## 1 Introduction  
Dyes are widely used in various industries, such as textiles, plastics, paper, and leather production [1, 2]. The large amounts of dye effluents occurred from these factories can infiltrate (approximately 85%) and accumulate into ground water and surface water systems [3]. These dye effluents, by transferring into drinking water, have caused significant problems, such as a threat of human health owing to mutagenic, carcinogenic, and toxic characteristics of dyes [1, 2]. Therefore, it is necessary to evaluate and investigation methods to eliminate dyes from aqueous environment. Technologies including physical, chemical, and biological processes for dye removal have been studied [1, 4, 5]. The chemical process has its own problems, such as the production of harmful intermediates and a high cost of operation [6]. In addition, the biological process has a disadvantage including the limits for removal efficiency due to a synthetic origin and stable molecular structure of dye, occurrence of sludge, and production of secondary harmful compounds [1, 3]. Currently, the physical processes, including activated carbon, ion exchange, and membrane filter, appear attractive because of their high performance, and facile reaction process [4]. However, these physical processes also possess limits, such as high cost of material and equipment [7]. As a promising adsorption-based approach, biochars (BCs) can be effectively used for dye treatment. BCs produced from waste bioresource residues, such as wood, leaves, sawdust, weeds, straws, and microalgae, are inexpensive and chemically and mechanically stable adsorbents [8; 9; 10]. BCs can be easily modified using various approaches, such as surface oxygenation and preparing their composites with other materials, to increase the adsorption efficiency [11; 12; 13]. In particular, the pyrolysis temperature is one of the crucial parameters for biochar production and may affect its characteristics with regard to dye treatment. According to the previous literature, BCs produced at different pyrolysis temperatures possessed different properties (e.g., metal crystallization, functional groups, surface area, pH,pzc (the pH at point of zero charge), and porous structure) [2; 10; 14; 15]. Hence, seaweed-, vermicompost-, cabbage-, wheat straw-, pulp-, and paper sludge-derived BCs from previous studies were used for dye removal (e.g., methylene blue, congo red, crystal violet, etc.) [2; 4; 8; 16; 17]. [PERSON] et al. (2016) reported that there were possible mechanisms such as hydrogen bond formation, \(\pi\)-\(\pi\) interaction, electrostatic attraction, and cation exchange for MB and CR dye removal using vermicompost-derived BCs [2]. In addition, the adsorption by activated carbons governed a higher surface area and mesoporous nature [4]. There are some difficulties for the treatment of dyes possessing different characteristics (e.g., cationic and anionic property). In particular, characteristics of these dye effluents could be dependent on the production process, season, and era. Therefore, BCs having various characteristics can be advantageous for the simultaneous treatment of dyes mixtures. Thus, this study focused on the adsorptive elimination of cationic and anionic dye mixtures (i.e., methylene blue and congo red) as cationic and anionic dye in water using BCs produced at various pyrolysis temperatures. In order to evaluate the adsorption efficiency of the dye, we tried to investigate the optimal and possible condition using response surface methodology (RSM).