# Removal of Methylene Blue by Metal Oxides Supported by Oily Sludge Pyrolysis Residues
## 1 Introduction  
Oily sludge is mainly composed of minerals, mineral oil, microorganisms, and water. According to surveys, the amount of newly generated oily sludge in oilfields in China is about 3.0 \(\times\) 10\({}^{7}\) t per year [1], and with the deepening of the oil development process and the continuous progress of the mining process, the output of oily sludge is also increasing. Oily sludge has been listed as hazardous waste and can pose great threats to human health and the environment, and its treatment and reuse have been continuing for years worldwide [2; 3; 4]. If oily sludge is directly landfilled or stacked, not only is the surrounding environment polluted, but oil resources are also seriously wasted. Nowadays, oily sludge treatment has become an urgent issue in China. After years of extensive research, both domestic and foreign scientific communities have formed avariety of treatment technologies, such as extraction treatment, conditioning treatment, solidification treatment, biological treatment, high-temperature pyrolysis treatment, and the incineration of oily sludge. Among them, the pyrolysis of oily sludge has the advantages of thorough treatment, a good volume reduction effect, a high resource recovery rate, a flexible recovery method, and the ability to solidify heavy metals in oily sludge [5]. Therefore, pyrolysis treatment is currently the sludge treatment technology with the greatest application prospects and development space [6; 7; 8]. However, previous research on pyrolysis treatment technology for oily sludge was mainly focused on the pyrolysis process and the yield, as well as the property analysis and utilization of the pyrolysis oil gas. Research on pyrolysis residue is less common, even though pyrolysis residue contains incompletely recovered oil resources and residual heavy metal elements, etc. ; thus, improper handling will cause secondary pollution. Therefore, the treatment and reuse of pyrolysis residues has become a bottleneck restricting the development of pyrolysis technology. Pyrolysis residue, mainly composed of ash and carbon, is the solid residue left in the reactor after the oily sludge is pyrolyzed. The mass fraction of carbon can reach 35-50%, which has good separation and recycling values [9]. Due to the abundant carbon elements in the pyrolysis residues, carbon nanotubes may even be formed during high-temperature pyrolysis [10]. However, the adsorption effect of pyrolysis residue itself is small compared with that of other adsorption materials. Therefore, pyrolysis residues can be loaded with metal oxides to obtain composites with high specific surface area and abundant pores [11; 12]. In addition, a large amount of printing and dyeing wastewater is produced in China. According to statistics, the amount of discharged textile dye wastewater in China exceeds 2 \(\times\) 10\({}^{9}\) t per year, accounting for approximately 11% of the national wastewater discharge, which has become a major threat to environmental security [13; 14]. Methylene blue is widely used in fabric dyeing as a cationic dye [15]. Because methylene blue has the characteristics of large output, strong toxicity, and high biological carcinogenicity [16; 17], it has caused great harm to ecological safety and human health [18; 19]. In recent years, many researchers have prepared adsorbents to remove methylene blue by loading metal oxides. For example, [PERSON] et al. [20] found that biochar/iron oxide composites can be successfully produced by green synthesis for the removal of methylene blue. [PERSON] et al. [21] used ZnO, MgO, and TiO\({}_{2}\) particles as additives to load into the pores of biochar used to prepare BC-T and BC-TMZ; the results show that the prepared BC-T and BC-TMZ exhibit high removal capability of methylene blue (MB). [PERSON] et al. [22] found that the preparation of spherical montmorillonite loaded with silver nanoparticles to remove methylene blue pollutants can achieve better reusability.