Article pubs.acs.org/est
Modeling Nonequilibrium Adsorption of MIB and Sulfamethoxazole by Powdered Activated Carbon and the Role of Dissolved Organic Matter Competition Kyle K. Shimabuku,* Hyukjin Cho, Eli B. Townsend, Fernando L . Rosario-Ortiz, and R. Scott Summers Department of Civil, Environmental, and Architectural Engineering, University of Colorado, 428 UCB, Boulder, Colorado 80309, United States S Supporting Information *
ABSTRACT: This study demonstrates that the ideal adsorbed solution theory−equivalent background compound (IAST−EBC) as a stand-alone model can simulate and predict the powdered activated carbon (PAC) adsorption of organic micropollutants found in drinking water sources in the presence of background dissolved organic matter (DOM) under nonequilibrium conditions. The IAST−EBC represents the DOM competitive effect as an equivalent background compound (EBC). When adsorbing 2-methylisoborneol (MIB) with PAC, the EBC initial concentration was a similar percentage, on average 0.51%, of the dissolved organic carbon in eight nonwastewater impacted surface waters. Using this average percentage in the IAST−EBC model yielded good predictions for MIB removal in two nonwastewater impacted waters. The percentage of competitive DOM was significantly greater in wastewater impacted surface waters, and varied markedly in DOM size fractions. Fluorescence parameters exhibited a strong correlation with the percentage of competitive DOM in these waters. Utilizing such correlations in the IAST−EBC successfully modeled MIB and sulfamethoxazole adsorption by three different PACs in the presence of DOM that varied in competitive effect. The influence of simultaneous coagulant addition on PAC adsorption of micropollutants was also investigated. Coagulation caused the DOM competitive effect to increase and decrease with MIB and sulfamethoxazole, respectively.
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and PAC physicochemical properties).3,10−12 Since the DOM composition varies in aquatic environments, the proportion of DOM constituents that compete with target adsorbates can be highly variable. PAC and DOM interactions are also complicated by the concurrent use of PAC with chemical coagulants that can remove large portions (10 to 50%) of the DOM. The ideal adsorbed solution theory (IAST)13 was originally developed to model competitive adsorption in systems with known adsorbates. It was later extended to account for DOM competition by representing the magnitude of the competitive effect of all DOM constituents as an equivalent background compound (EBC).14 The IAST−EBC method has successfully modeled micropollutant adsorption in natural waters including pesticides4,14,15 as well as taste and odor compounds.5,10,11 However, there are several limitations. In particular, the IAST− EBC approach has historically been restricted to modeling systems under equilibrium conditions (i.e., contact times of several days or more), which has limited its practical use in water treatment plants where PAC is unlikely to reach equilibrium with organic contaminants because the effective
INTRODUCTION Powdered activated carbon (PAC) can be an effective water treatment technology for the control of organic pollutants, such as naturally occurring taste and odor compounds (e.g., 2methylisoborneol (MIB)) and synthetic organic contaminants (e.g., the antibiotic drug sulfamethoxazole (SMX)) found in discharges to natural waters.1,2 PAC is well suited to treat contaminants that are found intermittently in drinking water sources, such as MIB, which occurs during episodic algal blooms,2 because capital costs associated with PAC are low as it can be fed into existing treatment processes (e.g., rapid mix, presedimentation) and PAC can be applied only when needed. However, operating costs can be expensive if high doses are required.2 Background dissolved organic matter (DOM), which is ubiquitous in drinking water sources, negatively impacts PAC treatment efficiency. DOM is a complex mixture of organic molecules that vary in size and chemical structure and suppresses PAC performance through direct competition and pore blockage.3−6 One method to characterize the nature of DOM is fluorescence spectroscopy, which is a simple and sensitive technique that can provide information about DOM source and reactivity.7−9 The DOM character (e.g., molecular size, hydrophobicity) is an important factor that determines the extent of its competitive effect, although other system parameters are also important (e.g., DOM concentration, pH, © 2014 American Chemical Society
Received: Revised: Accepted: Published: 13735
July 20, 2014 November 3, 2014 November 4, 2014 November 4, 2014 dx.doi.org/10.1021/es503512v | Environ. Sci. Technol. 2014, 48, 13735−13742
Environmental Science & Technology
Article
Table 1. Water Quality Parameters for Phase II Waters
a
water
pH
DOC (mg/L)
SUVA (L/m/mg)
spectral slope
average MWw (Daltons)
FI
peak C/UVA254 (RU-cm)
>1kD - BEM BEM 3 mg/L BEM 7 mg/L Barker Reservoir
