Adsorbability and photocatalytic degradability of humic substances in water on Ti-modified silica.

From the viewpoint of development of a removal agent for humic substances, we prepared Ti-modified silica gel, SiO2-Ti, from titanium alkoxide and microsized silica gel. The prepared silica agent was investigated in adsorption and photocatalytic degradation of humic substances in water. In these experiments, four humic substances, commercially available Wako humic acid (Wako-HA), Nordic aquatic humic acid (Nordic-HA), Nordic aquatic fulvic acid (Nordic-FA), and Suwannee river fulvic acid (Suwannee-FA), were used, and Freundlich constants (KF and 1/n) and photodegradation rates were evaluated. Wako-HA, which has the highest aromaticity ratio [Ar-OH]/[COOH] and molecular weight, had the highest adsorbability (KF=17.5 (mg/g)(L/mg)(1/n), 1/n=0.67) but the lowest photodegradability (<80%). On the other hand, Suwannee-FA, which has the lowest aromaticity, [Ar-OH]/[COOH] ratio, and molecular weight, afforded lesser adsorbability (KF=7.1 (mg/g)(L/mg)(1/n), 1/n=0.39) but the highest photodegradability (>99%). Nordic-HA and Nordic-FA afforded adsorbabilities similar to that for Suwannee-FA, and medium photodegradabilities between those for Wako-HA and Suwannee-FA. Adsorption and photodegradation capacities of SiO2-Ti were improved with increased Ti content and phosphorescence emission amount, respectively. From XRD analysis, we found that the structure of anatase-type TiO2 features the Ti modifiers of SiO2-Ti. Therefore, humic substance molecules effectively interact with the Ti modifiers and are decomposed by OH radicals generated in situ. We hope that SiO2-Ti will be used as a photodegradation catalyst in water purification plants.