Ammonia-nitrogen wastewater generated from complex refinery and chemical enterprises is detrimental to environment, as they can cause not only an offensive smell, but also eutrophication that is fatal to aquatic life. With poor biodegradability, the wastewater in such industry is often characterized as high ammonia-nitrogen concentration of 50–200 mg/L and high salt content (calculated by chloride ion) of 1g/L-25.59g/L(Yin-fang 2010). Therefore, it is difficult for traditional sewage treatment plants to treat this wastewater, which increases the huge pressure of environmental control and economic cost of the refinery and causes potential danger to the environment.
The commonly used technologies for treating high chlorine ammonia-nitrogen wastewater mainly include biological degradation (Mulder et al. 2006; Kim et al. 2008; Zhang et al. 2008; Ruiz et al. 2003; Fang et al. 1993; Schmidt and Bock 1997; Li et al. 2018; Strous et al. 1997; Jing and Lin 2004; Zhang et al. 2013), physical-chemical method(Kurama et al. 2002; Ozturk et al. 2003; MIN et al. 2009), physical adsorption (Huo et al. 2012; Zhou and Boyd 2014), advanced oxidation (Lin et al. 2009; Lin et al. 2009; Wang et al. 2008; Shavisi et al. 2014; Zhou et al. 2016), and chemical precipitation (El Diwani et al. 2007; Huang et al. 2017; Huang et al. 2014; Shaban et al. 2017). These technologies have great constraints for treating wastewater containing high level of ammonia-nitrogen. Biological methods for treating ammonia-nitrogen wastewater rely largely on microorganisms, which have stringent requirements for environmental conditions, such as wastewater toxicity, C/N ratio, salt concentration, and operational parameters(Zhang et al. 2018). Besides, wastewaters containing high chlorine ammonia-nitrogen are easy to dehydrate microorganisms, making them lose the cell activity and integrity. Advanced oxidation technologies, such as ultrasonic and photocatalysis, usually require high costs, high energy consumption and complex operational skills. With respect to adsorption technology, although the operating conditions are relatively simple, there are a variety of competitive ions that can form adsorption with ammonia-nitrogen in the high chlorine ammonia-nitrogen wastewater, resulting in poor removal efficiency of ammonia-nitrogen, and the desorption of the adsorbent itself is also a difficulty. The chemical precipitation uses magnesium compounds and chemical substances containing phosphate to react with ammonia-nitrogen to generate magnesium ammonium phosphate and remove the ammonia-nitrogen. This method will use a large number of magnesium compounds and phosphorus-containing substances, but the reaction is not complete, which will make a large amount of phosphorus into the environment, causing re-pollution of water bodies.
Electro-catalytic technology that can achieve organic degradation through electron transfer has attracted increasing attention as a promising approach for wastewater treatment, a owing to its easy operation,, minimal generation of secondary pollution, remote control, a wide range of operating temperature, rapid start-up, small foot-print, and high removal efficiency of contaminants (Ding et al. 2018; Ye et al. 2016). In particular, electrochemical processes exhibit a superiority in eliminating nitrogenous contaminants to biological processes since NH4+-N is readily oxidized at the anode while nitrates and nitrites can be reduced at the cathode(Li et al. 2010). At the same time, hydrogen is produced on the cathode, which can be collected as an energy source. Although electrochemical technology has disadvantage of high power consumption, compared with physical, chemical, and biological technologies, it exhibits superiority in the treatment of ammonia-nitrogen wastewater with high toxicity, poor biodegradability, and high salt content.
As one of the major mechanisms for electrochemical ammonia removal from wastewater, direct anodic oxidation proceeds via sequential dehydrogenation of adsorbed ammonia on electrode surfaces when a sufficiently positive potential was applied to the electrode. Therefore, electrode materials are the key factors in the treatment of ammonia-nitrogen wastewater by electro-catalytic technology. The majority of research on the direct oxidation of ammonia has been carried out using platinum (Pt) (Duong et al. 2018). The modification of Pt catalysts with other electrode materials, such as Ti/SnO2 + Sb/PbO2, has been well documented for efficient removal of ammonia-nitrogen in coking wastewater(Ma et al. 2012). Electrochemical technology using Ti/RuO2-IrO2 as an anode can synergically remove total ammonia-nitrogen (TAN) and nitrite in circulating aquaculture wastewater(Ruan et al. 2016). SnO2 electrode was found to be efficient for removal of organic matter, however, its lifespan was short; Similarly, toxic Pb can be dissolved from PbO2 electrode, resulting in secondary pollution of wastewater.
RuO2-IrO2-based catalyst is considered as an excellent electrode material in terms of ammonia oxidation, owing to its stability, high electrical conductivity, and excellent chlorine-evolution performance(He et al. 2013). However, the RuO2-IrO2 electrode has relatively poor corrosion resistance, and its service life is still short in industrial practical application. By contrast, Pd is not only more cost-effective, but also has better corrosion resistance, compared to Pt. Pd-SnO2/C has been reported to exhibit good electrical conductivity and electro-catalytic performance(Hameed 2017).With this regard, the modification of Ru-Ir electrode with Pd-Sn has been explored by combining the catalytic performance of Sn and corrosion resistance of Pd, to significant enhance the electro-catalytic performance of the Pd-Sn-based catalyst derived from Ru-Ir electrode. However, to date, the effect of modification of Ru-Ir electrode with Pd-Sn on electro-catalytic treatment of high chlorine ammonium-nitrogen wastewater has not been studied.
Furthermore, in the process of electro-catalytic oxidation of the high chlorine ammonia-nitrogen wastewater, reaction conditions can affect the performance of electro-catalytic ammonia-nitrogen, which can provide theoretical guidance for electro-catalytic treatment of high chlorine ammonia-nitrogen wastewater. In general, the main factors affecting the industrial application of electro-catalytic treatment of high chlorine ammonia-nitrogen wastewater include chloride ion concentration, initial concentration of ammonia-nitrogen, current, voltage, electrode material, pH, temperature, types and content of organic matter, suspended matter etc(Chen et al. 2007; Candido et al. 2013; Liu et al. 2009). For instance, Ma et al. prepared the Ti/SnO2 + Sb/PbO2 anode to investigate the effects of operating parameters including current density, anode material, pH and the concentration of chloride by orthogonal array experimental design and reported that the anode material, current density, and chloride concentration have significant influences on the ammonia removal(Ma et al. 2012). Kim et al. evaluated the effects of the pH and the chloride ion in the solution, kinds of anodes on the electrolytic decomposition of ammonia and revealed that the performances of the electrode were totally in the order of RuO2 ≈ IrO2༞Pt in both the acid and alkali conditions and the ammonia decomposition was the highest at a current density of 80 mA/cm2(Kim et al. 2006). Furthermore, the Ti/RuO2/IrO2, the Pt/ITO electrode, and the BDD anode were also applied to the treatment of ammonia-nitrogen wastewater(Wang et al. 2016; Liu et al. 2012; Alcocer et al. 2018). However, the effect of chloride ion concentration, pH, and temperature on the electro-catalytic treatment of high chlorine ammonia-nitrogen wastewater by the Pd-Sn modified Ru-Ir electrode remains largely unknown.
Within this context, this study is aimed to evaluate the electro-catalytic effect of the modification of Ru-Ir electrode with Pd-Sn on treating high chlorine ammonia-nitrogen wastewater. More specifically, this study is aimed to : (1) investigate the chlorine evolution performance using the modification of Ru-Ir electrode with Pd-Sn; (2) evaluate the influence of operational conditions on the removal of ammonia-nitrogen, kinetics and apparent activation energy; (3) explore the possible denitrification process in the electro-catalytic treatment for high chlorine ammonia-nitrogen wastewater.