# Removal of Methylene Blue from Aqueous Solution by Bone Char
## 1 Introduction  
Methylene blue (MB) is a cationic thiazine dye, widely used as a chemical indicator, dye, biological stain, and drug [1; 2; 3]. However, unbridled emissions of methylene blue into surface and underground water will harm human beings and other creatures because of its toxicity to neural tissue, reproductive system, and skin [4]. Therefore, the removal of methylene blue from effluent is important for environmental and human health.  
In recent years, photocatalysis has been considered as an effective method for the oxidization of methylene blue into nontoxic products [5]. With large numbers of surface active sites, nano-sized photocatalysts have been extensively studied [6]. However, the suspension composed by nano-sized photocatalysts and methylene blue solution can block light penetration, reducing the photocatalytic efficiency. In addition, nanoparticles are difficult to reclaim, so the secondary contamination may hinder their large-scale industrial application. To solve these problems, millimeter-sized porous materials such as activated carbon [7], diatomite ball [8], vermiculite [9], and molecular sieve 5A [10] have been developed to support nano-sized photocatalysts. Moreover, the application of nanofiltration membranes [11; 12] may be a potential solution for the separation of nano-sized photocatalysts from the solution.  
A good catalyst support has to possess the advantages of large specific surface area, fast and strong adsorption to the pollutant, and reversible adsorption in room temperature to promote photocatalysis. In the list of global greenhouse gas emission contributors, agriculture ranked the secondhighest, so more attention must be allocated to the utilization of agricultural waste to form sustainable solutions [13]. As one of the biggest wastes in livestock products, animal bone has to be recycled and utilized to meet the requirements of sustainable development. Meanwhile, the technological solutions to recycle waste must be conducive to sustainable industrial and social development [14].  
In this paper, bone char, a low-cost material with large specific surface area, was used to study its adsorption performance for methylene blue, which may be a promising support material for photocatalysts. Bone char (BC) is made from cheap bovine bone waste products with a porous hydroxyapatite structure. It has been highly regarded as a green (nontoxic), effective, and easily regenerable adsorbent to remove hormones and methylene blue dye [15; 16; 17; 18]. The performance of adsorbents and separation materials could be fine-tuned with various treatments such as pyrolysis treatment [19] and solvent treatment [20]. These two solutions have also been studied in our previous report [15].  
The effects of bone char size, contact time, and adsorption temperature on the removal rate of methylene blue have not been studied previously; however, these are important factors to consider in the photocatalysis process. Thus, the study of these effects is the main research purpose of this work. The adsorption mechanisms of bone char for methylene blue using kinetic, isotherm, and thermodynamic theories were also studied. Finally, the ion-exchange interaction mechanism between bone char and methylene blue was proposed.