Ionizable Organic Contaminants Research Articles

“PAW” a smart analytical process assessing lipophilicity of solutes in mixtures

BackgroundThe analysis of mixtures of contaminants remains a challenging task in many fields, including water quality and waste management. For example, the degradation of industrial waste such as plastics, leads to complex mixtures with hundreds of organic contaminants and often non-referenced analytes. In such cases, non-targeted or effects-based analyses provide complementary information to classical targeted-analyses, regarding contaminants nature or properties (molecular mass, lability, toxicity). In this study, a novel analytical method is proposed to characterise mixtures of unknown organic contaminants, with a focus on the lipophilicity of solutes. ResultsThe proposed process, named “PAW“ (Partition of Aqueous Waste), aims at the quantification of octanol-water partition coefficients (POW) of mixed organic analytes. The process is based on sequential liquid-liquid partition equilibria. The output result is a lipophilicity histogram of the solutes, screened according to the chosen detection method. The process quantifies the distribution of analytes as a function of their octanol-water partition coefficients, without requiring any identification or prior knowledge. The PAW process is applicable with various detectors (UV–Visible, total carbon, liquid scintillation, etc.) allowing to focus on specific families of contaminants (e.g. organic solutes, colloids, 14C-bearing, etc.). Experimental proofs of concept are proposed, illustrating process implementation and possible fields of application. The first example deals with purity analysis of synthetic radiolabeled compounds. The second example aims the monitoring of cellulose degradation and quantification of the lipophilicity of degradation products. SignificanceThe PAW analytical process seems especially useful for characterisation of mixtures containing both hydrophilic and lipophilic compounds, e.g. neutral and ionizable organic contaminants, hardly characterisable simultaneously by chromatographic methods. It could be complementary to more detailed targeted or screening analysis of samples and effluents. For example it may help assessing the composition and environmental fate of mixtures of unknown analytes, thus facilitating waste management or mitigation strategies.

Calibration comparison between two passive samplers -o-DGT and POCIS- for 109 hydrophilic emerging and priority organic compounds

The Polar Organic Chemical Integrative Samplers (POCIS) is the most widely used passive sampler for hydrophilic compounds, but unsuitable for certain ionic organic contaminants. The Diffusive Gradient in Thin-Film technique (o-DGT) has shown positive results for both ionic and hydrophilic compounds. However, a calibration step is now needed to evaluate kinetic constant of accumulation for a wide range of molecules.In this study, o-DGT and POCIS were compared for the sampling of three families of micropollutants of potential risk to aquatic environments: 53 pesticides, 36 pharmaceuticals and 20 hormones. A calibration experiment was conducted to compare the kinetic models and constants from a scientific and practical perspective. The results are discussed in a single table that summarizes the performance of both passive samplers for the 109 compounds of interest. The advantage of o-DGT is that it allows linear accumulation for 72 compounds versus only 33 with POCIS. The mean times to equilibrium obtained with o-DGT are higher than those obtained with POCIS. These results confirm that the presence of a diffusion gel delays the achievement of equilibrium during compound accumulation. Therefore, o-DGT can be considered for situations where POCIS cannot be used due to non-linear accumulation over a typical 14-day deployment period. However, overall sampling rates and mass transfer coefficients also appear reduced with o-DGT, which is explained by the smaller exchange surface area, as well as the consideration of an additional diffusive layer in this device. This paper also showed that the most appropriate membrane to sample polar compounds with o-DGT was a polyethersulfone polymer with a pore size of 5 μm.

Natural manganese sand activates sodium hypochlorite to enhance ionic organic contaminants removal: Optimization, modeling, and mechanism

Although sodium hypochlorite acting as an oxidant has been investigated for the role it plays in the degradation of organic contaminants, little attention has been paid to its activation and efficient utilization. In this study, natural manganese sand (NMS) was verified to be effective for activation of sodium hypochlorite (NaClO). Due to the generation of O2−, the removal efficiency of ionic organic contaminants in NMS/NaClO system was 1.9–4.1 times higher than that in NMS or NaClO alone. Hence, NMS activated NaClO system performed ~96.6 % contaminants removal efficiency at a wide pH range (pH 5–9). Kinetic modeling yielded that the NMS dosage was more important than NaClO dosage. Long-term stability was observed in the presence of various salts (bicarbonate, sulfate, phosphate, and chloride). Characterization results revealed that electron transfer among NMS, NaClO, and organic contaminants was responsible for NaClO activation. Then NaClO-based Fenton-like process was proposed by tracing the degradation intermediates of methyl orange (MO) and generations of reactive oxygen species in the MO/NMS/NaClO system. This study presents the potential of NMS to activate NaClO and enhance ionic organic contaminants removal from aquatic environments.

Effects of pH and gallic acid on the adsorption of two ionizable organic contaminants to rice straw-derived biochar-amended soils

The existing form of ionizable organic contaminants (IOCs) could affect their adsorption characteristics to soil and biochar. In this study, 2 IOCs, namely, sulfadiazine and imazalil, were selected to study their adsorption by rice straw-derived biochar-amended soils, as well as the effect of pH and gallic acid on their adsorption. The results showed that the soil adsorption isotherms of the two ionizable organic contaminants could be fitted well by a linear equation and the Freundlich equation, and r2 was more than 0.80. The adsorption coefficient (Kd) in the three kinds of soil ranged from 0.262 to 4.07 L kg−1 for sulfadiazine and from 3.11 to 96.5 L kg−1 for imazalil. After the addition of biochar, the adsorption of sulfadiazine and imazalil in the soil increased. The adsorption of sulfadiazine by biochar gradually decreased with the increase in pH; the adsorption of imazalil increased when the pH increased from 2 to 5 and then gradually decreased with increasing pH. Gallic acid enhanced the adsorption of the two IOCs to pure soil and biochar-amended soil.

Effects of Binary Mixtures and Transpiration on Accumulation of Pharmaceuticals by Spinach.

Many pharmaceuticals are present in reclaimed wastewater and effluent-dominated water bodies used to irrigate edible crops. Previous research has shown that plants irrigated with reclaimed wastewater can accumulate pharmaceuticals. However, plant-driven processes that contribute to differences in accumulation among compounds are not well understood. Here, we tested the effects of exposure to mixtures on spinach accumulation and metabolism of four psychoactive pharmaceuticals found in reclaimed wastewater: carbamazepine, fluoxetine, amitriptyline, and lamotrigine. Coexposure of plants to carbamazepine and fluoxetine or amitriptyline decreased accumulation of the toxic carbamazepine metabolite 10,11-epoxycarbamazepine. Furthermore, we tested a simple transpiration-based accumulation model and found that transpiration is a strong predictor for accumulation of the studied compounds. Amitriptyline accumulated to a larger extent than predicted from transpiration alone, and we suggest the possibility that a transporter protein may be involved in its uptake. Our findings highlight the need to consider plant physiology and mixture effects in studying accumulation of polar and ionizable organic contaminants and their metabolites.

PH-dependent sorption of sulfonamide antibiotics onto biochars: Sorption mechanisms and modeling

It remains a challenge to precisely predict and control environmental behaviors of ionizable organic contaminants (IOCs) due to their species change relative to pH and because of the lack of appropriate models to illustrate the underlying pH-dependent mechanisms. We studied the pH-dependent sorption behavior of five sulfonamide antibiotics (SAs) as typical IOCs with different pKa values towards a series of biochars as representative sorbents with well-characterized surface structures. After subtracting the contribution of the speciation effect using a classical speciation model, up to three unexpected enhanced sorption peaks could be found and regulated by the pKa,SA of the SAs and the pKa, BC of the biochars. The mono H-bond formation between the two pKa,SA of the SAs (pKa,SA1 is from NH2, pKa,SA2 is from SO2NH), and the biochar surface functional groups with comparable pKa values generated two peaks. Another peak around the middle between pKa,SA1 and pKa,SA2 appeared due to the aromatic π bonding-enhanced dual H-bond. All of these peaks were quantitatively separated by a novel two-compartment model, which was developed by capturing the characteristics of pH-dependent sorption. The quantified hydrogen bonding among different SAs elucidates the effectiveness and limits of the pKa equalization principle to predict the strengthening of hydrogen bonding at the solid-aqueous interface. This work recognizes the quantitative relationship among the structure, sorption, and H-bond interaction of biochars and guides the prediction of the fate of IOCs in the environment and the development of remediation options.

Sorption of ionizable organic chemicals to carbonaceous adsorbents: Solution pH change and contributions of different species

The adsorptive performance of carbonaceous materials (CMs) for the removal and sequestration of ionizable organic contaminants (IOCs) is heavily dependent on the pH-controlled speciation of the objective chemicals. To date, there is no effective method for measuring the sorption of IOC species at a solution pH near the IOC pKa, which is usually of environmental relevance and essential for validating sorption models. In this study, a unique “pH dynamics method” was developed. The method relates the uptake of the IOC species with the solution pH changes during sorption on the basis of the proton mass balance in the adsorptive process. The effectiveness of the method to quantify the sorption of IOC species at a pH near the pKa was validated by determining the extents and mechanisms of the pH change in a series of sorption systems employing graphite, biochar, carbon nanotube and activated charcoal as representative CMs, and 4-methylbenzoic acid, 2,4,6-trichlorophenol, 4-methylaniline and quinoline as model IOCs. Moreover, with the derived species uptake using the pH dynamics method, the effectiveness of already-reported speciation models for predicting species sorption at a pH around the pKa was verified for the first time. It was shown that the speciation models, including the linear speciation model and nonlinear speciation model, were convincing for estimating the sorption of individual species at the pH around the chemical pKa when the collected uptake of individual species could be reliably predicted. This research advances the application of pH dynamics for quantifying the uptake of individual species of IOCs and enhances the understanding of the speciation models and carbonaceous materials in managing risks of IOCs.

Enhanced adsorption of anionic toxic contaminant Congo Red by activated carbon with electropositive amine modification

Anionic ionizable toxic organic contaminants, such as pesticides, herbicides, pharmaceuticals and dyestuffs, are widely detected in aqueous and can exert specific toxicity in organisms. They are hard removed by traditional adsorbents with negative surface charges and hydrophilic property. To solve that bottleneck problem, this study synthesized a carbon-based electropositive absorbent via surface activation and radical impregnation. The monolayer adsorption capacity and favorability of the novel adsorbent increased 4.2 and 16 times compared with activated carbon, evaluated by Congo Red. The adsorption kinetics could be described by pseudo-second-order equation, dominated by chemisorption and shortened 75% equilibrium time. The excellent adsorption behavior of synthesized material was attributed to created and bridged electropositive quaternary ammonium structure onto activated carbon, identified with XPS and FT-IR. Additionally, the abundant mesoporous structures of modified carbon provide more tunnels and facilitate adsorption of Congo Red, identified by AutoSorb-iQ. With the batch experiments of pH and ion strength influence, the absorbent behaved well in acidic low ion strength conditions and attenuated by coexisting cations. The study provides a high efficiency, large capacity and low cost materials to remove anionic ionizable organic contaminants in aqueous.

Plant-Induced Changes to Rhizosphere pH Impact Leaf Accumulation of Lamotrigine but Not Carbamazepine

Many ionizable organic contaminants (IOCs) are present in treated wastewater used to irrigate edible crops and accumulate in plants under field conditions. Phytoavailability of IOCs with pKa values between 4 and 9 may be affected by the pH of the rhizosphere (the water and soil within 2–3 mm of the root surface). Plants can alter rhizosphere pH by 2–3 units in either direction in response to nutrient availability. The effects of plant modulation of rhizosphere pH on IOC accumulation have not been previously reported. Here we provide direct evidence that plant-driven changes in rhizosphere pH impact accumulation of an IOC in plant leaves. Using a modified hydroponic system, we found that rhizosphere pH was higher by 1.5–2.4 units when plants received only nitrate rather than a combination of nitrate and ammonium. Plant-driven changes to rhizosphere pH altered accumulation of lamotrigine (pKa = 5.7) but not carbamazepine, a non-ionizable contaminant. Lamotrigine accumulation in leaves correlated strongly wi...

Bioconcentration model for non-ionic, polar, and ionizable organic compounds in amphipod.

The present study presents a bioconcentration model for non-ionic, polar, and ionizable organic compounds in amphipod based on first-order kinetics. Uptake rate constant k1 is modeled as logk1=10.81logKOW+0.15 (root mean square error [RMSE] = 0.52). Biotransformation rate constant kM is estimated using an existing polyparameter linear free energy relationship model. Respiratory elimination k2 is calculated as modeled k1 over theoretical biota-water partition coefficient Kbiow considering the contributions of lipid, protein, carbohydrate, and water. With negligible contributions of growth and egestion over a typical amphipod bioconcentration experiment, the bioconcentration factor (BCF) is modeled as k1 /(kM + k2 ) (RMSE = 0.68). The proposed model performs well for non-ionic organic compounds (log KOW range = 3.3-7.62) within 1 log-unit error margin. Approximately 12% of the BCFs are underpredicted for polar and ionizable compounds. However, >50% of the estimated k2 values are found to exceed the total depuration rate constants. Analyses suggest that these excessive k2 values and underpredicted BCFs reflect underestimation in Kbiow , which may be improved by incorporating exoskeleton as a relevant partitioning component and refining the membrane-water partitioning model. The immediate needs to build up high-quality experimental kM values, explore the sorptive role of exoskeleton, and investigate the prevalence of k2 overestimation in other bioconcentration models are also identified. The resulting BCF model can support, within its limitations, the ecotoxicological and risk assessment of emerging polar and ionizable organic contaminants in aquatic environments and advance the science of invertebrate bioaccumulation. Environ Toxicol Chem 2018;37:1378-1386. © 2018 SETAC.

Spectroscopic properties of an amphiphilic naphthalimide derivative with high surface activity and detection of trace salt in the water as a fluorescent sensor

A novel amphiphilic fluorescent sensor bearing high surface activity, 4-(3-Dimethyl-benzyl propylamine ammonium)-N-dodecyl-1,8-naphthalimide chloride (DADNC), has been designed and used to rapidly detect trace salt in the pure water as an “on-off” fluorescent sensor. The surface activity and high sensitivity to trace salt ions and organic contaminants were measured. The average particle sizes of the molecular micelles of DADNC were 31–55nm in diameter. The fluorescence of DADNC is highly sensitive to trace salt ions and ionic organic contaminants. The aggregation and disintegration of DADNC in the presence of ionic micelles showed sensitive “on-off-on” fluorescence switching. There were good linear relationships between its fluorescence intensities and the salt concentrations at very low salt concentrations from 1×10^−8 to 1×10^−5mol/L. This system could be used as an “on-off” fluorescence sensor for the detection and the quantitative determination of ionic substrate or anionic organic compounds at ultralow ion concentration. It has potential applications in the recognition and detection of the trace amounts of the salt ions and organic contaminants in the drinking water and pure water.

Reactive mineral removal relative to soil organic matter heterogeneity and implications for organic contaminant sorption

Soil organic matter (SOM) is generally treated as a static compartment of soil in pollutant fate studies. However, SOM might be altered or fractionated in soil systems, and the details of SOM property/composition changes when coupled with contaminant behavior are unknown. In this study, a mild acid treatment was adopted to remove reactive minerals and partially remove SOM components. After acid treatment, biomarker signatures showed that lignin-derived phenols were released and black carbon (as suggested by benzene-polycarboxylic acids) and lipids were enriched. The biomarker information was consistent with common bulk chemical characterization. The sorption coefficient Kd for PHE was two times higher after acid treatment, whereas Kd for OFL was three times lower. The organic carbon normalized sorption coefficient KOC values for PHE were higher for soils after acid treatment, indicating stronger interactions between PHE and SOM. The linear regression line between Kd and fOC for OFL showed lower intercepts and slopes after reactive mineral removal, suggesting a decreased contribution of minerals and reduced dependence on SOM. These results were attributed to the release of polar compositions in SOM accompanied by reactive mineral removal. Our results suggest that the mobility of ionic organic contaminants increases, whereas that of hydrophobic organic contaminants decreases after acid treatment with respect to reactive mineral depletion. This study emphasized that new insights into the coupling of SOM dynamics should be incorporated into organic contaminant behavior studies. SOM molecular biomarkers offer a useful technique for correlating SOM composition and sorption property changes.

Fast and slow adsorption of carbamazepine on biochar as affected by carbon structure and mineral composition.

The influence of carbon structure and mineral composition of biochar on fast adsorption and slow adsorption of ionic organic contaminants (IOCs) is still unclear. This study used carbamazepine (CBZ) as adsorbate, peanut-shell-derived biochars produced at different charred temperatures, and the corresponding acid-washed biochars as adsorbents, to investigate the adsorption kinetics as affected by carbon structure and mineral composition of biochar. The adsorption of CBZ on amorphous (loose) carbon was lower than aromatic (condensed) carbon, but the former mainly contributed to the fast adsorption of CBZ. The hydrophobic and π-π interactions were likely the predominant adsorption mechanisms of CBZ on biochar. The ratio of CBZ fast and slow adsorption fractions (ffast/fslow) significantly reduced after acid-wash treatment of the biochars. The X-ray diffraction and Fourier transform infrared spectra indicated that minerals, including CaCO3, KAlO2, and quartz, were likely responsible for the fast adsorption of CBZ by the hydrogen bonds between CBZ and -OH on the surface of minerals. These results are useful for better understanding of the environmental behavior and prediction of the environmental risks of IOCs in biochar-amended soils.

Modeling the pH-mediated extraction of ionizable organic contaminants to improve the quality of municipal sewage sludge destined for land application

A model was developed to assess the impact of adding acids and bases to processed municipal sewage sludge (MSS) to mobilize contaminants, facilitating their removal from sludge by flushing prior to land application. Among 312 organic contaminants documented to occur in U.S. MSS, 71 or 23% were identified as ionizable organic contaminants (IOCs), contributing a disproportionately large fraction of 82% of the total mass of sludge-borne contaminants. Detected IOCs included 57 pharmaceuticals and personal care products, 12 perfluorinated compounds, one surfactant and one pesticide. Annually, about 2000t of IOCs were estimated to be released to U.S. soils via land-application of MSS. A partitioning model developed to assess the impact of pH on hydrophobic sorption revealed that between 36 and 85% of the mass of individual classes of IOCs potentially could be desorbed from MSS via pH adjustment and flushing. Thus, modeling results suggest that a sequential pH treatment [acidic (~pH2) followed by basic (~pH12) treatment] has the potential to reduce the burden of harmful IOCs in MSS applied on U.S. land by up to 40±16t annually. This approach may serve as a cost-effective treatment process that can be implemented easily in existing sludge treatment infrastructure in the U.S. and worldwide, serving to significantly improve the quality of MSS destined for land application.

Contribution of coated humic acids calculated through their surface coverage on nano iron oxides for ofloxacin and norfloxacin sorption

Sorption of organic contaminants on organo-mineral complexes has been investigated extensively, but the sorption contribution of mineral particles was not properly addressed before calculating KOC, especially for ionic organic contaminants. We measured the surface coverage of a humic acid (HA) on nano iron oxides (n-Fe2O3) in a series of synthesized organo-mineral complexes. The contribution of the coated HA to ofloxacin (OFL) and norfloxacin (NOR) sorption in HA–n-Fe2O3 complexes was over 80% of the total sorption with the surface coverage of 36% and fOC of 1.6%. All the coated HA showed higher sorption to NOR and OFL in comparison to the original HA, suggesting HA fractionation and/or physical re-conformation during organo-mineral complex formation. The decreased KOC with multilayer coating may suggest the importance of site-specific interactions for OFL sorption, while the increased KOC with multilayer coating may suggest the importance of partitioning in hydrophobic region for NOR sorption.

Understanding and Modeling Removal of Anionic Organic Contaminants (AOCs) by Anion Exchange Resins

Ionic organic contaminants (OCs) are a growing concern for water treatment and the environment and are removed inefficiently by many existing technologies. This study examined removal of anionic OCs by anion exchange resins (AXRs) as a promising alternative. Results indicate that two polystyrene AXRs (IRA910 and IRA96) have higher sorption capacities and selectivity than a polyacrylate resin (A860). For the polystyrene resins, selectivity follows: phenolates ≥ aromatic dicarboxylates > aromatic monocarboxylates > benzenesulfonate > aliphatic carboxylates. This trend can be explained based on hydration energy, the number of exchange groups, and aromaticity and hydrophobicity of the nonpolar moiety (NPM) of the anions. For A860, selectivity only varies within a narrow range (0.13-1.64). Despite the importance of the NPM of the anions, neutral solutes were sorbed much less, indicating synergistic combinations of electrostatic and nonelectrostatic interactions in the overall sorption. By conducting multiple linear regression between Abraham's descriptors and nature log of selectivity, induced dipole-related interactions and electrostatic interactions were found to be the most important interaction forces for sorption of the anions, while solute H-bond basicity has a negative effect. A predictive model was then developed for carboxylates and phenolates based on the poly parameter linear free energy relationships established for a diverse range of 16 anions and 5 neutral solutes, and was validated by accurate prediction of sorption of five test solutes within a wide range of equilibrium concentrations and that of benzoate at different pH.

Using Polyacrylate-Coated SPME Fibers To Quantify Sorption of Polar and Ionic Organic Contaminants to Dissolved Organic Carbon

A passive sampling method using polyacrylate-coated solid-phase microextraction (SPME) fibers was applied to determine sorption of polar and ionic organic contaminants to dissolved organic carbon (DOC). The tested contaminants included pharmaceuticals, industrial chemicals, hormones, and pesticides and represented neutral, anionic, and cationic structures. Prior to the passive sampler application, sorption of the chemicals to the fibers was characterized. This was needed in order to accurately translate concentrations measured in fibers to freely dissolved aqueous concentrations during the sorption tests with DOC. Sorption isotherms of neutral compounds to the fiber were linear, whereas isotherms of basic chemicals covered a nonlinear and a linear range. Sorption of acidic and basic compounds to the fiber was pH-dependent and was dominated by sorption of the neutral sorbate species. Fiber- and DOC-water partition coefficients of neutral compounds were both linearly related to octanol-water partition coefficients (log Kow). The results of this study show that polyacrylate fibers can be used to quantify sorption to DOC of neutral and ionic contaminants, having multiple functional groups and spanning a wide hydrophobicity range (log Kow = 2.5-7.5).

Adsorption of ionizable organic contaminants on multi-walled carbon nanotubes with different oxygen contents

Multi-walled carbon nanotubes (MWNTs), which are considered to be promising candidates for the adsorption of toxic organics, are released into aqueous environment with their increasing production and application. In this study, the adsorption behaviors of five structurally related ionizable organic contaminants namely perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA), perfluorooctanesulfonamide (PFOSA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 4-n-nonylphenol (4-NP) onto MWNTs with different oxygen contents (3.84–22.85%) were investigated. The adsorption kinetics was investigated and simulated with pseudo-second-order model. The adsorption isotherms were found to be fitted with Freundlich model and influenced by both the properties of organic chemicals and the oxygen contents of MWNTs. As adsorption capacity decreases dramatically with the increasing of oxygen contents, the MWNTs with the lowest oxygen contents possess the highest adsorption capacity among four MWNTs. For the MWNTs with the oxygen contents of 3.84%, the adsorption affinity related with hydrophobic interaction and π-electron polarizability decreased in the order of 4-NP>PFOSA>PFOS>2,4-D>PFOA. Furthermore, the adsorption characters of five contaminants were affected by solution pH and solute pKa considering electrostatic repulse force and hydrogen bonding, which showed the adsorption of MWNTs with lower oxygen content is much sensitive to solution chemistry.

Contribution of Different Sulfamethoxazole Species to Their Overall Adsorption on Functionalized Carbon Nanotubes

Antibiotics pose environmental risks, but their adsorption mechanisms are still unclear. Identifying the contributions of different mechanisms is vital in predicting antibiotic environmental behavior and consequently understanding their environmental risks. This study used functionalized carbon nanotubes (CNTs), namely hydroxylized (MH), carboxylized (MC), and graphitized (MG) multiwalled CNTs, as adsorbents and sulfamethoxazole (SMX) as an adsorbate to study the adsorption mechanisms of ionizable organic contaminants on solid particles. At pH around 3.7, SMX always showed the highest adsorption on different CNTs and the adsorption followed the order of MH > MG > MC. Combining the results on SMX specie analysis, the pH-dependent adsorption is well-explained by hydrophobic and electron-donor-acceptor interactions. The adsorption of neutral SMX is always dominant by contributing generally over 80% to the overall adsorption. A significant contribution of cationic SMX at pH < 3.5 suggested significant contribution of hydrogen bonds to SMX adsorption. The strength of bisphenol A (BPA) inhibiting SMX adsorption was dependent on the concentration ratio of BPA to neutral SMX, instead of to the overall SMX concentration. This result emphasized the importance of identifying the dominant mechanisms or species for the adsorption of antibiotics.

Characteristics of phenol and chlorinated phenols sorption onto surfactant-modified bentonite

Surfactant-modified bentonite was synthesized by replacing adsorbed Na+ with long-chain alkyl quaternary ammonium cation, hexadecyltrimethylammonium bromide (HDTMAB). The sorption isotherms of phenol, p-chlorophenol, and 2,4-dichlorophenol were modeled according to the Langmuir and Freundlich equations. The Langmuir isotherm was found to describe the equilibrium adsorption data well. The mechanisms and characteristics of sorption of these ionizable organic contaminants onto surfactant-modified bentonite from water were investigated systematically and described quantitatively. The sorption properties are affected by the treatment conditions, such as amount of organobentonite, and the properties of organic compounds. Results indicated that adsorption of phenols from water was in proportion to their hydrophobicities, which increased with chlorine addition (phenol<p-chlorophenol<2,4-dichlorophenol). Sorption isotherms of these phenols were typically nonlinear. Both adsorption and partition contribute to the sorption of investigated phenols to organobentonite. The separate contributions of adsorption and partition to the total sorption of these compounds to organobentonite is analyzed mathematically. Results indicate that the partition effect is weak and linear with contaminant concentration, whereas the adsorption effect is more powerful and nonlinear with contaminant concentration. The sorption of phenols onto organobentonite was dominated by adsorption at low concentrations and partition started to dominate at high concentrations, making the organobentonites powerful sorbents for organic contaminants over a wide range of concentrations.