###### Abstract  
Chemically activated biochars prepared from sorghum distillers grain using two base activators (NaOH and KOH) were investigated for their adsorption properties with respect to ammonium nitrogen from aqueous solution. Detailed characterizations, including scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetry (TG), and specific surface area analyses, were carried out to offer a broad evaluation of the prepared biochars. The results showed that the NaOH- and KOH-activated biochars exhibited significantly enhanced adsorption capacity, by 2.93 and 4.74 times, respectively, in comparison with the pristine biochar. Although the NaOH-activated biochar possessed larger specific surface area (132.8 and 117.7 m\({}^{2}\)/g for the NaOH- and KOH-activated biochars, respectively), the KOH-activated biochar had higher adsorption capacity owing to its much higher content of functional groups. The adsorption kinetics and isotherms of the KOH-activated biochar at different temperatures were further studied. The biochar had a maximum adsorption capacity of 14.34 mg/g at 45 \({}^{\circ}\)C, which was satisfactory compared with other biochars prepared using different feedstocks. The adsorption process followed pseudo-second-order kinetics, and chemical adsorption was the rate-controlling step. The equilibrium data were consistent with the Freundlich isotherm, and the thermodynamic parameters suggested that the adsorption process was endothermic and spontaneous. Consequently, this work demonstrates that chemically activated biochar from sorghum distillers grain is effective for ammonium nitrogen removal. sorghum distillers grain; biochar; chemical activation; ammonium nitrogen adsorption; adsorption kinetics and isotherms +
Footnote †: journal: Applied Physics Letters  
_Applied Physics Letters_  
related to eutrophication, including acidification of freshwater ecosystems, hypoxic water conditions, and toxicity to benthic organisms and fish [5]. Moreover, ammonium nitrogen can result in the generation of genotoxic nitrosodimethylamine [6]. Therefore, for public health and environmental safety, the removal of ammonium nitrogen from contaminated water is urgently required. Different technologies have been developed to remove ammonium nitrogen from wastewater, including biological treatments [7], ion exchange [8], adsorption [9], electrodialysis [10], and reverse osmosis [11]. Among these technologies, adsorption has gained extensive attention owing to its obvious advantages, such as high efficiency and a simple process [12, 13]. Another advantage of adsorption for ammonium nitrogen removal is that a wide range of adsorbents is available. Reported adsorbents for ammonium nitrogen removal include agricultural by-products [9], zeolite [6], resin [14], and hydrogel composites [15]. Currently, there is growing interest in developing effective adsorbents from low-cost renewable biomass resources, owing to their sustainability and economic efficiency. Biochar is a carbon-rich solid prepared from biomass pyrolysis, especially low-cost biomass, under oxygen-limited condition. Owing to its favorable characteristics, including high porosity and cation exchange capacity, biochar shows good adsorption performance towards many pollutants from industrial, agricultural, and domestic waste waters [16]. Therefore, biochars prepared from different sources, such as plant biomass, corn cob, and giant reed, have been studied as effective and low-cost adsorbents for ammonium nitrogen [9, 12, 17]. More importantly, biochar after the adsorption of ammonium nitrogen can be further employed as slow-release fertilizer, which realizes the reutilization of the used adsorbent [17, 18]. Sorghum distillers grain (SDG) is a solid waste from white spirits industry using sorghum as the raw material. Sorghum is a drought-tolerant plant that can easily adapt to arid climates. Due to its easy cultivation and high productivity, sorghum is widely used for liquor production.