Photochemical degradation and mineralization of 4-chlorophenol

Abstract

Intention, Goal, Scope, Background

Since the intermediate products of some compounds can be more toxic and/or refractory than the original compund itself, the development of innovative oxidation technologies which are capable of transforming such compounds into harmless end products, is gaining more importance every day. Advanced oxidation processes are one of these technologies. However, it is necessary to optimize the reaction conditions for these technologies in order to be cost-effective.

Objective

The main objectives of this study were to see if complete mineralization of 4-chlorophenol with AOPs was possible using low pressure mercury vapour lamps, to make a comparison of different AOPs, to observe the effect of the existence of other ions on degradation efficiency and to optimize reaction conditions.

Methods

In this study, photochemical advanced oxidation processes (AOPs) utilizing the combinations of UV, UV/H₂O₂ and UV/H₂O₂/Fe²⁺ (photo-Fenton process) were investigated in labscale experiments for the degradation and mineralization of 4-chlorophenol. Evaluations were based on the reduction of 4-chlorophenol and total organic carbon. The major parameters investigated were the initial 4-chlorophenol concentration, pH, hydrogen peroxide and iron doses and the effect of the presence of radical scavengers.

Results and Discussion

It was observed that the 4-chlorophenol degradation efficiency decreased with increasing concentration and was independent of the initial solution pH in the UV process. 4-chlorophenol oxidation efficiency for an initial concentration of 100 mgl^-1 was around 89% after 300 min of irradiation in the UV process and no mineralization was achieved. The efficiency increased to >99% with the UV/H₂O₂ process in 60 min of irradiation, although mineralization efficiency was still around 75% after 300 min of reaction time. Although the H₂O₂/ 4-CP molar ratio was kept constant, increasing initial 4-chlorophenol concentration decreased the treatment efficiency. It was observed that basic pHs were favourable in the UV/H₂O₂ process. The results showed that the photo-Fenton process was the most effective treatment process under acidic conditions. Complete disappearance of 100 mgH of 4-chlorophenol was achieved in 2.5 min and almost complete mineralization (96%) was also possible after only 45 min of irradiation. The efficiency was negatively affected from H₂O₂ in the UV/H₂O₂ process and Fe²⁺ in the photo-Fenton process over a certain concentration. The highest negative effect was observed with solutions containing PO₄ ^≡ ions. Required reaction times for complete disappearance of 100 mgl^-1 4-chlorophenol increased from 2.5 min for an ion-free solution to 30 min for solutions containing 100 mgl^-1 PO₄ ^≡ ion and from 45 min to more than 240 min for complete mineralization. The photodegradation of 4-chlorophenol was found to follow the first-order law.

Conclusion

The results of this study showed that UV irradiation alone can degrade 4-CP, although at very slow rates, but cannot mineralize the compound. The addition of hydrogen peroxide to the system, the so-called UV/H₂O₂ process, significantly enhances the 4-CP degradation rate, but still requires relatively long reaction periods for complete mineralization. The photo-Fenton process, the combination of homogeneous systems of UV7H₂O₂/Fe²⁺ compounds, produces the highest photochemical elimination rate of 4-CP and complete mineralization is possible to achieve in quite shorter reaction periods when compared with the UV/H₂O₂ process.

Recommendations and Outlook

is more cost effective to use these processes for only purposes such as toxicity reduction, enhancement of biodegradability, decolorization and micro-pollutant removal. However, the most important point is the optimization of the reaction conditions for the process of concern. In such a case, AOPs can be used in combination with a biological treatment systems as a pre- or post treatment unit providing the cheapest treatment option. The AOP applied, for instance, can be used for toxicity reduction and the biological unit for chemical oxygen demand (COD) removal.