Adsorption of Ammonium Nitrogen from Aqueous Solution on Chemically Activated Biochar Prepared from Sorghum Distillers Grain

: 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 di ﬀ raction (XRD), thermogravimetry (TG), and speciﬁc surface area analyses, were carried out to o ﬀ er 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 di ﬀ erent temperatures were further studied. The biochar had a maximum adsorption capacity of 14.34 mg / g at 45 ◦ C, which was satisfactory compared with other biochars prepared using di ﬀ erent 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 e ﬀ ective for ammonium nitrogen removal. ﬁtted the Freundlich model well. The thermodynamic parameters demonstrated that the adsorption process was endothermic and spontaneous. The maximum adsorption capacity was found to be 14.34 mg / g at 45 ◦ C, which was satisfactory when compared with other biochars prepared from di ﬀ erent feedstocks. This work demonstrates that chemically activated biochar from sorghum distillers grain is a novel and e ﬃ cient adsorbent for NH 4 + -N removal.


Introduction
Water pollution has become a major concern, and water pollution control has been regarded as a priority for both society and public authorities [1]. The chemical compositions of natural water resources are primarily determined by industrial, domestic, and agricultural practices, geology, climate, and other natural and anthropic sources [2]. The anthropogenic pollutants, such as nitrogen compounds, metals, pesticides, pharmaceuticals, personal care products, etc., have a critical effect on the quality of water resources [3]. Among them, ammonium nitrogen is a common water contaminant and is generally released into the environment through industrial effluent and agricultural drainage [4]. Excess ammonium nitrogen in water leads to a series of ecological problems in aquatic environments

Materials and Chemicals
Sorghum distillers grain (SDG) was collected from a sorghum distillery in Kinmen, Taiwan province, China. The sample was dried at 65 • C for 12 h in an oven (VOS-30A, STIK, Shanghai, China) and was ground to a 60-80 mesh particle size using a high-speed multifunction crusher (800, Yongkang Tianqi Shengshi Industry and Trade Co., Ltd., Jinhua, China). KOH, NaOH, 3,5-dinitrosalicylic acid, and petroleum ether were of analytical grade and were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). NH 4 Cl was a guaranteed reagent obtained from Macklin Co., Ltd. (Shanghai, China). Biological reagents, including cellulase and amylase, were purchased from Sigma-Aldrich.

Preparation of Biochar
Three grams of SDG were impregnated in 10 mL 30 g/L NaOH (or KOH) solution for 12 h and oven-dried at 105 • C overnight. Afterward, the SDG samples were ground and placed in a tube furnace (SK2-1-12, Tianye, China) for pyrolysis. The pyrolysis experiments were performed at different temperatures (550, 650, and 750 • C) with a heating rate of 5 • C /min in a sweeping nitrogen gas atmosphere for 2 h, according to the previous study [21]. After pyrolysis, the obtained biochar was washed with excess deionized water three times. The washed biochar was then oven-dried at 105 • C for approximately 5 h and weighed for a yield calculation. For comparison, dried SDG without chemical activation was also pyrolyzed at 650 • C for 2 h with a heating rate of 5 • C /min to obtain an activator-free biochar.

Ammonium Nitrogen Adsorption Experiments
To evaluate the ammonium nitrogen adsorption performance of the biochar samples, a series of adsorption experiments were conducted using a 100 mL conical flask containing 25 mL NH 4 Cl aqueous solution. Typically, 0.1 g biochar was added to 25 mL 100 mg/L NH 4 Cl aqueous solution, and the mixture was placed in an electrically thermostatic shaker (HNY-100B, Tianjin Honour Instrument Co., Ltd, Tianjin, China) at 200 rpm at 30 • C for 6 h. The pH of the NH 4 Cl aqueous solution was adjusted in the range from 3 to 10 by adding NaOH and HCl. After adsorption, the residual ammonium solution was filtered through a 0.45 µm membrane, and the ammonium concentration in the solution was analyzed using the Nesslerization method with the help of a UV-vis spectrophotometer (Shimadazu UV-2500, Japan) at 420 nm [23]. The ammonium adsorption capacity was calculated based on the following equation: where C 0 is the initial ammonium nitrogen concentration (mg/L), C t is the residual ammonium nitrogen concentration (mg/L) after biochar adsorption at time t, V (mL) is the solution volume for the adsorption experiment, and m (g) is the adsorbent mass used in the adsorption experiment. For the adsorption kinetic study, ammonium concentrations at different time points (e.g., 0.5, 1, 1.5, 2, 3, 4, 5, and 6 h) were determined. For the adsorption isotherms, 0.1 g biochar was added to 25 mL NH 4 Cl aqueous solution with different concentrations (50,100,200,400, and 600 mg/L). All the adsorption tests were performed in triplicate.

Composition Analyses of Sorghum Distillers Grain
The ash content was determined by placing the dried SDG samples in a muffle furnace at 600 • C for 2 h. The crude lipid content in the SDG was analyzed by Soxhlet extraction with petroleum ether for 5 h [24]. The solvent was then removed by evaporation under vacuum at 40 • C. The crude protein was determined using the Kjeldahl procedures as described in Association of Official Agricultural Chemists (AOAC, Washington, DC, USA, 1995, 16th ed.) guidelines. The cellulose and starch contents in the SDG were measured by enzymatic methods using cellulase (Sigma-Aldrich) and amylase Appl. Sci. 2019, 9,5249 4 of 16 (Sigma-Aldrich), respectively. The glucose released after enzymatic saccharification was estimated spectrophotometrically by the 3,5-dinitrosalicylic acid (DNS) method [25]. The element contents (carbon, hydrogen, and nitrogen) in the SDG were obtained using an elemental analyzer (Vario EL III, Elementar, Germany).

Characterization of Biochar
XRD patterns were obtained from an Ultima IV X-ray diffractometer (Rigaku, Japan), which was operated at 35 kV and a scanning speed of 20 • /min in the 2θ range from 10 to 70 • using Cu Ka radiation (k = 1.5418 Å). FTIR spectra were obtained using a Nicolet Avatar (Thermo Electron, San Jose, CA, USA) and the potassium bromide technique, in a proportion of 1:100 (100 mg of KBr approximately). For zeta potential determination, the biochar sample (0.01 g) was firstly ground to pass through a 200 mesh sieve and well dispersed in 100 mL distilled water (at pH ranging from 3.0 to 10.0, adjusted using NaOH or HCl) by 10 min ultrasonic treatment. The zeta potential was analyzed in triplicate with the help of a Zetasizer Nano ZS instrument (Malvern Instruments Ltd., Worcestershire, UK). The thermal stability of the biochar samples was measured by employing a TG instrument (TA SDT 650, New Castle, DE, USA) with a heating rate of 10 • C/min from 25 to 800 • C under the protection of N 2 . The specific surface area of the biochar samples was analyzed using an Automated Physisorption and Chemisorption Analyzer (Tristar 3000, Norcross, GA, USA). The samples were degassed under vacuum at 300 • C for over 3 h before measurements. The morphology of the biochar was observed using a SEM instrument (JSM-5600LV, JEOL Ltd., Tokyo, Japan) operated at a voltage of 20 kV. Boehm titrations of the biochars were conducted according to the protocol described previously [26].

Characterization of Sorghum Distillers Grain
The compositions of raw materials have an important effect on the properties of the obtained biochar. Composition analyses of the SDG sample showed that it contained 24.6% ± 0.7% starch, 19.6% ± 0.4% cellulose, 14.9% ± 0.5% protein, 4.4% ± 0.2% crude lipid, and 6.4% ± 0.3% ash. Although the SDG was the residue of sorghum after fermentation, it still had a high content of starch, which was mainly caused by the relatively low hydrolysis efficiency by microbes during fermentation [19,27]. The high contents of cellulose and ash mainly come from the rice husk, which is added to loosen the solid sorghum medium and improve the heat transfer for fermentation and distillation [19]. The elemental analysis suggested that the SDG mainly contained C (44.1%), H (6.5%), and N (2.9%) because of its high starch, cellulose, and protein contents. Accordingly, the presence of high carbon element content makes the SDG a potential feedstock for biochar production. Figure 1a shows the NH 4 + -N adsorption capacity and yields of biochar prepared using NaOH and KOH as activators, as well as those of biochar prepared without chemical activation. The NH 4 + -N adsorption capacity of the biochar without activation was about 2.06 mg/g, which was in agreement with the result reported by Gai et al. when they analyzed the ammonium nitrogen adsorption capacity of biochar obtained from corn straw [28]. After activation by NaOH and KOH, the biochar samples exhibited significantly enhanced adsorption capacity of 6.03 and 9.77 mg/g, respectively, indicating that the chemical activation facilitated ammonium nitrogen adsorption. The biochar without activation showed obviously higher yield than the two chemically activated biochars. This result can be attributed to the intensified pyrolysis reactions caused by the base catalysts, which convert the solid carbon substances into gaseous carbon volatiles [29]. Although the two activated biochars had similar yields (31.29% and 30.47%), the adsorption capacity of KOH-activated biochar was higher than that of NaOH-activated biochar. Thus, KOH-activated biochar was chosen as the adsorbent for the following adsorption experiments.  Figure 1b shows the yield and NH4 + -N adsorption capacity of KOH-activated biochar obtained at different pyrolysis temperatures. It was found that increasing the pyrolysis temperature resulted in a decline in biochar yield, which could be explained by the release of more volatile matter during the pyrolysis at a higher temperature [28]. However, the NH4 + -N adsorption capacity of the biochar improved when the pyrolysis temperature was increased. The biochar obtained at 550 °C exhibited an adsorption capacity of about 7.67 mg/g, while the value increased significantly to 12.02 mg/g when the pyrolysis temperature was increased to 750 °C. Thus, the NH4 + -N capacity of KOH-activated biochar is influenced by the pyrolysis temperature. The pyrolysis temperature has a critical effect on the specific surface area of the prepared biochar and, accordingly, the adsorption capacity of the biochar [28]. The specific surface areas of the KOH-activated biochar pyrolyzed at 550, 650, and 750 °C were 95.9, 117.7, and 155.3 m 2 /g, respectively. This result is consistent with those of other studies reporting a high surface area in biochar pyrolyzed at 650-800 °C [30,31]. The low surface area of biochar pyrolyzed at a lower temperature is caused by the formation of tars, which prevents pore formation during the pyrolysis process. However, when pyrolysis is conducted at a higher temperature, pores are generated easily due to the volatilization of tar components [30].  Figure 1b shows the yield and NH 4 + -N adsorption capacity of KOH-activated biochar obtained at different pyrolysis temperatures. It was found that increasing the pyrolysis temperature resulted in a decline in biochar yield, which could be explained by the release of more volatile matter during the pyrolysis at a higher temperature [28]. However, the NH 4 + -N adsorption capacity of the biochar improved when the pyrolysis temperature was increased. The biochar obtained at 550 • C exhibited an adsorption capacity of about 7.67 mg/g, while the value increased significantly to 12.02 mg/g when the pyrolysis temperature was increased to 750 • C. Thus, the NH 4 + -N capacity of KOH-activated biochar is influenced by the pyrolysis temperature. The pyrolysis temperature has a critical effect on the specific surface area of the prepared biochar and, accordingly, the adsorption capacity of the biochar [28]. The specific surface areas of the KOH-activated biochar pyrolyzed at 550, 650, and 750 • C were 95.9, 117.7, and 155.3 m 2 /g, respectively. This result is consistent with those of other studies reporting a high surface area in biochar pyrolyzed at 650-800 • C [30,31]. The low surface area of biochar pyrolyzed at a lower temperature is caused by the formation of tars, which prevents pore formation during the pyrolysis process. However, when pyrolysis is conducted at a higher temperature, pores are generated easily due to the volatilization of tar components [30].

Effect of Adsorption pH on the Adsorption Capacity of Biochar
The pH value of NH 4 + -N solution has a critical effect on the charge distribution of biochar and the electrostatic or molecular interactions between the biochar and NH 4 + -N, which accordingly determine the NH 4 + -N adsorption capacity of biochar. The effect of adsorption pH on the adsorption capacity of KOH-activated biochar is presented in Figure 2. It was found that the adsorption capacity increased with increasing pH value from 3.0 to 9.0, and further enhancement of the pH resulted in a sharp decrease in adsorption capacity. Similar results have been documented by other researchers [32,33]. The low adsorption capacity at low pH may be caused by the low negative zeta potential of biochar at low pH as shown in Figure 2. Since ammonium nitrogen is a positive ion, biochar with a high negative zeta potential facilitates adsorption through electrostatic attraction. However, when the solution pH value is increased to 10.0, most of the NH 4 + ion can be transformed into the NH 3 (aq) form as the NH 4 + ion has a pKa of about 9.3 [33]. Therefore, at this pH value (10.0), the electrostatic attraction mechanism was no longer effective, which resulted in the observed reduction in the adsorption capacity [32]. It is notable that the pH value of biochar in water without adjustment was about 8.4 and the adsorption capacity of biochar at this pH was almost the same as the highest value at pH 9.0. In order to simplify the adsorption process, the following adsorption experiments were conducted without pH adjustment (pH about 8.4).

Effect of Adsorption pH on the Adsorption Capacity of Biochar
The pH value of NH4 + -N solution has a critical effect on the charge distribution of biochar and the electrostatic or molecular interactions between the biochar and NH4 + -N, which accordingly determine the NH4 + -N adsorption capacity of biochar. The effect of adsorption pH on the adsorption capacity of KOH-activated biochar is presented in Figure 2. It was found that the adsorption capacity increased with increasing pH value from 3.0 to 9.0, and further enhancement of the pH resulted in a sharp decrease in adsorption capacity. Similar results have been documented by other researchers [32,33]. The low adsorption capacity at low pH may be caused by the low negative zeta potential of biochar at low pH as shown in Figure 2. Since ammonium nitrogen is a positive ion, biochar with a high negative zeta potential facilitates adsorption through electrostatic attraction. However, when the solution pH value is increased to 10.0, most of the NH4 + ion can be transformed into the NH3(aq) form as the NH4 + ion has a pKa of about 9.3 [33]. Therefore, at this pH value (10.0), the electrostatic attraction mechanism was no longer effective, which resulted in the observed reduction in the adsorption capacity [32]. It is notable that the pH value of biochar in water without adjustment was about 8.4 and the adsorption capacity of biochar at this pH was almost the same as the highest value at pH 9.0. In order to simplify the adsorption process, the following adsorption experiments were conducted without pH adjustment (pH about 8.4).

Adsorption Kinetics
The adsorption kinetics of KOH-activated biochar (produced at 750 °C) at various adsorption temperatures are presented in Figure 3a. It was found that the adsorption trends for different temperature were similar. Specially, the first 2 h were attributed to the rapid adsorption stage for NH4 + -N, which was owing to the exposed surface, porous network structure, and the high amounts of vacant adsorption sites. The adsorption capacity (Qt) increased from 11.58 to 14.34 mg/g when the adsorption temperature increased from 25 to 45 °C. Wang et al. [34] investigated the adsorption of NH4 + -N onto progressively oxidized maple wood biochar and found that the maximum adsorption capacity was 10.1 mg/g. Zheng et al. [20] prepared biochar from anaerobic digestion residue, with an NH4 + adsorption capacity of 8.37 mg/g. Zhu et al. [22] prepared activated carbon from avocado seeds through methanesulfonic acid activation and found that its NH4 + -N adsorption capacity was 5.4 mg/g.
The adsorption process was further fitted using the pseudo-first-order and pseudo-second-order models, which are expressed in Equations (2) and (3), respectively:

Adsorption Kinetics
The adsorption kinetics of KOH-activated biochar (produced at 750 • C) at various adsorption temperatures are presented in Figure 3a. It was found that the adsorption trends for different temperature were similar. Specially, the first 2 h were attributed to the rapid adsorption stage for NH 4 + -N, which was owing to the exposed surface, porous network structure, and the high amounts of vacant adsorption sites. The adsorption capacity (Q t ) increased from 11.58 to 14.34 mg/g when the adsorption temperature increased from 25 to 45 • C. Wang et al. [34] investigated the adsorption of NH 4 + -N onto progressively oxidized maple wood biochar and found that the maximum adsorption capacity was 10.1 mg/g. Zheng et al. [20] prepared biochar from anaerobic digestion residue, with an where Qe (mg/g) is the equilibrium amount of NH4 + -N adsorbed by the biochar, Qt (mg/g) is the amount of NH4 + -N adsorbed by the biochar at time t (min), and k1 (1/min) and k2 (g/(mg·min)) are the rate constants of the pseudo-first-order model and the pseudo-second-order model, respectively. The plots of Qt against t are presented in Figure 3a, and their fitting curves using the two models are also given in Figure 3a. Table 1   The adsorption process was further fitted using the pseudo-first-order and pseudo-second-order models, which are expressed in Equations (2) and (3), respectively: where Q e (mg/g) is the equilibrium amount of NH 4 + -N adsorbed by the biochar, Q t (mg/g) is the amount of NH 4 + -N adsorbed by the biochar at time t (min), and k 1 (1/min) and k 2 (g/(mg·min)) are the rate constants of the pseudo-first-order model and the pseudo-second-order model, respectively. The plots of Q t against t are presented in Figure 3a, and their fitting curves using the two models are also given in Figure 3a. Table 1 shows the results of the nonlinear regression analysis of the two kinetic models. The results in Figure 3a show that the theoretical adsorption capacities (Q e,cal ) using the pseudo-second-order kinetic model were closer to the experimental adsorption capacities (Q e ) than those using the pseudo-first-order kinetic model. The pseudo-second-order kinetics model had higher correlation coefficient values (R 2 ) than the pseudo-first-order model ( Table 1). These results suggest that the adsorption of the NH 4 + -N on KOH-activated biochar followed a pseudo-second-order kinetics model. Besides this, the values of both Q e,cal and k 2 increased when the temperature was raised from 25 • C to 45 • C, suggesting that the adsorption process was intensified by elevating the adsorption temperature. The relationship between temperature and the rate constant can be described by the Arrhenius equation, which is expressed as follows: where k 2 (g·mg −1 ·min −1 ) is the adsorption rate constant of the pseudo-second-order model, A (min −1 ) is the pre-exponential factor, and R (8.314 J/(K·mol)) and T (K) are the ideal gas constant and the absolute temperature, respectively. The activation energy (E a ) magnitude can provide an insight into the adsorption type [35]. In this work, the value of E a for the adsorption of NH 4 + -N on KOH-activated biochar was 47.01 kJ/mol after calculation, indicating that the adsorption is a chemical adsorption dominant type owing to the high E a value (>40 kJ/mol).

Adsorption Isotherm
In order to understand how NH 4 + -N is adsorbed onto the biochar adsorbent, as well as to define the equilibrium data, the adsorbed amounts and residual NH 4 + -N concentration in the solution after adsorption were fitted using both Langmuir and Freundlich isotherms. The Langmuir model (Equation (5)) indicates monolayer adsorption on a homogenous surface, while the Freundlich isotherm (Equation (6)) elucidates nonideal adsorption on heterogeneous surfaces and multilayer adsorption [36,37]: where Q e is the amount of NH 4 + -N absorbed (mg/g) and C e is the equilibrium concentration (mg/L) of NH 4 + -N in aqueous solution after adsorption. k L (L/mg) and Q m (mg/g) are the Langmuir constant and the monolayer maximum adsorption capacity of NH 4 + -N onto biochar, respectively.
The Freundlich constants k F and n are the multilayer adsorption capacity (mg/g) and the adsorption intensity, respectively. The plots of the adsorbed NH 4 + -N amounts on biochar versus the residual NH 4 + -N concentration in the solution at different temperatures are given in Figure 3b, as well as the fitting curves using the two isotherm models. The fitting parameters of the biochar adsorption equilibrium data using the two models are shown in Table 2. As shown in Figure 3b, the adsorption process was represented by the Freundlich model very well, and the correlation coefficient (R 2 ) values were all higher than 0.99 ( Table 2). The values of 1/n in the Freundlich model were lower than 0.1, suggesting that the adsorption process is favorable [35,38]. The value of 1/n can be further divided into five levels, and when the value is between 0.1 and 0.01, the adsorption is categorized as a strongly favorable level, indicating the good adsorption performance of NH 4 + -N on biochar [39]. Thermodynamic parameters, including Gibbs free energy ∆G 0 (kJ/mol), entropy ∆S 0 (J/(K·mol)), and standard enthalpy ∆H 0 (kJ/mol), can be used to estimate the temperature effect on NH 4 + -N adsorption onto the biochar and give an idea about the adsorption behavior and mechanism. The thermodynamic parameters can be obtained from the Freundlich model using the following equations [40]: where R is the gas constant, the value of which is 8.314 J/(K·mol); T (K) is the absolute temperature; and n and k F (µmol/g) are Freundlich constants. The slope and intercept of the plots of ln k d versus 1/T are used to calculate the values of ∆H 0 and ∆S 0 . Table 2 also presents the thermodynamic parameters calculated from Equations (7)- (9). The negative values of ∆G 0 demonstrated the spontaneous nature and the feasibility of the adsorption of NH 4 + -N on KOH-activated biochar. In addition, the ∆G 0 values became more negative with increasing adsorption temperature, confirming that adsorption is more favorable at higher temperature [35,41]. Meanwhile, the positive value of ∆S 0 indicated the affinity of the biochar for NH 4 + -N. The positive ∆H 0 reflected the endothermic adsorption of NH 4 + -N onto biochar [42], which is in agreement with the kinetics results (Section 3.2.3) that increased temperature favors the adsorption.

Scanning Electron Microscopy (SEM)
The morphology of the biochars activated by KOH and NaOH at 650 • C was characterized by SEM, and the results are representatively presented in Figure 4 with different magnifications. It was observed that both of the two biochars had an obvious porous structure, confirming the pore-forming effect of the two activators. The porous structures of the two biochars from the SEM results were also in good agreement with their high SAA values (132.8 and 117.7 m 2 /g for NaOH-and KOH-activated biochars, respectively). The nitrogen adsorption-desorption isotherms of the NaOH-and KOH-activated biochars are presented in Figure 5. It was found that the isotherms belonged to type IV (IUPAC classification), indicating that the two biochars had mesoporous structure [43,44]. When the P/P 0 was higher than 0.95, the increase in the adsorption-desorption isotherms of NaOH-activated biochar was more obvious compared with that of KOH-activated biochar, indicating more macropores in the NaOH-activated biochar [45]. From a higher-magnification observation (Figure 4c,f), NaOH-activated biochar had a thin and fractured pore wall structure, while KOH-activated biochar exhibited a continuous porous structure with relatively thick pore walls. Since the two biochar types were obtained under the same temperature, the obvious difference in porous structures between them was mainly attributed to the different activation effects of the two activators [29]. Besides this, there was no significant difference in the porous structures of the two biochars after NH 4 + -N adsorption, suggesting good mechanical properties of the prepared biochars [12].
Appl. Sci. 2019, 9, x 10 of 15 Besides this, there was no significant difference in the porous structures of the two biochars after NH4 + -N adsorption, suggesting good mechanical properties of the prepared biochars [12].

X-Ray Diffraction (XRD)
The XRD patterns of the NaOH-activated biochar and KOH-activated biochar before and after NH4 + -N adsorption are shown in Figure 6. All the four biochar samples exhibited analogous diffractogram patterns with a broad peak in the range of 20-30°. The broad peak was attributed to the characteristics of amorphous carbon, which was disorderly stacked up by carbon rings [46].  Besides this, there was no significant difference in the porous structures of the two biochars after NH4 + -N adsorption, suggesting good mechanical properties of the prepared biochars [12].

X-Ray Diffraction (XRD)
The XRD patterns of the NaOH-activated biochar and KOH-activated biochar before and after NH4 + -N adsorption are shown in Figure 6. All the four biochar samples exhibited analogous diffractogram patterns with a broad peak in the range of 20-30°. The broad peak was attributed to the characteristics of amorphous carbon, which was disorderly stacked up by carbon rings [46]. .

X-Ray Diffraction (XRD)
The XRD patterns of the NaOH-activated biochar and KOH-activated biochar before and after NH 4 + -N adsorption are shown in Figure 6. All the four biochar samples exhibited analogous diffractogram patterns with a broad peak in the range of 20-30 • . The broad peak was attributed to the characteristics of amorphous carbon, which was disorderly stacked up by carbon rings [46]. However, two sharp peaks at 28 • and 41 • assigned to sylvite (KCl) were found from the XRD pattern of KOH-activated biochar [12,47]. The sylvite formed in the biochar was presumably from the activator KOH. After adsorption, these peaks disappeared (Figure 6), which was mainly because of the ion exchange of K + and NH 4 + during the adsorption process [48].
Appl. Sci. 2019, 9, x 11 of 15 Figure 6. XRD patterns of NaOH-activated biochar before (a) and after (b) ammonium nitrogen adsorption and KOH-activated biochar before (c) and after (d) ammonium nitrogen adsorption. Figure 7 shows the thermogravimetric curves of SDG and NaOH-activated and KOH-activated biochars. The weight loss of SDG started when the temperature was increased to 200 °C, and over 60% weight loss was observed at the temperature of 500 °C, indicating that the SDG contained a large number of volatile substances [21]. However, no weight losses were observed for the two biochars until the temperature reached 350 °C, and their total weight losses (to 800 °C) were less than 25%, suggesting good thermal stability of the two prepared biochars. By contrasting the two biochars, it was found that the temperature at which the main weight loss started for NaOH-activated biochar was slightly higher than that for KOH-activated biochar. This result can presumably be attributed to the higher content of functional groups in the KOH-activated biochar (FTIR results), which tend to decompose during TG measurements. Weight (%) Figure 6. XRD patterns of NaOH-activated biochar before (a) and after (b) ammonium nitrogen adsorption and KOH-activated biochar before (c) and after (d) ammonium nitrogen adsorption. Figure 7 shows the thermogravimetric curves of SDG and NaOH-activated and KOH-activated biochars. The weight loss of SDG started when the temperature was increased to 200 • C, and over 60% weight loss was observed at the temperature of 500 • C, indicating that the SDG contained a large number of volatile substances [21]. However, no weight losses were observed for the two biochars until the temperature reached 350 • C, and their total weight losses (to 800 • C) were less than 25%, suggesting good thermal stability of the two prepared biochars. By contrasting the two biochars, it was found that the temperature at which the main weight loss started for NaOH-activated biochar was slightly higher than that for KOH-activated biochar. This result can presumably be attributed to the higher content of functional groups in the KOH-activated biochar (FTIR results), which tend to decompose during TG measurements.

Thermogravimetry (TG)
number of volatile substances [21]. However, no weight losses were observed for the two biochars until the temperature reached 350 °C, and their total weight losses (to 800 °C) were less than 25%, suggesting good thermal stability of the two prepared biochars. By contrasting the two biochars, it was found that the temperature at which the main weight loss started for NaOH-activated biochar was slightly higher than that for KOH-activated biochar. This result can presumably be attributed to the higher content of functional groups in the KOH-activated biochar (FTIR results), which tend to decompose during TG measurements. . Figure 7. Thermogravimetric curves of sorghum distillers grain (SDG), NaOH-activated biochar, and KOH-activated biochar.

Fourier Transform Infrared Spectroscopy (FTIR)
The NH 4 + -N adsorption capacity of biochar depends not only on the specific surface area, but also on the contents of functional groups on its surface. The FTIR spectra of NaOH-activated and KOH-activated biochars are shown in Figure 8. Interestingly, NaOH-activated biochar exhibited a significantly different FTIR spectrum compared with the biochar activated by KOH. No obvious absorptions were found from the FTIR spectrum of NaOH-activated biochar in the range of 4000-400 cm −1 , indicating the relative absence of functional groups on its surface. However, the KOH-activated biochars presented obviously enhanced absorption peak amount and intensity: a broad band at about 3420 cm −1 attributed to hydroxyl group stretching vibration [12,17], two bands at 1590 cm −1 and 1384 cm −1 assigned to the stretching vibration of asymmetric and symmetric COO-of deprotonated carboxylate functional groups [49], and a 1045 cm −1 band caused by a C-O-C stretching mode [17]. Thus, the FTIR results indicated that the KOH-activated biochar had much more functional groups, e.g., hydroxyl groups and carboxyl groups, which are critical for the adsorption of NH 4 + -N by electrostatic interaction [50]. Moreover, Boehm titrations were applied to analyze the oxygen-containing functional group contents of the two obtained biochars. The result showed that carboxyl and lactonic groups were present in KOH-activated biochar at 0.21 mmol/g and 0.08 mmol/g, respectively, much higher than the levels in NaOH-activated biochar (0.03 mmol/g and 0.02 mmol/g). Physical and chemical adsorption are two main factors for effective NH 4 + -N adsorption by biochar [17]. The physical adsorption capacity depends mainly on the adsorbent porous structure, since the adsorbate is captured through nonselective Van der Waals forces [17]. Nevertheless, chemical adsorption relates to the properties of the adsorbate and the contents of functional groups on the adsorbent surface. Thus, for physical adsorption, the adsorption capacity relies primarily on the specific surface area of the adsorbent, which quantitatively represents the porous structure [21].
Here, the NaOH-activated biochar had higher specific surface area compared with the KOH-activated biochar but lower NH 4 + -N adsorption capacity, suggesting that physical adsorption is not the main factor in NH 4 + -N adsorption by the biochar. According to the FTIR results, KOH-activated biochar has a much higher content of functional groups compared with NaOH-activated biochar and, accordingly, has higher adsorption capacity. Therefore, chemical adsorption is the main reason for the better NH 4 + -N removal ability of KOH-activated biochar, which was also confirmed by the activation energy from the kinetics results.
carboxylate functional groups [49], and a 1045 cm −1 band caused by a C-O-C stretching mode [17]. Thus, the FTIR results indicated that the KOH-activated biochar had much more functional groups, e.g., hydroxyl groups and carboxyl groups, which are critical for the adsorption of NH4 + -N by electrostatic interaction [50]. Moreover, Boehm titrations were applied to analyze the oxygencontaining functional group contents of the two obtained biochars. The result showed that carboxyl and lactonic groups were present in KOH-activated biochar at 0.21 mmol/g and 0.08 mmol/g, respectively, much higher than the levels in NaOH-activated biochar (0.03 mmol/g and 0.02 mmol/g). Physical and chemical adsorption are two main factors for effective NH4 + -N adsorption by biochar [17]. The physical adsorption capacity depends mainly on the adsorbent porous structure, since the adsorbate is captured through nonselective Van der Waals forces [17]. Nevertheless, chemical adsorption relates to the properties of the adsorbate and the contents of functional groups on the adsorbent surface. Thus, for physical adsorption, the adsorption capacity relies primarily on the specific surface area of the adsorbent, which quantitatively represents the porous structure [21]. Here, the NaOH-activated biochar had higher specific surface area compared with the KOHactivated biochar but lower NH4 + -N adsorption capacity, suggesting that physical adsorption is not the main factor in NH4 + -N adsorption by the biochar. According to the FTIR results, KOH-activated biochar has a much higher content of functional groups compared with NaOH-activated biochar and, accordingly, has higher adsorption capacity. Therefore, chemical adsorption is the main reason for the better NH4 + -N removal ability of KOH-activated biochar, which was also confirmed by the activation energy from the kinetics results.

Conclusions
Two biochars from sorghum distillers grain were prepared through KOH and NaOH activation followed by pyrolysis for the first time. They both presented enhanced NH 4 + -N adsorption capacity compared with the original biochar, owing to the significantly enhanced specific surface area after activation. The KOH-activated biochar had higher adsorption capacity than did the NaOH-activated biochar because of its much higher content of functional groups. The kinetics of NH 4 + -N adsorption onto KOH-activated biochar followed a pseudo-second-order model, and the adsorption equilibrium data suggested that NH 4 + -N adsorption fitted the Freundlich model well. The thermodynamic parameters demonstrated that the adsorption process was endothermic and spontaneous. The maximum adsorption capacity was found to be 14.34 mg/g at 45 • C, which was satisfactory when compared with other biochars prepared from different feedstocks. This work demonstrates that chemically activated biochar from sorghum distillers grain is a novel and efficient adsorbent for NH 4 + -N removal.