Electrochemical degradation and extraction capability of magnesium wastes in sewage treatment
Introduction
Magnesium alloys have received much attention in the automotive, aerospace, electronics, and biomedical industry due to their high specific strength, light weight, and natural biodegradation [1], [2], [3], [4], [5], [6]. However, as the use of Mg-based materials continues to increase, more Mg-related wastes are generated by the industry causing potential environmental threat [7]. Therefore, in order to protect the environment and utilize natural resources more effectively, it is desirable to identify means to recycle Mg wastes.
Dissolution of magnesium in an aqueous solution proceeds by the following reaction: Mg + 2H2O → Mg2 + + 2OH– + H2 ↑ [8], [9]. Hydrogen, one of the products, is a clean energy source to minimize the use of fossil fuels, the so called low-carbon strategy. In fact, there have been recent reports on the use of Mg wastes to generate hydrogen via degradation in NaCl solutions and seawater added with citric acid [10], [11]. Furthermore, dissolution of Mg generates Mg2 + and OH− besides H2. When those ions are saturated in the solution, magnesium hydroxide precipitates form. Magnesium hydroxide has many interesting properties such as large surface area, high endothermic decomposition temperature, and smoke suppression. It is often used in flame-retardant composites and the removal of azo-dye pigment from sewage is based on the adsorption/aggregation mechanism [12], [13], [14], [15]. Hence, it is possible to utilize the degradation products of Mg alloy wastes in sewage treatment.
Different from stainless steels in chloride-containing aqueous solutions, magnesium alloys do not show appreciable passivation in the anodic polarization curves. Consequently, the anodic dissolution current density increases rapidly and normally has a large value even though the applied voltage is quite small [16], [17]. On the other hand, the phenomenon can be exploited in that low-voltage anodic polarization can be applied to degrade Mg wastes. In this pilot study, AZ31 Mg alloy samples are immersed in a sodium chloride solution and a constant voltage of 500 mV is applied. The solid degradation product consists of mainly Mg(OH)2 which can be subsequently utilized to remove methyl orange (MO) from simulated organic dye sewage. The method enables recycling of Mg wastes to lessen the environmental impact and utilize natural resources more effectively.
Section snippets
Experimental details
The AZ31 Mg alloy (Mg-3 wt.%Al-1 wt.%Zn) was cut into square samples (20 × 20 × 5 mm3), mechanically ground by 1200 grit SiC paper, ultrasonically cleaned in pure alcohol, and dried by nitrogen. The electrochemical experiments were conducted on a Zahner Zennium electrochemical workstation using the three-electrode technique. The Mg sample with an exposed surface area of 0.5 cm2 was immersed in a 3.5 wt% NaCl solution. The Ag/AgCl electrode (saturated KCl) was the reference electrode and a platinum bar
Results and discussion
AZ31 is a common commercial magnesium alloy and it has been shown that the anodic polarization current density increases rapidly and becomes larger in small-scale polarization relative to the free corrosion potential [18], [19], [20]. It thus means that accelerated electrochemical degradation can be carried out with high efficiency and low energy consumption. Fig. 1 depicts the current evolution of the AZ31 Mg alloy in the NaCl solution under a constant voltage. As expected, the degradation
Conclusion
A novel strategy is proposed to turn Mg wastes into useful extraction agents to lessen the environmental impact and an electrochemical method inspired by the electrochemical corrosion behavior of Mg alloy is demonstrated. In this pilot study, AZ31 Mg alloy is immersed in a sodium chloride solution and a small voltage (500 mV) is applied to accelerate degradation. The degradation products are composed of mainly nanostructured magnesium hydroxide which is utilized in the simulated sewage
Acknowledgments
This work was supported by City University of Hong Kong Applied Research Grant (ARG) No. 9667122, Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No. CityU 11301215, Natural Science Foundation of China No. 51301004, and Shenzhen Science and Technology Research Grant No. JCYJ20150828093127698. The authors also thank Mr. Xiang Peng (City University of Hong Kong) for giving us precious suggestions in our experiments.
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2018, Materials and DesignCitation Excerpt :Consequently, exploring new approaches to developing high-efficiency catalysts for the degradation of organic contaminants have become the focus of study in the wastewater treatment field. Over the years, considerable degradation approaches and catalysts have been reported, such as flocculation by catalysts, adsorption onto active materials and catalytic or photocatalytic degradation by nanostructured materials [4–11]. Current approaches to degrading or decolorizing the organic wastewater contaminants generally include the reduction reaction or Fenton oxidation process by zero-valence metals, especially zero valent iron powders, which have attracted increasing industrial interests due to their low cost, efficient degradation activity and nontoxicity [12–17].