Adsorption of organic chemicals on graphene coated biochars and its environmental implications

2.1 Materials

All chemicals used in this work were of analytical grade. Synthetic graphite powder (particle size 20 mm), was purchased from Sigma-Aldrich and used as received. Peanut shell biomass was obtained from Dhahran, a local market in Saudi Arabia. All of the biomass feedstock was milled by a high speed rotary machine, to obtain a particle size in the range of 40–60 mesh and later dried at 383 K for 7 h to remove the moisture prior to pyrolysis. Two types of organic chemicals, phenol and methylene blue, were obtained from Sigma Aldrich Chemicals, with purity >90%. The physicochemical properties of selected chemicals are listed in Table 1.

Table 1

Selected physicochemical properties of phenol and methylene blue dye.

Properties	Phenol	Methylene blue
IUPAC name	Phenol	3,7-bis(dimethylamino)-phenothiazin-5-ium chloride
Chemical structure
Molecular formula	C6H6O	C16H18N3SCl
Molecular mass	94.11 g mol-1	319.85 g mol-1
Solubility	83 g l-1	35.5 g l-1
pKa	9.95	–
Wavelength	510 nm	665 nm

2.2 Preparation of graphene suspension

Synthetic graphite powder was used to prepare stable graphene suspension. Graphene suspensions were prepared by adding 2 g of graphite powder to 200 ml of deionized water. The graphene suspensions were sonicated in an ultrasonic homogenizer (Model 300 V/T, Biologics, Inc.) with an output frequency of 20 kHz for 1 h at pulse intervals of 12 min. The resulting suspensions were used for impregnation of biomass to achieve graphene-coated biochar.

2.3 Preparation of biochar

Milled peanut shell biomass (feedstock) was used for the preparation of biochars. For graphene-coated biochars, 40 g of the peanut shell biomass was dipped following a dip-coating procedure [14, 35] and rinsed for 1 h using a magnetic stirrer, after which dip-coated biomass was removed and oven dried at 105°C. The graphene-treated biomass was placed in a quartz tube inside a muffle furnace (MF 21GS, Jeio Tech, Seoul, Korea) to produce the graphene-coated biochar through slow pyrolysis in a N₂ environment at temperatures of 300°C and 500°C for 2 h. In addition, corresponding untreated peanut shell biomass was also used as feedstock to produce biochar without graphene coating with the same pyrolysis conditions. The resulting biochar samples were washed with deionized water several times to remove impurities, oven dried and sealed in glass containers for further testing. The untreated and graphene-coated biochar samples are referred to as B300, GB300, B500 and GB500, respectively.

2.4 Adsorption experiment

Batch sorption techniques were used to determine the adsorption capacity of the chars by keeping them in phenol and methylene blue dye solutions. About exact weights of 1±3 mg each biochar were used in 4 ml glass vials at room temperature (23±1°C). The prepared samples and blank solutions (without prepared sorbate) in the controlled environment were mixed properly for 24 h on a reciprocating shaker.

Once the initial evaluation of experimentation was completed, further isotherm studies were carried out on unmodified biochar (B300 and B500) and graphene-coated biochar (GB300 and GB500) in the presence of organic chemicals. For this purpose, 4 ml of each biochar sample was mixed with chemical solutions of varying concentrations (1–50 mg l^-1). To obtain the equilibrium and accuracy, samples were kept in the dark for 24 h in a rotary shaker. All samples were under observation during the mentioned time period and no apparent degradation was found. Later, all samples were centrifuged at 2500 rpm for 10 min and concentrations of the supernatants were evaluated on a UV-vis spectrophotometer.

2.5 Quantification of sorbate

The concentrations of sorbate in the supernatants were quantified using a UV-vis spectrophotometer (UV-2401, Shimadzu, Japan) 510 nm for phenol and 665 nm for methylene blue dye, respectively. Sorbed amounts of phenol and methylene blue dye on tested biochars were calculated as the difference between the initial and final aqueous solution concentrations. Sorption experiments were conducted in duplicate. A difference of <3% was observed between the initial and final equilibrium concentrations of sorbate in blank solutions.

2.6 Data analysis

The adsorption capacity of tested biochars at the equilibrium state was calculated as follows:

where q_e (mg g^-1) is the sorbate adsorption capacity, V (l) is the volume of solution, W (g) is the weight of biochar, C₀ and C_e (mg l^-1) are the initial and equilibrium concentrations of sorbate, respectively.

Adsorption isotherms were fitted using the Freundlich and Langmuir models with Sigma Plot 10.0.

where K_F (l g^-1)ⁿ is the Freundlich adsorption coefficient and n is an index of isotherm nonlinearity which is related to the heterogeneity of sorption sites. Q_e (mg g^-1) and C_e (mg l^-1) are equilibrium solid-phase and aqueous-phase concentrations, respectively. K_L (mg l^-1) is the Langmuir model adsorption coefficient, and Q⁰ (mg g^-1) is the Langmuir model adsorption capacity parameter.

Because the number of parameters used in the two models were not the same, the coefficient of determination (r²) could not be compared directly. The adjusted radj2 was calculated and compared:

where m is the number of data points and p is the number of parameters in the fitting equation.

The sorption coefficient K₀ was defined as:

3.1 Characterization of biochar

The surface and bulk elemental characterizations of biochars were determined by using X-ray photoelectron spectroscopy (XPS) and a CHN analyzer. A strong correlativity was determined between the calculated H/C and O/C ratios. H/C and O/C ratios, decreased at higher temperatures following decarboxylation, decarbonylation and dehydration during heat treatments (pyrolysis) [6]. The amount of hydrogen and oxygen atoms per carbon leads to a strong decline in reaction mechanisms. Surface elemental composition of biochars carbon, nitogen and oxygen, as suggested by XPS, also changed after graphene coating (Table 2). The yield, ash content, bulk and surface elemental composition, atomic ratio and surface area of untreated and graphene-coated biochar produced from peanut shell at two different temperatures (i.e., 300°C and 500°C) are listed in Table 2. The C content of the biochar increased with increasing temperature, while O and H contents decresed with increasing temperature, suggesting an enhanced degree of carbonization of chars following decarboxylation, dehydration and decarbonylation during pyrolysis [6]. The increase in temperature caused a decrease in biochar yield from 34% to 29% and 31% to 26%, for non-treated and treated biochars, respectively, which may be anticipated to be the decomposition of lignin and cellulosic contents in feedstock [9]. The reduction in weight loss was due to the removal of CO, CO₂, H₂O, H₂ and CH₄ from feedstock at >600°C [36]. Higher heat treatment temperature (HTT) could also result in loss of organic matter (OM). At higher HTT, ash content increases due to the distribution of OM and mineral combustion residues [37].

Generally, the higher pyrolysis temperature increased the surface area in comparison to lower temperatures. The higher HTT biochar shows a greater surface area because of removal of O and H carrying functional groups of main aromatic -CO, ester C=O, aliphatic alkyl-CH₂ and phenolic -OH groups during pyrolysis [38]. Additionally, the increase in surface area of chars at higher temperatures was mainly due to the dismissal of volatile matter, which results in increased micropore volume. There was an increase in pore volume of obtained biochars. A decrease in pore size, internal pores and an increase in porosity was mainly due to escape of volatiles during carbonization.

3.2 Fourier transform infrared spectroscopy analysis

Fourier transform infrared spectra of biochars and graphene-coated derived biochars are shown in Figure 1. The difference in surface functionality groups of biochars produced at two different temperatures and graphene-coated biochars can be observed by Fourier transform infrared spectra. The biochar properties were greatly influenced by HTT as suggested by characterization. According to Coates [39] spectral interpretations, the OH group of bonded water was assigned to the band at 3337 cm^-1. The peaks at 1375 cm^-1, 2854 cm^-1 and 2922 cm^-1 were related to -CH₂, while the bands at 1506 cm^-1, 1616 cm^-1 and 1734 cm^-1 were assigned to the C=C–C aromatic ring, C=C and C=O aromatic ring and C=O ester bond, respectively. There was an expansion in aromatic C-N and C-H bonds which was observed at 1024 cm^-1 and 1242 cm^-1. The band at 871 cm^-1 indicates the existence of contiguous bond which were depicted as an aromatic C-H bond.

Figure 1

Fourier transform infrared (FT-IR) spectra of biochars.

There was a significant decrease in peaks at 1504–1630 cm^-1 in biochars produced at 500°C due to a decrease in lignin content. The peak at 3337 cm^-1 represented the effectual hydrogen bonding. However, the hydrogen bonds from B300 and GB300 became anemic and finally decreased in B500 and GB500. The absorption bands at 2854 cm^-1 and 2922 cm^-1 indicated the existence of long linear aliphatic chain -CH₂ groups, which were decreased in B500 and GB500, and diminished in B300 and GB300. These consequences indicated a reduction in polar functional groups with an increase in pyrolysis temperature [38]. The band at 1616 cm^-1 (aromatic C=C and C=O) continues in all of the biochars, and ultimately showed a decrease in B300 and GB300. The greatest loss was observed in C-O, -CH₂ and -OH functional groups in biochar at 500°C, which was also obvious from their elemental composition. Relatively lower values of H, O and H/C in B500 and GB500 than those in B300 and GB300 revealed the significant elimination of polar functional groups of polar functional groups (C-O and -OH). Thermal demolition of lignin and cellulose in feedstock may conclude in hydroxyl -OH, aliphatic alkyl -CH₂, aromatic C=O and ester C=O functional groups in biochars [38].

3.3 Graphene-coated biochar

The production of biochar engineered nanoparticles could improve biochar performance in wastewater treatment, carbon sequestration and soil fertility enhancement [14]. The graphene coating could be considered for the existence of some structural reformations or small residual amounts of oxygen-containing functional groups, as suggested by the slow pyrolysis [40]. The amount of coating on the biochar surface could not be quantified since it is a carbon based material. There was a greater increase in C content, which suggested that graphene coating was fabricated on the biochar surface smoothly and uniformly and so char kept the original morphological structure. A similar effect was obtained for coal and carbon nanotube products [41–44]. As shown from characterization, the physicochemical properties (e.g., proportions, porosity and surface area) of the biochars were increased with treatment of graphene. Elemental composition indicated an increase in C content of biochar. Graphene has a significant property of high thermal stability, which can be imply to biochar samples. Generally, char samples could be partitioned into three stages on the basis of thermal degradation. These are the reflecting loss of water in the 1st stage, i.e., in the range of 50°C–100°C, the degradation of surface functional groups, i.e., the 2nd stage in the range of 100°C–350°C and the final stage at temperatures higher than 350°C, when the carbon skeletons begin to vanish. Thus, the thermal stability of biochar could be protected by graphene coating. Carbon sequestration is often recommended to be an efficient way by means of land application; the enhanced stability and efficient way for carbon sequestration gives it an improved carbon sink for large scale applications to mitigate global warming.

3.4 Sorption of phenol

Figure 2 shows the adsorption isotherms obtained from different biochars with initial phenol concentrations from 1 mg l^-1 to 50 mg l^-1. Sorption capacities of GB300 and GB500 suggested via Langmuir isotherm modeling (36.63 mg g^-1 and 46.2 mg g^-1, respectively), were higher than those of their corresponding untreated biochars, (10.09 mg g^-1 and 18.19 mg g^-1, respectively). The F>Prob in ANOVA confirms that the adsorption of phenol was a significant factor. The sum of the squares used to estimate factors affect and Fisher’s F ratios (defined as the ratio of mean square effect and the mean square error) are also represented in Table 4. The determined sorption values were not relatively higher than those of some carbonaceous waste materials which have already been reported previously and additional work will be required to optimize the graphene-coated biochar nanocomposites and their synthesis in order to improve sorption capacities for organic chemicals. The graphene treatment enhanced the biochar sorption capacities, as well as their surface functional group contents and surface areas. Among all of the derived biochars, GB500 demonstrated a higher adsorption capacity due to its highest surface area (468 m² g^-1). Biochars obtained at HTT 300°C resulted in reduction of the sorption ability, and that was consistent with their surface areas 176 m² g^-1 and 214 m² g^-1. Hence, the decrease in sorption capacities could be linked to the temperature of pyrolysis and surface areas of biochars. In lower HTT biochars, reason for the smaller surface area could be the formation of some hydrophobic macromolecules that got over the surface of the char. It has been also suggested that adsorbents do not purely acquire homogenous sites of biochar, whereas sorption can occur at multi-region sites depending on functional groups at various sites of biochars. The fitting parameters of adsorption isotherms and ANOVA analysis are listed in Tables 3 and 4, respectively.

Figure 2

Adsorption isotherms of phenol and methylene blue on biochars. The left side stands for untreated biochars and the right side stands for graphene-treated biochars.

3.5 Sorption of methylene blue

When the sorption isotherms of methylene blue for GB300 and GB500 were compared to the original biochars, an increased sorption capability to methylene blue dye was observed, as indicated by the sorption constant, i.e., log K_d. Furthermore, ANOVA analysis to determine adsorption characteristics was found to have a significant effect as shown in Table 4. The sorption isotherms of GB300 and GB500 were somewhat higher than those of their corresponding non-coated biochars (17.67 mg g^-1 and 16.11 mg g^-1, respectively), suggesting the existence of graphene on the biochar surface, hence enhancing its sorption capability to adsorb dye molecules greatly. Sorption isotherms suggested the quicker uptake of methylene blue on the graphene coated biochars. This enhanced capability was the result of larger pore volumes of graphene coated biochars; as a result, the interaction time was increased to detain the initial dye uptake. This suggests a stronger affinity of organic molecules to the COOH functionality on coated biochars as compared to non-coated biochars. In this work, the sorption mechanism could be attributed to a) sorption onto high affinity binding sites or b) sorption to biochar itself as the graphene sites become filled. The increased adsorption of dye molecules by the graphene-coated biochar could be assigned to the strong interactions between the graphene sheets on the biochar surface and dye molecules through the π-π interactions [45].

3.6 Adsorption mechanisms

In particular, the sorption process corresponded with that of activated carbon adsorption. A chemical phenomenon between functional groups on char surfaces and organic molecules was an additional significant driving force for the sorption of organic molecules. The chars include many groups comprised of oxygen or nitrogen such as -OH, -NH₂, C-O, C=O, etc. In the case of phenol adsorption, a Lewis acid, -NH₂ is a kind of basic group on char surfaces, and consequently, phenol interacts with -NH₂ through acid-base interaction. Other groups such as -OH, C-O and CO can act with organic chemicals via hydrogen bonds. In addition, the potent electron-donating ability of the -OH group stimulates the aromatic ring in organic molecules to be a π-electron rich system. Therefore, in the adsorption mechanism, the aromatic rings of organic molecules were likely to form π-π stacking. However, the fundamental interactions between the organic molecules and functional groups are more efficient to the enhancement of adsorption capacity.