Catalytic ozonation of phenol in water with natural brucite and magnesia

Abstract

Natural brucite and magnesia were applied as catalysts in catalytic ozonation of phenol in this work. It was found that both brucite and magnesia had remarkable accelerations on degradation of phenol and removal of COD in water. On this basis, effective and feasible routes for catalytic ozonation of phenol in water were proposed. The influence of initial pH value, radical scavengers and reaction temperature were investigated. The results revealed that there were different ozonation mechanisms in two systems: molecular ozone direct oxidation mechanism was proved in catalytic ozonation with brucite, and hydroxyl radical mechanism was demonstrated to play a main role in catalytic ozonation with 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.