Limitation of K₂CO₃ as a Chemical Agent for Upgrading Activated Carbon

Abstract

The chemical activation of a carbon precursor with KOH generally results in an activated carbon (AC) with a high specific surface area. However, this process generates a large volume of wastewater that includes dissolved alkali metals, existing mainly as K₂CO₃. Thus, wastewaters with a high concentration of dissolved K₂CO₃ can potentially be used in place of KOH as a chemical agent. In the present study, to reduce the thermal stability of K₂CO₃, which decomposes at temperatures greater than 891 °C, K₂CO₃ was chemically impregnated into carbon precursors prior to activation of the precursors. The thermochemical properties and activation efficiency of the carbon precursors treated with K₂CO₃ were compared with those of carbon precursors treated with KOH. Analysis by XPS indicated that C–O–K complexes formed on the surface of the carbon precursors; in addition, their peak intensities were approximately the same irrespective of the chemical agent used. However, the specific surface area of the K₂CO₃-impregnated AC was 2162 m²/g, which was ~70% of that of the KOH-impregnated AC (3047 m²/g) prepared using the same K/C molar ratio of 0.5. XRD results confirmed that both K₂CO₃ and KOH transformed into KHCO₃ and K₄H₂(CO₃)₃·1.5H₂O during the impregnation. The peak intensities of these compounds in the XRD pattern of the K₂CO₃-impregnated carbon precursors were two times greater than those in the pattern of the KOH-impregnated carbon precursors. These compounds eventually transformed into K₂CO₃, which hardly participated as a chemical agent at the temperature used in the present study (850 °C). Therefore, recrystallisation of K₂CO₃, even during the impregnation, appeared to adversely affect the degree of activation. Nevertheless, the specific surface area of the K₂CO₃-activated AC was still ~1.6 times greater than that of the untreated carbon precursor (1378 m²/g), suggesting that the use of wastewater as a chemical agent is feasible for resource recycling.

1. Introduction

Activated carbon (AC) has many reactive sites on its surface; it is therefore widely used as an adsorbent in the medical, automotive, water purification, and air-quality-control industries [1,2]. The AC market in Korea is divided into price-leading and technology-leading markets [3]. In particular, the demand for AC with a specific surface area greater than 2000 m²/g has been increasing rapidly because of strengthened air-quality regulations. According to the Korea International Trade Statistics [4], the amount of high-specific-surface-area AC imported in 2018 was ~5000 tonnes (~9.1% of the total imported AC) and originated from two specific countries (i.e., the United States and Japan).

Both physical and chemical activation processes are used to prepare AC. However, harsher conditions (e.g., a longer residence time and a higher loading of reagents) are necessary to prepare AC with a high specific surface area. This phenomenon is more pronounced with physical activation, where pores are developed by oxidative gases (i.e., steam and CO₂). Chemical activation has advantages over physical activation with respect to being conducted at a lower activation temperature and providing a greater production yield [5,6,7]. Potassium salts such as KOH and K₂CO₃ have been widely used in the manufacture of AC, most commonly using KOH. It has been found that AC prepared by KOH is highly microporous with higher specific surface area. However, the high costs associated with chemical activation have been pointed out as a drawback and limitation, stemming from the excessive use of chemicals and the generation of byproducts such as wastewater by the cleaning processes [8,9]. To address this aspect, some researchers suggest the possibility of recycling the chemical, but KOH is transformed as carbonate by reacting with CO₂ during the activation [8,9,10]. Due to this reason, washing water recovered alkali metals mainly in the form of K₂CO₃, which is normally a less effective activation agent [11].

The melting points of KOH and K₂CO₃, which are commonly used as chemical agents for treating AC, differ greatly. KOH melts at 380 °C and decomposes at 769 °C, whereas K₂CO₃ melts at 891 °C and decomposes at >1200 °C. As such, K₂CO₃ is less reactive than KOH as an agent during the activation process because of its high thermal stability [8,12,13], as documented by Lu et al. [14]. Even when the activation temperature is less than 830 °C, the Gibbs free energy (G°) of K₂CO₃ would not thermodynamically favor its reaction with carbonous materials:

K₂CO₃ + 2C → 2K + 3CO, G° = 91.6 kJ/mol

(1)

K₂CO₃ + C → K₂O + 2CO, G° = 205.0 kJ/mol

(2)

However, Hayashi et al. [12,13] have confirmed that wet impregnation of K₂CO₃ lowers its decomposition temperature to 890 °C. This effect has been verified on various biomasses (i.e., almond, coconut, oil palm, pistachio, and walnut shell) with specific surface areas as high as 2000 m²/g after impregnation with K₂CO₃ at 800 °C [15].

Even if chemical activation has advantages over physical activation with respect to production yield and reaction temperature, it still requires a large amount of chemicals and generates large volumes of wastewater. Thus, using wastewater with a high concentration of K-containing compounds as a chemical agent would be an attractive method to reduce the manufacturing costs of AC. However, most previous studies on the surface modification of AC have focused on optimizing conditions by controlling the ratio of agents; consequently, only the effects of appropriate agents have been identified. Even though carbon precursors have been impregnated with K₂CO₃ to upgrade the adsorption performance of the resultant AC, the literature contains little research on why K₂CO₃ is less effective than KOH [11].

In the present study, activation reaction characteristics with an existing alkali metal were compared for reuse of a K-containing compound in the form of K₂CO₃ in alkaline wastewater. To overcome the unfavorable thermochemical properties of solid-state K₂CO₃, we induced easier surface bonding of alkali-metal cations on the precursor surfaces via impregnation. The effects of introducing an alkali metal onto the surface of the carbon precursors, especially the activation properties of the alkali metal, were characterized. The results of this study are expected to suggest an approach for economically feasible chemical activation through effective utilization of high-concentration alkaline solutions generated in the AC washing process.

2. Materials and Methods

2.1. Preparation of Carbon Precursors and Chemical Activation

The carbon precursor in this study was commercial AC (JIG-SC-2040 BT, JAYEON SCI., Korea), which was produced from wood and exhibited a specific surface area of approximately 1378 m²/g. It has relatively high moisture (13.0%) and volatile (18.2%) contents while fixed carbon is about 63.4%, which is different from that of coconut-based AC [16]. For chemical activation, two types of K compounds (KOH, K₂CO₃) were used. Prior to the chemical activation, ~30 g of AC was mixed with 200 mL of K compound solution and then impregnated in a rotary evaporator (N-1300, Eyela, Japan); the K/C molar ratio was varied. After evaporation of the solution, the samples were dried at 105 °C for 24 h in preparation for activation. Five grams of impregnated carbon precursor was placed in a tubular furnace under flowing N₂, and the temperature of the furnace was increased at 5 °C/min until the activation temperature (850 °C) was reached; the activation temperature was then maintained for 2 h. After the activation was terminated, the sample was washed with distilled water several times until the leachates were neutral to remove impurities.

2.2. Analytical Methods

To identify thermal properties of activators and carbon precursors impregnated, thermogravimetric analysis (TGA) was performed. All of the samples were heated to 1000 °C at a rate of 1 °C/min under a N₂ atmosphere. To quantify the amount of K impregnated onto the carbon surface, we used X-ray fluorescence (XRF) spectroscopy (Shimadzu, Japan). The surface properties of the carbon were characterised by XPS (K-Alpha+, Thermofisher, Waltham, MA, USA) and X-ray diffraction (XRD, SmartLab, Rigaku). The specific surface area was measured by N₂ adsorption (ASAP-2010, Micromeritics, Norcross, GA, USA) at −196 °C. We confirmed the pore distribution by varying the relative pressure (P/P₀) of the specimen after pretreatment and calculated the sub-segment distribution using the Dubin–Astakhov formula and the Barrett–Joyner–Halenda method for intermediate and large-scale machining. The porous size distribution (PSD) was calculated using the density functional theory.

3. Results and Discussion

3.1. Changes in Thermal Stability of Chemical Agents

Figure 1 shows the TGA results for the commercial AC used as a carbon precursor, the chemical activation agents (KOH and K₂CO₃), and the AC impregnated with chemical agents. The weight losses of the specimens activated using KOH and K₂CO₃ reflect the different thermal properties of the activating agents. K₂CO₃ did not function as an activating agent at 850 °C because of its high thermal stability relative to that of KOH. However, the initial point of weight loss, which indicates the decomposition temperature, for AC impregnated with K₂CO₃ shifted ~200 °C lower than that of K₂CO₃. In fact, the TGA curves for the AC samples impregnated with agents varied depending on the carbon-to-agent ratio in the mixture. Thus, the TGA curves for this type of mixture are generally plotted along with the thermal properties of the two materials. However, at temperatures greater than 750 °C, the AC impregnated with K₂CO₃ exhibited a greater weight loss than the sample containing untreated AC. Given that K₂CO₃ exhibits negligible weight loss until 891 °C because of its thermal stability, the greater weight loss of the K₂CO₃-impregnated compared with that of the untreated AC indicates that the thermal properties of K₂CO₃ changed as a result of the impregnation. These results indicate that K₂CO₃ dissolved in wastewater from the chemical activation with KOH can be reused through the impregnation process.

3.2. Changes in Surface Properties of Carbon Precursors Impregnated with KOH and K₂CO₃

3.2.1. XPS Analysis

Table 1. XPS survey analysis of the KOH/K₂CO₃-impregnated carbon precursors.

Preparation of Carbon Precursors	K/C Ratio	C 1s (285.15 eV)	K 2p (294.25 eV)	N 1s (401.11 eV)	O 1s (532.73 eV)
KOH impregnation	0.5	27.1	9.17	N.D	63.7
K2CO3 impregnation	0.2	77.5	0.99	0.89	20.6
	0.4	34.0	17.2	1.09	47.7
	0.5	23.9	8.19	N.D	67.9

shows the XPS results used to verify the surface characteristics of the impregnated AC precursors. The XPS results confirm that the O and K contents of the samples increased with increasing amount of impregnated alkali metal. Because the surface properties of the carbon precursors are associated with an oxidation–reduction reaction during the activation process [17,18], an increase in O, which is an electron donor, on the carbon surface will strongly affect the pore development of the carbon matrix. Kopyscinski et al. [19] and Quyn et al. [20] have both proposed that C–O–K bound during the alkali-metal (i.e., K₂CO₃) impregnation is reduced to C–K at the surface of the carbon body, accompanied by the generation of CO. Lee et al. [21] and Punsuwan et al. [22], who qualitatively and quantitatively measured the gases emitted during the KOH activation of AC, also confirmed that CO is the main syngas (K₂O + C → 2K + CO at 800–850 °C [17,18]).

To more closely investigate these binding properties of the carbon precursors, we deconvoluted the K and O peaks in the XPS spectra (

Table 2. Deconvolution of K 2p and C 1s XPS spectra of the KOH/K₂CO₃-impregnated carbon precursors.

Agent	K/C Ratio	K 2p3/2		K 2p1/2		C 1s
		Ko (293.0)	KCB (295.7)	Ko (294.5)	KCB (296.6)	C=C (284.4)	C–C (285.9)	C–O (287.1)	C=O (288.5)	COOH (289.7)
KOH	0.5	49.6	15.9	25.7	8.88	51.7	22.5	7.59	10.2	7.95
K2CO3	0.2	53.9	11.5	25.9	8.68	71.7	16.8	5.65	3.62	2.15
	0.4	54.2	10.3	30.9	4.54	56.3	14.5	6.79	8.36	14.1
	0.5	50.0	15.2	26.3	8.59	50.8	23.7	7.62	10.4	7.40

K_o: K–O groups, K_CB: potassium surrounded by carbonate, Numbers in parentheses represent binding energy in eV.

). The K peak was deconvoluted into two peaks with binding energies of 293.0 eV (K 2p_3/2) and 295.6 eV (K 2p_1/2). The peaks for each of these main peaks were further deconvoluted into two peaks, which were identified as the K component of K–O bound to the surface (K_o) and K surrounded by carbonate (K_CB) [19]. These deconvolution characteristics are similar for the K 2p_3/2 and K 2p_1/2 peaks of carbon precursors impregnated with KOH and K₂CO₃. Deconvolution of the C 1s spectra for both KOH-impregnated AC and K₂CO₃-impregnated AC reveals peaks attributable to C=C (284.4 eV), C–C (285.9 eV), C–O (287.1 eV), C=O (228.5 eV) and COOH (289.7 eV). The intensity of the deconvoluted C 1s peaks varies depending on the K/C ratio but is independent of the agent at the same K/C ratio of 0.5. Thus, the increase in the specific surface area after activation of carbon precursors impregnated with KOH and K₂CO₃ at the same K/C ratio would be similar irrespective of the agent.

3.2.2. XRD Results

Figure 2 shows the results of the XRD analysis of the carbon precursors impregnated with KOH and K₂CO₃ at a K/C molar ratio of 0.5. Similar XRD peak patterns are observed for the carbon precursors impregnated with the different activating agents. The main peak is attributed to KHCO₃; however, weak peaks of K₄H₂(CO₃)₃ were also observed. Although K₄H₂(CO₃)₃ is known as an intermediate composition of KHCO₃ crystals exposed to a CO₂ atmosphere [23,24], it eventually transforms to KHCO₃: K₄H₂(CO₃)₃·1.5H₂O_(s) + CO_2(g) ⇌ 4KHCO₃ + 0.5H₂O_(g). Nevertheless, the large differences in the specific surface areas of the upgraded ACs impregnated with KOH and K₂CO₃, as shown in Figure 3, are difficult to explain. Because KHCO₃ also transforms into K₂CO₃ at temperatures greater than 140 °C (2KHCO_3(s) ⇌ K₂CO_3(s) + H₂O_(g) + CO_2(g)) [25], the amounts of K₂CO₃ formed on the carbon precursors would adversely affect the degree of activation. Given that the peaks of K₄H₂(CO₃)₃ and KHCO₃ are more intense for the carbon precursor impregnated with K₂CO₃ that for that impregnated with KOH, the K₂CO₃-impregnated sample would favor the formation of K₂CO₃ during activation. Because K₂CO₃ is thermally stable to 891 °C, it hardly participated as a chemical agent at the highest temperature used in the present study (850 °C), which is why the specific surface area of the carbon precursor impregnated with K₂CO₃ was lower than that of the precursors impregnated with KOH.

3.3. Chemical Activation with KOH and K₂CO₃

For chemical activation, carbon precursors were prepared by being physically and chemically mixed with KOH and K₂CO₃. The specific surface area of the AC after chemical activation with KOH was superior to that of the AC after chemical activation with K₂CO₃, irrespective of the mixing method (Figure 3). The specific surface area of the AC activated with KOH at a K/C ratio of 0.5 increased from 1400 m²/g to 3047 m²/g, which is ~30% greater than that of the AC activated with K₂CO₃ (2162 m²/g) under otherwise identical conditions. These results clearly show differences in the thermal properties of the ACs treated using KOH and K₂CO₃. However, the specific surface area of the AC impregnated with K₂CO₃ at the same K/C ratio was ~24% greater than that of the AC mixed with K₂CO₃ physically. Thus, the physical properties of K₂CO₃ were changed during the impregnation (Figure 1), enabling the K doped into the carbon precursors to more easily intercalate into the carbon matrixes.

Figure 3 shows the N₂ isotherms and PSD for the ACs treated using KOH and K₂CO₃ at various K/C ratios. The shape of the N₂ isotherms and PSDs varies with the K/C ratio; however, no substantial differences are observed between the ACs treated using these two agents. Nevertheless, AC impregnated with K₂CO₃ at a twofold K loading during the activation process was similar to that of AC treated with KOH at a K/C ratio of 0.25. These results reflect the relatively low reactivity of K as a chemical agent when impregnated as K₂CO₃ compared with that of K impregnated as KOH at the same K/C ratio.

4. Conclusions

In the present study, the reactivity of K₂CO₃ as a chemical agent was compared with that of KOH to evaluate the feasibility of recycling alkaline wastewaters generated as a byproduct during chemical activation. Wet impregnation with K₂CO₃ on carbon precursors effectively lowered the thermal stability of K₂CO₃, as indicated by a decrease in its melting point. However, the specific surface area (2162 m²/g at K/C = 0.5) of the K₂CO₃-impregnated AC was still only ~70% of that of the KOH-impregnated AC (3047 m²/g at K/C = 0.5). The XPS and XRD results showed similar surface binding properties (C–O–K) and potassium compounds (KHCO₃, K₄H₂(CO₃)₃) in the precursors impregnated with these two agents (K₂CO₃ and KOH). However, the intensity of KHCO₃ peaks in the XRD patterns differed dramatically dependent on the agent used. This result indicated that the degree of formation of C–O–K complexes, which participate in the activation reaction, strongly affected the carbon surface even if the binding energies and peak intensities were similar. Consequently, the specific surface area of the K₂CO₃-impregnated AC was lower than that of the KOH-impregnated AC even though the high thermal stability of K₂CO₃ had been overcome. Nevertheless, the specific surface area of the K₂CO₃-activated AC was still ~1.6 times greater than that of the carbon precursor (1378 m²/g), suggesting the possibility of resource recycling by reusing wastewaters as a chemical agent.