Date palm (Phoenixdactylifera) leaves, called fronds, are pinnate, compound leaves spirally arranged around the trunk. The fully mature leaf is 4 m long, but ranges from 3 to 6 m, and is 0.5 m wide at the middle midrib that narrows toward both leaf ends. Date-palm tree residues are one of the most significant natural fibers, obtained by the annual palm trees pruning as an agricultural practice. Worldwide, there are approximately 105 million palm trees, and it is estimated that more than 3,675,000 tons of residue are produced seasonally [1]. As these agricultural and horticultural residues are plentiful, widespread and easily accessible, researchers are currently studying their application as raw-material substitutes [2]. Several burning approaches are used to convert the agricultural waste into useful value-added products, which could decrease the cost of waste disposal. There are challenges of using conventional heating vis-a-vis the laser-based combustion. Recently, Gawbah et al. (2018) reported the preparation of silica and silicon carbide from wheat roughage and sesame seed cake by Nd: YAG laser as the combustion source [3, 4]. In addition, rice husk ash, which is rich in silica (92–97%), can be an economically feasible raw material to produce silica gel and powders [5]. Date palm fibers have high cellulose and low lignin content which ensures good mechanical strength; hence, used for making bricks with good thermal characteristics [6], and for producing medium density fiberboard [7].

Laser techniques are being utilized in combustion control and diagnostics [8]. The thermodynamic requirements of a high-compression ratio and high-power density are well fulfilled by laser ignition [9], which can promote the combustion operation and reduce pollutant fashioning [10]. Lasers have appropriate high-power with low divergence for focusing down to the desired size with enough power density to burn samples at high pressure. Lasers with high-power and beam pointing stabilities are fundamental for producing a heating spot at a constant temperature in a fixed position [11].

In general, ceramics are known as polymorphs, with the same type of chemistry but various crystalline structures. Traditionally, ceramics are classified as clay-based materials, and can be known as inorganic and non-metallic matters that are heat treated during processing or use. They tend to be crisp, hard and inert, with ionic or covalent bonding. Portlander is a beneficial prototype for hydrous and cement at high pressure because of its structural and chemical simplicity [12]. These materials produced from the organic materials (agricultural waste) which we treated by the method that we utilized. In this paper, we focus on utilization of laser technology to convert agricultural waste into a useful value-added product; particularly, date-palm-tree residues in producing minerals, portlandite, graphite, which could decrease the cost of waste disposal in the same time.

Date-palm-tree leaves and fibers samples were collected from the institutional garden of Sudan University of Science and Technology in Khartoum, Sudan. Samples were washed with distilled water to clean off stucked soil, debris and other contaminants, and air-dried at room temperature. Dried samples were then ground into powder by using a mortar and pestle. One gram of each sample was placed in a crucible and combusted the in air using an Nd: YAG laser (DORNIER medTech Medilas 5100 fibertom, UK) beam, with 1064 nm wavelength, 40 W output power, power density about 3.258 KW/m², continues mode (CW), and combustion time 50 s. The laser beam was delivered by a single-mode fiber optic cable; with a length of 60 cm and a diameter of 125 μm; the distance between the sample and the fiber-optic cable end was 1 cm. Due to the small spot size of the laser beam, the combustion operation was performed point-by-point in an open air to obtain the ash. The laser was located on a holder, while the crucible was rotated every 50 s, carefully, for approximately 5 mm. This step was done again several times again for accuracy, before the investigations. The experimental work was carried out in the Institute of Laser, Sudan University of Science and Technology, Khartoum, Sudan. Only solid residues after laser combustion have been characterized in this work. Whereas gaseous products were also observed during laser combustion, they were not characterized. These gaseous products may be harmful to the environment; it is recommended to be characterized in future studies.

The non-combusted and combusted palm fiber (sample 1 and sample 2, respectively) and non-combusted and combusted palm leaf (sample 3 and sample 4, respectively) were examined using X-Ray Diffractometer (XRD) (Shimadzu, MAX_X, XRD-7000, Japan), Cu K_α with a scanning speed of 1000°/min [13]. The data were collected for a two-theta (2θ) range of 10°–80° at a step size of 0.0002°. The samples were prepared by careful grinding, before the XRD measurements, using an agate mortar homogenization. XRD patterns were analyzed using the Materials Data, Inc., MDI Jade 0.5 Match Program. Chemical groups in the non-combusted and combusted samples were identified by a Fourier Transform InfraRed (FTIR) spectrometer (Satellite FTIR 5000, Japan). The samples were mixed with dry potassium bromide powder (KBr) at a ratio of 1:100. Through applying adequate pressure, the tablet was prepared for scanning. The FTIR spectra for the samples were collected, in the 400–3500 cm⁻¹ wavenumber range, using an FTIR spectrometer.

The XRD patterns of the palm fiber are depicted in Figure 1. The broad peak displayed here is attributed to an amorphous structure, and some of the other peaks indicate the crystalline species, as indicated in Table 1. The spectra of palm-fiber samples, before and after combustion analyzed by the Match Program are depicted in Figure 1. This figure confirms the existence of non-crystalline silica in the two samples, between 10° and 30° at a 2θ range of 21–23° (see Tables 1 and 2).

For the date-palm-tree fiber sample before combustion (sample 1, Fig. 1); carbon compounds such as acetamide (C₂H₅NO), dinite [NR] (C₂₀H₃₆), ikaite CaCO₃H₂O, hatrurite (Ca₃SiO₅) and Ca₄Si₂O₆(CO₃)(OH)_2, were observed. However, for date-palm-tree fiber sample after the combustion procedure (sample 2, Fig. 1), other new compounds including acetamide (C₂H₅NO), caoxite (C₂H₆CaO₇), caoxite (C₂H₆CaO₇), caoxite (C₂H₆CaO₇), portlandite (Ca(OH)₂), and graphite-2H(C), were observed. Notably, for sample 2, the combustion procedure using laser is highly effective in forming products, which appear in combusted samples such as portlandite (Ca(OH)₂), and graphite-2H(C) at 2θ values of 43.897°, 64.261°, 77.379°, and 77.525°, respectively (see Tables 1 and 2).

The XRD patterns of the date-palm leaves, before combustion (sample 3) and after combustion (sample 4) are shown in Figure 2 displaying the spectra of samples (3 and 4) analyzed by the Match Program [3]. This confirms the existence of non-crystalline silica in all the samples in the broad peak range of 2θ = 21° – 23° for the samples. However, some of the phases that did not appear in sample 3 were formed in sample 4, such as formicaite (Ca(HCO₂)₂), lonsdaleite (C) and cristobalite (SiO₂) at 2θ peaks of 37.555°, 43.799°, 64.169° and 77.296°, respectively (Fig. 2). Moreover, some phases that appeared in sample 3 spectrum disappeared in sample 4 which were moganite (SiO₂) and howlite (C₆H₄(CO)₂C₆ H₄), at the 28.529° and 40.685° peaks, respectively (Fig. 2). The X-ray analyses of the palm leaves are presented in Tables 3 and 4. Phases that appeared in the sample 3 XRD spectrum but, disappeared in sample 4 this may be due to high temperature and high pressure from the focusing of laser photons through the combustion process.

The infrared spectrum reveals the entire molecular-structure information of the investigated samples. From the intensities and frequencies of some of the spectral bands, it is possible to predict the types of functional groups that exist in the molecules. The FTIR spectra for samples 1 and 2 are depicted in Figure 3. The absorbance peak around 660, 420 cm⁻¹ corresponds to the bending and stretching of the siloxane bonds (Si–O–Si) [3]. The peaks at approximately 1032 cm⁻¹ in both samples, and at 1150 cm⁻¹ in sample 2 (1260–700 cm⁻¹) indicate the possible stretching vibration for alkane C–C [14]. In general, the peaks at 1437 cm⁻¹ appear because of the C–O stretching of carbonate [15]. Moreover, the absorbance band at the 1420 cm⁻¹ peak indicates C–O vibrations or C–H deformation in the samples [16]. In general, the 1422 cm⁻¹ peak appears because of the C–O stretching of carbonate [3]. The peak at 1650 cm⁻¹ is indicated the aromatic stretching in palm-fiber structure and increases in sample 2 [17]. The weak band at 1506–1510 cm⁻¹ is because of aromatic C–H. The absorbance peak around 2350 cm⁻¹ is attributed to –C≡N– (Nitrites) and –C≡C– (Alkynes) compounds [16]. The peak in sample 1, around 2924 cm⁻¹, can be indicated the C–H stretching modes; the peak around 940 cm⁻¹ is indicated to the asymmetric stretching of the CH₂ group.

The FTIR spectra of the palm-leaves samples (samples 3 and 4) were investigated, revealing several graphene and silicon bonds (Fig. 4). The new absorbance peak around 2926–2843 cm⁻¹ in sample 4, which did not exist in sample 3, can be indicated the C–H stretching modes. The absorbance peak around 2343 cm⁻¹, because of the –C≡N– (Nitrites) and –C≡C– (Alkynes) compounds [3, 14, 16], increases after combustion (sample 4). In addition, the additional peak formed in sample 4 at 1733 cm⁻¹ maybe refers the carbonyl group (C=O) [17]. The strong peak at 1630 cm⁻¹ is indicated the aromatic stretching in the palm-leaf structure, and decreases after the combustion process [18]. The absorbance band at the 1427 cm⁻¹ peak may indicate C–O vibrations or C–H deformation in the samples [19]. In general, the 1422 cm⁻¹ peak appears because of the C–O stretching of carbonate [20]. The strong absorbance peak at around 1110 cm⁻¹ is indicated the Si–O–Si antisymmetric stretching mode, and the peaks from 790 to 470 cm⁻¹ match siloxane bond (Si–O–Si) stretching and bending vibration [3, 14].

These results indicate that the laser heat has successfully modified the date-palm-tree residues samples by the interaction of laser photons with the molecules. When the high-power laser beam hits the agricultural waste, laser photons absorption and interaction occur. The main laser parameters that can be controlled during combustion include the wavelength, laser power, beam diameter, combustion time, and beam mode. In general, infrared wavelength produces a primarily thermal effect, but to obtain the highest productivity, high laser power is needed. So the power density of the laser is the most important parameter.

It was found, for example, that the combustion of one gram of agricultural waste could be completed in 50 s and 40 W by laser while 10 g required 1.5–10 min and 300–800 W power by microwave and at least 2 h with 1500 W power for conventional heating for 10 gramsminutes and 300–800 W power [21, 22]. Nd: YAG laser is equally effective but is less expensive in energy consumption than the oven for combustion, so it has both positive environmental and economic impacts. Although in recent years fiber lasers have become widespread and are gradually replacing solid-state lasers, Nd: YAG laser has more advantages in the combustion process; it is represented in that the fiber lasers are limited at output peak power of a few kilowatts where increasing the laser peak power leads to fiber damage due to nonlinear effects and high peak power at the output fiber. Despite semiconductor lasers having high-efficiency Nd: YAG Laser overtakes them with its high power.

In comparison, the conventional combustion using electrical heater it is difficult to control burning and furnace consumes more electrical power with more time of burning. Microwave also consumes more electrical power and it is not burning dry material easy and it takes more time and there is wave hazard around the microwave system. By using laser it saves the electrical power and the parameters (power, power density, combustion duration and wavelength) are under control as the results of XRD the crystalline structures of the products from the combustion may by be changed using different power and different combustion duration.

Basically; laser sources have a very small waist; this caused difficulty in combusting and gives a small amount of production. It is expected that using beam expander to increase the expansion of the combustion area and increase the output power will increase the efficiency of combustion. Additional optimization opportunities exist in making the sample move in a production line (moving belt) with a fixed laser source, which would allow the products of combustion in a reasonable amount. Several researchers have done significant investigations on the treating of a great amount of material with laser and the influence of laser parameters. In some set-ups, material move either on a special conveying belt with a constant speed over an area illuminated by a laser beam [23] or by the free-falling method [24]. In another method, a rotating laser beam sweep is installed [25]. Some researchers used a set-up containing many lasers [26, 27]. In addition, to provide the optimum laser parameters (beam spot size, optical power densities, intensity, etc) optics such as lenses beam expander were used [28–30], as well as the passing velocity and distance between laser and material can be also adjusted.

In future, the transformation of a large amount of agricultural wastes using laser-induced combustion; can be an economically method to obtain raw material to produce a lot of useful and high-value materials.

In summary, date-palm-tree fibers and leaves were combusted, using an Nd: YAG laser (1064 nm) with 40 W output power for 50 s. Two different procedures were utilized to identify the crystal structure and functional groups of the date-palm-tree fibers and leaves and their products after combustion. The first was XRD, which utilized to investigate the crystal structure. The XRD patterns of the combusted date-palm-tree fibers and leaves reveal the presence of crystalline materials such as portlandite (Ca(OH)₂), and graphite-2H(C). The second method was FTIR, which utilized to investigate the chemical groups of the four samples. It was found that the palm-fiber samples contained siloxane, aromatic, nitrites, and alkynes. In contrast, the palm-leaf samples contained graphene, silicon, nitrites, alkynes, carbonyl, siloxane, and aromatics. The gaseous products of this combustion method, apart from CO₂ (GHG), may be harmful to the environment; it is recommended to investigate them in future research. The present study provides a foundation for future research to apply laser technology in waste treatment to contribute to a better environment.