Study of synthesis and characterization of raw bagasse, its char and activated carbon prepared using chemical additive

The role of the agricultural sector in human and economic development cannot be overemphasized. The impact of agricultural solid waste on human and animal well-being as well as on the environment is substantial, which is mainly due to ignorance during managing agricultural solid waste. A lot of lignocellulosic biomass is created every year causing environmental issues and hence, one can convert agro-waste into net worth products. This step can promote the utilization of sustainable raw materials in a proper way (Hon & Siraishi 2000; Obi et al. 2016).

Due to an incessant demand for commercial activated carbon (AC) for industrial applications, the market cost of commercial activated carbon reached $1,100–1,500/tons. Hence, the preparation of activated carbon from cheap and easily available raw materials can make it cost-effective (Hock & Zaini 2018). One such agricultural waste is sugarcane bagasse (SB) which is used to synthesize biomass waste-based activated carbon in this study.

SB is the residue left after the extraction of sugarcane juice and it contains 35% of cellulose, 25% of hemicellulose and 22% of lignin (Rezende et al. 2011). SB is treated as a potential material for energy recovery. Once it has been processed properly, it can be used as a carbon-neutral energy material (Ramajo-Escalera et al. 2006; Hofsetz & Silva 2012; Dantas et al. 2013). SB is used as a fuel in sugar plants for the generation of steam and in ethanol distillation (Nunes et al. 2020). It is used in the production of derived fuels using gasification and rapid pyrolysis (Naik et al. 2010). It is also used in the paper and pulp industry for the manufacturing of corrugated boards (Samariha & Khakifirooz 2011). Due to its wide range of uses, it is no longer considered a waste nowadays. Due to high lignocellulosic content, its usefulness in the form of char and activated carbon is capturing researchers’ attention. The microporous structure of any activated carbon is due to its high cellulose and low lignin content (Misran et al. 2018; Dwiyaniti et al. 2020).

The term ‘char’ represents the solid residue that forms after thermal degradation of carbon-rich agricultural waste in limited oxygen conditions. It consists of organic material with a composition varying from barely carbonized agricultural waste at low temperatures to a highly carbonized material at high temperatures. The physical and chemical characteristics of biochar depend on the temperature used for pyrolysis and the type of biomass used. It can be used to improve contaminated soil quality, soil productivity, removal of toxic metals and production of bio-fuel, etc. (Kumar & Bhattacharya 2018). Activation of biochar can also be found useful in removing particular contaminants from water.

There are various methods used for the removal of pollutants removal from liquid or gaseous phases like membrane filtration, precipitation and coagulation, reverse osmosis distillation, adsorption, ion-exchange, photochemical degradation, biological degradation and chemical oxidation (Rashed 2013; Raut et al. 2021). Amongst these methods, the adsorption of organic as well as inorganic impurities with activated carbon is gaining importance and has been widely used (Harry & Francisco 2006). It has been found to be an economical and effective treatment method because of no sludge formation (Ioannidou & Zabaniotou 2007).

In chemical activation, the precursor was impregnated with a chemical agent followed by activation. The activating agents that can be used are H₃PO₄, ZnCl₂, KOH, NaOH, Na₂CO₃, K₂CO₃ and H₂SO₄ (Hayashi et al. 2002; Demiral et al. 2008; Kalderis et al. 2008; Gardare et al. 2015; Ghani et al. 2017). The AC is washed with hydrochloric acid followed by deionized water and dried for further use.

This study discusses the synthesis and characterization of char and ZnCl_2-made activated carbon. The effect of activation temperature and impregnation ratio on yield and iodine number was studied and discussed. The raw bagasse (precursor), char and activated carbon were characterized by iodine number, CHN analysis, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA-DTA) and Brunauer–Emmett–Teller (BET) surface area and elemental analysis energy-dispersive spectroscopy (EDS).

This study focuses on activated carbon preparation and its comparison with raw bagasse and bagasse char. Such types of comparative studies based on raw stage, char stage and activated carbon stage have not been extensively studied earlier, hence this study discusses the thermochemical conversion of bagasse into char and activated carbon.

Raw SB was collected from Manas Agro Industries (Umrer, India). Analytical (AR) grade chemicals used were dry ZnCl₂ (EMPLURA), Na₂S₂O₃ (EMPLURA), Merk Life Science, Pvt Ltd (Worli, Mumbai), resublime iodine (Q24904), KI (Loba Chemie Pvt. Ltd, Jehangir Villa, Colaba, Mumbai), HCl & starch (Fisher Scientific, Thermo Electron LLS India Pvt, Sion (East), Mumbai), purchased from Vaishnavi Scientific & Trading Company (Ayodhya Nagar, Nagpur, India). Instruments used were heating mantle (Bio Technics India, Mumbai), electric muffle furnace (Tempo Instruments, Mumbai), pH meter (Deluxe pH-101), digital balance (Citizon) and laboratory oven (BTI).

The raw SB, its char and activated carbon were characterized by iodine number which is an indication of the micro-porosity of activated carbon. It is determined as per ASTM: D4607-94 standard method. CHN analysis was carried out by using a ThermoFinnigan analyzer available at the Department of CIL, Punjab University, Chandigarh. The thermal decomposition of material was tested by TGA by using DTG-60 simultaneous DTA-TG apparatus. The functional group availability of the surface of activated carbon was recorded by FTIR by using an IR-Affinity-1 spectrometer. The amorphous or crystalline nature of activated carbon was studied by powder X-ray diffraction studies (XRD) by using ‘Rigaku’ Miniflex diffractrometer. The surface structure of activated carbon was studied by scanning electron microscope (JEOL-6380A).

From Table 1, it is observed that activation reduces the mass of produced carbon and char. The percentage of mass loss increases with the increase in ratio and temperature. As soon as the activation temperature increases, the yield of activated carbon decreases, as shown in Table 1. The decrease in yield indicates the evolution of more volatile matter at higher temperatures (Foo & Hameed 2011). The same observation was reported by Lua & Ting (2004). Loss in weight also represents the reaction mechanism carried out between lignocellulosic material and chemical agent (Angin et al. 2013). In ZnCl₂ activation, the aqueous solution is entered into the carbon skeleton to produce pores at a temperature above its melting point. The reaction between the carbon atoms and ZnCl₂ is promoted in the inner layers of the carbon. The use of ZnCl₂ as a chemical agent generally increases the carbon content through the formation of aromatic graphitic structures (Hock & Zaini 2018). During activation, it has been observed that around 400 °C of the removal of most volatile matter gets started like CO and CO₂ due to the effect of activation. During the initial stage of thermal degradation from 400 to 600 °C, zinc chloride is completely dehydrated from water molecules taken by bagasse (Bouchemal et al. 2015). Also, it takes off H₂ and O₂ atoms from the carbon structure to form H₂O molecules (Cartula et al. 1990). The activated carbon with the highest yield of 31.87% was obtained at 600 °C for the ratio 1:1. It has been observed from the experimental work that as the impregnation ratio increases, yield decreases. This is because at higher ratios, swelling and elasticity increase. This indicates that there may be an additional oxidation reaction occurring during impregnation with the removal of volatile matter (Hamza et al. 2015). The yield of prepared chars also decreases with increasing activation temperature. The percentage yield and loss of activated carbon and char at different temperatures are presented in Table 1.

Iodine numbers of activated carbon measure the approximate surface area and micro-pore volume. The iodine number of activated carbon and char was determined in the laboratory as per ASTM D 4607-94 standard method (Standard Test Method 2006). The milligram of I₂ adsorbed on the surface of 1 g activated carbon when the iodine concentration of residual filtrate is 0.02 N. All the required solutions of I₂, 10% HCl, sodium thiosulfate, potassium iodide and starch indicator were prepared in the laboratory with distilled water.

In this method, 1 g of activated carbon sample was taken in a 250 mL conical flask. Ten mL of 5% of HCl solution was added to it. The flask was then kept on a hot plate for boiling for 30 s. The flask was allowed to cool and 100 mL of 0.1 N iodine solution was added to it. The conical flask was immediately covered and shaking of contents for 30 s was carried out. The contents were filtered using filter paper immediately after the shaking period. The initial 20–30 mL of filtrate was discarded and a further 50 mL of filtrate was titrated with 0.1 N sodium thiosulfate by using starch as an indicator until the solution changes to colorless.

It can be seen from Table 2 that, in the case of SB char, the iodine number increases as the impregnation ratio increases from 1:1 to 1:3. The bagasse char gives iodine numbers of 496.17, 500.81, 504.12 and 510.75 mg/g at 600, 700, 800 and 900 °C, respectively. The maximum iodine number (510.75 mg/g) was found at 900 °C. In the case of activated carbon, the iodine number decreases first from 1:1 to 1:2 ratio from 600 to 800 °C. However, at 900 °C, as the impregnation ratio increases from 1:1 to 1:2, the iodine number increases and from 1:2 to 1:3, it decreases. The highest iodine number (1,140.69 mg/g) was obtained at 900 °C for a 1:2 ratio. Hence, the optimum impregnation ratio (bagasse: zinc chloride) was found to be 1:2. Char without activation provides a lesser iodine number than activated carbon (SB-Zn₂-900). This proves that activation by using chemical reagents enhances iodine numbers and adsorption capacity.

All the prepared samples at four different temperatures with three impregnation ratios were tested for iodine number. However, the char and activated carbon with the highest iodine number are only characterized further for FTIR, XRD, SEM and BET. Hence, based on the iodine number determined, SBC900 and SB-Zn₂-900 (1:2 ratio at 900 °C) were found to have the highest iodine number and therefore further characterization was carried out for FTIR, XRD, SEM and BET.

Elemental analysis works on the basic principle of oxidation of C, H, N and S into gases like CO₂, H₂O and SO₂ that are then separated by the chromatographic method. In this method, an accurately weighed sample was introduced in the furnace in the presence of a constant supply of inert gases (He). Oxygen gas is injected into the inert gas stream with a sample and allowed to combust in a tin capsule. The exothermic reaction takes place with the evolution of combustion gases which raised the furnace temperature from 1,000 to 1,800 °C within a short time. The combustion gases then swept through a series of oxidation columns containing agents like chromium oxide, tungsten trioxide, cobaltic oxide, etc., which ensures complete combustion of C, H, N and S into CO₂, H₂O and SO_2. The unreacted oxygen was then consumed by metallic copper present in the reduction column which was maintained at 700 °C and nitrogen-containing components were reduced to N₂. Separation of individual gases is carried out in the chromatographic column containing porous polymer maintained at less than 100 °C. Each element was then quantified by the signal generated during its sequential passage through a thermal conductivity detector (TCD) yielding a peak in abundance at different times (Michael et al. 2017). Elemental analysis was carried out on a ThermoFinnigan analyzer available at the Department of CIL, Punjab University, Chandigarh. The CHN analysis of raw bagasse, its char and activated carbon (SB-Zn₂-900) is shown in Table 3.

The thermal decomposition of raw material was tested by TGA-DTA analysis. TGA was carried out on DTG-60 simultaneous DTA-TG apparatus (SHIMADZU CORP. 00812), available at RTMNU, Nagpur. In this analysis, samples were heated in an inert environment like nitrogen (flow rate 20 mL/min) at a controlled rate of 10 °C/min up to 900 °C. The temperature was raised at a constant rate for a known weight of sample and changes in weights were recorded as a function of temperature at different time intervals. The plot of change in weight with respect to temperature is plotted as a thermogram.

FTIR spectra of ZnCl₂ activated carbon shows peaks around 3,600–3,700 cm⁻¹ which corresponds to free phenolic –OH stretching which is attributed to the –OH group of alcohols, carboxyl and phenols. This band is not observed in char corresponding to structure decomposition and hydroxyl group removal. The bands obtained between 2,857 and 3,000 cm⁻¹ are due to symmetric and asymmetric C–H vibrations. The band at 2,926.38 cm⁻¹ shows methylene C–H stretching. This peak does not exist in char. The band around 1,747.58 cm⁻¹ shows the presence of a C–O double bond group. The band at 1,555.27 cm⁻¹ informing aromatic C = C ring. The peaks at 1,000–1,200 cm⁻¹ show the existence of phenolic and alcoholic groups which are identified in raw bagasse, char and activated carbon. The peaks around 600–900 cm⁻¹ denote the existence of an aromatic ring structure which is observed in raw bagasse, char and activated carbon. The transmittance at 1,000–1,300 cm⁻¹ was observed in raw bagasse only which corresponds to C–O stretching in ethers. After activation of bagasse, C = O stretching in aldehydes, ketones and C–O stretch in ethers disappeared in the same fashion as reported by Guo & Lua (2000). At high temperature C–O and C = O groups were removed, forming polyaromatic structures (Li et al. 2008; Abdul Hamid et al. 2014).

Structural analysis was carried out by powder XRD studies. XRD study was carried out by using ‘Rigaku’ (Miniflex) diffractrometer, available at RUSA, Nagpur. XRD studies show the amorphous or crystalline nature of materials. X-ray scattering is an analytical technique which gives information about the crystallographic structure, chemical composition and physical properties of materials. These techniques are based on detecting the scattered intensity of an X-ray beam hitting a sample as a function of incident and scattered angle, polarization and wavelength or energy (Pradhan 2011).

The surface morphology of activated carbon was studied by scanning electron microscope. The SEM gives an indication of the nature of porosity. A SEM is a type of electron microscope that scans a sample with a high-energy electron beam. The electrons interact with the atoms of the sample thereby producing signals which tell about the surface structure of the sample, its composition and other properties such as electrical conductivity. In the SEM technique, the sample was coated with gold for good conductivity. Surface morphology was obtained by magnifying the real image 1,200 times (Pradhan 2011). The scanning electron microscope used was the JEOL-6380A model available at VNIT, Nagpur.

BET surface area analysis is the multi-point measurement of specific surface area through gas adsorption analysis in which an inert gas nitrogen is continuously flown over a solid sample of activated carbon. Small gas molecules adsorb to the solid substrate and its porous structures due to weak van der Waals forces, forming a monolayer of adsorbed gas. This monomolecular layer and rate of adsorption can be used to calculate the specific surface area of a solid sample (Pradhan 2011). It was determined by QUANTACHROME (Nova-Touch) surface area analyzer available at RTMNU, Nagpur.

EDS is an analytical technique used for the elemental analysis or chemical characterization of a sample. It was determined with SEM by the JEOL-6380A instrument available at VNIT, Nagpur.

The EDS analysis of raw bagasse, char and activated carbon is shown in Table 5. It was revealed from the EDS studies that raw bagasse contains 56.25% C, 42.22% O, 0.79% Pt, 0.42% Si and 0.33% Al. SB after carbonization contains 81.87% C, 16.72% O, 0.44% Pt, 0.53% Si and 0.45% Ca. Activated carbon produced after chemical activation with zinc chloride contains 74.50% C, 12.27% O, 0.22% Pt, 0.04% Si, 12.17% N, 0.52% Cl and 0.20% Zn. These results revealed that after carbonization the char and activated carbon met the requirement of 65% C as per SNI No. 06-3730-1995 (Guo & Lua 2000; Adlim et al. 2021).

The comparative study of raw material, activating agent used, condition of activation, CHN analysis, iodine number and BET surface area with previous research studies is shown in Table 6.

This study investigated the preparation of activated carbon from naturally available bio-waste material SB. In this study, the characterization results of CHN analysis, FTIR, XRD, BET surface area and SEM analysis of raw bagasse, bagasse char and zinc chloride-impregnated activated carbon were compared. From the results, it was observed that more activation temperature gives lesser yield of the product. It means product yield is inversely proportional to the activation temperature. The iodine number gives an indication about the porousness of activated carbon. Higher iodine numbers mean highly porous carbon. The iodine numbers of SB-Zn₂-900 and char at 900 °C were found to be 1,140.69 and 510.75 mg/g, respectively. The CHN analysis data reveal that activation of raw material can transform into activated carbon with carbon % of 42.049, 48.150 and 80.126 for raw bagasse, its char and activated carbon. The FTIR study investigated that raw bagasse and char shows very less functional groups. The appearance of functional groups occurred by chemical activation at various temperatures. The activated carbon was found to contain aliphatic, aromatic and oxygen-containing functional groups. From SEM micrographs, it is concluded that there are very less pores in raw bagasse, some pores were seen in char and a large number of pores were observed in SB-Zn₂-900. It means activation with the chemical agent can impart porousness in the carbon. The BET surface area of SB-Zn₂-900, char and raw bagasse was found to be 1,386.58, 514.270 and 0 m²/g, respectively, which shows raw bagasse does not exhibit adsorption properties. On the other hand, char has less and activated carbon has a larger adsorption capacity. The optimum ratio of impregnation and activation temperature was found to be 1:2 at 900°. Elemental analysis was also carried out by EDS and showed that char contains 81.87% C whereas activated carbon contains 74.50% C. However, SB can produce good surface area carbon that can be utilized for further adsorption studies.

All the data are available in the manuscript

E.R.R. carried out the collection of biomaterials, synthesis, characterization and preparation of manuscript. M. A. B. and A. R. C. carried out the concept and guidance of research work.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.