## 1 Introduction  
Environmental pollution from heavy metals or dyes has been progressively worsening, owing to rapid industrial growth, and has now become a worldwide issue [1]. Therefore, eco-friendly, cost-effective, and high-efficiency adsorbents for heavy metal- or dye-contaminated water are urgently required. Recently, biochar has attracted considerable attention for environmental protection, owing to its unique physicochemical properties [2, 3, 4]. Biochar is a carbonaceous material made by pyrolyzing biomass in the absence of oxygen, and has garnered a significant interest as a less toxic, environmentally friendly, and inexpensive material [5, 6, 7, 8, 9]. Additionally, biochar has the advantages of a high specific surface area, a well-developed mesoporous structure, enriched surface functional groups, and mineral components, making it an excellent adsorbent for removing contaminants from aqueous solutions. Compared to traditional activated carbon, biochar is a new cost-effective and efficient sorbent candidate. A higher temperature is required for the production of activated carbon, whereas the production of biochar, which is derived from agricultural biomass and solid wastes, is cheaper and requires less energy [10, 11, 12]. Previous studies have shown that biochar is incredibly effective at removing various organic and inorganic pollutants, such as heavy metals, dyes, and polycyclic aromatic hydrocarbons [13, 14, 15, 16]. [PERSON] et al. used modified biochar to remove malachite green from polluted aqueous environments [17]. Among the various biochars, rice-bran-derived biochar has been widely studied as a raw material for biochar synthesis for use as an adsorbent material, due to its chemical stability and local availability [18; 19; 20].  
The physical and chemical properties of biochar depend on several factors related to the pyrolysis temperature and the milling process. Ball milling is a mechanochemical process that utilizes mechanical energy to induce chemical and structural changes in materials, and is now considered a green technology for pristine biochar modification [21; 22]. [PERSON] et al. studied mesoporous ball-milled Fe-loaded biochar for enhanced sorption of reactive red [23]. Pyrolysis is the thermochemical conversion of agricultural organic waste, and the pyrolysis temperature is related to the biochar yield and its physicochemical characteristics [24; 25; 26].  
However, the effects of pyrolysis and ball milling on the physicochemical and adsorption properties of rice-bran-derived biochar remain unexplored. Rice bran is a major agricultural waste in Korea, and the potential applications of this biomass are currently under study. Thus, in the present study, we investigated the effect of pyrolysis temperature and ball-milling time on the physicochemical properties of rice-bran-derived biochar. Moreover, we demonstrated the effects of pyrolysis temperature, ball-milling time, biochar dose, and contact reaction time on the Rhodamine B (RhB) removal efficiency of the rice-bran-derived biochar. RhB is widely used in textile production and paper printing, as well as in the biological, analytical and optical sciences [27; 28]. RhB can cause burning of the eyes and throat, chest pain, cough, and headache in humans [29; 30]. However, RhB is a common contaminant in waste water; therefore, removing RhB from waste water is critical not only for fundamental studies but also for aquatic life and human health.