Understanding and application of an electroplating sludge-derived catalyst with an active texture for improved NO reduction
Graphical abstract
Introduction
Electroplating sludge is produced after the treatment of electroplating wastewater, which is rich in various transition metals, such as Ni, Cu, Zn, and Fe, and organics, such as surfactants and brighteners (Du et al., 2015). Every year, there was 100,000 ton of electroplating sludge produced in China (Li et al., 2010). These sludges are precious resource of transition metals. In our previous report, electroplating sludge was successfully recycled as a hybrid of mixed-metal oxides and carbon (J. Zhang et al., 2014b). Generally speaking, metal‑carbon hybrids were reported to be effective in nitric oxide (NO) reduction, and their performances were closely related to their structures and components.
Firstly, the structure of our catalyst was similar to multi metal-supported carbon (J. Zhang et al., 2014b), and its temperature (300 °C) for 100% reduction of NO was 200 °C lower than that of activated carbon-supported metal (Illan-Gomez et al., 1995). A compact mixture of carbon and metals resulted in better NO removal than a loose contact (Castegnaro et al., 2013; Wang et al., 2008), i.e. binding manner of carbon-metal interface determined the temperature for effective NO reduction. Secondly, the chemically-mixed carbon inherent in the catalyst was more available (80%) for NO reduction than physically-mixed carbon (30%) (J. Zhang et al., 2014b). Different carbon had the order of activity as: activated carbon > charcoal > real soot > synthetic soot (Nejar et al., 2007; Xue et al., 2008), indicating that carbon species also controlled activity. Thirdly, the catalyst contained various transition metals (Fe, Ni, Cu and Zn), and multi-metal catalysts were more active than single-metal one (Li et al., 2009; Nejar et al., 2005; Nejar and Illán-Gómez, 2007). Therefore, the sludge-derived material should have special textural components and interface required for efficient NO reduction.
The texture has been well established as resulting from the geometry of particle inter-stacking, and the textural pattern is determined by particle performance and the manner of binding (Yun and Pinnavaia, 1995). Previous works showed that carbon and metal were the basic textural units of carbon-supported-metal catalyst for NO reduction (Singoredjo et al., 1993; L. Zhang et al., 2014a). However, the questions of what an active texture is and what its functions are for effective NO reduction have not yet been satisfactorily answered. Increasing the amount of active textural component and interface area was expected to result in a long-term NO removal at low temperatures. Therefore, it was quite necessary to identify a simple and effective method to produce catalyst with an enhanced amount of ideal texture.
The carbon component in carbon-supported catalyst can be obtained by pyrolyzing biomass, graphite and other organic-containing materials. It was reported that the porous structure of sludge adsorbed organics from wastewater (Devi and Saroha, 2016; Huang et al., 2017). Thus, organic wastewater can mediate the organic content in sludge. Furthermore, the pyrolysis of transition metals (such as Fe) with organics can produce a textural interface between the metal and the carbon (Williams and Horne, 1994; Zhang et al., 1999). Taken together, these properties indicate that the texture for effective NO reduction can be potentially modified by pyrolysis after immersing the sludge in organic wastewater.
In this work, we modified texture of electroplating sludge with aromatic species in phenol wastewater by immersing and subsequent pyrolysis, and found that the obtained catalyst had an improved efficiency of NO reduction. Since this research was a primary investigation, no O2 was involved in most NO-reduction experiments. However, the catalyst still showed its potential to resist the effects of O2 and SO2 on NO reduction. Besides, the catalyst was made up of multi-metal-oxide (Me) core and graphene-oxide-like shell (GOL), which formed continuous texture of O-Me-GOL. This active texture was highly effective in oxygen and electron transfer during NO reduction.
Section snippets
Material preparations
The electroplating sludge was supplied by Shanghai Xinsheng Electroplating Co., Ltd., China. The phenol wastewater (COD = 5965 mg/L, pH = 8.7) was collected from Shanghai Baosteel Group. It mainly contained 610 mg/L of phenol and 350 mg/L of ammonia nitrogen. The dried sludge (ES, 6.0 g) was immersed in the phenol wastewater (600 mL) under vigorous stirring for 24 h, and then collected by filtration and dried at 105 °C. The obtained solid was denoted as ESc. Besides, Fe(OH)3 ((Fe)) was
Activity enhancement by pretreatment of phenol wastewater
Fig. 1A compares NO reduction by ES and ESc as a function of temperature. All hybrids showed similar temperature-dependent NO-reduction profiles. For example, the reduction increased from 4.0 to 92.8% as the temperature increased from 200 to 400 °C when ES was used. The temperature at which 50% of NO was removed (T50) was 320 °C for ES (Table 1). After modification with phenol wastewater, the activity was obviously enhanced, especially between 100 and 300 °C. Specifically, the performance of
Conclusions
The reveal of active textures for effective NO reduction suggested the following two valuable implications. Firstly, the texture theory indicated the reason why graphene oxide/graphene containing catalyst showed enhanced activity, since graphene structure facilitated electron transfers between catalyst and target molecule. Secondly, the identification of active texture suggested a novel strategy for the high-value-added utilization of waste, including transition metal waste (such as
Acknowledgments
This project was financially supported by National Nature Science Foundation of China, NO. 21707087, 21477071 and 91543123. Research grant from Science and Technology Commission of Shanghai Municipality, NO. 16DZX2260601. Program for Innovative Research Team in University No. IRT13078. We appreciate Instrumental Analysis & Research Center of Shanghai University for the help of sample characterizations.
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