Catalytic ozonation of phenol in water with natural brucite and magnesia
Introduction
Recently, treatment of industrial wastewaters has attracted much attention of many countries’ governments and lots of environmental experts. The pollution of wastewaters is mainly owed to the presence of organic contaminants such as PAH's, dyes, halogenated hydrocarbons, phenolic compounds, etc. [1], [2]. Amongst these pollutants, phenols are widely used in petroleum, petrochemical, coal conversion, and pharmaceutical industries and confirmed as refractory pollutants by the US EPA for their highly toxic and carcinogenic properties [3], [4], [5].
To remove phenol and its compounds from water, kinds of methods are applied according to literatures appeared. They include: wet peroxide oxidation [1], [6], catalytic wet air oxidation [7], [8], [9], adsorption [5], and ozonation [10], [11]. Especially, ozonation is a widely used method to mineralize organic pollutants in water. However, phenols usually could not be oxidized efficiently in ozonation and their byproducts are always more toxic and resistant, such cases will lead to a low mineralization level [12]. To settle this problem, technologies of O3/UV, O3/H2O2 and catalytic ozonation, which are advanced oxidation processes (AOP), were proposed. In these processes, ozone could be decomposed to generate hydroxyl radicals, which have higher reaction rate constants with phenols [13], [14], but those agents such as UV and H2O2 always present some drawbacks: high costs, turbidity of water, etc. [15], [16], [17]. As another advanced oxidation process, catalytic ozonation could also achieve the same effects on phenols degradation. According to literatures, many catalysts have been used in catalytic ozonation, such as translation metal oxides [15], [18], [19], metals ions [20], [21]. Assalin et al. [18] have studied the degradation of phenol in catalytic ozonation with Mn(II) and Cu(II), and he concluded that these two ions could apparently accelerate the phenol degradation rate. Okawa et al. [21] revealed that the presence of metal ions would lead to certain catalytic effects on decomposition of phenol with ozonation in acetic acid. However, it is well known that those translation metal ions may lead to a secondary pollution in water.
Recently Takehira et al. [22] has found that Cu/Fe/Al mixed oxide catalysts prepared by calcining hydrotalcite precursor showed high activity for the mineralization of both phenol and oxalic acid by ozone in aqueous solution. And as we know, just as hydrotalcite, natural brucite is also a kind of low-cost and environmental friendly alkali mineral with a moderate solubility in water [23]. Thus in consideration of Takehira's work and our former studies [24], natural mineral brucite and its calcined-product (magnesia) without any purification are introduced as catalysts in ozonation of phenol in this paper. It was found that both brucite and magnesia have prominent catalytic effects on the degradation of phenol and the removal of chemical oxygen demand (COD) in water. On this basis, economically feasible ozonation processes on degradation of phenol in water were proposed.
Section snippets
Preparation and characterization of catalysts
Natural mineral brucite was exploited from Fengcheng of Liaoning province in China. Bulky brucite was crushed and sieved into 100 mesh size, and its main chemical composition is: Mg(OH)2, 94.71%; SiO2, 2.85%; others [24]. From the gravimetric curve of brucite (Fig. 1), it can be observed that the first dehydration of brucite completes at about 450 °C. Thus to obtain magnesia, 100-mesh brucite was calcined at 450 °C for 6 h. This powder sample was characterized by X-ray diffraction (XRD) (Fig. 2) on
Catalytic effects of catalysts
In order to test the effects of brucite and magnesia on phenol degradation and COD removal from water, series of ozonation experiments with and without catalysts were carried out at 25 °C. Fig. 3 shows the evolution of the residual concentration of phenol in ozonation experiments. It can be observed that both brucite and magnesia have acceleration effects on the degradation of phenol in some extent. For instance, at 15 min the decomposition rate of phenol has been enhanced by proportion of 55% in
Conclusions
Main conclusions gained in this paper are presented as following:
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Natural minerals brucite and magnesia have remarkable catalysis on the degradation of phenol and removal of COD.
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Neither phenol nor its decomposed-products are adsorbed much by brucite and magnesia during ozonation.
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pH values of water are alkali in catalytic ozonation process with brucite and magnesia, and the alkaline environment is the main factor leading to the catalysis.
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Two catalytic systems have different catalytic mechanisms:
Acknowledgements
The authors gratefully acknowledge the support from the National Natural Science Foundation of China (Nos. 50474036 and 20673056), the Environmental Protecting Science Foundation of Jiangsu Province (No. 2007016), and the Open Analysis Foundation of Nanjing University.
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