Page - 158 - in Advanced Chemical Kinetics

4.3.Photocatalyticdegradation 4.3.1.Kinetics The0.5MZnO-FA-Sep is furtherused toanalyze the rateof reactions in the initial concentra- tion range of MO from 3.27 to 24.5 mg L�1(Figure 4A). The data from the photocatalytic activities isanalyzed for the followingrateexpression ln C0 C ¼kt (17) whereC0 is takenas theequilibriumconcentrationofMO(mgL �1) afterdarkadsorption.The linearity in the plot of ln(C0/C) versus t results confirmed the validity of pseudo-first-order kinetics.Therate-constants (k,min�1) are calculated fromtheslopesof the lines. For the initial concentration of 3.27 mg L�1, 88% degradation and 85% decolorization are achievedwithin20minwhereasonly44%(degradation)and43%(decolorization)MOremoval are obtained even at 70min for the concentration of 24.5mgL�1.Similarly, k values decrease from0.122to0.036min�1 (fordegradation)andfrom0.121to0.056min�1 (fordecolorization)as the initial concentrationofMOincreases from3.27 to24.5mgL�1(not shown). SincemoreMO moleculesareexpectedtobeadsorbedto thesurfaceof0.5MZnO-FA-Sepas the initial concen- trationofMOincreases, thesorptionofbothOH�andO2willbereduced.Thus, thenumberof bothphotogeneratedholesand•OHradicals is suppressedandresulted in lowerphotocatalytic efficiencies.Meanwhile,morephotonsareabsorbedinbulksolutionsbythehighconcentrations ofMO.Thiscreates theshortageofphotons toactivate thesupportedcatalyst. Additionally, Langmuir-Hinshelwoodmodel is successfully applied to estimate the relation- ship between the photocatalytic degradation rate and the initial concentration of organic contaminantsby the followingrearranged form[49,50]. 1 R ¼ 1 kKC0 þ1 k (18) where R is the rate of decolorization (or degradation), K is the adsorption coefficient ofMO ontothe0.5MZnO-FA-Sep(Lmg�1),k is thereactionrateconstant (mgL�1min�1).The linear correlation [0.997 (for degradation) and0.998 (for decolorization)] in the plot of (1/R) against (1/C0) proved the validity of themodel for the as-prepared supported catalysts (Figure 4B). Accordingly,bothadsorptionofMOto0.5MZnO-FA-Sep(K)andphotocatalyticdegradation ofMOby0.5MZnO-FA-Sep (k) are calculated as 0.066Lmg�1 and0.200mgL�1min�1 (for degradation)and0.177Lmg�1and0.154mgL�1min�1 (fordecolorization). 4.3.2.Comparisonwithbinary composites The ternary-supported catalyst (0.5MZnO-FA-Sep) is also comparedwith the binary ones (0.5MZnO-Sepand0.5MZnO-FA)and0.5MZnO(Figure4C).After20mindarkadsorption, supportedcatalystsshowlowerMOpercentagesremaininginthesolutionthanthe0.5MZnO (97% for decolorization and 91% for degradation). Meanwhile, 0.5MZnO-Sep displays the Advanced Chemical Kinetics158