Entrapped zinc oxide and titania nanoparticles in calcium alginate beads for the removal of Methylene Blue (MB). Adsorption properties and photocatalytic stability

In this work, zinc oxide (ZnO) and titania (TiO2) nanoparticles were entrapped in calcium alginate (Ca-Alg) beads to form a composite photocatalytic adsorbent material. The adsorption and photocatalytic properties of ZnO and TiO2 Ca-Alg beads were investigated for different amount of encapsulated nanoparticles. In particular, 2% w/w, 5% w/w, 7% w/w of ZnO and 2% w/w, 5% w/w, 10% w/w of TiO2 calcium alginate beads were synthetized and Methylene Blue (MB) was selected as target pollutant. Adsorption batch test revealed the existence of a maximum removal percentage of MB (at equilibrium conditions) according to the nanoparticles concentration: 54% and 40% of MB removal was obtained for 2% w/w of ZnO and 5% w/w of TiO2 respectively. Moreover, it was also observed that the kinetic of the process improves increasing the amount of nanoparticles in the beads. The pseudo-first-order kinetic model was used for fitting and it reproduces very well the behavior of experimental data. Photocatalytic batch tests revealed that, in the range of time analysed, negligible photocatalytic activity was recorded for ZnO Ca-Alg beads, while an intense photocatalytic activity was observed for TiO2 Ca-Alg beads. In the latter case the stability of the alginate structure was compromised as detected by a spectrophotometric analysis.