Adsorption of pyrolysis oil model compound (phenol) with plasma-modified hydro-chars and mechanism exploration

doi:10.21203/rs.3.rs-3146956/v1

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Adsorption of pyrolysis oil model compound (phenol) with plasma-modified hydro-chars and mechanism exploration

https://doi.org/10.21203/rs.3.rs-3146956/v1

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published 16 Nov, 2023

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Phenol is one of the important ingredients of pyrolysis oil, contributing to the high biotoxicity of pyrolysis oil. To promote the degradation and conversion of phenol during anaerobic digestion, hydro-chars with high phenol adsorption capacity were produced. The phenol adsorption capabilities of the plain hydro-char, plasma modified hydro-char at 25 ℃ (HC-NH₃-P-25), and plasma modified hydro-char at 500 ℃ (HC-NH₃-P-500), and their adsorption kinetics and thermodynamics were explored. Experimental results indicate that the phenol adsorption capability of HC-NH₃-P-500 was the highest. The phenol adsorption kinetics of all samples followed the Pseudo-second-order equation and interparticle diffusion model, indicating that the adsorption rate of phenol was controlled by interparticle diffusion and chemistry adsorption simultaneously. By DFT calculations, π-π stacking and hydrogen bond are the main interactions for phenol adsorption. It was observed that an enriched graphite N content decreased the average vertical distance between hydro-chars and phenol in π-π stacking complex, from 3.5120 Å to 3.4532 Å, causing an increase in the negative adsorption energy between phenol and hydro-char from 13.9330 to 23.4181 kJ/mol. For hydrogen bond complex, the average vertical distance decreased from 3.4885 Å to 3.3386 Å due to the increase in graphite N content; causing the corresponding negative adsorption energy increased from 19.0233 to 19.9517 kJ/mol. Additionally, the presence of graphite N in the hydro-char created a positive diffusion region and enhanced the electron density between hydro-char and phenol. Analyses suggest that enriched graphite N contributed to the adsorption complex stability, resulting in an improved phenol adsorption capacity.

Plasma modification

hydro-char

adsorption

phenol

density functional theory (DFT)

Hydro-chars modified with plasmas at 500 ℃ exhibits the largest phenol removal rate
The highest N doping in hydro-char achieved at 500 ℃ modification
Plasma-doped N in hydro-chars is mainly graphite N and pyridine N
Increase in graphite N is beneficial for the phenol adsorption
Adsorption force for phenol mainly consists of π-π stacking and hydrogen bond

Pyrolysis oil is a byproduct from organics pyrolysis process, for feedstock such as biomass wastes or sewage sludge, the pyrolysis oil is usually characterized with high water content and complex ingredients, which limits its usage. The widely employed treatment methods for the pyrolysis oil include physic adsorption (Zhou et al., 2019), coagulation sedimentation (Zhu, 2006), chemical degradation (Chu et al., 2012), and biology method (Lu et al., 2009). Among these methods, the biology method is especially promising, due to the advantages of none secondary pollutant, low cost, and wide applicability (Torri and Fabbri, 2014). However, the pyrolysis oil contains rich toxic compounds such as phenol, which is harmful to the biological processes. Yue et al. (2019) found that the addition of biochar is beneficial to the degradation of pyrolysis oil during its co-anaerobic digestion process. Because the adsorption of pyrolysis oil by biochar decreased the toxicity of pyrolysis oil in anaerobic digester and increased the pyrolysis oil degradation efficiency. Hence, developing char materials with low-cost and high adsorption capacity is essential for biodegradation of pyrolysis oil. Further studies have proven that the effect of biochars can be further enhanced by tuning the specific surface area and pore size, doping N element, etc.(Jin et al., 2020). Yang et al. (2014) found that the enhanced adsorption ability of the aminated modified carbon to phenol was closely related with the N-containing functional groups on the surface of carbon materials; more specifically, pyridinic and pyrrolic groups provided p electron for the π-π conjugate ring between phenol and the carbon material. Zeng et al. (2015) reported that the hydrogen bond and complexation between phenol and biochars were the important driving force for adsorption. In addition to modification method, to produce a proper char from both performance and economic points of view with satisfied properties for pyrolysis oil or phenol adsorption purpose is also highly desired.

Sewage sludge (SS) is the by-product of the wastewater treatment, and it is one of the most pressing environmental problems to deal with today, it usually has a high water content and contains rich organic substances, pathogens and heavy metals (Verlicchi and Zambello, 2015). The conventional disposal methods such as landfill, compost, and incineration (Fytili and Zabaniotou, 2008) are not suitable for its sustainable management, as these methods caused secondary pollution, waste of resources and/or land occupation (Smith et al., 2009). Hence, applicable SS treatment methods are highly desired. Hydrothermal carbonization is a potential SS treatment method, which can adopt to the high moisture character of SS, and lead SS to be partially carbonized at an environment of high pressure, temperature of 200–300°C under a moisture saturated state. The produced SS hydro-chars are carbon-rich materials. Compared with the pyrolysis reaction, the hydrothermal reaction requires a relatively low energy consumption; especially hydro-chars prepared from hydrothermal processes can achieve a high efficiency in maintaining the functional groups on the surface (He et al., 2013), which is benefit for its usage. To further enhance the performance of hydro-chars, modification methods such as acid modification, alkaline modification, air modification, and plasma modification (An et al., 2021) are usually adapted. Among these plasma modification has the advantages of no secondary pollution and being convenient (Gupta et al., 2015). An et al. (2022) reported that the plasma modification can not only increase the oxygen functional groups on hydro-chars, but also increase its specific surface area and porosity. Yi et al. (2019) employed two-step method to modify biochars, their first step was ultrasonic impregnation in KOH at ambient temperature and the second step was low-temperature plasma modification with a duration of 0.5 h, which can increase the specific surface area of biochar from 3.8 to 275.3 m²/g. To further improve the porous structures of the hydro-char, the hydro-char can be produced by blending high lignocellulosic biomass such as wheat straw (Deng et al., 2017), it was suggested that the developed porous structures could enhance the adsorption capacity of phenol, which can reduce the biotoxicity to favor the degradation of pyrolysis oil to produce the methane. Generally, Nitrogen doping is regarded as an effective method to promote sorption ability of hydro-char (Cheng et al., 2020). Various N doping methods have been employed to modify the carbon materials to improve the adsorption capacity of phenol (Jin et al., 2020; Zhou et al., 2018). However, an effective N doping method should be guided by the identification of the efficacious N species, which is not revealed yet.

The interactions at solid-liquid interfaces are complex during the adsorption process, due to the complex interactions of adsorbent morphology, guest molecule type, and properties of solvent. It is a challenge to understand the interface adsorption mechanism via experimental methods, therefore the dominant mechanism for phenol adsorption by the char materials is not clear yet. The Density Functional Theory (DFT) is a powerful tool to investigate the interface reaction, which is widely used to describe the structural, electronic, and energetic properties of the interaction between the adsorbent and the guest molecule. DFT analysis is also helpful for understanding the impacts of different N species on adsorption process, so that to improve the hydro-char modification process.

In this study, phenol was used as the pyrolysis oil model compound; its adsorption by the SS derived hydro-char was investigated both experimentally and via DFT analysis. The capability, adsorption kinetics, and adsorption thermodynamics of hydro-chars modified by different methods are compared, so that the interaction mechanism between phenol and hydro-chars can be determined and most efficacious N species can be identified.

2.1 Materials

In this study, the similar hydro-char preparation method has been taken from previous research (An et al., 2023), according to which SS and wheat straw (WS) were used together for hydrothermal carbonization process. The SS was taken from Zhuyuan SS Treatment Plant, Shanghai. The WS was from Henan Province. Both the SS and the WS were dried for 12 hours at a temperature of 105 ℃, then milled to 100 mesh. All the chemicals in this study were from Sigma-Aldrich and are analytically pure.

2.2 Hydro-char preparation

The above milled SS and SW were blended by mass ratio of 1:1 to prepare hydro-char with a hydro-thermal reactor. The preparation parameters are listed in Table 1. Inside the hydrothermal reactor, the ratio of water to dry mass was 4:1 and residence time was 30 min after the temperature reached 200 ℃. Deionized water with an ohmic resistance of 18 MΩ·cm was used in the experiments. The hydrothermal reactor was run under water vapor saturated state, namely its pressure is the saturated one at the corresponding temperature. Finally, the reactor was cooled to room temperature and the separated solid was dried for 12 hours at 105 ℃, and the hydro-char was obtained. The hydro-char samples were ground into powder on a disk-rotating mill and passed through a 100-mesh sieve; then, the prepared hydro-chars were modified by a two-step method. The first step is plasma modification, during this step NH₃ was used as a carrier gas and it was applied for the whole duration time of 20 min, and temperature was kept at room temperature (25℃) and 500 ℃, respectively for the two hydro-char samples. The second step was acid wash with 10% HCl solution and remove salts and to increase the specific surface area and porosity. The samples are named following a rule of “raw material + modification method”. For example, HC-NH₃-P-25 means hydro-chars modified at 25 ℃ with plasma under an NH₃ purge.

Table 1

Production parameters of different hydro-char samples
Sample	SS¹:WS²	Preparation temp./time		Plasma treatment and N-doping			Washing step
Sample	SS¹:WS²	Temperature (℃)	Time (minute)	Temperature (℃)	Time (minute)	Nitrogen source	Washing step
HC	1:1	200	30	-	-	-	Washed by HCl solution of 10wt.%
HC-NH₃-P-25	1:1	200	30	25	20	NH₃
HC-NH₃-P-500	1:1	200	30	500	20	NH₃
*SS¹: Sewage sludge; WS²: Wheat straw

2.3 Hydro-char characterization

The C, H, O, N, S elements’ contents of hydro-chars were analyzed via a VarioEL-III type elemental analyzer (Elementar Analysensysteme, Hanau, Germany). The specific surface area and pore size distribution of hydro-char were identified by the N₂ isothermal adsorption curve at a temperature of 77 K via Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH). A Fourier transform infrared spectrometry of version 8.0 (Nicolet Instruments Corporation, USA) was employed to evaluate the functional groups on the hydro-char surface in the frequency range of 400–4000 cm^− 1. The valence state of the elements was analyzed via X-ray photoelectron spectroscopy (XPS, Thermo Fisher, USA).

2.4 Batch adsorption experiment

In order to understand the adsorption behaviors and capacity of hydro-chars for phenol, a series of phenol solutions with different concentrations (200, 160, 120, 80, 40, 0 mg/L) were prepared and 50 mL of each of these phenol solutions was mixed with 50 mg HC, HC-NH₃-P-25 and HC-NH₃-P-500 hydro-char, respectively; and the mixtures were put into brown bottles to avoid the influence of the light. Three parallel samples were prepared for each kind of phenol concentration with a brown bottle without hydro-char prepared as a blank control sample. All the adsorption experiments were conducted in a shaking bed, the adsorption temperature was 5, 15, 25 and 35 ℃, respectively, and the shaking speed was 150 r/min. For each experiment, 1 mL samples were picked at the time of 5th, 10th, 15th, 20th, 25th, 30th, 35th, 40th, 45th, 50th, 55th, 60th, 90th, 120th, 180th, 240th, 300th, 360th, 420th, 480th, 540th, 600th, 1200th, 1800th, and 2400th min, respectively. All the picked samples were filtrated via a 0.22 µm filter membrane for later test.

2.5 Adsorption capacity analysis

The concentration of phenol in each picked sample was measured by a liquid chromatograph (Agilent 1260B). The packed column was C18, and fluid was the mixture of methanol and water (7:3). The temperature of column oven was set to 25 ℃. The sample volume was 50 µL, flow rate was 1 mL/min, detecting wavelength was 210 nm. The phenol adsorption capacity is calculated as:

$${Q}_{t}=\frac{{V}_{e}({C}_{0}-{C}_{t})}{m}$$

Where V_e (L) represents the volume of the reactor, C₀ (mg/L) is the initial concentration of phenol, C_t (mg/L) is the balance concentration of phenol in the reactor, m (g) is the dosage of different hydro-chars, Q_t (mg/g) is the phenol adsorption capacity of hydro-char.

2.6 Phenol adsorption kinetics and thermodynamics analysis

The experimental results for phenol adsorption are fitted by pseudo-first-order equation, pseudo-second-order equation, and interparticle diffusion model, respectively.

Pseudo-first-order equation (Liu et al.,2008):

$${q}_{t}={q}_{e}(1-{e}^{-{k}_{1}t})$$

Pseudo-second-order equation (Garcia-Delgado et al., 1992):

$${\text{q}}_{\text{t}}\text{=}\frac{{\text{k}}_{\text{2}}{\text{q}}_{\text{e}}^{\text{2}}\text{t}}{\text{1+}{\text{k}}_{\text{2}}{\text{q}}_{\text{e}}\text{t}}$$

Intra-particle diffusion model (Bulut and Aydın, 2006):

$${\text{q}}_{\text{t}}\text{=}{\text{k}}_{\text{p}}{\text{t}}^{\text{0.5}}$$

Where t is the reaction duration, q_t (mg/g) is the adsorption capacity at time t, K_p (mg/(g/min^0.5) is the interparticle diffusion adsorption rate constant.

The isothermal adsorption curves are fitted by Langmuir and Freundlich models, respectively.

Langmuir isothermal adsorption equation (Duff et al., 1988):

$${\text{q}}_{\text{e}}\text{=}\frac{\text{Q}{\text{R}}_{\text{l}}{\text{C}}_{\text{e}}}{\text{1+}{\text{R}}_{\text{l}}{\text{C}}_{\text{e}}}$$

Where q_e (mg/g) is the adsorption capacity of unit mass adsorbent at equilibrium state; Q (mg/g) is the saturated adsorption capacity of a single layer; C_e (mg/g) is the concentration of adsorbed matter in the adsorption solutions at an equilibrium state; R_l is a constant related to temperature or enthalpy change during adsorption.

Freundlich adsorption isothermal equation (Appel, 1973):

$${q}_{e}={R}_{f}{C}_{e}^{\frac{1}{n}}$$

Where R_f and n are adsorption equilibrium constants; C_e is the solute concentration at adsorption equilibrium; q_e is the adsorption capacity at adsorption equilibrium.

To understand phenol adsorption potential by hydro-chars, the enthalpy changes are calculated by. The adsorption process is supposed to be ideal. Thermodynamic functions are calculated by Gibbs equation.

$${ln}{K}_{d}=-\varDelta \text{H}/\text{R}\text{T}+\varDelta \text{S}/\text{R}$$

$$\varDelta G=-RT{{ln}{K}_{d}}^{}$$

$$\varDelta G=\varDelta H-\varDelta ST$$

Where ΔG is the standard adsorption Gibbs free energy change; ΔH is the standard adsorption enthalpy change; ΔS is the standard adsorption entropy change; R is the gas constant with a value of 8.3145 J/ (mol·K); T is the absolute temperature; K_d is the adsorption balance constant.

2.7 Mechanism analysis

DFT simulation was employed to reveal the phenol adsorption mechanism by O-containing functional groups and N-containing functional groups on surface of hydro-chars and show the changes before and after modification process. The flake-structured hydro-char was built according to the element analysis, XPS analysis and FITR analysis results. Because the number of benzene rings of the hydro-char has little influence on the calculation results, the reported hydro-char structure typically included 25 benzene rings to ensure calculation accuracy and reduce the calculation time (Chen and Yang, 1998; Montoya et al., 2001). In this study the hydro-char model with 25 benzene rings was set-up. All density functional theory (DFT) calculations were performed by using the ORCA program package (wB97X D3 def2-QZVP) (Cheng et al., 2021; Neese, 2012). The adsorption configuration, independent gradient model based on Hirshfeld partition (IGMH), electrostatic potential (ESP), and electron transfer between hydro-char and adsorbed phenol were analyzed by multiwfn3.7 and VMD(Lu and Chen, 2012; Lu and Chen, 2022).

Adsorption energy can be calculated as:

$$Eads=E\left(hydro-char+Phenol\right)-E\left(hydro-char\right)-E\left(Phenol\right)$$

where E (hydro-char + Phenol), E (hydro-char) and E (Phenol) represent the energies of the hydro-char, phenol, and their complexes, respectively.

3.1 Adsorption kinetics and isotherms analysis

The adsorption capacity of hydro-chars for phenol increased as increasing contact time and reached the plateau at 540 min while eventually reached saturation at around 1200 min for the different hydro-char samples, as shown in Figure. 1. The phenol sorption capacity vs. time behaviors of three hydro-chars are similar. With the increase in adsorption time, the adsorption capacity increased fast in the first 120 min for plain hydro-char (HC) and HC-NH₃-P-25 but for HC-NH₃-P-500 this time was 90 min; the adsorption capacity in this stage occupies around 80% of the largest adsorption capacity. After that the adsorption capacity increased slowly and this stage took about 500 min, which was caused by the occupation of available adsorption sites by the previously adsorbed phenol. In the third stage, the adsorption reached equilibrium state at around 1200 min.

The HC-NH₃-P-500 hydro-char showed the largest adsorption capacity for phenol (Q_t=177.17 mg/g), followed by HC-NH₃-P-25 (Q_t=47.56 mg/g) and the HC (Q_t=24.36 mg/g). The reason can be that the modified hydro-chars have larger specific surface area and porosity, and higher N element content. It was reported that a larger specific surface area can provide more active sites for adsorption, and hydro-char containing N functional groups would provide more adsorption sites than other kind of hydro-char (Feng et al., 2021). The extremely high adsorption capacity of HC-NH₃-P-500 for phenol (Q_t=177.17 mg/g) is higher than most of previously researches (da Silva et al., 2022; Inkoua et al., 2022), showing its superiority and potential wide usage.

3.1.1 Adsorption kinetics analysis

Experimental data were fitted by classic kinetic models, namely, Pseudo-first-order equation and Pseudo-second-order equation, as shown in Figure.1. According to the pseudo-first-order model, diffusion dominates the adsorption process. While the pseudo-second-order model assumes that the process is a chemical adsorption. According to the fitting results of all the experimental data, the R² values of pseudo-second-order equation are much larger than those of pseudo-first-order equation, indicating the chemical adsorption is dominant.

As shown in Figure. 1b, this study employed the interparticle diffusion model to describe the diffusion mechanism for hydro-char adsorption of phenol. The fitted curves exhibited a multi-slop feature, and the slop values varied a lot, which indicates that the phenol adsorption onto hydro-char particles is controlled by multi factors (Yu et al., 2001). Usually, the phenol adsorption onto hydro-char particles consists of three steps: membrane diffusion (K₁), interparticle diffusion (K₂), and balanced adsorption (K₃), where the first step was dominated by external transport resistance, and the last two steps were dominated by interparticle diffusion (Yu et al., 2001). In the adsorption process, firstly, the phenol transported through the flow boundary on the hydro-char particle and touched its outer surface; then, phenol diffused into the internal pores of hydro-char particles. Finally, the adsorption equilibrium state was reached. The experimental results shown in Figure. 1 indicate that the membrane diffusion and interparticle diffusion dominated the adsorption process.

3.1.2 Isothermal adsorption behaviors

The adsorption isotherms of phenol onto the hydro-char carbon were obtained for four different temperatures (5, 15, 25, and 35 ℃) (Figure.S1). The isotherm reflects the relationship between equilibrium adsorption concentration and concentration at a certain temperature, and the experimental data were fitted with Langmuir (Eq. 5) and Freundlich (Eq. 6) isothermal models for the four isotherms with initial phenol concentration changing from 20 ~ 200 mg/L. As illustrated in Table 2, the correlation coefficients of the Freundlich model with R² = 0.9780–0.9993 fitted better than the Langmuir model (R² = 0.9330–0.9875).

Langmuir isothermal model is suitable for the single molecule layer adsorption system (Mallampati et al., 2015), and Freundlich isothermal model is more suitable for the muti-layer adsorption of high concentration adsorbate system (Hamdaoui and Naffrechoux, 2007). The calculated Langmuir and Freundlich constants and regression coefficients suggesting the multi-layer adsorption of phenol on hydro-chars, which is consistent with chemical adsorption.

Table 2

Langmuir and Freundlich constants and regression coefficients for the adsorption of phenol onto hydro-char.
Hydro-char types	Temperature	Langmuir			Freundlich
Hydro-char types	(℃)	R_l	Q_m	R²	R_f	n	R²
HC	5	0.2195	18.2461	0.9694	1.1749	2.1891	0.9965
	15	0.3006	23.0876	0.9611	1.6523	3.8655	0.9873
	25	0.4369	32.6045	0.9330	2.4744	2.3202	0.9780
	35	0.6470	51.7600	0.9507	3.5263	2.2306	0.9799
HC-NH₃-P-25	5	0.1817	32.1593	0.9734	0.6773	1.6343	0.9924
	15	0.3489	34.2059	0.9875	1.7373	2.0342	0.9993
	25	0.7227	44.3374	0.9808	4.3344	2.5434	0.9890
	35	0.9605	48.2663	0.9605	6.0164	3.0941	0.9975
HC-NH₃-P-500	5	0.1928	83.8261	0.9517	1.8496	1.6483	0.9815
	15	0.4531	113.2750	0.9399	2.1011	2.3010	0.9860
	25	0.7589	154.8776	0.9568	3.3608	3.1696	0.9965
	35	0.9404	162.1379	0.9437	3.9312	3.2286	0.9899

It can be seen from Table 2 that, the phenol adsorption capacities increased with the increase in temperature for all of hydro-char samples, indicating the improved affinity between the hydro-chars and phenol as temperature increases. This phenomenon also proves the chemical adsorption of hydro-chars for the phenol.

3.1.3 Adsorption thermodynamic study

To understand the adsorption potential, the adsorption enthalpy change (ΔH), adsorption entropy change (ΔS), and adsorption Gibbs free energy change (ΔG) are calculated according to Eq. (7) to (9). The results are shown in Table 3.

Table 3

The calculated thermodynamic parameters for phenol adsorption onto the hydro-char
Sample	Temperature (°C)	∆G (J•mol^− 1)	∆S (J•mol^− 1)	∆H (J•mol^− 1)
HC	5	-547.96	-144.00	39450.27
	15	-1918.05
	25	-3533.01
	35	-4802.82
HC-NH₃-P-25	5	-2969.40	-104.15	25987.23
	15	-3947.09
	25	-5237.50
	35	-6010.81
HC-NH₃-P-500	5	-5562.27	-97.37	21547.72
	15	-6283.41
	25	-7850.42
	35	-8285.72

Based on the highly negative ∆G in Table 3, the phenol adsorption would happen spontaneously. ∆S is the sum of changes in solute (phenol) adsorption entropy and solvent (water) desorption entropy, which are determined by the interaction between solid and solute/solvent and the volume of molecule.

In this hydro-char and phenol system, negative values of ∆S in Table 3 indicate the system became more ordered due to the limited motion of adsorbed solute after adsorption. And the order degree of system increased with the increase in adsorption capacity. In Table 3, in the temperature range of 5–35℃, all the hydro-char samples had a negative ∆H, which corresponded to the heat release. Hence, it is supposed that a higher temperature will depress the adsorption. While in the temperature range of 5–35 ℃, the adsorption capacity increased with temperature, for the increased temperature promoted the phenol diffusion in solutions within this temperature range.

The adsorption driven forces between solid adsorbent and organic matter mainly includes van der waals forces, hydrogen bonding forces, ligand exchange, dipole moment forces and chemical bonding forces (Shuang,.et al 2012). In the adsorption reaction, the thermal effects of different driven forces are different. Hence, the adsorption enthalpy change is an indicator to justify the adsorption mechanism. Generally, the adsorption heat of physical adsorption is smaller than 20 kJ/mol, while those of chemical adsorption can be larger than 20 kJ/mol (Zhou, 2012). In this study, all the hydro-char samples had an adsorption heat greater than 20 kJ/mol within the temperature range of 5–35 ℃, which indicates chemical adsorption is dominant, and the adsorption capacity of the hydro-char HC-NH₃-P-500 developed in this study is quite promising, to seek the adsorption conditions and further increase the adsorption capacity, DFT simulation is employed to explore the phenol adsorption mechanism of different hydro-chars.

3.2 DFT simulation

3.2.1 Hydro-char structure model

The FTIR and XPS characterization results of hydro-chars are shown in Fig. 2. As shown in Fig. 2a, the peak of 3340 cm^− 1 indicates the -OH in chemically bound water and hydroxyl groups of SS. Compared with HC, HC-NH₃-P-25 and HC-NH₃-P-500 have less -OH. Because the thermal stability of chemically bound water and hydroxyl groups is poor, the high-energy plasma and acid wash lead to the loss of hydroxyl in chemically bound water (Lu et al., 2013). The peak of 2918 cm^− 1 and 2850 cm^− 1 are attributed to the vibration of CH_x groups. The decrease in signal means the compound has been oxidized or transferred to other aromatic structures (Demirbas, 2004). The loss of -OH and CH_x contributed to the increase of aromatization degree of hydro-chars, which is clearly shown in Table S1. The wave number of 1650 cm^− 1 to 1250 cm^− 1 exhibits the protein structure in hydro-chars, which is the main macromolecular organic compounds. The wave number of 1650 to 1580 cm^− 1 represents the N-H bending vibration of 1° amine, and the wave number of 1335 − 1250 cm^− 1 is related to the C-N stretching vibration of atomization amine (Yin et al., 2015). According to the N content of hydro-chars as shown in Fig. 2 and Table S1, the N doping was successfully achieved with plasma modification at 500 ℃. The peak at 1033 cm^− 1 indicates the C-O vibration of aliphatic amine and polyphenol, and the peak of 600 cm^− 1 to 800 cm^− 1 reflects aromatic structure and heterocyclic atom (Lu et al., 2013).

To further identify the surface chemical structure and types of N element, XPS analysis was conducted. As shown in Fig. 2b, HC, HC-NH₃-P-25, and HC-NH₃-P-500 have two types of N, namely, pyridine N (397.7 eV) and graphite N (401.1 eV). The N content in HC-NH₃-P-500 was 4.72% (Table S1), which was much higher than N content of HC and HC-NH₃-P-25. The XPS results validate the successful N doping in hydro-char HC-NH₃-P-500 and the N type was mainly pyridine N and graphite N.

Based on the FTIR and XPS results, it can be deduced that hydro-char surfaces are rich in -OH and -COOH functional groups simultaneously. Based on the above analysis, the hydro-char structure was set up, which included -OH, -COOH, pyridine N atom, and graphite N atom on the base surface and edge area of hydro-chars. According to the experimental results, the HC-NH₃-P-25 and HC-NH₃-P-500 had higher phenol adsorption capacities compared to HC, which may be due to their high pyridine N and graphite N contents. Also, the highest phenol adsorption capacity corresponded to the highest graphite N for the HC-NH₃-P-500, it suggested that the graphite N play an important role in the phenol adsorption process. To approve this theoretically, in this study, two hydro-char models were set up with different graphite N contents, including a HC-NH₃-P-A structure (Fig. 2f1) with 25 benzene rings, a -OH, a -COOH, a pyridine N atom and a graphite N atom, and the HC-NH₃-P-B structure (Fig. 2f2) with 25 benzene rings, a -OH, a -COOH, a pyridine N atom and two graphite N atoms.

3.2.2 DFT calculation

According to the geometry optimization of phenol and carbon structure, it was observed that phenol and hydro-char structure exhibited two types of interactions with each other, namely π-π stacking and hydrogen bond. For π-π stacking, increasing the graphite N content in the carbon structure resulted in a decrease in the average vertical distance between phenol and hydro-char, as shown in Fig. 3a and Fig. 3b. Specifically, the average vertical distance decreased from 3.5120 Å to 3.4532 Å. This decrease indicates a closer proximity between phenol and hydro-char as the graphite N content increases. As a result, the phenol adsorption capacity of hydro-chars is enhanced. Furthermore, the DFT results confirmed the presence of a hydrogen bond between phenol and the hydro-char structure (Fig. 3c and Fig. 3d). This hydrogen bond formation caused the average vertical distance between phenol and hydro-char to decrease from 3.4884 Å to 3.3386 Å.

Based on the DFT results, the calculated energy of adsorption (EAD) for the π-π stacking complex between hydro-chars and phenol changed from − 13.9330 to -23.4181 kJ/mol for HC-NH₃-P-A and HC-NH₃-P-B, respectively. For the hydrogen bond complex, the EAD changed from − 19.0233 to -19.9517 kJ/mol for HC-NH₃-P-A and HC-NH₃-P-B, respectively. The negative values of the adsorption energies indicate spontaneous adsorption on the surface of hydro-chars. Additionally, a more negative adsorption energy corresponds to a stronger interaction between the hydro-char facet and the phenol molecule (Bahamon et al., 2021). The simulation results clearly demonstrate the existence of π-π stacking and hydrogen bond interactions between phenol and hydro-chars, and the presence of graphite N in hydro-chars promoted the formation of π-π stacking and hydrogen bonds between hydro-chars and phenols and increased the negative adsorption energy. These results implied that the enhanced π–π stacking and hydrogen bond interactions between hydro-chars and phenols are the main factors contributing to the increased phenol adsorption capacity facilitated by graphite N in hydro-chars. This mechanism plays a crucial role in enhancing phenol adsorption and cast light on how to modify the chars for this purpose.

For better understanding the reactivity and selectivity of the hydro-chars to phenols, the IGMH model(Liu et al., 2021) was applied to identify and quantify the net electron density gradient attenuation. In this study, the hydro-char structure and phenol were defined as two separate fragments. As shown in Fig. 4a, Fig. 4b, Fig. 4e and Fig. 4g, the formation of π-π stacking between hydro-char and phenol was evident due to the distinct green color in the corresponding region. This π-π stacking resulted in a decrease in the average vertical distance between hydro-char and phenol. Additionally, the hydrogen bond was observed to form in the vicinity of the carboxyl group, as shown in Fig. 4c, Fig. 4d, Fig. 4f, and Fig. 4h. However, the increase in graphite N content caused the phenol fragment to move closer to the central graphite N. This finding implied that graphite N played a beneficial role in the adsorption of phenol, aligning with the results obtained from the adsorption experiments.

The Molecular Electrostatic Potential (MESP) map is a valuable tool for comprehending the intricate details of intermolecular interactions, particularly electrostatic interactions within complex systems based on electronic charge distribution among atoms. In Fig. 5, the MESP surfaces of the hydro-char/phenol complex are presented. Comparing Phenol-HC-NH₃-P-A (Fig. 5a and Fig. 5c) with Phenol-HC-NH₃-P-B (Fig. 5b and Fig. 5d), it is evident that both the π-π stacking and hydrogen bond interactions exhibit larger positive diffusion regions and electron density regions in the latter case. This observation indicates a more stable adsorption system resulting from the increase in graphite N content. The larger positive diffusion region suggested a higher concentration of positive charges, while the electron density region signified a higher concentration of electrons (Cheng et al., 2021). These findings point towards an enhanced stability of the adsorption system facilitated by the higher graphite N content. Consequently, the increased stability contributed to an improved phenol adsorption capacity.

In this study, N was doped in hydro-chars via plasma modification and different N contents obtained at 25 and 500 ℃. The hydro-chars were used for phenol adsorption and the interaction mechanism at the adsorption interfaces between phenol and different hydro-char samples were explored experimentally and via DFT analysis. The following conclusions can be drawn:

The plasma modification at 500 ℃ produced most effective hydro-char HC-NH₃-P-500, it was doped with more graphite N than HC-NH₃-P-25. In addition, HC-NH₃-P-500 showed the highest phenol adsorption capacity (Q_t=177.17 mg/g).
The Freundlich model can fit the adsorption results better, indicating the phenol adsorption onto hydro-chars is a multi-layer adsorption, or chemical adsorption. The phenol adsorption onto hydro-chars also satisfies the interparticle diffusion model and pseudo-second-order equation with high regression coefficients (R²), indicating the adsorption rate is governed by both interparticle diffusion and chemical adsorption at the same time.
DFT results suggest that the interaction between hydro-chars and phenols include π-π stacking and hydrogen bonding interaction. And the increase in graphite N promotes the adsorption energy between hydro-chars and phenol, and the electrostatic potential as well, which finally increases their adsorption capacity, and adsorption stability between adsorbent and adsorbate complex.

The above findings can provide valuable support to the optimization of hydro-char modification process for pyrolysis oil compound especially phenol adsorption.

Acknowledgment

This research work was financially supported by Shanghai Municipal Science and Technology Commission (Grant No. 20230712900) and Shanghai Science and Technology Innovation Action Plan (Grant No.22dz1208200).

Ethical Approval:

Not applicable.

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Not applicable.

Competing Interests:

The authors have no relevant financial or non-financial interests to disclose.

Author Contributions:

Qing An: Conceptualization, Methodology, Investigation, Data Curation, Writing - Original Draft

Dezhen Chen: Conceptualization, Writing - Review & Editing, Resources, Supervision, Funding acquisition

Yuzhen Tang: Investigation, Resources

Yuyan Hu: Investigation, Resources

Yuheng Feng: Investigation, Resources

Kezhen Qian: Investigation, Resources

Lijie Yin: Investigation, Resources

Funding:

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Adsorption of pyrolysis oil model compound (phenol) with plasma-modified hydro-chars and mechanism exploration

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Abstract

Figures

Highlights

1. Introduction

2. Materials and methods

2.1 Materials

2.2 Hydro-char preparation

2.3 Hydro-char characterization

2.4 Batch adsorption experiment

2.5 Adsorption capacity analysis

2.6 Phenol adsorption kinetics and thermodynamics analysis

2.7 Mechanism analysis

3. Results and discussions

3.1 Adsorption kinetics and isotherms analysis

3.1.1 Adsorption kinetics analysis

3.1.2 Isothermal adsorption behaviors

3.1.3 Adsorption thermodynamic study

3.2 DFT simulation

3.2.1 Hydro-char structure model

3.2.2 DFT calculation

4. Conclusions

Declarations

References

Supplementary Files

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