Colloidal stability and surface chemistry of aqueous tetravalent uranium nanoparticles: Effects of 4-nitrocatechol adsorption

The colloidal stability of uranium(IV) nanoparticles (U(IV)-NPs), which influences the subsurface U migration from contaminated sites, can be altered by the adsorption of natural organic matters possessing surface-binding groups, such as catechols. Therefore, in this study, the effects of catechol binding groups on the colloidal stability and surface chemistry of U(IV)-NPs in anoxic aqueous solutions were probed using 4-nitrocatechol (nCA) over a wide pH range. Multiple complementary investigations including zeta-potential measurements, controlled acid-base titration, adsorption isotherm analysis, and surface complexation modeling revealed that nCA significantly modified the colloidal behavior of the intact U(IV)-NPs-which were stable only in acidic solutions (pH similar to 2)-by shifting the isoelectric point to a lower-pH region in a concentration-dependent manner via strong adsorption complying the Langmuir isotherm model. Interestingly, the nCA-containing colloidal solutions stabilized at high pH and enabled U(IV)-NP redispersion. This colloidal behavior originated from the protonation/deprotonation capability of UOH sites on UO2(cr)-like primary particles, as evidenced by the two pK(a) values determined using the surface ionization model (5.0 +/- 0.6 and 7.4 +/- 0.7), as well as the strong inner-sphere and mixed monodentate/bidentate complex formation with nCA, as signified by the observed surface-enhanced infrared absorption effects and one-or two-site complexation modeling results.