Interactions between photodegradation components
© Abdollahi et al.; licensee Chemistry Central Ltd. 2012
Received: 22 June 2012
Accepted: 28 August 2012
Published: 11 September 2012
The interactions of p-cresol photocatalytic degradation components were studied by response surface methodology. The study was designed by central composite design using the irradiation time, pH, the amount of photocatalyst and the p-cresol concentration as variables. The design was performed to obtain photodegradation % as actual responses. The actual responses were fitted with linear, two factor interactions, cubic and quadratic model to select an appropriate model. The selected model was validated by analysis of variance which provided evidences such as high F-value (845.09), very low P-value (<.0.0001), non-significant lack of fit, the coefficient of R-squared (R2 = 0.999), adjusted R-squared (Radj2 = 0.998), predicted R-squared (Rpred2 = 0.994) and the adequate precision (95.94).
From the validated model demonstrated that the component had interaction with irradiation time under 180 min of the time while the interaction with pH was above pH 9. Moreover, photocatalyst and p-cresol had interaction at minimal amount of photocatalyst (< 0.8 g/L) and 100 mg/L p-cresol.
These variables are interdependent and should be simultaneously considered during the photodegradation process, which is one of the advantages of the response surface methodology over the traditional laboratory method.
Advanced oxidation processes (AOPs) are physicochemical procedures, which designed to remove environmental organic and inorganic pollution. Photocatalysis, the current interest of AOPs, is applied for decontamination the pollutions [1–4]. The photocatalysis, under suitable light illumination, produces hydroxyl radical (●OH) and hole (h+) which are powerful and non-selective oxidants to degrade a variety of organic compounds [5–7]. Since the photocatalytic degradation (photodegradation) is dependent on several parameters including irradiation time, pH, photocatalyst and pollution concentration, it need to study the relationship between the variables during the process [8, 9]. In the design of experiments, the independent variables are controlled to determine the relationship to an observable phenomenon . The single variable (one-variable-at-a-time) method considers the photodegradation process as a projection while the multivariate method generalizes the observation of the photodegradation . Therefore, the multivariate, which, increases the dimension of the system and produces more generalized results is preferred in comparing with the single variable approach. Recently the semi-empirical methods were used as an efficient technique to apply multivariate modeling for the photodegradation by response surface methodology (RSM) [12–18], however, no study has yet been conducted on the parameters interaction. This work looks at the parameters interaction of p-cresol photodegradation as a sample of organic pollution in present of ZnO as a photocatalyst by the RSM. The interaction between irradiation time, pH, photocatalyst loading, and p-cresol concentration (as variables) were investigated during the photodegradation process.
Independent variables and their levels employed in the central composite design
Level of Variables
Analysis of the results
The model validation
Interaction of variables
Figure 3 shows, the simultaneous behavior of p-cresol photodegradation variables during irradiation time. It may be observed from Figures 3a, b and c, that there are no clear interactions between irradiation time with pH, irradiation time with p-cresol and pH with p-cresol. Therefore, these variables can be independently investigated.
The study of four photodegradation variable’s behavior including irradiation time, pH, amount of photocatalyst and p-cresol concentration, experiments were designed by central composite design (CCD). The design was performed to obtain actual responses. The actual responses were fitted with linear, two factor interactions (2FI), cubic and quadratic model by RSM to obtain an appropriate model. The model was validated by analysis of variance (ANOVA). The obtained visual results from the validated model demonstrated that there is no clear interaction between irradiation time with pH, p-cresol with irradiation time, and pH with p-cresol. Therefore, these variables can be independently investigated. However, the component of photocatalyst amount interacted with other variables as following. The component had interaction with irradiation time under 180 min of the time while the interaction with pH was above pH 9. Moreover, photocatalyst and p-cresol had interaction at minimal amount of photocatalyst (< 0.8 g/L) and 100 mg/L concentration of p-cresol. Therefore, these variables should be simultaneously considered during the photodegradation process.
The authors would like to express acknowledgement to Ministry of Higher Education Malaysia for granted this project under Research University Grant Scheme (RUGS) of No. 04-01-04-SF0470.
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