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

: Biochar (BC) has been a viable resource in producing functionalized carbonaceous materials beneficial in soil fertility, environmental remediation


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 biomass (lignocellulosic crops and vegetables) or waste biomass (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, thermochemical is the most attractive technique. Methods such as pyrolysis, gasification and liquefaction are utilised to produce desired products dependent 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 desired product yield and selectivity. According to 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 sodium compounds promoted charring of solid fraction [8]. To the best authors knowledge, this is the first study that performs catalytic pyrolysis of green pea waste at different basic sodium carbonate catalyst ratios. Accordingly, the derived BC material from waste biomass shall demonstrate excellent structural properties enhancement such as pore properties, functional groups henceforth improving its application efficacy.
This study highlighted the synthesis of BC from green pea peels using pyrolysis technique at 500 °C in anaerobic environment. The influence of incorporating additive, i.e., sodium carbonate catalyst, on physicochemical characteristics of biochar was evaluated.

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

Production of BC over Na2CO3 Catalyst
The peels were crushed to particles of sizes less than 212 um 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 peas waste at 0%, 10%, 20% and 30% was dissolved in 50 mL of deionised water. 10 g of GPP 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 designated as R0%, R10%, R20% and R30%. The yield of BC produced is calculated from Equation (1).

Influence of Catalyst on Biomass Conversion, Yield, and HHV of Char
The pre-treatment with Na2CO3 catalyst promoted simultaneous thermal degradation of green peas pods and deduction of char yield as in Figure 1 [10]. The reduction of 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 basic Na2CO3 catalyst facilitates the sodium ions 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 lowtemperature catalytic conversion [10]. Furthermore, the reduction of char yield can be attributed to the promotion of liquid and gaseous products brought about by 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.

Surface Morphology of the 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 pre-treatment 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's 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 at increasing sodium carbonate appears more degraded, indicating the activation of the produced biochar.

BC's Surface Area and Pore Properties
The pore outcomes of BC produced under impregnation of biomass with Na2CO3 catalyst at different ratios are in Table 1. The increase in carbonate ratio enhanced the improvement of 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 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 volatile compounds' catalytic cracking, 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 adsorbate and adsorbent [15]. The BC pore size distribution in Figure 3, ranged from mesoporous to macroporous. The suited BCs applications are in the mode; mesoporous for pollutant decontamination and catalyst supports, microporous for nutrient retention in the soil and macroporous for aeration of the soil [6].

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 vibration. A peak at 1249 cm −1 is due to aromatic methoxy from the hemicellulose. The Peaks at 879 cm −1 and 616 cm −1 ascribe to O-H vibration stretching and aliphatic and aromatic C-H stretching, 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 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 solution to split off the acetyl groups attached to hemicellulose [14].

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. With increasing the quantity of Na2CO3 catalyst at R20%, graphite peak appeared at 26.4° at miler 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 application whereas those that have experienced immense impact of activation can be suited for wastewater treatment technologies.

Conclusions
In this work, the authors successfully studied the effect of the impregnation of Na2CO3 additive on BC's chemical and physical properties. An increase in the amount of additives reduced the BC yield whereas the calorifc 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 crystallographic carbon structure with changes in 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 proper via mesoporous BC.