs interaction at different pH was investigated for 6 PPCPs through equilibrium dialysis, under the same conditions of DOM and pH as our previous study. Association with DOM was confirmed for the more hydrophobic PPCPs at high pH. The results suggest the binding was driven by i) the presence of carboxylic groups of PPCPs, ii) high pH shifting the structural configuration of DOM, making it more suited to bind some of the PPCPs. A non-linear change of binding capacity with increasing DOM concentration was also observed among the tested PPCPs.

Influence of sulfide, chloride and dissolved organic matter on mercury adsorption by activated carbon in aqueous system

Using activated carbon (AC) as thin layer capping to reduce mercury (Hg) released from contaminated sediment is a feasible and durable remediation approach. However, several aqueous factors could greatly affect the Hg fate in the aquatic system. This study thus intends to clarify the influences on Hg adsorption by AC with the presence of sulfide, dissolved organic matter (DOM), and chloride. The lab-scale batch experiments were divided into two parts, including understanding (1) AC adsorption performance and (2) Hg distribution in different phases by operational definition method. Results showed that the Hg adsorption rate by AC was various with the presence of sulfide, chloride, and DOM (from fast to slow). Hg adsorption might be directly bonded to AC with Hg-Cl and Hg-DOM complexes and the rate was mainly controlled by intraparticle diffusion. In contrast, “Hg + sulfide” result was better described by pseudo-second order kinetics. The Hg removal efficiency was 92–95% with the presence of 0–400 mM chloride and approximately 65–75% in the “Hg + sulfide” condition. Among the removed Hg, 24–29% was formed into aqueous-phase particles and about 30% Hg was adsorbed on AC with 2–20 μM sulfide. Increasing DOM concentration resulted in more dissolved Hg. The proportion of dissolved Hg increased 31% by increasing DOM concentration from 0.25 to 20 mg C L− 1. Simultaneously, the proportion of adsorbed Hg by AC decreased by 47%. Overall, the presence of chloride increases the Hg adsorption by AC. In contrast, the presence of sulfide and DOM causes a negative effect on AC adsorption.

Does dissolved organic matter or solid peat fuel anaerobic respiration in peatlands?

Solid-phase soil organic matter is the largest carbon (C) pool in peatlands and, as such, has long been assumed to be the primary substrate driving anaerobic respiration in these systems. However, radiocarbon data from previous field samples suggest that dissolved organic matter (DOM) plays a key, and often dominant, role in fueling heterotrophic respiration across a variety of peatlands. In this study, we manipulated available C sources under laboratory conditions to empirically determine the primary C source – solid-phase peat or DOM – fueling anaerobic respiration at surface and deep depth increments within two bogs and a poor fen in northern Minnesota. We found that increasing DOM concentration from 0 to 50% during anaerobic incubations significantly increased rates of surface (25–50 cm) methane (CH4) production, but not carbon dioxide (CO2) production, indicating that DOM acts as a primary driver of surface methanogenesis in peatlands. Contrary to our expectations, this response was consistent across all three sites despite differences in plant communities and biogeochemical characteristics. However, we observed no effect of DOM availability on CH4 or CO2 production at deeper depths (75–100 or 150–200 cm). The lack of response of CO2 production to DOM manipulation highlights the sensitivity of surface CH4 production to changes in the DOM pool quality and quantity, which are likely to occur under future climate change scenarios. However, CH4 production in deeper peat appears to be limited by additional factors beyond labile C availability, at least on the timescale of the laboratory incubations.

Homo-Conjugation of Low Molecular Weight Organic Acids Competes with Their Complexation with Cu(II).

Dissolved organic matter (DOM) controls the bioavailability and toxicity of heavy metals in aquatic environments. The observation of decreased conditional binding constants with increasing DOM concentration is not well documented, which may result in significant uncertainties in heavy metal behavior modeling and risk assessment. We used eight low molecular weight organic acids (LMOC) with representative structures to simulate DOM molecules. The interactions between LMOC molecules resulted in the decreased Cu(II)-LMOC binding with increasing LMOC concentrations, but higher pH values than theoretical calculation after mixing LMOC solutions of different pHs. We thus proposed homoconjugation between LMOC molecules through negative charge-assisted H-bond ((-)CAHB). A mathematic model was developed to describe Cu(II)-LMOC complexation ( KC) and LMOC homoconjugation ( KLHL). The increased competition of LMOC homoconjugation over Cu(II)-LMOC complexation, as suggested by the increased ratios of KLHL/ KC, resulted in the apparently decreased Cu(II)-LMOC binding with the increased LMOC concentration. Similar concentration-dependent binding was also observed for DOM. With the identified homoconjugation between DOM molecules, some of the literature data with concentration-dependent behavior may be re-evaluated. This is the first work that quantitatively identified homoconjugation among organic molecules. Both the modeling concepts and results provide useful information in investigating the environmental roles of DOM in mediating metal bioavailability and transport.

Contrasting Effects of Dissolved Organic Matter on Mercury Methylation by Geobacter sulfurreducens PCA and Desulfovibrio desulfuricans ND132.

Natural dissolved organic matter (DOM) affects mercury (Hg) redox reactions and anaerobic microbial methylation in the environment. Several studies have shown that DOM can enhance Hg methylation, especially under sulfidic conditions, whereas others show that DOM inhibits Hg methylation due to strong Hg-DOM complexation. In this study, we investigated and compared the effects of DOM on Hg methylation by an iron-reducing bacterium Geobacter sulfurreducens PCA and a sulfate-reducing bacterium Desulfovibrio desulfuricans ND132 under nonsulfidic conditions. The methylation experiment was performed with washed cells either in the absence or presence of DOM or glutathione, both of which form strong complexes with Hg via thiol-functional groups. DOM was found to greatly inhibit Hg methylation by G. Sulfurreducens PCA but enhance Hg methylation by D. desulfuricans ND132 cells with increasing DOM concentration. These strain-dependent opposing effects of DOM were also observed with glutathione, suggesting that thiols in DOM likely played an essential role in affecting microbial Hg uptake and methylation. Additionally, DOM and glutathione greatly decreased Hg sorption by G. sulfurreducens PCA but showed little effect on D. desulfuricans ND132 cells, demonstrating that ND132 has a higher affinity to sorb or take up Hg than the PCA strain. These observations indicate that DOM effects on Hg methylation are bacterial strain specific, depend on the DOM:Hg ratio or site-specific conditions, and may thus offer new insights into the role of DOM in methylmercury production in the environment.

Effects of molecular size fraction of DOM on photodegradation of aqueous methylmercury

This study investigated the photodegradation kinetics of MeHg in the presence of various size fractions of dissolved organic matter (DOM) with MW < 3.5 kDa, 3.5 < MW < 10 kDa, and MW > 10 kDa. The DOM fraction with MW < 3.5 kDa was most effective in MeHg photodegradation. Increasing UV intensity resulted in the increase of photodegradation rate of the MeHg in all size of DOM fractions. Higher rates of MeHg degradation was observed at higher pH. For the portion of MW < 3.5 kDa, the photodegradation rate of MeHg increased with increasing DOM concentration, indicating that radicals such as singlet oxygen (1O2) radicals can be effectively produced by DOM. At higher portion of MW > 3.5 kDa, the inhibition of MeHg degradation was observed due to the effect of DOM photo-attenuation. Our result indicates that radical mediated reaction is the main mechanism of photodegradation of MeHg especially in the presence of MW < 3.5 kDa. Our results imply that the smaller molecular weight fraction (MW < 3.5 kDa) of DOM mainly increased the photodegradation rate of MeHg.

Influence of Dissolved Organic Matter on Tetracycline Bioavailability to an Antibiotic-Resistant Bacterium

Complexation of tetracycline with dissolved organic matter (DOM) in aqueous solution could alter the bioavailability of tetracycline to bacteria, thereby alleviating selective pressure for development of antibiotic resistance. In this study, an Escherichia coli whole-cell bioreporter construct with antibiotic resistance genes coupled to green fluorescence protein was exposed to tetracycline in the presence of DOM derived from humic acids. Complexation between tetracycline and DOM diminished tetracycline bioavailability to E. coli, as indicated by reduced expression of antibiotic resistance genes. Increasing DOM concentration resulted in decreasing bioavailability of tetracycline to the bioreporter. Freely dissolved tetracycline (not complexed with DOM) was identified as the major fraction responsible for the rate and magnitude of antibiotic resistance genes expressed. Furthermore, adsorption of DOM on bacterial cell surfaces inhibited tetracycline diffusion into the bioreporter cells. The magnitude of the inhibition was related to the amount of DOM adsorbed and tetracycline affinity for the DOM. These findings provide novel insights into the mechanisms by which the bioavailability of tetracycline antibiotics to bacteria is reduced by DOM present in water. Agricultural lands receiving livestock manures commonly have elevated levels of both DOM and antibiotics; the DOM could suppress the bioavailability of antibiotics, hence reducing selective pressure on bacteria for development of antibiotic resistance.

Effect of Dissolved Organic Matter from Wheat Straw or Swine Manure on the Cu Adsorption in Three Chinese Soils

Batch equilibration experiments were conducted to evaluate the effects of dissolved organic matter (DOM) from wheat straw (DOMw) and swine manure (DOMs) on copper (Cu) adsorption and behavior in Haplic Phaeozems, Haplic Acrisol, and Eutric Fluvisol in China. Results showed that the Cu adsorption isotherms were well fitted with both Langmuir and Freundlich equations. The Cu maximum potential adsorption capacity of the three soils followed the order of Eutric Fluvisol > Haplic Phaeozems > Haplic Acrisol. DOMw and DOMs increased the Cu adsorption capacity in Haplic Phaeozems and Haplic Acrisol, and the promoting role of DOMs on Cu adsorption was obviously higher than that of DOMw. Increasing DOM concentration of DOMw and DOMs promoted the Cu adsorption in Haplic Phaeozems and Haplic Acrisol. However, this promoting effect weakened with increasing DOM concentration. Moreover, DOMw and DOMs inhibited the Cu adsorption in Eutric Fluvisol, and this inhibitory effect significantly increased with increasing DOM concentration. The results may be used to assess the potential environmental contamination of the studied soils and to control the application of organic fertilizers.

Uptake of Hg2+by picocyanobacteria in natural water from four Andean lakes

Fil: Dieguez, Maria del Carmen. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Patagonia Norte. Instituto de Investigacion En Biodiversidad y Medioambiente; Argentina

Removal of dissolved organic matter (DOM) from raw water by single-walled carbon nanotubes (SWCNTs)

Dissolved organic matter (DOM) in raw water is a precursor for the formation of disinfection by-products (DBPs) during chlorination at a water treatment plant in southern Taiwan. The presence of DOM also has a large impact on the behavior of nanoparticles. The adsorption kinetics of single-walled carbon nanotubes (SWCNTs) was investigated in the presence of DOM as a possible technique for the pretreatment of DBP precursors. pH, ionic strength and temperature are major factors in adsorption tests. Experimental results indicated that adsorption reached equilibrium after 120 min. Additionally, the adsorption capacity of SWCNTs increased with an increasing DOM concentration and decreased with an increasing ionic strength. The optimum adsorption model out of a selection including the Modified Freundlich equation, the Pseudo-first-order equation and Pesudo-second order equation, was the Modified Freundlich equation model. The intraparticle diffusion equation model was also examined. The adsorption process was controlled by pore diffusion that was confirmed by the fits of intra-particle diffusion model. The maximum adsorption capacity was 26.1–20.8 mg/g at 4°C and 45°C. Isotherm adsorption results demonstrated that the adsorption of DOM onto SWCNTs was favorable at a high initial DOM concentration and at low ionic strength, pH and temperature. These findings highlighted the importance of pretreatments and the presence of DOM on the adsorption behavior of nanomaterials.

Investigating aggregation in Suwannee River, USA, dissolved organic matter using diffusion‐ordered nuclear magnetic resonance spectroscopy

Aggregation of the Suwannee River (USA) dissolved organic matter (SRDOM) is studied using nuclear magnetic resonance spectroscopy. Diffusion-ordered spectroscopy identifies two distinct bands, one corresponding to smaller components and the other to larger components. At lower concentrations (500 ppm), the dissolved organic matter (DOM) is present mainly as small components; however, with increasing concentration, the larger components become more pronounced as a result of aggregation. Calibrations indicate the small materials behave in a manner similar to that of maltodextrins of approximately 180 to 950 Da or proteins of 100 to 1000 Da. The aggregated species show behavior similar to that of maltodextrins of approximately 1000 to 21,000 Da or proteins of 1050 to 70,000 Da. The mean diffusivity of the aggregated components in SRDOM is consistent with that of maltodextrins of approximately 4500 Da and proteins of approximately 8000 Da at the highest concentration measured. At the lowest concentration (closest to environmental concentrations), little to no aggregation is observed. Diffusion profiles show an increase in large-molecular-weight material, with a simultaneous decrease in small-molecular-weight components with increasing DOM concentration. This suggests aggregates in SRDOM may be weak dispersive associations of low-molecular-weight material. Additionally, with a decrease in temperature, aggregates show faster diffusion, suggesting a tighter, more condensed arrangement. Further evidence supports DOM aggregation as conglomerations of numerous components in DOM rather than a more organized self-association. Carboxyl-rich alicyclic molecules (CRAM) play a prominent role in aggregation of SRDOM and, to a lesser extent, material-derived from linear terpenoids (MDLT). The role of lignin and carbohydrates is less clear, although at least some of the lignin is present as macromolecules and tends to show interactions with both the MDLT and CRAM components.

Effect of dissolved organic matter source on phytotoxicity to Lemna aequinoctialis

The effect of dissolved organic matter (DOM) on metal toxicity to aquatic organisms has been reported. Biotic ligand model (BLM) can account for this factor to predict metal toxicity. However, few attempts have been made to assess the effect of the DOM on metal phytotoxicity to duckweeds. The objectives of this study were to examine the effect of DOM on copper toxicity to the duckweed Lemna aequinoctialis, and to determine if DOM concentration alone, regardless of DOM source, is an acceptable input parameter for the BLM for copper. Nine different DOM isolates from nine different sites in Japan were used in this study. A significant difference was observed between the lowest and the highest copper binding capacity. Phytotoxicity for copper decreased with increasing DOM concentration. These observations support use of the copper biotic ligand model (BLM) with AFA% (active fulvic acid percent) as a regulatory tool to predict copper phytotoxicity on duckweeds.

Partitioning, Desorption, and Dechlorination of a PCB Congener in Sediment Slurry Supernatants

Partitioning and desorption played specific roles in the dechlorination of 2-chlorobiphenyl (2-CIBP) in sediment slurry supernatants, which are suspensions of dissolved organic matter (DOM). In short-term experiments, the partition coefficient (Kp) was related to the apparent dechlorination rate constant. The Kp value (160 L g(DOC)(-1)), which is independent of the DOM concentration, was determined based on the decrease of the apparent rate constant with the increase of the DOM concentration. In the long-term experiments, the overall rate of dechlorination can be described with a two-compartment model. The time constant for the sediment compartment was related to Kp and the desorption rate constant (k(d)). The k(d) value (0.21 h(-1)) was determined based on the decrease of the time constant values with an increasing DOM concentration. The use of DOM suspensions allowed a short time for equilibrium. Separation of the aqueous and DOM phases was not needed due to the dechlorination of 2-CIBP. The fundamental relationships between overall dechlorination rate constant and properties of the contaminant and sediment were established.

Adsorption kinetics of natural dissolved organic matter and its impact on arsenic(V) leachability from arsenic-loaded ferrihydrite and Al-ferrihydrite

The present work was broadly divided into two parts: first, the efficiency of synthetically prepared ferrihydrite and Al-ferrihydrite to adsorb dissolved organic matter (DOM) was tested as a function of time, pH and ionic strength and its effects on specific surface area was found. In the second part, the effect of DOM concentration and solution pH was studied to elucidate the influence of this two parameters on As(V) leachability from As(V) loaded ferrihydrite and Al-ferrihydrite. It was found that both pH and ionic strength had significant influence on DOM adsorption. The behavior and magnitude of adsorption was significantly different for both the materials. In general, the efficiency of Al-ferrihydrite to adsorb DOM was better compared to ferrihydrite. Results indicated that ligand exchange was the dominant interaction mechanism for DOM adsorption on ferrihydrite and Al-ferrihydrite. The study also suggested a strong potential for DOM to mobilize As(V) from the studied materials. At pH 5.0, DOM concentrations of 2.5 and 5.0 mg C/L had negligible effect on As(V) leachability. With increasing DOM concentration from 10 to 30 mg C/L, increased the As(V) leachability into the solution. Net release from ferrihydrite was up to 1.07 mg/g (5.75%) of the total adsorbed arsenic. The corresponding figure for Al-ferrihydrite was 1.47 mg/g (6.30%). As the primary mechanisms for the arsenic release from solid phases we identified two different types of mechanisms i.e., dissolution of the solid phase and competition between arsenic and organic anions for sorption sites.

Effect of organic fertilizers derived dissolved organic matter on pesticide sorption and leaching

Incorporation of organic fertilizers/amendments has been, and continues to be, a popular strategy for golf course turfgrass management. Dissolved organic matter (DOM) derived from these organic materials may, however, facilitate organic chemical movement through soils. A batch equilibrium technique was used to evaluate the effects of organic fertilizer-derived DOM on sorption of three organic chemicals (2,4-D, naphthalene and chlorpyrifos) in USGA (United States Golf Association) sand, a mixed soil (70% USGA sand and 30% native soil) and a silt loam soil (Typic Fragiochrept). DOM was extracted from two commercial organic fertilizers. Column leaching experiments were also performed using USGA sand. Sorption experiments showed that sorption capacity was significantly reduced with increasing DOM concentration in solution for all three chemicals. Column experimental results were consistent with batch equilibrium data. These results suggest that organic fertilizer-derived DOM might lead to enhanced transport of applied chemicals in turf soils.

Measuring the bioavailability of two hydrophobic organic compounds in the presence of dissolved organic matter

Bioavailability of benzo[a]pyrene (BaP) and 3,3',4,4'-tetrachlorobiphenyl (TCB) was studied in natural lake water containing dissolved organic matter (DOM). Lake water was diluted to give a dissolved organic carbon (DOC) range of 1 to 20 mg/L. Partition coefficients for the model compounds were assessed at different DOM concentrations and over time with three different methods, namely equilibrium dialysis and reverse-phase and liquid-liquid extraction. In addition, biological partition coefficients were estimated from the difference in the bioconcentration of the model compounds in Daphnia magna in the presence and absence of DOM. Results showed that bioavailability of the model compounds was reduced by the presence of DOM. The equilibrium dialysis method gave the best estimates for bioavailability of the model compounds when compared with biologically determined values. Both the reverse-phase and the liquid-liquid extraction overestimated the bioavailable fraction. The more pronounced overestimation of bioavailable fraction of TCB suggested that the sorption of TCB was not only lower but the interaction was also weaker than that of BaP. Increasing DOM concentration produced lower partition coefficients and the effect seemed to be more pronounced when measured by the reverse-phase and the extraction methods.

Dissolved organic matter effects on the performance of a barrier to polycyclic aromatic hydrocarbon transport by groundwater

In order to contain the movement of organic contaminants in groundwater, a subsurface sorption barrier consisting of sand or clay minerals coated with a cationic surfactant has been proposed. The effectiveness of such a sorption barrier might be affected by the presence of dissolved organic matter (DOM) in the groundwater. To study the impact of DOM on barrier performance, a series of batch experiments were performed by measuring naphthalene and phenanthrene sorption onto sand coated with cetylpyridinium chloride (CPC) and bentonite coated with hexadecyltrimethylammonium bromide (HDTMA) in the presence of various concentrations of DOM. The overall soil–water distribution coefficient ( K*) of naphthalene and phenanthrene onto CPC-coated sand decreased with increasing DOM concentration, whereas the K* of the compounds onto HDTMA-coated bentonite slightly increased with increasing DOM concentration. To describe the overall distribution of polycyclic aromatic hydrocarbons (PAHs) in the systems, a competitive multiphase sorption (CMS) model was developed and compared with an overall mechanistic sorption (OMS) model. The modeling studies showed that while the OMS model did not explain the CPC-coated sand experimental results, a model that included competitive sorption between DOM and PAH did. The experimental results and the modeling study indicated that there was no apparent competition between DOM and PAH in the HDTMA-coated bentonite system, and indicated that in groundwater systems with high DOM, a barrier using HDTMA-coated bentonite might be more effective.