Influence of Impregnation of Sodium Carbonate Catalyst on Physicochemical Properties of Biochar ^†

Abstract

Biochar (BC) has been a viable resource in producing functionalised carbonaceous materials beneficial in soil fertility, environmental remediation, and energy recovery. In this study, the influence of Na₂CO₃ on the pyrolytic conversion of green pea peels to BC was examined to assess BC’s yield and physicochemical characteristics. Experiments were conducted in batch processes at a constant temperature of 500 °C by wet impregnation using 0%, 10%, 20%, and 30% (w/w) ratios of Na₂CO₃ catalyst to biomass weight. Characterisations of the produced BC materials were conducted by bomb calorimetry, SEM, BET, FTIR, and XRD. Accordingly, catalyst incorporation by pretreatment resulted in a decrease in yield while increasing the porous development. The specific surface area at a constant temperature increased from 1.006 to 17.7 m² g⁻¹, and the pore diameter decreased from 173.1 to 9.283 nm. The oxygenated functional groups negatively correlated with the increase in the amount of catalyst, whereas the calorific values of the synthesised materials increased from 20.313 kJ g⁻¹ to 25.479 k Jg⁻¹ when the catalyst impregnation ratio was enhanced from 0% to 30%, indicating that BC produced from catalysed processes demonstrates better energy recovery potential.

1. Introduction

Industrialisation and urbanisation have widened gaps in energy demands among the populace [1]. These have caused the utilisation of fossil fuels with draconian environmental consequences, such as pollution, global warming, and acid rain [2]. Alternative renewable options, such as biomass, wind, hydro, and solar have been sought. The former exhibits high potential ranging from abundance to multiple energy products [3]. Categorically, biomasses are either virgin biomasses (lignocellulosic crops and vegetables) or waste biomasses (municipal solid waste and sludge).

Thermochemical and biochemical methods can produce multiple energy forms. Biological techniques face resistance from natural cell walls to microbial and enzymatic breakdown [4]. On the other hand, the thermochemical technique is the most attractive one. Methods, such as pyrolysis, gasification, and liquefaction, are utilised to produce the desired products depending on operating conditions [5]. Pyrolysis is the most reliable method to obtain pyrolysis products, such as bio-oil, syngas, and biochar [6]. This process involves heating biomass from ambient to peak temperature and holding it at that temperature for the required residence time in anoxic conditions [7].

Pyrolysis is a complex process influenced by many factors, such as feedstock composition, water content, inherent materials, temperature, heating rate, residence time, and catalytic additives [8]. Numerous studies have focused on the roles of temperature, heating rate, and residence time as significant influencers of biochar’s morphological and physicochemical properties [9]. However, additives are more flexible in catalytic pyrolysis to obtain the desired product yield and selectivity. Research by W. Jun et al. investigated the catalytic roles of six inorganic compounds, i.e., NaCl, Na₂CO₃, NaOH, TiO₂, Na₂SiO₃, and zeolite, and the findings revealed that sodium compounds promoted the charring of solid fraction [8]. To the best of the authors’ knowledge, this is the first study that performs a catalytic pyrolysis of green pea waste at different basic sodium carbonate catalyst ratios. Accordingly, the derived BC material from waste biomass demonstrates excellent structural property enhancements, such as pore properties and functional groups, thus improving its application efficacy.

This study highlighted the synthesis of BC from green pea peels using the pyrolysis technique at 500 °C in an anaerobic environment. The influence of incorporating an additive, i.e., sodium carbonate catalyst, on the physicochemical characteristics of biochar was evaluated.

2. Materials and Methods

2.1. Materials

Green pea peels (GPPs) were procured from the local market at Alexandria, and nitric acid (37% Darmstadt, Germany), sodium carbonate, potassium hydroxide, nitric acid (Fine chem, Mumbai, India) were of high analytical grades.

2.2. Methods

2.2.1. Production of BC over Na₂CO₃ Catalyst

The peels were crushed to particles of sizes less than 212 μm using an electric blender (Jumbo Nouval, China). BC was produced by the procedures below. The quantity of Na₂CO₃ in the weight of catalyst to green pea waste at 0%, 10%, 20%, and 30% was dissolved in 50 mL of deionised water. A total of 10 g of GPPs was soaked in the basic sodium solution and oven-dried until no weight change was realised. The dried biomass and the Na₂CO₃ catalyst were placed in a tightly enclosed ceramic crucible. The crucible was heated at a temperature of 500 °C, ramping rate of 27 °C min⁻¹ for 1 h in an anaerobic atmosphere created by purged nitrogen gas.

A brown solid was neutralised using 0.1 M HNO₃ and then washed with distilled water until it obtained a neutral pH and was designated as R0%, R10%, R20%, and R30%. The yield of BC produced was calculated from Equation (1).

$$\mathrm{B}\mathrm{C} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\left(\mathrm{\%}\right)=\frac{\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{B}\mathrm{C}}{\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{G}\mathrm{P}\mathrm{P}}\times 100$$

(1)

2.2.2. Characterisations of Biochar

BC was characterised by Scanning Electron Microscope (JEOL JSM-6360LA, Tokyo Japan), BET surface area analyser (Microtrac MRB Belsorp, Tokyo Japan), Fourier Transform Infrared Spectroscopy (Shimadzu 8400s, Kyoto Japan), and X-ray Diffraction (Bruker D2 Phasor, Tokyo Japan). For fuel properties, Oxygen Bomb Calorimeter (Labtron, Camberley UK) was used to determine the Higher Heating Value (HHV).

3. Results and Discussions

3.1. Physicochemical Properties of BC

3.1.1. Influence of Catalyst on Biomass Conversion, Yield, and HHV of Char

The pretreatment with Na₂CO₃ catalyst promoted simultaneous thermal degradation of the green pea pods and the deduction of the char yield as in Figure 1 [10]. The reduction in char yield from 33.22% at R0% to 23.77% at R30% can be credited to the swelling effect during impregnation and the catalytic role of Na₂CO₃ during pyrolysis [8]. The addition of the basic Na₂CO₃ catalyst facilitates the sodium ions in penetrating the peels’ biomass structure and breaking intermolecular hydrogen bridges. This induces a mixture of green pea peels and a catalyst to gain higher reactivity under the pyrolysis process to undergo low-temperature catalytic conversion [10]. Furthermore, the reduction in char yield can be attributed to the promotion of liquid and gaseous products brought about by the additional cracking and interaction with volatile hydrocarbons [11]. For the case of oxygen bomb calorimetry tests, the calorific value of the BC material increased with sodium carbonate impregnation indicating the potential of the resultant catalyst to acquire more potential energy.

3.1.2. Surface Morphology of BC

The visual disparity in the remnant structure of BC produced at varying sodium carbonate catalyst ratios was identified from micrographs prepared from SEM. The untreated BC (R0%) possessed a melted wall with hardly any recognisable pores, as in Figure 2a [12]. The BC produced after sodium carbonate pretreatment in Figure 2b–d has a bulky shape degraded with the production of pores. The creation of pores is attributed to the formation of volatiles and tar during pyrolysis. It is noteworthy that the visibility and clarity of the pores increase with the increased amount of sodium carbonate catalyst used during impregnation. Furthermore, the surface of BC with increasing sodium carbonate appears more degraded, indicating the activation of the produced biochar.

3.1.3. BC’s Surface Area and Pore Properties

The pore outcomes of BC produced under the impregnation of biomass with the Na₂CO₃ catalyst at different ratios are in

Table 1. BET surface area, pore volume, and mean pore diameter of the produced BC.

Parameter	BET Surface Area (m2 g−1)	Average Pore Diameter (nm)	Total Pore Volume (cm3 g−1)
R0%	1.007	174.090	0.044
R10%	3.005	16.002	0.012
R20%	11.510	15.643	0.045
R30%	17.700	9.283	0.041

. The increase in the carbonate ratio enhanced the improvement of the BET surface area, while the mean pore diameter experienced a negative correlation. Furthermore, the total pore volume exhibited an inconsistent trend with the addition of the catalyst. The phenomenon can be ascribed to transforming the organic chemical structures of green pea peels by impregnation with Na₂CO₃ and pyrolysis processes [13,14]. Moreover, the basic catalyst initiates catalytic cracking of volatile compounds, further enhancing the production of volatiles at low temperatures creating pores.

The N₂ adsorption and desorption studies of the BC are revealed in Figure 3. The BC isotherm was convex towards the relative pressure axis (p/p°) over most of its entire length. This is synonymous with an uncommon Type III isotherm, according to IUPAC nomenclature, that exhibits strong lateral interaction between the adsorbate and adsorbent [15]. The BC pore size distribution in Figure 3 ranged from mesoporous to macroporous. The suited BC applications are in the following modes: mesoporous for pollutant decontamination and catalyst supports, microporous for nutrient retention in the soil, and macroporous for aeration of the soil [6].

3.1.4. Functional Properties of the Produced Char Materials

The FTIR spectrum of the produced BC is represented in Figure 4. The infrared spectra revealed a strong, broad band in the range of 3400–3600 cm⁻¹, ascribing to the O–H stretching of water and phenol. Other significant peaks are observed at 1624 cm⁻¹ in the range of 1600–1630 cm⁻¹ attributed to aromatic C=C and C=O skeletal vibrations. A peak at 1249 cm⁻¹ is due to aromatic methoxy from the hemicellulose. The peaks at 879 cm⁻¹ and 616 cm⁻¹ are ascribed to O–H vibration stretching and aliphatic and aromatic C–H stretchings, respectively [16]. The BC produced in the presence of sodium carbonate and at higher ratios encountered a reduction in the intensity and shape of prominent functional groups. The band assigned to hydroxyl weakened and became less intense. This is attributed to the role of impregnation by sodium ions to weaken hydrogen bridges and make them susceptible to thermal degradation [10]. On the other hand, the peak encountered at 1249 cm⁻¹ is not exist in the other BC samples because the methoxy of the aromatic ring undergoes hydrolysis and the side chain fraction dissolves, causing disappearance at higher ratios [17]. This observation can be further confirmed by the ability of basic alkali solutions to split off the acetyl groups attached to hemicellulose [14].

3.1.5. XRD Diffraction Pattern of the Produced BC Materials

The spectral analysis of BC produced from green pea peels under the different ratios of sodium carbonate catalyst impregnation is shown in Figure 5. The disparity in strength and position of the peaks indicate diversity in the formation and emergence of different crystals [18]. The BC at R0% revealed two peaks at 28.1° and 40.4° at the plane of (111) and (520) (JCPDS 049-1720), suggesting the emergence of carbon oxide and C60 fullerite crystals at 500 °C, respectively. The same crystals emerge in the BC at R10% with decreasing intensities. By increasing the quantity of the Na₂CO₃ catalyst at R20%, a graphite peak appeared at 26.4° at a miller plane of (002) (JCPDS 008-0415), whereas the BC at R30% indicated synthesis of C8 carbon (JCPDS 082-0619). The BC with fullerite crystal can be well suited for biomedication applications, whereas those that have experienced the immense impact of activation can be suited for wastewater treatment technologies.

4. Conclusions

In this work, the authors successfully studied the effect of the impregnation of the Na₂CO₃ additive on BC’s chemical and physical properties. An increase in the amount of additives reduced the BC yield, whereas the calorific properties increased. The pore properties of the specific surface area increased, whereas the mean pore diameter decreased. Furthermore, the oxygen-enriched functional groups were reduced, and they were changes in the crystallographic carbon structure with changes in the impregnation of sodium carbonate. The microporous BCs are suitable for nutrient retention and adsorption, while macoporous chars are appropriate for aeration. Pollutant remediation could be performed properly via mesoporous BC.