Participation of\nStrong H‑Bonding to Acidic\nGroups Contributes to the Intense Sorption of the Anionic Munition,\nNitrotriazolone (NTO) to the Carbon, Filtrasorb 400

5-Nitro-1,2-dihydro-3H-1,2,4-triazin-3-one (“nitrotriazolone,”\nNTO) is an insensitive munition compound used in modern weaponry.\nIt poses a potential threat to soil and water quality at relevant\nsites due to its physical properties that cause high mobility in the\nenvironment. NTO is polar and predominantly monoanionic (NTO<sup>–</sup>) at environmental pH (p<i>K</i><sub>a1</sub> = 3.64 and\np<i>K</i><sub>a2</sub> = 11.06; determined by two independent\nmethods in this study). Nevertheless, NTO<sup>–</sup> sorbs\nstrongly to the carbon, Filtrasorb 400 (Freundlich coefficient <i>K</i><sub>F</sub> = 1.26 × 10<sup>4</sup> L<sup><i>n</i></sup> μmol<sup>1–<i>n</i></sup> kg<sup>–1</sup> at pH 9.5). We present evidence that sorption\nis contributed by the interaction of NTO<sup>–</sup> with functional\ngroups of similar acidity on the carbon to form exceptionally strong,\nnegative charge-assisted hydrogen bonds, (−)CAHB, written (−N<sup>–</sup>···H<sup>+</sup>···<sup>–</sup>O–), where −N<sup>–</sup> is the deprotonated\nN of NTO, and <sup>–</sup>O– represents a deprotonated surface\ngroup. Behaviors consistent with (−)CAHB include (1) a “hump”\nin the pH-sorption profile centered around pH 4, where maximal complexation\nis expected to occur; (2) an apparent ∼2.4-unit upward shift\nin p<i>K</i><sub>a1</sub> due to the enhanced H-bond strength;\n(3) the consumption of a proton from water to form the complex; and\n(4) sorption suppression by competing (−)CAHB-capable solutes.\nAnion exchange played a minor role. The findings help advance our\nunderstanding of weak acids sorption by carbonaceous materials.