## 1 Introduction  
The discharge of dye-containing wastewater into water bodies causes serious water pollution. For examples, pollutant dyes such as methylene blue (MB), alizarin yellow R (AYR) and neutral red (NR) have been widely used in food, textiles, cosmetics, papermaking, coatings, printing, pharmaceuticals and other industries [1]. Due to their long-term persistence, dye-containing wastewater hinders light penetration, weakens photosynthesis of aquatic plants and algae, resulting in reductions in dissolved oxygen levels and leading to the eutrophication of waters [2]. Furthermore, organic dyes with their complex aromatic structures have been associated with carcinogenesis in aquatic animals [3]. The existence of organic dyes in aquatic environments is highly dangerous to aquatic animals and plants [4]. In addition, the bioaccumulation of organic dyes can cause severe allergies and skin irritations to human beings [5]. Therefore, it is necessary to develop facile and reliable approaches for the safe and efficient removal of these harmful pollutants from dye-containing wastewaters before they are discharged into environmental waters. Various physio-chemical technologies used for the treatment of organic dyes, such as chemical oxidation, electrochemical degradation, adsorption and advanced photocatalytic degradation,etc., have been extensively investigated and implemented [6; 7; 8; 9]. Among these approaches, adsorption has attracted extensive attention for its simple operation and cost-saving properties [10]. Nevertheless, efficient and economical adsorbents with good handling properties are always required [11]. In particular, natural product-based adsorbents that are derived from renewable and biodegradable resources are more attractive because of their environmentally friendly, inexpensive and sustainable features [12]. Biochars (BCs) have been considered one of the most preferred adsorbents for contaminant removal [13], and they have attracted much attention because of their wide and low-cost raw sources available [14]. Many previous studies reported the preparation of biomass-based adsorbents using various methods, such as covalent grafting of functional organic units, doping heteroatoms or metal elements [15; 16; 17]. BCs prepared by direct pyrolysis have been also widely investigated. For instance, BCs obtained from pyrolysis of persimmon peel at 700 \({}^{\circ}\)C could be applied to the adsorption of MB [18]. Currently, most pyrolysis experiments have been conducted at relatively higher temperatures, which may result in secondary pollution to the environment. The pyrolysis of biomass is often responsible for the production of toxic substances such as dioxins and polycyclic aromatic hydrocarbons (PAHs). Dioxins and PAHs are the main byproducts of the thermal decomposition of lignocellulosic, and the discharge of these dioxins and PAHs increases as the pyrolysis temperature increases [19]. There has been some concern over the emission of PAHs, since they are associated with human teratogenesis, cancer or mutations due to their bio-accumulative effects [20]. A decrease in the discharge of PAHs and dioxins can be achieved by using lower pyrolysis temperatures [21; 22]. Definitely, the discharge of some toxic volatile components may occur even under low-temperature pyrolysis conditions. Importantly, the exhaust gases can be effectively eliminated by using a vacuum pump together with a gas cleaning system [23]. Therefore, low-temperature, flash-vacuum pyrolysis technology appears to be relatively more environmentally friendly. Interestingly, the reservation of heteroatomic substances in BCs may be beneficial to its highly-efficient adsorption [24]. In 2017, the world produced about 5.7 million tons of persimmons, and the yield of persimmon production in Asia was 5.1 million tons, confirming the significant production of persimmons and hence the possible production of large quantities of persimmon peel (PP) [25].