Preparation and characterisation of graphitic biochar materials derived from rose oil industry waste via different pyrolysis durations and ball milling for advanced composites

The valorisation of waste from rose-based industrial products produces value-added substances and paves the way for advanced composites. The rose waste generated from the essential oil industry of the Taif rose (Rosa damascena trigintipetala Dieck) is significant, and its management or disposal is a source of concern. In this study, it was valorised to produce a value-added nanomaterial. The synthesis of biochar nanoparticles via high-energy ball milling has gained tremendous research interest in recent times because of its low cost and eco-friendliness. Ball milling is a solvent-free technology with strong potential for waste volatilisation and eco-sustainability through the production of engineered biochar nanoparticles. Different biochar samples were produced and characterised to harness the synergistic combination of biochar production and ball milling. They were prepared at a constant pyrolysis temperature of 300 °C by varying the pyrolysis times for 2 h, 5 h, and 10 h. The characterisation results showed that pyrolysis confirms a high content of carbon, minerals, graphitic structure, novel morphology and chemical characteristics attached to the biochar surface controlled by different pyrolysis durations. These properties were further enhanced by ball milling for 10 h. The results showed that ball milling enhanced the porosity, surface area, surface functional groups, visible light absorption, crystallinity, and carbon content, and these were accompanied by a reduction in the particle size and mineral impurities. The engineered biochar can be an important tool, with promising potential in novel composites for water purification and energy harvesting.


Introduction
The consumption of Rosa damascena Mill.has witnessed significant industrial growth in the production of rose oil and water [1].These materials are extensively used in the fragrance, pharmaceutical, food, and cosmetic industries.The majority of essential oil production, accounting for 80%-90%, is carried out by Bulgaria and Turkey [2].However, other countries have also entered the oil industry.For instance, Saudi Arabia produces rose oil from the Taif rose (Rosa damascena trigintipetala Dieck), which has gained recognition owing to its exceptional quality and fragrance [3,4].The widely employed traditional distillation method requires approximately 3500-4000 fresh rose petals to produce one kilogram of essential oil, resulting in substantial waste generation [5].Rose waste generated from the essential oil industry is not effectively managed; it has no biochar nanostructures, including increasing the surface area, reducing the particle size, introducing oxygenfunctional groups, and enhancing the adsorption and catalytic activities [33].However, to the best of our knowledge, there is no existing literature on graphitic biochar derived from waste in the rose oil industry.Hence, the present study aimed to produce renewable graphitic biochar using a low-temperature pyrolysis method with varying durations and heteroatom contents.Furthermore, this study explored the potential of biochar as a precursor material for the synthesis of nanosized graphitic carbon with novel morphologies using a high-energy ball-milling technique.This study focused on three goals: (i) revealing the biochar properties produced from the rose waste of the rose essential oil industry under low pyrolysis, (ii) observing the effects of the time variation of low-pyrolysis biochar on its physical and chemical properties, and (iii) presenting a potential inexpensive biomass precursor to graphitic carbon to produce new renewable products on the nanoscale using the ballmilling technique.

Material preparation
In order to explore the properties of the biochar, both graphitic biochar and milled biochar were manufactured and evaluated.Rose waste samples used for graphitic biochar production in this study were collected from the Al-Qurashi factory for rose oil production in the Taif region of Saudi Arabia.The rose material was thoroughly cleaned from dust and contamination using ethanol, washed three times with deionised water (DI), and airdried for 24 h.Then, the samples were placed in a hot air oven at 80 °C for 10 h [34].In the next step, 100 grams of the dried samples was thermally heated in a muffle furnace (Nabertherm, Germany) at 300 °C using a closed crucible (limited oxygen content) with varying times of 2, 5, and 10 h at a ramp rate of 5 °C /min.The resulting biochar was dispersed in 10 ml of deionised water and sonicated for 15 min, followed by the same drying step mentioned above.The biochar samples were stored in sealed containers before the next stage of ball milling.Graphitic biochar prepared at 300 °C with varying times of 2, 5, and 10 h were labelled as GB2, GB5 and GB10, respectively.Then, carbon nanostructures with large surface areas and porous structures were prepared using dry high-energy ball milling (PM 400; Retsch, Germany).For this purpose, 5 g of carbonised powder obtained by pyrolysis with the duration of 2, 5 and 10 h was placed in a ball-milling machine for 10 h at a speed of 200 rpm, producing samples labelled as BMGB2, BMGB5 and BMGB10, respectively.To avoid the contamination problem, the jars and balls were coated with hard ceramic materials.This technique mechanically deforms the solid sample into very fine nanomaterials in powder form [36].A diagram of the manufacturing setup for the graphitic biochar production (GB2, GB5, GB10) and milled biochar (BMGB2, BMGB5, BMGB10) is shown in figure 1.

Material characterisation
Detailed physical and chemical investigations of the prepared samples were performed using various analytical techniques.The morphology, porous structure, and chemical composition were investigated using scanning electron microscopy (SEM) with energy-dispersive x-ray spectroscopy (EDS) analyses using a JEOL-7600F FESEM, Japan.For Fourier-transform infrared spectroscopy (FTIR), graphitic identification was analysed using a Raman microscope (DXR-I, Thermo Scientific, USA) with a 532 nm laser as the excitation source at a power of 7 mW.The structural pattern was obtained using a Rigaku Ultima IV x-ray diffractometer (Japan) with Cu Kα radiation (λ = 0.15418 nm).The solid-state reflection and absorption spectra were recorded using a UV-visible spectrophotometer (Shimadzu UV3600, spectrophotometer, Japan).

Results and discussion
In comparison to the existing reports, this research adopts an optimised strategy and provides up-to-date knowledge on pyrolysis at lower temperatures under different annealing durations, as well as providing a model for targeted application.
3.1.Characterisation of samples prepared from rose-waste biochar using the ball-milling method 3.1.1.Chemical composition EDS analyses were performed to effectively highlight the purity of the rose-waste biochar by determining its qualitative and quantitative elemental composition.The EDS results for the graphitic biochar and ball-milled biochar are presented in figure 2 and table 1 From the EDS results for the biochar sample prepared using the pyrolysis method under limited O 2 , carbon and oxygen were the major components, and the carbon content decreased with the increase in pyrolysis time, whereas the oxygen content increased.The loss of carbon content may be due to carbon burnout upon exposure to longer pyrolysis hours under a limited oxygen supply.The carbon content remaining after pyrolysis provides the required surface and porosity of the graphitic biochar, whereas the O composition dictates the biochar polarisation potential, as discovered in the findings of [45].
At a pyrolysis time of 2 h, the graphitic biochar had 65.32% carbon content.However, at longer pyrolysis times of 5 h and 10 h, decreases in the carbon content were observed, respectively.Ball milling was found to increase the carbon content of the biochar while reducing the oxygen content.The presence of mineral impurities was also minimised.The ball-milled biochar with 5 h of pyrolysis time had the highest carbon content of 71.27%, with a balance of 26.92% and 1.82% for constituent oxygen and calcium, respectively.Two hours of pyrolysis proved insufficient for the biochar yield and purity, with more impurities than the others, such as Al, K,  Ca, and oxides.This was attributed to the incomplete liberation of volatile matter with a reasonable thermal resistance under these conditions.
Pyrolysis for 10 h produced a lower carbon yield but possessed a higher amount of oxygen (31.61%).A higher oxygen content is an indicator of a higher heating value (CHAV) biochar [46,47].All the biochars produced possessed 0% silica content, which is desirable as silica causes wear in equipment and produces soft and sticky ash with a strong tendency to cause corrosion and affect the heating rate of systems [48].Interestingly, the naturally embedded heteroatoms (K, Al, Ca, etc) in the biochar structure play a pivotal role as electron donors, thereby maintaining the surface hydrophilicity and superior cyclic stability.

Surface morphology
The data for the surface morphology of the obtained biochar samples prepared by low pyrolysis under a limited O 2 supply at different pyrolysis times are presented in figure 3 (a)-(i).The figure shows the micrographs of the biochar samples at 2, 5, and 10 h of pyrolysis at a magnification of × 1500.The morphology was further studied at a higher magnification of × 30000, as shown on the right-hand side of the figure.It is evident from all the biochar samples that pyrolysis at 300 °C for 2-10 h influenced the morphology of the resulting biochar.All the biochars exhibited a granular and irregular structure, synonymous with the predominantly amorphous phase, as observed in the XRD results.The presence of hollow crevices and interstices was noticeable in all samples, indicating the loss of volatile matter from the rose waste during pyrolysis due to gasification.The pore size of the biochar increased with the increase in pyrolysis time from 2 to 5 h, due to the further loss of volatiles in the subsequent 3 h.After 5-10 h of pyrolysis, the pore size and pore size distribution decreased, which can be attributed to the collapse of pores and increased crystallinity [49].The micrographs show that each biochar sample comprises hollow tunnels formed during volatile gasification.The produced biochars were composed of high-quality agglomerated particles.Particle agglomeration results in the formation of larger pores and an increase in the surface area.For hard biomass, a pyrolysis temperature of 300 °C produced biochar with a flat surface and no visible defects [27].However, the biochar from rose waste produced more porous surfaces, particularly after 2-5 h of heating.The biochars from rose waste exhibited a lower bulk density and higher particle density than those of their parent or raw biomass.This trait is important for the potential application of biochar as an efficient host matrix to enhance the dispersion of nanoparticles, the stability of hydrated salts, the adsorption and diffusion of moisture and other active species, the mass transfer capability of associated composites, and the reinforcement of biopolymer composites.This is possible because of their ultrahigh surface area, pore volume, and hierarchically interlinked porous framework [50,51].
Intriguingly, SEM analysis revealed that the graphitic biochar was barely porous and composed of microscale particles, which is a well-known attribute of pyrogenic carbonaceous materials in special morphology.However, the ball-milled biochar displayed ultrafine particles with diameters of less than 100 nm.The major purpose of ball milling is to reduce the particle size and enhance both the internal and external surface areas of the biochar, with minimal influence on the opening of new pores or expansion of existing pores.This is consistent with the results reported previously [52].Thus, the microstructure and hierarchical porous structure coupled with the rich heteroatoms make self-doped biochar a potential candidate for energy storage devices.
The surface morphology and topology of the ball-milled biochar samples, obtained by subjecting the graphitic biochar to ball milling for 10 h, were investigated to determine the effects of ball milling on the morphology of the biochar, as depicted in the SEM micrographs in figure 4(a)-(i).The images show that the biochar ball-milled for 10 h possessed an ordered, definite, and more developed porosity than the graphitic biochars.In addition to the significant size reduction, the ball milling of biochar was observed to influence the shape of the biochar particles.The irregularly shaped particles of the graphitic biochar were transformed into spherical cottony shapes after ball milling for 10 h for all the synthesised samples.In addition, the particle size of the biochar decreased with the increase in pyrolysis time from 2 h to 10 h for all the ball-milled biochar samples.Although the milled graphitic biochar nanostructures agglomerate and become cluster forms, they offer novel morphology with accessible surface areas, enabling them to impact the photocatalytic and adsorption properties of ecofriendly nanocomposites.These clusters of biochar nanostructures also provide abundant microporous sites, giving them a unique feature for wanted applications [53,54].

Surface functional groups
The FTIR spectra of the rose-waste biochar produced by pyrolysis under limited O 2 supply are presented in figure 5(a).The broad spectral band observed in the range of 3000-3500 cm −1 was assigned to the -OH band belonging to the carboxyl and hydroxyl functional groups.This peak appeared broadly and distinctively in the spectra of the biochar pyrolysed for 2 h in limited O 2 , whereas it disappeared as the pyrolysis time reached 5-10 h.One distinct peak was observed, with a peak centre of 1712 cm −1 attributed to the C=O stretching.The presence of O 2 during pyrolysis (though in a limited amount) is responsible for the liberation of most of the surface oxygen present on the graphitic biochar, with the exception of the C=O stretching functional group at a wavelength of 2341 cm −1 corresponding to carboxyl or amide groups.This group predominantly constituted the carbon and oxygen content detected in the EDS results (table 2).This finding was consistent with the results obtained by Gupta et al [55,56].
Surface analysis using FTIR was conducted on various biochar samples exposed to ball milling for 10 h to determine the inherent surface functional groups of the materials.The effects of pyrolysis on the raw rose waste and ball milling of the freshly prepared biochar were investigated.Figure 5(b) shows the FTIR spectra for the ball-milled biochar samples at different pyrolysis times.Four distinct peaks were observed, with peak centres of 1326 cm −1 , 1624 cm −1 (, 2326 cm −1 and 2931 cm −1 .The peak at 1326 cm −1 is attributed to the OH in-plane bending, which corresponds to the presence of a tertiary alcohol or phenol [57]; the 1624 cm −1 peak indicates the presence of C=C stretching, which indicates the presence of lignin in the biochar [58]; 2326 cm −1 indicates the presence of groups with triple bond structures such as C ≡ N (nitriles) and C ≡ C (alkynes) groups on biochar [59]; and 2931 cm −1 is assigned to the hydrogen-bonded OH groups of dimeric COOH groups [60].
Generally, all the ball-milled biochars had similar functional groups, albeit at different intensities.The decrease in peak intensities with increasing pyrolysis time is a clear indication that the surface volatile chemical species liberated from the biochar during pyrolysis increased as the pyrolysis time increased.The -OH hydroxyl single bond at 3000-3500 cm −1 was not detected in the spectra of any of the synthesised biochars.This may be attributed to the presumption that all the single-bond hydroxyl-containing species in this range were released as volatiles during pyrolysis.Similar results were reported by Savitri et al [58] on biochar produced from oil palm empty fruit bunches for Mn 2+ removal from aqueous wastewater.The presence of functional groups makes the samples have more affinity with or be chemically more reactive to other species in their surroundings [8].Our results exhibit that the introduction of functional groups was present without chemical treatment because of the ball-milling in the air.This might be useful for several related applications including fillers and gas absorption.

Raman spectra
From the results obtained for the Raman shift of the biochar prepared by low pyrolysis under limited O 2 (figure 6(a)), the microstructure of the biochars produced two broad and slightly overlapping peaks centred at 1369 cm -1 and 1580 cm -1 representing the D-band and G-band, respectively.The ID/IG ratios for all the samples were less than 1.0, indicating a high level of graphitisation of the synthesised biochars.To produce highly graphitic biochar, the pyrolysis time should be increased while maintaining a constant pyrolysis temperature.This result indicates that more sp 3 carbon bonds were broken and transformed into sp 2 bonds [30,61].
Raman spectra indicating the formation of carbon species based on the evolution of the biochar produced via ball milling at different times are shown in figure 6(b).The spectra were characterised by two apparent peaks ascribed to the sp 2 in-plane carbon domain.The D and G peaks indicated the presence of in-plane vibrations of the highly ordered sp 2 hybrid graphitic carbon structure and the sp 2 hybridisation of the disordered amorphous carbon structures, respectively.The extent of graphitisation or defect formation of the produced biochars on the ball milling of the biochar produced after different pyrolysis times was also measured using the ID/IG ratio.This indicated the transformation of carbon from amorphous to ordered crystalline graphite.The ID/IG ratio increased from 0.71 to 0.88, as the pyrolysis time increased from 2 h to 5 h.This increased ratio is an indicator of the lower transition of biochar from a disordered amorphous state to a crystalline graphite structure due to the  increased pyrolysis time.A similar result was obtained for the wood biochar monolith [62].However, a further increase in the pyrolysis time to 10 h reduced the ID/IG ratio to 0.76 and also resulted in a reduction in the areas of both the D and G peaks.This is attributed to the high-level graphitisation of biochar and lower defect formation on the biochar; however, the lower peak is a sign of a lower biochar yield.A shift in the D and G peaks observed at lower wavenumbers of 1360 cm −1 and 1572 cm −1 , respectively, is related to changes in the molecular chemical bond length in the biochar.This also proves that ball milling alters the chemical structure of the biochar.

XRD analysis
The XRD patterns for the un-milled graphitic biochar produced at pyrolysis times of 2, 5, and 10 h are presented in Fig. 10.All the biochars displayed a characteristic broad peak at 2Θ centred around 21°.This peak indicates the presence of predominantly amorphous carbon in all the samples.Sharp and distinct peaks appear in the diffractogram of each sample, indicating the presence of a crystalline form of carbon and other impurities in the biochar.The intensity of the amorphous carbon decreased, which increased the pyrolysis char, indicating further degradation of the cellulosic component in the biochars.A sharp peak and 2Θ value of 24.5°appeared in all samples, known as the turbostratic graphite-like carbon developed as a result of the parallel orientation of adjacent layered planes of carbon crystallites [63].The calcite (Ca) peak was noticeable for all samples at 2Θ values of 30° [2,64].The calcite peak intensity decreased with the increase in pyrolysis time from 2 to 10 h.This conforms with the Ca composition obtained from the EDS analysis in table 1, where the percentage weight of Ca decreased from 3.54% to 1.37% with the increase in the pyrolysis time at 300 °C.Gupta et al [64] attributed the peaks at 2Θ values of 24.5°to quartz silica on the biochar.This peak was most noticeable in all the biochar samples, indicating that rose biochar is devoid of SiO 2 in its framework.This validates the elemental composition obtained from the EDS results; no silica was detected in the biochar samples at any pyrolysis time.
XRD analysis was conducted on the biochar produced by pyrolysis at 300 °C for different pyrolysis times to study the biomass gasification level and the associated level of crystallinity.The XRD powder patterns for the ball-milled biochar samples produced at different pyrolysis times are shown in figure 7(b).From the figure, it is observed that the XRD powder patterns of the graphitic and ball-milled biochar were similar and displayed high levels of amorphicity.A significant decrease in the intensity of the amorphous carbon diffraction peak was observed after ball milling for 5 h.This is an indicator of the change in the physicochemical properties of the biochar resulting from the excessive shear force and friction during ball milling [45].The amorphous peak also broadened during ball milling, which translates to a significant reduction in the particle size.The intensity of the crystalline carbon graphite and trace mineral peaks increased after ball milling, implying an increase in the overall crystallinity of the biochar.This result is in agreement with the observation from the SEM micrographs and thus establishes that the ball milling process enhances the performance of the surface properties of the biochar.

Optical analysis
The optical features of the produced ball-milled biochar were evaluated using UV-vis diffuse reflectance spectroscopy (UV-vis DRS), while the bandgap energy level of each sample was determined from the extrapolated linear part of the Tauc plot obtained using the Kubelka-Munk function (equation ( 1)).
where α is the absorption coefficient, h is the Plank constant, v is the incident light frequency, hv is the photonic energy, Eg is the optical bandgap energy, A is the transition probability parameter, and n represents the nature of the transition, with a value equal to ½ for a direct allowable transition.The Eg of each sample was determined by drawing a line tangential to the plot of ( ( )) h 2 a n against hv to the point at which it touched the energy axis [65].
The optical bandgap energy of the ball-milled biochar is shown in figure 8(a).The figure shows that the bandgap of the ball-milled biochar decreased with the increase in the pyrolysis time at 300 °C.A decrease in the Eg occurred as a consequence of the hybridisation of the σ and π orbitals, which invariably indicated an increase in the hydrodynamic diameter of the samples [66].The reduction in bandgap improves the response of ballmilled biochar to visible light, increases the electron-hole separation, produces vacant sites, improves the lifetime of photogenerated carriers, and inhibits photogenerated electron-hole recombination [22,67].
Figure 8(b) shows the optical properties of the ball-milled biochar samples measured using UV-Vis DRS.Peaks were observed for different ball-milled biochar samples at wavelengths in the range of 250-800 nm.The UV absorption peak below 380 nm indicated that all the ball-milled biochars produced had a strong absorption potential for UV light.It is also observed from the figure that the absorption of visible light increased with the increase in the pyrolysis time of the ball-milled biochar at 300 °C, of which 10 h of pyrolysis time produced biochar with the highest visible light absorption.Graphitic biochar is known to show the distinct absorption of visible light owing to its black colour, which affects its photocatalytic efficiency.This setback is typically alleviated by the formation of composites with other substances or semiconductors [28].According to [29], an overly thick layer of biochar tends to obstruct visible light absorption, leading to a decrease in the photocatalytic performance.Therefore, the appropriate compositing of biochar with other materials, such as metal oxides, can  enhance the photocatalytic activity of biochar and retain its polymeric structural framework.Our findings in this study confirm that the enhancement of visible light absorption of ball-milled biochar is not only dependent on the small size but also on the large number of small pores on the surface influenced by pyrolysis duration.This demonstrates that ball-milled graphitic biochar synthesised after several hours of pyrolysis at 300 °C is more suitable for photocatalytic applications.

Conclusions
The effect of the low-temperature pyrolysis duration time on the morphology, elemental composition, and structural properties of graphitic biochar from Taif rose oil industry waste was demonstrated.The characterisation results showed that pyrolysis confirms a high content of carbon, minerals, graphitic structure, novel morphology, and chemical characteristics attached to the biochar surface controlled by different pyrolysis durations.Graphitic biochar produced at a pyrolysis temperature of 300 °C for different times of 2, 5, and 10 h was ball-milled for 10 h to enhance the physicochemical properties of the biochar material.Our methodology successfully converted this type of biomass into new products.From the characterisation results, it was established that the ball-milling process led to a significant improvement in morphology, porosity, graphitic crystallinity, surface functional groups, and visible light absorption.Ball milling of biochar was also found to be responsible for the conversion of biochar microparticles to the nanoscale, accompanied by a reduction in the mineral impurities present in comparison with the unmilled samples.Thus, it is concluded that ball-milled biochar from rose waste precursors has special properties that might be quite useful and show strong potential for applications in water treatment, photocatalysis, energy conversion and storage, adsorptive separation processes, and heterogeneous catalysis.

Figure 1 .
Figure 1.Experimental setup of the biochar and milled biochar preparation.

Table 2 .
Chemical composition summary of the biochar and milled biochar samples.