Novel insights into date pit-based activated carbons: H3PO4 and KOH activation

Maryam Ali Al-Kaabi; Nabil Zouari; Mohammad A. Al-Ghouti

doi:10.25259/AJC_20_2025

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Original Article

2025

:18;

202025

doi:

10.25259/AJC_20_2025

Novel insights into date pit-based activated carbons: H₃PO₄ and KOH activation

Maryam Ali Al-Kaabi, Nabil Zouari, Mohammad A. Al-Ghouti^,*

Department of Biological and Environmental Sciences, College of Arts and Sciences, Qatar University, State of Qatar, Doha. P.O. Box: 2713

* Corresponding author: E-mail address: mohammad.alghouti@qu.edu.qa (M.A. Al-Ghouti)

Received: 2025-01-07, Accepted: 2025-04-10, Epub ahead of print: 2025-10-03, Published: 2025-10-30

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Abstract

Different activated carbon preparations were obtained from date pits (DPs). The DPs were chemically activated and then pyrolyzed at various temperatures and were chemically impregnated either by H₃PO₄ or KOH, then pyrolyzed at 500°C or 700°C, respectively, under nitrogen gas flow in a tube furnace. The prepared absorbents were examined using different techniques, such as X-ray diffraction (XRD), electron dispersive X-ray (EDX), scanning electron microscope (SEM), Fourier transform infrared (FTIR), Brunauer-Emmett-Teller (BET), and carbon-hydrogen-nitrogen (CHN) elemental analysis. The results indicated that the prepared materials provided abundant functional groups on the surface and developed pore structures. The research verified the successful carbonization of DPs, producing a material of high carbon content. Structural analysis also found that there were different forms, crystalline and amorphous phases, and thermal stability. The highest BET surface area was obtained with carbonized DPs, chemically activated by H₃PO₄ at 500°C, and the specific surface area increased from 3.042 to 382.8 m²/g. In comparison, the carbonized DPs activated by KOH at 700°C increased from 3.042 to 113.9 m²/g. These results emphasize the significance of activated carbon produced from DPs for a variety of uses.

Keywords

Biomass

Carbon materials

Carbonization

Date pits

Physicochemical characterizations

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1. Introduction

In the “Aceraceae” family, the date palm, or Phoenix dactylifera L., is the most important and valuable fruit tree [1]. It is found in dry, humid, and subtropical regions worldwide, mainly in the area extending from the Middle East to North Africa. Date production was increased to meet the growing demands, which touched about 9.8 million tons in the year 2021 [2], considering that 10% of the overall weight of dates consists of date pits (DPs) [3-5]. This led to a yearly production of more than 980 thousand tons of waste as DP. The date production in Qatar reached 27,000 tons in 2021, in addition to approximately 10,000 tons of dates annually [6].

The latest research shows the use of agricultural waste as a source of activated carbons (AC) with high adsorption potential. [7]. ACs have an activated carbon compound with a high surface area, high porosity, and many surface functional groups. The formation of pores is done during the activation step, which is crucial. The two most common methods of activation are physical and chemical methods.

Both chemical and physical differ in terms of processing methods and energy requirements. Chemical activation is relatively easy to implement and control. It has attracted the most significant interest among the several activation techniques in industrial uses and research, as it generates better yields at lower temperatures. Many chemicals have been used to explore the activation, such as KOH, ZnCl₂, H₃PO₄, K₂CO₃, NaOH, and HCl [8,9]. Potassium hydroxide and phosphoric acid are relevant in commercial applications because of their marked improvement in the porosity and adsorption volume of activated carbon [10]. The processes of activation and carbonization are key steps in the production of activated pyrolyzed carbonization under inert conditions and at higher temperatures, as thermal decomposition increases the carbon content of carbonaceous materials. The activation step improves the pore diameter, surface area, pore volume, and AC porosity [9]. Date pit (DP) is chemically activated with KOH and H₃PO₄, producing AC with mesoporosity-like ZnCl₂ activation [11]. Among those, chemical activation by H₃PO₄ enhances the pore volume and the surface area [12], is easy to recycle and recover into the process, and is eco-friendly [13].

Many researchers performed chemical activation of AC by either H₃PO₄ or KOH. The combined effects of high temperature and chemical oxidation treatment led to a rise in the the surface area [Brunauer-Emmett-Teller (BET)]. For example, DPs chemically activated by 85% H₃PO₄ and then carbonized at 650°C in a muffle furnace exhibited a surface area increase by a factor of 100, raising to 316.9 m²/g, with a pore volume increase to 1.167 cm³/g [14]. Belhamdi et al. produced activated carbon from DPs by H₃PO₄ activation, and the BET surface area increased from 0.098 to 1040 m²/g [15]. Merzougui et al. prepared DPs AC chemically activated by KOH and pyrolyzed at 800°C; BET was 1032 m²/g with a total volume of pores of 1.21 cm³/g, compared to activated carbon without chemical activation, having 0.65 cm³/g and 640 m²/g as total volume of the pore and BET, respectively [11].

In this paper, activated DPs were prepared using two different chemical activators, H₃PO₄ and KOH, as a newly proposed method. The exhibited developed characteristics were evaluated and demonstrated using advanced techniques, including Energy-Dispersive X-ray Spectroscopy (EDX), Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA), Brunauer-Emmett-Teller (BET) surface area, X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Raman, and elemental testing of carbon, hydrogen, and nitrogen (CHN), showing their originality and their potential in applications in wastewater treatment.

2. Materials and Methods

2.1. Material

The DPs were obtained locally from the Qatari market. A 50% (w/w) KOH solution was prepared from potassium hydroxide (KOH) pellets obtained from VWR, and phosphoric acid (H₃PO₄, 85% w/v) was obtained from Sigma-Aldrich. Deionized water (DW) was used in all preparations.

2.2. Activation and carbonization of DPs using H₃PO₄ and KOH

2.2.1. Raw DPs (RDP) preparation (S1)

First, the RDP were rigorously washed with DW to guarantee the removal of all materials stuck in DPs. Then, they were oven-dried overnight at 105°C, grounded, and then sieved using 500 µm sieves. Finally, plastic containers are used to store the samples named RDP (S1) until further use [16-19].

2.2.2. Carbonized DP-AC Preparation (S2, S3, and S12)

RDPs were carbonized inside the tube furnace at two temperatures: carbonization at 500°C named DP-AC-500°C (S2) and carbonization at 700°C named DP-AC-700°C (S3), under 99.9995% puriﬁed nitrogen gas flow, while the sample named S12 (DP-AC-Ar-700°C), was the DPs carbonized by tube furnace at 700°C, under 99% puriﬁed argon gas flow. First, the internal temperature of the tube furnace was calibrated. Then, the location and dimension of the hot zone’s constant temperature were determined.

The temperature was raised from the ambiance to the carbonization temperature of 500°C or 700°C with 30 mL/min nitrogen or argon flow. The holding period was 2 h, and the heating rate was maintained at 10°C/min. With the same nitrogen or argon flow, the system was brought down to an ambient temperature [20].

2.2.3. Chemical activation of DPs using H₃PO₄ (DP-H₃PO₄)

Two types of activated DPs were prepared: the “rinsed then dried” DP-H₃PO_{4 -}rinsed then dried (S4) and the “dried then rinsed” DP-H₃PO_{4 -}dried then rinsed (S6). After being crushed and sieved, the DPs were blended with concentrated H₃PO₄ (85%, w/v), corresponding to a 1.71 g/mL density, with a mass ratio of 1:4 (w/w). The used H₃PO₄ volume per 100 g dry material was 275.2 mL. The mixture was continuously agitated at 85°C for 6 h to guarantee H₃PO₄ access to the interior of the DP material. For DP-H₃PO₄ rinsed and then dried (S4), the mixtures were filtered using a 63 μm sieve, and then the activated DP samples were washed at least five times with DW to ensure the removal of the residual H₃PO₄. To prepare the DP-H₃PO₄ dried and then rinsed (S6), the mixtures underwent centrifugation for 10 min at 10,000 rpm; then the activated DPs were dried for 48 h at 105°C. After chemical activation, the DP samples were thoroughly washed at least five times with DW. The process of DW-washing was conducted until the pH value of the rinsed water was between 6-7 for both samples S4 and S6. Drying of the activated DPs was always performed at a temperature of 105°C overnight, followed by cooling in desiccators and eventually storage in sealed plastic bottles [18,21].

2.2.4. Carbonization of H₃PO₄ activated (ACDPs-H₃PO₄)

Activated carbon date pit (ACDP)-H₃PO₄ rinsed then dried (S5), and ACDP-H₃PO₄ dried then rinsed (S7) were carbonized at 500°C inside a tube furnace under a flow of 99.9995% pure nitrogen gas. First, the internal temperature of the tube furnace was calibrated. Then, the location and dimension of the hot zone’s constant temperature were determined. The temperature was raised from the ambient to the carbonization temperature of 500°C or 700°C with 30 mL/min nitrogen gas flow. The holding period was two h, and the heating rate was maintained at 10°C/min, with the same nitrogen gas flow [18,20-22].

2.2.5. Chemical activation of DPs using KOH (DP-KOH)

Two types of activated DPs were prepared: DP-KOH rinsed then dried (S8), and DP-KOH dried then rinsed (S10). Chemical activation of DPs using potassium hydroxide (KOH) was performed using a 1:4 (w/v) Char to KOH impregnation ratio. The required KOH volume per 100 g dry material was 400 mL of 50% KOH (w/w). The mixture was blended at 85°C for 6 h to guarantee KOH access to the interior of the DP material. For DP-KOH rinsed and then dried (S8), the mixtures were filtered using a 63 μm sieve and then washed with DW. While the DP-KOH dried and then rinsed (S10), the mixtures were centrifuged at 10,000 rpm for 10 mins, then dried for 48 h at 105°C, and washed repeatedly with DW until removal of residual KOH. The washing of the activated product with DW was carried out for both samples S8 and S10 until the pH of the rinsing solution reached 6-7 [17]. The activated DPs were dried again at 105°C overnight, followed by cooling in desiccators and eventually storage in sealed plastic bottles.

2.2.6. Carbonization of KOH-activated DPs (ACDP-KOH)

ACDP-KOH rinsed, then dried (S9), and ACDP-KOH dried, then rinsed (S11), was carbonized in a tube furnace under a flow of 99.9995% pure nitrogen gas at 700°C. First, the internal temperature of the tube furnace was calibrated. Then, the location and dimension of the hot zone’s constant temperature were determined. The temperature was raised from the ambient to the carbonization temperature of 700°C with 30 mL/min nitrogen gas flow. The holding period was two h, and the heating rate was maintained at 10°C/min with the same nitrogen gas flow [17,20].

2.2.7. Characterization of the prepared activated DPs

Various instruments were used to analyze the physical and chemical properties of the prepared adsorbent. FTIR (Spectrum 400, PerkinElmer) was employed to identify functional groups present on the adsorbent’s surface. Surface morphology and characterization were examined using a Nova Nano SEM (Nova Nano SEM 450, ThermoFisher) equipped with a Bruker EDX detector. The thermal stability was determined by the Thermogravimetric Analyzer model Pyris 6 TGA from PerkinElmer. The surface area and pore distribution were analyzed by BET from the Aim Sizer AM301. The XRD form Empyrean Alpha 1 was used to study the adsorbent lattice’s crystalline and possible defect structures.

3. Results and Discussion

3.1. The BET (Brunauer-Emmett-Teller) surface analysis of the synthesized DP adsorbents

The performance of adsorbents is mainly dependent on the available area as well as the porosity of their surface during the process of adsorption. The surface area (BET) and the pore distribution in the prepared adsorbents were examined using the adsorption-desorption isotherm measurement for nitrogen gas at 77 K, explained by barret, Joyner and Halenda (BJH) techniques. Figure 1 compares the BET of DPs and the value of the various prepared DPs. The BET of some of the activated DP was significantly enhanced. Results of Table 1 and Figure 1 show that the BET surface area of DP-AC (S2, S3 & S12) is greater than that of raw date pits (RDP). The pore volume of RDP was 0.006800 cm³/g, and the surface area was 3.042 m²/g, similar to that reported by Al saad et al. for the DPs (0.007987 cm³/g and 2.72 m²/g, respectively) [23]. Moreover, Krishnamoorthy et al., Belhamdi et al., and Wakkel, et al. , reported a smaller specific surface area (SBET) of 0.027 m²/g, 0.098 m²/g, and 0.27 m²/g respectively, but with bigger pore volumes [14, 15,24].

Comparison of the total pore volume and BET (surface area) of the prepared DPs.

Table 1. Textural properties of DPs and prepared adsorbents.

Sample no.	Sample name	Surface area (S_BET) m²/g	Total pore volume cm³/g
S-1	RDP	3.042	0.006800
S-2	DP-AC-500°C	79.376	0.065462
S-3	DP-AC-700°C	129.359	0.097288
S-4	DP-H₃PO₄ rinsed then dried.	2.648	0.006465
S-5	ACDP-H₃PO₄ rinsed then dried.	382.853	0.336215
S-6	DP-H₃PO₄ dried then rinsed.	6.054	0.030378
S-7	ACDP-H₃PO₄ dried rinsed.	150.954	0.144624
S-8	DP-KOH rinsed then dried.	2.030	0.004663
S-9	ACDP-KOH rinsed then dried.	40.135	0.024016
S-10	DP-KOH dried then rinsed.	2.044	0.004727
S-11	ACDP-KOH dried rinsed.	113.921	0.087895
S-12	. DP-AC-Ar-700°C	2.455	0.012010

The DP-AC-700°C (S3) exhibited a higher surface area (129.359 m²/g) than that DP-AC-500°C (S2) (79.376 m²/g), while DP-AC-700°C -Ar (S12) exhibited BET of 2.455 m²/g. The maximum surface area BET was 382.8 m²/g with a pore volume of 0.336215 cm³/g for ACDP-H₃PO₄ rinsed and then dried (S5). Similar characteristics were reported by [14], with a BET of 316.9 m²/g and a pore volume of 1.167 cm³/g, for the chemically activated DP by 85% H₃PO₄ and carbonized at 650°C. The BET rose by a factor of 100 due to the synergistic effects of chemical oxidation treatment and the high temperature. The lowest surface area (BET) was 2.030 m²/g for DP-KOH rinsed, then dried (S8). Furthermore, the total pore volume single point of the prepared DP-AC was higher than RDP. The highest total pore volume was found at 0.336215 cm³/g for ACDP-H₃PO₄ rinsed then dried (S5), and the lowest one was 0.006800 cm³/g for RDP. The total pore volume increased by 50 times because of the synergistic effects of chemical oxidation treatment and the high temperature. Therefore, the results indicate that the sorption properties of activated DP are significantly enhanced with the increase in surface porosity. The BET of the AC prepared from El-Oued DPs using H₃PO₄ was 125.86 m³/g, and 0.039 cm³/g for the pore volume [25]. In RDP chemically activated by H₃PO₄ at 400°C, the BET was 725 m²/g, and the total pore volume was 0.31 cm³/g, while at 450°C, they were 952 m²/g and 0.36 cm³/g, respectively [18]. The SSA (specific surface area) of activated DPs by H₃PO₄ (DPH) was 1040 m²/g [15]. However, the BET of the activated DPs by H₃PO₄ varied from 794 m²/g to 1707 m²/g [26].

The highest specific surface area obtained using KOH as an activation agent of DP AC-DP-KOH (dried, then rinsed) was 113.921 m²/g. Merzougui et al. prepared activated carbons from DPs with KOH (9 mmol) and heat to 800°C, showing a greater BET of 1032 m²/g and a total pore volume of 1.21 cm³/g [11]. Nasir et al. reported chemical activation of date palm fiber-based biochar with KOH and then pyrolysis at 750°C, exhibiting a pore volume and surface area of 0.062068 cm³/g and 1220.2755 m²/g, respectively [27].

Merzougui et al. prepared activated carbon from DPs using KOH (9 mmol) and heating to 800°C and achieved a BET of 1032 m²/g and a total pore volume of 1.21 cm³/g [11]. Nasir et al. reported hydroxide chemical activation of date palm fiber-based biochar KOH, followed by pyrolysis at 750°C, enhanced the pore volume and surface area to 0.062068 cm³/g and 1220.2755 m²/g, respectively [27].

3.2. Effect of temperature on the pore size and surface Area

With increased temperature, the interaction between KOH and carbon becomes more pronounced, and the surface area increases to 1032 m²/g at 700°C. This reaction favors the development of both micropores and mesopores, which increases the total porosity of the activated carbon [11].

Da Silva et al. investigated the effect of KOH concentration and temperature during the activation of Brazil nutshell-derived activated carbon on its surface area and pore structure. As in the other studies, they noted that temperature increase improves the formation of micropores and mesopores and overall porosity. There is a possibility that excess metallic potassium may form, which can weaken the structure and cause a decline in the overall surface area of the material. This is true for very high temperatures [28].

Xu et al. studied the influence of temperature on the pore structure and area of activated carbon surface formed under the treatment of H₃PO₄. Their findings suggest that the surface area reached its maximum value of 1547 m²/g at a temperature of 400°C. The surface area increased with the temperature rise from 300°C to 400°C. On the other hand, temperatures above 400 °C reduced the overall surface area. This might be due to the breakdown of the structure and the buildup of residue. Moreover, at temperatures between 300 to 350°C, the pores were dominated by small micropores. As the temperature rose to 450°C, the predominant form shifted to mesopores and reached 60.46% at the peak level. Porosity was drastically reduced as the temperature was increased beyond the previously mentioned range because most of the pores started shrinking and collapsing. H₃PO₄-treated activated carbon under a heat range of 600 to 800°C displays interlinked networks of pores, which augment the overall pore area and expand the mesoporous structure [29].

Effects of temperature on pore volume in terms of KOH activation, using a carbonization temperature over 700°C, results in the material having a broader pore size distribution, which allows the effective adsorption of both smaller and larger molecules. For H₃PO₄ activation, the prominence of mesopores increases, especially after reaching a temperature of 700°C. But if the heating temperature is further increased, the phosphate residues can start to interpose themselves into the structure, which will likely block several of the pores, thus marginally lowering the overall surface area [11].

3.3. Classification of porous materials

According to Figure 2 and Table 2 related to the porous materials IUPAC classification, the average pore radius of BJH desorption revealed that the DPs were mesoporous Type 5, and the types of hysteresis loops are H3. DP-AC at 500°C and 700°C were Mesoporous Type 4, H3. Table 2 shows all samples’ isotherm types and IUPAC classification for prepared samples, indicating that the prepared DP samples are porous, and SEM micrographs support the conclusion. The isotherms were type I according to nitrogen adsorption and desorption results of seeds chemically ACs. Date seeds produced by chemical activation with phosphoric acid have a specific surface area (BET) of 942,14 m²/g; as a result, an increase in the acidity, such as H₂SO₄/H₃PO₄ ratio, the number of micro- and mesopores increased. Furthermore, because of carbon combustion within the ash structure, elevated temperatures led to a greater degree of microporosity. The activated materials’ sorption properties were improved by their high surface porosity [30].

Isotherms of nitrogen at 77°K adsorption and desorption of all prepared DPs.

Table 2. Isotherm types and IUPAC classification of prepared DPs.

Sample	Sample name	Isotherm types	IUPAC classification on pores
S-1	RDP.	Type 5, H3	Mesoporous (C < 2)
S-2	DP-AC-500°C)	Type 4, H3	Mesoporous (C > 80)
S-3	DP-AC-700°C	Type 4, H3	Mesoporous (C > 80)
S-4	DP-H₃PO₄ rinsed then dried.	Type 5, H2b	Mesoporous (C < 2)
S-5	ACDP-H₃PO₄ rinsed then dried.	Type 4, H3	Mesoporous (C > 80)
S-6	DP-H₃PO₄ dried then rinsed.	Type 5, H3	Mesoporous (C < 2)
S-7	ACDP-H₃PO₄ dried rinsed.	Type 4, H4	Mesoporous (C > 80)
S-8	DP-KOH rinsed then dried.	Type 5, H2b	Mesoporous (C < 2)
S-9	ACDP-KOH rinsed then dried.	Type 4, H4	Mesoporous (C > 80)
S-10	DP-KOH dried then rinsed.	Type 5, H2b	Mesoporous (C < 2)
S-11	ACDP-KOH dried rinsed.	Type 4, H3	Mesoporous (C > 80)
S-12	DP-AC-Ar-700°C	Type 5, H2b	Mesoporous (C < 2)

Eucommia Ulmoides Oliver (EUO) wood tar has been thermochemically reacted with KOH, resulting in the formation of mesoporous AC. Two samples showed H4 hysteresis loops with Type IV adsorption-desorption isotherms, implying the existence of mesopores. The remaining samples exhibited isotherms of type I adsorption-desorption, indicating the existence of micropores [31]. The isotherms demonstrate a mesopore-structured type IV H3 hysteresis loop according to BET analysis of cashew nutshell chemically activated by KOH and pyrolyzed at 600°C [32].

3.4. Analysis of morphology and textural structure by SEM characterization

The textural morphology and structure of the prepared adsorbents were characterized by SEM before and after the chemical and thermal activation. The magnifications of the scanning electron microscope were X500, X1000, X2000, X5000, and X10000. The SEM images illustrated in Figure 3 clearly show that the surface of the RDP (S1) is relatively smooth with a little wrinkle, while DP-AC-700°C, either by nitrogen gas (S3) or argon (S12) gas, shows no significant difference in morphology. The carbonized DP-AC-500°C (S2) has a much rougher surface than DP-AC-700°C (S3). The textural morphology of DP activated with H₃PO₄ showed distinct features, as shown in Figure 4. Rough, uneven, and more porous surfaces were observed in S4 than in the S6 samples. The pores produced can be understood from the fact that DPs were activated with H₃PO₄ through oxidation, which resulted in cleaving carbonate and phosphorus compounds intercalating, leading to pore formation. It indicates that the H₃PO₄ activation procedure is efficient, while DP-H₃PO₄ dried and then rinsed (S7) are much rougher than rinsed and dried S5. Figure 5 demonstrates a significant difference in the morphology of DP-KOH rinsed then dried (S8) and DP-KOH driedthen rinsed (S10). DPs activated through KOH were noted to have smoother textures. The structures of the activated DPs, which were etched by KOH oxidation, were found to have even shapes with honeycomb pores, which indicates a significant increase in porosity. During oxidation processes with KOH, carbonate was produced as well as the intercalation of potassium compounds, which led to pore formation. It indicates that the KOH activation process was also efficient in improving the characteristics of DP. ACDP-KOH rinsed and then dried (S9) showed similar results to those of the activated one with honeycomb structures, but with some broken walls. DP-KOH and ACDP-KOH dried and rinsed showed an uneven honeycomb pore structure.

SEM images of RDP (S1), DP-AC- 500°C (S2); DP-AC- 700°C (S3); and DP-AC- AR-700°C (S12) at different magnifications 500x-2,000x.

SEM images of results of the prepared H3PO4 DPs and carbonized S4, S5, S6, and S7 at different magnifications 500x-2,000x.

SEM images of results of the prepared KOH DPs and carbonized S8, S9, S10, and S11 at different magnifications 500x-2,000x.

Al saad et al. mentioned that the burned DPs had a higher porosity than the raw material, according to SEM micrographs, showing a structure with numerous holes ranging in size from 2 to 17 μm, which are mostly generated by released hot gases [23]. All organic and volatile matters are lost, which leads to the creation of a carbonaceous substance with an advanced pore structure at 500°C [23,33].

The SEM images of the prepared DP-H₃PO₄ show many more cavities of various shapes and sizes on its external surface, in comparison to the almost uniform surface of the DPs. The reason for this could be the evaporation of H₃PO₄ during the activation of DPs, which then causes the creation of space and cavities. These exterior pores serve as the main access points for the micropores and mesopores of the internal adsorbent surface [15]. Girgis and El-Hendawy generated modified active carbons with H₃PO₄ and showed a “phosphate skin” that covers the internal structure of an adsorbent and guards against excessive burn-off throughout the process of acid activation [34]. As a result of this treatment, the adsorbent’s specific surface area becomes low [34,35].

After 1 h of activation of the DPs by KOH, pores were developed with homogeneous honeycomb structures with a diameter of approximately 5 μm, highly porous, even, and well-defined, which improved their entrapment and adsorption capacities [36]. Cashew nutshell-activated carbon produced through KOH activation and pyrolyzed at 600°C clearly showed the formation of a honeycomb-like porous surface [32].

3.5. Study of the elemental composition of the adsorbents by EDX

EDX is the most used technique to study the elemental composition of materials [23]. Figure 6 shows that the highest carbon percentage in dry matter was 98.5% in the carbonized DP-AC -700°C (S3) and the lowest was 60.12% for DP-H₃PO₄ rinsed then dried (S4) The EDX spectrum also showed that DPs contains C and O elements, and the C percentage was 75.1% in the dry matter.

EDX (Energy dispersive X-ray analysis) results of RDP (S1), DP-AC- 500°C (S2); DP-AC- 700°C (S3) and DP-AC- AR-700°C (S12).

DP-AC -700°C (S3) showed a higher carbon content (98.5%) than DP-AC -500°C (S2) (81.5%). The EDX spectrum of DP-AC -500°C (S2) showed its C, O, and K elements components, while DP-AC -AR-700°C (S12) contained a lower carbon composition (86.7%), lower than that by nitrogen gas, as shown in Figure 6.

ACDP- H₃PO₄ dried then rinsed (S7) showed the presence of phosphorous while ACDP- H₃PO₄ rinsed then dried (S5) does not contain phosphorous elements. EDX spectrum of DP-H₃PO₄ dried then rinsed (S6) showed O and C elements that were common to DPs, and the extra P elements of DP H₃PO₄ are from its synthesis by employing H₃PO₄ as an activator. ACDP-H₃PO₄ rinsed then dried (S5) contains higher carbon content (89.8%) than dried then rinsed (76.8%), as presented in Figure 7.

EDX (Energy dispersive X-ray analysis) results of the prepared H3PO4 DPs and carbonized S4, S5, S6, and S7.

ACDP-KOH dried and then rinsed (S11) contains Potassium, while the ACDP-KOH rinsed then dried (S9) are potassium free. The EDX image of DP-KOH (dried and rinsed) (S10) revealed contents of O and C elements that were similar to DPs, as well as the extra K element of DP-KOH, which was a result of employing KOH as an activator. ACDP-KOH rinsed then dried (S9) has higher carbon content (91.8%) than dried then rinsed (86%), as demonstrated in Figure 8.

EDX (Energy dispersive X-ray analysis) results of the prepared KOH DPs and carbonized S8, S9, S10, and S11.

The EDX spectrum suggests that DPs are primarily composed of oxygen and carbon, with 47.3 and 51.04 %, respectively. Chloride, potassium, calcium, and magnesium elements also exist, but with concentrations less than 1%. When the DPs were burned, the percentage of carbon increased to 91%, suggesting that the DPs were almost completely converted into carbonaceous material [23]. The presence of various elements was shown by EDX analysis of ACs derived from spent arabica coffee grounds soaked with H₃PO_4, including P, which might be included in the structure of carbon or create a “phosphate skin” on the materials’ surface [35]. EDX of raw material (Sargassum muticum) and two porous carbons (PCs) prepared by two different activation methods using KOH (KPC) and H₃PO₄ (PPC), respectively, showed that raw Sargassum muticum contains oxygen, carbon, and potassium that constitute 32.28%, 48.33%, and 1.19%, respectively. PPC contains basically oxygen, carbon, and phosphorus that constitute 33.92%, 59.29%, and 6.79%, respectively, while KPC contains basically carbon, oxygen, and sodium, calcium constitutes 40.28%,55.94%, 2.44%, and 1.34%, respectively [37]. Karakehya showed, by using energy dispersive X-ray spectroscopy (EDS), that the external surface of the Lycopodium clavatum spores (LCSs) saturated with chemical activator KOH and then pyrolyzed at 800°C was mostly composed of carbon (81.2%) and oxygen (18.3%) [38]. Following the EDS study, carbon content reduces as activator KOH ratios increase, while oxygen content increases, because of the strong reaction between KOH and the precursor [31].

3.6. Study of the crystalline structure of the adsorbents by using XRD

XRD analysis was conducted to investigate the prepared DP-based adsorbent crystalline structure. The Rigaku X-ray diffractometer was used to conduct the measurement. The Cu-K radiation was applied at room temperature, using an electric current and voltage of 40 mA and 40 kV, respectively, and 1.789 A° for the X-radiation wavelength. The prepared adsorbents were continuously scanned from 5.0 to 90.0 degrees at a 2θ angle. Powered XRD data which is shown in Figure 9; is representative of the DPs and non-carbonized samples S1, S4, S8, and S10 while Figure 10 represents the carbonized samples either at 500°C or at 700°C (S2, S3, S5, S6, S7, S9, S11 and S12).

XRD patterns of S2, S3, S5, S6, S7, S9, S11, & S12 derived from carbonized DPs.

XRD patterns of S1, S4, S8, and S10 derived from DPs.

From the XRD spectra, the broad peak for adsorbents S5 and S7 can be observed between the angle (2θ) of 5° to 12°, another peak can be observed between the angle (2θ) of 17° to 27° for the adsorbents S2, S3, S5, S7, S9, S11 and S12 except for adsorbent S6 in which the peak between the angle (2θ) of 12° to 27° can be observed among the angle 2θ of 38° to 48°.

Actually, the XRD technique helped understand the analysis of certain amorphous components of lignocellulose’s and the changes in cellulose’s crystalline structure [39]. Sharp peaks in the XRD pattern suggest the presence of a crystalline form, whereas broad peaks suggest an amorphous nature of the material [39,40]. The width and the length of peaks were employed to identify the size of the crystal [41]. The anomaly in some XRD spectra is due to the existence of impurities [39,42].

As shown in Figure 9, the synthesized adsorbents S4 prepared by the H₃PO₄ activation have a clear intense crystalline diffraction and sharp peak at 2θ = 15.9°, 18.2°, 20°, 23.7°, 25°, 32°, and 43° which indicates that crystalline carbon is the most dominant component of the product. There is also an intense peak near 2 θ= 15.9°, as well as relatively weak diffraction peaks at 2θ= 13.8°, 26.4°, and 32°, as well as a unique peak at 2θ= 18.2°. For adsorbents, S8 and S10 DP chemically activated by KOH (rinsed then dried) and (dried then rinsed) respectively have almost the same sharp peaks but S10 exhibits sharper peaks than S8 with less intensity for the following 2θ =15.9°, 20°, and 32°, while 2θ = 11.8° have more intensity than for H₃PO₄. The DPs show almost the same sharp peaks but with intensity for 2θ =15.9°, 20°, 23.7°, 25°, and 32°.

Close examination of the intensity showed that after the addition of acid, the intensity significantly changed, indicating that potential changes to the structure of the carbon lattice might have happened [43]. AC performed from spent arabica coffee grounds that have been phosphoric acid (v) impregnated, showed fuzzy and broad peaks at 2θ = 15°–30° in the XRD spectra, indicating that the majority of the carbon is amorphous. The XRD analysis of Epoxy composites reinforced with date palm fiber showed two peaks at approximately 2 θ = 16° and 22°, demonstrating that the raw fibers exhibited an amorphous form attributable to the presence of hemicellulose [44,45]. XRD patterns of AC generated by activating date seeds with KOH showed broad peaks between the angle (2θ) of 20° to 27° [39].

Sargassum muticum was chemically activated with KOH and H₃PO₄ to produce PCs, and according to XRD patterns, the PCs-H₃PO₄ have a noticeable intense peak of crystalline diffraction at 2θ =20–30, which suggests amorphous carbon as the main component of the product. There is also a weak diffraction peak near 2θ = 43° and a strong diffraction peak near 2θ = 25°, demonstrating the massive, irregular, turbostratic graphite crystalline structure of the corresponding carbon materials. KOH has a strikingly different shape from H₃PO₄ based on XRD analysis. The curves of KOH have a sharper peak than those of H₃PO₄ [37]. The peak width indicates the graphitization degree, and the intense and narrow peak indicates good crystallinity [37,46]. Therefore, KOH exhibits better electrical conductivity than H₃PO₄ [37].

The XRD patterns of synthetic cashew nutshell AC pyrolyzed at 600°C and chemically activated with KOH display two large noisy humps without clearly defined peaks except from a small one at 2 θ = 43 and 26 which can be correlated with (100) and (002) and diffraction plane. As a result, both adsorbents exhibit a mostly amorphous form [32]. Two broad reflection peaks were observed in the XRD patterns at around 2θ = 25° and 2θ = 45°for both AC adsorbents AC11 and AC105, which were produced from nut shells in Brazil, using 1:1 and 1:0.5 weight ratios, respectively. These are usually disordered aromatic ring structure peaks, revealing the materials’ amorphous features [28].

The XRD spectroscopy of AC mesoporous created by chemical activation of wood tar EUO (Eucommia ulmoides Oliver) with KOH, revealed the existence of diffraction peaks at 2 θ = 43, graphite skeleton Lattice plane (100), with low intensity and a large range. This finding proposed that the AC had a strong indeterminate structure and was structurally disordered [31].

3.7. Thermogravimetric (TGA) Analysis of the ACs

The thermal stability of the DPs and the prepared activated carbon samples was examined using TGA analysis using a Pyris 6 TGA PerkinElmer instrument (thermogravimetric analyzer) at a temperature range of 28- 640°C. The TGA profile of RDP (S1) is displayed in Figure 11. As the temperature increased, the weight gradually decreased. In temperatures ranging from 50°C to 150°C, the first step represents a 5.35% loss of weight, which refers to moisture release. The second step represents a 48% weight reduction in the range from 150°C to 350°C. Cellulose and other main components’ decomposition are responsible for weight loss. Weight loss of 37% was observed between 350°C and 640°C due to hemicellulose and lignin decomposition. Thermal decomposition of RDPs conducted by Al-Saad et al. in the range of 30°C to 800°C, between 50°C and 200°C, a small peak was detected, which could be attributable to volatile compounds and/or evaporated humidity, which represents an 8% loss of weight. The primary decomposition process was found between the temperatures 200°C and 400°C. This process is separated into two steps: between 230°C and 333°C, a sharp peak was observed, which refers to 40.4% decomposition of the DPs, and a second peak, corresponding to 19.6% of the absorbent’s decomposition, occurs between 330°C and 380°C. The remaining adsorbent for the adsorption process is 32.9% [23]. The reported breakdown temperatures for hemicellulose, cellulose, and lignin range from 210°C to 320°C, 315°C to 400°C, and 150°C to 900°C, respectively [23,47].

Figure 11.

TGA profile RDP (S1), DP-AC- 500°C (S2), DP-AC- 700°C (S3), and DP-AC- AR-700°C (S12).

The loss of weight for the carbonized samples was 12.5% for S2, 10% for S3, 20% for S5, 20% for S7, 23% for S9, 17% for S11, and 15% for S12. TGA results proved that DP and prepared date pit activated carbon (DPAC) have good thermal stability and chemical stability.

Figure 11 represents the DP-AC-500°C (S2) and DP-AC-700°C (S3), and DP-AC-AR-700°C S12 with argon. During the carbonization at 500°C, the weight gradually decreased with increasing temperature, in the range from 28°C to 100°C. The first step shows a 5.5% loss due to moisture loss. Between the temperatures of 100°C and 640°C, the breakdown of some cellulose and other main components, as well as some hemicellulose and lignin in the second stage, results in a 7% weight loss. For carbonization at 700°C, the weight gradually decreased with increasing temperature, in the range from 28°C to 100°C. The first step represents a 4% loss of weight corresponding to the loss of moisture. The degradation of some cellulose and other major components, as well as some hemicellulose and lignin, leads to 6% weight loss in the second step at a temperature between 100°C and 640°C. The TGA model was utilized to examine the DP-AC thermal stability for temperatures ranging from 25°C to 850°C [48]. A weight reduction of 8.3% was found within the range temperatures from 25°C to 165°C, mostly attributed to the evaporation of volatile gases and moisture. DPAC weight loss was found to be 20% within the range of temperature of 165°C to 800°C because of lignin, cellulose, and hemicellulose breakdown to gases and tar [48]. From Figure 11, it is clear that in temperatures between 50°C and 275°C, the first step involved a 10% weight loss (release of moisture). Weight loss of 50% is observed during the second step, which occurs in temperatures ranging from 275°C to 325°C. The degradation of cellulose and other main components might cause this weight loss. The third step demonstrates a 28% reduction in weight at temperatures between 325°C and 640°C, with slow loss of weight due to hemicellulose and lignin decomposition.

Activated biochar was produced from the waste of pomelo peels by H₃PO₄ chemical activation. PPAB lost 10.5% of its weight as the temperature was elevated from 25°C to 300°C, and 21.5% of its weight was lost as the temperature increased from 300°C to 800°C, demonstrating that the PPAB organics matter was rapidly degraded [49]. TGA profile of DP-H₃PO₄ dried then rinsed (S6) is shown in Figure 12. In temperatures ranging from 50°C to 100°C, the first step represents a 6.5% weight loss due to the release of moisture. The second step showed a weight loss of 43% in temperatures between 100°C and 640°C in the second step.

Figure 12.

TGA profile of the prepared H₃PO₄ DPs adsorbents S4, S5, S6, and S7.

The TGA profile of DPKOH rinsed then dried (S8) is also shown in Figure 13. In temperatures between 50°C and 250°C, the first step represents a 12% weight loss. The second step showed a 60% weight loss at temperatures between 250 and 350°C. The third step represents an 18% weight reduction over the temperature interval between 350°C to 640°C, with a gradual loss of weight as a result of the breakdown of lignin and hemicellulose.

TGA graph of prepared KOH DPs adsorbents S8, S9, S10, and S11.

TGA profile of DP-KOH dried and then rinsed (S10) shows that in temperatures ranging from 50°C to 250°C, the first step represents a 12% weight loss. During the second step, a reduction in weight of 58% is observed for the temperature between 250°C and 350°C. The third step involves a weight reduction of 14% for the temperature between 350 and 640°C. The TGA curves of ACs generated from LCSs (Lycopodium clavatum spores) with KOH activation methods revealed a considerable breakdown that initiated at 150°C and continued up to 500°C, with most of the weight loss of around 90%. However, at temperatures above 600°C, a very minor mass loss occurred. At 1000°C, the main mass of the K81, K82, and C81 samples was around 51.5, 69.3, and 84.9 wt%, respectively. The LCSs that had undergone heat treatment (C81) had greater thermal stability than samples K81 and K82, the KOH-activated carbons [38].

3.8. Raman Spectrum study of adsorbents

The Raman spectrum was employed by using a DXR Microscope from Thermo Fisher Scientific using 532 nm a wavelength as the source of excitation, with the power of the laser of 10 , using 10X microscope objectives with a range from 35 to 3,500 cm^-1, and a 100 times scan. Raman Spectroscopy is a non-invasive analytical method that offers comprehensive insights into chemical composition, phase characteristics, polymorphism, crystalline, and molecular interactions. Infrared spectroscopy and Raman spectroscopy were used for the DPs, carbonized, and chemically activated adsorbents. The G bands indicate the graphitization degree, whereas the local defects and disordered sample properties are represented by the D bands [50,51]. In Raman scattering, the D band is commonly associated with a carbon structure in a disordered state. The D band intensity is proportional to the number of defects; as the number of defects rises, the D band intensity correspondingly increases [39,42,52]. Carbon compounds often show broad bands in the region of 1300-1600 cm^-1. The D band, having a wavenumber of about 1350 cm-1, attributed to nanocrystalline carbon, correlates to the breathing modes vibrations of the aromatic carbon ring. On the other hand, the G band at wave number 1580 cm^-1 sp² carbon stretching vibration of carbon pairs, present in the carbonaceous rings and chains structure, is associated with amorphous carbon materials [53]. The D and G bands are typical peaks in the carbon-based materials spectra, and they represent the graphene oxide pattern [23,54]. The D band is not indicative of the chemical composition of the carbon compounds; instead, it represents the sizes and defects in the lattice characteristics of carbon. The D band’s position is mostly dependent on the laser beam’s wavelength, whereas the G band represents the stretching of C-C bonds within the sp² system [23,55].

The results of the Raman spectrum analysis are presented in Figure 14. The patterns of Raman scattering of RDP (S1) did not show any of the two peaks (D and G), and the third peak was not found in the RDP spectra, indicating that AC is formed after DPs are burned or chemically activated. The spectrum of Raman showed strong D and G bands at 1320-1380 cm⁻¹ for the D band and at 1590-1600 cm^-1 for the G band, as well as a peak observed at around 2870–3010 cm⁻¹. Figure 14 shows a rise in the D and G bands’ intensity. However, the D and G bands’ locations remained unchanged. In addition, for the tested materials, the peak heights and the intensity bands ratio of ID/IG, as the peak heights represent vibrational intensity, where the G band corresponds to sp² stretching in ordered graphitic regions, and the D band is linked to sp² breathing vibrations from structural defects. A higher ID/IG ratio indicates more defects, while a lower ratio reflects greater crystalline order [56]. The DP-H₃PO₄ dried then rinsed (S6) exhibited the highest intensity. DP-AC- 700°C (S3) and DP-AC- AR-700°C (S12) exhibit almost the same intensity as the ACDP-KOH rinsed then dried (S9). This may be due to the same carbonization temperature of 700°C. The same outcomes were reported by Al-Saad et al., Mallick et al., and Zhang et al. at around 1588-1590 cm⁻¹ for the G band and 1348-1350 cm⁻¹ for the D band [23,39,50].

Raman scattering patterns of carbonized DPs and chemically activated S2, S3, S5, S6, S7, S9, S11, and S12.

For the DP-H₃PO₄ dried then rinsed (S6), C-H bond out-of-plane vibrations in various aromatic structures lead to a shift in Raman spectra. It could also be related to strong P-C stretching as a result of the production of organophosphorus compounds, respectively [30,57,58]. Ali et al. found that the peak intensity increased as the H₃PO₄ concentration in the acid mixture increased. During the thermophysical treatment, these functional groups would have provided possible pollutant adsorption active sites or surface reaction activation in aqueous/gaseous phases. Additionally, the detected peaks within the range between 1100 cm^-1 and 1300 cm^-1 were indicative of phosphorous compounds, specifically P=O and P–O, which are present in chemically activated material and cause vibrations. In addition, conjugated systems such as keto-enol, diketone, and ketoester exhibit C=O stretching vibrations, and were assigned a sharp peak within the range between 2420– 2430 cm^-1, 2880 cm^-1 CH₂, C-CH₃, aromatic C-H, 1600 cm^-1 Aromatic/hetero ring, Ketone, Carboxylic acid, C=C 1300 cm^-1 Carboxylate salt, Nitro, Aromatic azo [30].

The Raman spectra of activated carbons of LCSs (Lycopodium clavatum spores) prepared by KOH activation exhibited an amorphous structure with G and D bands. The two distinct peaks, associated with the G band of graphite domains and the D band for amorphous domains, were found to be sited in the ranges of 1330–1350 cm^-1 and 1590–1600 cm^-1, respectively. Overlapping peaks showed that the AC sample was primarily amorphous, whereas the separate peaks were indicative of highly organized activated carbon components [38].

3.9. The FTIR analysis of the adsorbents.

FTIR spectroscopy or FTIR analysis, commonly referred to as Fourier Transform Infrared spectroscopy, is an analytical method used to detect polymeric, organic, and occasionally inorganic materials. The FTIR analysis was carried out for the DPs, carbonized DP, and chemically activated DP to illustrate the functional groups present by using the Spectrum 400 FTIR from PerkinElmer with a KBr pellet within the spectrum 4000 to 400 cm^-1 and 4 cm^-1 spectral resolution. It was essential to analyze the adsorbent’s chemical structure by FTIR analysis since the adsorbent’s surface chemistry significantly impacts the capacity of the adsorbents [59]. The spectrum of FTIR reveals different variations in band intensity and shape (Figures 15 and 16). The FTIR spectra of the prepared DPs, summarized in Table 3, indicated the existence of C-H stretching alkane and asymmetric aliphatic C-H stretch over the DP surface appearing at 2923 cm^-1: the presence of C-H stretching vibration alkane; symmetric aliphatic C-H methylene at 2853 cm^-1, presence of unconjugated C=O in xylan (hemicellulose); Ester (carbonyl group); Lactone.; Aldehyde; Cyclopentanone; C=O anhydride; Ketone; Carboxyl groups at 1744 cm^-1; stretching of C-O in hemicellulose and cellulose; Symmetric COO^-; C-O-H deformation in cellulose at 1029 cm^-1. The FTIR spectra show no significant difference between carbonized DP, DP-AC-700 700°C (S3), and DP-AC-AR-700°C (S12).

FTIR spectrum of S1, S4, S6, S8, and S10.

FTIR spectrum of S2, S3, S5, S7, S9, S11, and S12.

Table 3. FTIR wave number and assignment functional group for the prepared DPs adsorbents.

Sample ID	Wavenumber cm^-1	Assignment
S1: RDP	2923	C-H stretches alkane. Asymmetric aliphatic C-H stretch (methylene CH₂ or methyl CH₃).
	2853	C-H stretching vibration alkane. Symmetric aliphatic C-H methylene. C–H stretches in the methylene and methyl groups.
	1744	Unconjugated C=O in xylans (hemicellulose). Ester (carbonyl group). Lactone. Aldehyde. Cyclopentanone. C=O anhydride. Ketone. Carboxyl groups.
	1029	C-O stretching in hemicellulose and cellulose. Symmetric COO^-. C-O-H deformation in cellulose.
S4: DP-H₃PO₄ rinsed then dried.	3362	O-H Stretch alcohol. Stretch aliphatic primary amine N-H.
	2923	C-H stretches alkane. Asymmetric aliphatic C-H stretch (methylene CH₂).
	1742	Unconjugated C=O in xylans (hemicellulose). Ester (carbonyl group). Lactone. Aldehyde. Cyclopentanone. C=O anhydride. Ketone. Carboxyl groups.
	1009	C-O group. C=C alkene. C-OH stretch.
	937	C=C alkene. C-OH stretch. C-H.
	870	C-H deformation of alkene group. C–H deformation in cellulose.
	804	C=C alkene. C-H deformation.
S6: DP- H₃PO₄ dried then rinsed (S6)	2923	Ester (carbonyl group). Lactone. Cyclopentanone. C=O anhydride. C=O aldehyde. Ketone.
S6: DP- H₃PO₄ dried then rinsed (S6)	1593	C=C stretching unsaturated ketone. C=C stretching cyclic alkene. C=C alkene or aromatic. N-H bending amine.
S8: DPKOH rinsed then dried (S8)	3340	O-H Stretch alcohol. N-H stretches aliphatic primary amine.
	1007	C=C alkene. C-OH stretch.
	868	C-H deformation of alkene group.
	809	C=C alkene.
DP-KOH dried and then rinsed (S10)	3349	O-H Stretch alcohol. N-H stretch aliphatic primary amine.
DP-KOH dried and then rinsed (S10)	1011	C=C alkene. C-OH stretch. C-O stretch.

FT-IR spectra of DP-H₃PO₄ rinsed then dried (S4), summarized in Table 3, showed the presence of stretch aliphatic primary amine N-H; stretch alcohol O-H over DP surface appearing at 3362 cm^-1: presence of alkane C-H stretch; asymmetric aliphatic C-H stretch at 2923 cm^-1, presence of unconjugated C=O in xylan (hemicellulose); Ester (carbonyl group); Lactone; Aldehyde; Cyclopentanone; C=O anhydride; Ketone; Carboxyl groups at 1742 cm^-1, presence of C-O group; C=C alkene; C-OH stretch at 1009 cm^-1 and 937 cm^-1, presence of C-H deformation of alkene group; cellulose’s C–H deformation at 870 cm^-1 and presence of C-H deformation at 804 cm^-1.

FTIR spectra of DP- H₃PO₄ dried then rinsed (S6), summarized in Table 3, displayed two peaks only at 2923 cm^-1 and 1593 cm^-1, and the former one representing stretching of C=C in an unsaturated ketone; C=C stretching cyclic alkene; N-H bending amine, and C=C aromatic or alkene. FTIR spectra of DP-KOH dried and then rinsed (S10), as summarized in Table 3, displayed two peaks only at 3349 cm^-1 and 1011 cm^-1, and the former one representing C=C alkene, C-OH stretch, and C-O stretch. FTIR spectra of DP-KOH rinsed then dried (S8), as summarized in Table 3, exhibited the existence of aliphatic primary amine N-H stretch, and O-H Stretch alcohol at 3340 cm^-1, existence of C-O group; C=C alkene; C-OH stretch at 1007 cm^-1, existence of C-H deformation of alkene group at 868 cm^-1, presence of C=C alkene at 809 cm^-1.

Wakkel et al. investigated cationic dye adsorption in an aqueous solution using RDP [24]. The FTIR spectrum of DPs revealed a wide band at 3396 cm^-1 likened to the existence of O-H stretching vibration. It correlates to both bonded and free OH groups (carboxylic acids, alcohols, and phenols), as in lignin, cellulose, and pectin [24,60,61]. Aldehyde molecules C–H stretching is correlated for two neighboring bands at 2922 and 2852 cm^-1. The carbonyl groups (–COOH, –COOCH₃) associated with stretching vibration C=O are observed in 1745 cm^-1 and may be allocated to esters or carboxylic acids [24,62,63]. Xylans (hemicellulose) have a predominance of unconjugated C=O. The alkene group C=C bending mode is indicated by the band 1616 cm^-1. The two peaks shown at 1437 cm^-1 and 1375 cm^-1 are due to the deformation of C-H in hemicellulose/cellulose and lignin / carbohydrates, respectively. The adsorption band at 1240 cm^-1 is coupled with the C-O stretch and syringyl ring present in lignin and xylans. The bands at 1055 cm^-1 and 1156 cm^-1, respectively, correspond to the C-O stretch and C-O-C vibration of ester, carboxylic acid, and alcoholic groups present in cellulose and hemicellulose [24,59,64,65].

The main functional groupings found on RDP’s surface were described to be alkane (C–H), alcohol (O–H), aldehyde, ester, and ketone C=O and carboxylic group (COOH), all of which are directly influenced by adsorption mechanisms processes [24,66].

The activated carbon of El-Oued DPs of Southern Algeria activated by H₃PO₄ at 400°C to treat wastewater exhibited the following peaks: 3400 cm^-1 (N-H amine and imine (valence), 2900 cm^-1 (C–H vibrations in methyl and methylene and groups valence), 2350 cm^-1 (C=N nitriles (valence), 1570 cm^-1 (N=O (valence), 1700 cm^-1 (C=O aldehyde and acetone groups (valence), 1450 cm^-1 C= C Aromatic (valence), 1320 cm^-1 (C-O alcohols and ether groups (valence), and 1091.6 cm^-1 (C-C groups (valence) [25].

The hydroxyl groups of the phenolic function are present in activated carbon derivative from date seeds activated by H₃PO₄ [67], and the carboxylic function gives the surface adsorbents an acidic character. The presence of carbonyl functions provides the surface of the adsorbents with a basic character. The only difference between the activated date seeds and the DPs analyzed is the intensity of the peaks. The primary bands that have been recorded are 1449 cm^-1, 1620-1635 cm^-1, 2362 cm^-1, 2922-2927 cm^-1, and 3436-3444 cm^-1. The existence and vibration of free O-H groups account for the bands found in the range 3436 cm^-1 and 3444 cm^-1 (OH). The bands detected between 2922 cm^-1 and 2923 cm^-1 correspond to the symmetrical valence vibrations and C-H bonds symmetrical in the alkyl group. The peak at 2927 cm^-1 was also found, indicating that C-H was present in both raw and activated date seeds. The vibration of the triple bond of the nitriles C-N can be assigned to the bands absorbed at the 2362 cm^-1 interval. The C = C bonds of aromatics vibration can be assigned to the bands found in the 1620-1635 cm^-1 range. H₃PO₄-activated date seeds include additional bands (1635 cm^-1). The 1149 cm^-1 band has been assigned to the C-O bond vibration, while the C-O elongation is responsible for the broadband at 1149.5 cm^-1. This band was also found in the activated carbon of date seeds, which contained phosphorus and phosphocarbon compounds. The asymmetric elongation of aliphatic P-O in P = OOH, elongation of aliphatic P-O-C, and elongation of aromatics P-O-C could correspond to the 972 cm^-1 band that was only clearly seen in H₃PO₄ activated carbon spectrum. Because H₃PO₄ was used in the activation step, the functional group of P-O exists. The bands at 667 cm^-1 and 568 cm^-1 link to the C-H of aromatic rings [67].

Belhamdi et al. prepared adsorbents from DP and DPH (DPs activated with H₃PO₄) [15]. Strong and broad bands observed in the 3000–3500 cm⁻¹ region are correlated to hydroxyl groups O-H stretching mode, accompanied by hydrogen bending, based on functional group qualitative data on DP and DPH surfaces. The bands found at 2870 cm^-1 and 2923 cm^-1 are due to the symmetric and asymmetric stretching of aliphatic: –CH, –CH₂, and –CH₃. The bands of the region 1640-1700 cm^-1 correspond to the stretching vibrations of lactone and C=O bonds, which are associated with carboxylic acid groups. The ring stretching vibration of the aromatic C]C is assigned to 1452 cm⁻¹ and 1583 cm^-1 bands [15,68]. Methyl compounds –CH₃ stretching vibrations are identified at the band 1386 cm^-1. The stretching vibrations of C–O are responsible for two bands in raw biomass at 1158 cm⁻¹ and 940 cm^-1. The stretching vibration of P-O included in the group P-O-C is attributable to the strong band at 1163 cm ^-1 as a result H₃PO₄ (aromatic bond) activation process [15,69] , P–OR ester compounds are assigned to the weak band at 881 cm^-1. The aromatic C-H out-of-plane bending mode is attributed to the bands at 823 cm⁻¹ for DPH and 875 cm^-1 for DP, which indicate the existence of aromatic benzene rings.

In the FT-IR spectrum of AC, the peak observed at 1220–1080 cm⁻¹ is tentatively linked to several phosphorus-containing species, such as O-C stretching vibrations in P–O–C groups of aromatic compounds, hydrogen-bonded P=O, and P–O–P bonds in polyphosphate chains, and P=OOH which were typical of phospho-carbonaceous and phosphorus compounds present in H₃PO₄ activated carbons [70].

The peak intensity increased as the concentration of H₃PO₄ was increased, indicating the orthophosphoric acid oxidative nature, due to the production of phosphorus - and/or functional groups containing oxygen [20,30,71]. In the region of 1000-1300 cm^-1, a broad transmission band due to the spectrum of the oxidized carbon was attributed to the group of C-O stretching from phenols, ethers, alcohols, esters, and/or acid functional groups. It was also the specific peak of phosphocarbonaceous and phosphorous compounds, which were found on the phosphoric acid-activated surfaces. However, the transmission bands in this range may have overlapped due to the coexistence of oxygen- and phosphorus-containing functional groups. This complicates the identification of a specific functional group. The stretching mode of hydrogen-bonded P=O was attributed to the peak that showed up at 1180–1280 cm^-1. It could also have represented the O-C stretching mode in the P=OOH or P-O-C (aromatic) groups, based on the literature [30,72]. A weak peak was observed in all activated samples corresponding to conjugated C-O stretching at 1118 cm^-1. The wavelength of the transmission in the region of 1100-1000 cm^-1 had a significant signal in raw OFA and varied with acid composition [20,30]. Adsorbents AC1, AC2, and AC3 each had a signal at 1000 cm^-1, which was connected to P-OP chain (polyphosphate) symmetric vibrations or acid phosphate esters ionized P⁺-O. It could be connected to PO₂ and PO₃ symmetric stretching in complexes of phosphate-carbon or asymmetric stretching of P-O-C, P–O stretching (aliphatic and aromatic), P=OOH, bending of P–O–P, P–OH asymmetric stretching, in polyphosphates [30,72]. A peak observed at 673 cm⁻¹ in the lower region is associated with S-O stretching vibrations, arising from sulfuric acid functional groups on the surface. When raw OFA was subjected to a mixture of acids with a higher H₃PO₄ concentration, in the samples with high acid concentration, the peak location was further pushed to lower regions. The existence of stretching P-C and P=S in organophosphorus compounds is likely responsible for the overlapping out-of-plane deformation vibrations of C-H in various aromatic structures [30,57,73]. Because the activated adsorbents have a high content of orthophosphoric acid, the existence of a tiny peak lower than 600 cm^-1 confirmed the presence of a P⁺-O ionized bond (deformation mode), sulfanylidene phosphane ion, and orthophosphate salt [30,58]. The raw oxidized fatty acid (OFA) was physiochemically activated, which caused the elimination of the majority of carbonyl functional groups.

The FTIR patterns of synthetic AC generated from cashew nutshell and activated with KOH then pyrolyzed at 600°C showed a wide newfound peak at 3401 cm^-1 O-H stretching. The intensity of the C-H vibration bands (1369 cm^-1, 1450 cm^-1, 2860 cm^-1, and 2926 cm^-1) has significantly decreased, whereas the strength of the bands associated with bending vibrations of aromatic out-of-plane C-H increased at 754 cm^-1 and 875 cm^-1. These findings point to the thermal instability and chemical alterations of functional groups containing oxygen during activation, as well as a greater aromatization product [32,74].

The activated carbon adsorbents AC11 and AC105, which were produced from nut shells (Brazil) and KOH using weight ratios 1:1 and 1:0.5, respectively, O-H bond stretching vibrations from phenols, alcohols, and carboxyl attributed to the detected bands at 3433 cm^-1 (AC11) and 3438 cm^-1 (AC105). The stretching vibrations of aromatic rings C/C peaks were at 1559 cm^-1 for AC11 and 1564 cm^-1 for AC105. As a result of carbohydrate dehydration during pyrolysis, these aromatic rings form. The peaks at 1064 cm-1 (AC11) and 1058 cm-1 (AC105) are correlated to hydroxyl groups and the C-O stretching vibrations bonds in compounds such as carboxyls, phenols, or alcohols [28]. Du et al. proposed that the porous carbon network was etched by KOH during the activation process, which caused the functional groups C-O, O-H, and C-O [75].

KOH activation procedure through one-step and two-step procedures was used to create AC from Lycopodium clavatum spores (LCSs). The FTIR spectra of K81, K82, and C81 samples were almost identical for the region range from 3300 cm^-1 to 3650 cm^-1, which correlated to stretching vibrations of the absorbed hydroxyl group. The bonds C-N and C-O vibrations in organic materials are represented by the peaks at 1555 cm^-1 and 1624 cm^-1. In contrast, the wide bands between 900-1300 cm^-1 relate to stretching vibrations of the C-O single bonds in esters, ethers, phenols, hydroxyl, and alcohol groups [38].

The O-H stretching vibration, which might be linked to the alcohols, phenols, or carboxyl groups, was the existence of an absorption band of the activated carbon at 3440 cm ^-1. The C-O bonds stretching vibration in alcohols, phenols, ethers, or esters induced the absorption band at 1050 cm^-1. Additional functional groups with oxygen, such as carboxyl and ester groups, were advantageous in enhancing the wettability of activated carbons, which may increase storage capability [31,76].

3.10. CHN (Carbon, Hydrogen, and Nitrogen) composition of the adsorbents

The CHN (carbon, hydrogen, and nitrogen) composition was analyzed by using Flash 2000 & CN Soil–Thermo-Fisher (Table 4). The results show that the highest N content was found in carbonized DP-AC- 500°C (S2) and DP-AC- 700°C (S3) with 2.42% and 2.12%, respectively. RDP, DP treated with H₃PO₄, and KOH had the highest H content. The carbonized sample contained less H. The DP-AC-700°C (S3) had the highest C content at 86.01%. The DP-AC-700°C (S3) was found to be higher than DP-AC-500°C (S2) and DP-AC-Ar-700°C (S3). DP- H₃PO₄ dried then rinsed (S6) has a higher carbon content than DP-H₃PO₄ rinsed then dried (S4), with 63.076% and 45.845%, respectively. However, the carbonized AC-DP-H₃PO₄ has almost the same content of 76.062% and 78.770%, respectively.

Table 4. CHN analysis of the prepared DP adsorbents.

Sample	Sample Name	N %wt	C %wt	H %wt
S-1	RDP	1.144	46.312	6.799
S-2	DP-AC-500°C	2.423	79.722	2.533
S-3	DP-AC-700°C	2.128	86.011	1.291
S-4	DP-H₃PO₄ rinsed then dried.	0.335	45.845	6.564
S-5	ACDP-H₃PO₄ rinsed then dried.	1.160	78.770	2.776
S-6	DP-H₃PO₄ dried then rinsed.	0.588	63.076	5.644
S-7	ACDP-H₃PO₄ dried rinsed.	0.806	76.062	2.564
S-8	DP-KOH rinsed then dried.	0.126	41.160	6.673
S-9	ACDP-KOH rinsed then dried.	0.926	79.075	1.715
S-10	DP-KOH dried then dried.	0.092	39.620	6.445
S-11	ACDP-KOH dried rinsed.	0.347	77.647	1.213
S-12	DP-AC-Ar-700°C	1.247	75.941	1.374

DPKOH rinsed then dried (S8) and DP-KOH dried and then rinsed (S10) have almost similar content of 41.160% and 39.620%, respectively. However, the carbonized chemically activated by KOH AC-DP-KOH have almost similar content, 79.075% and 77.647%, respectively.

For the RDP, the total carbon was 51.61%; the total hydrogen was 6.74%; and the total nitrogen was 0.52% [77]. Gao et al. reported the CHN content of the AC developed from pine wood sawdust, which was chemically activated by H₃PO₄, and reported 49.09% (C), 6.05% (H), and 0.33% (N) [70]. DP chemically activated by KOH and pyrolyzed at 800°C contained 0.43% (C), 75.04% (H), and 0.46% (N) [16].

4. Conclusions

In conclusion, this study successfully prepared and characterized porous DP activated carbon using H₃PO₄ and KOH as chemical activators. It has been established that the surface area, along with the pore volume and all other structural parameters, improved significantly. Activated with H₃PO₄, the sample produced the best results, having a BET surface area of 382.8 m²/g and a pore volume of 0.336 cm³/g. The results from the elemental analysis confirmed the presence of carbonized DP with more than 86% at 700°C. The characterization analysis of the structure determined that the material was comprised of both crystalline and amorphous phases, depending on the temperature of carbonization. The analysis of thermal stability as well as the functional groups of the prepared DPs confirmed that the activation and carbonization steps were achieved successfully. In summary, the primary aim of improving the properties of DP-activated carbon is accomplished, highlighting the possibility of using it in many applications.

Acknowledgment

The authors would like to thank Qatar University for granting access to its advanced analytical facilities. Specifically, the SEM, TGA, Raman, and CHN analyses were performed at the Central Laboratories Unit; the BET analysis was conducted in the Chemistry Department; and the EDX analysis was carried out at the Center for Advanced Materials (CAM). The authors also would like to thank ExxonMobil Research Qatar (EMRQ) for the sponsorship of this research.

CRediT authorship contribution statement

Mohammad A. Al-Ghouti and Nabil Zouari: Conceptualization, Supervision, Visualization. Maryam A. Al-Kaabi, Nabil Zouari, Mohammad A. Al-Ghouti: Formal analysis, Investigation, Methodology, Validation, Writing-Reviewing and Editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Declaration of Generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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1. Introduction

2. Materials and Methods

2.1. Material
2.2. Activation and carbonization of DPs using H3PO4 and KOH

3. Results and Discussion

3.1. The BET (Brunauer-Emmett-Teller) surface analysis of the synthesized DP adsorbents
3.2. Effect of temperature on the pore size and surface Area
3.3. Classification of porous materials
3.4. Analysis of morphology and textural structure by SEM characterization
3.5. Study of the elemental composition of the adsorbents by EDX
3.6. Study of the crystalline structure of the adsorbents by using XRD
3.7. Thermogravimetric (TGA) Analysis of the ACs
3.8. Raman Spectrum study of adsorbents
3.9. The FTIR analysis of the adsorbents.
3.10. CHN (Carbon, Hydrogen, and Nitrogen) composition of the adsorbents

4. Conclusions

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