Comparative Study of Phenolics Degradation Between Biological and Photocatalytic Systems

Phenol and phenol compounds are toxic organics that can be found in many industrial wastewaters. Biological wastewater treatments are considered to be the most convenient methods owing to their efficiency and low economic cost. Nonetheless, many organic pollutants are refractory to bacterial degradation. Photocatalytic methods can be an interesting alternative as pretreatment to improve biodegradability and reduce toxicity of industrial effluents. The goal of this study was to compare and combine $TiO2$-photocatalysis with constructed wetlands to obtain a low-cost method for the treatment of phenolic wastewater. The degradation of phenol was studied by means of $TiO2$-photocatalysis (solar and UV-lamp) in batch reactors. The degradations of phenol and two of its photocatalytic degradation intermediates, catechol and hydroquinone, were studied in wetland reactors with and without two wetland plants: common reed (Phragmites australis) and papyrus (Cyperus alternifolius). The application of pseudo first-order kinetics to the elimination of phenol in the wetland reactors provided high correlation coefficients $(R2=0.85–0.99)$⁠, allowing the comparison of the biological and photocatalytic methods. Although higher concentrations of phenol $(250–400mg∕l)$ could be treated, the elimination of $50ppm$ was usually accomplished in batch experiments in less than $15h$⁠, the time in which low or nil solar radiation is available for $TiO2$-photocatalysis. As a consequence, this concentration can be considered to be the upper limit for the wetland influent. The degradations of catechol and hydroquinone showed higher rate constants $(0.2–0.4h−1)$ than that of phenol (about $0.15h−1$⁠), particularly in the reactor with common reed $(1–2h−1)$⁠. The degradation of phenol by the photocatalytic methods was three to four times faster than those obtained with the wetland reactors. By using solar $TiO2$-photocatalysis, concentrations of phenol up to $100ppm$ were reduced down to $16ppm$ and $27ppm$ of phenol and hydroquinone, respectively, in about $7h$⁠. However, it was toxic. When this sample was continuously $(38ml∕min)$ added to wetland reactors with common reed, phenol and hydroquinone concentrations were below their detection limits (⁠$1ppm$ and $2ppm$⁠, respectively). Solar $TiO2$-photocatalysis is a promising technique for the treatment of phenol but its application is limited to daytime periods with appropriate weather conditions. Constructed wetlands can also eliminate phenol and phenolic compounds without these limitations, but the toxicity of the influent must be as low as possible. The combination of both methods can provide a low-cost method for the treatment of phenolic wastewater.