A simple method to prepare carbon-based mesoporous materials by coal gasification of fine slag and its application in phenol adsorption

Phenol is a common organic pollutant that is difficult to degrade and widely exists in all kinds of wastewater. In this study, an economical and environmentally friendly alternative process for phenol-containing wastewater has been developed using porous nano-adsorption material (PNAM) prepared from coal gasification fine slag. The morphology, crystal structure, surface functional groups, gap structure, and specific surface area of PNAM were characterized by SEM, XRD, FT-IR, and BET. The effects of adsorbent dosage, temperature, pH, and reaction time on adsorption were further investigated. In addition, the adsorption kinetics, thermodynamics, and adsorption mechanism were explored. The results show that the surface area of PNAM is high, up to 602 m2 g−1, and the pore volume is 0.507 cm3 g−1. Adsorption processes mainly occur in mesopores between 2 and 5 nm, including physical and chemical adsorption, and here chemical adsorption plays a significant role. The adsorption rate of phenol in a 1000 mg l−1 simulated phenol solution by PNAM reaches 96.14%, while the unit adsorption capacity is 32.045 mg g−1. As a result, it is expected that employing coal gasification fine slag to prepare adsorption materials for phenol-containing wastewater treatment may be an economically feasible and environmentally sustainable strategy.


Introduction
Phenol compounds are highly toxic organic pollutants to humans and animals and widely exist in petrochemical, pharmaceutical, dye, pesticide, and other industries [1][2][3].Due to the improper treatment of phenolic compounds, they have caused great damage to the environment and increasingly aroused wide attention [4,5].Among the common phenolic compounds, phenol has always been the focus of environmental pollution control owing to its large quantity and wide distribution.
In the past decades, a variety of methods have been tried to treat phenol wastewater, including membrane treatment [6], biochemical processes [7], extraction [8], precipitation [9], etc.But these methods have some shortcomings that restrict their application.The instability and leakage of liquids are the main defects of membrane treatment [10].Biochemical treatment [11] is only applicable to low-concentration wastewater and has high requirements for inlet water quality.Extraction possesses a good effect on phenol removal, but the high price and nature of the extraction agent determine the high cost of water treatment [12].Precipitation will cause secondary pollution due to the large amount of solid waste formed in the process of treatment [9].
Adsorption is another widely used method for the treatment of wastewater containing phenol pollutants.The removal rate of phenol was more than 99.92% by using nanosilicon particles prepared from industrial waste onion skin [13].The feasibility of treating phenolic compounds in wastewater was investigated by utilizing iron nano-zeolite (Fe-NZ) as an adsorbent, indicating that the maximum adsorption capacity of Fe-NZ for phenol (Ph) was 138.7 mg g −1 [14].By analyzing the regeneration properties of the organic pollutant load Fe-NZ, the phenol desorption efficiency remained at 46.3% even after 10 sorption-desorption cycles.Although the adsorption method has the advantages of a simple process and a small equipment investment, its high cost of materials restricts its industrial application.It is a challenge to develop an adsorbent with low cost, high adsorption capacity, and environmental protection potential for removing phenol from industrial wastewater.
Coal gasification fine slag (CGFS) is a kind of solid waste generated in the process of gasification and is a kind of mesoporous material [15][16][17].During the process of coal gasification, the remaining unburned carbon in the fine slag has been activated into porous carbon by various gases in the gasifier at a high temperature, which has a high specific surface area [18].In this study, CGFS was selected as a raw material to prepare mesoporous material, which was further used for phenol adsorption.
The objective of this study is to prepare porous materials by using CGFS and evaluate its adsorption effect in phenol-containing wastewater.The PNAM was characterized by SEM, BET, FT-IR, XRD, etc.The influence of the amount, temperature, pH, and reaction time on the adsorption effect was studied.Further, the adsorption mechanism was explored through thermodynamic and kinetic studies.

Materials and chemicals
In this study, CGFS was obtained from a chemical plant in Yulin, Shaanxi Province, China.The phenol, concentrated sulfuric acid, and sodium hydroxide used in the experiment were purchased from Tianjin Kemeiou Chemical Reagent Co., Ltd All the reagents can be directly used without further treatment.

Preparation process of modification
Due to the complex composition of coal, there are many impurities in coal gasification slag produced at 1200 °C.The acid-base activation method was used to prepare porous nanometer adsorption material (PNAM).In the preparation process, acid and base are used to remove most metal oxide and silicon oxide impurities from CGFS, respectively.Figure 1 shows the preparation process of PNAM.Impurities can be substantially removed through acid-base soaking, then activated in a tubular furnace.The adsorption property of the sample was improved during the process of modification.
The specific steps for PNAM preparation are as follows: (1) 10 g CGFS and a 100 ml sulfuric acid solution of 10 wt% were stirred on a magnetic agitator for 3 h to fully leach the metal oxides in the material.After the reaction, the solid-liquid mixture was filtered, and the sample was repeatedly washed to remove surface ions until it became neutral.The sample was then placed in a drying box at 120 °C and dried for 12 h.(2) The dried sample was mixed with a 100-ml sodium hydroxide solution of 20 wt% on a magnetic stirrer for 3 h.The purpose of this step is to fully remove SiO 2 impurities.After the reaction, strain the mixture, then rinse the sample repeatedly with distilled water until it becomes neutral again.The sample was again placed in an oven at 120 °C for 12 h.(3) After acid and alkali treatment, the internal impurities of the samples can be substantially removed.Finally, the sample was activated for 2 h at 150 °C under nitrogen protection, then cooled to room temperature, and the obtained sample is called PNAM.

Adsorption experiment
A typical case study has been carried out with 50 ml of phenol solution (concentration of 1000 mg l −1 ) and 1.5 g of PNAM with pH 7 adjustment placed in a beaker.The mouth of the beaker has been sealed with a sealing film.After 30 min of magnetic stirring with a rotational speed of 50 r min −1 at a 30 °C reaction temperature, the reactants were transferred to a centrifuge tube.The solid-liquid separation was performed using a centrifuge with a rotational speed of 12000 r min −1 .The aliquot was taken, and the concentration of phenol was then measured by an ultraviolet spectrophotometer (UV).The adsorption capacity of phenol is calculated by the following formula: q refers to the adsorption capacity of phenol per unit PNAM (mg/g).
V is the volume of the reaction solution (ml); C0 refers to the concentration of phenol solution before adsorption (mg/l); Ce refers to the concentration of phenol solution after adsorption (mg/l); M refers to the mass of the adsorbent used (g).

Characterization method
The adsorption mechanism of phenol depends on the physical and chemical properties of the adsorbent [19].In this study, scanning electron microscopy (SEM) was used to analyze the morphology of the samples; x-ray diffraction (XRD) was used to identify the elemental and crystal structures of the samples; Fourier infrared spectroscopy (FT-IR) was used to observe the functional group form of the samples; The textural properties and pore size distribution of PNAM were determined by N2 physisorption measurements, which were obtained from a Micromeritics APSP 2460 instrument at 195.7 °C.Before measurements, the adsorbent was degassed at 300 °C for 2 h.

Physicochemical properties of PNAM
The SEM image of CGFS is depicted in figure 2(a).After the high-temperature gasification, CGFS have abundant channels, but these channels are blocked by metal or non-metal oxides formed in the gasification process [15].As a comparison, figure 2(b) shows the channels of PNAM.By using the acid-base activation method, pores become unblocked as the impurities blocked in channels are greatly removed.Table 1 depicts the BET analysis results of CGFS and PNAM.The raw material is formed in the gasification process at 1200 °C.During the gasification process, abundant pore structures and a large specific surface area are formed.Further, through acid-base treatment, the specific surface area of the material is further improved, and finally, the material PNAM with a large specific surface area and pore volume is formed, which is favorable for adsorption.The above results indicate that the activating method chosen in this study is effective and feasible.The nitrogen adsorption-desorption curve and the curve of pore size changing with pore volume are shown in figure 3.According to the International Union of Pure and Applied Chemistry (IUPAC) classification [20], the curves showed type IV isotherms with hysteresis loops, indicating mesoporous structures presented on PNAM with a wide pore size distribution (3-8 nm), which is related to the coal gasification process.Further, PNAM formed an H4 hysteretic ring in the range of P/P0 0.4-1.0,indicating that the catalyst has a disordered mesoporous structure and a wide pore size distribution.It can be determined that mesoporous materials play an important role in the adsorption process of PNAM, including providing a channel for the migration of adsorbents and providing adsorption sites.
The FTIR spectra of PNAM materials are shown in figure 4(a).Through infrared spectral analysis, information about functional groups on the surface of the materials can be obtained.The surface of PNAM is rich in functional groups.The wave number at 3435 cm −1 is generally an O-H stretching vibration peak or an   -NH 2 antisymmetric stretching vibration peak.However, due to the high intensity of the O-H stretching vibration peak, it can be determined that the wave peak here is mainly caused by the -NH 2 antisymmetric stretching vibration peak [21].NH 2 functional groups are beneficial to the adsorption of Lewis acids such as phenol.The crest at 2027 cm −1 is caused by C≡C.The peak of 1641 cm −1 is caused by C=C stretching vibration, and the peak is in the range of 1640±20 cm −1 can be inferred to be the vibration peak of cis-vinyl.The crest at 1128 cm −1 is caused by C-O stretching vibration, and the C-O bond is conducive to the adsorption of phenol.Finally, the bands located between 621 cm −1 can be attributed to benzene derivatives [22], which have the same aliphatic ring as phenol and also promote phenol adsorption.The existence of these adsorption peaks indicates that the material has abundant functional groups, most of which can promote the adsorption of phenol [23].In the adsorption experiment of phenol by PNAM, chemical adsorption will also occur due to the presence of functional groups, in addition to physical adsorption.The structure of the prepared adsorbent (PNAM) was determined by x-ray diffraction (XRD).The XRD pattern (figure 4(b)) of the as-prepared product shows six peaks corresponding to (100), ( 101), ( 112), (005), ( 104) and (002) of faces CaAl 2 SiO 8 (quartz phase of SiO 2 : JCPDS-46-1045; graphite carbon: JCPDS-26-1077; silicate anorthite (CaAl 2 Si 2 O 8 ): JCPDS-20-0452).The characteristic peak is stronger at 26.4°(101), indicating that the content of SiO 2 is higher in the coal gasification fine slag.The above analysis of the XRD pattern of PNAM shows the wide dispersion characteristics and non-crystal structure, which are due to the presence of different inorganic minerals in coal gasification fine slag.As per the result analysis from tables 1 and 2, PNAM shows improved coal gasification fine slag is chiefly amorphous due to the fact that the inorganic materials have entirely melted and chilled during the process of high-temperature treatment of coal gasification.
PNAM prepared by the acid-base activation method possesses a large specific surface area and pore volume, which can provide sufficient adsorption sites for phenol adsorption.Furthermore, the abundant functional groups on the surface of PNAM also promote the adsorption of phenol.

Adsorption effect of phenol
The prepared PNAM was utilized for the adsorption of a 1000 mg l −1 phenol solution.The influences of the amount, pH, temperature, and reaction time on the adsorption effect were investigated, and the results are shown in figure 5.
The amount of adsorbent is one of the most important factors affecting the adsorption effect.In general, the adsorption effect is positively correlated with the amount of adsorbent.However, considering the cost and other factors, it is necessary to determine the appropriate amount of adsorbent.Figure 5(a) shows the influence of the amount of adsorbent on the removal effect of phenol.As the adsorbent usage increased, the adsorbent adsorption quantity increased gradually.This is due to the fact that a high amount of adsorbent can provide a more specific surface area and more active sites [24].However, the saturated adsorption capacity of the adsorbent decreases with an increase in the amount of adsorbent [25].In order to ensure an adsorbent with high saturated adsorption capacity and high adsorption efficiency, the rate between adsorbent and solution is determined to be 3:100 for further research.
Figure 5(b) reveals the influence of temperature on the phenol removal effect.With the increase in temperature, the adsorption capacity of phenol by PNAM first increases and then decreases; the turning point occurs at 30 °C.Because of the low temperature of 10 °C-30 °C, it can't provide enough energy for molecular thermal motion.With the increase in temperature, the adsorption sites are conducive to contact with phenol molecules, resulting in an increase in material adsorption capacity.However, since the adsorption process is exothermic (3.2.2 proves this), increasing temperature is not conducive to adsorption.Therefore, when the temperature is higher than 30 °C, the concentration of phenol after adsorption gradually increases, and the adsorption effect becomes worse.The optimum adsorption temperature was determined to be 30 °C.
Solution pH has a strong influence on the adsorption process [26], as shown in figure 5(c).With the increase in pH, the adsorption capacity of phenol by the adsorbent increases first and then decreases.When pH is 7, the adsorption capacity of phenol reaches its maximum.This is because in an acidic solution, with an increase in pH value, the concentration of hydration cations decreases, which can increase the electrostatic attraction between the positively charged surface of the adsorbent and the negatively charged surface of phenol, thus improving the removal efficiency.Under alkaline conditions, more hydroxyl radicals compete with the negative charge of phenol, reducing its adsorption effect [27].Therefore, the optimum pH was determined to be 7.
Figure 5(d) reveals the effect of reaction time on phenol removal capacity.It can be easily found that, with the reaction going on, the removal amount of phenol gradually increases, but the increase range gradually decreases.The gradual decrease in adsorption capacity is related to the adsorption sites of the adsorbent.At the beginning of the experiment, the adsorbent had more adsorption sites, so the adsorption rate was faster.With the process, a large number of adsorption sites are gradually occupied, and the adsorption rate decreases until the adsorption equilibrium is reached in 70 min [26,28,29].Hence, under the conditions of a pH of 7, a temperature of 30 °C, a ratio of adsorbent to phenol solution of 3:100, and a full reaction for 80 min, the removal amount of phenol in the phenol solution was 961.358 mg L −1 , and the adsorption amount of the unit adsorbent reached 32.045 mg g −1 , indicating the great potential of PNAM in phenol adsorption.
The most important criteria influencing the use of an adsorbent are efficiency and cost.For that reason, the adsorption capacities of some adsorbents reported in the literature for phenol adsorption were determined and listed in table 2. As compared with the reported data in table 2 [30][31][32][33][34][35][36][37][38][39][40] the maximum adsorption capacity of PNAM for phenol is much higher than that of the adsorbents described in the literature.This suggests its potential use in actually removing phenol from wastewater.

Adsorption kinetics and thermodynamic analysis 3.3.1. Adsorption kinetics study
It can be concluded from the previous experiments that PNAM has a high adsorption rate for phenol and a significant adsorption effect.In order to study its dynamical mechanism, three dynamical models, Lagerren pseudo-first-order kinetics (PFO), Ho-McKay pseudo-second-order kinetics (PSO), and intra-particle diffusion (IPD), are used for analysis [41][42][43].The equations are as follows: 2 2 Where qt (mg/g) is the adsorption capacity of phenol by unit PNAM at time t.q e1 and q e2 represent the theoretical equilibrium adsorption capacities of PFO and PSO, respectively.k 1 (min −1 ) and k 2 (min −1 ) are the rate constants of PFO and PSO dynamics, respectively, and k 3 is the IPD diffusion rate constant.D is the model constant related to the thickness of the convenient layer.If D is 0, it means that the internal diffusion rate is affected by a single factor.According to the experimental conclusions in the adsorption process of phenol, the kinetics of the adsorption process were studied, and the fitting results were shown in table S1 and figure 6.
As shown in figure 6(a), the adsorption rate gradually decreases with time and finally reaches equilibrium adsorption at 70 min.The faster adsorption rate in the initial stage may be due to the large number of active sites available on the adsorbents at the beginning of the adsorption reaction.With the progress of the adsorption reaction, the driving force caused by the concentration gradient gradually decreases due to the accumulation of phenol on the surface adsorption sites, and finally, the adsorption rate gradually decreases.According to the kinetic parameters in table S2, compared with the quasi-first-order kinetic model, the quasi-second-order kinetic model has a higher R2 value.Figure 6(b) can better describe the adsorption process.At the same time, the equilibrium adsorption capacity calculated in the quasi-second-order kinetic model is more consistent with the experimental equilibrium adsorption capacity. Figure 6(c) shows that the adsorption process mainly includes external transfer, intra-particle diffusion, and adsorption analysis equilibrium.

Thermodynamic analysis of adsorption
In order to study the effect of temperature on the adsorption of phenol, the adsorption experiments at 303.15K, 313.15K, and 323.15K were analyzed, as shown in figure 6(d).It can be seen from the figures that the adsorption capacity of PNAM for phenol decreases with the increase in temperature, which indicates that the adsorption behavior of PNAM for phenol is exothermic.In order to further study the influence of temperature on adsorption, detailed thermodynamic data were used to analyze the adsorption behavior.Thermodynamic parameters include enthalpy change (ΔH 0 ), entropy change (ΔS 0 ) and Gibbs free energy (ΔG 0 ), which could be calculated by the following equation [44]: ( ) Where K C is the equilibrium constant, q e (mg/g) is the theoretical equilibrium adsorption capacity, C e is the phenol concentration at equilibrium, R is constant (8.314J mol −1 K −1 ), and T is the absolute temperature (K).
According to the experimental data at 303.15K, 313.15K, and 323.15K, the relationship between lnK C and 1/ T was plotted, as shown in figure 6(d).ΔH 0 and ΔS 0 were calculated according to the slope and intercept of the curve, and ΔG0 was further calculated, as shown in table S2.The value of ΔH 0 is less than zero, indicating that the adsorption of phenol is an exothermic process, so low temperatures are conducive to its adsorption.The positive value of ΔS 0 reflects that the confusion degree of the adsorbent at the interface between solid and liquid increases during the adsorption process, indicating that increasing temperature is conducive to the adsorption reaction.In addition, ΔG 0 was negative at all reaction temperatures, indicating that the adsorption of phenol was spontaneous.ΔG 0 decreases with increasing temperature, which indicates that the adsorption efficiency is higher at lower temperatures.Therefore, it can be concluded that the adsorption of phenol on PNAM is an exothermic spontaneous process, and high temperatures are not conducive to the adsorption process.

Exploration of the adsorption mechanism
To explore the relevant adsorption mechanism, an in-depth study on phenol adsorption is conducted from the perspectives of influencing factors, characterization analysis, adsorption kinetics, adsorption thermodynamics, etc.The results show that the adsorption process included physical adsorption and chemical adsorption.Thermodynamic studies show that the ΔH 0 value is less than zero during the adsorption process, indicating that the adsorption of phenol is an exothermic process and that low temperatures are conducive to its adsorption.At different reaction temperatures, ΔG 0 is negative, indicating that the adsorption of phenol is spontaneous.
ΔG 0 decreases with increasing temperature, which indicates that the adsorption efficiency is higher at lower temperatures.Therefore, the adsorption of phenol on PNAM is a spontaneous exothermic process, and high temperatures are not conducive to adsorption.By analyzing the pore size distribution of PNAM and the nitrogen adsorption-desorption curve, it can be determined that material adsorption mainly occurs in mesoporous materials and that the adsorption process is multi-layer adsorption.Combined with the results of kinetic studies, the adsorption process conforms to the pseudo-first-order and pseudo-second-order kinetic models at the same time.Therefore, it is determined that the adsorption process of phenol by PNAM is a coexistence of physical adsorption and chemisorption, and chemisorption is the main process.
Based on the above analysis, the adsorption process of phenol by PNAM is depicted in figure 7. Firstly, phenol was attracted to the surface of the adsorbent under electrostatic action.Except for a small amount of phenol that accumulated on the surface of the material, most of the phenol entered the material through the pore to complete the adsorption.The C-O, aromatic ring, and -NH 2 functional groups on the surface of the adsorbent and in the pore channels can provide abundant adsorption sites for phenol, resulting in low local phenol concentrations around the adsorbent.Under the action of concentration difference, phenol molecules continue to move to the PNAM material to complete adsorption until adsorption equilibrium is reached.

Conclusion
A method of preparing porous adsorption material using solid waste coal gasification slag was proposed for the adsorption of phenol.
(1) The preparation of PNAM by the acid-base activation method can effectively increase the specific surface area and pore volume of materials so as to provide more adsorption sites.The obtained PNAM has abundant functional groups, most of which can promote the adsorption of phenol.
(2) Under the conditions of a pH of 7, a temperature of 30 °C, a ratio of PNAM to phenol solution of 3:100, and full reaction for 80 min, the unit PNAM adsorption capacity reached 32.045 mg g −1 , indicating PNAM with strong phenol adsorption potential.
(3) Kinetic and thermodynamic calculations manifest that the adsorption of phenol by PNAM is a spontaneous exothermic reaction.The adsorption process includes physical adsorption and chemical adsorption, and chemical adsorption is the main process.
In conclusion, this study develops a cheap method to prepare adsorption material (PNAM) by using coal gasification slag, realizing the high-value utilization of solid waste.The application of PNAM in phenol adsorption proves that it is an economical, feasible, and environmentally friendly method for the treatment of phenol-containing wastewater by adsorption and is expected to offer theoretical reference for the industrial application.

Figure 3 .
Figure 3.The pore size distribution and nitrogen adsorption-desorption curve of PNAM.

Figure 5 .
Figure 5.Effect of different factors on adsorption of phenol: (a) the ratio of adsorbent to reaction solution; (b) reaction temperature; (c) pH; (d) reaction time.

Table 2 .
Comparison of the effects of PNAM and absorbents in the literature.