Enhanced Photocatalytic Activity Of Nano-scaled TiO₂by Natural Schorl

Nowadays，many kinds of semiconductor materials have been developed as thephotocatalysts for the degradation of organic pollutants, among which nano-scaledTiO₂is the most widespreadly used and environment-friendly one, due to its higherphotocatalytic activity, non-secondary popution and low cost. However, the lowquantum effieieney and narrow light response range confine its development.Therefore, in this work, natural schorl was employed to immobilize nano-scaled TiO₂on its surface and enhance TiO₂photocatalytic activity through its self-polarizationelectric field. The composite photocatalyst was obtained via Sol-Gel method andcharacterized by X-Ray diffraction （XRD） and scanning electron microscope （SEM）.The effect of preparation parameters include sintering temperature, schorl contentand sintering time on the photocatalytic activity of TiO₂/schorl were carefullyexplored. The process parameters of the discoloration of an active dye, MethylOrange （MO） was optimized by means of the response surface methodology （RSM）based on central composite design （CCD）. Then, the kinetics and thermodynamics ofthe photocatalytic system were carried out and analyzed.The results indicated that the TiO₂/schorl composite with3%schorl contentsintered at550oC for2.5h exhibited the optimum photocatalytic activity for thediscoloration of MO and discoloration ratio could reach91.2%. In comparation, thediscoloration of MO was investigated in different systems including UV/schorl,UV/TiO₂, UV/TiO₂-schorl and visible/TiO₂-schorl. The comparative resultssuggested that the photocatalytic activity could be enhanced by natural schorl andmechanism was discussed. Moreover, the COD removal of methyl orange couldreach100%under some certain condition, which implied MO could be completelymineralized and degraded into CO₂and H2O.The results of one-factor experiments showed that MO discoloration ratioincreased with reaction time increasing and MO initial concentration decreasing,while, there existed optimum values for photocatalyst dosage and solution pH. On this basis, the process optimization was implemented by RSM. The experimentaldesign, mathematical modeling and optimization were performed with the DesignExpert8.0.7.1software （Stat-Ease, Inc.） in this work. The significance of a secondorder polynomial model for predicting the optimal values of MO discoloration wasevaluated by the analysis of variance （ANOVA） and3D response surface plots forthe interactions between two variables were constructed. Based on the modelprediction, the optimum conditions for the photocatalytic discoloration of MO byTiO₂/schorl composite were determined to be3×10-3mM MO initial concentration,2.7g/L photocatalyst dosage, solution pH6.6and43min reaction time, with themaximum MO discoloration ratio of98.6%. Finally, a discoloration ratio of94.3%was achieved for the real sample under the optimum conditions, which was veryclose to the predicted one, implying that RSM is a powerful and satisfactory strategyfor the process optimization.Kinetic studies showed that the discoloration of MO using TiO₂/schorl asphotocatalyst followed first order reaction kinetics in different conditions. Theimpacts of MO initial concentration （C0）, photocatalyst dosage （S）, pH value andreaction temperature （K） on reaction rate constant （k） were studied, respectively, andthe mathematical relationships between k and C0, S, K, pH were constructed,respectively, as follows: k=C0-0.6327, k=exp（1.0521S-0.177S2-4.9763）, k=0.0996K-26.102and k=exp（0.7876pH-0.0559pH2-6.2034）. In addition, the relationshipbetween lnk and1/T was well linear, according with Arrhenius equation, and thereaction activation energy （Ea） was obtained as14.01kJ·mol-1. Then, the relatedthermodynamic parameters （△H,â–³S andâ–³G） were calculated.