Elucidating Adsorptive Fractions of Natural Organic Matter on Carbon Nanotubes Mohamed Ateia,* ,† Onur G. Apul, ‡,§ Yuta Shimizu, † Astri Muflihah, † Chihiro Yoshimura, † and Tanju Karanfil ∥ † Department of Civil and Environmental Engineering, Tokyo Institute of Technology, 2-12-1-M1-4 Ookayama, Tokyo 152-8552, Japan ‡ Department of Civil and Environmental Engineering, University of Massachusetts Lowell, Lowell, Massachusetts 01854, United States § School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, Arizona 85259, United States ∥ Department of Environmental Engineering and Earth Sciences, Clemson University, Anderson, South Carolina 29625, United States * S Supporting Information ABSTRACT: Natural organic matter (NOM) is a heteroge- neous mixture of organic compounds that is omnipresent in natural waters. To date, the understanding of the adsorption of NOM components by carbon nanotubes (CNTs) is limited because of the limited number of comprehensive studies in the literature examining the adsorption of NOM by CNTs. In this study, 11 standard NOM samples from various sources were characterized, and their adsorption behaviors on four different CNTs were examined side-by-side using total organic carbon, fluorescence, UV−visible spectroscopy, and high-performance size-exclusion chromatography (HPSEC) analysis. Adsorption was influenced by the chemical properties of the NOM, including aromaticity, degree of oxidation, and carboxylic acidity. Fluorescence excitation−emission matrix (EEM) analysis showed preferential adsorption of decomposed and terrestrial- derived NOM compared to freshly produced and microbial-derived NOM. HPSEC analysis revealed preferential adsorption of fractions in the molecular weight range of 0.5−2 kDa for humic acids but in the molecular weight range of 1−3 kDa for all fulvic acids and reverse-osmosis isolates. However, the smallest characterized fraction (MW < 0.4 kDa) in all samples did not adsorb on the CNTs. 1. INTRODUCTION Natural organic matter (NOM) is ubiquitous in fresh waters and is found in concentrations that range from 1−2 (mg of C)/ L to 40 (mg of C)/L depending on the source and climate. 1,2 The presence of NOM, which is a mixture of complex polyelectrolytes, during water treatment processes presents a broad range of problems, such as an increase in chemical demands, the formation of disinfection byproducts, the occurrence of taste and odor problems, and the fouling of activated carbons and membranes. 3, 4 Therefore, several processes have been employed to remove NOM from water, including coagulation, membrane filtration, ion exchange, advanced oxidation processes, and adsorption. 5 Carbon nanotubes (CNTs), with their high surface areas, hydrophobicity, porosity, and rapid sorption kinetics, have been explored as one of the next-generation adsorbents. 6,7 Over the past decade, extensive studies have investigated applications of CNTs in the adsorption of several organic and inorganic water contaminants. 8,9 Furthermore, the interactions between CNTs and NOM have been examined in some studies. 1,10−17 Mechanisms for the adsorption of NOM components by CNTs include hydrophobic interactions, π−π interactions, hydrogen-bonding interactions, and electrostatic interac- tions. 10,18 These interactions are influenced by (1) the characteristics of the CNTs (e.g., surface area, pore volume, and surface functionalities), 10,14,19 (2) the NOM molecular structure and composition (e.g., molecular weight and size), 13,19 and (3) the solution chemistry (e.g., pH, water temperature, and ionic strength). 1,10 It was also reported that the conformation and composition of the NOM remaining in water will change as a result of adsorption processes due to NOM fractionation. 15,20 However, there are no comprehensive studies in the literature examining the adsorption of NOM by CNTs. The limited observations in previous studies are due to a number of factors: (1) different Received: March 9, 2017 Revised: May 21, 2017 Accepted: May 24, 2017 Published: May 24, 2017 Article pubs.acs.org/est © 2017 American Chemical Society 7101 DOI: 10.1021/acs.est.7b01279 Environ. Sci. Technol. 2017, 51, 7101−7110