PRODUCTION AND CHARACTERIZATION OF ACTIVATED BIOCHAR OF CÁSSIA FISTULA L. LEAVES

A new activated biochar was produced by carbonizing Cássia fistula L. leaves and its characterization was performed. Scanning Electron Microscopic (SEM) images of the activated biochar showed morphology with numerous irregular cavities and pores. BET surface area and total pore volume of produced adsorbent were 192 m2.g and 0.108 cm.g, respectively. Mean porous diameter of the produced biochar was 2.263 nm, characterizing a mesoporous material. Crystallinity and functional groups of the adsorbent were determined by X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy analyses, respectively. The pH of zero charge point (pHzcp) observed was equal to 10, demonstrating that the activated biochar has a negative surface, which facilitates the adsorption of positively charged compounds in water and wastewater treatment. Tree leaves of Cássia fistula L. represent a promising raw material for activated biochar production, given their availability and characteristics of the adsorbent produced. The use of tree leaves for activated biochar production can reduce the operational costs of adsorption process, besides providing the use of a residue for a more noble purpose.


INTRODUCTION
Conventional activated carbon is a known porous material with high specific surface area, fast adsorption kinetics, and high adsorptive capacity (Saka, 2012). However, due to its high cost of production and regeneration, largescale application is limited with an increase in interest in organic matrix alternatives. These matrixes are in line with sustainable development and reduce the total cost of production of activated carbons (DURAL et al., 2011;ROVANI et al., 2016).
Agricultural and biomass residues are considered cheap and abundant raw materials with high potential for adsorption applications (NOR et al., 2013, KANKILIÇ et al., 2016AHMED, 2017). In addition, the conversion of these residues into value-added products such as activated carbon could solve environmental problems such as accumulation of agricultural waste, water and air pollution, as well as being efficient in the adsorption of pollutants (Nor et al., 2013). In the last years, numerous studies have reported the use of activated carbon obtained through the use of residues of different biomasses, such as rice husk (KALDERIS et al., 2008), tobacco residues (KILIC; APAYDIN-VAROL; PUTUN, 2011), orange peel (KOSEOGLU; AKMIL-BASAR, 2015), apple bagasse (ROVANI et al., 2016), Phragmites australis (KANKILIÇ et al., 2016;AHMED, 2017) and shell of fruits (ISLAM et al., 2017). Among the various biomass residues studied, tree leaves have also been considered as potentially promising raw material for the production of activated carbon (SUMATHI et al., 2009;NOR et al., 2013;KANKILI et al., 2016;AHMED, 2017 Tree leaves are composed basically by three different compounds: cellulose, hemicellulose and lignin. The lignin is the main responsible by the adsorption process due to its high carbon content and molecular structure similar to bituminous coal (SUHAS et al., 2007;ROSS;POSSETI, 2018). Recent studies have indicated satisfactory results in the removal of pollutants by the adsorption process with leaves of Posidonia oceanica L. (DURAL et al., 2011), palm (Pinus brutia Ten.) (DENIZ; KARAMAN, 2011;SOLIMAN et al., 2016), Ginkgo (Ginkgo bibola) (ZHU et al., 2018), and pineapple (Ananas comosus) (BELTRAME et al., 2018). However, more researches are necessary to evaluate different conversion techniques of these biomasses into adsorbents, and its characterization.
Cássia fistula L. is a species native to India, found in several countries such as Mexico, South Africa, China and Brazil (VIEGAS JR. et al., 2006). Some parts of Cássia fistula L. trees (stem bark, branches, pods, and internal pods mass) have been the focus of studies on production of low cost adsorbents (Senniappan et al., 2016;Sikri et al., 2018). However, adsorbents produced with leaves of Cassia fistula L. have not been studied in adsorption processes.
In the present study a new activated biochar was produced from leaves of Cassia fistula L. using the physical activation. The produced adsorbent was characterized by structural, textural and morphological analysis. The greater novelty of the present study is the production of an activated biochar from a low cost residue using only physical activation technique. This activation type is considered a sustainable technique, since chemical activation is not ecologically correct due the possibility of production of numerous undesirable, dangerous, and corrosive compounds. Furthermore, physical activation allows greater control over the development of microporosity and it eliminates the use of chemicals, reducing process costs and associated pollution.

Production of the Activated Biochar
A cleaning step on leaves of Cássia fistula L. (collected after their natural fall) was performed with distilled water in order to remove any residue and particulate matter. The cleaned leaves passed by two drying stages: at room 803 R. gest. sust. ambient., Florianópolis, v. 9, n. esp , p. 800-815, fev. 2020 temperature and in an oven for 48 hours at 60 ± 5°C. Posteriorly, the material was crushed and sieved to obtain a homogeneous mixture. The activation process was realized by carbonization, with a heating rate of 10ºC.min -1 until 800ºC (held for 1 hour). Afterward, the produced activated biochar was stored in an airtight container for later use.

Activated Biochar Characterization
The carbonization process of Cássia fistula L. leaves was evaluated by a after 24 h and the pHPZC value was determined as the point where initial pH was equal to final pH.

Thermogravimetric Analysis
The behavior of the thermal degradation of the material produced from the leaves of Cassia fistula L. is presented in Figure 1.

Granulometric Analysis
The particles mass fraction retained in each sieve of the Tyler series were determined based on the granulometric analysis of the activated biochar. The particle size distribution curve of the produced adsorbent was obtained, as presented in Figure 2. Approximately 77% of the particles were retained between sieves of 100 and 60 mesh, with an average diameter of 0.15 and 0.25 mm.
Particles with mean diameter lower than 0.1 mm corresponded to 23% of the sample mass. This fraction of smaller particles is desired in adsorbents since the adsorption process efficiency is favored (SEKAR et al., 2004).

Morphological Analysis -SEM and EDS
Scanning Electron Microscope (SEM) analysis of the activated biochar sample is essential to understand its global morphology and microscopic porous structure, as shown in Figure 3. The adsorbent produced from leaves of Cassia fistula L. has a porous structure, with some irregular channels and, in some parts,  Regarding the EDS analysis of the activated biochar, the main elements quantified in descending order were: carbon (72%), calcium (16.8%), and oxygen (10%), as presented in Figure 4. The contribution of carbon and oxygen elements in the activated biochar represents a percentage above 80%. Similar results were obtained by Ravulapalli and Kunta (2017) with an adsorbent produced of Ficus racemosa peel. The high percentage of these elements confirms the carbonaceous characteristic of agricultural by-product adsorbents.  (REN et al., 2019).

Porous Structure Characterization
SBET and pore volume of the produced adsorbent by Cássia fistula L.
leaves were 192 m².g -1 and 0.108 cm³.g -1 , respectively. These results corroborate with conclusions of the SEM images, where the biochar morphology presented more aggregated particles and consequently a larger surface area. The produced activated biochar presented a mean porous diameter of 2.263 nm and was classified as a mesoporous adsorbent, in the transition range between micro and mesoporous. This study revealed that the surface area of the activated biochar produced from Cássia fistula L. leaves was higher than those found in adsorbents prepared with different leaves, such as, Posidonia oceanica L. (38.9 m².g -1 ) (Dural et al., 2011), Pinus brutia Ten leaves activated with sulfuric acid (64.12 m².g -1 ) (Soliman et al., 2016), and Ananas comosus leaves activated with phosphoric acid (25 m².g -1 ) (BELTRAME et al., 2018).

Structural Analysis -FTIR and XRD
FTIR analysis ( Figure 5) was used to evaluate and identify major functional groups contained in the structure of Cássia fistula L. in natura and the produced biochar. The bands at 3,415 cm -1 characterize vibrations in O-H groups presented in the cellulose. In the in natura sample it was possible to identify the presence of methylcellulose by detecting υ (C-H) and δ (C-H) strongly electronegative in the ortho position in absorption bands located in the spectrum at 2,920 cm -1 . After carbonization, it was observed the advance of a new peak of 875 cm -1 , corresponding to the angular deformation outside the plane of C-H, possibly by celluloses degradation. It was observed the band shift in the in natura sample of 1,618 cm -1 to 1,417 cm -1 in the adsorbent. They are correspondent to the C=O and C-O bonds present in the lignin structure, as well as the interactions with the 808 R. gest. sust. ambient., Florianópolis, v. 9, n. esp , p. 800-815, fev. 2020 carboxylate group. The peaks of 707 cm -1 and 1,041 cm -1 represent angular deformation of C=C and axial deformation of C-O, respectively. The interactions between the phenolic O-H bonds present in the lignin structure, as well as the interactions with the unsaturated aliphatic structures, are the functional groups responsible for adsorption process (CALVETE et al., 2010;CARDOSO et al., 2011).
Similar bands between 1161-1027 cm -1 (C-O elongation vibration in cellulose and hemicellulose) and 1632 cm -1 (vibrations of lignin) were found in the study by Maaloul et al. (2017) with peanut shell adsorbent. Cardoso et al.  and 30° indicate the presence of crystalline cellulose in the samples (KEILUWEIT et al., 2010). The crystallinity degree depends on the molecular arrangement of biomass and may vary mainly by the amount of lignin, cellulose, and hemicellulose from each plant (Maia et al., 2016). It is important to note that the produced adsorbent has a high degree of crystallinity and small peaks in the regions below 20º, representing highly amorphous species (PECHYEN et al., 2007). The peaks around 35 and 45º correspond to reflections of disordered 809 R. gest. sust. ambient., Florianópolis, v. 9, n. esp , p. 800-815, fev. 2020 micrograph structure, which are characteristics of activated carbon representing the mineral phase of carbon (SCHETINO et al., 2007).

Measurement of pH of zero charge point (pHZCP)
The zero charge point of the produced activated biochar is the pH value in which the superficial charge is equal to zero, shown in Figure 7. In the experiments with initial pH between 3 and 11, it was observed a plateau for measured final pH (buffer effect), with the pHZCP equal to 10. The activated biochar of Cássia fistula L. leaves has a basic surface which is negatively charged, facilitating the adsorption of positively charged molecules. Based on these results it is possible to affirm that the contact of the adsorbent with a solution with pH less than 10 is able of promoting a negatively charged state, where a large number of cations can be absorbed. This process can be explained by the electrostatic attraction between the charge generated on the activated biochar surface and the cationic group of the solution.

CONCLUSIONS
A new activated biochar was produced by carbonizing Cássia fistula L.

leaves. Scanning Electron Microscopic (SEM) images of the activated biochar
show morphology with numerous irregular cavities and pores. BET surface area and total pore volume of produced adsorbent were 192 m².g -1 and 0.108 cm3.g -1 , respectively. Mean porous diameter of the produced biochar was 2.263 nm, characterizing a mesoporous material. The high content of C (72%) and O (10%) was observed in biochar. EDS analysis imply that the raw material is a good precursor for activated biochar production. FTIR spectrum indicated the presence of carboxylate groups, which could increase the effectiveness of adsorption of several pollutants. XDR analysis provided the fingerprint of the crystalline solids presented in adsorbent, mainly crystalline cellulose. The pH of zero charge point (pHZCP) was equal to 10, demonstrating that the activated biochar has a negative surface, which facilitates the adsorption of positively charged compounds in water and wastewater treatment. Tree leaves of Cássia fistula L. represent a promising raw material for activated biochar production, given their availability and characteristics of the adsorbent produced. The use of tree leaves for activated biochar production can reduce the operational costs of adsorption process, besides providing the use of a residue for a more noble purpose.