Effect of calcination temperature on the catalytic performance of CoFe2O4/Nitrogen doped sludge based activated carbon in activation of peroxymonosulfate for degradation of coking wastewater

A novel supported heterogeneous magnetic catalyst CoFe2O4/N-doped sludge based activated carbon (CoFe2O4/N-SAC) was prepared by polymer network gel method for the first time. The physicochemical properties of the materials were characterized by means of XRD, SEM, TEM, VSM and XPS techniques. The prepared catalyst is applied to the heterogeneous activation of peroxymonosulfate for degradation of coking wastewater, and the effect of calcination temperature on the catalytic activity was investigated. The result reveals that the catalyst shows the highest catalytic activities under the calcination temperature is 800 °C with the TOC removal rate of coking wastewater is 84.31%.


Introduction
Over the past period of time, the advanced oxidation processes (AOPs) based on peroxymonosulfate (PMS) have been widely concerned for degradation of coking wastewater because the sulfate radical (SO 4 − •) is an oxidizing agent with a high ability to oxidize organic pollutants due to its higher oxidation-reduction potential and longer half-life period (the oxidation-reduction potential and half-life period of SO 4 − • are 2.6-3.1eV and 4s, respectively), and its use avoids some shortcomings of traditional Fenton and ozone, which both use hydroxyl radicals (•OH) to degrade organic pollutants with the half-life of •OH is short and the redox potential is low (the half-life and redox potential of •OH were 1µs and 1.8~2.7 eV, respectively) [1][2] . Transition metal ions combined with persulfate to form SO 4 − • is a highly efficient method for the degradation of organic pollutants and Co 2+ is the best catalyst for activating persulfate [3] .
However, the detrimental impact of Co 2+ lost with the effluent on environment and human health is a serious concern. Thus it is essential to develop heterogeneous Co catalysts [1] . coupled with PMS have been investigated [4][5] and the spinel-structured nano-ferrite CoFe 2 O 4 has been widely used in wastewater treatment because of its excellent catalytic performance [6] . Activated carbon has been used as a carrier for heterogeneous catalysts because of its huge specific surface area [7][8][9][10][11][12] . In addition, the research has been shown that doping heteroatoms such as N into the carbon network can create more active sites and bring new properties such as higher selectivity towards reduction reaction [13][14] . Sludge contains a large amount of organic matter, which can be used as raw material for the preparation of sludge based activated carbon (SAC) [15] . China), and DI water was used throughout.

Characterization of catalyst
The crystal structure of catalysts was analyzed by a powder X-ray diffractometer (XRD, RigakuD/max-2000) with monochromatic Cu Ka radiation (45 kV, 50 mA).
The morphology of the catalysts was characterized by a    [16] .

XPS
In order to investigate chemical states and the compositions in the content on the surface of the SAC Relatively intensity (a.u.) Binding energy (eV) nitrogen and nitrogen oxides [17] , respectively. In addition, the content of pyridine nitrogen increases with the increase of temperature when the calcination temperature is lower than 800 °C, but declines when the calcination temperature further increases to 900 °C, while the content of pyrrolic nitrogen and graphitized nitrogen increases with the increase of calcination temperature. This may be attributed to the transformation from pyridine nitrogen to pyrrolic nitrogen at higher temperature [18] .   [19] . The minimized coercivity ensures that the CoFe 2 O 4 /N-SAC does not become permanently magnetized after exposure to an external magnetic field, which in turn permits the composite particles to be re-dispersed without aggregation when the magnetic field is removed [20] . The inset in Fig.4 affirms that the composite could be easily separated and reused from solution by applying an external magnetic field, which is beneficial to its practical application in wastewater treatment.  The results show that the effect of the calcination temperature on TOC removal is significant. The TOC removal rate of the coking wastewater increases from 50.34% to 80.53% with the increase of temperature from when the temperature is further increased to 900 o C.
Cobalt ferrite has an ideal inverse spinel structure that contains different types of crystal faces in which the (111) plane exposes exclusively tetrahedral Fe 3+ sites, the (100) face expose the octahedral Co 2+ /Fe 3+ sites. The chemical components exposed on the surface depended on the  [22] demonstrated that pyridine nitrogen, containing a pair of lone electron, could effectively convert the valence band structure of carbon materials and enhance the π state.
Long et al [23] found that graphitized nitrogen was the active site of catalysis. Sun et al [13] proved that graphitized N play a dominant role for PMS activation by theoretical calculation. Guo et al [14] proved that pyridine nitrogen was the main active site in the oxygen reduction reaction of nitrogen-doped carbon materials by template catalyst.
In this experiment, the result shows that pyridine nitrogen was the highest at the calcination temperature of 800 o C, and the catalytic activity is best at that calcination temperature, which could be concluded that the pyridine-N is the main active site in this catalytic reaction and this conclusion is consistent with Guo's report.