Environ. Sci. Technol. 2008, 42, 1227–1231
Effects of Powdered Activated Carbon Pore Size Distribution on the Competitive Adsorption of Aqueous Atrazine and Natural Organic Matter L I D I N G , †,§ V E R N O N L . S N O E Y I N K , * ,†,§ B E N I T O J . M A R I Ñ A S , †,§ Z H O N G R E N Y U E , ‡,§ A N D J A M E S E C O N O M Y ‡,§ Department of Civil and Environmental Engineering, University of Illinois at Urbana–Champaign, Urbana, Illinois, Department of Materials Science and Engineering, University of Illinois at Urbana–Champaign, Urbana, Illinois, and Center of Advanced Materials for the Purification of Water with Systems, University of Illinois at Urbana–Champaign, Urbana, Illinois
Received May 5, 2007. Revised manuscript received September 11, 2007. Accepted November 6, 2007.
The pore size distribution (PSD) of adsorbents has been found to be an important factor that affects adsorption capacity for organic compounds; consequently, it should influence competitive adsorption in multisolute systems. This research was conducted to show how the PSD of activated carbon affects the competition between natural organic matter (NOM) and the trace organic contaminant atrazine, with a primary emphasis on quantifying the pore blocking mechanism of NOM competition. Isotherm tests were performed for both atrazine and NOM from a groundwater on five powdered activated carbons (PACs) with widely different PSDs. The capacity for NOM correlated best with the surface area of pores in the diameter range of 15–50 Å, although some NOM also adsorbed in the smaller pores as evidenced by a reduction in capacity for atrazine when NOM was present. Kinetic tests for atrazine on PACs with various levels of preadsorbed NOM showed that the magnitude of the pore blockage effect by NOM was lower for PACs with higher surface area of pores with diameter in the range of 15–50 Å. Therefore increasing pores in the size range where NOM adsorb can reduce the extent of the pore blockage competitive effect on the target compound atrazine. The effect of PSD was further studied with a flow-through PACmembrane hybrid water treatment system, in which experimental results successfully verified model simulations by the COMPSORB model.
Introduction Activated carbon is mainly used in water treatment to remove organic contaminants such as taste- and odor-causing compounds and synthetic organic chemicals. Natural organic matter (NOM) present in all natural water bodies competes with target contaminants and significantly increases the * Corresponding author tel.: +1 217 333 4700; fax: +1 217 333 6968; e-mail:
[email protected]. † Department of Civil and Environmental Engineering. § Center of Advanced Materials for the Purification of Water with Systems. ‡ Department of Materials Science and Engineering. 10.1021/es0710555 CCC: $40.75
Published on Web 01/18/2008
2008 American Chemical Society
required dose of activated carbon and treatment cost. NOM competes via two major mechanisms, direct site competition and pore blockage (1–3). Small, strongly adsorbing molecules of NOM with size comparable to that of the target compound are mainly responsible for direct site competition, thereby reducing the adsorption capacity for the target compound. Larger NOM molecules adsorb in large pores and reduce the effective pore diameter, thus, decreasing the rate of adsorption of smaller molecules that must pass through these pores to reach smaller pores. The extent to which NOM competes with target compounds depends on the characteristics of the NOM (4, 5), the physical/chemical properties of powdered activated carbon (PAC) (6), and the initial concentration of the trace compounds relative to that of NOM (7, 8). Single solutes have been shown to preferentially adsorb in pores with diameter similar to their molecular size (9), and to undergo size exclusion in pores with average diameter smaller than 1.7 times the molecule’s second-widest dimension (10). One type of NOM has been reported to adsorb in pores with a size range of 30–100 Å (11). Hopman et al. (12) studied the effect of NOM preloading on the removal of atrazine and found that a carbon with only relatively small micropores adsorbed little NOM due to size exclusion, and NOM preloading resulted in little reduction of atrazine adsorption capacity. In contrast, a carbon with larger micropores adsorbed higher levels of NOM, which in turn resulted in a reduction in the adsorption capacity for atrazine. Pelekani et al. (13–15) found that NOM could cause a large reduction in the atrazine adsorption capacity of carbon containing only small micropores. Pelekani and co-workers (15) also found that direct site competition was the dominant mechanism responsible for capacity reduction when the competing compound was of similar size to the target contaminant, and an increase in molecular size of the competing compound shifted the competition mechanism from direct competition to blockage of the pore mouth of a predominantly microporous carbon. Li et al. (1) showed that constriction of internal carbon pores caused a reduction in the rate of diffusion of target compounds. By comparing two carbons (2, 16), the extent of reduction in diffusion coefficient caused by adsorption of the same concentration of pore-blocking compound (in mg/g of PAC), was found to be less pronounced for the carbon with larger volume of mesopores. The objective of this study was to quantify the effect of PSD on the competitive adsorption effects of NOM. A series of carbons made from one raw material by the same manufacturing process, and a new carbon developed from another raw material activated by a different method, were used to study the removal of atrazine from natural water. Both direct competition for sites and pore blockage were quantified using the COMPSORB kinetic model (2).
Materials and Methods Water. Experiments were performed with distilled–deionized (DDI) water as organic free water (OFW) and groundwater from Clinton Water Works (CWW), Clinton, IL. CWW water was collected at the plant pump station before any treatment and stored at 4 °C in a stainless steel barrel. Prior to an experiment, the water was warmed to ambient temperature and passed through a nylon membrane filter with a nominal pore size of 0.45 µm (OSMONICS, Minnetonka, MN) to remove suspended solids. The dissolved organic carbon (DOC) concentration of CWW water, measured with a VOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Adsorption capacity for CWW NOM as a function of surface area for pores with diameter in the range of 15–50 Å.
FIGURE 1. Pore size distribution of the five PACs.
FIGURE 2. Atrazine and NOM adsorption isotherms. Phoenix 8000 TOC analyzer (Tekmar-Dohrmann, Cincinnati, OH), was 7.0 ( 0.2 mg/L. Adsorbents. Five PACs were used in this study. Four of them (SA UF, SA Super, W20, W35) are commercial products from NORIT Netherlands, B.V., Amersfoort, Netherlands, made from the same raw material, but with different levels of activation. According to the manufacturer, the particle diameter of SA Super, W35, and W20 was 10, 20, and 25 µm, respectively, whereas the diameter of SA UF was measured by us as 6 µm. The fifth carbon (Pellet II) was produced by activating an agglomerated carbon black impregnated with a cellulose-ZnCl2 solution in a nitrogen atmosphere in the Department of Materials Science and Engineering laboratory, University of Illinois at Urbana–Champaign (patent pending). It features a high surface area in the mesoporous region. It was machineground and sieved through US Mesh Sieve size No. 400 (37 µm 1228
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diameter opening) prior to use. In the kinetic model, the diameter of all carbons was assumed to be 6 µm in order to simplify the modeling effort. This assumption changes the value of the diffusion coefficients but it does not affect the quality of the fit of the kinetic curve. All carbons were stored in glass bottles inside a desiccator. Prior to use, the carbon was taken out of the desiccator, oven-dried overnight at 105 °C, and placed back in the desiccator for cooling. Trace Organic Compound. The herbicide atrazine radiolabeled with Carbon 14 (Syngenta Crop Protection, Inc., Greensboro, NC) was used as the target trace compound. A stock solution prepared by dissolving 12.4 mg/L of atrazine in DDI water (stored at 4 °C) was used to prepare all test solutions. The concentration of Carbon-14 was measured with a liquid scintillation analyzer (Tricarb Model 1600A, Packard Instrument Co., Downers Grove, IL) to obtain atrazine concentrations. Isotherm Experiments. Atrazine isotherm tests were conducted in OFW and in CWW water using the conventional bottle-point technique (17). The experimental procedures were described previously (1). Samples were taken from these bottles after a 7-day contact time and analyzed. The DOC concentration was also measured in the tests performed in CWW water. Adsorption Kinetic Experiments. Two types of atrazine adsorption kinetic tests were carried out: (a) adsorption on fresh carbon in OFW and (b) adsorption on carbon preloaded with NOM from CWW. Experimental procedures and analytical methods used were described previously (18). Hybrid Adsorption/Filtration System Experiments. Experiments were performed with a flow-through hybrid adsorption/membrane reactor. Reactor design and operating conditions were the same as those used in a previous study (18). Pore Size Distribution Analyses. PSD analyses were performed using N2 gas adsorption at 77 K with an Autosorb-1 Volumetric Sorption Analyzer controlled by Autosorb-1 software (Quantachrome Corp., Boynton Beach, FL). All samples were degassed at 150 °C until the outgas pressure rise was below 5 µHg/min prior to analysis. The BrunnauerEmmett-Teller (BET) surface areas were determined from the N2 adsorption isotherm. The total pore volume was estimated from the amount of nitrogen adsorbed at the relative partial pressure P/Po ) 0.95. The nonlocal density functional theory (DFT) model was applied to the N2 adsorption data at 77 K to get pore size distributions and micropore volumes (19). The pores were divided into four groups according to the International Union of Pure and Applied Chemistry classification: macropores (>500 Å), mesopores (20–500 Å), secondary micropores (8-20 Å), and primary micropores (
