Trihalomethane, dihaloacetonitrile, and total N-nitrosamine precursor adsorption by carbon nanotubes: the importance of surface oxides and pore volume†
As drinking water sources become increasingly impaired, enhanced removal of natural organic matter (NOM) may be required to curb formation of disinfection byproducts (DBPs) upon chlor(am)ination. While carbon nanotubes (CNTs) can adsorb NOM, their properties for DBP precursor adsorption have not been elucidated. Nine types of CNTs were assessed for trihalomethane (THM), dihaloacetonitrile (DHAN), and total N-nitrosamine (TONO) precursor adsorption. Batch isotherm experiments were completed with lake water and, to simulate an impaired condition, effluent from a wastewater treatment plant (WWTP). Adsorption varied with CNT type and dose, with TONO precursors having the highest percent removals from WWTP effluent (up to 97%). Physicochemical properties of CNTs were characterized by gas adsorption isotherms and X-ray photoelectron spectroscopy and numerical models were developed to identify CNT properties driving DBP precursor adsorption. The models fits were strong (R2 > 0.92) and indicated removal of the three precursor types increased with percent carboxyl groups (p < 0.01) and, for TONO precursors only, cumulative pore volume (CPV, p = 0.001). A multicollinearity analysis suggested surface oxides – particularly carboxyl groups – on the CNTs increased CPV, presumably by increasing electrostatic repulsive forces, which enhanced microporosity sufficiently to overshadow any repulsion of DBP precursors from negatively charged surface oxides. A size exclusion analysis revealed all CNT pores were accessible to TONO precursors, while THM and DHAN precursors had limited access to the smaller micropores. These findings provide a framework to modify CNTs to optimize adsorption of DBP precursors and demonstrate the potential of CNTs for TONO precursor removal.