Introduction

Various environmental problems have attracted more and more attention this century [1,2,3]. The greenhouse effect is a significant ecological problem mainly caused by the excessive emission of CO2 due to the burning of fossil fuels [4,5,6]. The intensification of the greenhouse effect can result in the rise in sea level, sharp weather changes, and frequent natural disasters [7]. Therefore, the development of CO2 utilization and storage technology has become a hot research topic in recent years [8]. Several technologies were developed for CO2 capture, including chemical absorption with amine or amino-based absorbent and membrane separation [9,10,11]. However, these technologies showed some disadvantages, such as the corrosion of equipment and high cost.

Water pollution, especially the organic dye polluted water, is another environmental problem that needs to be solved urgently. Organic dyes are extensively used in textile, printing, and papermaking industries [12, 13]. The accidental discharge of toxic dye wastewater is harmful to human health [14, 15]. Recently, membrane separation, solvent extraction, and chemical precipitation techniques have been developed for wastewater treatment [16,17,18,19]. The main problems of these technologies are high operation costs and low treatment efficiency. Thus, seeking technology with high efficiency and low cost is highly necessary to solve the state-of-the-art environmental issues.

The adsorption method is regarded as one of the most efficient methods that can be used not only to capture CO2 but also to remove dyes in polluted water. Compared with other methods, the adsorption method has the advantages of low energy consumption, environmental friendly, and simple operation process [20]. Various adsorbents have been developed, including metal–organic frameworks, covalent organic frameworks, zeolites, porous carbon, etc. [21]. Among these adsorbents, porous carbon adsorbent is favored by researchers because of their large specific surface area, adjustable pore structure, lower preparation cost, and abundant source [22, 23]. The abundant pore structure makes these adsorbents promising materials in CO2 capture or dye adsorption [24,25,26]. Meanwhile, these reusable carbon materials can be used not only as the adsorbent for CO2 and dyes, but also as catalysts, supercapacitor, etc. [27,28,29]. Thus, developing high-performance carbon materials has become a research focus.

Recently, more and more attention has been paid to the high value-added products prepared from biomass-derived material [30,31,32]. Porous carbon adsorbent can be obtained from biomass-derived materials, such as coconut shell [33], lotus stem, maple leaf, garlic peel, and nutshell [34, 35]. Using these biomass-derived materials to prepare porous carbon adsorbents can not only reduce the production cost but also promote the comprehensive utilization of biomass resources. Citric acid is a valuable chemical product with high content of carbon elements, which can be extracted from biomass resources, such as the peel of grapefruit, orange, etc. With the maturity of industrial fermentation technology for citric acid production, there are more and more studies on the application of citric acid [36]. Moreover, potassium citrate can be used as an activator to prepare porous carbon. It was reported, when poplar catkin was activated by potassium citrate, the surface area of the prepared carbon material reached 2186 m2 g−1 [37]. So far, there are no studies on using potassium citrate-derived porous carbon adsorbent to capture CO2 and adsorb dyes simultaneously.

Here, we reported an effective strategy to prepare a porous carbon from potassium citrate with a high specific surface area (1476 m2 g−1) and pore volume (0.951 cm3 g−1), as well as abundant oxygen functional group on its surface. The effect of carbonization temperature on the adsorbent structure was investigated. When used as the adsorbent for CO2, PCMCA-900, which was prepared at 900 °C, exhibited the highest CO2 adsorption capacity of 7.67 mmol g−1 at 0 °C and stable recycling stability. In addition, the PCMCA-900 showed the high Congo red adsorption of 652.3 mg g−1 in water.

Experimental section

Materials

Analytical grade potassium citrate monohydrate (PCM) was bought from Tianjin Damao chemical reagent company and used directly without purification.

Preparation of porous carbon adsorbent

First, 3 g PCM was heated at 300 °C for 30 min in a tubular furnace under the N2 atmosphere. Then, the temperature of the tubular furnace was raised to carbonization temperature ranging from 700 to 900 °C and kept at the carbonization temperature for 30 min. The yield of the obtained residual was about 68%. Finally, the black solid residual was repeatedly washed with distilled water to neutral. The obtained PCM-derived carbon adsorbent was dried at 120 °C for 1 h and named PCMCA-X, where PCMCA indicated the PCM-derived carbon adsorbent and X presented the carbonization temperature. For instance, when the carbonization temperature was 900 °C, the obtained adsorbent was named PCMCA-900. The preparation procedure is shown in Fig. 1a.

Fig. 1
figure 1

a Preparation process of PCM-derived hierarchical porous carbon adsorbent; b–d SEM images and e–g TEM images of PCMCA-X

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Characterization

A scanning electron microscope (SEM, Phenom ProX) was used to observe the surface morphology of the prepared adsorbent. The graphitization degree and crystal structure of the samples were analyzed by Raman spectrometer (RS, T6400) and X-ray diffractometer (XRD, D8). The elemental composition on the surface of the adsorbent was characterized by an X-ray photoelectron spectroscope (XPS, ESCALAB 250) using Al Kα radiation. A Quantachrome Autosorb-1C-BP analyzer was used for nitrogen adsorption/desorption. The specific surface area was calculated by the BET method, and the pore size distribution curve was calculated by density functional theory (DFT).

CO 2 adsorption measurement

A static CO2 adsorption experiment of adsorbent was carried out on the Quantachrome Autosorb-1C-BP analyzer from 0.01 to 1 bar for 5 h at 0 °C, 25 °C, and 50 °C, respectively. Before the CO2 adsorption experiment, the adsorbent was degassed at 250 °C under a vacuum of 1 × 10–6 mbar for 1 h. The reusability of the adsorbent was examined with the same instrument at 0 °C for 10 cycles.

Congo red adsorption measurement

The CR adsorption performance of the adsorbent was conducted in a batch adsorption process. During the isotherm experiments, 10 mg adsorbent was added into 100 mL CR solution with different concentrations (50, 100, 150, 200, 200 and 300 mg L−1). Then, the mixture was shaken in a thermostatic shaker at 35 °C, which is a temperature close to the actual application environment that is often used in adsorption research [38, 39]. After equilibrium adsorption for 25 h, the suspension was filtered with a 0.22 µm syringe filter. The adsorption capacity (Qe) of the adsorbent was calculated using the following equation:

$${Q}_{\mathrm{e}}=\frac{\left({C}_{0} -{C}_{\mathrm{e}}\right) \times V}{m}$$
(1)

where C0 (mg L−1) is the initial CR concentration, Ce (mg L−1) is the equilibrium CR concentration, V (mL) is the solution volume, and m (mg) is the adsorbent weight. In the kinetic experiments, the effect of the adsorption time (from 0 to 25 h) on the CR adsorption was examined with the initial concentration of 50, 100, and 200 mg L−1. An ultraviolet spectrophotometer (JC-UT 2000) was used to determine the CR concentration at the wavelength of 497 nm. The recyclability experiment of PCMCA-900 was carried out according to the procedure reported in literatures [40].

Results and discussion

Morphology and nanostructrue of PCMCA-X.

To illustrate the influence of the carbonization temperature on the structure of the PCM-derived carbon materials, an SEM examination of the morphology was carried out first, and the results are shown in Fig. 1b–d. The obtained PCM-derived carbon materials showed similar morphology. These carbon materials were the aggregate of some nanosheets and irregular hollow boxes. The formation of these structures is related to the template effect from K2CO3 [27]. With the thermal decomposition of PCM, K2CO3 and carbon can be produced, and the K2CO3 particles are usually coated by carbon. After the removal of K2CO3 during the following water washing procedure, the irregular hollow box-like carbon can be obtained. In PCMCA-700 and PCMCA-800, since the carbonization temperature is relatively low, some irregular hollow box-like carbon can be found. While in PCMCA-900, due to the thermal decomposition of the irregular hollow box-like carbon at high temperature, the aggregate of nanosheets can mainly be observed. These irregular nanosheets can also be found from the TEM images (Fig. 1e–g), and with the increase of carbonization temperature, it is more conducive to the formation of micropores.

Characterization of PCMCA-X.

Figure 2a shows the Raman spectrum of the prepared samples, from which the D peak at 1340 cm−1 and the G peak at 1592 cm−1 can be seen [41]. The ratio of ID/IG reflects the defect degree of the carbon material. The higher the ratio, the more defect structures in the carbon material. The largest ID/IG of PCMCA-900 indicates that it has more defect structures. These defects are caused by a large number of micropores generated by the increase in carbonization temperature. The XRD patterns of the prepared samples are recorded in Fig. 2b. There are two broad diffraction peaks at 2θ of 26° and 43°, which correspond to the (002) and (101) crystal planes of amorphous carbon, respectively [42].

Fig. 2
figure 2

a Raman curves; b XRD profiles; c N2 adsorption/desorption isotherms and d pore size distribution curves of the prepared samples

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The textural properties of PCM-derived carbon materials were determined by the N2 adsorption/desorption method. As shown in Fig. 2c, PCMCA-700 shows a type-IV isotherm with an H4 type hysteresis loop, which means that PCMCA-700 is a mesoporous carbon material [43]. The isotherms of PCMCA-800 and PCMCA-900 exhibited a typical type I isotherm. Compared with PCMCA-800, the isotherm curve of PCMCA-900 presents relatively high nitrogen adsorption in the low pressure range (P/P0 < 0.05), which indicates the formation of a more microporous structure in PCMCA-900 [44]. Meanwhile, a wide adsorption knee can be observed in the isotherm curve of PCMCA-900, demonstrating that the mesopores are existing in PCMCA-900 [45,46,47]. Pore size distribution diagrams of PCM-derived carbon materials indicate that most of the pores of PCMCA-700 are distributed in the pore diameter range from 2 to10 nm, while PCMCA-800 and PCMCA-900 process somewhat broader pore size distribution with the most of their pores distributed in the pore diameter range from 0.5 to10 nm (Fig. 2d). Meanwhile, the percentage of the pore diameter around 0.6 nm in PCMCA-800 is a little higher than that of PCMCA-900, which may affect the CO2 adsorption capacity of PCMCA-800 at low pressure.

To illustrate the difference in textural properties of the three PCM-derived carbon materials, detailed textural parameters of the three adsorbents are listed in Table 1. The BET surface areas of PCMCA-700, PCMCA-800, and PCMCA-900 are calculated to be 506, 1032, and 1476 m2 g−1, respectively. PCMCA-900 shows the highest micropore volume (Vmicro) of 0.497 cm3 g−1 and total pore volume (Vtotal) of 0.951 cm3 g−1 among the three adsorbents. During the investigation of CO2 adsorption, it has been reported that the volume of narrow micropores (the pores distributed between 0.7 and 0.9 nm, P0.7–0.9) can significantly affect the adsorption result. Usually, the relatively strong interaction between CO2 and the surface of narrow micropores can greatly improve the CO2 adsorption capacity [48, 49]. It can be observed from Table 1 that the P0.7–0.9 in PCMCA-900 is 19.21%, which is the highest among the three adsorbents. Thus, the high surface areas and suitable pore volume distribution of PCMCA-900 can provide more active surface sites and diffusion channels for adsorption, which can enhance the CO2 and CR adsorption performance of PCMCA-900.

Table 1 Detailed textural parameters of PCM-derived carbon materials
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The surface element state of the PCM-derived carbon materials were identified by XPS. As shown in Fig. 3a, two peaks appearing at 285 eV and 533 eV, which suggests the main elements in PCM-derived carbon materials are C and O [50]. In Fig. 3b, the high resolution spectra of C 1 s can be further fitted into four separate peaks at 284.3 eV, 285.3 eV, 286.5 eV, and 288.6 eV, which was attributed to C–C (C1), C–OH (C2), C=O (C3), and COOH (C4), respectively [51, 52]. The percentage of each component is listed in Table 2. The percentage of the O-containing functional groups (C2, C3, and C4) of the PCMCA-900 sample is highest among the three examined samples, indicating that more O-containing functional groups were introduced in PCMCA-900. With the introduction of O-containing functional groups, the surface properties of carbon materials can be greatly changed, which is beneficial to improve the adsorption performance of carbon materials [53]. In Fig. 3c, The high-resolution spectra of O 1 s can be divided into four separate peaks at 531.5 eV, 533.5 eV, 533.6 eV, and 535.0 eV, which can be assigned to carbonyl, ketone or lactone groups (O1), ether or alcohol groups (O2), carboxyl group (O3) and oxygen in water (O4), respectively [54, 55]. The detailed percentage of these components is presented in Table 3. The total percentage of O1 and O2 in the PCMCA-900 sample is the highest among the three samples. Since O1 and O2 are responsible for alkalinity, and O3 is responsible for acidity, the high percentage of O1 and O2 in the PCMCA-900 sample indicates that PCMCA-900 shows more alkalinity properties than the other two carbon materials [25]. The presence of these O-containing functional groups can improve the adsorption properties of PCM-derived carbon materials [51].

Fig. 3
figure 3

a XPS full spectra and high-resolution spectra of b C 1 s and c O 1 s of the prepared samples

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Table 2 XPS data of C 1 s composition of PCM-derived carbon adsorbents
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Table 3 XPS data of O 1 s composition of PCM-derived carbon adsorbents
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CO 2 adsorption analysis

To compare the CO2 adsorption performance of the PCM-derived carbon adsorbents, the CO2 isotherms of PCMCA-X were examined at 0 °C, and the corresponding results are presented in Fig. 4a. The static CO2 adsorption capacities of the three adsorbents at 0 °C and 1 bar are 4.12 mmol g−1, 6.52 mmol g−1, and 7.67 mmol g−1, respectively. The difference in the static CO2 adsorption capacity of the three adsorbents can be related to their different textural properties. PCMCA-900 has the highest surface area and suitable pore volume distribution, which can not only provide more CO2 adsorption sites but also provide more CO2 diffusion channels, leading to the best CO2 adsorption performance of PCMCA-900 among the three adsorbents. Moreover, as shown in Fig. 4b, SBET of the PCMCA-X materials has a strong influence on the CO2 capture with a high R2 of 0.954, indicating the higher specific surface is favor for improvement of CO2 capture at 0 °C [52].

Fig. 4
figure 4

a CO2 adsorption isotherms and b SBET vs CO2 uptake with a fitting line of PCMCA-X samples at 0 °C. c Qst of PCMCA-900. d CO2 capture mechanism of PCMCA-900. e CO2 uptake capacities at various temperatures (0 °C, 25 °C, and 50 °C) at 1 bar and f recyclability of PCMCA-900

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To illustrate the binding affinity between the PCMCA-900 adsorbent and CO2, the isosteric adsorption heat was calculated according to the method provided in the literature [52]. The isosteric adsorption heat value of the prepared PCMCA-900 adsorbent is in the range of 22.37 to 27.47 kJ mol−1 in Fig. 4c, which indicates the adsorption of CO2 on PCMCA-900 adsorbent is a typical physical adsorption process [56]. Thus, the physisorption with relatively low isosteric adsorption heat between CO2 molecule and the surface of the porous carbon, as illustrated in Fig. 4d, led to the good adsorption performance of PCMCA-900 [57].

Temperature is an important factor affecting the CO2 adsorption performance of adsorbents. Thus, the influence of adsorption temperature on the CO2 adsorption performance of PCMCA-900 was explored as well. It can be observed from Fig. 4e that the CO2 adsorption capacity of PCMCA-900 decreased from 7.67 to 2.11 mmol g−1 when the adsorption temperature increased from 0 to 50 °C [58]. The adsorption capacity of CO2 decreases with the increase of adsorption temperature. This phenomenon is due to the fact that, when the temperature increases, the surface adsorption energy and molecular diffusion rate of CO2 increase, resulting in the instability between CO2 and adsorbent and the decrease of CO2 adsorption capacity [25, 59].

To investigate the recyclability of PCMCA-900, the successive adsorption–desorption experiment was applied. At each cycle, PCMCA-900 was degassed at 250 °C for 1 h under vacuum. Then, PCMCA-900 was cooled to 0 °C to start the next cycle. 10 cycles were carried out to evaluate the recyclability of PCMCA-900. It can be seen from Fig. 4f that, after 10 cycles of CO2 adsorption and desorption, the CO2 adsorption capacity decreased by less than 5% (7.67 mmol g−1 to 7.29 mmol g−1), demonstrating the high recyclability of PCMCA-900.

During these years, materials used for CO2 adsorption have been continuously developed [52, 56, 57]. Table 4 listed some reported materials that used for CO2 adsorption. It can be observed that most materials exhibited high CO2 adsorption ability with the CO2 adsorption capacity between 5 and 7 mmol g−1 at 0 °C. PCMCA-900 showed relatively better CO2 adsorption capacity of the 7.7 mmol g−1 at 0 °C. Meanwhile, to prepare PCMCA-900, only potassium citrate was needed. Expensive reagents or activator such as sodium lignosulfonate NaNH2 are not need. The preparation cost of PCMCA-900 is relatively lower than other listed materials. Thus, PCMCA-900 is a relatively good adsorbent for CO2 adsorption.

Table 4 Comparison of the CO2 adsorption capacity of the prepared adsorbent with the reported materials
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Congo red adsorption analysis

Since PCMCA-900 showed good adsorption performance in the CO2 adsorption, PCMCA-900 was further used in the study of CR adsorption. The adsorption kinetics of the PCMCA-900 for the removal of CR was examined first, and the results are shown in Fig. 5a. The CR adsorption ability of the PCMCA-900 is greatly affected by the CR initial concentration. When the CR initial concentration was increased from 50 to 200 mg L−1, the CR adsorption capacity of the PCMCA-900 increased from 297.7 to 408.5 mg g−1. In general, adsorption can be divided into physical adsorption and chemical adsorption. To demonstrate the adsorption mechanism of PCMCA-900, the pseudo-first-order and pseudo-second-order kinetic models were used to illustrate the adsorption mechanism [62, 63]. The pseudo-first-order and pseudo-second-order kinetic models can be described in the following equations:

Fig. 5
figure 5

a CR adsorption kinetic by the PCMCA-900 (Conditions: initial concentrations: 50, 100, 200 mg L−1; dosage: 0.1 g L−1; adsorption: 1500 min; temperature: 35 °C). b Adsorption isotherms of CR at 35 °C by PCMCA-900 (Conditions: initial concentrations: 50 to 300 mg L−1; dosage: 0.1 g L−1; temperature: 35 °C). c Effect of pH values on the CR adsorption capacity of PCMCA-900 (Conditions: initial concentrations: 100 mg L−1; dosage: 0.1 g L−1; temperature: 35 °C). d recyclability of PCMCA-900. e adsorption scheme for CR of the PCMCA-900

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$$\mathrm{ln}\left({Q}_{e}-{Q}_{t}\right)=\mathrm{ln}{Q}_{e}-{K}_{1}t$$
(2)
$$\frac{t}{{Q}_{t}}=\frac{1}{{K}_{2}{Q}_{e}^{2}}+\frac{1}{{Q}_{e}} t$$
(3)

where Qe (mg g−1), Qt (mg g−1), K1 (min−1), and K2 [g (mg min)−1] are corresponding to the adsorption capacity at adsorption equilibrium, the adsorption capacity at time t (min), the adsorption rate constant of the pseudo-first-order, and the adsorption rate constant of the pseudo-second-order, respectively [64]. The simulated results are shown in Table 5. When the isothermal adsorption kinetic data of CR were simulated by the pseudo-second-order model, the obtained correlation coefficient (R2) was greater than that of the pseudo-first-order model. Meanwhile, the calculated adsorption capacities at adsorption equilibrium (Qe,cal) based on the pseudo-second-order model are much closer to the experimental data than that based on the pseudo-first-order model. These results indicate that the pseudo-second-order model is more suitable for describing the CR adsorption behavior of the PCMCA-900. Therefore, the kinetic study results demonstrate that the overall adsorption kinetics of CR by PCMCA-900 is mainly a chemisorption-controlled process and governed by the diffusion time of adsorbate into the narrow pores [65,66,67].

Table 5 CR kinetic adsorption fitting parameters
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To illustrate the equilibrium state of CR molecules in the liquid and solid phase at a constant temperature, Langmuir and Freundlich isotherm models were used, which can be, respectively, represented by the following equations [68]:

$$\frac{1}{{Q}_{\mathrm{e}}}=\frac{1}{{Q}_{\mathrm{m}}{K}_{\mathrm{L}}}\frac{1}{{c}_{\mathrm{e}}}+\frac{1}{{Q}_{\mathrm{m}}}$$
(4)
$$\mathrm{log}{Q}_{\mathrm{e}}=\mathrm{log}{K}_{\mathrm{F}}+\frac{1}{n}\mathrm{log}{c}_{\mathrm{e}}$$
(5)

where Qe (mg g−1), Qm (mg g−1), Ce (mg L−1), KL (L mg−1), KF ((mg g−1)(L mg−1)1/n), and 1/n refer to the adsorption capacity of the adsorbent for the adsorbate at adsorption equilibrium, the maximum adsorption capacity of the adsorbent for the adsorbate, the concentration of adsorbate in solution at adsorption equilibrium, the equilibrium constant in the Langmuir isotherm model, the equilibrium constant in the Freundlich isotherm model, and the adsorption intensity, respectively [69]. It can be seen from Fig. 5b and Table 6 that the Langmuir model is more suitable for simulating the experimental data due to its high R2 value of 0.999. Since the Langmuir model is related to the monolayer adsorption, the isothermal adsorption results illustrated that CR molecules are adsorbed on the PCMCA-900 surface in the monolayer form at the equilibrium state.

Table 6 Langmuir and Freundlich isotherm of CR on PCMCA-900
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Since the pH value can affect the chemical properties of adsorbent and adsorbate, the effect of pH value on the CR adsorption performance of PCMCA-900 was studied, and the results are presented in Fig. 5c. The CR adsorption capacity of PCMCA-900 is strongly affected by the pH value. With the increase of the pH value of the CR solution, the CR adsorption capacity of PCMCA-900 decreased clearly. The CR adsorption capacity of PCMCA-900 at the pH value of 4 was 652.3 mg g−1, which was about 2 times higher than that at the pH value of 10. This phenomenon can be explained by the different concentrations of H+ and OH− ions in CR solution at different pH values. When the pH value of CR solution is lower than 7, with the decrease of solution pH value, the H+ ion concentration increases, and the protonation of the PCMCA-900 surface results in the increase of the CR adsorption [65]. When the pH value of the CR solution is higher than 7, the concentration of OH− ion in the CR solution increases, which can compete with the CR molecule to adsorb on the PCMCA-900, leading to the decrease of the CR adsorption capacity of PCMCA-900 [70].

The recovery of PCMCA-900 is of great importance for its potential application. The recyclability of PCMCA-900 is shown in Fig. 5d. It can be observed that after 10 times recycling, the CR adsorption of PCMCA-900 still reached 315.7 mg g−1, which decreased less than 10% compared with the first-time use. This result indicates that PCMCA-900 is a long-term adsorbent for the removal of CR from the solution.

Figure 5e presents the possible adsorption mechanism of PCMCA-900. The adsorption of CR is mainly due to electrostatic attraction and ion exchange [71]. The benzene ring in CR molecule can interact with porous carbon through π–π stacking, thus facilitating the adsorption of CR. In addition, the –NH2 group in CR molecule and –OH group in PCMCA-900 can form hydrogen bonds, which is helpful for CR adsorption [72].

Comparison of the CR adsorption capacity of the PCMCA-900 with the reported materials is represented in Table 7. Due to the difference in adsorption conditions, the adsorption ability of different materials can only be roughly compared. It can be observed that the PCMCA-900 and C/MnCuAl–LDOS has relatively good adsorption ability due to their high adsorption capacity of higher than 300 mg g−1. However, the PCMCA-900 showed relative advantages on the aspect of low preparation cost and simple preparation procedure. Thus, the PCMCA-900 is a promising adsorbent for the removal of CR in wastewater.

Table 7 Comparison of the CR adsorption capacity of the prepared adsorbent with the reported materials
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Conclusions

In summary, a porous carbon adsorbent was prepared from PCM by a simple carbonization method. The obtained PCM-derived carbon adsorbent, PCMCA-900, shows a high specific surface area of 1476 m2 g−1 and a relatively suitable pore size distribution, resulting in excellent performance on CO2 capture and CR adsorption. The highest CO2 uptake reached 7.67 mmol g−1, and the highest CR adsorption capacity reached 652.3 mg g−1 when PCMCA-900 was used. Therefore, the preparation of PCMCA-900 provided a promising method to utilize biomass-derived material, and developed a high-performance adsorbent, which has the potential application in solving the problems of the greenhouse effect and dye wastewater pollution.