Reusing Cow Manure for the Production of Activated Carbon Using Potassium Hydroxide (KOH) Activation Process and Its Liquid-Phase Adsorption Performance

: In this work, cow manure (CM) was reused as a potential precursor in the production of activated carbon (AC) using a potassium hydroxide activation process at di ﬀ erent temperatures (i.e., 500, 600 and 700 ◦ C). The optimal activated carbon from cow manure (CM-AC) with high speciﬁc surface area ( ca. 950 m 2 / g) was further investigated for its adsorption performance in the removal of a model compound (i.e., methylene blue) from aqueous solution with various initial concentrations and adsorbent dosages at 25 ◦ C. It was found that the resulting AC could be an e ﬀ ective adsorbent for removal of cationic dye from aqueous solution in comparison with a commercial coal-based AC. Based on the observations of the energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy (FTIR), the CM-AC adsorbent has a stronger interaction with the cationic compound due to its more oxygen-containing complex on the surface. Furthermore, the adsorption kinetic parameters ﬁtted using the pseudo-second order model with high correlations were in accordance with their pore properties.


Introduction
Water pollution caused by organic pollutants (e.g., dye, pesticide) is a serious issue because it can degrade water quality and render it toxic to public health and the environment. In general, there are two advanced treatment processes for removal of contaminants from the water bodies [1]. One belongs to destructive processes such as ozone oxidation and the Fenton method [2]. Other processes are recuperative. Among the recuperative process, adsorption may be the most used method due to its features of easy operation, high efficiency and simple design [3]. Therefore, carbon adsorption by granular activated carbon (GAC) or powdered activated carbon (PAC) has been considered as one of the best available technologies in the water and wastewater treatment processes for removal of organic pollutants [4,5]. However, the adsorption process is not a low-cost method because commercial activated carbon (AC) products have higher prices and the regeneration or disposal when they are exhausted will generate additional issues. In recent years, the use of low-cost precursors in the preparation of AC for the removal of various types of pollutants from water and wastewater has been reviewed [6][7][8][9][10][11]. Obviously, utilizing organic wastes or residues as AC precursors poses several advantages such as climate change week. The methods for characterizing its main thermochemical properties were similar to the previous studies [26,39], showing that the dried CM is composed mainly of volatile matter (about 80 wt%) with 42 wt% carbon. The potassium hydroxide (KOH; Merck Co., Darmstadt, Germany) was used to impregnate CM to produce AC using chemical activation. As methylene blue was commonly selected as a probe compound for determining the adsorption capacity of AC quickly [37], this cationic dye, which was purchased from Merck Co., was used as an organic pollutant target in the work. In order to compare the adsorption performance of the resulting AC with commercial AC, a product (Filtrasorb-300; Calgon Carbon Co., Moon Township, PA, USA) for liquid-phase applications was used to remove methylene blue (MB) at the same adsorption conditions.

Impregnation and Activation Experiments
The preparation of AC from CM was carried out using the KOH activation process referred to in a previous study [40]. In the present study, the CM impregnated with KOH (KOH-CM) was described as follows: 10 g of CM was mixed with 5 g of KOH in 150 cm 3 of de-ionized water and stirred using a magnetic bar at about 75 • C for 30 min, and then decanted into the supernatant prior to being dried at about 105 • C for 24-48 h. Subsequently, the KOH-CM (about 3 g) was activated by passing nitrogen gas (500 cm 3 /min) at a heating rate of about 10 • C/min up to the specified temperature (i.e., 500, 600, and 700 • C, respectively). Similarly to the procedures in the impregnation with KOH, the AC product was poured into 3 N HCl solution, then heated at about 75 • C for 30 min and finally rinsed with deionized water (150 cm 3 ) three times to remove the residual KOH and inherent ash minerals effectively. Prior to the determination of pore properties, the resulting AC products (denoted as CM-AC-500, CM-AC-600 and CM-AC-700) were dried at about 105 • C for 24 h.

Characterization of Resulting and Commercial ACs
The pore properties of the resulting AC (CM-AC) products and the commercial AC (Filtrasorb-300) were determined by its N 2 adsorption-desorption isotherms at the temperature of liquid nitrogen (i.e., −196 • C) on the Micromeritics ASAP 2020 accelerated surface area and porosimetry system. Based on the analytical methods applied, the types of pore characterization included specific surface area, pore volume and pore size. For example, the specific surface area of the resulting AC can be obtained using the Brunauer-Emmett-Teller (BET) method in the relative pressure (P/P 0 ) range of 0.05-0.30. The total pore volume is indicative of the liquid nitrogen amount adsorbed at P/P 0 of about 0.99 [41]. According to the values of the BET surface area and total pore volume, the mean pore diameter or width can be further estimated by assuming the cylindrical geometry [42].
In order to elucidate the elemental compositions of AC on the surface, energy dispersive X-ray spectroscopy (Swift ED3000, Oxford Instruments, Abingdon, UK) was conducted to observe their contents of carbon, oxygen and other elements. The data could be associated with the interaction between the AC adsorbent and target adsorbate in the liquid-phase adsorption system. Using the potassium bromide (KBr) pellet technique, the Fourier infrared spectrometer (FT/IR-4600, JASCO Co., Tokyo, Japan) was further used to measure the functional groups on the surface of the resulting AC (CM-AC) and the commercial AC (Filtrasorb-300) in the range 4000-400 cm −1 .

Adsorption Performance Experiments
Following the adsorption experiments referred to in a previous study [43], the batch adsorption kinetics of the CM-AC-700 were carried out in a 3-L mixing tank with four baffles. In the work, the data on the adsorption uptake of methylene blue (MB) by the MC-AC-700 from aqueous solution (2 liters) were obtained at the fixed solution temperature of 25 • C and agitation speed of 200 rpm under the determining adsorption process parameters, including initial MB concentrations (i.e., C 0 = 5, 10, 15 and 20 mg/L) and AC dosages (i.e., 0.1, 0.3 and 0.5 g, based on the solution volumes of 2 L, which were abbreviated by 0.1, 0.3 and 0.5 g/2 L). During the adsorption experiments, an aliquot solution (about 10 cm 3 ) was taken out at specified intervals (i.e., 5, 10, 20, 30, 40, 50 and 60 min).
After filtrating with mixed cellulose esters membrane filter with a size of 25 mm, the analysis of dye concentration (i.e., C t ) in the remaining filtrate solution was immediately measured with a UV/Visible spectrophotometer (U-2900, Hitachi Co., Tokyo, Japan) at the maximum absorption wavelength (i.e., 661 nm). It should be noted that about half of the solution sample after the filtration was first discarded to dye the filter completely. The adsorbed amount of MB (q t , mg/g) was obtained by the AC mass, solution volume (i.e., 2 L), and the difference between C 0 and C t . On the other hand, this work also compared the adsorption performance of a commercial AC (i.e., Filtrasorb-300) with the CM-AC-700. The adsorption conditions were fixed at an initial MB concentration of 10 mg/L, AC dosage of 0.3 g/2 L, agitation speed of 200 rpm, and temperature of 25 • C. Table 1 listed the pore properties of the resulting ACs (CM-AC) and commercial AC (Filtrasorb 300), including surface area, pore volume and average pore diameter. Obviously, an increase in the activation temperature created the higher pore properties due to the development of more micropores. In addition, CM-AC-700 possessed favorable pore properties by comparison with Filtrasorb-300. For instance, the BET surface area of CM-AC-700 (i.e., 946 m 2 /g) is slightly higher than that of Filtrasorb-300 (i.e., 660 m 2 /g). Based on the data of surface area, regardless of the calculation methods, the ratio of that of Filtrasorb-300 to that of CM-AC-700 is close to 0.70. Using the data of total pore volume, the ratio of that of Filtrasorb-300 to that of CM-AC-700 was 0.68, close to 0.70. This result gave an indication of the pore size difference between CM-AC-700 and Filtrasorb-300. As mentioned above, we confidently inferred that the contribution to total pore volume of Filtrasorb-300 should be mostly from micropores. These results were in accordance with their average pore diameters (3.04 vs. 2.32 nm, listed in Table 1). As shown in Table 1, the BET surface area values of the resulting AC products (CM-AC-600 and CM-AC-700) are higher than that (445 m 2 /g) of biomass-based AC prepared from plant (Jatropha curcas L.) root with similar KOH activation conditions [44]. For large adsorbate molecules having similar dimensions to MB, CM-AC-700 could be preferable to remove them from the aqueous system due to the steric sizes of adsorbate molecules [45]. To describe pore structures of the AC studied, the adsorption-desorption isotherms of nitrogen (N 2 ) at liquid nitrogen temperature (i.e., −196 • C) is the commonly used method [41]. Figure 1 showed the N 2 adsorption-desorption isotherms of CM-AC-700 and Filtrasorb-300. Obviously, these ACs belonged to type I isotherms due to their microporous feature (i.e., pore width < 2 nm) [46]. Because of its high potential for adsorption, the micropore filling occurs significantly at P/P 0 of < 0.05 [46]. On the other hand, the hysteresis loops occurred at the isotherms from the adsorption to desorption Processes 2019, 7, 737 5 of 12 route, especially for CM-AC-700. It is well known that the loop is associated with mesoporous solids, where capillary condensation occurs at higher relative pressures. As listed in Table 1, these isotherms ( Figure 1) also confirmed that CM-AC-700 has larger pore properties with wider pore size in comparison with Filtrasorb-300. Furthermore, the hysteresis loop pattern can be correlated with corresponding pore shapes. According to the classification of adsorption hysteresis loops recommended by the International Union of Pure and Applied Chemistry (IUPAC), the isotherms in Figure 1 belong to the type H4 loops [41], which are given by many ACs and some other microporous adsorbents with slit-shaped pores.

Pore Properties of Resulting and Commercial ACs
Processes 2019, 7, x FOR PEER REVIEW 5 of 12 mesoporous solids, where capillary condensation occurs at higher relative pressures. As listed in Table 1, these isotherms ( Figure 1) also confirmed that CM-AC-700 has larger pore properties with wider pore size in comparison with Filtrasorb-300. Furthermore, the hysteresis loop pattern can be correlated with corresponding pore shapes. According to the classification of adsorption hysteresis loops recommended by the International Union of Pure and Applied Chemistry (IUPAC), the isotherms in Figure 1 belong to the type H4 loops [41], which are given by many ACs and some other microporous adsorbents with slit-shaped pores.  Obviously, the fast decrease in residual MB concentrations (Ct/C0) occurred at a limited time. It implies that strong interactions between the cationic dye and the resulting AC exist. In this work, the pseudo-second order model was used to fit the adsorption system [47]. In order to fit the adsorption kinetic data, its linear form can be expressed as:

Adsorption Performances of AC
where qt is the adsorbed amount of MB at time t (mg/g), qe is the adsorbed amount of MB fitted at equilibrium (mg/g), k is the rate constant (g/(mg.min)), and t is the time (min). Furthermore, the time required for the adsorbent (i.e., CM-AC) to take up half as much adsorbate (i.e., MB) as occurs at equilibrium (i.e., t = t1/2 as qt = qe/2) can be obtained by the following equation.
The fitted values of kinetic parameters for the adsorption system were listed in Table 2 and 3. Obviously, the kinetics of MB adsorption into CM-AC followed this model with high correlation coefficients (>0.99). As listed in Table 2, the fitted adsorption capacity (i.e., qe) decreased as the initial MB concentration (i.e., C0) increased. The decrease of qe with increasing adsorbent dosage implies the exhaustion of adsorption onto excessive adsorbent due to the limited MB molecules. On the other hand, the rate constant values significantly increased at higher adsorbent dosage because of the increase in more adsorption sites provided. This result agrees with previous studies with relation  Figures 2 and 3 show the adsorption performances of CM-AC-700 for MB removal at the specific adsorption conditions under various initial MB concentrations and AC dosages, respectively. Obviously, the fast decrease in residual MB concentrations (C t /C 0 ) occurred at a limited time. It implies that strong interactions between the cationic dye and the resulting AC exist. In this work, the pseudo-second order model was used to fit the adsorption system [47]. In order to fit the adsorption kinetic data, its linear form can be expressed as:

Adsorption Performances of AC
where q t is the adsorbed amount of MB at time t (mg/g), q e is the adsorbed amount of MB fitted at equilibrium (mg/g), k is the rate constant (g/(mg·min)), and t is the time (min). Furthermore, the time required for the adsorbent (i.e., CM-AC) to take up half as much adsorbate (i.e., MB) as occurs at equilibrium (i.e., t = t 1/2 as q t = q e /2) can be obtained by the following equation.
to the adsorption of dyes onto activated carbon and clay [45,48,49]. On the other hand, the fitted adsorption capacity (seen in Table 3) indicated an increasing trend as the initial MB concentration (i.e., C0) increased. This result can be expected because more adsorbate molecules were adsorbed at the equilibrium condition onto the adsorbent without exhausting its adsorption.   (Table 3).  (Table 2). to the adsorption of dyes onto activated carbon and clay [45,48,49]. On the other hand, the fitted adsorption capacity (seen in Table 3) indicated an increasing trend as the initial MB concentration (i.e., C0) increased. This result can be expected because more adsorbate molecules were adsorbed at the equilibrium condition onto the adsorbent without exhausting its adsorption.   (Table 3).  (Table 3). The fitted values of kinetic parameters for the adsorption system were listed in Tables 2 and 3. Obviously, the kinetics of MB adsorption into CM-AC followed this model with high correlation coefficients (>0.99). As listed in Table 2, the fitted adsorption capacity (i.e., q e ) decreased as the initial MB concentration (i.e., C 0 ) increased. The decrease of q e with increasing adsorbent dosage implies the exhaustion of adsorption onto excessive adsorbent due to the limited MB molecules. On the other hand, the rate constant values significantly increased at higher adsorbent dosage because of the increase in more adsorption sites provided. This result agrees with previous studies with relation to the adsorption of dyes onto activated carbon and clay [45,48,49]. On the other hand, the fitted adsorption capacity (seen in Table 3) indicated an increasing trend as the initial MB concentration (i.e., C 0 ) increased. This result can be expected because more adsorbate molecules were adsorbed at the equilibrium condition onto the adsorbent without exhausting its adsorption. Figure 4 shows the adsorption kinetic profiles of CM-AC-700 in comparison with that of the commercial AC at the same conditions, that is, initial MB concentration of 10 mg/L, CM-AC-700 dosage of 0.3 g/2 L, agitation speed of 200 rpm, and temperature of 25 • C. As listed in Table 4, CM-AC-700 exhibited a better adsorption performance for the removal of MB from the aqueous solution as compared to the commercial AC (Filtrasorb-300). This difference between the fitted values of adsorption capacities (i.e., q e ) can be associated with the interactions between the adsorbate and the adsorbent, including the pore properties of the adsorbent for physical adsorption and the electrostatic attraction interaction between the cationic dye and the negatively charged surface [44]. The observations using energy dispersive X-ray spectroscopy ( Figure 5) showed a higher content of oxygen on the CM-AC-700 surface in comparison with the adsorbent Filtrasorb-300, thus enhancing its polar nature (i.e., hydrophilicity, acidity) due to more oxygen complexes or groups on the surface [50]. These surface oxygen complexes and oxygen-containing functional groups (e.g., carboxylic, carbonyl, phenolic groups) could arise from its lignocellulosic constituents and ashes (consisting mainly of silica, alumina, alkaline and alkaline earth metals) in the starting materials (i.e., CM). The carbon contents of the resulting carbon materials were further analyzed using an elemental analyzer and showed a range from 60 to 65 wt%, while that of the commercial activated carbon (Filtrasorb-300) demonstrated a range up to about 90 wt%. These results were consistent with the energy dispersive X-ray spectroscopy analyses ( Figure 5). Therefore, the interaction between the cationic dye (MB molecule) and the adsorbent CM-AC-700 via stronger van der Waals forces or other binding forms also plays a vital role in the liquid-phase adsorption. On the other hand, Figure 6 depicted the Fourier transform infrared spectroscopy (FTIR) spectra of CM-AC700 and Filtrasorb-300. The peaks of CM-AC-700 at about 3440, 2400, 1630, 1380, 1180 and 620 cm −1 could be mostly associated with functional groups containing oxygen (i.e., O-H, C=O, C-O) [51], which were more significant than those of Filtrasorb-300. For example, there is a broad absorption peak at 3440 cm −1 in the CM-AC-700, which is assigned to the stretching vibration of O-H. In addition, a sharp peak at about 3440 cm −1 may correspond to the bending band of O-H. The interaction between the cationic dye and the adsorbent CM-AC-700 via stronger van der Waals forces or other binding forms also plays a vital role in the adsorption process.  (1) and Table 4.  symbols: experimental data, full lines: calculated from Equation (1) and Table 4.  (1) and Table 4.

Conclusions
The thermochemical conversion of cattle manure can be positively related to climate change mitigation (reduction of greenhouse gas emissions) and waste and wastewater management (improvement of environmental quality). In this work, based on the experimental results of the chemical, pore and adsorption properties for the resulting and commercial ACs, cattle (cow) manure could be an excellent precursor for producing AC at different temperatures (i.e., 500, 600 and 700 °C) using a KOH activation process. The conclusions can be itemized as follows:

Conclusions
The thermochemical conversion of cattle manure can be positively related to climate change mitigation (reduction of greenhouse gas emissions) and waste and wastewater management (improvement of environmental quality). In this work, based on the experimental results of the chemical, pore and adsorption properties for the resulting and commercial ACs, cattle (cow) manure could be an excellent precursor for producing AC at different temperatures (i.e., 500, 600 and 700 °C) using a KOH activation process. The conclusions can be itemized as follows:

Conclusions
The thermochemical conversion of cattle manure can be positively related to climate change mitigation (reduction of greenhouse gas emissions) and waste and wastewater management (improvement of environmental quality). In this work, based on the experimental results of the chemical, pore and adsorption properties for the resulting and commercial ACs, cattle (cow) manure could be an excellent precursor for producing AC at different temperatures (i.e., 500, 600 and 700 • C) using a KOH activation process. The conclusions can be itemized as follows: - The nitrogen adsorption-desorption isotherms of the resulting ACs showed that an increase in the activation temperature created the higher pore properties due to the development of more micropores. - The optimal AC product (i.e., CM-AC-700) showed its microporous feature with the pore properties of a BET surface area of 946 m 2 /g and total pore volume of about 0.72 cm 3 /g, which are close to those of common commercial ACs. - The resulting AC could be an effective low-cost adsorbent for the removal of cationic dye (i.e., methylene blue, MB) from aqueous solution based on the fittings of the pseudo-second order model. - The resulting AC exhibited a better adsorption performance for the removal of MB from the aqueous solution as compared to the commercial AC. - The cow manure-derived adsorbent has a stronger interaction due to its larger pore properties and more oxygen-containing complex on the surface based on the observations of energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy (FTIR).