Synthesis and Characterization of Activated Carbon from Agrowastes for the Removal of Acetic Acid from an Aqueous Solution

In this study, activated carbons prepared from agrowastes by chemical activation were used to remove acetic acid from an aqueous solution through a batch process. The prepared adsorbents were characterized by SEM, XRD, FT-IR, and point of zero charge (pHpzc). The effects of adsorbent dosage, initial concentration, and contact time were considered. Equilibrium data was tested using Langmuir, Freundlich, Temkin, and Frenkel–Halsey–Hill models. The degree of adsorption of acetic acid increased for both adsorbents as contact time, and adsorbent dosage and initial concentration were increased. The adsorption data were described well by the (Freundlich=Frenkel–Halsey–Hill) models with the highest regression coefficient of R 2 = 0.9961 and R 2 = 0.9951 for Rice Husk Activated Carbon (RH-AC) and Potato Peels Activated Carbon (PP-AC), respectively. This suggests a multilayer through the existence of a heterogeneous pore distribution in the adsorbent surface. Kinetic data agreed well with pseudosecond-order ( R 2 = 0.999 and R 2 = 0.994 ) RH-AC and PP-AC, correspondingly. This indicates that the adsorption process was chemisorption in nature. The regeneration studies showed that the adsorbents prepared could be renewed and reused before losing their adsorbing affinity for acetic acid.


Introduction
Manufacturing industries still remain the backbone of the economy in both developing and developed countries. However, of late, rapid industrial growth has resulted into disastrous environmental damage and pollution. Effluents originating from manufacturing industries flowing into water bodies have result into surface and groundwater pollution [1]. Studies done in Egypt and Malaysia, for example, have detected a large array of chemical pollutants of industrial origin exceeding the permissible limits for wastewater. Among wastewater pollutants are organic and inorganic chemicals, heavy metals, petrochemicals, chloroform, and bacteria [2][3][4]. Most of the organic pollutants normally experience an oxidation process producing carboxylic acids such as dicarboxylic acids as final products. Maleic, acetic, and oxalic acids are well-known examples of dicarboxylic acids. Research findings have shown that high levels of these acids in potable water result into despicable health problems to human beings such as kidney stones, uremia, corrosion of enamel and mouth, vomiting, and hematemesis, just to mention a few [5].
Acetic acid, in particular, is among the organic compounds recently reported in wastewater as a contaminant despite it being a key component very useful in various industrial processes such as the manufacturing of highgrade phosphate fertilizer, as a rust proofing agent of iron metals, and as baking powder as well as phosphate syrup used in soft drinks or as a water softening agent [1]. There are several techniques which are used in order to remove organic pollutants in water, main of which include the following: phytoremediation, bioremediation, reductive and oxidative process, separation by membranes, liquid-liquid extraction, and adsorption [6]. Of all these processes, adsorption still remains the best and widely recognized method of purification for very good reasons. First, it is simple and easy to operate. Second, it is cheap and so far the most effective purification technique for various organic pollutants from water and wastewater. In addition, it has been observed to produce high-quality treated effluents [7][8][9]. On adsorbent materials, there are a variety of them such as silica gel, zeolites, synthetic adsorbents (resins), clays, activated alumina, and activated carbon which are used in the treatment processes of industrial wastewater containing chemical pollutants [10]. However, activated carbon is the most preferred adsorbent used to eliminate harmful chemical compounds from wastewater including acetic acid at low concentrations [11]. This is due to its high adsorption efficiency, its porous structure, and the presence of high surface area [12].
Generally, carbonaceous materials are typically prepared from materials rich in elemental carbon such as petroleum residues, peat, fossil coals, lignite, and coke, all of which are highly costly and nonrenewable [13]. As an alternative, investigators have successfully prepared affordable activated carbons from various sources of agrowastes [14][15][16].
There are two routes which can be adopted for synthesizing activated carbon for wastewater treatment, namely, physical and chemical. Physical activation starts with carbonization of the materials at elevated temperatures ranging from 500°C to 900°C in an inert atmosphere. The carbonized material is then activated by subjecting it to oxidative gases like carbon dioxide, oxygen, and nitrogen at temperatures ranging from 800°C to 1000°C [17]. The first step is performed to remove noncarbon elements while the final step is aimed at attaining surface porosity. Chemical activation, on the other hand, combines carbonization and activation into a single step, with the raw material being impregnated with a chemical activating agent and then activated in an inert atmosphere [18]. Chemical activation enjoys more advantages over physical activation. These include a lower activation temperature, a larger carbon yield, and more porosity [19].
Various published reports indicate that some researchers have prepared excellent activated carbon adsorbents from agricultural by-products and have successfully used them for the removal of inorganic pollutants in aqueous solutions [20][21][22][23]. Currently, there are no published research findings that we are aware of that have reported the use of rice husks and Irish potato peels as carbonaceous materials for the elimination of organic contaminants specifically acetic acids. While emphasizing on the need to eliminate the undesirable health effects of industrial wastewater on human beings and in quest to meet the suitable requirements of effluent quality set by the Malawi Bureau of Standards and Environmental Regulatory Agency, this research, presents a groundbreaking discovery of the use of low-cost activated carbons prepared from agrowastes through chemical activation processes to remove acetic acid in an aqueous solution.

Materials and Methods
2.1. Chemicals and Reagents. All chemicals used were of analytical grade. The phosphoric acid (H 3 PO 4 ), sodium hydroxide (NaOH), and hydrochloric acid (HCl) were supplied by Sigma Aldrich (Germany) while nitric acid, (KNO 3 ), and ethanoic acid (CH 3 COOH) reagents were provided by Minema Chemicals (PTY) Ltd (South Africa). All required solutions were made by dissolving the corresponding reagent in double-distilled water.
2.2. Source of Raw Materials. The precursors (rice husks and Irish potato peels) were collected from Songani and Zomba Free Market in Zomba, Malawi. The precursors were firstly washed with running tap water until the color of the materials was clear and then were washed in deionized water to completely remove the remaining surface impurities. The precursors were sun-dried for a day and then oven-dried at a temperature of 80°C overnight. The dried samples were then ground using an electric blender and were sieved to obtain precursor powder of particle size less than 500 μm. The powders were kept in air-tight container pending use.

Preparation of Adsorbents.
The activated carbons were successfully prepared through chemical activation using 40% orthophosphoric acid (H 3 PO 4 ) as an activating agent ( Figure 1). The impregnation ratio of 1 : 2 (solid/liquid) was adopted and then left in an electric oven at a    The mixture of KBr to adsorbent was set at 1 : 10 mg. The samples were examined, and spectra in the region of 4000 to 400 cm -1 were documented. X-ray diffractometer (Siemens) was used to assess crystalline phase (Model No.: 10190376, Germany). The scanning was conducted at a diffraction angle of (2θ) with a range of 10°to 90°. The surface structure of the sorbent produced was characterized by FESEM (JEOL JSM-7100F Japan).

Batch Adsorption Studies.
The adsorption studies were carried out in the batch process using 250 mL conical flasks at room temperature (25°C). 50 mL volumes of working adsorbate concentrations were prepared from acetic acid stock solution by dilution with double-distilled water to get the desired concentrations. The effect of initial adsorbate concentration (1, 1.25, 1.5, 1.7, and 2 M), adsorbent dose (0.1-0.8 g), and contact time (10-140min) was studied. The preferred mass of the adsorbent was measured using an electronic balance and transferred into a 250 mL conical flask containing 50 mL acetic acid. The contents were then agitated at a constant speed of 120 revolutions per min (rpm) for 120 min. At the end of each desired time, the samples were filtered in 0.22 filters to determine the concentration of the acetic acid left in the solution after the adsorption process had occurred. 10 mL of the filtrate was drawn and transferred into a clean conical flask; 2-3 drops of phenolphthalein indicator were added and then titrated with 0.1 M NaOH until the end point was attained. The process was repeated three times for every sample, and mean for residual concentration was then calculated. The removal percentage and adsorption capacity (Q e ) of acetic acid adsorbed by adsorbents were calculated by equations (1) and (2) below, respectively.
where C o (mg/L) is the initial concentration of acetic acid in solution and C e (mg/L) is the liquid concentration of acetic acid left in the solution after the adsorption process has occurred. W is the mass of adsorbent (g), and V is the volume of the solution (L). This implies that, as the pH of the solution decreases below pHpzc, the adsorbent surface gets protonated due to excessive competitiveness of hydrogen ion, promoting the adsorption of pollutants such as anions. So, if the pH of the solution is above pHpzc, hydroxide ions dominate in the solution resulting into deprotonation of the sorbent surface with negative charge, thereby favoring uptake of impurities such as cations [24]. pH pzc of the activated charcoal was almost equivalent to neutral point. Therefore, carbon samples were appropriate for the treatment of water containing acetic acid.

Results and Discussions
3.2. X-Ray Diffraction. Figure 2(b) shows the X-ray characteristics of the produced carbon materials. The findings demonstrate Bragg peak intensity in the 2θ = 20-30°range for both activated carbons. The presence of the hump indicates a high level of disorder, which is an indicative characteristic of activated carbons. Additionally, the absence of a strong peak indicated that there were no identifiable minerals in the carbon materials produced [15]. Similar findings were also reported in literature by several researchers who successfully derived agro-based carbonaceous materials [14,15]. 3.4. FESEM Results. FESEM micrographs were taken before adsorption of acetic acid and are shown in Figure 3. The micrographs indicated several cavities, high porous structure, and numerous interconnected pores that provide access to internal areas to adsorb acetate ions. The development of excellent surface characteristics of the produced activated carbons could be associated with chemical activation of the precursors at elevated temperatures (500°C) with the presence of an activating agent (40% H 3 PO 4 ), resulting in the substantial elimination of volatile materials and stimulating the opening and formation of additional pores (micropores and mesopores) [20]. This also implies that the presence of cracks enhanced the surface area of the synthesized carbon. For the availability of the high porous structure, the findings indicate that the carbonaceous materials were effective for acetic acid removal in aqueous solution.  Figure 4(a) indicate that as the contact time increased, the adsorption capacity of acetic acid increased from 0.95 to 2.11 mg/g and 1.04 to 2.268 mg/g thus for PP-AC and RH-AC, respectively. This could be elucidated because as time was prolonged, more adsorbates were accommodated on the active sites of the adsorbents until equilibrium was attained [28]. This pattern of results was observed for 120 min; thereafter, a plateau was seen to appear, and no significant adsorption of acetic acid was noticed. The decrease in adsorption capacity as the dosage was increased could be associated with the fact that at high dosages, the available acetate ions are not enough to cover the present vacant sites on the adsorbents resulting into low adsorption capacity [1].

Effect of Initial Adsorbate
Concentration. The influence of initial adsorbate concentration on the equilibrium adsorption capacity was also assessed, and the results for both adsorbents are displayed in Figure 4(c). The adsorption capacity for RH-AC increased from 0.11 to 0.25 mg/g, and the adsorption percentage decreased from 94 to 60% with an increase in initial adsorbate concentration. For similar initial concentrations, PP-AC the adsorption ability increased from 0.08 to 0.24 mg/g and adsorption percentage decreased from 90 to 54%. This visibly indicates that the mass gradient acts as a driving force for the adsorption process, and that the rise in adsorbate molecule concentration bulk in solution at constant adsorbent dose does boost the mass transfer process [29]. The reduction in percentage removal is due to the fact that at higher concentration, more acetate ions remain in the solution unadsorbed due to saturation of binding sites [30]. This occurs due to an increase in 3.6. Adsorption Isotherms. Adsorption isotherms are mathematical models commonly used to describe the characteristics of the adsorption process between adsorbent and adsorbate phases when the adsorption equilibrium is achieved at a constant temperature. To appreciate the adsorption capacity of agro-based activated carbon prepared for the uptake of acetate ions from an aqueous solution, four isotherms, namely, Langmuir, Freundlich, Temkin, and Frenkel-Halsey-Hill were used to validate the data obtained in adsorption studies of acetic acid.
3.6.1. Langmuir Isotherm. The Langmuir mathematical isotherm model assumes that the adherence of the adsorbate to the adsorbent occurs on a homogenous surface by monolayer adsorption without any interaction between adsorbed molecules or ions. The linearized form of Langmuir's mathematical model is denoted by the following equation [31]: where Q e is the monolayer adsorption capacity of the adsorbent (mg/g), C e is the equilibrium concentration (mg/L), Q m is the maximum adsorption capacity that can be taken up per mass of adsorbent (mg/g), and b (L/mg) is the Langmuir constant associated with the rate of sorption energy between adsorbate and adsorbent. The important characteristics of Langmuir isotherm can also be expressed in terms of a dimensionless constant separation factor or equilibrium parameter R L, which is explained as follows [32]: where b is the Langmuir constant and C o is the initial concentration of acetic acid. The value of R L indicates the type of the isotherm to be either unfavorable (R L > 1), linear (R L = 1), favorable (0 < R L > 1), or irreversible (R L = 0) [33].
The constants of this model were calculated from the plot of C e versus C e /Q e in Figure 5(a), and the values calculated are shown in Table 1.   log where K f ((mg/g) (L/g) 1/n ) and n are the Freundlich constants that are connected with the adsorption capacity and intensity, respectively. The coefficients of this mathematical model were calculated from the intercept and slope of the linear plot of log C e versus log Q e in Figure 5(b). The values attained are listed in Table 1.

Temkin
Isotherm. Temkin isotherm model postulates that because of indirect adsorbate/sorbate interactions, the heat of adsorption of the atoms adsorbed would linearly decrease with coverage. It also suggests that the adsorption process is characterized by a uniform distribution of binding energies at the adsorbent surface [35]. The Temkin isotherm linearized equation is given as equation (6) below:  Table 1. 3.6.4. Frenkel-Halsey-Hill Isotherm. This model is suitable for multilayer adsorption and heteroporous adsorbents. The Halsey adsorption isotherm in linear form is signified as per the equation below [36]: where Q e is the quantity of adsorption at equilibrium and C e is the adsorbate concentration at equilibrium. K H and n are Halsey's isotherm constants and are calculated from the slope and intercept of the plot ln Q e versus ln C e in Figure 5(d) with attained results are displayed in Table 1.
From Table 1, in terms of correlation coefficient (R 2 ) values obtained from the four isotherm models, the applicability of the isotherms is ranked in the order (Freundlich=-Frenkel-Halsey-Hill)>Temkin>Langmuir. The best fit is attained by Freundlich and Frenkel-Halsey-Hill isotherms with (all values of R 2 > 0:99) for both adsorbents suggesting the applicability of these isotherms in fitting the experimental data over other tested mathematical models under similar conditions. The results indicate that the adsorption of acetic acid followed a multilayer through the existence of a heterogeneous pore distribution in the adsorbent surface as Freundlich and Frenkel-Halsey-Hill isotherm postulations imply [36]. Furthermore, the efficacious fitting of the experimental data by Frenkel-Halsey-Hill model also confirms that the adsorbents were heteroporous in nature. Similar observa-tions were also reported in different studies for adsorption of acetic acid from aqueous solution using zeolite [37]. Table 2 provides the summary of the main results attained in the study and those reported in literature.

Kinetic Models.
Two kinetic models, namely, pseudofirst-order and pseudosecond-order, were used to identify the adsorption process. The linear equations of firstorder were elucidated using the equation below [38]: where q e (mg/g) and q t (t) are the adsorption abilities at equilibrium and at the time, respectively, and k 1 (min -1 ) is the rate constant. The values of k 1 and q e were determined from the slope and intercept of the plots ln ðq e − q t Þ versus t, respectively, in Figures 6(a) and 6(b). The linearized form of second-order can be written as follows [26]: where k 2 (g/mg/min) is the main rate constant of pseudosecond-order adsorption. The values of k 2 and q e are calculated from the slope and intercept of the linear plots t/q t against t, in Figures 6(c) and 6(d). The results obtained for both kinetic models are shown in Table 3. The regression coefficients obtained for pseudosecond-order were higher for both adsorbents (RH-AC and PP-AC) compared to those of pseudofirst-order. This means that the adsorption data conforms to this kinetic model, which is an indication that the rate-limiting reaction of the acetic acid adsorption process was chemisorption dealing with valence force through electron sharing [20]. The applicability of kinetic models is confirmed through the sum of error squares as given by the following: where N is the number of data points. The R 2 values which resulted from the linear plot were > 0.99 for both adsorbents.
The higher R 2 value and lower value of SSE shown in Table 3 for the calculated for pseudosecond-order indicates a better fit, and that the adsorption process followed pseudosecond-order kinetic model. Similar findings were also reported by [35]. The maximum adsorption capacity for the adsorption of acetic acid by RH-AC and PP-AC adsorbents was found to be 0.25 mg/g and 0.24 mg/g, respectively.
3.8. Regeneration and Reusability. Regeneration studies were carried out to determine the suitability of the derived adsorbents in practical applications and the economic possibility of the adsorption process. The alkaline 0.1 M sodium hydroxide and distilled water were selected and used as desorbing agents. 0.2 g of the adsorbents was transferred into a 250 mL conical flask containing 1.5 M acetic acid which was agitated at 120 rpm for 90 min to adsorb acetic acid. The adsorbents were separated from the acids and then washed with sodium hydroxide using a magnetic stirrer for 10 min to remove physically adsorbed acetic acid followed by distilled water and finally oven-dried at 105°C for 2 h. The adsorbents were then utilized in the next cycle of adsorption studies. The above process was repeated for four times, and a very slight change was observed.

Conclusion
The current study has found out that activated carbons prepared were effective for acetic acid removal. The activated carbons had the same point of zero charge, many functional groups were excellent for adsorption, and they were amorphous, thus according to the characterization studies. Increases in contact time, adsorbate concentration, and adsorbent dosage improved the removal of acetic acids. The adsorption data followed two isotherms, namely, Freundlich and Frenkel-Halsey-Hill while kinetic data conformed to pseudosecond-order models. Reusability tests revealed that the adsorbents may be reused numerous times without losing their adsorbing affinity, thus allowing for multiple uses.

Data Availability
Data for this paper has been included in the manuscript and is available upon request to the corresponding author.

Conflicts of Interest
The authors do solemnly declare that there is no any conflict of interest as the research was not conducted for commercial or financial purposes.