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.