Pore blockage effect of NOM on atrazine adsorption kinetics of PAC: the roles of PAC pore size distribution and NOM molecular weight
Introduction
Activated carbon has been widely used in drinking water treatment to remove dissolved organic compounds, including background natural organic matter (NOM) and a number of synthetic trace organic compounds. However, due to a limited understanding of the mechanisms of competitive adsorption, there is yet the need for the development of an accurate model that will predict trace organic compound adsorption from natural water. Previous studies often use the ideal adsorbed solution theory (IAST) [1] to model the adsorption equilibrium of a multiple solute system, which assumes equal access of all adsorbates to all adsorption sites and interaction of adsorbates through a single mechanism: direct competition for adsorption sites. It has been realized only recently that NOM affects trace organic compound adsorption not only by directly competing for adsorption sites but also by blocking carbon pores [2], [3], [4], [5].
A common example of the pore blockage effect of NOM in continuous flow adsorption systems is the “preloading” phenomena found in fixed bed GAC columns. Due to their slower adsorption kinetics, NOM compounds move down the column faster than trace organic compounds. As a result, the GAC at the effluent end of the bed is preloaded with NOM before it is contacted with the trace organic molecules. In fact, all the adsorption systems in which activated carbon is held in the system while water continuously flows through are vulnerable to the pore blockage effect. In these systems, including granular activated carbon (GAC) adsorbers, floc blanket reactors with powdered activated carbon (PAC) addition, and PAC/membrane systems, the activated carbon is partially loaded with NOM and trace organic compounds before more trace organic compounds enter the system, resulting in pore blockage when the surface concentration of NOM gets high enough. The ‘preloading’ effect in GAC adsorbers is usually attributed to occupation of adsorption sites by NOM or to pore blockage by NOM that makes adsorption sites in small pores practically unavailable to trace organic compounds [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Therefore, most competitive adsorption research on the preloading effect has focused primarily on its impact on the adsorption equilibrium of trace organic compounds [2], [3], [4], [5], [15], [16]. However, a recent mechanistic study by Li et al. [17] showed that, in addition to practically reducing adsorption capacity, pore blockage by large molecules had a strong effect on the adsorption kinetics of atrazine. It was also found by Lebeau et al. [18] that both adsorption capacity and the surface diffusion coefficient of atrazine decreased as the PAC age in an immersed microfiltration system increased. The result is that higher carbon doses are required to achieve given treatment goals compared to those predicted by batch isotherm and kinetic tests using fresh carbon.
Since adsorption equilibrium is rarely reached in continuous flow adsorption systems such as PAC/membrane reactors and GAC columns, it is critical to fully understand the effect of NOM on adsorption kinetics of a trace organic compound so that meaningful, accurate design tools may be developed. However, NOM in different natural waters varies so much that it can have dramatically different competitive adsorption effects [19]. One of the most important characteristics of NOM that affects adsorption is molecular weight. Moreover, the heterogeneity of activated carbon surface is also an important factor in competitive adsorption. The pore size distribution of activated carbon relative to the molecular sizes of adsorbates (e.g. trace organic compounds and NOM) has been found to determine the dominant mechanism of competitive adsorption [15], [17], [20], [21]. Therefore, an accurate evaluation of the roles of NOM molecular weight distribution (MWD) and PAC properties is necessary for optimizing system design and operation to achieve maximum pollutant removal.
The objectives of this study are to: (1) demonstrate the pore blockage effect of NOM on adsorption kinetics of trace organic compounds in batch as well as in continuous flow systems; (2) determine the effect of NOM surface loading on adsorption kinetics of trace organic compounds; (3) evaluate the roles of NOM molecular weight and carbon pore size distributions in pore blockage. This information is needed to improve our understanding of the competitive effect of NOM, which is necessary for better modeling of competitive adsorption of trace organic compounds in natural water. The results from this study will also help water utilities choose the best adsorbent based on NOM characteristics and the physical/chemical properties of the target compound.
Section snippets
Water
Organic-free water was obtained by passing deionized water through a NANOpure ultrapure water system (Barnstead, Dubuque, Iowa). The dissolved organic carbon (DOC) concentration of the organic-free water is lower than 0.3 mg/L. A central Illinois ground water (referred to as GW) and two surface waters were used to study the effect of NOM. The GW was taken from the source immediately before use. It was treated with a greensand filter to remove dissolved iron and manganese and filtered through a
Effect of NOM preloading on atrazine adsorption kinetics: batch experiments
Batch atrazine adsorption kinetic tests were conducted using PAC preloaded with NOM in natural water to determine the effect of NOM preloading on atrazine adsorption kinetics. Doses of 4, 8, 12 and 16 mg/L of PAC A and 2, 4, 8.3 and 12 mg/L of PAC B were preloaded in 2 L of fresh Lake Decatur water (FLDW) for 4 days to obtain a range of NOM surface loading. Following the same procedure, 4, 8, 12 and 20 mg/L of PAC A was preloaded with the one year old Lake Decatur water (DLDW), and 2 and 4 mg/L of
Conclusions
By studying the effect of preloading PAC with natural water, it was found that atrazine adsorption kinetics of PAC could be severely impeded by NOM molecules through the pore blockage mechanism. When PAC was preloaded in natural water, the surface diffusion coefficient of atrazine decreased with increasing surface loading of NOM on PAC, and was reduced by up to more than two orders of magnitude depending on the PAC dose or NOM surface concentration. The comparison of the two PACs showed that
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Current address: Department of Chemical Engineering, Yale University, 9 Hillhouse Avenue, New Haven, Connecticut 06510.