Evaluation of NH + 4 Adsorption Capacity in Water of Coffee Husk-Derived Biochar at Different Pyrolysis Temperatures

Ammonium ( NH + 4 ) is a pollutant that can be harmful to the water environment. The purpose of this study is to access NH + 4 removal capacity from water by coﬀee husk-derived biochar. The properties of biochar prepared at diﬀerent temperatures (300, 450, and 600 ° C) were determined including TOC, and pH, pH pzc , functional groups of H + /OH − , cation-exchange capacity (CEC), and the characteristics of groups of organic matter (FT-IR spectrum) were identiﬁed and evaluated. The trend of NH + 4 adsorption equilibrium and kinetics of biochar have been studied. The experimental design of adsorption equilibrium was carried out by exposing biochar to a NH + 4 solution at diﬀerent concentrations, ranging from 0 to 50mg NH + 4 /L for 12 hours. Kinetic surveys were carried out when biochar was exposed to a solution containing 8.3mg NH + 4 /L for a varying length of time. The results showed that Langmuir and Freundlich models and the pseudo-second-order kinetic model are suitable to explain the NH + 4 adsorption equilibrium and kinetics on the biochar forms derived from coﬀee husk. Biochar derived from coﬀee husk prepared at lower pyrolysis temperature has a higher adsorption capacity. The results suggest that the biochar could be used as an adsorbent ammonium from water.


Introduction
Nitrogen is present in natural water in the forms of organic nitrogen, ammonia, nitrate, and nitrite. In most cases of raw wastewater, nitrogen is normally present in the organic nitrogen and NH + 4 . Ammonia also can be naturally generated in the environment [1]. e presence of NH + 4 at high concentrations can contribute to eutrophication and the subsequent devastation of aquatic life. e residual amount of ammonium in water causes ecological issues in relation to eutrophication, acidification of freshwater ecosystems, and in anoxic conditions, poisoning benthic organisms and fish [2]. According to Eddy, the ammoniac tolerance in freshwater fishes ranges from 0.07 to 2.00 mg/L [3]. At a dose of more than 33.7 mg of ammonium ion per kg of body weight per day, it influences the metabolism by shifting the acid-base equilibrium, disturbing the glucose tolerance, and reducing the tissue sensitivity to insulin [4]. erefore, removal of nitrous ammonium in polluted water is necessary. Many methods of removing NH + 4 from water have been used, including ion adsorption and exchange, which is more effective than the others, yet of high cost [5]. Looking for new renewable materials that can be used in ion adsorption and exchange is a crucial research topic. Biochar prepared from agricultural waste is a useful material due to its efficiency, low cost, environment friendliness, and availability in large quantity [6]. Adsorption of inorganic pollutants by biochar is the result of (i) ion exchange, (ii) electrostatic attraction, or (iii) surface precipitation [7]. In the case of ammonium, mostly electrostatic exchange and interaction is used; moreover, Khalil et al. stated that biochar's surface chemistry is more important than the material's surface area [5,8]. However, surface chemistry properties are controlled by the pyrolysis condition, especially temperatures, heating, and heat-keeping time [9]. Ammonium adsorption capability by biochar which was prepared with varying pyrolysis temperatures among research is not always alike [10]. ere have been studies which use biochar derived from corn plants, red oak (Quercus rubra), maple trees, and wheat plants to adsorption of ammonium [10].
Vietnam is the No. 1 producer of robusta coffee in the world, accounting for more than 40% of the global output in the 2019-2020 marketing year [11]. Coffee husks are the major solid residues from the processing of coffee, for which there are no current profitable uses, and their adequate disposal constitutes a major environmental problem. Dak Lak is a leading coffee-producing province in Vietnam with over 200,000 ha of coffee fields, and its annual coffee production reaches 450,000 tons. Together with about 250,000 tons of coffee husk to be discharged, it is a valuable material, yet has not been used efficiently [12].
us, innovative techniques and products for the profitable and adequate use of this type of residue are being sought. We have been also interested in the adsorption of metal ion, textile dyes, etc. on different materials [13][14][15]. In this work, we are reporting the assessment of NH + 4 in the water adsorption capability of biochar derived from coffee husk at different pyrolysis temperatures. e composite samples were obtained from the coffee peeling process. Coffee husk was dried at 60°C for 24 hours and stored in polyethylene bags [16].

Chemicals.
All chemicals used in the study were of analytical grade. Storage solution concentration was NH + 4 1000 mg/L. Sample water was distilled water and was purified by the model EASYpure II RF from ermo Scientific, USA. Instruments must be cleaned by being filled with nitric acid for 24 hours and then cleaned by demineralized water [17].

Experimental Design.
e biochar modulation refers to the research of Yoo et al., where processed coffee husk was furnaced in a Naberthem P330 furnace at 300, 450, and 600°C [18]. e heating rate was set to 10°C.min −1 . Once the desired temperature was reached, the temperature was kept constant for 2 hours and the samples were let to cool in the oven overnight. e biochar was then pressed through a plastic sieve (hole diameter of 1 mm) to make it homogeneous and was stored separately in polyethylene (PE) containers in dark at 4°C [18]. Biochar samples were analyzed and used to conduct equilibrium and kinetics experiments. Parameters including recovery efficiency and surface functional groups were determined. Analyses were conducted on these biochar samples to determine characteristics of surface functional groups such as pH and pH pzc [19], total organic carbon (TOC) [20], functional group H + / OH − [21], and cation-exchange capacity (CEC) based on the Walkley Black method. Changes in biochar's functional groups were analyzed by reflectance spectroscopy FT-IR-4700 type A with 350-4000 cm −1 resolution.
Equilibrium experiment of NH + 4 ion adsorption on biochar refers to the work of Khalil et al. and Xue et al. [5,22]. e experiment was performed in 50 mL polypropylene tubes, and 0.3 g biochar was mixed with 30 mL of NH + 4 solution. e concentration of the diluted NH + 4 solution varied between 0 and 50 mg·L −1 . Two drops of chloroform were added to prevent microbial activities. e initial pH of the solutions was adjusted to 5.0-5.5 by adding either dilute HCl or NaOH solutions (pH value is close to that of natural surface water sources). e mixtures were then shaken by using a GFL3015 orbital shaker with frequency 150 round-per-minute (rpm) in 12 hours (which is the time for the NH + 4 adsorption to reach the equilibrium, determined by preliminary experiments.). After that, the solutions were not adjusted during this experiment. e solid settlement was separated from the mixture by using a DLAB DM0636 centrifuge at 4000 rpm for 15 min. en, the remaining solution was filtered through a 0.22 µm filter. NH + 4 was determined according to ISO 7150-1: 1984 (E). Langmuir and Freundlich adsorption isotherms models were used in evaluating the suitability of experimental data.
Ion adsorption kinetics survey was conducted by mixing 0.3 g biochar and 30 mL of 8.3 mgNH + 4 /L solution. e mixture was then shaken at 150 rpm. All the samples went through shaking intervals 5; 10; 15; 20; 30; 40; 60; 90; and 120 minutes and then were filtered to conduct NH + 4 analysis. Pseudo-first-and second-order kinetic models were used to consider experiment data and to estimate kinetic parameters.

(i) Productivity efficiency of biochar is
where m 0 (g) is the initial coffee mass before being furnaced; m b (g) is the biochar mass after being furnaced. (ii) pH pzc of biochar is where pH i : initial pH value; pH f : pH value after biochar is added to the 0.01 M KCl solution (they were shaken for 2 h and allowed to settle for 48 h).
Plotting ΔpH according to the initial pH, pH pzc is where the pH curve overcomes ΔpH � 0 [19].
2 International Journal of Agronomy (iii) Adsorption equilibrium: Adsorption capacity, mg/g: where C 0 (mg/L) is the initial NH + 4 ion concentration, C i (mg/L) is the adsorbed NH + 4 concentration at equilibrium, V(L) is the NH + 4 solution volume, m (g) is the adsorbent mass (biochar), And q i (mg/g) is the NH + 4 adsorption capacity at equilibrium. Langmuir isothermal equation: where q 0 (mg.g −1 ): NH + 4 maximum adsorption capacity at equilibrium; K L (L·mg −1 ): Langmuir adsorption constant.
e abovementioned equation has the form of y � ax + b, and therefore, it could be solved by curve fitting y � 1/q i and x � 1/C i to yield R 2 , q 0 . Freundlich isothermal equation: where n F is the Freundlich isothermal constant for NH + 4 adsorption intensity; K F is the Freundlich isothermal adsorption constant for adsorption capacity.
is equation also has the form of y � ax + b, and as all the parameters are known, the plot of logq i against log C i can be drawn.
(iv) Adsorption kinetics: Pseudo-first-and second-order (PFO and PSO) reaction models were usually used to study the adsorption mechanism, in order to assess adsorption kinetics parameters.
Pseudo-first-order kinetics equation: Ln(q e −q t ) is plotted according to t. Pseudo-second-order kinetics equation: e abovementioned equation has the form of y � ax + b, and therefore, it could be solved by curve fitting y � 1/q t and x � 1/t to yield R 2 , q e is the NH + 4 adsorption capacity at equilibrium (mg/g), q t is the NH + 4 adsorption capacity at time t, k 1 (1/ minute) and k 2 (g/mg .minute) are the pseudo-first-and secondorder kinetics constants, and t (minute) is the adsorption time.

Data Processing. Control and replicate samples (n � 3)
were employed in order to minimize error sources. e analysis evaluated the accuracy and precision of the parameters calculated using the methods described in the previous section. SPSS 20.0 was used to determine variance's homogeneity, and root mean square of the results was then calculated with p value <0.05 using Tukey's test post hoc if Sig >0.05 or Tamhane if Sig <0.05.

Influences of Chemical Synthesis Temperatures on Productivity Efficiency and Surface Chemistry of Biochar.
e study showed that chemical synthesis temperatures affect productivity efficiency and surface chemistry of coffee husk biochar; Table 1 illustrates that, with increasing temperatures of 300, 450, and 600°C, recovery efficiency (%H) decreased at 51.4, 34.9, and 30.7%, respectively. is was due to the reduction of volatile substances in biochar [8]. e results were consistent with those from the work of Yavari et al. which reported a decrease in recovery efficiency when biochar pyrolysis temperature rises from 300 to 700°C [23,24]. Biochar acquired at 300°C was statistically significantly different to the ones at 450 and 600°C, while biochar produced at 450 and 600°C was not statistically significantly different.
is was related to the production of organic acids and phenolic compounds due to thermal decomposition of cellulose and hemicellulose compounds in the pyrolysis temperature range of the study [23] and the removal of water, hydrocarbons, H 2 , CO, and CO 2 during the carbonization process [8]. %TOC of biochar samples had statistically significant differences. pH values at 7.59, 9.16, and 9.69, respectively, to the biochar pyrolysis temperatures 300, 450, and 600°C, the pH values were also statistically significantly different (Table 1). is is due to the partial transformation of organic matters into ashes, which releases alkali salts and increases the pH level of biochar [25].
Similar to pH, pH pzc values at 7.2, 8.2, and 9.5, respectively, were also statistically significantly different. With the increase in biochar pyrolysis temperatures, the number of acid functional groups (H + ) decreased and the values of H + functional groups were statistically significantly different. e number of base functional groups (OH − ) increased at 11.17, 11.24, and 12.50 mmolOH − .g −1 , respectively, to the increasing biochar pyrolysis temperatures, and the value at 600°C was statistically significantly different from the ones at 300 and 450°C, whereas the values at 300 and 450°C were not.
is observation matched the conclusion reached by Mukherjee et al. and Yang et al. el. in regard to biochar originating from plant [26,27]. CEC values were 309, 290, and 266 mmol.kg −1 , respectively. Decreasing CEC values were statistically significantly different with increasing biochar pyrolysis temperatures, and a similar conclusion was stated by Fidel et al. [10].

International Journal of Agronomy
Analyzing the relationship between the parameters, Table 2 showed that the biochar pyrolysis temperatures were correlating and proportional to pH, pH pzc , and mmolOH − and reversely proportional to %H, mmolH + , %TOC, and CEC.
Analysis results by FT-IR reflectance spectroscopy, Figure 1, showed that the featured through series at 3251 cm −1 and 1575 cm −1 of biochar 300°C clearly indicated the presence of − OH and COO − groups, the results showed that the carboxylase group decreased with increasing biochar pyrolysis temperatures, and similar conclusions were stated by Fidel et al. [10] and uy and Do [28]. is suggested a decrease in the polar organic functional groups with an increase in pyrolysis temperature [8]. Series at 1076 cm −1 were assumed to be due to the decrease of C-O in connection with the increasing pyrolysis temperatures [29]. An aromatic hydrocarbon peak (789 cm −1 ) was observed at 450 and 600°C, which was in accordance with the study by Lugovoy et al. showing that lignin decomposition occurs at 280-500°C [30]. e interchangeable results were concluded by Park et al. who also studied biochar derived from coffee husk produced at 300, 400, 500, and 600°C [31]. + 4 in Water of Biochar. Results of the NH + 4 adsorption equilibrium experiments of biochar derived from coffee husk produced at varying temperatures showed that adsorption capacity increased with increasing the initial NH + 4 concentration in all 3 biochar forms and they all reached saturation in the experimental condition ( Figure 2). Increasing the initial NH + 4 concentration resulted in competition among cation in the solution, which promoted adsorption chance and capacity [5], specifically when raising C 0 from 0 to 50 mgNH + 4 · L − 1 , the maximum capacity at equilibrium (saturated) was 1.8, 1.4, and 1.1 mg·g −1 , respectively, to biochar produced at 300, 450, and 600°C ( Figure 2). One-way ANOVA on data of the adsorption showed that ammonium adsorption values varied and there were statistically significant differences in initial ammonium concentration ranging from 0 to 25 mg·L −1 . Ammonium concentrations from 25 to 50 mg·L −1 were statistically insignificantly different. is indicated the adsorption process which reached saturation. e same explanation is also used for the case of biochar at 450 and 600°C ( Figure 2). e data (Figure 2, Table 3) showed that Langmuir and Freundlich models were suitable to describe the NH + 4 adsorption, with R 2 greater than 0.95 in all the 3 biochars, and maximum adsorption values are 1.5, 1.2 and 0.9 mg g −1 , respectively, to 300, 450, and 600°C biochar, which fitted the experiment values. Other studies also reported that the ammonium adsorption onto some biochars followed both the Langmuir and Freundlich models [32,33]. R 2 values of the Langmuir model were 0.97, 0.95, and 0.96, respectively, which were slightly less than those of the Freundlich model, 0.97, 0.99, and 0.97, respectively. erefore, the Freundlich adsorption isotherm appeared to be more favorable in describing the adsorption of ammonium onto the coffee husk-derived biochar surface than the Langmuir model. e Freundlich model indicated the heterogenous nature of the biochar surfaces. In light of the heterogeneity nature of the biochar surface, ammonium adsorption would occur at multiple layers onto sites where nonuniform energy distribution could occur. Similar observations were also found in the study of Fan et al. [8,34]. e adsorption capacity decreased with increasing biochar pyrolysis temperatures [8,28]. e decrease in adsorption capacity may be due to reduction of organic groups such as -COOH and -OH on the surface of biochar samples (Figure 1). e study used biochar produced from maple trees at 500°C and generated a Langmuir maximum adsorption capacity of 0.99 mg NH + 4 .g −1 at pH 5.9 [35]. In the study of uy and Do, ammonium adsorption capacity on biochar derived from coffee husk was relatively high after 6 hours with maximum 2.8 mg·N.g −1 biochar [28]. e higher adsorption capacity can be attributed to the biochar being prepared at 350°C, but the heat retention time is much shorter, only 1 hour.

Adsorption Equilibrium NH
Langmuir adsorption constantly follows a downward trend with increasing temperatures, and the same conclusion is found in the work of Fidel et al., on certain biochar [10]. In terms of the Freundlich model, n F greater than 1 indicates exposure to the adsorption (for n F represents the surface exchange intensity or surface heterogeneity) [5].
According to the work in [6], better adsorption of the NH + 4 of biochar with lower pyrolysis temperatures was assumed to be related to cation-exchange capacity (CEC) with greater quantity carboxylase groups on the biochar's surface, where adsorbing positions locate, and this finding fits the results of this study. e results showed that NH + 4 adsorption on coffee husk biochar occurred mainly by cation H + on functional groups containing surface oxygen; therefore, high adsorption capacity means low pyrolysis temperature. Additionally, the adsorption capacity shrinks with increasing temperatures because of the increasing competition among ions in organic minerals (K, Fe, Zn, Ca, and Mg) in biochar with increasing temperatures [36]. Similar results were also found in the study of Fidel et al., which suggested that, with increasing pyrolysis temperature, the NH + 4 adsorption capacity of biochar decreased [10].
is finding was also mentioned by Gao et al. who studied the ammonium adsorption capability of biochar from peanut shells, corn cobs, and cotton tree trunks at 300, 450, and 600°C and kept in 2 hours [33]. e study's result matched with the work of of Begum et al. of pyrolyzed woodchips at 700°C whose result was 0.96 mg.g −1 [32].
To investigate the main factors influencing biochar's ability to adsorb NH + 4 − N, correlations between q 0 , some properties of biochar, biochar pyrolysis temperature, and CEC of biochars were analyzed, Table 4. Maximum NH + 4 adsorption capacity q 0 was positively correlated with the total organic content in the biochar (r 0.986) ( Table 4). is indicated that the removal of the organic functional groups with increasing pyrolysis temperature induced the decreasing NH + 4 adsorption capacity of biochar. CEC seemed to be the dominating factor influencing the NH + 4 adsorption capacity of biochar. e q 0 values were positively correlated with CEC (r 0.954). at is to say, the biochar with higher CEC values had larger NH + 4 adsorption capacity. e q 0 values were negatively correlated with biochar pyrolysis temperature, t°C (r −1, 000) so that NH + 4 adsorption capacity of the biochar also decreased with increasing pyrolysis temperatures.

e NH +
4 Adsorption Kinetics of Biochar. e NH + 4 adsorption in the water process of biochar with varying pyrolysis temperatures and exposure time periods, Figure 3, showed that adsorption kinetics occurs in 3 kinetic phases: the fast phase, slow phase, and ultraslow phase. e adsorption was dramatic within the first 15 minutes in 300, 450, and 600°C biochars. e adsorption capacity reached 82.0, 83.8, and 84.2%, respectively, to the 3 biochars and then slowed down and reached the equilibrium after 45 minutes. e speed of the reaction can be explained in the fast phase, where some anionic groups with negative charges were located on the surface of the adsorbent such as -COO − . ese groups rapidly interacted with ammonium ions. en, the NH + 4 went into a slow process, which represented the adsorption in the internal part of the biochar [5]. So, the NH + 4 adsorption on biochar goes through (hoặc chia thì quá khứ) two reaction processes.  International Journal of Agronomy According in ANOVA analysis, differences in average experiment results showed, that after 30 minutes, the increasing NH + 4 adsorption capacity was not statistically significant (p < 0.05) in all the 3 biochars (Figure 3). is showed that the process reached a saturated adsorption state.
Simulations from the first-order and second-order models described the experimental data fairly well with R 2 values larger than 0.89. However, only the pseudo-secondorder kinetic model with q e derived from the model and the experiment was satisfactory; specifically, the modelled   (Table 5). erefore, using the pseudo-second-order kinetic model to explain the NH + 4 adsorption kinetic of biochar derived from coffee husks was suitable. e same was also confirmed from the finding of Khalil et al. [5]. is also means that the kinetics was controlled by valency-related adsorptions by sharing or ion-exchanging between adsorbents and adsorbates [37]. e k 2 parameter did not behave in a particular trend when increasing the pyrolysis temperature from 300 to 600°C.

Conclusions
Coffee husks acquired from Krong Buk Town (Dak Lak Province, Vietnam) were used to transform into/produce biochar. Recovery efficiency and surface physical-chemical properties of the biochar (TOC, pH, pH pzc , H + and OH − groups, and CEC) at different temperatures were determined. e results showed that biochar pyrolysis temperatures were positively correlated with pH, pH pzc , and mmolOH − and negatively with %H, mmolH + , %TOC, and CEC. e study showed biochar derived from coffee husks with lower pyrolysis temperature had higher NH + 4 adsorption capacity. Both Langmuir and Freundlich isotherm models fit the NH + 4 adsorption process with R 2 in range 0.91-0.98. Time to reach adsorption equilibrium ranged from 15 to 30 minutes. e pseudo-second-order kinetic model was used to explain NH + 4 adsorption kinetics of coffee husks to derive biochar. In conclusion, a new idea of researching on the application of biochar produced from coffee husks in water treatments by NH + 4 adsorption is based.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare no conflicts of interest.