The effects of dissolved natural organic matter on the adsorption of synthetic organic chemicals by activated carbons and carbon nanotubes

Understanding the influence of natural organic matter (NOM) on synthetic organic contaminant (SOC) adsorption by carbon nanotubes (CNTs) is important for assessing the environmental implications of accidental CNT release and spill to natural waters, and their potential use as adsorbents in engineered systems. In this study, adsorption of two SOCs by three single-walled carbon nanotubes (SWNTs), one multi-walled carbon nanotube (MWNT), a microporous activated carbon fiber (ACF) [i.e., ACF10] and a bimodal porous granular activated carbon (GAC) [i.e., HD4000] was compared in the presence and absence of NOM. The NOM effect was found to depend strongly on the pore size distribution of carbons. Minimal NOM effect occurred on the macroporous MWNT, whereas severe NOM effects were observed on the microporous HD4000 and ACF10. Although the single-solute adsorption capacities of the SWNTs were much lower than those of HD4000, in the presence of NOM the SWNTs exhibited adsorption capacities similar to those of HD4000. Therefore, if released into natural waters, SWNTs can behave like an activated carbon, and will be able to adsorb, carry, and transfer SOCs to other systems. However, from an engineering application perspective, CNTs did not exhibit a major advantage, in terms of adsorption capacities, over the GAC and ACF. The NOM effect was also found to depend on molecular properties of SOCs. NOM competition was more severe on the adsorption of 2-phenylphenol, a nonplanar and hydrophilic SOC, than phenanthrene, a planar and hydrophobic SOC, tested in this study. In terms of surface chemistry, both adsorption affinity to SOCs and NOM effect on SOC adsorption were enhanced with increasing hydrophobicity of the SWNTs.     

Characterization of natural organic matter adsorption in granular activated carbon adsorbers

The removal of natural organic matter (NOM) from lake water was studied in two pilot-scale adsorbers containing granular activated carbon (GAC) with different physical properties. To study the adsorption behavior of individual NOM fractions as a function of time and adsorber depth, NOM was fractionated by size exclusion chromatography (SEC) into biopolymers, humics, building blocks, and low molecular weight (LMW) organics, and NOM fractions were quantified by both ultraviolet and organic carbon detectors. High molecular weight biopolymers were not retained in the two adsorbers. In contrast, humic substances, building blocks and LMW organics were initially well and irreversibly removed, and their effluent concentrations increased gradually in the outlet of the adsorbers until a pseudo-steady state concentration was reached. Poor removal of biopolymers was likely a result of their comparatively large size that prevented access to the internal pore structure of the GACs. In both GAC adsorbers, adsorbability of the remaining NOM fractions, compared on the basis of partition coefficients, increased with decreasing molecular size, suggesting that increasingly larger portions of the internal GAC surface area could be accessed as the size of NOM decreased. Overall DOC uptake at pseudo-steady state differed between the two tested GACs (18.9 and 28.6 g-C/kg GAC), and the percent difference in DOC uptake closely matched the percent difference in the volume of pores with widths in the 1-50 nm range that was measured for the two fresh GACs. Despite the differences in NOM uptake capacity, individual NOM fractions were removed in similar proportions by the two GACs.     

An overall isotherm for activated carbon adsorption of dissolved natural organic matter in water

Design and analysis of activated carbon processes in water treatment often requires the adsorption isotherm for dissolved natural organic matter (NOM). Of the isotherm models available, the Summers and Roberts (SR) equation, capable of describing the adsorbent dose effect with the fewest parameters, has been successfully used to normalize NOM isotherm data. In this study, we show that the adsorbent dose in the SR equation can be eliminated as an intermediate variable and the initial concentration effect on NOM adsorption is then described explicitly. Comparing with the original SR equation, the derived isotherm equation is in a form more amenable to analysis. To ensure that the prediction is physically attainable, we introduced the limiting adsorption capacity by taking the adsorbent pore volume and size exclusion into consideration. Subsequently, we develop a simple relationship that can be used to determine the minimum adsorbent usage required for any desirable level of treatment. By comparing with extensive isotherm data previously published by Li et al. [2003a. Polydisperse adsorbability composition of several natural and synthetic organic matrices. J. Colloid Interface Sci. 265(2), 265-275], we demonstrated that the isotherm equation derived herein yields predictions that agree with the much more complicated fictive component-ideal adsorbed solution theory (IAST)-based model for NOM from different sources and over a range of initial concentrations.     

Effects of activated carbon characteristics on the simultaneous adsorption of aqueous organic micropollutants and natural organic matter

The overall objective of this research was to determine the effects of physical and chemical activated carbon characteristics on the simultaneous adsorption of trace organic contaminants and natural organic matter (NOM). A matrix of 12 activated carbon fibers (ACFs) with three activation levels and four surface chemistry levels (acid-washed, oxidized, hydrogen-treated, and ammonia-treated) was studied to systematically evaluate pore structure and surface chemistry phenomena. Also, three commercially available granular activated carbons (GACs) were tested. The relatively hydrophilic fuel additive methyl tertiary-butyl ether (MTBE) and the relatively hydrophobic solvent trichloroethene (TCE) served as micropollutant probes. A comparison of adsorption isotherm data collected in the presence and absence of NOM showed that percent reductions of single-solute TCE and MTBE adsorption capacities that resulted from the presence of co-adsorbing NOM were not strongly affected by the chemical characteristics of activated carbons. However, hydrophobic carbons were more effective adsorbents for both TCE and MTBE than hydrophilic carbons because enhanced water adsorption on the latter interfered with the adsorption of micropollutants from solutions containing NOM. With respect to pore structure, activated carbons should exhibit a large volume of micropores with widths that are about 1.5 times the kinetic diameter of the target adsorbate. Furthermore, an effective adsorbent should possess a micropore size distribution that extends to widths that are approximately twice the kinetic diameter of the target adsorbate to prevent pore blockage/constriction as a result of NOM adsorption.     

Adsorption of synthetic organic chemicals by carbon nanotubes: Effects of background solution chemistry

With the significant increase in the production and use of carbon nanotubes (CNTs), they will be inevitably released into aquatic environments. Therefore, the fate and transport of CNTs in aqueous solutions have attracted extensive attention. In the present work, the effects of natural organic matter (NOM), solution pH and ionic strength on adsorption of three synthetic organic chemicals (SOCs) by both pristine and surface functionalized single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) were investigated. The three SOCs (phenanthrene, biphenyl, and 2-phenylphenol) with different planarity, polarity, and hydrogen/electron-donor/acceptor ability, representing typical scenarios for the SOC-CNT interactions, were employed as probe molecules. Among the three background solution characteristics examined, NOM showed the most significant effect on SOC adsorption, while solution pH and ionic strength exhibited minimal or negligible impacts. The presence of NOM greatly suppressed the SOC adsorption by CNTs, and the impact on the SWNTs was higher than that on the MWNTs. The planarity and hydrophobicity of SOCs were two important factors determining the effects of NOM, solution pH and ionic strength on their adsorption by CNTs.     

Application of powdered activated carbon for the adsorption of cylindrospermopsin and microcystin toxins from drinking water supplies

Cylindrospermopsin (CYN) and microcystin are two potent toxins that can be produced by cyanobacteria in drinking water supplies. This study investigated the application of powdered activated carbon (PAC) for the removal of these toxins under conditions that could be experienced in a water treatment plant. Two different PACs were evaluated for their ability to remove CYN and four microcystin variants from various drinking water supplies. The removal of natural organic material by the PACs was also determined by measuring the levels of dissolved organic carbon and UV absorbance (at 254 nm). The PACs effectively removed CYN and the microcystins from each of the waters studied, with one of the PACs shown to be more effective, possibly due to its smaller particle diameter. No difference in removal of the toxins was observed using PAC contact times of 30, 45 and 60 min. Furthermore, the effect of water quality on the removal of the toxins was minimal. The microcystin variants were adsorbed in the order: MCRR > MCYR > MCLR > MCLA. CYN was found to be adsorbed similarly to MCRR.     

Modeling the behaviors of adsorption and biodegradation in biological activated carbon filters

This investigation developed a non-steady-state numerical model to differentiate the adsorption and biodegradation quantities of a biological activated carbon (BAC) column. The mechanisms considered in this model are adsorption, biodegradation, convection and diffusion. Simulations were performed to evaluate the effects of the major parameters, the packing media size and the superficial velocity, on the adsorption and biodegradation performances for the removal of dissolved organic carbon based on dimensionless analysis. The model predictions are in agreement with the experimental data by adjusting the liquid-film mass transfer coefficient (k(bf)), which has high correlation with the Stanton number. The Freundlich isotherm constant (N(F)), together with the maximum specific substrate utilization rate (k(f)) and the diffusion coefficient (D(f)), is the most sensitive variable affecting the performance of the BAC. Decreasing the particle size results in more substrate diffusing across the biofilm, and increases the ratio of adsorption rather than biodegradation.     

The relation between activated carbon adsorption and water quality indexes

In order to estimate the adsorbability by activated carbon of organic compounds dissolved in aqueous solutions, the contribution of individual atoms to the adsorption process is calculated. The contribution of carbon, bromine and chlorine atoms is positive, that of oxygen atoms is negative, that of hydrogen atoms is very small and that of nitrogen atoms is influenced by the kind of functional group in which they are contained. The dominant factors governing adsorption are the numbers of carbon and oxygen atoms in a molecule. The activated carbon's effectiveness for adsorption of organic compounds dissolved in solutions can be predicted from such water quality indexes as total organic carbon, total organic nitrogen, total organic halogen an total oxygen demand.     

Experimental study on adsorption capacity of an activated carbon-based adsorption water chiller

The objective of this study was to evaluate the adsorption capacity of a working pair for an adsorption water chiller. Activated carbon fibre–methanol, activated carbon fibre–ethanol and activated carbon pallet–ethanol were used as an adsorbent–adsorbate pair in this study. The experiment was conducted using a stainless steel adsorber, 110?mm diameter by 150?mm height, filled with adsorbent and transparent plastic evaporator, 100?ml capacity, filled with adsorbate. The experiment was performed by isobaric adsorption in the temperature range of 10–100°C at the evaporator temperature of 20°C (water chiller). An experimental investigation showed that the activated carbon fibre–methanol pair has the highest adsorption capacity (0.44?kg/kg) compared to the activated carbon fibre–ethanol and activated carbon pallet–ethanol pairs. The finding revealed that uniform structure and large surface area of adsorbent as well as low boiling point and large latent heat of adsorbate had highly significant effects on adsorption capacity. The effect of time and adsorber temperature on adsorption capacity is also discussed in this study.     

Removal of VUV pre-treated natural organic matter by biologically activated carbon columns

A potential alternative water treatment process using VUV (185 nm+254 nm) irradiation followed by a biological treatment is described. The system uses sufficient VUV radiation (16J cm(-2)) to significantly enhance the production of biologically degradable moieties prior to treatment with biologically activated carbon (BAC). Two similar activated carbons were used, one virgin and one taken from a water treatment plant with an established biofilm. The VUV-BAC process decreased the overall dissolved organic carbon (DOC) concentration of a natural water sample by 54% and 44% for the virgin carbon and previously used BAC, respectively. Furthermore, VUV-BAC treatment decreased the trihalomethane (THM) formation potential (THMFP) by 60-70% and the haloacetic acid (HAA) formation potential (HAAFP) by 74%. The BAC systems effectively removed the hydrogen peroxide residual produced by VUV irradiation. Although nitrite formation can result from VUV treatment of natural organic matter (NOM), none was detected before or after BAC treatment.     

Assessment of organic chlorinated compound removal from aqueous matrices by adsorption on activated carbon

Adsorption on activated carbon is currently the most frequently used technology to remove organic chlorinated pollutants from wastewaters. The present study examines the ability of five commercially available types of activated carbon to remove organic chlorinated compounds from the effluent of a chemical plant. The various types were tested on the basis of Freundlich adsorption isotherms for 14 pure organic chlorinated compounds, of molecular weight ranging from that of dichloromethane (MW=84.93 gmol(-1)) to hexachlorobenzene (MW=284.78 gmol(-1)). The best was selected and used in a laboratory fixed-bed column to assess its removal efficiency with respect to the tested organic chlorinated compounds. Removal efficiency was always higher than 90%. These results provide information necessary to optimize scale-up from the pilot plant to the real one.     

Formation of assimilable organic carbon (AOC) and specific natural organic matter (NOM) fractions during ozonation of phytoplankton

Ozonation of natural surface water increases the concentration of oxygen-containing low molecular weight compounds. Many of these compounds support microbiological growth and as such are termed assimilable organic carbon (AOC). Phytoplankton can contribute substantially to the organic carbon load when surface water is used as source for drinking water treatment. We have investigated dissolved organic carbon (DOC) formation from the ozonation of a pure culture of Scenedesmus vacuolatus under defined laboratory conditions, using a combination of DOC fractionation, analysis of selected organic acids, aldehydes and ketones, and an AOC bioassay. Ozonation of algae caused a substantial increase in the concentration of DOC and AOC, notably nearly instantaneously upon exposure to ozone. As a result of ozone exposure the algal cells shrunk, without disintegrating entirely, suggesting that DOC from the cell cytoplasm leaked through compromised cell membranes. We have further illustrated that the specific composition of newly formed AOC (as concentration of organic acids, aldehydes and ketones) in ozonated lake water differed in the presence and absence of additional algal biomass. It is therefore conceivable that strategies for the removal of phytoplankton before pre-ozonation should be considered during the design of drinking water treatment installations, particularly when surface water is used.     

Effect of NOM, turbidity and floc size on the PAC adsorption of MIB during alum coagulation

The effect of natural organic material (NOM) and turbidity on the powdered activated carbon (PAC) adsorption of the odour compound 2-methylisoborneol (MIB) was evaluated during alum coagulation. The character of the flocs, in terms of their size and fractal dimensions (Df), was used to interpret the observed adsorption behaviour of MIB during the coagulation process. As the alum dose was increased, the adsorption of MIB decreased. This was determined to be due to the size of the flocs, with larger flocs incorporating PAC into their structure, reducing the efficiency of mixing, and the bulk diffusion kinetics for the MIB molecule. The presence of turbidity also reduced MIB adsorption due to the formation of larger flocs. The character of NOM was found to have a greater influence on the adsorption of MIB than the floc structure.     

Effects of natural organic matter model compounds on the transformation of carbon tetrachloride by chloride green rust

Interest has grown in the use of reactive minerals for natural and engineered transformation of ground water contaminants. This study investigated how the structural properties of 10 model compounds representing natural organic matter (NOM) influenced their adsorption to chloride green rust (GR-Cl), and how this adsorption affected rate constants for transformation of carbon tetrachloride (CT) by GR-Cl. The affinity of benzoic acid, phthalic acid, trimesic acid, pyromellitic acid, and mellitic acid for the GR-Cl surface generally increased in the order of increasing number of carboxylic acid functional groups, increasing acidity of these functional groups, and increasing charge density. For NOM model compounds that had phenolic functional groups (p-hydroxybenzoic acid, α-resorcylic acid, and caffeic acid), the affinity for the GR-Cl surface was greatest for caffeic acid, which had two adjacent phenolic functional groups. Some NOM model compounds had experimentally determined Langmuir maximum adsorption capacities (q_{max−Langmuir}) greater than those calculated based on external surface area measurements and the size of the NOM model compound, suggesting adsorption to internal as well as external sites at the GR-Cl surface for these compounds. Rate constants for CT transformation by GR-Cl generally decreased as the affinity of the NOM model compounds (estimated by Langmuir K values) increased, but there was no statistically significant correlation between Langmuir parameters (i.e., K and q_{max−Langmuir}) and rate constants, perhaps due to significant adsorption of some NOM model compounds to sites that were not accessible to CT, such as interlayer sites. Unlike the other NOM model compounds, caffeic acid, which adsorbed to a significant extent to the GR-Cl surface, increased the rate constant for CT transformation. The influence of NOM on rate constants for CT transformation by green rusts should be considered in ground water remediation planning.     

A model for predicting dissolved organic carbon distribution in a reservoir water using fluorescence spectroscopy

A number of water treatment works (WTW) in the north of England (UK) have experienced problems in reducing the dissolved organic carbon (DOC) present in the water to a sufficiently low level. The problems are experienced in autumn/winter when the colour increases and the coagulant dose at the WTW needs to be increased in order to achieve sufficient colour removal. However, the DOC content of the water varies little throughout the year. To investigate this further, the water was fractionated using resin adsorption techniques into its hydrophobic (fulvic and humic acid fractions) and hydrophilic (acid and non-acid fractions) components. The fractionation process yields useful information on the changing concentration of each fraction but is time consuming and labour intensive. Here, a method of rapidly determining fraction concentration was developed using fluorescence spectroscopy. The model created used synchronous spectra of fractionated material compared against bulk water spectra and predicted the fraction concentrations to within 10% for a specific water. The model was unable to predict fraction concentrations for waters from a different watershed.     

Reconsidering the quantitative analysis of organic carbon concentrations in size exclusion chromatography

The evaluation of the molecular size distribution of natural organic matter (NOM) in aquatic environments via size exclusion chromatography (SEC) is important for the understanding of environmental processes such as nutrient cycling and pollutant transport as well as of technical water treatment processes. The use of organic carbon (OC) detectors has become popular in recent studies due to improved availability and quantification possibilities, which supposedly are superior to those of ultraviolet (UV) detectors. A set of 12 NOM samples was used to demonstrate the limitations of online OC detection (OCD) when analyzing complex aquatic organic matter. A novel evaluation approach for SEC data is introduced by combining the information from UV absorbance (UVA) and OCD chromatograms as well as offline total OC (t-OC) and dissolved OC-specific UVA (SUVA) measurements. It could be shown that about 70% of certain OC components were not detected with the OCD system used in this study. For the investigated samples, these types of carbon accounted for up to 72% of the t-OC, i.e. for such NOM samples quantification by OCD is not possible or at least highly questionable. The addition of an oxidant improved the overall oxidation efficiency only slightly. Most likely NOM that predominantly consists of polysaccharides and features a nominal molecular weight of 150 kg/mol or more was responsible for low OCD yields. For future applications, a further improvement of the OCD system would be worthwhile so that quantitative analytical data on the molecular size distribution of NOM and its structural characteristics such as the SUVA distribution can be obtained.     

活性炭吸附去除重金属研究进展

介绍了国内外学者应用活性炭吸附去除包括:Cr(Ⅵ),Cu2 ,Zn2 ,Cd2 ,Pb2 ,Mn2 ,Hg2 以及混合重金属离子在内的各种废水的处理效果与影响因素,并对相应的吸附机理进行了讨论,以促进活性炭吸附去除重金属离子的进展。     

Modeling equilibrium adsorption of organic micropollutants onto activated carbon

Solute hydrophobicity, polarizability, aromaticity and the presence of H-bond donor/acceptor groups have been identified as important solute properties that affect the adsorption on activated carbon. However, the adsorption mechanisms related to these properties occur in parallel, and their respective dominance depends on the solute properties as well as carbon characteristics. In this paper, a model based on multivariate linear regression is described that was developed to predict equilibrium carbon loading on a specific activated carbon (F400) for solutes reflecting a wide range of solute properties. In order to improve prediction accuracy, groups (bins) of solutes with similar solute properties were defined and solute removals were predicted for each bin separately. With these individual linear models, coefficients of determination (R²) values ranging from 0.61 to 0.84 were obtained. With the mechanistic approach used in developing this predictive model, a strong relation with adsorption mechanisms is established, improving the interpretation and, ultimately, acceptance of the model.     

Simplification of the IAST for activated carbon adsorption of trace organic compounds from natural water

Recent studies have shown that the ideal adsorbed solution theory (IAST) coupled with the concept of equivalent background compound (EBC) can be simplified for describing trace organic compound adsorption from natural water, provided that the adsorbent surface loading is dominated by competing natural organic matter. The resulting simplified IAST has been used to reduce the complexity of kinetic models for various dynamic adsorption processes. In order to be correctly applied, however, the simplified IAST requires some additional clarification and a quantitative evaluation of the deviation caused by the simplifying assumption. In this study, we derive a simple equation that relates the relative deviation of the simplified IAST directly to the molar ratio of EBC and trace organic compound surface loadings and their Freundlich isotherm exponents. We then verify the simplified IAST using the original IAST and experimental isotherm data from the literature for trace organic compounds at various initial concentrations in natural water. By further assuming that the adsorbed amount of the EBC is substantially greater than what remains in solution, a new pseudo single-solute isotherm equation is derived and a simple relation is subsequently established between the carbon dose and the remaining trace compound concentration. The results show that the adsorption capacity and relative removal of a trace organic compound at any carbon dose can be estimated directly with the simple equations developed here and data from a single isotherm experiment for the target compound conducted in the natural water of interest.