1 Introduction

Porous carbon (PC) material is most commonly used as an adsorbent because of its high surface area and large-scale production possibility [1,2,3]. Activated carbon and biochar are two typical kinds of PC materials, and they have been widely applied in the adsorption removal of nutrients [4], heavy metals [5, 6], antibiotics [7], dyes [8] and so on. Pore structure is one of the most important features of PC considering its applications in adsorption processes involving organic compounds in liquid phase [9]. Although the development of micropores smaller than 2 nm can lead to high surface area, they restrict mass transfer and reduce pore accessibility to large adsorbates [10]. For example, the high molecular weight organics contained in wastewater may cause micropore blockage and reduce the adsorption capacity of PC, while mesopore (2 nm < diameter < 50 nm) is the main transfer artery for large molecules, higher mesoporosity may benefit for the adsorbates transfer from liquid solutions to solid adsorbents [11,12,13,14]. Thus, recently, much attention has been paid to the preparation of PC with a reasonable distribution over the diameter ranges of both micropores and mesopores.

Activation has been proved as a powerful way of enhancing porosity of PC and improving its performance as an adsorbent [15,16,17]. Chemical activation is a single-stage process that includes impregnation of raw materials using activating agents such as KOH, K2CO3, NaOH, ZnCl2, H3PO4 and H2SO4 prior to heat treatment in an inert atmosphere, and these activating agents can improve the pore distribution and increase the surface area [18, 19]. The activation temperature depends on the type of activating agents, for example, 400–500 °C for H3PO4 and 500–700 °C for ZnCl2 [20]. The use of chemicals results in risks of equipment corrosion and secondary environmental pollution during disposal [21, 22], and these factors have led to a greater number of studies on physical activation that occurs through oxidation with an oxidizing gas (CO2 or water vapor). However, physical activation always takes place at higher temperatures (800–1000 °C), leading to a huge consumption of energy [21, 23]. Thus, exploring a new way to carry out physical activation at lower temperatures for shorter periods of time will be a desirable feature in the field of activated carbon industry.

Traditionally, physical activation consists of two main stages: carbonization of raw materials under an inert atmosphere, followed by the heat treatment under the oxidizing gas atmosphere such as water vapor, CO2 and air at an elevated temperature ranging between 800 and 1000 °C [18]. Unlike two-step procedure, in one-step procedure, the temperature is ramped up from room temperature to the required activation temperature, and both carbonization and activation steps are carried out simultaneously in the presence of oxidizing agents. One-step activation is preferable because of the reduced production costs in terms of time and energy [24]. For example, Yang et al. reported that activated carbon with high BET surface area of 1667 m2 g−1 can be directly prepared from coconut shell by one-step CO2 activation at 900 °C for 140 min [25]; Gonçalves et al. found that compared with two-step CO2 activation, one-step activation resulted in a higher BET surface area of the activated carbon using bagasse/molasses pellet as the raw material [26].

Recently, CO2 has become the preferred oxidizing gas because it facilitates control of the activation process due to its lower reactivity than water vapor at high temperature [27,28,29]. Moreover, it contributes to both water-saving and offering a potential solution for the alleviation of greenhouse gas emitted from industrial facilities [30, 31]. The essence of CO2 activation is the partial gasification of the carbonaceous materials with CO2, resulting in the removal of carbon atoms and simultaneously producing pores [25]. There have already been some published papers related with CO2 gasification of biomass chars in the presence of alkali and alkaline earth metals (AAEMs) as catalysts, which could change electron-cloud distribution of the surface atoms, promoting chars to participate in the gasification reaction and inhibiting the graphitization or condensation of char [32, 33]. For example, Perander et al. found that the gasification rate of the spruce wood char increased linearly with an increase in the concentration of Ca or K [34]. Lv et al. reported that AAEM species existing in biomass increased the char reactivity, resulting in the decrease of initial gasification temperature and the increase of the peak gasification value [35]. It was found by Lahijani et al. that activation energy of the gasification reaction between pistachio nut shell char and CO2 can be reduced by 53 kJ/mol by adding 5 wt% NaNO3 [30]. However, these previous studies focused on the gasification behaviors of chars, such as carbon conversion rate, gas yield, gasification reactivity and kinetics, there was no information about the pore structure of the solid porous char. Sadhwani et al. took the surface area of char into consideration when studying the AAEMs-catalyzed CO2 gasification kinetics of pine char, the emphasis of their study was the validation of the random pore model (RPM) to predict the gasification process, and there was a lack of the data about the evolution of pore volume and pore size distribution [31]. Overall, there has been limited published work aimed at the preparation of PC via AAEMs catalyzed CO2 activation.

It is a general knowledge that seawater (SW) is naturally high in Na, Mg, Ca and K salts [36, 37], thus SW could be a cost-effective source of AAEMs for catalyzing CO2 activation. However, there has been no report on this topic. In the present study, sawdust (SD), a common kind of low-cost biomass material, was impregnated with SW and then used for the preparation of micro-mesoporous carbon. The main contributions of the work are as follows: (1) the effects of SW impregnation on thermal decomposition and activation behaviors of SD under CO2 were clarified using thermogravimetric analyzer coupled with a Fourier transform infrared spectrometer (TG-FTIR); (2) the evolution of the porous structure of the final products was summarized based on N2-adsorption/desorption data, and the surface functional groups were characterized using FTIR; (3) the relationship between the pore structure and the adsorption capacity of the product to oxytetracycline (OTC), a most common antibiotic detected in aquatic environments, was established, and the adsorption equilibrium isotherms of OTC by the optimum product were determined. In order to more fully understand the work, the graphical abstract is shown in Fig. 1.

Fig. 1
figure 1

Graphical abstract of the study

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2 Experimental

2.1 Materials

Poplar SD was dried at 100 °C for 24 h, then ground and sieved into particle sizes of 80 to 120 mesh prior to use. Seawater (SW) was collected from South China Sea and filtered through 0.22 μm membranes to eliminate dirt before use. The concentrations of sodium (10.8 g L−1), potassium (399 mg L−1), magnesium (1.31 g L−1), calcium (421 mg L−1) in SW were determined by an inductively coupled plasma mass spectrometry (ICP-MS) (iCAP Q, Thermo Scientific, USA).

2.2 Preparation of porous carbons

SD was mixed with SW under a ratio of 100 g:1L, then the mixture was agitated in a shaker at 180 rpm for 12 h. After impregnation, the mixture was vacuum filtered and the solid was dried in an oven at 100 °C for 24 h (labeled as SW–SD). SW–SD was heated at a temperature increasing with a heating rate of 5 °C min−1 to a final temperature of 700 °C under pure CO2 atmosphere with the flow rate of 200 mL min−1 in a tube furnace (CTF12/65/550, Carbolite, Great Britain). The final temperature was maintained for 5, 20, 40 and 60 min, respectively. The products were cooled under CO2 flow, then washed with 0.1 M HCl solution and distilled water to a neutral pH, followed by drying at 105 °C for 8 h. The final products were denoted PCSW/CO2/700/05, PCSW/CO2/700/20, PCSW/CO2/700/40 and PCSW/CO2/700/60. The yield of product was calculated from the following equation:

$$Yield\left( {\text{\% }} \right) = \frac{{M_{{{\text{Carbon}}}} }}{{M_{{{\text{SD}}}} }} \times 100$$
(1)

where MCarbon is the mass of the product, and MSD is the mass of SD used as raw material. For comparison, porous carbons were also obtained from SD without SW impregnation under N2 and CO2 atmosphere at 700 °C for 20 min according to the same process, and the products were labeled as PCN2/700/20 and PCCO2/700/20, respectively.

2.3 Characterization of porous carbon

Pore characteristics of the samples were analyzed with N2-adsorption/desorption using a gas adsorption apparatus (Autosorb-iQ, Quantachrome Instruments, USA) at − 196 °C, and BET surface area (SBET) and the successive pore size distribution from micropores to mesopores were calculated using Brunauer–Emmett–Teller (BET) method and density function theory (DFT), respectively. The total pore volume (VTotal) was estimated from liquid volume of N2 adsorbed at the saturation of relative pressure. Horvath–Kawazo (HK) and Barret–Joyner–Halenda (BJH) method were used to deduce the micropore volume (VMicro) and the mesopore volume (VMeso), repectively. The average pore diameter (DAvg) was calculated as DAvg = 4VTotal/SBET. The surface functional groups were investigated by FTIR (TENSOR II, Bruker, Germany).

2.4 Adsorption study

2.4.1 Comparison of adsorption capacity

Aqueous solution of OTC was prepared with concentration of 100 mg L−1. 10 mg of PCSW/CO2/700/05, PCSW/CO2/700/20, PCSW/CO2/700/40 and PCSW/CO2/700/60 were added to each aqueous solution (20 mL) contained in 50 mL Erlenmeyer flasks. Adsorption equilibrium was reached after the Erlenmeyer flasks agitated in a shaker at 180 rpm and room temperature for 24 h. The suspensions were filtered by 0.45 μm membrane filters and the filtrates were diluted to a suitable concentration. The concentrations of OTC in the diluted filtrates were determined using a UV–Vis spectrophotometer (DR5000, HACH, USA) at the maximum wavelengths (λ) of 268 nm. OTC removal efficiency [RE (%)] was calculated from the following equation:

$$RE\left( {\text{\% }} \right) = \frac{{C_{0} - C_{{\text{e}}} }}{{C_{0} }} \times 100$$
(2)

where C0 and Ce (mg L−1) are the initial and equilibrium liquid-phase concentrations of OTC.

2.4.2 Adsorption equilibrium isotherms

The optimal PC sample (10 mg) selected based on Sect. 2.4.1 and aqueous solution (20 mL) of OTC contained in 50 mL Erlenmeyer flasks were agitated in a shaker at 180 rpm and room temperature for 24 h. The initial concentrations of OTC ranged from 20 to 120 mg L1. The concentrations of OTC in the suspensions were determined as described in Sect. 2.4.1. OTC uptake per unit mass of porous carbon at equilibrium [qe (mg g1)] was calculated from the following equation:

$$q_{{\text{e}}} = \frac{{C_{0} - C_{{\text{e}}} }}{W}V$$
(3)

where C0 and Ce (mg L−1) are the initial and equilibrium liquid-phase concentrations of OTC. V (L) is the volume of the solution and W (g) is the mass of porous carbon used. The equilibrium data were modelled using Langmuir, Freundlich and Temkin isotherm models and the model equations are shown in Table 1 [38].

Table 1 Linear and non-linear forms of Langmuir, Freundlich, and Temkin isotherm models
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2.5 TG-FTIR analysis

Thermal behaviors and gaseous products obtained during the thermal treatments of SD and SW-SD was studied using a TG-FTIR instrument that consists of a simultaneous thermal analyzer (STA 2500 Regulus, NETZSCH, Germany) and a Fourier transform infrared spectrometer (TENSOR II, Bruker, Germany). For comparison, SD was also pre-treated with deionized water according to the same process of SW impregnation, and the product was denoted as W-SD. SD, SW-SD and W-SD (about 13 mg) were heated from 35 °C to 1000 °C at CO2 flowing rate of 70 mL min−1 and the heating rate of 20 °C min−1. The volatiles released during heating were detected by FTIR, and the spectra was recorded at 4000–600 cm−1 with a resolution of 4 cm−1. To minimize secondary reactions, both the transfer pipe and the gas cell were heated at a constant temperature of 200 °C.

3 Results and discussion

3.1 TG-FTIR study

3.1.1 Thermal behavior

Figure 2 shows the TG and derivative thermogravimetry (DTG) curves of SD, SW-SD and W-SD. The characteristic parameters obtained from the TG and DTG curves such as the temperature ranges of the weight losses, the maximum weight loss rates (Rmax) of the weight loss stages, and the corresponding temperature (Tmax) are shown in Table 2. For SD, the weight loss below 200 °C indicated the loss of moisture, and the major weight loss appeared within the temperature of 220–410 °C, with the maximum weight loss rate peak at 365 °C, indicating the decomposition of hemicellulose and cellulose. The weight loss rate became quite slow above 410 °C because the violent cracking of organic compounds finished at this stage. Instead, the polycondensation of carbonaceous matters in the residue became the primary reaction, leading to the release of small molecules at a slow rate. The final weight loss stage above 840 °C indicated the gasification reaction between the solid char derived from SD and CO2 atmosphere.

Fig. 2
figure 2

TG and DTG curves of SD, SW-SD and W-SD.

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Table 2 Characteristic parameters obtained from TG and DTG data for samples
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There were also four stages in the weight loss process of SW-SD, but its weight loss rate became obviously higher than that of SD above 320 °C, and the maximum weight loss rate peak of SW-SD appeared at 344 °C. The major weight loss process of SW-SD finished at 370 °C, indicating that AAEMs in SW were helpful for the decomposition of the bio-polymers (mianly hemicellulose, cellulose and lignin) in SD at low temperatures. Besides, the residue rate of SW-SD was higher than that of SD between 390 and 845 °C, suggesting that the AAEMs also benefited the formation of solid char. The above results were similar to those in a previous study [39]. The maximum weight loss rate temperature of the final weight loss stage of SW-SD was 890 °C, which was 45 °C lower than that of SD, proving that the AAEMs in SW had catalytic effects on the gasification of SD char by CO2.

The thermal decomposition process of W-SD can also be divided into four stages, which were evaporation, pyrolysis, polycondensation and gasification. The temperature of the maximum weight loss rate of W-SD was 385 °C, which was 20 °C higher than that of SD, indicating that the thermal stability of SD was enhanced by water-washing. Moreover, W-SD had a lower residue rate than SD above 390 °C. In addition, Rmax of CO2 gasification of W-SD char was only 0.14% °C−1, which was the lowest among these of the three samples. It is interesting to note that the above mentioned changing trends were just opposite to these of SW-SD. The results were due to the fact that the removal of the intrinsic water-soluble AAEMs from SD by water washing reduced the catalytic effect on promoting thermal decomposition, char formation and CO2 gasification [35, 40].

3.1.2 Analysis of gaseous products

The volatiles evolved from the thermal decomposition of SD, SW-SD and W-SD were analyzed by FTIR in real time. As can be seen from Fig. 3a, the Gram-Schmidt curves shows the variation of the yields of the volatile compounds with temperature. SD curve had two peaks centered at 348 and 932 °C while SW-SD curve had two peaks centered at 343 and 886 °C. For W-SD, the two peaks appeared at 377 and 905 °C. The Gram-Schmidt peaks of the samples matched well with their DTG peaks. 3D (absorbance-wavenumber-temperature) spectra of the volatiles generated from SD, SW-SD and W-SD can be seen from Fig. 3b, c, respectively. When the temperature is fixed, absorbance at different wavenumber can be obtained to study the volatile components released at this moment. Figure 4 shows the FTIR spectra at the peak temperatures for SD and SW-SD. The gaseous products composed of a variety of molecules were evolved from SD due to the thermal cracking of the bio-polymers (mainly cellulose) at 348 °C. The peaks centered at 3575 and 1515 cm−1 represented –OH bond stretching vibrations in H2O. The band between 3015 and 2650 cm−1 indicated the stretching vibration of C–H in aliphatic hydrocarbons. The presence of CO led to the presence of two subtle peaks at 2174 and 2102 cm−1. The most remarkable absorbance peak at 1772 cm−1 belonged to C=O stretching in carboxylic acids, ketones and aldehydes. The peak at 1394 cm−1 can be assigned to the stretching vibration of C–H, indicating the release of aliphatic hydrocarbons. The peaks at 1177 and 1110 cm−1 can be attributed to the stretching vibration of C–O–C in ethers. With the temperature increased to 932 °C, only two sharp adjacent peaks attributed to CO can be observed. This was because almost all the organics in SD were decomposed at this high temperature, and the violent gasification of SD char by CO2 led to the generation of CO and the formation of pores in char. SW-SD and W-SD spectra at 343 and 377 °C show that the gaseous components were not altered due to SW impregnation and water washing, but the relative contents of these components were changed. CO was also the only gaseous product evolved from SW-SD and W-SD at 886 and 905 °C, indicating the occurrence of gasification of SW-SD and W-SD char.

Fig. 3
figure 3

a Gram–Schmidt curve of SD, SW-SD and W-SD at the heating rate of 20 °C min−1, b 3-D infrared spectra of gaseous products from SD, SW-SD and W-SD.

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Fig. 4
figure 4

FTIR spectra for gaseous products from SD, SW-SD and W-SD corresponding to typical temperatures.

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Since CO was the only product generated during CO2 activation, the evolution history of CO absorbance as a function of temperature are presented in Fig. 5a. CO produced at the low temperatures was caused by the breakage of ether bonds and C=O bonds during the pyrolysis of SD, SW-SD and W-SD [41], while a large amount of CO released above 650 °C indicated the occurrence of activation reactions. Figure 5b shows the variation of the difference between the absorbances of CO released from SW-SD (AbsSW-SD), W-SD (AbsW-SD) and SD (AbsSD) with temperature. As can be seen, SW impregnation led to two positive peaks at 700 and 890 °C, indicating that more CO was released from SW-SD than from SD at these two temperatures, and a larger amount of CO represented a higher degree of CO2 activation. In contrast, less CO was produced from W-SD than from SD during the CO2 activation stage until the temperature reached as high as 905 °C. Thus, SW-SD was more suitable than SD and W-SD for the preparation of PC via CO2 activation, and 700 °C was selected as the final activation temperature in the follow-up experiments in the aim of energy-saving. Besides, this temperature was much lower than the commonly used CO2 activation temperatures (800–1000 °C).

Fig. 5
figure 5

a Evolution curves of CO from SD, SW-SD and W-SD at the heating rate of 20 °C min−1, b variation of the difference between the absorbances of CO released from SD, SW-SD and W-SD with temperature.

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3.2 Characterization of porous carbons

N2-adsorption/desorption isotherms of the PCs are shown in Fig. 6. The adsorption isotherms of the samples show the remarkable nitrogen uptakes in the low pressure region (P/P0 ≤ 0.05), standing for the presence of micropores (diameter ≤ 2 nm). The adsorbed volumes at high relative pressures after the initial filling suggested the presence of mesopores (2 nm < diameter < 50 nm). The isotherms of PCN2/700/200 and PCSW/CO2/700/05 exhibited relatively sharp knees and they appeared to be of type I according to the IUPAC classification, reflecting the predominance of microporosity. The isotherms of PCCO2/700/20, PCSW/CO2/700/20 PCSW/CO2/700/40 and PCSW/CO2/700/60 displayed type IV isotherms that had more rounded knees pointing to a widening of microporosity that even extended to mesoporosity. Moreover, the distinct hysteresis loops can be observed from the isotherms of PCSW/CO2/700/20 PCSW/CO2/700/40 and PCSW/CO2/700/60, indicating the existence of abundant mesopores with an interconnected multistage channel structure.

Fig. 6
figure 6

N2-adsorption/desorption curves of porous carbons.

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Pore characteristic parameters of the samples measured by the N2-adsorption/desorption isotherms are shown in Table 3. PCCO2/700/20 had a lower SBET than PCN2/700/20 because CO2 etching effect led to the expansion of micropores into mesopores. PCSW/CO2/700/05 had a SBET of 572 m2 g−1, and it consisted mainly of micropores, indicating that a highly microporous carbon can be prepared from SW-SD in only 5 min. This was because the gasification reaction can be significantly promoted by the AAEMs contained in SW. The increase of activation time from 5 to 20 min led to the formation of both more micro- and mesopores, thus PCSW/CO2/700/20 had the highest SBET of 600 m2 g−1 among the samples. Note that SBET of PCSW/CO2/700/20 was 1.8 times higher than that of PCCO2/700/20, proving that AAEMs in SW played an important role in promoting pore formation during CO2 activation. With the activation time increasing to 40 and 60 min, VMicro decreased while VMeso increased because CO2 etching of original micropore walls led to pore widening and formation of mesopores. It is worth noting although PCSW/CO2/700/60 had a relatively low SBET of 490 m2 g−1, it had the highest proportion of mesopore (65%), and its Davg reached as large as 3.78 nm. Thus it can be said that PCSW/CO2/700/60 was a typical micro-mesoporous carbon. The comparison of the pore characteristics of PCSW/CO2/700/60 with some micro-mesoporous carbons is listed in Table 4.

Table 3 Pore characteristics of porous carbons
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Table 4 Comparison of the pore characteristics of PCSW/CO2/700/60 with some micro-mesoporous carbons
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To find out the detail effects of CO2 activation and SW impregnation on porous structures of the products, DFT pore size distribution curves of PCN2/700/20, PCCO2/700/20, and PCSW/CO2/700/20 were compared. As can be seen from Fig. 7, PCN2/700/20 had a strong peak at 0.54 nm and a distribution between 1.20 and 1.50 nm. The porous structure of PCN2/700/20 was determined by the natural properties of the raw material SD because no reactive gas was introduced during the heating process. Compared with PCN2/700/20, PCCO2/700/20 had a much weaker peak at 0.63 nm, and a weaker distribution between 1.14 and 1.48 nm. However, the distribution of PCCO2/700/20 curve within the diameter ranges of 0.64–0.90, 1.48–2.03, and especially 2.25–4.50 nm were stronger than PCN2/700/20. The results indicated that reaction between CO2 and char led to micropore widening and formation of more mesopores. PCSW/CO2/700/20 had obviously more micropores than PCCO2/700/20 within the range of diameter smaller than 0.61 nm, and it also an even stronger distribution than PCN2/700/20 between 1.14 and 1.48 nm. Moreover, PCSW/CO2/700/20 contained more mesopores with diameter of 3.50–4.24 nm than PCCO2/700/20. The above results proved that CO2 activation of SD led to the expansion of original micropores into mesopores compared with carbonization of SD under inert atmosphere, but pre-impregnation of SD with SW was apparently beneficial to the formation of both more micro- and mesopores during CO2 activation.

Fig. 7
figure 7

DFT pore size distribution curves of PCN2/700/20, PCCO2/700/20, and PCSW/CO2/700/20.

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To determine the influences of activation time on the porous structures of the products, DFT pore size distribution curves of PCSW/CO2/700/05, PCSW/CO2/700/20, PCSW/CO2/700/40 and PCSW/CO2/700/60 are compared in Fig. 8. PCSW/CO2/700/20 had stronger distribution than PCSW/CO2/700/05 within the whole diameter range, except the peak at 0.51 nm obviously decreased. Therefore, more micro- and mesopores were created in the carbon matrix with the activation time increasing from 5 to 20 min. The strongest peak shifted from 0.51 to 0.58 nm when the activation time increased to 40 min. In addition, the distribution between 1.16 and 1.50 nm decreased while more pores were formed with the diameter between 2.32 and 4.40 nm. Activation time extended to 60 min led to stronger distribution within the ranges of 0.65–0.90, 1.81–2.02 and especially 2.32–4.40 nm. However, the maximum peak value of PCSW/CO2/700/60 curve appeared at 0.63 nm and it was much lower than that of the other three curves.

Fig. 8
figure 8

DFT pore size distribution curves of PCSW/CO2/700/05, PCSW/CO2/700/20, PCSW/CO2/700/40, and PCSW/CO2/700/60.

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To characterize the surface groups on the porous carbons, FTIR spectra of PCSW/CO2/700/05, PCSW/CO2/700/20, PCSW/CO2/700/40 and PCSW/CO2/700/60 were recorded (Fig. 9). As can be seen, activation time had no significant influence on the position and intensity of the adsorption peaks. The strong peak at 3448 cm−1 was caused by the presence of O–H and bands around 2928 and 2853 cm−1 indicated the presence of C–H stretching in aliphatic hydrocarbons. The peak at 1628 cm−1 can be attributed to the stretching vibration of C=O conjugated to the aromatic rings. The peak at 1070 cm−1 demonstrated the presence of C–O–C in ethers [42].

Fig. 9
figure 9

FTIR spectra of PCSW/CO2/700/05, PCSW/CO2/700/20, PCSW/CO2/700/40, and PCSW/CO2/700/60.

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3.3 Adsorption study

The removal efficiencies of PCSW/CO2/700/05, PCSW/CO2/700/20, PCSW/CO2/700/40 and PCSW/CO2/700/60 for OTC are shown in Fig. 10. Interestingly, although PCSW/CO2/700/60 had lower surface area compared with other samples, it had the highest removal efficiency of 47.64% under the same experimental condition. Figure 11a, b show that the qe values of the samples had positive correlation with their VMeso and DAvg, respectively. Moreover, the higher corresponding correlation coefficient R2 of 0.9942 suggested that qe had a better linear relationship with DAvg than with VMeso.

Figure 10
figure 10

Removal efficiencies of PCSW/CO2/700/05, PCSW/CO2/700/20, PCSW/CO2/700/40 and PCSW/CO2/700/60 for OTC.

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Thus, DAvg can be possibly used as an index for the prediction of adsorption capacity of a PC sample for OTC.

Fig. 11
figure 11

Linear relationships of qe with a VMeso and b DAvg

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PCSW/CO2/700/60 had a much better adsorption performance than other samples, thus the adsorption isotherm study was focused on PCSW/CO2/700/60. The adsorption isotherm is shown in Fig. 12a, as can be seen, the increase in initial concentration of OTC resulted in the increase of adsorption equilibrium, this was because the initial concentration provided a driving force for OTC transferred from the aqueous phase to the surface of PCSW/CO2/700/60. Figure 12b–d shows the linear fitting of adsorption equilibrium data by Langmuir, Freundlich and Temkin isotherm models, and the best fitting model was determined by the highest R2 value. R2 values and the isotherm parameters calculated from the slopes and intercepts can be seen in Table 5. Langmuir model had the best fitting, suggesting the formation of OTC monolayer on the PCSW/CO2/700/60 surface and no further adsorption occurred after the formation of the monolayer, and there was no OTC molecules transmigrated in the plane of the neighboring surface. Moreover, each OTC molecule had similar enthalpy and activation energy [43]. Based on the Langmuir constant, the maximum monolayer adsorption capacity of PCSW/CO2/700/60 for OTC was calculated to be 100 mg g−1. Comparison of OTC adsorption capacities of PCSW/CO2/700/60 and other absorbents can be seen from Table 6.

Fig. 12
figure 12

a Adsorption isotherm of OTC on PCSW/CO2/700/60, linear fitting of equilibrium isotherm data by b Langmuir, c Freundlich, and d Temkin isotherm models.

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Table 5 Fitting parameters for Langmuir, Freundlich and Temkin isotherm model
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Table 6 Comparison of OTC adsorption capacities of various absorbents
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4 Conclusion

  1. 1.

    Seawater impregnation benefited the decomposition of sawdust at low temperatures and the formation of char. More CO was produced from seawater impregnated sawdust than from the untreated sawdust under CO2 at the 700 and 890 °C, indicating a higher degree of activation. 700 °C was selected as the activation temperature for the purpose of energy-saving.

  2. 2.

    Because of seawater impregnation, BET surface area and total pore volume of porous carbon increased from 331 m2 g−1 and 0.238 cm3 g−1 to 600 m2 g−1 and 0.402 cm3 g−1, respectively, without changing the activation condition. A longer period of activation time led to the formation of remarkably more mesopores.

  3. 3.

    Micro-mesoporous carbon with a BET surface area of 490 m2 g−1 and an average pore diameter of 3.78 nm can be prepared from seawater impregnated sawdust after 60-min activation. Its adsorption equilibrium data for OTC followed Langmuir model with a maximum monolayer adsorption capacity up to 100 mg g−1.