Interactions of humic acid, quinoline, and TiO2 in water in relation to quinoline photocatalytic removal

To expand the still-rare studies about the effect of humic acids (HA) on th e photocatalytic removal of water organic pollutants over TiO2, quinoline w as selected, considering the previously investigated aromatics. Dark adsorp tion measurements have shown that the amount of quinoline (initial concentr ation (C-Q)(o) = 0.17 mmol L-1) adsorbed at equilibrium on TiO2 (50 m(2) g( -1); 2.5 g L-1) corresponds to only ca. 0.012 molecule nm(-2). By contrast, HA (Aldrich Na salt) was found to completely cover TiO2 for the initial ma ss concentration (C-HA(m)). = 81 ppm, assuming a flat structure for adsorbe d HA and a mean molar mass of 4 kg. Notwithstanding, HA did not hinder quin oline adsorption. Without TiO2, the fraction of quinoline chemically bound to HA was only ca. 1.5% for (C-HA(m))(o) = 87 ppm; however, a weak quenchin g of quinoline fluorescence by HA illustrated the existence of looser inter actions. The photocatalytic (lambda greater than or equal to 340 nm) initia l removal rate, r(o), of quinoline ((C-Q)(o) = 0.077 - 0.77 mmol L-1) witho ut HA was in agreement with a pseudo-first-order Langmuir-Hinshelwood kinet ics, however, with an adsorption constant much higher than that deduced fro m dark adsorption. This difference suggests that the quinoline molecules in volved should be not only those present in the monolayer, in accord with pr evious considerations for other poorly adsorbed compounds (J. Cunningham et al. In Aquatic and Surface Photochemistry; Helz, G. R. et al., Eds.; Lewis Publishers: Boca Raton, FL, 1994, p 317). For (C-HA(m))(o) greater than or equal to 12 ppm, r(o) was also pseudo-first-order at least initially, and its decrease was consistent with HA UV-light absorption (inner-filter effec t) more than with the competition for the active species between HA and qui noline. For (C-HA(m))(o) = 6 ppm, r(o) was slightly, but significantly, hig her than without HA. This net, as yet unreported, favorable effect of HA, w hich was counterbalanced at higher (C-HA(m))(o) /[TiO2] ratios, was shown n ot to arise from HA photoexcitation and is tentatively suggested to be caus ed by the sequestration of a fraction of quinoline molecules close to the T iO2 surface owing to adsorbed HA.