Synthesis of nitrogen-doped carbon nanotubes-FePO4 composite from phosphate residue and its application as effective Fenton-like catalyst for dye degradation
Graphical abstract
Introduction
Phosphate residue is formed after metal surface phosphorization and is regarded as hazardous waste due to its strong acidity, high phosphorus content and the presence of some heavy metal ions in the sludge from the phosphating solution (Juchi and Huang, 2010, Kuo, 2012, Narayanan, 2005, Petschel, 1996). Additionally, the accumulation of metals and excess PO43 − from phosphate residue can easily cause soil acidification and water eutrophication, resulting in serious environmental pollution (Lurling and van Oosterhout, 2013, Li et al., 2010, Meinikmann et al., 2015). The conventional methods for treating the phosphate residue are landfill, and solidification and vitrification, which are expensive and environmentally unfriendly, as a large amount of energy is consumed (Dogan and Karpuzcu, 2010, Meinikmann et al., 2015, Reeve, 2007, Xiao et al., 2008). More importantly, the useful metals contained in the sludge are discarded. Therefore, it is important that a more reliable and economical strategy be developed to recover the major components of phosphate residue. Yue et al. (2014) recently regulated the composition of phosphate residue by adding hydrochloric acid to change the pH value, as the components exhibited different phase states as a function of pH. When the pH value is 1–3, the FePO4 remains in the solid phase while other phosphates including Zn3(PO4)2 and Ca3(PO4)2 are dissolved. However, this leaching process using hydrochloric acid has a very low throughput. Thus, it is imperative to select a suitable acid for the leaching process. Considering the redox potential and the simplicity of the extraction process, phosphoric acid (H3PO4) could be used (Molina-Sabio et al., 1995, Shamsuddin et al., 2016).
On the other hand, the organic waste produced in industrial processes has been another significant environment and health issue. The Fenton-like catalytic process, as a green remediation technology used to degrade dyes using various catalysts, has drawn intensive attention (Khataee and Zarei, 2011, Sheydaei et al., 2014, Wang et al., 2005). In such a process, the transition metal (Fe, Mn, etc.) is considered an effective catalyst for generation of radicals, such as •OH, O2•−, •OOH, which can decompose organics into water, CO2 and easily degradable compounds. So far, a variety of heterogeneous iron-based materials have been widely used in Fenton-like technology due to their abundant resources, high catalytic activity and possibility of recycling after reaction (Cruz et al., 2017). Moreover, as the major component of phosphate residue, iron phosphate has many advantages in catalysis (Fedorková et al., 2010, Guo et al., 2015), water purification (Hamayun et al., 2014), electrochemical performance (Jegal et al., 2013, Yue et al., 2014) and environmental compatibility (Jiang and Jiang, 2012, Liu et al., 2008). For these reasons, it would be very powerful if the materials collected from waste residues could be used to remediate other environmental concerns. For example, Lin et al. (2010) fabricated silica-supported FePO4 catalysts for oxidative bromination of methane and achieved 50% methane removal. Nevertheless, the catalytic activity of FePO4 in the Fenton-like process is quite low, because the reduction reaction from Fe(III) to Fe(II) is limited by its kinetics, which need to be improved by utilization of ultraviolet (UV) light combined with carbon-based materials (Jung et al., 2012, Li et al., 2012, Nitoi et al., 2013, Peng et al., 2017).
Carbon-based materials, such as nitrogen-doped carbon nanotubes (NCNTs) (Wang et al., 2011, Yao et al., 2016a), graphene (Liu et al., 2013) and graphitic carbon nitride (Li et al., 2017, Ma et al., 2017), have recently been demonstrated to be promising Fenton-like catalysts, which have been used to efficiently improve the adsorption and catalytic performance in organics removal due to their high specific surface and excellent capability for electron transfer. Iron encapsulated in boron and nitrogen co-doped carbon nanotubes showed an increased activity as a catalyst in oxidative degradation of various organics (Yao et al., 2016b). It has been demonstrated that heteroatom doping (oxygen, nitrogen and sulfate) bonded and concentrated on the edges of the carbon layers results in the formation of unsaturated carbon atoms and defects at the edge of the carbon planes (Duan et al., 2015a, Duan et al., 2015b, Hlekelele et al., 2016). These sites with a high concentration of unpaired electrons play a key role in promoting the recycling of Fe(III)/Fe(II), thereby accelerating the production of •OH from H2O2.
Inspired by the above aspects, we explored an effective method to recover FePO4 from phosphate residue and further synthesize nitrogen-doped carbon nanotubes by the chemical vapor deposition method. To the best of our knowledge, such an approach has not been reported in any previous publications. The synthesized NCNTs-FePO4 composite has shown strong catalytic capability to remove organic dye in aqueous solution.
Section snippets
Materials
Phosphate residue was supplied by an auto components company in Zhejiang, China. Na2CO3 (analytical reagent (AR), ≥ 99.8%), hydrogen peroxide (AR, ≥ 30%), hydrofluoric acid (AR, ≥ 40%), phosphoric acid (AR, ≥ 85%) and diethylamine (AR, 99%) were obtained from Sinopharm Chemical Reagent Co. Ltd., of Shanghai, China. Rhodamine B (RhB) (AR, ≥ 95%) was purchased from Sigma Aldrich Co., USA. Other chemicals used in the present work were of analytical grade and used without further purification. Deionized
Recovery of FePO4 from phosphate residue
The composition of the phosphate residue is mainly consisted of FePO4, Zn3(PO4)2 and Ca3(PO4)2. In the acid leaching process, the purity and morphology of the obtained FePO4 strongly depend on the acid leaching time. XRD patterns (Appendix A Fig. S1) revealed that the diffraction patterns of samples can easily be ascribed to the monoclinic FePO4·2H2O phase (PDF#33-0666). The sharp and narrow diffraction peaks of the samples indicate that the material has excellent crystallinity (Zhou et al.,
Conclusions
In this study, an effective phosphate residue recovery method was proposed, which could effectively purify 98.14% of FePO4 and be used to fabricate a heterogeneous photo-Fenton-like catalyst. It was found that NCNTs-FePO4 has good catalytic performance in RhB degradation in combination with UV and H2O2. The kinetic studies indicated that the catalyst dosage, H2O2 dosage, and pH have a great influence on RhB oxidative degradation. The stability and recyclability results showed that the catalytic
Acknowledgments
This work was supported by the Science and Technology Development Foundation of Pudong New Area (No. PKJ2014-Z03), Dawn Program of Shanghai (No. 09SG54), Material Science and Engineering Key Subject of Shanghai Polytechnic University (No. XXKZD1601), and Gaoyuan Discipline of Shanghai-Environmental Science and Engineering (Resource Recycling Science and Engineering. The authors acknowledge the use of equipment, and the scientific and technical assistance of Research Center of Resource Recycling
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