Carbon materials based on lignin and their environmentally friendly applications

doi:10.21203/rs.3.rs-5034064/v1

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Carbon materials based on lignin and their environmentally friendly applications

https://doi.org/10.21203/rs.3.rs-5034064/v1

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The green revolution concept has accelerated the pace of innovation to avoid the use of materials that can harm the environment. In this work, the valorization of lignin and olive wastes was studied. Olive stone used to prepare activated carbons which are modified by lignin. The carbons were characterized by TGA-DTA, FTIR spectroscopy, adsorption-desorption of N₂ at 77K, Boehm titration and the measure of pH_PZC. FTIR results show the presence of some groups that confirm that carbons based lignin can be used as fertilizers. BET results show micro- mesoporous structure of carbons materials, they might be used as electrodes. Boehm titration shows the increase of phenolic groups in modified carbon, which confirms that lignin can be used as bio-composites. The thermal analyses show the high stability after lignin coating so the carbon based on lignin can be used as a green biofuel.

Lignin

olive stone

activated carbon

valorization

adsorption

Cellulose and hemicellulose, lignin is a highly abundant biopolymer. As a major component of lignocellulosic materials, lignin accounts for a significant proportion of biomass including wood and agricultural residues. It has great potential for utilization as a green raw material or as an additive in various industrial applications, it can be used as an adhesive or tanning agent and it is used for paper industry (Tong H et al .2019). According to literature, much of the lignin produced by the paper industry is consumed as a fuel and the annual production of industrial lignin in the world is more than 70 million tons (De Wild P.J et al.2012, Suhas, P.J.M et al.2007).

Other applications such as production of activated carbons (ACs) and carbon fibers from lignin have also gained much interest in the last few years. Lignin has been proven to be a good source for producing activated carbons using chemical activation methods. In addition to that, lignin has been used as a precursor or feedstock for the preparation of high-performance electrochemical energy materials and components such as electrodes, electrolyte additives, membrane separators, and binders.

Therefore, using waste biomass to produce activated carbons for water treatment is a potential method for comprehensive utilization of biomass waste. Olive stone is a waste, which is largely produced in the Mediterranean countries as Tunisia, where there are more than 60 million olive trees (Hannachi H et al.2007). Tunisia is the world’s second largest olive oil producer and during olive oil production process approximately 35–40 kg of olive pomace is released per 100 kg of olive, it represents roughly 10% by weight of olive (Sellami F et al. 2008). The use of this solid waste as a precursor for the preparation of activated carbon produces not only a high-quality adsorbent with low inorganic content for the purification of contaminated environments, but also contributes to minimizing the environmental impact of solid wastes.

Recently, the modification of carbonaceous materials seems particularly promising for improving the absorbents affinity. The literature review shows that the modifications of AC surface improve the properties by different treatment applications such as carbon dioxide adsorption (Shafeeyan MS et al. 2010, Chiang Y-C et al.2017), removal of siloxanes from landfill gas (Gong H et al. 2007), removal of contaminants from aqueous solutions (Yin C.Y et al.2007, Suhas et al.2016).

In addition, the porous structure of activated carbon depends on the pore size: meso- and macropores allow the diffusion of charged species whereas micropores favor the accumulation of charged species so carbon materials prepared from olive stone can be used as electrodes (Jaouadi M. 2020).

The main objective of this work was to develop a new method for olive waste and lignin management by preparing activated carbon which modified by coating silica and lignin. But to our knowledge, studies focusing on preparing and modification of activated carbon by lignin are few. The prepared composites could use as an adsorbent, fuel or for energy storage (used as an electrode). The thermal behavior, the structure, the porosity, and the general composition were studied using TGA-DTA, FTIR spectroscopy, adsorption-desorption of N₂ at 77K, Boehm titration and the measure of pH_PZC.

2.1 Synthesis of activated carbon and activated carbon coated by silica

In our previous work, an activated carbon was prepared from olive stones and modified by silica (Jaouadi, M., 2021). The carbon materials are referred in the text as “AC’’ and “AC-Silica”.

2.2 Modification the prepared carbon materials by lignin

1 g of industrial lignin (Supplied by a nursery in Soliman-Tunisia, 33% of humic acid and 67% fulvic acid) was added to 1 g of AC and/ or AC-Silica, the mixture was stirred at 60°C during 12h. The composite AC-lignin was washed using distilled water.

2.3 Physico-chemical characterizations

The pH_PZC values were determined by a mass titration method proposed by M.V Lopez and al. (Lopez-Ramon, M.V et al. 1999). Various amounts of carbon materials were put in 10mL of 0.1mol L^− 1 NaCl solutions (prepared with boiled water). The bottles were sealed and shaken in a thermostat shaker overnight, the equilibrium pH values of the mixtures were measured, and the limiting pH was taken as the pH_PZC. The functional (acidic) groups present on the surface were quantified by Boehm‘s titration (Strelko V et al.2002). Experimentally, 0.1 g of carbon materials powder was mixed with 20 mL of 0.02 mol.L^− 1 aqueous solutions composed by either NaOH, Na₂CO₃ or NaHCO₃ reactants. The mixtures were stirred for 48 h at a constant speed of 150 rpm at room temperature. Then, the suspensions were filtered through 0.45 µm membrane filters (Millipore 405314). To determine the contents of oxygenated groups, back-titrations of the filtrate (10 mL) were performed with standardized HCl (0.02 mol. L^− 1). The numbers of acidic sites were calculated by assuming that NaOH neutralizes the carboxyl, phenolic and lactonic groups, while Na₂CO₃ neutralizes the carboxyl and lactonic groups, and NaHCO₃ neutralizes the carboxyl groups only. The characterization of prepared AC and AC-Si powders by Fourier transforms infrared (FT-IR) spectroscopy were performed using a Spectrum 100 FTIR spectrometer (PerkinElmer- France). The transmission spectra were recorded using KBr pellets. All spectra were obtained in the 4000 − 400 cm ^− 1 range at a resolution of 4 cm^− 1. The thermal analyses (TGA-DTA) of carboneous materials were carried out with a SETARAM apparatus. Experiments were performed under an argon gas at a heating rate of 5°C min^− 1 in the temperature range of 25-1100°C by using 17.5 mg of each sample. The specific surface areas and pore size distributions were obtained from the adsorption/desorption isotherms of N₂ at 77 K using an automatic sorptometer (ASAP 2020, Micromeritics, USA). Prior to each measurement, the sample was degassed during 48h at 250°C for AC and 100°C for AC-Si, AC-lignin and AC-Silica-lignin to avoid the decomposition of the surface groups. Then, specific surface areas of prepared powders were calculated from the Brunauer-Emmett-Teller (BET) equation.

3.1. Characterization of the carbon materials

3.1.1. Boehm titrations and pH_PZC

The pH_PZC value of AC (2.25) is in agreement with the one (i.e. 3.7) measured by Clark et al. (Clark, H.M. et al. 2012) for an activated carbon prepared by H₃PO₄ activation of coffee residues at 350°C.

The pH_PZC of AC increased after silica coating (2.6). For such low values (pH_PZC < 4.5, (Kosmulski M. 2002), amphoteric silanol groups are deprotonated and the net surface charge is negative over a wide pH range. The low pH_PZC values for all carbon materials are generally associated to the presence of acidic carboxylic functional groups.

The pH_PZC values were confirmed by the acidic character of activated carbons surface measured by Boehm titration‘s method. Table 1 indicates the amounts of carboxylic, phenolic, lactonic and acidic groups present on as-prepared carbonaceous surfaces. The AC-Silica and AC- Lignin-Silica surface present the highest amount of acidic groups. The increase in the content of oxygenated groups after the Si coating is mainly due to the formation of additional carboxylic groups.

According to Jaouadi et al. (Jaouadi, M et al. 2021) the increase of the content of oxygenated groups after the Si coating is mainly due to the formation of additional carboxylic groups that are beneficial for the hydrophilicity and wettability of carbonaceous surfaces, the AC-Silica and AC-lignin-Silica can be used as electrodes for electrochemical devices especially for supercapacitors.

AC has a small quantity of phenolic groups, which confirms that after preparation of activated carbon from olive wastes (olive stone) the phenolic groups were removed. The study of Monetta et al. (Monetta P et al. 2012) confirmed that olive wastes when applied, as a soil amendment produce a high increase in polyphenol levels, so preparing an activated carbon from olive stones will remove phenolic compounds.

Table 1

pH_PZC, total surface acid groups of the carbon materials (mmol/g : determined by Boehm titrations).
	pH_PZC	Carboxylic	Phenolic	Lactonic	Acidic
AC	2.25	0.06	0.07	0.06	0.19
AC-Silica	2.6	1.3	0.8	0.6	2.08
AC-Lignin	2.2	0.08	0.1	0.02	0.2
AC-Lignin-Silica	2.4	1.6	1	0.04	2.63

3.1.2. FTIR analyses

FTIR spectra of AC, AC-Silica, AC-Lignin and AC-Lignin-Silica are shown in Fig. 1. For all the samples, the stretching vibrations of -OH groups (υ_OH), observed around 3500 cm^− 1, are attributed to the adsorbed residual water on carbons and are related to the hydrogen bond formed by the association of O-H, and the peak at 3290 cm^− 1 were verified to be the hydroxyl association belongs to carboxylic acid. For AC-Silica-Lignin and AC-Lignin the high intensity of this υ_OH band could be related to the high amount of phenolic groups in this material. For both samples, the band at 1700 cm^− 1 (υ_C=O) is assigned to the C = O stretching vibrations of ketones, aldehydes, lactones or carboxyl groups (Strelko V. 2002). The broad band at 1000–1300 cm^− 1 (maxima at 1190–1200 cm^− 1) has been assigned to C-O stretching in acids, alcohols, phenols, ethers and/or esters groups (Jaouadi M et al.2021). This band is particularly more intense for AC-Si. The strong C-O-C stretching bands (υ_C−O−C) were also observed from 1100 to 1200 cm^− 1 for AC-Silica, indicating the presence of aromatic ether or ester bond. The presence of Si in AC-Si composites is confirmed by the weak band located at 955 cm^− 1, ascribed to Si-O-H asymmetry stretching and bending vibration, and bands at 1090 and 650 cm^− 1 corresponding to Si-O-Si asymmetric and symmetric stretching, respectively (Jaouadi, M et al. 2021).

The peak at 1030 cm^− 1 appeared in the AC-Silica-Lignin and AC-Lignin was ascribed to the C-OH and C–O-C stretching of the side groups and glycosidic bonds, respectively [18]. The intensity at 1461 cm^–1 in both carbon materials modified by lignin was attributed to the methoxy C–H bending and C–C stretching in the aromatic skeleton ( Shi Z et al.2019). It can be found that the characteristic peaks corresponding to lignin appeared in AC-Lignin and AC-Silica-Lignin, these phenomena confirmed the reaction between lignin and carbon materials.

Lignin contains phenolic component and the presence of several functional groups (OH, COOH, and C = O), lignin has the potential to be valorized for bio-composites and antioxidant materials (Umme M. A et al. 2021). These are useful in the agroindustry as additives, coating agents, adsorbents, plant growth stimulators for food production, packaging materials, and fertilizers (Maurice N.C et al. 2020). Therefore, carbon materials modified by lignin probably play the same role of lignin.

Another side, oxygen-containing functional groups found in carbon material surfaces play a vital role in the adsorption. Notably, the peaks of the -OH, -COOH groups displayed the adsorption of heavy metal ions due to the possible formation of complexes with them (Zawadzki J.1989).

3.1.3. Porosity characterization

Figure 2 shows the N₂ adsorption–desorption isotherms (at 77 K) of AC, AC-Silica, AC-Lignin and AC-Silica-Lignin. The isotherms of AC, AC-Silica exhibit an IV–type profile that reflects the existence of micro- and mesopores. The isotherms of AC-Lignin and AC-Silica-Lignin are type I and typical of mainly microporous activated carbons. In the AC-Lignin and AC-Silica-Lignin composites the volume has sharply decreased compared to AC and AC-Silica (Table 2).

The BET specific surface area (Table 3) was decreased after lignin reaction with AC and AC-Silica (from 957.26 to 642.5 and from 902 to 375. 12 m².g^− 1, respectively) because the pores were either collapsed or blocked after strong reaction with lignin. The AC-Silica-Lignin composite showed a low specific surface, which indicated that free pores of activated carbon were occupied by silica upon modification and pores of AC-Silica, was also covered by lignin upon reaction. Probably the component of lignocellulosic precursors found in lignin primarily responsible for the microporosity of activated carbons (Suhas, P.J.M et al. 2007).

According to Wang et al. (Wang X et al.2018) lignin has been considered as the substrate of adsorbents because of its low price and easy modification, most importantly, its ability to accelerate the adsorption rate of heavy metal ions (Wang X at al.2021). The prepared activated carbon can be used as adsorbents or as electrodes for energy storage. The mesoporous structure is expected to improve the electrochemical performance by enhancing the ionic mobility, while micropores are more efficient for the charge accumulation. In the same way, the wettability of the electrode may be affected by the size of electrolyte ions in regards to the pore size of the surface (Chmiola J et al. 2006).

Table 2

Porosity characteristics of AC, AC-Lignin, AC-Silica and AC-Silica-Lignin.
Samples		Pore volume V_tot (cm³.g^− 1)	Micropore area (m².g^− 1)	Mesopore area (m².g^− 1)	External area (m².g^− 1)	Micropore volume V_m (cm³.g^− 1)
AC	957.26	0.59	601.36	355.64	355.89	0.27
AC-lignin	642.5	0.11	565.35	77.15	77.16	0.28
AC-Silica	902	0.57	550.27	351.73	351.83	0.25
AC-Silica-Lignin	375.12	0.24	304.99	70.13	70.13	0.15

AC-Silica, AC-Lignin and AC-Silica-Lignin.

3.1.4 TGA-DTA analyses

The thermal stability of electrode materials is a significant parameter. Hence, TGA/TDA measurements were carried out for AC, AC-Silica, AC-Lignin and AC-Silica-Lignin powders as presented in Fig. 3. The weight losses carbon materials are mainly embodied in the temperature range over 120°C and attributed to the decomposition of carbon frameworks (Zawadzki J. 1989). The weight losses of AC, AC-Silica, AC-Lignin and AC-Silica-Lignin are established in three stages; when the temperature is below 150°C, the weight loss of ~ 20% is caused by the desorption of water. From 150 to 500°C, the loss is about ~ 10% and results from the decomposition of oxygenated functional groups. From 500 to 1100°C, the weight loss is about 13, 20, 20 and 14% for AC, AC-Silica, AC-Lignin and AC-Silica-Lignin, respectively. This weight loss corresponds to the decomposition of the organic compounds. It is noticed that DTA curves of AC, AC-Silica samples present very similar decomposition profiles. The endothermic transition at 120°C, due to water vaporization, is followed by an exothermal transition at 450°C. For AC-Lignin and AC-Lignin-Silica, the exothermal transition at 450° was disappeared after reaction with lignin.

The DTA curves of carbon materials modified by lignin (AC-Lignin and AC-Lignin-Silica) analyzed in this work, shows an endothermic peak between 400 and 1000°C, which may be assigned to decarboxylation reactions (Jaouadi M. 2020). Lignin has special features such as thermal stability and can be used a compost.Industrial lignin is currently not fully utilized. A large amount of industrial lignin is discarded as waste or burned only as a low-value fuel.

In short, the addition of lignin in AC and in AC-Silica enhanced the overall thermal stability of the adsorbents, and confirmed that lignin was successfully grafted onto the carbon materials (AC and AC-Silica). On the other hand, carbon materials modified by lignin can be consumed as a fuel. The main advantage of coating of lignin into carbon surfaces is to reduce the high ash fractions and to increase the energetic density. Hence, Tunisia and other emerging countries, lacking sources of classic fossil fuels can base energy policy on economic and sustainable alternatives like agropellets.

Several authors described the thermal degradation as a progressive decomposition of their cellulose, hemicelluloses and lignin (Ouatmane A et al.2000).

Olive wastes and lignin were key compounds and have a great importance for the development of sustainable biomass valorization and they were successfully developed. FTIR results show the presence of some active functional groups, which play a vital role in the adsorption and confirm that modified carbon by lignin, can be used as plant growth stimulators and fertilizers. Boehm titration shows the increase of phenolic groups in carbon based lignin, which confirms that lignin has the potential to be valorized for bio-composites and antioxidant materials. The thermal analyses show the high stability after lignin coating so the carbon based on lignin can be used as a green biofuel. Carbon based on lignin can be used as electrodes for energy storage.

Funding

This work was supported by research program of the Tunisian Ministry of Higher Education

Conflicts of interest/ Competing interests

The authors declare no competing interests

Availability of data and materials

All data generated and analyzed during the current study are included in this article

Code availability

Not applicable

Author’s contributions

Mouna Jaouadi: Material preparation, Materials characterization, original draft preparation, Reviewing and supervision.

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You are reading this latest preprint version

Carbon materials based on lignin and their environmentally friendly applications

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials and methods

2.1 Synthesis of activated carbon and activated carbon coated by silica

2.2 Modification the prepared carbon materials by lignin

2.3 Physico-chemical characterizations

3. Results and discussion

3.1. Characterization of the carbon materials

3.1.1. Boehm titrations and pH_PZC

3.1.2. FTIR analyses

3.1.3. Porosity characterization

3.1.4 TGA-DTA analyses

4. Conclusion

Declarations

References

Status:

Version 1

Carbon materials based on lignin and their environmentally friendly applications

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials and methods

2.1 Synthesis of activated carbon and activated carbon coated by silica

2.2 Modification the prepared carbon materials by lignin

2.3 Physico-chemical characterizations

3. Results and discussion

3.1. Characterization of the carbon materials

3.1.1. Boehm titrations and pHPZC

3.1.2. FTIR analyses

3.1.3. Porosity characterization

3.1.4 TGA-DTA analyses

4. Conclusion

Declarations

References

Status:

Version 1

3.1.1. Boehm titrations and pH_PZC