# Adsorption Separation of Various Polar Dyes in Water by Oil Sludge-Based Porous Carbon
## 1 Introduction  
Oily sludge (OS) is a typical solid waste generated in the petroleum processing industry during various stages, including extraction, transportation, storage, and refining [1]. However, with China's increasing demand for crude oil, the annual production of oily sludge is projected to exceed 6 million tons by 2024, accounting for an estimated 3.15% of the total output, culminating in a historical inventory of approximately 143 million tons [2]. Generally, OS contains harmful pollutants such as heavy metals, polycyclic aromatic hydrocarbons, and oily compounds [3, 4]. The accumulation of OS is not only limiting the development of the petroleum and petrochemical industries but also harmful to the environment [1, 5]. Dyes in wastewater have become a significant source of water pollution due to the textile, paper, and printing industries [6]. When released into the environment, organic dyes have various harmful effects on humans and aquatic organisms, and the biodegradation process is slow and complex, releasing possible carcinogenic products [7]. Therefore, treating and utilizing OS is a key issue for the petroleum processing industry to address. The commonly employed harmless treatment methods for OS include solvent extraction [8], ultrasonic irradiation [9], incineration [10], stabilization/solidification [11],and biodegradation [12; 13]. However, these technologies are limited by large land requirements, high costs, leakage, and air pollution, which hinder their development and adoption. Porous carbon (C) is a promising adsorbent material due to its high specific surface area and abundance of functional groups [14; 15; 16]. It has been successfully utilized to adsorb various pollutants, including bisphenol A [17], phenol [18], Cr-(VI) [14; 15], Pb(II) [16], Cd(II) [19; 20], phosphates [21], organic dyes [22; 23], and antibiotics [24]. Therefore, preparing oils sludge-based porous carbon (OSC) is an effective strategy for harmless treatment and resource utilization of OS. Compared to expensive activated C and other porous C materials, OSC is a more economical and environmentally friendly option [25]. However, OS lacks active functional groups, which are essential adsorption sites, thereby limiting the adsorption properties of the as-prepared OSC [17]. Typically, activating agents such as ZnCl\({}_{2}\), H\({}_{3}\)PO\({}_{4}\), and KOH are used to activate C-based materials, enhancing their surface area, porosity, and functional groups [26]. For instance, [PERSON] et al. [14] activated OS with KOH, achieving a remarkable 97.36% removal rate for Cd ions. Similarly, [PERSON] et al. [1] synthesized a novel pyrolyzed carbon (PC) by catalytically pyrolyzing OS with phosphoric acid at 411 \({}^{\circ}\)C for 59 min. The results demonstrated that the prepared PC exhibited excellent hierarchical porous properties and a high adsorption capacity for methylene blue (MB) of 322.89 mg/g. Similarly, [PERSON] et al. [18] prepared high-performance activated C with a microporous structure by activating OS with KOH, achieving a BET specific surface area of 2263 m\({}^{2}\cdot\)g\({}^{-1}\), total pore volume of 1.37 cm\({}^{3}\cdot\)g\({}^{-1}\), micropore volume of 1.004 cm\({}^{3}\cdot\)g\({}^{-1}\), and a phenol removal rate of 87.8%. The adsorption process was found to be spontaneous and exothermic. Moreover, ZnCl\({}_{2}\) is widely used as a chemical activator due to its effectiveness in producing a larger specific surface area [27]. [PERSON] et al. [28] examined the adsorption efficiency of ZnCl\({}_{2}\)-activated carbon derived from tamarind wood for removing Cr (VI) from water. The study revealed a maximum adsorption capacity of 28.019 mg/g, with the adsorption process being governed by a combination of pore diffusion, film diffusion, and particle diffusion. The results indicated that the adsorption was feasible, spontaneous, and endothermic, suggesting that activated tamarind wood is a viable material for effective Cr (VI) removal from aqueous solutions.