Environ. Sci. Technol. 2008, 42, 7431–7437
Removal of Dissolved Organic Matter by Anion Exchange: Effect of Dissolved Organic Matter Properties T R E A V O R H . B O Y E R , * ,†,‡ PHILIP C. SINGER,† AND GEORGE R. AIKEN§ Department of Environmental Sciences and Engineering, CB 7431, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7431, Water Resources Division, United States Geological Survey, 3215 Marine Street, Boulder, Colorado 80303
Received March 11, 2008. Revised manuscript received June 25, 2008. Accepted July 14, 2008.
Ten isolates of aquatic dissolved organic matter (DOM) were evaluated to determine the effect that chemical properties of the DOM, such as charge density, aromaticity, and molecular weight, have on DOM removal by anion exchange. The DOM isolateswerecharacterizedasterrestrial,microbial,orintermediate humic substances or transphilic acids. All anion exchange experiments were conducted using a magnetic ion exchange (MIEX) resin. The charge density of the DOM isolates, determined by direct potentiometric titration, was fundamental to quantifying the stoichiometry of the anion exchange mechanism. The results clearly show that all DOM isolates were removed by anion exchange; however, differences among the DOM isolates did influence their removal by MIEX resin. In particular, MIEX resin had the greatest affinity for DOM with high charge density and the least affinity for DOM with low charge density and low aromaticity. This work illustrates that the chemical characteristics of DOM and solution conditions must be considered when evaluating anion exchange treatment for the removal of DOM.
Introduction It has been previously shown that anion exchange treatment using a magnetic ion exchange (MIEX) resin is very effective at removing dissolved organic matter (DOM) from raw drinking waters (1-5). The subsequent coagulant demand, chlorine demand, and formation of trihalomethanes and haloacetic acids were all decreased as a result of the substantial removal of DOM by MIEX resin. Recent work by the authors evaluated the removal of a well-characterized aquatic DOM isolate by MIEX resin and four conventional polymeric anion exchange resins (6). For the DOM isolate investigated, it was shown that anion exchange is the operative mechanism of removal of DOM by the polyacrylic and polystyrene anion exchange resins, that polyacrylic resins exhibit a greater removal of DOM than polystyrene resins, and that MIEX resin, a magnetically enhanced polyacrylic resin, performs similarly to conventional polyacrylic resins. * Corresponding author. E-mail:
[email protected]. † University of North Carolina at Chapel Hill. ‡ Current address: Department of Environmental Engineering Sciences, University of Florida, P.O. Box 116450, Gainesville, FL 326116450. § United States Geological Survey. 10.1021/es800714d CCC: $40.75
Published on Web 08/28/2008
2008 American Chemical Society
Due to the heterogeneity of aquatic DOM, however, additional work is required to demonstrate conclusively that anion exchange is the operative mechanism of removal for an array of DOM with different properties and to determine the effect of these properties on the anion exchange process. DOM properties that are reported to influence anion exchange include the distribution of carbon between aromatic and aliphatic domains, acidity, and molecular weight (7, 8). It is well-known that the chemical characteristics of aquatic DOM are affected by the source and age of the organic precursor material and by the techniques used to concentrate and isolate the DOM (9-11). Organic matter derived from higher plants is characterized by high aromatic carbon content and low nitrogen content. In contrast, DOM derived from algae, bacteria, and aquatic plants is enriched in aliphatic carbon and organic nitrogen. Isolation techniques often make use of DOM properties, such as acidity, hydrophobicity, and molecular weight, to obtain DOM fractions that are chemically well-defined. In this work, DOM isolates were chosen to represent a range of chemical properties. The objective of this research was to determine which DOM properties have the greatest influence on the removal of DOM by anion exchange. This is important for understanding fundamental interactions between DOM and anion exchange resins and for maximizing the removal of DOM during water treatment to improve treatment outcomes. DOM properties of interest included the charge density, aromatic and aliphatic carbon content, molecular weight, and nitrogen content. The specific aims of this work were to quantify the effects of inter- and intra-DOM properties on the stoichiometry of anion exchange and on the removal of DOM by MIEX resin. Inter-DOM properties were evaluated by comparing several well-characterized DOM isolates, whereas intra-DOM properties were evaluated by studying a single DOM isolate under different solution conditions.
Materials and Methods DOM Isolates. Ten aquatic DOM isolates from seven different locations were studied. The DOM isolates were classified as hydrophobic acid (HPOA), fulvic acid (FA), or transphilic acid (TPIA) as described in the literature (12, 13). Suwannee River fulvic acid (SRFA) and Pony Lake fulvic acid (PLFA) were purchased from the International Humic Substances Society (IHSS). Fort Lauderdale, Florida hydrophobic acid and transphilic acid (FtLHPOA and FtLTPIA) were isolated at the University of North Carolina at Chapel Hill. Coal Creek fulvic acid (CCFA), Florida Everglades site F1 fulvic acid, hydrophobic acid, and transphilic acid (EF1FA, EF1HPOA, and EF1TPIA); Pacific Ocean fulvic acid (POFA); and Williams Lake hydrophobic acid (WLHPOA) were obtained from the U.S. Geological Survey (USGS). Chemical properties of the DOM isolates are listed in Table 1. Table S1 in the Supporting Information (SI) provides a brief description of the source of each aquatic DOM isolate. Ultraviolet (UV) absorbance was measured on a Hitachi U-2000 spectrophotometer using a 1 cm quartz cell. Dissolved organic carbon (DOC), total dissolved nitrogen (TDN), and dissolved inorganic carbon (DIC) were measured on a Shimadzu TOC-VCPH total organic carbon analyzer with a TNM-1 total nitrogen measuring unit. All samples were analyzed in duplicate with average values reported; the relative difference (defined as the absolute difference divided by the mean) between DOC, TDN, and DIC duplicates was
