## 1 Introduction  
Antimony (Sb), a hazardous element that is toxic to human health, causes several diseases such as respiratory irritation, pneumoconiosis, high blood cholesterol, low blood sugar, and cancer [1, 2]. Due to the high toxicity and hazardous of Sb, the World Health Organization (WHO) and some countries regulate its standard concentration in drinking water: WHO, 20.0 \(\mu\)g/L [3]; the United States Environmental Protection Agency, 6.0 \(\mu\)g/L [4]; and European Union, 5.0 \(\mu\)g/L [5]. The Korean government does not provide guidelines to regulate Sb concentration in drinking water, but its concentration in wastewater is controlled to below 0.02 mg/L. Sb is predominant in nature as Sb(OH)\({}_{3}\) and Sb(OH)\({}_{6}\)\({}^{-}\), and Sb(III) and Sb(V) are usually present in reducing and oxidizing water environments, respectively [6]. The stability and solubility of Sb(V) is higher than that of Sb(III), which prevails in water bodies by mostly remaining in an oxidizing environment [7]. Therefore, more efforts are needed to develop efficient technologies to remove Sb(V) from aqueous solutions. Because of the toxicity of Sb, various studies including coagulation, membrane separation, electrodeposition, bioremediation, and adsorption have been performed to remove Sb from aqueous solutions [8]. Among them, adsorption has been considered sustainable and efficient for Sb removal because of its high efficiency, cost-effectiveness, and simple operation [9]. The efficiency and cost of the adsorption process depend highly on the selection of adsorbents, and various adsorbents have been explored in removing target contaminants under different environmental conditions. Because of its porous structure, large specific surface area, abundant functional groups on the surface, and high stability, application of biochar in environmental remediation has attracted the interest of many researchers [10]. A wide range of organic materials, such as agricultural byproducts, sawdust, animal manure, and sludge, can be used as raw materials to produce biochar [11]. Biochar has been extensively applied to adsorb heavy metals such as Cd, Cu, Cr, and Zn [12]. Biochar has attracted the attention of many researchers as an adsorbent alternative to activated carbon, which has been most widely used but is costly [13]. The efficiency of heavy metal removal by biochar is influenced by the physicochemical properties of biochar, biochar dosage, pH, water temperature, and initial concentrations and types of heavy metals [14]. However, research on biochar-mediated Sb removal has only been attempted recently [1; 8; 9; 10; 15; 16]. Nascent biochar is relatively ineffective in removing anions and oxyanions such as fluoride, phosphate, Se, As, and Cr; hence, modifying the biochar to enhance its anion adsorption capacity has been explored. Fe impregnation has been an inexpensive and highly effective method in enhancing the biochar adsorption capacity for As removal [17]. Since Sb and As belong to group 15 in the periodic table, Sb exhibits many similar chemical characteristics and behaviors as As in natural water [8]. In addition, Fe oxides and hydroxides have been reported to strongly bind to Sb(III) and Sb(V) [18]. In our previous study [19], cattle manure pyrolyzed in the presence of Fe produced valuable energy fuels (gas and oil) and biochar, which effectively removed selenate. Animal manure has been applied to cropland as an organic fertilizer for replenishing the N, P, Ca, and K, but their excessive application has resulted in environmental and public health issues such as water pollution and the occurrence of pathogen, odors, and bugs [20; 21]. Therefore, more efficient, inexpensive, and environmentally sustainable methods for treating them have been explored, and thermal-chemical processes including anaerobic digestion, combustion, gasification, and pyrolysis have been considered a promising technology to treat animals manure and produce biomass energies [20].