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.     

Effect of natural organic matter on powdered activated carbon adsorption of trace contaminants: characteristics and mechanism of competitive adsorption

Batch adsorption experiments using powdered activated carbon (PAC) to remove trace synthetic organic chemicals (SOCs) from water containing natural organic matter (NOM) were conducted. The percentage of SOC removed at any contact time and at any PAC dose was observed to be independent of the initial SOC concentration. Equations derived from the ideal adsorbed solution theory and the pore surface diffusion model validated this observation. For the strongly adsorbing SOCs (simazine and simetryn), the percentage of SOC removed was independent only at low initial SOC concentrations. The NOM fraction competing with the weakly adsorbing SOC (asulam) constituted a larger percentage of the total NOM than that competing with the strongly adsorbing SOCs. Although the adsorptive capacities of the SOCs were greatly reduced in water containing NOM compared with those in pure water, the change in the pore diffusion coefficient was insignificant. Therefore, NOM competed with the SOCs for adsorption sites, reducing the adsorptive capacity, but the amount of NOM loading was not so severe that it blocked or filled the pores, hindering the internal diffusion of the SOCs.     

Impact of solution chemistry on viral removal by a single-walled carbon nanotube filter

This study investigates the effectiveness of a single-walled carbon nanotube (SWNT) filter for removal of viruses from water. MS2 bacteriophage viral removal was examined over a range of environmentally relevant solution chemistries, spanning various ionic strengths, monovalent and divalent salts, pH, and natural organic matter (NOM) concentrations. Viral removal by the SWNT filter was governed by physicochemical (depth) filtration. The removal of viruses increased at higher ionic strengths (NaCl) due to suppression of repulsive electrostatic interactions between viruses and SWNTs. Addition of divalent salts, however, had varying impacts. While CaCl₂ increased virus removal, likely due to complexation of calcium ions to viral surfaces, addition of MgCl₂ reduced viral removal by the SWNT filter. Solution pH also had significant impact on viral removal as the interactions between viral particles and SWNTs changed from attractive below the virus isoelectric point (about pH 3.9) to repulsive at higher pH. Suwannee River NOM was shown to be detrimental to filter viral removal. Reduction of viral removal by NOM was attributed to adsorption of NOM macromolecules to viruses and SWNTs, thereby resulting in steric repulsive forces. Modifications of the filter to incorporate thicker SWNT layers mitigate the negative impacts of NOM on filter performance. This study has shown that while it is possible to attain high levels of viral removal over a broad range of solution chemistries, the extent of viral removal will be highly dependent on the specific solution chemistry of the treated water.     

天然有机物及无机盐对膜蒸馏过程的影响

系统地研究了天然有机物(NOM)、离子强度、溶液pH以及Ca~(2+)对膜蒸馏过程的影响.结果表明:溶液pH、水中有机物的组成及含量、水中无机盐的种类及含量均对膜蒸馏过程产生重要影响.腐殖酸溶液膜蒸馏过程的膜通量是逐渐衰减的;当溶液中无Ca~(2+)存在时,离子强度、腐殖酸的浓度及溶液的pH对膜蒸馏过程影响不大;当溶液中有Ca~(2+)存在时,Ca~(2+)与腐殖酸易形成络合物从而导致腐殖酸凝胶沉积,这是膜蒸馏过程中膜通量衰减的主要原因,且高pH的腐殖酸溶液比低pH的更易发生凝胶现象,导致膜污染.     

Determination of hydrophobicity of NOM by RP-HPLC,and the effect of pH and ionic strength

A reverse phase high performance liquid chromatography method for the determination of the hydrophobicity of natural organic matter (NOM) was developed by modification of an existing method based on elution with acetic acid/acetonitrile. The modified method separates the NOM into two well-defined groups according to their retention on a C18 column. The recovery was 101±11% for samples containing between 4 and 40 mg/L dissolved organic carbon (DOC). The method was tested for effects of ionic strength up to 0.8 eq. /L. Testing the method on eleven well-described NOM samples at pH 4.7 and 7.0 revealed the strong pH dependency of the hydrophobicity. The effect of pH on the hydrophobicity could be predicted from the proton binding properties of the NOM. Application of the method to NOM samples fractionated by HPSEC demonstrated considerable variations in hydrophobicity among the fractions, particularly at low pH.     

Adsorption of trihalomethanes from water with carbon nanotubes

Commercial carbon nanotubes (CNTs) were purified by acid solution and were employed as adsorbents to study adsorption of trihalomethanes (THMs) from water. The properties of CNTs such as purity, structure and nature of the surface were greatly improved after acid treatment which made CNTs become more hydrophilic and suitable for adsorption of low molecular weight and relatively polar THM molecules. The adsorption of THMs onto CNTs fluctuates very little in the pH range 3-7, but decreases with pH value as pH exceeds 7. A comparative study between CNTs and powdered activated carbon (PAC) for adsorption of THMs from water was also conducted. The short time needed to reach equilibrium as well as the high adsorption capacity of CHCl3, which accounts for a significant portion of THMs in the chlorinated drinking water, suggests that CNTs possess highly potential applications for THMs removal from water.     

Pore distribution effect of activated carbon in adsorbing organic micropollutants from natural water

Adsorption isotherms of organic micropollutants in coexistence with natural organic matter (NOM) were analyzed to evaluate the impacts of pore size distribution of activated carbon (AC) on the competition effects of the NOM. Single solute adsorption experiments and simultaneous adsorption experiments with NOM contained in a coagulation-pretreated surface water were performed for four agricultural chemicals and three coal-based activated carbons (ACs) having different pore distributions. The results showed that, for all the carbons used, the adsorption capacity of the chemicals was reduced distinctly in the presence of NOM. Such a reduction was more apparent for AC with a larger portion of small pores suitable for the adsorption of small organic molecules and for the agricultural chemicals with a more hydrophilic nature. Ideal adsorbed solution theory (IAST) incorporated with the Freundlich isotherm expression (IAST-Freundlich model) could not interpret the impact of NOM on the adsorption capacity of the chemicals unless a pore blockage effect caused by the adsorption of NOM was also considered. By taking into account this effect, the adsorption isotherm of the chemicals in the presence of NOM was well described, and the capacity reduction caused by the NOM was quantitatively assessed from the viewpoints of the site competition and the pore blockage. Analytical results clearly indicated that pore blockage was an important competition mechanism that contributed to 10-99% of the total capacity reductions of the chemicals, the level depended greatly on the ACs, the chemicals and the equilibrium concentrations, and could possibly be alleviated by broadening the pore size distributions of the ACs to provide a large volume percentage for pores with sizes above 30 A.     

Calcium accumulation on activated carbon deteriorates synthetic organic chemicals adsorption

The accumulation of calcium on biological activated carbon (BAC) and their effects on adsorption of synthetic organic chemicals (SOCs) were studied using BAC, which have been operated for 5 (BAC5.0) and 3.5 (BAC3.5) years in a pilot-scale water purification plant, and granular activated carbon (GAC) preloaded with fulvic acid and/or calcium. The major inorganic material accumulated on BAC was calcium. The amounts of calcium on BAC5.0 and BAC3.5 were 36.6 and 29.7 mg g(-1), respectively. Seventy-one percent of calcium existed as calcium carbonate in both BACs. BAC5.0 had higher amount of accumulated calcium than BAC3.5 even though both BACs have already exhausted for NOM in the influent in 1-year operation, suggesting that calcium carbonate gradually accumulated on BAC even after the 3.5 years of operation. The isotherms of GAC preloaded with fulvic acid and/or calcium clearly indicated that the calcium accumulation on GAC reduced adsorption capacity for simazine. The conclusion also confirmed by significant recovery of adsorption capacity of both BACs by acid-washing to remove accumulated calcium from BACs. The difference of adsorption capacity between BAC3.5 and BAC5.0 was caused not only by the difference of adsorbed NOM but also the difference in the amount of accumulated calcium.     

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.     

Study of pH effects on the evolution of properties of brown-water natural organic matter as revealed by size-exclusion chromatography during photocatalytic degradation

This study shows the effect of pH on the photocatalytic degradation of natural organic matter (NOM). The experiments were carried out in batch reactor (a solar UV-light simulator) with Degussa P-25 titanium dioxide (TiO₂). The NOM degradation was followed by size-exclusion chromatography for dissolved organic carbon (DOC), ultraviolet absorption and fluorescence-detection (SEC-DOC, SEC-UV₂₅₄ and SEC-Fl_254/450). Changes in pH values affected the adsorption of NOM onto TiO₂, but did not affect the photodegradation sequence of NOM. For high or low pH values, the degradation of the NOM preferentially removed the larger molecular size fraction in comparison to the middle and small molecular size fractions, resulting in the relative increase of these smaller fractions. This sequence of NOM degradation leads to the evolution of the formation potential for disinfection by-products (DBPs). Specifically, the trihalomethanes and halogenated organic compounds formation potential (THMF and AOXFP) decreased steadily.     

Role of morphology in the aggregation kinetics of ZnO nanoparticles

The aggregation kinetics of two types of ZnO nanoparticles were investigated under various conditions. Distinct differences in aggregation kinetics were observed between the two ZnO particles. The aggregation of the nearly spherical ZnO (denoted as Me ZnO) exhibited strong dependence on the ionic strength (IS) of the solution; while minimal influence of IS was seen on the irregularly shaped ZnO (mixture of slab-like and rod-shaped particles, denoted as Mk ZnO) in the IS ranged tested. It is postulated that Mk ZnO possesses a critical coagulation concentration (CCC) below the lowest electrolyte concentration tested (1 mM NaCl) due to the interactions between various surfaces. The CCC of ZnO was found to be a function of pH; the CCC increased significantly as the pH was further away from the point of zero charge. Natural organic matter (NOM) was found to substantially hinder the aggregation of both types of ZnO particles (above 10 mg/L for Me ZnO and above 1 mg/L for Mk ZnO). A Langmuir adsorption model was used to describe the NOM to ZnO nanoparticle adsorption isotherms. To our knowledge, this is the first study to report the effect of particle morphology on nanoparticle aggregation, which outlines the importance of accounting morphology into environmental transport assessment of nanoparticles.     

碳纳米管水泥基材料耐久性理论探讨

碳纳米管是近几年国内外纳米材料界研究的热点。在建设工程中,水泥基材料的耐久性问题日益突出。本文在介绍碳纳米管的结构性能和水泥基材料的耐久性问题的基础上,分析了多壁碳纳米管改善水泥基材料耐久性的可能性,得出拟解决的关键问题是多壁碳纳米管在基体中的均匀分散和其与基体的相容性以及水泥基复合材料的耐久特性和机理,为工程应用奠定基础。     

NOM removal by adsorption onto granular ferric hydroxide: Equilibrium, kinetics, filter and regeneration studies

Adsorption onto granular ferric hydroxide (GFH) with subsequent in-situ regeneration is studied as a new process for natural organic matter (NOM) removal from groundwater. Adsorbent equilibrium loadings of 10-30 mgDOC g(-1)GFH(-1) are obtained, whereas the non-adsorbable DOC fraction amounts to 1.5 mgL(-1) for all investigated groundwaters. The larger and UV-active NOM fractions (mainly fulvic acids) are well adsorbed while the smaller molecular fractions are poorly or not adsorbed. However, kinetic studies show that the smaller and medium-sized fulvic acids are removed first. The equilibrium is strongly dependent on pH but only weakly on ionic strength, pointing to ligand exchange as the dominant adsorption mechanism. With regard to NOM structure, prerequisites for adsorption onto GFH are both a minimum number of functional groups and a molecular size small enough to enter the GFH pores. NOM breakthrough curves are successfully simulated using the LDF model (homogeneous surface diffusion model (HSDM) with linear driving force approach for surface diffusion) and experimentally determined mass transfer coefficients. Regeneration of loaded GFH is possible either by use of NaOH or oxidatively by H(2)O(2). The optimal quantities and concentrations are determined.     

Pore blockage effect of NOM on atrazine adsorption kinetics of PAC: the roles of PAC pore size distribution and NOM molecular weight

Natural organic matter (NOM) in natural water has been found to have negative effects on the adsorption of various trace organic compounds by activated carbon through two major mechanisms: direct competition for sites and pore blockage. In this study, the pore blockage effect of NOM on atrazine adsorption kinetics was investigated. Two types of powdered activated carbon (PAC) and three natural waters were tested to determine the roles of PAC pore size distribution and NOM molecular weight distribution in the pore blockage mechanism. When PAC was preloaded with natural water, the pore blockage effect of the NOM was found to cause a reduction of up to more than two orders of magnitude in the surface diffusion rate of atrazine compared to simultaneous adsorption of atrazine and NOM with fresh PAC. The surface diffusion coefficient of atrazine for preloaded PAC decreased with a decrease in PAC dose or an increase in NOM surface concentration. Because of the pore blockage effect of NOM, a 30% drop in atrazine removal was observed in a continuous flow PAC/microfiltration (MF) system after 7 days of contact compared to the removal predicted from the batch isotherm test. Large micropores and mesopores were found to play an important role in alleviating the effect of pore blockage. A PAC with a relatively large fraction of large micropore and mesopores was shown to suffer much less from the pore blockage effect compared with a PAC that had a much smaller fraction of large pores. Natural waters with different NOM molecular weight distribution caused different extent of pore blockage. The NOM molecules with molecular weight between 200 and 700 Dalton appeared to be responsible for the pore blockage effect.     

Effect of NOM on arsenic adsorption by TiO(2) in simulated As(III)-contaminated raw waters

The effect of natural organic matter (NOM) on arsenic adsorption by a commercial available TiO(2) (Degussa P25) in various simulated As(III)-contaminated raw waters was examined. Five types of NOM that represent different environmental origins were tested. Batch adsorption experiments were conducted under anaerobic conditions and in the absence of light. Either with or without the presence of NOM, the arsenic adsorption reached steady-state within 1h. The presence of 8 mg/L NOM as C in the simulated raw water, however, significantly reduced the amount of arsenic adsorbed at the steady-state. Without NOM, the arsenic adsorption increased with increasing solution pH within the pH range of 4.0-9.4. With four of the NOMs tested, the arsenic adsorption firstly increased with increasing pH and then decreased after the adsorption reached the maximum at pH 7.4-8.7. An appreciable amount of arsenate (As(V)) was detected in the filtrate after the TiO(2) adsorption in the simulated raw waters that contained NOM. The absolute amount of As(V) in the filtrate after TiO(2) adsorption was pH dependent: more As(V) was presented at pH>7 than that at pH<7. The arsenic adsorption in the simulated raw waters with and without NOM were modelled by both Langmuir and Frendlich adsorption equations, with Frendlich adsorption equation giving a better fit for the water without NOM and Langmuir adsorption equation giving a better fit for the waters with NOM. The modelling implies that NOM can occupy some available binding sites for arsenic adsorption on TiO(2) surface. This study suggests that in an As(III)-contaminated raw water, NOM can hinder the uptake of arsenic by TiO(2), but can facilitate the As(III) oxidation to As(V) at TiO(2) surface under alkaline conditions and in the absence of O(2) and light. TiO(2) thus can be used in situ to convert As(III) to the less toxic As(V) in NOM-rich groundwaters.     

Adsorption of natural organic matter oxidized with ClO2 on granular activated carbon

The paper describes the influence of the oxidation of natural organic matter (NOM) molecules with chlorine dioxide (ClO2) on granulated activated carbon (GAC) adsorption. In order to determinate the influence of ClO2 dosage on the NOM adsorption on GAC two parallel pilot scale experiments were performed. The raw water was treated respectively with 0.2 and 0.4 mg ClO2 L(-1) followed by the adsorption on GAC filters. Experiments were total organic carbon (TOC) measurements and size exclusion chromatography (SEC) controlled. The molecular weight distribution of NOM in the filtration bed outlet demonstrates that the low molecular weight molecules are less retained than the higher molecular weight components of NOM. It is shown that low molecular weight NOM causes less ClO2 demand. The oxidation of NOM molecules and very high capacity of GAC filter bed for NOM components can be used to control high ClO2 demand.     

Characteristics of competitive adsorption between 2-methylisoborneol and natural organic matter on superfine and conventionally sized powdered activated carbons

When treating water with activated carbon, natural organic matter (NOM) is not only a target for adsorptive removal but also an inhibitory substance that reduces the removal efficiency of trace compounds, such as 2-methylisoborneol (MIB), through adsorption competition. Recently, superfine (submicron-sized) activated carbon (SPAC) was developed by wet-milling commercially available powdered activated carbon (PAC) to a smaller particle size. It was reported that SPAC has a larger NOM adsorption capacity than PAC because NOM mainly adsorbs close to the external adsorbent particle surface (shell adsorption mechanism). Thus, SPAC with its larger specific external surface area can adsorb more NOM than PAC. The effect of higher NOM uptake on the adsorptive removal of MIB has, however, not been investigated. Results of this study show that adsorption competition between NOM and MIB did not increase when NOM uptake increased due to carbon size reduction; i.e., the increased NOM uptake by SPAC did not result in a decrease in MIB adsorption capacity beyond that obtained as a result of NOM adsorption by PAC. A simple estimation method for determining the adsorbed amount of competing NOM (NOM that reduces MIB adsorption) is presented based on the simplified equivalent background compound (EBC) method. Furthermore, the mechanism of adsorption competition is discussed based on results obtained with the simplified EBC method and the shell adsorption mechanism. Competing NOM, which likely comprises a small portion of NOM, adsorbs in internal pores of activated carbon particles as MIB does, thereby reducing the MIB adsorption capacity to a similar extent regardless of adsorbent particle size. SPAC application can be advantageous because enhanced NOM removal does not translate into less effective removal of MIB. Molecular size distribution data of NOM suggest that the competing NOM has a molecular weight similar to that of the target compound.     

The effect of ozonation on natural organic matter removal by alum coagulation

Natural organic matter (NOM) was extracted from a moderately colored, eutrophic surface water source (Forge Pond, Granby, MA), and fractionated into quasi-homogeneous fractions. Fulvic acid (FA) and hydrophilic neutrals (HN) were the two most abundant NOM fractions that were isolated. Adsorption affinity of the isolated NOM fractions on preformed aluminum hydroxide flocs increased with increase in specific organic charge of the fractions, except for the two most highly charged fractions, FA and hydrophilic acids (HAA), which showed less adsorption affinity than expected based on their specific organic charge. Prior ozonation of FA and HN fractions resulted in a decline and an increase, respectively, in their adsorption affinity on aluminum hydroxide surface. Prior ozonation of Forge Pond raw water resulted in a progressive decline in dissolved organic carbon (DOC) removal by alum coagulation with increase in ozone dose. It appeared that ozone applied to raw water reacted preferentially with the humic fraction of NOM, resulting in the detrimental effects of ozonation on subsequent NOM removal by alum coagulation being magnified. Forge Pond raw water was pre-coagulated to remove humic substances. Ozonation of the pre-coagulated water demonstrated the beneficial effects of ozonation on the removal of non-humic NOM through alum coagulation. A strategy for staged coagulation with intermediate ozonation was proposed for waters containing both humic and non-humic NOM for maximum DOC and specific UV absorbance at 254nm (SUVA) removal.     

Application of carbon nanotube technology for removal of contaminants in drinking water: A review

Carbon nanotube (CNT) adsorption technology has the potential to support point of use (POU) based treatment approach for removal of bacterial pathogens, natural organic matter (NOM), and cyanobacterial toxins from water systems. Unlike many microporous adsorbents, CNTs possess fibrous shape with high aspect ratio, large accessible external surface area, and well developed mesopores, all contribute to the superior removal capacities of these macromolecular biomolecules and microorganisms. This article provides a comprehensive review on application of CNTs as adsorbent media to concentrate and remove pathogens, NOM, and cyanobacterial (microcystin derivatives) toxins from water systems. The paper also surveys on consideration of CNT based adsorption filters for removal of these contaminants from cost, operational and safety standpoint. Based on the studied literature it appears that POU based CNT technology looks promising, that can possibly avoid difficulties of treating biological contaminants in conventional water treatment plants, and thereby remove the burden of maintaining the biostability of treated water in the distribution systems.     

The adsorption of cyanobacterial hepatotoxins from water onto soil during batch experiments

Public health concerns associated with cyanobacteria, both chronic and acute, arise from their ability to produce toxins. Rural communities within Australia and those in developing countries require an inexpensive and low-cost method for removing toxins from drinking water. A candidate technology is bank filtration. Adsorption of cyanobacterial hepatotoxins was measured in batch studies to determine the applicability of bank filtration as an efficient removal strategy. Five soils with different physicochemical properties were collected from regions around South Australia. The soils were mixed with either nodularin or microcystin-LR in distilled water and buffered solutions (pH 4.8, 6.9 and 9.1). Additionally, nodularin was mixed in unbuffered solutions (pH 4.8 and 8.9). The three soils with the high clay and/or organic carbon contents (Paringa A, McLaren Flat and Lakes Plains) had the higher nodularin adsorption coefficients, ranging from 0.2 to 16.59 L kg-1. Soil suspensions in acetate buffer (pH 4.8) generally produced significantly higher nodularin adsorption coefficients, when compared to the other buffer systems. The background interference from the ionic strength of the buffers, however, made interpretation of the effect of pH on toxin sorption difficult. Increases in solution ionic strength, from freshwater to seawater, resulted in corresponding increases in the nodularin adsorption coefficients for all sites, except the sandy Hallett Cove site. The implications for bank filtration are that higher water pH values and lower salinities will enhance the in situ mobility of the toxins, resulting in an increased distance of filtration through the river bank before toxin free water could be abstracted for human consumption.