Degradation of phenol in wastewaters via heterogeneous Fenton-like Ag/CeO2 catalyst
Graphical abstract
Introduction
In the last two decades wastewater treatment has become a very important concern and several approaches were considered to reduce organic contaminants in order to fulfil the more stringent laws and regulations [1], [2], [3]. Among the pollutants generated by industrial processes, a special attention is focused on the organic refractory compounds that are very difficult to remove by means of conventional technologies. This challenging task has led to the development of wastewater treatments such as catalytic or non-catalytic wet air oxidation (CWAO and WAO respectively) [4], [5] and wet peroxide oxidation (CWPO and WPO, respectively) [4], [6], [7], [8], [9], [10], [11]. WAO and CWAO require high temperature (400–600 K) and pressure (0.5–20 MPa) [4] and although they show good potential, mainly in terms of biodegradability enhancement [12], these techniques are limited due to their severe reaction conditions and high operating costs. The conventional CWPO is the homogeneous Fenton reaction in which hydrogen peroxide and an iron (II) salt are used under mild conditions. The main drawbacks of the Fenton process are the limited range of pH (3–5) in which the system is active and the separation of the homogeneous catalyst (soluble iron species) after the treatment [6], [8], [13]. Specifically, the range of optimal pH for reaction is a limitation also for the heterogeneous Fenton process, and for this reason there is no major benefit compared to homogeneous reaction. However, it is recognized that the important sludge production and the separation of the homogeneous catalyst (soluble iron species) after the treatment are the main disadvantages for homogeneous Fenton reaction that can be overcome using the heterogeneous process. These disadvantages have promoted the development of heterogeneous Fenton process and in recent years this technology has captured a special attention by the scientific community [7], [8], [14], [15], [16], [17], [18], [19].
Several studies were conducted over different solid catalysts, such as supported or unsupported metal oxide [20], [21], [22], [23], [24], [25], zeolites [26], [27], [28] and activated carbon or carbon nanotubes [29], [30]; however the efficiency of the process is still unsatisfactory compared to the requirements [4], [7].
Phenol is one of the most common organic water pollutants and it is an intermediate in the oxidation pathway of high molecular weight aromatic hydrocarbons. It is frequently used as a model pollutant for organic recalcitrant contaminant in advanced wastewater studies due to its toxicity and its poor biodegradability [4], and several data about its degradation in wastewater treatment are also available [4]. For this reasons, in the present work phenol has been chosen as model pollutant in the evaluation of the catalytic activity of silver-based catalysts over a Fenton-like process. Compared to WAO and CWAO, the mild conditions used in the heterogeneous Fenton-like process (ambient pressure and temperature lower than 100 °C), might also justify the use of more expensive catalysts. Silver is known to be an efficient partial oxidation catalyst and it is industrially used for the epoxidation of ethylene [31] and for the oxi-dehydrogenation of methanol to formaldehyde [32]. Other applications in which Ag has shown remarkable performances include NOx abatement, ammonia oxidation and oxidation of methane, carbon monoxide and organic volatile compounds [33]. The use of Ag deposited on ceria was also found to increase the rate of carbon gasification compared to other noble metals [34] establishing a sort of enhanced metal-support interaction. Our recent experience on silver-based catalysts for soot removal [35] and the use of gold-based catalysts in the degradation of phenol [36] stimulated us to investigate in detail the behaviour of silver deposited on metal oxides such as CeO2, ZrO2 and Al2O3 in the oxidation of phenol via a Fenton-like process with the aim of understanding the potential role of silver in this reaction.
Section snippets
Characterization of materials
Silver doped catalysts (5 wt%), were prepared by incipient wetness impregnation of CeO2, ZrO2 and Al2O3 (Grace Davison), with an aqueous solution of the nitrate salt, AgNO3 (Aldrich). They were dried at 373 K overnight and calcined in air at 773 K for 3 h. The catalysts are indicated as Ce5Ag, Zr5Ag and Al5Ag.
Textural and morphological characteristics were measured by conventional techniques: B.E.T., X-ray diffraction, high-resolution transmission electron microscopy as reported elsewhere [35].
Materials
Table 1 summarizes surface area characteristics of the bare supports and silver-supported catalysts. It can be seen that the addition of silver results in a decrease of surface area at the loading used in this study. This is typically observed upon addition to high-surface area supports of oxides possessing high specific weight and low porosity [38].
Reduction characteristics as measured by H2-TPR confirm the presence of Ag2O and Ag over ceria and zirconia/alumina respectively. Hydrogen
Conclusion
The degradation of phenol over silver doped systems has been investigated. In summary, a combination of morphological characterization with activity tests, shows that the addition of silver, as a surface promoter, modifies the mechanism of phenol oxidation observed in bare supports, by promoting a higher degradation under mild conditions. Ceria based catalyst results to be the more efficient system due to its capability to maintain the silver in a higher oxidation state, necessary to initiate
Acknowledgments
The authors are grateful to Acquedotto Poiana Spa, Cividale for the financial support.
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