Abstract
The efficient mineralization of high-concentration aqueous aniline (HCAA) is an issue needing to be resolved. In this study, a hybrid process of ozonation and electrochemical oxidation (ECO) was proposed for improving the mineralization of HCAA (1000 mg·L−1). The results indicated that chemical oxygen demand (COD) removal by the hybrid process was far greater than that of a single ozonation or ECO process, revealing that the hybrid process might avoid low efficiency in late ozonation and initial ECO. Thus, a subsequent combination effect clearly existed. In this hybrid process, ozonation stage time was selected as 60 min for optimal COD removal. The main products of the ozonation stage were maleic and succinic acids, with declining pH which was beneficial to the following ECO stage. Nitrite and nitrate formed during ozonation, which acted as electrolytes for the ECO stage, in which maleic and succinic acids were fully degraded and pH thus increased. Moreover, the aniline degradation mechanism of the hybrid process was deduced, demonstrating the superiority of this hybrid process. Finally, more than 95% COD removal was achieved, which met the COD limit requirement and achieved pH control simultaneously, according to the discharge standards of water pollutants for dyeing and finishing of the textile industry in China (GB 4287–2012).
Highlights
Aniline is commonly used by the chemical industry in several processes, such as for producing synthetic paints, antioxidants, herbicides, and varnishes [1,2,3]
The initial electrochemical oxidation (ECO) stage might result in electrode fouling as a consequence of the adsorption of polymeric oxidation products on their surfaces, which can lead to low chemical oxygen demand (COD) removal in initial times of single ECO (Figure 3) [36]
Examination of pH evolution over time during ozonation, ECO, and hybrid processes showed that, due to aniline oxidation products, including carboxylic acid, organic alde6 of 13 hyde, or alcohol, pH significantly decreased with ozonation reactions carried out without buffer conditions, which coincided with preliminary results (Figure 4) [37]
Summary
Aniline is commonly used by the chemical industry in several processes, such as for producing synthetic paints, antioxidants, herbicides, and varnishes [1,2,3]. This chemical exhibits significant accumulation in nature due to its low degradability [4,5,6]. Advanced oxidation processes (AOPs) have attracted increasing interest in recent years, owing to their high-degradation efficiency and short treatment period for organic wastewater treatment [19,20,21]. Among AOPs, ozonation and electrochemical oxidation (ECO) processes are widely used in wastewater treatment. ECO exhibits excellent degradation of refractory compounds in aqueous solution [25,26,27,28], but in the initial ECO stage, high-concentration aniline polymerization frequently occurs [29,30,31], which lowers anode oxidation ability. HCAA was degraded by a hybrid process, with the purposes of: (1) comparing the effects of the single processes and hybrid process; (2) investigate and optimize the key factors of the hybrid process; and (3) explore the decomposition mechanism of HCAA by the hybrid process
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