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Final Report: Tailoring Activated Carbon Surfaces for Water, Wastewater and Hazardous Waste Treatment Operations
EPA Grant Number: R828157Title: Tailoring Activated Carbon Surfaces for Water, Wastewater and Hazardous Waste Treatment Operations
Investigators: Karanfil, Tanju , Kilduff, James E.
Institution: Clemson University , Rensselaer Polytechnic Institute
EPA Project Officer: Krishnan, Bala S.
Project Period: June 1, 2000 through May 31, 2002 (Extended to July 31, 2004)
Project Amount: $223,978
RFA: Exploratory Research - Environmental Engineering (1999)
Research Category: Engineering and Environmental Chemistry
Description:
Objective:Granular activated carbon (GAC) treatment has been proven to be an excellent option for the removal of a broad range of synthetic organic chemicals (SOC) from drinking water sources and industrial process and wastewaters. Despite the demonstrated effectiveness of GAC treatment, design of technically and economically optimal treatment systems remains difficult. Perhaps the greatest challenge to meeting treatment objectives with an effective design is to accurately incorporate the impacts of “system heterogeneity” on the adsorption process. System heterogeneity is manifested in several ways. The organic and inorganic characteristics of each individual water or wastewater may vary substantially depending on source. Moreover, the characteristics of GAC may vary greatly, depending on source material and mode of activation. Each of these variables prevents the ready transfer of design parameters between GAC systems, necessitating individual consideration of each adsorber design.
The overall objective of this research project was to conduct a systematic investigation to examine how activated carbon surface chemistry influences adsorption of priority pollutant SOC and natural organic matter (NOM) from complex solution matrices. The specific objectives were to: (1) investigate how sorbate molecular properties impact adsorption by activated carbons with different surface chemistry; (2) examine the role of carbon surface chemistry on competitive (i.e., simultaneous adsorption and preloading) adsorption interactions between the target SOC and NOM that is ubiquitous in natural waters; and (3) provide a rational basis for selecting or preparing activated carbons for removal (and in some cases regeneration) of SOC and NOM from aqueous solutions.
Summary/Accomplishments (Outputs/Outcomes):The results obtained in this research project indicated that characteristics of both adsorbate (i.e., the molecular size and geometry) and activated carbon (surface hydrophilicity, pore volume, and pore size distribution in micropores) control adsorption of SOC from dilute aqueous solutions typical of water and wastewaters. Trichloroethylene (TCE) adsorption increased as the carbon surface hydrophilicity decreased, indicating that water competition is an important factor for this compound, and likely for other SOC. When the correlations between the TCE uptake and the pore volumes in different regions of carbon micropores (e.g., 5-8 Å, 6-8 Å, 7-10 Å and < 10 Å) were examined, the best correlation for TCE was obtained in the pore region of 5-8 Å. This also was consistent with the gas phase adsorption results, which suggested that TCE molecules adsorbed in a flat orientation and had the ability to access the deep region of carbon micropores. Therefore, in addition to carbon surface hydrophobicity, the molecular geometry of the adsorbate is an important factor controlling the SOC adsorption. The important pore size region for a target compound adsorption will depend on its geometry.
Oxygen sorption experiments showed lower oxygen uptake by carbons heat-treated under hydrogen as compared to those heat-treated under vacuum, indicating that hydrogen treatment effectively stabilized the surfaces of various carbons tested in this study . Kinetic studies showed that oxygen chemisorption was affected by both carbon surface chemistry and porosity. The results indicated that oxygen chemisorption initially started on high energy sites in the mesopore region without any mass transfer limitation; thus, a constant oxygen uptake rate was observed. Once the majority of these sites were utilized, chemisorption proceeded on less energetic sites in mesopores as well as sites located in micropores. As a result, an exponential decrease in the oxygen uptake rate was observed.
Uptake of phenol by graphite and regeneration by methanol extraction were measured to evaluate irreversible adsorption of phenols to carbon surfaces. The emphasis of this work was to identify the role of oxidative coupling, which has been invoked to explain irreversible phenol sorption by activated carbons. Graphite was chosen as a model carbon surface to eliminate the potentially confounding effects of microporosity present in other types of carbonaceous sorbents. The isotherm data were well described by the Langmuir-Freundlich isotherm from pH 3 to pH 9. At pH 12, measured uptakes were higher than expected based on model predictions, suggesting the occurrence of an adsorption mechanism besides physisorption.
One oxidative coupling product, 2,2'-dihydroxybiphenyl, was obtained exclusively after adsorption at pH values above 7 and appeared both in aqueous solution and in the methanol regenerant solution. The fraction of total uptake that was not recoverable by methanol extraction decreased with increasing phenol concentration in solution, suggesting preferential sorption by high-energy sites. However, absolute irreversible adsorption increased with phenol concentration in solution. Both fractional irreversible adsorption and 2,2'-dihydroxybiphenyl oxidative coupling product recovery as a function of pH and contact time demonstrated that irreversible sorption of phenol by graphite could not be explained by an oxidative coupling mechanism alone.
Several pathways have been employed to systematically modify two GACs for examining adsorption of NOM from natural waters. Increases in the carbon supermicropore and mesopore volume (i.e., > 1 nm) increased the NOM uptake, if the surface chemistry was favorable. The isotherms normalized on a surface area basis showed the significance of carbon surface chemistry on the NOM uptake. At neutral pH, adsorption of negatively charged NOM molecules was favored by basic and positively charged surfaces, whereas the NOM uptake was minimized when the surface had acidic characteristics. High temperature ammonia-treatment of oxidized carbons considerably enhanced the NOM uptake, mainly because of the increase in accessible surface area and surface basicity. Iron-impregnated carbons indicated an enhanced affinity of dissolved organic matter (DOM) toward iron-laden carbon surfaces if the surface was not negatively charged. Overall, the results showed that high mesopore volume and positively charged surfaces (e.g., basic or iron-impregnated) are essential to maximize the DOM uptake by activated carbons during drinking water treatment.
In terms of competition between SOC and NOM, TCE adsorption by activated carbon previously loaded (“preloaded”) with NOM was reduced significantly in comparison to as-received carbon. Modification of granular activated carbon surface chemistry was evaluated as a means to preferentially reduce the uptake of NOM foulants and thus increase the uptake of TCE by preloaded carbons. Heat treatment (up to 1,000°C under nitrogen) and oxidation (concentrated nitric acid) produced large changes in the surface chemistry of wood- and coal-based activated carbons without extensively changing their pore structures. Heat treatment of both coal- and wood-based activated carbons at 1000°C under nitrogen optimized TCE uptake in single solute systems and under preloading conditions. Such treatment reduced surface acidity to a minimum, and maximized uptake of both TCE and NOM, suggesting that it is more important to maximize the ability of TCE to compete effectively with both NOM and water molecules than to minimize NOM uptake at the levels investigated in this study (5-30 mg dissolved organic carbon/g GAC). The heat treatment strategy was effective in minimizing the effects of preloaded DOM having a wide range of molecular properties. The impact of preloading also was negligible when the wood- and coal-based carbon surfaces were highly acidic, because NOM loading was low; however, the uptake of TCE by such surfaces was too low to be economically attractive. For a family of carbons having undergone similar treatments (e.g., nitric acid oxidation) to varying extents (e.g., different reaction times and temperatures), the uptake of TCE correlated with surface acidity, increasing with decreasing acidity. Surface acidity, however, should be used with caution as a predictor of carbon performance under preloading conditions, especially when carbons have been activated under widely different conditions.
A model was developed using an approach based on the Ideal Adsorbed Solution Theory (IAST) to predict TCE adsorption by GAC preloaded with natural DOM isolated from three surface water sources. The IAST model was formulated for a bi-solute system in which TCE and DOM single-solute uptakes were described by the Langmuir-Freundlich and Freundlich isotherms, respectively. Consistent with previous work that identified low molecular weight species as the most reactive with respect to preloading effects (i.e., reducing target compound uptake), the low molecular weight components of the polar (hydrophilic) and nonpolar (hydrophobic) DOM fractions, isolated using ultrafiltration (1 kDa molecular weight cutoff membrane), exhibited significant competitive effects. Furthermore, the effects of these fractions on TCE uptake were similar; therefore, they were considered together to represent a single “reactive fraction” of DOM. Based on this finding, isotherms for the less than 1-kDa low molecular weight DOM fraction of the whole water were measured, and molar concentrations were computed based on an average molecular weight determined using size-exclusion chromatography. The IAST model was modified to incorporate surface area reduction resulting from pore blockage by DOM, and to reflect the hypothesis that TCE molecules can access adsorption sites that humic molecules cannot, thus preventing competition on these sites. The model was calibrated with data for TCE uptake by carbon preloaded with the less than 1-kDa low molecular weight DOM fraction, and was verified by predicting TCE uptake by carbon preloaded with whole natural waters, for both constant GAC dose (hence, constant DOM loading) and variable GAC dose (hence, variable DOM loading) TCE isotherms. Preloading by DOM reduced volume in GAC pores having widths smaller than 1.25 nm (likely accessible only to TCE) to a greater extent than total pore volume, suggesting preferential blockage of micropores. Such preferential pore blockage may explain, in part, why increased DOM loading decreases the fraction of the total surface area on which no competition between TCE and DOM occurs.
Journal Articles on this Report: 8 Displayed | Download in RIS Format
| Other project views: | All 25 publications | 8 publications in selected types | All 8 journal articles |
| Type | Citation | ||
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Dastgheib SA, Karanfil T. Adsorption of oxygen by heat-treated granular and fibrous activated carbons. Journal of Colloid and Interface Science 2004; 274(1):1-8. |
R828157 (Final) |
not available |
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Dastgheib SA, Karanfil T, Cheng W. Tailoring activated carbons for enhanced removal of natural organic matter from natural waters. Carbon 2004;42(3):547-557. |
R828157 (Final) |
not available |
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Karanfil T, Dastgheib SA. Trichloroethylene adsorption by fibrous and granular activated carbons: Aqueous phase, gas phase and water vapor adsorption studies. Environmental Science & Technology 2004;38(22):5834-5841. |
R828157 (Final) |
not available |
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Kilduff JE, Karanfil T. Trichloroethylene adsorption by activated carbon preloaded with humic substances: effects of solution chemistry. Water Research, Volume 36, Issue 7, April 2002, Pages 1685-1698. |
R828157 (2001) R828157 (2002) R828157 (Final) |
not available |
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Kilduff JE, Srivastava R, Karanfil T. Preloading of GAC by natural organic matter: effect of surface chemistry on TCE uptake. Characterization of Phosphorus Solids VI Studies in Surface Science and Catalysis 2002;144:553-560. |
R828157 (2002) R828157 (Final) |
not available |
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Kilduff JE, Mattaraj S, Wigton A, Kitis M, Karanfil T. Effects of reverse osmosis isolation on reactivity of naturally occurring dissolved organic matter in physicochemical processes. Water Research 2004;38(4):1026-1036. |
R828157 (Final) R828045 (Final) |
not available |
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Wigton A, Kilduff JE. Modeling trichloroethylene adsorption by activated carbon preloaded with natural dissolved organic matter using a modified IAST approach. Environmental Science & Technology 2004;38(22):5825-5833. |
R828157 (Final) |
not available |
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Pimenta ACD, Kilduff JE. Oxidative coupling and the irreversible adsorption of phenol by graphite. Journal of Colloid and Interface Science 2006;293(2):278-289. |
R828157 (Final) |
not available |
environmental chemistry, analytical measurements, adsorption, toxics, solvents, ecosystem protection/environmental exposure and risk, waste, water, bioavailability, chemistry, drinking water, engineering, fate and transport, industrial waste, synthetic organic chemicals, granular activated carbon, hazardous waste treatment,
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Ecosystem Protection/Environmental Exposure & Risk, Water, Scientific Discipline, Waste, RFA, Drinking Water, Chemistry, Hazardous Waste, Fate & Transport, Environmental Chemistry, Hazardous, Engineering, industrial waste, treatment, water quality, wastewater treatment, other - risk management, fate and transport, monitoring, granular activated carbon
Progress and Final Reports:
2001 Progress Report
2002 Progress Report
Original Abstract