Insight into the Synergistic Effect of Adsorption-Photocatalysis for the Removal of Organic Dye Pollutants by Cr-Doped ZnO.

The adsorption of dye molecules is an important process for the photodegradation removal of dye pollutants. In this work, a semiconductor photocatalyst of Cr-doped ZnO nanorods (Cr-ZnO NRs) was synthesized, and its adsorption-photocatalysis synergy (APS) effect was investigated for anionic methyl orange (MO-) and cationic methylene blue (MB+). The detailed thermodynamic information (including adsorption maximum capacity qmax, adsorption equilibrium constant Kads and adsorption efficiency AE %) and dynamic information (including adsorption rate constant ka, degradation rate constant kd and degradation efficiency DE %) were obtained to evaluate the different reaction performances for MO- and MB+. With qmax(MB+) = 40.59 mg g-1 > qmax(MO-) = 15.95 mg g-1, ka(MB+) = 20.61 min-1 > ka(MO-) = 4.62 min-1, and AE(MB+) = 40% > AE(MO-) = 9%, Cr-ZnO NRs showed much superior adsorption performance for MB+ than MO-. With kd (MB+) = 0.0430 min-1 > kd (MO-) = 0.0014 min-1 and DE(MB+) = 98% > AE(MO-) = 20%, Cr-ZnO NRs also showed much superior photodegradation performance for MB+ than MO-. The APS mechanism of Cr-ZnO NRs is revealed to be multiple π-π interactions and stronger electrostatic attractions dominant for enhanced adsorption of MB+ and higher AE and more photocatalytic active species dominant for enhanced photodegradation of MB+. The APS was furthermore characterized and verified by zeta potential analysis, Fourier transform infrared investigation, and fluorescence imaging. The results indicate that Cr-ZnO NRs are promising adsorbent and photocatalyst candidates favorable for positive MB+ than negative MO-. Such an APS investigation can effectively help to improve the photodegradation treatment performance of photocatalysts for dye pollutant removal.